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Mile shooting cannon?

A place to ask general spud cannon related questions.
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Unread postAuthor: SpudFarm » Tue Mar 30, 2010 2:33 pm

Oh I can't explain what a pain it is to load and operate a big hybrid. Heavy barrel needs to come off at every shot, then the change of burst disk (piston eventually but not many of them around) screw the barrel back on and aim it. Then fuel it, this can also be a pain if you have a small compressor and a tiny leak somewhere.

You also have the massive transport problem you get from a big steel comstruction. You simply get tired of firing the gun in a matter of days.

I would not do that if I where you. My hybrid is my love but it still gets only about four shots EACH YEAR!
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Unread postAuthor: velocity3x » Tue Mar 30, 2010 6:00 pm

Ragnarok wrote:Mathematically speaking, a golfball needs to be pulling about 500 m/s at the muzzle in order to be capable of the ~536 metres that equates to.


How does the math explain some golfers driving GB's to distances ranging from 471m to 626m?

http://hypertextbook.com/facts/2004/CrystalCuevas.shtml
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Unread postAuthor: Ragnarok » Tue Mar 30, 2010 6:46 pm

velocity3x wrote:How does the math explain some golfers driving GB's to distances ranging from 471m to 626m?

Tailwinds - add in a fast tail wind, and drag is significantly reduced.

In still air, to get a 540 m distance, you do need velocities that relate to about 500 m/s, with backspin having relatively little effect.

The point here to bear in mind here is the inefficiency of a golfball at those velocities. A golfball launched at 500 m/s will be at 400 m/s only 20 metres later - taking into account launch angle and the extra height the golfball will have over one launched at 400 m/s , that extra 100 m/s might add 30 metres.

Very much a case of diminishing returns - in still air, a 200 m/s golfball will go about 360 metres. To increase that by half, you need about two and a half times the velocity.
These diminishing returns mean that the 70-80 m/s of a good golf drive can still go a long way, and an improvement in aerodynamic conditions is far more important for distance than velocity.

When I do my calculations, they assume still air, as those figures are great deal more practical than what can be done under the freak conditions world record drives are set under.

Gippeto wrote:How about a picture then?

If I had wanted to, I could have shown you CAD models of pretty much this a while ago, but I'm keeping the specifics of this project under wraps until I've had the opportunity to actually complete it.
As and when I'm done, I'll make the designs available - I'm sure there are enough people who could make use of them in their own projects.

Also, it is still a prototype. There is a minor stability problem that needs fixing. While I've managed to reduce fin drag by over an order of magnitude and improve consistency, there's a velocity range in which its stability falls slightly outside of parameters - and as Sod's law would have it, the intended muzzle velocity is within that range.

I think I've fixed the problem with a minor redesign, but I'll have to do some experimental testing to confirm that.
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Unread postAuthor: velocity3x » Tue Mar 30, 2010 7:28 pm

Ragnarok wrote:Tailwinds - add in a fast tail wind, and drag is significantly reduced.


For sake of discussion....If winds aloft are 20 knots, doesn't that have the same net effect as increasing initial velocity by only 7.25 meters per second as the GB launches from the tee?
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Unread postAuthor: saefroch » Tue Mar 30, 2010 7:38 pm

No. It just decreases drag.
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Unread postAuthor: Ragnarok » Tue Mar 30, 2010 8:30 pm

velocity3x wrote:For sake of discussion....If winds aloft are 20 knots, doesn't that have the same net effect as increasing initial velocity by only 7.25 meters per second as the GB launches from the tee?

Not even slightly. As I've already explained, gains in initial velocity are usually fairly inconsequential, as that velocity is fast lost.
What a tailwind does is reduce the equivalent airspeed over the projectile - and that consequentially reduces drag by a huge factor.

The difference is not the same as if it were a faster projectile, but as if it were a more aerodynamic projectile.
I'll give a mathematical example. However, we'll simplify the maths required - aerodynamic drag requires the use of calculus, and given that simpler maths will demonstrate the point just as well, it seems pointless to make it more complex than it needs to be.

Any student who paid attention in physics should be familiar with the equation s=ut+(at<sup>2</sup>)/2, for calculating displacement when acceleration is constant. We'll be using this.

Take a 120 m/s object that's slowing down at 10 m/s<sup>2</sup>, and a 100 m/s object that's slowing down at 5 m/s<sup>2</sup>.
The 120 m/s object will come to a halt after 12 seconds and 720 metres. The 100 m/s object will come to a halt after 20 seconds and 1000 metres - in spite of the fact it was initially slower, the fact that it was decelerating more slowly meant that it went further.

The same thing applies to range. When drag is involved, often the most efficient solution to improving range is to reduce the drag, rather than increase the muzzle velocity.

To refer briefly to the darts I mentioned, most of the effort has gone into making the dart as aerodynamic as possible, rather than increasing the muzzle velocity. Not that they'll be slow, but they'll still have over 95% of that initial velocity left when they're 400 metres clear of the muzzle.

Why bother making it fast in the first place if you can't keep it fast?
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Unread postAuthor: D_Hall » Wed Mar 31, 2010 10:10 am

Ragnarok wrote:Several?

Mathematically speaking, a golfball needs to be pulling about 500 m/s at the muzzle in order to be capable of the ~536 metres that equates to.
I know there are a handful of golfball hybrids that are up to that velocity, but they're not exactly numerous.


I'm wondering if your drag coefficient is off in your math.

I say this this past Xmas I took my GB gun (a simple pneumatic) into the office to goof off with. Myself and others were having a lot of fun shooting at the turret off an old WWII destroyer. Said turret is about 200 meters away (yes, it's been measured with a laser rangefinder). It wasn't easy to hit (hook!), but it made a satisfying (if underwelming) noise when we did manage to hit it. More significant to the conversation, when we missed.... Well, the ball still had a LOT of energy and went a LONG ways past the turret (never found any of the balls that missed).

Point being that you'd have to go a long way to convince me we weren't in the 500 meter range on shots wherein we elevated the barrel (we could hit the turret by pretty much just aiming straight at it and praying that the ball didn't curve too much).

Muzzle velocity for that gun is approx 600 fps (measured with commercial chrony).
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Unread postAuthor: velocity3x » Wed Mar 31, 2010 11:09 am

Not long ago, I had the weekend use of a radar crew with equipment. Things didn't go as planned so, we tracked a shot from my GB gun for something to do. They tracked one GB shot at 364 m/s and after a minute of calculation, the computers produced a graph. According to the radar crew, the GB shot had the potential of 1 mile. Is Doppler Radar that untrustworthy?
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Unread postAuthor: POLAND_SPUD » Wed Mar 31, 2010 11:27 am

I'd rather say that the radar crew was untrustworthy... did they enter the cd value of a golfball ??
well the question is 'what is the cd value of a golfball'... better than that of a sphere (~0.46). But even with 0.1 GGDT predicts max range of 1094 m at 369m/s of muzzle velocity
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Unread postAuthor: velocity3x » Wed Mar 31, 2010 3:25 pm

POLAND_SPUD wrote:But even with 0.1 GGDT predicts max range of 1094 m at 369m/s of muzzle velocity


As for GGDT predictions .....Mr D_HALL (who is very familiar with GGDT) said the following about shots from his gun at 600fps:
D_HALL wrote:Point being that you'd have to go a long way to convince me we weren't in the 500 meter range on shots wherein we elevated the barrel


GGDT also predicts that a 600fps shot will have a max range of 250m yet, he believes the shots were in the 500m range. (Much farther than his own calculations predict). There seems to be some discrepancy between theory and real world testing. Mr D_HALL......your opinion, please.
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Unread postAuthor: MrCrowley » Wed Mar 31, 2010 3:46 pm

From experience, with GGDT, I had to use a cd value of 0.25 to get the (measured) range of my golf ball cannon.

edit: fixed cv value
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Unread postAuthor: ramses » Wed Mar 31, 2010 4:31 pm

I think it should be considered that GGDT is a um... Gas Gun Design Tool, and not a full out external ballistic simulator. Rag is working with his own proprietary tool that hasn't been released yethow's that going? . I would recommend you go to the wiki and use the very old version of LRC (the spreadsheet), as it was actually designed to compute range.

Not saying anything against D_Hall, but IIRC the external ballistics part of his calculator was pretty much an afterthought, and he acknowledges that it isn't very accurate.

EDIT: I apologize, D_Hall, I made the rather stupid assumption that requiring more input automatically resulted in a more accurate output. Although LRC can do crosswinds, rag says it has a flaw, so "take the results with a grain of salt"
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Unread postAuthor: POLAND_SPUD » Wed Mar 31, 2010 5:16 pm

@volocity3x
I don't know the exact cd value of golfballs and it's the most important thing here... anyway I find GGDT pretty accurate... I guess he made a guesstimate, it doesn't seem that bad if you use cd value of 0.35... anyway definitely not if you considering how difficult it is to guess the exact distance
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Unread postAuthor: D_Hall » Wed Mar 31, 2010 6:51 pm

ramses wrote:I think it should be considered that GGDT is a um... Gas Gun Design Tool, and not a full out external ballistic simulator. Rag is working with his own proprietary tool that hasn't been released yethow's that going? . I would recommend you go to the wiki and use the very old version of LRC (the spreadsheet), as it was actually designed to compute range.

Not saying anything against D_Hall, but IIRC the external ballistics part of his calculator was pretty much an afterthought, and he acknowledges that it isn't very accurate.


First, let it be said that long before I wrote GGDT, I used to write external ballistics codes for a living. It is a topic I was considered a "Subject Matter Expert" at by the Department of Defense.

Is the ballistic calculator in GGDT "up to snuff?" At once no and yes.

No, because it is nowhere near what I am capable of writing (or in fact, have written in the past).

Yes, because it is totally adequate for the kinds of trajectories one will normally see with a pneumatic gun. Does it suck in the transonic? You betcha, but not many guns are pushing transonic (and certainly the 600 fps demonstrated by my gun is safely subsonic). Does it do cross winds? Nope, but that's a second order affect at best for those who are doing their shooting in nice weather (ie, how many of us ever go outside for a day of shooting in high winds?). Blah blah blah. The ponit being that for the conditions most of our projectiles will experience there is one primary source of error.... Drag coefficient.

And it's that error that prevents me from putting too much work into an external ballistics calculator. I mean, if you can't nail your drag coefficient down to closer than 10% - and I guarentee most of ya'll can't(*) - then what's the point of worrying about a potential 0.01% error caused by the rotation of the Earth (which I've coded for in the past as well)? There's just no point in beating myself up modeling second and third order effects when there's a huge unknown associated with a first order effect.

Cutting to the chase... While I would never call the ballistics calculator of GGDT a rigorous external ballistics tool, undertand that I say that from the perspective of one who used to write such codes for the military. So while it ISN'T a rigorous tool, that doesn't mean that it isn't adequate for the task at hand under "nice day conditions." It is.

So when GGDT's predictions are WAY off observed data for subsonic shots fired in nice weather I will say this: By far the dominant source of error is going to be an incorrect drag coefficient; not the methodologies/etc. used by GGDT's external ballistics calculator.



(*) Even if you use published data for a sphere there are effects due to surface finish and the like.
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Unread postAuthor: Ragnarok » Wed Mar 31, 2010 7:55 pm

D_Hall wrote:I'm wondering if your drag coefficient is off in your math.

I'm using a Cd of 0.30, which gets modified relative to Mach number. Mass is taken as 45 grams, diameter as 42.7mm.
Those are the most reliable figures I can find for golf-balls, including a few tests of my own that corroborate that.

Point being that you'd have to go a long way to convince me we weren't in the 500 meter range on shots wherein we elevated the barrel

Well, here's some interesting pictures for you.

Here we can see the first 200 metres of the trajectory of a normal golf-ball (Blue), a golf-ball with backspin (Red), and a golf-ball with a 10% reduction in drag (Green) - all have a 200 m/s muzzle velocity, and IIRC, a 35 degree launch angle.
Image
Based on how much of that you can see, it looks like those balls could fly over 500 metres.

Now we'll complete that graph with the rest of the trajectories:

Image

... not one makes it to 400 metres. It's hard for the human eye to make out the range and velocity of a fast moving small object that's soaring through the sky, and it's very easy to make mistakes extrapolating, particularly where poor ballistic coefficients are involved.

As a result, I'd generally sooner trust a simulated range - even if it was calculated with rough figures - than a guessed one.
Of course, if you should ever happen to find a golf-ball that far out (assuming still conditions), then I'll take apart the calculations to find where they went wrong - but I'm pretty confident that you won't ever find a golf-ball that far out. Based on your muzzle velocity, I'd expect a range of up to about 350 metres in still air.

ramses wrote:Rag is working with his own proprietary tool that hasn't been released yet.

Sure am. Currently, it takes about the current list into account: Drag and how it changes with Mach number, Lift, Atmospheric conditions and how altitude changes them, Wind, Latitude, Coriolis effect, Earth curvature, Basic projectile stability - and possibly one or two other things I've forgotten to mention. (Although, in this case, I wasn't calculating anything from wind onwards).

Provided you've given it the right numbers, its results are about as reliable and accurate as you could sensibly hope for - as D_Hall explains above, drag coefficients are not an exact science even at the best of times.

But, even accepting that no simulation can be perfect, that doesn't stop the LRC having the edge over the GGDT tool - in this case, the LRC's more sophisticated transonic and supersonic modelling is very relevant.
Spheres have a very wide transonic range, from about Mach 0.5 to Mach 1.5 - with a perfect sphere, D_Hall's 600 fps WOULD be marginally transonic. Golfballs are slightly less egregious as far as that goes, with a transonic range of about Mach 0.6 to Mach 1.3, but that's still a major consideration.

I would recommend you go to the wiki and use the very old version of LRC (the spreadsheet), as it was actually designed to compute range.

The old version of the LRC has an inherent flaw in the way it calculates drag that can cause errors in the predicted range. Take its outputs with a pinch of salt.

How's that going?

It's been on hold for some time now, because I'm busy.

... okay, I'm also lazy.
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