Piston ejection systems ... question

[rocketmom106]

I've built a PML Tethys for my Level 1 Cert attempt at the end of this month. Are any vent holes used for the piston ejection system? Where would they be placed? The first flight will be on an H238 which will take it up around 1,000 feet. Are vent holes used for higher altitude flights only?

[sylvie369]

Vent holes are intended to keep the internal (ground level) pressure inside a rocket from pushing the rocket apart at its separation point when the outside pressure is lower (at altitude).

At 1000 feet I don't think there's really any chance that you're going to have an issue with that. On the other hand, there's no harm in drilling a small vent hole, if for no other reason than so that you don't forget to do so when you DO fly high enough to need one.

I guess there really should be a hole in the airframe anywhere that pressure might build up inside, so you could argue that you need one above the piston and one below it, but I've never done that, nor have I seen it done. If the piston is loose enough to slide properly, the air would (*I think*) be able to move past it. The piston does NOT need to seal the airframe - the ejection charge will certainly push it out even if there is leakage around it.

The only place I'd think you should NOT put the vent hole is right where the piston sits, as the piston would block the hole and render it useless.

That's my two cents' worth, based on nothing more than what makes sense to me. Be sure to talk about it with the person supervising your Cert.

You need to also make sure that the separation point of the rocket is "sticky" enough to prevent drag separation, which is a completely separate issue. You should be able to lift the entire rocket by the nose cone.

[TD Sky]

Just one 1/8" hole thru the wall of the airframe, about 5" down from the top of the airframe (so that the shoulder of the nosecone is not blocking it) should work to keep the pressure differential to a minimum. This vent hole really has nothing to do with the piston ejection system. All my rockets are PML and in my experience, the stock piston system works just fine. Hope your cert flight goes well.

[JimJarvis50]

Vent holes are intended to keep the internal (ground level) pressure inside a rocket from pushing the rocket apart at its separation point when the outside pressure is lower (at altitude).

At 1000 feet I don't think there's really any chance that you're going to have an issue with that. On the other hand, there's no harm in drilling a small vent hole, if for no other reason than so that you don't forget to do so when you DO fly high enough to need one.

I guess there really should be a hole in the airframe anywhere that pressure might build up inside, so you could argue that you need one above the piston and one below it, but I've never done that, nor have I seen it done. If the piston is loose enough to slide properly, the air would (*I think*) be able to move past it. The piston does NOT need to seal the airframe - the ejection charge will certainly push it out even if there is leakage around it.

The only place I'd think you should NOT put the vent hole is right where the piston sits, as the piston would block the hole and render it useless.

That's my two cents' worth, based on nothing more than what makes sense to me. Be sure to talk about it with the person supervising your Cert.

You need to also make sure that the separation point of the rocket is "sticky" enough to prevent drag separation, which is a completely separate issue. You should be able to lift the entire rocket by the nose cone.

I use pistons alot, and I would agree with everything mentioned above. I use 1/8" holes for venting above the pistons. For your case, you might go with something slightly smaller.

Jim

[Handeman]

I've had a PML Callisto since 2003. I've never put a vent hole in it and have flown it on F to H motors with altitudes over 2,200 feet (LDRS drag race was the latest). I've never had an issue with seperation from pressure. In fact, I flew my El Juan with no vents to 6,810 ft and had no problems, of course it doesn't seperate easily.

[rocketmom106]

So...the vent holes are a function for air pressure and not for the piston performance?... I just didn't want to compromise the piston by adding vent holes (or not adding them).

I was also wondering about the ejection charge being too strong or not strong enough for the piston to work properly? Is this a valid concern for an H238 in a 4" airframe?

[BsSmith]

I was also wondering about the ejection charge being too strong or not strong enough for the piston to work properly? Is this a valid concern for an H238 in a 4" airframe?

Not really, just fill up the ejection well if your worried about it. I fill it up all the way on all my rockets and I have never had a rocket blow up from an ejection charge.

[H_Rocket]

If you go through the instructions of the PML kits they list suggested powder loads for piston equipped rockets. It is usually somewhat less as once you install the piston the volume that is pressurized by the charge is markedly smaller than the whole bay.

Also, keep in mind that if you are flying Quantum Tube, the coeeficient of expansion is different than that for the phenolic piston. This means you may need to be prepared to sand the piston down on the field on a very cold day.

Finally, have some methodology to clean the gunk out of the body tube as it will build up and make the piston sticky.

[talkin Monkey]

Also, keep in mind that if you are flying Quantum Tube, the coeeficient of expansion is different than that for the phenolic piston. This means you may need to be prepared to sand the piston down on the field on a very cold day.

Very true, after sanding the pistons down to fit the QT nice at room temperature, I popped the QT with piston installed outdoors (WI winter) for a couple hours (a chest freezer works almost as well for summer building too BTW). The piston might as well have been welded in at that temperature! Quite a bit of 120 grit power sanding to get the piston sliding nice and happy at both temps. Moral of the story, better to do this at home then at the launch.

I also install my pistons "upside down" with the bulkhead towards the ejection charge. Just like the pistons in a combustion engine. I'm sure someone will chime in with..."The Link"...and agree or dis-agree opinions. There's also A LOT of other ejection methods that work just as well if not better and require little or no modifications to the kit you're building.

As far as venting the airframe goes, why not? A 3/32" hole drilled on the rail guide side won't be too obvious and can't hurt for a cert 1 flight. Nothing worse than mid flight pre-mature separation.

The BP, put it all in and shake it down so it makes contact with delay grain. They don't supply enough to break a rocket as far as I know.

[Jeff]

Yes - the vent holes are for relief of pressure. The piston will work the same with or without a vent hole. As others have said, just make sure the piston slides up and down fairly easily and doesn't feel jammed.

[sylvie369]

Very true, after sanding the pistons down to fit the QT nice at room temperature, I popped the QT with piston installed outdoors (WI winter) for a couple hours (a chest freezer works almost as well for summer building too BTW). The piston might as well have been welded in at that temperature!

I simply don't fly QT here in Wisconsin in winter anymore, and my latest PML build was one I ordered with phenolic instead of QT. Several times I brought a QT-based rocket out in February or so, and found it far too tight to fly.

My next piston will be installed the other way around as well, and I wish I'd remembered to do that on my last one. I do think that backwards is better.

I don't know if anyone answered your question about the vent holes "compromising" the piston. The answer is that reasonable sized vent holes (the suggestions here look fine) will not cause any problems. Don't worry about that. Just make sure your piston slides smoothly, and that your separation point is secure enough.
(why does that sound dirty?)

Paul

[rocketmom106]

Thanks everyone for the help! Vent hole is now installed. Below is a picture of the finished rocket. And, thanks for the comments about these QT frames in the winter...our club has a nice size field which is available in the winter (until farmer puts in crops) which is the perfect size for this rocket but the temperatures in NE Ohio in January and February will definitely factor into the problems you noted with the frame and the piston!

Thanks again...see ya on the other side of Level 1...

[tbrogan]

You might want to bring along some baby powder. I put a small amount down the body tube and rub some on the piston itself. Makes it slide nice and smooth. Just a thought, take it or leave it. Good luck with the cert.

Tom

[DMcCauley]

Whatever you do, ENSURE that you have some protective wrap on the webbing that attaches the piston to the motor mount (larger of the webbing). Nomex or Kevlar will work well here.

Otherwise, after a few flights, you'll find that webbing to be seriously damaged from burning up via the ejection charge.

[Handeman]

Whatever you do, ENSURE that you have some protective wrap on the webbing that attaches the piston to the motor mount (larger of the webbing). Nomex or Kevlar will work well here.

Otherwise, after a few flights, you'll find that webbing to be seriously damaged from burning up via the ejection charge.

You are correct, but it think it will take more then a couple of flights. I've been flying my PML Callisto since 2003 and have at least 50 flights on it. The large webbing is showing some damage, but it's still solid.

Of course as I write this I'm thinking back to someone's Callisto at LDRS that snapped the webbing at the motor mount and came down in two peices. Fortunately the body tube came down flat so no core sample.