MEMS temperature regulator hitches a ride into space


By David Raths

NASA’s current space mission was designed to demonstrate that good things come in small packages - and part of its plan is to showcase the role MEMS can play in space.

On March 22, using a small Pegasus rocket, the space agency launched ST5, short for Space Technology 5, which contained three cake-size satellites that work together to measure the Earth’s magnetic field using small boom-mounted magnometers. Each of the ST5 microsatellites weighs approximately 55 pounds compared to more than a ton for typical satellites.

On a 4-inch-square section of one of the satellites sits a radiator composed of tiny comb-shaped motors powered by electrostatic charges that open and close microscopic shutters to regulate the temperature of that area of the satellite. This radiator system, known as Variable Emittance (Vari-E) Coatings for Thermal Control, can be adapted to a broad range of temperatures during flight, its designers say.

ST5’s 90-day flight is part of a larger NASA effort called the New Millennium program, created to identify and test miniaturized components expected to contribute to the development of ever-smaller and less expensive satellites.

A small radiator placed on the skin of a micro-satellite uses MEMS-based technology to regulate the satellite’s temperatures. Photo courtesy of Johns Hopkins University Applied Physics Laboratory
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“Launch costs are extraordinary, so they’re moving to microsatellites,” explained Ann Darrin, the program manager for the Vari-E project at Laurel, Md.-based Johns Hopkins University Applied Physics Laboratory. “Well, without MEMS, you’re not getting there.”

One technological challenge is that as satellites get smaller, it becomes more difficult to keep their temperatures constant. MEMS-based devices hold great promise for thermal management in space because they offer reductions in size, power use, volume and weight, said Donya Douglas, NASA’s lead thermal engineer on ST5.

“As you start to shrink the satellites, you need a thermal management system that’s lightweight and dynamic,” Douglas said. The MEMS technology allows scientists to adjust the shutters to control the temperature of the satellite. For instance, when the satellite is facing the sun, project managers can close the shutter doors to reflect the heat.

Darrin said the Vari-E project is record-setting on several fronts. “This is the first fully qualified MEMS space project and the first to sit on the skin of a satellite,” she said. “It’s definitely the most complex MEMS device sent into space yet,” she added.

The project should serve to demonstrate that MEMS-based technology can reliably be used in space, she stressed. “People perceive small as frail, but these devices are not fragile,” Darrin said. “So we’re getting people over the hump as far as their thinking about MEMS.”

Designed over a period of seven years by a team of 10 researchers from APL and NASA Goddard Space Flight Center, the Vari-E device was fabricated by Sandia National Laboratories in Albuquerque, N.M., using a five-layer micromachining process called Summit V.

“Sending MEMS into space is not trivial,” Darrin said, adding that the Vari-E team had to overcome some significant design challenges. The shutters had to be built to survive the launch and work in the harsh environment of space. To protect the devices from grit or moisture, the scientists enveloped them in a clear polymer rugged enough to sit on the outside of a satellite in space.

NASA’s Douglas said that data coming back from the microsat indicates the Vari-E is functioning well. “We have completed tests of all its operational modes and will compare how it performs in space with the simulated ground testing we did earlier.”

At the end of the mission, researchers will assess whether there’s been any degradation in performance of the MEMS device caused by exposure to the space environment. If the ST5 experiments prove successful, the Vari-E devices may be the prototype for radiators that cut the overall weight of future microsats by several pounds.

Both Douglas and Darrin said the Vari-E radiator wouldn’t have commercial application outside the space program. Yet Darrin said the significance of the project is that it may lead to an expanded role for MEMS in space.

“We’re creating a path for future space missions to use MEMS,” she said. “It will be used for things like beam steering and mirror movement.”

In fact, she noted that the James Webb Space Telescope, which is being built now to replace the Hubble telescope, relies on MEMS-based microshutter arrays for use with its near infrared spectrograph. The microshutter arrays, which are approximately 100 times lighter than a conventional solution, are designed for the spontaneous selection of a large number of objects in the sky and the transmission of light to the spectrograph detector.

“They will learn from what we’ve done,” Darrin said. “The journey is what’s most important.”