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mycrofte 11-03-2019 08:54 AM

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This is an old thread but seemed appropriate.

Suddenly occurred to me there was no need to build a separate baffle when I am already building an engine tube. So, for my last few rockets, I have been designing one right into an extended tube.

jetlag 11-04-2019 07:50 AM

By your sketch, it looks to me like that baffle will burn up after just a few flights. Not enough room for flame or heat dissipation.


ghrocketman 11-04-2019 08:53 AM

+1 to what jetlag just said.
That thing will be TORCHED in less than 5 flights.

mycrofte 11-04-2019 09:08 AM

Originally Posted by jetlag
By your sketch, it looks to me like that baffle will burn up after just a few flights. Not enough room for flame or heat dissipation.


The drawing was more for concept than dimensional accuracy. But most baffles seem to be about 2". I have never done a test burn to see how far the flame comes out during ejection.
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tbzep 11-04-2019 09:21 AM

I built a AAA Aviation Fuels Cherokee G with a similar motor tube baffle. Even with heavy wall tubing, the holes in the motor tube directed jets of hot gases and eventually burned the body tube at the location of each motor tube baffle hole.

mycrofte 11-05-2019 05:24 AM

I don't plan on anything that big.

tbzep 11-05-2019 07:03 AM

Originally Posted by mycrofte
I don't plan on anything that big.

My point is that it even burned through heavy wall tubes, not the size of the rocket. I've built D powered rockets that were much larger.

The ejection gases need to be contained in a way that they do not get directed toward an outside wall unless it is protected with a heavy coupler, preferably fish paper or something that will take a long time to ablate.

mycrofte 11-06-2019 05:40 AM

Might be a good idea to cut an inch long section of tube and glue in where the holes exhaust. At least doubling the body tube there would last longer.

vulcanitebill 11-09-2019 10:28 PM

I have buit baffles of that type in 29mm motor mount tubes. I use a longer tube, at least 12" long, glue a 29mm wood disk in the front of the tube and then a solid bulkhead of airframe diameter with big slots or openings cut in the perimeter. I build an eyebolt into this to attach the recovery harness to. Then I cut some rectangular openings in the side of the motor tube just behind the bulkhead, usually a couple of rectangular openings on opposite sides of the tube. The part of the tube that remains gets glassed so it can still resist pull from the recovery harness.

I didn't know how large the openings needed to be but I figured that an Aerotech RMS could blow through a 24mm or 29mm tube so I calculated the area of a 24mm tube and try to make the slots and openings add up to at least that much area.

I've not had a problem with this but my rockets haven't had more than a half dozen flights each. I'm not concerned about burning out the airframe because the openings in the motor tube are far enough in front of the motor that the flames probably won't reach that far.

I don't think a baffle makes sense in a very short rocket. I think the best strategy would be a kevlar chute protector.

ghrocketman 11-10-2019 10:35 AM

The ejection flame front of typical SU BP motors goes at least 8" if not more.

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