A nice upgrade to existing satellite engine technology comes out of Georgia Tech, where researchers have developed a design that allows the engine to optimize available power, much like the transmission of a car. Thus the engine can burn at full throttle in ‘first gear,’ maximizing acceleration, while dropping into a much more economical gear for long-term space operations. “You can really tailor the exhaust velocity to what you need from the ground,” says team leader Mitchell Walker.
The engine at work here is known as a Hall effect thruster, a plasma-based propulsion system that operates with xenon, a gas that is injected into a discharge chamber where its atoms become ionized. The electrons that are stripped from the outer shell are trapped in a magnetic field, while the heavier xenon ions are accelerated out into space by an electric field. What Georgia Tech has introduced is better control over the exhaust stream through an enhanced electric and magnetic field design.
Image: Georgia Tech’s enhanced engine uses a novel electric and magnetic field design that helps better control the exhaust particles. Ground control units can then exercise this control remotely to conserve fuel. Credit: Georgia Institute of Technology.
A key factor is specific impulse (ISP), which measures how much thrust is produced per unit of fuel in each second of an engine’s burn. Looked at another way, ISP is a measure of how many seconds one pound of propellant can produce one pound of thrust. As specific impulse (stated in seconds) rises, it takes less fuel to produce a given amount of thrust, and the amount of payload compared to propellant can also rise.
In a telephone interview, Walker told me that the Georgia Tech work is not about creating a new engine but pushing existing engines into regimes in which they normally don’t function well:
We took an engine that normally runs at 2500 seconds and we backed it down to 1000 seconds. In other words, we traded exhaust velocity for more thrust. When we did that the engine efficiency dropped from 65 percent down to about 25 percent — the engine did not like to run there. What we’ve been able to do is to focus ions that would otherwise crash into the chamber wall to create huge efficiency losses. That drives the efficiency of the engine back up while running at 1000 seconds.
All of which is good news for various space missions, since the design — modified from a donated Pratt & Whitney satellite engine — can reduce onboard fuel needs by 40 percent, thus freeing up space for payload. An already efficient engine thereby becomes more stingy still with its fuel using proven technologies. Remember that Deep Space 1, launched in 1998, tested out ion propulsion using xenon, producing only one-fiftieth a pound of thrust at full throttle, but with high specific impulse.
For that matter, the European Space Agency’s SMART-1 lunar mission used solar-electric methods, with the electricity generated from its solar panels being used to accelerate xenon ions. Today’s low thrust ion engines are so efficient that they can run for months or years. Researchers at the Jet Propulsion Laboratory operated an NSTAR thruster for a continuous 30,352 hours — these engines are workhorses — and both power and specific impulse are being improved in ion thruster designs like NEXT, the NASA Evolutionary Xenon Thruster.
A good backgrounder on ion propulsion from New Scientist is here, with details on NEXT. The classic book on the subject is Robert Jahn’s The Physics of Electric Propulsion (McGraw-Hill, 1968). A 2006 paperback edition from Dover brings this core text back onto bookstore shelves.