Even though spacecraft represent an ultra modern technology, the essential physics that drives their motion dates way back to the seventeenth century and Sir Isaac Newton. Newton’s third law tells us that for every action there is an equal and opposite reaction. So when you fire a gun the bullet moves forward and the gun recoils backwards. Rocket engines operate on the exact same principle. Propellant is thrown out of the back and because momentum has to be conserved, the rocket moves forwards.
However, not all rockets are created equal. An engine that throws its propellant out very quickly generates far more momentum and therefore far more thrust for each kg of propellant used, than one that does so slowly. Each kilogram of propellant you burn in space requires about ten kilograms of propellant to get it up there, so efficiency in propellant use is something that space engineers care very much about.
The problem with conventional chemical rockets is that there is a limit, set by thermodynamics, to how fast gas can be ejected which is around 3000 ms-1. Another way to eject propellant is to ionize a gas and use an electric or magnetic field to accelerate it. This can achieve exhaust velocities of ten or even a hundred times higher than those of a chemical rocket, so the precious propellant is used far more efficiently. This makes plasma (ionised gas) thrusters the technology of choice for manoeuvring satellites or long space missions.
A new type of plasma thruster has recently been developed by Professor Christine Charles at the Australian National University. This HDLT (Helicon Double Layer Thruster) offers several advantages over conventional plasma thrusters not least of which is its simple robust design that should offer much higher reliability than existing units.
Unlike a conventional ion drive, the HDLT ejects both positive ions and electrons in its exhaust. This avoids charging effects on the spacecraft and also eliminates the need for separate electron ejecting neutralisers, that are one of the most common sources of failure on existing ion thrusters.
The HDLT has attracted attention around the world and currently forms part of a collaboration between Ariane rocket manufacturer ASTRIUM, the satellite arm of the European Aeronautic Defence and Space (EADS) and the ANU. But one of the problems with building revolutionary space systems is how to test them. It costs many millions of dollars to launch any spacecraft and naturally no one wants to be the first to run an untested manoeuvring thruster on their satellite, however promising it may be.
The answer to this is to build a facility on Earth that can simulate the environment of space well enough to be able to refine the thruster design and establish its reliability. Recent funding from the Australian Government will allow the HDLT team to do just this.
The Advanced Instrumentation and Technology Centre at Mount Stromlo Observatory is rapidly becoming one of the most advanced astronomical instrument and spacecraft assembly facilities in the country. So it makes perfect sense to build the new simulation facility there, along with many other space related facilities.
Essentially the new test facility will be a large tank that can accommodate the test thruster and be pumped down to a vacuum approaching that of space. But this is a more complicated task that it may first appear. The problem with a plasma thruster is that it spews ionised gas out into the chamber. If this accumulates in any quantity it makes the environment within the chamber less space-like and distorts the results of the tests. Consequently, the test facility has to have the fastest and most efficient pumps available to maintain good vacuum as the thruster runs. Achieving such a vacuum begins with simple rotary pumps then turbo molecular pumps, like miniature jet engines, then finally cryopumps. A cryopump works by cooling an active area down to close to absolute zero so that any gas that comes into contact with this liquefies or freezes instantly. This is a very effective way to remove residual gas and troublesome contaminants like water and oils from a high vacuum system.
Another stress that spacecraft face is thermal cycling. As they pass the sunlit side of the Earth they heat up then as they are plunged back into the freezing cold of the Earth’s dark side they cool down again. This can happen every few minutes depending on the orbit and the resulting repetitive expansion and contraction can easily cause components to fail. To simulate this in the test facility, the tank will contain huge shrouds that can be heated or cooled with liquid nitrogen. In this way a prototype device can be exposed to such thermal cycling then removed and examined for signs of failure. A luxury that engineers never get on spacecraft in actual orbit.
The new test facility is expected to be operational by the end of 2012 so that actual flight testing of the HDLT thruster can begin in 2013/14.