Opening the Next Frontier
by Anthony Tate

Part 8: Heat, temperature, and cooling.


Now, in a Nuclear Thermal Rocket (NTR) we have to get the heat out of the nuclear reactor part of the engine and into the gas we plan to shoot out the back. In a solid core reactor this is done using conduction. IE, you drill hundreds of holes right through your reactor core and pump the gas through them. The gas picks up heat by rubbing along the inside of the reactor, and then blasts out the back. This worked great, but as I said above, you can't run the reactor very hot, since it melts.

With a gas core reactor, you can use a combination of conduction, where the hot reactor gas rubs against the cold fuel gas, and convection, where small amounts of the hot core gas mixes with the cold fuel gas. This is more efficient than conduction alone, with the huge problem that now you are leaking radioactive core gas out of your rocket. This is bad for a lot of reasons.

Luckily for us, there is a third way of moving heat around, and that is radiative. Under most conditions, radiative heating is very small compared to conduction or convection, and can be ignored. However, the inside of a GCNR is not 'most conditions.'

The way this works is simple. If you turn on an electric stove element, and put your hand off to one side of it but not touching, you still feel the heat. That is radiative heat transfer working.

In a GCNR, the core is run SO hot, it lights up like a lightbulb, and then gets much, much, much hotter. The energy being given off goes above red hot, even goes above white hot, until the core is blazing away in the deep ultraviolet. Yes, it gets so hot you can't see it any more.

At those huge temperatures, the normally small radiative heat transfer mechanism grows until it is easily big enough to get the energy from the core into the reaction gas all by itself. You no longer need to mix the two gases together, and you can keep them separate. But how can we do that, if the core is so super hot?

The answer is fused silica.

Silica is very transparent to ultraviolet light. If we treat the core like a real lightbulb and put a dome of fused silica glass around it, the glass lets basically all of the ultraviolet energy shine right through. Even though it seems impossible, the smart fellows back in the 70's actually built test models of this type of system and made it work. Given the technology we have today, we can make fused silica of such perfect transparency that this works great.

A GCNR with one of these bulbs in it is called a nuclear lightbulb. With today's technology we can build these pretty easily.

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Contents:

1: The Frontier Spirit

2: What went wrong.

3: Where do we go next?

4: So, why aren't we going?

5: Dealing with the Devil

6: A brief technical interlude

7: So how good is Nuclear, anyway?

8: Heat, temperature, and cooling.

9: But isn't this dangerous?

10: Prometheus would be proud of us.

11: Ok, that all sounds nice, but this is just fantasy, right?

12: But isn't this just too big?

13: But doesn't this thing make nuclear waste?

14: Conclusions