Fitting two altimeters in a 3" body

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Sep 7, 2016
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Need some help from more experienced minds. I am building a 3" body rocket and I am trying to figure out how to fit two altimeters in the e-bay. I really want the redundancy of two but I can't figure out how to fit them both and keep everything secured. The idea is to fit two Stratologger CF altimeters, two rotary switches, and two nine volt batteries. Any suggestions? Thank you
More info needed. How long is the ebay? Threaded rods - how far apart?

I am not a fan of redundant altimeters, but it seems like you should be able to fit 2 SLCF systems in 3". Consider LiPos instead of 9V. There are many threads on choosing a LiPo for the SLCF.
I don't like putting the9V batteries on same side of sled as altimeter in case they break loose during a rough landing.

That said, can you use al longer coupler and lengthen the bay? Or 2 couplers with w wide switch band to hold both securely?

Not sure how much added weight becomes an issue (going for altitude, etc.).
I agree with cavecentral on placing batteries and altimeters on opposite sides of the sled. Its never happened to me, but I have seen the damage (carnage might be a better word) a loose battery can do inside an avbay. Just use a long bay with threaded rods and you should have plenty of room.
Only problem with a long ebay is one side is going to have to have longer wires to one of the bulkhead so you have enough slack to wire-in the altimeter
and seal up the ebay.

I faced the problem with a 3" Wildman rocket I stretched the lower and upper bays 10 years ago. In reality now the rocket "could" fly a Loki 54mm M motor.
I got a longer coupler for the ebay, I believe 10" and for the life of me, I couldn't fit an MAWD, an ARTs II and a BeelineGPS with the associated batteries and such. Besides that, the keyswitch dang near went all the way across the ebay. I went with the MAWD and BeelineGPS, got one flight to Mach 1 and 10k on an L
and let the rocket sit thereafter because I had concerns flying such a large rocket with one altimeter.

Now, with the smaller devices, when I get around to it I should be able to get two deployment devices in there, get a new nosecone so I can move the tracker in there and
go fly with redundancy.

I was toying with doing an L3 with the rocket in the new configuration but the expected altitude is above the waivers of any nearby sites plus I'd have to construct the
documentation. I have absolutely no doubt the rocket could handle it as I constructed it entirely with Cotronics 4525B epoxy (except the fillets were PowerPoxy Weld)
I have a "shorter" keyswitch and could use an EggTimer Quantum for wireless activation of a second altimeter.

I fired off a question to TRA concerning wireless switches/devices like the WiFi switch Cris Cerving offers. I have small projects I connect up the battery and use a wifi switch to
"turn-on" the devices. According to NAR rules one has to have "everything" on a switch. Since the EggTimer offerings default to off when the battery is connected, a mechanical
switch is superfluous for deployment electronics. Connect the battery to the device, seal up the ebay, get the rocket on the pad, turn the device on wirelessly, when one is
satisfied the device(s) cycled properly and are ready, then insert the igniter.

Oh, I was sent a nice thank-you email that my question was good and going to be considered at a BOD meeting. I believe it's good to consider because if folks are flying like
I describe above, short of stripping everyone's ebays for switches, it's danged near impossible to enforce. I take my rockets to the RSO say the one with the EggTimer TRS and
the rocket is quiet, no beeping , the TRS has power but nothing is activated. I have several with Featherweight mag switches and the only thing with a mag switch is it defaults
to "on". I did exhaustive bench testing with "contained" ematches that showed nothing happened when I swiped the magnet rapidly to turn off the deployment device before it cycled. It was now in a standby mode. Kurt
I don't have it here (it now belongs to my buddy,) so no pix, but I was able to fit two GWiz LCx's, four 9 volt Duracells, and two screw switches into a 7" long, 3" av bay. Accelerometers on one side of the sled, batteries and switches on the other, small rubber grommet thru the sled to protect the wiring. Little tight, but they fit, so you shouldn't have a problem with a couple of Stratologgers.
The standard av-bay sled can sometimes be a limitation for space, for example on a recent 1.6" build the altimeter I wanted to use was almost the same width as the all threads which created a problem, on top of the width the height of the altimeter was an issue then as well. My solution was to mount the altimeter sled off center and at right angles to the all thread. A similar method could be used to mount two alts side by side in a 3" bay, making sure the sled is fitted so it cannot rotate around the all thread is important. The sled could also have a vertical between the allthreads on each end, and room permitting the backside of the sled could still be used to mount the LiPo's. As for putting the LiPos on the back of the sled, I don't feel its absolutely necessary due to the fact that my method of securement is such that if the battery breaks loose due to an event then most likely the altimeter was toast as well.

I would put the two batteries end to end on the side of the sled the all-thread passes and between the two pieces of all-thread. I would mount the two stratologgers on the other side, side by side. The only problem I foresee is if the depth of the stratologgers doesn't allow them to fit on that side.
In that case I'd try a single threaded rod down the center between the two SLs and mount the batteries on the shallow side.

P.S. Rich, that's really a sharp looking setup!
For building ideas on 3in fiberglass rockets...see the sticky at top of high power.

How to build a 3in DarkStar or any fiberglass rocket. There is an index, so you can locate things without reading all.

here's some altimeter bay links...showing how to mount 2 altimeters and 2 batteries, leaving one side of sled wide open for GPS or trackers.....there are many ways, this is just one of them.

Good luck with your build! :wink:
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It takes some mental Lego efforts, but I do two altimeters, two switches, and two LIPO batteries in a 54mm AV bay. Usually an RRC3 and an RRC2 or Eggtimer quark.
Here are some pics of my 3" Darkstar 8" AV bay, with dual altimeters, 1 SL100 and 1 SLCF. I use zip ties through holes drilled into the sled to mount the 1S Li-Po batteries, one each side. This setup uses a magnetic switch per altimeter, one at each end of the sled. Overall it is a well balanced design. The AV bay lids are dual charge-wells with the e-match exiting through the bottom of the wells directly into the AV bay, which is sealed with an o-ring.

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Cool, Nylon screws to float the mag switch.


I floated the mag switch on nylon washers and screws. Stupidhead here did the single all-thread just to see if I could do it. After I mocked it up I realized with
that all-thread abutting the metal screws, the switch would be shorted out! Enter in the nylon washers and screws. To be more sure, I did solder the wire
connections to the mag switch and I resolve never to do this again. I also did it in a modified Aerotech HV Arcas but that tube diameter is larger. Kurt

(The final iteration has an aft notched thrust block for the battery and double sided sticky tape along with a ziptie through two holes in the board to secure
the battery for flight.)
2 Stratologgers on 2 Additive Aerospace 54mm sleds mounted on 2 rods. Ech has mounts for 9v battery. Trimmed off excess off the lengths not needed. See the micro switches once sold by PML. I just use a mini screwdriver through the 3/4" vent hole to turn them on.
Ebay is only 8" long.

The Additive Aerospace sleds are the way to go if you don't have a 3D printer. If you want one of the altimeters to be a Raven and want to keep your wiring time and real estate down...the Simple Circuit is the way to go!

Here's an example of my very similar to version of the Additive Aerospace sled that incorporates a Raven + Simple Circuit, Altus Metrum Easy Mini, 2x 160 mAh LiPo's, and two PCB screw switches all in a very compact 3.2" wide by 3.5" long package. This all fits inside of my 3.375" ID coupler (for my customer 3.725" OD FG rocket), but could easily be downscaled to fit inside of a 3" coupler. The only soldering I needed to do was on the two switches.


Need some help from more experienced minds. I am building a 3" body rocket and I am trying to figure out how to fit two altimeters in the e-bay. I really want the redundancy of two but I can't figure out how to fit them both and keep everything secured. The idea is to fit two Stratologger CF altimeters, two rotary switches, and two nine volt batteries. Any suggestions? Thank you

A couple things... Baro altimeters like the CF do mot need to go in to the bay longitudinally. You can put them in horizontally since barometric pressure can be read at any angle. Second. Consider 3.7V 750 mAh LIPO batteries. They will fire off an e-match without any difficulty. Third. a 3" airframe is plenty large enough. I've gotten an MARSA24, Stratologger, a beeline tracker, and an HDWingCam plus 2 9V batteries in one.

This photo shows tow startologgers, an MT4, and an HD WingCam in a 3" airframe. Batteries were on the other side of the sled.
Altimeters, Timer, and Camera.jpg
I also make and sell my Pancake Altimeter Sled specifically for the purpose of fitting an Altus Metrum Easy Mini in a length-restricted bay.

Pancake Plate for Altus Metrum Easy Mini


These will work if you use 10-32 all thread spaced 1.5" on centers. It only consumes 2" of length, and allows you to fit a fully redundant altimeter system inside of 4" of 54mm coupler, complete with batteries and switches!
I fit an Adept22, RRC2+ two micro switches and two 9 volts in a 2.6" bay, 6" long. Two Stratologgers should be pretty easy. Use the small 1/2" aluminum channel from Lowes and put a piece of G10 on each side of the channel. Batteries on one side, altimeters on the other. Your threaded rod runs through the channel. I've since rebuilt the av bay for that rocket to accommodate one RRC2+ and one lipo. I epoxied the boards directly to the threaded rods and put a screw switch on the edge of the sled where it will sit close to the coupler.
Another option is use tubing in between the two pieces of G10. I just did this to build a bay in the nose cone of a Wildman Sport. It will fit a RRC2+, 9 volt and screw switch.
Lots of options out here outside the box.
I'll get some pictures of them and do a better write up. I hate trying to type on my phone.

So here are some pictures of my double decker boards. They ain't pretty, but they get the job done.

sled 2.jpg sled 1.jpg sled 3.jpg sled 4.jpg

First and second picture is the old sled I was going to use in my level 1 bird. Also my first electronics sled. I decided against full redundancy in lieu of simplicity. 1/2" channel on the ends. It was setup for a single 1/4" rod through the middle and the channel was tapped and attached to the lids via #6 screws. This turned out to be a real PITA to get everything assembled. It worked with only one lid attached to the sled, to keep it from spinning on the rod, but still. The micro switches in the middle are both closed when the screw is tightened. I never flew this setup but I have a similar homemade screw switch (one switch) setup in my Cowabunga and it works great, hung on a power line all night and was still beeping the next day until I got there to turn it off. Only thing is both altimeters turn on at once. Then you have to discern all the beeps at the same time. The other side is bare now but was setup for the Adept and the RRC2+ (all I had at the time). So it will easily hold 2 Stratologgers.

Third picture is what the first two evolved into. The flip side of the board has a place for a LiPo and Jolly Logic Altimeter 3. The charge canister is a piece of blowgun attached with a piece of #10 threaded rod and JB Weld. I recently had one come loose so I have moved onto something else. The resistors are there just in case the ematches short so the LiPo wont fry my RRC2+. I have a 6 position connector epoxied to the board for quick connect/disconnect. All of my boards were going to have this setup, along with a "daughter" board with the idea being less tightening and loosening of the screws in the terminal and less removing the altimeter from the board; but now I have more altimeters than I have rockets that need them. This one has a MissileWorks screw switch epoxied in between the two boards and is nearly up against the side of the coupler so it is real easy to find

Last picture is the board for my Wildman Sport nose cone, no wires yet. This is the 9 volt side, the other side is only 2 standoffs for a RRC2+ so no reason to post that. The metal thing flopped on there is a piece of aluminum that keeps the 9 volt from running off. I have two pieces of tubing running through the board and the G10 is epoxied to those. My nose cone has 2 #10 thread rods epoxied into it. This slides right down into it. Before it housed 4 oz of weight, this bay will weigh around 6 oz so I should be spot on for stability for baby H's. I ditched the blowgun in exchange for a .308 casing, cut off right behind the neck. It will hold 1 gram plus the ematch and some dog barf. 357 Magnum proved a big on the small side, as did 45 ACP.

Like I say, not pretty, but maybe you can get an idea.

Mikey D
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I managed to get two Ravens, four batteries and microswitches and four charge wells (internal to the bay) into a 54mm for fully redundant dual deploy. Sorry, but I don't have a pic. 75mm should be easy :wink:

Batteries were the 120mAh and 180mAh 2S LiPos.

Two lengths of SS rod (ends threaded) were run down the inside of the coupler, but very close to the edge. Sled was a sandwich of 0.5/3.2/0.5mm G10 and slid in with the SS rods near the outer edges. The 3.2mm was slightly narrower, leaving a groove down either side of the sled.

Charge wells were internal (machined aluminium, sized to fit 2.5ml centrifuge vials) to maximise the space for recovery harnesses and chutes.

Mental note: Must remember to take photos before flying rocket next time in case it gets lost (second flight in this case)