BlueComet24 Posted August 10, 2016 Posted August 10, 2016 I got my 12.7mm BP rocket motors working. Here is a cross-section of approximately what the motors look like, except I now make the bulkhead go to a couple millimeters from the top of the tube, so it's about the length of the ID of the tube. I plan on drilling a passfire hole through the bulkhead to light a header or ejection charge, and I'm wondering if this could weaken the bulkhead enough that my motors could CATO. They used to CATO before I started ramming them on a solid post and made the bulkhead longer - I'm not sure which of these changes (could be both) solved the CATO issue, and unfortunately I can't test it now because I have a healing wrist and can't hammer hard enough to make new motors. I plan on drilling passfires in the range of 3-7mm diameter, but usually about 5mm or less. I'm thinking that what matters most is that there is some solid bulkhead around the edges to hold the pressure and fuel in, much like a snap ring. Would drilling a passfire (or replacing the bulkhead with a delay comp of the same length) make my motors more likely to CATO?
NeighborJ Posted August 10, 2016 Posted August 10, 2016 Blue, that passfire hole should not need to be any larger than 3mm or 1/8 in. It should be in the center of the bulkhead and the bulkhead should be about the same length as the I'd. If you stay within these parameters I can't foresee any issues. I'm not sure why you would need a larger opening unless you plan to use 1/4" time fuse. I've tried drilling multiple passfire holes for different effects and found that if the holes were too far from center it would blow out. So now if I need to pass fire to multiple devices I drill a 1/8" hole thru the bulkhead then drill a larger hole(1/4") inside the first but only 1/2 way into the bulkhead. That ensured that the hot jet of flame will light anything inside the cavity. The other little bit of insurance I add is a small hole thru the side of the motor casing above the bulkhead this will also encourage even more flame thru the passfire opening. This is a trick used in multiple stage rocket motors to prevent a motor from ejecting before lighting the next stage. I'm sure someone will object to it for some reason I've overlooked but it's what works for me.
BlueComet24 Posted August 10, 2016 Author Posted August 10, 2016 As you said, I wouldn't need a passfire much larger than 3mm unless I'm using 1/4" time fuse, which I rarely use. I was thinking 5mm because that would be large enough to fit tape-wrapped visco, but I could just drill it out to 5mm halfway, as you suggested. Sounds like I should be all good, then. Thank you.
NeighborJ Posted August 10, 2016 Posted August 10, 2016 I'm not quite sure why you would need to seal the passfire hole to the fuse. I always want the flame to jet out past the fuse, this way I can be sure the pressure in the passfire hole won't prevent the flame from reaching the fuse. In TRs PGI seminar on YouTube he states that you do not need the fuse to reach all the way down to the top of the fuel. It only needs to be slightly in the hole to keep it centered over the jet of hot flame.
BlueComet24 Posted August 10, 2016 Author Posted August 10, 2016 The tape wrapped around the fuse isn't to seal the passfire hole. It's there because of how I construct my ball shells, and provides the delay. It doesn't stick very far out of the shell, so I could drill the top half of the passfire wider and place the fuse in it. I would avoid sealing the passfire hole both because I don't want to create back pressure and because I want flame to get through to the top of my motor where I plan on having a BP charge to separate the shell from the rocket and a fuse to detach the stick from the motor.
NeighborJ Posted August 11, 2016 Posted August 11, 2016 I gotcha, we're on the same page. I fuse my small mortars in that manner as well. I'll use the delay on the outside of the shell, that way I can splice the quick match directly to the time fuse that way I don't need to rely on the lift charge to light it.
Seymour Posted August 12, 2016 Posted August 12, 2016 I prefer to use a delay which is slow and sparky for night time rockets, or something like fast BP for day time rockets (as it provides some extra thrust). This delay is pressed a little bit above the spindle so that it burns after the core burning section. With end burners you will want a slow burning delay above the fast fuel to allow it to coast to apogee for the parachute. However all this does not translate over very well at all if you are casting your fuel and are not pressing. I've always drilled out near the side of bulkhead, because this is where the fuel/delay is consumed last, so less is wasted. I cannot think of any times that I've had motors work with intake bulkheads but cato with drilled ones. However I am only talking about 19mm diameter and 25mm diameter (1lb and 3lb?) and cannot speak for larger motors.
OldMarine Posted August 12, 2016 Posted August 12, 2016 I use Caleb's bulkhead/passfire rammer and have found that if you don't press in a tube diameter height bulkhead they'll blow out if you use a fast delay. Recovered the two that blew last weekend and it wasn't a Cato event but rather blown bulkheads.
BlueComet24 Posted August 13, 2016 Author Posted August 13, 2016 Care to explain? I'd like to try these motors without bulkheads, but they might just CATO.
NeighborJ Posted August 13, 2016 Posted August 13, 2016 You'll never know unless you try. We're not asking you to jump off a bridge here. Don't let your bulkhead be a barrier. Get rid of the clay I bet you'll stay.
dagabu Posted August 14, 2016 Posted August 14, 2016 I never use a bulkhead, there is really no need. If timing is what you want, back drill the comp to the timing and be done with it. Of the thousands of sticks I watched fall from the sky this past week, all of them that had clay in the tube were high speed lawn darts. I watched as a 4 year old girl was missed by a few feet! I am more now than ever opposed to anymore clay than is absolutely necessary in a rocket.
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