Turns out there is a good use for the multiplication table for 0...
Let's put an end to the "Base Drag Hack"
- Thread starter Buckeye
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It is launch day, grasping my latest oddroc build prepped for flight I begin to walk with it in plain sight. Overcome with fear there is only one thing left to do: PRAY!
"The sim is my Shepard: I shall not fear. It maketh me bare down in fluid dynamic texts: it leadeth me beside the far far away pads. It restoreth my confidence: it leadeth me in the paths of pure rocket science, for Barrowman's sake.
Yea, though I walk through the valley of rocket eating trees, I will fear no instability: for thou art with me, my half inch stainless rod and thy base drag hack, they comfort me. Thou preparest a table before me in the presence of a really really tough RSO, thou anointest my rail with WD 40; my battery life runneth over.
Surely rocket science and pure logic shall follow me all the days of my life: and I will dwell in the house of computer simulation forever.
Amen."
I am now ready to fly Crazy Train. Fear is the mind killer.
"The sim is my Shepard: I shall not fear. It maketh me bare down in fluid dynamic texts: it leadeth me beside the far far away pads. It restoreth my confidence: it leadeth me in the paths of pure rocket science, for Barrowman's sake.
Yea, though I walk through the valley of rocket eating trees, I will fear no instability: for thou art with me, my half inch stainless rod and thy base drag hack, they comfort me. Thou preparest a table before me in the presence of a really really tough RSO, thou anointest my rail with WD 40; my battery life runneth over.
Surely rocket science and pure logic shall follow me all the days of my life: and I will dwell in the house of computer simulation forever.
Amen."
I am now ready to fly Crazy Train. Fear is the mind killer.
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Nudge nudge, tell me more!
user 30299
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Can you suggest a reasonable and acceptable form of validation for Buckeye's CFD model? Short of him
having to do a wind tunnel test. Past wind tunnel tests that establish general form/pattern for base drag,
or CFD results from other CFD software programs?
And is it possible the Drag Hack turns out to just be coincidental?
Navy 5" gun fire control was my last gig. Yes - base drag + rifling-induced spin. Spin precesses the round in the direction of the rotation a bit, but they do not tumble. Fire control compensates.
Cheers and THX for the links.
Maybe a 6-DoF Flight Computer ?
I dunno ... thats what makes this such an interesting thread ...
-- kjh
lakeroadster
When in doubt... build hell-for-stout!
One of his guns is in the Bahamas/Bermuda/somewhere and on an "abandoned stuff" show.
lakeroadster
When in doubt... build hell-for-stout!
Barbados...
The 16-inch gun used during Project HARP, along with a smaller gun in Barbados.
If you don't like it, don't use it. Just because you think you are right, does not give you the right to tell others what to do.
Base drag works if use correctly.
Base drag works if use correctly.
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It's all about the base drag. Gas bleed rules! GDS is rad. Answering the age old question of the base drag hack could be harmful to your health if you work for the wrong employer.
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Has anyone given an explanation of why the vortex drag behind a flat plate - the aerodynamic basis of the hack - should be thought analogous to that behind a rocket of any shape other than a flat plate? I do not see the leap of that model from saucers to traditionally-shaped rockets explained in Bruce Levison's articles. Why should a rocket with any sort of streamlining at all kick up such a draggy vortex behind it? The airflow around a short, stubby rocket that is still rocket-shaped (as opposed to a saucer, etc.) is not remotely the same as that around a flat plate.
It seems likely to me that there is some relevant amount of vortex drag at the back of the rocket, but probably nowhere near that of a flat plate. Further, how much vortex drag actually develops at the base of a rocket under thrust, with the low-pressure region behind the rocket being at least partially filled with exhaust expanded to ambient pressure? Given that cg of solid-motor rockets moves forward as propellant depletes - increasing stability - the base vortex drag would seem to become more relevant as the rocket is growing more stable already, so I question the predictive value of any such model when based on - and applied to - the conditions of launch itself.
There are several comments in this thread along the lines that the hack explains why short, stubby rockets are stable, but I'm not seeing how it explains anything at all as opposed to being simply a rule of thumb to make calculated stability margin come out with a number that makes another rule of thumb happy. This is actually how I use it: to provide some comfort to those with an overriding faith in the 1-caliber rule. (Well, I used to put more stock in the hack, back before I read the original articles and found they were based on the vortex drag of a flat plate, and especially back when I was a 1-caliber stickler myself.)
I also see several comments about how the hack works in terms of accurately predicting necessary nose weight for stability, and I'm wondering how that is known. Were your rockets flown experimentally with differing amounts of nose weight to determine what made them stable, and the hack predicted a similar weight as turned out to be needed? If not, how was the hack validated in your cases? If the answer is simply that the weight the hack predicted was needed for 1 caliber was used, and that the rocket when flown this way was stable, I'm not seeing how it is known that what was predicted by the hack was the minimum needed for stability. If the cg was actually behind cp without the hack applied, but the rocket flew stably, well then I see your point, but has this been the case for anyone?
The hack probably does model shallow saucers quite well.
It seems likely to me that there is some relevant amount of vortex drag at the back of the rocket, but probably nowhere near that of a flat plate. Further, how much vortex drag actually develops at the base of a rocket under thrust, with the low-pressure region behind the rocket being at least partially filled with exhaust expanded to ambient pressure? Given that cg of solid-motor rockets moves forward as propellant depletes - increasing stability - the base vortex drag would seem to become more relevant as the rocket is growing more stable already, so I question the predictive value of any such model when based on - and applied to - the conditions of launch itself.
There are several comments in this thread along the lines that the hack explains why short, stubby rockets are stable, but I'm not seeing how it explains anything at all as opposed to being simply a rule of thumb to make calculated stability margin come out with a number that makes another rule of thumb happy. This is actually how I use it: to provide some comfort to those with an overriding faith in the 1-caliber rule. (Well, I used to put more stock in the hack, back before I read the original articles and found they were based on the vortex drag of a flat plate, and especially back when I was a 1-caliber stickler myself.)
I also see several comments about how the hack works in terms of accurately predicting necessary nose weight for stability, and I'm wondering how that is known. Were your rockets flown experimentally with differing amounts of nose weight to determine what made them stable, and the hack predicted a similar weight as turned out to be needed? If not, how was the hack validated in your cases? If the answer is simply that the weight the hack predicted was needed for 1 caliber was used, and that the rocket when flown this way was stable, I'm not seeing how it is known that what was predicted by the hack was the minimum needed for stability. If the cg was actually behind cp without the hack applied, but the rocket flew stably, well then I see your point, but has this been the case for anyone?
The hack probably does model shallow saucers quite well.
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Nice story and a great win. However, yeah, some more info would be helpful to this thread.
Your sustainer doesn't look all that stubby and with ample fins. What's the L/D? What prompted you to add the hack? If you do it all the time on every rocket, then kudos to you for being consistent.
What's the stability margin with and without the hack? Care to post a RockSim file?
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Ok, I think that's doable.
ThirstyBarbarian
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Check out the base drag on my Warlock. You can see the smoke in the vortex right under the aft centering ring. I love this picture!
As said before,....And, I would pay to see someone swing that on a rope.
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ThirstyBarbarian
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You are right that the hack does not explain why short stubby rockets are stable. It’s more like what you described — a way to make the 1-caliber rule of thumb happy.
How much would you pay to see it swinging on a power line above a road?
That was a typo. Rope, not road.
I can see Buckeye's point. The trailing RocSim "Cone" is a method, but currently it does not offer any fundamental understanding and I have not seen any guide lines as to how large the "Cone" should be. (Has it been calibrated? There could be a science project here, if someone wants to do the work.) With that being said I am not sure if anything has been said as to whether a critical speed off the rod has been addressed. I had a short stubby Christmas ornament rocket years ago that was suppose to be stable according to RocSim, but in actual flight it was unstable. However, inspection of the RocSim analysis showed that it was below the critical speed for stability coming off the guide rod.
ThirstyBarbarian
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The cone is supposed to be the same diameter as the base of the rocket. And the length is supposed to be the diameter times pi. That’s how we know it’s science, because of pi. Who uses pi, except math and science people? Otherwise, we would just say “about 3 times as long as it is wide”. Pi, ergo science.
By the way, pi times the diameter is also equal to the circumference. So you could say the cone is one diameter wide and one circumference long. Isn’t science amazing!
stealth6
insert witty tagline here
lakeroadster
When in doubt... build hell-for-stout!
The 2nd Newsletter, 158, defines the size of the cone.... 
... which is as @ThirstyBarbarian describes it.
... which is as @ThirstyBarbarian describes it.
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Calculations will never make an unstable rocket stable. They can only tell you whether or not you can expect it to be stable.
No. That's the only way to validate the calculations.
A totally independent CFD analysis reaching the same conclusions is as close as we can get without a wind tunnel.
I'm not sure what that means.
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Is it possibly a "mathematically correct result derived by incorrect lines of reasoning" ?
The statement is italicized because Wikipedia provided the best way to put my thoughts in words.
It's not a common occurrence, but it does happen in the world of math, engineering, science and technology.
Or another way to look at it is how many times have you needed to troubleshoot
problems with your car or truck, with all sorts of indicators pointing to that one
part, only to replace the part and find out that was not what caused the problem.
For those of you that work around machinery, how many times have you been told
by the equipment operator the problem is a bad bearing. It may be a "bad" bearing,
but it's really due to a bowed (bent) shaft.
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user 30299
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How did you get it down from that "electrified tree" ?
cbrarick
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Hmmm. science. I love pi! Especially cherry pi!
seriously, why are we trying to disprove something we know works, and can be proven using both math and visual (sorry your rocket is on the electrical 'tree')
I would instead ask the question, "why did the OP's work not result in short stubby rockets being stable?
I really would like to congratulate the OP in his efforts. CFD is real;y hard to do. However, when the results do not predict what has been tried and true, you must go back to the original model and ask why. There is something in his model that doesn't work with short, stubby rockets. I'd suggest he has a good model for "standard" rockets but it has a limitation - short and stubby ones. And beyond that, we might even suggest that this error is endemic in the software we use. I'm sure if we dumped a NASA like budget into fixing the problem we could. Unfortunately this is only a hobby.
back to flying short stubby rockets............ :>
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I suggested this here in the past and a got a cold reception. Length of the rocket is a better measure of how large the rocket is than diameter/caliber, and therefore a better definition of what a significant margin is. With most stubby rockets, there is simply no need to explain a problem because there isn't one. If the needed margin does become a smaller fraction of the length of a rocket, that's interesting, if it becomes smaller because your unit is a wrong variable, that's just a mistake.
In the case of saucers, etc. where it appears the margin would be negative, I still think it's a factor of the forward shape or edges, not the base in some kind of vacuum, for one thing the motor pressure kind of messes that up. My experience was a finless rocket that had a sizable base but no blunt forward shapes, and as far as I could tell there was no increase in stability in that case, using wind testing and flight. I didn't try pushing it to failure, maybe retry on the away cell.
The "hack" seems a little strange way to adjust for some kind of math error, but my main problem with it is the reasoning in the paper, which mixes a common error regarding stubby rockets with actual big questions.
In the case of saucers, etc. where it appears the margin would be negative, I still think it's a factor of the forward shape or edges, not the base in some kind of vacuum, for one thing the motor pressure kind of messes that up. My experience was a finless rocket that had a sizable base but no blunt forward shapes, and as far as I could tell there was no increase in stability in that case, using wind testing and flight. I didn't try pushing it to failure, maybe retry on the away cell.
The "hack" seems a little strange way to adjust for some kind of math error, but my main problem with it is the reasoning in the paper, which mixes a common error regarding stubby rockets with actual big questions.
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What he said. I didn't in the post earlier, but I've elsewhere called it an "error in denominator selection," one of at least a couple handed-down rules of thumb that are practiced in this hobby and turn out to be poorly-founded when you dig down to figure out where they came from and why.
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