[F-FT] More zipperless piston commentary

Well, I had a chance to launch with the folks from MDARS yesterday
(thanks to Kevin and the gang). I did several more launches of my
'zipperless piston' design before a had a failure that will require some
rework...
I did several launches (3-4) on H-I motors with intentionally short
delays. In every case, the design worked 'well', but in 1 case the
piston went farther than intended because the tie wrap was a bit loose.
I should add the for my first couple of launches (previous to this date)
I had pulled the tie-wrap as tight as I could with a pair of pliers. I
recently obtained a real 'tie wrap tool' from Fry's, and had the tension
set a bit too loose, so the tie wrap ended up not as tight.
For my final launch with the design, I used an I435T, but the piston
shattered at ejection due to the bending forces. The good news is that
the body tube is undamaged, so I simply have to replace the
coupler/bulkhead assembly, so (overall) I'm ahead of the game, but it
means I still have some improvements to make. On the other hand, for
'nominal' flights this is probably a good design, as I doubt if I will
be intentionally launching with short delays too often .
Somebody at MDARS suggested using the bulkhead plate with vertical
'spars', such that looking at the end of the assembly would be like
looking at an 'X' or an '*'. This way, there is maximum area for the
exhaust gases to disperse, plus the stress force is distributed a bit
better than on a slotted coupler. I'm going to take a look and see how
difficult that would be to do.
I'm a bit concerned about the smoothness of such a configuration as a
'piston', but I'll have to experiment to find out.
Any additional comments on this would be appreciated.
David Erbas-White
Reply to
David Erbas-White
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David,
I recently built a piston in a 12" diameter body tube. The body tube section was about 30 inches long and the piston was about 5 inches long. I used 1/4" ply for the piston bulk head. I tested it on the ground with 6 grams of black powder. It kicked a 15 lb chute 10 feet into the air. The most amazing thing was the fit was tight enough on the piston and body tube that the piston did not leave the body tube. The piston was propelled foward by the black powder with enough force to eject the chute but was held in the body tube by the suction created by the tight fit.
If you can design your piston system such that there is more volumn in the body tube then gas created by the ejection charge and the fit is close enough to maintain a good seal the piston will not leave the body tube.
Bob Heninger Glendale, AZ
Reply to
Bob Heninger
Cut backon the BP. Glass the inside of the piston & review whatever else I said in my previous post on the subject.
When you put a tie wrap on a wire, if it is to tight, it can cut through the wire or insulation or cause the wire to break later. As a result, most tie wrap guns don't make them REAL tight.
Reply to
Phil Stein
I used the standard amount of BP supplied with the motor, which is generally identically the same -- so no extra used on this flight to distinguish it from any other. Further, the tube area of the D-Region Sandhawk has a fair amount of volume, so I'm a bit surprised by the pressure produced.
I'd glass the inside of the piston except that I've never done glassing before -- maybe this is the right place to start . However, I'll be a bit surprised of the glassing adds a HUGE amount to the shear forces produced that caused the phenolic piston to crack -- perhaps others can chime in here.
You're only other comments were a) to add holes on the top of the piston (in my mind, that defeats the purpose of this design), b) to use a steel cable (I prefer the Kevlar, and the 'slip fit' adds some amount of shock aborbtion), and c) to ensure that the slots are wide enough (according to my math they work out to the same area as a 2" tube, so they should have been sufficient -- on the other hand, probably several larger holes in the "Space Ship One" pattern would have made a better choice).
I know, did many years on military/aerospace projects. The gun came adjusted to the 'weakest' position, and I'm gradually experimenting with increasing the pressure on the tie-wrap gun (it's adjustable).
David Erbas-White
Reply to
David Erbas-White
I cut back on BP when using a piston. Someone suggested it when I first started and I've always done it and it's worked well. Not sure of the technical reasons. It seems like the area you need to pressurize is the same with ot without it.
Maybe someone else here can enlighten us on that.
I've flown a lot of pistons and have had several crack or even lose pieces. I think it is from the shot they take when the 'chute deploys. I've written some stuff on glassing & if you would like to see it, I'd be happy to dig it up & send it to you, It's pretty easy.
My piston (as I thought your did also) stops before it is all the way out of the body tube. The holes are not exposed until the top of the piston is sticking out of the body tube. At this point, the pressure from the charge has done its job and has nowhere else to go. It vents the pressure that remains after the charge has done it's job.
I think I mentioned that I liked your idea of using the Kevlar better.
I know what you mean. This is where you don't want to inject fracures or weak points into the brittle phenolic. Glassing helps avoid the possibility of fractures.
Good - you have the 'deluxe' adjustable model.
Reply to
Phil Stein
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Pages 3 and 4 talk about why you use less BP with a piston vs non-piston. I have used their numbers and have never had a piston break or shed parts and it always has deployed for me (knock on wood)
-Aaron
Reply to
Aaron
Great info. It looks like what I need is an accurate method of measuring BP in sub-gram quantities.
Any suggestions?
David Erbas-White
Reply to
David Erbas-White
PML's recommendation for the amount of BP to use only applies to PML's piston. Recovery systems that use a different type of piston, such as the one I use, are not sensitive to "too much" BP (unless you use so much that the airframe blows apart).
Dean
Reply to
Nobody
er....a scale?
PML sells a scoop that measures out 1g of BP. I normally use combinations of 0.5 and 1.0 scoops. Anything smaller then 0.5 gram I don't worry about. I have also found that my 1lb container of FFFFg BP is going to last me a very.....very long time.
-Aaron
Reply to
Aaron
Ammo reloaders use a small single beam cheapo balance. I don't know much about reloading but I have seen one. Try web sited dedicated to reloading ammo. I think that's what you want. Another possinilty is that mabe you'll find a scoop that is the right weight - also used by reloaders.
Reply to
Phil Stein
I use a 5-0-5 from RCBS (made by Ohaus). 500 grain (NOT gram). 1/10 grain adjustments.
Be aware the volumetric scoops are designed for firearm BP ONLY. Do not "weigh" non-firearm BP, or any other propellant, with a scoop; the amounts are totally dependent upon correct powder density. BP firearms are very tolerant of significant overcharging, rockets are not.
Reply to
Gary
Like I said, I'm not a reloader. Shouldn't the scoops come with some info about that? I have some 3x & 4x. maybe I'll throw a scoop of each on the triple beam & compare weights. Do you think there's much difference between the two? The 1lb cans look to be filled about the same amount but I realize my eyeball isn't well calibrated.
Reply to
Phil Stein
Yes, the scoops should have info on what can be "measured" with them. The powder throwers I have work with ffg and fffg rifle powders, they throw very similar weights of those two. I don't use ffffg and have not tested a thrown charge of it.
I was primarily warning about "other" black powders which are available for fireworks, cannons, etc, or even homemade BP. They are not suitable for a firearm BP volumetric thrower (scoop).
When in doubt, weigh it out.
Except, of course, when you are using a BP substitute. They are volumetrically equivalent, not mass equivalent.
Reply to
Gary
I use a syringe body retired from epoxy use to measure powder charges. Get one that's graduated in CC's. (a 5CC syringe is good) CC's are closely equivalent to grams. The ones I use have graduations between the CC's, which will get you .2 grams. It's close enough for what you want to do. It's always worked great for me.
Reply to
J.A. Michel
Well, if you actually go to a store and ask for them, ask for a BP powder measure. Most are adjustable, but there are fixed charge "scoops" as well. They are almost universally "set" for 2f and 3f powders, that is, they are marked in grains and will throw a weight of 2f or 3f BP pretty close to the mark. Many will argue with me, but "pretty close" is fine for BP firearms.
The substitute BP makers complicated all this. Their powders are closely equivalent to BP IF YOU USE A POWDER MEASURE to throw a "volume" of the substitute. IIRC, an equal volume of Pyrodex R weighs about half what a real BP charge weighs. Weighing a BP substitute charge can get you in a bind quickly unless you realize this. That's why I try to throw around "volumetric" as much as possible.
Reply to
Gary
Years ago I picked up a nice little brass BP measure from a local gun shop, which measures in grains. I convert using approx. 15 (actually 15.4324) grains per gram.
The measure looks a lot like this one:
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...Rick
Chad L. Ellis wrote:
Reply to
Rick Dunseith
A piston is leakless during operation. Other systems have big leaks, especially around a chute after a payload coupler is ejectedd.
Tech Jerry
Reply to
Jerry Irvine

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