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wwattles

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Does anyone have any experience with surface coatings for the INside of a fin can to protect it from exhaust gases? I'm thinking of trying some of that paint for the exterior of barbecues, but it says not to use it for areas that are going to be exposed directly to the flames. The exhaust nozzles are nearly in direct contact with the body tube, and are recessed about 1 1/2 inches into the back of the rocket, so 24mm F-72's are going to burn through my fin can if I'm not careful.

Any thoughts?

WW
 

Zippy

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You could try automotive high heat paint. No garunties though.
 

billeblurzz

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I have built several Launch Pad kits that have recessed motor tubes, some with 2 and 3 motor clusters. I use a silver tape that comes in 11/2" wide size and has a backing that you peel off to apply simliar to carpet. This tape can be bought at hardware stores and sometimes is used to seal air ducts or rain gutters. It is thin and reflects the heat from motors and lasts for several launches. You just keep checking for the gradual deterioration of the tape and peel off and replace when necessary. It protects the body tube very nicely. Just line the inside of the fin can with it.;)
 

LarryH

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You could try spreading JB-Weld real thin all around the area(not gonna be an easy job!)... stuffs rated to 500 degrees, and is recommended by motor retention ring manufacturers to attach thier retention rings to your motor mount because of it's high heat rating
 

powderburner

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I fly 99 percent low-power stuff, so don't really have experience at anything big. But I coat the inside of the BT with CA, or thin epoxy, or at least a good swab-down with thinned white glue, and I have not had any problems. Radiated motor heat is probably the real problem to watch here, as the ejection charge only flashes up to peak temperatures for an instant. You can relieve the heat build-up just a bit by using a solid centering ring at the front of the motor mount (for a gas seal) and using a highly perforated CR at the rear, or even just some balsa centering struts.
As far as recessing your entire motor so the nozzle is 1 & 1/2 inches inside the rear of the BT, you may be pushing your luck. Are you familiar with the Krushnik effect? Motor nozzles that are set too far inside a small chamber end up simply recirculating (swirling) the gasses inside the BT, pumping that circulation vigorously but losing a GREAT deal of net vehicle thrust. You may want to re-think locating those nozzles so far forward.
Taking a wild guess here, but maybe you are trying to use a cluster, you have a lot of motor weight in the tail, and you are trying to relieve the aft-c.g. problem by moving the motors forward a bit? If so, put the motors back where they belong and use nose ballast.
 

wwattles

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Alas, this is the ubiquitous Estes Phoenix. Actually, it's my Quadri-Phoenix (4x24mm mounts), so the CP is WAY forward because of the huge fin area. I've already got about 3/4 lb of lead weights stuck up in the nose.

Changing out the foil tape shouldn't be a problem, since the motor mounts themselves are inside of a friction-fit coupler that is slid in and out as necessary (makes changing engines and mount sizes a snap). I picked up some tape yesterday, and already have it lining the coupler tube.

Larry, on my previous flights, I've used a thin coating of 2-ton epoxy, and even that singed through after 2 flights, leaving a couple very weak areas in the fin can.

Thanks for the help guys - I'll let you know how it goes after this Saturday's launch.

WW
 

powderburner

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wwattles, is the burn-through/scorching occurring next to the motors or behind them? I thought you had a cluster that was set far enough forward that the nozzles were still 1 & 1/2 inches inside the rear of the BT, and that rear edge of the BT is what was getting singed.
As far as the configuration of the model Phoenix missile, yes it has lots of fin area in the middle and towards the forward end. I can see that with a cluster in the tail, you would end up with a ton of nose ballast. Maybe it's time (at least for this model rocket) to single-up the motor, and not quite fly to Mars. Save your F-cluster (that sounds obscene, doesn't it?) for another time.
 

wwattles

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It's occurring behind the motors, because the BT-50 motor mounts are practically right up against the inside of the fin can/body tube. The burn-through is all throughout the aft end of the body (and I can see the swirling effect with my naked eye as it launches - it's definitely there).

On a slightly related note, the Frankenstein effect is also very apparent on this rocket because I've got a "stretch" module that I put in that is basically a normal ogive nose cone with a 24" shoulder. Does a good job of throwing the CG way forward again. That lets me put the motor mount further back on that configuration so it doesn't burn through. This truly is a modular rocket based upon an extremely over-engineered main body tube.

WW
 

wwattles

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By the way, I've got a bunch of pictures that are still in my camera - gotta finish the roll before I can get them developed and posted.

WW
 

powderburner

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As long as we are stretching, ogiving, re-motoring, and making other changes, I take it that you are not concerned with scale appearance of your Phoenix. If that's the case, why not move the c.p. aft some more by trimming some of that forward fin (cut it down to a strake?) or adding on a bit of aft fin span, or chord, or both. Then you could move your motor mounts back a bit and avoid the entire burn-through problem?
 

wwattles

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The thought has crossed my mind, but the goal of the project is to be able to still return to the original "stock" appearance at any time. The 3 mods I've made (variable nose cone weights using velcro'ed lead strips, longer nose cone, and variable motor mount configurations) can all be returned to the original parts.

Yeah, it's screwy, but it's sort of a learning rocket - it's a test platform on performance enhancers, I suppose.
 

wwattles

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I tried billEblurzz's idea for the foil tape, and it worked well, except for a small tear that was in the tape when I installed it. At that particular point, the heat quickly built up under the tape, destroyed the adhesive, and caused it to burn under the tape. Very interesting. But other than that, it looked to be none the worse for wear. I had to rub off all the motor residue, but that was easy with a baby wipe. Thanks for the suggestions!

WW
 
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