Doubie Shot build.

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Jimbo TRA 2142
TRF Supporter
Nov 18, 2016
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Ancillary parts are starting to show up, so I guess it's time to start building my Madcow Double Shot. This is my first time on a forum, so be gentle with me.


Awesome, was looking hard at the double shot but decided to hold off and focus on some other projects.
Will be watching!
Ok, let's organize the next few days.

First the mods:
Index all tubes and couplers
Cut each main BT 12.5" from rear for future rear separation
Cut extra 6" 54mm coupler for 2" piston and 4" rear coupler
Tape couplers in place as apprpriate
Drill six 9/64" holes 1/2" from every 54mm tube end including NC an EB band
Countersink 54mm tube holes
Jig and grind LE of upper fins (lower fins come pre-beveled)

Now for the standard stuff:
Drill holes and arrange mounting for motor retention with 10-32 socket bolts and shock cord in CRs
Cut foam board fin guide and widen pre-cut slots to fit
Clean and sand all fin area surfaces
Line all ducks up and mix epoxy to; mount fwd CR to 38mm MM and insert from front, insert fins and tape exterior fin BT joints,
fillet fin/BT/MM, install rear CR w/shock cord
With excess epoxy or a new batch glue nuts on the inside of every coupler using screws to hold them in place
Glue NC bulkhead above nuts on NC coupler

This should complete the assembly of the second stage (except for the EB)

:pop: I got one too, but I don't think I'm going to be touching it for a few months... too many other projects.
Ancillary parts are starting to show up, so I guess it's time to start building my Madcow Double Shot. This is my first time on a forum, so be gentle with me.

Welcome. Congratulations on the retirement, (hopefully I'm not too far behind you).

That's a pretty impressive TRA number, you've been around awhile.
Where did you do your TRA flying? Lucerne?

As for being new of the forum...hey, we all were at one time.
Anyone gives you the business, ignore 'em.
Listen to MaxQ, his TRA number is almost a low as yours. You might want to be careful, I still haven't decided if he's a pud-knocker or not! :wink:
I got my BFS Double Shot yesterday, unboxed it today. Will definitely require some cleanup, only two of the CR ID's is right for the tube it's supposed to go around, some of the ODs have some burrs, and only one of the 6 fin slots was the right size to slide a fin into. But nothing that a few seconds of sanding each problem area can't fix.

This is my first 2-stage. The sustainer makes perfect sense to me[*] as does the lower part of the booster. But I am curious how people tackle the ISC part of this kit. There's the 4" coupler, the actual transition, and the long 54mm upper coupler, those parts are obvious. Ring-wise the 4"-to-54mm-coupler CR makes sense (and was one of the two CRs that 'just worked', the other was one of the two 54-to-38 CR's). But then there are two coupler bulkplates, one on the 4" side and one on the 54mm side. So these would just float around inside the ISC tubes. As I try to come up with a reasonable way to have access to charges at the top and bottom of the ISC, I'm leaning towards epoxying the 4" plate at the bottom of that coupler and epoxying the 54mm coupler tube to the top of the transition, but not epoxying the 4" coupler to the bottom of the transition, instead this part would be riveted and is where I'd separate things to get access to the electronics, as well as the upper 54mm coupler bulkplate, which would have my charge well for stage separation. So my thought is I'd prepare my booster electronics, then could attach my electronics assembly to the lower plate using the eye-bolt as the bolt (I do this in other rocket bays), prep the charges, then slide the upper bulkplate up into the 54mm coupler tube (which would also slide the 4" coupler into the transition), then install the rivets and have a completed ISC. Quick sketch attached. The main down-side I see to this approach is the 54mm bulk plate won't make a perfect seal since this part doesn't get epoxied in, but I'd rather this not be lost in the 54mm coupler tube since I would like to have a charge well here. How do others handle this?

[*] Okay, getting the ignitor wire down to the motor in the sustainer is another question, the 38mm MMT in the 54mm tube doesn't leave a lot of space, doesn't help that I plan on using an AeroPack RA38 retainer as well, though I could also go with an MD54 and top retention of the motor as I do on my 54mm MD. I've heard mention of using brake line tubing to create the wiring channel but never really looked into it before, is that tubing small enough to work here?

I see Mad Cow has them back in stock. They have a number of kits that were out of stock from the BF sale back in stock.
The big decision with a 2-stager build is where the airstart controller will reside.

If you put it in the interstage/booster it makes the build easier, but you have the issue that an early drag separation might pull your igniter out of the sustainer motor. You also can't use a separation charge. Putting it in the sustainer makes more sense, but you have to figure out where to put it.

If you put it in the standard AV bay, you have to run a wire down to the interstage somehow for the ignition, and possibly the separation charge if you do it from that controller. The problem is that it's ahead of the drogue sepration point, so you have to somehow have the wire come loose so it doesn't hang up the drogue separation. This probably means running it down the outside of the rocket, possibly through some kind of tube that can be clipped to each separation point so it comes off at separation.

The other option is to create a second AV bay between the sustainer motor and the drogue, and run the igniter down an internal tube to the interstage. That makes it look cleaner from the outside, but you have to be able to take apart that AV bay, so it complicates the airframe build because you need to cut the body tube to add the second AV bay, plus that's more pieces you need to build. You also need to secure the bottom of this break (the motor side) so that it doesn't come apart, and the top part too if your drogue break is going to be at the bottom of the "normal" AV bay. It needs to be strong enough so that the shock cord forces aren't going to pull it apart... you don't want the piece with your motor casing coming apart at apogee. It also adds weight to your sustainer.
My initial plan was to handle stage separation and booster deployment from an altimeter in the ISC (this booster is single opening, so if it did DD I'd use a Chute Release for the main), and handle ignition and sustainer DD from the sustainer's avionics bay. I'm waiting to receive my Black Monday order from MW so I haven't seen exactly how big the RRC3+RTx ends up being, but I'm hoping that will fit in the 54mm x 7" bay. As you point out Cris that does bring up the question of how the ignition wire will break during main separation, this is another sustainer topic I'm curious about, I'm hoping there's some reasonable connector that won't come undone during booster acceleration but would ideally disconnect before damaging the attached wires so that it could be reused without having to replace every time.

On a bigger sustainer I'd be all for making a hatch in the motor area and putting a staging altimeter down there (ideally both separation and ignition), leaving the booster and sustainer with typical DD altimeters. But no way there's room for that in a 54mm tube with 38mm MMT. :)

Cris, you don't happen to have anything that would do a burnout + X sec charge firing (or else launch detect + X sec), followed by apogee, would you? If not, any chance a future Quantum update could do that? Would be nice if I didn't need one of the fancier >2 output altimeters in the booster when it only needs to fire two charges, it's just not the usual apogee+main charges.

FWIW, I did pretty detailed length/weight measurements on every part in my Double Shot kit to make my starting OR model. Attached is what I have that is purely RW/Madcow's kit, no personalization yet. The 4 Cg-customized parts are the ones I figured wouldn't be balanced, like the NC, transition and slotted tubes, all other weights were set by adjusting the material density for each item. 124oz of bits and 100" of length makes it my longest and heaviest rocket so far, also, of course with recovery, electronics and epoxy it will only get heavier. It occurred to me that the third 54mm coupler bulkhead may have been intended to seal off the NC coupler rather than as use in the ISC, but then it would leave nothing to seal off the 54mm coupler tube in the ISC which seems like a bad idea to me. So I have it placed down in the ISC as per my drawing above.

View attachment Stock Rocketry Warehouse Double Shot.ork

Edit: BTW, OP if you'd rather I make my own thread rather than adding to yours just let me know. No intention to hijack, but I also figured I wouldn't create a 'competing' thread unless you wanted this one to be specific for yours.
I haven't totally decided how I'm going to build my Double Shot, but I'm leaning towards putting the separation & booster chute in the interstage coupler, and putting the airstart controller and DD deployment controller in the standard AV bay. I'll have to run the igniter wire outside somehow, though. Tracker goes in the nose.

The Quantum airstart code is going to handle the two channels as separate timers including altitude@time and/or velocity@time qualifications, as well as breakwire support. There will also be an option to have the drogue channel act like an actual drogue and deploy at apogee, for cases in which you're going to put it into the interstage and have it do the separation (with the Main channel) and pararchute on the booster. It's currently in the flight test stage... it will be a few months before it's released.
CJ did a great build thread on a two stage and provided really detailed options on location of hardware and software.

CJ used a jst connector to separate the wiring.

Your options of motor eject in the booster and a chute release sound intriguing.

On those few two stage flights I have put up, I used an altimeter in the ISC, motor eject backup for the booster. For the sustainer, I used the eBay for altimeters that triggered a separation charge and subsequently lit the motor.

Good luck with the flight. I'm subscribed.
OK, sorry. I'll try again.
Almost two years later.
I built the upper stage, got my L1 and then had an ejection event during boost for my L2 at BALLS a week laterL 1 a.jpg . The kevlar got behind a fin and zippered the fin slot. I'll be rebuilding the fin can, documenting this time. BTW, everything else survived unscathed but for some playa dust. These fiberglass tubes, couplers and NC's are tough! Had I filleted the the fins, the cord wouldn't have got under the fin and I would be moving on to the 1st stage.

Some notes:
Working on a spring release for deployment, it was inverted on the second flight and generated the failure.
I need to work on the camera mounting (the Micro SD card unmounted on acceleration).
Flourescent T-shirts spiral cut make great, cheap streamers.
Rails and their buttons are awesome!


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