cluster baffle help

[astronboy]

Hi Gang,

I am building a BT-80 based Astron Super Ranger using 3 clustered 24mm D12-7s. I would like to build in an ejection baffle. My current plans call for a 3" long tube coupler for BT-80 tubes, with 4 baffle plates having 4 holes each. I will build this as a unit, and then glue it into place in the body tube.

My question is this: how big do I make the holes? I have heard that the combined area of the baffle holes should be at least 1/2 of the area of the inner diameter of the engine. I also know that BP engine delays can vary from engine to engine. Do these delays affect the overall area of the baffle holes?

Thanks ahead of time...

 

[Stones]

It's my experience that the baffle holes should equal that of the MMT. 24mm MMT (.95 I.D.) using 4 holes would be 1/4 (.25) in. holes in each plate. The fact that you're using 3 motors does figure in a larger ejection pressure at the baffle if they all go off at the same time.
With a BT-80 (2.6 in.), and using a 3 engine cluster, I would drill the holes at 3/8 (.375) in. Not having done a cluster configuration, I can't speak from experience.
Also, I would think that 3 plates would be plenty to catch the ejection particles. Just my 2 cents.

 

[astronboy]

THanks Stones, that makes perfect sense. How far should the baffle be from the end of the motor mount tube? What prevents the baffle from burning up?... do you coat it with anything?

 

[Stones]

Is the airframe one(1) length of BT or two(2)? If two(2), attach the coupler/baffle per usual, between the two(2) lengths of BT. If a single length of BT, I would attach it somewhere towards the nosecone end of the BT, giving ample room for all the recovery apparatus(chute, shockcord, etc.).
I coat the baffle plates and the inside of the coupler with a thinned out layer of epoxy. You'll need to coat the plates before you install them into the coupler. Insert the baffle plates into the coupler, one at a time, and cement them in place with epoxy around the edges. Space them out evenly in the coupler. Before you cement the coupler in the BT, you might want to coat the inside of the BT also, below the coupler.
Pic is of a recent baffle I did for a BT60. Plates are of a different design but, you get the idea.

 

[jetra2]

I haven't had much experience with baffles of the type you a talking about, but I'll throw in my two cents worth. What I have heard is that the disks are only designed to catch the burning particles of BP from the ejection charge and duct the cooled, not burning air to the recovery device. So, from that, you would want two, maybe three more centering rings. On the one closest to the motors, have it have holes on the outside, close to the tube. the second one, have just one central hole, 18 to 24mm in diameter. Then on the optional third one, four large holes near the outside. You want the hot air to go through a maze to get to the chute, catching about 99% of the burning particles. There is also a simpler design. You have two or three bulkheads. On the bulkheads, cut out approx. 1/2 to 3/4 of inch off. Then arrange them above the motor mount, alternating which side the cut is on.

Like this: __
__
__

The lines are the bulkheads. Just imagine the body tube on the sides, and the motor tubes at the bottom.

Ok Ok maybe that was four or five cents worth, but just trying to help.

Jason

 

[jetra2]

dang, it messed the drawing up. You get the idea thugh, don't you?

Jason

 

[Stones]

Here's a pic showing what jetra2 is talking about, as far as alternating the plates.

Baffle plates

 

[astronboy]

Thanks Guys, all of this is great help and answers a lot of my questions. As this is a Ranger, the main BT is 19" long, scaled to BT-80. Minus 3.5" for the triple motor mount tubes, and 1.5" for the upperbulkhead shoulder, leaves me 14" for the baffles and recovery systems. I figure that I sould leave 8-10" for the recovery system, so that would be 4-6" between baffle and the motors. As this is a BT-80, I am thinking of lengthening the motor mount tubes as stuffer tubes, and leaving 2 inches between the triple stuffers and the baffle. Any thoughts?

 

[rbeckey]

Ejection gases cool and slow down upon expansion. The more room you have between those baffles and the end of the motor tube, the less wear and tear on the baffles.
With triple ejection charges, pressurizing the tube enough to deploy the recovery system is not going to be a problem. I'd add a stuffer for a SINGLE motor, but not three. Your problem is going to be keeping that triple charge from overpressurizing the baffle system. Consider using 1/8 inch lite ply for the plates. I'd coat them with thin slow cure epoxy to strenghten them.

 

[astronboy]

Again, thanks. I am way ahed of you on the materials dept. I am using 1/8: lite ply covered with epoxy. Maybe I should leave the stuffers off and just make the motor mount tubes 3.5" long... allowing the full BT-80 diameter between the tubes and the baffles... 7-8" above?

 

[KermieD]

I'm lost on this terminology. Probably something where I'm gonna go "oh, duh" when you tell me, but please explain.

 

[rbeckey]

It is an extension of the motor mount toward the front of the rocket. It is used to decrease the volume of tube the ejection charge must pressurize, assuming that it has acentering ring at the end.

 

[Stones]

Originally posted by rbeckey
It is an extension of the motor mount toward the front of the rocket. It is used to decrease the volume of tube the ejection charge must pressurize, assuming that it has acentering ring at the end.

Actually, and I may be way off on this one but, once the nose cone is in place on the BT, isn't the pressure in the tube nominal at any length?
I.e., I have a tube that is 12" long and a tube that is 36" long. Both have a nosecone closing off one end and the other end is open. If I blow in the open end of either tube, I don't think it would take any more blowing pressure to eject the nosecone from the 36" tube as it would from the 12" tube. The atmospheric pressure in either length of tube should be the same. Does this make sense?

 

[rbeckey]

I guess that it depends on the total volume. Adding the same amount of overpressure to volumes of different sizes does not have the same result. Imagine setting off a firecracker in a barn as compared to setting one off under a coffee can. The air inside a body tube can be compressed to a certain degree before the NC is pushed out. Less volume, less possible compression. Also, the tube is not truely sealed. The nozzle can let pressure bleed off if given time. A smaller space gives a sharper pressure rise, popping the NC before the pressure bleeds out of the nozzle.
Of course, that's just my opinion, I could be full of it.

 

[powderburner]

I think he is right on track in his explanation, certainly from an engineering point of view.
I know I do lots of things conservatively, mainly as a result of my engineering training, but at least I have never had a nose cone fail to deploy when it was supposed to. I use stuffer tubes on all models that are based on large diameter tubes. And I coat the insides of the stuffers with a swabbed-in coating of epoxy.

 

[jflis]

Originally posted by Stones
Actually, and I may be way off on this one but, once the nose cone is in place on the BT, isn't the pressure in the tube nominal at any length?
I.e., I have a tube that is 12" long and a tube that is 36" long. Both have a nosecone closing off one end and the other end is open. If I blow in the open end of either tube, I don't think it would take any more blowing pressure to eject the nosecone from the 36" tube as it would from the 12" tube. The atmospheric pressure in either length of tube should be the same. Does this make sense?

You are correct that with either a 12" tube or a 36" tube, the amount of pressure needed to blow off the cone is the same.

HOWEVER...

The 36" tube has 3 times the volume as the 12" tube (3 times as long, same diameter, for this example). Therefore, given a fixed amount of ejection gas from the ejection charge, the 36" tube will have (roughly) 1/3 the pressure of the 12" tube (eg: it would take 3 times as much ejection powder to presurize the 36" tube to the same pressure as the 12" tube)

An ejection charge only gives off so much gas. It is the expansion of this gas that produces the pressure.

 

[rbeckey]

Here is a question related to baffles and distance from the MM. I have noticed that I could probably get away with one sheet of wadding to protect the chute in my Mean Machine. I assume because the chute is so far away from the business end of things that lots of cooling and expansion have gone on in the long body tube, even though there is no baffle. I certainly have never had a chute even singed after many flights. Anyone else notice the same thing with this rocket?

 

[astronboy]

Based on everyone's advice, here is the badffle so far: I have coated the inside of the coupler with 12 hr epoxy, and both sides of each baffle. In the pic, one baffle has been expoxied in, and a another will be ready to be added today.(with a 45 degree rotation to the holes).

 

[MarkABrown]

I hope your plate still fits after all of that epoxy dries. The epoxy on the inside coupler wall will subtract a little bit from the inside diameter. Also, if you've coated the endgrain of your plate, then the bond between the already epoxied inner wall and your plate won't be as strong. Luckily, you don't need alot of strength to stand up to the ejection gases.

 

[astronboy]

Hi mark,

I was worried about both of these issues, so... the baffles were made a bit undersized. They were loose, but now fit snugly into the coupler. Also, I did not coat the edges of the baffle plates fo these two reasons. Great minds think alike!!

 

[astronboy]

OK, I am ready to place the second baffle. How critical is the 1/2 between plates? Is 3/4"too far?

 

[astronboy]

Here is the finished baffle, ready for installation: I drilled a 1/16" hole near the edge of the top baffle and knotted some 400# kevlar through the hole. The knot is epoxied on the inside of the top baffle.