1/2 Scale PML Patriot

[AlexBruccoleri]

I recently bought one of the old PML 1/2 Scale Patriot kits with a fully glassed airframe and the CPR Max upgrade. I normally view rockets kits as a "bag of parts" and I customize the electronics, and airframe attachments etc. On this kit, I was thinking of following the instructions as close as possible for nostalgia, but I am worried about the pistons. Does anyone have any experience/advice for 7.5" PML pistons and their overall CPR Max design for the larger kit?

Cheers!

 

[jd2cylman]

I’ve got a unglassed version that’s been in my basement for 20 years. Maybe someday I should build it…

 

[cwbullet]

I have one also. Never built it.

 

[heada]

I built one stock including CPRMax and pistons. It works well and on the dozen or so flights I had, without issue. Last flight the apogee event fired at ~3000 ft but no separation. Came in on a flat spin and landed in a tilled corn field. The fin can was trashed but the rest was fine. Couldn't determine why there was no separation and I decided to part out that rocket into a few other projects I had rather than repair it. Overall the pistons held up well and worked. Had to clean the BP residue out after each flight or it'd feel gritty and not slide well. The edges were starting to chip some but thats normal with PML phenolic. It's an area you'd want to watch but that's about it. I know the pistons and QT tube can have issues with temp cycling but I only launched at the same site and only in the summer so no issues there.

Since I was flying at the AMA in Muncie IN(national RC headquarters), there are RC planes flying at the same time near us. The RC guys kept over flying our launch site and we kept asking them to stay in their area since we had to shutdown each time they did it. Club president and LCO knows I'm prepping the 1/2 scale Patriot on an AMW K600WW. He asks if he can do a little something. I ask if it'll be good and he says "yeah, I think so". I put it out on the pad and make everything ready. After 10 minutes, here comes a pair of scale P51 planes over flying our launch. He waits until they turn to return to their site and calls a quick countdown on the patriot. It roars into the air and after burnout we can hear the 2 pilots yell from 1/2 mile away "holy sh**! What was that!?!" 10 minutes later they come over to talk. The convo goes something like "You know what would happen to your plane if it hit our rockets?" And they said "Yeah, they'd be totally destroyed." Few seconds later the RC guys ask "What would happen to your rockets?" And the reply was "not a da** thing" They stopped over flying our site after that.

 

[AlexBruccoleri]

I built one stock including CPRMax and pistons. It works well and on the dozen or so flights I had, without issue. Last flight the apogee event fired at ~3000 ft but no separation. Came in on a flat spin and landed in a tilled corn field. The fin can was trashed but the rest was fine. Couldn't determine why there was no separation and I decided to part out that rocket into a few other projects I had rather than repair it. Overall the pistons held up well and worked. Had to clean the BP residue out after each flight or it'd feel gritty and not slide well. The edges were starting to chip some but thats normal with PML phenolic. It's an area you'd want to watch but that's about it. I know the pistons and QT tube can have issues with temp cycling but I only launched at the same site and only in the summer so no issues there.

Since I was flying at the AMA in Muncie IN(national RC headquarters), there are RC planes flying at the same time near us. The RC guys kept over flying our launch site and we kept asking them to stay in their area since we had to shutdown each time they did it. Club president and LCO knows I'm prepping the 1/2 scale Patriot on an AMW K600WW. He asks if he can do a little something. I ask if it'll be good and he says "yeah, I think so". I put it out on the pad and make everything ready. After 10 minutes, here comes a pair of scale P51 planes over flying our launch. He waits until they turn to return to their site and calls a quick countdown on the patriot. It roars into the air and after burnout we can hear the 2 pilots yell from 1/2 mile away "holy sh**! What was that!?!" 10 minutes later they come over to talk. The convo goes something like "You know what would happen to your plane if it hit our rockets?" And they said "Yeah, they'd be totally destroyed." Few seconds later the RC guys ask "What would happen to your rockets?" And the reply was "not a da** thing" They stopped over flying our site after that.

Thanks for the detailed reply and neat story. Did you invert the piston as some on this forum have mentioned?

 

[heada]

No, I built them stock. I've built several PML kits with the pistons and each time it was stock. They were all on fiberglassed phenolic except my first one which was QT. It seemed to me that the QT based one was much more sensitive to BP residue than the phenolic. I always went for a tight fit so that I could lift the rocket by the piston and it would slowly slide out at just a few inches per second.

Good build process and keeping them clean worked very well for me.

 

[AlexBruccoleri]

No, I built them stock. I've built several PML kits with the pistons and each time it was stock. They were all on fiberglassed phenolic except my first one which was QT. It seemed to me that the QT based one was much more sensitive to BP residue than the phenolic. I always went for a tight fit so that I could lift the rocket by the piston and it would slowly slide out at just a few inches per second.

Good build process and keeping them clean worked very well for me.

Okay excellent. One follow-up question. Did you protect or replace the nylon straps with anything more flame retardant?

 

[heada]

Again, pure stock. The nylon sees a little signs of heat exposure but in my experience something else fails long before the nylon. Plus the nylon has some stretch to it whereas kevlar doesn't and the pistons need that stretch to help absorb the ejection force

 

[AlexBruccoleri]

Again, pure stock. The nylon sees a little signs of heat exposure but in my experience something else fails long before the nylon. Plus the nylon has some stretch to it whereas kevlar doesn't and the pistons need that stretch to help absorb the ejection force

Okay understood. I am thinking of going with kevlar-wrapped nylon, but good to know that is not likely necessary.

 

[heada]

If you're going to do kevlar sleeve over the nylon, make sure the kevlar is longer than the nylon. I normally go for 10% longer or so.

 

[UhClem]

There is/was a fully built one available for adoption from DARS. I tried to talk myself into it but it just wasn't built to my preferences (I prefer an anti-zipper setup.) so I passed.

View attachment 498020

As for the nylon strap, it helps if you put a little wadding/dog barf just in front of the motor. In the motor tube if it is long enough.

 

[heada]

The kit I had was 75mm MMT so motor ejection wasn't a concern and anything outside the pistons never saw any flame/heat.I'm trying to imagine a way to use dog barf inside the pistons to help protect the nylon and I'm coming up blank. Only place I could see would be in the ejection wells of the CPRMax but that'd just help to contain the BP and do nothing to protect the nylon straps.

 

[AlexBruccoleri]

The kit I had was 75mm MMT so motor ejection wasn't a concern and anything outside the pistons never saw any flame/heat.I'm trying to imagine a way to use dog barf inside the pistons to help protect the nylon and I'm coming up blank. Only place I could see would be in the ejection wells of the CPRMax but that'd just help to contain the BP and do nothing to protect the nylon straps.

One thing to do is cover the ejection wells with a nomex or kevlar cloth, and have a kevlar cord attached to the cloth to keep it from falling away.

 

[jimzcatz]

To answer the original question, don't use the piston. That's a throwback from PML which I always discarded

 

[AlexBruccoleri]

To answer the original question, don't use the piston. That's a throwback from PML which I always discarded

Why?

 

[boomtube-mk2]

You might be a "Rocket geek" if you have been stopped multiple times by law enforcement for having a 1/2 scale Patriot rocket strapped to the top of your Ford Fiesta.
And you might be a "Rocket geek" if the motor for that 1/2 scale Patriot rocket has more horsepower than the Ford Fiesta it is strapped to the top of.
But you are absolutely a "Rocket geek" if you actually know a mathematical formula for converting Newton Seconds into Horsepower.

 

[UhClem]

You are beyond geek, or have studied engineering, if you know that they aren't comparable. One is a unit of work and the other isn't.

 

[Bat-mite]

You are beyond geek, or have studied engineering, if you know that they aren't comparable. One is a unit of work and the other isn't.

Horsepower would be more equivalent to Newtons than Newton-seconds, am I correct?

 

[heada]

The PML piston system when paired with their CPR setup works well. The pistons protect the parachutes and since they contain the BP gases, they require less BP to give a successful deployment. On a kit without pistons, there are easier/cheaper methods to provide protected deployment like NOMEX blankets but if they're included in the kit, then I would build them as instructed.

When I have heard of people having issues with the piston system, it is due to temp cycling on QT based systems (too loose or too tight since the last season), not cleaning them properly or using too much BP. When used in a fiberglassed phenolic setup, with CPR doing the deployment events and the correct amount of BP (ground test!!) then they work very well and as expected.

 

[OZRoc]

To answer the original question, don't use the piston. That's a throwback from PML which I always discarded

I started out MPR with PML pistons and have never looked back. All builds since then have incorporated pistons where warranted. Am now in the process of an L3 7.5" half scale Aerobee Hi 12.5' long. Both deployments will be piston. Ground tests so far with CO2 have been favourable.

As @heada just stated there are problems with QT and temperature (+ humidity). I generally tended to seal the pistons with CA to factor out one problem.
Cheers

 

[UhClem]

Horsepower would be more equivalent to Newtons than Newton-seconds, am I correct?

You can get from horsepower to N-m/s.

As for pistons, I like them but not with that horrible Quantum tube. The difference in thermal expansion means that in cold weather the piston will seize up.

 

[AlexBruccoleri]

The PML piston system when paired with their CPR setup works well. The pistons protect the parachutes and since they contain the BP gases, they require less BP to give a successful deployment. On a kit without pistons, there are easier/cheaper methods to provide protected deployment like NOMEX blankets but if they're included in the kit, then I would build them as instructed.

When I have heard of people having issues with the piston system, it is due to temp cycling on QT based systems (too loose or too tight since the last season), not cleaning them properly or using too much BP. When used in a fiberglassed phenolic setup, with CPR doing the deployment events and the correct amount of BP (ground test!!) then they work very well and as expected.

Another follow-up. What are your thoughts on just using Kevlar cord in the Piston instead of Nylon? Do you think the elasticity of Nylon is important to connect the piston to the main body?

 

[mustcomedown]

After 10 minutes, here comes a pair of scale P51 planes over flying our launch. He waits until they turn to return to their site and calls a quick countdown on the patriot. It roars into the air and after burnout we can hear the 2 pilots yell from 1/2 mile away "holy sh**! What was that!?!"

So the P51s violated the no-fly zone and were engaged by a Patriot battery? Nice.

 

[heada]

Another follow-up. What are your thoughts on just using Kevlar cord in the Piston instead of Nylon? Do you think the elasticity of Nylon is important to connect the piston to the main body?

In most cases I'd say you can swap kevlar for the nylon without concern. For the pistons in PML kits, the nylon on the deployment side is fairly short, like only a few feet longer than the length of the BT its in. You could swap for kevlar but you'll have to increase the length from say 6ft to 20ft to make sure the pistons don't snap back into the BT.