Thursday, October 15, 2015

A Brief Primer on NTR Designs

As y'all probably know, I'm quite fond of the Nuclear Thermal Rocket. It has high performance and can be made using current technology (they actually tested some back in the 60s). I legitimately believe it's one of the best options for manned exploration beyond low Earth orbit.

Of course, NTRs are a pretty broad category. That term encompasses everything from solid core designs like NERVA to the nuclear light bulb. So, here's a brief primer on some of the basic designs, and some of their important features.

First, the open-cycle, solid core design.

It's fairly simple: pass hydrogen (or whatever fuel, but hydrogen is usually best) over the fuel rods of an operating nuclear reactor. The hydrogen heats up and is expelled out the rocket nozzle at high velocity. This is what the NERVA designs and their Soviet counterparts used. Thrust to weight is decent; much worse than a chemical rocket, but far superior to an ion engine. Specific impulse is around 800-900 seconds, maybe up to 1100 if you use better materials. The main limitation is the reactor temperature; above a certain temperature, your fuel rods will melt. Most concepts I've seen have the reactor operating at a maximum temp of about 3,000 K.

Next, the closed cycle, solid core design. I haven't seen much written about this one at all. Essentially, you separate your propellant and the radioactive bits (such as by putting a transparent window between them). Assuming the reactor operates at 3,000 K, most of you emissions are going to be in the infrared.
So, the radiation is going to pass through an IR transparent window, and then heat up the propellant. The advantage of this is that you don't risk having radiation in the exhaust. However, that's already a low concern for solid core designs, and closed-cycle solid is going to be heavier and have less efficient energy transport. I doubt you'll see one of these built anytime soon.

Next is the liquid-core NTR. Something of an intermediate step between solid and gas core designs; you heat up the uranium hot enough that it liquefies; this lets you get hotter temperatures and more energy out of your propellant. Of course, you have to contain the fuel; either by using a closed cycle design, or some other method. Possibly spinning the reactor at high speed like a centrifuge (the uranium will go to the outside, while the lighter hydrogen stays in the middle). Again, I haven't seen as much written about this one, though Atomic Rockets has a bit on it.
(picture via Atomic Rockets page)

 Performance would probably be somewhere inbetween solid and gas core designs, ballpark about 1,500-2,000 seconds of isp.

Next come the gas core designs. First, the open cycle. Heat up your fuel so hot it gasifies (well over 5,000K, possibly even over 20,000 K). Keep your fuel confined somehow (by centrifuging again, or injecting the propellant and fuel carefully), you'll still lose a bit anyway. Let the propellant pass directly over the fuel, so your heat transfer is very efficient. Above 4500K (the gas core is much hotter), diatomic H2 will dissociate, giving you another 40% or so bonus on your specific impulse.

Specific impulse for this one could be well north of 3,000 seconds (I've seen figures near 10k isp quote). Decent thrust, too. Shame about the radioactive exhaust.

Which is where the closed cycle gas core comes in. Also known as the nuclear lightbulb.

As you can see, it's pretty complex. Not only do you have to have some sort of material transparent to UV radiation (which is what your gasified fuel is going to be emitting), but you also have to keep it from melting. Absent finding a material that can withstand 50,000K temperatures, you have to cool it somehow. Luckily, LH2 is cryogenic, so you can just use your propellant as coolant. The performance is going to be a bit lower than an open-cycle design (since your weight is higher, and you'll have a bit less efficient heat transfer), but it more than makes up for this by not spewing fission products out the exhaust. In my opinion, this and NERVA are the ways to the future.