I also just used the 54mm MD Aeropack retainer in a new 54mm MD rocket.
As Kyle said you can just drill, tap and use some machine screws. I epoxied some self clenching nuts onto the inside the the Aeropack and have had it in and out of the rocket no less than a few dozen times at this stage. I sized it for the biggest case I plan to use, a Loki 54\2800 with extended bulkhead. But I can put anything in it with some simple adapters or even threaded rod; I am launching it this weekend at NYPOWER on a 1 grain CTI 54mm case. If I was planning on using more shorter motors then I would drill multiple mounting holes which would not be a problem, and do away with the adapters.
4-40 self clenching nuts epoxied inside
View attachment 263723
Had to make an adapter to go from Loki to Aeropack thread sizes
View attachment 263724
Flat head machine screws countersunk to sit flush and use a T10 head
View attachment 263722
Michael,
If your airframe goes all the way from the Aeropack down over the top 6-8 inches of the motor case (or the complete motor case) I do not see this to be a problem. The aerodynamic forces on the airframe would be distributed to the motor case just like a coupler tube would see. Because of the fact that your Aeropack retainer is removable, you won't have to worry about getting the threads on the bulkhead stuck in the retainer and having the bulkhead spin inside the case. The airframe will hold the motor centered, and as long as the Aeropack adapter is mounted on the center line of the case and the entire assembly slides smoothly inside the airframe, thrusting the motor against the retainer should not cause any problem what so ever in my opinion.
The only weak point (if any) I can see in this type of design would be the screws holding the airframe to the retainer, or perhaps the airframe material or retainer itself around where the screws are. I honestly don't think that you'd have a problem, but there's one way to find out. Mount the retainer in the airframe, put a large wooden dowel up the back end, stand it all upright and balance yourself on top. That last part's the hard part. ;-) If you can still use the motors thrust ring, that would be the safest approach, but it will stick out into the air stream a bit.
Those screws might be your ultimate thrust retention point I think. If you have enough screws that are strong enough to hold the expected G forces X airframe weight, then you should be good to go. Let me know if you need any help in figuring that out. Nice work btw. This is very similar to what I am planning for Airfest this year, but I'll be using the Loki Lock Collar in place of the Aeropack retainer to hold the motor to the airframe. Also check out the tail cone below. There's still a few ways I can do that, not sure which one just yet, but I'd like to remove the motor cases thrust ring for a more flush design since even the low profile thrust rings still have a bit larger OD than the airframes out there.
However if the retainer were to be mounted permanently, what you need is the bulkhead Lock Collar I am trying to work on. Otherwise the threads could get locked and when you go to unscrew the motor from the adapter and the bulkhead could spin inside the case, since it's "floating". CTI forward bulkheads are floating as well, but they are under compression which keeps them from spinning.
What I am missing on the bulkhead lock collar pictured below are the internal threads that will screw onto the 54mm Extended Bulkhead. The bottom .200" on the ID are not threaded so that when the set screws dig into the threads, those threads won't be engaged on the bulkhead. You'd screw down the collar so it is tight on the motor case and then tighten up the set screws so it doesn't move. This will hold the bulkhead firmly in place so that it does not spin inside the case. At least that's the plan. One of it's purposes was so the motors using the extended bulkhead would work with the Aeropack MD retainer.
If that works as planned, the next step is to drill and tap holes in the top of the lock collar to act as the rear bulkplate of an electronics bay. You'd screw in your all-thread into these holes and mount a bulkplate at the top end of the bay. The troublesome thing here is, FG coupler tube that is centerless ground like most all of them are, doesn't hold a good tolerance on the wall thickness. What you end up with is a lip sticking out on the thick wall side and a flush fit on the other. The only fix I can see right now is to have a sloppy fit so the coupler tube can be properly centered. This may not even be a problem though because most tubing is 2.150" ID and the motor cases (Loki) are 2.127"OD so there's some amount of play right there which should compensate and equal things out.
blackjack2564 said:
Depends on pressure & thrust. Some small motors & you can get aways with it.
Jim, every 54/4000 and 76/12000 I have tested has put the full thrust load of the motor directly on the load cell through the forward closure. That is how I have always done all my testing. As long as aerodynamic forces are not applied directly to a forward closure itself, pushing on the closure in this manner should never be a problem in my opinion. Do the math and it'll tell you the same thing. Looking at the photos I posted, if you were to attach the airframe directly to the forward closure assembly and the airframe was not sleeved down the top end of the motor case, all bending or flexing at the CP would put the force directly on the closure retention, something it was never designed for, unless perhaps you have a bolted case MD rocket like you'd see at BALLS. I had to cobble some scrap parts together and this was all I had. :-(
Note- the CTI P-8000 is designed to mount from the forward closure, albeit a composite case.