We are all painfully familiar with the cost of filling the tank – even with a hybrid, visiting the gas pump isn't fun. Similarly, NASA is familiar with the high cost of launching vehicles from Earth, as it takes about $10,000 per pound to launch things into space.
But, what about propulsion once the craft is up there? Satellites, spacecraft, and deep-space probes require thrusters to keep them in place or push them wherever they need to go – not an easy task when space, payload, and cost are incredibly finite.
NASA’s answer to these issues has been the development of ion propulsion engine technology, and projects like NASA's Evolutionary Xenon Thruster (NEXT) Project at the Glenn Research Center. The NEXT Thruster was recently sanctioned to be shut down after running for more than 48,000 hours, or somewhere around five and half years, as a test of the system.
The test was performed in a high-vacuum chamber where the engine used about 870 kg of xenon propellant.
The NEXT Thruster uses Xenon propellant, which is used for most electric propulsion systems. Xenon is an advantageous propellant because it’s inert, so it is non-reactive and non-contaminating to ground test facilities or spacecraft surfaces, and it has a high Atomic mass unit (AMU), which results in a high thrust-to-power ratio.
Also, Xenon propellant is highly condensable (about twice the density of water at high pressure), making it ideal when space is limited. Using Xenon propellant, this thruster technology could readily change a spacecraft velocity over the duration of the thruster lifetime by greater than 10 km/second.
The amount of impulse produced would have consumed more than 10,000 kg of conventional rocket propellant in similar applications. Ion propulsion is nothing new to NASA, but with NEXT, they are hoping to take science and observation to further and more challenging destinations than ever before.
The Key to Ion Propulsion
NEXT is an ion propulsion system that includes a 7-kw-class thruster intended to operate on solar electric power. Mike Patterson, principal investigator for the NEXT project, explains the electric propulsion process as accelerating charged particles (ions) with electric fields.
“With electric propulsion, your energy source comes from electricity," explains Patterson. Electric propulsion doesn’t produce very much thrust because the engine is not expelling a lot of propellant, but the exhaust velocity is very high – up to 90,000 mph with NEXT.
“Because we produce relatively low thrust, we have to operate for longer periods of time to get the same total impulse,” Patterson adds. Though the thruster requires a longer period to get where it’s going, the efficiency is greatly improved with this method of propulsion – hence the long-duration test. “Since we’re no longer energy limited by the chemical propellant, we’re only energy-limited by the amount of available solar power that can fit on the spacecraft.”
Ion propulsion engines, like the NEXT Thruster, have already been employed on numerous commercial spacecraft, as well as two previous NASA missions – Dawn and Deep Space 1. “The NEXT Thruster is the evolutionary higher-power version of the technology flown on those missions,” says Patterson.
NASA science missions go through a competitive process for technology. “They might have 30 proposals for one mission, from those 30 you have a down-select to maybe 3 for what’s called a 'Phase A study,' and then they’ll down-select one to go to 'Phase B.' From that, you’ll end up flying one mission four or five years later,” explains Patterson. NASA science missions are risk-adverse, so the researchers choose the lowest risk, lowest cost system to perform their objective.
NEXT, though it has been on the docket a few times, is still fairly new technology, making the risk higher than other, more tested technologies. “The easy stuff, the low-hanging fruit has been picked out of the solar system, so we’re getting to the point if you want to conduct a high-value science, you have to start relying on advanced propulsion, like NEXT,” says Patterson. “[NEXT] is the natural progression beyond the ion propulsion system flying on the Dawn Mission. NEXT has been identified as critical for missions in NASA’s Decadal Survey, which has identified planetary science missions that need to be performed over the next decade.”