| Superthree Scratch Built Three Stage Rocket |
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| Forced to retire my Supergee,
I needed a replacement. The Supergee was a 2-stager that I flew
regularly at launches. It was minimum diameter, and flew on 18mm
motors. Thus, the Superthree was born not only to take its place, but also to meet the additional challenge of having a 3-stager that could fly on 18mm motors - no 24mm boosters allowed. Historically, 3-stage model rockets have always used either the no-longer-available 18mm B14-0 or the 24mm D12-0. Kits such as the old Farside, in their instructions, called out the B14-0 as the only suitable first stage motor. Later kits, such as the Comanche, use the 24mm D12-0 staged to 18mm motors in the second and third stages. The goal here was to build a 3-stage rocket that could use the 18mm B6-0 and C6-0 booster motors in the first stage. These are medium thrust motors, and have enough oomph for most 2-stage rockets, but are quite marginal for 3-stagers. The moderate thrust dictates a light rocket and a long launch rod. Minimum diameter is key to getting it light. 3/16" launch lugs allow the use of a 4' long launch rod which gives the rocket a bit more time to get up to speed before free flight. 1/16" thick fins also helped save on weight. Extensive checking in Rocksim convinced me that this would work. |
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| As
the rocket progressed, I got a little bit of cold feet. The
stability margin was a bit thin, so I hedged and added about 10g of
nose weight. The base of the cone was hollowed out, and a slurry
of epoxy and lead shot were used as ballast. A piano wire loop,
for connecting the chute and shock cord, was anchored in the
slurry. Rocksim indicated the stability was improved and that the rod speed was still good |
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At left is a good look at the motor hook on the first stage. A wrap of tape will secure it to the motor. It ensures the booster stage separates with the motor when the second stage lights. |
At
right is the Superthree on its maiden flight. Everything worked
like a charm. It staged a B6-0 to an A10-0T to an A8-5 with
streamer recovery. All three parts came back in good condition.
January 11, 2009 |
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