Advanced Nuclear Thermal Propulsion Design for 90 Days Trip to Mars

Ultra Safe Nuclear Technologies (USNC-Tech) has delivered a design concept to NASA as part of a study on nuclear thermal propulsion (NTP) flight demonstration. Fully Ceramic Micro-encapsulated (FCM™) fuel will enable a safer nuclear rocket with double the performance of chemical rockets.

Despite doubling the ISP (fuel efficiency in space) with nuclear thermal, it is possible for large chemical rockets to also achieve 60-90 day trip times to Mars from Earth. SpaceX is developing orbital refueling and the Super Heavy Starship. A fully refueled Super Heavy Starship could achieve a massive delta-V that would enable a superior orbital transfer and faster trip times than were ever achieved when small vehicles are boosted to orbit on smaller rockets.

However, it would still be good to fully develop safe nuclear thermal propulsion. There will be space missions where this capability will be useful.

There was previous work on nuclear thermal propulsion starting with the NERVA project back in the 1960s. They actually built the hardware. There has been numerous projects to update nuclear thermal propulsion. The technology would clearly work and it is a matter of actually fully funding and pushing through to putting actual systems in space. There have also been nuclear thermal designs to launch massive payloads from the earth. The nuclear materials is enclosed so there would be no radiation problems.

A problem is that nuclear thermal only doubles or in some cases triples the ISP that is possible from chemical rockets. This means making beginner nuclear thermal systems that are smaller than the chemical rockets may not give any benefit. SpaceX mass producing fully reusable rockets and have orbital refueling means that any nuclear rocket has a higher bar to clear for any advantage. SpaceX driving launch costs down also makes it more challenging to introduce a clearly superior new space propulsion technology.

FCM Fuel Kernel
The fuel packaging starts with a Uranium Fuel Kernel measuring less than 1 mm across.

TRISO Particle Fuel
The kernel is coated with special layers designed like tiny pressure vessels. The layers contain fission products inside and ensure mechanical and chemical stability during irradiation and temperature changes. This is called a TRISO Particle. Developed in the 1960’s for gas-cooled reactors, TRISO has enjoyed continued international development resulting in an excellent starting point for the Ultra Safe reactor.

FCM™ Fuel
Ultra Safe Nuclear’s breakthrough is encasing the TRISO Particles within a dense Silicon Carbide matrix, which we call Fully Ceramic Micro-Encapsulated Fuel, or FCM™ Fuel

Pellet Stacks
FCM™ Fuel Pellets are stacked…

Graphite Blocks
…and placed into Graphite Blocks. Graphite is the moderator which slows down neutrons and increases the likelihood that neutrons will cause fission reactions in the fuel. Cooling channels are built into the Graphite Blocks. Cold helium flows through the cooling channels and picks up heat.

Reactor Core Cartridge
The Reactor Core is made up of several hundred such Graphite Blocks with a few tons of fuel. Various openings and channels are used for control rods and coolant flow.

The Reactor can be adapted for space propulsion and for ground based nuclear power. High temperature capabilities make it ideal for hydrogen production, space reactors and space nuclear propulsion.

SOURCES- Ultra Safe Nuclear Technologies (USNC-Tech), NASA
Written By Brian Wang,

43 thoughts on “Advanced Nuclear Thermal Propulsion Design for 90 Days Trip to Mars”

  1. Exactly, Bjorn.

    You send all the cargo ahead of the people. You have enough AI onboard to assemble and construct housing, powerplants, greenhouses, etc. You make sure that all checks out before any people board a rocket.

    I've seen too many movies. The crew are injured on the way to Mars, they can't build a structure, and everyone dies. Mars is already deadly enough. They need to be able to get there fast and chill for a second upon arrival.

    There's absolutely no need to send large volume cargo and people on the same trip. We don't do that on Earth, do we?

    You want to send a person from Shanghai to Los Angeles, you put them on a jet. You want to send cargo from/to the same places, you use a ginormous container ship. Slower, but much more efficient.

  2. This is why you start with a Moon colony first and work out the kinks. If something goes wrong, you can return the colonists in days, not months.

    I still think any Mars mission is doomed to failure. If someone else wants to go, more power to them. Not me though. Not until we have the Starship Enterpise. Maybe then.

  3. Well if TRISO fuel can support higher fuel element temperatures than the alternatives then it has a shot at being used in a NTR. Cost is less of an issue for a reactor that is run for 10 minutes every two months.

    TRISO seems to be reliable in a normal reactor up to 1600C.
    The Zr-C coating for Timberwind fuel elements melts at 6,000C.

    So Timberwind wins and this is probably yet another PPT reactor.

  4. NERVA and Timberwind and every other NTR are open cycle engines, you just rub cryogenic Hydrogen against 1000C fuel elements and let it fly out the open hole at the bottom of the reactor.

    That's why crazy durable fuel element material is so important. I'm not sure a MSR could get hotter than Cermet nuclear fuel and in the end the extra weight of the salt would probably reduce the thrust to weight ratio of the engine (which is already awful compared to chemical rockets).

  5. That's not what I said read the comment again. Yes rocket equations change when you have different loads, rocket performance, etc.

  6. The original NERVA rigs didn't have their fuel elements made THAT small did they? How did they get sufficient heat transfer?

    (Actual question, I have never seen the inside of a nuclear rocket.)

  7. Just trying to visualise how you'd get sufficient propellent mass flow and heat transfer through a molten salt bath without losing all your salt out the nozzle in a spray of fine droplets.

    I was thinking of something that looked like a coal power station boiler

  8. "For all the rest, SpaceX Starship will work just fine."

    … indeed, and no one is ever going to need more than 640KB or RAM.

  9. ISP is directly tied to core temperature which is completely dependent on cladding material and propellant choice.

    Doesn't have too much to do with enrichment % but by design all these NTR's are very very high enrichment to reduce the mass of Uranium in the core and to sustain a reaction in the presence of neutron losses.

  10. " If you look at the manned version it's designed to carry a lot of people not cargo."

    Agreed, it is. I fully expect that to change before it gets built, to a more practical, "Line all the walls with cargo for radiation shielding, and have a reduced crew live in the center for the trip." And I think they'll ditch that silly picture window, too.

    Oh, and the colony won't have a huge glass dome, either. More likely Quonset huts buried under sandbags.

    This is my expectation of where the engineering practicalities will lead, regardless of the graphic artists' renderings.

    If you'll look back a while, I was saying thumbs down on 301 stainless, too, and recommending 304. Eventually Musk did the right thing in that regard. He'll come around on transit times, too.

  11. Earth is more than enough for us. What we have are inefficient, polluting technologies and poor social structures of production. And a lot of them are by choice.

    Those problems are made worse with a growing rejection of efficient solutions that are also less polluting. Like the rejection of nuclear power in favor of the inefficient and polluting "renewables" alternatives, which end up feeding our dependence on carbon. Or the rejection of ocean fertilization, which would make life much better for a whole swath of living creatures including us.

    Or what to say about the still popular yet delirious notion that socialism/leftism represents an actual solution to anything?

    That's an example of long lived mass delusion that keeps on being liked despite the evidence.

    So I won't have my hopes up concerning the future being any easier for the forward thinking, actual progressive humans.

    In fact, kicking the rear of humanity's reactionaries is one of the best arguments for outwards expansion to space, the same as emigration to the Americas was a few centuries back.

  12. What good is all that extra cargo if you don't have the people to use it? You need a lot of people to grow a colony. Most of the cargo that will be sent to Mars will be done by unmanned starships dedicated to bringing supplies not people. If you look at the manned version it's designed to carry a lot of people not cargo. Getting them to Mars faster if the technology is there is no way a waste of money. If we were using SLS to get us there cargo would be a priority but Starship won't cost anywhere near as much per launch. Nuclear propulsion is supposed to be twice as efficient so you won't need extra fuel to get people there faster or for deceleration. Lining the entire ship with cargo and water to protect from radiation would be too heavy and a waste of space. I think your using rocket equations based off technology from the 1960s, rocket equations change get with the modern times.

  13. I'm talking about getting people to Mars, too. And I'm telling you, as somebody who has dreamed of space colonization since I was a child in the 60's, who helped found a college chapter of the L-5 society, and who has been looking at the technology necessary for this all my life from an engineer's perspective:

    If you're a colonist you're not going to waste fuel getting to Mars fast.

    It's always going to be more practical to arrive via a low energy trajectory with more material support. You'll always prioritize more equipment, more emergency stores, over a few months transit time. It's just a few months, and you'll be on Mars the rest of your life, why would you beggar your self to save a few months travel time?

    Cargo is radiation shielding. You can pair up Starships on the trip over, go bolo with a tether, and spend the trip acclimating to Martian gravity.

    I think you just don't appreciate the rocket equations, just how much cargo capacity those few months cost you. Those would be the most expensive saved months of your entire life, and you'd be paying for them for the rest of your life.

  14. Earth does just fine for humans, any problems are caused by people. We need to colonize Mars at some point, better to do it now than later.

  15. Large amounts of cargo sent to Mars will be unmanned, therefore the time it takes to get there is not so important. I'm talking about getting people to Mars in as little amount of time as possible. This is obviously important to reduce time spent in Zero G and exposure to radiation.

  16. I'd be surprised if there were any major accumulations of "mostly acetylene" on Titan. The stuff is only metastable. Cryogenic solid acetylene would be a nasty explosive.

  17. "not to mention the fact that the bubbles would be void space that would tend to shut the system down or make it oscillate in output."

    Sounds like negative feedback to me; The bubbles expand as they get hotter. And the faster you spin the salt pool, the better the salt is confined.

    I'd be more concerned about the stability of the salt in such a reducing environment. Might lose fluorine over time.

  18. If we had a magical teleportation machine, sure, we'd use it.

    But if a rocket can get you to Mars in weeks, it can get you to Mars with a hundred times the payload in months, at far less cost. Because it's not a magic teleportation machine. It operates by physics.

    Colonists aren't tourists, they prioritize practicality. Burning huge amounts of fuel to reduce the trip time is not something a colonist would do. It would be a fundamentally stupid thing for a colonist to do.

  19. Not an expert. Just looking for more info.
    I assume the design is inherently safe due thermal dilation, and that when the core sections are in the closest proximity to each other that the maximum temperature is self-regulating.
    Only a small percentage of the fuel will ever be consumed before refueling becomes necessary.
    The fuel in the center of the reactor burns more efficiently than the outer sections- requiring the core to be reconfigured occasionally, in order to get more uniform consumption before refueling is required.
    I assume that these core sections are going to be mounted on some sort of accordion structure, so that the reactor can be throttled.
    After time, I assume the fuel pellets will begin to crack or "corrode" due to the conversion of active elements within them.
    I don't anticipate this sort of technology ever being used in Earth's atmosphere, and probably not even in LEO, but this would be a good design for long hauls in deep space, perhaps starting at Lunar Gateway.
    Does this sound about right?

  20. How about a cylindrical rotating pool of molten salt, with hydrogen bubbling through it? No shortage of surface area…

  21. Tbh, colonization of mars just sounds insane. Earth can barely provide for humans on earth. The technology needed to establish a manned base so far away and with so many known and unknown variables is decades away. Robots ok, people? not so soon. Elon is delusional on that regard or is just a marketing strategy.

  22. Hadn't heard of the Poodle project.

    Wikipedia, as usual, is the place to start

    the design, while highly efficient, can produce thrust levels of perhaps 1.3 to 1.5 N

    TRW maintained a fairly active development program known as Poodle from 1961 to 1965, and today the systems are still often known as Poodle thrusters. The name was a play on the larger systems being developed under Project Rover, which led to NERVA. In April 1965 they ran their testbed engine for 65 hours at about 1500 °C, producing a specific impulse of 650 to 700 seconds (6.5 to 7 kN·s/kg).

  23. So, would this be a vat of molten salt with platinum or tungsten pipes running though it with the hydrogen propellent running within?

  24. That's ridiculous of course reduced travel time is useful for colonization. If we had the technology to travel to mars in an hour we would have been there by now with millions of people. Three month trips means less time in zero gravity and less radiation exposure. It also means more days spent on Mars.

  25. It would be good to have an emergency fast travel system to get to Mars. For now, though, in terms of colonization, cargo over speed might be the winner, so I'm in line with the comments saying that so far. However, in the future, when tourism becomes a thing and people want to move around quickly, it's better to have quick interplanetary turnaround times.

  26. From an ISRU perspective Solid-Core NTRs of any design are kind of pointless. The favoured propellant, liquid H2, requires throwing away 88% of the mass of water (for example) which seems grossly inefficient mass-wise. Except if we sourced it from somewhere that already has plenty of free H2 floating around. The Gas Giants are too deep to source it, except maybe Uranus, but some 66 billion tonnes of gaseous H2 is available on Titan. With a background temperature of 94 K, chilling it down into liquid at 20 K isn't as energy intensive by a large factor as cracking it out of water anywhere else, then chilling it down.

    Boosting it from the surface of Titan could just use an NTR, but a 1:8 mix of acetylene-hydrogen makes for a decent propellant for free, given the dunes of Titan are probably mostly acetylene. No oxidiser required and about 260 seconds Isp at Titan's surface (up to 420 seconds in vacuum.)

  27. Since a NTR needs only H as working fluid, the Moon would
    even be better, easier to reach and less deep gravity well.

  28. At high heat flux, the inside of these pellets will be much hotter than the surface. You might indeed get a higher ISP with a molten salt design where heat transfer is better.

  29. For ultra-fast delivery of a few kilogram packages within the Solar System, laser pushed sails may have a brighter future.

    You only need to have them at both sides, and then you can send things between almost every planet in a matter of days or weeks.

    But given they tend to require continuous focusing of the thrusting laser per package, the cost won't be cheap.

    For all the rest, SpaceX Starship will work just fine.

  30. Yeah, yeah, trip to Mars yada yada. Exoindustrialization makes trips to mars much easier, safer, and cheaper. A well shielded, highly redundant, and roomy cycler station between the orbits of Earth, and Mars would make trips safe, and comfortable. Freight, and spare parts could be tethered to the surface of the station. The station could be for the most part a well shaded comet, all the hydrogen makes a very good radiation shield, and be used as reaction mass.
    This "engine" would be better used in moving asteroids, and comets to the moon-earth system, or anywhere else in the solar system. Set up mining, and use less valuable chemicals as reaction mass, usually oxygen, CO2, or CH4. Heat from the reactor could be used to refine mined materials.
    You'd likely need to redesign for maximum hours of operation. You might even want to refuel the things. On the other hand, it might be easier to give them enough reaction mass, and let them cancel their rotational momentum relative to the sun, and say goodbye to all the fission daughters.

  31. I think reduced travel time to Mars has little use for a colonization project. Colonists will basically always prioritize additional cargo over reduced trip time, and cargo can be used as radiation shielding. 

    But higher ISPs can allow greater cargo capacity at the same trip time, so that's good.

    Though I suppose having rapid delivery of urgent, light cargo, even away from ideal orbital timing, would be useful.

  32. A Mars base would be a great place to launch a nuclear rocket as the NIMBYs won't leave Earth out of fear of solar and cosmic radiation. If anything, if Musk's plans come to fruition, then nuclear rockets are more likely to be developed.

  33. Hotter is better, hottest is best. A molten salt reactor running at a much higher temperature could give higher ISP resulting in higher velocity or lower mass ratio.

    A radioactive isotope engine could also be cheaper and smaller than a reactor engine. Since you don't need a lot of thrust once you are off the earth you could build a small engine. If I remember right the rockets were called "POODLE". You could get ISP of about 800 which is about the same as a nuclear engine without any of the complexity and almost none of the radiation.

Comments are closed.