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Piston Ejection

  • TimDawson
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2 weeks 4 days ago - 2 weeks 4 days ago #10850 by TimDawson
Piston Ejection was created by TimDawson
Thank you to "The Powers That Be" for setting this up. As suggested in the meeting, I'll start with my new love of piston ejection.

I've always been a Baffle Guy.  If the tube was BT-60 or above, it got a baffle.  Baffles do have their issues. They don't work in small tubes. Even after shaking the tube out, it is surprising how much build-up can occur.  And after a couple dozen flights, the tube can degrade where the baffle is, from the hot debris constantly stopping in the same area (I've yet to launch the same rocket a couple dozen times).

Then I built my first high power kit, an AIM-120 from PML that came with 4 inch plastic body tubes and piston ejection.  In building the piston system, I saw how easy it would be to apply to even the smallest of body tubes.  I had four rockets set up with pistons at the club launch on the 7th, from BT-50 up to the 4 inch AIM-120, and all four worked great.

The basics of a piston is a kevlar cord with one end at the motor mount and the other at the piston.  The Cord comes up through the middle of the piston bulkhead.  Form a loop and a knot in the cord so that it can't get back through the bulkhead and glue or epoxy to help.  The cord should be as long as the shock cord as it needs time to slow down in open air from the ejection thrust.  The shock cord is attached to the loop that was made.

For BT-50, BT-20 and BT-5, use a spent Estes engine of the right size for the piston.   Other than that, any coupler will work.  The bulkhead should be wood.  Glue/epoxy the crap out of the bulkhead to piston connection, but be careful to keep the piston clean.  There are tricks for sealing and sanding paper, if you really want to get deep.

Things that would stop you from using a piston are obstructions inside the tube, such as the Estes paper shock cord mount or beads of glue running down the inside of the tube from over-gluing the motor mount into place.

With piston ejection, you want to keep things clean.  After launches, I run a paint-gun cleaner down through the rocket.  I examine and clean the piston, if necessary. I wont hesitate to use a little light sandpaper on the piston.  It should be as smooth to a lot smoother then the nosecone.

Attached is a pic of my latest build, a BT-50 sized WAC Corporal with a "D" Size Piston.
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Last edit: 2 weeks 4 days ago by TimDawson.
The following user(s) said Thank You: KennB

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2 weeks 3 days ago #10852 by deangelo54
Replied by deangelo54 on topic Piston Ejection
Great writeup. Thanks for sharing your knowledge. Years ago I was having a problem where the E9-4 engines I used on my scratch built rockets were not strong enough to pop the chute. I jury rigged a piston from items I had in my field box and to this day it still works fine. At the time I thought I invented it but later found out that it had already been developed. I attached a photo of my piston.
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1 week 5 days ago #10863 by TimDawson
Replied by TimDawson on topic Piston Ejection
One last update:
I launched 4 rockets Saturday, the 20th. Two with baffles and two with pistons.  The pistons both worked great but one of the baffles was directly involved in a failed deployment on my Thor-Agena.  Baffles filter hot material out of the hot gas but they don't really cool or stop the hot gas, and I didn't use enough safety wadding.  A seam/edge of the nylon chute got sealed enough to prevent deployment but not seared enough to ruin the chute.  The exact same chute worked perfectly with the piston of my new Titan 34C (no wadding what so ever).

A chute or shock cord could be seared by hot gas and material coming out the tube, since it is free to just come shooting out.  But, since the shock cord starts at the end of the long kevlar cord, the chute and shock cord are usually very far away from the escaping gases.

Attached is the Titan on chute.  You can make out the piston between the chute and main body.  From the piston, elastic shock cord to the chute and nose cone, kevlar cord to the body.
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1 week 5 days ago #10864 by deangelo54
Replied by deangelo54 on topic Piston Ejection
In my piston rocket I start out with Kevlar from the forward centering ring. From that I connect to an elastic cord about an inch before the end of the body tube. This is to prevent zippering. The elastic shock cord connects to the aft end of the piston. The top of the piston is elastic cord to the chute and nose cone. keep up the good work.

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