PML Piston Info

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OC_Rocket_Man

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Can anyone guide me to a thread(s) or outside page(s) or YouTube video(s) that has something in-depth to the PML piston system?

I've searched within TRF, YouTube, Google etc but I not finding anything with any meat.

For instance, I can't even tell if the piston leaves the stuffer tube during the initial ejection charge (obviously it will once the main canopy hits line stretch/inflation).

Thanks!
 
Can anyone guide me to a thread(s) or outside page(s) or YouTube video(s) that has something in-depth to the PML piston system?

I've searched within TRF, YouTube, Google etc but I not finding anything with any meat.

For instance, I can't even tell if the piston leaves the stuffer tube during the initial ejection charge (obviously it will once the main canopy hits line stretch/inflation).

Thanks!

I’ve used them on a lot of rockets. The section of cord attaching the piston to the motor mount is plenty long to exit the body tube. The piston is PML phenolic with a plywood bulkplate. The instructions for their piston system are online. Here’s a link to FAQs that includes the pistons.
https://www.publicmissiles.com/PMLRecoveryComponentsFAQ.pdf

Here’re the piston instructions:
https://publicmissiles.com/images/PistonInstructionsSheet.pdf
 
I’ve used them on a lot of rockets. The section of cord attaching the piston to the motor mount is plenty long to exit the body tube. The piston is PML phenolic with a plywood bulkplate. The instructions for their piston system are online. Here’s a link to FAQs that includes the pistons.
https://www.publicmissiles.com/PMLRecoveryComponentsFAQ.pdf

Here’re the piston instructions:
https://publicmissiles.com/images/PistonInstructionsSheet.pdf

So when the ejection charge detonates, does it actually eject the whole assembly? Or does the piston assembly actually get pulled out by the laundry lines?

I'm ordering a modified PML kit tonight. Just trying to edumacate myself.

Once I have the kit in hand I'll start a build thread.
 
It ejects the piston from the airframe, once the piston clears the end the ejection gases are released. A properly sized charge won't cause the piston to slam into the end of its strap too hard.
 
So when the ejection charge detonates, does it actually eject the whole assembly? Or does the piston assembly actually get pulled out by the laundry lines?

I'm ordering a modified PML kit tonight. Just trying to edumacate myself.

Once I have the kit in hand I'll start a build thread.

As long as the charge is sized correctly and the piston and body tube are clean and sized correctly the piston exits the body tube easily. I test mine by blowing into the motor mount tube and the piston pops out like a champagne cork.
The guide that comes with the kit will tell you how much black powder to use. It was always accurate for my rockets. Pistons use much less than rockets without pistons and protect the parachute very well.
 
Pistons can work very well. There are several issues though that I just pointed out in another thread:

* if you are building with quantum tube it contracts a lot in cold weather which can bind the piston if not accounted for

* the inside of the body tube needs to be cleaned after each flight to allow the piston to move freely, the bp residue can cause it to bind (Steve alluded to this by saying the body tube must be clean)

* the PML instructions on how to build the piston form a very weak system to retain the d-ring. I learned to modify mine to make a loop in the tubular nylon that did not require one end to be epoxied to the piston bulkhead

* one of the d-rings that PML ships failed for me during a hard deployment, I would use a quick link or some other method instead of the d-ring

As Steve says you do need less bp than a traditional system and they do protect the chute very well. They seem to be more reliable in that you don't need to 'burrito' the chute with a Nomex blanket and if the piston ejects, the chute will always eject. I never had one fail to eject the chute.


Tony
 
When I did my L3, I used a piston. It was a close-tolerance fit to the air frame in a 4" rocket. In my initial ground test, the piston didn't come out of the tube, although the cone and the chute ejected just fine. Further, when I looked into the tube, the piston had hardly moved. Hmmm. So, I repeated the ground test with exactly the same result.

Being a chemical engineer, I decided to determine how much gas was actually being produced, and to cross check the results against some of the black powder charge size estimators. As it turned out, the charge I was using (maybe 1.5 or 2 grams - it's been a while), was enough to move the piston a foot or so when the gases were hot. Then, as the gases cooled, the vacuum sucked the piston back to within an inch or so of the starting position. All consistent with the calculated results.

Since PML pistons are looser than the piston I was using, it's likely that it won't get sucked backwards, but it is not a given or even necessary that it get blown out of the tube. Now,
I put a little vent at the base of the piston so that at high altitude, the piston doesn't move upward as the air behind it expands.

Jim
 
Lots of great info...thanks!

I've read the kit instructions, the PML FAQs, various TRF threads etc. I don't have the kit in hand, so I'm having to go off imagination for some of it. Like in my mind I could see the ejection charge being enough to get the nose cone and laundry out but not actually ejecting the piston itself. As Jim said this can be the case in some piston systems.

One thing about my kit I noticed is the addition of a stuffer tube. I will have a 4" QT body tube, but a 3" interior phenolic stuffer tube that goes to the motor tube and acts as the piston sleeve. I not sure if this was done to eliminate the binding issues experienced when the QT was the sleeve, or to decrease the volume of gas expansion needed, or both. Obviously it does both, but I wonder if eliminating the binding was the primary goal? One thing it does do is make it easier for me to use rail buttons versus guides if I desire.
 
One thing about my kit I noticed is the addition of a stuffer tube. I will have a 4" QT body tube, but a 3" interior phenolic stuffer tube that goes to the motor tube and acts as the piston sleeve.
Interesting -- did it come this way from PML or was this a third-party change? What kit was it?
 
Lots of great info...thanks!

I've read the kit instructions, the PML FAQs, various TRF threads etc. I don't have the kit in hand, so I'm having to go off imagination for some of it. Like in my mind I could see the ejection charge being enough to get the nose cone and laundry out but not actually ejecting the piston itself. As Jim said this can be the case in some piston systems.

One thing about my kit I noticed is the addition of a stuffer tube. I will have a 4" QT body tube, but a 3" interior phenolic stuffer tube that goes to the motor tube and acts as the piston sleeve. I not sure if this was done to eliminate the binding issues experienced when the QT was the sleeve, or to decrease the volume of gas expansion needed, or both. Obviously it does both, but I wonder if eliminating the binding was the primary goal? One thing it does do is make it easier for me to use rail buttons versus guides if I desire.

I haven’t seen a PML kit with the stuffer tube so I cannot say how well that works, but a phenolic piston inside of a phenolic stuffer tube should respond better to temperature changes.
As far as completely ejecting the piston, every PML kit I built did. In fact, if you use too much BP the piston snaps back and breaks the piston skirt.
 
Interesting -- did it come this way from PML or was this a third-party change? What kit was it?

It comes this way from PML. It's the 4" Striker. I'm on my phone so I can't post my .rkt file, but here's a link to Rocket Reviews with a parts breakdown and drawings. https://www.rocketreviews.com/rocksim-file-6567.html

I'm having my kit extended 8.5" for the BT and the stuffer tube to make something like a 1:1.75 (not a real scale rendition though by any means) of a South African Mokopa ATGM.
 
It comes this way from PML. It's the 4" Striker.
Oh, OK. I think they used the stuffer tube so they could extend the piston travel into the nose cone, since the rocket is pretty squat. All the other kits I'm aware of just have the piston inside the airframe with no stuffer tube.
 
Oh, OK. I think they used the stuffer tube so they could extend the piston travel into the nose cone, since the rocket is pretty squat. All the other kits I'm aware of just have the piston inside the airframe with no stuffer tube.

You're right about the squat rocket thing. I see the stuffer tube setup in the Striker, BumbleBee and Little Lunar Express. All 4" squat designs.
 
I have another question about the piston setup now that I have a bunch of this rocket assembled, and didn't feel it warranted a new thread.

My piston recovery tube is 26.5" in length. The motor tube seals into the aft end of the recovery tube maybe displacing 1", likely less. That would give the aft end of the piston 25.5" of travel to clear the tube. The other day the coupler that will eventually become the piston slid down to the bottom of the recovery tube...and it sure didn't want to slide back up freely. That got me thinking that I probably would NOT want the piston/laundry to get moved all the way aft during liftoff as 25.5" of travel for ejection seems excessive and that it can create complications.

How much piston travel is necessary to create the positive pressure I am looking for to eject the nose cone/deploy the laundry?

I feel like I need to mount a block of some sort that leaves plenty of room for the canopy/JLCR but otherwise limits the piston travel aft. Whether that's just a dab of JB Weld, or a 3"-2.6" centering ring that I dremel out the center further to make it 3"-2.85". But you know how feelings are. LOL.

I have attached the Rocksim file to illustrate my situation better than my words can describe.

View attachment MokopaRev2sand.rkt
 
The other day the coupler that will eventually become the piston slid down to the bottom of the recovery tube...and it sure didn't want to slide back up freely. That got me thinking that I probably would NOT want the piston/laundry to get moved all the way aft during liftoff...
The second part doesn't concern me, but if the piston's too tight it can jam. Sounds like you want to sand it down a little until it moves freely, and make sure that it moves freely before each flight.

Phenolic pistons in a QT airframe can jam up if it's cold, but I think your internal tube is also phenolic, so you won't have that problem.
 
The second part doesn't concern me, but if the piston's too tight it can jam. Sounds like you want to sand it down a little until it moves freely, and make sure that it moves freely before each flight.

Phenolic pistons in a QT airframe can jam up if it's cold, but I think your internal tube is also phenolic, so you won't have that problem.

Once I have the piston truely assembled I'll sand it for free travel. It actually traveled freely down the tube as is, just got stuck at the bottom and required my wife shoving her arm down the 3" tube with masking tape on the tip of her fingers (which was humorous actually as her arm got stuck).

But if there's no reason for a full 25" of travel I can't see a benefit of allowing it to do so. If nothing else, so the parachute and JLCR don't get scraped across the tube for that distance.
 
But if there's no reason for a full 25" of travel I can't see a benefit of allowing it to do so.
I've flown several pistons and I've never done anything to keep the piston from getting as close to the motor as it could, without any problems. If you pack the shock cord loosely below the chute, that will keep the chute fairly far forward. I'd be a little worried that anything you put in to limit the piston travel might get caught by the piston strap and either rip out or stop the piston from moving.
 
Once I have the piston truely assembled I'll sand it for free travel. It actually traveled freely down the tube as is, just got stuck at the bottom and required my wife shoving her arm down the 3" tube with masking tape on the tip of her fingers (which was humorous actually as her arm got stuck).

But if there's no reason for a full 25" of travel I can't see a benefit of allowing it to do so. If nothing else, so the parachute and JLCR don't get scraped across the tube for that distance.

You should be able to just blow into the motor mount and have the piston pop right out like a spit wad. A well fitting piston slides easily with the motor out and slides slowly with the motor in place.
As far as length of travel, having the piston start it’s travel with less volume will result in it having faster acceleration at first due to the smaller volume being pressurized. That’s part of the reason that pistons function so well with much less powder compared to airframes without pistons.
Raising the piston and increasing the initial volume will result in less jerk and less acceleration but may require more BP to work. I’ll be interested to hear your results.
 
I haven't used the piston tube like you are, but I've been flying a PML Callisto for 15 years and have never done anything to keep the piston from going as far down the tube as it can. I still only use 0.5 g of 4F black powder. That means I don't use all the supplied powder from 29/40-120 reloads, much less HPR reloads. You might want to use more like 0.75g but some ground testing would be in order.

I would think you would want the piston as deep in the tube as possible. That puts as much air above it as you can get which will compress to a higher pressure when the piston reaches the top of the tube then if it starts from the middle of the tube. That should give you a more positive ejection and push the nose cone off with a little more force.
 
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