Photo of Swamp Gas (25 inch diameter saucer rocket) in workshop

First Flight of Swamp Gas

The time finally came…St. Louis Rocketry Association’s second night launch at Buder Park, and the first of what will hopefully become an annual tradition: the Halloween Spooktacular. We had perfect weather for this type of event – cool but not too cold, clear skies and low wind. It was time to fly Swamp Gas.

The flight went well, I thought – none of the bad things I thought might happen, happened. I had really been concerned with two classes of failures: unstable flight and a descent rate that was too fast. Unstable flight was a concern because I had no way of simulating the flight, and I had scratch built this rocket just by looking at photos of other saucer rockets. It ended up being somewhat flatter than the other saucers I was looking at (principally Art Applewhite’s saucer) and I didn’t know how that would affect stability. I also didn’t really know what orientation of the rocket was going to be stable coming down – how it “prefers” to fall, in other words. Falling edge-first would be a worst-case scenario in terms of how hard it would hit the ground.

Here are my observations:

  • Stability seemed good under thrust, but the saucer then got wobbly as the thrust tapered off. The motor I used was an AeroTech F23-4 FastJack, which I picked because it was the largest motor I had in the box that had the shortest delay (4 seconds) – more on the motor delay aspect below. It has a very linear thrust regression, starting at just below 50N at ignition and tapering off immediately. You can see in the video how it started wandering near the end of the burn. It also didn’t go very high at all, which tells me that a G motor would be very reasonable for this rocket. When I fly it again I will probably use something like a G79 which has a higher thrust, higher impulse, and a much flatter thrust curve.
  • I had originally planned to put a small drogue parachute in the space between the motor tube and the body tube, above the upper centering ring, but after trying several different chutes, I couldn’t find any way to fit anything in that space. What I then decided to do was run the shock cord back into the motor mount and rig up a rear-ejection system, in which the spent motor casing would end up hanging below the saucer and hopefully encourage it to fall in a nose-up orientation. This failed for a number of reasons. First, even with a 4-second delay, the rocket was back on the ground before the ejection charge went off. Second, the shock cord (60 pound test kevlar thread) snapped easily when the motor ejected. Fortunately, the rear-ejection scheme wasn’t needed at all, since the saucer flipped right over and fell nose-down naturally, which resulted in a safe landing on the grass. When I fly again I will use conventional motor retention and no ejection charge.
  • When I abandoned the drogue parachute idea, I took the nose cone I had and glued it into the motor mount with CA. However, that nose has a solid shoulder and a loop to attach a shock cord, which takes up almost an inch of the length of that tube that I expect I’ll need to fit a G motor. So I may need to try to coax that nose cone out of there somehow and replace it with one that has a hollow shoulder.
  • The top skin of the saucer is made of poster board, which looked good in the workshop (see photo at the beginning of the video) but started wrinkling up right away when I took it outdoors, even though it wasn’t especially damp. I don’t think there’s anything I can do about it now, but if I build another saucer I’ll want to look for some other material for that.
  • The lighting worked great. Just a few days before the launch, I thought of another lighting mode to add, where the ring lit up and then randomly dropped individual pixels until it was dark again. That introduced a very weird bug where a cluster of pixels was lighting up random colors during the “drop-out” loop. After much debugging, it turned out that initializing the random number generator with randomSeed() made that behavior go away. I still have no idea how those are related.
  • Swamp Gas was the hit of the launch. If we do the Halloween night launch again next year, I will definitely bring it back!

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