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).

Neil

On Oct 19, 3:20 pm, "Neil" 
wrote:
> 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).
>
> Neil

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.

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'.

On Mon, 19 Oct 2009 23:20:42 +0100, "Neil"
 wrote:

<snip>

>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).

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.

>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?

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.

"spacecadet"  wrote in message 
news:4JmdnQNabpSUQEDXnZ2dnUVZ8gOdnZ2d@bt.com...
>I suggest you go along to a club flight if there is one near you. All 
>knowledge will then be yours.

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.

> 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.

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.

> 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.

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.

Neil

"shreadvector"  wrote in message 
news:ae06dde3-3219-4985-bdf7-e0449e596fcf@b3g2000pre.googlegroups.com...

> Complete details are on the Aerotech website.

Thanks.  I found them now you said they were there.

Neil

"Terry"  wrote in message 
news:njnrd515buag2u08qcmb2umfd65s6ne591@4ax.com...
>
> 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.


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.

Neil

Neil wrote:
> "spacecadet"  wrote in message 
> news:4JmdnQNabpSUQEDXnZ2dnUVZ8gOdnZ2d@bt.com...
>> I suggest you go along to a club flight if there is one near you. All 
>> knowledge will then be yours.
> 
> 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.
> 
>> 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.
> 
>  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.
> 
If you email the webmaster you will get a reply but give them a chance 
they wear many hats.