Inverse Engineering

Project
823 flights

Post-flight Analysis

Great set of launches! Alberto Gassol and I launched a total of four successful rockets. I made three identical 6" ABS rockets with four J300 engines in each and Alberto made a minimum diameter cardboard rocket powered by one J300 engine. The first flight was perfect except the parachute ripped off when it deployed causing the rocket to come down ballisticly. The second flight was Alberto's. The rocket left the rail and flew straight up but disintegrated just before the engine cut out. The third flight veered sharply when it left the rail and the rocket took a horizontal path South. It's still a mystery why that happened. A possible explanation would be that one of the engines failed to light but if that had been the case the rocket would have spun in circles. There was no weight in the rocket to shift to cause a catastrophic CG shift. The last rocket was a little off vertical and took off extremely quickly. The only reason I can think of is that one or more of the engines had a stress fracture in the fuel causing an increase in the surface area burning and thus more thrust. It seems unlikely however, that this could happen without the engine exploding due to over-pressurization.
The large rectangular holes cut into the fin sections made a marked improvement in the overall performance of the rockets. When the rockets were recovered it was clear that the exhaust temperature going through the fin section was substantially reduced and the damage to the plastic was minimal.
Also tested was one static J300 engine with steel wire wrapped around the inside of the engine casing. This was in an effort to prevent any cracking due to stress on the fuel. The static was not successful.
Click here to view Alberto's launch report.

Construction

6" ABS pipe was used for the fin section and parachute section. The bulkheads were made from 6" ABS caps held together by 1/4-20 fully threaded aluminum rods.
Four J300 engines were used in each rocket for a total combined output of 4800 Newton seconds.
Fins were made from matt board covered balsa wood attached to the fin section with #10 fully threaded aluminum rods.
The ejection device was chemical and the parachute was made from 2 mil. plastic sheeting with brass grommets.
Total take-off weight was 32.0 lbs.

Pros