Understanding baffles

[Gunstar]

I've never used a baffle in a rocket I've built but I'm thinking about it in BT-60 and BT-80 based rockets, and trying to understand how they work. It looks to me like the idea is to create a non-linear path to prevent hot particles from the ejection charge from reaching the parachute and damaging it. But doesn't the ejection charge also put out hot gasses? It doesn't look to me like a baffle would help against that. Are the gases just not as hot as I'm thinking?

 

[Steve Shannon]

I've never used a baffle in a rocket I've built but I'm thinking about it in BT-60 and BT-80 based rockets, and trying to understand how they work. It looks to me like the idea is to create a non-linear path to prevent hot particles from the ejection charge from reaching the parachute and damaging it. But doesn't the ejection charge also put out hot gasses? It doesn't look to me like a baffle would help against that. Are the gases just not as hot as I'm thinking?

You’re exactly correct in how it works. The hot gases cool very quickly as they expand and don’t damage parachutes like hot particles do.

 

[kuririn]

Understanding baffles​

Yes, this is a conundrum.
Because by definition, if something baffles, it defies understanding.
But we must soldier on and do the best we can.

 

[les]

Also, there is some initial cool air inside the baffle. The hot gases first need to push that cool air out.
Some baffles include a steel wool type pad to also help absorb the heat.

 

[Back_at_it]

You are correct. The baffle is there to stop the burning embers from hitting the recovery system. The hot gases generally are not enough to do damage to parachute and shock cord but I have seen some minor melting of some of the thinner Mylar type streamers over repeated use.

As long as your baffle system blocks a direct path from the motor to the recovery system you are good to go. I run baffles in more than half of my rockets. I find them perfect for BT55 up to BT80. Below that I still use wadding. Above that I use fire blankets mostly although I did do a baffle in my 3" Big Red Max. Below are a couple of examples.

This first one is from Qualman rockets. They are a really nice laser cut baffle that comes in most of the common body tube sizes.



Next is this twin tube design. These work well if the body tube allows you to get two decent sides tubes in there beside each other. These are for a BT80 body tube.

What I like about these is that you can make them very compact. The one I built for my Big Red Max was only 2.5" long.







This final baffle is the one I use most often. This baffle has 3 or 4 over lapping plates inside. Each plate covers about 60% of the opening so the embers don't have a direct path. FOr me these are the easiest to make and work great as long as you have enough body tube length to allow them.



Here is a illustration of the inside of this style baffle.

 

[mh9162013]

I've been working on designing my own baffles and here are my observations from personal experience and online research:

- All else being equal, longer baffles will last longer than shorter ones. But that means they take up more space in the rocket and can adversely affect stability depending on where the baffle is located.
- Shorter baffles can work just fine, but they may not last as long as a longer baffle and/or cause increased wear and tear on the rocket. But the benefit of shorter baffles is reduced weight and a smaller impact on the rocket's stability or weight distribution.
- The more "maneuvering" the hot ejection gasses and burning bits have to do, the better. But too much (in too short of a baffle) could potentially lead to increased wear and tear on the baffle/rocket as well greater stresses placed on the rocket's airframe.
- Baffles work even better when paired with reusable wadding.
- Baffles have a tendency to trap bits and pieces from the ejection charge, like small bits of clay. So when choosing a baffle and motor mount design, keep in mind that you'll want to have a way to remove the "built up bits" from the rocket. This might mean using a baffle design that's less likely to trap particles and placing the top centering ring (the one closest to the nose) at the very end of the motor mount tube. This will make it easier for small bits to be shaken out of your rocket.

 

[lakeroadster]

Every time I see a photo or diagram of a baffle, I think, why not a piston? Same or smaller overall dimensions, less weight, self cleaning and in regard to recovery gear protection, same end result.

Am I missing something?

 

[mh9162013]

Every time I see a photo or diagram of a baffle, I think, why not a piston? Same or smaller overall dimensions, less weight, self cleaning and in regard to recovery gear protection, same end result.

Am I missing something?

Pistons are more complex and have moving parts, right?

 

[Steve Shannon]

Every time I see a photo or diagram of a baffle, I think, why not a piston? Same or smaller overall dimensions, less weight, self cleaning and in regard to recovery gear protection, same end result.

Am I missing something?

Personally, I like pistons. They’re like hard deployment bags, completely protecting the chute from hotness. As long as the right care is taken they work really well, requiring less BP and completely protecting the chute. I like them in phenolic or all fiberglass rockets where the body tube and piston have the same thermal expansion characteristics. I’m not wild about them in cardboard rockets or mixed materials such as phenolic pistons in quantum tube bodies, but I have done both.
However, I don’t think pistons are self cleaning, at least not completely. Each time an ejection charge goes off it deposits material on the inside of the body tube where the piston rides. But it’s relatively easy for the flyer to clean.

 

[tsmith1315]

Pistons are more complex and have moving parts, right?

Pistons are less complex, just a coupler with a bulkhead attached. You don't mount the coupler in place, so the ejection charge moves the piston to force the parachute out.

I've had one rocket with piston eject and it worked perfectly.

 

[mh9162013]

Pistons are less complex, just a coupler with a bulkhead attached. You don't mount the coupler in place, so the ejection charge moves the piston to force the parachute out.

I've had one rocket with piston eject and it worked perfectly.

Sounds a lot more complex to me, given the moving parts.

Perhaps I'm splitting hairs...

 

[Back_at_it]

Every time I see a photo or diagram of a baffle, I think, why not a piston? Same or smaller overall dimensions, less weight, self cleaning and in regard to recovery gear protection, same end result.

Am I missing something?

I've only owned two rockets that had piston ejection systems and I will admit that they worked ok when the rocket was new but I needed to be extremely diligent with cleaning the rocket body and the piston after each flight. The body tube needed to be absolutely spotless or I'd have issues with ejection. After a few partial deployments I decided they were more trouble then I wanted to deal with.

I rebuilt both rockets with baffles and they have each flown a dozen times without incident. The baffle system I put in place was actually substantially lighter then the dual shock cord, piston system I removed. On my IO I dropped almost 4oz of total weight by eliminating it.

For me baffles are a simple easy way to protect the recovery system. No need to worry about moving parts or the maintenance that goes along with those parts. Build it once and forget about it. After each flight I shake the rocket to get out any misc. junk that might have gotten into the baffle. I inspect each rocket after flying and have a couple of rockets with 40+ flights and the baffles work perfectly.

In the years I have been using them I've only had two fail. One was a fiber design that I bought from a vendor and the other was a plate style that I built myself from Balsa. Since it was almost 30 inches from the ejection I wrongfully assumed that balsa would be strong enough. It lasted maybe 6-7 flights then the lower two plates failed on an E motor. These days I only use Plywood for the plates.

 

[Steve Shannon]

Sounds a lot more complex to me, given the moving parts.

Perhaps I'm splitting hairs...

Pistons can fail. They can jam in place if they cant or if a shroud line or fold of material catch between the piston and the body tube.
Some baffles can also fail if an ejection charge cap plugs a vent hole. The design with alternating 60% discs look immune to that though.

 

[mh9162013]

In the years I have been using them I've only had two fail. One was a fiber design that I bought from a vendor and the other was a plate style that I built myself from Balsa. Since it was almost 30 inches from the ejection I wrongfully assumed that balsa would be strong enough. It lasted maybe 6-7 flights then the lower two plates failed on an E motor. These days I only use Plywood for the plates.

Was that balsa reinforced in any way? Also, how thick was that balsa?

I ask because I'm thinking about using balsa in my 3-half-moon baffles. But these baffles are for 13mm and 18mm BP engines.

 

[Back_at_it]

Was that balsa reinforced in any way? Also, how thick was that balsa?

I ask because I'm thinking about using balsa in my 3-half-moon baffles. But these baffles are for 13mm and 18mm BP engines.

Not sure at this point exactly what the size was but I can say with 90% certainty that it was either 3/32 or 1/8 Balsa as that is the most common balsa I buy. As for reinforcement, the plates get a decent fillet then everything (wood and the inside of the baffle) gets a coat of whatever wood glue I'm using on the project for a little extra stiffness but mostly for fire protection. This can be seen in the photos I posted previously. You can see the glue on the plates and inside the baffle.

Mine lasted a few flights on C11 and D12 motors. I don't know if the E motor ejection is that much stronger than the others but I feel like it would have failed on a D motor as well. I have no experience with balsa baffles on 13mm and 18mm motors but I know that 1/16 ply will live almost forever in that application.

 

[Paul Howard]

I'm a big fan of Baffles, Parachute Pistons & Buckets and Parachute Blankets. I agree with thought/comments on "Pistons & Buckets" - they do require some diligence in cleaning and testing before each flight since they often have a tendency to build friction from sooty build-up. One of the reasons I prefer all the aforementioned is to go away from "recovery wadding" that I had burn an ember all the way to the ground twice last year and has done so in the past. Since fire-risk has increased practically everywhere, I decided to reduce fire-risk as much as possible. I also always carry at least an extra quart of water (not intended for drinking until after launch) beyond drinking water and a mini-fire-extinguisher.

 

[sinbintd1]

A good source for baffles and other mods, you can contact him for custom work too:

https://www.ebay.com/str/badboyrocketry
https://www.ebay.com/itm/325294575102?hash=item4bbd114dfe:g:wiQAAOSwDediwlvM

 

[Back_at_it]

A good source for baffles and other mods, you can contact him for custom work too:

https://www.ebay.com/str/badboyrocketry
https://www.ebay.com/itm/325294575102?hash=item4bbd114dfe:g:wiQAAOSwDediwlvM

I've bought from Bad boy rockets a few times. Really nice stuff at a decent price.

That said I am not a fan of that style of baffle. Of all of the Baffles I have tried those were the least effective at stopping partials from burning holes in the recovery gear.

 

[bjphoenix]

I've always worried that some baffle designs would provide too much resistance, maybe that's not a problem.

I built a few baffles in 2.6" diameter airframes with 29mm mounts. I used a long motor tube, cut some holes in the side of the motor tube near the front and reinforced the parts that were left with fiberglass. I used 2 centering rings near the back and one solid disk on the front end of the motor tube, with some notches cut around that front disk. The the gases had to come out the holes in the side of the motor tube, then go forward through the notches in the disk. The front disk also had an eyebolt for the shock cord. Seemed to work well for G powered rockets.

I didn't want to put all that much stuff into a BT60 rocket for C motors so I've been using normal wadding. After more reading and some tests I have been using a crepe paper pompom in BT55 and BT60 and it seems to work very well.

I applaud people who have developed a simple, light, effective baffle, especially for LPR. It removes a source of worry and hassle.

 

[Dan Griffing]

I've never used a baffle in a rocket I've built but I'm thinking about it in BT-60 and BT-80 based rockets, and trying to understand how they work. It looks to me like the idea is to create a non-linear path to prevent hot particles from the ejection charge from reaching the parachute and damaging it. But doesn't the ejection charge also put out hot gasses? It doesn't look to me like a baffle would help against that. Are the gases just not as hot as I'm thinking?

I tried using a section of stainless steel scrub pad as a baffle and it cooled the BP ejection gases so much that normally vaporized BP combustion products condensed on it and reduced the gas volume too much to be effective for deploying the parachute.

I had recorded slow mo videos of both instances and a BP ejection charge generates a big fireball to generate the ejection gases. When your baffle effectively absorbs this fireball, you have a lot less gas to push out your parachute.

The chemical reaction for BP is

10 KNO3 + 3 S + 8 C → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2.
But the components are separate and granular and require time to decompose and react. Quite often you will find soot in your parachute compartment which indicates that the charcoal carbon hasn't been able to combust. The cooling that takes place by using the stainless steel scrubber baffle cools down the BP fireball before its had time to react.

 

[dhbarr]

What I like about the moon baffles is that you can shake out crud but don't have to scrub. Not aware of any other hard design for which that's true

 

[Sooner Boomer]

What I like about the moon baffles is that you can shake out crud but don't have to scrub. Not aware of any other hard design for which that's true

There's what I call a "T" baffle. The motor mount extends up and connects to a solid bulkhead. The bulkhead has "nibbles" taken out of the rim; the motor mount tube has a series of vent holes near the bulkhead. With a little bit of shaking and rolling, you can get the burned bits to fall out through the "nibbles". Kinda like this, except the holes would be at the edge -
https://www.rocketryforum.com/media...-and-bulkhead-for-baffles-and-fin-set.152855/

 

[mh9162013]

What I like about the moon baffles is that you can shake out crud but don't have to scrub. Not aware of any other hard design for which that's true

In addition to to what Sooner Boomer said, another design is where one of the baffle plates has holes along its edge where it touches the end of the baffle casing/coupler. So by tilting the baffle and shaking it, the bits can fall out without getting caught up on an edge or rim.

 

[mh9162013]

I tried using a section of stainless steel scrub pad as a baffle and it cooled the BP ejection gases so much that normally vaporized BP combustion products condensed on it and reduced the gas volume too much to be effective for deploying the parachute.

I had recorded slow mo videos of both instances and a BP ejection charge generates a big fireball to generate the ejection gases. When your baffle effectively absorbs this fireball, you have a lot less gas to push out your parachute.

The chemical reaction for BP is

10 KNO3 + 3 S + 8 C → 2 K2CO3 + 3 K2SO4 + 6 CO2 + 5 N2.
But the components are separate and granular and require time to decompose and react. Quite often you will find soot in your parachute compartment which indicates that the charcoal carbon hasn't been able to combust. The cooling that takes place by using the stainless steel scrubber baffle cools down the BP fireball before its had time to react.

This is very interesting, but does it apply to small LPR rights, such as those using a BT-20 or BT-50 main body tube?

From my observations and testing, a baffle in a BT-20 or BT-50 rocket of reasonable length (think Estes Yankee, Wizard, Generic E2X, etc.) cannot "over cool" the ejection charge from a standard 18mm BP engine. Instead, it delays the time it takes for the hot gasses and bits to reach the recovery system that needs protection from the ejection gasses and bits.

 

[Dan Griffing]

This is very interesting, but does it apply to small LPR rights, such as those using a BT-20 or BT-50 main body tube?

From my observations and testing, a baffle in a BT-20 or BT-50 rocket of reasonable length (think Estes Yankee, Wizard, Generic E2X, etc.) cannot "over cool" the ejection charge from a standard 18mm BP engine. Instead, it delays the time it takes for the hot gasses and bits to reach the recovery system that needs protection from the ejection gasses and bits.

I was using the stainless steel scrubber baffle mounted at the end of a 38mm motor mount in a 4" Apogee Zephyr rocket with a dual deployment modification, with 2g BP using a spent motor to test the motor ejection. The baffle started just an inch or two from the end of the rocket motor, so there wasn't much volume for the BP combustion to take place before it went through the SS baffle.

Without the baffle there was enough BP combustion gas to energetically separate the two rocket halves, but with the baffle, there was not.

I videoed the ejection fireball with the spent motor mounted in a 38mm motor mount without the rocket, both with and without the SS baffle tube mounted on the end and saw a big fireball when there was no baffle, but only a much smaller smoke puff with the baffle.

In summary, I found that its too easy to make a baffle that works too well in protecting the parachute from the BP combustion products.

 

[SolarYellow]

Figured this would be a good place to ask.

Do the typical half-moon style lite ply baffles, in particular those sold by Apogee as kits, normally have a service life that requires replacement? There's another thread going on where someone is working on developing his own, fairly close-coupled design, and he's having some charring that makes them a consumable. I'm thinking about using one in a future project where I can afford to have the baffle several (5+) diameters of the 24mm tube ahead of the longest engine I'll use. Wondering whether it will be wise to glue it in, or if I should look at making it replaceable.

 

[Back_at_it]

It really depends on the material that you use to create the baffle. I have the half moon baffle design made out of light plywood plates in rockets that have 30+ flights on them with no visible degradation in the structure of the baffle. Yes they are dirty but I see no signs of wear and I have not found the failure point of one yet

I do coat the exterior and interior of the baffle along with the plates with wood glue for a little protection and that seems to help.

 

[mh9162013]

Figured this would be a good place to ask.

Do the typical half-moon style lite ply baffles, in particular those sold by Apogee as kits, normally have a service life that requires replacement? There's another thread going on where someone is working on developing his own, fairly close-coupled design, and he's having some charring that makes them a consumable. I'm thinking about using one in a future project where I can afford to have the baffle several (5+) diameters of the 24mm tube ahead of the longest engine I'll use. Wondering whether it will be wise to glue it in, or if I should look at making it replaceable.

Technically yes, but in practice, it depends. My half-moon design is based off those sold by Apogee, but I would imagine that eventually, all baffles will wear out and need to be replaced. The only question is will another critical component of the rocket wear out first or will the rocket otherwise be lost before the baffle's life ends?

Like Back_at_it mentioned, how long a baffle lasts will depend on what you use to create it. But there are other factors to consider, such as the baffle's physical design and its proximity to the ejection charge.

A baffle will last longer if it's made from more durable materials (from a fire-resistance perspective). But where you place the baffle and how freely gases and hot bits can flow through the baffle will also affect its life.

My experience with baffles is only in BT-20 and BT-50 sized rockets. But from my observations, it seems like wear and tear due to heat exposure (hot gasses and burning bits) have a bigger impact on baffle life than physical forces on the baffle (like built-up pressure). And because you only want to reinforce your baffle so much, I think your "weight budget" is better spent on coating the inside of the baffle with epoxy or glue to make it more heat resistant than using a stiffer baffle material.

For example, in my BT-20 removale baffle design, the half-moon pieces are made out of 1/16" balsa, paper, and JB Weld. Yet after 8 test launches, they show no signs of fatigue or failure. Yet the coupler's housing (the yellow engine spacer) is showing signs of not lasting that much longer. However, the burn marks on the yellow engine spacer are only showing up in parts of the baffle that were NOT coated with glue or epoxy to provide heat/fire protection. I'm doing more tests to learn more about how to make baffles last longer, as well as test them in more extreme conditions, ie next to a C engine instead of an A engine.

In summary: the further away your baffle is from the ejection charge, the more room your baffle has inside to allow the flow of hot gases and burning bits, the more fire resistant materials and building techniques you use, the longer you baffle will last. But if you push these variables so far, your baffle will become excessively heavy and likely overkill for your application. It may also be of a certain size or in a certain location that doesn't work well for your rocket's design and performance expectations.

Here's my take: if you can build your baffle to be removable, do it. Even if your baffle is built so robustly, it'll last thousands of launches, it's still nice to be able to remove your baffle to clear up any potential blockages or crud that builds up. Some build up is nice that it can help insulate your baffle from the heat. But too much means inhibited gas flow and extra weight carried by your rocket. Another benefit is that if you can tie your shock cord to the baffle, a removable baffle is a great way to replace and/or inspect your shock cord.

A removable baffle also provides flexibility to use any type of baffle. For instance, right now, I'm thinking about having 2 types of baffles: a performance baffle and an endurance baffle. The former will be as light as possible and likely only last 3-6 launches. But the endurance baffle will be built more robustly and should last more than 20 launches (or for all practical purposes, last forever). But it may be anywhere from 50% to 75% heavier.

 

[SolarYellow]

You inspired a brainstorm. Hold it in with a motor retainer. Especially on a non-MD rocket, there's room. You could even go deluxe and use a motor block below it with an Estes screw-cap motor retainer to hold it in the front end of the MMT.

Back in the day, I used to paint the fuel tank compartment of my model airplanes with epoxy thinned with alcohol. Seems like this would be a good application of that technology.

 

[Paul Howard]

Technically yes, but in practice, it depends. My half-moon design is based off those sold by Apogee, but I would imagine that eventually, all baffles will wear out and need to be replaced. The only question is will another critical component of the rocket wear out first or will the rocket otherwise be lost before the baffle's life ends?

Like Back_at_it mentioned, how long a baffle lasts will depend on what you use to create it. But there are other factors to consider, such as the baffle's physical design and its proximity to the ejection charge.

A baffle will last longer if it's made from more durable materials (from a fire-resistance perspective). But where you place the baffle and how freely gases and hot bits can flow through the baffle will also affect its life.

My experience with baffles is only in BT-20 and BT-50 sized rockets. But from my observations, it seems like wear and tear due to heat exposure (hot gasses and burning bits) have a bigger impact on baffle life than physical forces on the baffle (like built-up pressure). And because you only want to reinforce your baffle so much, I think your "weight budget" is better spent on coating the inside of the baffle with epoxy or glue to make it more heat resistant than using a stiffer baffle material.

For example, in my BT-20 removale baffle design, the half-moon pieces are made out of 1/16" balsa, paper, and JB Weld. Yet after 8 test launches, they show no signs of fatigue or failure. Yet the coupler's housing (the yellow engine spacer) is showing signs of not lasting that much longer. However, the burn marks on the yellow engine spacer are only showing up in parts of the baffle that were NOT coated with glue or epoxy to provide heat/fire protection. I'm doing more tests to learn more about how to make baffles last longer, as well as test them in more extreme conditions, ie next to a C engine instead of an A engine.

In summary: the further away your baffle is from the ejection charge, the more room your baffle has inside to allow the flow of hot gases and burning bits, the more fire resistant materials and building techniques you use, the longer you baffle will last. But if you push these variables so far, your baffle will become excessively heavy and likely overkill for your application. It may also be of a certain size or in a certain location that doesn't work well for your rocket's design and performance expectations.

Here's my take: if you can build your baffle to be removable, do it. Even if your baffle is built so robustly, it'll last thousands of launches, it's still nice to be able to remove your baffle to clear up any potential blockages or crud that builds up. Some build up is nice that it can help insulate your baffle from the heat. But too much means inhibited gas flow and extra weight carried by your rocket. Another benefit is that if you can tie your shock cord to the baffle, a removable baffle is a great way to replace and/or inspect your shock cord.

A removable baffle also provides flexibility to use any type of baffle. For instance, right now, I'm thinking about having 2 types of baffles: a performance baffle and an endurance baffle. The former will be as light as possible and likely only last 3-6 launches. But the endurance baffle will be built more robustly and should last more than 20 launches (or for all practical purposes, last forever). But it may be anywhere from 50% to 75% heavier.

That's all very well articulated and totally makes sense, and I've been really liking the other thread you have going with the removable BT-20 size baffle experiments. Thanks!