Stuffer Tube and Engine Baffle

[LiteMeUp]

Hi,

Thought I'd ask - be gentle... Building an upscale USS America, setup for a 29mm (Series 115 tube) in a Series 275 tube. The sustainer consists of 2 sections, the lower section is 20" long. My plan is to run the 115 series tube the entire length + 1" for a coupler/baffle I'm making (coupler/baffle will stick out 1.5" for top section). The baffle will be based on the one Apogee instructs about making (will be 6" long, with 6 - 6" BT-5 tubes. I'll have roughly 10.5 inches of space between motor mount and where baffle starts.

I'll leave 1/2 - 3/4" clearance at the bulkhead ring and top of the 115 tube for the gases to make their turn, same clearance at bottom of baffle for the BT-5's...

I'll have roughly 10" for chutes, etc in the upper section. Anyone see any problems arising (hoping I explained myself well enough). TIA

[Intruder]

Are you making a baffle that looks like this?

[LiteMeUp]

Are you making a baffle that looks like this?

Yes, that's what I'm "building".... Except add an engine mount to the bottom of it (all one tube) and there's about 10" of space in between the mount and the buffer. The buffer will stick out about 1.5" out of the lower section to become the coupler for the top section.

[Intruder]

I've never flown composites, so I don't know how hard they eject or how long the baffle will last. I do know that it will sound like a baby rattle after the first flight though. IMHO, a baffle modeled after something like this would be a little easier to construct and get the soot out of.

[LiteMeUp]

I've never flown composites, so I don't know how hard they eject or how long the baffle will last. I do know that it will sound like a baby rattle after the first flight though. IMHO, a baffle modeled after something like this would be a little easier to construct and get the soot out of.

I'm flexible, I do see your point after the first launch and having a baby rattle using the BT-5 tubes, also like your suggestion in design... :) Thanks much!!!!

[Intruder]

Check out this link (http://www.apogeerockets.com/site_map.asp) and then scroll down to "Issue #129" about 3/4 way down the page. It has a lot of information on different types of baffles.

I personally use a baffle similar to the "S-Turn" design in "Figure 9", except I use semi-circles for all three plates. Very effective, strong, easy to construct, and it's easy to shake out all of the crud from launch.

Hope this helps. :)

[Intruder]

Oh, one more thing. When you start your project be sure to post LOTS of pics. :D

[LiteMeUp]

Check out this link (http://www.apogeerockets.com/site_map.asp) and then scroll down to "Issue #129" about 3/4 way down the page. It has a lot of information on different types of baffles.

I personally use a baffle similar to the "S-Turn" design in "Figure 9", except I use semi-circles for all three plates. Very effective, strong, easy to construct, and it's easy to shake out all of the crud from launch.

Hope this helps. :)

It helps. :) I was looking at Issue #188 to come up with mine, and yes I will start posting pictures once I start on it. :D

[Bob Kaplow]

The baffle design I've used on several models can be found here:

http://www.nira-rocketry.org/Document/Phantom4000.pdf

My baffle rule is pretty simple: preserve the cross sectional area at each stage of the baffle. I'll drill 4 holes in the centering ring, each half the diameter of the stuffer tube. I'll use 16 holes, each 1/4 the diameter of the MMT to vent that tube.

The concept of a baffle is pretty simple. Hot gasses don't burn chutes, burning particles do. Any baffle should be designed like a darkroom light trap. Particles traveling in a straight line can't get through, and are blocked somewhere along the path. The ejection gasses can still get through the obstructions.

And yes, a baffle will start to sound like a baby rattle after a few flights.

[Thomas Malthouse]

What about just taking 3 little squares of fiberglass widow screeening and glue the squares in so the different directions of the holes will prevent all the particles from coming through. However, you can just shake out the particles out the motor tube unlike wire mesh. I am thinking about retrofitting all my rockets ths way so do you think it will work? We have plenty of window screening lying around.

[Intruder]

What about just taking 3 little squares of fiberglass widow screeening and glue the squares in so the different directions of the holes will prevent all the particles from coming through. However, you can just shake out the particles out the motor tube unlike wire mesh. I am thinking about retrofitting all my rockets ths way so do you think it will work? We have plenty of window screening lying around.

I think that, if it can work, it would be almost impossible to get the right balance of airflow and particle retention. If your mesh is too coarse, airflow would be ok, but the finer particles would just pass right on by. If it's too fine, it would catch the smaller particles, but I think it would cake up and plug up the baffle rather quickly. If that happens, the BT would overpressurize and, well, the end result won't be pretty.

Just my 2 cents

[Solomoriah]

I have a bypass-tube baffle in my New Centurion, which also has interchangeable engine mounts; the latter helps in getting the crud out. I recommend, when using a baffle, putting the upper centering ring as close to flush with the top of the engine tube as possible, for this same reason.

[Bob Kaplow]

What about just taking 3 little squares of fiberglass widow screeening and glue the squares in so the different directions of the holes will prevent all the particles from coming through. However, you can just shake out the particles out the motor tube unlike wire mesh. I am thinking about retrofitting all my rockets ths way so do you think it will work? We have plenty of window screening lying around.

Years ago I tried using a metal wire screen. It burns! I'll bet fiberglass would do the same.

And again, you need something opaque enough to block light to catch the particles. I don't think screen would do it, even if it didn't burn up the first time ejection hit it.

[Mark II]

I like the idea of a completely removable and replaceable baffle. Basically, it uses any of the better designs described in Apogee POF#129, but it is a self-contained unit that sits between the top of the motor and a stop ring (similar to a thrust block) glued in just ahead of it. It is either built inside a tube coupler, or inside a smaller tube that fits into the airframe or motor tube with centering rings. The baffle unit is not permanently glued into the airframe, but is slid in ahead of the motor. It can be designed to either fit inside an extended motor tube or else into the portion of the airframe just ahead of the motor mount. In the first case, the stop ring is at the top of the motor tube. If, on the other hand, you outfit the rocket with a removable motor mount, then the baffle unit can be an extra component that fits in ahead of the mount. In either case, you slide the baffle in from the aft end of the rocket, ahead of the motor (or mount); it is held in place by the stop ring in front and the motor behind it. When you remove the motor (or the entire mount), you just push the baffle out after it with a dowel inserted from the front end of the rocket. You can then clean out the baffle between flights, and eventually replace it when it wears out. No more side wall burn-throughs, no more baby rattles, no more blocked baffles and no more extreme surgery to remove and replace a blown baffle.

You can even design the baffle unit so that it can be completely dismantled and cleaned itself after every few flights. The parts would be held together with small screws passing through the upper and lower ends of the baffle.

I discussed this idea in a thread in the old version of TRF, but I have made a few tweaks in the concept since then.

MarkII

[Solomoriah]

The closer the baffle is to the engine, the more damage it suffers. Now I'll admit, that's the only downside I've been able to see with your idea, but it is a downside.

[LiteMeUp]

Mark II,

Looking around on the net, found a couple examples of what your referring to. It would almost be like a changeable motor mount but extended further to hold a baffle also. Using a centering ring expoxied at base with "T" nuts, you'd undo the screws and entire motor mount/baffle assemble would come out. I may have over simplified the explanation, but seems like an option.

[Mark II]

The closer the baffle is to the engine, the more damage it suffers. Now I'll admit, that's the only downside I've been able to see with your idea, but it is a downside.
The tube or coupler that contains the baffle doesn't have to be the same length as the baffle; it can be longer. The baffle can be located at the upper end, with space below it to provide some distance from the ejection charge well or top of the motor. In this case, you could place another stop ring inside the container tube, right below the baffle. Then you can take the assembled baffle and just pop it into the top of the "carrier tube." When it is inserted into the airframe, the baffle will then be sandwiched in between this lower stop ring and the upper one that I described earlier. The upper stop ring will need to be wide enough to not only prevent the carrier tube from moving forward, but also the baffle itself. Not an insurmountable, or even difficult problem. In between flights, the carrier tube containing the baffle can be pushed out, and then the baffle itself, which is not glued in, can be knocked out of the carrier tube and cleaned, inspected, replaced, etc.

I hadn't thought about this latest twist in the design, but your feedback, Solo, prompted me to outline it. (It was always implied in the design, but it was sitting in the back of my mind, waiting for the right opportunity to be summoned forward. :D ) Thanks for the reminder! ;)

If, because of the rocket's overall design, the baffle still needs to be located directly above the motor, then its vulnerable surfaces should be coated with a decent layer of epoxy. (This is going to be the case with the rocket that I'm building now.) When the baffle is removed, it can be recoated as needed; this is especially true if the baffle has been designed to be able to be disassembled for maintenanace. In any event, the baffle becomes a replaceable component and not a permanent structure inside the rocket. In the thread where I talked about this in the old TRF, Jim Flis mentioned that all baffles, regardless of design, will eventually fail. With a replaceable baffle, this failure does not need to spell the end of the rocket's usable life. In fact, because the baffle can be removed and cleaned or maintained periodically it should last considerably longer than a permanently-installed one.

I guess I should say that I have been on a kick recently, thinking about ways to "modularize" the model rocket. I'm not looking at following the Mach-1 Industries (http://www.oldrocketplans.com/pubs/Mach-1/Mach-1_Catalog.pdf) route, but rather, I'm exploring ways to make the rocket's internal components serviceable and replaceable, just like parts on the exterior are (such as the fins and the nose cone). This would be most practical with large format low-power rockets and the whole range of mid-power rockets. (Interestingly, this is the same range of designs for which ejection baffles are most practical.) It would be difficult to implement in smaller LPRs, and high power rockets require too much in the way of structural integrity to do much more in this area than they already do. What I am trying to get to is a rocket that consists of an empty tube or tubes, with fins and external decorations attached, but with the ability to have all of the internals configured in various ways for assorted flight plans.

In some ways, rocketry has been at least halfway there for quite some time. We have always been able to swap out recovery devices (parachutes and streamers) to suit various conditions, to add accessory payload sections and booster stages, and to use motors with a range of total impulse levels. Now we are starting to see interchangeable motor mounts. Pretty soon we will see replaceable ejection baffles and shock cord anchors. The trick is to design the rocket and its swappable components in such a way that they are all compatible and have the rocket be stable with all manner of component combinations. And do this without making the rocket look like a Frankenstein's monster of mismatched parts and unsightly external bolts, fasteners and patches. We would not want to do that neccesarily with all of our rockets, but it can be done, and we already have some of those concepts and techniques now.

Well, there I go again, babbling on and on and hijacking the thread. Sorry about that. :o I really need to get on the stick and put together a build thread or two.

MarkII

[Solomoriah]

I have baffles in a few of my rockets, but the closest one to the engine is 7" away; my New Centurion has almost a foot of clearance, and the Celestial Navigator's baffle is about two feet from the engine. So I expect it will be a while until I see a failure of any of my baffles.

BUT... now you have me thinking about it. Instead of a stop ring, you could use retaining screws (like my New Centurion, through the outer wall into the engine mount proper), or possible rig the stop ring at the bottom of the assembly (like high power engine thrust rings). Then the problem becomes keeping the assembly in the body tube at ejection.

I'm experimenting with rear-ejection stuffer tube designs that need no wadding... that's my solution, I guess. I like rear ejection because the relatively tough nose cone takes the first hit, rather than the relatively fragile fins. Getting reliable deployment is the challenge.

[blackshire]

I have baffles in a few of my rockets, but the closest one to the engine is 7" away; my New Centurion has almost a foot of clearance, and the Celestial Navigator's baffle is about two feet from the engine. So I expect it will be a while until I see a failure of any of my baffles.

BUT... now you have me thinking about it. Instead of a stop ring, you could use retaining screws (like my New Centurion, through the outer wall into the engine mount proper), or possible rig the stop ring at the bottom of the assembly (like high power engine thrust rings). Then the problem becomes keeping the assembly in the body tube at ejection.

I'm experimenting with rear-ejection stuffer tube designs that need no wadding... that's my solution, I guess. I like rear ejection because the relatively tough nose cone takes the first hit, rather than the relatively fragile fins. Getting reliable deployment is the challenge.

A thought: With rear ejection, you could also more easily use an externally-mounted shock cord than is the case with front (nose cone) ejection, since it wouldn't have to be run as far *up* the body tube to be anchored at the rocket's Center of Gravity. This would allow the rocket to touch down in a horizontal attitude to spread out the impact loads, and its increased drag in that attitude would also slow down its descent rate.

The shock cord could be tied to the rear end of the ejectable motor mount, and a slot cut out of its rear centering ring (with gently rounded corners to prevent snagging/chafing the shock cord) would allow you to "fan fold" (fold up lengthwise, that is) the shock cord along the length of the motor tube and to one side of the slot.

Another possibility would be to locate the motor mount's Center of Gravity with an expended motor installed and then anchor the *middle* of the shock cord (halfway between the 'chute and the rocket body) to the CG. The two lengths of shock cord could be wound around the motor tube so that they would unwind in opposite directions after ejection (the way that the steel cables of a pair of "yo-yo" de-spin weights unwind from a spinning sounding rocket, satellite, or upper stage in space). The unwinding would reduce the 'chute's opening shock loads as well as reduce the "twang" on both shock cord anchor points.

[Solomoriah]

I don't use external shock cords. I do rear ejection to make a smoother rocket (no nose cone line) and using an external shock cord would not be smooth.

As I said, still working it out.

[LiteMeUp]

Well, this is what I finally went with, I still need to lay epoxy on, this is in a 275. It's not complete and you'll have to follow along in my explanation - and of course I probably made it more difficult then it needs to be.. :)

First picture shows the actual baffle, it's made out of 6 BT-5 tubes, I have two supports in the middle setup for a 115 BT. Top piece consists of bulkhead ring that the 6 BT-5's go thru.

Second picture shows the baffle slid onto the 115, I'll have about 11 inches between the top of the motor mount and the bottom centering ring. I'll give myself about 5/8" between the tubes and rings. Although it may not look like it in the picture there is plenty of room for the air to travel.

Third picture time to use your imagination, I took the 275 tube and made a coupler 6.25 inches long. This will be my baffle casing, the two centering rings on the left I'll glue together and enlarge the centering hole to allow baffle setup to slide out. I was thinking of placing 4 "T" nuts in the combined centering rings. Baffle will then come in from the top and get screwed on... Think it will work? :confused:

[Mark II]

OK, I'm having a bit of trouble understanding your design. Where is the ejection gas coming in, what route is it taking, and where is it going out?

MarkII

[Niteowl]

OK, I'm having a bit of trouble understanding your design. Where is the ejection gas coming in, what route is it taking, and where is it going out?

MarkII

If you look at the center picture, imagine the motor off to the right, mounted at the end of the stuffer tube.

The gasses flow to the left, up the stuffer tube and exit at the gap between the stuffer tube and the BT-5 mounting bulkhead.

The gasses then travel back to the left and enter the BT-5 tubes near the bottom centering ring.

The gasses then travel back to the left through the BT-5 tubes exiting through the bulkhead.

At least that's what I see. Looks operational, maybe a little overkill, but operational.

[Mark II]

Ah, OK - now I see it. Thanks!

BTW, I see four pictures, not three. I wasn't sure which one you were calling the "middle one," but now I see what you mean.

LiteMeUp - how far will you need to reach in to loosen and tighten the screws holding in the baffle? You look like you've got a design there, though. Nice job!

MarkII

[tbzep]

...maybe a little overkill, but operational.

A little? :p

[LiteMeUp]

A little? :p

Sorry, already admitted I probably made it more complex then in needed to be. :p MarkII - I'll need about an 12" magnetic screw driver, aligning it shouldn't be a problem. LOL