NASA NASA-LLIS-0944: Lessons Learned – Pyrovalve Anomaly Studies

Description of Driving Event:

Starting in 1995, WSTF tested approximately 50 pyrovalves in order to provide insights into the Mars Observer and certain other spacecraft anomalies. The pyrotechnic blow-by past the ram was measured and characterized and overall valve speed and other physical properties were characterized. Pyrotechnic/propellant interaction testing also was accomplished; testing included simulation of the Telstar 402 and LandSat 6 failures, verification that the Mars 98 pyrovalve and actuation scenarios were safe, evaluation of the Interim Control Module actuations, and tests that characterized pyrotechnic induced propellant reaction rates. Some major safety implications are summarized below.

• Most of the valves with O-ring seals on Telsar 402, Landsat 6, and the Mars Observer produced significant hot pyrotechnic blow-by. (These were manufactured by one company.)

• Telstar 402: Flight telemetry indicated that the propellant system was breached immediately following initiation of the second parallel pyrovalve. Actuation of the first valve introduced helium-saturated hydrazine into the evacuated downstream manifold. This created a very unfavorable scenario that greatly increased the chances of ignition. The second pyrovalve was liquid locked and some hydrazine foam may have been produced. Both WSTF and Lockheed Martin duplicated the scenario and both encountered explosions that were characteristic of the flight anomaly.

• LandSat 6: This was very similar to the Telstar 402 anomaly. Significant differences included a small variation in the time between the actuation of the first and second parallel pyrovalve, and the materials from which that the pyrovalves were made. Telstar 402 used titanium valves and the LandSat 6 used stainless steel valves. Both types of valves caused similar explosions in WSTF testing.

• Interim Control Module: This system used monomethalhydrazine (MMH) and no explosions occurred in WSTF tesing, probably due to the fact that MMH is much less sensitive than hydrazine.

• Mars 98: These tests were accomplished using separate hydrazine and nitrogen tetroxide systems and used very low blow-by valves (utilizing interference fit rams rather than O-ring seals). No system pressure increases were observed that were above water baseline test data. This indicated that blow-by was below the threshold for ignition of hydrazine, and that this type of valve no reactivity with nitrogen tetroxide (as was observed with some titanium valves from OEA.)

Engineers at Langley Research Center developed a test which measures the amount of energy required to actuate a valve by using a weight drop tower and then measures the energy output of the pyrotechnic charge using a volume chamber. Comparing these energies determines if the valve is over or under-powered. It was determined that the Mars Observer and Telstar 402 valves were so over powered that they frequently broke chunks of titanium out of the ram stop at the end of the stroke.

Configuration Management

Energetic Materials - Explosive/Propellant/Pyrotechnic

Flight Operations

Flight Equipment

Hardware

Parts Materials & Processes

Risk Management/Assessment

Safety & Mission Assurance

Spacecraft

Test Article

Test & Verification