Two stage with small gap between motors- Question

[swatkat]

So I finished building a modified STM-012 with twin 24x95mm mounts (Estes "E" size). For the maiden flight though, I'd like to do a D12-0 to D12-7. I've got a ton of the 1 inch spacers to fill up the 25mm delta between the motor sizes. However, all the BP staged rockets I've done in the past have all been two motors taped directly together. Is the 1 inch gap between the motors going to be an issue or will the sustainer light reliably?

 

[Micromeister]

So I finished building a modified STM-012 with twin 24x95mm mounts (Estes "E" size). For the maiden flight though, I'd like to do a D12-0 to D12-7. I've got a ton of the 1 inch spacers to fill up the 25mm delta between the motor sizes. However, all the BP staged rockets I've done in the past have all been two motors taped directly together. Is the 1 inch gap between the motors going to be an issue or will the sustainer light reliably?

Gap staging REQUIRES that there be at least two 1/4" vent holes 180degrees apart to allow the cold air column above the booster motor be vented allowing the burn thru particles & super hot gas flow into the upper stage motor nozzle. If the holes are not in place the cold air column will cause the upper stage to seperate long before the upper stage come up to temperature to ignite the BP.

 

[OverTheTop]

You can use a tube to couple the two motors. Apogee Components has one of their Peak of Flight newsletters discussing it I think. Have a dig on the Apogee website for it.

From what I remember it works better with small vent holes near the forward motors to allow some gases to escape.

 

[GlenP]

So I finished building a modified STM-012 with twin 24x95mm mounts (Estes "E" size). For the maiden flight though, I'd like to do a D12-0 to D12-7. I've got a ton of the 1 inch spacers to fill up the 25mm delta between the motor sizes. However, all the BP staged rockets I've done in the past have all been two motors taped directly together. Is the 1 inch gap between the motors going to be an issue or will the sustainer light reliably?

For a 1 inch distance, I don't think it really matters much. Try cutting a 1 inch length of empty motor casing to use as an extension to the D engine, super glue it to the booster engine to match the length of the motor mount, and then tape it up to the sustainer motor with the cellophane tape like you usually do for CHAD type staging. When you use the longer motors they are touching, right? You are not talking about a longer than 1 inch gap staged mod where you split the STM012 at the transition with the sustainer 10 inches or so away from the top of the booster engine? If they don't light with the 1 inch extension CHAD way, then try adding a couple of vent holes to the extension and through the entire engine mount and body tube, but it sounds like you probably don't need them.

 

[swatkat]

Thanks all. I'll add the vent holes. The gap is only that created by the 70mm D being shorter than the 95mm E. An E12-0 or E9-0 in the booster would be fine (touching/tape-able to the sustainer motor) The MMT on the booster extends all the way up to touch the base of the sustainer motor.
Photo attached.

 

[OverTheTop]

FYI the vents allow the gases to move forward (and then vent), carrying particles of burning BP forward for ignition of the upper motor.