Steve Jackson Games Forums Binary propellant and Standard TL 8 guns?
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 09-22-2017, 09:57 AM #31 Anthony   Join Date: Feb 2005 Location: Berkeley, CA Re: Binary propellant and Standard TL 8 guns? The problem with solving for half velocity is that GURPS 1/2D range bears pretty much no relation to the actual range at which projectile velocity is halved. __________________ My GURPS site and Blog.
 09-22-2017, 10:32 AM #32 mlangsdorf   Join Date: Aug 2004 Location: Austin, TX Re: Binary propellant and Standard TL 8 guns? Google docs are easy to share: https://docs.google.com/spreadsheets...it?usp=sharing You can't edit it, but you should be able to duplicate it and then edit the copy. How it works: You put the bullet diameter in mm in cell C1, bullet mass (in grams) in C2, muzzle velocity in m/s in C3, and the bullet aspect ratio* in H1. The spreadsheet will take a guess for the coefficient of drag** and do a lot of math. Eventually, it will spit out the results in cells D6 to D9: 1/2d range and max range, for firing at a 10 degree angle (basically horizontal) and at a 45 degree angle (maximum possible range). It uses 400 rows to calculate the first 4 seconds of travel time in 0.01 second increments, and uses another 600 rows to calculate the rest of the flight time. deltaT for those rows is variable based on muzzle speed, and you can scale the deltaT by putting a value in F10 ("DelTFudge"). The deltaT is based on the flight time of the projectile in a vacuum, so it's usually longer than necessary and you can usually shorten the deltaT to get more precise results. By default, it calculates shots from 0 m above ground level, at 10 degrees and at 45 degrees. You can change the height and vary the angle with cells C5 and C4. Cells P1-Z6 have some sample values for various projectiles, with the numbers for mass and muzzle velocity taken from wikipedia. Compared to GURPS standard, pistol bullets have roughly 2x the 1/2d range, but 2/3rds the max range; rifles are with +/- 20%. Wikipedia and GURPS High-Tech disagree on a lot of those numbers, so I don't know. * This is 1 for round shot like cannonballs, 2 for pistol bullets and shotgun slugs, and 3 or more for rifle bullets but see the note on coefficient of drag. ** Coefficient of drag varies with bullet shape and velocity. I'm assuming a static CoD based on the numbers from the wikipedia article and some rough curve fitting. You can adjust the CoD by putting a value in H10 ("CoD Fudge"). If anyone has better data on CoD I'd love to see it; I know you can theoretically do stuff with a ballistics coefficient but it's been a long, long time since fluid dynamics and I can't resolve the equations on wikipedia. __________________ Read my GURPS blog: http://noschoolgrognard.blogspot.com
 09-22-2017, 09:02 PM #33 hal   Join Date: Aug 2004 Location: Buffalo, New York Re: Binary propellant and Standard TL 8 guns? I was able to download it as an XLSX file without a problem. The next step for me is to try and figure out what the formulas you are using are, what the numbers are used in the formula, and maybe recreate it sufficiently enough that I can write a VBA code or VB.NET code to do the same thing. The good news is that I have Doug's updated balistics sheet, and I have yours, and that should keep me busy trying to understand what they're doing. My theory is that if your methodology and Doug's methodology show two different paths to the same goal, I can maybe use one method to check against the other. It may well be they're both enough alike to be the same for all intents and purposes. I don't know. Thanks both of you for the spreadsheets. :)
09-22-2017, 09:12 PM   #34
hal

Join Date: Aug 2004
Location: Buffalo, New York
Re: Binary propellant and Standard TL 8 guns?

Quote:
 Originally Posted by Anthony The problem with solving for half velocity is that GURPS 1/2D range bears pretty much no relation to the actual range at which projectile velocity is halved.
I'm not sure that what you wrote is what I thought you wrote...

I wasn't expecting that the velocity would be halved as such. Using an actual example...

A round designated as .45 Colt, developed and released for public sales in 1873 that weighed 255 grains, and had a muzzle velocity of 855 feet per second, its average damage worked out to be 8.5 points, or 2d6+1 damage.

In order to reduce the average damage to 8.5/2 or 4.25...

The velocity must now become: 440 feet per second. Not half the velocity to be sure, but that's what I had to change the velocity value to get a damage of 4.25.

I'm guessing however that this is not necessarily what you meant. Am I right?

09-23-2017, 12:24 AM   #35
Johnny1A.2

Join Date: Dec 2008
Re: Binary propellant and Standard TL 8 guns?

Quote:
 Originally Posted by Anthony Note that, had ultra-tech been written completely from scratch in 2007 rather than being based on earlier work, I suspect liquid propellant guns wouldn't even be in there; liquid propellant guns were a hot topic briefly in the early 90s, and thus made sense for inclusion in GURPS Vehicles, but appear to have subsided back into obscurity since then. The big theoretical benefit of liquid propellant is that it lets have variable muzzle velocity relatively easily. This is a significant benefit for artillery (with fixed muzzle velocity, at close ranges you're forced to choose between low angle shots that may not clear near obstacles, or very high angle shots that have extreme travel time), but is not terribly useful for direct fire weapons.
It also adds complexity. I have a suspicion that liquid-propellant direct-fire weapons would (at least in their earlier incarnations) have a lot of reliability and maybe safety issues to deal with.
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09-23-2017, 02:06 AM   #36
Anthony

Join Date: Feb 2005
Location: Berkeley, CA
Re: Binary propellant and Standard TL 8 guns?

Quote:
 Originally Posted by hal I'm guessing however that this is not necessarily what you meant. Am I right?
Yes. GURPS 1/2d range incorporates elements of both damage and accuracy, and thus doesn't accurately represent either one.
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09-23-2017, 06:59 AM   #37
johndallman

Join Date: Oct 2010
Location: Cambridge, UK
Re: Binary propellant and Standard TL 8 guns?

Quote:
 Originally Posted by Anthony Yes. GURPS 1/2d range incorporates elements of both damage and accuracy, and thus doesn't accurately represent either one.
In 3e, the Acc bonus was lost at 1/2D range, but this is not the case in 4e. Or did you mean some other aspect of accuracy?
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09-23-2017, 07:01 AM   #38
Fred Brackin

Join Date: Aug 2007
Re: Binary propellant and Standard TL 8 guns?

Quote:
 Originally Posted by hal I'm not sure that what you wrote is what I thought you wrote... I wasn't expecting that the velocity would be halved as such. Using an actual example... A round designated as .45 Colt, developed and released for public sales in 1873 that weighed 255 grains, and had a muzzle velocity of 855 feet per second, its average damage worked out to be 8.5 points, or 2d6+1 damage. In order to reduce the average damage to 8.5/2 or 4.25... The velocity must now become: 440 feet per second. Not half the velocity to be sure, but that's what I had to change the velocity value to get a damage of 4.25. I'm guessing however that this is not necessarily what you meant. Am I right?
As a normal thing Gurps firearms damage _is_ linear with velocity. The not-quite matches in your examples are possibly rounding issues.
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09-23-2017, 07:16 AM   #39
mlangsdorf

Join Date: Aug 2004
Location: Austin, TX
Re: Binary propellant and Standard TL 8 guns?

Quote:
 Originally Posted by hal I was able to download it as an XLSX file without a problem. The next step for me is to try and figure out what the formulas you are using are, what the numbers are used in the formula, and maybe recreate it sufficiently enough that I can write a VBA code or VB.NET code to do the same thing.
My spreadsheet is a straightforward application of the drag formula and freshmen physics and introductory calculus. I can break it down for you.

Calculate the static portions of the drag equation as -0.5*1.23*(object coeffecient of drag, roughly 0.47/object aspect ratio)*cross sectional area/mass. 1.23 is the density of air, more or less. Calculate subsonic drag static portion as 1.4* the previous value.

For each row, calculate instant velocity as the square root of the sum of the squares of the previous row's horizontal and vertical velocity, and current angle of the inverse tangent of vertical velocity/horizontal velocity. Calculate instant horizontal and vertical acceleration as the square of instant velocity times static portions of the drag equation (using the subsonic number if instant velocity is below the speed of sound) times the cosine or sine of current angle, and add the static acceleration due to gravity for the vertical accel. Instant horizontal and vertical velocity are the previous horizontal or vertical velocity plus instant horizontal or vertical acceleration times delta-T, and total distance traveled in the horizontal and vertical are the sums of horizontal and vertical velocity times delta-T.

I calculate 1/2d range as the horizontal distance when instant velocity has dropped below muzzle velocity * square root (1/2), since that velocity gives you half damage in Doug's formula.

With a little approximation of the co-effficient of drag, the current spreadsheet gets the maximum range for the RIA M2A1 105mm howitzer within 10 yards, and the maximum height and range of a 5"/38 cannon within 2%, so I'm pretty satisfied with it. I just need to speed it up a bit.
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