PML Piston question

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AfterBurners

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I have the PML 3" Bull Pup with the piston ejection. Great kit BTW. Anyway just wanted to get some feed back on the fit of the pistion0 I know some of you guys coat the piston with thin CA and sand. Is this necessary or should I just the piston "as is" and sand it until it slides smoothly inside the air frame? I was wonder how the fit of piston changes as the humidity changes? I know there have been issues with card board tubing. but do those issues apply to QT under similar circumstances and weather conditions?
 
I've heard the best place to install a piston is right here-

158
 
I've heard the best place to install a piston is right here-

158

x2, first thing I remove.

Having said that I have seen a lot of people leave them in so this topic is likely one of those many split rocketry topics, and will come down to a personal preference. My logic for pulling mine is it is just one more thing to fail and has two failure points I have witnessed. The strap breaking and the piston getting stuck. I figure if the chute is placed at the correct place on the recovery harness and packed properly then it will get out into the air stream...at least in theory.

Besides if I left mine in then I would of not been able to cram the 38/720 case with my I600 and that would just be downright disappointing!
 
I have 2 PML kits with piston and never applied CA. Had one land in a canal and when dried it still worked fine. I'd leave it alone.
 
I spent maybe 15 minutes sanding the piston and fits nice. Moves freely without much resistance. Just not sure how freely it should slide? There's no side to side play like its sloppy or neither is it so tight it doesn't move.

I got the primer coats on the rocket and ready for the body work and sanding. So far so good.:)

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There's side to side play like its sloppy or neither is it so tight it doesn't move.

Yes. ;)

I've only had experience with it in my Sumo and the last time I launched my Bullpuppy. The Sumo was fine until one day the piston decided to stick in there and wouldn't budge, so I had to scrub it's launch that day. Once I got it home, I managed to free it (took quite a lot of effort). Then I proceeded to sand it a lot and fit it well (but not loose since that would probably mean I've taken too much material off)...I even used a marker to make line-up marks to decrease chances of it being stuck due to being off-round vs. the tube - Bullpuppy shouldn't have this though due to the plastic bt.

For my Bullpuppy, I used rail buttons and after installing them, realized I had forgotten about the piston. $#@!%! Luckily I didn't install the button too far up and also had a hollowed the nose (with allthread through it), so I flipped the piston over and managed to fit the chute 80% in the bt and 20% in the nose. Worked well the last time I launched it (announcer said it flew up straight and beautiful) and had good chute; however it landed about 40' in a tree and got scuffed up and lost a canard in the retrieval process. Note that any one trying to retrieve this out of a tree should be aware it can be dangerous coming down due to it's weight and sharp fins.
 
Ken - I meant to say there's "NO" side to side play. I'm terrible at making typos....

For paint I'm going with Duplicolor gloss white with a red tip nose cone to add some color and make it "pop"

I'll spray about 5 coats of paint and probably 7 coats of clear...of light coats and not laying anything down until the last few coats...wet sand and hand buff.

Just a note I noticed that the Polecat 4" Bull Pup the canard fins are actually slotted to fit inside the nose cone and as they mentioned on their site that this is how a "scaled" version should be...pretty interesting.
 
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I fly glass..
I like pistons,, I use them almost always. ..
The fit should be free,, not sloppy but as free as you can get without making any slop...
The tube the piston runs in must be cleaned after each flight...
A piston won't work smoothly in a tube with residue on it from an early charge...
The reason I carry both male and female eyes are to make pistons...

Teddy
 
I fly glass..
I like pistons,, I use them almost always. ..
The fit should be free,, not sloppy but as free as you can get without making any slop...
The tube the piston runs in must be cleaned after each flight...
A piston won't work smoothly in a tube with residue on it from an early charge...
The reason I carry both male and female eyes are to make pistons...

Teddy

"FREE" meaning it should slide with some help...but not so free that you can put it in and it will drop by itself to the bottom of the airframe?
 
One thing we see commonly with PML rockets and their pistons is that heat and cold cause a fair bit of expansion or contraction, so don't preload the recovery system till on the field and you have trial fitted the piston and make sure it moves smoothly. We had a couple of pistons stick last summer resulting in failed main chute deployments, both times it was very hot outside.
 
One thing we see commonly with PML rockets and their pistons is that heat and cold cause a fair bit of expansion or contraction, so don't preload the recovery system till on the field and you have trial fitted the piston and make sure it moves smoothly. We had a couple of pistons stick last summer resulting in failed main chute deployments, both times it was very hot outside.

Thank you for the good advice. How is the piston affected by cooler weather. After my last launch at Lucerne in June I'm over the summer launches for now
 
Thank you for the good advice. How is the piston affected by cooler weather. After my last launch at Lucerne in June I'm over the summer launches for now

Afterburners,
Seeing as how you're flying in So Cal, you should be OK. Quantum tube really doesn't like below freezing temps. It shrinks enough to really stick the piston. Sand it to fit for your coldest days of flying and clean the tube after each flight and you should be OK. My PML Tethys had a piston until I flew it at MWP in below freezing weather. Ended up splitting the airframe all the way down the side and broke completely off. So there was enough airframe to install a coupler and put a new bodytube on. I took this as an opportunity to ditch the piston and just use nomex. Good luck.

Adrian
 
The problem is that the coefficient of expansion for Quantum Tube is substantially different than for the phenolic piston. So when things get frosty the BT shrinks faster than the piston - hence gets stuck. My personal way around that is to lay the BT someplace cold. I use my basement entry when it is quite cold to shrink. I then sand the piston to fit then. Warming up is not a problem. I have also had luck with the freezer.

As Teddy alludes to I use a very short lead on my piston so I can disconnect it and clean the BT. I also use the piston upside down so the open end is up. It seems to give me more room to pack. Finally I glass the inside of the piston or coat it with epoxy as the phenolic is brittle and the skirt will chip away over time.

P.S. - I like pistons for recovery. But then I also like the CPR3K, so you might consider the source here. :wink:
 
I just built a PML Explorer and kind of wondered the same thing, but I tend to like to build kits completely per the manufacturer's guidelines first. I saw a some negative things said about them on the forum, but multiple club members here told me they had never had an issue with them, so I just went for it.
 
I just built a PML Explorer and kind of wondered the same thing, but I tend to like to build kits completely per the manufacturer's guidelines first. I saw a some negative things said about them on the forum, but multiple club members here told me they had never had an issue with them, so I just went for it.

I always build per instructions. I never used piston recovery and wanted to try it out. It doesn't get too cold here but when it does I'll re check the fit and sand if necessary.
 
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I've been flying a PML Callisto with a piston since 2003. We fly on farm fields so we fly from Oct. to Apr. when the crops aren't in. It is Virginia so it isn't real cold, but what was said about sanding the piston to work on the coldest day is correct. If it works in the cold, it will work in the heat too. The problem you will have is the Quantum tube get very brittle in cold. After 10 years, I finally flew it on an exceptionally cold day with the chute that came with it instead of a larger chute I usually use. I managed to snap the tube in half on landing.

Your chute size should be inversely proportional to the temperature when flying Quantum tubing in freezing temps.

As for the piston, never had a problem with that. I do take some BP out of most motors I fly with. I only use 0.5g of BP. The hobbyline 29/40-120 come with .7g and the HPR motors are usually 1.5g. In either case, the piston is so efficient that a minimum amount of charge is needed.
 
"FREE" meaning it should slide with some help...but not so free that you can put it in and it will drop by itself to the bottom of the airframe?

Perfect,,, exactly...
If it were "free" enough to fall through on it's own,, there would be a little slop...
Any slop and they'll be a little blow by,, you'll burn your expensive chute...
After you burn your expensive chute you'll resort to a bit of barf over the piston...
After you resort to barf over the piston,, on a flight the barf will get caught between the piston and airframe wall and wedge the piston....
Please don't ask me how I know this,,, lol...
No slop....

Teddy
 
The plastic Quantum tubing is poor HPR material choice these days - weak, brittle, and thermal expansion/contraction. The upside is that it paints nicely. I had a PML Ariel, and nearly every flight had an issue with the body tube. I like cardboard and FG. The canvas phenolic from MAC looks nice, too.
 
I found this web page to be helpful.

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

As mentioned several times before it is important to keep the tube and piston very clean.

I am sorry I forgot to mention this..
This is 100% correct..
I had all of my pistons with the bulkhead up..
My buddy Steve M showed me a different web page explaining this..
I turned all of my pistons over so that the bulkhead is down,,
now the pistons CG is toward the energy exerted on it..
They seem to work better,, more consistently....

Al's right too,,
I use a relatively short piece of Kevlar ( 1/3 of the entire harness )
below the piston and 2/3 of Nylon above the piston......

Teddy
 
It will work fine. Heck it was designed that way. The only challenge you might see is cleaning the body out as the stock PML motor strap is a bit short to get it out of the way when you want to swab out the tube.
 
It will work fine. Heck it was designed that way. The only challenge you might see is cleaning the body out as the stock PML motor strap is a bit short to get it out of the way when you want to swab out the tube.

I agree completely. I have 2 PML kits with pistons. Never had to sand the fit nor have I reversed the piston. Why would PML instruct to build it that way if it was going to fail or not worthy of functioning? Only thing I did to mine was put some "thinned" epoxy on face of piston to protect from ejection charge and easy clean up.
 
I flew pistons in all of my rockets for years before I found out the correct orientation of the piston...
It will work just fine...
I think the video I saw was on that site mentioned here in this thread....
It really is interesting to see...
No need to turn that around if you don't want to though...
It'll still work...

Teddy
 
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