Improving Reentry Hazard Prediction: The Reentry Breakup Recorder
Improving Reentry Hazard Prediction: The Reentry Breakup Recorder
The behavior of reentering debris and the environment through which it passes is violent and unpredictable. An Aerospace-designed and -built device looks to improve on knowledge about these dangers.
Humans have been placing objects in orbit for 58 years. Much important information has been gleaned from these missions, including the truth of an old adage: “What goes up, must come down.” All objects in orbit—satellites, rocket bodies, and debris—will eventually return to Earth. This might happen very quickly after launch, or take thousands of years. For the most part, there is no controlling when and where these objects will fall. When an object reenters, the atmosphere imparts tremendous heating and stress, and unless a vehicle is designed for this, it will likely break up and burn up. However, not all reentering debris burns up. Estimates are that as much as 40 percent of a vehicle’s dry mass (that is, no liquids or gasses) survives, and some of these remaining fragments can be hazardous to people on the ground.
In the late 1960s, the Aerospace Vehicle Atmospheric Survivability Project (VASP) and associated Vehicle Atmospheric Survivability Tests (VAST) were conducted to determine the reentry survivability and condition of payload elements. These tests indicated that space hardware breaks up at lower altitudes than models had predicted, and that many debris fragments could survive, falling through airspace and impacting the ground. The lower breakup altitude meant that the area across which the debris landed (the debris footprint), was shorter than expected, an important consideration for designers of future missions where hardware would be directed to land in a safe area. The fact that some debris could survive reentry and be hazardous to people on the ground was also a driving factor toward improving reentry hazard predictions.
Experience and knowledge gained during the VASP tests affected how the reentries of several vehicles were analyzed and addressed. In 1978, the Soviet Cosmos 954 reconnaissance satellite reentered, bringing with it the small nuclear reactor used to generate electrical power. The VASP data helped estimate the ground area where radioactive debris might be recovered. In 1979, Skylab, the United States’ first space station, reentered—it remains the largest single reentry event of an uncontrolled vehicle. Only a very few fragments from Skylab were found on the ground, and once again, the VASP data helped in understanding the footprint and where debris might be found. VAST data have also helped to calibrate reentry breakup and hazard models for the space shuttle’s external tank, and to characterize hazards associated with potential reentry scenarios for NASA’s Cassini, Galileo, Ulysses, and the Mars Science Laboratory deep-space missions, all of which carried radioactive power sources.
While the reentries of Skylab and Cosmos 954 generated widespread publicity, the impact of their debris in remote areas encouraged the perception that survival of, and danger from, large, hazardous space debris fragments was not common.
Then, in January 1997, a 250-kilogram stainless steel object landed 45 meters from a farmer’s house in Texas. At approximately the same time, a smaller, lightweight fragment brushed the shoulder of a woman walking in a park near Tulsa, Oklahoma (luckily, she was not injured). Both objects, in addition to a large pressure sphere and a cylindrical combustion chamber, were remains of the second stage of a launch vehicle that had carried a U.S. satellite into orbit nine months prior.
This event refocused public perception about reentering debris, raising awareness that such debris does not always burn up, and could therefore present a hazard to people and property on the ground.
Trajectory reconstructions of the tank’s reentry supported the lower breakup altitudes predicted from the VAST/VASP tests. It also highlighted significant disagreements in the reentry safety community about what happens when an object reenters. Some believed that the results of the VASP tests were an anomaly and not generally applicable. This disparity between predicted and observed reentry characteristics pointed out the need to better model the reentry environment and the behavior of space hardware passing through that environment. Enter the Aerospace Reentry Breakup Recorder (REBR).
The Reentry Breakup Recorder
The ideal way to resolve the differences between predicted and observed reentry events was to record data on a vehicle as it reentered the atmosphere and was broken apart by aerodynamic heating and loads. The Aerospace-designed and -developed REBR was conceived to perform just this task. There were a number of challenges that had to be overcome if REBR was to be successful, including:
* During reentry, recording devices would be exposed to the same severe environment as the host vehicle.
* As the host vehicle disintegrated, its aluminum structure would melt, a plasma sheath would surround the vehicle, and the environment in the vicinity of the recording device would make transmitting data very uncertain.
* For an orbit-decay reentry, the host vehicle could reenter without prior warning anywhere on Earth that was under the vehicle’s orbit. Thus, the communication system had to be available worldwide, 24 hours a day, seven days a week.
* Launch providers would not support carrying an extra large object or large mass to orbit, so the device had to be small and lightweight. Also, the device could not contain any parts that might inadvertently activate and threaten the success of the launch or payload.
* Launch providers would be less likely to carry the device if it imposed constraints or requirements on a mission.
In 2001, Aerospace’s Center for Orbital and Reentry Debris Studies and the corporation’s Concept Design Center developed preliminary designs for REBR that addressed these challenges. The idea was to build a small, lightweight vehicle with these characteristics:
* A heat shield to protect the data recorder and electronics during reentry
* No moving parts or stored energy other than batteries for power
* Self-stabilizing in hypersonic, supersonic, and subsonic flight regimes
* No requirements on the host vehicle other than it return to Earth within the lifetime of the device’s batteries
* No transmission of data during reentry breakup; instead, recording of data prior to and during breakup, and transmission only after the device is released from the host vehicle
* Data transmission via the Iridium system, which offers global coverage around the clock. Further, since operators might not be available 24/7, data would be routed to a ground-based web server so that it could be received whenever reentry occurred.
Aerospace filed an application for a patent on REBR in 2003, which was granted in 2005.
REBR was further developed with NASA’s Ames Research Center. Ames suggested the suitability of NASA’s Mars Microprobe reentry vehicle shape for this application and conducted wind tunnel testing to verify its subsonic stability properties. NASA also performed preliminary heat shield sizing for an initial candidate material, satisfying requirements for Iridium and GPS frequencies. NASA/Ames participation in early balloon testing verified that the proposed communications via Iridium were possible.
In 2008, the European Space Agency (ESA) agreed to provide a ride to space for REBR aboard its Automated Transfer Vehicle 002, or ATV-2, flight to the International Space Station (ISS). After delivering supplies to the ISS, ATV-2 would separate and reenter the atmosphere over the South Pacific for disposal. ESA wanted data to confirm its estimates of the breakup and accompanying risk analysis for the ATV.
The initial plan was to send two REBR devices to the ISS aboard ATV-2, and both would reenter aboard that vehicle. One REBR would remain in its foam-lined container to provide more protection from the uncertain reentry environment, while the second would be removed from its container by astronauts and attached to a plate at the rear of the vehicle where the engines were located. As that plan was being developed, the Japanese Aerospace Exploration Agency approached Aerospace and asked if one of the devices could be used to collect similar information during reentry and breakup of its H-II Transfer Vehicle 2, or HTV2, another ISS supply vehicle. Aerospace and ESA agreed to this plan.
Two REBRs were delivered to the ISS in February 2011. One was removed for subsequent attachment to ATV-2 and the second was strapped inside of HTV2 to await release from the ISS.
On March 28, 2011, HTV2 was released from the ISS and two days later executed a controlled motor firing designed to send the vehicle and any surviving debris into the South Pacific. As planned, HTV2 reentered, aerodynamic heating and loads gradually increased as the vehicle encountered denser atmosphere, the aluminum melted, structures failed, and the vehicle disintegrated.
REBR was awakened early in this process as HTV2 encountered fringes of denser atmosphere, and, as HTV2’s exterior was heated and loads increased, REBR recorded accelerometer, rate gyro, and temperature sensor data. As the breakup proceeded, temperatures increased to the point where the plastic bolts holding the two halves of REBR’s copper housing together melted. REBR was released and fell free from the disintegrating HTV2 and its debris field.
At this point, REBR was about 66.6 kilometers above the ocean and travelling at about Mach 23. Over the next 2.5 minutes, it gradually slowed to a subsonic velocity at an altitude of 31 kilometers, falling straight down. As planned, it dialed the Iridium system and began transmitting its data—the first ever recorded on an unprotected object as it reentered the atmosphere. REBR landed in the ocean and continued to provide data until its batteries failed, some 17 hours later, resulting in a very expensive phone call.
The second REBR, aboard ATV-2, failed to communicate, but REBR’s placed on two subsequent flights, one aboard HTV3 and a second on ATV-3, performed well.
REBR provided definitive information on satellite breakup, including verification that significant reentry breakup events occur between 66–84 kilometers, well below the breakup altitude predicted by some models, but consistent with the VASP and VAST data. Data also suggested that tumbling of the host vehicle on reentry seems to have minimal effect in terms of the altitude at which breakup occurs, which is also consistent with prior data. These results have helped bring into agreement reentry breakup and reentry hazard models worldwide.
The Future for REBR
While the first REBR flights have provided macroscopic data on breakup, more detailed data on the heating of a space vehicle’s structure is required to optimize space hardware designs that will provide predictable results on reentry. A second REBR design, REBR-Wireless, has been developed to do just that.
While REBR recorded data from its internal temperature and pressure sensors, REBR-Wireless will collect data from internal and external sensors that send data via wired and wireless links to the REBR device, where it is recorded for subsequent broadcast. REBR-Wireless test flights are expected to be flown in 2016, during which astronauts will attach temperature sensors inside the host vehicle so that the readings reflect as closely as possible the temperatures experienced outside the body of the host vehicle. REBR will also record data from a pressure sensor to detect when the airtight containment of the host vehicle is breached, while an acoustic sensor will listen for other significant events via sounds transmitted through the spacecraft’s structure. All of these data are expected to provide new insights on the reentry breakup process, help refine reentry hazard prediction models, and further efforts to design breakup-enhancing features into future spacecraft.
The author thanks Michael Weaver and Douglas Moody of the Aerospace Vehicle Systems Division for their review, consultation, and suggestions on aspects of this article.
Aerospace Report No. TR-2008(8506)-3, “Reentry Breakup and Survivability Characteristics of the Vehicle Atmospheric Survivability Project (VASP) Vehicles,” (The Aerospace Corporation, El Segundo, CA, 2008).
H. Ailor and M. A. Weaver, “Reentry Breakup Recorder: An Innovative Device for Collecting Data During Breakup of Reentering Objects,” 5th IAASS Conference (Versailles-Paris, France, Oct. 17–19, 2011).
H. Ailor, M. A. Weaver, A. S. Feistel, and M. E. Sorge, “Reentry Breakup Recorder: Summary of Data for HTV3 and ATV-3 Reentries and Future Directions,” 6th European Conference on Space Debris (Darmstadt, Germany, April 22–25, 2013).
S. Feistel, M. A. Weaver, and W. H. Ailor, “Comparison of Reentry Breakup Measurements for Three Atmospheric Reentries,” 6th IAASS Conference (Montreal, Canada, May 21–23, 2013).
A. Weaver and W. H. Ailor, “Reentry Breakup Recorder: Concept, Testing, Moving Forward,” AIAA 2012-5271, AIAA Space 2012 Conference, Pasadena, CA, Sept. 11–13, 2012.
Related publication: Crosslink, Fall 2015, Understanding Space Debris