Plugged Closure Problem

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Jan 21, 2009
Reaction score
Penns Creek, PA
I just got my RMS 38/720 motor in the mail today (From ROL Auction). It came with a plugged forward closure.

Now I noticed something with the Plugged closure, it is a pain to put in the smoke element. The o-ring between the delay grain and the delay well is so tight, that the air in the well can't exscape.

The only way I figured to seat everything together, is slowly tighten and loosen the closures together with everything assembled. However, I am concerned how reliable this is... I know that since the closure is plugged, blowby is not an issue... but if that delay grain ignites from the other end... it may cause problems. Also, I am afraid that constant loosening and tightening the closures under stress may damage the threads...

Any suggestions or comments?
Well, how about this - you don't need the delay grain to function right? So drill a 1/16 hole through it and then fit it in, then take some epoxy and inject some in there so you don't get it burning like a core burner. that way gasses won't get by and start a core burner.

I think that using the tighning of the closures to seat the delay would be asking for trouble. This would place a a lot of force on the forward seal disk (or insulator). With a phenolic liner, this force would be transfered to the aft o-ring which can probably handle it. Probably.

With a paper liner, you would probably deform the insulator disk or crush the liner. Resulting in a lack of pressure on the forward o-ring. The o-ring must be compressed in order to seal reliably.

Have you tried seating the delay o-ring in the bottom of the ejection well and then inserting the delay grain? This will probably be a struggle but perhaps easier than the usual method. Put the delay into the insulator without the spacer (which would get crushed) and slide in. Then use a dowel or something else about the same diameter as the delay element with a flat surface to push on the element. After it is in place, then you can add the spacer.

Disclaimer: I don't have a plugged closure and have never tried this, but it should work.
On our sugar motor plugged wells we use a very small piece of wire resting on the wall of the well, slide the delay in and pull the wire out. The wire allows the trapped air to escape. If you are worried about both ends burning, wipe the top end with red RTV or grease to inhibit it.
It's real easy, put the o-ring in the bottom of the well then push the delay into the o-ring while it is seated in the bottom. This way, you are only compressing 1/16 of an inch of air. If you put the o-ring on the delay and then push it in you will have to compress +1 inch of air. PITA.


Geoff is right. I now fly about 80% plugged on RMS and do it that way ...

The order of assembly for the forward closure after you lightly grease inside the delay well, sides and top is:
1) Small rubber washer.
2) Greased O-ring.
3) Delay element (push firmly into o-ring).
4) Delay insulator/liner
5) Delay spacer.

Just make sure you can get the last 3 pieces together before you start and it isn't hard at all.

HTH, --Lance,