4 ms·

Yup, that's exactly the problem. Once you have it incorporated into an overall rocket (and aren't just looking at the engine in isolation), the gains are small

Yup, that's exactly the problem. Once you have it incorporated into an overall rocket (and aren't just looking at the engine in isolation), the gains are smaller. And the engine and fuel are significantly more costly and risky. The fuel to fill up a Falcon 9 costs only a few hundred thousand bucks. The fuel to fill up a nuclear thermal rocket ... would be astronomical in cost.

EthanHeilman 6y ago
>The fuel to fill up a nuclear thermal rocket ... would be astronomical in cost. Perhaps I have it wrong, but my basic understanding of a nuclear thermal rocket is that it uses a reactor to energize a reaction mass like water. The water should be cheap, and the nuclear fuel should be expensive, but how much of the nuclear fuel would be reusable? Would the total lifetime cost be less than a Falcon 9 assuming you replaced the reaction mass?
- CydeWeys 6y ago
  Nuclear fuel is very expensive to process and manufacture, and is not reusable. In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone. The reactor itself is going to be very expensive too; doing anything nuclear just fundamentally ramps up the cost a lot vs building e.g. a Merlin engine. The hydrogen propellant is cheap enough, sure. I'm specifically talking about the nuclear fuel itself as being expensive.
  - EthanHeilman 6y ago
    >In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone Let's say you are using a nuclear thermal rocket to ferry material from lunar orbit to mars orbit. You can just replace the propellant in Mars or Lunar orbit and reuse the reactor and reactor fuel. Processing H2O to H2 on the moon and pushing it the moon's gravity well shouldn't be too expensive. I guess my specific question is: How often would you need to replace the nuclear fuel? Could you run this as a breeder reactor and breed your own fuel?
    - CydeWeys 6y ago
      It's an interesting question. The more fuel you send up there, the longer it'll last, but also the more mass you'll be wasting by constantly having to carry it around. I suspect that it would not be practical to bring along enough nuclear fuel to make many Earth->Mars round-trips on every one of those round trips. Land-based nuclear power plants, for what it's worth, are refueled every ~1.5 years. Nuclear-powered aircraft carriers go two decades, but they're not concerned about carrying around lots of extra mass. Practically speaking I don't think you could refuel a nuclear-powered spacecraft in space. This is a very hard operation to do and everything involved is extremely radioactive. I don't think it could reasonably be done, either by robots or by people. Not until we have entire shipyards in space like we do on Earth, anyway. It takes 3 years to refuel a nuclear-powered aircraft carrier, that's how complicated the reactor tear-down process is (and you do need to tear the reactor down to replace the fuel).
      - ufmace 6y ago
        IIRC, Naval reactors usually use Uranium enriched to much higher levels that civilian nuclear plants, both to get higher power output for smaller reactor size, and to not need refueling as often. Reactors designed to operate in space may do the same sort of thing. I wonder what other kind of optimizations and/or workarounds might be required for a real space-capable reactor.
        CydeWeys 6y ago
        Whatever it is, it's going to be insanely expensive. That's just guaranteed. Honestly it's probably just better to use a larger conventional rocket. Yes in theory the nuclear engine might be more mass-efficient, but it won't be more cost-efficient, and that's really what engineering optimizes for. This is something SpaceX is proving to be really good at that others, e.g. Robert Zubrin, just don't get. He keeps proposing alternatives to the Starship design that, while more physically efficient, would actually cost a lot more money overall to do the same job.
        6y ago
credit_guy 6y ago
> would be astronomical in cost. The actual nuclear fuel is not expensive at all. The online calculator [1] shows that to enrich uranium to 20% (HALEU grade, which is what DARPA is looking into), you pay about $6500/kg. At this concentration, Uranium has an energy density of about 800 GJ/kg [2]. The NERVA engine had a power of about 1.1 GW and provided about 246 kN of thrust, which is about 3.4 times the thrust envisioned for the Starship upper stage [3], so assuming you burn only 50% of the uranium at only 50% efficiency (that's very conservative), you can run the engine for 3 minutes. If you want to run this engine for 5 hours (how many trips to Mars and back would that be?), the fuel cost would be $650k. Most likely this would be well below the fuel cost for the Starship upper stage. The cost of the cryogenic hydrogen is a different story, but even that one should not be a dealbreaker. [1] https://www.uxc.com/p/tools/FuelCalculator.aspx https://www.uxc.com/p/tools/FuelCalculator.aspx [2] https://energyeducation.ca/encyclopedia/Energy_density https://energyeducation.ca/encyclopedia/Energy_density [3] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
- DuskStar 6y ago
  > 246 kN of thrust, which is about 3.4 times the thrust envisioned for the Starship upper stage AFAICT Raptor is planned to have ~2,000 kN thrust [0], and Starship has more than one. And your metrics for fuel use are a bit off to me - better to assume that all of the fuel in the reactor is used with no possibility to recycle, because no one's going to be happy with you landing with a used engine. (And the value of a used engine is pretty significantly negative anyways - just look at the decommissioned naval reactors at Hanford, or the used fuel rods sitting in pools across the country) Similarly, a 50% burn rate is incredibly optimistic - commercial reactors normally manage around 6.5%, and while I believe naval reactors are higher, it's by a factor closer to two than ten. For NERVA I'd expect something more like 0.1%, because it'd be operating for such a short time. (minutes or hours instead of years) Combined, that puts the fuel cost in the $10,000,000-$100,000,000 range to operate for 5 hours, instead of $100,000-$1,000,000. Or, coming from a completely different direction, from reading this [1], specifically page 99, NRX/EST had 176kg of highly enriched uranium for a 1,055MW reactor. At ~$30k/kg for 85% enrichment from your calculator, that means $5,280,000. Yes, modern NTR designs don't use HEU, but that means much lower power densities and thus more uranium. 0: https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor 1: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199200...

EthanHeilman 6y ago

>The fuel to fill up a nuclear thermal rocket ... would be astronomical in cost. Perhaps I have it wrong, but my basic understanding of a nuclear thermal rocket is that it uses a reactor to energize a reaction mass like water. The water should be cheap, and the nuclear fuel should be expensive, but how much of the nuclear fuel would be reusable? Would the total lifetime cost be less than a Falcon 9 assuming you replaced the reaction mass?

CydeWeys 6y ago
Nuclear fuel is very expensive to process and manufacture, and is not reusable. In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone. The reactor itself is going to be very expensive too; doing anything nuclear just fundamentally ramps up the cost a lot vs building e.g. a Merlin engine. The hydrogen propellant is cheap enough, sure. I'm specifically talking about the nuclear fuel itself as being expensive.
- EthanHeilman 6y ago
  >In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone Let's say you are using a nuclear thermal rocket to ferry material from lunar orbit to mars orbit. You can just replace the propellant in Mars or Lunar orbit and reuse the reactor and reactor fuel. Processing H2O to H2 on the moon and pushing it the moon's gravity well shouldn't be too expensive. I guess my specific question is: How often would you need to replace the nuclear fuel? Could you run this as a breeder reactor and breed your own fuel?
  - CydeWeys 6y ago
    It's an interesting question. The more fuel you send up there, the longer it'll last, but also the more mass you'll be wasting by constantly having to carry it around. I suspect that it would not be practical to bring along enough nuclear fuel to make many Earth->Mars round-trips on every one of those round trips. Land-based nuclear power plants, for what it's worth, are refueled every ~1.5 years. Nuclear-powered aircraft carriers go two decades, but they're not concerned about carrying around lots of extra mass. Practically speaking I don't think you could refuel a nuclear-powered spacecraft in space. This is a very hard operation to do and everything involved is extremely radioactive. I don't think it could reasonably be done, either by robots or by people. Not until we have entire shipyards in space like we do on Earth, anyway. It takes 3 years to refuel a nuclear-powered aircraft carrier, that's how complicated the reactor tear-down process is (and you do need to tear the reactor down to replace the fuel).
    - ufmace 6y ago
      IIRC, Naval reactors usually use Uranium enriched to much higher levels that civilian nuclear plants, both to get higher power output for smaller reactor size, and to not need refueling as often. Reactors designed to operate in space may do the same sort of thing. I wonder what other kind of optimizations and/or workarounds might be required for a real space-capable reactor.
      - CydeWeys 6y ago
        Whatever it is, it's going to be insanely expensive. That's just guaranteed. Honestly it's probably just better to use a larger conventional rocket. Yes in theory the nuclear engine might be more mass-efficient, but it won't be more cost-efficient, and that's really what engineering optimizes for. This is something SpaceX is proving to be really good at that others, e.g. Robert Zubrin, just don't get. He keeps proposing alternatives to the Starship design that, while more physically efficient, would actually cost a lot more money overall to do the same job.
        6y ago

credit_guy 6y ago

> would be astronomical in cost. The actual nuclear fuel is not expensive at all. The online calculator [1] shows that to enrich uranium to 20% (HALEU grade, which is what DARPA is looking into), you pay about $6500/kg. At this concentration, Uranium has an energy density of about 800 GJ/kg [2]. The NERVA engine had a power of about 1.1 GW and provided about 246 kN of thrust, which is about 3.4 times the thrust envisioned for the Starship upper stage [3], so assuming you burn only 50% of the uranium at only 50% efficiency (that's very conservative), you can run the engine for 3 minutes. If you want to run this engine for 5 hours (how many trips to Mars and back would that be?), the fuel cost would be $650k. Most likely this would be well below the fuel cost for the Starship upper stage. The cost of the cryogenic hydrogen is a different story, but even that one should not be a dealbreaker. [1] https://www.uxc.com/p/tools/FuelCalculator.aspx https://www.uxc.com/p/tools/FuelCalculator.aspx [2] https://energyeducation.ca/encyclopedia/Energy_density https://energyeducation.ca/encyclopedia/Energy_density [3] https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA

DuskStar 6y ago
> 246 kN of thrust, which is about 3.4 times the thrust envisioned for the Starship upper stage AFAICT Raptor is planned to have ~2,000 kN thrust [0], and Starship has more than one. And your metrics for fuel use are a bit off to me - better to assume that all of the fuel in the reactor is used with no possibility to recycle, because no one's going to be happy with you landing with a used engine. (And the value of a used engine is pretty significantly negative anyways - just look at the decommissioned naval reactors at Hanford, or the used fuel rods sitting in pools across the country) Similarly, a 50% burn rate is incredibly optimistic - commercial reactors normally manage around 6.5%, and while I believe naval reactors are higher, it's by a factor closer to two than ten. For NERVA I'd expect something more like 0.1%, because it'd be operating for such a short time. (minutes or hours instead of years) Combined, that puts the fuel cost in the $10,000,000-$100,000,000 range to operate for 5 hours, instead of $100,000-$1,000,000. Or, coming from a completely different direction, from reading this [1], specifically page 99, NRX/EST had 176kg of highly enriched uranium for a 1,055MW reactor. At ~$30k/kg for 85% enrichment from your calculator, that means $5,280,000. Yes, modern NTR designs don't use HEU, but that means much lower power densities and thus more uranium. 0: https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor 1: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19920005899.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199200...