PML Ejection Piston

[exprditer789]

I am looking at some PML kits and I see ejection piston.So my question is what is a ejection piston.Alex

 

[JAL3]

I have just one PML kit and it flew Yesterday. The build thread and flight report are here: https://www.rocketryforum.com/showthread.php?t=451

As to the piston, it is basically a coupler tube with a bulhead that substitutes for wadding or a Nomex sheet and pushes the chute out.

It has worked for me but I have to sand it after each flight to make it fit the BT again. I was glad to try it once but would probably not elect to go this way in the future. I would just buy the kit and substitute and Nomex shield.

 

[als57]

The piston discription given above is correct.

Two things to note:

1) You need to sand the coupler down enough that it is very loose. If you have to sand it after each use then you didn't reduce the diamater enough to begin with.

2) The black nylon strap glued to the engine mount will burn thru eventually. Replace it with 1/2" tubular kevlar.

Lots of folks build their PML kits without the piston. The choice is yours.


Al

 

[GregGleason]

Another consideration regarding pistons.

Greg

 

[rockie]

As said its a question of personal preference.

I would probably never had any problems with the piston in my PML rocket but i left mine out to eliminate one source of potential problem and used a nomex sheild instead.

There is a saying, "if somthing can go wrong, it will"

 

[Gillard]

I have a couple of PML kits that are built with the pistons and have never had a problem and really like the pistons - its a personal choice thing.

 

[degreaser]

T
2) The black nylon strap glued to the engine mount will burn thru eventually. Replace it with 1/2" tubular kevlar.

I have a PML Ariel that I'm beginning to build. What's a good source for tubular kevlar?


Thanks!

~Don

 

[bobkrech]

Please read GlenGleason's reference on pistons. It provides actual proof that most pistons are installed backwards.

You should reverse the piston opposite to the PML instructions so the the parachute goes in the piston cup, and the bulkhead faces the ejection charge. Once this change is performed, you should not will not have any deployment problems in a phenolic or fiberboard airframe.

However you will have a sticking problem with a Quantum Tube airframe unless you have large clearances around the piston, or don't launch in cool or cold weather. Quantum tube has a very high coefficient of thermal expansion compared to plywood or conventional airframe. A piston that slides well in the summer will jam in the winter.

See https://www.rocketmaterials.org/research/temp/ and https://www.rocketmaterials.org/research/quantum_tube/ for details.

Doc's website has real data, so you don't have to speculate why your piston sticks in cold weather. The CTE of Quantum Tube is 100 ppm per degree F or 0.0001" per inch diameter per degree Fahrenheit. In simple terms this is at least a factor of 10 times fiberglass or phenolic.

Bob

 

[aerostadt]

I had 2 models that used the nomex blast shield. On one of my 4" Dia. models with a 10' chute the ejection charge blew around the nomex shield and did not blow the chute all the way out. Thereafter, I converted both the 4" Dia. model and a relatively short 5" Diam. with a 10' chute from the nomex shield to pistons. The piston models worked excellent last summer.

Bob

 

[hardinlw]

Working with a TARC team trying to use a streamer for the payload recovery and a parachute for the booster recovery provided an interesting insight. I suggested ground testing the ejection system to make sure both recovery devices would pop out without tangling. To that end, I provided ejection charges that could be placed into a dummy engine so we did not have to waste an engine for each test. The ejection charge was sized to match an Aerotech F52 charge. I actually loaded about 2/3 of the charge supplied with the F52 to make sure there was some margin. We ran several tests and could not get the parachute wrapped in a nomex heat sheild to come out. The nose cone popped off and pulled the streamer out, but nothing they did could encourage the parachute to come out. Apparently the momentum of the nose cone was pulling the streamer out and the ejection charge was just blowing by the parachute without blowing it out.

This caused me to recall that the instructions with my Aerotech Astrobee D which had one chute for the nose section and another for the booster called for packing the chute for the bottom section on top of the chute for the top section. Apparently they had run into situations where the bottom chute did not pop out unless situated so that the nose section being blown off pulled it out.

With that in mind, we tried tying a fine line from the heat shield to the nose cone to see if it would pull the chute out as well. It twisted sideways and jammed. Ultimately, I showed them a PML piston setup and they built one and installed it. Now both chute and streamer are ejected with great authority. Now if we could just fold a streamer so that it slows the payload section down to the desired descent velocity.

 

[bobkrech]

Larry

The AT Mirage is another rocket that is designed to recover in two parts, each with its own chute. Unless you pack the payload chute in the airframe before you pack into the booster chute, the booster chute will not get pushed out of the airframe and the booster will core sample. After this happened to me, I joined both halves with a 30' length of 1/2 elastic cord and never had another recovery problem.

You can't do this quick fix for the TARC problem, so there's a real advantage using a piston cup instead of a Nomex chute protector. You can put anything in piston cup and it will get pushed clear of the rocket when the ejection charge fires.

Bob

 

[hardinlw]

Larry

You can't do this quick fix for the TARC problem, so there's a real advantage using a piston cup instead of a Nomex chute protector. You can put anything in piston cup and it will get pushed clear of the rocket when the ejection charge fires.

Bob

Amen to that. In fact, the first time they flew a piston the parachute did not open because the force of the ejection charge blew the piston out so hard that it yanked the 1/8" tubular kevlar cord between the motor mount and the piston right out of the 1/8" light ply piston bulkhead, sending the piston up the shock cord and parachute shroud lines to reef the chute. No damage on landing and replacing the 1/8" light ply with 1/4" aircraft plywood has eliminated that particular failure mode.

I had always assumed the ejection charge was blowing the parachute out and the nose cone was only going along for the ride. It appears that it is really the opposite that is happening, the nose cone is getting blown off and its inertial yanks out the chute.

 

[Handeman]

I had always assumed the ejection charge was blowing the parachute out and the nose cone was only going along for the ride. It appears that it is really the opposite that is happening, the nose cone is getting blown off and its inertial yanks out the chute.

That's exactly correct. I've had several flights with a 4" Thug and Hobbyline motors where the charge pushed the nose cone off, but not hard enough to stretch out the shock cord. The chute never left the BT. Makes for some hard landings. I also had one flight where the rocket never arched over, it stopped in a vertical position, blew the nose cone, chute deployed and it landed with all the dog barf still in the BT.

 

[als57]

I have a PML Ariel that I'm beginning to build. What's a good source for tubular kevlar?


Thanks!

~Don

Performance Hobbies has 3/8 and 1/2 tubular kevlar. I've also seen it on Wildmans site.

Al

 

[degreaser]

Performance Hobbies has 3/8 and 1/2 tubular kevlar. I've also seen it on Wildmans site.

Al

Thanks for the response. I found some over at https://www.commonwealth.net/rocketstore/rocketryframes.html as well.