Piston Ejection

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flying_silverad

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Is it practical to build smaller airframes (BT20-BT70) with piston ejection? Has anyone had any luck with it?
 

sandman

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My thoughts are that in the smaller sizes, say up to a BT-55 size, a piston ejection might just be impractical and just more complicated than it needs to be.

For BT-60 on up, sure! Try it.

With the smaller sized the KISS principal seems more appropriate.

sandman
 

mtmind

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I'm with sandman, the little rockets do just fine with wadding and adding a piston means moving parts. Wadding does a great job in small rockets because the wads fill the space nicely. On larger rockets, though, ejection gasses can blow around the wadding since the wadding doesn't as effectively fill the space. This can happen around chute protectors too if they're not evenly packed into the tube. When the tube gets big enough that the wadding is not effective enough it can pay to put in a piston system since they fit the size tube they're made for every time. Personally, I think wadding works OK up to Bt-60 but larger than that a piston may be nice to keep the chute from getting scorched.

Peace :cool:

Mike
 

KenParker

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I agree with sandman on this one... except for the part of trying to use a piston system on what I perceive to be paper based body tube airframes (BT60 and up). For smaller rockets, I prefer dog barf or flameproof nomex shields.

The rockets that I have built that have piston ejection systems all have PML Quantum (aka... plastic) tubing for the main airframe. The pistons are made out of phenolic components, which slide very well inside the Quantum tubing.

In my experience, putting a piston system inside an airframe made out of paper or phenolic tubing.... I think that would be prone to jamming. Piston systems cause the airframe body tubing to take a lot of punishment from the ejection charges, and the body tubes get very dirty very quickly. With the plastic Quantum tubing, the inside of the body tube can be cleaned very effectively and returned to a very slick condition. I think the paper based tubes would quickly get pitted and scored by the ejection charges, and wouldn't clean up very well. That would cause them to get gritty and wouldn't let the piston slide freely. JMHO.
 

Micromeister

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Flying:
I agree with all that's been said, however there are other alternatives to estes wadding, Dog barf insulation and even Nomex ejection shields.
First for MMX to Bt-50 standard size bodytube models, particularly hi performance P/D, S/D and all flexwing glider booster models a 1/2" or 1" long Styrofoam Plug can be used in place of the wadding. I make all my plugs by using a 3" piece of the size tube i want the plug to fit, soaking it in CA. then sharpening one edge like a gasket cutter. find a dowel that fits inside the cutter tube, cut a piece about 6 inches long, place the dowel & cutter combination on a piece of any type styrofoam. Pressing down lightly and turn the cutter tube through the foam. push the plug out with the dowel. by leaving the dowel in the cutter while turning you won't cursh the cutter tube. when it cutter dulls resharpen just like you would any other gassket cutter.

permanent teflon ball wadding. These easy to make wadding balls stay with the model. for Bt-20, 50 and 60 size models I use 15 to 30 - 12" strips of 1/2" or 3/4" plumbers teflon theard sealing tape. layed in a circular pattern over a 12" piece of 70lb kevlar line. after all the strips are layed out, tie the kevlar in a square knot. baby powder the heck out of both sides of the tape on a cookie sheet or something. pick the ball up, shake out the baby powder. attach the ball or balls to the models shock cord just outside the body tube. to prep for flight. shake the ball out a little the stuff into the body, followed by the shock cord, chute and nosecone. I'm using this system in my Super Vega with a custom nylon 24" chute, as well as several other smaller models like the Hurcules 2 stage sustainer, and Warp-II sustainer.
For BT-80 models I've used 3 of these 30 piece balls tied together at the same point on the shock line. Worked just fine.
 

Juerg

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Pistons work down to 18mm diameter and are the best protection you can have.
However pistons require a hard surface to slide on. Paper tubes don't have this hard surface, but flushing them out with thinned epoxy helps. The epoxy soaks into the paper and converts it into a kind of phenolic when hardening.
If you don't do this, the paper of spiral wound tubes will come loose sooner or later and block the piston.

Juerg
 

teflonrocketry1

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Originally posted by Micromister
permanent teflon ball wadding. These easy to make wadding balls stay with the model. for Bt-20, 50 and 60 size models I use 15 to 30 - 12" strips of 1/2" or 3/4" plumbers teflon theard sealing tape. layed in a circular pattern over a 12" piece of 70lb kevlar line. after all the strips are layed out, tie the kevlar in a square knot. baby powder the heck out of both sides of the tape on a cookie sheet or something. pick the ball up, shake out the baby powder. attach the ball or balls to the models shock cord just outside the body tube. to prep for flight. shake the ball out a little the stuff into the body, followed by the shock cord, chute and nosecone. I'm using this system in my Super Vega with a custom nylon 24" chute, as well as several other smaller models like the Hurcules 2 stage sustainer, and Warp-II sustainer.
For BT-80 models I've used 3 of these 30 piece balls tied together at the same point on the shock line. Worked just fine.
I wonder where he got that idea?

Bruce S. Levison, NAR #69055, A.K.A. Teflon Rocketry
Besides Teflon ball wadding, I have also made streamers and parachutes from this material which holds up to the ejection charge!
 

Micromeister

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Originally posted by teflonrocketry1
I wonder where he got that idea?

Bruce S. Levison, NAR #69055, A.K.A. Teflon Rocketry
Besides Teflon ball wadding, I have also made streamers and parachutes from this material which holds up to the ejection charge!

Bruce:
Not to steel your thunder but I've been using Teflon plumbers tape (Ball) wadding since 1983 or 84, The first rocket I remember flying one in was an Estes Nomad BT-50/20 airframe I think. Worked so well, my son and I have used the method in sport models ever since. Talking them up to everyone and anyone we met. They are a welcome remedy for the Estes wadding blues, they are my own idea.

Teflon streamer's are great for Micro-maxx models, Which I've been doing since they (Micro-Maxx) were introduced in 1999.

Your supplier for the heavier (thicker) expanded PTFE (Teflon) 2" wide material from McMaster-Carr is doing a great job on some of the larger LPR models, thanks for the tip.

Haven't tried your PTFE Chutes yet but, I'm researching the suppliers you give me. These chutes are a very cool idea, and absolutely totally yours:)
 

lalligood

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Originally posted by bfraley
Here's a thought on pistons.

https://www.deltavrocketry.com/piston.htm

It's written about an HPR recovery system but the installation orientation is applicable to smaller rockets as well...
bfraley,
Excellent experiment! Thanks for sharing. I do have one question though:

Did you happen to test with the open end of the piston facing the ejection charge BUT with additional weight near the aft end to (re)compensate for the balance of the piston?

Your diagrams show that it requires the bulkhead to be on the aft end but it seems possible that you took a tiny 'leap of faith' to draw your conclusion that, to quote your site, "Heavy side towards the charge", doesn't it? I have a PML Phobos with the piston built per PML's instruction but I could easily attach small weights near the aft end... It'd be an easy modification to make ;)

Thank you for your input on this!
 

Juerg

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Nice article.
However I tend to disagree with the theory of piston orientation.
It actually is just the other way round!
If you are using an inverted piston (bulkhead down) you may run in all sorts of problems because of the rather weak skirt end facing forward:
- parachute fabric may be caught between piston and airframe
- the weak forward end will be much more prone to binding on slight imperfections of the tube than the "bulkhead-reinforced" end

The much stiffer bulkhead end will greatly reduce these problems.

Also the pistion is not in an environment where you will see anything like
the corrective forces drawn in the article. Deviations of the piston are in fractions of degrees maximum (remember, there is a good fit!) so the effect of charge pressure on deviation can be neglected.
Only friction forces are important.
If the piston starts to bind, this allways will be on a forward edge of the piston. This point then gets the turning point, with lots of forces acting.
The reinforcing bulkead helps a lot to withstand these forces without deviation.
If you invert the piston, pressure forces will still act on the bulkhead, way behind the (potentially binding) forward edhe of the piston. If the piston "turns" in the tube and one edge starts binding, pressure force will increase binding even more. This is like bushing something comapred to pulling. If you are pushing you have to keep correcting the track while pulling something in general does mean it is just trailing your path.
Bottom line: Pistons should have the bulkhead forward!

(drawings will follow, hard to explain in words)
 

bfraley

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This always seems to be a difficult concept for most people to grasp as the inverted orientation is counter intuitive.

I won't re-explain the article, but I will point out the example that caused me to start exploring pistons. I was ground testing my ejection charges with the piston pictured at the start of the article and I had several jams. I cleaned, lubricated, adjusted the fit etc. and I still managed to get it to jam. One jam was severe enough that the connection points of the parachute bay to the altimeter bay bulkhead broke the bulkhead!

As I was trying to figure out what was causing the problem I dropped the piston through the parachute bay tube while the tube was standing on the ground it slid down the tube, but rattled all the way down. I repeated this and got the same results. Then I turned it around and it slipped right through without significant contact with the wall of the tube.

After doing the experiements described in the article, I redid my ejection testing multiple times and had success every time with no jams. The rocket has flown successfully with the piston 7 times with no jams or issues with the piston.

I will also point out that inspite of some initial reservations, a number of people in my club have starting using this method and there have been no piston related failures since.

This is also why bullets are heavier at the aft end. Once in flight a bullet will be stabilized by gyroscopic effects, but when it is in the barrel it can wobble even with the sides in contact with the rifling.

I don't think it will be productive to re-articulate the article in a message board. All I can say is, the info is there. Try it and see what your results are.

Brittain
 

bobkrech

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It's counter-intuitive (at first) to think that having the bulkhead nearer the ejection charge is the better method, but when you analyze the forces and mechanisms of jamming, it makes sense.

In addition to bfraley's applied force explanation and his videos, when the bulkhead is positioned nearest the ejection charge, the interior of the piston is not pressurized. When the bulkhead is towards the ejection charge, the hot ejection charges has to pass between the piston wall and the BT. While the tubes may exand due to the heat, the higher pressure will tend to expand the BT and collapse the piston, further reducing the probability of piston jamming.

Conversely if the piston is configured with the bulkhead near the NC, internal pressurization of the piston by the hot ejection gases will cause both mechanical and thermal expansion of the piston diameter. If the initial fit is tight, this is another potential jamming mechanism.

If properly packed, the piston can be used as a sabot for the parachute similar to the plastic wadding/shot holder in a shotgun shell.

Bob Krech
 

aksarben10

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Originally posted by flying_silverad
Is it practical to build smaller airframes (BT20-BT70) with piston ejection? Has anyone had any luck with it?
I have used, with great luck, foam plugs in 18mm and 24mm tubes. I guess they could be considered pistons but I had always call them plugs. Anyway the are just a foam plug cut from pink or blue foam. You can punch them out with a brass tube or the like. then just push them down into the tube with the parachute or streamer on top no other wadding needed.

Scott
 

Juerg

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I still stick to my statement but I'd like to point your attention to the potential source of differences: I am not talking about a piston that has space to wobble or for airflow going past it. Now if the piston fits nicely, there is no wobbling of the piston, no force balance moving around etc.
And let's face it, on top of the piston there is a parachute, this parachute will void all stability considerations anyway because it is a disturbing mass of unknown properties sitting right on top of the piston and influencing it heavily.

I am also not talking about pistons that can expand under pressure, my pistons are strong material pistons only.
I don't dispute the fact that under such conditions (loose/weak piston) a inverted piston is more "stable", but that's only the less important part of the story. Such designs will work for very small pistons only, anything bigger than 3" will fail if to loose or weak.

My concern is how the piston recovers from binding/friction and there the "conventional" piston is looking better.

It's not my intent to "pick" on that article but maybe to help even improve it.
Give me some time to put all of that together into a pdf that I'll post here.

Possibly, as often, there is no "universal truth" but one has to look at size and materials also...
 

powderburner

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I am no piston expert, I don't use them, I really don't even fly high-power (my number of HP launches can be counted on one hand).

Why wouldn't you want to just put a bulkhead at both ends of the piston? That way, both ends are reinforced, you could run a continuous piece of threaded rod right through the middle and tie on both ends.
 

Juerg

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Powderburner: That would be the best possible piston, however sometimes you are happy about the extra space inside of the piston.

Brittain: One mistake on your article in Fig 6c: The pressure inside the piston skirt actually would force the piston back to be straight on the left side (pressure acting in all directions, red arrows!)
What you have been observing on your test is flow phenomena around the piston.
 

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