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The economic benefits of the Variable-Pitch Screw Launch system

Conceptual illustration of the Variable-Pitch Screw Launch system showing a launch vehicle (white) being accelerated by an adaptive nut (orange) that magnetically couples to variable pitch screws in an evacuated tube. Credits: Phil Swan and Alastair Swan.

The Variable-Pitch Screw Launch (VPSL) system, is a revolutionary ground-based electromagnetic launch technology that leverages magnetic coupling and variable-pitch leadscrews to accelerate payloads to very high exit velocities (e.g., >11,000 m/s) at a fraction of the cost of traditional chemical rockets. In a paper authored by Phil Swan and Alastair Swan of the Atlantis Project, details are presented on how VPSL overcomes limitations of existing mass drivers, such as the switching constraints of linear motors and rail wear in railguns. Phil Swan appeared on The Space Show last January to discuss the concept with Dr. David Livingston.

The capital cost of a VPSL system scales with the square of exit velocity (ΔV2), a significant improvement over the exponential cost growth of chemical propulsion (exp(ΔV/ΔVe )) and the cubic scaling (ΔV3 ) of some linear motor components in mass drivers. The authors present results from a parametric model that estimates a $33 billion USD capital cost (2024 dollars) for a human-rated system capable of accelerating vehicles to escape velocity for Mars missions, positioning VPSL as a game-changer for cost-effective space exploration.

As humans begin to explore and develop space beyond low Earth orbit (LEO), missions to the Moon, Mars, asteroids and beyond will demand significantly higher delta-v than those needed for LEO operations, especially for human round-trips, which nearly double the velocity requirements. High delta-v missions also reduce crew exposure to cosmic radiation and optimize provisions, but the rocket equation—where fuel mass grows exponentially with delta-v—makes traditional rockets increasingly expensive. VPSL is presented as a scalable, infrastructure-based solution that mitigates these costs, offering both economic and environmental benefits. By reducing reliance on chemical propellants, it aligns with global climate goals, marking a pivotal shift toward sustainable spaceflight.

As a starting point for economic considerations the Swans provided a historical context for exploration costs (in 2020 USD) of the Apollo Program ($257 billion), Space Shuttle ($197 billion) and the International Space Station annual costs ($500 million per-person-year; total of $150B to date); with an estimate that the Artemis Program will cost $93 billion through the end of FY2025 (likely over $100 billion by the time Artemis III returns to the Moon according to ChatGPT). Since the dawn of human spaceflight these programs demonstrate the immense financial burden associated with traditional (chemical rocket) spaceflight, yet their broader benefits—economic stimulus, technological innovation, and geopolitical prestige—justify the investment. The aim of VPSL is to reduce these costs dramatically.

The analysis then moves to a cost comparison of all rocket systems using empirical data that show an exponential relationship between launch cost and delta-v reflecting the “tyranny of the rocket equation” where higher velocities require exponentially more fuel, driving up costs for missions beyond LEO, which will become increasingly important as global space agencies push out into the solar system toward high delta-v destinations.

The paper contrasts the economics of rockets with mass drivers where the latter scale as the cube of the velocity (ΔV3) due to increased power demands at higher velocities. VPSL avoids this by converting electrical energy into rotational energy in screws, then transferring it magnetically to the payload, minimizing expensive pulsed-power electronics. For example, scaling a traditional mass driver from 100 m/s to 10,000 m/s increases costs by a million-fold as ΔV3 dominates, but a well designed VPSL mitigates this issue.

Cost curve generated from a digital twin computer model for the Variable Pitch Screw Launcher (dark blue) versus empirical curve fit for all-rocket systems (light blue) showing significant cost savings. Credits: Phil Swan and Alastair Swan

The specific implementation of a VPSL system is presented with an architecture targeting a 22-year Mars outpost program, with launches during Mars transfer windows. The payload is human-rated, assuming fit crews and acceleration couches, and is designed with sufficient capacity for life support, power generation systems, and rocket propulsion for in-space maneuvering as well as decent to the Martian surface.

This VPSL system includes a 774 km submerged floating underwater section, an 83 km underground ramp curving upward, and a 122 km aeronautically supported elevated tube with the exit aperture at an altitude of 15 km. The entire 979 km launching conduit would be evacuated to minimize drag with air locks at both ends, and face East to take advantage of the Earth’s rotation. For a Mars transfer orbit the exit velocity was calculated to be 11,129 m/s taking into account the Earth’s rotation.

VPSL system scale compared to the Hawaiian Islands, the site under consideration for implementation. Credits: Phil Swan and Alastair Swan

VPSL outperforms rockets for high delta-v missions, leveraging fixed infrastructure costs and low marginal launch costs. It’s quadratic cost scaling and sustainable design make it a transformative option when compared to rockets for high delta-v missions.

I reached out to Phil Swan after his appearance on The Space Show to discuss VPSL and he graciously agreed to participate in an interview with me via email to dive deeper into some of the challenges for implementation of the architecture of the Mars mission. His outstanding responses below are backed up with rigorous engineering reasoning and I thank him for his time collaborating with me on this post.

Many of my interview questions arose from public feedback he received from over 125,000 YouTube views of his presentation on VPSL at the International Space Development Conference last May (Section F of the paper). This approach will hopefully help ascertain what actions are needed to realize the system as well as further engineering development needed to advance it’s technical readiness level. The first two questions involve funding mechanisms for implementation.

SSP: There didn’t appear to be a funding mechanism proposed for the VPSL system although there were a few references to features that would provide incentives for investors. Do you envision the project to be funded by private venture capital, governmental sources or a combination through public/private partnerships?

PS: Our funding strategy is designed to attract private investment through a phased development approach, where some liquidity and financial flexibility is offered by allowing employees and early-stage investors to sell shares to later-stage investors as key technical and engineering milestones are met, similar to staged investment rounds in deep-tech ventures. It would be like many other tech startups where for many years the company’s primary focus is growth as opposed to profits. While we anticipate private venture capital to play a significant role, we are also exploring potential government grants or public-private partnerships to support critical advancements. Revenue generation from early-stage prototypes and other technologies we develop along the way may provide additional funding streams, but the most significant returns will come when we enable affordable interplanetary spaceflight.

SSP: The $33.3B price tag included capital and operations costs but I did not see research and development included. While your calculations show that VPSL costs are very competitive and environmentally beneficial when compared to rockets, this system will require significant development costs to reach TRL 9. Do you have an estimate of the R&D budget?

PS: We anticipate the R&D budget to be 10% of the total estimated capital and operational costs. Our research and development efforts thus far have led to substantial reductions in the estimated costs, so strategic investment in R&D can drive down capital expenditures and improve overall system profitability. For example, a while ago our R&D work led to an improvement where we placed grapplers on both sides of the screws instead of just on one side. This innovation dramatically reduces the forces transmitted to the brackets that support the screws. In this sense, R&D serves as a cost-reduction mechanism. If we do the right amount of R&D and focus it on the most important problems, it could end up paying for itself.

SSP: The remainder of interview questions probe deeper into issues identified through public feedback in Section F of your paper. With respect to constructing a 979 km long vacuum tube and designing fast-acting doors to maintain vacuum while allowing high-speed exit of the vehicle, what are the specific engineering requirements and cost estimates for designing and maintaining fast-acting airlock doors capable of sealing a vacuum tube after a vehicle exits at 11,129 m/s, and how do these compare to existing vacuum systems like LIGO (Laser Interferometer Gravitational-Wave Observatory)?

PS: To exit the tube, the vehicle will pass through an already open fast-acting door first, and that door will start closing immediately. The other end of the airlock is covered with a burst disk. The ambient air pressure at the airlock’s altitude (15km) is around 12000 Pa and the pressure inside the tube is 5 Pa. When the vehicle breaks through the burst disk, the rarified outside air will start travelling into the tube at the speed of sound. The fast-acting door needs to finish closing before the ambient air rushing into the tube reaches it. The math in the model estimates that to meet these requirements the airlock needs to be at least 288 m long if the fast-acting door is engineered to close in 1 second. I should add that the fast-acting door can be backed by a second slower door that is designed to achieve a better vacuum seal.

After the vehicle exits, a new membrane needs to be stretched over the end of the tube to from a new burst disk, and then the airlock needs to be pumped down again from 12000 Pa to 5 Pa. Our current model estimates that it will take 10 minutes and cost 312 dollars to pump the air out of the airlock each time we cycle it.

For LIGO, the exterior pressure is roughly 100,000 Pa and its interior pressure is 1.33 × 10⁻⁷ Pa to 2.67 × 10⁻⁷ Pa – which is a vacuum that it has maintained for 25 years. That’s a ratio of ~7e11 to 1. For VPSL, the exterior pressure is 12000 Pa and it has an interior pressure of 5 Pa for a ratio of only ~2.4e3. So, in one sense, LIGO’s vacuum engineering problem is eight orders of magnitude harder than the problem for VPSL. So, what we’re proposing here falls comfortably within established engineering capabilities. But, VPSL introduces operational dynamics that LIGO does not face – such as repeated venting and sealing at the airlocks and high-speed vehicle interaction. So, in another sense, we will be facing some new challenges that LIGO doesn’t have to deal with.

SSP: To address skepticism about sourcing materials robust enough to endure the high speeds, heat, and magnetic forces cost-effectively, you asserted that the choice of steel and aero-grade aluminum would have sufficient engineering margins when compared to rockets. What are the maximum stresses, thermal loads, and electromagnetic forces experienced by steel screws and aluminum tubes at peak speeds, and can existing manufacturing processes scale these materials to a 979 km system without cost escalation?

PS: This question assumes that extreme forces or heat are unavoidable, but that’s not how we approached the problem. From both an engineering and architectural perspective, we began with the constraints of existing materials and designed a system that stays within those limits.

For example, let’s start with the mechanical stresses. If we want a launcher for sending missions to Mars, this creates a requirement – we will need to launch vehicles at a speed of ~11,129 m/s relative to the surface of the spinning Earth. This is the speed at which the maximum mechanical stresses will occur.

The idea is that the spinning screws drive the adaptive nut. It’s basically a leadscrew and nut with a certain gear ratio. To figure out what that ratio needs to be, we first need to figure out how fast we can turn the screws without exceeding the stress limits of existing affordable materials. To ballpark that, we know that the yield strength for M2 High-Speed Steel can reach 1,300 to 2,200 MPa. But let’s assume we use a cheaper steel with a yield strength of 700 MPa and a density, ρ, of 7850 kg/m3. If we apply an engineering factor of 1.5, then we can set the maximum stress, σ, that we want to see in the steel to a value of 467 MPa. The rate that you can spin a spinning pipe without exceeding this level of stress is

[ref] where ω is in radians-per-sec, and ri and ro are the inner and outer radii in meters. Multiplying ω by ro gives the max rim speed of 404 m/s. This is a value similar to what the tips of airliner fans blades reach during takeoff.

From this value we can calculate the maximum slope of the screw flights, which is 11129/404=27. This means that the total force of the screw flights needs to be ~27 times higher than the force you need to accelerate the spacecraft, sled, and adaptive nut.

Since the coupling is magnetic, you can work out the coupling force across the “airgap” per square meter (see math in above linked paper). This works out to be 795775 N/m2, or less than 1 MPa (about 1/500th the internal tensile stress due to the centrifugal forces).

While you didn’t ask about this in the question, I feel that it’s important to mention that for this to work the screws and rails need to be very straight. To achieve that we will need automatic alignment actuators and something akin to an ultra-high-precision GPS system to achieve the required straightness.

You also asked about heating. This is a good question to use to validate the practicality of a launch architecture. For example, if a launcher was 1000 km long and it was made up of 1 million 1-meter segments, and each of those segments heated up by, say, 5 degrees each launch, then you could estimate how much energy was being dissipated as heat rather than being converted into kinetic energy—and it could be a lot. If each segment weighed one ton, heated up by 5°C, and had the heat capacity of water (about 4,200 J/kg·°C), then the total energy lost to heat would be:

1,000,000 segments × 1,000 kg × 5°C × 4,200 J/kg·°C = 21,000,000,000,000 J. That’s 2.1 × 10¹³ joules, or about 5.8 gigawatt-hours of energy lost to heating per launch.

By comparison, the kinetic energy of a 10-ton spacecraft (10,000 kg) in low Earth orbit at 7.8 km/s is:

(1/2) × 10,000 kg × (7,800 m/s)² ≈ 3.0 × 10¹¹ joules

So, the energy lost to heating in this example would be about 70 times greater than the kinetic energy delivered to the payload. In other words, such a launcher would not be very energy efficient.

In other architectures, this heat is generated because the segments rapidly convert energy from one form to another in the process of accelerating the vehicle, and such high-power conversions invariably generate heat. But the VPSL doesn’t rapidly convert energy from one form to another. The kinetic energy in spinning screws is directly channeled into the kinetic energy of the vehicle through what is essentially a magnetic worm gear. So, the screws and guideway will not heat up significantly during a launch because they are not heated up by the process of rapid high-power energy conversion.

Now there is still some friction that generates heat. Even a train on rails will generate some heat due to friction between its wheels and the rails, but the friction and heat generation associated with magnetic levitation systems is low enough that most people think of them as being “frictionless” – even though that’s not entirely true – maglev tracks and magnetic bearings are really just “very low” friction technologies.

SSP: Concerns were raised about potential eddy currents from the spinning screws and electromagnetic interactions causing energy losses and heat buildup which could reduce efficiency. In view of your acknowledgement that more engineering work is needed to quantify these interactions, have you calculated the magnitude of eddy current losses in a VPSL system at peak velocity, and have you designed experiments or computer code to run simulations or small-scale tests to determine how effective uniform magnetic fields and laminated components would be in reducing these losses?

PS: There are devices that are designed to use Eddy currents for braking, and there are technologies, such as magnetic bearings and maglev trains, that are designed to generate far less friction and wear than their mechanical counterparts. We’ve certainly designed devices to explore the limits of the low-friction high-speed magnetic levitation, but given the high speeds involved, we’ve chosen to implement these designs later on our prototyping roadmap. For one of them, we worked with a well-credentialed Ph.D. and an ASME Fellow in the field of rotordynamics and magnetic bearings. We shared our concerns with him about venturing into uncertain or poorly understood engineering territory. He reassured us that he was not aware of any engineering or physics reasons why our proposed technology would not work, and wrote us a letter of support where he stated, “I am confident in the merits of the proposed research.” That said, pushing beyond the speeds already achieved with maglev trains, the world-record-holding magnetic levitation rocket sled track at Holloman Airforce Base, energy storage flywheels, etc. certainly will involve doing more research and experimentation.

In addition to building physical prototypes, we plan to license advanced engineering software and bring on specialized talent to develop a multi-physics simulation using finite element analysis (FEA) techniques. These simulations will be validated through data collected from instrumented small-scale prototypes. They will give us more visibility on a wide variety of performance metrics.

SSP: Regarding fast-acting components, to ensure operational reliability and test real-world applicability of existing technology to VPSL’s extreme speeds, how reliably can electromagnetic grappler pads and actuators maintain synchronization and stability at speeds up to 11,129 m/s, and what are the failure rates of similar systems (e.g., magnetic bearings) under comparable conditions?

PS: It becomes easier to maintain synchronization as the vehicle approaches the muzzle of the launcher because the screw geometry changes more slowly at the muzzle end. Near the beginning, the geometry changes quickly and the grapplers need to reposition more rapidly, but the forces that they need to manage are also much smaller. If you haven’t yet seen Isaac Arthur’s video, “Mass Drivers Versus Rockets”, you should check it out. It has some good clips that show how the screw geometry changes and how the grapplers reposition during a launch.

Compared to ball and roller bearings, magnetic bearings exhibit extremely low failure rates in industrial use due to the lack of mechanical contact. Although, I suppose there must be some failures due to, for example, defective solid-state electronics in the controllers, power surges, corrosion of wires, fouling of sensors, etc.

Getting the failure rate to the level we need it to be at is a well-understood engineering exercise – like perfecting jet engines or building fault tolerance into hard drives. You need to test, iterate, and apply good engineering practices—refinement, redundancy, early fault detection, and so on. We will be building upon a substantial amount of experience that already exists within other industries – we’re not starting from scratch here.

SSP: You mentioned that to maintain investor confidence, you had a roadmap for developing the technology using a combination of physical prototypes and simulated “digital twin” prototypes. To address scalability physics and ensure the system can handle larger payloads effectively, how does magnetic field strength and consistency vary across a 979 km screw system compared to a small prototype, and what payload mass thresholds trigger performance degradation in digital twin simulations?

PS: Magnetic fields are not generated by the launcher’s guideway or screws’ flights (there are fields inside the magnetic bearings and electric motors that support and spin the screws though). Magnetic fields are generated by the adaptive nut and the sled. The strength of the fields between the grappler pads and the screw flights does peak as the vehicle approaches the muzzle end of the launcher. The strength of the fields between the sled and the guideway’s rail is constant during forward acceleration, and then it jumps up to its peak when the vehicle is on the ramp. Some of the small-scale prototypes will explore the same peak field strengths so that we can avoid surprises later as we scale up.

The system’s cost is expected to scale linearly with payload mass and payload mass will not trigger performance degradation. But if we were to go in the other direction, and scale down too far, that may introduce challenges – particularly with respect to vehicle stability and thermal protection during reverse reentry.

In the paper we said that cost scales with the square of delta-v – which is a lot better than the way that chemical rocket cost scales with the exponent of delta-v. However, we haven’t really explored how cost will scale at speeds much beyond 11,129 m/s. If we try to go much faster than that we’ll probably start running into material limits. Switching to more exotic materials will likely alter the cost-versus-delta-v relationship. We certainly do not want to suggest that the technology can scale up to the speeds needed for interstellar travel or anything like that.

SSP: To validate the economic and efficiency claims of VPSL when compared to existing rockets using energy data, have you done a detailed breakdown of the system’s $33.3 billion capital cost compared to the lifecycle cost of a chemical rocket program delivering 9.6 million kg to Mars, showing how much energy is saved by regenerative braking in real-world conditions?

PS: Yes, the capital and operating costs are computed by code within the digital twin and this includes the power savings from regenerative braking. While there has been some analysis of chemical rocket costs, much of our discussion in the public sphere revolves around addressing and correcting overly optimistic claims—particularly those made by Elon Musk—which are often repeated uncritically by some space enthusiasts. For example, this paper attempts to demonstrate that based on empirical evidence there is clearly an exponential relationship between cost and delta-v. As a widely circulated quote attributed to Peter Diamandis says, “Our brains are wired for linear thinking in an exponential world, and its causing us a great deal of strife.”

Personally, I haven’t felt it was in our best interests to publish a study that emphasizes how prohibitively expensive a permanently manned outpost—or a city—on Mars would be using chemical rockets. While some people argue that attempting to settle Mars is fundamentally misguided, I personally don’t share that view since I believe in the potential of launch infrastructure.

But if you think that rockets are the only option available to us, then right now the cost-per-kg to Mars is on the order of 1.2 million USD. While many are excited about Starship and the potential of full reusability, we’re far more cautious about its ability to fundamentally change the cost of spaceflight for the delta-v’s and mission durations required for one-way and round trips to Mars. We’ve shared some of our reasoning and the available data on this – see: https://youtu.be/Apu6nDahjB4 and https://youtu.be/GvqAM9p4hss. In the absence of a game-changing development, sending a million tons to Mars with chemical rockets will cost on the order of ($1.2 x 106 /kg )(1 x 109 kg) = $1.2 x 1015, or 1200 trillion dollars. This isn’t the kind of problem that will “fix itself” anytime soon through experience curve effects.

SSP: Related to your preferred site of Hawaii’s Big Island, the ongoing legal, cultural, and logistical hurdles encountered by Caltech and the University of California in getting approval to build the Thirty Meter Telescope (TMT) seem insurmountable. Native Hawaiian groups and environmentalists who consider Mauna Kea a sacred site have caused a decade of delays. The telescope’s future is still uncertain so a project of the scale of a VPSL system seems very challenging. While your plan to engage with the community in a respectful and productive manner by clearly communicating benefits to the indigenous people like economic opportunity and cultural legacy make sense, it has likely already been tried by the TMT team. Have you identified specific alternate coastal sites with high elevation, low latitude, and access to large bodies of water that may not present such difficult environmental and cultural challenges?

PS: The summit of Mauna Kea is a culturally sensitive area. For many Native Hawaiians, the numerous telescopes located there are seen as an incursion on sacred land. Additionally, the U.S. military has used portions of the mountain’s slopes for training exercises, causing ecological damage. As a result, the local population is particularly sensitive to further disruption and, in many cases, would prefer the mountain be restored to its original, undisturbed state.

The VPSL system’s acceleration segment would be located offshore and underwater, while the ramp portion would be on the island but almost entirely contained within a tunnel. The tunnel would exit well below the summit—away from the existing observatories—through a small opening situated to avoid culturally significant sites. The elevated, evacuated launch tube would be a temporary structure, deployed every two years for a few weeks during Mars transfer windows.

A potential path forward could involve a three-way agreement: the launcher could be used to deploy multiple space telescopes. These offer a path to eventually phase out the existing summit observatories without impacting the scientific community that relies on them. In return, the Hawaiian community would agree to permit the construction and limited use of the launcher, for example during Mars transfer windows and on a few other occasions.

Over time, the Hawaiian people may come to see the launcher not only as a less intrusive alternative but as a source of enduring pride—an opportunity to contribute to humanity’s next great era of exploration. Rather than diminishing their culture, it could elevate it, building upon the proud legacy of the Polynesian navigators who first discovered and settled the islands. This vision, however, must be informed by dialogue with Native Hawaiian leaders and cultural practitioners – not just outreach – to ensure the project is shaped in a way that reflects and respects their values. In this way, Hawai‘i’s role in space exploration could be seen as a modern extension of their deep tradition of voyaging and discovery.

But, if Hawaii choses to pass on the opportunity, there are many alternative sites around the world that would suffice. Developing and characterizing alternative sites simply hasn’t received priority yet.

SSP: What are the projected environmental impacts (e.g., land use, wildlife disruption) and cultural consultation costs for siting a VPSL system on Hawaii, and how do these compare to alternative sites like desert-mountain regions in terms of construction feasibility and community acceptance?

PS: We don’t expect there to be a significant amount of environmental disruption but with an ecology that’s very sensitive, we will need to be careful. The launcher is underwater and should not impede marine life. The ramp is within a shallow tunnel, so it shouldn’t affect ecologies on the surface, but we’d need to come up with a good plan for dealing with the excavated material generated during tunneling. I expect that birds would tend to avoid the elevated evacuated tube. Vehicles will exit the system far offshore and at an altitude of 15 km, so they shouldn’t generate a lot of noise. Rockets, on the other hand, generate a lot of noise and a lot of pollution from their exhaust. By eliminating the need for rocket launches, VPSL’s net benefit to the environment would be enormously positive.

To close out, we view VPSL not just as an engineering challenge, but as a test case for a new kind of sustainable, infrastructure-led approach to spaceflight – one grounded in realism, openness to critique, and collaborative development.

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Minerva Space Settlement and University of Space Exploration

Conceptual illustration of the Minerva Space Settlement in orbit around Jupiter’s moon Ganymede. Credits: Minerva Project Team

Space Settlement Progress typically features the latest advancements in technology that are enabling the settlement of space.  This post will be a little different.  When attending the International Space Development Conference last May I was impressed by a team of students from Highschool Colegiul National Andrei Saguna in Romania, who had conceived of a space settlement in orbit around Jupiter’s satellite Ganymede which they call Minerva.  The project was an entry in the National Space Societies’ Space Settlement Contest, and for which they won a second place award for 9th graders.  While admiring their poster I was approached by Maria Vasilescu, who proudly described their project and agreed to collaborate with me on this post. She spoke perfect English, shared marketing materials (key chains, buttons and bookmarks with QR codes linking to their website) and explained that the primary purpose of Minerva would be a deep space location for a University of Space Exploration.  I was intrigued by the concept and was struck by Maria and her teammates’ enthusiastic vision of humanity’s future in space.  I wanted to know more about what motivated this group of teenagers to come together and create such an imaginative project, as youths like them will be future pioneers on the High Frontier.  Maria agreed to coordinate with her team on an interview via email about Minerva.

The Minerva Project Team and their poster session at ISDC 2023, a second prize winner for 9th graders of the NSS Space Settlement Contest. Credits: Minerva Project Team: clockwise from lower right: Bodean Mircea-Sorin, Ana Radus, Andrei Ioan Prunea, Alexandra Nica, Alexandra Maria Nemes, Maria Vasilescu

SSP: How did the team come up with this Minerva concept?

Minerva: We took inspiration from our school which gave us a lot of opportunities to which we owe a lot and we wanted to build such a university in the final frontier.

SSP: You mentioned stumbling across some obstacles during your journey but sticking together by motivating each other.  Is this an experience you feel comfortable sharing?

Minerva: One of the hardest things was to think about all the aspects that go into making a space settlement as ninth graders, such as the form [Forum on the website], which was decided in the last week, or the economical part. But we managed to meet often and brainstorm to come up with better ideas.

SSP: You said that the project helped you discover your true selves. Can you explain how this came about?

Minerva: We developed ourselves and our passions and we found out what we like because it covers a broad area of subjects beyond science. We managed to see by which area we are drawn to and enjoy actually researching.

SSP: You’ve stated that one of the reasons for building Minerva is to invent new lifestyles different from those that exist on Earth. How do you envision lifestyles changing in space?

Minerva: The university can prepare you for life in space, which will be an important part in the humans’ future, therefore we don’t want to invent new lifestyles, but incorporate space in the ones that already exist.

SSP: You’ve proposed auctioning a Minerva NFT to fund your efforts and future experiments.  Would this be the sole source of financing for the project, and will it be sufficient?  What about simply charging tuition for the USE?

Minerva: Everything on our settlement is given and made by us for the people so they don’t need to have money to buy material things. And because we have worked to make almost everything renewable and green, the funds MinervaNFT will bring are more than sufficient for everything else. And as for tuition, we feel like putting students through an exam such as the one that defines their attendance to USE is stressful enough as it is. However, the students will need to pay for the transport from Earth to the settlement.

SSP: There does not appear to be any trade or economic activity on Minerva, only academic studies. Students may choose to return to Earth or stay on the space station after they complete their studies. If they stay, have you considered the possibility of graduates developing and marketing other industries such as software development, robotics, mining water from Ganymede as rocket fuel, intellectual property on life support systems, or many other potential industries that could arise from scientific innovation that would take place on a space settlement? Or would this be totally an academic institution?

Minerva: It is not a totally academic institution because we have two thirds of the ship which will be occupied by students that remained on the settlement. But here, you don’t need money, everything being provided by us, so people don’t work for money, they work to occupy time, for enjoyment. If they do develop other industries, it will be fully for the greater good of humanity and the future of our kind, not for money.

SSP: The location chosen for Minerva is very challenging from an engineering perspective.  Although Ganymede is not deep in Jupiter’s magnetosphere, and has its own magnetic field which could help mitigate exposure, the location will still have high levels of radiation if unprotected, which complicates the design because much more mass is needed to provide adequate shielding to be safe for humans.  In addition, travel times to Jupiter are quite long even with improved propulsion which you’ve indicated would be as high as four years for students wanting to make the journey.  Finally, solar energy at Jupiter’s remote distance from the sun requires that photovoltaic arrays be enormous to provide sufficient energy. A good compromise might be the asteroid Ceres, which is believed to be 25% water and does not have a magnetic field generating high radiation like what would be experienced at Jupiter.  Others have proposed this asteroid as a good destination for space settlement.  Why not locate the settlement in a more accessible and hospitable environment that might reduce costs? 

Minerva: The main reason we chose such a far away location is precisely because we want to explore as much as possible of the cosmos. It’s not that we don’t want a closer location, it’s just that we know very little about Jupiter and its surrounding moons and further and this university can offer humanity an opportunity to explore it and send the research back to Earth. At the same time, we have taken the radiation into consideration and just how today’s spaceships have protection against it, so how [sic] our settlement, but ten times more efficient.

SSP: The sources of power for Minerva include solar arrays and nuclear fission, but you excluded fusion energy because it is currently experimental.  By the time it will be technologically possible to travel to Jupiter and establish infrastructure that far out in the solar system, we will have developed fusion energy for use on Earth as well as in space.  The preliminary design work for a Direct Fusion Drive for rapid transit to the outer planets has been started by Princeton Satellite Systems and the Fusion Industry Association just came out with their third annual report stating that the industry has now attracted over $6 billion in investment.  When it is feasible to begin work on Minerva, fusion power sources will likely be available. Will you be updating your project plan as new technologies become available? 

Minerva: Of course, we are sure that many aspects of our settlement can be improved by future developments in science, engineering and many other fields. As much as possible, we will incorporate them into our settlement. As mentioned in our paper, when talking about technological advances that may happen, we have to keep up with innovation and incorporate them to help us fulfill every need when travelling to space.

SSP: You raised the concern that Earth is approaching a major crisis with population growth putting a strain on Earth’s vital resources.  You also said that the purpose of the space community is to sustain humanity if Earth’s environment became unfavorable for life.  In selecting the location of Minerva, when considering Mars and its orbital distance, you said that even though it fulfills most of your requirements “…the disadvantage of Mars its it proximity to Earth…” and it “…is too close to our planet in order for us to choose it as the proper placement for the spacecraft.”  Why must Minerva be distant from Earth if the planet is in crisis in the future and why isn’t the orbit of Mars, at 56 million kilometers, considered not far enough away?

Minerva: Mars wasn’t a viable option because, as we have stated before, the purpose of the USE is to gather information and scientific news that can only be found in the farther cosmos. We already know a lot about Mars and planets in close proximity to Earth, we want to venture further, discover and experiment with more than we already have.

SSP: Some surveys say that young people live in fear of the future due to climate change.  Many media outlets amplify this doom and gloom.  However, some economists point out that using the United Nation’s own data from the Intergovernmental Panel on Climate Change, with the predicted increase in temperature by the year 2100, global GDP will be reduced by only 4% to deal with climate related impacts.  Although it is clear that we should eventually reduce our dependance on fossil fuels this is not an existential threat.   Plus, technological innovation continues to improve efficiency in resource utilization, energy development and agriculture, enabling higher standards of living notwithstanding increasing population growth. 

The viewpoint that the Earth is in “crisis” is closely aligned with Elon Musk’s motivation, who believes it is urgent that we become a multiplanetary species, to have a “Plan B” in case of a planetwide catastrophe.  Jeff Bezos has a different perspective, that heavy industrial activity could be moved off world to preserve the Earth’s natural environment and to improve humanities’ standard of living though utilization of unlimited space resources.  

Gerard K. O’Neill saw the promise of space settlement as a way to solve Earth’s problems through the humanization of space.  He saw it as a way to end poverty for all humans, provide high-quality living space that would continue to grow robustly, to moderate population growth without war, famine, dictatorship or coercion; and to increase individual freedom.  Does your team share the same anxiety about the future as other young people: that life on Earth is doomed and therefore, we need to build Minvera as a sanctuary to preserve humanity?  Or do you see it as one among many options for space settlement to improve life on Earth and beyond, as outlined in O’Neill’s vision?

Minerva: We see Minerva as a place where people that are smart and passionate about space have a chance to make scientific discoveries that would be impossible to do on Earth. Aligned with Gerald O’Neil’s [sic] view, we believe that humans should expand into space whether it is as a Plan B or by harvesting resources from other planets or celestial objects. With the help of Minerva, the smartest children of their generation will be able to experience these scenarios and be closer to the future. We don’t see Minerva as a Plan B for humanity, students that have finished their 4 years being able to return to earth, but rather as a place where people can enjoy a stress free and enjoyable environment. Therefore Minerva is preparing smart youngsters to be able to take advantage of any of the two cases. If they choose to remain on Earth, the knowledge that they acquired while in the USE will definitely increase humanity’s survivability against the existential threats mentioned.

SSP: You’ve created a survey [what was earlier referred to as a “Form” and can be found at the “Forum” link on the Minerva website] for anyone to express their opinion about your project and the prospect of living in space.  Will you use this feedback to improve your project? 

Minerva: Maybe in the future, yes. We have encouraged people to complete the survey honestly and there’s always place for improvement for anything. And the second reason is to observe humanity’s view on such a settlement. In creating such a complex space settlement, you need to align your view with the society’s beliefs, them being the ones who will eventually populate it.

SSP: Does your team expect to remain engaged with the project as you progress in your education and after you eventually establish your careers here on Earth?

Minerva: It was certainly an experience we will treasure for a long time, but not everything has to be drawn out. I think this project took a lot of work and effort and we want to invest into something new, see this contest from as many angles as possible while we can. This project like no other can incorporate so many aspects of society from which you can discover your biggest passions. Talking to everyone in our group, we found that each one of us enjoyed a different part of the project and we believe that that was the key to our win. We were all doing something we are passionate about and therefore worked even harder for the final result. Now that we’ve learned what topics intrigue us, we can start doing even more work in that domain. We believe that this project is the perfect opportunity and will open numerous doors in any future career path. We strongly recommend this contest to anyone wondering whether they should put their effort into it or not.

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Space development on the Moon, Mars and beyond featured in 2023 NIAC Phase I Grants

Conceptual illustration of an oxygen pipeline located at the lunar south pole. Credits: Peter Curreri

This year’s list of NASA Innovative Advanced Concepts (NIAC) Phase I selections include a few awards that look promising for space development. For wildcatters (or their robotic avatars) drilling for water ice in the permanently shadowed craters at the lunar south pole and cracking it into hydrogen and oxygen, Peter Curreri of Houston, Texas based Lunar Resources, Inc. describes a concept for a pipeline to transport oxygen to where it is needed. Clearly oxygen will be a valuable resource to settlers for breathable air and oxidizer for rocket fuel if it can be sourced on the Moon. The company, whos objective is to develop and commercialize space manufacturing and resources extraction technologies to catalyze the space economy, believes that a lunar oxygen pipeline will “…revolutionize lunar surface operations for the Artemis program and reduce cost and risk!”.

Out at Mars, Congrui Jin from the University of Nebraska, Lincoln wants to augment inflatable habitats with building materials sourced in situ utilizing synthetic biology. Cyanobacteria and fungi will be used as building agents “…to produce abundant biominerals (calcium carbonate) and biopolymers, which will glue Martian regolith into consolidated building blocks. These self-growing building blocks can later be assembled into various structures, such as floors, walls, partitions, and furniture.” Building materials fabricated on site would significantly reduce costs by not having to transport them from Earth.

A couple of innovations are highlighted in this NIAC grant. First, Jin has studied the use of filamentous fungi as a producer of calcium carbonate instead of bacteria, finding that they are superior because they can precipitate large amounts of minerals quickly. Second, the process will be self-growing creating a synthetic lichen system that has the potential to be fully automated.

In addition to building habitats on Mars, Jin envisions duel use of the concept on Earth. In military logistics or post-disaster scenarios where construction is needed in remote, high-risk areas, the “… self-growing technology can be used to bond local waste materials to build shelters.” The process has the added benefit of sequestration of carbon, removing CO2 from the atmosphere helping to mitigate climate change as part of the process of producing biopolymers.

Graphical depiction of biomineralization-enabled self-growing building blocks for habitats on Mars. Credits: Congrui Jin

To reduce transit times to Mars a novel combination of Nuclear Thermal Propulsion (NTP) with Nuclear Electric Propulsion (NEP) is explored by Ryan Gosse of the University of Florida, Gainesville.

Conceptual illustration of a bimodal NTP/NEP rocket with a wave rotor enhancement. Credits: Ryan Gosse

NTP technology is relatively mature as developed under the NERVA program over 50 years ago and covered by SSP previously. NTP, typically used to heat hydrogen fuel as propellant, can deliver higher specific impulse then chemical rockets with attractive thrust levels. NEP can produce even higher specific impulse but has lower thrust. If the two propulsion types could be combined in a bimodal system, high thrust and specific impulse could improve efficiency and transit times. Gosse’s innovation couples the NTP with a wave rotor, a kind of nuclear supercharger that would use the reactor’s heat to compress the reaction mass further, boosting performance. When paired with NEP the efficiency is further enhanced resulting in travel times to Mars on the order of 45 days helping to mitigate the deleterious effects of radiation and microgravity on humans making the trip. This technology could make an attractive follow-on to the NTP rocket partnership just announced between NASA and DARPA.

Finally, an innovative propulsion technology for hurling heavy payloads rapidly to the outer solar system and even into interstellar space is proposed by Artur Davoyan at the University of California, Los Angeles. He will be developing a concept that accelerates a beam of microscopic hypervelocity pellets to 120 kilometers/s with a laser ablation system. The study will investigate a mission architecture that could propel 1 ton payloads to 500 AU in less than 20 years.

Artist depiction of pellet-beam propulsion for fast transit missions to the outer solar system and beyond. Credits: Artur Davoyan

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A brief history of starship pioneering

The photon rocket on an interstellar voyage exploring exoplanets. Credit: © David A. Hardy / www.astroart.org

Eventually we will get to the stars. It may not happen in our lifetime but its going to happen some day. Adam Crowl has provided a nice historical review of the interstellar pioneers from the last century that worked out the physics of the starships that will take us there. He does this in a chapter he wrote for James and Gregory Benford’s ground-breaking anthology Starship Century which was based on the findings of the 100‐Year Starship Symposium seeded by a DARPA solicitation and executed by NASA back in 2011.

Crowl begins the story with the early days of rocketry pioneered by Tsiolkovsky determining the rocket equation and Goddard and others experimenting with liquid fueled rockets. Tsiolkovsky was the first to come up with the idea of a generation starship (sometimes referred to as a worldship) when he realized that existing chemical propellants would be insufficient to fuel a space ship for interstellar travel.

Artist depiction of an interstellar worldship. Credits: Michel Lamontagne / Principium, Issue 32, February 2021

More practical interstellar craft don’t come on the scene until after WWII when advanced propulsion concepts really take off. The possibility of harnessing light to “push” a rocket, feasible because photons carry momentum, first appeared in science fiction. As it turned out, physicists realized that to generate the needed thrust with light pressure would require enormous amounts of energy, the waste heat of which would vaporized the vessel. Nevertheless, the photon rocket was still being discussed as late as 1972 when I first saw the rendering at the top of this post by David Hardy in the book he coauthored with Patrick Moore called Challenge of the Stars. Fast forward to today, Dr. Young K. Bae’s Photonic Laser Thruster shows great promise if it can be scaled up for interstellar travel.

Diagram depicting the layout of the Photonic Laser Thruster. Credits: Young K. Bae, Ph.D.

In the latter half of the last century, as the physics of nuclear energy and laser technology progressed, we see a proliferation of many concepts for star travel, including various forms of fusion rockets, laser sails, antimatter propulsion and my personal favorite, the Bussard ramjet. Conceived by the physicist Robert Bussard in 1960, the ship eliminates the need to carry fuel by collecting hydrogen from the interstellar medium using a magnetic field as a ram scoop and compresses the gas to fusion temperatures to create thrust. Crowl summarizes some of the physical limitations of the original concept and discusses several physicist’s alternative designs to address them.

One concept that didn’t make it into Crowl’s piece was developed recently by Leif Holmlid and Sindre Zeiner-Gundersen. Called the laser induced annihilation drive, it uses a pulsed laser to initiate “antimatter-like” annihilation reactions in hydrogen fuel producing high velocity K meson elementary particles at relativistic speeds to generate thrust.

Diagram of a laser-induced annihilation generator for space propulsion. Credit: Leif Holmlid and Sindre Zeiner-Gundersen, Acta Astronautica 23 May 2020

When I asked Crowl if he had any updates to some of the starship propulsion concepts he sent me an article penned by an unknown author for Medium that came up with another alternative to address the limitations of the original Bussard Ramjet. The author, who goes by the pseudonym “deepfuturetech”, reminds us like Crowl discussed in his piece, that the cross section ( i.e. the probability that a given atomic nucleus or subatomic particle will undergo a nuclear reaction in relation to the species of the incident particle) of the Bussard ramjet proton-proton fusion reaction is too low to be useful. Deepfuturetech proposes a different fusion mechanism via (p,n) reactions which involve a nucleus capturing a proton and subsequently emitting a neutron. These type of reactions have higher cross sections and could be tested in reactors in the near future. Further analysis is needed to confirm whether these reactions could produce neutrons at sufficiently low energy cost to enable profitable hydrogen fusion.

Artist depiction of a Bussard ramjet. Credits: NASA

Incidentally, Crowl talked about many of these starship concepts at a subsequent Starship Century Symposium held in 2013 by the Arthur C. Clarke Center for Human Imagination in collaboration with the Benford brothers who shared the highlights from their Starship Century anthology summarizing scientific results from the 100‐Year Starship project. You can also get a “Deeper Future View” of his independent research on interesting items not typically covered by the mainstream science media at his blog Crowlspace.

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The thorium molten salt reactor for Earth and space applications

Schematic of the Thorium Molten Salt Reactor for space propulsion applications. Credits: Ajay Kothari / The Space Review

President Joe Biden recently signed into law a sweeping climate bill that will have very little (if any) impact on addressing global warming (a reduction of 0.028 degrees F by 2100). While there are tax credits in the bill for construction of new nuclear power plants over the next 10 years, only two are planned to add to the existing 93 facilities operating today which provide 18% of the U.S. energy production. Most of the funding in the bill is targeted at tax credits for EVs and incentives for renewable sources such and wind and solar which are subject to interruption. Nuclear energy holds enormous promise to offset the carbon emissions associated with fossil fuel energy production and can provide reliable base load power, but it is still plagued by negative public perceptions related to safety and the potential for weapons proliferation.

Is it time to reimagine our approach to sourcing clean energy in general, and nuclear power in particular while at the same time addressing climate change? Ajay Kothari thinks so – by research and development and eventual commercialization of nuclear power plants fueled by thorium rather than uranium. Dr. Kothari describes his vision in the August 1, 2022 issue of The Space Review. He believes that this powerful and sustainable power source “…will solve the world’s energy problem a thousand times over with zero carbon dioxide emission during operation, and it may be the cheapest form of energy production for us.”

“One ton of thorium is roughly equivalent to five million barrels of oil”

Thorium is abundant in the Earths crust making it relatively cheap and therefore, more affordable. It is only slightly radioactive, far less so then uranium and does not contain fissile material making it much safer and easier to moderate (i.e. switch off) in the case of an accident. This would prevent meltdowns unlike conventional reactors which have coolants that operate at much higher pressures and need far more complicated engineering safeguards to prevent disasters.

Thorium molten salt reactors are inherently safe. Flibe Energy is designing a Liquid Fluoride Thorium Reactor (LFTR) and according to the company’s website, “…any increase in operating temperature reduces the density of the salt which in turn, causes the reaction to slow and the temperature to fall. LFTR is also designed with a simple frozen salt plug in the bottom of the reactor core vessel. In the event of power loss to the reactor, the frozen salt plug quickly melts and the fuel salt drains down into a storage tank below – causing a termination in the fission process.”

Once developed for energy production on Earth, the same technology has applications in space. While it would not be used in a booster during launch, a molten salt thorium reactor upper stage, like that shown in the illustration above, could provide an efficient 700 second specific impulse by heating hydrogen as fuel for advanced propulsion for the next few decades until fusion energy comes on line. An added benefit would be that the upper stage reactor could also be used to provide energy at the destination, for example on the Moon or Mars.

“One kilogram of thorium taken from Earth [to the Moon] … can support a 2.6 thermal megawatt plant for a year.”

A thorium reactor was developed at Oak Ridge National Laboratory (ORNL) back in the 1960s but was never commercialized after the then Atomic Energy Commission favored plutonium fast-breeder reactors.

Diagram of the thorium fuel cycle in molten salt reactors. Credits: Flibe Energy

There are challenges to overcome. For example, the thorium fuel cycle is complicated and still produces some radioactive waste, but far less and with much shorter half life when compared to conventional uranium nuclear reactors. But the benefits of this clean, abundant and affordable energy source could make investment by the public and private sector worth the effort.

“With US reserves at 595,000 tons of thorium, we have enough to last us 600 years at current rates.”

Kothari has been a long time proponent of Thorium reactors. He recently gave a talk on the molten salt thorium reactor via Zoom for the University of Maryland now available on YouTube. You can also hear an in-depth discussion of the technology on The Space Show when he was a guest back in October 2021 and when he returns to the show September 13, 2022.

Dr. Kothari agreed to take a deeper dive with SSP into what he calls “Thor – The Life-Saver” through an email interview. If you have questions I didn’t cover about thorium molten salt reactors please leave a comment.

SSP: Dr. Kothari, thank you for taking the time to answer my questions. With respect to the public’s fear of nuclear power in general, the safety of thorium molten salt reactors is certainly an argument in favor to the technology. But aren’t there still risks of nuclear proliferation?

AK: We have more than 400 reactors in more than 40 countries worldwide. We found ways to have countries develop their reactors but have proliferation controls. This idea, the TMSR, creates no Plutonium, and would be easier to monitor. Besides, whether we want [to] or not, other countries WILL do it. Many are. Also we can develop the technology for ourselves and [for] friendly countries OR at the very least, USE IT FOR OURSELVES! How can we deny this incredible opportunity for our (US) populace? Is that fair?

SSP: Flibe Energy appears to be the only U.S. company pursuing LFTR technology. Chicago based Clean Core is focusing on thorium-based fuels to be used in existing pressurized heavy-water reactor designs. What do you think of these two company’s approaches and are you aware of any other thorium reactor development efforts in the U.S, either in private industry or academia?

AK: MIT is developing tech to resolve some of the TMSR issues that would be quite helpful [SSP found this story from MIT Nuclear Reactor Laboratory on deployment of its “…nuclear reactor (MITR) and related testing apparatus as a proving ground for the materials and processes critical to molten-salt-cooled reactors.”]. Others are shown in the chart below with some of them being US based (bottom right).

Color coded map showing global molten salt reactor technology development activities and the sponsoring country/entities. FHR= Fluoride salt-cooled high-temperature reactor. LEU = Low Enriched Uranium. HEU= Highly Enriched Uranium. TRU=Transuranic wastes, i.e. heavier elements than Uranium. Credits: Oak Ridge National Laboratory

SSP: How difficult would it be to adapt this technology for space propulsion and power applications and is it so far off that fusion energy may be available by the time development efforts come to fruition?

AK: In my opinion, …. controlled fusion may be 100, 200 or 50 years away. We have a valley of death … between now and then. This TMSR can fill the gap but can also be used for space propulsion as my diagram above shows. Sure, the TRL of it needs to be brought up, but that’s what we are here for. It would be less heavy than [the] NERVA idea, especially if the chemical processing plant is separated and U233 is used for space propulsion rather than Th232. This would be the idea. The rate of fission is then controlled by the graphite rod moderators/controllers.

SSP: China has been working on a LFTR since 2011 and was recently cleared to start operating the reactor which is a direct descendent of the original experimental design that ORNL studied in the 1960s. It would appear that the Chinese have a significant head start. Is this concerning?

AK; Absolutely. All I can say is that we are idiots.

SSP: One of the disadvantages of thorium reactors is that large upfront costs are needed due to the significant amount of testing and licensing work for qualification of commercial reactors. The reactors also involve high fuel fabrication and reprocessing costs. How would you address these issues to attract investors?

AK: This idea really is a golden nugget, so to speak. The way to attract investors is to bring the TRL up with government (DoE, NASA and DoD) funds. When the light at the end of the tunnel is seen by investors, they will jump in with both feet. It may still be 5-10 years away but if we do not do it soon, (1) it will always remain so, and (2) some other country (China) or many other countries will DEFINITELY move ahead of us!

SSP: Another disadvantage is the presence of a significant level of gamma ray emissions due to Uranium-232 in the fuel cycle. How will this be dealt with safely?

AK: The Gamma ray radiation occurs from Protactinium 233 absorbing another neutron (before it Beta decays) to become Pa234 If it is separated in a chemical processing plant, it would remain easier to handle. From Wiki[pedia]: “The contamination could also be avoided by using a molten-salt breeder reactor and separating the 233Pa before it decays into 233U)”.

SSP: What regulatory and policy changes are needed to realize this technology in the U.S.?

AK: [The] NRC and DoE should allow smaller (~2 MW) size experimental reactors at Universities and research institutions right now.

SSP: On a related note, what efforts can leaders in private industry, academia and government undertake to begin the research and technology needed to commercialize thorium molten salt reactors.

AK: There are a few uncertain items in this nuclear process that Universities, small businesses and government research institutions can resolve. Government agencies need to fund SBIR/STTR type of initiatives to address the following technical issues:

  1. The sustainability of the heat exchangers whether they are to be made of Hastelloy-N or some other composite. This characterization is needed w.r.t. neutron flux intensity, temperature reached and time exposed (in months to years)
  2. The same as above for reactor containment vessels and pipes carrying the hot molten salt.
  3. Chemical separation for in-line or off-line work for Protactinium and U233.
  4. Tritium mitigation ideas (probably using CO2 in closed loop for electricity generation) or sequestration of it for later use in fusion when and if available. Designing and demonstrating tritium separators are key elements of DOE’s solid fuel MSR program at both universities and national laboratories
  5. Gamma ray mitigation or reduction

Thorium doesn’t spontaneously undergo fission – when an atom’s nucleus splits and releases energy that can generate electricity. Left to its own devices it decays very slowly, giving off alpha radiation that can’t even penetrate human skin, so holidaymakers don’t need to worry about sunbathing on thorium-rich beaches.

We don’t have as much experience with Thorium. The nuclear industry is quite conservative, and the biggest problem with Thorium is that we are lacking in operational experience with it. When money is at stake, it’s difficult to get people to change from the norm.

Irradiated Thorium is more dangerously radioactive in the short term. The Th-U cycle invariably produces some U-232, which decays to Tl-208, which has a 2.6 MeV gamma ray decay mode. Bi-212 also causes problems. These gamma rays are very hard to shield, requiring more expensive spent fuel handling and/or reprocessing.

Thorium doesn’t work as well as U-Pu in a fast reactor. While U-233 an excellent fuel in the slow-neutron regime, it is between U-235 and Pu-239 in the fast spectrum. So for reactors that require excellent neutron economy (such as breed-and-burn concepts), Thorium is not ideal.

Proliferation Issues

Thorium is generally accepted as proliferation resistant compared to U-Pu cycles. The problem with plutonium is that it can be chemically separated from the waste and perhaps used in bombs. It is publicly known that even reactor-grade plutonium can be made into a bomb if done carefully. By avoiding plutonium altogether, thorium cycles are superior in this regard.

Besides avoiding plutonium, Thorium has additional self-protection from the hard gamma rays emitted due to U-232 as discussed above. This makes stealing Thorium based fuels more challenging. Also, the heat from these gammas makes weapon fabrication difficult, as it is hard to keep the weapon pit from melting due to its own heat. Note, however, that the gammas come from the decay chain of U-232, not from U-232 itself. This means that the contaminants could be chemically separated and the material would be much easier to work with. U-232 has a 70 year half-life so it takes a long time for these gammas to come back.

The one hypothetical proliferation concern with Thorium fuel though, is that the Protactinium can be chemically separated shortly after it is produced and removed from the neutron flux (the path to U-233 is Th-232 -> Th-233 -> Pa-233 -> U-233). Then, it will decay directly to pure U-233. By this challenging route, one could obtain weapons material. But Pa-233 has a 27 day half-life, so once the waste is safe for a few times this, weapons are out of the question. So concerns over people stealing spent fuel are largely reduced by Th, but the possibility of the owner of a Th-U reactor obtaining bomb material is not.

Molten Salt Reactors

See our full page on Molten Salt Reactors for more info.

One especially cool possibility suitable for the slow-neutron breeding capability of the Th-U fuel cycle is the molten salt reactor (MSR), or as one particular MSR is commonly known on the internet, the Liquid Fluoride Thorium Reactors (LFTR). In these, fuel is not cast into pellets, but is rather dissolved in a vat of liquid salt. The chain reaction heats the salt, which naturally convects through a heat exchanger to bring the heat out to a turbine and make electricity. Online chemical processing removes fission product neutron poisons and allows online refueling (eliminating the need to shut down for fuel management, etc.). None of these reactors operate today, but Oak Ridge had a test reactor of this type in the 1960s called the Molten Salt Reactor Experiment [Wikipedia] (MSRE). The MSRE successfully proved that the concept has merit and can be operated for extended amounts of time. It competed with the liquid metal cooled fast breeder reactors (LMFBRs) for federal funding and lost out. Alvin Weinberg discusses the history of this project in much detail in his autobiography, The First Nuclear Era [amazon.com], and there is more info available all over the internet. These reactors could be extremely safe, proliferation resistant, resource efficient, environmentally superior (to traditional nukes, as well as to fossil fuel obviously), and maybe even cheap. Exotic, but successfully tested. Who’s going to start the startup on these? (Just kidding, there are already like 4 startups working on them, and China is developing them as well).

Loss of coolant accident consequences are significantly different than for Light Water Reactors

– Low driving pressure and lack of phase change fluids

– Guard vessels employed on some designs

– Planned vessel drain down to cooled, criticality-safe drain tanks on some designs

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Dennis Wingo’s strategy for development of cislunar space and beyond

Image credit: NASA/Goddard/Arizona State University

The Cislunar Science and Technology Subcommittee of the White House Office Science and Technology Policy Office (OSTP) recently issued a Request for Information to inform development of a national science and technology strategy on U.S. activities in cislunar space.

Dennis Wingo provided a response to question #1 of this RFI, namely what research and development should the U.S. government prioritize to help advance a robust, cooperative, and sustainable ecosystem in cislunar space in the next 10 to 50 years?

In a prolog to his response Wingo reminds us that historically, NASA’s mission has focused narrowly on science and technology.  What is needed is a sense of purpose that will capture the imagination and support of the American people.    In today’s world there seems to be more dystopian predictions of the future than positive visions for humanity.  We seem to be dominated by fear of “…doom and gloom scenarios of the climate catastrophe, the degrowth movement, and many of the most negative aspects of our current societal trajectory.”  This fear is manifested by what Wingo defines as a “geocentric” mindset focused primarily within the material limitations of the Earth and its environs.

“The question is, is there an alternative to change this narrative of gloom and doom?”

He recommends that policy makers foster a cognitive shift to a “solarcentric” worldview: the promise of an economic future of abundance through utilization of the virtually limitless resources of the Moon, Asteroids, and of the entire solar system.  An example provided is to harvest the resources of the asteroid Psyche which holds a billion times the minable metal on Earth, and to which NASA had planned on launching an exploratory mission this year but had to delay it due to late delivery of the spacecraft’s flight software and testing equipment.

Artist rendering of NASA’s Psyche Mission spacecraft.  Credits: NASA/JPL-Caltech/Arizona State Univ./Space Systems Loral/Peter Rubin

Back to the RFI, Wingo has four recommendations that will open up the solar system to economic development and address many of the problems that cause the geocentrists despair. 

First, we should make the Artemis moon landings permanent outposts with year long stays as opposed to 6 day “camping trips”. This should be possible with resupply missions by SpaceX as they ramp up Starship launch rates (assuming the launch vehicle and lander are validated in the same timeframe, which seems reasonable). Next, we need power and lots of it – on the order of megawatts.  This should be infrastructure put in place by the government to support commerce on the Moon.  By leveraging existing electrical power standards and production techniques, large scale solar power facilities could be mass produced at low cost on Earth and shipped to the moon before the capability of in situ utilization of lunar resources is established.  Some companies such as TransAstra already have preliminary designs for solar power facilities on the Moon.

Which brings us to ISRU.  The next recommendation is to JUST DO IT.  This technology is fairly straightforward and could be used to split oxygen from metal oxides abundant in lunar regolith to source air and steel.  Pioneer Astronautics is already developing what they call Moon to Mars Oxygen and Steel Technology (MMOST) for just this application.

Conceptual illustration of the Lunar OXygen In-situ Experiment (LOXIE) Production Prototype. Credits: Mark Berggren / Pioneer Astronautics

And lets not forget the wealth of in situ resources that could be unlocked via synthetic geology made possible by Kevin Cannon’s Pinwheel Magma Reactor.

Conceptual depiction of the Pinwheel Magma Reactor on a planetary surface in the foreground and in free space on a tether as shown in the inset. Credits: Kevin Cannon

Of course there is water everywhere in the solar system just waiting to be harvested for fuel and life support in a water-based economy.

Illustration of an ice extraction concept for collection of water on the Moon. Credits: George Sowers / Colorado School of Mines

Wingo’s final recommendation is industrialization of the Moon in preparation for the settlement of Mars followed by the exploration of the vast resources of the Asteroid Belt.  He makes it clear that this is more important than just a goal for NASA, which has historically focused on scientific objectives, and should therefore be a national initiative.

“…for the preservation and extension of our society and to preclude the global fight for our limited resources here.”

With the right vision afforded by this approach and strong leadership leading to its implementation, Wingo lays out a prediction of how the next fifty years could unfold. By 2030 over ten megawatts of power generation could be emplaced on the Moon which would enable propellant production from the pyrolysis of metal oxides and hydrogen production from lunar water.  This capability allows refueling of Starship obviating the need to loft propellent from Earth and thereby lowering the costs of a human landing system to service lunar facilities.  From there the cislunar economy would begin to skyrocket.

The 2040s see a sustainable 25% annual growth in the lunar economy with a burgeoning Aldrin Cycler business to support asteroid mining and over 1000 people living on the Moon.

By the 2050s, fusion reactors provide power and propulsion while the first Ceres settlement has been established providing minerals to support the Martian colonies.

“The sky is no longer the limit”

By sowing these first seeds of infrastructure a vibrant cislunar economy will enable sustainable settlement across the solar system. A solarcentric development mythology may be just what is needed to become a spacefaring civilization.

Artist’s concept of an O’Neill space colony. Credits: Rachel Silverman / Blue Origin

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Converting orbital trash to treasure with CisLunar Industries’ Micro Space Foundry

Illustration of orbital debris recycling. Instead of deorbiting after a few missions, debris removal spacecraft can refuel themselves with metal propellant using the Micro Space Foundry extending the lifespan and lowering costs. Credits: CisLunar Industries

CisLunar Industries is developing an innovative way to clean up Earth orbit by recycling spent rocket stages and other orbital debris using their Micro Space Foundry (MSF). In a March 2 presentation to the Future In-Space Operations telecon, CisLunar CEO Gary Calnan described the technology and markets for the MSF, development of which was funded by an SBIR/STTR grant from NASA. There is a vast untapped value chain of metals high above our heads. Over the last 60 years as satellites have been launched into space, the used upper stages have been cluttering up low Earth orbit and beyond. But the trash has value because it is useful material in orbit that has already incurred the launch cost.

The system works by robotically cutting aluminum feedstock off of derelict satellites and then processing the metal through the MSF using electromagnetic levitation furnace technologies originally proven on the ISS for virtually contactless metal recycling and reuse in a weightless environment. The MSF spits out rods of “fuel” to feed a Neumann Thruster on the debris removal spacecraft, which can then be powered to deorbit the target satellite and move on to its next destination. Rinse and repeat. The architecture has the potential to change the economics of the cislunar economy by harvesting a valuable in situ resource while cleaning up Earth orbit at the same time.

The Neumann Thruster, invented by Dr. Patrick “Paddy” Neumann, is an electric propulsion system for in-space use which is a highly adjustable, efficient and scalable method for moving satellites where they are needed. The Neumann Drive uses solid metal propellant and electricity to produce thrust via a pulsed cathodic arc system analogous to an arc welder. Neumann, who created the company Neumann Space to commercialize his invention, explains the physics behind the thruster in a video of an early prototype.

CisLunar Industries has other applications planned for the MSF in an emerging in-space ecosystem. In addition to extruding metallic rods as propellant, the system can fabricate long tubes for large-scale space structures or wires for additive manufacturing enabling an in-space commodities value chain and creating demand for processed metals.

Conceptual illustration of the MSF core processing unit, utilizing a modular design to enable lower cost flexible deployment and multiple products in an emerging cislunar economy. Credits: CisLunar Industries

So how mature is the technology? CisLunar has already demonstrated component validation in the lab taking the system to TRL 4. You can see a video documenting the experiment at timestamp 35:54 here. A parabolic flight to run an experiment in simulated weightlessness is scheduled for later this year. Actual in-space end-to-end demonstration with a Neumann Thruster is planned in 2024 via an agreement with Australian space services company Skykraft.

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Update on the Photonic Laser Thruster and the interplanetary Photonic Railway

Diagram depicting the layout of the Photonic Laser Thruster (PLT). Credits: Young K. Bae, Ph.D.

SSP reported last year on the promise of an exciting new Photonic Laser Thruster (PLT) that could significantly reduce travel times between the planets and enable a Phonic Railway opening up the solar system to rapid exploration and eventual settlement. The inventor of the PTL, Dr. Young K. Bae has just published a paper in the Journal of Propulsion and Power (behind a paywall) that refines the mathematical underpinnings of the PLT physics and illuminates some exciting new results. Dr. Bae shared an advance copy of the paper with SSP and we exchanged emails in an effort to boil down the conclusions and clarify the roadmap for commercialization.

Illustration of a Photonic Railway using PLT infrastructure for in-space propulsion established at (from right to left, not to scale) Earth, Mars, Jupiter, Pluto and beyond. Credits: Young K. Bae.

In the new paper, Dr. Bae refines his rigorous analysis of the physics behind the PLT confirming previous projections and discovering some exciting new findings.

As outlined in the previous SSP post linked above, the PLT utilizes a “recycled” laser beam that is reflected between mirrors located at the power source and on the target spacecraft. Some critical researchers have argued that upon each reflection of the beam off the moving target mirror, there is a Doppler shift causing the photons in the laser light to quickly lose energy which could prevent the PLT from achieving high spacecraft velocities. The new paper conclusively proves such arguments false and confirming the basic physics of the PLT.

There were two unexpected findings revealed by the paper. First, the maximum spacecraft velocity achievable with the PLT is 2000 km/sec which is greater than 10 times the original estimate. Second, the efficiency of converting the laser energy to the spacecraft kinetic energy was found to approach 50% at velocities greater than 100 km/s. This is surprisingly higher than originally thought and is on a par with conventional thrusters – but the PLT does not require propellent. These results show conclusively that once the system is validated in space, the PLT has the potential to be the next generation propulsion system.

I asked Dr. Bae if anything has fundamentally changed recently in photonic technology that will bring the PLT closer to realization. He said that the interplanetary PLT can tolerate high cavity laser energy loss factors in the range of 0.1-0.01 % that will permit the use of emerging high power laser mirrors with metamaterials, which are much more resistant to laser induced damage and are readily scalable in fabricating very large PLT mirrors.

With respect to conventional thrusters, he said the PLT can be potentially competitive even at low velocities on the order of 10 km/s, especially for small payloads. This is because system does not use propellant which is very expensive in space and because the PLT launch frequency can be orders of magnitude higher than that of conventional thrusters. Dr. Bae is currently investigating this aspect of the system in terms of space economics in depth.

The paper acknowledges that one of the most critical challenges in scaling-up the PLT would be manufacturing the large-scale high-reflectance mirrors with diameters of 10–1000m, which will likely require large-scale in-space manufacturing. Fortunately, these technologies are currently being studied through DARPA’s NOM4D program which SSP covered previously and Dr. Bae agreed that they could be leveraged for the Photonic Railway.

Artist’s concept of projects, including large high-reflectance mirrors, which could benefit from DARPA’s (NOM4D) plan for robust manufacturing in space. Credits: DARPA

I asked Dr. Bae about his timeline and TRL for a space based demo of his Sheppard Satellite with PLT-C and PLT-P propellantless in-space propulsion and orbit changing technology. He responded that such a mission could be launched in five years assuming there were no issues with treaties on space-based high power lasers. There is The Treaty on the Prevention of the Placement of Weapons in Outer Space but I pointed out that the U.S. has not signed on to this treaty. Article IV of the Outer Space Treaty states that “…any objects carrying nuclear weapons or any other kinds of weapons of mass destruction…” can not be placed in orbit around the Earth or in outer space. Dr. Bae said “We can argue that the [Outer Space] treaty regulation does not apply to PLT, because its energy is confined within the optical cavity so that it cannot destroy any objects.  Or we can design the PLT such that its transformation into a laser weapon can be prevented.”

He then went on to say: “For space demonstration of PLT spacecraft manipulation including stationkeeping, I think using the International Space Station platform would be one of the best ways … I roughly estimate it would take $6M total for 3 years for the demonstration using the ISS power and cubesats. The Tipping Point [Announcement for Partnership Proposals] would be a good [funding mechanism] …to do this.”

Once the technology of the Photonic Railway matures and is validated in the solar system Dr. Bae envisions its use applied to interstellar missions to explore exoplanets in the next century as described in a 2012 paper in Physics Procedia.

Conceptual illustration of the Photonic Railway applied to a roundtrip interstellar voyage to explore exoplanets around Epsilon Eridani. This application requires four PLTs: two for acceleration and two for deceleration. Credits: Young K. Bae

Be sure to listen live and call in to ask Dr. Bae your questions about the PLT in person when he returns to The Space Show on March 29th.

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Wind Rider propellentless space drive for rapid transit across the solar system

Conceptual illustration of the Wind Rider plasma magnet drive: Credits: Brent Freeze

When humanity eventually moves out into the galaxy to settle new worlds, we will need to take stock of potentially habitable planets capable of sustaining life as we know it to identify potential new homes. The James Webb Space Telescope will have the capability to search for exoplanets in the habitable zones of stars in our local neighborhood by using spectroscopy to reveal biosignatures in the planet’s atmosphere as starlight filters through it when transiting across the disk of the host star. But to discern more detail on the surfaces of these distant new Earths, much more powerful methods for imaging will be needed.

One such method could be to utilize a solar gravitational lens (SGL), a property arising from the Theory of Relativity where large gravitating masses bend light resulting in the possibility of a natural telescope capable of very powerful magnification and significant angular resolution. This would require placing a detector beyond 550 astronomical units from the sun. Such an instrument could potentially resolve the size and shape of continents adjacent to oceans on exoplanets orbiting TRAPPIST-1 or other nearby stars. Located 40 light years away, this star is an ultra-cool red dwarf with seven rocky planets, three of which are in the habitable zone where liquid water can exist.

But getting out to this distance with conventional rockets would take over a hundred years. Voyager 1 is currently over 150 AU from the sun and was launched back in 1977. Enter the Wind Rider plasma magnet drive. A pathfinder mission using this concept to demonstrate the technology of a mission out to the SGL to image planets in the TRAPPIST-1 system will be presented by Brent Feeze, an AIAA mechanical engineer, in a poster session at the American Geophysical Union meeting this month. Calculations show that a spacecraft using this drive could sprint to the SGL focal plane in about eight years. The Wind Rider was also described recently by Alex Tolley on Centauri Dreams.

Originally conceived by John Slough at the University of Washington under a NASA NIAC grant from 2004 – 2005, the system is a propellentless drive that works by creating a rotating magnetic field that traps the charged particles in the solar wind to create a large circular electric current, inducing a large scale magnetosphere. Thrust is imparted to the craft via magnetic fields, analogous to the coupling induced in an electric motor. Unlike a solar sail, the trajectory of the craft is a straight line out from the sun toward its destination, with no gravity assists from other planets and a rapid acceleration to a velocity approaching that of the solar wing (400 km/s).

Jeff Greason, Board Chairman of the Tau Zero Foundation covered the technology during a presentation at the Tennessee Valley Interstellar Workshop back in 2017. He called it a “Ridiculously high thrust to weight magnetic sail” that by chemical propulsion standards is “blindingly fast”. Greason was looking into how Tau Zero could help support a small technology demonstrator on a ride share launch but it would have to be on payloads headed out toward cislunar space to be free of the Earth’s magnetosphere which deflects the solar wind.

Alex Tolley: “If it works as advertised, it would open up the solar system to exploration by fast, cheap robotic probes and eventually crewed ships.”

To be able to image a potentially new world for interstellar settlement is an exciting technology. The hardware required is not expensive and the scientific payoff of such a mission would be valuable from an astrophysical perspective. However, what we already know about the TRAPPIST-1 system is that life as we know it would have a tough time getting started and persisting because these worlds are bathed in intense ultraviolet radiation as they orbit within a range of about 3 – 6 million kilometers from TRAPPIST-1. That said, a pathfinder mission of a Wave Rider to send imaging equipment to the SGL could help prove the technology for rapid transit to the outer solar system as well as validating imaging techniques which could be used on more promising exoplanet candidates for eventual settlement. And expanding our knowledge of planetary systems in the galaxy would be icing on the cake.

TRAPPIST-1e – JPL Travel Poster. Beautiful but life is unlikely due to intense radiation from stellar winds: Credits: Jet Propulsion Laboratory

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Saving Earth and opening the solar system with the nuclear rocket

The NERVA solid core nuclear rocket engine. Credits: NASA

James Dewar believes it is time to reconsider the solid core nuclear thermal rocket, like what was developed in the 1960s under the NASA’s Nuclear Engine for Rocket Vehicle Application (NERVA) Project, as a high thrust cargo vehicle for opening up the solar system and for solving problems here on Earth. A tall order, as he explained in his appearance on The Space Show (TSS) October 26, because nuclear propulsion within the atmosphere and close to the Earth was taken off the table by NASA over 60 years ago and research on nuclear rockets was put on ice after 1973 until recently. Dewar worked on nuclear policy at the Atomic Energy Commission and its successor agencies, the Energy Research and Development Administration and the Department of Energy. He has documented his views in a paper linked on TSS blog.

What is old could be new again. NERVA had a very light high power solid core reactor with Uranium 235 fuel in a graphite matrix creating nuclear fission to heat hydrogen to produce rocket thrust. The specific impulse (efficiency in conversion of fuel to thrust) of the first iteration of NERVA was about 825 seconds, or almost twice that of chemical rockets. More efficient versions were on the drawing board. The compact design (35×52-inch core) lends itself to low development costs and would be inexpensive to fabricate and operate. It has the potential to lower launch costs significantly and research could pick up where it left off nearly 50 years ago.

So why is NASA announcing development of new nuclear thermal propulsion systems for missions to Mars in the distant future? The reactor cores like those used in Project NERVA are known technologies that can it be adapted for other useful applications and it can be done safely on Earth. There could be a large niche market for energy production in remote rural areas such as Alaska or Canada, or supplementing base load utilities during power disruptions due to severe weather events. With their high operating temperatures, these reactors can replace fossil fuel power generation for manufacturing industries that require process heat such as steel/aluminum or chemical production, which cannot be powered efficiently by wind or solar energy. There may also be a cost advantage and environmental benefit to replacing carbon based fuels for powering maritime oceangoing vessels.

“Even the Greens may support it…What if a reestablished program included making a nuclear propelled 1000-foot tanker sized skimmer to rid the oceans of plastic?”

Additionally, a nuclear reactor of this type could service manufacturing centers in both space and on Earth. It could inexpensively launch satellites and provide power for environmental and solar weather stations to monitor and protect Earth’s health. Dewar even thinks that the solid core nuclear reactor could be used to address the growing global problem of industrial waste by melting it down to its chemical constituents and then separating out commercially valuable components from the actual waste prior to permanent disposal. The low launch costs of the nuclear rocket may actually make disposal of waste off Earth economically feasible. Whole clean industries could spring up around these process centers. So this type of reactor could address many national goals and objectives rather than just crewed missions to Mars or deep space.

But what about the elephant in the room? Safety, radiation and fear of all things “nuclear”? Would the public support ground based testing if a NERVA type solid core nuclear thermal rocket program were restarted? Dewar covers this in detail in his book The Nuclear Rocket, Making Our Planet Green, Peaceful and Prosperous. As reported by the EPA in 1974, “…It is concluded that off-site exposures or doses from nuclear rocket engine tests at [the] NRDS [Nuclear Rocket Development Station] have been below applicable guides.”

What about regular launches of a nuclear rocket in the Earth’s atmosphere? First, the launch range proposed would be in an isolated ocean area over water to eliminate the possibility of failure or impact in populated regions. Second, the nuclear core would be enclosed in a reentry vehicle type cocoon for safe recovery in the event of an accident. Third, the nuclear engine is envisioned as an upper stage and would not be “turned on” until boosted high in the stratosphere, thus emission of gamma rays and neutrons from the fission reaction would not be any different then the radiation already impinging on our atmosphere from cosmic and solar radiation.

“…the best way to banish fear is for citizens to profit from the program.”

There is also the potential for the U.S. and its citizens to profit from this venture. Dewar suggests a governance framework for creating a public/private corporation in which the private sector is in charge, but leases assets from NASA and DOE. The government would support the venture via isolated testing sites, providing technical advice, supplying the uranium fuel and security to guard against potential nuclear proliferation. The public/private partnership would be set up to incentivize citizen participation through stock purchases and distribution of dividends in addition to providing jobs and funding the missions.

“Another source of funding would exist beyond the government or private billionaires: the public now has access”

Dewar concludes his paper with an inspirational statement: “…a new space program emerges based on science, not emotion, one that maximizes the technology for terrestrial applications, one that neuters the rocket equations and democratizes the space program, allowing citizens to participate and profit, and one that ever integrates Earth into the Solar System.”