How Nuclear Engines Work
By: Paul May
Tags: Engine, Nuclear, Science, Energy, Rockets

Photo by Christian Wiediger on Unsplash

If you have read the news lately you may have read about the nuclear accident at Nyonoksa that occurred in Russia recently. The cause of this was attributed to a failed test of nuclear-powered engine believed to be for the 9M730 Burevestnik cruise missile. This missile is claimed to possess unlimited range when in flight due to its use of a nuclear engine. But how do they work and where did they come from?

History

The idea of using nuclear power to provide motive force for a vehicle is in fact not new and something that many nations have looked at over the years. Some examples are:

There was always found to be issues with the usage of many of these, covering things like nuclear proliferation risks, accident mitigation (e.g. if there’s a car crash how do you stop a reactor spilling its contents everywhere), shielding and subsequent weight problems. The weight problem was found in many of the manned vehicles because it was simply not possible to provide enough shielding to protect the aircrew and still have the aircraft take off.

This weight issue is mitigated in maritime vessels because of the larger weights they can carry and in applications where there are no crew (such as missiles and spacecraft) there is no need for shielding once in flight (the launch sites may require shielding for the ground crew). In fact spacecraft and rovers often utilise radioisotope thermoelectric generators (RTG’s) in areas where batteries or solar panels do not suffice. RTG’s work by using the heat from the decay of radioactive material to power and keep their electronics warm (e.g. Curiosity Rover). As a side note RTG’s are used terrestrially such as in unmanned lighthouses in Soviet territories.

Photo by SpaceX on Unsplash

But how does a nuclear reactor give a missile or aircraft unlimited range?

To explain this, we have to look at how rockets for space travel work. At their most basic the engine is a small compartment where fuel and oxygen are burnt within and the resulting hot gases expand and leave through a purposefully placed hole. The gases leaving propel the engine (and attached craft) in the opposite direction.

The reason for this is due to Newton’s Third Law of Motion which states that:

“For every action, there is an equal and opposite re-action.”

You may have seen this in science class in school where a teacher on a wheeled chair sets off a fire extinguisher propelling themselves to the other end of the class. The gas coming out of the extinguisher causes a force to be exerted on the teacher and they travel away. The main take away from this is that if you were to increase the volume of gas being expelled, the weight of the material being expelled or the speed that it is expelled at then you will increase the opposing force.

Photo by Stacy Olivier on Unsplash

How can you increase the speed of the rocket exhaust?

The speed the exhaust gases leave the engine is determined by the amount of energy the heat the burning fuel imparts to it. If you burn more fuel you get more energy given to the gases, but you also generate more exhaust gases, so you also increase the total mass of material coming out the back.

Another way is to make the fuel burn more efficiently (i.e. make it burn much much hotter) or contain more energy. The downside is that high energy rocket fuels are often expensive and very dangerous (e.g. they will corrode materials or present temperature hazards such as liquid oxygen).

These downsides are often acceptable in the field of space travel where a key point in designing the fuel and engines for space travel is to make the fuel go as far as possible for the same level of performance. If you can make a fuel that imparts the same thrust for half the weight of fuel that an old system uses then the rocket needs to carry less fuel, will be lighter and much cheaper to get into space. It also means that the space craft can burn its engines for longer giving it a longer operation life if it is a satellite (often when satellites are decommissioned it is because they are low on fuel and not because of another fault).

How does this work for aircraft?

The same ideas for space travel work for aircraft in our atmosphere, where the less fuel you need to travel a certain distance (either by engine or fuel design) then the further it can travel before refuelling extending its range.

Aircraft also have a significant benefit that they can suck in air to combine with the fuel in the engine meaning they do not have to carry the oxygen with them (as space rockets must do because there is no air in space) reducing the amount of weight they need to carry. This further extends their range for an equivalent system where they have to carry both oxygen and fuel to burn.

Photo by Herry Sutanto on Unsplash

So why do missiles use rocket engines then?

This is because rockets can be a lot simpler than the complex ones that commercial aircraft use and are often easier to keep stored for long periods in a state ready to be quickly used.

With conventional aircraft engines there is a need for complex fan assemblies (look at the front of a commercial airline engine) and ducting to take the incoming air and compress it so the engine can burn it with fuel. There is also a limit to the speed conventional aircraft engines can achieve before their moving parts fail or the heat of the fuel causes them to melt.

As I said before if you can make the fuel burn hotter then more energy is available for the exhaust gases to absorb before they come out of the engine. The downside is that the engine must be capable of containing and operating with such engines running.

How would nuclear engines work?

In a nuclear reactor the fuel is broken apart by neutrons in a controlled manner. By doing this large amounts of energy is released in amounts much greater than a similar mass of even the most advanced rocket fuel can contain.

During the cold war three main designs of nuclear rocket engine emerged: solid, liquid and gas core engines. While each operates slightly differently the basic principle is to feed a propellant (the material we want to heat and expel out of the engine as exhaust gases) into the reactor, have it absorb large amounts of the heat contained within and then emerge at high speed from the engine, propelling the engine forwards.

Solid-Core Nuclear Rocket

This is the most traditional and simple design. Here a nuclear reactor core has the propellant fed around it to pick up the heat and is then fed out of the rocket exhaust nozzle. The fuel in the core is kept solid and the propellant does not come into direct contact with it and the exhaust contains no radioactive material. This is important in a terrestrial design as you generally do not want the exhaust from a nuclear rocket to leave radioactive fallout behind it.

The main limitation of this design is that because the fuel must be kept solid there is a limit to how hot the reactor can get before it will melt. This limits the amount of energy it can generate and therefore the maximum speed it can give a set volume of propellant.

Liquid-Core Nuclear Rocket

Here the nuclear fuel can achieve much higher temperatures because it can melt becoming a liquid. The reactor itself is rotated at high speeds in order to keep the fuel “stuck” to the sides of the reactor and not out of the back of the engine. This is important as you do not want unused fuel being lost, reducing the efficiency of the engine, and you would be leaving radioactive material behind.

However, it would be exceptionally hard to design the reactor so no radioactive material is released while taking advantage of the higher temperatures the liquid fuel can generate.

Gas-Core Nuclear Rocket

This design takes the temperatures even higher by having the fuel boil and become gaseous. To keep the fuel inside the engine a vortex is required which contains the gases (think of a tornado) which is complex to do as it also requires the engine to feed nuclear fuel into this vortex to keep the reactor running and also propellant to provide thrust without it all become mixed and lost out of the engine.

Nuclear Engine Usage on Earth

As we have seen there are three basic designs, but it is likely that the solid-core reactor will be the one utilised. This is because it is less complex, is safer to use and will not cause radioactive fallout to be emitted from the engine.

The engine can also be more easily moved so that it is contained elsewhere, and the heat is sent to the engine through a heat exchange system, which further reduces chances of nuclear material emerging from the engine.

The downside is that the reactor will cause a lot of damage if the rocket crashes and are much more expensive than conventional engines, but if utilised to carry a nuclear warhead these downsides are probably trivial to the result of a nuclear blast.

The designs I’ve shown so far also all use a propellant carried on the craft to feed the engine, this limits the range to the amount of propellant carried. But if adapted to use the outside air as the propellant the range is only limited by the useful life of the reactor (where the nuclear fuel is used up).

The most likely design modification would be to a ramjet and is the most likely version used in the 9M730 Burevestnik cruise missile that may have been the cause of the recent nuclear engine failure.

Nuclear Ramjets

In a conventional jet engine the outside air must be compressed before the engine can add fuel and burn the resulting mix. This adds a lot of complexity.

What a ramjet does is use the forward motion of the craft to compress the air without the use of fans. This greatly simplifies the engine, lightening and making it more (better to fit into a missile housing that can be carried by other vehicles), but requires the engine to be travelling above a certain speed to ensure the air is compressed enough. To get it up to that speed a booster rocket engine is often used which can be jettisoned later (similar to a JATO used on aircraft).

Summary

So, we have looked at:

A short History of nuclear engines

The application of nuclear reactors to travel

The basic nuclear engine designs

Discussed the most likely design to be used on earth

I hope you enjoyed reading this article as much as I enjoyed writing it. Until next time!