Description of Driving Event:
The propellant tanks on the Genesis spacecraft were filled with isopropyl alcohol (IPA) prior to modal and acoustic vibration testing in order to obtain the proper spacecraft dynamic response. IPA was used as a referee fluid in place of the hydrazine propellant because of its similar mass, as well as for safety reasons. When the Genesis tanks were drained about ten days later, levels of particulate and non-volatile residue (NVR) were found that significantly exceeded the cleanliness specification. Both the particulate levels and the NVR were brought back into specification by filling the tank with IPA and draining it several times, followed by vacuum drying.
A cleanliness demonstration test was subsequently performed on an Atlas Centaur propulsion system hydrazine tank that was of a design similar to Genesis'. The Atlas tank was also found to be out-of-specification with respect to particulate contamination. After the tank was exposed to the IPA cleaning fluid for about a day, it was also found to be out-of-specification on NVR. A total of 16 Atlas tanks were subsequently returned to the vendor.
The Genesis tank is a titanium sphere with a diaphragm fabricated from AF-E-332 rubber material. The diaphragm is incorporated to ensure gas-free propellant expulsion. When empty, the diaphragm conforms to the lower wall of the tank. As the tank is filled, the diaphragm reverses toward the upper dome of the tank. The tank is cleaned at the piece-part level and subjected to a final cleaning at the assembly level. The ability of cleaning fluids to remove particulate from the assembled tank during this final cleaning may have been limited by the fact that the vendor used only 1 liter of IPA for this procedure. Furthermore, the vendor has asserted that transportation of a "clean" tank may result in particulates being released from the faying surfaces near where the diaphragm is attached to the wall and/or in generation of particulates due to rubbing of the diaphragm on the tank wall. It is likely that the out-of-specification particulate seen in the Genesis and Atlas tanks was a result of imperfect cleaning and/or this self-generation mechanism.
Further materials testing verified that the NVR contamination was a substance leaching out of the rubber diaphragm material following prolonged exposure to IPA. This AF-E-332 diaphragm material has been used in this application since 1974, and contact with hydrazine does not produce degradation of the material or performance problems in propulsion systems. Exposure to IPA for periods of over 4 hours, however, can produce excessive NVR.
Similar tank vendor cleaning processes were used on the Deep Impact project, but it was found that conservative propulsion system filter sizing mitigated the threat of high particulate levels.
References: Genesis Tank NVR Contamination Interim Report, (contractor document), September 15, 2000.
Additional Key Words: cleanliness verification, decontamination, environmental test, alcohol flush
Following delivery, diaphragm-type propellant tanks may exhibit particulate contamination levels that exceed the vendor's cleaning tag results and also the customer specification. Additional particulate contamination may be generated by subsequent transportation of the tank and/or the system.
Isopropyl alcohol can leach out-of-specification amounts of NVR contaminant from AF-E-332 diaphragms in propulsion tanks when exposed for periods significantly longer than 4 hours.
- Conservatively sized propulsion system filters should be used immediately downstream of diaphragm propellant tanks in order to accommodate particulate levels which may exceed the specification levied on the tank.
- Vendor procedures for cleaning of diaphragm propellant tanks should be carefully reviewed to ensure that adequate quantities of cleaning fluid are being used to ensure the internal surfaces are fully wetted and agitated.
- Propellant tank cleaning procedures should limit the time the diaphragm is exposed to isopropyl alcohol to no more than 4 hours. Exposure should be followed immediately by vacuum drying of the tank.
- Do not introduce isopropyl alcohol into diaphragm-type propellant tanks for extended periods during system test unless the tanks are subsequently re-cleaned, verified clean, and vacuum dried.
Evidence of Recurrence Control Effectiveness:
Documents Related to Lesson:
Additional Key Phrase(s):
- Flight Equipment
- Parts Materials & Processes
- Pressure Vessels
- Test Article