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 TYPICAL FIRING MODES
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: 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 TESTING
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

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 FIRING CIRCUIT DESIGN TESTING
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
12/4/2018

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
12/4/2018

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
12/4/2018

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
12/4/2018

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.
12/4/2018