Baffling questions

My question is with regards to a drilled-plate baffle: basically, which end is up, or does it not matter? The baffles that I've gotten from Uncle Mike's Rocket Shack or Jon Rockets didn't come with instructions. If I want to add an eye bolt for recovery attachment, and I want that eye bolt to be to one side, then it seems easier to attach it to the baffle plate that has the holes in the center, rather than the one with holes around the edge. It would be more structurally sound. OTOH, it seems like flow through the baffle might make more sense entering through the middle and exiting around the periphery, and might be more effective blowing the cone out. Any thoughts on this?

Without doing a set of trials, your guess is as good as anyone's. IMHO, I don't think that it will make that much difference one way of the other WRT flow.

That said, attach the baffle the way that makes the most sense (or is easiest) for this rocket.

Also, you don't have to use a screw eye, You can form a loop of heavy braided Kevlar and glue it to the inside of the baffle with epoxy. Or you could glue a small block of wood inside the coupler where the screw eye could be screwed into. That way the block is sharing the load stress, not just the baffle plate.

Greg

We are talking low-power here.... As long as your shock cord attachment is stronger than your friction fit nosecone you are good. Just make sure you have a long enough cord so the shock is minimized and does cause damage to your airframe assembly.

I'm not sure what you're saying - I would think that the shock cord attachment would need to be able to support the weight of the rocket body under chute, plus some "shock" factor. But that's not really relevent to my question (which way is up on the baffle)...

GregGleason said:

Also, you don't have to use a screw eye, You can form a loop of heavy braided Kevlar and glue it to the inside of the baffle with epoxy. Or you could glue a small block of wood inside the coupler where the screw eye could be screwed into. That way the block is sharing the load stress, not just the baffle plate.

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I like both of these suggestions. I have used the kevlar loop in a few builds and it has held up very well. Adding a block glued to the inner wall of the coupler and up against the baffle disk to screw an eye into sounds really strong. And no, I don't think it matters which end is up as far as pushing off the nosecone (pressure is pressure), but I tend to put the side with the holes around the periphery towards the motor so that the solid center gets the brunt of any ejection particles.

kj

kjohnson said:

... And no, I don't think it matters which end is up as far as pushing off the nosecone (pressure is pressure), but I tend to put the side with the holes around the periphery towards the motor so that the solid center gets the brunt of any ejection particles.

kj

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That's a good point. Holes on the periphery of the bulk plate will be less likely to have the "great balls o' fire" intrude inside the coupler at ejection. So holes in the center to the top (closer to nose cone).

Greg

GregGleason said:

That's a good point. Holes on the periphery of the bulk plate will be less likely to have the "great balls o' fire" intrude inside the coupler at ejection. So holes in the center to the top (closer to nose cone).

Greg

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Agreed, that was what I'd thought. Doing it that way also adds somewhat to the baffling, assuming you used a stuffer tube to come up close to the lower baffle plate. Gases and particles have to flow outward from the stuffer tube to the periphery ring of holes, then back toward the center to exit out the top. Sort of like adding an extra chamber to a muffler. But I'd thought I'd read that the periphery holes were better placed on the top to let the gasses vent around the parachute (which might have just been someone's opinion on the internet...), that's why I asked the question.

And those are good thoughts for cord mounting. The couple of baffles I've used had the eye bolt double nutted against the plate with washers, but that is admittedly kind of heavy. The rocket I'm building now needs nose ballast anyway, but on smaller LPR stuff, I can see the detriment.

EXPjawa said:

My question is with regards to a drilled-plate baffle: basically, which end is up, or does it not matter?

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Typically, when I build with this type of baffle, I put the bulkhead with the holes to the outside of the disk on bottom. This gives a large, uninterrupted area for the ejection gasses to impinge upon. But I could be exactly wrong. Centauri used this technique quite a bit. Some research at JimZ's web site should tell you how they did it.

For the Centuri Orion, the bulk-plate with the perimeter holes were shown to be placed in the forward position.



Greg

GregGleason said:

For the Centuri Orion, the bulk-plate with the perimeter holes were shown to be placed in the forward position.

View attachment 256433

Greg

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HA! See - I was right; I was wrong.

Well, I guess that's the heart of the question - I've seen that was how they did it. But does it have to be that way, or is it acceptable to build it the other way around? It looks like a number of people are doing the opposite way and I'm guessing that it works for them...

EXPjawa said:

Well, I guess that's the heart of the question - I've seen that was how they did it. But does it have to be that way, or is it acceptable to build it the other way around? It looks like a number of people are doing the opposite way and I'm guessing that it works for them...

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i thinks it's acceptable to build it either way.

Greg

EXPjawa said:

Well, I guess that's the heart of the question - I've seen that was how they did it. But does it have to be that way, or is it acceptable to build it the other way around? It looks like a number of people are doing the opposite way and I'm guessing that it works for them...

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The intent of a baffle (of any design) is to prevent hot/burning particles from hitting the parachute and recovery system. There's a column of "cold" air on top of the baffle that will (hopefully!) push the nosecone off and recovery gear our, before the hotter gasses expand up the tube. To me, it seems like having the holes to the outside gives a large, solid target for the ejection charge particles to strike, with reduced chance of them making it up into the baffle. Aerotech uses a baffle (what is the hole pattern?), and include steel wool for additional "insurance".

sooner.boomer said:

The intent of a baffle (of any design) is to prevent hot/burning particles from hitting the parachute and recovery system. There's a column of "cold" air on top of the baffle that will (hopefully!) push the nosecone off and recovery gear our, before the hotter gasses expand up the tube. To me, it seems like having the holes to the outside gives a large, solid target for the ejection charge particles to strike, with reduced chance of them making it up into the baffle. Aerotech uses a baffle (what is the hole pattern?), and include steel wool for additional "insurance".

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The Aerotech system is less of a baffle and more of a vent. The best way I can describe it here is as two concentric circles, smaller solid forward, larger with hole aft, connected in a T arrangement with a cross of plastic.

I would not use this without wadding or nomex. It will stop some of the hot particles and gasses but certainly not all.

watermelonman said:

The Aerotech system is less of a baffle and more of a vent. The best way I can describe it here is as two concentric circles, smaller solid forward, larger with hole aft, connected in a T arrangement with a cross of plastic.

I would not use this without wadding or nomex. It will stop some of the hot particles and gasses but certainly not all.

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If you use the mesh with the Aerotech system, can't be beat. I have/had (lost 2) 9 Aerotech kits all 12 plus years old and the elastic shock cords and chutes are in perfect shape yet. Never used any other form of chute protection other than the "baffle". My Initiator has 50+ flights on it, most others 25+. None of the chutes even have a black mark on them. As for using any other baffles, I don't, got lots of dog barf for LPR and use chute protectors for the other MPR, HPR.

As for the baffle design discussed in this thread I would go with the periphery holes up. It should work either way, but why direct the hottest gases/particles toward the wall of the body tube, when they can flow up first into the baffle and be directed to the sides of the wall that is doubled up especially with a thin walled body tube?

rcktnut said:

If you use the mesh with the Aerotech system, can't be beat. I have/had (lost 2) 9 Aerotech kits all 12 plus years old and the elastic shock cords and chutes are in perfect shape yet. Never used any other form of chute protection other than the "baffle". My Initiator has 50+ flights on it, most others 25+. None of the chutes even have a black mark on them. As for using any other baffles, I don't, got lots of dog barf for LPR and use chute protectors for the other MPR, HPR.

As for the baffle design discussed in this thread I would go with the periphery holes up. It should work either way, but why direct the hottest gases/particles toward the wall of the body tube, when they can flow up first into the baffle and be directed to the sides of the wall that is doubled up especially with a thin walled body tube?

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Oh yes, absolutely it works well as a complete system.

My comment about adding wadding or nomex was in response to the notion of using the mount vent alone as a baffle. Any of the three added to the mount vent will be fine.