Why a Nuclear Powered Rocket is a Bad Idea
Taking a critical look at the enthusiasm for nuclear rocket engines in the media. Looking at the pros and cons of nuclear rocket engines.
There is newfound enthusiasm for nuclear powered space rockets. Wired magazine is reporting on BWX Technologies and NASA working together to develop a nuclear rocket engine.
“You can do chemical propulsion to Mars, but it’s really hard,” says Emrich. “Going further than the moon is much better with nuclear propulsion.”
Further the article says:
“Many space exploration problems require that high-density power be available at all times, and there is a class of such problems for which nuclear power is the preferred — if not the only — option,” Rex Geveden, a former NASA associate administrator and CEO of the power generation company BWX Technologies, told the National Space Council in August.
I see several problems with these claims. There are no mentions of either Blue Origin or SpaceX which specifically has plans to go to Mars using chemical propulsion, something the interviewed people in this article go rather far in dismissing as a viable option. Somebody better head over to Elon Musk and tell him, he is wasting billions on something that will not work.
My issue with these guys is that we are seeing a repeat of the same bullshit that has kept space exploration stuck for decades. It is the same kind of thinking that brought us the exorbitantly expensive Space Shuttle. It is NASA and space geeks pursing sexy high tech hardware without any regard to practicality, simplicity or cost.
The Space shuttle was such a complicated solution that it ended up costing significantly more than a rocket with zero reuse.
One of the problems with the people working at NASA and its subcontractors is that they are primarily trained to think in terms of technology and not cost. E.g. it has been remarked that engineers at SpaceX actually have intimate knowledge of the cost of the parts they put together, which is unusual in the old space industry accustomed to cost plus subcontracting.
Another important observation they are overlooking is that costs tend to rise much faster as a function of complexity, while costs tend to be linear with respect to scale. Meaning it is often simply cheaper to build a larger space ship than a more sophisticated one.
Before shooting down the idea of using nuclear engines, let us at least explore the advantages and talk about technical challenges.
The Allure of Nuclear Rocket Engines
Two things are of profound importance to space rockets:
- Thrust to weight ration, TWR. This is basically how powerful the rocket engines are relative to the weight of the rocket. In car speak this is equivalent to how much horse power you got relative to the weight of the car. Naturally a car with lots of horse power weighing very little can accelerate very fast.
- Specific impulse, Isp. This is how efficient the rocket engines are. Basically how much propellant you need to spend to increase velocity. In car speak this is equivalent to gas mileage.
You can read a more in depth guide to these concepts in my Gas mileage and horsepower on a space rocket story.
The importance of these factors depends a lot on what your rocket stage is doing. Isp (propellant efficiency) is rather irrelevant if you cannot produce enough thrust to lift the rocket up from the launch pad.
While out in space, even a minuscule force will increase the velocity of a space craft.
It is important to understand this distinction. Nuclear rockets engines are extremely efficient and so newbies playing space simulation games like Kerbal Space Program will strap on a big nuclear rocket engine on their first stage only to discover that their dream rocket cannot even push itself off the ground. Disillusioned they conclude nuclear engines are crap.
Actually they aren’t, you just cannot use them for the booster stage (first stage of a multi-stage rocket). For that you need powerful kerosene fueled chemical rocket engines. These produce immense thrust and can push a massive rocket off the ground. However these things don’t use their fuel efficiently. Thus they quickly go empty.
The beauty of nuclear engines is when you get into space. A space craft with nuclear engines are extremely fuel efficient, meaning they spend very little propellant to increase their velocity. That means nuclear engines can cut the travel time in half going to Mars, in principle.
Problems with Nuclear Rocket Engines
However nuclear engines are not easy to work with. They produce enormous amount of heat once the chain reaction of nuclear fission gets started. This places major challenges on creating metals and cooling systems to deal with it. This has always been a challenge in developing rocket engines. Nuclear engines just make it worse.
While a nuclear rocket engine is in operation the reactor core gets cooled by the fact that the propellant absorbs heat and emits it form the rocket. This way the rocket sheds heat. But once you run out of propellant you have a problem. The reactor core has nowhere to shed heat, and you risk melting your whole rocket.
The solution is to fire up the core only briefly and then shut it down before all the fuel is spent. The last fraction of the fuel is then spent cooling the reactor core.
This illustrates some of the complexities of running a nuclear rocket. It is not like a nuclear submarine which has access to cooling liquid all around it being in the ocean. A rocket is in a vacuum which does not transport heat easily. You cannot start and stop a nuclear engine at leisure. You need to plan it carefully. Should you forget to retain enough propellant for cooling the reactor core, you are screwed.
Lessons from SpaceX
Before drooling too much over the extreme efficiency and technical marvel of nuclear rocket engines, it is worth having a sober look at what actually made SpaceX such a success.
SpaceX did not use any esoteric technology, quite the contrary. The Falcon 9 rocket engine was basically well established 1960s style rocket engine technology modeled and manufactured with modern tools. Instead of using slide rules and thousands of welds SpaceX used modern 3D modeling tools and used 3D printing extensively in manufacturing.
It was a bit like making a more optimal version of Henry Ford’s T ford using 3D modeling tools, 3D printers and carbon fibre composites.
It may not be what get a technology geek exited but it worked. It allowed SpaceX to make cheap and reliable designs. It allowed them to design rockets in a fraction of the normal time and build them at a fraction of the normal cost.
It gave SpaceX a rocket which was considerably cheaper than the Space Shuttle even when it was not being reused. Being cheap allowed them to iterate many times to eventually nail reusability, brining costs down further.
This isn’t even a new discovery by SpaceX. It is really what the Russian space program had been doing for decades. The Russians built simple design and iterated over these designs aggressively brining down costs and creating reliable solutions.
SpaceX is just taking this a step further, by being willing to use more modern design and manufacturing techniques.
Why Nuclear Rocket Engines are a Dumb Idea
Going nuclear throws all these hard earned lessons overboard. You end up with a system with a lot of complexity and inflexibility.
Chemical rockets may be inefficient but they are flexible:
- You can relatively easily refuel in orbit.
- Feedstock to produce rocket fuel can be obtained a multitude of place: on asteroids, on the moon and on Mars.
- You can easily start and stop a chemical rocket, which makes them safer and more flexible to operate. Got a problem, just turn off the turbo-pump.
- The fuel can usually be handled by humans without too big concern for toxicity and pollution.
Nuclear rocket engines are the opposite. Loading up with new fuel is going to be a complicated operation. Obtain nuclear fuel on asteroid, the Moon or Mars? Forget about it! That is going to be far into the future before we can do that effectively.
With chemical rockets there is an opportunity to start building space infrastructure for manufacturing of fuel. This could be in orbit with material obtained from asteroids as well as on the moon or Mars. With a nuclear powered space future, we kill the incentive to start building up such an industry.
This is unfortunate because it is exactly the kind of training wheels we need. The first stepping stones before we begin more advance manufacturing in space. Nuclear engines will forever tie us to using a sophisticated industrial based located on earth.
But the future of space exploration is in establishing based outside of earth and being building up a self sufficient industry. When you start building up an industrial society from scratch it would be madness to base it off nuclear power.
You are several months of travel away from a sophisticated industrial base on earth. You are not going to have sophisticated tools, machines and nuclear physicists on site. You need simple robust technology which you have some measure of hope of being able to maintain and fix without too specialized knowledge and advance tools.