Testing Piston Launcher with Air Compressor.

[shockie]

I'm doing some ZVPL research and one of the most difficult problems is friction fitting the model rocket engine into the top of the piston tube.
You don't want it so tight bit at the same time you don't want it to loose either.

Down thru the years people have used various tapes on the aft end of the rocket engine, plastic collar clamps. And clamps with limit line lanyards with pull pins.

So to test various piston tube to engine friction fitting ideas I was thinking of attaching an air compressor to the aft end of the piston tube, and gradually raise the internal pressure unto the engine pops off.

I was thinking that if I use a model of the same weight and shape, the entire model would pop off and go x amount dustance .
Then I could do a comparsion of which friction fitting technique was optimal by the distance the model goes

Comments and suggestions are welcome

 

[Alan15578]

I'm doing some ZVPL research and one of the most difficult problems is friction fitting the model rocket engine into the top of the piston tube.
You don't want it so tight bit at the same time you don't want it to loose either.

Down thru the years people have used various tapes on the aft end of the rocket engine, plastic collar clamps. And clamps with limit line lanyards with pull pins.

So to test various piston tube to engine friction fitting ideas I was thinking of attaching an air compressor to the aft end of the piston tube, and gradually raise the internal pressure unto the engine pops off.

I was thinking that if I use a model of the same weight and shape, the entire model would pop off and go x amount dustance .
Then I could do a comparsion of which friction fitting technique was optimal by the distance the model goes

Comments and suggestions are welcome

Please post pictures and diagrams of the collar clamps, limit line lanyards, and pull pins.

 

[Sandy H.]

I'm doing some ZVPL research and one of the most difficult problems is friction fitting the model rocket engine into the top of the piston tube.
You don't want it so tight bit at the same time you don't want it to loose either.

Down thru the years people have used various tapes on the aft end of the rocket engine, plastic collar clamps. And clamps with limit line lanyards with pull pins.

So to test various piston tube to engine friction fitting ideas I was thinking of attaching an air compressor to the aft end of the piston tube, and gradually raise the internal pressure unto the engine pops off.

I was thinking that if I use a model of the same weight and shape, the entire model would pop off and go x amount dustance .
Then I could do a comparsion of which friction fitting technique was optimal by the distance the model goes

Comments and suggestions are welcome

I have never been a 'real' competition flier, but if I decided to do a club level competition, I would try to at least be kind-of competitive. I've done an LPR piston launcher once, probably used 3-5 times total that day. It was fun and I absolutely think it is worth trying, just for fun.

I like the idea of gathering data (I'm a data junkie) by doing ground tests and I think the concept of measuring the distance doesn't seem bad to me. I'm assuming it would be easier if using a constant pressure (i.e. play for a bit and decide 20psi is the right number, then run all tests at 20 psi) would make the data more easily reduced. I'm sure there are people here who could do some calculations about the amount and rate of gas generation for a BP motor and maybe they could tell you what the right pressure would be, but I'm not that guy for sure. I'm assuming that the temperature change due to the BP motor burning vs. just an air compressor might be negligable, as the time required to heat up the tape or something like that would be long vs. the time the rocket was still attached, but I could be wrong.

Anyway, when I did it, I pretty much just used blue tape to make the fit to the motor and I made it a tighter fit than a nosecone, but less than friction fitting a motor - much closer to a nosecone fit. I never iterated or tried different methods, but the performance increase was pretty obvious to me at the time, while not necessarily fleshed out. The reason the launcher was used more than twice was that I made 2 flights and then 3 other people asked if they could use it, so it appeared to be a good idea at least.

I think this would be a very fun project to do and hope you post lots of information as you go through it and I hope those with more experience than me comment and give suggestions.

Sandy.

 

[shockie]

Doing some additional research with past R&D reports, Trip Barber suggested 3 psi, the Alway Brothers used the Weiss/Vincent data and derived about 3.3 psi for an 18" bt-5 FHPL and 1/2A3 motors, and Chris Flanigan derived values of 2.5 psi for a 34" bt-5 and A3 and 3.5 psi for an A10, again for FHPL.

My study is going to look at ZVPL as FHPL have been banned from FAI competition.

So I'm going to have to get a air regulator the air compressor.

 

[shockie]

Please post pictures and diagrams of the collar clamps, limit line lanyards, and pull pins.

hi Alan Jones.

This plastic clamp has serrated teeth so you can press it together and creates quite a strong holding force. I used this at the Flyoffs at NARAM-52 in Cincinnati(??) on a FHPL and it worked very well. There was a noticeable POP when the engine released from the piston tube.

The next 2 pics shows a pull pin method. This is from back in the 70's

the 3rd pic shows Geoff Landis' original ZVPL from circa 73-74. It uses a static limit line/lanyard piece of cord. . This gave away to a piston ring stop in the aft end of the piston tube. I have devised a dynamic limit line that consists of a length of elastic paracord attached to kevlar line, that I hope will alleviate the sudden deceleration of the position tube, and potential loss of momentum. As instead of a hard stop, it is a softer, gradual, linear resistance, which should in theory, dampen and moderate the deceleration and loss of momentum at rocket piston sepration.

The next 3 pics shows Chris Flangan's experiment piston hold down clamps.

In the recent past Kevin Kuczek has used a pull-pin on his piston launcher, but in talking with him about it, he disclosed that it has issues and he's abandoned it for the time being. I only have 1 picture of his pull-pin piston clamp, and the resolution isn't good enough to determine how it is designed and works. I haven't asked Kevin for any additional info about it .

 

[shockie]

I have never been a 'real' competition flier, but if I decided to do a club level competition, I would try to at least be kind-of competitive. I've done an LPR piston launcher once, probably used 3-5 times total that day. It was fun and I absolutely think it is worth trying, just for fun.

I like the idea of gathering data (I'm a data junkie) by doing ground tests and I think the concept of measuring the distance doesn't seem bad to me. I'm assuming it would be easier if using a constant pressure (i.e. play for a bit and decide 20psi is the right number, then run all tests at 20 psi) would make the data more easily reduced. I'm sure there are people here who could do some calculations about the amount and rate of gas generation for a BP motor and maybe they could tell you what the right pressure would be, but I'm not that guy for sure. I'm assuming that the temperature change due to the BP motor burning vs. just an air compressor might be negligable, as the time required to heat up the tape or something like that would be long vs. the time the rocket was still attached, but I could be wrong.

Anyway, when I did it, I pretty much just used blue tape to make the fit to the motor and I made it a tighter fit than a nosecone, but less than friction fitting a motor - much closer to a nosecone fit. I never iterated or tried different methods, but the performance increase was pretty obvious to me at the time, while not necessarily fleshed out. The reason the launcher was used more than twice was that I made 2 flights and then 3 other people asked if they could use it, so it appeared to be a good idea at least.

I think this would be a very fun project to do and hope you post lots of information as you go through it and I hope those with more experience than me comment and give suggestions.

Sandy.

Sandy: the pressure is variable over time just like the thrust is variable over time. What I'm more interested in is, which type of friction device will pop off at what pressures. The time that the friction device stays attached at a particular pressure is as important as how much friction the device impartsto the engine.

you don't want so much friction or normal force in the case of gripping devices, that it loses momentum and reduces the separation acceleration and velocity.

So uou have what I will call a dwell time : the time the rocket/piston stays attached at max piston syrike

as fas as I know no one has every researched this. I'm looking at using mylar tape, versus masking tape(smooth versus rough), length of tape on the engine, 1/4",3/8 or 1/2" on the aft part of the engine. And I'm also looking at different grits of sandpaper strips attached via double sided tape around the aft end of the motor. The finer the grit the less friction there is.

I've also designed and 3d printed what I call a fly away clamp that is released by a pull pin at piston full stroke. It has a relatively smooth internal surface but is grips the engine very good. I almost can't pull the engine out of its grip. So that's a powerful normal force. It would be interesting to see this same device with a rough internal surface.

so I may end up with a combination of a gripping device with x amount of friction.

 

[Sandy H.]

Sandy: the pressure is variable over time just like the thrust is variable over time. What I'm more interested in is, which type of friction device will pop off at what pressures. The time that the friction device stays attached at a particular pressure is as important as how much friction the device impartsto the engine.

you don't want so much friction or normal force in the case of gripping devices, that it loses momentum and reduces the separation acceleration and velocity.

So uou have what I will call a dwell time : the time the rocket/piston stays attached at max piston syrike

as fas as I know no one has every researched this. I'm looking at using mylar tape, versus masking tape(smooth versus rough), length of tape on the engine, 1/4",3/8 or 1/2" on the aft part of the engine. And I'm also looking at different grits of sandpaper strips attached via double sided tape around the aft end of the motor. The finer the grit the less friction there is.

I've also designed and 3d printed what I call a fly away clamp that is released by a pull pin at piston full stroke. It has a relatively smooth internal surface but is grips the engine very good. I almost can't pull the engine out of its grip. So that's a powerful normal force. It would be interesting to see this same device with a rough internal surface.

so I may end up with a combination of a gripping device with x amount of friction.

Awesome! I look forward to seeing this develop!

Sandy.