- This topic has 17 replies, 8 voices, and was last updated 19 years, 1 month ago by
Bruce R. Schaefer.
Bruce R. Schaefer.-
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Warren B. Musselman
I’ve used pistons in 29mm through 54mm birds, but I’ve never used them in anything larger.
At the last launch I flew at (June I think), my 4″ project didn’t properly deploy the main and suffered some damage to the airframe and nose. The sheer pins blew, but the laundry got hung up halfway out of the tube. Post mortem looked to me like the charge blew by the laundry and nomex pad, blew the nose, and didn’t give enough ooomph to the laundry to get it out of the tube. It’s also possible that the laundry hung up on a fragment of sheer pin or a fracture that I found later in the airframe occured during deployment rather than impact.
Given all that, I’ve been considering adding a piston to the rebuilt 4″ bird. What are folks’ experiences with using pistons on 4″ and larger diameter rockets? Obviously, I’m concerned with the piston hanging up, something that I fear to be a greater risk, perhaps illogically, due to the larger diameter tubing and the somewhat sloppy fit I’ve experienced with 29-54mm airframes and pistons using GLR and Hawk Mtn filament wound and PML quantum and phenolic airframe tubing and phenolic pistons.
IF you’ve used pistons on 4″ and larger birds, how do you size them to prevent them jamming as they slide? PML’s standard piston length seems to be equal to or slightly shorter than the OD of the airframe. My own logic dictates it should be at least slightly longer than the OD to prevent cocking and jamming. Thoughts? Differences with larger diameter tubing?
While we’re on this topic, I’m also working out the design of my 5″ L3 project – a greatly modified Performance Rocketry Competitor 5 (longer, different nose, fins, longer motor mount, and a camera bay. The stock coupler material is exceptionally tight in terms of OD match to the airframe ID, but still slides easily. Does anyone have any background using pistons with the convolute wound PR fabric fiberglass tubing?
Warren
Doug Gerrard
I have used pistons on the 6″ OD BDCR on every flight except its first flight when I thought all I needed was a deployment bag. The bag came out of the rocket but the parachute stayed in the bag. I have used the piston on every flight since without a problem. The length of the piston is 6″ with the plate in the middle and I would describe the fit as “loose”. If you put it in an empty tube it slides down freely on its own weight. If there is a bulkhead plate at the bottom end you have to push it down. You can check it out at Oktoberfest if you want to see it.
Doug
elviss_boy
On my denali I have a piston for my main. It worked great and did not jam through ground testing and 1 flight. Mine is the leanth of the OD and in my opinion thats long enough so I would suggest doing a piston on your bird and will work great!
Chris LaPanse
My BBX uses pistons, and they work perfectly every time.
Honestly, they are very similar to those in smaller birds – just keep the inside of the rocket relatively clean, and you’ll be fine.
Anonymous
Pistons work ideal in filament wound glass tubing. The tubes are slick, don’t get dinged (which can cause a piston to hang up), etc. I use pistons whenever possible, and almost always in glass rockets. I’ve gone up to 5″, but if it was a FWFT I’d fell comfy using them in a 12″ tube. ALWAYS go at least one caliber for your piston — i.e. a 5″ rocket needs a 5″ (or longer) piston.
I’ve used pistons in 3″ glass rockets dozens of times, 4″ rockets many times, and 5″ rockets 6-7 times. No worries- except when the stitching broke on my piston leash, but that is another thread…
denverdoc
Warren,
I’ve been leery of that newfangled technology, but looking at the kit I won at LDRS–2.6 inch diameter AT sumo, Gonna give it a roll/slide whatever. I’ve followed some treads re upside down, v mid mount, v conventional. I’d go inverted or about a 1/3 so as not to rob too much cargo room, and use graphite powder for a dry lube.
John SWarren B. Musselman
There is a major difference in fit between Hawk Mtn or GLR filament wound and their corresponding coupler material and the Performance Rocketry tubing and its corresponding couple material. The first fits pretty sloppy and slides easily through the airframe. The second is an incredibly tight fit. It certainly can be inserted and moved through the airframe material, but it takes some force and effort to do so. I’m wondering…. I hate to sand down such a nice good fit and make it sloppy.
Warren
denverdoc
not to sound sarcastic, but the third chioce is use use the PR stuff (provided you can wait that long 🙄 ) and grind it down ever so gently on a belt sander with fine sandpaper for a perfect fit. I think it should be ground a fraction small, not sloppy, and one can use flilament tape or whatever for a small build-up if weather conditions require.
Personally, I have asked the really basic question of myself and a few others I trust, is why do bigger parts seem more prone to seizure, when dry fitting or on entry into the larger hole. Sometimes the parts are ever so slightly out of round, but I cant even count the number of times when a seemingly good fit either balked at insertion only to drop in smoth as can be, or after sliding in just so perfectly, seized on withdrawal. I know off center forces can be a big problem, but even when I drob a BH in and bang on that as best dead center as I can do, it still is stuck.
In addition to strap wrenches, I have had to use angle iron, 2 by 4 etc to knock parts apart or out, even when the parts were all inside the house for days at as time. Mystery to me, and one reason I’ve been leery of pistons, even while knowing that the instantaneous pressure increase with BP is a lot greater force than I can generate manually.
JohnAnonymous
Never once have I had a piston jam. I’ve had ~200 High Powered flights, and off the cuff I’d say 90% of them had at least one piston and 80% of them had two pistons. I also fly almost exclusively w/ filament wound glass. I’d be less chipper about pistons if I flew phenolic.
There is a school of thought out there that a quick blast, like from a charge, can “inflate” the cardboard or phenolic sleeves of the pistons behind the bulkhead, potentially causing it to seize. I don’t believe that, but I have read it several places. Glass, properly fit, slides wonderfully. You have to clean out the fouling on the tubing after every flight, but that is easy — I use Windex, a paper towel, and a dowel.
Even in my highest performance rockets, where space and length are premiums, I use a piston for the main. If the laundry doesn’t get out, you are outta luck. I use a long strap that I tuck into the base of the piston, so I don’t think I’m adding any length anyway…
Doug Gerrard
Never once have I had a piston jam.
Same here, I’ve never had a piston jam. I do prefer the “looser” fit and I was wondering, what is the problem with a loose fit? I still use some kind of protection in the form of Kevlar or Nomex pad. My thinking is like this; The piston is to get the laundry out of the rocket and NOT to protect it. That is the job of the Nomex pad.
Doug
Warren B. Musselman
Doug Gerrard
elviss_boy
Chris LaPanse
Anonymous
denverdoc
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