Build Thread - Wildman Jr. Two-Stage

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To make this easier, I'm going to buy an extra length of airframe tube and build up the interstage as shown here.
I've ended up buying extra bits of coupler and tubing with almost every two-stager I've ever built.

At some point, years ago, I realized it was just going to be advantageous to have extra tubing on-hand for things like this, so I bought a full stick of tubing and a full stick of coupler tubing in every diameter I use frequently in both FG and cardboard (2 sticks of everything in 4" - that size definitely gets the most use). It is a lot cheaper (per inch) to buy the whole stick and shipping it all at once is cheaper overall than buying short pieces over and over again. I just cut off lengths that I need and a stick usually lasts me a couple years for these purposes. It is a bit of an expense up front, but, really, for the cost of maybe a good 3" FG kit, you could set yourself up with all the spare tubing you would need and have it on-hand to experiment with, especially for scratch builds.
My old ISC was just a coupler with a bulkhead set below the sustainer nozzle. That bulkhead was also where I had my all thread for my sled. Nothing hard.

I tried the closed ISC and later the separation charge. Easy enough. But with the charge, I had a wiggle of the sustainer. Sometimes it was big and sometimes small. It might not matter but I have seen some sustainers light during that wiggle. And that always scared the crap out of me. The whole "what if". After conversations with a few "in the know", the design was changed. I used drag separation from then on and never had any issues.

As for timing, the best I found was one second after booster burn out. This gave the booster time to separate and fly out of the way before sustainer came up to pressure. It was always nice to see that tracking smoke from the booster flying out of the way and the sustainer motor smoke as the motor hit.

Conduit, I always used the very thin aluminum tubing. Doesn't need to be anything substantial. But, I always plugged the top by wrapping masking tape around the igniter and pulling it tight. Didn't want ejection charge gasses escaping causing a failed recovery.

I am currently working on another two stage. After being gone from the hobby for over 10 years, it's time to get another one flying.

Photo of my ISC. It's the upside down blue painted piece.


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Installed the motor retainers and aft centering rings to the motor tubes.

The left one has a GLR Slimline, and is the booster mount.

The right one, the sustainer motor mount, has a Kosmo Slim. Like a slimline, but even slimmer. I originally bought it for my 4" Mega Alpha, but it didn't fit the phenolic motor mount tube I was using for it. I remembered I had it the other day, and it turned out to be perfect for the upper stage of this rocket, taking up as little space as possible where I also need to install the sustainer igniter conduit in the space between a 38mm motor mount and 54mm airframe tube.

I've thought about it, but I am concerned about accidentally crimping the end.

I have always belled the end. Used a automotive flaring tool and filed the end smooth. My first sustainer booster section, I left the conduit long and the recovery harness got wrapped around it somehow and crimped it over. Reaching into a 3.9" tube with a cutoff wheel on a flexible Demel tool was not fun. On my second one, I cut the end so it was just below the motor tube and flared the end to stop from getting a bend in the ignition wires.
If anyone is wondering, it has been too cold the past couple of days for epoxy. I am concerned that low temperatures, especially overnight, would disrupt the full cure. The last fins are probably going to get installed on Saturday, when it will be reasonably warm again.
I can’t tell from your pictures whether you are planning internal fillets or not.
I find it way too difficult to do internal fillets with a 38mm motor mount and 54mm airframe. The space between the motor mount tube and airframe tube is really narrow, and there would be added difficulty with the sustainer, since the motor mount is a couple of inches up the body tube.
I agree that trying to do internal fillets by leaving the aft centering ring off is rather impossible with a Junior. I generally inject the internals. Perhaps it’s overkill for a Junior, but I just saw a rocket fail a certification flight when a fin came loose. It had nice external fillets but no internals. You need to build for your field, but at the hard fields I fly on, I wouldn’t trust not having internals.
I find it way too difficult to do internal fillets with a 38mm motor mount and 54mm airframe. The space between the motor mount tube and airframe tube is really narrow, and there would be added difficulty with the sustainer, since the motor mount is a couple of inches up the body tube.
Nice build Antareas and thanks for sharing. I would like to second the Jmhepworth comment on injected internal fillets. I just finished a Dark Star Jr 2-stage build and that's what I did for internal fillets. Worked great even for such a small space to work with. I just had to pay special attention to clean up anything that oozed out past the aft CR on the sustainer so as not to interfere with the interstage coupler fitting.
PSA for anyone who wants to try this on their rockets.

Rustoleum bright coat metallic silver paint looks great, providing the most chrome-like finish for the best price that I'm aware of. However, what I have found when using it is that it MUST NOT be applied on top of itself. No second coats, no touch-ups. If you try to touch up bright coat silver metallic, you will end up with an orange peel texture that looks all the more unsightly for how shiny it is.

If you find a spot where you still see your primer after applying the silver and letting it dry, you have to weigh whether to live with it or sand the whole thing down and try again.

I did figure this out by experiment - I sanded down half a tube while leaving the silver on the other half. The sanded down part of the tube ended up smooth and shiny, while the half that was already silver got orange peel.

This does not appear to apply to the gold version, which seems to accept touch-up coats just fine.
I posted this in a new thread in the Techniques forum.

Long story short, I have to redo all the silver.