If not piston deployment, then what???

I just used piston deployment for the first time in my Level 1 certification flight last weekend. Here are my comments:

1. I don't think I'd 'trust' the standard elastic that is/was supplied for use with the piston on the engine side. I replaced mine with heavy-duty Kevlar.
2. It is/was 'difficult' to sand the piston to be 'just right'. By that I mean that I wanted it 'snug', but not 'too snug', which meant a few passes with the sandpaper, then re-testing it. And it makes a difference what temperature you fly it at, so you might need to 'touch it up' for the first few flights.
3. There's a bit of grit on the inside of the tube after launching, which means you need to clean the tube out so that the piston works well for subsequent flights. Not a big deal, but it's something you need to be mindful of.

That having been said, I don't think there's a better way to completely protect the chute -- the chute looks like I just took it out of the bag.

David Erbas-White

The only real problems with piston recovery i've seen is when you use a phenolic piston with quantum tubing AKA drainpipe.

With the expansion rates of the two materials being different they have the nasty habit of jaming in cold or damp weather.

Damian

Which is why you should check it before flight. Every rocket should have some things checked before each flight - it's no biggie.

Advantages:

1. A piston pushes the recovery system out of the rocket. The other option is to hope the nose cone drags it out.

1. A piston protects the recovery system from fire and heat. The fire and heat is on one side of the piston. The nylon parachute is on the other side. I have a great video that illustrates this. An ejection charge was test fired in an unpainted fiberglass tube. Carefull examination shows the piston sliding and pushing the chute out, and you see a big fireball.

If done right a piston is great. But, just like assembling a motor incorrectly, if it isn't done right then you've probably created a worse problem.

If the fuselage is plain cardboard then I cannot recommend a piston. Plain cardboard is soft and absorbs crap that is difficult to impossible to remove.

The piston needs rigid walls that cannot bend, fold or otherwise deform. A deformed piston can jam. Imagine what would happen in your car if a piston deformed.

I start with a phenolic coupler tube. Plain cardboard is too soft and absorbs crap that is difficult to impossile to remove.

Fill the coupler tube with two-part foam to make it non-deformable. After the foam has cured, cut to size (about 1/2 the fuselage's diameter). Coat the exposed foam on the ends with epoxy to protect the foam from the BP blast. I cut a hole in the center for the TN to slide through. (Coat the hole with epoxy, too, otherwise the foam will tear apart.)

My pistons slide on the tubular nylon cord. The pistons are not fixed to a point. They don't snap to a stop and rip something apart. Sometimes one will slide several feet along the TN before drag and gravity stop it.

If the tube is 30" long then place at least 30" of TN (covered with Nomex) under the piston. The piston should be able to clear the fuselage before it needs to slide on the TN (or Kevlar or whatever you use).

A piston should freely slide in the fuselage. Hold the fuselage tube vertical and the piston should freely slide to the bottom. I needed to remove a layer of paper from the (flex phenolic) piston for it to slide freely, in part because the fuselage wasn't perfectly round.

Sabot?

There isn't a single recovery methodology that is perfect for all rockets. What size rocket and what recovery scheme (single deployment, dual deploy, etc.) are you using? What is the fuselage material? What is the diameter?

Dean

Piston deployment has a possibility of the piston getting sticky, or getting cocked in the body and stuck, or getting a shroud line or something caught between the piston and the body (thus getting stuck once again).

Well... I like pistons a lot. People who don't prefer things like Aerotech-style baffles or putting the chute in a kevlar bag.

Piston deployment is the best method to ensure safe ejection of your recovery system. Wadding, Baffles, Nomex or Kevlar pads are also viable options, but there is a better protective seal between the motor and chute when using the piston. And as stated in another comment, with the piston, after deployment, you cant even tell that the chute was subjected to the ejection gases.

-- Jeffrey L. Miller Anchor Parachutes

Opinion? Fact? Strongly stated opinion = fact?

How about: Piston deployment is the best method to ensure occasional lawndarts due to complete failure of the recovery system to deploy?

I personally do not like them as add weight and another thing to mess up. I prefer the Kevlar balnket method as is lighter and I so far have had no problems. These are strictly my opinions from my policy of KISS.

I like pistons. I've never had a failure caused by them. Although I build rockets larger that 4". I've never used a piston on one larger than that. It doesn't mean that you couldn't.

I like pistons but admit that there are numerous things that can make them jam. One thing not covered by other posts in this thread is the potential for jamming on the excess glue fillet the motor mount is glued in. If the piston is jammed down hard by acceleration, that can happen. After finding that the piston in my PML Tomahawk was binding at the very bottom, I hit upon the idea of slipping an extra coupler on top of the motor mount assembly as a spacer. 30 flights and never a jammed piston.

Larry

What a wonderful solution. I'll use that on the next project.

Thanks, Peter

I've been using that method for protecting the main ejection charge. A

2.5" high piece of coupler sits between the electronics bay's top, where the charge is located, and the piston. The coupler creates a shelf for the piston, preventing the piston from contacting the charge or wiring during ascent. The spacer is removeable so that it can be cleaned or replaced.

Dean

This should be in the FAQ.

Ahh, but you clearly state an opinion based on empirical evidence, not an opinion presented as fact.

My (somewhat snarky) comment was not meant to be a serious response.

baffle systems work great to protect the recovery equipment. no moving parts, nothing to clean. weight-wise, I think baffles are lighter than piston systems.

This should be in the FAQ.

I feel their is no better way for anything,

only the way the rocketeer likes his/her best :-)

I use parachute cannon methods.

my parachute tube is really the smooth bore of a BP cannon made of rocket tubing.

the ejection charge in main deploy is the cannon charge.

I make my chute fold up in a nomex blanket totaly covering it, then that just fits snugly in the cannon muzzle.

This becomes the cannons wadding, and is inserted muzzle loaded and seated on top of the Black Power Charge.

the recovery harness cord is placed on top, and the nose cone fited with shear pins or shear tape. Very little space should be between the nomex/chute/harness "plug" and the nose cone. If there is, use more harness or a larger chute.

the charge should be sized to blow everything out.

this method does not rely on the nose cone to pull out the chute, but the cannon charge to blow everything out.

blow it out, or up.

Are you trying to imitate Jerry or something? 8-)

Just to clarify - Aerotech uses a baffle system and they do clog and need to be cleaned over time. THere are other types of baffles that do not have this 'feature.'