Friday, June 19, 2009

It's not 'research' but what is it?

Last year, I ordered a 3-pack of F24 RMS loads that was missing a ton of parts including 2 of the 3 delay assemblies. I contacted AeroTech and, after emailing some evidence, they promptly replaced the reload pack. I quickly decided that I really liked this motor. Later in the year, I picked up two 3-packs of F39's on the cheap from a hobby store that was closing. It soon dawned on me that, with the grain and nozzle, I could use the F24's for my Hat of Death or Whirlygig-24. I salvaged the rest of the components from the F39's and flew the F24s. All I needed was a complete delay column, it didn't matter that the delay grain was the wrong one.

However, I now had extra F39 grains and nozzles. I previously had good success repackaging grains from a G77 reload, one at a time, into an F20 LMS case. So, I wondered if I could go the other way by taking the two F39s and making nominally a G78.

I'm a tube pack rat and found a convolute wound tube that fit the grains, less liners, perfectly. Could I be so lucky? Yes! It also fits perfectly in a spent 29mm SU case. I took a used Ellis Mountain G case (I told you I'm a pack rat), whacked off the forward end, cleaned it out, and I was on my way.

Since chamber pressure is proportional to the propellant burn area divided by the nozzle throat area, all I had to do is double the throat area. Luckily, the required diameter is a skosh over that of the eroded throat of the used motor. But, this is still an experiment. I know how to size nozzles vs. core area, but have no rule of thumb for slotted grains like those used in the F39s. But what the heck, all I'll be risking is a saucer.

Attached is a photo of the components so far. I cut the liner, which is about as thick as an Estes E9, a bit long. I'm going to epoxy in an internal plywood bulkhead (shown) and then add an outer epoxy plug.

I figure this will be close to a 99G74 (using actual the tested F39 numbers vs. the marketing ones). It should fly at MDRA's July launch.