Headlines

Assessing Solar Cells

Spacecraft designers will soon have a new tool to help in the selection and qualification of solar cells. The Advanced Solar-Cell On-orbit Test (ASCOT) has generated five years' worth of data concerning the radiation degradation of solar cells in space. Aerospace was closely involved in the experiment design and test-article selection and analyzed the data.

(Boeing Space Systems)

ASCOT contains a variety of advanced gallium-arsenide solar cells as well as standard silicon cells and thin-film flexible solar cells. The goal, said Dean Marvin of the Space Technology Directorate, is to assess how each withstands extended exposure to space radiation. Because of its particular elliptical orbit, ASCOT accumulates radiation dose much more rapidly than a satellite in a typical low Earth orbit. In fact, ASCOT's silicon reference cells have accumulated the equivalent radiation exposure of 2000 years in a 450-nautical-mile polar orbit and 600 years in geosynchronous orbit.

Long-term radiation tests such as ASCOT hold great scientific and engineering value, Marvin said, providing data that cannot be obtained through accelerated studies on the ground. For example, solar cells are typically qualified for space by studying the effect of protons and electrons at a few selected energies, but in actuality, the space environment presents a continuous spectrum of energies. Furthermore, ground tests typically assume that the effects of all particles in the spectrum are additive—an assumption that may not be accurate in all cases. Finally, the ground tests expose the cells to many years' equivalent radiation in just a few hours—a procedure that can obscure annealing effects and introduce high-dose-rate artifacts. ASCOT, on the other hand, provides reliable long-term on-orbit data that can be used to validate the ground-based testing procedures.

Successful Launch of NRO Satellite

The National Reconnaissance Office (NRO) successfully launched its GeoLITE satellite from a Delta II rocket on May 18, 2001, from Cape Canaveral. This was the first time an NRO satellite was launched on a Delta II. Aerospace provided critical support throughout the launch campaign, from source selection to early-orbit operations.

(NRO)

The GeoLITE program broke from traditional NRO practices, said Tom Darone of the Advanced Technology Group, moving instead toward a more streamlined acquisition of the spacecraft and a more commercial procurement of the launch vehicle. The satellite was designed and built in less than four years at a cost of $130 million.

The new approach initially limited Aerospace's involvement to the satellite itself, but late in the program, Aerospace was asked to certify the Delta II launch vehicle as well. In doing so, engineers grew concerned about the stability of the rocket's third-stage engine because it was unclear how GeoLITE's 2000 pounds of propellant would affect the spinning stack. Aerospace performed independent analyses of the third-stage control and the fluid dynamics of the propellant tank; these analyses were key to the Air Force's decision to fly.

The actual launch was delayed one day to replace four of eight flex hoses in the first-stage rocket engine. These hoses became suspect when routine factory testing yielded leaks in two similar hoses. Aerospace worked to understand the factory failure, verify that the replacement hoses were flightworthy, and exonerate the hoses that weren't replaced.

GeoLITE will perform a dual mission: For the first 15 months of its nine-year mission life, it will test an advanced laser communications system. Afterward, it will provide operational support data to the military using a conventional ultrahigh-frequency transmitter.

Beaming Power to Satellites

Batteries add mass, cost, and uncertainty to satellite missions—but such concerns may soon be a thing of the past. A recent study by Aerospace indicates that lasers can be used to beam power from an orbiting space structure to a constellation of satellites. What's more, preliminary results show that with the expected advancements in optical technologies, total system cost can be lower than that of traditional power systems.

The concept builds upon the Air Force Research Laboratory's PowerSail, a large free-flying thin-film solar array. By adding solid-state lasers, thermal controllers, and optical telescopes to this platform, Aerospace researchers were able to create a satellite—the PowerSat—with optical power-beaming capability. Several architectures are being investigated to determine the cost and performance advantages of each.

One model found that just two PowerSats could provide a full-time energy supply to a constellation of 12 low-Earth-orbit satellites. In this configuration, each mission satellite keeps its existing solar array, but loses its battery and most of its power-management system. This arrangement substantially reduces the mass, volume, and cost of each satellite. Moreover, the power available to the mission satellites can be varied in real time, enabling better power optimization based on the requirements of the constellation payloads.

Researchers Track Mir's Reentry

After 15 years in orbit, the Mir space laboratory took a fiery plunge toward Earth, and Aerospace was on hand to examine the event. The Center for Orbital and Reentry Debris Studies (CORDS), under the direction of William Ailor, has been gathering data to characterize how Mir broke up and what parts survived reentry.

(CNN)

Mir disappeared into the South Pacific, so the prospects for examining debris are naturally limited. But Ailor's team has gathered useful details from a wide variety of sources—including tracking data, video clips, eyewitness reports, and radar measurements. Using this information, the researchers have been reconstructing Mir's final descent. NASA has contracted Aerospace to perform this analysis.

The information should hold immediate benefits for designers and operators of space systems, who need to understand what will happen to a satellite or space vehicle when it reaches the end of its mission life. Knowledge of how complex structures disintegrate during reentry can also help designers optimize satellite configuration. "If you have critical components, and you really want to make sure they get cooked on the way down," said Ailor, "you can put them in places where you know they'll disintegrate. Similarly, if there are things you want protected, you can keep them on the interior, keep heating to them low."

Ailor further noted that satellite operators are increasingly being asked—though not yet required—to examine reentry risk. In fact, Aerospace has been working with NASA to develop thresholds and guidelines for satellite deorbiting. "Basically, if your footprint on the ground is bigger than eight square meters, you'll need to exercise a controlled deorbit," Ailor said.

CORDS also helped NASA in reconstructing the demise of the Compton Gamma Ray Observatory, which was successfully brought down in June 2000 (see Crosslink, Winter 2000/2001). Of course, that observatory weighed only 17 tons, whereas Mir—the "granddaddy of them all," as Ailor puts it—weighed about 140 tons. Moreover, the observatory left orbit in a very precise manner, coming down "right on the money," said Ailor. Mir, in contrast, fell slightly ahead of its mark, primarily because Russian controllers were very conservative in their calculations.

Aerospace provided technical support for the reentry. Wayne Hallman is leading the Mir trajectory reconstruction effort.

Controlling Jitters in Space

The first active scientific experiment arrived onboard the International Space Station—thanks, in part, to The Aerospace Corporation. The Middeck Active Control Experiment II (MACE-II) was launched aboard the space shuttle on September 8, 2000, and delivered to the Space Station for operation by the Increment One crew in early 2001. Aerospace provided support in the integration, planning, and flight preparation of the experiment.

Developed by the Air Force Research Laboratory and the Massachusetts Institute of Technology, MACE-II is an on-orbit demonstration of advanced control technologies for suppressing unwanted vibration in space vessels. The goal, said David DeAtkine of Aerospace's Houston office, is to create "smart" structures that can maintain attitude despite unexpected vibration, impacts, and mechanism failures.

MACE-II is a 1.5-meter-long device that floats free in a pressurized compartment. The unit has gimbals at each end and reaction wheels in the middle. One gimbal creates vibrations, which are detected and counteracted through complex computer algorithms, keeping the other end steady. Most important, these algorithms can modify themselves or "adapt" when they sense changes in the characteristics of the system—without human intervention. As a result, the technique should allow future spacecraft to continue performing their missions even as their subsystems degrade and fail, effectively extending mission life.

MACE-II is a reflight of MACE-I, which flew on the space shuttle in 1995. The current device, DeAtkine said, is much more sophisticated, employing smarter algorithms that can account for more types of failure modes and disturbances. The experiment has been operated successfully by the Increment One and Increment Two crews and has been generating useful data, which are being examined.

Space-Based Laser

Aerospace personnel supporting the Space-Based Laser Integrated Flight Experiment (IFX) were called upon to provide more than technical assistance last year. When a commercial laser vendor unexpectedly withdrew, the Air Force found itself without a laser for its Beam-Control Risk-Reduction Test Bed, a platform for identifying and reducing overall risk in the IFX program. With no viable alternative, the Air Force asked Aerospace to supply a replacement laser. Within five months, Aerospace built and delivered a low-power tunable solid-state laser that met or exceeded all mission requirements, saving the program at least $500,000.

To Summer 2001 Table of Contents