Help with BP charge placement!

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Well-Known Member
May 10, 2011
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OK... I am midway through my Shadow and Flame project, and I am trying to figure out where to put my electronics bay. I just bought a LOC electronics bay, but I dont know where to put it...

I was thinking I could have the rocket seperate at the fincan. This improvised drawing kind of explains it...

I could use some insight from people who have done this... SHould I have the nose come off for the chute, or have it seperate at the fincan, or what?

Thanks in advance!:)
Sorry, that drawing looks stinky at the size it pops up at. Hold your cursor over it and enlarge it, and it looks better.

It is entirely illegible at normal size....:(
Well, you could do either but for this placement I would go with NC deploy so you don't have as much space to pressurize... if you are going for DD you would want to do both NC and fincan.
Oh, and another question for recovery... Should I have the fincan and the upper section come down on thier own chutes, or should I have them both connected to two big chutes?

NC or fincan... Same space to pressurize. (I think...) What difference would it make?:confused:
Am I seeing this right? Dual charges for a single chute?

Also, is the payload section popping off, or is it a zipperless fincan?
The payload section would pop off the fincan so the chutes could come off.

Yes, thats two charges. Altimeter with radio-controlled backup. If one pops, it will proabably blow the other one along with it, but if it doesent I will pop it with the backup ground-controlled deploy. (is this a good idea or a bad idea?)
Look at it this way. Your initial altimeter pops the two sections apart, but the chute does not come out. The radio controlled unit will not help in this situation, it will just fire the charge into open air. You're better off deploying with the altimeter only, and maybe motor back up. I would personally go with only the altimeter.

No matter what, it's hard to detect Apogee from the ground, and then deploying with the Pratt unit. Depending on your alititude, reception may not be good enough at altitude to make it worthwhile.
altimeter with no backup? I dont think I really want to do that...

Could I use a piece of 1.5" tubing to make a little "BP holder" with a cap, with the BP inside it so it wouldent go when the altimeter one goes? That might work a little better. That way if it pops and the chute is still in there, it pops, blows its cap off and pushes the chute out. That sounds a little better to me...

Heres a new version with that. The blue is a LOC 1.5" tube, and the green is the plywood cap.

Your thoughts?
What are you using for the electric matches?
With DaveyFire's N28'B you can leave on the plastic thingamabob, put a wrap or three of tape around them and press on an appropriate length of the tube (BT-5?) that CrapperHeads come in. Fill with BP and tape the end of the CrapperHead tube. Seal with a light coat of epoxy over both ends and that will ensure the charge doesn't go off except when it's told to. It also confines the BP just long enough for it to go POP instead of pfflltt.
I dont really know...

Whats the best for firing off Missile Works altimeters?:confused:
Any low current ematch such as the Daveyfire N28B is fine. They will fire off any altimeter or other timing device. I think they go for about $2 apiece but I bought a case so I paid a little less. I use them for cluster ignition too, after a nice dip in pyrogen. Don't get the N28F's though as they are higher current and not as well suited to use from air born electronics. The Daveyfires have really long wires so you can just screw them to the terminals on your bay and place the charge anywhere you want, above or below the 'chute depending on the placement you think is best.
How far back of the nose cone is your bay? You need your ports a ways back. I do at least 2 body tube diameters from the nosecone or any transitions. If not you could get early/late deployment issues.

Also, If you haven't done anything yet I would convert to drogueless dual (My favorite recovery method). You would shift your altimeter bay to about 6" after the coupler for room for a tether. This would split the rocket at the top. Then at your set height you would deploy the main out of the NC end. My favorite method.

This thread shows my drogueless dual setup.

My point is this : If you are using single, apogee deployment only, I do not suggest using the Pratt Unit. If by any chance you get button happy, or flinch during lift off and trip the unit, you have an avoidable, SERIOUS problem.

With a ground activated unit such as the Pratt, it would be hard to sense apogee at altitude. And if you did, it would be a gamble as to whether or not the unit would receive at such an altitude. The unit is meant to be used at a lower altitude where timing is not nescessarily crucial, and is more preference based than that of an apogee event.

My suggestion on a single altimeter is not bad, a lot of people use it. It will be hard for you to do it the first time, but once you do you'll get more comfortable with it. Just repeat "Arm the altimeter, Arm the altimeter" as you walk out to the pad and are setting it up, or stick a paper towel in the vent hole. RRC2's are a very reliable unit when used with DaveyFire's. So good that I am contemplating getting another one to use in my Magnum, instead of the PICO AD2 that I am not too confident in...
On the motors I am flyin' this thing on this year, and at LDRS, it will only go about 750'-1K'. Not that high. I bet you could see it at that altitude... Wouldent it be easy enough to set the box on a table untill you are ready to pop it?:D Thats that problem solved:D

The whole point of the Pratt unit is backup to the altimeter. Better to have an altimeter with ground backup than an altimeter on its own, IMHO

The recovery section isnt built yet, so I can put it just about anywhere. So it isnt good to have it right next to the nose? Where should I put it? Dead center?

People trust their altimeters stand alone all the time. They work when you construct the bays properly and get the vent holes right. If you are relying on the backup, then what happens when the backup fails. If you are using the RRC2- Jim make a great unit and it won't fail you if you get everything installed. Why go through all the trouble. Trust your electronics.

Backups can never hurt, as I saw. Someone up at my club had an altimeter with the same Pratt unit I want to put in there as backup, and it shot up, didnt deploy, and he pressed the button and sure enough I think it saved the rocket from lawn darting into the line of cars:eek: Well, maybe im exagerating by saying "into the line of cars" (there were only 12 of the things at the launch... Pretty small target :D), but it would have toataled the rocket... It was waaaay the heck up there! :eek:

Ill put the other electronics in as though I were relying only on them, and then put the reciever in there in case the worst should happen. I would hate to see my months of hard work on this thing go down the drain because of some electronics problem.:eek: :( It certainly cant HURT the rocket, can it?

So where in the tube do I put em? Dead center, a foot below the cone? Where does it get the best flow?
Well, if you designed it with the bay somewhere you would have an idea of what the bay will do to your CG and stability. You draw it out toward the nose and your CG goes forward and you become overstable. Back further and you might go unstable. I go at least two BT's back of the nosecone, maybe more. Use three properly sized and spaced ports for your static ports. Air can get in/out better and lessens the chance of a false deployment. But first you have to decide what recovery you are going to use...then you can see where to put the bay and how to wire it up. Just be sure to keep the wires all together. Don't want to mis-wire and have no charges go off.

"static ports"? Do you mean holes drilled in the tube for air to get in/out? (im new to this electronics thing... Sorry for the stupidity ;))

This thing weighs a ton anyway, I really doubt a quarter pound electronics bay will push it over stable/unstable...

I think ill go with single stage deploy, but I would like to have the capability to do drougeless dual also. For the first few flights, it will be going under 1K, so dual deploy would be overkill IMHO. Id rather go with single deploy to make it easier for those flights.

How about I cut the payload section in half exactly, so the bit above the nose cone can just have a big shock cord for drougless dual deploy, and the space in between the coupler and fincan would hold the dual 56" chutes for single deploy/main recovery. How does that sound? Kinda like this crude drawing... Have the electronics bay inside a coupler tube for my Staples mailing tube, with the electronics in there.
a quarter of a pound is definantly enough to push it over, remember the torque rules, it only applies a quarter of a pound of force at the CG, once you move away more force is applied because you gain leverage.

I wouldnt do the backup, you are going to have too many things to do and something will get messed up. Just do the one and make sure you are doing it right. The RRC is an extremly reliable altimeter
I guess I disagree... I like backups. Things screw up. Everything has screwed up at one point or another. NASA still screws up. RCC altimeters could screw up. Having a backup just makes me feel safer, and theres really not much that could convince me otherwise. Some people put TWO receivers in there along with an altimeter... I dont think it will be that hard to put one more piece of simple electronics in there to keep the rocket from lawn darting should the altimeter fail.

There is going to be an RCC2X altimeter in there with a Pratt ground controlled deploy reciever in there as backup. end of story. It makes me feel better, and that is worth installing two more wires to me. Thats just the way I see it.:)
I would normally think this was a good idea but I am kinda skeptic of the Pratt device... The way I see it, its more things to go wrong or break. If I were doing this, I would just use the RRC, I have only seen one altimiter fail, but the only reason for tht is not having enough BP in there to eject the chute. The drogue went fine.

Just an opinion,


Good luck whatever you do!
Obviously we have seen different things happen, so we obviously disagree on the topic:D ;)

Your skeptic of the Pratt thingy because (I assume...) you have seen most altimeters work on thier own.

I like the Pratt thingy because I have seen it save rockets carrying over $500 worth of stuff:eek:

I know someone who doesent fly a single big rocket without the Pratt reciever in it (or two units, if its a really big rocket), and it has served him well on several occasions. He has advised me to do the same, and after seeing that unit save (more or less) his 7" V2 (along with an AMW baby M casing...), I dont blame him! I think its a good idea to put at least one in there... Along with a transmitter to locate the rocket and an altimeter for normal deployment...:D I trust his advice 110%, and I think ill follow it. It cant hurt.

Besides, between my dad and me each double checking the wiring, I bet we can get it right.

Thanks for the concern, though.

I think I have made up my mind on the backup issue, so I dont think we really need to discuss it much further.

So, anyways... heres another crude drawing... Should I put the electronics in the fincan coupler, or should I put them a little below the cone? The drawing is pretty self-explanitory.:D
My vote is payload section.

That thing is 10 pounds! What will you fly it on as a L1?
Yep. nice big 10-12 pounder....:eek: :cool: Ive got my work cut out for me sewing the second parachute for this thing... The friggin things take up 4 square yards of cloth apiece!!!!:eek:

Im thinking an I357T for the first few flights... We just got 3 of those in the mail, and they want to be burned:D Rsim says 750'. Should be a good low and slow to test it out before I see about buying a J350:D It also says it could fly on an I161, but I think that might be a little too low power for it... What do you think? Could it fly well on the I161 or should I stick to the higher thrust motors?

My dad is still working on his cert, so I cant do a J350 unless I find a third party to help... That shouldent be too hard. I already know someone who will proabably help once it flys once or twice on Is.

But low and slow Is should be great fun for testing it out, and taking pictures:D Even if I cant fly it on a J350 at LDRS, a few Is will definetly satisfy my HPR needs:D I have only ever flown Hs, so even a few Is will be an accomplishment if it all goes without a hitch. And if it doesent go without a hitch, itll be a little lower so I can see apogee with my radio controll:D
that's pushing dad's 10# Bullpup went 882' on a 38-480 I285R...straight up, too.

I'd say at least use an I300T.
But the I357T would be OK? If it is, I guess ill go with that for the first two flights. What diameter was said bull pup?

Im sure I can find more ways to burn I161s....:D
How much is the Pratt unit anyways?

I think an I357 would be OK.... Don't take my word on that though, I'm a MPR guy. :)
They are OOP, so I am getting an unused one from some guy who bought it for the same purpose, but never flew it. $75. Not too bad, considering what it can do.:)

I357T is is, then!
I dont know man, for a 10lb rocket I wouldnt use an I at all, I guess if you really wanted to you could try it but it would be a risky flight.
How much did Bill Spadafora's L2 rocket (the one that worked:D The 5th attemt I think) weigh? It only went like 600' on a J... so I guess we cant compare that to this with an I.

So are you reccomending that I go with a J350 for the first flight, or a bigger I (I366R or something :confused: )? Heres the rsim file with the simulation data. It might help in deciding a little bit... It says the I357T will get it to 750', but I forget about what it said with the weathercock...

Well see when the time comes. If its nice and calm I might want to fly it on an I, and if it isnt, well, ill wait for the next day:D :eek: .

But would it be a BAD idea to have the first flight be a J350 when I have never flown an I before?

I will obviously be flying Is at LDRS, but a J in the first flight of my biggest rocket to date.... Is that reccomendable?:confused: