Or at least I am sure they are questions that have been answered many times and possibly in many ways before, it's just that I'm having a little trouble finding the answers at the moment...
Since my first (and so far only) model bit the dust the other week, I have decided to reincarnate the salvagable parts (fins, nose, motor mount) into some new tubes. But to be a bit more interesting about it (and since it was apparently a model of a multistage vehicle and had two sets of fins anyway) I thought I would turn it into a two-stage model.
First I was going to light the upper stage electronically. I couldn't quite work out where I was going to put the electronics. I'd built a payload bay for an altimeter I'd had for a present, and I'd thought of packing it in there. Then I was a bit confused about how to reliably run wires between two parts of the rocket designed to separate. I read about a bit, and discovered that the gubbins is most often located in the void between the motor tube and the main body. That space in my model is tiny, 24mm motor - BT60 body tube, so I can't do that. So I've gone over to a design that lights the upper stage using the ejection charge of the booster. Today's design has the motor tub in the booster stage extended forward so it slots over the protruding end of the motor in the upper stage. (Just thinking about that, I'll have to add a slot to accommodate the engine hook). I've done it that way for two reasons: first to try to get as much of the stuff to hit the back of the upper stage motor as possible; and second to try to protect the recovery device (probably a streamer) from the ejection charge and from the initial exhaust of the sustainer motor; and third (three reasons) because I know no better.
Question 1: Is that protection likely to be enough, or will I have to shield the streamer some other how as well? Question 1b: In knowing no better, have I missed a problem with doing it this way?
At the moment I'm half convinced that lighting the sustainer motor limits me to BP motors for the upper stage. For now, that's not so much of a big deal, but I just know that later on I'm going to want to send the model as high as I possibly can.
Question 2: Is it possible to light composite motors using some chemical means from the ejection charge of another motor? E.g. can I light a fuse either with an ejection charge, or off the top of a delay element which has enough oomph to light a composite motor (e.g. Aerotech AP) if I stuff the other end up into the motor core?
The next problem is that my simulations show that in all but the stillest conditions a BP motor for the booster doesn't get it flying quickly enough soon enough for it to fly reliably straight up, so I'd probably want to use a composite motor for the booster from day one. The only trouble is, I can't get motors with zero delay to light up the upper stage. The motors I have used (F24W-7 & F24W-4) in flying the model before C-day have delay elements which differ in length. I suppose I can take one of those, e.g. the 4s element, and shorten it further, e.g. with a flat bottomed drill bit, to get the delay I want (as close to zero as possible without going negative).
Question 3a: I am asking to go up in flames by drilling out the delay element material? (I suspect not, since I have seen other references to it but no details). What do I need to be careful with? Should I limit myself to doing it by hand to restrict the cutter speed? Question 3b: The difference in length between a 7s delay element and a 4s delay element is about 2.3mm. Does that mean if I take another 2.3mm off the 4s element I end up with a 1s element? (The elements are much thicker than 0.75mm/s burn rate would suggest, but I think the part that burns while the motor is lit probably burns much faster than the rest). Or are the two elements likely to be of a different composition?
That'll do for now.
Thanks for all the helpful answers (I hope).