Project "Shorty R" - less than 1 foot high BT-60 rear-eject

Here is one I've been tinkering with for a while now. Started as spare parts from my Magnum build. I have all the parts on hand except for a streamer.
What I would like to do is build a short rocket with the Vagabond fin set that is rear eject. After drawing it up in OR, I really like it.

The issue right now is that I have ZERO experience with rear-eject and I'm jumping into a custom design. Any kind of feedback would be greatly appreciated as I don't really know how well this is going to work.

Here is the file:
View attachment Shorty_R_v1.ork

Here is a snapshot of the design:


Looks cool to me!

Cabernut,
Would break some of the concepts down for ye, but I would never be able to do better than Mr Rocket himself:
https://www.apogeerockets.com/Advan...deo_82?zenid=c00c137afe794e8e45e87c7cae0c6974

Given that you have a short stubby rocket and will be packing the recovery material aftward, would make sure you have enough weight in the forward end so that the rocket is stable (ie: CG is more forward than the CP) and it sims to be stable as well.

I love his videos... Lots of good info on the Apogee site. I wish he went into a little bit more detail on that one.

With about 8-10g of nose weight, I can keep it around the 0.8 to 1.0 caliber range depending on engine which should be good for a short rocket based on what I've read.

I think I'll fly it once and see what breaks, build a 2nd one with improvements. Only thing I'm really worried about is the HDPE nose cone warping on ejection. HDPE has a low melting point of 266F where the polystyrene nose cones melting point is 464F. With this design, the ejection gases are going to fill up the nose cone while ejecting the rear module.

Perhaps kill two birds with one stone, coat the interior of the nose cone with 5-min epoxy, giving it a bit more weight as well as protecting it from heat...

I was going to do the usual front-eject at first but that left hardly any room between the motor and the recovery. After running a BP ejection force calculation, assuming my inputs into the excel file are correct and assuming an equivalent 0.3g BP ejection from an 18mm estes motor going off in ~3 cubic inches of volume, it should produce upwards of 300 psi at ejection. Sounds like trouble. The extra volume from the nose cone should help tremendously.

Once I launch it, less "should" and more "because". The proof of the pudding is in the eating.

I built a "prototype" to test out the design to see what breaks and therefore needs tweaking. Launched it today on an A8-3 which is barely enough according to the sims. What I found out was this: the forward CR split upon eject. There were burn marks on the aft-facing side which would lead me to believe the CR stuck a bit and came loose after/during eject?

Here is the launch video:
[youtube]mUip7G1Nxps[/youtube]

And some pics of the damage:


I'm curious as to how this happened. Any thoughts?

sorry to hear of the damage, but on the bright side it was stable.
Rex

As of now I'm thinking of doing three things:

1. add about an inch to the motor tube, where the cord will wrap around in order to keep it behind the hot ejection gases instead of in front.

2. omit the "CR" in front of the rear pod as the lip on the tailcone is sufficient to hold it, like a thrust ring.

3. beef up the forward CR as it gets the most stress.

question, what material did you use for the ring? think I would have used 1/16" birch ply if for no other reason than for wear resistance (from sliding in the tube.
Rex

Is the nose cone rear solid? The ejection pressure is bound to be high. I made a mistake on my 2nd rear ejection design in kind of assuming a small ejection volume is desirable. The desired range is just a little lower than conventional eject.

I've been having issues with light ply centering rings being up to the task of eject pods. Some CA soaked around the edges should help. Since my rocket is larger I'm adding some epoxy reinforcement with each repair too. I have managed to break birch ply ones but only during a CATO or crash.

The ring was made from 1/8" plywood, seems strange that it would split. Only thing I can think of is that there was a bit of epoxy residue that I had sanded down, which may have provided enough stickyness to grab the edge in a split second?

The nose cone was cut open to allow for more internal volume.

I only have some medium CA at the moment I may have to find some thin and try that as well.

Here is the interior, the balsa ring in the middle has been sanded down as it's not needed. That probably also provides less restriction for the gas to blow the pod out the back.


I have glued on a 1" section of 1" heavy tubing(from aluminum foil roll) to keep the shock cord away from ejection gases as well as less chance of tangle, I hope.


Here is the updated .ork file
View attachment Shorty_R_v4.ork

In the video, it seems to arc-over very quickly after burn out. Might try a B6-4 on the next flight to get it up higher and give the chute a chance to open.

After watching the frame-by frame of the ejection, here is a new theory as to what happened:

Upon ejection it takes 3 frames to move 4 feet, or ~ 1/10th of a second. It also appears the forward CR is intact until it reached the end of the cord and snaps back with such force as to knock back into one of the fins and take a chunk out. Since where it broke was exactly where the 220lb kevlar cord was attached, It appears more likely to be too much bang for 4 feet of travel with a sudden stop. As for the burn marks seen, I can barely see a "flame" still coming out of the top of the engine for at least 1 frame after the bounce-back, thereby leaving the black marks afterward. I could probably add another 4 feet of kevlar cord, it would fit in fine and provide some breathing room for the ejection.

TL;DR version: too short of a cord broke the CR, need some shock absorption or longer cord!

Currently it's a 5ft section of cord, but after install only 4 ft of travel. Any suggestions other than adding an extra 4ft?

Update! Fully successful launch today on a C6-5. I had doubled the kevlar cord length to about 7 ft in addition to the modifications in post #10. That was the ticket. Everything is intact and in good shape. Great stable launch with full deploy and soft landing.

Sorry no pics or video as my phone was in use at the time.

This rocket is now one of my favorites. Now that the bugs have been worked out. Here is the latest version that is verified to work, with real-world finished weight and CG.

View attachment Shorty_R_v6.ork

Neat!

Update: This little rocket has been performing flawlessly and is becoming one of my favorites.

Flies well on B6-2 and C6-3. I finally got a video of a successful flight.

[youtube]4lNjgSM3HV8[/youtube]

What a great rocket! Seems like it'd be hard to lose..those are my favorite haha