BP motor consistency

[Winston]

I don’t, but you’re really good at finding interesting links. Go look on the downloads page at Tripoli.org. There are only four issues per year so it should not take too much time.

Found it.

Tripoli Report Vol 29 No 3 Summer 2018

TRIPOLI MOTOR TESTING INVESTIGATION PYROTECHNIC DELAY ELEMENT TIMING IN COMMERCIAL HIGH POWER ROCKET MOTORS

OR

Why You Should Use An Altimeter

by Alan Whitmore

In the report I don't see any mention of ambient temperatures during the tests and while I see mention of some tests being done at 5500 ft ASL and 11 ft ASL locations, no altitude spec I can see is associated with any individual motor test result.

Anyway, to avoid potential copyright issues I'll include below only the paragraph found early in the issue claiming to summarize the test and the results. While I agree with the suppositions providing excuses for delay element variability in that paragraph, a possibly important variability factor, delay element initial temperature isn't even mentioned in the study and due to the lack of the test site altitude being specified for each motor test the altitude-based variability isn't measured either.

"Elsewhere in this issue, Alan Whitmore has submitted a report on the variability of rocket motor delay/ejection charges, which provides an excellent example of how information faithfully supplied by reliable manufacturers can be unintentionally deceptive, if not further considered. The essential truth revealed by the study is that while delay grains are well understood and characterized by testing, their burn rate will vary depending on altitude-related atmospheric pressure. Accordingly, delay times for those grains test-burned at ground level at any given site will vary depending on the elevation of the site, and may also be assumed to perform differently in an actual rocket flight where the altitude is progressively increasing (hopefully!). The actual delay will therefore be affected by the flight profile/apogee. Also, as most realize, if there is an ignition anomaly on the pad, the delay grain may light up before the propellant fully does, or viceversa. In these cases, the defined delay time may not appear to match how that delay actually performs. Thus, one cannot necessarily treat the rated delay time as gospel, and in most cases, moderate aberrations of the actual delay should not be the source of the flier getting upset with the manufacturer."

30% of the 40 certified commercial motors in that test series failed to meet NFPA 1125 requirements.

BTW, if I had to guess, with field use and the associated motor user technique and launch system variability, I'd say motor ignition consistency and chamber pressure curve variability could be major, related factors affecting the delay time for any delay element exposed to combustion chamber gases from ignition onward.

Since this exposure does not happen in BP motors it doesn't excuse their delay variability. In that case, I'd expect that all sorts of potential manufacturing variabilities would be at fault.

 

[Winston]

Because this experiment was so easy to do, just for the halibut back in late June of 2009 I recorded the audio from motor ignition to ejection during a club launch of Estes Generic E2Xs flying on A8-3s. None of them had their decals on them for some reason which eliminated that potential drag variability factor caused by very badly applied decals and it was a pretty calm day so slant range variations due to weathercocking weren't that great.

Speed of sound timing compensation for the ejection charge report was made based upon Rocksim's predicted altitude (~220 ft). Timing analysis was done using the fantastic, free audio tool Audacity.

This was not a delay accuracy only test, just a very rough overall motor uniformity test. If I still had the audio files I could go back and just measure the delays, but I don't.