Nuclear Submarines and Nuclear Rocket Propulsion

We were very interested in other uses of nuclear fission technology (besides in power plants). There are two areas in particular we learned about: nuclear submarines and nuclear-powered rockets.

Submarines are now powered by nuclear reactors almost exclusively, we learned, as they allow the subs to stay underwater for such long periods (the fuel doesn't require oxygen as a combustion engine would!) and thus allow great stealth. This site has a great video on the subject; we also had a couple books.

It was funny, a couple weeks later: I was reading the children a book about dolphins, and it said they used high-pitched sounds to bounce off objects to help them navigate, a technique called echolocation. As we'd talked about this before with our Night unit, I asked, "What other animal uses echolocation?" Seb immediately shot his hand in the air and stopped me almost mid-sentence. "Nuclear submarines!" he said. "Oh, and bats." Yes, Sebby thinks nuclear submarines are animals. How sweet. :)

The information we had on nuclear rockets was sketchier. We read about the Orion project, after WWII, which explored the idea of using nuclear bombs to propel rockets. It was eventually scrapped in favor of the Saturn rocket series (chemically propelled) but we kept finding references to the fact that "nuclear technology is also used in rockets today." However, we couldn't find many details about this, even on the NASA site!  Luckily I have a physicist brother that I can turn to. Hooray for Karl! I reproduce our conversation here, not because I think it will be particularly interesting to most, but in case someone who doesn't have a physicist brother is searching for the same information.

Karl's answers appear in blue:

Karl, 

Is there an easy way to explain our current nuclear rocket technology? We learned about the Orion project in the 1950s-70s, which proposed using tiny fission bombs to propel a rocket. After that project was cancelled, we learned, nuclear rocket propulsion was mostly abandoned---but then we read that the Russians have been using nuclear power for their rockets for some time, and that the New Horizons spacecraft also uses nuclear power for some things. But we can't find details. We assume it's not the tiny nuclear bomb thing, and the boys surmised that maybe it was a mini nuclear reactor, like they use in nuclear submarines. Do any rockets use this, and if so, how? (They don't have propellers for the turbines to turn, do they?)

First, I'd never heard of the Orion project.  It sounds dangerous!  I'm not surprised it never came to fruition.

We tried to read about the New Horizon spacecraft, and it talked about radioisotope technology, but very vaguely. Does this mean they AREN'T using a reactor, but instead just using the natural radioactive decay of plutonium to create heat? And then what is the heat used for? And wouldn't that be too slow/weak of a power source to fuel a rocket? There was a bunch of stuff on the NASA site about how safe! and not scary! this technology is, but not much about how it actually worked.

Every use of radioisotopes harnesses their heat.  In a fission reactor, you get a lot of Pu and a lot of heat, and you superheat steam with it and drive a turbine which turns an electric generator.  You are correct, there are no turbines on spacecraft.

If no steam and no turbines, what is left?  There are thermopiles and thermovoltaics.

Thermopiles are multiple thermocouples in series and parallel to generate the needed voltage and current (Seebeck effect).  An example of a thermopile is in your gas fireplace if you have one.  The little rod poking into your pilot light is a thermopile, and it generates enough electricity to open the gas valve.  Your fireplace will run even with the power out. 

A thermovoltaic works like a photovoltaic (photocell) where the "light" is very long wavelength (IR) light from the hot radioisotope.  These are less common. 

Some spacecraft that use thermopiles: Cassini, New Horizons, Curiosity. 

When we say the spacecraft are powered by these devices, we refer to generating electricity for the on-board systems: communication, navigation, attitude control, etc.  Not propulsion.  They are sent on their way using conventional rocket propulsion.  The entire future trajectory is set in the first few minutes of flight!  It does use small jets of hydrazine for attitude correction. 

The boys also wanted to know, are there any plans to use the "tiny bomb" type of rocket in the future? Or to use a fission reactor, if they aren't already using them?  

No plans that I know of, and I'd be surprised to ever see it. 

Karl.

Next we wondered:

Thank you! Very helpful. We are disappointed there is no nuclear propulsion method. Wouldn't that be able to get us much farther into space, if it could be done?

And Karl answered:

I don't really know how much better that would work.  Propulsion is all about Newton's 2nd law: action, reaction.  The only way you can go forward in space is to send some mass backward.  An explosion sends some mass backward, very fast.  But not all the energy coming from the explosion can be turned into motion.  A more spectacular explosion does not result in any more motion. 

If you want to get the most bang for the buck, so to speak, I believe you want to be gentle.  That is, you want to get the maximum eventual speed for a given amount of propellant since you can only take a finite amount of propellant with you.  I believe the best engine in that sense is the ion engine.  A heavy gas atom is ionized and accelerated through a potential.  As it exits the engine, an electron is added to the ion to make it neutral again, and therefore not attracted back to the spacecraft.  Therefore, the entire mass of the gas atom minus the tiny fraction that is the electron mass is converted into forward momentum. 

It takes years for these to accelerate to an appreciable speed, but they will get us farthest into space because of the efficient use of the propellant.  (To get away from earth's gravity, you do need to do something violent to create enough acceleration at first.) 

Karl.

So! There you have it. Thank you Karl, for a concise explanation of nuclear-powered rockets that anyone can understand! After all, it's not rocket science! (Ha ha! Sorry, I can't help myself. We kept that joke going for several days. We had a book called This is Rocket Science that set us off.)

Here is a bit more information, from the NASA site.

And an interesting video about a small reactor for deep space exploration.