Scratch building baffles

[n5wd]

A baffle is a device that is used in a rocket between the motor and the rest of the airframe - it can replace wadding or "dog barf", the materials used to prevent the hot particles of black powder that are created by the ejection charge from damaging the recovery equipment. A good baffle design will allow the gasses from the ejection charge to implement the recovery scheme without resistance while preventing any of the damage caused by burning BP particles from reaching the recovery laundry.

Several manufacturers produce baffles, either as an integral part of a kit, or as a stand alone product, but these are available mostly for low and medium powered kits. Baffles are good to use in larger projects, as well - the only difference is that you might need to scratch build one yourself, as I found myself doing in a recent contest.

I'm a TARC (Team America Rocketry Challenge) team sponsor besides being a BAR, myself, and a member of DARS, the Dallas Area Rocket Society. One of the DARS mentors that's been working with my TARC teams for the second year, Jack Sprague, challenged other DARS members to see if they could re-create the TARC challenge and do as well as the high school kids - in this case, scratch build a rocket that flies an altimeter and a single egg (lying in a horizontal position when the rocket is ready to fly) to an altitude of 750 feet, in a flight lasting 45 seconds, and return the egg, unbroken. Several prizes were at stake. Last year, I was pretty busy so I didn't get a chance to take up Jack's challenge to the DARS members, but this year was going to be different. Besides, teaching the kids by demonstrating to them how things are done can be an effective teaching strategy - more on that later.

My challenge bird was going to be a 3" rocket, about 45 inches tall, based on the booster tube, nosecone, and fin pattern of an Estes Big Daddy. Because it was going to be stretched for a payload and altimeter section, I chose to separate the rocket at the end of the stock booster and instead of using a plain coupler, to install a baffle at that point for three reasons:
1. a baffle's upper disk provides substantial structural support for the coupler sticking up out of the booster, and
2. a baffle compliments the desire for a "zipperless" design - since the shock cord is attached at the very end of the coupler, there's no chance for a zipper to occur.
3. Since this is an engineering project/contest, I wanted control over the weight of the rocket as much as possible - not needing wadding or dog barf eliminates one variable in the weight for each flight.

To construct the baffles, I used two 3" bulkhead discs, and two leftover 24mm motor tubes.

Two one-inch holes were drilled into the discs across one axis of the disc. Into those holes, epoxy one-half of a 3.5" 24mm motor tube. Repeat the same process on the other bulkhead disc. Be liberal in your application of epoxy - these tubes are going to bear the brunt of your ejection charge.

The two sub-assemblies are mounted together by fitting the tubes into an "X" pattern with enough overlap so that the particles have to make an "S" to get out the other side. Epoxy on the inside aspect of the bulkhead discs insures that nothing burns - every other flight or so, just shake out the baffle for any particles that might remain.

The only tricky part is getting the assembly to fit correctly into the tube - a spare piece of tubing can be used as a jig when fitting the two assemblies together or you can tack the two together with CA and then go in and glop the epoxy as required.

After about 15 flights on my challenge rocket, the baffle has shown to work perfectly - no burnt laundry and though it's managed to be dragged across a couple of parking lots by our Texas wind, no damage to the booster's top portion and no zippers.

Oh, the contest? I think 748 feet and 44.85 seconds is good enough to take home the Red River Rocketry Merlin that Jack's offering for 1st prize in the DARS "Are you as good as a High Schooler" contest. We'll see if anyone beats the 2 point score (zero is perfect) at the April club meeting.

 

[Sartori42]

Wayne,

I know this isn't in the Beginner forum, but please pardon my beginner questions....

1. I'm not sure what I'm seeing in the final photo. Does the entire top section come off when the ejection charge blows?

2. Can you explain (briefly) what a zipperless design is? I know what a zipper is.... There are ways to completely avoid this?

3. Does the baffle come out of the rocket at any time during a flight or does it stay in the BT?

Thanks.

Steven

 

[Micromeister]

Very elegant design Wayne!
Simple yet looks like it'll work wonderfully and last a good amount of time. It also looks like it can be adapted to just about any size set of tubes allowing use in just about any size bird. Great work!

While 15 flights is good, Have you been using this design long enough to give an idea of the number of flights we can expect before it'll need replacing?

Thanks for sharing, a very interesting baffle configuration.

 

[Peartree]

Wayne,

I know this isn't in the Beginner forum, but please pardon my beginner questions....

1. I'm not sure what I'm seeing in the final photo. Does the entire top section come off when the ejection charge blows?

2. Can you explain (briefly) what a zipperless design is? I know what a zipper is.... There are ways to completely avoid this?

3. Does the baffle come out of the rocket at any time during a flight or does it stay in the BT?

Thanks.

Steven

Wayne can weigh in when he get the chance but some of your answers are pretty straightforward.

1) What you are seeing is probably the top of the baffle that was shown in earlier photos. Something is covering the openings in this pic but I'm not quite sure what it is.

2) When parachutes open on a fast moving rocket of a typical design the rocket rapidly changes direction and the shock cord is pulled tightly while it is still 'bent' around the edge of the body tube. This change of direction focuses so much force at this one point of contact that the shock cord tears through the body tube and leaves a ragged scar that resembles the unzipping the front of your jacket therefore... 'zippering' the tube.

A zipperless design is one of several methods used to prevent this from happening.

3) The baffle stays in (and is generally glued in/built in) the BT but redirects the hot ejection gases in such a way as to prevent any hot/burning particles from the ejection charge from reaching the recovery system and burning them.

 

[n5wd]

While 15 flights is good, Have you been using this design long enough to give an idea of the number of flights we can expect before it'll need replacing?

Thanks for the kind words! The design goal was to make a baffle that should not need replacing... I want it to be fully functional as long as the rocket is flyable and not have to be maintained except for the occasional shaking out of particulate matter. That's why I chose a design that had no filtering such as the pot scrubbers and why there's such a liberal coating of epoxy.

I'll letcha know how that works when I break it!

 

[El Cheapo]

The question I have is how do you actually get the burnt particals out of the baffle. Seems like no matter what way you tip the rocket the particles won't be able to exit through the tubes.

 

[n5wd]

I know this isn't in the Beginner forum, but please pardon my beginner questions....

Not to worry about that - there were several sections where this could have been posted - too bad there's not an easy way to cross-reference them into the other sections.

1. I'm not sure what I'm seeing in the final photo. Does the entire top section come off when the ejection charge blows?

What you're seeing is the rocket, separated at the booster, with the payload bay and the nose cone lying on the ground. Yep, I separate the rocket right above the booster section so that the entire top section (consisting of the parachute well and the tether that ties the booster and the upper airframe together, the payload and altimeter, and the nose cone) comes down separately. That allows a softer landing for the payload (the booster's tether is longer, so it lands first, then a second or two later, the payload lands) as well as separating the rocket into two roughly equal weight parts (much like you'll see in dual deployment systems).

The "covering" that Peartree is seeing is an optical illusion caused by the flash... what you're seeing is the end of the baffle with the screw eye on top, with the tether attached.

2. Can you explain (briefly) what a zipperless design is? I know what a zipper is.... There are ways to completely avoid this?

Yeah - if you can prevent the tether from ripping into the body tube in the first place, you can prevent zippers. Some folks put a tennis ball or a wad of duct tape on the tether at the point where it would meet the edge of the body tube, so that the tether never touches the BT and isn't able to rip the tube causing the zipper. That wouldn't work for me since there wasn't a lot of room in my booster section.

The baffle at the end of the booster means that the tether never goes into the booster's body tube - it can't because the baffle is there. With the tether attached only on the end of the baffle, zipper problem for the booster is solved (though that leaves the upper airframe to potentially have a zipper, but they're easier to repair/replace than a booster section).

3. Does the baffle come out of the rocket at any time during a flight or does it stay in the BT?

There's no reason why a baffle can't be removable, as long as you can achieve that air-tight seal on the outside of the baffle so that the ejection gasses go through the baffle instead of around it (gas, like water and electricity, seeks the path of least resistance). I've prototyped a removable baffle system and found that achieving that seal would be the biggest problem.

But to answer your question directly - in this design, the baffle is epoxied in so that it should never be removed.

 

[n5wd]

The question I have is how do you actually get the burnt particals out of the baffle. Seems like no matter what way you tip the rocket the particles won't be able to exit through the tubes.

You're correct that you probably won't ever get all of the particles out, but we're not talking about a great amount of mass here - the particles would have to completely fill up the 3" round (in this case) void between the bottom bulkhead and the bottom of the top tubes before it would stop the baffle from working, and that's not going to happen any time soon - those 29mm motors are expensive! And you're going to get some amount of the particles out by the shaking, so.....

 

[El Cheapo]

I've been making my own baffles using three offset semicircles for quite some time from BT5 all the way to my largets of BT60 out of both layered cardstock, epoxied cardboard and epoxied balsa. I've even come up with a very simple design for the Fliskits Caution Rocket.

Other than on a short airframe or dual deploy, at what point in diameter or motor size does one stop using baffles and switch over to fire resistant blankets. I do have a 5.5" diameter nomex that I use on a 3" Mercury but it doesn't seem to be large enough as I've burnt the chute. I know have a 9x9 out of a Madcow kit I'm going to use on it.

 

[n5wd]

Other than on a short airframe or dual deploy, at what point in diameter or motor size does one stop using baffles and switch over to fire resistant blankets.

I don't know that there's a good answer to that one, EC - my current design is a 3" tube, and I'm also in the process of converting a 3" BSD 38 Special that zippered on its last flight, into a baffled zipperless booster using the same kind of baffle as presented here.

The next in the build pile is a 4" Binder Samurai that's going to be built the same way - with a baffle (I'll probably be able to use 3 or 4 of the 1" holes in the bulkheads instead of the 2 I'm using the with the 3" model) but otherwise plan on the same design upscaled.

IS there an upper limit? Don't know.

 

[El Cheapo]

on the 3" Thor and 4" Samarai, are you flying those with dual deploy? The Samurai is a monster.

 

[Tom W]

Wayne,

If you were to slide your baffle into a permanently secured sleeve and attach it with screws down to a centering ring (that's the hard part) sealing it with some high temp silicone sealer (which is easy to remove if the parts are sealed ahead of time) could seal the inner sleeve (baffle) to the outer sleeve (coupling).

Thanks for sharing.

Tom W.

 

[n5wd]

on the 3" Thor and 4" Samarai, are you flying those with dual deploy? The Samurai is a monster.

Originally, the 38 Special (almost a Thor) was my single-deploy Level 1 bird, but in it's new incarnation, both it and the Samurai will be dual-deploy.

 

[Tom W]

Have you made any more flights with this rocket?

How's the baffle holding up?

Thanks,

Tom W

 

[n5wd]

Have you made any more flights with this rocket?

How's the baffle holding up?

We're up to about 20 flights so far. Hopefully this summer, I'll put it through its paces with a few G's and maybe an H or two.. (It normally flies on a Roadrunner F60-7). That'll stress it a bit more.

 

[Tom W]

Thanks for the update...keep us posted.

 

[Handeman]

Wayne,

If you were to slide your baffle into a permanently secured sleeve and attach it with screws down to a centering ring (that's the hard part) sealing it with some high temp silicone sealer (which is easy to remove if the parts are sealed ahead of time) could seal the inner sleeve (baffle) to the outer sleeve (coupling).

Thanks for sharing.

Tom W.

Just wondering, does it have to be sealed against the body tube? As long as the baffle is a snug fit so hot particles can't get through, does if matter if it's gas sealed or not? After all, the point of the baffle is to pass gas but not the hot solids.

 

[luke strawwalker]

Just wondering, does it have to be sealed against the body tube? As long as the baffle is a snug fit so hot particles can't get through, does if matter if it's gas sealed or not? After all, the point of the baffle is to pass gas but not the hot solids.

You might have, ummm... phrased that a bit differently... Doesn't sound too good...

Depends on what you mean by 'sealed'. I've never seen a baffle that wasn't 'glued in' and of course, gluing it in creates a seal (enough of one to consider it one piece, anyway.) I suppose you COULD just friction-fit a baffle, but why would you want to?? Guess that would make removal a LOT easier if it ever wore out (if the model outlasts the lifetime of the baffle, that is) but then it opens up another can of worms-- what if it shifts aft under thrust, especially if the parachute slides down the tube at liftoff and knocks it down the tube-- it would probably be too close to the engine at ejection and take a hard hit from the ejection charge... plus the aft CG shift during powered flight could do some bad things to the stability of the rocket...

Besides, if you want a sliding baffle, just make it solid (no passing gas) make it slightly looser fit in the BT and call it an ejection piston...

Later and good luck! OL JR

 

[n5wd]

Just wondering, does it have to be sealed against the body tube? As long as the baffle is a snug fit so hot particles can't get through, does if matter if it's gas sealed or not? After all, the point of the baffle is to pass gas but not the hot solids.

Oh, Handeman.... never feed someone straight lines like that, ok?

I confess... I've been bitten by the high power / bigger rocket bug and haven't been doing anything but 3" rockets for this last season. In this configuration, the baffle sits at the top of the booster section, and is the attachment point for the tether going up to the middle airframe. As such, it's very well epoxied in, since it has to take the full force of the ejection and separation and stay together, else the rocket comes down in separate parts. In my configuration, I never dreamed of not sealing both of the the baffle bulkheads in well, both top and bottom.

That said, I suppose you could have a baffle that allowed the gas from the ejection charge to seep around the edges of the bulkheads. But I don't know how you'd insure that none of the hot particles tried to take the same route.

Besides the sealing effect, forcing the gas through your path (be it a mesh or, in my case, the "S" bends of the baffle tubes) another benefit of liberal doses of epoxy is the burning resistance to the hot particles.

I'm sure someone could/probably has designed a removable baffle system that's not air-tight... I just don't remember seeing one.

 

[Handeman]

Actually I was expecting a lot more "comments" about that line. I'm disappointed

I've never used a baffle that wasn't permenantly attached. Maybe I was reading Tom W's post wrong, but it sounded like he was talking about inserting a baffle and attaching it by screwing it to the forward CR. I think a removable baffle would work, I just don't think it could or should be the shock cord anchor.

 

[luke strawwalker]

Actually I was expecting a lot more "comments" about that line. I'm disappointed

I've never used a baffle that wasn't permenantly attached. Maybe I was reading Tom W's post wrong, but it sounded like he was talking about inserting a baffle and attaching it by screwing it to the forward CR. I think a removable baffle would work, I just don't think it could or should be the shock cord anchor.

Ah, Ok, I see what you're getting at... It would be easy enough, especially with the design showed in this thread, to make it two piece with a removeable top... all one would need is to run all-thread rod up from the bottom piece with a couple nuts jammed together to keep the top piece from being pushed down too far and closing off the ports, and then another nut screwed down on top to hold the top half in place on the all-thread after installation. Technically speaking, the upper piece doesn't have to be 'sealed' to the tube sides, but you WOULD want a good close fit to prevent leakage and hot particles from sliding past it between the edge of the upper disk and the tube wall. Put a coupler in and eliminate the jam nuts, and create a small 'step' on the tube wall to help the upper disk seal and give something for the upper disk to tighten down on.

The lower piece would still be epoxied in, I suppose... OL JR

 

[KDRaven]

I did a coupling tube baffel that used all thread that went through the botton,middle and top plates and used nuts to space them apart and to hold them tight on the coupling tube and then slid the coupling tube into the the rocket and used screws to hold the CT to the bottom BT tube. That way the entire baffel unit can be removed for inspection or cleaning.

 

[TheAviator]

Other than on a short airframe or dual deploy, at what point in diameter or motor size does one stop using baffles and switch over to fire resistant blankets. I do have a 5.5" diameter nomex that I use on a 3" Mercury but it doesn't seem to be large enough as I've burnt the chute. I know have a 9x9 out of a Madcow kit I'm going to use on it.

Quick hint: Nomex protectors should be 3x the diameter of the airframe ID.

I've seen pictures of a 5.5" zipperless baffle before. Really there's no limit as long as you make it durable enough to survive whatever you're going to throw at it.

 

[Tom W]

I just finished up a similar baffle for my Estes Fat Boy that I plan on testing in a few weeks.

Because of weight considerations I've elected to reduce the number of inlets and outlets to just one each.

While I'm confident that the two 180 degree turns will stop any hot particles from getting to the chute I'm not sure that there is enough volume in the system to keep very hot gases from coming in contact with the recovery system.

Because of this reasoning I've extended the exhaust tube all the way to the nose cone.

It did add a significant amount of weight to the rocket so most initial testing will be with an AT SU D10-5. I orginally planned on using an AT RMS D13-4 but I'm not confident that the red cap on the ejection charge won't block the gases in the motor tube (the baffle is barely 1/2 inch above the original top CR.

If the design works I'll have to clone the Fat Boy but I'll stretch it out a 1/2 inch or so to make sure the red cap has somewhere to go.

Tom W

 

[n5wd]

Have you made any more flights with this rocket?

How's the baffle holding up?

Guess it's time for an update - my TARC-task bird didn't fly as much this summer as I had hoped, due to some scheduling problems. But, we're up to about 30 flights, now, without any apparent problems. The baffle continues to work well.

Getting into the TARC flying season pretty soon, so I'll have an opportunity to fly it some more.