Project Orion and the Forgotten Potential of Nuclear Pulse Propulsion
Overview of in-space propulsion
There are three main types of spacecraft propulsion: chemical, non-chemical and advanced propulsion technology.
Every spacecraft propulsion system works using conservation of momentum and Newton’s third law. Basically, momentum is always conserved, and for every force there is a reaction force of equal magnitude acting in the opposite direction.
Today, typical rockets and spacecraft exhibit this by obatining energy through chemical reactions to create a hot gas which, as it expands, is expelled through a nozzle. The mass chucked out the back is known as the reaction mass.
The other less common system is non-chemical propulsion, also known as electric propulsion. Rather than using chemical reactions to obtain energy, electric propulsion uses electrical energy to directly accelerate the reaction mass using electromagnetic forces. In this case, the reaction mass is a stream of ions.
There are many different ways of implementing electric propulsion but they broadly fall under three types: ion thrusters, electrothermal thrusters and electromagnetic thrusters.
Performance metrics
The two most important metrics for propulsion systems are thrust and specific impulse. Let’s start with thrust.
Thrust is fairly intuitive. It is a measure of instantaneous force.
Basically at a point in time, what is the maximum force a given propulsion system can exert on a spacecraft? Force is proportional to the rate of change of momentum (Newton’s second law), so the higher the thrust, the greater the acceleration.
Specific impulse (ISP) is the ratio of the thrust produced to the mass of propellant used. It is a measure of efficiency. The higher the ISP, the more efficient the propulsion system is.
Chemical propulsion uses a lot of mass, but does not require a great deal of energy supplied since the energy is obtained through the mass itself undergoing chemical reactions. So it obtains very high thrusts, but has relatively low ISPs.
Non-chemical/electric propulsion must fire ions out at very high speeds, so requires a lot of energy to work. So it provides relatively low thrusts but uses very little mass so has incredibly high ISPs.
Nuclear pulse propulsion (NPP) falls into the third category: advanced propulsion systems. The reason why it is so exciting, is because it achieves outrageously high thrusts whilst having a significantly higher ISP than chemical rockets.
For context, the Merlin engine which SpaceX uses has a thrust of 411 kN and an ISP of 342 s. The NPP system below was predicted to have a thrust as high as 40 MN and an ISP on the order of 10,000 s!
How does it work?
In the 1950s, scientists wanted to see whether the power of the atomic bomb could be harnessed for “more peaceful uses”. This is when nuclear pulse propulsion was first hypothesised, in the form of Project Orion.
The goal of Project Orion was to create a propulsion system that could allow for a manned interstellar mission to Mars.
It works on the principle of detonating small nuclear pellets close to a spacecraft and have the resulting propellant plasma push the spacecraft forward.
The design consists of a pusher plate connected to propellant magazines via a series of shock absorbers. The propellant magazines store nuclear pulse units/pellets, where each unit has a nuclear fission device at its core.
The units are ejected through a hole in the pusher plate, and then the fission device explodes.
The acceleration caused by the plasma hitting the pusher plate would be on the order of 50,000 g , where g is the acceleration due to gravity on the surface of the Earth (9.8 ms^-2).
This acceleration is far too high for a human to withstand, so the shock absorbers aim to store the energy of the pressure plate and gradually transfer the momentum to the payload.
In essence, you are literally using exploding atomic bombs behind your ship to accelerate it.
It seems barbaric, but unlike many outlandish propulsion methods in the third category, NPP is highly feasible with today’s technology. Not only that, but as described above, the numbers are remarkable.
So what happened?
While the system may work in theory, its development has been blocked for several reasons.
As one might expect with something that has “nuclear” in the name, the main challenge is actually a very political problem: the Partial Test Ban Treaty of 1963.
This treaty banned the use of nuclear explosions in space. That alongside funding problems, there was little to salvage of Project Orion.
In fact, the US did try to make an exception in the treaty for in-space propulsion systems, but Soviet fears about military applications stopped this from occurring.
Despite this, many projects have tried to work around this by designing the system in such a way that the thermal power dissipation is decreased by using much smaller fission pellets for detonation, as opposed to “full nuclear bombs” proposed in Project Orion.
The most notable of these projects are Project Longshot, Project Daedalus, and MiniMag Orion. However they are all much smaller in scale than Project Orion.
Most of the other problems arise from typical challenges associated with containing nuclear systems.
For example, exposure to many nuclear blasts of a long period of time would likely lead to ablation (erosion) of the pusher plate mentioned earlier. Although, experiments and calculations indicated that a steel pusher plate sprayed with an oil would not ablate at all.
I find it a great shame how Project Orion ended. There was huge amounts of potential, but the recurring theme of “fear everything nuclear” prevented it from flourishing.
I believe that with our modern day advancements in nuclear physics, as well as computer simulations for propulsion system testing, nuclear pulse propulsion could be humanity’s best method for traversing space.
