Ejection Baffles

How do these work?
Not as in how well, but rather what is the mechanism involved?
From what I have seen (very little) it appears that the material is a metal mesh like stuff , i.e., steel wool "fluffed up" as it were. I'll call it the Baffle Medium (BM)
What I imagine is that the BM is loose enough (because we all know that loose BM is much better than the other kind ) to allow the flow of the pressure of the ejection charge to move through it, but as it is a fine metal, it absorbs the heat. What comes out the other side, while most likely not cool, is enough so that it doesn't melt whatever you are using for recovery.
So is there anything special one needs to do to use this technology? Would you recommend single ought? Double ought? Maybe I need to go heavier rather than lighter? Something other than steel wool?

Thanks,
Greg

Another kind of baffle uses plates with alternating openings to cause cooling of the not gasses. It also traps the hot particle below the baffle. Picture three bulkhead plates separated by two pieces of coupler tube 1 inch long. Each plate has approx. 1/3rd of its area cut off one side, and the plates are stacked with the couplers between and with the openings on the opposite side as the one directly below.

Alternately, stack three bulkhead plates and tape the edges together, then drill four holes in a symmetrical pattern. Ideally, the area of the holes should be about the same as the area of the motor. Build a stack of plates separated by coupler sections, and twist the plates so that the holes in one are not aligned with the holes in the one directly below.

I prefer this kind of baffle because the ydo not become clogged with burnt particles from the ejection charge. I have used both kinds on BT 60 and larger tubes, Estes D motors through AT G motors. There is no reason I know of that it cannot work at any power level.

Another type of baffle I used once involves plugging the end of an extended motor tube and making rows of holes in it from the top down to a couple of iches above the top of the motor. The top centering ring has holes drilled through it and is mounted above the topmost row of holes in the motor tube. The hot particles stay mostly in the motor tube, and the hot gasses cool as they expand in the space between the motor tube and the body tube, then through the holes in the centering ring. This type of baffle adds little or no weight.

On a large diameter rocket I installed a bulkhead plate with holes in its perimeter and attached half an aluminum soda can to the botton center with JB Weld. The motor mount extended into the can about an inch. The hot gasses and particles hit the inside of the can, were redirected back downward, expanded and cooled in the large space between the BT and MMT, then up through the holes in the plate.

I have never had a scorched chute with any of these designs.

Check out issue #129:
https://www.apogeerockets.com/education/newsletter_archive.asp

Great article about baffles!

don't use fine steel wool, it' burns rather nicely
the aerotech stuff is stainless steel mesh, look for "chore boy" in stores(in the pot scrubber section)

also be carefull with baffles in tubes smaller than bt-60, I have seen more than one case of the tube overpressurising and blowing apart, there is a century(?) tech report describing baffles and that phenomina in more detail.

Originally posted by 11Bravo
How do these work?
Not as in how well, but rather what is the mechanism involved?
From what I have seen (very little) it appears that the material is a metal mesh like stuff , i.e., steel wool "fluffed up" as it were. I'll call it the Baffle Medium (BM)
What I imagine is that the BM is loose enough (because we all know that loose BM is much better than the other kind ) to allow the flow of the pressure of the ejection charge to move through it, but as it is a fine metal, it absorbs the heat. What comes out the other side, while most likely not cool, is enough so that it doesn't melt whatever you are using for recovery.
So is there anything special one needs to do to use this technology? Would you recommend single ought? Double ought? Maybe I need to go heavier rather than lighter? Something other than steel wool?

Thanks,
Greg

Click to expand...

NOT steel wool. You can light it with a match. This would be a Bad Thing between an ejection charge and your chute. Must be the bigger "Chore Boy" stuff.

Not sure what "size" AT uses, but it works OK. They say to stretch it out to 6" length. I have one I compressed down to 2" in the MMT and the flow-through is still fine. I'll let you know about heat results after testing.

It catches burning junk. Making it removeable for cleaning is a good idea I've seen offered. IMO, AT should make their baffle screw-on/off. I'd buy them for my birds if they did. I'd like to see a nice removeable retro-fit version.

Copper catches heat better than steel. These are available at stores also (or so I'm told, I haven't found either around here yet).

We (FlisKits) are working on several baffle kit designs from BT-50 through BT-70 size.

In the case of the BT-50 baffle, this will consist of 3 bulkheads with a slice taken out (looks like a "D" shape), assembled in an alternating pattern inside a stage coupler.

The other sizes will make use of a pair of rings with 1/4" holes cut in a circumference in a manner that they do not line up when assembled.

None of our designs use any form of steelwool as our experience shows they cause more problems with clogging than any help they provide in cooling the gasses.

Our experience is that, with the baffle located near the top of the rocket, the cool air in the body tube cools the ejection *gasses* sufficiently to protect the parachute. The real culprit are the hot particles. These are blocked and trapped by the baffle.

I have a Deuce (2 ejection charges per flight) with one of our baffles in it and I am 100% after about 10 flights, so they look good.

I have newly designed rings coming in, in the next week or so. Assuming they work as predicted we expect to have kits available in late september/early october.

keep watch!

jim

First, I knew that finer and finer steel wool burns better and better. When I went to the winter warfare school at Ft. Greeley, AK, one trick they showed us to start fires was to use #0000 steel wool and unroll a roll, roll it back up with dryer lint sandwhiched in, and touch it with a 9v battery to light.
Second, I had already downloaded the Apogee newsletter that deals with baffles; I haven't read it yet and it just plain slipped my mind.
I'm sitting here looking at this 3 foot tall grey crayon and brianstorming (BSing ) ideas.
Thanks all for the replies and the info,

Greg

The AT "cooling mesh" tends to catch the ejection particles. If you are using the reloadable system, make sure that you retreive the red ejection cap after flight. Most of the time it will fall out when removing the hardware. Sometimes it takes some tapping on the tube. But, you should systematically use a rod of some sort (hanger) to dislodge the build up.

Originally posted by jflis
None of our designs use any form of steelwool as our experience shows they cause more problems with clogging than any help they provide in cooling the gasses.

Click to expand...

That's a rather strong statement, Jim, especially with the number of successful ejections in AT birds. You've done some testing?

Originally posted by 11Bravo
Would you recommend single ought? Double ought? Maybe I need to go heavier rather than lighter? Something other than steel wool?

Click to expand...

I just found some copper Chore Boys. I got two to try. They're lighter than the AT mesh, so I'll use them both.

What I thought of when comparing these with the AT mesh: metal lathe tailings. Pick up the longest tailings from a metal lathe, soak them in solvent to get the cutting oil off, rinse them with nail polish remover to get the last of the solvent off and dry them, and tangle them into a mass. Beware sharp edges.

I am presenrtly putting together a bag of parts to build a bird specifically to use removeable mesh, so I can measure it before and after several launches to see how much stuff it accumulates.

Originally posted by ssthor
The AT "cooling mesh" tends to catch the ejection particles. If you are using the reloadable system, make sure that you retreive the red ejection cap after flight. Most of the time it will fall out when removing the hardware. Sometimes it takes some tapping on the tube. But, you should systematically use a rod of some sort (hanger) to dislodge the build up.

Click to expand...

Most of the time, the mesh will out live the rocket. However, when you plaay it safe and you keep that 50-100 dollar rocket so long and have so many launches that the baffle clogs up, then what? It seems a lot more logical to use something of exactly the same quality that will always outlast the rocket. That way, you will not have to retire your 50 dollar rocket that you have cared for since infancy to a display stand.

Originally posted by DynaSoar
That's a rather strong statement, Jim, especially with the number of successful ejections in AT birds. You've done some testing?

Click to expand...

Enough testing to know that baffles with steel wool eventually clog and those with out do not.

Will the steel wool help cool the gasses more? Certainly. Is that extra cooling needed? Again, my experiences says "no". It's a trade off.

Mind you, my experience is with BP motors (which our baffles are designed for).

A "strong statement"?, not really. Just providing what my experience has shown me is all.

Well, as they say (they being me and people like me): "It's an empirical question".

So here's the bird: 18" BT50, 4:1 tangent ogive PNC50, three balsa fins, 3" x 1/8" lug, kevlar anchor epoxied mid-body, and elastic shock cord. Brand new Rockethead 12" mylar chute.

2.75" D hook engine mount through the body, anchored by an inch of BT50 with a slit in it covering the hook slot. Inside that is a 20/50 engine mount sized exacly like a D, to fit the hook, but will contain any regular 18mm motor using another hook built into the adaptor.

The bird weighs 62 grams loaded without motor. 20 grams of that is the copper Chore Boy baffle, rolled to 1" diameter and 6" long. The removeable 18mm motor mount allows access to the baffle.

I'll launch it on various 18mm motors, checking weight for accumulation and chute for burn marks. I'll use no wadding. If there's significant weight gain, I'll remove the baffle, weigh it and the bird separetly to determine where it gained.

The nose is a tight fit, and I'm not going to change that. If the baffle clogs up too much, this should fail.

Parameters and notes will be recorded on the bird; initial numbers and CG already are.

create a replaceable/removable heat sink by sandwiching a piece of window screen between 2 centering rings with holes in the middle...this will grad the hpt particulate while allowing the gases to go into the baffle where there will be cooled even more....this heat sink is just placed between the top of the engine and the baffle...so you cna remove it to clean it out.....

I have actually used circular pipe screens as heat sinks/particulate catchers on small bp models.....

Just so happens my latest project will be using a baffle. Here's a 2.6 version using the "AT type" stainless scrubbing pad for the innards.

Originally posted by shockwaveriderz
create a replaceable/removable heat sink by sandwiching a piece of window screen between 2 centering rings with holes in the middle...this will grad the hpt particulate while allowing the gases to go into the baffle where there will be cooled even more....this heat sink is just placed between the top of the engine and the baffle...so you cna remove it to clean it out.....

I have actually used circular pipe screens as heat sinks/particulate catchers on small bp models.....

Click to expand...

{forehead smack}

I shoulda knowed. Available by the little envelope full at the water bed (and stuff) store. Darn near disposable. Tuck one up in there before the motor. Check it before the next flight. If it's fried or clogged, replace it.

Not all by itself, of course, but it'd definitely help.

Now, *there's* a thought...

why *not* "all by itself"?

It would be interesting to see if you could use a ball of steel wool like those chute protectors. Attach it to the shock line with a length of kevlar and just push it down the tube ahead of the parachute for each flight.

Originally posted by jflis
...
It would be interesting to see if you could use a ball of steel wool like those chute protectors. Attach it to the shock line with a length of kevlar and just push it down the tube ahead of the parachute for each flight.

Click to expand...

Hmmm....food for thought...
My only concern would be how to keep the "ball" together. Also, would the heat absorbed by the wool, which is contacting the chute, cause melting/burning?

I think its amazing that in the year 2004, we still don't have any data on how hot the ejection charge gases really are..... or worse, if the gases are funneled through a baffle of X or Y design, how much the gases are cooled......

If some 16-18 yr wants to do an NAR R&D report and win that $1000 cash prize next year at NARAM heres a good idea for you... Use thermocouples at different dstances from the ejection charge to measure the drop in temperatue of the gases with length and diameter...then try baffles....

The old centuri baffle was no mor than what? 1-1.5 in length ? I wonder what the gas temp change was in that 1-1.5" .......perhaps if one was made longer, then it would cool the gases that no wadding at all would be needed....

I noticed that my Mean Machine that ejects at the end probably doesn't really need wadding. At the beginning of this thread I mentioned several styles of "solid" baffles. I had always theorized that baffles worked on the same principles as firearm sound supressors. Gasses are slowed and cooled by expansion and turbulence. I doubt that the baffle material absorbs much heat. It doesn't really have time. In the case of ejection, burning particles are also a big part of the problem, and I bet that if you stop the actual flame front and the hot particles, that is 90% of the battle. The steel wool probably doesn't actually save the recovery system by absorbing heat, but by stopping the flame front and the hot particles. Wooden baffles work just as well, and their capacity to absorb heat quickly is very small compared to steel wool. In a small diameter rocket wadding is more like a piston, pushing the laundry out in front of it and keeping the heat and burning particles below it.

Or maybe not. Just food for thought.

Originally posted by shockwaveriderz
I think its amazing that in the year 2004, we still don't have any data on how hot the ejection charge gases really are..... or worse, if the gases are funneled through a baffle of X or Y design, how much the gases are cooled......

If some 16-18 yr wants to do an NAR R&D report and win that $1000 cash prize next year at NARAM heres a good idea for you... Use thermocouples at different dstances from the ejection charge to measure the drop in temperatue of the gases with length and diameter...then try baffles....

The old centuri baffle was no mor than what? 1-1.5 in length ? I wonder what the gas temp change was in that 1-1.5" .......perhaps if one was made longer, then it would cool the gases that no wadding at all would be needed....

Click to expand...

You took the data right out of my calculations.

It's a whole new century and we're still using and arguing over last century's technology. AND adding more layers of it rather than testing for what works and trying new things.

The Great Old Farts found what worked. We need to find what Works Better. Anyone who actually wants to do real research and prove what's better can call on me for advice on methodology, controls and sadistics (statistics).

We have the technology. We can rebuild this hobby. Make it faster. Stronger. Lighter. Just as long as we don't have to watch that poor DynaSoar doing somersaults at landing week after week.

oh lordy...

...I'm an old fart...

I think rbeckey and shockwaveriderz have some valid observations about how baffles work. I am not sure if they really cool the ejection gas very much, but I think they can do a good job of mixing it with other internal air and diluting the heat before it gets to the plastic 'chute.

Anyway, for what it's worth, I threw in some phots of one of the standard baffle designs that I have settled on. I believe it separates particles by flinging them outward when the ejection flow has to cut around corners -- the particles end up trapped in the odd corners and pockets. Occasionally I will see some indication that gasses are making it all the way through the baffle (at least, they are hot enough to wilt the 'chute a little bit).

On to the attachment.

The baffle body (top left of prev phot) is nose-down and has the fwd retaining ring already in place. The baffles are laid out left-to-right in the sequence that I installed them and the 1/4 inch spacers are shown at the top right. The baffle plates were made from scrap poster matting board (because I can't find a vendor that will sell solid bulkhead plates without pre-slotting them to pieces to use as 'universal' centering rings). Holes were punched with a paper punch and are 1/4 inch in diam. Soaking the baffle plate with CA turns it into a pretty tough little piece of structure.

The assembled baffle looks like this (you are looking at the front end this time):

I pre-glue the inside of my rocket's BT with thinned white glue in the zone where I will want my baffle finally located. I usually have to swab the glue around in there with a Q-tip stuck into the end of a straw to be able to reach eight to ten inches inside. Don't forget to attach your tether (kevlar, or braided nylon, or elastic band) to the baffle before it is installed.

After waiting for the first glue to dry, I dry-fit-check the baffle to make sure it fits snugly and peel off a little paper (from the outside of the baffle) if needed. I coat the outside of the baffle with a thin layer of thinned white glue and let dry. I push the baffle into place with a big honkin dowel (think curtain rod size) and drip some more very thin white glue around the perimeter joint. It soaks on through the joint and bonds things quite well. After all that is dry I follow up with a good internal fillet of normal-strength white glue on that same perimeter.

Never had one blow out (yet). Never had one burn up (yet). Never had one let go of the tether (yet).

now, *that's* a nice design. It looks very effective and still rather light weight.

When I install baffles, I like to install them as close to the top of the rocket as I can and still have room for the parachute and cords. Three reasons for this:

1) Keeps the weight of the baffle forward, increasing stability

2) Keeps the laundry weight forward, increasing stability

3) Provides the largest amount of ambient tempurature air that the ejection has to travel through before encountering the baffle (helps in cooling)

I also try to keep baffles forward for the same reasons as Jim. It also occured to me that if you consider wadding as a type of piston, it explains why it is more difficult to protect the chute in larger diameter rockets. It is hard to get enough wadding in there to efficiently seal the ejection particles and flames behind the wadding. I much prefer baffles in BT60 and up sizes.

Originally posted by powderburner
(because I can't find a vendor that will sell solid bulkhead plates without pre-slotting them to pieces to use as 'universal' centering rings)

Click to expand...

If you order a centering ring from BMS and specify a size of 0 for the MMT, you get a bulkhead.


Bill

Originally posted by jflis
oh lordy...

...I'm an old fart...

Click to expand...

Embrace it, Jim. It provides you with an excuse for wider latitudes of behavior. And that amuses the kids no end.

Plus, you can join AARP (Advanced Age Rocket People). Then you can get one of those little electric scooters so you don't have to walk to retrieve your rockets.

Wave your cane and hold it high,
you're an old fart, so am I.
Sound off,
one, two
sound off,
What? Who?
Was I saying something?
Where's my FSI catalog?

Just and FYI, FlisKits has announced a "Coming Soon" for our baffle kits in this thread.

jim