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Those options are already opening up without Nuclear Thermal. The problem with NTR is the engines are heavy, and the necessary shielding and heat radiators are

Those options are already opening up without Nuclear Thermal. The problem with NTR is the engines are heavy, and the necessary shielding and heat radiators are very heavy. So the high ISP is mostly lost to higher dead mass requirements. If (when) Starship proves it can be fully refueled in-orbit, it obviates every potential advantage of Nuclear Thermal rockets. Starship will be able to make 90 day trips to Mars, carry heavy payloads in excess of 100 tons to the Martian or lunar surfaces, and do this all without requiring specialized landers and with zero radiation and other regulatory concerns.

m4rtink 6y ago
Sure, starship is a perfect first step (finally!) but nuclear propulsion in some form will almost certainly be used in mid to long term - you can do a lot with orbital refueling but the double plus good ISP can't be ignored.
- valuearb 6y ago
  I’m still skeptical. Nuclear just has too many characteristics that limit its ability to make use of that ISP. Starship will have a dry mass around 85 tons including about 9 tons of engines, payload of 100 tons, and 1,200 tons of methane/LOX fuel. At an ISP of 380 that gives it a deltaV of around 9.8 Km/sec. A similar NTR based Starship with a 750 ISP requires a lot more dry mass. The engines will mass at least 100 tons just to provide 1/15th the thrust of Methane based Raptors. That’s ok in space, given you can just fire them longer but they will need hundreds of tons of radiators and shielding. And more mass to insulate the hydrogen tanks. A dry mass of 500 tons would give the NTR a deltaV of 10.7 Km/sec. That’s better, but it comes at a price. Your NTR Starship can’t land on Mars, it doesn’t have enough thrust. So you need dedicated landers, increasing dry mass further. And it’s hydrogen leaks, so you lose some of that DeltaV on a long voyage. And you can’t use anything other than hydrogen, or your ISP drops precipitously, and you can’t use any reactive fuels that will corrode your engine. NTR designs need dramatically higher thrust to Weight ratios to ever become competitive with chemical rockets, even in deep space.
  - Tuna-Fish 6y ago
    > but they will need hundreds of tons of radiators and shielding. NTRs do not need radiators, they can keep cool using the prop. And the mass of shielding is measured in hundreds of kg to single-digit tons, not hundreds of tons. (You can very effectively reduce your shielding needs by putting the engine on a spar.) The rest of your concerns are still valid. NTRs seem like they would work better if you are going to asteroids, or other such targets that don't have an atmosphere you can use to brake with, but I do not see the appeal for trips to Mars, where a more compact atmospheric-capable ship can shed half the trip Δv using a heat shield.
    - rbanffy 6y ago
      > they can keep cool using the prop They stay hot after shutdown and you'll still need to cool them to avoid damage. If you use your propellant/coolant for that, the ISP will drop further.
      - Tuna-Fish 6y ago
        You plan for that in your burn, essentially doing a very low-powered long tail in it. Yes, doing this will slightly reduce Isp, but assuming you can alter the size of the throat on your bell, this effect is just single-digit seconds on the entire burn, compared to the "hundreds of tons of radiators" the GP referred to.
        rbanffy 6y ago
        A variable geometry nozzle would help a lot here, by keeping a lower volume of propellant to still be ejected at a high speed. May be a good idea after all.
    - valuearb 6y ago
      You need heavy radiators or your entire ship quickly becomes uninhabitable when the engine is running. We’ve never built a NTR capable of shedding all its heat through its propellant. In fact, we’ve never operated one in space, only on Earth where cooling is trivial.
mabbo 6y ago
But why not use starship to carry a heavy but efficient ntr engine up, and use that for the trips to and from Mars? Build a special-purpose starship (like is planned for the moon landing) that replaces the rap-vacs with NTRs instead. Boom, you don't need an oxygen tank anymore, creating more living space. This ship can't land, but if you're heading to the belt for mining or on a manned trip near Venus or Jupiter, you weren't going to anyway.
- valuearb 6y ago
  You can’t replace Raptors with NTRs because of size/thrust mismatch. A NTR with the same vacuum thrust as a Raptor would weigh at least 80 times more, and likely be far larger. Then you have shielding and radiators, it’s never gonna be able to lift off the Moon. And you aren’t saving space losing the oxygen tank. First you will need that space for hydrogen propellant which doesn’t have the density of Methane. Worse is it also offset by requiring expensive (in energy and cost) additional cooling for Hydrogen.

m4rtink 6y ago

Sure, starship is a perfect first step (finally!) but nuclear propulsion in some form will almost certainly be used in mid to long term - you can do a lot with orbital refueling but the double plus good ISP can't be ignored.

valuearb 6y ago
I’m still skeptical. Nuclear just has too many characteristics that limit its ability to make use of that ISP. Starship will have a dry mass around 85 tons including about 9 tons of engines, payload of 100 tons, and 1,200 tons of methane/LOX fuel. At an ISP of 380 that gives it a deltaV of around 9.8 Km/sec. A similar NTR based Starship with a 750 ISP requires a lot more dry mass. The engines will mass at least 100 tons just to provide 1/15th the thrust of Methane based Raptors. That’s ok in space, given you can just fire them longer but they will need hundreds of tons of radiators and shielding. And more mass to insulate the hydrogen tanks. A dry mass of 500 tons would give the NTR a deltaV of 10.7 Km/sec. That’s better, but it comes at a price. Your NTR Starship can’t land on Mars, it doesn’t have enough thrust. So you need dedicated landers, increasing dry mass further. And it’s hydrogen leaks, so you lose some of that DeltaV on a long voyage. And you can’t use anything other than hydrogen, or your ISP drops precipitously, and you can’t use any reactive fuels that will corrode your engine. NTR designs need dramatically higher thrust to Weight ratios to ever become competitive with chemical rockets, even in deep space.
- Tuna-Fish 6y ago
  > but they will need hundreds of tons of radiators and shielding. NTRs do not need radiators, they can keep cool using the prop. And the mass of shielding is measured in hundreds of kg to single-digit tons, not hundreds of tons. (You can very effectively reduce your shielding needs by putting the engine on a spar.) The rest of your concerns are still valid. NTRs seem like they would work better if you are going to asteroids, or other such targets that don't have an atmosphere you can use to brake with, but I do not see the appeal for trips to Mars, where a more compact atmospheric-capable ship can shed half the trip Δv using a heat shield.
  - rbanffy 6y ago
    > they can keep cool using the prop They stay hot after shutdown and you'll still need to cool them to avoid damage. If you use your propellant/coolant for that, the ISP will drop further.
    - Tuna-Fish 6y ago
      You plan for that in your burn, essentially doing a very low-powered long tail in it. Yes, doing this will slightly reduce Isp, but assuming you can alter the size of the throat on your bell, this effect is just single-digit seconds on the entire burn, compared to the "hundreds of tons of radiators" the GP referred to.
      - rbanffy 6y ago
        A variable geometry nozzle would help a lot here, by keeping a lower volume of propellant to still be ejected at a high speed. May be a good idea after all.
  - valuearb 6y ago
    You need heavy radiators or your entire ship quickly becomes uninhabitable when the engine is running. We’ve never built a NTR capable of shedding all its heat through its propellant. In fact, we’ve never operated one in space, only on Earth where cooling is trivial.

mabbo 6y ago

But why not use starship to carry a heavy but efficient ntr engine up, and use that for the trips to and from Mars? Build a special-purpose starship (like is planned for the moon landing) that replaces the rap-vacs with NTRs instead. Boom, you don't need an oxygen tank anymore, creating more living space. This ship can't land, but if you're heading to the belt for mining or on a manned trip near Venus or Jupiter, you weren't going to anyway.

valuearb 6y ago
You can’t replace Raptors with NTRs because of size/thrust mismatch. A NTR with the same vacuum thrust as a Raptor would weigh at least 80 times more, and likely be far larger. Then you have shielding and radiators, it’s never gonna be able to lift off the Moon. And you aren’t saving space losing the oxygen tank. First you will need that space for hydrogen propellant which doesn’t have the density of Methane. Worse is it also offset by requiring expensive (in energy and cost) additional cooling for Hydrogen.