A new era of spaceflight? Promising advances in rocket propulsion


The United States Defense Advanced Research Projects Agency (Darpa) has recently put into service three private companies, Blue Origin, Lockheed Martin and General Atomics, to develop nuclear fission thermal rockets for use in lunar orbit.

Such a development, if realized, could usher in a new era of spaceflight. That said, this is just one of the many exciting avenues in rocket propulsion. Here are a few more.

Chemical rockets

The standard means of propulsion for spacecraft uses chemical rockets. There are two main types: solid fuel (like the solid rocket thrusters on the space shuttle), and liquid fuel (such as the Saturn V).

In both cases, a chemical reaction is used to produce a very hot, highly pressurized gas inside a combustion chamber. The engine nozzle provides the only outlet for this gas which therefore expands out of it, providing thrust.

The chemical reaction requires a fuel, such as liquid hydrogen or powdered aluminum, and an oxidizer (an agent that produces chemical reactions) such as oxygen. There are many other variables that ultimately determine the efficiency of a rocket engine as well, and scientists and engineers are always looking to get more thrust and fuel mileage out of a given design.

Recently, private company SpaceX conducted test flights of their prototype Starship launcher. This vehicle uses a “full flow staged combustion engine (FFSC)”, the raptor, which burns methane for fuel and oxygen for the oxidizer. Such models were tested by the Russians in the 1960s and the US government in the 2000s, but none have yet flown into space. The engines are much more fuel efficient and can generate a much higher thrust-to-weight ratio than traditional models.

Thermal fission rockets

The nucleus of an atom is made up of subatomic particles called protons and neutrons. These determine the mass of an element – the more protons and neutrons, the heavier it is. Some atomic nuclei are unstable and can be split into several smaller nuclei when bombarded with neutrons. It is the process of nuclear fission, and it can release a huge amount of energy. As nuclei decay, they also release more neutrons which crack more atoms – producing a chain reaction.

In a nuclear fission thermal rocket, a propellant, such as hydrogen, is heated by nuclear fission to high temperatures, creating a high pressure gas in the reactor chamber. As with chemical rockets, this can only escape through the rocket nozzle, again producing thrust. Nuclear fission rockets are not envisioned to produce the kind of thrust needed to lift large payloads from the Earth’s surface into space. Once in space, however, they are much more efficient than chemical rockets – for a given mass of thruster, they can accelerate a spacecraft to much higher speeds.

Nuclear rocket engine transported to the test stand at Jackass Flats, Nevada, in 1967.

Nuclear fission rockets have never been launched into space, but they have been tested on the ground. They should be able to shorten flight times between Earth and Mars from about seven months to about three months for future crewed missions. However, obvious drawbacks include the generation of radioactive waste and the possibility of launch failure which could result in the dispersal of radioactive material over a large area.

A major challenge in engineering is to miniaturize a reactor enough to fit a spaceship. There is already a booming industry in the production of compact fission reactors, including the development of a fission reactor which is smaller than an adult human.

Electric propulsion

A must for science fiction, true ionic entrainments generate charged particles (ionization), accelerate them using electric fields and then trigger them from a propellant. The propellant is a gas such as xenon, a fairly heavy element that can be easily charged electrically.

Image of a NASA ion thruster.
NASA Deep Space Ion Booster 1.

As the charged xenon atoms accelerate out of the thruster, they transfer a very small amount of momentum (the product of mass and speed) to the spacecraft, providing gentle thrust. Although slow, ion drives are among the most fuel-efficient methods of propelling spacecraft, which could take us further. Ionic drives are commonly used for attitude control (changing the direction a spacecraft is facing) and have been considered for desorb old satellites.

Current ion engines are powered by solar cells, which makes them effectively powered by solar energy and requiring very little propellant. They were used on Esa SMART-1 Mission to the Moon and Bepi-Colombo Mission on the way to Mercury. NASA is currently developing a high power electric propulsion system for the Lunar gate, an outpost that will orbit the Moon.

Solar sails

While propulsion generally requires a propellant of some description, a “greener” method relying solely on sunlight itself.

Image of the solar sail used on Ikaros.
Ikaros solar sail.
Pavel Hrdlička, Wikipedia, CC BY-SA

Sails are based on the physical property of conservation of momentum. On Earth, we are used to seeing this momentum as a dynamic pressure of air particles blowing into a listening while navigating, propel a ship forward. The light is composed of photons, which do not have mass, but which have momentum and can transfer it to a sail. Since the energies of the individual photons are very small, an extremely large sail size is required for any appreciable acceleration.

The speed gain will also depend on the distance from the Sun. On Earth, the power received from sunlight is about 1.3 kW per square meter. If we had a wing the size of a football field, that would equate to 9.3 MW, providing very low acceleration even to a low mass object.

The solar sails have been tested by the Japanese IKAROS spaceship who successfully flew by Venus and the Planetary Society Lightsail-2, which is currently orbiting the Earth.

One way to improve efficiency and reduce the size of the sail is to use a laser to propel the spacecraft forward. Lasers produce very intense beams of photons that can be directed at a sail to provide much higher acceleration, but would need to be built in Earth orbit to avoid a loss of intensity in the atmosphere. Lasers have also been proposed as a way of deorbing space trash – laser light can slow down a piece of orbital trash, which would then fall out of its orbit and burn in the atmosphere.

The development of nuclear fission rockets may excite some and worry others. However, as private companies and national space agencies increasingly commit to a sustainable human presence in space, these alternative means of propulsion will become more mainstream and have the potential to revolutionize our nascent space civilization. .

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