PML Piston question.

[Shade]

I am starting to build my first PML Rocket, AMRAAM - 3.
My question is how snug or loose should the piston fit in
the Quantum tube. It is slightly snug now, that seems
odd to me I would figure you would want a looser fit.

What do you all say?

 

[cwbullet]

Pistons fit snug enough for no gas to get by but still move.

 

[als57]

A bit loser is recomended. Quantum tube shrinks/expands noticably with temperature changes.

Al

 

[DAllen]

Some people love them. Some hate them. I am with the latter. Leave out the piston and buy a chute protector or d-bag which will have the same effect and a lot less hassle. I fly in the winter here in Michigan and pistons can be a royal pain when it gets cold out.

Another thing I don't like is how much do you sand off the piston? How loose is good/bad? I donno. As a result, I cut mine off the cord and tied off the lower cord to the upper and flew without.

-Dave

 

[Crazyrocket]

I put my QT in the freezer and once it was cold, I put in the piston. I kept sanding the piston until it would fall through the tube by its own weight (slowly). I want to make sure that it would be good in the winter. I ultimately removed the piston on my AMRAAM 3 and just use nomex.

 

[hardinlw]

The piston should fall through the tube under its own weight. QT shrinks more than the piston when it gets cold, so you should check on the field. Sand the piston, not the inside of the QT. You also have to keep cleaning out the airframe tube to keep the piston moving freely. All this is extra work, but it does protect the parachute wonderfully. Almost all of my high power rockets have pistons and none of the pistons have jammed. I have, however, had a few unkind thoughts when having to clean out airframes or sand down pistions on the field. The sanding part almost always happens in sub-freezing conditions and those thoughts are particularly unkind.

 

[Shade]

Thanks for all the input!

I have a good idea on where to go with it now, it definitely needs some
sanding. Before I purchased the rocket, I had read all the opinions on
pistons. I have never flown a pistion ejection rocket before even in LPR. So
I will try it with this one. I can always cut it off later.

I am planning on installing PML HAMR system for motor retention. I figure if I
like I can make my own on my lathe later. I figure I will see their design first.

 

[Shade]

Some people love them. Some hate them. I am with the latter.
-Dave

Dave I think I am starting to fall into the latter group also... :dark:

... I have, however, had a few unkind thoughts when having to clean out airframes or sand down pistions on the field. The sanding part almost always happens in sub-freezing conditions and those thoughts are particularly unkind.

Well I was verbalizing those unkind thoughts tonight sanding down that piston
and after six years in Uncle Sam's Canoe Club I can cuss with the best of them.
I did finally dig the 1" belt sander out of the shop attic and finished up with it,
less cussing. I pretty much took off the entire outer layer of kraft paper from the
tubing.

 

[phil]

I sand mine down so they'll slip through under their own weight. This can leave the piston wall a bit thin and brittle so I reinforce the inside with with an offcut of CT, or several layers of a suitable BT.

Phil

 

[cwbullet]

This is about what I do also - moves on own weight.

 

[Pantherjon]

I like the piston..I put mine in my Matrix and it has worked fine..However, if given the chance to do again I would put the piston in upside down so it is like a cup..This will give you more room in the tube to pack your shock cord and parachute..Just remember to attach averything before you launch it!

 

[hardinlw]

One further trick I've learned is to make sure the piston cannot bind on a blob of glue on the inside of the airframe. Normally the strap securing the piston to the motor mount occupies enough volume that the piston cannot squeeze down and bind on the fillet of epoxy forward of the last centering ring; however, on larger diameter rockets, this is a possibility. I simply cut a 1" length of coupler tube and push it down on top of the forward centering ring before the glue dries. Obviously, this must be done before building up the piston on the end of the strap. Now the piston has a nice hard stop and can't possibly bind for that reason.

 

[Shade]

One further trick I've learned is to make sure the piston cannot bind on a blob of glue on the inside of the airframe. Normally the strap securing the piston to the motor mount occupies enough volume that the piston cannot squeeze down and bind on the fillet of epoxy forward of the last centering ring; however, on larger diameter rockets, this is a possibility. I simply cut a 1" length of coupler tube and push it down on top of the forward centering ring before the glue dries. Obviously, this must be done before building up the piston on the end of the strap. Now the piston has a nice hard stop and can't possibly bind for that reason.

That's a great idea, since I am waiting on my Motor retention system to show up I have not installed the MMT yet.

Thanks for the idea I will use it.

 

[KDRaven]

I have the PML Callisto in the QT and I built it with the piston.
Mine is the 38mm version MMT with the H.A.M.R. system and I made up adapter rings to be used when I fly 29mm motors.
The first thing I did was not to use the nylon strap to be used with the piston but replaced it with a Kevlar strap. I did use the included strap for the shock cord.
I also trimed off 12" of body tube to shorten the body down to the length of the IO rocket and used this lenght for a Ebay and nose cone collar(for the use of an altimter when using pluged motors)

One thing about the QT and the use of the piston is that yes ejection residue does coat the walls of the QT but I remove it via a dowl rod with a wet rag attached to the end of the rod and scrub it out and then running a dry rag down it to dry it out (I dont wory about having the BT swell from the water)

I do like my Callisto and have even added a 18" lenght of QT to the top of my Ebay for DD use. So far it has flown on the G339, G64 and G69 motors. I want to put an "I" or even a "J" in it some time and see how high it will go.

 

[sylvie369]

I simply cut a 1" length of coupler tube and push it down on top of the forward centering ring before the glue dries. Obviously, this must be done before building up the piston on the end of the strap. Now the piston has a nice hard stop and can't possibly bind for that reason.

An after-the-build alternative is to install two or three of those removable rivets at about the same point.