Why can’t we build nuclear powered rockets in order to reach the moon or mars and the beyond?
We have nuclear powered subs, aircraft carriers etc.
I read the US tried in the early years of the Cold War but cancelled the program.
What gives?
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The previous designs are known as solid core nuclear thermal reactors. You take a mixture of uranium and graphite and push it through something like a pasta maker to make long fuel rods with holes through them for the hydrogen fuel to travel through.
Hydrogen is a wonderful fuel because its atoms are very light and therefore the rockets that use it have a high fuel efficiency – known as “specific impulse” – about double that of chemical rockets.
That’s the upside. There are a bunch of downsides:
* The temperature of the core is limited by the materials in the core; they have to stay cool enough so they don’t melt. In the NERVA tests, they generally ran at about 2400 K, while chemical rockets run at about 3500 K.
* The nuclear core is heavy and the shielding to protect the rocket from radiation is also heavy. That makes the thrust/weight for the engine pretty poor, and therefore the engine performance is worse.
* Hydrogen is very non-dense and therefore requires bigger and heavier tanks.
* As soon as you turn on the engine, it becomes very radioactive. For the NERVA tests all of the testing had to be controlled remotely and there was a special building required so they engines could be disassembled without irradiating the workers.
* You need significantly enriched uranium, at least 20% on up to 90% depending on the engine design. This is highly controlled because you can make weapons from 20% enriched.
With most of the designs, the resulting performance you get isn’t really much better than what you get with chemical rockets for most scenarios.
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