Headlines

Aerospace Aids Shuttle Investigation

Scientists from The Aerospace Corporation provided technical support and analyses to NASA earlier this year in its investigation of the space shuttle Columbia accident. William Ailor, director of Aerospace's Center for Orbital and Reentry Debris Studies (CORDS), testified before the Columbia Accident Investigation Board in a closed session March 13 on the history of space hardware reentry and breakup and what can be learned about the breakup from debris recovered on the ground. Ken Holden, general manager of the Aerospace Launch Verification Division, briefed board members May 21 on the corporation's basic launch verification process.

More than 80,000 pieces of debris—roughly 40 percent of the shuttle's mass—were recovered from the Columbia. Here, they are placed within an outline of the shuttle, indicating where they were on the vehicle before breakup.

The disintegration of the Columbia occurred February 1 during the reentry phase of the Space Transportation System (STS)-107 mission. The resulting debris field has characteristics similar to those seen for other reentry breakups, Ailor said. Aerospace has been involved in analyses of reentry breakups for many years and established CORDS in 1997 to lead this work.

Ailor's testimony covered the kinds of evidence of the cause of the accident that might have survived the extreme reentry environment and included recommendations for how individual pieces of debris and the distribution of debris within the debris field might help reconstruct events leading to the accident. The investigation board invited Ailor to provide a similar briefing to a public session, broadcast live on CSPAN March 17. Aerospace scientists, including Ailor, Douglas Moody, Gary Steckel, and Michael Weaver, later visited the hangar where the recovered debris was cataloged to evaluate the debris and provided recommendations for analysis.

During his briefing to the board, Holden described the elements of the launch verification process, which Aerospace uses to provide unbiased independent technical assessments to support all Air Force space launches. "Unparalleled Aerospace scientific and technical capabilities for analyses and modeling and simulation provide the Air Force with a second opinion on virtually every technical issue," he told the board. "The effectiveness of Aerospace's role is significantly enhanced by contractors willing to listen to a second opinion and an Air Force customer that puts mission success above any other objective."

Nondestructive Inspection

Two of the Space Based Infrared System (SBIRS) satellites are designed to operate in highly elliptical orbits. These units employ optical solar reflectors constructed of thin back-surface reflecting tiles bonded onto heat-rejection radiator panels. After several of these tiles "dimpled" during thermal vacuum testing (see photo, left), Aerospace developed a unique thermographic inspection technique for noncontact inspection of the panels. The technique uses infrared imaging to record the thermal pattern of a test object as it cools down after a rapid but mild heating of the surface. Areas that have an underlying void or debond—which impedes heat transport away from the surface—appear brighter than well-bonded areas because they retain heat longer.

The contractor evaluated several different methods of doing the inspection and concluded that the Aerospace thermograph approach "was the best available," said Harry Yoshikawa of the Space-Based Surveillance division. The contractor requested that Aerospace perform the inspection.

The inspection revealed significant voids in the adhesive below the deformed tiles (see photo, right), resulting in the need to rebond approximately 70 percent of the tiles. If the tiles had not been rebonded, the voids would have impeded the heat rejection capability of the panels, causing electronic components to overheat and reducing mission life. The Aerospace technique "is a highly reliable inspection approach and has saved the program much time and money," said Yoshikawa.

Improving GPS Theater Support

In preparation for Operation Iraqi Freedom, the 14th Air Force tasked the 50th Space Wing to develop and deploy an extended type of GPS support to sustain an intensive precision munitions push. Aerospace supported the 2nd Space Operations Squadron (2SOPS) by developing an innovative tactic to enhance theater accuracy and integrity.

As explained by P. J. Mendicki of the Navigation Division, the new technique is a variation of the GPS enhanced theater support (GETS), which was implemented just a few years ago. Using traditional GETS, field personnel would contact 2SOPS with a generalized target location and a strike time window. The 2SOPS office would predict which satellites would be overhead, monitor their performance, and update their broadcast navigation message. The system worked well, but the improvements were short-lived, lasting only about an hour, and planning required adequate advanced notice. "Traditional GETS," said Mendicki, "is very limiting—we can't do it 24/7. Just a few years ago, round-the-clock enhancement wasn't a major concern, because GPS-guided weapons weren't as prolific as they are today."

Aerospace proposed a new approach. "We know when satellites will be visible to the theater, and we control our contact schedule, so why not proactively schedule uploads to maximize theater performance?" Mendicki asked. Thus, those satellites approaching the area would be uploaded with a new navigation message shortly before entering the theater of operations. "Rather than do it ad hoc, or on the fly, we made it a routine scheduled activity, which helped smooth out operations." As an added bonus, he said, "the new approach allows war planners to attack targets of opportunity," such as those that began the air campaign; the old GETS approach could not.

Aerospace went to 2SOPS with the proposal, and within four days the 2SOPS team tested this new tactic with the operational GPS constellation. The results were so promising that the technique was implemented 48 hours later in support of the opening salvos of the air campaign. Throughout Operation Iraqi Freedom, in which thousands of GPS-guided munitions were employed, the GPS in-theater accuracy was improved by more than 20 percent. "It worked out very well," Mendicki said.

Mendicki has since been researching whether the technique would yield similar results in other theaters, and how it might be applied during two simultaneous conflicts. "Geography may limit our support to other theaters," he said, "but overall, it looks good."

Preventing Pogo on Titan IVB

A Titan IVB rocket successfully launched a Milstar satellite from Cape Canaveral on April 8, 2003. Prior to launch, mission planners were concerned that a so-called Centaur longitudinal event (CLE) could occur during the mission, leading to pogo (undamped dynamic instability), structural failure, and mission loss. Aerospace undertook extensive analysis and test activities to help the contractor identify the source of the problem and adopt corrective action.

Program managers were concerned because an Atlas/Centaur mission in September 2002 experienced dynamic levels much greater than expected and twice as great as a prior identical mission, said Ken Holden, general manager of the Aerospace Launch Verification Division. Moreover, the specific launch vehicle configuration had not been tried before. "The Titan/Centaur for the Milstar mission was the first and only time we had to use Atlas/Centaur RL 10A-4-1A rocket engines to support a Titan mission," explained Holden. "All other Titan/Centaur missions used RL 10-3-3 engine configurations." Potential impacts for Titan/Centaur were amplified because its propellant system was different from that on Atlas/Centaur, which provided an interactive capability to offset CLE and pogo.

(Russ Underwood, Lockheed Martin)

The Air Force, Aerospace, and contractor team conducted additional hot-fire tests on the Atlas/Centaur RL 10 engines. Those tests revealed that under certain conditions, the engine would produce dynamic resonant frequencies through a phenomenon best described as "rotating cavitation." The team then sought ways to limit the risks from rocket engine cavitation and dynamics. "These extremely complex assessments involved the interplay of possible engine dynamics with the Centaur structure and with the Milstar spacecraft's structure," said Holden. "It was ultimately concluded that the initial flight profile for Centaur's mission could result in cavitation in one or both of the RL 10s and that that might lead to undamped dynamics." Aerospace and the contractor agreed that a mission profile could be designed that would avoid engine cavitation and would not affect mission reliability or accuracy. This was accomplished by increasing inlet pressures to the engines and adjusting the fuel mixture ratios to avoid conditions associated with cavitation, he said.

The Milstar satellite was safely delivered to orbit well within required accuracy. Initial flight data indicate that the Titan booster and Centaur upper stage performed near nominal throughout the mission.

To Summer 2003 Table of Contents