In-situ: Fixing Only What Needs to Be Fixed
The launching and operating of space-based assets is considered a “one strike and you’re out” endeavor. Should a problem occur during on-orbit operations, repair is usually so prohibitive in terms of cost, time, and complexity that it is easiest to write off the asset and either replace it or abandon the capability. An anomaly occurring during launch usually results in destruction of both the launch vehicle and payload; repair is not possible at all.
Identifying potential problems with launch vehicles and satellites during construction can potentially save millions of dollars and avoid critical capability loss; further, even greater savings in cost and time can be realized if problems are identified and rectified early in the build process.
During the build and testing phases, space system engineers go to great lengths to simulate the space environment the hardware will be operating in, to ensure that both launch vehicle and satellite will operate as designed.
One of the more effective ways of discovering the cause of an anomaly is to take the suspect hardware apart and examine it. However, this approach is time-consuming and expensive, as it requires not only disassembly and analysis, but repair and reassembly as well. A more efficient approach would be to determine and solve anomalies in place, without the need to disassemble space-system components.
Recent testing of a major U.S. space program satellite revealed an anomaly and illustrated the potential efficiency of in-situ inspection and repair. An attitude-control thruster on the satellite became stuck during a test and would not expel propellant. Should this occur on-orbit, the satellite would be unable to change the direction it was pointing. Further, this thruster type is used on many different systems, some already deployed. For those systems still being constructed, removing each thruster from each satellite for disassembly, analysis, and, if needed, repair would have impacted launch schedules and constrained budgets.
Engineers at The Aerospace Corporation developed a technique that would permit the thrusters to be examined in place, and to replace or repair only those that were defective.
Identifying the defective thrusters proved challenging, since it required examining small springs within the thrusters; the springs themselves were contained inside metal housings and were surrounded by dense wire coils. Aerospace engineers used a portable microfocus x-ray source and ran tests to determine the correct combination of beam parameters, film type, and shielding that would produce the most useful image. Then, they developed the imaging protocol, designed the physical set-up of the imager and the thrusters (still in place on the satellite), designed the image-analysis process, and demonstrated the process on a satellite that was being constructed. Both the government and the contractor building the satellite were impressed with the Aerospace approach, and the government directed Aerospace and contractor personnel to use the technique on many space vehicles currently being built.
Dozens of thrusters have been examined using the Aerospace in-situ x-ray technique; to date, only one thruster has been found to be defective. Because the other 15 thrusters on the tested spacecraft did not have to be removed to be inspected, the Aerospace technique resulted in a significant savings in both cost and time. Additionally, since these thrusters are used in many different space programs, the technique can be used to inspect them all, potentially resulting in even more savings. Finally, this technique has allowed the potential thruster problem to be so well understood that Aerospace engineers have been able to recommend a protocol to apply to thrusters that are already in space, to minimize the potential for failure in those units.
The cost savings realized on the program for which the problem was first identified was $10 million. It is not known what the savings will be on other systems, but it will be significant considering that inability to detect and repair such problems before the system is deployed can result in mission failure.