29mm Dual Deployment

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Well-Known Member
Sep 11, 2009
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I'd like to try dual deployment on a 29mm minimum-diameter altitude model I'm working on. It'll be going to altitudes of around 6k on Is and 5k feet on H motors, so apogee-deploy by motor ejection is not a viable option.

First, an incredibly stupid question. Would it be possible to have the motor eject the drogue at apogee and then use a timer to eject the main? Perfectflite's staging electronics claim delays of 'at least 60 seconds' possible; even from 6000 feet, 45-50 seconds (from apogee) at 90-100 fps* will bring it down to 1500 feet or so.

* Yes, I know that's rather high for deploying the main, but this is a 10-ounce rocket with kevlar shock cords so it can survive it.

This is very appealing to me because the staging electronics are half the cost and size of the smallest dual-deploy altimeters, and I'd rather only deal with one ematch than two. However, because I can't know exactly how fast it will descend, it'll only be accurate to within around 500 feet or so of altitude, rather than knowing that the main will deploy exactly at 1000 feet or whatnot.

Second, how do I go about making such a tiny av-bay? I cannot use the standard design with a sled because the electronics are almost exactly the same width as the tube and thus must be centrally located. Also, the tiny bulkheads - 1" diameter - might not be able to hold three separate rods (two thread rods + eyebolt).

The ffive units I'm currently considering are the PerfectFlite HiAlt45K, Entacore AIM USB, Adept DDCS2, Perfectflite MAWDand Missile Works RRC-2. All are between 2.75" and 4.25" long.

I have two options for the coupler: use two connected Blue Tube couplers, 1.01" internal diameter and a total of 8" long, or BMS coupler tubing, any length that I choose and 0.977" (can be peeled a bit) internal diameter. Does it particularly matter which I use?

Or, would it be better to use 29mm tubing for the av-bay and attach the bulkheads to couplers, as seen here? That would give more room for the internal stuff, but be trickier to set up.

I'll be using one of the mini 6v or 12V camera batteries; how should I mount that in the altimeter bay?
The perfectflite time will actually go to 6000 seconds as seen in the manual snapshot below. So you could work with that. A few sims with Rocksim and you could get close enough to use it

Also you can buy extra long delays for many AT H and I motors. I think the one I used in the I-200 was 18 sec.

Here are some shots of a 29mm bay I did using a section of airframe for the sled. Having a curved shape gave it the needed room.
Motor for apogee and altimeter for main and having a record of how high it went.

Threaded rod gets glued to side of NC and passes under battery and sled. There is room for a tracker on the blank area of sled, put in backwards with the antenna going up into the NC.

There is just enough room left in front for the NC shoulder to fit. There was no point to DD this one as it only has a 12in chute and drops like a stone. It falls just as fast with the chute as with out, the chute stabilizes the fall, where with out it tends to tumbler and side slip,carrying it much further horizontally. It's gone over 8000 on a I-200, but you really need a tracker to find it!

Picture 3.png




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The sled fits into the coupler ,battery will not.

The edge of sled prevents the battery from moving. Battery JUST fits in the 29mm tube. And the NC butts up to front edge of battery sandwiching it in and keeping it from moving.

Bulkplate glued to coupler.
Coupler glued into 29 tube.
tracker taped to sled fits all the way into coupler.
Altimeter fits 29 tube. [Perfectflite HiAl 45]
Battery butts up to sled.
NC butts up to battery. Tape hold NC on. You could use small screws, I did not want to screw into carbon.
Threaded rod glued onto side of NC slide under everything and through BP.
It requires alot of dry fitting before glueing anything, but it works!

If your using camera batteries, make sled longer and attach to that or put it where my tracker goes. I could not because a 9v won't fit.

Pic below shows sled in backwards so you can see how the edge of sled stops battery. Placed the correct way I could not get a shot showing this because the altimeter is flush with that edge. Here you can see the notch filed in the end of sled to clear the eyebolt nut in bulkplate when it inserted correctly.

Above post shows correct way all fits together.





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About 2/3s of AT 54mm motors have X (18-second) delays listed. No 29s or 38s, though. Simmed on the I200W, optimum delay is about 12 seconds, so I can use medium or long delay.

The Raven is nice and the size is good, but the price is high. I don't need accelrometers, and $149 is a lot for me to drop on an altimeter for 0.1" of extra space...

I wouldn't feel comfortable with the Pico; the site makes it clear that it's for users experienced in avionics and I am not.

I'm leaning more and more towards the timer. It's easier and cheaper than the others, fits easily with a battery in a 4" coupler, and even if I'm an idiot and it doesn't fire the main, then the drogue is still deployed. Because hitting the sod at 90 fps is a hundred times better than hitting at 900 fps.

Dry weight of my rocket will be about 8 oz (227 g); with a 29/360 or Pro29-6G casing it's up to just about 13-14 oz. I'll use a 6" or 8" chute, or 4" x 40" streamer for the drogue and an 18" chute for the main.
I guess I'd be scared about accurately calculating the time to apogee and the time to mains deployment. Having an active system that is monitoring for apogee and main deployment altitude in real-time removes those uncertainties.
To be honest I would use a Perfectflight Hialt45k or RRC2 mini rather then a timer.You could still use motor eject for your drogue and have the alt do the main but if your going to do DD you mine as well use the alt to fire your drogue and main it will be more precise then motor ejection and a timer.And both those alts don't cost much more then a perfect flight minitimer.Just my 2 cents but if I were going to do a DD flight I would use an alt rather then a timer.