piston question

[vref15]

for those who use pistons---

I ground tested my L-3 today and it uses a piston. the thing is, i expected the piston to come flying out the end, but it just made a pop, cut 4 shear pins and the chute and nose cone went flying. Im assuming that the piston pressurized the tube and sent the nose cone flying. it worked great except the piston was still at the bottom of the tube. should this be a great concern since it worked, i used 2 grams of ffffg. did the shear pins keep the piston from working like it should?

 

[snowman2]

I ground tested my L-3 today and it uses a piston.

Please provide the diameter and length of the tube with the parachute/piston.

A description of the piston's design would help, too.

i used 2 grams of ffffg.

Without knowing the diameter and length of the tube it is impossible to determine if that is enough, too much or too little.

A photo would be worth a 10*10*10 words.

 

[vref15]

its 6" tube 24 inches long. Im using a public missiles piston. course the tube is pretty full with a cert 3 parachute and 40' of 3/4" tubular nylon strap. Im cutting 4 shear pins 2-56 from missile works.

 

[Rocketlady]

for those who use pistons---

I ground tested my L-3 today and it uses a piston. the thing is, i expected the piston to come flying out the end, but it just made a pop, cut 4 shear pins and the chute and nose cone went flying. Im assuming that the piston pressurized the tube and sent the nose cone flying. it worked great except the piston was still at the bottom of the tube. should this be a great concern since it worked, i used 2 grams of ffffg. did the shear pins keep the piston from working like it should?

I used a piston in my L3 bird as well. The piston didn't come out of the tube, either, but did send the laundry flying quite a ways. No, the shear pins wouldn't keep the piston from working any better than they did.

According to the PML site, for a 6" dia X 30" area they recommend 1.4mg to 1.7gm BP. Based on the size info also given with the four shear pins, 2gm ffffg is sufficient.

 

[snowman2]

Im using a public missiles piston.

I assume that means the piston is attached to one length of cord on the bottom and a separate length on the top. My pistons have a slot through them and slide on the tubular nylon.

its 6" tube 24 inches long.

My 6" diameter rockets' parachute tubes are longer. I use 3 #2 screws for shear pins. The cones are fiberglasss, which means they easily cut the pins. The cones recover separately, which means they can be kicked off with a lot of force without risk of damage to the rocket. I use 4 grams of BP.

One thing I learned after a couple of recovery failures is to blow it out or blow it up. The amount of powder that worked okay on previous flights wasn't enough on the flights when the rocket failed to separate. During flight tubes flex and deform, and more force might be needed than what is determined to work during ground testing.

i expected the piston to come flying out the end, but it just made a pop, cut 4 shear pins and the chute and nose cone went flying.

How far did the cone and chute get kicked?

If the piston does not slide on the cord, how much cord is on the bottom side of the piston?

Based on my calculations and experience, 2 grams of BP in a 6" diameter rocket with 4 #2 pins seems to be right on the edge of success/failure.

There are multiple factors that affect the amount of needed powder. One is how loosely the piston fits the tube. My pistons are pretty loose. Tighter pistons can require less powder.

Try the ground test more than once. I'd also recommend a test with 3 grams.

 

[vref15]

the chute and nose cone flew around 10 feet into a backstop, it probably would have gone further,it was moving pretty fast. the piston is a real good fit, it will slide all the way down the tube if dropped in, and it seals very well.
the strap on the bottom of the piston doesnt slide, its secure and it has more than enough room to exit the tube.
i tested it twice once with 1 1/2 grams and the second with 2 grams, both times the same result, a great seperation but piston hasnt moved very much.
one thing i failed to mention, im using the piston upside down, because of a thread on piston stability that i read, states that the piston is more stable weighted side down. I wonder if that has anything to do with it.
I will try the test again with 3gm to see what happens.


thanks everyone for your imputs

 

[vref15]

3 grams was too much, it scared me with the force it came apart, but the piston still stayed in the tube, weird. the nose cone nearly went throught the canvas backstop. i guess im going to settle for middle of the road and use 2 1/2 grams. ive used pistons before and never seen this.

 

[agrippo]

Pretty interesting results of your ground testing.

You might want to send an email to Andy with PML at andywaddell @ publicmissiles.com with all the details and see what he says. I'd guess he has seen just about all there is to see concerning pistons and can give you a very good explanation.

Andrew Grippo

 

[Handeman]

Just a quick guess, but does your piston have a loose seal? I'm thinking that when the charge goes off, the pison moves enought to equalize the pressure on both sides of it. When the nose cone pops off because of the high pressure, the gas from the charge is cooling and leaking around the piston and thats why it doesn't make it out of the BT.

Just a thought.

 

[vref15]

the piston fits snug, but there is a small amount of gap where the strap goes throught the center , i guess pressure could leak throught that. I guess i could use some masking tape wrapped around the strap to better seal it.

 

[Tom Rouse]

This is something we experienced in the ARLISS program.
Bear with me in my attempt to describe it-
In ARLISS we use a 6 inch payload section. Inside that 6 inch diameter X 36 inch long tube is our bulkhead with the ejection charge on top of it. Also, right on top of the bulkhead is the ARLISS carrier, which is a coupler 12" long with a bulkhead plate on the bottom, 1 inch up. Thus, its a 12 inch piston. The students payloads are inside that 12 inch piston/carrier. Right on top of that 12 inch piston/carrier is the PML 6 inch nosecone. Tight fit.
Pius Morozumi and I did a lot of ground testing and what we discovered was as follows-
Our nosecones flew across the yard and the piston/carriers just barely moved. The explanation is that the energy from the ejection charge is transferred to the nosecone and the piston stays there and the nosecone goes.
Remember those steel balls hanging on strings in a rack that you bounce back and forth with different quantities of steel balls at each try? You can take one and it will transfer all the energy to the other side of the rack and send one moving. This is the same as the transfer of energy in the rockets piston to the nosecone.
The same as croquet balls. Hitting one ball and transferring all the energy to the other object.
This is what often happens with pistons.

Adding more pyrogen will get the nosecone flying further and still not guarantee that the piston will come out! Try it.

Cheers
Tom

PS- Did I explain it clear enough?

 

[Fore Check]

Wow, I never heard of this one. But Tom's description makes some sense......

 

[jrcox]

Tom

Interesting observation, what did you do to change the setup and eject the piston/payload section?

John

 

[Tom Rouse]

To deal with the scenario- the nosecone flying off and the piston/carrier staying behind, we tried different amounts of Black Powder to see what happened. Our volume area was quite small for the BP since the piston/carrier was sitting against the bulkhead plate. A 1 gram charge separated the nosecone but left the piston/carrier in the rocket. We tried 2 grams and it was too violent. But the piston/carrier came out, although Pius recorded 50 G's of force in the separation.
So, you should experiment with different amounts to see what works best for you.

(I changed over to CO2 and it now works fine. No "shock", just a whoosh.)

 

[jrcox]

Thanks Tom

The CO2 solution makes a lot of sense, I would assume that a CD3 builds pressure inside the rocket much slower than a BP charge, allowing the canister to get pushed out after the nose cone is ejected.

John

 

[vref15]

Tom,guys,

thanks, what you said was pretty much what i was thinking was happening, just couldnt figure out how to describe it. It just went against what i thought should happen, but i guess as long as the result is what is intended eg. nose cone off, laundry out, then mission accomplished.

 

[JimJarvis50]

I had this happen (the piston not coming out) when I ground tested my L3. I couldn't believe it, so I tried it again. Still didn't come out. Still couldn't believe it. So, I put pen to paper.

Similar to what Jeff described, my piston is a very close fit, so there is minimal leakage around the outside of it (the piston takes several minutes to "fall" through the tube when the end of the tube is sealed). So, the idea was to look at the pressures on either side of the piston to see where the piston is when they balance. For a ground test or low altitude deployment, this would be 1 atmosphere (assuming the nose cone came off).

The first thing I did was calculate the moles of gas produced by burning the black powder. I had to find the composition of black powder itself and the composition of the reaction products. Not a problem.

Then, I had to calculate the maximum temperature that might be achieved (recognizing that the actual gas will cool very quickly as the piston moves and will not stay at this maximum temperature). I back-calculated a value from the black powder/pressure equations in ROL. I got an assumed temperature of something on the order of 2400 F iirc.

Then, based on the volume (moles) of gas produced and a maximum temperature of 2400F, I calculated that the piston would move about 8" or so up the 4" tube, or about half of the way out of the tube. This movement would give a pressure of 1 atmosphere behind the piston. If the pressure on the other side of the piston was about the same (i.e., low altitude as in the ground test after the nose pops off), that's where the piston would stop, or at least, that would be the equilibrium point. I further calculated that when the gases cooled down, the piston would be sucked back to within an inch or so of its starting location (assuming no leakage). A couple grams of black powder doesn't really produce much gas at ambient temperature.

In the case of a loose piston, I think it could easily be pulled out by the momentum of the nose cone and chute. For a gas-tight piston, though, I think the differential pressure controls the movement, and I concluded that the piston did exactly what it should have done.