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31 votes

Why do nuclear rockets (e.g. NERVA) have such poor Thrust-to-Weight ratios?

At their core nuclear rockets working by heating a working fluid and running it out a nozzle are still constrained by the same physics as a chemical rocket where exhaust temperature cannot be much ...
GremlinWranger's user avatar
21 votes

Why do nuclear rockets (e.g. NERVA) have such poor Thrust-to-Weight ratios?

The one exception to this fact is Project Orion Not quite. Project Timberwind was a solid-core NTR using a pebble-bed reactor design that combined high Isp with a moderate T/W of 30. The DUMBO NTR ...
Starfish Prime's user avatar
20 votes
Accepted

Can nuclear thermal engine designs be shutdown in such a way as to be restartable, in space?

At least for the NERVA, after shutdown, a residual, thrust-generating cooldown flow of propellant was run through the reactor. From the engine specs 3.1.1.1.5.1.4 Shutdown and Cooldown Shutdown ...
Organic Marble's user avatar
16 votes
Accepted

How does propellant flow work in a nuclear thermal rocket?

You have the same problem in any rocket. The pressure at the pump outlet must be higher than the chamber pressure. What you are missing, is that the density of your propellant drops by heating, both ...
Rikki-Tikki-Tavi's user avatar
9 votes

Can nuclear thermal engine designs be shutdown in such a way as to be restartable, in space?

Is it currently possible to shut down an actual rocket (not test reactor with external cooling) while in space and then restart it, without melting significant parts of the engine? If so, how? It was ...
Starfish Prime's user avatar
8 votes

How much thrust can a nuclear thermal rocket provide?

The American NERVA program developed a design expected to produce up to 333kN (75klbf) of thrust in a 6.8 ton package, for a thrust to weight ratio of about 5:1. It was never completed or flown. NERVA ...
Russell Borogove's user avatar
7 votes
Accepted

Is it possible to use zinc powder as a NTR propellant?

I suggest you look into the chemistry of clay slip and slurries. For example, porcelain slip is very fluid and can pour through distribution channels in molds to form fine features, then sets up as a ...
Kengineer's user avatar
  • 1,748
7 votes

Why was the NERVA program dropped?

I know a man who worked on it. All my knowledge of the nuclear program comes from him. The design objective was a single engine to be reused (fancy that--they're talking about reusable rockets in ...
Joshua's user avatar
  • 864
5 votes

How are nuclear thermal reactors designed for cosmic rays in space?

A couple of things: The reference OP provided for cosmic rays states: Cosmic Ray Composition: Cosmic rays include essentially all of the elements in the periodic table; about 89% of the nuclei are ...
codeMonkey's user avatar
  • 1,548
4 votes
Accepted

Nuclear thermal rocket working fluid performance

The relation is quite straightforward. The energy injected into a unit of propellant before it evaporates and spent on evaporating it is minuscule comparing to energy applied to the propellant as gas;...
SF.'s user avatar
  • 55k
3 votes

How are nuclear thermal reactors designed for cosmic rays in space?

Neutron flux created by cosmic rays would be small and would actually make control of the reactor slightly easier in terms of neutron economy. The more non-prompt neutrons are involved in a reactor, ...
Jonathan Ray's user avatar
1 vote
Accepted

NTR performance of hydrazine compared to ammonia?

I ran across this chart on Twitter. I do not know its provenance so take with a large dose of hydrazine salts. If the chart is correct, hydrazine is somewhat superior to ammonia, especially at low ...
Organic Marble's user avatar
1 vote

Is there any way to get better performance from an NTR with non-H2 propellant?

To copy the key part of this answer (and this related earlier answer)... an important figure of merit in a rocket engine is the "characteristic velocity": $$c_* \propto \sqrt{\frac{T_t}{M_w}}...
Starfish Prime's user avatar

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