PML Pterodactyl Jr - piston ejection versus baffle

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Bob in Phoenix

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I am building a PML Pterodactyl Jr. Four inch diameter, Quantum tube pre-slotted, G-10 fins, 38 mm engine mount, all very nice. The kit comes with a piston ejection system. But I like baffles. So finally, a question: Should I use the piston ejection system, or should I install my favorite baffle?

The pistol ejection system employs its own shock cord. One end gets epoxied to the motor tube, then forward through a slot in the forward centering ring. The other end goes through a slot in the piston bulkhead, through a D ring, back through the slot, and is then epoxied to itself. This shock cord is long enough for the piston to be pushed completely out of the body tube. Is that how it is supposed to work? This seems to make sense to me; the piston out of the body tube would facilitate cleaning the inside of the Quantum tubing and the piston itself.

Use of the piston system would, however, remove any possibility of using rail buttons. The head of the T-nut would prevent the piston from traveling any further forward. I do have some stand off rail guides I could epoxy to the tube. I could still use a 1010 rail.

Your suggestions will be appreciated.

Bob in Phoenix
 
I am building a PML Pterodactyl Jr. Four inch diameter, Quantum tube pre-slotted, G-10 fins, 38 mm engine mount, all very nice. The kit comes with a piston ejection system. But I like baffles. So finally, a question: Should I use the piston ejection system, or should I install my favorite baffle?

The pistol ejection system employs its own shock cord. One end gets epoxied to the motor tube, then forward through a slot in the forward centering ring. The other end goes through a slot in the piston bulkhead, through a D ring, back through the slot, and is then epoxied to itself. This shock cord is long enough for the piston to be pushed completely out of the body tube. Is that how it is supposed to work? This seems to make sense to me; the piston out of the body tube would facilitate cleaning the inside of the Quantum tubing and the piston itself.

Use of the piston system would, however, remove any possibility of using rail buttons. The head of the T-nut would prevent the piston from traveling any further forward. I do have some stand off rail guides I could epoxy to the tube. I could still use a 1010 rail.

Your suggestions will be appreciated.

Bob in Phoenix
I would ditch the piston
 
+2, ditch the piston. All my poop pipe rockets with pistons eventually jammed and crashed. Use the piston parts to make a baffle and enjoy.

Pterodactyl is a handsome rocket!
 
I have a couple of smaller PML kits that use pistons, but I tend to leave them out now when building PML kits. The Quantum tubing is pretty sensitive to weather changes, and it can be problematic to get the piston to slide easily in different temperatures. I see no issues with installing your favorite baffle, or just keep both out and use parachute protectors instead.

If you use the piston, you can use rail guides instead of buttons. Also, wipe out the inside of the tube after each flight to keep residue from building up inside the tube and binding the piston.
 
I like pistons. They require much less BP and eject the parachute much more positively. I’ve never had one jam, but I make sure they slide freely before each flight. I clean the inside of the body tube and if necessary sand the outside of the piston. As others commented the difference in the coefficients of expansion for the phenolic piston and the Quantum Tube body tube definitely can cause problems if not recognized and adjusted for each flight.
On a short design like the Pterodactyl (I have flown two different ones) the upper rail button can still be mounted below the piston skirt. The rocket will be just fine leaving the rail. There really doesn’t need to be a long span between rail buttons.
 
If you like baffles, use a baffle. You might modify the supplied coupler and piston anchor for part of it.

Ordinarily I automatically leave out the piston with electronics. The #2 reason would be available space. The Pterodactyl Jr is a little short, so you may have to have some recovery extend into in the nose if you use some longer motors with a baffle or a piston. I usually don't pack recovery in the nose unless I have to.
 
If you like baffles, use a baffle. You might modify the supplied coupler and piston anchor for part of it.

Ordinarily I automatically leave out the piston with electronics. The #2 reason would be available space. The Pterodactyl Jr is a little short, so you may have to have some recovery extend into in the nose if you use some longer motors with a baffle or a piston. I usually don't pack recovery in the nose unless I have to.
Mounting the piston backwards gives you a little more volume.
 
Mounting the piston backwards gives you a little more volume.

Good idea. It's hard to explain just how much flexibility you have when building one of these, or how the parts can be repurposed if you are inclined. If I have a rocket that is more work than it should be (to fly), I tend to avoid it, despite how much I like it, so for a whoosh pop, no electronics rocket like my Pterodactyl Jr, I just tend to gravitate to simple.

My (Pterodactyl Jr) is my go-to for flying H-550's, usually with a chute release. Seems like a match made in heaven. It always gets the college kids to look to see what made that noise (and where it went).

It's a pretty durable rocket, and I like flying it.
 
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The piston systems work but they require you to be meticulous with cleaning the tube and the piston after every flight. you really can't fly it multiple times without doing a serious deep clean. Because of this and a few near fail recoveries, I removed the piston from my IO and AMRAAM rockets. I replaced both with baffles and have never had an issue since.

At 4in in diameter I'd seriously consider using a fire blanket. I start using them at the 3 inch mark and they have worked very well for me. If it comes down to piston vs baffle then I'm going baffle everytime.
 
I like pistons. My L2 was a PML Black Brant X. I used basswood standoffs for my rail buttons and carefully sized the screw length to just make a little contact into the Quantum tube. For my L3, I used a fiberglass piston for my stretched Blue Iguana. You do need to clean the tube after every flight. I talc the pistons before flight. I also drill small vent holes (which you should do anyway) so when pushing the pistons down, the air can more easily vent out. I have several other piston models along with hybrids of LOC and PML parts. I just finished a PML Andromeda that I lengthened so I could use dual pistons (like my PML Eclipse does.) With both of these, I used the Acme Al rail guides.
 
I love the Pml piston systems.
I've never had any kind of issue with them, and have never really done much internal cleaning, either. However, I do make certain that the pistons move freely before EVERY flight.
 
I have a PML Callisto that is 20 years old and the piston is beat up, but still working fine. I installed rail buttons. Just install them with the piston out of the rocket and make sure there is enough room above the piston to install the chute and nosecone.
What is nice about the piston is it only uses 0.5g of powder, no matter how much is supplied with the reload.
 
I have both and I prefer one over the other, NOT. Depends on the application. If it’s a rocket I fly often, neither used. If it’s a rocket I fly often, having a clear easy view from end to end, cleaning, motor case removal (especially if stuck) and repair is SO much easier without a piston or baffle. I also highly recommend they be secured with rivets and not permanently attached.

Also, be careful in cold weather with a piston and quantim tubing. I guess summer too for that matter. Strange things happen with that stuff. Don’t take it from your heated car to the -10 and expect it to work.
 
Baffle. Pistons are fine, but you need to clean them. If you don't, they'll eventually bind and then you get a lawn dart (personal experience). With a baffle you can just shake out any detritus and you're good to go.

That said, I only use baffles on BT-80 and smaller. From 3" and up it's nomex.
 
I have not had a piston failure or get jammed. With the Patriot and amraam from my PML kits. Clean it and send it. I also did 4 small vent holes on the patriot and on the AMRAAM as well. But I don’t disagree about putting it backwards.
 
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