JCRL
Well-Known Member
I would like to kitbash an Estes Viking by swapping out the stock fins with trapezoidal or elliptical fins. Can anyone point me in a direction for how to determine the correct fin size for a given rocket?
Determining Fin Size?
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The simplest (assuming your fins are in approximately the same place, and of the same quantity), make the area of the new fins approximately the same as the old fins. Next would be the cardboard cutout method (least accurate, and most conservative), followed by the Barrowman equations (slightly better), and then OpenRocket or Rocksim (improves upon the Barrowman equations).
Even if not the same number, such as going from the Viking's five fins down to four, if you keep the total area equal, each new fin having 5/4 if the area of the originals, you should be fine. If you start with five or more and go down in number this is conservative because of diminishing returns above four fins. If going up in number just keep the area per fin the same and that'll conservative, because the returns do not diminish to zero.
But really, OpenRocket or RockSim is the way to go. If you want to keep the CP in just the same place, for instance to do an apples-to-apples performance comparison, it's either that or a wind tunnel.
But really, OpenRocket or RockSim is the way to go. If you want to keep the CP in just the same place, for instance to do an apples-to-apples performance comparison, it's either that or a wind tunnel.
JCRL
Well-Known Member
I'm running altitude and speed performance comparisons, so I guess I need to learn OpenRocket.
Becoming proficient with the sim programs is certainly worthwhile but can be a bit of a learning curve. Mr. Stine documented some rule-of-thumb dimensions for a simple clipped delta fin based on body tube diameter. If memory serves, span is 2x the diameter and the root edge is 1.5x . Please go right now and buy, beg, borrow (not steal) a copy of The Handbook of Model Rocketry.
https://www.airplanesandrockets.com/rockets/low-drag-design-american-aircraft-modeler-may-1968.htm
Another resource is Apogee's How-To pages: https://www.apogeerockets.com/
https://www.airplanesandrockets.com/rockets/low-drag-design-american-aircraft-modeler-may-1968.htm
Another resource is Apogee's How-To pages: https://www.apogeerockets.com/
I thought as much. (I'd seen a thread about testing a bunch of Vikings with different fin orientations and figured that was probably you.) Now, if you really want really apples-to-apples, you also need to adjust the mass and location of added weight to keep the CG and total weight constant as well as the CP. going down to four fins with about the same total area will reduce the total weight and shift the CG up. If there's nose weight to start with you can reduce it and, if necessary, move it. If not, you'll need to all "tail weight".
Sounds like rules of thumb wouldn't do. If you need help with OR just ask here; you'll get plenty of good helpful answers. (But not from me; I'm a RockSim guy.)
And Apogee's newsletter. There's a topic index to help find specific information.
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JCRL
Well-Known Member
I was reading through the Handbook of Model Rocketry last night before posting this in hopes that Mr. Stine would have a formula for fins. Your memory serves you correctly, the only fin he describes in detail is the clipped delta. I also found an equation which seems to calculate the volume of the rocket and gives the fin area in square inches. For three fins: 0.17*[(d+0.5)*L] where d=body tube diameter and L=body tube length. For four fins: 0.13*[(d+0.5)*L]
(d+0.5)*L doesn't give the volume or something proportional to it, since d isn't squared. Imagine a plane cutting the rocket lengthwise right in half; d*L would be the area of the rectangular intersection and (d+0.5)*L would be, well, a little more. Also, I hope Stein gives the units for this; for a 1.6" diameter that added 0.5 means a lot, but for a 38 mm tube it means nothing.
And, for my last bit of math geeking, the factors of 0.17 for for three fins and 0.13 for four could be replaced by 1/(2n) for both cases. But I'd be careful extending that because of the diminished returns I mentioned in post #4.
And, for my last bit of math geeking, the factors of 0.17 for for three fins and 0.13 for four could be replaced by 1/(2n) for both cases. But I'd be careful extending that because of the diminished returns I mentioned in post #4.
JCRL
Well-Known Member
Yeah, that was me. We were discussing starting a science fair for adults.
Right. I didn't go back and check because I was pretty sure.
My favorite part of "The Handbook of Model Rocketry" is the appendix with the Barrowman CP equations. This was from a public library copy, probably the Second Edition. I hope that appenix is in all subsequent editions. This will allow you to do precise fin sizing to place the CP where you want it. After you have mastered that skill, you can plug your design into a simulator and estimate how high it will go.
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