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3

The reason is that you have to carry a fission reactor, which is large & heavy, and a heat exchanger which is also probably fairly heavy. This is all especially heavy if you want to avoid the working fluid from ending up radioactive, and also if you want to avoid the payload from getting irradiated. If you're willing to forego some or all of this ...


30

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 higher than the melting point of nozzle (cooling the nozzle lets you cheat a bit), putting limits on how much energy can go into the fuel. Nuclear rockets get ...


21

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 used a quite different core design, and had predicted T/W ratio of 70. Still somewhat shy of a good modern chemical rocket, but with a much better specific ...


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