Another Cardboard Mailing Tube Build 3" rocket with 54mm motor mount.

[Off Grid Gecko]

Don't have a name for this rocket yet. Basically building this as my L2 fun flier. 54mm will give me a range from I to baby-L motors and some experience as I approach the Mach Barrier. Got most of my bulk components today to start building so decided it was time to open the thread.


The nosecone is a Madcow 3"x12.5" plastic ogive. It's surprisingly tough, I guess from the smaller diameter, but I still plan to fill the tip with foam and build a 2" payload bay into it where added weight and possibly some little experiments can be housed. One idea is to test new altimeters in a launch and then take the data from them to ensure they are working to snuff, testing various sensors and instruments, etc. Fun stuff.

These tubes are thick wall cardboard from mailing steel and aluminum stock for the HAAS lathe that I run for my buddy's machine shop.

I made a long coupler from a more traditional "heavy duty" mail tube that slips into a 5" section to create an electronics bay for dual deployment.

Fins were cut from 1/4" 3-ply with a veneer face on both sides and some time with the tablesaw yesterday. I'm planning to sand some shaping into these, but I realize that I actually own a planer and thought that might be a more precise and efficient way to bevel fins, so I may give that a go, not sure yet.





Slots were cut a few days ago and the fins popped in there very well and straight.

 

[Off Grid Gecko]

Screenshot from my openRocket sketchup

 

[Off Grid Gecko]

So, first problem I encountered, there seems to be about 4 hundreths of an inch slack between the the OD of the NC and the ID of the tubing. This may be a blessing in disguise as I knew the OD of the tubing would have a mismatch (it's very thick). I plan to sand a long and subtle bevel in to to closer match the NC OD, and that gap on the fitting gave me an idea.

I have some aluminum flashing that I can maybe slip in there to secure the top of the body tube and add stiffness. It would fill the gap, and provide a solid plate to drill some sheer pin holes into. I'm going to look into this today as I catch a few breaks in my work schedule.

Also on my purchase list were some centering rings, 54mm LOC tubing, a Motor retainer from AeroPack, and some rail buttons. This bird should share chutes with my DX3-4" rocket so I maybe saved myself some costs there, ahha.

 

[SolarYellow]

Does the weight of the actual mailing tubes match what's in the sim?

 

[Off Grid Gecko]

Does the weight of the actual mailing tubes match what's in the sim?

Yes I had the tubes so the weight was measured while I was drawing it up. I do need to recheck the NC after I cut the bottom off the shank.

I tend to weight everything out dry, then go back and retake measurements as things are being constructed and update my sim file as I go. On my L2 build the CoG came within 1/4" of the measurement on OR.

I've cut that aluminum flashing and did a test dry fit, certainly snugs up the NC fit a bit but still a nice easy slip fit that will require some tape. I think I'm going to lay it into the cardboard with epoxy. I sanded the mating surface with 60 grid to scratch it up good and proper and give better adhesion, but thinking the best way to get it in there and secure it before I start mixing. Once it cures I should be able to sand down the upper parachute bay to give me a nice taper to the NC diameter. I hope, I'll post some pics tonight of whatever progress I make.

Edit: actually the weights in OR are slightly heavier to allow for paint and primer.

 

[Off Grid Gecko]

long, frustrating day... but I got some rocket stuff done. I started off by putting that aluminum sleeve in the end of the pipe.




This worked out so well that I really don't know why I haven't tried it before. Next time I have a sloppy nose cone fit, I'm reaching for the flashing. Might even do it anyway and sand the ribs of the NC, hahaha. Nose cone slips in butter smooth with just a little bit of tension. It's quite nice.

Also glued one of the centering rings onto my MMT and got that cut to size. I think the extra 8" piece is going to work nicely for a cargo bay in the nose cone. payload.. whatever.

Then I sanded down the top of the tube so the NC would be flush with the OD of the tube, which went better than expected.



snug as a bug, though will need some filling and sanding in the rough patches. I put an extra wide region of CA at the top after taking this pic.



Worked out well enough I tried sanding some fin profiles by hand. That was not as nice, but they're good enough, they'll do.

 

[Off Grid Gecko]

Put my rail buttons in yesterday. TBH I'm going to stop using the 2-part apogee system but in this rocket with the long 3" tube it's going to be a bit of a pain to work around. I'll get thru it though.

To fish the flange into the tube I taped a neodymium magnet to the end of a stick. The tape secures the magnet as well as keeping epoxy off of it. Put the flange on the magnet, covered in epoxy, then reached into the tube the approx distance and looked through the airframe hole to locate and push it through. Once it was in the hole, I threaded the bolt into it from the outside and use pulling force to secure and clamp it.

Glued the forward centering ring to the MMT after slipping it past the little metal flange that's now inside my tube and then installed the aft rail button.. TBH this would be easier if I would just use a shorter MMT rather than a full 26" for a 54mm motor. But what the heck, I like a challenge. And I'll find it as I need to secure two centering rings at once in the tube and then I can start installing fins.... wish me luck, haha.

 

[Off Grid Gecko]

Going to be getting the fins mounted today. I'm curious about this superglue tacking method. I'm using titebond for all the attachment and inner fillets so it sets up and dries pretty fast anyway, but it would be nice to get them all in place at roughly the same time. Would save a lot of waiting for glue to dry before I can start running my epoxy fillets on the exterior.

I have about an inch at the back of the fin that isn't tabbed. Do I just do a squirt of CA on either side of that part and it's usually strong enough to keep the fin alignment so I can start on the next one? I mainly use CA for other things so it doesn't see a lot of use on my rockets other than touching up tube ends. Any help with this CA tacking idea would be appreciated.

 

[Off Grid Gecko]

All the Titebond has had a couple of days to set up and cure. Adding epoxy fillets today as time permits. Nothing fancy. Well mixed 5-min epoxy and some fairly quick finger work. I'm not a fan of huge fillets, guess i've not yet found a reason for them. I prefer a very tight seam close to the body. I guess if the fins were sticking up farther a fat fillet would provide some more stiffness for Mach 2 + flight to reduce flapping. The fastest motor I've been able to sim this on is about 1.8 and with 1/4" ply my calcs showed me that I should be fine well past that. Normally I have tube spirals filled at this point and have to scratch through the first primer layers to lay down the epoxy, but trying a little bit different order this time.



Purchased a bunch of hardware yesterday at all. The Shock cord mount is going to be installed on a separate baffle above the motor tube, that will be the last thing to go in the pipe. And I got a 3" hole saw so maybe I'll cut some of my bulkheads today as well. I couldn't find a 3-1/8" at the hardware store so I'll have to cut the bigger end-caps for the Ebay with a jigsaw.

 

[Off Grid Gecko]

How I cut baffles...



By some sourcery, hole saws tend to cut these very close, close enough for most of my cardboard rockets anyway. I had to do a little bit of touch-up sanding for the 3-1/4" "hole" to fit into the end of the tube. Like 1-2 points is all I had to take off, did it by hand on the belt sander.



These will be added to smaller holes cut with a 3-1/8" bit to make the end caps for the E-Bay. I cut 3 but I need to make another one so I can double up on the main anchor inside the fin can that will secure my harness. Part of me wants to make that piece removable. I'm thinking about inserting it into the tube and then securing it with 6 wood screws instead of using glue, but that's probably not the most sound Idea that I've ever had and likely to end up ripping the cardboard. Perhaps it will be an option on a bigger 4" tube made of fiberglass so I'll pass on this rocket. I like when things are removable as it makes maintenence later on a little bit easier, so always looking for new ideas to keep my birds flying if something happens to them.

I'm going to be using 5/16" eye bolts and I'll secure them onto my baffle plates after I get the eyes welded shut. I don't have a welder so I need to have a friend do it.

 

[Off Grid Gecko]

Got the motor retainer on finally. This is centering ring #4 on this rocket. aeropak is cemented to the CR with JBWeld. I had considered leaving the cap-screws in the wood so it would be removable. I couldn't get the threaded pieces in that ring without busting it (I busted the first one) because there simply isn't enough "meat" on a 54x75 centering ring to do the trick. They did work quite well to clamp the ring to the retainer while the JBWeld set up.

Dumped some gobs of glue in there to secure the ring and then turned the whole assembly upside down so they can form nice beads on the back side for a strong joint, though I'm confident there's more than enough here to hold that motor in, and 4 - 1/4" CRs should effectively transfer any thrust load I can put on it. I'm not planning on using a delay charge on this rocket at all, but at the very least it will only be a backup.



I've also been doing a lot of thinking today about the payload bay in the nose cone and how to secure everything up-front. My original idea was to foam in the tip of the NC. The leftover 54mm tube slides about 7.5 inches in from where I cut off the NC shoulder, but I will need some positive retention up front. I'm considering gluing everything in then using screws through the nose and into the forward plywood bulkhead (and residual glue/epoxy around it) to keep everything secure up there. My idea is to have this tube as a chamber and make experiment packs that will fit inside. Still tackling several issues on that front but plenty of rings and bulkheads cut today. I definitely shouldn't need to worry about the NC collapsing by the time I finish.

 

[Off Grid Gecko]

What scratch-built rocket would be complete without a scratch-built electronics bay and sled. 1/4-20 threaded rod running front to back along the 11x3 electronics bay. This will be the first time using this type of design but so far I'm really liking it. Very simple to construct and seems sturdy enough. I need to add some foam padding to the closing side for a snug fit but it should work quite nicely. What do you guys think?

 

[Off Grid Gecko]

Next time I think I'll prep the bay a little further first, but I think this will work fine. Scratched up the inside of the 3" nose as best I could and used gorilla glue to foam in the front and secure the tube and forward bulkhead... not exactly according to plan but it'll work just fine. I plan to slip a centering ring down in there to use as a backstop for some more threaded stock, allowing room (I hope) so that the payload bay is a fullsize 54mm tube with no obstructions inside, and whatever depth I can get. In any case, pretty sure this NC won't crush under pressure.