A slow motion illustration of a gap staged model rocket's failure to ignite the upper stage. This can happen if the booster motor pressurizes the booster body tube when the motor's propellant burns through.
There is a fraction of a second delay between the burnthrough of the lower stage motor and the ignition of the upper stage. This delay - roughly 0.001 second - is called the Barber delay (after Trip Barber, who studied it at MIT), and can sometimes be enough to cause the booster to blow off before the upper stage lights.
Note in this slow motion video you can actually see the flames from the booster motor shooting out the top of the booster section as it falls away.
Due to its momentum, the upper stage continues to coast, but since it has no motor burning, there is no ejection charge, and when the rocket reaches its apogee, it will arc over, and come in ballistic.
The chances of gap staging failure like this one can be greatly reduced by venting the booster stage. Small vent holes can be drilled either directly into the booster body tube, or through all of the booster centering rings, allowing the increased pressure to release out the back of the booster.
This was not my rocket, but I love the footage. The rocket did come in ballistic, but heads-up was called, and nobody was near it when it landed. Since it was made of wound paper, the upper section got crunched, and the rocketeer had to rebuild.
Video was shot by Joe Barnard, Barnard Propulsion Systems. His website is www.bps.space
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