PML Piston Ejection System

I'm currently working on a PML Hydra, this rocket has a piston ejection system. My question is, how reliable is this system and what are the pros and cons?
The reason I ask is that the nylon strap that holds the piston in is epoxied directly to the motor mount. How much flame can this take before it fails?
I guess my real question is, should I swap out this system for a traditional Kevlar cord?

My first step in building a PML rocket is to locate the piston parts and toss the bulkhead in my spare bulkhead box. The coupler chunk goes in the trash. I like KISS and having something that's temperature dependent & can stick with any residue is just another failure point

JMHO

One of the members of our club has a Tiny Pteradactyl with the piston setup and it has many flights on it. It seems like he is always doing slight adjustments due to the range of temps we can get, due the shrinking and expansion of Quantum Tube the rocket is made from.

Actually the PML piston system is quite reliable and effective. You hit on the one item of note, replace the motor strap with some Kevlar or find a way to replace the strap. It will fail after a while.

Some things that have made them more effective for me.

• Install them upside down. Seems to work better and provides a bit more packing space.
• If you know how, glass the inside of the piston, The phenolic is a bit brittle and will break over time. Or find a way to disconnect the piston (I do this). I found that having a removable piston allows me to clean the BT out.

And, of course, be aware the phenolic and Quantum tube expand differently. As such, when it gets cold out the piston may bind. I put the BT in the freezer and sand the piston to fit when the tube is as cold as possible.

You will find a range of opinions on the PML piston system ranging from outright hatred to everlasting love. I fall in the latter camp. The strap that comes with rocket model will work fine epoxied to the piston according to the PML directions. I have never had one wear out from repeated ejection charges. I have had a number of scratch designs that I have converted to the piston concept, so far up to 5" diameter. My 4" diameter Orbital Transport had a catastrophic crash on the first flight, because the ejection gases blew around the parachute/nomex blast deflector combination that failed to push the parachute fully clear of the body tube. I then went to the piston system and never had that particular failure mode again in that model (about the 10 next flights). Possibly this failure mode could be eliminated, if the nose cone had enough mass and inertia to pull the parachute out. You almost need to ground test to know if you have a nose cone heavy enough to pull the parachute out. Having become apprehensive of nomex blast deflectors, I ground tested my 5" diameter Gemini-Titan and found that the parachute hung up in the body tube. I went with a piston system and never had a problem on that model. Last summer I tried my 40 pound Atlas-Mercury on a piston system with a 5" diameter stuffer tube. It worked the first time. I generally check to see if the piston will slide easily beforehand. If it does not, I sand the outside of the piston and check for any residue on the piston tube.

I built a PML Callisto in 2003. I use the nylon strap and follow instructions. We fly in the winter when the crops aren't in so by 2005 I had the piston sanded down where it worked in any temp. Clean the BT of residue every 4 or 5 flights and that will help the most.

In 2014 I flew the Callisto on an exceptionally cold day and the BT broke on landing. I examined the nylon strap expecting to replace it when I added a coupler to repair the BT. Turns out it was in great shape. A little melted strands on the outside, but still plenty strong. I just fixed the BT and left the nylon strap, piston, etc in place. I've been flying it since and although I haven't kept track of the flights, a conservative estimate is 90 to 100 so far.

My opinion is to build the PML according to the instructions and don't worry about the nylon strap or the piston. Just make sure it slide loosely in the BT and don't OVER DO the ejection charge! On the Callisto I use the 0.7g in the Hobbyline motors but when I put AT HPR motors in it, I only use 0.7g of powder, not the whole amount supplied. Using the smaller ejection powder amounts may have contributed to the longevity of the rocket.

I have a 4" AMRAAM built stock with the piston and nylon strap. Its flown many times. I've never had to adjust the piston fit. Nylon strap is singed but perfectly fine. It's very easy to prep for flight. I didn't have to buy any Nomex for it nor do I have to transfer Nomex from another rocket.

For maintenance, after every few launches I wipe down the inside of the body tube with a baby wipe to get the residue out.

The only problem I've had is the piston snapping back. The body tube is unglassed phenolic, so every once in a while the piston snaps back and takes a chunk out of the tube. I just cut the damaged end off and reapply a yellow sticker stripe and I'm good to go again.

Thanks for all the feedback guys, I think I'll give the piston a shot. I can always remove it later if need be.

H_Rocket said:

Some things that have made them more effective for me.

Install them upside down. Seems to work better and provides a bit more packing space.

Click to expand...

Confirmed by experimentation:

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

Excerpt:

Bottom Line - Heavy side towards the charge!

To utilize this analysis to your advantage, you should make sure that your pistons have their heavy side towards the ejection charges. Most kits that are currently sold with pistons, show construction in the exact opposite manner.

I have a PML Amraam 2 that I built back in 2003 and I always fly it at least once at every launch - It has hundreds of flights on it. The key is to sand that piston down so it will slide with little effort at any temperature. I tend to reduce the amount of black powder on Aerotech reloads too. You just need enough powder to move that piston and push everything out...
If I built another stock PML kit, I would invert the piston and glass the inside of it. That phenolic WILL crack over time.

MALBAR 70 said:

I'm currently working on a PML Hydra, this rocket has a piston ejection system. My question is, how reliable is this system and what are the pros and cons?
The reason I ask is that the nylon strap that holds the piston in is epoxied directly to the motor mount. How much flame can this take before it fails?
I guess my real question is, should I swap out this system for a traditional Kevlar cord?

Click to expand...

I flew a Small Endeavour for my L1 way back when, and it had a piston. Flew great, and after all these years somehow it's still flyable... The piston has worked reliably every time and I have never seen a nylon strap fail in a PML kit.