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Most Efficient Projectile Shape

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Unread postAuthor: Ragnarok » Fri Nov 21, 2008 3:03 pm

jimmy101 wrote:Ragnarok: How accurate are your Cds and velocity and range calculations? How acurately do you thing they predict actual performance?

I'm having a few arguments with the main algorithm of my spreadsheet to make sure it's all up to a very good spec. There are a some small polishes to it I have yet to make which might change the end results very slightly.

I've tried to consider as many variables as possible - for instance, the variable air density with altitude, and how the speed of sound varies with this. That affects drag due to both air density, and mach number.
It also means if fired sufficently high and with the right drag characteristics, things are capable of hitting the ground at greater than their terminal velocity at ground level air density would suggest, as the projectile can build up a higher velocity in lower density air.

A semi complete list of advanced features beyond simple drag and gravity:
Earth Curvature; Air Density variations (currently good to about 11 km up, I'm considering working on sorting it to 71+ km); Variable Cd vs. Mach; Gravity Variation (if the user is really planning on shooting the moon); and for the user that's really feeling daft, the gravity of the Moon and Sun.

All of these advanced features can be turned off if the user desires, and that last one is probably going to be removed (because users will probably find having to put in a date, time and global co-ordinates to be a bit too bizarre), but I do things like that when I'm very bored. I blame _Fnord for suggesting a variable like that needed to exist in the first place.

I've not currently got the controlled spin features (like back spin) modelled, and I'm also trying to work out if I want to include the small natural lift of anything moving through the atmosphere, and buoyancy - which of course varies with air density and altitude. Probably both moot, but I'll likely do it anyway.

At the moment, I know I should working on cleaning the project up for release, but I'm wiling away time adding features I know really don't need to be there, because I want to see how much they don't need to be there. In many cases, the differences are less than a percent of a percent, but by then, I've already coded it.

Air density variation and Drag vs. Mach are two features that are reasonably worth having activated though, the latter being the more important.
Air density variation tends to increase the end result by a couple of percent (It only ever really increases the results, as it reduces drag at higher altitudes. I suppose it is possible a change in Mach number to increase Cd and overall lose out, but I doubt it)

Assuming the data the spreadsheet is given is all correct, and obviously allowing for environmental variations like the fact that wind speed varies considerably with altitude (a situation, that although it can be modelled, is very hard for the user to actually acquire the pre-requisite data to model it. I'm trying to find a way to approximate this.)

Essentially, there is less room for error in the calculation than the natural error of the cannons in question.

My understanding was that precise ab initio calculations for the aerodynamic performance of shape is basically impossible.

All the drag models I'm talking about are based on many hundreds of actual firings and experimental data, rather than scientific calculations. Indeed, the original version of the GI model was created before the scientific principles behind air flows were fully understood.

I am wasting further time on a system that's capable of predicting drag based on the projectile's shape, although as you say, an ab initio calculation of this that is totally accurate is impossible. However, again - it should be more accurate than the cannons.
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Unread postAuthor: psycix » Sat Nov 22, 2008 12:47 pm

A semi complete list of advanced features beyond simple drag and gravity:

Could I add the heating of both projectile and air at extreme velocities. Which results in different characteristics of the air around (and especially behind) the projectile, as the speed of sound changes with temperature.
Also thermal expansion or just completely burning away in the atmosphere may be concerned.
This normally doesn't play much of a role, but when firing stuff a freaking 71km high, it may. :D
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Unread postAuthor: Ragnarok » Sat Nov 22, 2008 1:50 pm

psycix wrote:Could I add the heating of both projectile and air at extreme velocities.

That's going to be a little hard to model, and will require a hefty understanding on the part of the user.

Also thermal expansion or just completely burning away in the atmosphere may be concerned.

Well, I was considering a big "Your projectile is going to catch fire" alarm, given previous examples that have been quoted of me (mostly when talking about golf balls/tennis balls being fired a mile), but it would require all sorts of bizarre information to do.

Basically, I need to try and add features without more demand on the part of the user to supply information. However, they may be considered as "super advanced features" for the really nerdy user.

Also the 71km high stuff is a couple of lines and small data table fix from the 11km stuff. But given the kind of random crap I've modelled in the program, I expect some other people are going to start experimenting wildly too.
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Unread postAuthor: jimmy101 » Mon Nov 24, 2008 12:20 pm

I wonder how much a bit'o wobble in the round affects things? When a real round exits it typically get kicked to one side or the other a bit. Crowning the barrel is supposed to reduce this but I would think there would still be some kick and the kick would vary from shot to shot. I would think that a small, and variable, amount of wobble would significantly affect the ability to precisely model the aerodynamics. Using experimental data (instead of ab initio) would help a lot but that just averages out the affect.

How much variability do you get in the experimental data? Is the shot to shot reproducability good to a couple percent? Obviously, the model can be no more accurate than the experimental data used to create (or calibrate of fine tune) it.
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Unread postAuthor: psycix » Mon Nov 24, 2008 3:49 pm

Ragnarok wrote:Well, I was considering a big "Your projectile is going to catch fire" alarm, given previous examples that have been quoted of me (mostly when talking about golf balls/tennis balls being fired a mile), but it would require all sorts of bizarre information to do.

Should send that one in to mythbusters: "Can you burn a tennisball in the atmosphere with a ridiculously strong spudgun?
Lets see what they can manage... :D


Oh btw, have you also checked the Cd of golfballs vs the mach number?
And what if we gave a part of a cylindrical round the surface of a golfball? (or something like that)
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Unread postAuthor: jackssmirkingrevenge » Mon Nov 24, 2008 4:57 pm

jimmy101 wrote:I wonder how much a bit'o wobble in the round affects things? When a real round exits it typically get kicked to one side or the other a bit. Crowning the barrel is supposed to reduce this but I would think there would still be some kick and the kick would vary from shot to shot.


A spin stabilised projectile is constantly yawing...

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Unread postAuthor: Fnord » Mon Nov 24, 2008 5:50 pm

Well, I was considering a big "Your projectile is going to catch fire" alarm, given previous examples that have been quoted of me (mostly when talking about golf balls/tennis balls being fired a mile), but it would require all sorts of bizarre information to do.


You could use a density calculation to let the program guess what material the projectile is made from (i.e., over a certain density you could only have either metal or stone of some type).

Then you just need to convert the muzzle energy lost due to drag into frictional energy on the projectile, guess the heat conductivity and specific heat and you're set, right?

Actually, the more I read over that, the less attractive it becomes, although getting a "Warning: Projectile on fire!" error would be awesome for the random guy who happens to find and download your program :)


If you want to get really ridiculous, you could account for the mass increase of the projectile due to the kinetic energy it contains (what with general/special relativity and all that). Wouldn't be hard, but still ridiculous.


Btw Ragnarok you never replied to my pm the other day (I don't know whether you intended to or not, but in case you forgot...).
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Unread postAuthor: Ragnarok » Mon Nov 24, 2008 6:51 pm

_Fnord wrote:Btw Ragnarok you never replied to my pm the other day (I don't know whether you intended to or not, but in case you forgot...).

Yeah, sorry - I do mean to reply, but I'm completely bogged down, and with a sprained wrist, extra typing is hardly first and foremost on my list of things to do.
I have also been up for over a day and a half right now, so as you can imagine, I really need sleep now - I can barely focus on these words for more than a second or two before they slowly fade again.

@psycix: Far faster than any conventional method could offer. The 'busters couldn't manage it.
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Unread postAuthor: jimmy101 » Tue Nov 25, 2008 11:49 am

jackssmirkingrevenge wrote:
jimmy101 wrote:I wonder how much a bit'o wobble in the round affects things? When a real round exits it typically get kicked to one side or the other a bit. Crowning the barrel is supposed to reduce this but I would think there would still be some kick and the kick would vary from shot to shot.


A spin stabilised projectile is constantly yawing...

Image

Yaw is a bit different than the lateral kick that most barrels give when the round exits. I believe basic yaw is pretty reproducable so its affect on the trajectory might not vary much from shot to shot. The muzzle kick might be (1) much less consistent and (2) of much larger magnitude.

Basically, it still comes down to "if you fire the same weapon with the same rounds how reproducable is the trajectory?" If the shot to shot variability is 5% then changing the shape of the round to improve performance by 2% doesn't really accomplish all that much. I seem to recal though that artillary shells are much more reproducable in their trajectories than 5%. More like 0.5%.
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Unread postAuthor: POLAND_SPUD » Thu Dec 04, 2008 2:43 pm

now something for those discussing the shape of the raindrop http://www.youtube.com/watch?v=X2g9vgTh_hY
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Unread postAuthor: jackssmirkingrevenge » Thu Dec 04, 2008 3:06 pm

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Unread postAuthor: Ragnarok » Thu Dec 04, 2008 3:38 pm

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Unread postAuthor: psycix » Thu Dec 04, 2008 5:41 pm

They are all very amusing, but they dont have much to do with aerodynamic shapes do they?
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Unread postAuthor: POLAND_SPUD » Thu Dec 04, 2008 5:56 pm

well if you think about it.... they are not really raindrops (they are droped from several cm)... so I am still a bit confused
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Unread postAuthor: Ragnarok » Thu Dec 04, 2008 5:59 pm

psycix wrote:They are all very amusing, but they dont have much to do with aerodynamic shapes do they?

Actually, it does. The important point is that water drops in free fall are not the "teardrop shape" they're often assumed to be.

The interesting part is that the water's surface tension exceeds the forces on it from drag.
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