Ejectable 38mm av-bay for booster deployments

[Adrian A]

I thought some people here might be interested in how I do avionics and deployments inside a minimum-diameter 38mm booster or 2nd stage rocket. I started doing it this way about 10 years ago, and it has worked well for me ever since.

To make the full multi-stage stack as rigid as I can, I minimize the number of airframe breaks. In my 3-stage 38 mm rocket I only have one for the booster/2nd stage separation, one for 2nd stage/sustainer separation, and one for sustainer deployments. I also prefer to separate stages immediately after burnout, to minimize drag losses. What this means for the 2nd stage is that there's an av-bay buried in the middle of the tube between 2 motors that needs to arm and ignite the 2nd stage motor on the aft end, fire a separation charge out the forward end, and then at apogee deploy a chute out the forward end also. In my rockets, the 2nd stage deployment hardware also needs to support the weight of the sustainer motor during the boost.



To make this easier, I use an N-shape floating av-bay structure that is deployed with the chute. Here is an av-bay for my booster that I made yesterday:


This is looking at the forward end, showing how the coupler tube is cut with an N-shaped cut, and the separation charge wiring. The open area where the wires are going is filled with the main chute and some harness. The connector is just a simple 0.1" pitch header male and female connector with wires soldered on and with heat-shrink tubing to provide some strain relief.

Here is the aft end, looking down at the top of the 38mm av-bay:

This photo was taken after I soldered on the brass nuts and the wires, and before I gooped it all up with JB Weld to glue the bulkhead in and strain relieve the wires. The bulkhead is a round circuit board I had made, that goes with the 38mm av-bay:



This av-bay provides power to the Raven via a magnetic switch. A tracker can also be squeezed into there if the battery and the Raven capacitor are moved over. It's 2" long. The active bulkhead with the power switch and battery charger are on the bottom and the top bulkhead just has passive electrical connections and is mostly there to provide structure and a place to attach the harness. The photo above is was taken before soldering anything on or gluing the top bulkhead into the structure.

Once the chute and harness is installed in the middle, inserting the parts into the airframe holds them together. The chute and the electronics are protected from the forward stage separation charge by the forward bulkhead, and then to deploy the chute, a deployment charge on the aft end of the av-bay ejects the whole thing forward and out of the rocket. The two pieces split apart and the chute falls out easily, without having to be extracted from a deep tube.

The photos above are what I'm using for the booster. But what about arming and igniting a 2nd stage motor? For the second stage I have the same basic hardware design, but add on a motor adapter piece that provides a screw switch for arming, a separation connector, and structural spacing between the motor and the deployable portion:


Shown in the photo above is the cylindrical motor ignition spacer and the round bulkhead which is attached to the back of the av-bay. The orientation is from the aft looking forward. On the top left is a 2-pin header connector that provides the separation between the av-bay and the motor igniter when the av-bay is ejected. On the top right is the arming screw switch. The two holes in the cylinder are for a 1.5" long aluminum female threaded spacer that goes through holes I drilled in the motor forward bulkhead and that screw into the airframe with 2 countersunk screws at both ends. The cylindrical part stays with the motor, and the round bulkhead is deployed as part of the av-bay. The cylinder also holds the av-bay deployment charge and harness. After I took this photo I glued the screw switch, wires, and header pins to the cylinder.

Here is another view of the rear bulkhead of the 2nd stage av-bay, with female 2-pin connector soldered on but not yet glued down:


On the left of the photo is a 1/4" round female-threaded spacer with #6 countersunk screws. I ended up using a narrower hex version so I could drill smaller holes through the end of the motor deployment bulkhead.

 

[manixFan]

For some reason I missed this when you first posted it, very cool solution. I do have several questions though, even if a bit late.

It looks like from the Rocksim file, nearly 1/2 of the 2nd stage motor extends past the end of the airframe and into the booster. Am I reading that correctly? The sustainer looks to be more typical, so the line just aft of the 2nd stage fins seems a long way from the end of the motor. I assume done that's to keep the lower section of the airframe as rigid as possible by keeping the booster airframe supported by the 2nd stage motor? Can I ask the length of the booster airframe?

It's also hard not to notice the custom circuit boards you are using. I especially like the harness attachment points on the passive side of the AV bay header. I hope you plan to make those additional components available when your release the Blue Raven. I used the 38mm AV bay at BALLS this year for the first time and really liked how it worked out in my MD rocket. Those separate attachment points for a harness would make assembly a lot easier than using the thru posts. I also really like the way you attached the female connector.

Very cool stuff, and it seems to make setup a lot easier for the smaller diameter since so much can be done outside the airframe.


Tony

 

[Adrian A]

For some reason I missed this when you first posted it, very cool solution. I do have several questions though, even if a bit late.

It looks like from the Rocksim file, nearly 1/2 of the 2nd stage motor extends past the end of the airframe and into the booster. Am I reading that correctly? The sustainer looks to be more typical, so the line just aft of the 2nd stage fins seems a long way from the end of the motor. I assume done that's to keep the lower section of the airframe as rigid as possible by keeping the booster airframe supported by the 2nd stage motor? Can I ask the length of the booster airframe?

You are reading that correctly. I had designed and built the 2nd stage airframe about 10 years ago for a CTI 6G XL or the Aerotech equivalent, but then I decided to go for the max Loki motor for this one, and the difference sticks out the back. It's more overlap that I would do on purpose otherwise. The booster is 43.75" long. The booster length was the last part of this rocket to get finalized, so I was able to cut it long enough to cover the extra 2nd stage motor length.

It's also hard not to notice the custom circuit boards you are using. I especially like the harness attachment points on the passive side of the AV bay header. I hope you plan to make those additional components available when your release the Blue Raven. I used the 38mm AV bay at BALLS this year for the first time and really liked how it worked out in my MD rocket. Those separate attachment points for a harness would make assembly a lot easier than using the thru posts. I also really like the way you attached the female connector.

Very cool stuff, and it seems to make setup a lot easier for the smaller diameter since so much can be done outside the airframe.


Tony

Thanks. The stock 29mm and 38mm av-bays come with 2.5" long threaded rods from McMaster-Carr. The next longest size is 12." The lack of convenience in lengthening the rods has been limiting the options in what I sell, but recently in all of my 29mm and 38mm builds I have liked adding another passive bulkhead on each end. So much so that I want to find a way to start selling that, at least as an option. The additional passive bulkhead makes it easy to attach harness on each end, and it also protects the active bulkhead from the shock and soot of the deployment charge.

I have also switched over to using a scratch-built charge holder that I screw onto the new extra bulkheads, and I have been getting cleaner and more consistent deployments than I was when I would seal all of my deployment charges into cardboard tubes. I start with an Aluminum female threaded coupler from McMaster and then drill out one end of it with an ID that matches the red plastic caps on the ematches. The other end is still threaded for an undercut flathead screw. I think they were 6-32 IIRC. I stick the ematch into the open end of the charge holder, which is a big improvement from when I was trying to put them into the base of the charge holder and threading the wires out the hole in the side. It's better to have the charge start burning from the open end into the charge holder so that everything gets ignited before it starts moving. Not to mention it's more convenient not to have to thread the ematch through any small holes. The red ematch cap makes a nice seal so the BP doesn't fall out, and it keeps the ematch head from shorting out against the Al charge holder, so I just leave them on.

Here are my 29mm additional passive bulkheads:


I just made these by drilling 3 holes into the extra 29mm passive bulkheads I have. They charge holders are 1" long, which is a good size for BP after they are drilled and there is a screw in one end.

 

[manixFan]

Ahh, interesting use of the passive bulkhead to protect the active side, that makes a lot of sense. Regarding the threaded rods, I bought several 3' lengths from Grainger (there is one near my house so I don't have to pay for shipping) and have been cutting them to length as needed. But I can see for production being able to buy them precut makes life a lot easier. (I checked and they do have 3" lengths available.) For users who would be interested in using the passive bulkheads, I don't think having to cut their own threaded rods would be a big deal.

Overall, I think the passive bulkheads would be a very useful addition to your lineup, for exactly the reasons you use them. So, you've got at least one vote to make them available!


Tony

 

[manixFan]

I also went back and reviewed your Stratospear thread and see you did use the passive bulkheads in the project. I know I'm not the only one who really appreciates all the time you spend on your write-ups, lots of good info.

IMHO, the wide variety of build threads are probably the most useful part of TRF. So great many builders are willing to put in the time to share their knowledge and experiences.


Tony

 

[Adrian A]

After losing my booster last weekend, I'm re-doing the ejectable avionics bay for it. Part of the reason I lost the booster is because I was concerned about cramming a tracker in next to the Blue raven in the avionics section, because when I did this a few weeks ago, I crushed a wire and got a short that let some magic smoke out. The threaded rods were already semi-permanently connected at the closed end, and I had to work the tracker and Blue Raven into the open end of the coupler, along the threaded rods, without any visibility once the bundle got most of the way in.

This time for the re-build, I'm going to accept an additional ~1/2" of total length in order to make the avionics separable from the chute section. A central bolt will connect them, using the same strategy that has been working well for me on connecting my chute cannon to the forward end of the sustainer electronics.

First, I start by cutting some coupler tube with some N-shaped cuts that make the chute compartment.

I used a Roto-Zip with a plastic and wood blade.

Next I made two round bulkheads, starting with 1/16" FG bulkheads I had available. The forward one will withstand the separation charge, and I sanded it down to fit inside the coupler tube. The bottom one will be bolted to the front of the av-bay. It needs to be a specific distance from the bottom so that the bolt will reach and I don't waste a bunch of space. If I had thought this through better I would have made the N-cut at the right height, but since I had already squared off the bottom of the tube, I decided to insert the bulkhead partway down. It's ugly, but I like that the bulkhead goes out to the OD of the coupler so when I bolt it down, at least half of it is fully supported by the coupler edge:


This is what it looked like after I JB-welded it in place. On days like this, I keep my kitchen oven on the "warm cookware" setting at about 140F to speed up the cure cycles. Next I glassed both bulkheads on both sides so that there's no way they will break free from the coupler tube. After I mixed the epoxy, I laid up one side and put it in the oven (with some parchment paper to catch drips, and then when it was cured about an hour later, the remainder of the epoxy in my chilly garage was still good for laying up the FG on the opposite side.

I cut little discs out of medium-weight FG cloth that are sized so that they wrap about 1/4" up the inside of the coupler. Here's how it looks now, after the FG is done and rough edges are sanded off.


The black bulkhead with the engraving at the bottom is what will close out the top of the av-bay. The av-bay threaded rods are also the deployment charge terminals. They stick up from the bottom. I will wire them up and then bolt chute compartment to the top of the av-bay with a screw that goes down through the center and screws into the 10-32 T nut that I glued in place today.

 

[kjhambrick]

Adrian --

I agree with manixFan.

I have been seeking parts for a flyable AV-Bay that keeps my electronics ( i.e. Blue Raven ) away from the ejection charges and it is a daunting task

I would DEFINITELY buy an AV-Bay Kit like you outline above per rocket !

Thanks !

-- kjh

 

[Adrian A]

I made some more progress today on the booster ejectable av-bay. It's 95% done.

First up: I modified a tracker to use a magnetic switch so that I can preserve battery charge while the av-bay is sealed up.



After this photo was taking I also took off the slide switch, which helps it nest in better against the av-bay's battery connector.

Next I worked on the pass-through for the separation charge. Below the 2-pin connector is just a female header with wires lap-soldered onto it. I soldered the other end of the wires to some brass 4-40 nuts. Some short segments of 4-40 Al threaded rod provide the pass-through to the bulkhead.



Here's what it looks like on the inside when those are tightened into place:



The other 2 holes are for the kevlar harness. Here's how it looks from the other side, where a small cardboard-potted charge will separate the sustainer:


After the Kevlar harness is looped through, I'll cover it with some duck tape to protect the harness and mostly-seal the holes. Seeing this close-up reminds me that I should chamfer the holes to reduce chafing on the cord.

 

[Adrian A]

At the other end of the av-bay is the anchor which will be bolted to the motor and to the booster airframe. It's just a basic 2" long coupler tube with a bulkhead that I glassed into place like the other bulkheads:



Inside of it is the coiled shock cord, a bolt to the forward closure of the motor, and a 1.5" long cross-bar of 1/4" Aluminum threaded with 8-32 on each end. The cross-bar will retain this anchor in the airframe and also anchor the shock cord. First I need to put everything together, measure carefully and then drill all the way through the airframe and this anchor. Then I'll trim the front end off of this booster tube and square it off.

Here are some photos of the anchor together with the ejectable av-bay:



The shock cord between the av-bay and the anchor is intentionally short and beefy (for me) because I'm actually counting on the shock of that cord getting suddenly tight to help encourage the chute compartment to split apart. It doesn't need much encouragement, but I suspect that a rapid and complete separation will help avoid chute tangles.


Altogether, what goes into the booster airframe besides the motor is 10.5 inches long and 161 grams. Actually it will be a few grams more once I add in the anchor cross-bar that I haven't done yet.