PML Piston ejection

[biziedizie]

Hi guys I bought my first PML and I was wondering what you thought of the piston ejection system. I bought the X-calibur and I was told that it's better to just use dog barf as it's more reliable. What do you think? I plan to run a G through I on this and I plan to have the rocket painted at a body shop so there's a few bucks going into this rocket and I would hate to loose it over an ejection issue.

Thanks!

Steve

 

[KenParker]

I have a PML AMRAAM 3 and a PML AMRAAM 4. Both have Quantum tubes and piston ejection. The AMRAAM 4 has two pistons, one for the drogue section and one for the main section.

You must be very careful to *thoroughly* clean the insides of the body tubes after each use. Since it is Quantum (plastic) tubing, this is relatively easy to do. You must also make sure that your pistons slide easily in the body tubes. You may have to sand them after you build them to get them to be loose enough to slide easily.

Following these simple guidelines, I haven't had a single problem or failure.

 

[Hospital_Rocket]

One thing to keep in mind with the Quantum Tube / Piston design. Figure out how cold it will be in the worst possible conditions you would be launching under. Simulate those conditions and sand the piston to work at that temperature. The reason is that the QT has a a different coefficient of expansion than the phenolic piston. The result is that if you build the rocket to work at 70 degrees and try to launch it in freezing weather, odds are the piston will bind. Trust me, sitting in 20 degree weather sanding a piston really bites.

 

[biziedizie]

Thanks guys! That was kinda what I was thinking! I like the idea of the piston ejection and I thought it would be cool to see it in action so I will try it this way.
Let's say that I changed my mind in winter conditions would the dog barf still work?
I would hate to see a shock cork burn!


Steve

 

[Hospital_Rocket]

The assembly instructions don't allow for removing the piston. So get ready to modify. I'd suggest taking the strap that holds the piston to the MMT and cutting it about 10" back from the piston. Then fold the ends over to create loops. A sewing machine and nylon thread would work well here. And then insert a quick link to join the two ends for piston use. For wadding you can attach the recovery shock cord to the piston strap in the same manner.

 

[biziedizie]

OK that sounds like it will work great! Now for something really dumb that I just did! I glued the MMT in and forgot to glue the strap in first! What should I do now? Can this be over come without any problems when I fly?

Steve

 

[Donaldsrockets]

With LOC kits, they have you glue the end of the shock cord to the inner airframe wall so in your case I think that may work also but make sure to throughly scuff up the quantum tubing to get a good bite and use a good blob of exoxy also.

As for the piston function, I have three PML kits with the pistons and on a total of 6 flights, I have never had a deployment problem.

Hope this helps.

Good luck

 

[omgb]

I have a Small Endevour that I've flown several times. The piston fit is key and it's difficult to judge what is too loose. If I had it to do over, I'd leave it out.

 

[Hospital_Rocket]

Steve

Did you glue the MMT to the rings w/o the strap or did you glue the whole mess into the airframe?

If it's the former, try this (No, I have not done it myself so this is just theoretical). Use a small drill and punch a series pf holes along the forward CR that sort of lay along the MMT. Basically recreate the slot in the original forward CR.

If it's the latter - ouch!

I scritched my head on this one and then came up with this.

Create a small extension to the MMT that looks somethung like a spool rocket and is just long enought to create the strap anchor like detailed in the directions. then glue the whole assembly in rigt in front of the MMT.

I don't think the LOC/Estes style mount will work as it would be tricky to get enough of a glue surface far enough down to ensure it would not get torn out and give you enough room to suff in the parachute and the piston.

A

 

[biziedizie]

Hospital_Rocket, yes I glued everything in already so I guess I'm kinda screwed! lol!
What I'm thinking at the moment is to forget about the piston ejection and just use the piston tube to mount the strap to and glue it just above the MMT. Does this sound like a good idea?
I plan to fly this as a single body tube and also as the complete thing. We checked it on Rocsim and it looks like it will work.

Thanks guys for the answers and the advice!

Steve

 

[omgb]

You could try this, make another centering ring making sure that the center hole is about 25% larger than the MMT. Slot it off to the side. Epoxy the strap through this slot. Now slather the underside of the ring with epoxy as well as the sides of the tube where the ring will rest. Insert the ring and push it into place. When it has cured, creat an epoxy fillet around the circumfrence. This should be more than secure enough and still permit you to use the piston. I haven't done this myself but it seems resonable and basically is an extension of the intended design so I can't see any big problems with this method.

 

[midpwrguy]

Biz. When I dont have room for a I-bolt. I'll use a coulper to epoxy the shock cord to. This way I dont need a I-bolt in the middle of my payload bulkhead.Witch gives me more room for the
terminals. For my deployment connections. So I think it would work just fine for what your trying to do.
At the time the piston was a good Idea. But now with nomex.
I feel that it's just one more thing that can go wrong. And there is times it will.

 

[midpwrguy]

This will give you an Idea of the room Or how little I had to work with.

 

[midpwrguy]

This is what it looks like.

 

[solrules]

Originally posted by Hospital_Rocket
One thing to keep in mind with the Quantum Tube / Piston design. Figure out how cold it will be in the worst possible conditions you would be launching under. Simulate those conditions and sand the piston to work at that temperature. The reason is that the QT has a a different coefficient of expansion than the phenolic piston. The result is that if you build the rocket to work at 70 degrees and try to launch it in freezing weather, odds are the piston will bind. Trust me, sitting in 20 degree weather sanding a piston really bites.

Ahh.....here's a piston story for you.

At Rockets for Schools, 2003 in Sheboygan WI, there was a major quantum tube/piston problem. The temperature was frigid, about 10-20 degrees, and all the level 1 kits were PML TFNC type piston ejection models. Turns out the day before the launch when everything was prepped indoors, all the pistons worked fine. Right when we went out to the launch pier, everything siezed. It was funny, because if you went into a heated car for 5 minutes, the pistons would work fine. We ended up having to cut out each piston on site (about 20).

In short, pistons are NOT good for anywhere there is a durastic temperature change between seasons. Good old dog puke is good enough for me.

 

[Todd Moore]

When building a QT airframe rocket, put the QT in a chest freezer before you do anything else. While you are letting it get nice and cold, sand the snot out of the piston. Get the tube out of the freezer, and put the piston inside of the tube. If the piston does not drop to the bottom of the tube under its own weight, then back into the freezer with it and keep on sanding.

Don't worry about making the piston 'too loose'. It's impossible to make it too loose. The piston is too long to cock, and it's not like making it loose is going to let all the hot gasses get around to your parachute. About the only danger is that you are making a brittle piece of phenolic a little thinner.

I have done several PML QT rockets like this, and I fly them in Michigan, where our flying season temps can range from -10 to +98, and the pistons always work.

If you don't like sanding, or don't like to do prepwork like this, just leave the piston out, and use a chute protector or dog barf. That works too.

 

[Batman]

Just an FYI regarding the post above. I read a review on EMRR that a guy actually had a shroud line get between the piston and the tube which jammed it and prevented deployment. If you over sand and create gaps like that, it could be a problem. Not very likely, but it could happen.

 

[EMRR]

The only time I have had a "piston" problem is when I didn't use a SU and shorted the BP for ejection charge. Use the full amount provided on RMS.

Other suggests are noted, about cleaning and sanding the black build up off.

Regards,
Nick

 

[shinbone]

I'm finishing a PML kit myself, and have been wondering if it makes any sense to run a few small screws through the tubing to keep the piston from nestling all the way down on the motor mount tube.
My thought is that the piston would only have, say, half as far to travel and there would be less likelihood of something going wrong.
Also, the weight of the piston itself as well as that of the chute would be shifted forward, enhancing stability.
Any thoughts?

 

[lalligood]

Originally posted by shinbone
I'm finishing a PML kit myself, and have been wondering if it makes any sense to run a few small screws through the tubing to keep the piston from nestling all the way down on the motor mount tube.
My thought is that the piston would only have, say, half as far to travel and there would be less likelihood of something going wrong.
Also, the weight of the piston itself as well as that of the chute would be shifted forward, enhancing stability.
Any thoughts?

I would say "don't bother". The piston doesn't go as far down into the tube as you might think--since the nylon strap on the back (that goes to the MMT tube) provides the support underneath. And even with the smallest PML kits, the piston & chute have such a small mass compared to the rest of the kit, it's really a non-issue.

I built my PML Phobos stock with the piston. Flown it a total of 5 times with motors ranging from a G35-4W to a Pro38 H153-10. Recovery has never been an issue. The *only* problem I've ever had with the kit was I had an usually short delay on a G35-4W (making it a motor problem really), causing the booster tube to collide with payload section after ejection. Shortened the booster tube & she was as good as new

 

[EMRR]

Ya could. I have had this same effect by using RailButtons on my PML kit. So there is no harm. I would even potentially improve CP/CG relationship.

Nick

 

[BlueNinja]

Yea, ditch the launch lug if you can and install a conformal rail lug on it with screws.

 

[havoc821]

If you didn't want to go the piston route, you could turn the piston assembly into a baffle by epoxying the bulkhead into the couple, drilling holes to vent the pressure (in the bulkhead that is) and put an eye bult in the center to attach parachute. With this method, you don't need wadding although I would put a little to be on the safe side. Hope this helps!