Transcript SAFER (main)

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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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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Pyro Isolation
Valve
SAFER PYROVALVE
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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 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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