Fins too close to engine blast?

I'm putting together a very simple D-engine model with a nosecone camera, and to increase stability I'm liking the idea of fins overhanging the body tube (see photo) - but I'm concerned as to whether this puts the (balsa) fins too close to the engine blast...

I can certainly imagine that if the rocket were going to be stationary while the engine is blasting, the fins would catch fire after a few seconds.. but assuming a quick lift-off, are the fins kept sufficiently cool during engine burn by 200+mph of air flowing over them to the extent that I don't really need to worry about them catching fire?

Many thanks,

Chuck

While I wouldn't necessarily design it that way, I don't think you'll have a problem there. The flame and exhaust shoots pretty nearly straight down out of the motor nozzle; there's very little spread within the first 1/2" or so like you've got there. If you want a little extra protection you can coat the vulnerable bits with a thin layer of epoxy, but I don't think you'll really need it.

Look at the fins on the old Astron Beta for comparison:


No worries.

You're more likely to see damage from the blast deflector spreading out the initial flame on ignition. Use a clothespin or something to raise the rocket an inch or so.

kj

I agree with kjohnson, the fins and body tube are more likely to be damaged by the initial jet of gas bouncing off the blast deflector than by the flame jet in flight. Place a wooden clothes pin about 10cm up from the blast deflector and let the rocket sit on that. If you don't have a clothes pin you can use a used motor. Just knock out the clay nozzle and slide the used motor down the launch rod and let the rocket sit on that.

Welcome to the forum and thanks for posting a very interesting design. I think the design approach you've chosen with fins swept back behind the airframe was quite common in the 1960's. I think you would get a lot of benefit from papering the fins in this case. Couple of sheets of printer paper cut to size on either side with watered down PVA glue would add significant stiffness with minimal added weight. Another strengthening technique I've used on balsa fins is model airplane tissue and dope.
I agree that the moment of ignition is the critical time and the 10 cm distance from the blast deflector that Zeus-cat mentioned should take care of the biggest scorching risk. I'm intrigued by the forward end of your model. Beautiful ! Turned balsa ? Will the camera mount in it ?

Wow, guys, thanks for the quick responses. Great, I'll go ahead with these fins. I tend to use a wire lug from electrical terminal block (photo) to keep rockets up the launch rod bit anyway (resting the bottom edge of the body tube on the lug setscrew has worked welll for me), so hopefully blastplate bounceback shouldn't be a prob - but many thanks for mentioning it.

Yes, it's turned balsa, then cut lengthwise and scooped out for the little U8 camera with a mirror hot-glued at 45 degrees. I thought a horizon cam might be fun, and wanted to send one up as high as poss on a D engine.

Thanks again for all the help.

neil_w said:

...Look at the fins on the old Astron Beta for comparison:

No worries.

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The inside edge of my Beta fins get a little singed each launch, even keeping it well away from the blast deflector. It’s modest though, took a few flights before burning through the paint. Your design is fine if you don’t mind this superficial damage.

Got no prob with a bit of superficial damage. My son and I reckon war wounds make a rocket look all the cooler.

car3107 said:

...

Yes, it's turned balsa, then cut lengthwise and scooped out for the little U8 camera with a mirror hot-glued at 45 degrees. I thought a horizon cam might be fun, and wanted to send one up as high as poss on a D engine.View attachment 341171

Thanks again for all the help.

Click to expand...

That is a nice bit of fabrication. How did you determine stability for this design ?

Thanks for the kind words. Been playing around with OpenRocket and this design produces a stability figure of somewhere between 1.1 and 1.4, depending on which diameter (payload nose or main tube) you use to calculate it. Btw, I've got a question to ask you guys about launch lugs but I'll start another thread.

car3107 said:

Thanks for the kind words. Been playing around with OpenRocket and this design produces a stability figure of somewhere between 1.1 and 1.4, depending on which diameter (payload nose or main tube) you use to calculate it. Btw, I've got a question to ask you guys about launch lugs but I'll start another thread.

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Very good. My vintage calibrated eyeballs were telling me "the airframe's a little short" but it wouldn't be the first time my old eyes lied. Looking forward to more progress and a flight report.

car3107 said:

Got no prob with a bit of superficial damage. My son and I reckon war wounds make a rocket look all the cooler.

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Indeed! Well said.

I saw in separate thread you did some masterful work hollowing out the nose cone for a camera. Not sure if you have the space for it, but you probably have the woodworking skills. since you have already hollowed out the cone, a third option (after a deep groove or standoff’s) may be a tunnel which would line up with some of the cuts you ALREADY have in the cone.

If (and it is a big IF) you have space and can perfectly line it up, you can place a “tunnel” THROUGH the nose, so only a small hole on th front of the nose that won’t affect stability much. You will have a standard small lug on the back.

Yes, you do have to line up the nose cone with the rear lug, but given nose cone is a completely separate part from body,shouldn’t be too hard.

I did this on Tank Killer ( https://www.rocketryforum.com/showthread.php?139310-Tank-Killer-update-11-15&highlight=Tank+killer ) and it worked really great.

Looking forward to flight report and your on board video!

If you do standoff’s, make sure to recalculate stability. They will act like fins, so position relative to CG may be an issue if too far forward. I am guessing as long as they are tailward of current CG you should be okay.

My son built a 24mm Near-MD that has fins that stretch WAAAY back from the base of the rocket and hasn't had a problem in (checking the flight log) a half dozen launches yet:



(that fin discolouration in the picture is actually his choice of rattle can colours for the back of the fins, not any kind of scorching...

That's a great nosecone. The biggest problem that many have with fins hanging off the back is they tend to be more susceptible to damage upon landing.

Hey, Babar, I knocked together a "Tank Killer" (or "Bazooka Round", as we've been calling it) myself recently where I, too, have the launch rod tunneling through the nose and payload bay. I guess great minds think alike and all that, eh? The point of the payload bay being set up like this, by the way, is to be able to have one camera facing downwards for the standard "down the airframe" shot, as well as a "horizon cam" on the other side. It weighs a ton and doesn't go very high (110m maybe?), but my son and I think it looks pretty cool, so who cares...?!

car3107 said:

Hey, Babar, I knocked together a "Tank Killer" (or "Bazooka Round", as we've been calling it) myself recently where I, too, have the launch rod tunneling through the nose and payload bay. I guess great minds think alike and all that, eh? The point of the payload bay being set up like this, by the way, is to be able to have one camera facing downwards for the standard "down the airframe" shot, as well as a "horizon cam" on the other side.View attachment 341204View attachment 341205 It weighs a ton and doesn't go very high (110m maybe?), but my son and I think it looks pretty cool, so who cares...?!

Click to expand...

You have hit on a couple of my favorites: outside the box styles done just for the fun of it and trying different approaches with cameras. Looking forward to your flight reports!

Thanks for all the help, guys. The launch went pretty well. Reasonably stable in 7mph breeze, but a bit of wobble, so less than ideal as the camera platform it was designed to be. Have sorted out a slight asymmetry in the nose mass, and am going up to a 4-fin setup for the next launch. Hopefully I'll get an awesome "straight-up-like-and elevator" horizon shot of the northeast London skyline out of this baby yet.

vid here:

https://youtu.be/90gEluuACg8