Space savers

A project to design micro electric propulsion thrusters for small satellites could help control overcrowding in low-Earth orbit and allow small satellites to journey further into space.

Researchers at Surrey Satellite Technology (SST) hope the thrusters will be used to bring satellites safely down to Earth, where they will burn up in its atmosphere.

The development could help solve the satellite industry’s growing problem of space junk, particularly in the low-Earth-orbit band between 700km and 900km, where debris and old or broken satellites can remain in orbit for decades before gravity pulls them back to Earth. The orbiting debris can impact and damage working satellites.

‘Basically we are self-policing,’ said Peter Shaw, one of the researchers on the SST project. ‘We are doing the right thing by not leaving satellites in space to clutter up important orbits.’

While de-orbiting will be the initial purpose for the thrusters, the researchers say they would like them to do much more in the future. Shaw said they hope micro electric propulsion thrusters will one day replace chemical thrusters and be the main power source for small satellites ranging in size from a TV set to a washing machine.

SST is focusing on a specific electric propulsion thruster — a pulsed plasma thruster — that uses plasma as its building block for electric propulsion.

‘There is a big advantage in using electric propulsion on very small satellites,’ Shaw said. ‘We measure thrust in aspects known as Isp, specific impulse. For a chemical thruster, it could get up to 350Isp or 400Isp. But with electric propulsion, you are looking at 2,000 -10,000Isp. So it’s a lot more fuel-efficient.’

Small satellites with electric propulsion would have more room to add components or could be launched for less cost. This means they could one day be used to do missions once reserved for large satellites.

Shaw said a limiting factor for small satellites with chemical thrusters is they are only able to hold a relatively small amount of fuel, which means they are essentially bound to low-Earth orbits.

‘When you get any further than those orbits you need a lot more fuel and, of course, there is not a lot of space on small satellites to give a lot more fuel,’ he said.

‘So if you want to put small satellites out into larger orbits, then you need to look at electric propulsion solutions.’

Building smaller satellites with electric propulsion presents many design challenges, however. It’s much easier to shrink the size of satellites with chemical thrusters, Shaw said, because it only requires shrinking the propellant tank and the feed lines to the thrusters.

‘With electric propulsion, you’re taking energy from solar panels, storing it in batteries and using that energy for your thruster’s device, and the big limiting factor is how much power you can get from your solar sails,’ he said.

‘For large satellites, getting power is not such a big problem. But when you’re talking about satellites the size of TVs or washing machines, you’ve got limited amount of solar array space and power and only certain types of electric propulsion work in this regime.’

Shaw said satellites with colloid thrusters are difficult to shrink because these thrusters work on hundreds of watts. ‘For pulse plasma thrusters it’s not a big problem,’ he said, ‘because they only use 10-50W of operating power.’

The SST researchers said they have already constructed some prototype pulse plasma thrusters and will test them in the vacuum chambers at Worcester Polytechnic Institute in Massachusetts this summer.

More than anything else, the main goal of the project is to develop an efficient satellite system that will make access to space more affordable.

The way to do this, the researchers reckon, is to increase fuel efficiency and reduce the size of satellites.

Shaw said this will bring exciting possibilities to small satellite developers such as SST.

‘At the moment small satellites are very good at contending with large satellites in low Earth orbit, but the big satellites still reign supreme when you are looking at the Moon or missions to look at the Sun,’ he said.

‘The big satellites are still the bullies of the playground and to give small satellites the option to go and contend with these big guys will mean big business for small satellites. It will mean small satellites can be used for good science missions as well.’