Light 'n Tight II - Av Bay Rebuild - Wild Child

Greg, I did not see the need for the screws (in fact they might cause a short when vibrating during thrust). Your design is so clean and the match wires are restrained better than most ebays I have seen pictures of. If you put a little slack in the wire, how would they pull out? Did you round off the edges of the milled out part of the "raceways" so no sharp edges exist?

accpool said:

Greg, I did not see the need for the screws (in fact they might cause a short when vibrating during thrust). Your design is so clean and the match wires are restrained better than most ebays I have seen pictures of. If you put a little slack in the wire, how would they pull out? Did you round off the edges of the milled out part of the "raceways" so no sharp edges exist?

Click to expand...

I did file off the edges after milling, so those should be good. It is possible that the screws could cause a short, so they may not be necessary. I may fly the first time without them and see how it goes before looking to see if it is actually a problem in need of solving.

I finished prepping the charge cups. On the earlier variant I put some laminating epoxy as a brass sealer. The purpose of the sealer was to help with post-firing cleanup. It didn't work so well. After researching sealants for brass, I found out that some waxes work ok for the task. I had some high temp wax used for mold release, so I used that. Regardless, they look nice at the moment.



Greg

In a bit of "works as advertised news", I checked to see if there was a clear path for the e-match. The light provides the answer.



Greg

Back to the thread ...

I ran into a problem when I mounted the MAWD onto the sled, it didn't fit well. I researched what could have gone wrong and I discovered that I had two sets of numbers for the screw centers to mount the MAWD. I used the wrong ones. So I will have to relocate the holes where it fits like it should.

Another problem was I had to model the battery clips to make sure that they would fit in the area of the av bay where there is the least amount of room for error, the battery bay.



I found out that I had to mill out a section for the clips to drop down into just a bit so that it would fit without any height adjustments to the work on the angle aluminum on the cap.

If I build it as designed, it should all fit. Barely.



I'm reminding myself that I am doing all of this close-quarter work because this av bay is about the same volume as a toilet paper tube.

Greg

After placing the MAWD mount holes where they didn't fit, I spent some time measuring and re-measuring to make sure I got it right. But I didn't feel confident it was going to fit unless I did a test first. So I made an analog of sorts of the sled, a mockup if you will, where I could test the parts before I started carving up the flight sled. So I sawed off a section of 1/8" 3-ply plywood and put it on the mill.



Now that I had that, it was time to see if the altimeter actually fit on the sled. After a getting all of the hex standoffs mounted, I placed the MAWD onto the standoffs and the first screw went on. The next screw that went on was the one diagonal from it, and if it didn't work it was going to be a fail. It went on just fine, and then the remaining two. It was a perfect and solid fit. At that point I was pretty relieved and happy.

Then I placed the battery clips into their holes and mounted a spent 9V battery in them to see how it fit.



The clips went in fine, and the location seemed to be okay WRT the MAWD, but upon closer inspection of the clips it looked like they were a bit far apart.



So, the plan is to move each side 3/64" (~1.2 mm) closer together so that the clips are more vertical and see how that looks.

Greg

I finalized the design of the components mounted on the sled, making provision for the aluminum angle brace. And it came down to fractions of millimeters to have everything fit in the allotted space.











I milled another mockup, and when I got everything mounted on the sled, it all fit as designed. However, after I removed the battery from the clips it was pretty toasty. My theory is that the aluminum angle brace (visible in pic 3) is in contact, albeit slight, with the battery clips (visible in pic 4) causing it to short. That would be a critical problem in flight, so I have to put some kind of insulation on the surface. I could use electrical tape or even paint it, but tape can "un-stick" and paint can get worn off. Any one have other ideas that might work well?

Greg

GregGleason said:

I milled another mockup, and when I got everything mounted on the sled, it all fit as designed. However, after I removed the battery from the clips it was pretty toasty. My theory is that the aluminum angle brace (visible in pic 3) is in contact, albeit slight, with the battery clips (visible in pic 4) causing it to short. That would be a critical problem in flight, so I have to put some kind of insulation on the surface. I could use electrical tape or even paint it, but tape can "un-stick" and paint can get worn off. Any one have other ideas that might work well?

Greg

Click to expand...

Do the braces need to be made from aluminum? (I know that's what you had on hand to fabricate the part with.) Could those parts be formed or milled from a non-conductive material? (Wood, plastic, resin, fiberglass sheet?)
Also I'm wondering if it's the brace attached to the bulkhead (as opposed to the one on the sled - see post #23) that could be making the electrical connection and causing the short across the clips. If that's the case, perhaps the clips themselves could be shortened more? Also, could small pieces of insulating foam be inserted between the braces and where they would come in contact with the clips? (Instead of painting/taping?)

rbelknap said:

Do the braces need to be made from aluminum?

Click to expand...

The short answer is "not necessarily". But aluminum meets the criteria.

rbelknap said:

Could those parts be formed or milled from a non-conductive material?

Click to expand...

The short answer is "yes".

But, the desirable characteristics of aluminum (strong and lightweight), coupled with the angled geometry to form a brace make it ideally suited in this application.
The major issue is that I barely have barely a 1/16" to work with between the altimeter and the clips, so the 1/16" x 0.50" angle aluminum is the only structural element I have confidence in to use. It will work, but I need some way to insulate it. If I had some angle with those dimensions made of G-10, that would be perfect. Wouldn't be as easy to mill, but it would work. That is part of the challenge of this design; making it robust and all fit in very cramped space. I'm thinking that after this exercise, larger av bay designs will be a breeze and take hardly any effort!

Then there is also the time budget. I've spent quite a bit time on this already and I just want to get on with it. I have to keep in mind I want a good design, not a perfect design.

rbelknap said:

Also I'm wondering if it's the brace attached to the bulkhead (as opposed to the one on the sled - see post #23) that could be making the electrical connection and causing the short across the clips.

Click to expand...

In the case of the test last night, it was not a factor since they were not part of the test. But you are exactly right in thinking that it could, and whatever treatment I apply to the battery brace on the sled I will have to do to the brace on the cap.

Greg

Greg- in the real world on the real Atlas we used what is called "Kapton tape" for insulation. Mine has aged horribly but you can find it on E-bay pretty easily. Fresh rolls are best.

fyrwrxz said:

Greg- in the real world on the real Atlas we used what is called "Kapton tape" for insulation. Mine has aged horribly but you can find it on E-bay pretty easily. Fresh rolls are best.

Click to expand...

Wow! I've heard of Kapton before, being used in the space program and all, but I didn't know why it was used.

The cool part is McMaster sells it!

https://www.mcmaster.com/#kapton-polyimide-tape/=tuov7c

I think I need to place an order.

Thank you Mr wrxz!

Greg

No-Thank YOU for your innovative design!

fyrwrxz said:

Greg- in the real world on the real Atlas we used what is called "Kapton tape" for insulation. Mine has aged horribly but you can find it on E-bay pretty easily. Fresh rolls are best.

Click to expand...

This sounds like what you'd want. I was thinking about "the tape they use to insulate electrical components" but couldn't remember what it was called.

Oh Kapton, my Kapton!

I finally got the Kapton tape applied to the aluminum angle battery anchors to keep the current where it needs to be. Then came the big moment of dry fitting the whole enchilada. There was literally no room for error. If it didn't fit it would be a big FAIL and would likely mean that it would not make this weekends launch window.

Here are the major pieces as they fit on the sled. BTW, just for reference, the major blue lines on the paper are one inch squares.



After orienting everything, this was the result: it fit! There was no binding or forcing, it all went in and closed without force, which was so not the experience I had before.



Now I need to solder up the leads to the battery clips and epoxy everything in.

Greg

Great job, and congratulations on the success. The attention to detail and your perseverance have been rewarded. Enjoy the Launch.

accpool said:

Great job, and congratulations on the success. The attention to detail and your perseverance have been rewarded. Enjoy the Launch.

Click to expand...

Thanks! It has been quite the journey.

Greg

As the launch day approached I got more concerned about the weather for Saturday. Weather reports indicated a 60% of rain and a low cloud base.

Aside from that, I had not completed a personal Flight Readiness Review and closeout of the av bay project. I didn't have a written procedure to follow and I hadn't bothered checking out the other sub-systems of the rocket for that matter. Aside from that, I had a nagging doubt that I was missing something. I felt like that a successful flight was possible, but it just didn't "feel right".

While there is the design and fabrication element for this project, there is also the procedural side to this as well. For example, in a match fit test on Friday, I discovered that while the matches fit like they were supposed to, the insulation had a high-coefficient of friction and that made them hard to turn in the match conduit. While it's a problem, it's by no means a show stopper. I just need to figure out what I can do to "slicken" things up a bit.

So while I am not quite there, I am close. When I finish this, I want to finish well. So even it it fails, I will at least have learned a lot and not lose sleep that I missed something.

Greg

After giving the issue of the match fit some thought, I thought the best solution would be to just open up the ports a bit more on the match conduit. I believe this will be enough to relieve the friction enough so that the fit issues fall within acceptable limits.



Greg

I was able to open up the e-match ports a bit more to accommodate the turns into the terminals. This way the wires will have a more "relaxed fit".



It looks like the tubes have some debris in them, but I cleaned them out with alcohol and a twisted paper towel. So they maybe internal scratches that are just catching the light. So I will need to recheck that.

I plan to do a fit check later to see if the wires can be better managed with this configuration.

On another note, I was able to remove and replace the MAWD unit easily from the base when milling. Before I had the mindset that this altimeter was going to be dedicated to this av bay. Now using this altimeter in other projects appear to be a possibility. So that is a bit of good news for this project.

Greg

One area of concern for me is the thin amount of wood between the "breech" area of the charge tube. Because the blanks used for the caps were 1/4" thick to make room for the battery bay, there is just a thin bit of wood at the base. To be precise it is a mere 0.085" thick. My fear is that over time, the recoil of the charges will weaken the wood and eventually cause it to fail. To mitigate the risk, I will plug the end with a 3/16" brass plug and place a brass collar around it to help distribute the recoil over a wider footprint.



Greg

I finally got the brass plugs and charge collars milled. Let me say I have a new respect for machinists. Most of the work I have done before has been in wood, so this was the first attempt into some beefy metal. The good thing is that brass, like aluminum, is considered a soft metal and even though it is denser than carbon steel, it's only 9% more.



Together the two collars weigh a paltry 2.18 grams, which is less than a dime.

Greg

I wanted to complete this part of the project today, so I first epoxied the plugs to the base of the tubes (I used Loctite 5-minute). I added a bit of milled fiber as an amendment to the epoxy, as well as some brass filings that was a byproduct of the milling operation. I then set the plugs aside to cure for an hour or two. Later, I scuffed up the base of the tube with 150 grit sandpaper and cleaned it off with alcohol. I then mixed a new batch of epoxy and joined the collar to the tube, then the tube to the cap. I followed the same process for the other one.



In the process, I dodged a bullet by nearly making a serious mistake. I had forgot that the tubes were not identical, as the bottom tube is designed to be a bit longer because it holds more BP than the top one. Thankfully, I glued the correct one to its cap before I discovered the potential problem.

I am glad that it turned out well, and that the caps are now complete.

Greg

Looking good.

One other thing that was of concern, was that the twist-and-tape method, while simple and effective, it is a pain to work with. The location of the switch terminal to the external switch port is rather hard to feed the wire through. It wouldn't be such the issue if there was more volume, but the hard turns in the small confines of this av bay make it problematic. I was thinking there has got to a better way. I came up with another way, but I don't know if it is better, yet.



The idea that I came up with is the screw switch, but that's not a new idea. What may be novel is that the configuration uses brass plates (acting as large, flat wires) to carry the current from the screw to a terminal point much closer to the MAWD's switch terminal. The way I plan to get the current from the end of the brass plate terminals to the MAWD switch terminal is to use a Novak 22 ga JST male-female pigtail. So if all goes well, all I will need to do is to connect the pair before buttoning up the unit.

Greg

I wanted to see if the screw that I had planned was going to be long enough, so I added the screw to the study.



Now it looks like I may need to mill the screw a bit, since I don't want it to extend any further than necessary.

BTW, this looks like the last major modification that I need to make to the av bay, so I may be seeing the light at the end of this tiny tunnel.

Greg

I finalized the design for the brass strips that will take one end of the JST pigtail. I wanted to accommodate a good solder site for the pigtail that would be more or less maintenance free. Hopefully I will be able to mill these out soon.



Greg

Now that the brass strips milled out, so I hope to get them soldered to the JST today. There is not a lot of mass with these strips. These are just K&S brass strips that are about 1/4" wide and 0.033" thick and a little over an inch long.

Once the soldering is done (I don't like soldering, and I'm not that good at it either), then they can be epoxied to the switch plank. I plan to cover the brass that faces the center of the av bay with strips of Kapton tape for insulation.



Greg

GregGleason said:

I milled another mockup, and when I got everything mounted on the sled, it all fit as designed. However, after I removed the battery from the clips it was pretty toasty. My theory is that the aluminum angle brace (visible in pic 3) is in contact, albeit slight, with the battery clips (visible in pic 4) causing it to short. That would be a critical problem in flight, so I have to put some kind of insulation on the surface. I could use electrical tape or even paint it, but tape can "un-stick" and paint can get worn off. Any one have other ideas that might work well?

Greg

Click to expand...

I was also inspired by the JCSalem design and use the A10 battery with clips in my MAWD. Spacing of the clips is critical for proper tension and contact. Yes, the battery easily shorts on the clips and/or your soldering and wiring. I wrap the battery cylinder in cellophane tape and ensure it does not get toasty each time I set it up. As for battery retention, I just use a zip tie.

That is a nice looking av bay Buckeye!

Greg

I finally got the wires soldered, which is a task I would gladly sub-out for a reasonable fee. Regardless, the connections are solid and they are able to adequately carry current through them.




After prepping the brass strips for epoxy, I test fit them to the switch plank. That was a good idea since I had to do a little corrective filing to remove excess solder. The red lead needed to go first since it didn't quite fit, so that the black lead would then key off of its position. After the epoxy cured for a few hours, I did some more finish sanding on the brass strips before adding the Kapton tape. I also milled the switch screw down to its proper ( or at the very least, its estimated) length.



Greg

Positioning the switch plate needs to be done with a fair amount of precision, because a degree of arc one way or the other will render the switch plank useless. I will be using two techniques to properly locate it.

The First
I will reference the switch plank based on the location of the sled supports. One of the features of the CAD software I use is a command that lets you "unroll" a curved surface. So I used that command to highlight on the reference points in the av bay.



The program did the unroll routine, but I did have to do a little cleanup on it. But it did provide a flat sheet that I could then print and place inside the av bay for "general reference".



The Second
Once I have the general location, I will need to fine tune it. For that I plan to tap the primary brass tapped with a 2-56 hole. Once it is close, I can then thread a screw into it to finalize the location. The switch screw is a 6-32, so the tapping process for it will "overwrite" the 2-56 screw, as long as I "hit" the same centerline.

Greg