Building PML's without pistons.

[sodmeister]

I've decided to build a 1/4 scale patriot and bull puppy with no pistons,on CTI 29 3gr.,
And 38 2gr.But instead use 10 or so ft. of kevlar from the mo.mnt.through a nomex chute
Protector to 15 or so ft.of the supplied shock cord.So i essentially would be running straight
Pipe.It's not that a piston isn't a good idea,i just don't like another moving part that may or may not fail,under any conditions!All comments would be appreciated.
Thanks,sod.

 

[Bill P]

I suggest don't limit your motor length, if at all possible. Today a two grain 38mm may sound like the biggest you intend, but tomorrow a three or four grain may sound even better.

 

[sodmeister]

I suggest don't limit your motor length, if at all possible. Today a two grain 38mm may sound like the biggest you intend, but tomorrow a three or four grain may sound even better.

Absolutely,i always have phenolic mo.mnts.in stock,which is another nice touch with CTI,the rear closure on the 29's act as a thrust ring and on the 38's they come built in with the motor.

 

[blackjack2564]

Pistons can be a great addition to a rocket,but I never personally cared for them. I have built several PML kits and even the AT Sumo and did not use any of the pistons. Simply felt that there would be less to go wrong. All rockets lived a long and uneventful life span with out the piston. Yours should too.

 

[Jeff]

I built a PML Tethys and instead of the piston I installed a U-bolt into the forward centering ring, and use a 20 foot cord with chute and cord protectors. Couldn't be simpler.

 

[JimJarvis50]

Just to weigh in on the other side, I believe that pistons increase the reliability of deployment. It is important for the piston to fit well and for the piston and tube to be clean. It is also important that there not be cords, rubber bands or the like to get jammed up in the piston. I was using rubber bands on the harness once to help control deployment, and I noticed that they got jammed in the piston. That's the only problem in nearly a hundred flights that I've ever had.

The main disadvantage that I have seen for pistons is that they take up a valuable few inches of room in the airframe. Sometimes, I just don't have the space. I also have rockets with breakwires where I can't use a piston. But where I can use one, I do. Less powder, less mess and better reliability.

Jim

 

[H_Rocket]

All rockets lived a long and uneventful life span with out the piston.

Don't believe this. No rocket around Jim has an uneventful life. In fact I have heard several of his rockets whimper as he walked up to them.

 

[DMcCauley]

Note the webbing connecting the motor mount to the bottom side of the piston (thicker webbing) is especially prone to heat damage from ejection charge (assuming motor ejection). You'll need to have nomex around this particular webbing or the webbing typically won't last too long. I have had several rockets fail in this fashion before i learned my lesson. PML states you don't need additional protection, however i don't agree with this statement.

 

[als57]

Note the webbing connecting the motor mount to the bottom side of the piston (thicker webbing) is especially prone to heat damage from ejection charge (assuming motor ejection). You'll need to have nomex around this particular webbing or the webbing typically won't last too long. I have had several rockets fail in this fashion before i learned my lesson. PML states you don't need additional protection, however i don't agree with this statement.

Replace the black nylon with a simular lenght of 1/2" tubular kevlar on the Patriot and all will be well. Mine's been flying for 6 years like that with no failures at all.

Al

 

[sodmeister]

Replace the black nylon with a simular lenght of 1/2" tubular kevlar on the Patriot and all will be well. Mine's been flying for 6 years like that with no failures at all.

Al

Absolutely,as i stated,i'd use 10' or so 1/2" kevlar,JB welded to mo.mnt.
1600 .lb tensile strength ought to hold!

 

[als57]

Absolutely,as i stated,i'd use 10' or so 1/2" kevlar,JB welded to mo.mnt.
1600 .lb tensile strength ought to hold!

Actually regular epoxy will work fine in this application.

The 1/2" tubular kevlar I've been using is rated about 4000 lb.

While I like long shock cords ; if your using the piston the Kevlar doesn't need to be much longer than the black strap (maybe a foot or so). If your not using the piston ; then enough kevlar to be a couple feet out the front of the booster is plenty.

Al

 

[titan]

I built a PML Tethys and instead of the piston I installed a U-bolt into the forward centering ring, and use a 20 foot cord with chute and cord protectors. Couldn't be simpler.

the pistons are very efficient but you can acheive the same results without them. my first tethys was with pistons and the second without which I converted to dual deploy and have flown it many times with flawless recovery. I have flown it on k700 to 8800 ft my favorite rocket.