Piston placement for ejection

[mariohm1311]

I have been stuck this afternoon with a problem regarding piston deployment. Let's say my piston is facing bulkhead end towards the motor mount. The ejection charge will push it with the parachute forward. Next thing that happens is that the piston, coupler end, hits the coupler of the rocket, pushing the forward half of the rocket and breaking the shear pins. This liberates the parachute.

I have several questions regarding this process:
1. Is it problematic that the piston is pushing against the coupler. I see no alternative, so I am pretty worried.
2. How durable is a fiberglass-reinforced cardboard piston? What do I do if I need to replace it later on, considering that the shock cord towards the engine side is impossible to detach, and the loop located at the piston is epoxied. Should I make the shock cord between the motor mount and the piston long enough that I can cut it several times for future replacements?
3. How should I treat the inside of the rocket to prevent damage? I was thinking about filling the spirals and sanding. Is that enough?

PS: I will attach a picture of the OR design, and the files. Feel free to make any changes and/or critique my work.

 

[mariohm1311]

Now that I'm at it... which side of the shock cord should be the longest, av-bay side or motor side?

 

[Steve Shannon]

Merry Christmas,
Most people install the piston so the bulkplate faces the parachute. The parachute will be between the bulkplate of the piston and the bulkplate at the end of the coupler for the payload bay. I have installed the piston with the bulkplate recessed to give me more room for a chute sometimes.
You don’t need to do anything about the spiral. Just wipe out the body tube to keep it clean. Pistons should slide easily. Sand it or the body tube if needed.
I’ve never had to replace a piston. I have replaced the phenolic couplers for pistons, but I reused the bulkplate. Even if I had to replace the bulkplate there’s enough length to the cord.
With PML kits, the strap used for the hot side of the piston is a special material that’s more flame resistant than tubular nylon. Don’t swap the two straps. I don’t think length matters much as long as the piston comes out of the “cylinder”
Many people tire of pistons and just connect the straps together, eliminating the piston entirely.

 

[mariohm1311]

Merry Christmas,
Most people install the piston so the bulkplate faces the parachute. The parachute will be between the bulkplate of the piston and the bulkplate at the end of the coupler for the payload bay. I have installed the piston with the bulkplate recessed to give me more room for a chute sometimes.
You don’t need to do anything about the spiral. Just wipe out the body tube to keep it clean. Pistons should slide easily. Sand it or the body tube if needed.
I’ve never had to replace a piston. I have replaced the phenolic couplers for pistons, but I reused the bulkplate. Even if I had to replace the bulkplate there’s enough length to the cord.
With PML kits, the strap used for the hot side of the piston is a special material that’s more flame resistant than tubular nylon. Don’t swap the two straps. I don’t think length matters much as long as the piston comes out of the “cylinder”
Many people tire of pistons and just connect the straps together, eliminating the piston entirely.

I have read that the piston should be more stable with the short side CoM-wise facing the charge (that is, inverted to how PML expects you to install it), but it seems that this is a minor effect.

Regarding my first question. I get that I shouldn't worry too much about the piston pushing against the coupler right? I get that this is common for how others install theirs, but I just wanted to confirm.

Finally, should I avoid attaching the cord to the eye bolt by the means of a quick-link? That would be preferable from a modularity point of view. On that same line, how secure would fixing the coupler to the airframe with screws be, compared to the traditional glue option?

I hope this amount of questions isn't too annoying, specially during these dates. Merry Christmas!

 

[Steve Shannon]

I have read that the piston should be more stable with the short side CoM-wise facing the charge (that is, inverted to how PML expects you to install it), but it seems that this is a minor effect.

Regarding my first question.
1. I get that I shouldn't worry too much about the piston pushing against the coupler right? I get that this is common for how others install theirs, but I just wanted to confirm.

2. Finally, should I avoid attaching the cord to the eye bolt by the means of a quick-link? That would be preferable from a modularity point of view.
3. On that same line, how secure would fixing the coupler to the airframe with screws be, compared to the traditional glue option?

I hope this amount of questions isn't too annoying, specially during these dates. Merry Christmas!

There’s no difference in “stability” for the piston. It’s sliding in a tube. As long as the body of the coupler is long enough that it cannot bind, it’ll slide true.
1. The piston will not press against the coupler. Either air (compressed ahead of the piston) or the parachute will press against the bulkplate at the end of the payload coupler. If you build without that bulkplate you’re making a mistake.
2. I would use a quick link. Why avoid it unless you’re up against a weight limit in which case leave the piston out too.
3. It depends on the screws and the material you’re using. I wouldn’t use screws. There’s nothing wrong with adhesives.

 

[Wallace]

I've never tried using a piston but have worked on a "prototype" system that used venting slots and a spring to keep it from completely exiting the tube. Never followed through with it though since I felt it just wasn't good engineering. Would have been difficult to remove for cleaning and after toying with it for a bit I felt it really wasn't necessary to contain the piston in the tube. The best advice I've read regarding actual piston design (at least in my mind) is to make it a "solid" design. In other words, bulkheads at both ends and possibly filled with 2 part foam. Reasoning is that an open ended piston will tend to expand with gas pressure increasing the likelihood of binding in the tube. Makes sense no?

 

[Steve Shannon]

I've never tried using a piston but have worked on a "prototype" system that used venting slots and a spring to keep it from completely exiting the tube. Never followed through with it though since I felt it just wasn't good engineering. Would have been difficult to remove for cleaning and after toying with it for a bit I felt it really wasn't necessary to contain the piston in the tube. The best advice I've read regarding actual piston design (at least in my mind) is to make it a "solid" design. In other words, bulkheads at both ends and possibly filled with 2 part foam. Reasoning is that an open ended piston will tend to expand with gas pressure increasing the likelihood of binding in the tube. Makes sense no?

The day I first met Crazy Jim, in 2005 (LDRS 24) he showed me his Bengal Tiger rocket as well as some others. One of them had a vented piston, maybe the Tiger.
I don’t think a phenolic piston body will expand enough to bind if it’s a a slip fit in the first place. I’ve never had one bind. You use less than half the ejection powder with a piston.

 

[mariohm1311]

The day I first met Crazy Jim, in 2005 (LDRS 24) he showed me his Bengal Tiger rocket as well as some others. One of them had a vented piston, maybe the Tiger.
I don’t think a phenolic piston body will expand enough to bind if it’s a a slip fit in the first place. I’ve never had one bind. You use less than half the ejection powder with a piston.

Less than half of the calculated amount to fill the entire volume at a given pressure right (traditional method)?

 

[Steve Shannon]

Less than half of the calculated amount to fill the entire volume at a given pressure right (traditional method)?

I just use the recommendations from PML. [emoji851]

 

[cbrarick]

I toss mine in the parts bin and use a nomex blanket - easier, doesn't stick or need dialed in...
Having said that, if it's your interest, go for it! Just ground test....preferably at the temp you're gonna fly at...

 

[rharshberger]

I toss mine in the parts bin and use a nomex blanket - easier, doesn't stick or need dialed in...
Having said that, if it's your interest, go for it! Just ground test....preferably at the temp you're gonna fly at...

+1, pitch em', too finicky to mess with IMO. What they are good at is pushing the laundry out if its packed incorrectly, I still prefer a nomex any day. Using half the powder of conventional methods really is not much of a savings, .6 vs .3 grams in a 1.6" rocket, meh, and its one more component I have to verify is in proper working order.
As for the piston orientation I can't remember where I read that bulkhead towards charge was a better orientation.

 

[blackjack2564]

The day I first met Crazy Jim, in 2005 (LDRS 24) he showed me his Bengal Tiger rocket as well as some others. One of them had a vented piston, maybe the Tiger.

It was the "rockets for mummies" [play on DOS for dummies] rocket you saw. PML 6in Ultimate Endeavor.
I did a hybrid piston to protect the phenolic fincan edge from cracking on landing. The piston only comes out of fincan about 2 inches. The vents prevent the "shock load" when it suddenly stops.



Here is the definitive research I found on pistons for chutes, they are used backwards, from what we think is normal. Open end to top....Bulkplate on bottom [or next to charge] with chute inside open top end. The standard opinion about piston stability is challenged by this research and shown to be incorrect....more stable used upside down...much like pistons in a car engine, where combustion is ignited on top forcing open end down towards crankshaft. Make your own decision.

Mine was done bulk plate at top as I wanted the solid disk to prevent tube distortion & breakage.
I never had motor eject. My piston was "pulled" out by apogee charge.....not "pushed" by a charge below from motor..
It was my first big rocket and many scoffed at my design, telling me the charge from above would force it down tube and jam it in there........they were all wrong....lol
PDF;

PISTON RESEARCH.pdf

 

[blackjack2564]

The best advice I've read regarding actual piston design (at least in my mind) is to make it a "solid" design. In other words, bulkheads at both ends and possibly filled with 2 part foam. Reasoning is that an open ended piston will tend to expand with gas pressure increasing the likelihood of binding in the tube. Makes sense no?

If used like described [upside down] in the PDF you won't have to worry about that. Nothing to expand.

{gosh...I've been around along time and saved more research than I can remember...lol
just about everything I ever read concerning rockets]

I too used to pitch them. Now since I'm doing extreme altitude research, I am becoming a believer in sticking a chute inside. I'm thinking of using boron to eject. Immune to altitude ,but extremely hot and burns anything not solidly protected. Further study shows sounding rockets that did this.{Loki-dart] and split the piston sleeve in half so it falls apart upon ejection assuring chute does not get stuck inside. The edges of piston cut the shear-pins....even better...and doing it this way [chute inside] keep from compressing/damaging chute.

 

[cbrarick]

Dang Jimbo, now you brought that sciency stuff into the discussion....
Are you planning a BALLS flight? I don't know of any standing waivers in the east that would require boron..

 

[mariohm1311]

It was the "rockets for mummies" [play on DOS for dummies] rocket you saw. PML 6in Ultimate Endeavor.
I did a hybrid piston to protect the phenolic fincan edge from cracking on landing. The piston only comes out of fincan about 2 inches. The vents prevent the "shock load" when it suddenly stops.

View attachment 369787 View attachment 369788 View attachment 369789

Here is the definitive research I found on pistons for chutes, they are used backwards, from what we think is normal. Open end to top....Bulkplate on bottom [or next to charge] with chute inside open top end. The standard opinion about piston stability is challenged by this research and shown to be incorrect....more stable used upside down...much like pistons in a car engine, where combustion is ignited on top forcing open end down towards crankshaft. Make your own decision.

Mine was done bulk plate at top as I wanted the solid disk to prevent tube distortion & breakage.
I never had motor eject. My piston was "pulled" out by apogee charge.....not "pushed" by a charge below from motor..
It was my first big rocket and many scoffed at my design, telling me the charge from above would force it down tube and jam it in there........they were all wrong....lol
PDF;

PISTON RESEARCH.pdf

Is that shockcord going through the piston without any fixture? As in, is the piston able to move freely along the shockcord?

 

[blackjack2564]

Is that shockcord going through the piston without any fixture? As in, is the piston able to move freely along the shockcord?

No it stops just as seen in picture.
There is a Y harness below attached to 2 eyebolts on top MM Centering ring .
Looking closely you see the black cord coming out is doubled. That is a loop going through piston and hooked up to Y-harness.
Shock cord is attached to that. I can untie loop to pull piston out for inspection/maintenance.
Length of Y harness controls how far piston will extend out of tube.[ for this use about 2inches,just enough to protect the end of tube from damage]

 

[blackjack2564]

Dang Jimbo, now you brought that sciency stuff into the discussion....
Are you planning a BALLS flight? I don't know of any standing waivers in the east that would require boron..

Yes Rick I am...but it has been set back for a while.

https://www.rocketryforum.com/threads/full-scale-sparrow-hv-arcus-2-stage-sounding-rocket.132380/

 

[JimJarvis50]

I wondered if anyone would remember that "piston research". I haven't seen it mentioned for a long time.

In my experience, it doesn't matter much how the piston is oriented as long as it is a smooth (not loose) fit to the body tube. I usually do not use hardware on my pistons. I just pass the harness through a hole in the bulkhead in sort of a friction fit. It doesn't matter at all if the harness moves a little in the hole.

I use pistons in every rocket I own. It's just my opinion, which I don't care to argue about, that pistons increase the reliability of the main coming out by at least an order of magnitude, because it is no longer necessary for the weight of the cone to be the thing that pulls out the chute. I'm not really sure if it would help in the configuration of the OP or not (I'm not sure what happens in the drawing to the left of the chute).

When I did my L3, I used a piston that had a nice fit to the body with very little air leakage. I did my ground test and the piston didn't come out. The chute came out fine though. Then, I looked in the tube and the piston was right next to the altimeter bay, essentially where it started from. Huh? So, I repeated the ground test and got exactly the same results. I then calculated the amount of gas formed and learned that the BP would produce enough gas to move the piston about 90% of the way out of the tube (i.e., not enough gas to completely eject the piston). Then, as the gas cooled, it sucked the piston back to about it's starting point. Mystery solved!

Jim

 

[Steve Shannon]

I disagree with that PDF. It misrepresents the physics of pressure and totally disregards the restorative force the body tube places on the piston skirt which constrains the piston from rotation in the first place. It’s only relevant in the case of a piston that is too short and/or undersized diametrically.

 

[mariohm1311]

No it stops just as seen in picture.
There is a Y harness below attached to 2 eyebolts on top MM Centering ring .
Looking closely you see the black cord coming out is doubled. That is a loop going through piston and hooked up to Y-harness.
Shock cord is attached to that. I can untie loop to pull piston out for inspection/maintenance.
Length of Y harness controls how far piston will extend out of tube.[ for this use about 2inches,just enough to protect the end of tube from damage]

I have trouble visualizing that... could you make a simplified drawing or send me some more pictures? Thank you in advance.

 

[blackjack2564]

Here ya go...look at pic looking into airframe.[above posts]
White is shock cord.
Black is the loop going through piston down to Y-harness.
Knot tying loop end together is outside of piston and the tails are taped flat.
Simply untie the loop and remove piston.
This piston was to prevent airframe zipper/damage from landing....not protect a chute.
It certainly DOES ensure chute comes out.
Holes in piston to prevent a vacuum when it is pulled forward by ejection charge & payload.
Drawing below: