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This is solving a different issue. Aerospike engines deal with the fact that rocket engines need to be different in an atmosphere vs in vacuum. Nuclear thermal

This is solving a different issue. Aerospike engines deal with the fact that rocket engines need to be different in an atmosphere vs in vacuum. Nuclear thermal rockets (NTRs) are not really envisioned for being used in the atmosphere, the risk of a blowup is too high. I consider them a wonderful technology which could really open up new possibilities, but we first need to nail down the challenge of affordably getting to orbit.

jhgb 5y ago
Except LANTR solves a very similar problem, which is the requirement for different performance parameters at the beginning and at the end of a rocket flight (high thrust and propellant density in first stage, low thrust and high Isp in the second stage) in a single stage vehicle. And I mentioned it because its way of operation is very similar to the thing describe above, i.e., injecting something extra into the nozzle, so it reminded me of it. Not because I consider it comparable to using aerospikes on Earth (I'm not convinced that's useful at all) but because this is one of the few such variable Isp options that appears to provide any useful benefits at least somewhere (even if it's not on Earth).
pfdietz 5y ago
The problem with NTRs in the atmosphere isn't the risk of blowup, it's that their thrust/weight ratios are just terrible. And in the atmosphere, it's early in the launch, so Isp is not important.
- panick21_ 5y ago
  I don't think it necessary true that NTRs have terrible trust/weight. Depending on what your 'fuel'. You potentially lose ISP of course, but your Thrust would go up. It also depends on the engine configuration, you could do much better in terms of weight and density.
  - pfdietz 5y ago
    Thrust of a rocket is a function of nozzle throat and exit areas and thrust chamber pressure, and is (ideally) independent of the density of the propellant. Engines used on a first stage typically have high thrust, but that's because they're designed with large throats, not because of the propellant they use. A dense propellant does allow the pumps to be smaller. But I don't think that's going to help much in a NTR, since the reactor itself is heavy. The problem with an NTR is the difficulty of getting high mass flow past very thin fuel elements (the thinness necessary for high heat transfer) without destruction of the fuel elements by that flow. The thin fuel elements also ensure some fission products will escape and be carried off in the propellant.

jhgb 5y ago

Except LANTR solves a very similar problem, which is the requirement for different performance parameters at the beginning and at the end of a rocket flight (high thrust and propellant density in first stage, low thrust and high Isp in the second stage) in a single stage vehicle. And I mentioned it because its way of operation is very similar to the thing describe above, i.e., injecting something extra into the nozzle, so it reminded me of it. Not because I consider it comparable to using aerospikes on Earth (I'm not convinced that's useful at all) but because this is one of the few such variable Isp options that appears to provide any useful benefits at least somewhere (even if it's not on Earth).

pfdietz 5y ago

The problem with NTRs in the atmosphere isn't the risk of blowup, it's that their thrust/weight ratios are just terrible. And in the atmosphere, it's early in the launch, so Isp is not important.

panick21_ 5y ago
I don't think it necessary true that NTRs have terrible trust/weight. Depending on what your 'fuel'. You potentially lose ISP of course, but your Thrust would go up. It also depends on the engine configuration, you could do much better in terms of weight and density.
- pfdietz 5y ago
  Thrust of a rocket is a function of nozzle throat and exit areas and thrust chamber pressure, and is (ideally) independent of the density of the propellant. Engines used on a first stage typically have high thrust, but that's because they're designed with large throats, not because of the propellant they use. A dense propellant does allow the pumps to be smaller. But I don't think that's going to help much in a NTR, since the reactor itself is heavy. The problem with an NTR is the difficulty of getting high mass flow past very thin fuel elements (the thinness necessary for high heat transfer) without destruction of the fuel elements by that flow. The thin fuel elements also ensure some fission products will escape and be carried off in the propellant.