NESC tech bulletin

National Aeronautics and Space Administration

www.nasa.gov

NASA Engineering and Safety Center Technical Bulletin No. 09-01

NESC-TB-09-01

Ram

Initiators

Primer

Chamber

Assembly

(PCA)

Shear Tube

Valve Body

Booster

Applicability

Pyrovalves are used frequently in propulsion systems built by 

NASA and industry.

Background

Four spacecraft propulsion system pyrovalve “no-fire” 

failures were recently investigated by the NESC (NASA 

Engineering and Safety Center). In all four cases, a normallyclosed pyrovalve failed to actuate during tests in which 

simultaneous firing of dual initiators failed to ignite the booster 

charge. In each failure, a common aluminum Y-shaped PCA 

(Y-PCA) manufactured by CONAX was used to mechanically 

accommodate the two initiators and to direct the individual 

output products of each initiator towards the booster charge. 

Booster charge ignition is intended to generate sufficient 

pressure to actuate the pyrovalve.

Findings and Conclusions

The primary finding from this investigation is that dual 

simultaneous firing (< 10 microseconds skew) is not as 

robust as a single firing and should be avoided. When close 

sequential firing of the redundant initiator is necessary, the 

NESC recommends the skew time between initiator firings 

should ideally be longer than the flow duration for single 

fire. Thus the NESC recommendation that use of the device 

should be constrained to single fire operation (or dual with 

skew greater than 2 milliseconds) to ensure robustness of 

booster ignition.

The NESC has forwarded this finding, and others related 

to critical PCA parameters and device qualification issues, 

to projects using the current design. In addition, the NESC 

contributed to a new stainless steel PCA design activity

initiated by the Mars Science Laboratory project. 

Recommendations outlining other follow-on tasks have been 

communicated to the NASA Pyrotechnic Working Group.

References

CONAX Y-PCA Booster Anomaly Investigation Report, NESC 

Document Number RP-08-111, NASA Technical Memorandum 

(TM) Number TM-2008-215548

This work was led by Michael Hagopian, Goddard Space Flight 

Center, and Andreas Dibbern, Kennedy Space Center. 

For information contact the NESC at www.nesc.nasa.gov

Failure of Pyrotechnic Operated Valves with Dual Initiators

Investigation of recent pyrovalve failures reveals timing of redundant initiator firings is crucial for reliable operation 

of pyrovalves. Simultaneous firing, i.e., within a very narrow time frame, was found to be the primary cause of the 

valve failing to operate. Testing of both single and dual initiators revealed important design characteristics affecting 

pyrovalve device performance. They include Primer Chamber Assembly (PCA) geometry and material properties, 

as well as operational effects on combustion product flow, and resulting energy transfer to the booster.

The CONAX PCA design has evolved slowly over time undergoing incremental changes to correct known issues. 

These piecewise changes were verified by limited test, without a full understanding of the overall system impact or 

effect on margins. Adequate system performance margins may be adversely affected in existing or future systems 

incorporating a pyrovalve actuated by simultaneous firing of dual initiators.

A normally closed pyrovalve

Unfired PCA sectioned at midline