L1/L2 2.6" Madcow DX3 Build

Nice work.

So I have two questions. Hopefully people see them and chime in. Don't be shy!

1. When mixing epoxy for fin fillets, is there an optimum ratio to aim for when adding microballoons? For instance, by weight, if I have one part epoxy (mixed) should I add 1 part microballoons? 1/2? 2? I'm going to be able to practice on two other MPR builds first, but I'd rather not have to experiment too much.

2. Concerning the alignment of airframe/av-band/payload bay, do most people rely on just the shear pins for alignment, or do they key the parts (probably on the av-bay band with cutout notches in the two airframe halves?) I don't think they're nessisary unless I had some other kinds of internals that depended on airframe alignment, but I was concerned that if one hole in a given set is 'off' of symmetric by even a little bit, I'd have difficulty assembling the parts when preping. I'd rather not have it be a mystery. The other option I considered was setting one hole of each set a bit further fore/aft of the others to mark it out as 'different' which should be pretty easy to eyeball when assembling.

Thoughts?

I draw a key/line to line up each part.

Since microballoons simply thicken the epoxy mix, it is up to you how much to add based on the consistency your are trying to achieve.

I think no matter how hard you try, it will be impossible to get your shear pins perfectly drilled so they have complete interchangeability through 360 degrees of rotation. For that reason people often key by offsetting on hole or making a mark of some kind on the airframe--either will work.

I was thinking more from a sandability standpoint for the microballoons, but I guess it makes more sense that you would want to get the consistency/thickness to 'optimal' in preference to sandability. Otherwise applying could be a nightmare. I've heard 'peanut butter' thickness before, but worried that might not penetrate and bond well. I imagine there is such a thing as 'too thick' but where is the line?

MCriscione said:

Last night i got the upper centering ring and kevlar strap epoxied on to the motor tube. I used Loctite 5 minute epoxy to place the ring on the tube, avoiding the area right near the straps. That locked the centering ring in place pretty quickly, and about 30 minutes later, I came back and added fillets and epoxied down the kevlar strap (with a light overlay to 'fillet' or 'feather' the edges of the strap into the motor tube. This part was done with 20 minutes BSI epoxy with System 3 colloidal silica added until it was a thicker peanut-butter-like easily spread consistency that wouldn't drip and run. The binder clips did a pretty good job of holding the shock cord in place without getting in the way of the epoxy.

View attachment 279801 View attachment 279802

The next steps was to mount the lower centering ring and the base of the motor retainer. (done tonight, pictures tomorrow)

I ran into a snag tonight laying out the backing board for the Eggfinder. I had intended to 'mount' it in the nosecone such that is was removable using an Estes 29mm motor retainer. However, I quickly realized that the 2S lipo battery that I had purchased won't fit through the available opening. I can't believe I didn't check that dimension closer when ordering. Now I'm stuck with either finding a new battery (either using a 9V or finding a smaller Lipo) or using some other nosecone mount, most likely with t-nuts and bolts of some kind. I really liked the easy accessibility of using a motor retainer, and I'll be a bit disappointed if I have to reject that part of the design, so I think at this point I'm looking for a different battery. Preferably something like a 2S with a cross-section that will fit inside a 1.125" diameter circle. I have two 1S nanotech 300mAh lipos that I could chain in series that could work, but one of those was supposed to run the Eggfinder (along with a secondary deployment channel 2s lipo) and I don't have a third. Arrrg, stupid shipping fees from Hobbyking!

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I think you probably should have tied an overhand knot to catch on aft side of the CR. You are relying solely on the epoxy to hold in place. Maybe drill a 1/16" hole from one side of the shock cord to the other and slide some 16 or 18 ga. steel wire through it. Wrap it around the motor tube...just for a little more insurance.

EeebeeE said:

I think you probably should have tied an overhand knot to catch on aft side of the CR. You are relying solely on the epoxy to hold in place. Maybe drill a 1/16" hole from one side of the shock cord to the other and slide some 16 or 18 ga. steel wire through it. Wrap it around the motor tube...just for a little more insurance.

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I'm not certain that I entirely agree. Firstly, there wouldn't have been room for any kind of knot, there's just not enough clearance. Even if there was though, I find it hard to believe that the epoxy wouldn't be sufficient. It has (in total) nearly 6 square inches of contact area. Even assuming a rather poor bond, I thought that the shear strength of epoxy was at least 500psi (when bonding fiberglass, per https://www.jcrocket.com/adhesives.shtml). That would equate to nearly 3000 lbs of force required to separate. The total rocket should be under 7 lbs, so we'd be talking about some shock force in excess of 400g's? I'd think that at that point, I'd have severe failures of other parts of my recovery harness And none of that accounts for the addition of silica or the tensile strength of the fillets.

Honestly, I rather thought I had gone a bit overboard with the double mount to distribute the force. Now I'm getting a bit concerned. Does anybody else share this feeling?

Evan, I'm not entirely clear on what you are suggesting. I'd have thought drilling any holes in the kevlar would weaken it. I think you've suggesting that and then threading some wire parallel to the MM tube, through each kevlar strand and back again? I'm really not sure I could get this to work, the kevlar is only about 1/16" thick at most. Probably less. At this point I'm not sure what I could do. Maybe some 1/16 'pins' (nails) inserted in holes drilled radially into the MM tube? I'd have to keep them flush with the ID of the motor mount though.

As I look again at what you did, I think you will be OK. With two connections you have redundancy built in and you also essentially double the strength of the mount. I take it this is one piece of Kevlar. If it is I suggest tying a loop knot where your other shock cord attaches so that in the event one of the bonds gives way, you still have the other bond holding. I had only looked at the mount showing one side, not both.

Fanning the Kevlar as you did at the end will help as a last resort. And your charges will be fairly small. Probably somewhere between 1.25 and 1.5 grams of BP.

I did not realize this kit had a 54mm tube....You could stick an L in this and kiss a few waivers. This is definitely a mach buster. Sim it with a CTI K1440.

Sorry to alarm you. My fault for not looking at the entire concept.

One thing to consider with microballoons. If you intend to breach Mach 1, you really don't want them. They make the fillets porous and weaker than if you used straight epoxy, and when you go transonic (About Mach 0.8 to Mach 1.2) you will run into a lot of turbulence that will put a lot of pressure on the fins. This rocket is designed for mach speeds so you won't experience as much flutter, but you will still get a little and you want to make sure you have exceptionally strong fillets.

Rocketry Warehouse and Apogee Rockets sell RocketPoxy. This is a very thick epoxy that does very well for fillets. It will take about 4 hours to set, but the fillets are smooth and extremely hard. It has a shear strength of over 14,000 PSI. It bonds very well to FG if you rough up the contact areas on the fins and the airframe. Do 2 fillets at a time by laying the rocket sideways, taping off the areas you do not want, smoothing the epoxy into place and letting it set. You end up with beautiful fillets that are nice and smooth.

EeebeeE said:

As I look again at what you did, I think you will be OK. With two connections you have redundancy built in and you also essentially double the strength of the mount. I take it this is one piece of Kevlar. If it is I suggest tying a loop knot where your other shock cord attaches so that in the event one of the bonds gives way, you still have the other bond holding. I had only looked at the mount showing one side, not both.

Fanning the Kevlar as you did at the end will help as a last resort. And your charges will be fairly small. Probably somewhere between 1.25 and 1.5 grams of BP.

I did not realize this kit had a 54mm tube....You could stick an L in this and kiss a few waivers. This is definitely a mach buster. Sim it with a CTI K1440.

Sorry to alarm you. My fault for not looking at the entire concept.

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Thank goodness, I'm very glad that it was just a misconception rather than a design error on my part. I agree, the charges will be moderately small, and yes, the 54mm motor mount option is relatively new to this kit. I should probably go back and emphasize that in the first post. Needless to say, you're right, with the right 54mm motor, this thing is going to bust any waiver I have nearby. I'll probably be sticking with I's and K's regardless of the mount size. I suppose I can dream of someday under the right conditions a Loki 1040 or 2050, but who knows if that would ever happen.

EeebeeE said:

One thing to consider with microballoons. If you intend to breach Mach 1, you really don't want them. They make the fillets porous and weaker than if you used straight epoxy, and when you go transonic (About Mach 0.8 to Mach 1.2) you will run into a lot of turbulence that will put a lot of pressure on the fins. This rocket is designed for mach speeds so you won't experience as much flutter, but you will still get a little and you want to make sure you have exceptionally strong fillets.

Rocketry Warehouse and Apogee Rockets sell RocketPoxy. This is a very thick epoxy that does very well for fillets. It will take about 4 hours to set, but the fillets are smooth and extremely hard. It has a shear strength of over 14,000 PSI. It bonds very well to FG if you rough up the contact areas on the fins and the airframe. Do 2 fillets at a time by laying the rocket sideways, taping off the areas you do not want, smoothing the epoxy into place and letting it set. You end up with beautiful fillets that are nice and smooth.

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I'll have to consider this. I haven't really seen this as a rocket that I'd fly more than once over mach, but regardless, it could happen. I understand there are a lot of weird and extreme effects that occur at and over mach 1 and I'll have to look into this one some more. I had been aware of the Cp shift and mach heating / ablation, but hadn't much considered it as an issue for the fillets. I can understand thought that if the paint is stripped away and off the fillets, that the microballoons would provide a porous surface that could be more erosive. It sounds like you're more concerned with the actual strength of the fillet though, rather than the surface finish. I can certainly understand wanting the strongest fillets possible (really regardless of speed, but especially transonic) but I had assumed the through wall mounting would take care of the vast majority of the strength related to 'attaching' the fin to the rocket. I thought that fillets, at least on non-minimum diameter rockets, were more for aerodynamics and visual appeal than strength. Do people add strengthening additives like silica or chopped carbon to fillet epoxy for TTW fins?

This is an old video but it shows the need for strong fillets at mach speeds.

[YOUTUBE]M_FCQ550770[/YOUTUBE]

So update time finally. February really got away from me as far as keeping active with rocket building. As mentioned by some others, there was a lovely stomach flu circulating that hit the house, first my daughter (mildly, thank goodness) and then myself. In addition, a number of colds for other family members made February a slow month.

Based on EeeBeeE's suggestion, I did end up ordering some Rocketpoxy to use for fin attachment/fillets. As a first time user, I decided to try it out first on another LPR that I'm building as a MPR. I got an Estes Big Daddy for Christmas that I immediately converted to a 29mm MMT and Plywood fins. That was pretty much all assembled by the time I got the Rocketpoxy, but it didn't have fillets yet. So I tried it out on there (with the included black dye) and it worked wonderfully. Tapped and then pulled with a small length of PVC. I think I used too large a diameter, but for the Big Daddy, being a bit fat seems to be in character with it's short and squat nature. They did indeed come out very smooth. I only had one that ended up with a void in it that I missed during the drying process that I will have to fill (probably with Elmers CWF)



After working with the Rocketpoxy, I decided to go ahead and use it to install the motor mount. Before I did though, I drilled some 1/8" holes along the fin slots to allow me to inject epoxy a bit more easily into the pockets I created on the motor mount. I dyed the epoxy again, so that I could see it more clearly inside the tube. It turns out that the applicator that I was using (syringe taped to a rod, with another rod taped to the plunger) ended up depositing more epoxy that I had originally intended. Thus the large black band at the forward CR. It's not as bad as it seems, the black dye turns a thin coat totally opaque, and makes it look like it's loaded up with epoxy, even though it's not.



Lastly, I got the switchband epoxied to the AV bay coupler, so that I could assemble the rocket and start placing marks for drilling the plethora of holes that are needed to make DD happen correctly. Next steps are epoxy in the nosecone coupler, then actually drill the 16 or so holes that I need to make. After all the air-frame holes are made, I need to bevel and rough the root edge of the fins, so that I can start attaching them. I suppose I need to rough up the airframe in those areas too, so the fillets have somewhere to stick. Anyway, more work to go, but it's making progress. Plus, I've nearly finished that Big Daddy, just have to figure/add nose weight, glue in the NC bulkhead, add rail buttons and prime/paint.

Speaking of rail buttons, do people drill holes and add the buttons prior to paint and then just mask the buttons, or do they drill holes, and temporally fill them with something during paint (rolled up piece of paper), and attach the rail buttons afterwards? I'm not sure how I'm feeling, but I think I'd rather put them on before paint. I'm just worried about some seeping past the masking and 'gluing' the rail button to the air-frame. They're supposed to be somewhat free-spinning right?

MCriscione said:

Speaking of rail buttons, do people drill holes and add the buttons prior to paint and then just mask the buttons, or do they drill holes, and temporally fill them with something during paint (rolled up piece of paper), and attach the rail buttons afterwards? I'm not sure how I'm feeling, but I think I'd rather put them on before paint. I'm just worried about some seeping past the masking and 'gluing' the rail button to the air-frame. They're supposed to be somewhat free-spinning right?

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I drill the holes before paint, but don't worry about getting paint in the holes. I just screw the buttons in place afterwards.

I think the paint helps old the screws in place, kind of like thread locker.

Handeman said:

I drill the holes before paint, but don't worry about getting paint in the holes. I just screw the buttons in place afterwards.

I think the paint helps old the screws in place, kind of like thread locker.

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That makes sense. Even if there was a lot of paint build up, I suppose that cleaning it out afterwards should be pretty easy. Alright, I'm sold, no buttons until after paint is on.

Your kevlar to MMT joint looks fine. That method is shown in the Wildman instructions for their kits. I believe CJ builds his that way, all the way up to heavy level 3's (IIRC he's mentioned that in other threads). It's a lot of surface area and epoxy. With the method you used, the entire MMT would have to tear out of the rocket. Between the centering ring joints and fin joints, a lot would have to fail for that to happen. Your harness would probably snap first.

Your external Rocketpoxy fillets look great.

You might find Rocketpoxy is too thick to inject well for internals. Some use West Systems to inject for this reason. I found Rocketpoxy is thick enough that the double-butter method gets a lot of epoxy at the fin-to-mmt joint. You spread a bunch of epoxy on the fin root, insert it into the slot and press it against the MMT, depositing a bunch of epoxy on the mount. Pull it back out. Spread a bunch more epoxy on the fin root and insert it into the fin slot again.

Since your FG tubing is semi-clear, you can add black die to the epoxy (like you did for your fillets). Then you can see the epoxy joint through the tube. Like this:



Those are double butter Rocketpoxy joints.

Once you get your fins on and the rocket loaded up for flight (laundry, av bay, GPS) would you post the total weight? I want to run some simple Thrustcurve sims to see what speeds and altitudes this will do. (I'm still thinking of buying one for myself).

Exactimator said:

Once you get your fins on and the rocket loaded up for flight (laundry, av bay, GPS) would you post the total weight? I want to run some simple Thrustcurve sims to see what speeds and altitudes this will do. (I'm still thinking of buying one for myself).

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I'd be happy to post that info, but it probably won't come along for a while. I'm actively building an OpenRocket file that will eventually have those weights, cg, etc. included, but it's obviously not complete yet either. I'll be posting that too, once it's finished. I suspect another month or so of sporadic, highly inconsistent building due to family/work/free time balance issues before then, which will still probably not see any paint applied.

Plans for 'injection' seemed to work out OK with the Rocketpoxy when I did the motor mount. I didn't picture it, but I used a 'syringe on a stick' method that worked pretty well. If you get it right after you first mix it and make sure your syringe nozzle is large enough, it seems to work. I used a curved tip 'oral' syringe (from Monoject) to do it, and didn't find it to be excessively difficult. I'll probably stick with that rather than the double dip, if only to keep the OD a bit cleaner prior to fillets. It probably doesn't matter / I could tape the surfaces off, but why not, I already drilled the holes.

I will say that I got a bit of practice for the AV-bay though. I've also been building a Madcow 1.6" Mini Frenzy, which I converted for Dual Deploy. And by converted, I mean I took the 4 inch long coupler that comes with the kit and jammed an Eggfinder Quark + battery + switch inside. It has given me a bunch more respect for those that manage to similar miracles with smaller diameter air-frames. I hope that the practice makes it easier to design/build the AV-bay for this rocket.

Maybe I'll post pictures of it if anybody is interested. It was a challenging conversion, at least it was for me. Not sure if it will have much practical use, but that rocket will give me some practice with dual deploy prior with MPR motors, prior to using DD on the DX3 (probably for both L1 and L2 attempts)

So I made my first 'big' mistake tonight. I finished gluing the nose cone coupler into the nose cone, dry fit and taped the rocket together. I had previously done this and marked all the hole locations and labled them. I then proceeded to drill all my holes. Or rather, I drilled nearly all as 1/16" pilots, which I then re-drilled to larger appropriate sizes.

When starting this re-drilling, I had intended to start with the Rivet holes, between the AV bay and the upper Payload section. This is where I would install the push-pin type rivets that came with the kit (which are not very well sized in my opinion, they're too long for the thin wall tubing). However, instead of drilling between the AV bay and the upper Payload section, I drilled the matching pilot hole on the other side of the AV bay where it mates with the fin can. This hole was supposed to be a shear pin hole, and a significantly smaller diameter. Well crap. Now I have a big (3/16?) hole where I wanted a much smaller one for a shear pin.

I suppose it's just another 'vent' hole for the AV bay, but I'd really rather it not be there. It'll unbalance the other vent holes and is way too large for a vent anyway. I'd like to 'repair' it somehow, but I'm not sure how. Either completely cover it up or reduce the diameter so that I can still use it for the shear pin. The hole is in both the air frame and coupler. I suppose I don't need to worry about the ejection charge pressure due to it's location, but I'm still concerned that any 'simple' repair is going to pop out if the tube flexes when landing, or maybe even on accent.

So I'm putting out the call. Hey Everybody! How do I fix this?!


No pictures tonight; perhaps tomorrow if I have a moment.

Epoxy and wax paper. 5 min epoxy will fill the hole. Use wax paper on both sides to form a smooth surface. Allow to cure. Sand. Done.

Been there, done that. Both the mistake you made, and the repair.

Paint covers lots of sins.

As a bonus, you'll have an extra hard plug to shear the pins when you redrill the holes.

I would try filling the hole with epoxy. Tape over one side of the hole (I would use masking tape) then fill with epoxy from the other side. Overfill just a tiny bit. After it's completely cured, pull the tape and sand flush on both sides. Now, the only thing I would be concerned about after that is if you need to re-drill in exactly the same spot. If I understand your last post, this was to be a shear pin hole. The epoxy repair may not drill cleanly and I'm not sure about the integrity of that hole over the long haul with shear pin use. I'm curious as to what others may suggest in this case.

I would drill out the airframe (not into the coupler) a bit bigger with a forstner style bit (say, 1/2") and then I would find some brass strips/flashing. Cut the metal to fit your larger diameter hole, with a bit of reveal around the edges. Add masking tape to the inside to act as a dam, and rough up your brass strip with some sandpaper wetted with epoxy. Fill in your hole with epoxy, add the brass strip and press into place, then barely overfill your hole with epoxy. Sticky tack poster adhesive is good to create this little dam on the outside. Wait until the epoxy is at the green stage and then pull the sticky tack and smooth out over the surface of your rocket. After everything cures sand both inside and outside. Now you have a spot where you can drill your 1/16" diameter vent hole and have something inside there that will give it some bite.

Edward

I was a bit hesitant to use just epoxy, thinking that a rough landing could cause it to 'pop' out of the hole it's filling. Thinking more though, perhaps that's not a concern. It is an adhesive after all. I just wasn't sure if it would have enough flexibility after hardening and contact area for adhesion to stay in place long term. I'm not so worried about putting a shear pin hole back in the same place, and I share Kendal's concern that the epoxy may be difficult to drill cleanly and may become a weak point over long term use.

If, as Edward suggested, I had some way to create a narrow 'reveal' on the ID of the airframe, then I'd give it a try with a brass shim plate, but I don't have a spot face drill that size, or the ability to drill from the ID in tube this narrow. The coupler is such a close fit with the air frame that I don't think the brass would easily fit between the two, and may cause issues during deployment (snagging), at least that's my fear.

To create a bit more contact surface, I'm considering adding a few notches around the perimeter of the hole, maybe 1/16 wide by 1/8 or 3/32 radially, in kind of a 6 or 8 point star pattern. More thoughts? Or is everybody in agreement that epoxy fill should cover me?

This is a fun read. Thanks for posting it this far and I look forward to reading more progress.

Mendal said:

This is a fun read. Thanks for posting it this far and I look forward to reading more progress.

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I'm glad you've enjoyed it. I feel bad that it's stretching out quite so much, I had really hoped to get this at least to primer by now, but have hit some delays.

I got the AV Bay lids drilled the other night, and now I get to start building the sled. I've arranged the bulkhead to allow me to accommodate a redundant altimeter in the future (i.e., space for an extra terminal block/charge well) but haven't drilled holes for them. I figure I'll do the same with the sled, setting it up to hold both my eggtimer and quark, though I probably won't fly with both.

MCriscione said:

I was a bit hesitant to use just epoxy, thinking that a rough landing could cause it to 'pop' out of the hole it's filling. Thinking more though, perhaps that's not a concern. It is an adhesive after all. I just wasn't sure if it would have enough flexibility after hardening and contact area for adhesion to stay in place long term. I'm not so worried about putting a shear pin hole back in the same place, and I share Kendal's concern that the epoxy may be difficult to drill cleanly and may become a weak point over long term use.

If, as Edward suggested, I had some way to create a narrow 'reveal' on the ID of the airframe, then I'd give it a try with a brass shim plate, but I don't have a spot face drill that size, or the ability to drill from the ID in tube this narrow. The coupler is such a close fit with the air frame that I don't think the brass would easily fit between the two, and may cause issues during deployment (snagging), at least that's my fear.

To create a bit more contact surface, I'm considering adding a few notches around the perimeter of the hole, maybe 1/16 wide by 1/8 or 3/32 radially, in kind of a 6 or 8 point star pattern. More thoughts? Or is everybody in agreement that epoxy fill should cover me?

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Epoxy should be more than enough without any extra treatment. I have done far worse, like spot putty or fiberglass cloth with CA glue. That was on a rocket that does 1200mph and 8000 feet.

However, I would avoid trying to drill that same spot for shear pins. That sounds like a pain.

Just now catching up on this build. I like the way you did the switch on your sled, I will incorporate something like that into my Hurricane 38 build when the time comes.

I am putting a bay in the nose cone of my 4" Cowabunga and have been searching for options for that. I like the idea of the retainer but it kinda limits the size on what you can put in there. I have found some alternatives. Look under your sink, the pipe that connects the drain to the P trap. Can't remember what it's called, I always called it a head pipe. They make one with a 1.52" ID. It is basically the same size as a 38mm MMT, and a near perfect fit in Madcow centering rings for 38mm motors. Lowes sales one with two ends for like 3 bucks. Cut it in half and you have 2 av bays. I got a mockup I'll just have to get the pics up.

I am actually gonna use some 3" conduit fittings (cheaper and lighter than PVC) in my build. Will have to trim em down a bit but no biggie. Will be the same basic idea as what you are doing with the retainer, just on a larger scale.

Looking forward to your finished project. I want a DX3 of my own on day.

Mike

So with nearly a MONTH delay due in large part to winter daycare sickness spread through the house and an overload of actual work (you know, the type that pays the bills) I was able to do some work over the last week or so. I'm going to post this in two separate posts, for simplicity when retroactively hyper-linking this whole thing when I'm done.

So first is the 'ugly'. Or rather the correction of my mis-sized shear pin hole, which was drilled quite a bit too large. I took the recommendations and just filled them with epoxy. I used blue painters tape to seal the inside surface and slightly overfilled the holes with rocketpoxy. The next day, a bit of sanding knocked the fill back down to be flush with the exterior surface. When I removed the tape from the inside, it didn't seem to have sagged at all, and didn't need any sanding.



In the photo above, I've partially inserted the AV-bay into the booster so that it would hold still long enough to photograph. Now I'll just have to drill a new shear pin hole just below this one, which will act as a key for this joint.

Although I've started to 3D model my AV-Bay in Solidworks so that I can play around with sled configurations without cutting up a whole lot of plywood, I'm not nearly ready to start building yet. So in the mean time, I decided that it would be fun to work on the recovery harness portion of the build.

This rocket will have two recovery harnesses, one between booster and AV-bay (lower) and one between AV-bay and nosecone (upper). Each harness needed a third 'loop' or attachment point about 1/3 of the way 'down' from the top end. In addition, because the Madcow kit came with a long length of 9/16" tubular nylon, the ends of the harness near ejection charges needs to be protected. For the ends near the AV-bay, this was accomplished with a few feet of Nomex shock cord protector. (Actually, in this case, it's one protector cut in half, bought from Sunward here).



I started by collecting supplies. Afterwards, I realized that I forgot to picture the welded d-rings that I used in some places. Also, not certain why I threw the quick link in there. I suppose I did use it occasionally to test fit things. I'll also be using a 1/8" quicklink for the drogue attachment. The colored rectangles are heat shrink, to protect the polyester thread that I'm using from both heat damage and from getting stitches caught on things or otherwise physically damaged. I also used them to color code a bit, red towards the AV-bay, blue for mid-point loops. In general I followed the John Coker method for sewing bridles when constructing the loops, even down to finding a speedystitcher, which worked VERY nicely. I'll note here that the stitcher came with some waxed polyester thread, which is what I used for the majority of construction. I had 100lb. woven Kevlar that I could have used (and did for attaching the Nomex, below) but choose not to. Per John's recommendation and my own web-research, it should be plenty strong, plus the waxed surface is much easier to work with than stitching with the knobby woven Kevlar I had available.

The upper harness was fairly straight forward, just TN with three sewn loops. I'll let John's video cover the basic end loops. I'm glad I practiced on a short spare piece before doing it for real. The stitching is easy, but getting evenly spaced, parallel/straight stitches is a good bit harder.



I've included a few photos of the middle loop here, where the nose cone will attach. Note, I plan to attach the Main chute to the top loop of the harness, as the greatest load is between the booster and the Main, and that will give it a direct path. With this doubled over construction, when you attach a quicklink to the loop, you are completely attached to the harness. Even if the middle loop stitching releases for some reason, worst case is the nose cone will slide down to the AV-bay.

For the lower harness, I wanted a bit more stand-off from the top of the booster before exposing nylon to ejection charges (for my level 1 flight, I plan to use either solely motor ejection for recovery or motor ejection as partial backup) I have a 'U' of kevlar installed in the booster (see earlier posts) but it doesn't extend much more than a few inches beyond the top of the booster. I didn't have another Nomex protector, but I did still have some additional 1/2" tubular kevlar, so I choose join the nylon harness at the lower end with approx 4' of kevlar to act as a heat resistant buffer zone.



To do this, I inserted the tubular kevlar into the end of the tubular nylon, and held it in place with some binder clips while sewing directly through the 4 layers. Due to the relative narrowness of the kevlar, I had to keep the stitches rather small, but I think they turned out alright. I don't think this introduces a significant weak spot in this harness (everything is quite over-sized for the weights/loads involved) but I'd be happy to hear feedback on this choice.

Lastly, the ends nearest the AV-bay.



The end loops were sewn as usual (with red heat shrink this time, because I had some and it matches!) but with a welded D-rings, as shown, to permit a bit more standoff and better protect the nylon from stray flame and heat. The Nomex sleeve was then placed on the harness, and positioned so that only a portion of the D-ring was exposed. It was only lightly secured in a single location using a double loop of some 100 lb. woven Kevlar 'thread' (what I use as a replacement to the elastic for my low-power / Estes builds) tied in a simple square knot. The Idea is to allow the Nomex to be slid away from the D-ring slightly so that a quick link can be passed through, but not be so high as to expose the nylon loop. I only secured it in one place because I think the Nomex is stiff enough to remain extended along the harness all on it's own, but also because I don't want any difference in elastic coefficient to cause stress on deployment. If it was pinned in two places and the materials stretch differently when stressed, that difference would cause strain at the pinned points. This avoids all that (if it's even a real issue and not just in my head). It's also slightly less work. :wink:

And finally, thanks to the 10 pictures per post limit, the final product:

Upper Harness:


Lower Harness:

Nice work on the repair and the sewing.

Banzai88 said:

Nice work on the repair and the sewing.

Click to expand...

Thanks Tom. I haven't needed to sew often in my life, but I'm still thankful that our middle school forced us to take a home economics course in 7th or 8th grade that was half sewing and half cooking. I'm proficient in both, but better with the cooking as I get to practice it far more often (nightly typically). I've sewn on loose buttons before and occasionally more challenging projects, but usually nothing this 'heavy'. I wish I had known about this 'speedystitcher' tool back when I was somewhat interested in leather work back in high school. It would have made quick work of some projects where I probably spent hours punching holes.