Easy questions regarding two-stage model

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
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).
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Drilling delays only works to shorten a longer delay down to NO LESS than the shortest delay sold for that motor. REASON: If the delay is shorter, there will not be enough material left to retain the extremely high pressure while the propellant is still burning.
Only drill on the combustion chamber side.
Complete details are on the Aerotech website.
Reply to
I suggest you go along to a club flight if there is one near you. All knowledge will then be yours. Failing that there is a lot more traffic on the UKRA public forum for which you can ask for a guest login without being a member. Failing that, you can gap stage BP motors up to 8" or so, above which you can use quickmatch. However, if your booster is only a bit longer than the motor, it's quite routine to expect it to light the sustainer and to use 'tumble recovery', i.e. no recovery at all. BTW it's not ejection charge from the booster, booster motors have no delay, it's just burn-through. AP is somewhat harder to light than BP so I don't know if it works with composite. Don't forget that altering a BP motor in any way is illegal in the UK. It's a prohibited 'act of manufacture'.
Reply to
IF you drill a delay, holding the drill in the fingers is quite satisfactory. Each 1/16" (roughly) is two seconds of delay time. However, see below.
You are correct in that the delay grain composition is different from that of the propellant. If you drill out 2.3 mm you will probably have flame blow through the delay element before the thrust phase is complete, because the remaining delay is too thin to handle the chamber pressure.
There's also the issue of whether the output will actually light a BP motor. Maybe it will...but when a lower-stage BP motor ignites an upper-stage BP motor, it's from the bits of burning propellant that fragment and are tossed into the nozzle of the upper stage. Hot gases alone may not do the job. That's why venting the space between stages can allow up to 25 cm or more between the stages.
Suggestion: Cluster two or three BP booster motors around the central booster, each in its own pod. Light all on the ground. Boosters in their pods will eject, booster in the middle will light the upper stage.
Reply to
I had been down to a couple (it was certainly the easiest way to get to a big enough, open enough field to fly without having to cross everything that it all wasn't going to end up stuck up a tree somewhere. I missed the last couple for various reasons, only some of which were not having a model to fly with anymore, and now there are no more (at least in that group) for some time, it seems. I'm sure I'll be there again.
From ukra.org.uk, how do I find the forum? I've spent a little while shuffling around the website and not found my way to it.
I may go for that. It is pretty small but feels quite weighty (to me) just to let it fall down by itself; it would be 70g plus the weight of the spent motor. And if I go for the suggestion of adding more motors to overcome the lack of a short-delay 24mm F motor to get it shifting quickly enough, it will be even heavier than that.
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I did do some designs like that, but then I thought it was getting a bit extravagant. Perhaps it is the easier thing to do after all.
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