4 ms·

You seem pretty knowledgable about this stuff. Wouldn't an increase of a factor of 3 in ISP, result in a non-linear increase in performance (deltaV) due to les

You seem pretty knowledgable about this stuff. Wouldn't an increase of a factor of 3 in ISP, result in a non-linear increase in performance (deltaV) due to less fuel requirements? How important is it that nuclear thermal propulsion can use water as most of its reaction mass for in situ utilization (ISRU)?

sudosysgen 6y ago
If the engine had the same mass, it would. But these engines require incredibly heavy shielding.
- CydeWeys 6y ago
  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.
        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...
- bryanlarsen 6y ago
  And they use H2 for reaction mass. Which means incredibly massive tanks because H2 is so much less dense and requires a lot of insulation to stay liquid.
  - EthanHeilman 6y ago
    Shouldn't that scale well in space as the shell of the H2 tank scales at the square whereas the volume scales at the cube? In general it seems like density should matter less than mass, but I've never attempted to contain and accelerate large quantities of liquid H2. Is insulation that much of a problem in the vacuum of space? I would think you could just cover it in a reflective material and call it a day.
    - bryanlarsen 6y ago
      Yes, I've been assuming that we're not getting our H2 from the bottom of a gravity well. If not, the trade-offs do change dramatically.
    - sudosysgen 6y ago
      The mass-fraction actually doesn't scale like that. While yes if you just wanted to cover your H2 tank it would scale with the square of side length, it's not just about covering it but mostly about structural matters. If you were to keep the same wall thickness your rocket would not be structurally sound.
    - CydeWeys 6y ago
      Another big problem is leaking and boil-off. There's a reason that no space mission longer than days uses LH2 as its propellant; it just doesn't last long enough, because of boil-off caused by solar heating and the hydrogen actually leaking out through the fuel tank. Insulation isn't 100% effective and it's hot in space near the Sun where we are, certainly much hotter than the 20K boiling temperature of LH2. Even if the insulation is 99% effective that's still significant heat flow at a temperature higher than 20K, and you have boil-off. So, yeah, it's fine for uses in stages to orbit, and it was even fine to use for the Apollo missions because it was only 3 days there, but anything longer than that and it's increasingly not suitable. > In general it seems like density should matter less than mass Mass is already incorporated into Isp. So, for the same total mass of propellant being exhausted at the same velocity, you definitely would rather have the denser fuel so that your fuel tank can be smaller and thus lighter.
credit_guy 6y ago
Not the OP, but the way the rocket equation works is that for the same ratio of gross to net mass if you double the exhaust velocity, you double the delta-v. So, it's exactly linear. It's actually a little sublinear, as you add the delta -v to the original v (which is the speed of the planet Earth plus the orbital speed of the vehicle in the LEO that starts the trip to Mars). So, if you double the delta-v, you don't actually double the deep space velocity. Where the rocket equation helps is in the net to gross mass ratio. Starship is expected to have a 9% ratio. If you multiply the ISP by 2.2, you would increase the ratio to 34% (as someone else in this thread mentioned). The Starship proposed net mass is 120 tons, that could go to 450 tons. For comparison the largest (cargo) airplane in the world, the Antonov AN-225 has a maximum takeoff weight of 640T, but 300 of that is fuel.

sudosysgen 6y ago

If the engine had the same mass, it would. But these engines require incredibly heavy shielding.

CydeWeys 6y ago
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.
        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...
bryanlarsen 6y ago
And they use H2 for reaction mass. Which means incredibly massive tanks because H2 is so much less dense and requires a lot of insulation to stay liquid.
- EthanHeilman 6y ago
  Shouldn't that scale well in space as the shell of the H2 tank scales at the square whereas the volume scales at the cube? In general it seems like density should matter less than mass, but I've never attempted to contain and accelerate large quantities of liquid H2. Is insulation that much of a problem in the vacuum of space? I would think you could just cover it in a reflective material and call it a day.
  - bryanlarsen 6y ago
    Yes, I've been assuming that we're not getting our H2 from the bottom of a gravity well. If not, the trade-offs do change dramatically.
  - sudosysgen 6y ago
    The mass-fraction actually doesn't scale like that. While yes if you just wanted to cover your H2 tank it would scale with the square of side length, it's not just about covering it but mostly about structural matters. If you were to keep the same wall thickness your rocket would not be structurally sound.
  - CydeWeys 6y ago
    Another big problem is leaking and boil-off. There's a reason that no space mission longer than days uses LH2 as its propellant; it just doesn't last long enough, because of boil-off caused by solar heating and the hydrogen actually leaking out through the fuel tank. Insulation isn't 100% effective and it's hot in space near the Sun where we are, certainly much hotter than the 20K boiling temperature of LH2. Even if the insulation is 99% effective that's still significant heat flow at a temperature higher than 20K, and you have boil-off. So, yeah, it's fine for uses in stages to orbit, and it was even fine to use for the Apollo missions because it was only 3 days there, but anything longer than that and it's increasingly not suitable. > In general it seems like density should matter less than mass Mass is already incorporated into Isp. So, for the same total mass of propellant being exhausted at the same velocity, you definitely would rather have the denser fuel so that your fuel tank can be smaller and thus lighter.

credit_guy 6y ago

Not the OP, but the way the rocket equation works is that for the same ratio of gross to net mass if you double the exhaust velocity, you double the delta-v. So, it's exactly linear. It's actually a little sublinear, as you add the delta -v to the original v (which is the speed of the planet Earth plus the orbital speed of the vehicle in the LEO that starts the trip to Mars). So, if you double the delta-v, you don't actually double the deep space velocity. Where the rocket equation helps is in the net to gross mass ratio. Starship is expected to have a 9% ratio. If you multiply the ISP by 2.2, you would increase the ratio to 34% (as someone else in this thread mentioned). The Starship proposed net mass is 120 tons, that could go to 450 tons. For comparison the largest (cargo) airplane in the world, the Antonov AN-225 has a maximum takeoff weight of 640T, but 300 of that is fuel.