J425 Minimum Diameter 38mm

[Flare]

So this will be a "built thread" rather than a build thread since this project is complete.

I got the idea to do a 38mm minimum diameter rocket back in the fall around September or October. I was preoccupied with getting my L2 among other things in the fall so didn't get around to anything more than OpenRocket and CAD until the start of the year when I began actual construction. I worked on it on and off until this month where it flew at the April 20th launch at URRG.

Starting off with the general overview:

The rocket is a fairly simple design. Nosecone, coupler, tube, with the avbay directly on top of the motor. The recovery is set up for single separation dual deploy. Airframe is fully fibreglass that I made myself, fins are wood with carbon laminated on top.

 

[Flare]

First up was the airframe. I did all the composites myself in my basement.

Body tube and nosecone:


The body tube used a steel chainlink fencepost from home depot as a mandrel. It's a perfect fit for 38mm airframe, 1.50" OD exactly. I use a layer of parchment paper spiral wrapped around the mandrel as a release layer. The body tube used 5 layers of 6oz plain weave glass fabric. The nosecone was done using the 3D printed mold with dual condom mold release method. I used three layers of light fibreglass sleeve from Soller composites for it. I then gave it a cast epoxy tip. The coupler (not pictured) was roll wrapped using the same fabric as the body tube but using a 3D printed mandrel to get the correct fit.

The fins were made by simply laying up two layers of 6oz plain weave carbon fibre on a sheet of 1/8 plywood. Then I cut the fins and beveled them by eye with a dremel. I tacked them using two 3D printed alingment jigs.

 

[Flare]

The avionics included an easymini and an eggfinder GPS, each with their own LiPo. The EasyMini gets a screw switch but the GPS just plugs in when I button the rocket up. The avbay has a slot through the middle for the shock cord to run through where it will be attached to the top of the DMS motor. More on that later. Here's a CAD screenshot:



Photo from launch day. EasyMini is on the other side.


I also included an RDF beacon taped to the shock cord. The beacon design is the same one that @Brainstormz123 sells here: https://www.rocketryforum.com/threads/custom-comspec-receiver-compatible-beacons.183577/ (If you are a flyer at URRG and want one of these beacons, I have several available in the 440hz/70cm band, NOT comspec compatible but usable with various amateur radio equipment. DM if interested.)

 

[Flare]

After filleting the fins with the legendary Temu epoxy, I set up the recovery configuration. This rocket was designed to use the single separation dual deploy method pioneered and well documented by the AeroPAC two stage 100k attempts and subsequently used by Kip's Mesos rocket. It has since gained a bit of popularity in the rocketry community on various sized rockets and I had some help from some other rocketeers, notably @flyguy614 who gave me a fantastic MS paint diagram of how he set his up. This method has both chutes in the same bay, but the main chute remains in the body tube with no tension on its line until ejected by its own charge.

On this rocket, the drogue is in the nose tip and the main is in the coupler directly above the avionics bay.



The recovery attachment on the nose was done by gluing the cord in with a small 3D printed frustrum piece to ease installation. The other end of the harness was fed through a hole drilled in the DMS forward charge well and potted in place with more epoxy.

The avbay then gets screwed in to the body tube right above the motor.


0.75g charges for both events were tested and worked exactly as intended.

 

[Flare]

Next up was painting, as launch day was approaching. First it got some sanding, filling, filler primer, and more filling and sanding. After that I was able to give it a decent paint job with a silver nose, white body, with some red accents. I like to always have a single vertical stripe on my rockets to help me align parts while assembling the rocket.



I also built a launch tower. It uses 1/2 electrical conduit tube and 3D printed end caps. Home depot ratchet straps connect at the top and framing nails are used as stakes. Here's the tower set up in my backyard as a test.



Next up is the launch.....

 

[robopup]

Great job! EDIT: And it's already flown, nice!

 

[Flare]

Launch day! I'll let the pictures do most of the talking here.

Set up in the tower:


Liftoff!


Took a bit of a right turn (downwind unfortunately) out of the tower but then continued straight up. This thing was FAST. It really zipped right out of sight like nothing else.



As we saw it fly off towards the east, quickly a couple of folks said things to the effect of "well, I hope you weren't expecting to get that back". I was unable to get any RDF pings, but upon checking the GPS I was indeed getting a radio signal with full GPS coordinates locked. The rocket had landed ~2.5 km east of the launch site in a neighbouring farm, luckily up on a hill. With some help from another club member we were able to drive to a road about 500m from the coordinates and walk to the landing site where the rocket was found with all chutes deployed and perfectly intact. The only casualty was the RDF beacon which had fallen off the shock cord leaving only its antenna still attached. I feel super lucky to have found this rocket considering how far it ended up from the launch site. The eggfinder GPS really came in clutch.



Maximum altitude from the EasyMini was 9,437 ft. The baro speed estimates are not that useful but by adjusting the OpenRocket sim to match time to apogee and apogee itself the maximum speed can be estimated to be approximately Mach 1.9. A new personal record for myself in both speed and altitude!

Overall this was a super fun rocket to work on. Got some fun ideas for future minimum diameter projects.

 

[FredA]

Nice rocket, but you need to work on your recovery.
Going 9500 feet up and landing over 8000 feet away says you have recovery problems.

Was this recovered outside of the waiver? Is that a good thing?

 

[cwbullet]

I like the design. Concur with FredA. Might want to try a Jolly Logic CR. I think it will fit.

 

[cbrarick]

stuff the main in the nose cone and fly it conventional dual deploy. Probably the most reliable measure. Not a fan of the chute release, In my experience as LCO for a fairly large club, those things are 50/50 propositions.

Nice flight, though! Do it again!

 

[kjhambrick]

Hmmm ...

Can you tell from the easyMini data whether your main deployed 'way up high' ?

Or did it work as expected ?

-- kjh

EDIT: That's a big, beautiful red flame !

But when I look closely at the rocket leaving the tower ( yeet.png ), I see something circular at an angle in the flame just at the tail.

Is there really something there or is it 'trick photography' ?

 

[David_Stack]

I'd be curious to see the GPS 'track' overlayed on Google maps or similar. Perhaps much of the 'distance from the pad' was actually amassed during the 'up' portion of the flight, since he did mention it took 'a bit of a turn' and headed downwind...

 

[dvdsnyd]

@Flare
Nice project! Would you have any more details about how you laid up your composites?
Dave

 

[Flare]

The distance it travelled along the ground is almost certainly due to the turn it took out of the tower (as well as the relatively high winds that day). It ended up going off at a decent angle. I unfortunately don't have a GPS track, only the final location. I'm going to have to do some investigating with my tower and see if I should maybe adjust the fit, I thought it was a nice sliding fit but I might need it to be more snug.

I would like to fly this again, maybe on a smaller 38mm motor, like the H550 just to give it another go. I'm pretty confident in this single break deploy scheme and wouldn't use a jolly logic, not a fan of those things especially for high performance. The way it's set up I couldn't modify it for regular dual bay deployment anyway.

@kjhambrick I have no idea what that dark circle is in the flame. I would assume some sort of camera trickery because nothing fell off the rocket. This is a still frame from a GoPro which has a bit of a fish eye effect that I edited so it doesn't seem so curved and it's also not particularly high res. I think it's probably just the back of the motor.

@dvdsnyd my composites are pretty standard. Wet layup, I use peel ply on the tubes but not the nosecone. The key when doing tubes is to keep tension on the fabric as you wrap. Doing nosecones with sleeves is one of the easiest things you will ever do with composites, too. Super simple and incredibly easy to get good results.

 

[Flare]

Also, for the record, the main chute did deploy at the correct altitude during descent.

 

[FredA]

High winds USUALLY result in a rocket weather-cocking into the wind off the pad and then rocket continues to fly into the wind.
This leaves the rocket upwind at apogee and allows the recovery to bring it back towards the pad.
So it was likely really far downrange at apogee.
Was it outside the waiver?

 

[cbrarick]

dark circle: i bet is whatever he used to hold his tower together at the top (and attach is straps to) and not a camera trick