Tube Fins and Open Rocket

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Screaminhelo

Shade Tree Rocket Surgeon
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I have been following threads by Neil_W and Thirstybarbarian and got to playing with some ideas inspired by them. I started out with the idea of something low and slow that would fly on an Estes F15.

In an effort keep weight down and drag high, I used tube fins but wanted them angled to make it look faster than it is. I used a combination of balsa fins to approximate the front and rear of the tube with a short tube in between. I also added a short ring fin around it all but the only way thatI know to include this is to use an inner tube so it will capture the weight but the aerodynamics won't be accounted for.

Here it is for your amusement and comment. The goal is to keep it around 1k feet on the F15.

View attachment Low_N_Slow_F.ork
 
Comments:

1) As you said, the ring fin is unaccounted for (aerodynamically) by OR. In the file it's 1/4" long... you didn't mean that, did you? It'd be near impossible to build such a thing. If, as I've read, tail rings have high drag, then it puts a big unknown into your calculations.

2) Obviously, OR can't simulate tube fins cut at angles. I tried to approximate it by setting the length of the tube fins equal to the average of the root and tip chords (3") and it knocked another 50' off the F15 apogee but I don't know if that's really the right way to do it.

3) The engine is really deeply recessed into the body. Is that for CG purposes? I'd be worried about the inside of the BT getting scorched.

4) I'd also probably be just a wee bit scared of the extremely narrow (0.25") tip of the tube fins. I'd almost certainly ruin them if I tried to cut that, but then I suck at tube cutting.

In general, I think the angled tube fins encased by a ring would look pretty cool.
 
1) I did make the ring fin 1/4 " on purpose, i thought that it would look cool. Like you said, it would be a challenge but not impossible.

2) You are probably closer aerodynamically than me. I don't mind loosing 50' on this one though, that is really the goal here.

3) The recessed motor does help with CG here. I am trying to avoid dead weight to help the F15 get it off the pad. After playing with a GDS build last year, I don't think that there will be more than cosmetic damage to the BT and I could add vents immediately aft of the motor to mitigate damage to the aft tube.

4) Yes, they would be delicate but it is only cardboard right? To paraphrase JFK, I chose delicate tube fins not because they are easy but because they are hard.

The ring fin was inspired by the Ringed Warrior thread. I thought that it would add to the contrast of a sleek, high performance profile on a slow, draggy rocket. This is largely for play right now, one of many designs that I have dreamed up in OR just to play with ideas and excercise my brain but this one is one that I will probably build one day in some shape, form or fashion.
 
In that case you are a more intrepid tube-cutter than I. :)

Should be interesting. I hope you can squeeze an altimeter in there so you can evaluate how accurate the simulation is. Does this need a base drag correction given its short stubbiness?
 
I think it's getting close but I am not sure what a resonable threshold for that is. The main goal of these little exercises is to explore how different design elements affect the performance of an airframe. This one may be worth building so that I can compare sim data with my Mk1 E observations.
 
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