"Shorty"  Launchable Ejection Testbed

Shorty is a small, heavy airframe with mediocre aerodynamics and a tiny engine.  It is intended to achieve very limited heights, thus moderate breadths.  This is so that I can launch it at home, observe its entire flight, and have a good chance of getting it back.

It is to be used to test ejection systems using smokeless powder as ejection charge.

I did not plan for it to go this low.  The poor performance is apparently due to engine design error.  Prime suspect is the nozzle throat diameter of 9/16 inch.  In retrospect this is probably too large to maintain a consistently high engine pressure, and thus the burn-rate for the charge of 100 grams KN/Sucrose propellant was inconsistent, seemingly bi-modal.

Click on this picture to download the (somewhat comical) video:

Beware!  This is a 5.5 megabyte download for only 15 seconds of video.
Probably not worthwhile unless you have fast internet access, or something else to do while it loads.

Notice that I almost dropped the camera when the rocket tilted over, still coughing.  I have occasionally had rockets take off with a vengeance toward the end of a coughing spell.  I was happy to see this one hit the ground.

Unfortunately, the movie ended just before the ejection charge fired.  It worked just fine.  Since the nose-cone was stuck firmly in the dirt, the body was ejected upward a foot or two, freeing the streamers used in this initial test.

Thanks to its low achievement, the airframe is intact, to be tested again soon.

Jimmy Yawn

Tested again, Soon!

Wednesday, and I finally got home a couple of hours before dark and rigged up another test of this brave little rocket.  The engine was nearly identical to the previous one, but burned slightly more fuel (120 grams) and had a 3/8 inch nozzle throat.

Click on the photo for a video of the launch - 1.3 meg file, about 3.5 seconds of video.

This time the engine fired well, despite a bit of excitement at the instant of ignition - the fireball that pops out is probably a chunk of black powder priming.  I really do need to refine this approach to priming, perhaps by consolidating these chunks of BP into some kind of pyrogen ignitor.

Unfortunately, the engine was the only thing that worked well.  Oh,  maybe the airframe too in that it did manage to go up, more or less.  But the ejection charge failed to fire, so there were no pretty streamers.  This time I was using BP for ejection, so it wasn't a problem with smokeless powder.  My guess is that the burn-through delay charge went out when the grain was spent and the pressure dropped.

So Shorty came down like a speeding bullet into the waist-high weeds.  I was distracted by the camera and did not notice exactly where.  He will sleep out in the field tonight.  I hope to find remnants tomorrow.

Jimmy Yawn