Plugged boosters

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Apr 12, 2009
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I have a clustered rocket with 7 18mm outboard boosters and one 24mm in the center. The boosters fit up against a bulkhead and would be C6-0s. My question is do I put an epoxy cap on the end of the motor? I saw in previous posts that this may not be legal as far as NAR is concerned. I was thinking I could stuff some flame proof wadding in the end of the motor. Would that work?


I successfully have plugged motors with epoxy with no ill effects. I also have plugged a G40 w/ejection charge with some cellulose and some tape. It worked great.

I don't know about the NAR rule on plugging w/ epoxy, so I can't comment. I would think, though, that the pressure from the ejection charge just goes right out the nozzle as a little bit of thrust, but not enough to really matter...

What's important to remember is that (1) the bulkhead has to be strong enough to contain the pressure, and (2) the motor retention has to be strong enough to prevent the motor from ejecting, once the propellant burns through. Flameproof wadding in the end of the motor won't help, though it might protect the bulkhead some against scorching.

Capping the propellant with epoxy, similar to the way -P motors are plugged with clay, does work. But, you'll lose any NAR insurance coverage for any potential mishaps, and you won't be allowed to fly a modified motor at an NAR sanctioned launch.

If you build the motor mount strong enough to meet (1) and (2) above, though, you won't have to worry about it.
Thanks for the insight.

I plan on using masking tape retention, which should be strong enough since there is no ejection charge in the motor.

So which is more flame proof, Dog barf or Estes TP?
Remember that when the booster blows through it's forward end it will have a lot more surface area of burning propellant. On plugged motors, the plug material inhibits the burning along the forward surface and prevents the blow through. With unplugged motors you will get a "kick in the pants" - a second thrust spike at the end. make sure the bulkhead in fron of the motors is strong and make sure the motors are very secure. Either wadding will work fine.
If you're going to a club launch, you may be able to get a collection together of Aerotech 18 x 20 motor hardware. You can put a D13 or D24 in each one and not put in the black powder ejection charge. This would give you boosters without voiding your NAR coverage, and give you the power of composites as well. Of course if you do this, you may want to airstart the composite boosters.


If you are going to plug your motors with epoxy, make sure you use a lot. In a day or two I will post a pic of what hapens when you don't get enough. I flew my Eye in the Sky 4 videocamera rocket on a central D12 and 3 C6 outboards. I plugged the C6s with epoxy and put them in the outboard tubes. I didn't drill vent holes because I thought there was no need to because they were plugged. It turns out that 2 of the 3 didn't get plugged good and the outboards exploded off the rocket because there was nowhere for the pressure to go except out. BTW, just because it says C6-0, that 0 doesn't mean no ejection charge, it means that ejection is immediatly after motor burnout. Just FYI. Good luck.
Originally posted by havoc821
BTW, just because it says C6-0, that 0 doesn't mean no ejection charge, it means that ejection is immediatly after motor burnout. Just FYI. Good luck.

well, not really. The "0" DOES mean "no" ejection charge. If you look in the end of a booster motor you can see the end-grain of the fuel. There is no delay and no ejection.

*however*, when the fuel burns, the wall between the combustion chamber and the end-grain gets thinner and thinner till it breaks through. When this happens, the expanding gasses travel the path of least resistance, namely the top of the motor, and it can produce a great deal of pressure *like* an ejection charge but far less powerfull

Oh, my bad. I thought it mean't it had a 0 sec delay for staging and stuff. Well, that clears up a lot of stuff. Thanks!
Originally posted by sramberg
Thanks for the insight.

"I plan on using masking tape retention, which should be strong enough since there is no ejection charge in the motor."

Sorry to have to burst your bubble... No matter how much tape you use, if you don't vent the burn thru gasses, your booster motor casings will be ejected. I have 3 models set up for B and C6-0 friction fit booster motos, Estes SR-71, Geo-Sat HLV and the Deep Space Transport, every single outboard booster motor casing is ejected at burnout. Which is fine by the way for sport flying!, As long as your not flying in a competition event motor casing ejection is perfectly legal, lots of commerical kits are designed to eject the casing. Ejectiing a motor casing is NOT a violation of the Model rocket SAFETY code, it is a violation of the pink book competiion rules, if the casing doesn't have an attached streamer.

Plugging the motor is also a violation of the Safety code and voids your insurance. Altering your motors in ANY WAY is never a goor idea. A little forethought will eliminate the need for such activities.

It is always better to vent the booster motors either out the side of the pod or back thru the centering rings and out the rear depending on the make-up of your model. a 1/4" paper punched hole is fine for b & c boosters, if your venting out the rear, back thur the centering rings a 1/8" slot on both sides of the ring work perfectly.

Another option is to use 19mm tubeing for the booster motor tubes, taping a 1/2" x 6" streamer to each casing. and let the launch office know to watch for the seperating casing.
Thanks again for the tips and info. The whole reason to post this was to learn from others experiences.

Mircomister you hit the nail on the head concerning ejecting the booster motor casings. I flew the rocket yesterday and had a near perfect flight. All eight motors lit and had a nice straight flight. Upon recovery I discovered that all 7 boosters had been ejected. I was wondering how I was going to get them out anyway, since they fit flush with the end of the motor tubes. I did find six of the seven motor casing so it was not all bad.

Congrats on the flight!
Wheres the photos of the little beasty????
Heres a pic of my 6 x C6 cluster altitiude model. I tape the motors in, the motors casing stay in by venting the burn-thru out the outboards with 1/4" vent holes which are covered with a tiny piece of 3m Magic tape during boost, Works very well, she has a 4% closed track at 666 meters (2,185 feet):D
Here is a couple of picture of the rocket in question. It is a kit from Fat Cat Rockets called the Frenzee. It was a good flight with all 8 motors lighting.
If I'm not mistaken, it means that there is an error of +/- 4% in his altitude recording.
Forecheck is correct:
for a flight to be "official" in an NAR Altitude competition the two station trackers must both close within 10% of each other. Meaning both tracking scopes spaced 1000 ft apart both saw the same or nearly the same achived altitude, or in this case saw the same puff of tracking powder at ejection.
I don't think plugging motors is a violation of the safty code:

You are not disassembeling the motor.
You are not altering the motors performance.
You are not risking injuring yourself.

This question was brought up at S&T once... had a lot of discussion... I don't recall what the final concensis was, but I will ask for you guys.
Please Doug ask again... maybe something has changed in the last year or so
I have 3 NAR "officials" in the club and the VP is a member of our sister club in VA... ALL have agreed plugging motors are a violation of the safety code. NOT to mention it doesn't work worth a durn anyway.
In the strictest sense you ARE altering the motors performance by adding weight to the motor lowering that motors lift capacity. Not so bad for D's and up, but could make a big difference in 10.5, 13 and ever 18mm motors.
I was told specifically by Tom Lyon his investigation with the insurance carrier indicated YES plugging (altering) motors Voids the NAR insurance. My question to you is Who do we listen to... S&T's defination or the insurance underwriter?
It was tough getting an underwriter, let's not screw it up for a non-sense reason. That million dollar landowner coverage is the difference between having a launch site and not for many clubs.

DO NOT PLUG MOTORS! it isn't worth the hastle.
Originally posted by Micromister
In the strictest sense you ARE altering the motors performance by adding weight to the motor lowering that motors lift capacity.

It would not change the motors performance or lifting ability any more than adding a rocket on top of the motor. And I know flying a motor without a rocket is a violation ;). So what is the difference then to adding weight on top of or outside the motor? In that sense... adding masking tape to friction fit the motor would be a violation.