Distance a baffle should be from the motor?

[Scode68]

How far from a motor should a baffle be? I'm going to build a few BT-60 size rockets and thought the baffles would be a good idea.

 

[Boosterdude]

I install the baffles up high in the airframe towards the nose. Of course you need to make sure you leave room for your shock cord and chute.

From what I can tell on my models the baffle will also last longer when it's further away from the motor.

 

[slogfilet]

Also, by placing the baffle as far forward as possible (while still allowing room for recovery gear), your CG stays farther forward. Not only does the weight of the baffle move affect the CG, it also ensures that the shock cord and parachute stay forward.

While I don't have anything to back it up, I would imagine that the baffle being farther away from the ejection blast would increase its longevity. Hey, speculation is free!

 

[rocketsmith]

Install the baffle as close to the nose as possible while still being able to get all the recovery apparatus in the airframe. Consider the biggest chute you would ever use and add 1 airframe diameter to the space required. It will make the baffle last longer and concentrate weight towards the nose helping the rocket be more stable. Most people don't consider this: as the motor fires up and generates thrust the parachute will slide back in the airframe making the center of gravity shift back and affecting stability. Keeping the chute by the base of the nosecone helps prevent that, in an otherwise stable rocket of course.

 

[Scode68]

I thought I read somewhere to have it at least 9" forward but can't remember where it was or why. I did think about the advantages of moving the CG and keeping the chute up top after deciding to install the baffle.

 

[n5wd]

I thought I read somewhere to have it at least 9" forward but can't remember where it was or why. I did think about the advantages of moving the CG and keeping the chute up top after deciding to install the baffle.

You're not going to be able to move the baffle forward far enough for the hot particulate matter to extinguish itself before it gets to the baffle, so you only NEED to clear a little more than the longest motor you'll use. But, as has been pointed out, moving the baffle forward moves the CG forward, which is usually a good thing.

 

[MarkII]

I always take the parachute and the approximate length of shock cord that I will be installing and pack it into the top of the airframe just like I was prepping the rocket for launch, and then insert the nose cone. Then I measure how much length this takes up. I position my baffles no more than 1/2 inch below that point. (1 inch in longer airframes.) Use a long dowel or a long thin ruler inserted up through the motor mount to determine where the bottom of the recovery package is after you have packed it like this and placed the nose cone on. (Remember that the shoulder of the nose cone requires some space, too.) That will give you an indication of how far into the top of the airframe you should recess the baffle when you install it.

I always coat all surfaces of my baffles with a smooth layer of epoxy when I assemble them (laminating or finish-cure epoxy works great for this) and then smear the inner wall of the airframe where the baffle will be with more epoxy when I install it. The baffle will be doing its job when it traps all of the burning particles of BP from the ejection charge and retains them within it. This means that there will be stuff inside the baffle that will burn for a little while after recovery system deployment. When you retrieve your rocket after it lands, don't be surprised when you put your hand on the part of the airframe where the baffle is located and it feels HOT! It will stay that way for awhile, too. The coating of epoxy on the baffle will help it to last a little bit longer, and the epoxy on the inner wall will slow down, or even prevent, burn-through of the airframe.

MarkII

 

[Micromeister]

Ditto Marks comments about where and how to locate your baffle. I usually use 1/2 to 3/4" below the installed recovery package.

Another tool I've been using on baffle plates is 2" stainless Steel foil tape on both surfaces UNDER the epoxy. This little addition has more then doubled the useful life of baffles in several of my smaller BT-50 models, which seem to burn through epoxy clad ply baffles in 15-25 flights, clustered models as well which seem to erode even quicker. Eventually these will fail as well, but the cost in time and material has been nothing compaired to adding live to the longevity of many of these models

 

[MarkII]

Another tool I've been using on baffle plates is 2" stainless Steel foil tape on both surfaces UNDER the epoxy. This little addition has more then doubled the useful life of baffles in several of my smaller BT-50 models, which seem to burn through epoxy clad ply baffles in 15-25 flights, clustered models as well which seem to erode even quicker. Eventually these will fail as well, but the cost in time and material has been nothing compaired to adding live to the longevity of many of these models

Good idea!

MarkII