A Speedy Guide of Upcoming Propulsion Technologies

Imagine a trip to Mars that lasts 100 days. Sounds pretty crazy, doesn't it? Considering that the current estimates for a trip to our celestial neighbour is well over 200 days. Well, a future where we can reach Mars in 100 days may be closer than we think.

The UK Space Agency recently made a major announcement that they were considering the use of nuclear-powered spacecraft, uniting with Rolls-Royce for this mission. Although this sounds suspiciously like technology straight out of Amazon's "The Expanse", there has actually been a significant amount of research done into nuclear propulsion at NASA in the past, and it holds a lot of promise.

Nuclear-powered spacecraft has been proven to be successful before, based on NASA and the Atomic Energy Commission's Nuclear Engine for Rocket Vehicle Application programme that built and tested nuclear rockets -on Earth- that current nuclear thermal power rocket designs are based on today.

To really understand how this works, we need to break down how a typical propulsion system, in a chemical rocket, works, and compare it to a nuclear-powered spacecraft.

There are a few key terms that you should be aware of before looking further into rockets. The main term is a propulsion system. A propulsion system is a machine that generates thrust, which is the main output we are looking for from a rocket. Thrust is the force that pushes the rocket forward. The propellant is a chemical substance that is used to make energy, that generates propulsion. In the case of a rocket, the propellants used are fuel and an oxidizer.

A key principle to consider in propulsion systems is Newton's Third Law, which states that every reaction must have an equal and opposite reaction.

A simple free-body diagram of a rocket

We are able to obtain thrust by harnessing the power of chemical reactions in a chemical rocket engine through the mixing of propellants, which react to form gases that are accelerated and ejected through a nozzle. This is the exhaust gas that exists in the engine, and due to Newton's Third Law, the opposite momentum force, thrust, pushes the rocket up. The nature of the chemical reaction depends on the category of rocket engine; it will either be liquid or solid.

In a liquid rocket, propellants are stored separately as liquid and mixed in the nozzle combustion chamber. However, they are made out of complex parts, and require cryogenic storage (ie. very cold).

On the other hand, a solid rocket involves mixing propellants and packing them into a solid mass. This mass will only burn after ignition and will burn until all of the propellant has disappeared. The ability to pack propellants into a solid mass means that a solid rocket is easier to handle than a liquid rocket, since it can wait for a long time before firing, and also have higher thrust, but with a shorter burn time.

"Imagine a future where the furthest reaches of our solar system, worlds such as Saturn and Pluto, are accessible to humans?"

Now that the initial explanation of propulsion has been explained, as well as the engines that make up a typical chemical rocket, we can compare it to nuclear-powered spacecraft.

A nuclear thermal propulsion (NTP) system works by using a liquid propellant, but instead of using oxygen, as with a standard liquid engine, the system uses hydrogen. This hydrogen will pass through a reactor core, where, inside the core, uranium atoms are split apart and release heat through a process called nuclear fission.

The process heats up the propellant and converts it to a gas that is finally expanded through a nozzle to product thrust. It works on a similar basis to a classic propulsion system, although with nuclear elements added to it.

NTP systems are actually more efficient than a typical chemical rocket and are more energy-dense. How do we know this? Based on the specific impulse, which is the amount of thrust you can get from a set amount of propellant. Think of this like if you are pushing a toy car, and the more force you put into pushing the car, the further it will go. A nuclear-powered rocket can last for 900 seconds with the same amount of fuel that it takes for a chemical rocket to last for 450 seconds. With the car analogy, it's like pushing two cars with the same

Demonstration of specific impulse

force, and whilst one of them stops after a few seconds, the other one keeps travelling. This means that NTP systems can travel further with less fuel.

The reason why NTP systems are more efficient is due to the absence of oxygen. Instead, hydrogen, which is a lighter gas, produces a lighter byproduct.

Although NTP systems are more efficient, they will not be used for launching from the Earth, since they are not able to produce the same amount of thrust as chemical rockets. So we still will need chemical rockets to work with NTP rockets.

However, once in the vacuum of space, NTP systems do not need to exert a force against Earth's gravity and can be used to enhance deep space missions. They can actually reduce travel times to Mars by 25%, to around 100 days, thus limiting astronauts' exposure to cosmic radiation, which is a significant challenge in the space medicine world.

One of the challenges of nuclear rockets is the safety aspect. After all, a reactor will be on board the spacecraft, which will contribute radiation to the astronauts on that mission. However, the reduction in time exposed to deep space radiation does outweigh this drawback.

Since completing initial testing into NPT systems in the 20th century with regards to nuclear-powered spacecraft, new fuels have been developed. Low-enriched uranium, a safer alternative to the 1960s choice of highly-enriched uranium, drives down testing costs and allows more rocket facilities to run tests. Engineers have been running tests on low-enriched uranium to examine its performance in harsh temperatures that an actual NTP system would be exposed to on its mission to Mars.

We have now discussed nuclear fission, where atoms are being forced apart. How about the opposite, where instead of atoms being split apart, they are squeezed together to create energy? This is called nuclear fusion and can be done by squeezing hydrogen atoms into helium. A great example of fusion is actually our Sun, which combines light in the form of plasma, which is a state of matter comprised of free electrons and atomic nuclei, to generate a huge amount of energy.

However, modelling our fusion processes off of the sun would be an impossible challenge, due to its enormous mass that would not be possible to replicate on Earth. Being able to even sustain fusion energy, and then miniaturise it to mimic a fission reactor would simply be a huge step.

This technology sounds interesting, but likely a few lifetimes away, though. Perhaps not! A research group at the Princeton Plasma Physics Laboratory is working on a Direct Fusion Drive, which may become a reality closer than you may think. Their design uses magnetic fields to shoot plasma particles, which is electrically charged gas, into space to create thrust.

Remember the discussion about specific impulse, where NTP rockets are about 900, and chemical rockets 450? Well, the specific impulse of fusion rockets would be about 10,000 seconds, whilst generating electricity for the spacecraft when far away from the sun. A fusion reactor would also likely be able to power a spacecraft's instruments way out in deep space. and accelerated out of the rocket to create thrust.

To give you some things to think about, a current mission to Saturn would take about eight years. However, with a direct fusion drive, this could take two years with human passengers. The Voyager spacecraft, which would be dwarfed by an astronaut-carrying rocket, took over three years to reach Saturn. This means a spacecraft with human cargo would take significantly longer. Imagine a future where the furthest reaches of our solar system, worlds such as Saturn and Pluto, are accessible to humans? Would that not be life-changing?

Phew- that was a lot of information! Can you imagine what it would be like to be responsible for developing a propulsion system? You can actually learn more about innovative scientists, past and present, who have contributed to this fascinating field below.

Yvonne Brill was a Canadian-American rocket scientist, known for her contributions to rocket propulsion systems, and inventing the fuel-efficient propulsion system to keep satellites in orbit. She pursued a degree in mathematics and chemistry at the University of Manitoba after being barred from engineering due to her gender. However, she went on to obtain a master's in chemistry from the University of California and ended up working at NASA on various projects, including overseeing the Space Shuttle Solid Rocket programme.