Motor Block replacement idea: A long bolt

While I source a 29mm/BT60 centering ring, I'm thinking about motor retention for my semi-upscale Solar Warrior (Stretch Warrior?)

One of the things that was almost universally suggested when I built my Argent was "Don't put the motor block in!" (into the 29mm motor mount) - the logic being that if I did, it would seriously limit my motor choices. So I didn't put the block in and I got a CTI 3-grain retainer and it's great. And if I really want to use a BP engine, I can wrap it with a bit of tape and epoxy the tape and it should be fine.

But on the Solar Warrior, I'm thinking maybe a 29mm BP engine would be fun and it sims out just fine and wrapping my own thrust ring seems like a pain. So I had a thought:

What if I VERY carefully drilled a hole and put a long, thin, plastic bolt through both the BT and the MMT? In other words, find the spot where the motor mount would be, mark it very carefully on the outside of the rocket, and then, VERY carefully, drill a hole, let's say 1/8" diameter, through the BT wall, through the MMT, and then out the other side. Then if you want to use a BP motor, you slide in a very thin bolt (aluminum would be awesome if I could source it), and if you want to use something longer, you just take the bolt out.


Thoughts? The only downsides I can think of are:
-It could weaken the tubes. This doesn't seem like a huge deal, especially if I strengthened them with some CA around the holes.
-It could give the ejection gases an "escape hole". But I don't think that 2x1/8" holes would reduce the pressure that much....would they?

I think it would be fine for typical black powder thrust. For composite motors simply make sure you use one long enough to get past the holes and those downsides disappear! Well I suppose that would be true if minimum diameter but you still have the outer holes in play. You could put a forward centering ring at the top of the motor mount to eliminate that gap, but also remember that high power rockets typically use 1x or 3x vents of roughly this size, so that part should be fine too.

I prefer thrust rings myself but I appreciate creativity and I do not see any reason your idea would not work.

Back in the mid-60's, I was scratching building a rocket and the local hobby shop had no motor blocks, so, I drill two sets of holes through the BT and MMT and inserted two brads into the holes. The brads formed an "X" and prevented the motor from flying up into the BT. I cemented the brads in place, then filed off the sharp points. It worked very well and I still have the rocket, or, what's left of it.

ActingLikeAKid said:

While I source a 29mm/BT60 centering ring

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These should do it. https://www.erockets.biz/semroc-centering-ring-115-to-16-pkg-of-6/ The IDs of ST-16 tubing and BT-60 are only .005 inch apart.

Don't forget that most motors have ejection charges placed along the centerline. Your bolt would have to be very carefully off-center or it could either be damaged by or otherwise possibly hinder the ejection charge. Also, if the bolt is plastic, it's probably going to melt and be hard to remove again.

I haven't done so myself, but I'd consider making the holes, essentially as you suggested, except then gluing (epoxy) some nuts to the inside of the body tube (between BT and MM). Then instead of using one long bolt, use two short machine screws that are just long enough to protrude into the MM tube, but not interfere with ejection (about 1/8". If you do this, I'd strongly suggest upgrading to 3 or 4 equally spaced holes instead of just 2. I can't remember how much clearance you have between a 29mm motor mount and BT-60, if it's not that tight, you could even re-enforce with a small block of balsa and put the nut right into that.

MCriscione said:

Don't forget that most motors have ejection charges placed along the centerline. Your bolt would have to be very carefully off-center or it could either be damaged by or otherwise possibly hinder the ejection charge. Also, if the bolt is plastic, it's probably going to melt and be hard to remove again.

I haven't done so myself, but I'd consider making the holes, essentially as you suggested, except then gluing (epoxy) some nuts to the inside of the body tube (between BT and MM). Then instead of using one long bolt, use two short machine screws that are just long enough to protrude into the MM tube, but not interfere with ejection (about 1/8". If you do this, I'd strongly suggest upgrading to 3 or 4 equally spaced holes instead of just 2. I can't remember how much clearance you have between a 29mm motor mount and BT-60, if it's not that tight, you could even re-enforce with a small block of balsa and put the nut right into that.

Click to expand...

On my 29mm to BT-60 mmt's I just use cardstock wraps as centering rings.