Rocket science

C&I Issue 5, 2016

In the 2015, Hollywood movie The Martian, the thruster used to power the Hermes spacecraft on its journey to Mars is a xenon-powered accelerator relying on an electric field for propulsion.

Today, the thrust generated on the 200 or so spacecraft using electric propulsion thrusters is very small because these devices operate at around 1 to 1.5 kilowatts – useful for adjusting the orbits of satellites above the Earth. However, an advanced ion thruster now promises to boost performance more than ten-fold, making it valuable for space exploration missions, including for a planned mission to capture and tow an asteroid back to lunar orbit.

‘We are developing a system that will operate at considerably higher power, somewhere between 10 and 15 kilowatts. We could combine multiple strings of these thrusters together to operate at 50 kilowatts or more,’ explains David Manzella, engineer at NASA’s Glenn Research Center in Cleveland, Ohio. ‘These types of systems could be used to cost effectively position assets such as landers or habitats or return stages in support of human exploration missions to Mars.’

Nasa’s prototype thruster, which uses xenon as fuel, will be transformed over the next three years into a space-ready thruster by Aerojet Rocketdyne of Redmond, Washington, US. Specifically, the new ion thruster is a ‘Hall-effect thruster.’ In this Hall-effect, electrons are magnetised such that they are trapped circulating within the thruster where they collide with heavy xenon atoms to generate additional ions and electrons. Ions are accelerated by an electric field created within the thruster, accelerating away from the spacecraft at velocities of 30,000ms-1. Ions thrusters only work in the vacuum of space, not in the Earth’s atmosphere.

NASA favours xenon as a propellant because it’s heavy and generates sizeable momentum when accelerated through an electric field. It is also relatively easy to ionise and has attractive storage properties. A xenon-powered ion thruster was used on the Dawn mission to the asteroid Vesta in 2011-2012, and that craft is now orbiting the proto-planet Ceres. Around 425kg of xenon was fitted in a spherical tank about 32 inches in diameter, but the power generated restricted the spacecraft’s size.

The Asteroid Redirect Mission is a mission under development that would launch in around 2020 and would look to return a 20t asteroid to the moon’s orbit in five years, requiring about 5000kg of xenon to complete the mission. The xenon would be stored in a tank 42 to 48 inches in diameter, perhaps 6 to 8 feet tall, according to Nasa estimates. The fuel economy of xenon in the new thruster is ten times higher than for chemical propellants – for a given mass, you get ten times more thrust.

‘The amount of propellant can be drastically lowered and because it costs on the order of $10,000/lb to put it into orbit, this could have a significant impact on the cost of doing space missions,’ Manzella explains.

Ultimately the device will need an input of power, which is expected to come from advanced solar arrays. These are large gossamer structures that efficiently fold up for launch and then deploy in space.

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