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Old 05-15-2013, 11:29 PM
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Gus Gus is offline
7/21/61
 
Join Date: Sep 2005
Location: North of Detroit
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I routinely gap stage 10.5 mm motors, both old Apogee motors and European motors. I also routinely gap stage Estes 13 mm motors. I have done a great deal of experimentation and high speed photography and here is what I have learned. (Doug, you and I discussed this in another thread ~4 years ago but I've experimented a lot since then so this info is slightly different)

Contrary to what G. Harry thought, staging, including gap staging has little to do with flying burning particles. In standard, non-gap staging, when motors are taped together booster burn-through results in a jet of flame entering the sustainer nozzle. Ignition depends on the nature of the flame jet and, more importantly, whether the force of the tape holding the motors together is enough to overcome the pressure buildup between the booster and sustainer long enough for the jet of flame to ignite the upper motor. Booster burn-through is an inconsistent process so the jet produced is quite variable.

The top of an Estes booster motor is simply the upper end of the pressed propellant slug. If you view it from above at ultra high speed burn through may occur in the middle, or closer to an edge. It may resemble a cross burning out from the middle of where the bars cross or be more like a widening circle. The variability of this burn through is completely random resulting in very different jets being produced. A booster that burns through closer to the edge will produce a flame jet which may not be centered near the sustainer nozzle opening. As the booster continues its burn-through the flame jet widens. If the tape can hold the motors together long enough for the flame jet to reach and enter the nozzle, then sustainer ignition will occur. In most cases taped together motors do a fine job and ignition occurs.

In gap staging, the tube between the booster and sustainer motors acts like fireworks Quick Match, propgating a ball of flame from booster to the sustainer almost instantaneously. A pressure wave of gas precedes the flame by milliseconds and, if not vented out the upper sides of the tube, will push the sustainer motor off the tube before ignition occurs. But what travels up and ignites the sustainer is not little burning embers, but a ball of flame that envelops the aft end of the sustainer motor causing it to ignite. Part of the flame ball also exits the vent holes. The ball of flame is long (in milliseconds) gone before noticeable flame begins exiting the sustainer nozzle.

This is why it is possible to reliably ignite gap staged European sustainer motors with VERY tiny nozzles, without any wick. Be aware, though, that European competition 10 mm booster motors are slightly different from Estes or old Apogee motors. Euro booster motors have a forward cap with a very tiny hole in the middle. When the booster motor burns through it is only a very tiny spurt of flame that exits the top end of the motor. Euro booster motors are designed to be augmented with a pinch of black powder on top that serves as the flame propgator for sustainer ignition.

The fact that gap staging depends on a moving ball of flame, rather than burning embers, allows you to do interesting things. You can actually gap stage around gentle corners. The sustainer motor does not have to be located in a straight line directly above the booster motor. My daughter's Little Joe 1A which she flew in Slovakia used this technique to have one of the 4 18mm booster motors located at their scale outward locations in the booster to ignite a 13 mm motor in the center of the base of the Mercury capsule (to simulate the escape motor). Photo of the LJ1A internals below. This system used standard Estes motors, no augmentation. Very reliable.

Final note. Wick is useful in a sustainer nozzle when you are trying to delay ignition by a second or so. The wick fills the sustainer nozzle preventing the ball of flame from directly igniting the sustainer. Instead, the wick ignites and then takes a second or so to ignite the sustainer. Useful if you are going for altitude, but it adds a whole extra failure mode and can be tricky.

Hope this info is useful,

Steve
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