2004 MAPLD International Conference Aerospace Mishaps and Lessons Learned STS-86 SAFER Keith E. Van Tassel Group Lead, P

1. 2004 MAPLD International Conference Aerospace Mishaps and Lessons Learned STS-86 SAFER Keith E. Van Tassel Group Lead, Pyrotechnics NASA – Johnson Space Center September 2004

2. MAPLD 2004 SIMPLIFIED AID FOR EVA RESCUE

3. MAPLD 2004 Astronaut Mark Lee is floating above the Shuttle's payload bay during the first test of the Simplified Aid for EVA Rescue (SAFER). SAFER is a device, attached to the bottom of the EMU backpack, that allows the astronauts to maneuver themselves around. It was developed as a self-rescue device to be used during the assembly and maintenance of the International Space Station in the event that a tethered astronaut breaks away during an EVA, and a Shuttle is not available to assist in the rescue.

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6. MAPLD 2004 Pyro Isolation Valve SAFER PYROVALVE 23

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10. MAPLD 2004 NSI PERFORMANCE CHARACTERISTICS • 1 AMP 1 WATT DEVICE • 3.5 AMP ALL FIRE • 1.0 AMP NO FIRE TYPICAL FIRING MODES • Capacitive Discharge: 20 to 40 V, 680 microfarads • Constant Current: 5 Amps

11. MAPLD 2004 The SAFER Flight Unit was activated during STS-86 Demonstration Test Objective (DTO) with EVA astronaut secured in foot restraints Post flight SAFER checkout identified that NSI had not fired • Pressure from the NSI operates a normally closed pyrovalve. The pyrovalve opens hermetically sealed nitrogen tank needed for propulsion. Subsequent trouble shooting revealed that the firing circuit did not provide enough current to guarantee that the NSI would fire 100% of the time. • Temperature increase in bridgewire (BW) due to current flow caused BW resistance to increase significantly • Increased resistance in constant voltage source firing circuit caused firing current to drop • Resulting firing current was below the minimum all-fire level of the BW • Current was at approximately the 60% fire level

12. MAPLD 2004 HOW DID THIS HAPPEN? COMMUNICATION • WHAT A PYRO ENGINEER MEANS WHEN HE SAYS CONSTANT CURRENT • DESIGN REVIEWS • FIRING CIRCUIT DESIGN TESTING • NSI “EMULATOR” • NUMBER OF ACTUAL TEST FIRINGS SCHEDULE PRESSURE COMMUNICATION

13. MAPLD 2004 COMMUNICATION • WHAT A PYRO ENGINEER MEANS WHEN HE SAYS CONSTANT CURRENT • Three basic types of firing circuits • Capacitive Discharge – most common at NASA • Constant Current – occasionally used • Constant Voltage - rare • Typical BW resistance is 1.05 Ohms. • Current flows through the BW causes the BW resistance to increase to around 1.6 Ohms • Since BW resistance changes during firing, a constant current firing circuit must be able to adjust the voltage to compensate for the resistance increase • Minimum all-fire level is 3.5 amps constant current. • Recommended constant current is 5.0 amps • Design engineer designed circuit providing 4.2 constant volts. • Since the bridge wire resistance increases when current is sent through it, the actual current ended up being around only 2.8 amps. • At 2.8 amps, you have approximately 60% chance of the NSI firing.

14. MAPLD 2004 COMMUNICATION • DESIGN REVIEWS • Design Reviews were brief • FIRING CIRCUIT DESIGN • Pyro Engineer did not have an opportunity to review firing circuit design • TESTING • Pyro engineers were not consulted about acceptable number of qualification firing tests. • Only two SAFER qualification test firings were performed. • These test firings were performed on the qualification unit only. • No firings were performed on any of the flight units • Typical system level qualification testing using pyros has a minimum of 15 firings • Other SAFER “firings” were done using a 1-ohm resistor to simulate the NSI. • Pyro engineer was not consulted about the use of an NSI simulator.

15. MAPLD 2004 RESULTANT ACTIONS REDISIGNED FIRING CIRCUIT Capacitive Discharge Detailed design review of new firing circuit with independent assessment DELTA CDR A formal, in-depth review with independent assessment DELTA QUALIFICATION TESTING AND ACCEPTANCE TEST FIRINGS 17 test firings – included firing all flight units as part of Acceptance Testing Monitor firing circuit current during tests EMI testing Bruceton test on firing circuit NSI USER’S GUIDE

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20. SAFER II DELTA CERT TESTING Propulsion and Power Division Keith E. Van Tassel SAFER II DELTA CERT TESTING NSI FIRING AND EMI TESTING May 19, 1998 Presenter: JSC - EP5/ Keith E. Van Tassel Subsystem Manager, Pyrotechnics

21. SAFER II DELTA CERT TESTING Propulsion and Power Division Keith E. Van Tassel • Recommendations for testing the NSI firing circuit • Testing on Cert Unit

22. SAFER II DELTA CERT TESTING Propulsion and Power Division Keith E. Van Tassel Recommendations for testing the NSI firing circuit • Testing on Flight Units

23. SAFER II DELTA CERT TESTING Propulsion and Power Division Keith E. Van Tassel Recommendations for testing the NSI firing circuit • Monitor and record capacitor voltage and NSI bridgewire current as a function of time on all firings. • The Qual Tests from the cert unit plus the Acceptance Tests from the Flight units will give you a total of 17 NSI firings of the NSI firing circuit

24. SAFER II DELTA CERT TESTING Propulsion and Power Division Keith E. Van Tassel • EMI/NSI Testing • EMI testing was performed on NSI Qualification • NSI will go off in the pin-to-case mode at 9 GHz with 686 milliwatts of power applied. • Firing time in this case takes approx. 97 seconds. • NSI will go off in the pin-to-pin mode at 9 GHz with 327 milliwatts of power applied. • Firing time in this mode takes approx. 66 seconds • Since average voltage of EMI is zero, the capacitive discharge will always work regardless of EMI frequency.