Light 'n Tight - Av Bay Build - Wild Child

About seven years ago, I thought about doing dual deploy to the point of buying a PerfectFlite MAWD. The long walks were a factor, along with the possibility of losing a rocket due to drift. It seemed like dual deployment was a good candidate to mitigate the risk of losing a rocket during recovery.

Finding a suitable rocket to fly it on has been somewhat of an issue, with nothing really grabbing me to execute a project.

I wanted something that I could fly "F" and "G" impulse on before going with bigger motors.

When Tim Lehr (aka, "Wildman") made the Wild Child available as a good MPR rocket with simple and clean lines (my build thread here) and turned out to be a good and robust flyer. When he later came out with a dual deployment option, it seemed like a natural contender for this arena.



Greg

One of the challenges of this build is the diminutive av bay. At a mere 100 mm long (a tad under 4"), space was going to be a premium.

I had to do a lot of calculating before hand to find out how much room I had to play with. The bottom line is that there is not much.

While there was room for the PerfectFlite unit to fit, it wasn't a perfect fit. The standard two U-bolts and two studs wasn't going to work well here, and I would be hard pressed to find room for a standard 9V battery.

One project that served as inspiration for mine is Jim Salem's build found here. He made good use of space and, more importantly, it worked.



Greg

The design template I was going for was an efficient use of space and using light-weight materials where it made sense. Light 'n Tight.

Instead of using two steel 1/4"-20 all threads to join the end caps I would use one aluminum 1/4"-20 all thread. Aluminum has a density of 2.70 g/cm3 whereas 316 stainless steel has a density of 7.99 g/cm3, so for equal volumes aluminum has 34% of the weight of 316 stainless steel. For the end caps I would use 0.50" plywood instead of the supplied composite plate. I chose plywood because it is light and sufficient strong and easy to mill to my specs.

I used Rhino as my CAD design tool tool so that would also feed into the CAM development. This was the end cap that I designed.



While it shows two hex nuts on the end, this was an early design consideration that fell by the wayside and wound up with just one.

Greg

It took a while to get the milling of the caps to come out how I wanted them to be. I chose to have 2 ports for wires in case I wanted to use two. Not sure I will, but it's easier to do it now than to go back and retrofit it. The hole for the 1/4"-20 all thread is offset from the axial centerline because things fit better. The hole is not too tight and not too loose. Just right for this application. And speaking of the centerline, I drilled a 1/16" hole there so I would have a physical reference.

For the charge well, I wanted to use a 9mm casing. So I went to a local gun store with a range and they said that once the brass hits the floor it's their property and they don't turn it over to folks because of their insurance. Sigh.

So it was off to Ace Hardware to get some brass tubing. I found some tubing that to was my liking, and at about 40 caliber, it was a bit bigger than the aforementioned 9mm.

I sawed off a section of the brass tubing and epoxied to some MDF for testing. I tested up to 0.8 grams (I really couldn't see using more than that) and it passed with flying colors.

I milled off some of the tubing for the av bay caps and epoxied them in place with 5 minute epoxy.



Greg

Here are the major parts to the av bays, sans the electronics:



When placed on the scale ...



... it's a little over 68 grams. So far so good.

Greg

For your next AV bay you could save a lot of weight by going with some smaller allthread


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conman13 said:

For your next AV bay you could save a lot of weight by going with some smaller allthread


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Click to expand...

and space

Another question that I needed to answer is how to connect the av bay recovery harnesses to the the av bay itself. Again, there is not much room for the classic U-bolt so that idea was a non-starter.

I could use a screw eye, but I wanted the load to go through the centerline of the all thread, so that idea was out.

Then I hit upon the idea of just using a loop of 1/8" thick 750 pound-rated Kevlar and have looped through both sides of the nut.

However, I didn't have a good way to do that so I designed and milled what I call a harness anchor plate.



The harness line is designed to go from the top of the plate, then underneath via the slot, then up and over the stud then over to the other side.

The harness anchor plate was milled from was 1/8" x 1" aluminum bar stock. My apologies for the pic being a little out of focus.

Greg

After milling the anchor plates, I had an idea of how long I wanted the all-thread stud to be, so I marked it with masking tape and bobbed and milled it off.

This is what it looks like installed, but obviously without the recovery harness installed.



Then I put it on the scale to see how much weight it gained. As you can see the av bay is now sporting its fashionable switch band.



Less than 10 grams added for this stage, so not too bad.

Greg

At the end of the day, the av bay is just a container so the altimeter can do what altimeters do. So on to the PerfectFlite MAWD.

The configuration for the MAWD is rather ... spartan.

The sled is 1/8" plywood (from Hobby Lobby) with 4-40 mounting hardware.

I bought some small gauge (IIRC 22 gauge) stranded wire from Radio Shack. I tinned the ends going into the MAWD per PerfectFlite's suggestion and soldered small washers to the other end to provide contacts for the battery (I got that idea from this website). As far as a battery housing, it's not going to have one. I am going to connect the terminals to the battery with masking tape. I am going to wrap a section of paper towel (which has a high electrical resistivity, which is good around electrical conponents) around the battery and gently "stuff" it in. The mass of the L1022 battery is low, so even under high g loads it's not going to move much.



On the backside of the sled I epoxied an aluminum tube.



Greg

nice idea on the harness plates! Live to see new ideas.


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Thanks!

I had to look up in my knot app to find a suitable know for the ends to secure it. I hope to do the rigging tomorrow.

Greg

The weekend was busy trying to get things just right. In the process I found out that there are many ways to mess things up if you are not paying attention.

For example, if you ever build an asymmetrical bay (e.g., off-center all-thread stud), you need to pay attention to it's orientation inside the stack. If you don't, bad things could happen when you start drilling holes. Another thing is when you drill into the av bay caps, make sure that there is nothing in the way (such as an all-thread stud). Thankfully, I dodged both those bullets but I deserved not to.

One of the decisions I needed to make was whether or not to include keys, those holes you drill and fill so you can have positive alignment. I didn't want to do the work, but I have been in the field long enough to know that it is one of those things that I will thank myself later if I do it now. So I milled holes to accommodate 2 x 10-24 screws. I tapped the holes and epoxied two screws in with 5 minute epoxy. I trimmed off the heads with a Dremel cutting disk and then ground the studs close to the airframe with a grinding bit.

Then I applied Trim MonoKote to the av bay and main bay to prepare for the 10 holes necessary for the dual deployment configuration.

• 2 x 2-56 tapped holes for Nylon screws in the lower (drogue) airframe for break plane
• 3 x 1/16" drilled port holes in the av bay switchband for pressure sensing
• 1 x 7/64" drilled wire switch hole in the av bay switchband for wire pass-through
• 2 x 2-56 tapped holes for stainless steel screws in the upper (main) airframe
• 2 x 2-56 tapped holes for Nylon screws in the upper (main) airframe to the nose cone for break plane

The big advice is keep things tight and taped so nothing moves during this process.

Here is a pic of the av bay in situ in orange and a black directional indicator.



Greg

The above photo wasn't very good on the keys/av bay area, so here is another one. These screws are made of steel, but aluminum would have been preferable, only because it is softer and easier to mill.



On a side note, I did the initial trim with the classic easy-to-break cutoff discs that we all know and love with the Dremel. It worked fine. I tried using one of the newer, larger reinforced cutoff wheel, because after all "newer is better". After a minute or two the disc deformed and gave itself an "un-designed" axis of rotation. Apparently the disk is a composite of some type and I exceeded the Tg of the epoxy in the disc. Who knew? Oh well. There's no school like the old school.

Greg

So here is the full stack.



And the mass is now over 600 grams, but not too much.



Greg

Save some weight by swapping to aluminum allthread. It should be way more than sufficient for the size you have used.

patelldp said:

Save some weight by swapping to aluminum allthread. It should be way more than sufficient for the size you have used.

Click to expand...

I agree. That is what I am using (see third post in this thread). The nuts are aluminum as well.

Greg

GregGleason said:

I agree. That is what I am using (see third post in this thread).

Greg

Click to expand...

Apologies...

Back to lurking...

No worries. I'm just glad you wrote something, since I'm starting to think that this thread is a bit of a "yawner" because of the lack of comments!

Greg

Nice av bay build Greg.
I like to see different ideas on how to build and setup av bays. Especially these small ones like the mini series, they get the noggin thinking when your limited on space.



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ksm2001 said:

Nice av bay build Greg.
I like to see different ideas on how to build and setup av bays. Especially these small ones like the mini series, they get the noggin thinking when your limited on space.



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Click to expand...

Thanks Ken.

"Yes" on getting the mind in high gear to explore all of the possible scenarios on getting things to fit and be user friendly. CAD helps, but there is no substitute in getting the parts together and see if it works in the "real small world". If it works, I can use this same setup on rockets with traditional paper tubes of the same diameter or larger.

Greg

Now it was time to add the Kevlar to the Harness Anchor Plates. Even that was a bit of a thought exercise.



The prime consideration was what kind of knot to use to secure the end. I have a knot app on my phone, so I consulted that and found one that I thought was best suited and it is called the Ashley Stopper Knot. I got the hang of tying the knot after a few tries.



The above pic shows how the harness will pull the load at the centerline of the all-thread when the harness is taut. I love it when a plan comes together.

Greg

It was time to run the ground test.

I installed the low-current BP starting devices by placing the heads in the brass wells. I added 0.64 grams of BP in the wells, then packed the remaining void with "dog barf" insulation. I tamped down the insulation with a 1/4" wooden dowel, then used a couple of layers of masking tape to secure the loads in the wells.

Although I prepped both ends, I only ran one wire out of the switch port for the first ground test since the port can only accommodate one wire. In this case it was the wire for the aft end for the drogue.



After the av bay prep was completed, I buttoned everything back up and went out to the testing area.

Greg

The test results generally went well. The break for the drogue bay could have been more energetic, but the the main bay break was spot on. So I'll bump up the charge size a bit for the drogue bay. That result is not too surprising since the drogue bay is a larger volume.

One surprise was how much BP soot invaded the av bay. I thought having the all all-thread finger tight was sufficient. That would be a "No".



It looks like one of the major leak paths was through the wire port. There were some minor leaks due to the tape impinging on the washer area. I wondered at the time if it was a good idea at the time and now I have my answer.



So I'll tighten it up and perhaps get some O-rings to see if that helps provide a better seal. I'll also put some goop on the wire port to seal it up.

So overall I would call the test a success, I do have some adjustments I need to make.

One other thing, the Nylon screws that were in the av bay and nose cone (the ones without the heads) were a pain to get out. Anyone have a good idea on how to get these out? :confused2:

Greg

GregGleason said:

Now it was time to add the Kevlar to the Harness Anchor Plates. Even that was a bit of a thought exercise.

View attachment 175179

The prime consideration was what kind of knot to use to secure the end. I have a knot app on my phone, so I consulted that and found one that I thought was best suited and it is called the Ashley Stopper Knot. I got the hang of tying the knot after a few tries.

View attachment 175180

The above pic shows how the harness will pull the load at the centerline of the all-thread when the harness is taut. I love it when a plan comes together.

Greg

Click to expand...

Shove an exacto knife into them and then use it as a screwdriver


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One other thing, the Nylon screws that were in the av bay and nose cone (the ones without the heads) were a pain to get out. Anyone have a good idea on how to get these out? :confused2:

Greg[/QUOTE]
I know I read a suggestion somewhere on TRF that someone threads the "outer" tube and leaves the "inner" tube smooth. That way you can unscrew the head if needed and just push the screw "shaft" into the void. Does that make sense?

I always enjoy and admire your attention to detail and the "out of the tube" thinking.

conman13 said:

Shove an exacto knife into them and then use it as a screwdriver


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Click to expand...

I did try that, and while it worked (sort of), I was wondering if there was a better way to do it.

accpool said:

I know I read a suggestion somewhere on TRF that someone threads the "outer" tube and leaves the "inner" tube smooth. That way you can unscrew the head if needed and just push the screw "shaft" into the void. Does that make sense?

Click to expand...

Yes, that is a tip that I might use. After a while, that is what my happen anyway because you will eventually destroy those threads doing it that way.

accpool said:

I always enjoy and admire your attention to detail and the "out of the tube" thinking.

Click to expand...

Thanks. I can be a tad on the OCD side, so being a perfectionist can sometimes be counterproductive to productivity. Sometimes I just have to force myself to do things the best I can so that through failure I might learn how to do it better. After all, it is just a hobby. Relax. Breathe. Enjoy. At the end of the day, it's more fun that way.

Greg

Launch Report: 2014-07-12

Well it was finally time to put the stack into the sky.

The morning came of the launch so I started prepping the av bay, which I thought would take 30 minutes. But things were not coming together. Things didn't fit. While I had done testing on components, I had never done an "all up" test with the av bay. I did a fit test with everything except the wires installed. There is so little room in the bay that the wires touch the side and create an enormous amount of friction. Not only that, I discovered that the switch port is NOT in the optimal location WRT to sled. So, the 30 minutes quickly became 2 hours until it was finally together, or at least where I thought it was flight worthy. I got the rest of the rocket prepped. I had no idea the a dual deploy prep would be so difficult and makes putting together an RMS reload seem like child's play by comparison. Well, I brought this on myself so I shouldn't complain. It was my first time and didn't have anyone to guide me through it, so it's all part of the rocket learning curve*.

I built the motor (an AT RMS G64-10W) with the delay installed for a secondary drogue event (just in case). I weighed it and the launch weight tipped the scales at 950 grams, making it by far the heaviest rocket that I have ever launched. I had it racked, then turned in the flight card and said I was ready for the button to be pushed.




Greg


* IMHO, before I went through all this trouble I believed that a successful dual deployment flight was a greater challenge than getting a Level 1 cert. I still think that way.

Liftoff!



It climbed and climbed, right up into the sun. At that point, no one could see anything. I waited and waited, and thought I heard a faint "pop". That must be the drogue. Then I heard another faint pop. That must be the motor eject charge. Finally, it appeared and the drogue was out. So far so good. I set the main to deploy at 700 feet and .. and ... and ... it's ... not going to happen. Thankfully, it landed on soil and not on the runway. I walked to where the rocket landed and the altimeter beeped out 1,444 feet as I an one of my sons walked back. It didn't appear that there was a "hang fire" for the main. So began the process to figure out why the main didn't deploy.

But the good news was that the drogue deployed and the rocket was undamaged.

Greg