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But only by a nose...
The rocket was far less stable than all sims and adding a simulation of base drag would have made the simulation even worse.
Gary, the Apogee newsletter explains it well for application on Rocsim. IIRC, you add a "zero weight" cone transition onto the aft of the rocket at a length of max diameter * pi.
If anyone has a link to this article, I'd like to read it!
Thanks,
Erik
How can adding base drag per the Apogee cone method adversely affect your stability margin? Base drag moves the CP further aft.
It would have increased the simulation error. OR showed the CP about 2/3 of the way back already, but actually it was almost at the middle of the rocket, at least at certain small angles of attack, as verified by wind testing and flight. I have another rocket with no regular fins but more of a defined "cone fin" which is a draggy shape like a funnel or saucer, but this is also larger compared to the airframe. That one has a CP near the rear. But the rocket in question used a plastic quart milk bottle (cut off) which is a very low drag shape except for the base drag and it had much less effect on stability. This was a case of a rocket that is not short but would be expected to have a substantial base drag effect anyway because the thick part of the rocket was short and at the rear.
My point is if you look at needed stability margin in terms of rocket length rather than diameter, this correction is not necessary. The Apogee paper doesn't consider any rockets with a negative stability margin until base drag is compensated for. I think it's this nonsense with "calibers" of stability that is responsible for the idea these rockets weren't stable in the first place.
Or perhaps the needed margin is a function of both length and diameter. For example:
.6 cals x 3" dia. = 1.8", is 1/10 of 18" length OR 1/10 of 3" dia. + 15" long
2 cals x 1.6" dia. = 3.2", is 1/10 of 32" length OR 1/10 of 1.6" dia. + 30.4" long
^^-- I think it's interesting but the examples they're using simulate as already stable without it. What would prove it is a rocket that simulates unstable until base drag is simulated, that flies stable. You're right my example is not conclusive it's a just a data point and definitely not a case of "all else equal". But if base drag stabilization is real, it applies to some extent to all rockets with base drag.
If you have a design that is 1" diameter and has a 1 cal (1") stability margin, and then change nothing but double the diameter so that 1" margin is now only .5 cal, is the rocket less stable? Not much if any. If instead the change was make it half as long, with the same motor and fins, with the usual designs both CP and CG become relatively closer to the center of the rocket. Even if the margin is now only .5 cal, the rocket is probably more stable than before because this CG/CP shift results in less movement of CP with AoA and the margin is still the same fraction of the rocket's length.
If I'm in the field and the rocket has CP marked on it and I balance it on my finger, it's a lot easier to eyeball the margin in calibers than any other unit, and would be very innaccurate to eyeball it in a fraction of the rocket's length unit. But calibers is not a fixed unit, nor a natural unit in terms of what matters. It is accepted that shorter rockets need less than long rockets, but people get nervous when the number is less than 1.
Attached is a file of my proposed rocket. That nose weight is 128 oz (8 pounds) giving the rocket a margin of about 2.2. It is barely "stable" at about 1.15 with an L2200 loaded. But if I remove the nose weight, the margin goes to -0.03 with that L2200 in the tube. What will happen then with trajectory? I need more concrete information based on facts, not opinion. Besides for the safety involved, I may want to use this as an L3 qualifying rocket someday. My original question about nose weight was just to get the conversation going but it has become more interesting now with the current discussion. Ergo my current question: Is this a stubby rocket and do I or not need to make some adjustment in Rocksim with that invisible transition on the aft end?? This file is not based on the final design by the way, but it's getting close.
View attachment 276891
The saucers that they show in the Apogee article are not stable in the least.
Please post the Rocksim file.
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