Clarification/rebuttal (however you want to look at it).

You can easily fly BP motors without a thrust ring. It is handy to have one, but not necessary. If you go without them, you will need to use tape to create a thrust ring at the rear of the motor and do a light friction fit into the motor tubes. A wrap or two of tape around the motor abd tube and you are good to go. I've done it dozens of times with BP motors. It does eventually char up the upper end of the motor tube, so I wouldn't do it this way if you plan to fly the thing a lot with shorter motors then fly with a longer one.

That said, if you know you will never go over a certain length motor, go ahead and put in a thrust ring and use spacers to protect the motor tube when you use shorter motors.

It is fine to mix ejection charges. A D12-3 will not eject at the exact same time as a C6-3, so it won't matter if they are "matched". As an example the D12-3 has a burn time of 1.65s and delay of 2.39s giving a total ignition to ejection time of 4.04s. A C6-3 has a burn time of 1.86s and delay of 2.47, with a total time of 4.33s. That's 0.29s difference, which doesn't sound like much, but the D12 has already kicked the laundry out before the C6 fires. These are the average NAR test times, so it's possible or even probably to get them even farther apart.

What you must do is make sure the extra ejections do not burn your shock cord because there won't be any wadding left to prevent direct contact of the charge. My son and I have often flown the Semroc Saturn 1B with two B6-2's and two B6-4's to make sure we did not have too strong of an ejection with all four going at once. It works great.

Once you decide on motor combination, make sure your core, or a couple of the outboards have the right ejection time. The rest can have longer ejection times so that you won't end up with a shotgun ejection.

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