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

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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

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MAPLD 2004

SIMPLIFIED

AID

FOR

EVA

RESCUE

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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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SAFER PYROVALVE

Pyro IsolationValve

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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

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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

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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

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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.

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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.

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RESULTANT ACTIONS

REDISIGNED FIRING CIRCUITCapacitive DischargeDetailed design review of new firing circuit with independent assessment

DELTA CDRA formal, in-depth review with independent assessment

DELTA QUALIFICATION TESTING AND ACCEPTANCE TEST FIRINGS17 test firings – included firing all flight units as part of Acceptance Testing

Monitor firing circuit current during testsEMI testingBruceton test on firing circuit

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