Ejection baffle question

[Che]

I'm thinking of making an ejection baffle for my Quest Magnum Sport Loader and was wondering if there are any guidelines or ratios to ensure that enough gas/air passes through the baffle to ensure that the recovery system is effectively ejected.

 

[jms]

I am not familiar with this rocket, but if you take the BT size, and then take a look at the baffles that FlisKits or Sunward provide, it might give you an idea. I don't remember running into any information on ratios or lengths, etc.

 

[El Cheapo]

-My boy and I just built one. I used what I call a cold air ejection baffle. It consists of two 13mm tubes going up through a bulk plate and two going out. I wont be on a computer for the next week and cant attach pics throu my phone but I bet someone can post a pic of what Im ttalking about.

 

[SAC of MMMSClub]

Che,

I have bought some and built some,

Most have what I have seen described as Salt & Pepper hole pattern.

One bulk head has one hole in the center and six holes in a array around it.

The other bulk head has 8 holes in an array around the out side edge.

Some come with a coupler tube to build it in and then slide/glue into your rocket you are building.

I have built some with out the tube and just used popsicle sticks on three sides to separate the two bulkheads.

I have drilled 1/8th holes on small ones and .25 on bigger ones.

You can also add a tuft of stainless "Chore Boy" scrubber to really make sure no burning particles get through.

The most important thing is to watch your Center of Gravity as you add them to the rocket either as a modification or upgrade.

You might have to add some nose weight to keep it forward 1.5+ cal. of the Center of Pressure

Scott Costigan
NAR 91379 L-1
[email protected]

 

[BEC]

If were to do one of those (interesting design, that) I'd use a BT-60 baffle from Semroc and just sand the edges of the assembly down a bit until it fit into that slightly smaller tubing. Effective, inexpensive, light weight....

 

[Che]

So, I had a bit of insomnia last night and about 1:30am I decided to use the scrap balsa from the kit to mock up a plan that kept plaguing my brain. Here it is (it's pretty rough), what do you think? Not sure if I am going to rebuild it as it is or in a coupler.

 

[Che]

I just sanded it to fit the body tube, crammed it half way down in there, and took a peek through to see what it looked like. The only light I could see was from the perimeter where my sanding wasn't perfect (a little overdone). Next I put the nose cone in and blew a puff of air through the aft end of the body tube and popped the nose cone out quite easily.

 

[Che]

I know it is an unconventional design, but I think it should get the job done and it seems to be easy enough to make. Plenty of airflow, lightweight, and fits a small body tube very easily, but the question still remains. Will it actually work?

 

[cgould]

That's pretty clever!

I've designed several sorts of baffles in my rockets. Yours *seems* to be something that could work.

I'd say ground test that baby using an el cheapo plastic chute. If the chute comes out alive and well, then it's a GO!

 

[Che]

Ground test? What sort of ground test?

 

[bjphoenix]

The baffles that I have built have been relatively direct. I usually plug the stuffer tube, cut some holes in the side of it, and cut holes in the front centering ring for the gases to go through.

When I did the first one I wasn't sure how big to make the openings so here is what I did- I figured that if a 29mm motor could get enough ejection gases through the area of a 29mm tube, then I would make sure that all of the openings in each stage of my baffle would add up to the area of a 29mm tube. If I was using a baffle for 24mm diameter engines then I would use the area of a 24mm motor tube.

 

[SAC of MMMSClub]

Che,

You did the ground test, :roll:it popped when you blew in the tube.

That is a neat design, and it looks to be very light. should work.

Scott Costigan
NAR 91379 L-1

 

[pick1e]

Cool! That's a clever design.

The only thing I'd be worried about is the vertical pieces getting a good seal against the body tube. This would be hard to do if you built it as a unit and glued it into the body tube. Easier to do if you assembled it inside the tube (lower ring, then uprights, then upper ring).

If you didn't have a good glue seal at those edges, some gas or particles could pass by the uprights along the tube wall like in my edited photo below.

 

[Che]

This is only a mock up that I worked up in the middle of the night laying in bed out of the scrap pieces from the fin sheet that came with the kit. I was just trying to get the feel for what was rolling around in my noggin and to put it into three dimensions. I think that I will try to make it to the the hobby store sometime this week and see if I can pick up some wood and make it out of better quality components.

 

[cgould]

Ground test? What sort of ground test?

It's where you test the ejection system on the ground to make sure all will work as planned before you send the rocket up. I use a spent single use motor casing, and put in a home-made (black powder) ejection charge canister that I plan to use in my upcoming dual deployment system. I fire this off with a launch controller to test ejection baffles, friction fit parts (as in the video below) as well as the charges themselves.

I like to test with the rocket sitting vertically to make sure there is enough pressure to do the job. In this video, I am testing using Triple 7, which is a bit different of a beast from black powder in its characteristics...

[YOUTUBE]lfWzwzLDIII[/YOUTUBE]

 

[cgould]

I should point out that this rocket also has one of my ejection baffle designs in it.

The anti-zipper mod to this rocket meant a change in the way the baffle has to work, so more tests were done with a cheap thin plastic chute to make sure the baffle still had its protective powers (it did).

-C

 

[qquake2k]

I've always used the "half moon" style baffle in my scratch built rockets. It's a design I learned here. I used to cut the discs out freehand, but lately have been cutting them with a hole saw. I epoxy the 1/4" pilot holes closed, them cut the discs a little bigger than half so that they overlap properly.

 

[qquake2k]

For my zipperless designs, I put the half moons in the motor tube, then make a baffle plate to be epoxied into the coupler with the recovery eyebolt in it.

 

[boomtube]

To the best of my knowledge there are no off the shelf baffle kits that will fit the Quest 40mm tube. If anybody would have one it would most likely be Semroc.

One problem I have noticed with baffles, is that they don’t seem to work very well if you can’t get at least six inches between the lower baffle plate and the top of the motor/s. Less than that and they allow hot gasses/particles to slip through.

If the body tube is long enough to allow for that 6” space with enough remaining for the chute then in my opinion a baffle will reduce the amount of wadding/ dog barf you need/use but not completely eliminate the need for some especially on small diameter rockets.

Then there is the fact that the “Magnum” is a twin motored cluster.
Normally with the so-called peppershaker style of baffle you would place the baffle plate with the holes that are around the edge, facing the motor. With clusters it seems to work better if the plate with the centered holes is facing the motors.

Baffles also make a convenient place to anchor a Kevlar shock cord.

 

[bradycros]

Here's how a baffle works.

The pressure wave from the ejection charge has to move up the motor tube (or body tube) because the motor hardware is mechanicly secured into place, blocking one of the two exits. The path of least resistance is up the motor tube (or body tube), through the baffle, into the parachute compartment, pushing out the parachute and nose cone to reach the only exit available to the pressure wave (unless there's a CATO and a new exit is created).

The path the pressure wave has to take is already full of cool air. As the pressure wave travels up the motor tube it's forcing the cool air up and out of the motor tube, through the baffle and then pushing out the parachute. The parachute is deployed by cool air and will be long gone before any hot gas from the ejection charge reaches the parachute compartment.

Think of the cool air as a piston and the hot gas as a connecting rod to the crank shaft (motors ejection charge).This process is going to happen the same way reguardless of were the baffle is placed. The baffle should be placed as far forward as practical to aid the CG/CP relationship and stability issues.

 

[bjphoenix]

I've always used the "half moon" style baffle in my scratch built rockets. It's a design I learned here. I used to cut the discs out freehand, but lately have been cutting them with a hole saw. I epoxy the 1/4" pilot holes closed, them cut the discs a little bigger than half so that they overlap properly.

I think this is an easy baffle to make. One of my first was done this way, in my Vulcanite. I cut 2 disks and put them in a coupler. In a 54mm tube I didn't have to cut away as much of the disk so there was more overlap. I used an eyebolt with a long threaded portion and extended it through both disks, with nuts and washers on both sides of each disk, and epoxied everything.

 

[Che]

great explanation bradycros.

I just put a baffle that I ordered from Apogee into a kit that I built a couple of weeks ago, I haven't flown it yet, but can't wait to give it a whirl. I wasn't thrilled with the design as I felt that the volume of gas able to get through the baffle might be restricted a bit due to the size of the holes, but we shall see.

 

[Rocket86]

Baffles are well worth the effort and save money in the long run, not to mention making pre-launch prep easier and more foolproof. I incorporated one into my Executioner tube coupler. It's light and made of two 3/32 balsa discs and four 'walls' that form the path through the baffle. The ejection gasses enter a slot in the bottom disc, then travel up, down, and back up again, exiting on opposite sides of the body tube. It is right at the CG of the empty rocket. I've flown it on Estes D's and E's so far and it works well.

 

[pick1e]

Here's how a baffle works.

The pressure wave from the ejection charge has to move up the motor tube (or body tube) because the motor hardware is mechanicly secured into place, blocking one of the two exits. The path of least resistance is up the motor tube (or body tube), through the baffle, into the parachute compartment, pushing out the parachute and nose cone to reach the only exit available to the pressure wave (unless there's a CATO and a new exit is created).

The path the pressure wave has to take is already full of cool air. As the pressure wave travels up the motor tube it's forcing the cool air up and out of the motor tube, through the baffle and then pushing out the parachute. The parachute is deployed by cool air and will be long gone before any hot gas from the ejection charge reaches the parachute compartment.

Think of the cool air as a piston and the hot gas as a connecting rod to the crank shaft (motors ejection charge).This process is going to happen the same way reguardless of were the baffle is placed. The baffle should be placed as far forward as practical to aid the CG/CP relationship and stability issues.

I would add that you want to block the burst of burning particles. Not only from the burning chunks of propellant but the hot clay pieces that cover the top of the propellant. These are going to fly faster than the hot gas IMHO.

 

[Che]

I would add that you want to block the burst of burning particles. Not only from the burning chunks of propellant but the hot clay pieces that cover the top of the propellant. These are going to fly faster than the hot gas IMHO.

That's why I like my design. The hot particles would have to go up, make a u-turn, and then a second u-turn to get to the recovery system. Hopefully, I will be able to make the baffle fit well enough so that there aren't any real opportunities for the hot particles to escape and not travel the intended path.

 

[BEC]

To the best of my knowledge there are no off the shelf baffle kits that will fit the Quest 40mm tube. If anybody would have one it would most likely be Semroc.

As I already pointed out, the BT-60 baffle from Semroc will fit with just a bit of sanding of the disks. The ID of BT-60 is about 0.07 inches more than the ID of 40mm Quest tubing (based on specs posted on their respective web sites).

One problem I have noticed with baffles, is that they don’t seem to work very well if you can’t get at least six inches between the lower baffle plate and the top of the motor/s. Less than that and they allow hot gasses/particles to slip through.

I have had absolutely no 'chute meltage using the Semroc BT-60 baffle in a Baby Bertha with about 2.5 inches (if that much - it's a tight fit) between the top of the motor tube and the base of the baffle. For a "half-moon" type I can see where more distance would be better.

Heck, I have maybe six inches between a three-motor cluster and a Semroc BT-60 baffle and again - no issues (in a Ranger clone) - at least so far.

<snip>

Baffles also make a convenient place to anchor a Kevlar shock cord.

Said Kevlar is supplied with Semroc baffle kits.

*standard disclaimer: no affiliation - just a happy customer*

All that said, it looks like the OP is brewing his own. I would only suggest coating the lower portion with something to forstall charring of the balsa. Even a coating of white glue will help.

 

[Che]

BEC - Good to know. Thanks.

 

[bradycros]

I would add that you want to block the burst of burning particles. Not only from the burning chunks of propellant but the hot clay pieces that cover the top of the propellant. These are going to fly faster than the hot gas IMHO.

Could you explain why you think that is what's taking place?

 

[pick1e]

Could you explain why you think that is what's taking place?

Momentum. The hot gas will slow down faster when pushing against the existing cool air. Particles will retain more of their initial velocity due to their density.

 

[hcmbanjo]

Last year I blogged about how to make your own baffles.
Mine were really clones of the old Centuri design.

https://modelrocketbuilding.blogspot.com/search/label/Baffle

The blog showed how to make a coupler from a body tube so it could fit any diameter tube you have.
This style is best for BT-55 through BT-70 size tubes and could certainly work in the Quest 40mm tubing.