Date: Jan 15, 2010 Author: Joan Zimmermann Source: MDA (
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by Joan Zimmermann/jzimmermann@nttc.edu
When the first satellite, Sputnik I, was launched into space in 1957, one of its most sophisticated sensor systems lay in the fact that it was little more than a nitrogen balloon. If the skin of the satellite were to be breached by a micrometeorite, the signal would have been quite simply "kaboom."
Today, most satellites are not so simple. Instead, they orbit the earth as semi-autonomous spacecraft housing a variety of control subsystems, including power systems, telemetry and attitude control equipment, and thermal control systems designed to protect the satellite's instruments against rigorous space conditions. All of these components add weight, which increases launch costs, and complexity, which can limit satellite lifetimes.
Thermal control systems—the devices that either heat or cool components aboard a spacecraft—pose problems for all agencies and companies that deploy satellites. MDA is no exception. Over the years, the agency has granted SBIR awards seeking innovation and advancement for radiators, heat pipes, sensors, and other systems that support space-based defenses. Two companies, ATEC, Inc. (College Park, MD), and Sensortex, Inc. (Kennett Square, PA), both have developed components of thermal control systems for MDA and recently participated in a dual experiment on the International Space Station (ISS).
Space needs:
In 2008, NASA sent a collection of materials into space to be tested for their ability to withstand ionizing radiation, atomic oxygen, extremes of temperature, high levels of ultraviolet bombardment, and other hazards that are found in low-Earth orbit. The test module, designed to evaluate current methodologies and materials, contained more than 400 new materials for potential use in advanced reusable launch systems and next-generation spacecraft. Samples included paint for lunar power systems and silicone rubber seals planned for NASA's new crew vehicle, Orion, which is currently under development.
Known as the Materials International Space Station Experiment (MISSE), the latest package (MISSE 6A and 6B) to be tested was launched and placed on the Columbus External Payload Facility of the ISS in March 2008. After a year-and-a-half's exposure to the hostile low-Earth-orbit environment, Space Shuttle astronauts retrieved the module during flight STS-128, which returned to Earth on September 11, 2009. ATEC and Sensortex, given their complementary technologies, were able to combine forces to devise an experiment that hitched a ride on MISSE-6, to measure the activities of passive and active thermal coatings, both of which are used to control temperature in spacecraft components.
ATEC's Angle:
In 2002, ATEC received a Phase I MDA SBIR award for the development of heat flux sensors, devices that estimate temperature and heat flow as a function of time. The sensors, developed with Professor Jungho Kim of the University of Maryland, were designed to measure high heat fluxes on one surface of a wall being exposed to laser energy by monitoring the heat flux and temperature on the other surface of the wall. Designed as part of an effort to understand the effect of laser energy on materials, the sensors also can be used in the monitoring of explosives or chemical reactions, or in chambers used in the vapor deposition of optical coatings.
An offshoot of this work led to ATEC's development of a heat-flux-based emissivity (HFBE) measurement technique, also co-developed with Professor Kim. (Emissivity pertains to a material's ability to emit absorbed energy.) This particular project employed commercially available, thin-film heat flux sensors to measure the emissivity of spacecraft thermal-control coatings in a space environment. The new technique was designed to take advantage of the low thermal capacitance of the sensors, which in turn could provide a better picture of how and where heat flows over time, as well as how it varies over the area of the coatings.
As a result, the HFBE method can pull multiple duties simultaneously, evaluating different coatings with differing characteristics on a single material. ATEC's lightweight thermal system has been lauded as a "breakthrough" by the U.S. Air Force. ATEC's emissivity sensor is a simple, lightweight, energy-efficient method for measuring emissivities through multiple surfaces; eliminating weight and reducing power consumption compared with conventional heater-plus-electronic-control systems.
Enter Sensortex:
The Air Force Research Laboratory's Materials and Manufacturing Directorate facilitated the integration of ATEC's technology with Sensortex's electrostatic-switched radiators (ESRs), devices that switch the heat transfer mode from conduction to radiation. The company's ESRs claim an origin in Phase I and II SBIRs with MDA. Sensortex developed lightweight, appliqué-type ESRs as a novel way to control radiated energy from a spacecraft, based on an existing rigid-structure device.
The appliqué that was developed during the MDA efforts was designed as a lightweight coating a few thousandths of an inch thick. Compared with conventional systems that require heaters and electronic control systems, ESR-based thermal control systems weigh less and consume minimal power (less than 2 milliwatts), enabling spacecraft to stay aloft longer, according to Sensortex. The ESRs also boast a very fast response time.
Of interest to both NASA and MDA, active thermal coatings can increase satellite lifetimes by 3 to 5 times compared to passive thermal control systems. To help confirm the applicability of the ATEC's HFBE system for testing active thermal coatings, the MISSE-6 experiment incorporated Sensortex's ESRs to do the job.
As of late 2009, the retrieved module was still undergoing data reduction at NASA, and results are pending. However, each technology stands on its own. Sensortex founder William Biter is planning to retire soon, but his company and its ESR products will likely find a new owner in the near future. ATEC continues to refine its thermal systems and to work on other MDA technologies, including ongoing efforts to develop lightweight, low-cost, ceramic-based high-heat-flux microchannel cold plates and air-cooled micro-groove heat sinks that are suitable for electronics cooling applications
