Minimum Diameter motor retention

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Well-Known Member
Jan 19, 2009
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I am planning a 29mm minimum diameter rocket with goals of mach one and one mile on MPR G engines. The plan is for a 22.5" 29mm BT, lathe-turned basswood nose cone and 1/8" ply clipped delta fins. (Wrasp says it'll do Mach 1.04 on a G80 and 5400' on a G40-10 if I can keep the drag down and the weight under 5 oz. w/o engine.) I've been looking around for ideas on motor retention but not sure of what might work best. I could use a block ring glued at about 6.5" (for an Ellis Mtn G35) with spacers for shorter engines and masking tape friction fit, but I'm wondering if anyone in TRF-land has any better thoughts? Keeping weight and drag down is pretty critical.

Thanks in advance.

I have run a few simulations of your design in RockSim, on a G80, I get a maximum speed of 770 mph and 4725 feet AGL for a minimun diameter 29mm airframe that long. So it is doubtful that you will achieve either of your goals. If you post more details like nose cone length and shape and fin dimmensions, I can create a more accurate RockSim version 5 file for you that can be viewed using the demo version of the software.

For motor retention you could build up an aft trust ring with 1/4 inch wide masking tape at the base of the motor. The motor would be friction fit into the airframe. What material are you using for the airframe?

Bruce S. Levison, NAR #69055
I get a maximum speed of 770 mph

I used wRASP with a 5 oz weight (w/o motor), a drag coefficient of 0.58 (no launch lug-tower launch), and a standard day (mach one is 759mph) and get Mach 1.04 at 4,400ft. Getting the mile is tougher. On a G40 it sims to 5123', but on a Aerotech G25 it sims to over 5400', but as far as I can tell, G25's are no longer made or sold. I have an Ellis Mountain long burn G35 from Magnum, but it's not in the wRASP database.

Airframe is a 22.5 length of 29mm motor tube from Aerospace Specialty products. (My RockSim demo version says it weighs about 2.1oz, but my mail scale has it weighing in at 1.5 oz) My plan is for a 5" ogive cone with a rounded tip made of basswood, fins are clipped delta w/ a 3.75" root edge and 1.75" base made of 1/8" aircraft ply. RockSim has the CP at about 22" from the front, which with a 3.8 oz motor may require about 1/2 oz of nose weight for one caliber stability. I could make the airframe shorter, but would probably need more noseweight to keep it stable.

I'll probably end up w/ a friction fit retention system but was wondering if anyone else had any ideas.


heck, just friction fit it ,as you may not get it back anyway at that speed and altitude.
You might want to consider some of the techniques used by competition modelers.....tying kevlar shock cord around the engine to aid in the friction fit.....its called a lariat loop....

The example shows the cord coming on the outside...I have used this same technique but kept the shock cord inside the body tube to reduce drag and it also works
see here:

also I use mylar or kapton tape to tape the engine to the body I put the fins forward of the base by at least 1/4"....
I tried to make a RockSim version 5 file that you could open in the demo version of the software. Is this close?

Bruce S. Levison, NAR #69055
I tried to make a RockSim version 5 file that you could open in the demo version of the software. Is this close?

Very close! My demo version had the CP at 21.768 " from nose, less than an half inch off. (The fins look a bit bigger, than on mine.) I used a mass object to simulate the motor and the CG moves back to about 20.5", making it stable. As long as I can get the CP identified, I can use nose weights to place the CG as needed.

As for motor retention, the Apogee Newsletter had an interesting idea for a U-shaped "cage" made out of thin piano wire, expoxied against the fins and covered by the fin fillets to minimize drag. Gives an extra measure of motor retention in addition to tape.