Rear ejection question

Ok, here's a scratch build using a 6" tube from my daughter's lava lamp packaging and a plastic lamp shade from another lamp. The fins are very light plywood; one of the key goals is to build something deceptively light.

Because the fins are relatively fragile and the nose will need some weight to keep the rocket stable, I decided to experiment with a form of rear ejection. The motor tube in this photo is 38mm, although I plan to step it down to 29mm and fly G & H motors. The other moon-shaped tube goes all the way through, adjacent to the motor tube the whole way. The plan is/was to glue a bulkhead somewhat above (11", to be exact) the forward end of this motor mount contraption. Then I would stuff a chute tightly in the moon-shaped compartment. Upon ejection, the pressure would push the chute out the back end. Then the rocket would land on its nose, protecting the fins.

I'm worried that inertia will cause the chute to "eject" at liftoff. I've considered this from the beginning, but as the build nears completion those nagging doubts are growing. What do you think? If I stuff the chute in very tightly (and maybe very tight dog barf below it), do you think it will be okay? If not, can you think of any method of retaining the chute during liftoff in a way that can be broken at ejection?

I have another idea that would use conventional ejection and run a line down the side of the rocket (and closing off the moon shaped tube), but that's aesthetically displeasing and breaks the experimental spirit I started with.

Any opinions/ideas?

Geof

The problem; one side of the rocket will weight more.

Thanks for the comment. I doubt it will be much of a problem. A 24" chute doesn't weigh much at all, and in any case the large fins should keep the flight path pretty straight. For example, consider "airplane" style rockets with two wings and one tail fin. Or a glider with a pop pod.

It's a useful thought, however, and it suggests that I should install the motor apparatus in an alignment where the chute is maximally opposite two fins (and therefore aligned with the 3rd). That would give the most "corrective" force to counterbalance the weight imbalance.

I've got several--well three ----rockets that use rear ejection. Two of them are boost gliders but the principals are the same. I eject the entire mmt with the chute attached to it. A long shock cord attached to the inside of the rocket is easy enough to do. When designing it you have to think about two really important things---first, you want to avoid flame damage ,so think about it as an entire system not just a part. Second , you want to avoid binding on the way out. The best way I've found is to put a bt between two bulkheads. this configuration avoids it getting cocked side ways on the way out. It needs to slide freely. Viewed from front to back it would be thrust ring/bulkhead-- bulkhead--bt---bulkhead---chute---bulkhead. The shock cord is mounted to the front of the assembly and attached to the inside of the nosecone. The rear bulkhead should fit snug enough to not fall out. One way to mitigate flame damage is to have the mmt slip over a smaller interior tube--sort of a reverse plunger idea

I did something like that once. My 'chute tube was really small and a kind of U-joint was used to avoid pressurizing the rocket interior. Worked only once of 3-4 flights. Yours might be better than that but still, I wish I had known a better way. It may help to wrap everything in a big wadding or blanket to form a unified piston. But has to be tight too, acceleration is 5 gravities or more. Also, fins don't really "guide" against inbalance, although it's not a sensitive as one might think.

With the whole mount ejection method, for some reason I've ended up thinking an internal bulkhead was needed, but if the nose cone is well-attached and the internal volume is not unusually large, shouldn't be needed. Especially avoid making the volume very small because the pressure will be ridiculous.

How many g's do you think I could make by swinging/shaking it around in my hands? An obvious strategy would be to do a test now: cram in a chute real tightly, then try to shake it out any way I could. I wonder how that compares to the forces at liftoff.

I just googled it and a high-g roller coaster gives 3.5-6.5 g's. I'm sure I can cram the chute in firmly enough that it wouldn't come out during a roller coaster ride.

My physics teacher was the basketball coach, and that was 30 years ago, so maybe someone can help me with the physics. My gut instinct is telling me that the rocket liftoff is different than a roller coaster because the rocket accelerates from 0 to 100mph extremely quickly, whereas a roller coaster changes velocities more slowly. However, my brain is saying that "g" is a unit of acceleration and the time-to-velocity is factored into the number. If the chute stays in on a 5-g roller coaster turn then it will stay in on a 5-g liftoff. Who's right, my gut or my brain?

If you plug the back of the chute-tube with a balsa tube connector and use a reasonable level of friction to hold it in place.
I don’t foresee any G-force the rocket is likely to encounter being able to dislodge it.

Can you add a bulkhead at the rear with a couple of shear pins?

Yeah, I've been thinking about that. Or maybe a paper wrapper over the tube end.

Have the entire mmt eject with the 'chute between the centering rings. Everything tied together or separate 'chute/streamers.

I built this a while back - two nice flights and recoveries so far.

https://www.rocketryforum.com/showthread.php?t=69837

I did something similar with a K550 in a Fatboy upscale. I used dual deployment and the rear ejection was the drogue. I built a cap for the aft end of the parachute tube with a coupler and coupler bulkhead and retained it with two polystyrene shear pins. That worked fine. You should tape over the shear pins with a metalized tape so any heat reflected by the blast deflector at liftoff won't melt them.

geof said:

My gut instinct is telling me that the rocket liftoff is different than a roller coaster because the rocket accelerates from 0 to 100mph extremely quickly, whereas a roller coaster changes velocities more slowly. However, my brain is saying that "g" is a unit of acceleration and the time-to-velocity is factored into the number. If the chute stays in on a 5-g roller coaster turn then it will stay in on a 5-g liftoff. Who's right, my gut or my brain?

Click to expand...

You're brain. A gee is a gee.

The shear pin idea sounds best to me, though I've never done a configuration like this. You'll still want to ground test it to make sure that the motor's ejection charge is adequate to break the pins. That gets into the force-pressure-volume-charge calculations that are typically only needed with dual deployment (since that's typically the only place you'd be using shear pins.) And I don't know if the size of the ejection charge is published. All of which is why you definitely need to ground test.

I like the basic idea here; I may have to steal it and try my own hand at the notion.

Here's another idea, which is way out of the box, and probably off the rails. In my crude model below, say that my two bits of rolled up copier paper are your two tubes, the Wendy's drive through napkin is your parachute, and my phone charging and data cable is actually a piece of cotton string or nylon fishing line. (What do you want from me during lunch hour with only the stuff on my desk?) The 'chute is held in place because it is draped over the string, until the ejection charge burns (cotton) or melts (nylon) through the string, releasing it. Cockamamie? Yes. Viable? Maybe.

mikemech said:

Have the entire mmt eject with the 'chute between the centering rings. Everything tied together or separate 'chute/streamers.

Click to expand...

This would be very much like a Fatboy version of the Estes Sizzler/Talon (2127) kit. Or a variation on the motor mount from the old Estes Space Shuttle (1467). With the Space Shuttle variation, you possibly could get the added advantage of getting the weight out of the rocket.

https://plans.rocketshoppe.com/estes/est2127/est2127.pdf

https://www.estesrockets.com/media/instructions/001467_SPACE_SHUTTLE.pdf