You nailed it.
The most experienced person I know with respect to high altitude multistage flights is Jim Jarvis. He does them safely year after year, and he got there by starting smaller and working up incrementally, testing one or two new things with each flight and keeping what has worked. I would study his threads on TRF. He never uses head end ignition. He makes BP work consistently. He thinks everything through instead of jumping on some bandwagon and he is very methodical.
Thanks Steve! By the way, I did use head-end ignition in 2019 with no other choice available, and will use it again this year.
If a charge is reasonably contained (i.e., a hard-sided container covered with tape or the like), then I tell people to increase the amount of BP they use by perhaps 30-50% at a 30K altitude. But, this is obviously an approximation based on experience.
I DO NOT tell people to use surgical tubing charges as they do not work at altitude unless the volume that they are in is small, which is often the case. However, testing shows that these charges do not work if the volume of the airframe is more than the very minimum. The developer of the technique, Tony A, agrees with this and has developed an alternative approach that does work. You can find his description of it here on TRF.
The method that I use is relatively simple. It's chief advantage is that 100% of the powder burns regardless of the altitude. Therefore, whatever happens in a ground test is what will happen at altitude - any altitude. The chief disadvantage is that the charge is very energetic (because all of the powder burns very quickly). I call it deployment by shotgun, and that's the feature Tony was trying to improve on with his design. But what I do works fine as long as you design accordingly. I wrote an article about the method some years back that I've posted on TRF many times, but it's been a year or so, so here it is again. I've used this on something like 10 flights above 50K, with 5 of them above 100K, and they are very reliable. I would always advise ground testing of any particular design.
One change or clarification of the information in the article is that the ematch should be placed on top of the powder and not at the bottom of the charge. This is particularly important for larger charges.
Jim
