Opening the Next Frontier
by Anthony Tate

Part 7: So how good is Nuclear, anyway?


VERY good. Fission and fusion and antimatter have the ability, once we build them, to get us anywhere in the Solar System with ease. Chemicals on the other hand can barely get us into orbit.

For this discussion, I will stick to nuclear fission. Fusion/antimatter is more complicated, and I like simple.

As I mentioned above, way back in the 60's NERVA and ROVER made nuclear powered rockets. These rockets were thoroughly tested and were able to generate as much as 250,000 pounds of thrust, with an Isp of 900 seconds or better. The best chemical fuels in use today are liquid hydrogen and liquid oxygen, the stuff burned by the three Main Engines on the Space Shuttle (SSME's). The SSME's produce a maximum of about 450 Isp.

NERVA did this using technology that still used vacuum tubes. And not because they 'sound better' than transistors.

Now, technically speaking all rockets that use a reactor to heat up a gas to make thrust are called Nuclear Thermal Rockets (NTR's). An NTR like NERVA is what is called a solid core NTR, since the reactor core was a heavy solid mass of ceramics.

The efficiency of any NTR is limited by the difference in temperature between the core and the gas. The bigger the difference, the more efficient the rocket is. I'll repeat that, because it is an important principal: A nuclear rocket is more efficient when the reactor runs hotter.

NERVA was pretty hot, basically running just barely under the temperature that would start the core ceramics melting. The smart guys who came up with this concept way back then were not satisfied with that however. They came up with an even more efficient system, in which the core of the rocket was not a huge solid mass of ceramic, but it was a cloud of Uranium HexaFluoride gas. Since the core started out as a cloud of gas, it couldn't melt! Therefore it could get much hotter than a solid core rocket, and would thus be much more efficient.

This idea was dubbed the Gas Core Nuclear Rocket, or GCNR for short. The way a GCNR kept the gaseous core in a single mass was the height of simplicity:

Imagine a pot of hot water. You stick in a spoon and begin stirring it in a circle, as fast as you can. Soon, a deep funnel shaped hole appears in the center of the water. If you then crack an egg into the pot, it settles quickly into the bottom of the funnel, which is called a vortex, and cooks all the way through without ever touching the pot itself. Now imagine the water is a buffer gas, and the egg is the Uranium Hexafluoride fissile mass. Simple, isn't it.

They built test models of the GCNR many years ago, and discovered a little problem. Since the core was a hot gas, when you pumped the fuel gas through it to get it hot, the radioactive core gas would leak out through the exhaust. This is a real problem. Luckily, they were able to figure out a way to get around this issue. To fully understand this concept will take a little explaining, but bear with me.

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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