Baffling question

[Sooner Boomer]

I guess I'm getting old and lazy. I'd rather be flying rockets than preping them. It just takes a bit of extra time and materials to build in or add on an ejection baffle. I mostly use the 3-D (or 4-D) style; a series of (just over) half-round bulkheads aligned so that the successive baffle is rotated 180 degrees from the previous.

So the question of the day is: "How far from the end of the motor should the first baffle be, and how far should the successive baffles be spaced?". I guess the best way would be to answer in body tube diameters (or motor mount diameters if you're building a rocket with an extended motor mount/stuffer tube).

 

[rharshberger]

The real question is how long to limit the motor by baffle placement. To protect my baffles I ditch the red cap (AT), and instead use masking tape to seal the ejection charge well, some of my baffles are less than 2 diameters off the top of the longest motor for that rocket and they seem to survive reasonably well (bottoms coated with thin layer of JB Weld). The other advantage to placing them forward of the motor as far as possible is to allow repairs to the fincan area in the event of damage, I can cut the area between the fincan and baffle out and use a coupler to splice back together if necessary.

 

[Sooner Boomer]

I coat the bottom baffle, with yellow glue, epoxy, or JB Weld - whatever I have on hand. One thing I'm trying out is the addition of small triangular gussets on the side of the baffle away from the engine. I've never had one blow out, but this is cheap insurance.

 

[BABAR]

As a non structural part, would a piece cut out of an aluminum can work?

 

[Sooner Boomer]

As a non structural part, would a piece cut out of an aluminum can work?

In theory it would, because it would be (fairly) heat resistant, but I would be reluctant to use it. Paper and wood are porous, and a very strong bond can be made with yellow glue. You'd need (plain) epoxy or JB Weld to glue aluminum. Don't know if it would be worth the extra cost and weight (I'm thinking low/mid power rockets).

 

[solid_fuel]

I’ve seen or heard of foil lined motor mounts so I would assume that a piece of aluminum foil glued to to bottom of the baffle might also give some protection against heat and small burning particulates. And be easier to work with than a piece from a soda can. Albeit the piece from the can would offer much better protection to the wood underneath.

 

[K'Tesh]

Generally speaking, I try to put my baffles as far forward as I can and still have enough space for the parachute and related hardware.

 

[Sooner Boomer]

Generally speaking, I try to put my baffles as far forward as I can and still have enough space for the parachute and related hardware.

How would you do it on an 18" body tube? I'm thinking of making a motor mount and baffle as one unit (two pieces glued together) and slid in from the aft end. Have to use epoxy or other glue that would not grab as it was slid in.

 

[jqavins]

Reading this has got me thinking, and it occurs to me that it might be a good idea to build the baffle into a coupler and not glue it, just friction fit or rivet, like a tiny ebay. The baffle, especially the first and second plates, will be taking a beating, so why not make it serviceable?

As a non structural part, would a piece cut out of an aluminum can work?

I have a hunch that a piece of soda can might melt or burn through more easily than a piece of 1/8" inch balsa; that stuff is crazy thin. I'd stick with treating the wood. (I wonder about treating it with waterglass.) Unless...

A piece of gorilla glass would do a great job, if you can get some and cut it (or get it cut) to shape. Any glass will take the heat easily, and gorilla glass is plenty strong.

Well, or fiberglass.

 

[Sooner Boomer]

As I mentioned, I've got a design with an 18" body tube. I think I'm making things harder than they need to be. I think I'll just order some coupler material from BMS, chop the body tube, and add the coupler with baffle disks to join the tube back together. I intend to offer this as a kit to the young son of a friend. I'll post plans here, too.

 

[gldknght]

As a non structural part, would a piece cut out of an aluminum can work?

Throw an aluminum soda can into a camp fire some time and watch what happens. Just make certain you don't breath the resulting smoke, it's toxic.

Aluminum will burn, and doesn't need to get that hot to do so.

 

[rharshberger]

However, due to the very short duration of an ejection charge flame a soda can aluminum shield should be fine.

 

[gldknght]

I wouldn't take the chance.

 

[solid_fuel]

Balsa will burn from a lower intensity flame than aluminum

 

[jqavins]

The first mention was of using just a piece of can, not soda can as a shield over wood. I wouldn't hesitate to use 1/16" aluminum, but those cans are only about 3.8 mils thick.

• If the can material is glued flat to a piece of wood, heat is conducted very, very easily through it and into the glue. It can withstand higher temperatures than the wood, but will not see any higher temperature than the wood would without it. It's a fine shield.
• If the can material stands alone, the heat is very poorly conducted along the metal so the peak temperature induced by a particle of burning BP may be a bunch higher. It may not, I don't know, but I i) wouldn't try it in flight without considerable testing, and ii) wouldn't bother testing it.
• 1/8" balsa is about 30 times thicker than the can material, so the can can tolerate virtually no degradation before a hole appears.

So, I'm not saying the can material will surely fail, your parachute will burn, and your rocket will crash; I'm just saying it seems like a really poor choice.