Open and shutter case

A tiny radiator to control heat emissions inside small satellites was launched aboard a test spacecraft last week, heralding a new generation of nano-satellites, claim its developers.

Created by researchers from the John Hopkins University Applied Physics Laboratory near WashingtonDC, the temperature control device — Variable Emittance Coating for Thermal Control (Vari-E) — is based on MEMS technology. The work was conducted in conjunction with NASA’s Goddard Space Flight Centre.

The system actively controls how much heat the spacecraft emits into space by opening or closing the shutters on the radiator. The new shutters are micro-machined on to a highly emissive silicon substrate.

Opening the shutters allows heat from the spacecraft to be dispersed in the form of infrared radiation, as it exposes a section of this special silicon that readily emits heat.

The 9x10cm radiator comprises 36 chips, each fitted with 72 individual microscopic shutters. The shutters are controlled by six tiny comb-shaped motors powered by electrostatic charges.

Robert Osiander, the project’s principal investigator, believes the device represents a complete change in the way heat is managed on satellites. ‘On-board a satellite you need the temperature to be kept absolutely steady at around 300K,’ he said. ‘The heat created by the electronics can affect the spacecraft’s performance and so it needs to be controlled extremely precisely.’

Larger spacecraft use a similar system but with much larger shutters, and removing heat only in a passive manner. According to Osiander, the removal of heat from micro and nano-satellites becomes even more important because the on-board electronics are so dense.

‘On a large spacecraft you use the batteries on-board to heat up the craft, with a fixed radiator cooling it at a constant and fixed rate. You heat or cool the spacecraft by powering the battery up or down, but in a small spacecraft you do not have the battery power to do that,’ he said.

This means that a new way of removing heat had to be developed, which controls the amount of heat emitted rather than how much heat is produced. One big advantage is that the device does not need much power to work and functions on small amounts of electrostatic charge drawn from the electronics from spacecraft control on-board the ship.

It works by automatically opening or closing the shutters to keep the temperature stable, but can also be operated by remote control if a temperature change of a certain amount is required.

A prototype of the device has been fitted to one of NASA’s three Space Technology 5 satellites, which were launched using a Pegasus rocket last Wednesday. These small satellites are being used to explore the Earth’s magnetic fields and test new space technology that could feature in future NASA missions.

According to Ann Derrin, the system’s program manager, this was the first time a MEMS device of this type has been flown on the outside of a satellite.

One concern that the team had about the device was its robustness, as dust and condensation could impact upon its performance. So the research team decided to encase the radiator inside a window made from a rugged polymer known as CP-1 to protect it from debris in space. The polymer is more cost-effective than crystal sapphire — which could provide the same protection — and does not affect heat emission.

‘People think that because it is small it is fragile,’ said Derrin, ‘but actually the less mass it has the more benefits it might have.’

To avoid wear and tear the shutters, which were extensively tested at the university’s laboratory before launch, do not touch when closed.

‘The use of these MEMS structures on satellites is fantastic,’ said Derrin. ‘It is an enabling technology that will allow for smaller NASA and ESA missions. The weight of these satellites is one of the major costs involved so if you can use MEMS in RF switches or telemetry they are so enabling that it opens up a whole new generation of satellite systems.’

Osiander added that while the prototype is testing only the heat control of a small section of the spacecraft, large areas of a nano-satellite could in future be covered with such a thermal control coating.