This laboratory in a leafy part of Hampshire is where defence and security firm Qinetiq develops and tests its ion engines - a technology that will take spacecraft to the planets, powered by the Sun.

The pressure, meanwhile, can drop to a millionth of an atmosphere. Using helium gas as a coolant, they can bring down the temperature in the vacuum chamber to an incredibly chilly 20 Kelvin (-253C). A big fluffy ion like iodine or cesium would be almost as good since big fluffy ions generally don't have strong chemical bonds to things.The test chamber is one of the biggest in EuropeĮngineer Neil Wallace peers into a huge vacuum chamber designed to replicate - as far as possible - the conditions of space.Ĭryogenic pumps can be heard in the background, whistling away like tiny steam engines. That said, the vanilla option is to use yet another noble gas, such as krypton, because the inertness is convenient. It's inherent to the breadth of electric propulsion concepts and should be called out as such. Zinc, magnesium, potassium, iodine, argon, nitrogen, krypton etc. In real design work for bleeding edge electric thrusters, there are just an awful lot of potentially valid propellants. This excludes helium but otherwise covers most of the periodic table. For a thruster with a specific impulse of 2000 s, any atomic mass greater than 15 amu (threshold at which 15 eV work of ionization is half the work of propulsion) is adequate to achieve reasonably high energy efficiency, in addition to a few elements below that mass which have lower ionization energies. The atomic mass is, on its own, relatively unimportant. Xenon is conveniently inert and easy to store and ionize.

Given a desired maneuver, requiring a certain impulse, higher specific impulse requires less reaction mass but more energy. There are a variety of atoms and molecules which can be accelerated to the desired exhaust velocity. The correct to first order explanation is that specific impulse is the parameter that is traded.

Xenon is also attractive, aside from the cost, due to its low critical temperature and pressure, and thus the possibility of storing it with moderately high density.Īlso note that with modern material science it is increasingly practical to use propellants other than inert gases, such as halogens or metals. Helium is a bad propellant for ion engines because it would likely result in very poor energy efficiency. With argon it should still be doable, but the engineering is more difficult and less mature, but still of interest. For helium, 8.3 and 24.6.Ī real ion engine involves more physics than this, but with xenon it's relatively easy to use a large fraction of input power on propulsion rather than on ionizing the propellant. To accelerate an argon ion to 20 km/s requires 83 eV and ionization requires 15.8 eV. To accelerate a xenon ion to 20 km/s requires 273 eV, and to ionize it in the first place requires 12 eV. Imagine ejecting individual ions at that speed. To achieve 2000 s specific impulse, a representative design target, the average speed of the exhaust needs to be 20 km/s. The thrust of ion engines is generally limited by the available power, so it's very desirable for them to stay somewhat close to highest theoretical efficiency. However it's possible to achieve higher specific impulse than is needed, so other tradeoffs matter. In an ion engine that's not the case even though with a limited accelerating voltage lighter molecules go faster. The achievable temperature is limited so lightest exhaust products are best. In a thermal rocket, the speed of exhaust products is determined by combustion chamber temperature and mass and shape of products.

Helium electric propulsion