HALT testing is not always in a chamber

Adam Bahret
Adam Bahret

When doing talks on HALT testing I frequently get the question “What if stresses other than chamber temp and table vibration could expose the failure mode? Can I use other stresses?” This is very important to acknowledge because a HALT chamber is only a piece of equipment. It should in no way limit the HALT investigation. A baseball bat is an acceptable HALT tool, High voltage is an acceptable HALT tool, Ultrasonic frequency is an acceptable HALT tool.

For those unfamiliar with HALT it is a reliability test method where the product is stressed up to, and beyond, it’s operating limits for the purpose of observing failure modes.  These observations are critical design input for rapid increases in design robustness.

I think a great example of using an innovative stress is a test method created during the development of the Rocketdyne-F-1 engine program. The rocketdyne F-1 engine was used in the NASA Saturn V Apollo program during the 60s and 70s. The F-1 remains the most powerful single-combustion chamber liquid-fueled rocket engine ever developed. It’s broad usage and long life in NASA’s history is a testament to it’s outstanding reliability.

Wernher Von Braun Chief architect of the F1 with a set of F1 engines on a Saturn V rocket

A primary area of concern during development was combustion instability that could put high strain on the nozzle. Testing demonstrated cases where catastrophic failure could occur. Progress on solving this was very slow and a threat to the success of the entire program. Although the HALT concept had not been formalized the HALT philosophy is what ensured the program’s success. Engineers developed a diagnostic technique of detonating small explosive charges (which they called “bombs”) outside the combustion chamber, through a tangential tube. The material used was C4 or black powder explosives.

I mean how much more HALT can you get than setting off explosives in your test sample! I hope someone found a way to turn the word “awesome” into an acronym for this test method because that is the only word that comes to mind. Accelerated Wide range Explosive Sampling Of Measured Energy or something like that.

These explosions were done while the engine was firing, similar to HALT  with operational unit under load. This allowed them to determine exactly how the running chamber responded to variations in pressure, and to determine how to nullify these observed fatal oscillations. Eventually through this iterative HALT process of extreme random stress, investigation, and design improvements the engine design became so stable that the nozzle would self damp artificially induced instability in 1/10 of a second. This stability was demonstrated by 1961 and the Saturn V program started soon after, ultimately putting the first man on the Moon.

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