motors in plugged motor tubes

For many years, I flew nothing but single engine rockets [with scant exceptions]. Now that I'm back in the hobby, I'm thinking of making a few cluster-able rockets, such as seven 29mm tubes, only three of which would go all the way into the parachute compartment [if anyone here remembers the old LOC Ultimate kit or the current THOY Nighthawk, it would work the same way].

My question is, if I use motors in the plugged tubes [the tubes that do NOT go into the chute compartment], what would I have to do to those motors to keep them from popping out of the bottom of the rocket at ejection? I'm thinking along the lines of dumping out the black powder ejection charge, or not installing it in reloadable motors. Will this work?

For the reloadable motors in the tubes that don't pass through to the parachute compartment, there are a couple of options.

If you are using hardware that has a forward closure with the ejection port, leave out the delay charge and also leave out the black powder ejection charge. Depending on the type of forward closure, it might be a good idea to stuff a piece of recovery wadding into the forward closure just to plug up the hole that would normally pass the hot gases to the ejection charge.

Another option is to buy a plugged forward closure. Again, leave out the delay and ejection charges when assembling the engine.

Hope this helps.

Well first I have to totaly disagree with Ken, sorry ken... But you always install the delay grain, even with a pluged forward closure. If you look at the certifaction charts they recommend a long delay grain.

Now I can't recall where I got this info, but I'v tryed it and worked very well on a 4 cluster flight with 3 of the motors being 18/20 motors. I built the motor as directed and then smear Dielectric (100 silicone) on the top of the delay grain and then in the ejection can area. My understanding on this is that it will not burn and stops the burning.. don't know... but it did work very well for me.

Now for the SU (single use) motors.. the best that you can do is run the pluged tubes into a tube that will vent through the aft center ring. You may wish to use more then one tube for this depending on the size of the motors.

I haven't looked at the rockets you are compaing to, and will later. Good luck with the rocket and keep us informed.

hmmmm..... Trying to learn here, and maybe figure out why I may have gotten what seems to be conflicting information.

I could understand using the delay grains in all the 29mm hardware except the 29/40-120. The delay grain in this hardware goes in the forward closure. What is the need for it? I was told by another Level 2 person that it was better to stuff that delay cavity with wadding rather than using the delay grain because it saved the rocket motor mount tube in the rocket wear and tear by avoiding the heat caused by the delay charge burning.

Although, if I understood your reply, you are saying that you always smear the delay charge with the Dielectric compound to keep it from burning? Perhaps the person I got info from on this didn't know that particular method and that's why he came up with the wadding method.

At any rate, I've used the wadding method on 24mm and 29/40-120 reloadable hardware quite a few times in rocket boosted RC gliders and it has always worked well for me.

Where can one obtain this Dielectric 100 silicone compound?

Dielectric can be bought at most auto parts stores. It's what auto mechanics use on wireing harness to keep out water, dirt and basicaly keep a good connection. a good size tube cost around $4.00. This is what I use when loading RMS motors.

One thing that troubles me about not using the delay grain is that I read somewhere that the delay grain works in the pressure of the motor. and with the delay removed there is more open space in the motor, would this change the characteristics of the motor? I'll look into this and get back to everyone.

Never leave out the delay grain!

The functioning of the motor depends on all the elements, including the delay grain. Note that the delay grain itself does not cause the ejection charge, that is done with the black powder added into the forward ejection charge well. The delay grain generates the tracking smoke that you see after the motor burns out so is always useful.

If you don't want the ejection charge, just leave out the black powder. If you have one, use a plugged forward closure, but this is not necessary. Remember that the motor will have burned out before the delay grain burns through so nothing should come through the forward closure.

Good comments from all. I can see how the lack of a delay grain could indeed cause a pressure change in the motor, as gases could and will escape from the ejection charge port unless it is well plugged with something that won't disintergrate under the pressure of hot gasses. Probably not a big problem for LPR loads, but it's something I would not try in HPR.

A plugged forward closure on a 54mm load would do the trick, but on 98mm loads, which do not use a delay grain, they still include a "delay type" grain that fits inside the upper closure and is used to produce tracking smoke, as previously mentioned.

Carl