Most effective baffle design?

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JGemini

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Hi everyone,
I'm currently working on a scratch build rocket in a BT50 tube, and I want to try implementing an ejection baffle in this one. I've never built/used one before, but the concept sounds appealing. I've seen all sorts of different designs online though (Apogee, Semroc, Qualman, etc.) and I was wondering if one design is considered 'better' than others, particularly for small tubes like BT50. I'm sure everyone has had varying experiences with any of them, but I'm just looking for some guidance.
Thanks,
Joey
 
One advantage of some designs over others is the ability to shake out the ejection charge debris the inevitably accumulates.
 
Any design that slows the gasses and allows them to cool before reaching the recovery system (parachute) but at the same time doesn't create enough back pressure to eject the motor is successful. Beyond that, personal preference.
 
Any design that slows the gasses and allows them to cool before reaching the recovery system
After reading this forum for a while my understanding is that "cooling the gasses" is not really the issue; rather, it is blocking/intercepting the hot and/or flaming particles that inevitably are mixed in with the gasses.
but at the same time doesn't create enough back pressure to eject the motor is successful.
That's the real rub. I have never been clear how to judge this in a baffle design. In fact, I have been so paralyzed by this question that I haven't yet put a baffle in any of my rockets. I realize that this is my own problem to deal with. :)

I will say I am fascinated by the huge variety of baffle designs.
 
After reading this forum for a while my understanding is that "cooling the gasses" is not really the issue; rather, it is blocking/intercepting the hot and/or flaming particles that inevitably are mixed in with the gasses.

That's the real rub. I have never been clear how to judge this in a baffle design. In fact, I have been so paralyzed by this question that I haven't yet put a baffle in any of my rockets. I realize that this is my own problem to deal with. :)

I will say I am fascinated by the huge variety of baffle designs.

Well, if each "layer" of the baffle has more open cross-sectional area than the last upstream restriction, you have more gas flow than a minimum-diameter or stuffer tube.

So if you have an 18mm motor tube in a 24mm body, say -- the first baffle should probably have not less than 18mm open cross-section ( you wouldn't want to use 13mm, the inside measurement; as you'd have gas expand then re-choke when it narrowed back down from the OD to the ID )

I'm probably not explaining myself well, just don't re-narrow the gas path while you're bending it 🙃
 
Yes, so consider the following scenarios:
  1. A minimum diameter rocket, or a rocket with full-length stuffer tube. No baffle design can meet the requirements.
  2. A BT55 rocket with a 24mm (BT50) motor mount. Cross sectional area of a BT55 is about 1.84*cross sectional area of BT50. Virtually (?) every baffle design I've seen has somewhat less 50% of cross sectional area open. Therefore, those baffle designs in a BT55 would all represent a constriction of the gas path.
  3. In your example of an 18mm motor in a 24mm body, the cross-sectional ratio is even worse: 1.77.
So, it would seem that unless your body tube is at least two clicks larger than the motor mount, you will be violating the "no constriction" rule.

And that is a big reason why I've never used a baffle.
 
I've not done this on BT50 but....on some of my rockets 38mm -3" I just tie on a/some Stainless steel scrubbing pads (from the 'Dollar store") to the nomex shock cord. It's easy to clean out any ejection debris or replace the pads. I did learn (the hard way) that stainless steel scrubbing pads do reduce the BP to gass generation ability 'some'.

Tony
 
Yes, so consider the following scenarios:
  1. A minimum diameter rocket, or a rocket with full-length stuffer tube. No baffle design can meet the requirements.
  2. A BT55 rocket with a 24mm (BT50) motor mount. Cross sectional area of a BT55 is about 1.84*cross sectional area of BT50. Virtually (?) every baffle design I've seen has somewhat less 50% of cross sectional area open. Therefore, those baffle designs in a BT55 would all represent a constriction of the gas path.
  3. In your example of an 18mm motor in a 24mm body, the cross-sectional ratio is even worse: 1.77.
So, it would seem that unless your body tube is at least two clicks larger than the motor mount, you will be violating the "no constriction" rule.

And that is a big reason why I've never used a baffle.

I just meant don't neck down tighter than the top of the motor, as a rule of thumb.
 
The purpose of the ejection charge isn't to flow up and out of the rocket, it's to pressurize the body tube. As the ejection gases expend, the rest of the air in the tube compresses. It is not necessary to have full free flow of the gases through the baffle as long as the pressurization can occur.
 
I baffle most of my rockets, 24mm and up. I have tried several of the commercial versions but they just look fairly restrictive, My favorite scratch built is the 3 slightly-more-than-half disks in a coupler. Works great for smaller rockets, just keep the bottom of the baffle at least 2 in. above the motor, preferably more.
 
Hi everyone,
I'm currently working on a scratch build rocket in a BT50 tube, and I want to try implementing an ejection baffle in this one. I've never built/used one before, but the concept sounds appealing. I've seen all sorts of different designs online though (Apogee, Semroc, Qualman, etc.) and I was wondering if one design is considered 'better' than others, particularly for small tubes like BT50. I'm sure everyone has had varying experiences with any of them, but I'm just looking for some guidance.
Thanks,
Joey
This was for BT-55 size tube, but I have used this design with BT-50 also, simple, works great. For BT-50 use two couplers, make the inside tubes about 1/4" shorter than the couplers. the bulkheads are just cast 30 min epoxy, set ends on masking tape and pour about 3/32" - 1/8" thick. For the inside tubes on BT-50 baffle, I used 10mm tubes from ASP. The ones in the photo are from a loc 1/2" launch lug tube.
18a1fae7-9fdf-4a08-8453-8d506ff9bea8.jpga98c33c4-7293-4f8f-9f46-e8c449b0b816.jpg
47f4ca93-bade-494c-a003-f70937595427.jpg
 
I baffle most of my rockets, 24mm and up. I have tried several of the commercial versions but they just look fairly restrictive, My favorite scratch built is the 3 slightly-more-than-half disks in a coupler. Works great for smaller rockets, just keep the bottom of the baffle at least 2 in. above the motor, preferably more.

I use this method in most of my smaller diameter rockets, just recently in an Estes Star Orbiter, where I added a kevlar "Y" harness to it for an anchor point. Shoved it down about half way to keep the chute from sliding all the way down the body tube.
For larger rockets, nothing beats a cold air baffle, which I put in my Nike Smoke recently.
Start with a stuffer tube/motor mount tube that extends up into a coupler.IMG_2592.jpg

The upper bulkplate on the coupler has four smaller tubes attached pointing down.
IMG_2590.jpg

When you assemble it, the stuffer tube fits up into the baffle, creating a path for the ejection charge to hit the center of the upper bulkplate, circulate back down, forcing cold air up into the upper part of the tube.
IMG_2594.jpg

I added an eyebolt to the upper bulkplate for an anchor point.
 

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IMHO I wouldn't go for a baffle. Wherever possible I use a piston and kevlar shockcord. Much easier to clean out after a flight and don't have to worry about gunk getting caught up in the baffle. And if the shockcord ever does break down near the fixture point I put in a guide tube through the motor mount so another one can be attached.
Cheers, Mark
 
Here's my favorite design. The purpose is 3-fold) Stop hot particle movement; 2) lightweight; 3) Use minimal space. I prefer stubby designs so #3 is important. These photos show the Baffle Vent Valve installed and ready for the upper body to be glued on, and the shot with the valve open shows it's operation (gravity is holding the valve open - the pic is upside down to make it easier to see the operation of the valve). This is a BT-80 build. However I've used this same design in both my 4" Lvl 1, Lvl 2, and my last BALLS build (L850 to 12k, mach 1.2). One of the big plusses is that the parachute bundle (burrito wrap) sits directly on top of the baffle and upon ejection, the valve itself gives the bundle a 'push' as the valve opens and gases expand. Also, the position of the Baffle in the body keeps the parachute bundle from sliding aft and messing with the CG.

The strings are kevlar (100# in this iteration - I use 250# in the 4" version). For the two valve pieces I mostly use heavy matte board (1/16") that is soaked in wood hardener. I use plywood if the rocket is expected to deal with high gee forces. So far I've kept my gee's under 25 or so. The valve has worked flawlessly every time.


Bottom.jpgTop.jpg
 
Wow. Lots of interesting stuff here. As I probably should have guessed, there doesn't seem to be a clear cut answer. Consensus seems to be 3 half moons are fairly reliable for small tubes, so that might be what I go with. I wonder if anyone has conducted any study on the various baffle types. Nothing in the NAR R&D database, so maybe a good topic for a future project. Thanks for your help.
 
Yeah I get lost here too. I like the idea but I'm afraid of ejection problems. The half moons ice heard of, as well as the scrubber. The 2 half tubed next to each other is another ive seen.


This makes me want to skip to piston ejection lol
 
I've had great luck with the half moon design. Just make sure they overlap slightly and are on opposite sides of the tube. Should really be more like 60% moons than half moons. The idea is that a burning particle can't go directly past one and get to the chute.

Normally 3 are enough but I have done 4 in longer rockets. See the pic below. Black outline is the coupler while the red is the baffles
 

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You can slide one down into a long tube from either end to install it, but if you are scratch building one try to use one in combination with a coupler between two body tubes. Much easier to install a baffle like a coupler.
 
So many designs! I recently had a burned parachute and I used wadding, probably not enough. Online the idea of a baffle. For the 3 .6 moons, how much space in body tube diameter between disks?

I've also seen the holey designs. https://qualmanrocketry.com/Baffle BT plywood.html

For heat proofing, some say coat with glue others have said silicone.
 

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I just got some of those baffles from Qualman earlier this week and I'm really impressed with the design. It's different than anything I've seen and looks to be extremely effective as I don't see any possible way a hot ember from the motor could make it up to the recovery system. If you are going to buy baffles, buy these.

As for other designs. The 3 or 4 half moon shapes on opposite sides of the tube has worked extremely well for me over the years. Normally 3 plates are enough but I will say that using 4 plates works better. I've not had a single issue when using rockets with the 4 plate design. If I'm building a shorter rocket, I have had to cut that down to 3 plates and it works but I have seen a couple of burn marks on chutes but nothing that would cause a failure. At least not yet.

As for spacing of the discs. I use the inner diameter of the motor as a guide. 18mm motors have a are well, 18mm or about 3/4 of an inch so I space them about 3/4 of an inch apart. 24mm motor are about 1 inch so I space them about 1 inch apart.

For materials. I have seen Balsa baffles blown apart but I don't know how they were constructed. I guess you could soak them in CA or epoxy and they would last. I personally use Basswood on smaller rockets and hobby ply on larger models. As for coating. Some use high temp spray paint. I personally just coat them in normal Tite Bond 2 glue.

There are a number of other designs but one that seems to be popular is shown below. I've found that this design is only effective in specific conditions. Rockets where the baffle can be placed far up the rocket body. By far I mean 36 plus inches from the motor.

I had one of these that I decide to use in a recent Brighton Clone build. That model has a stuffer tube that runs up about 17 inches. I placed the Baffle at the top of the 29mm stuffer tube with a 1 inch gap between the stuffer tube and the lower plate of the baffle. I also inserted the baffle with outside holes facing down. This would require the gases and embers to leave the stuffer tube and make it to the openings at the outer edge of the baffle then come back to center to exit the top. I figured that embers would be stopped as they won't have a chance to make direction change in the 1 inch gap between the stuffer and baffle.

So far that rocket has flown once with an issue. I guess time will tell if it continues to work.
 

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For the smaller rockets I'm ok with the holey type. Estes ejection charges for a 13mm is 0.4g BP so if your baffle is 5 or 6 inches up it works ok.
I feel the problem is those little holes are pretty restrictive, it tends to want to blow the baffle right up the tube.
An 18mm motor has 0.6g BP and a 24mm motor has 1g BP.
I use 1g to pop the nose off my 4 inch dual deploy rocket so 1g is quite a lot.
Baffle.jpg
Compare the area of the opening and you can see the half moon type is much less restrictive than the little holes
As for how far apart I would say at least 1/4 in. But if you put 4 baffles in a 1 1/2 in. long coupler they will be almost 1/2 inch apart. The bigger the tube the longer the coupler so the further apart they will be.
One thing , in your picture it looks like you added angles below the baffle plates. I would not do this, it would tend to bounce the burning particles up thru the baffle which is what you are trying to avoid.
As far as heat treating if you have a tube of silicone open and handy it would be the best. I tend to use what ever is handy, and wood glue seams to work ok.
 
For the smaller rockets I'm ok with the holey type. Estes ejection charges for a 13mm is 0.4g BP so if your baffle is 5 or 6 inches up it works ok.
I feel the problem is those little holes are pretty restrictive, it tends to want to blow the baffle right up the tube.
An 18mm motor has 0.6g BP and a 24mm motor has 1g BP.
I use 1g to pop the nose off my 4 inch dual deploy rocket so 1g is quite a lot.
View attachment 438187
Compare the area of the opening and you can see the half moon type is much less restrictive than the little holes
As for how far apart I would say at least 1/4 in. But if you put 4 baffles in a 1 1/2 in. long coupler they will be almost 1/2 inch apart. The bigger the tube the longer the coupler so the further apart they will be.
One thing , in your picture it looks like you added angles below the baffle plates. I would not do this, it would tend to bounce the burning particles up thru the baffle which is what you are trying to avoid.
As far as heat treating if you have a tube of silicone open and handy it would be the best. I tend to use what ever is handy, and wood glue seams to work ok.

I agree that the opening are much smaller but you are not really blocking the path of the burning embers. Regardless of the direction you install the baffle there is a direct path. The closer the baffle is to the motor the more likely the chance that you are going to catch embers bouncing off the tube or interior baffle walls and out the top.

By comparison the moon shape baffle requires the embers to change direction. A three piece design requires it to change direction once where a four piece would require it to change direction twice. The one factor that is important to remember with the moon shape is that the must overlap each other. If they are true 1/2 moons then there is the potential of something getting by.

As for back pressure. I was concerned when I first started using baffles as well but I've never had an issue with excessive pressure. Even in rockets with friction fit motors I've never had a motor push out or had a tube fail due to over pressure. I guess it could happen if the ejection was strong enough.


baffle comparison.jpg

Thinking about this a little bit more. If you added an additional plate to the hole style and made it a 3 plate system you would then be forcing the embers to change direction and it would be much more effective even when located close to the motor.baffle compare 2.jpg
 
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An idea for rockets that only have room for a 2 plate moon style would be to add a lip on the end of the plate that directs gas flow. Of course these would need to be long enough that there wouldn't be a direct shot past them from an angle.


Baffle 3.jpg
 

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