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Old 11-20-2016, 10:43 AM   #1
Wavefunction
 
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Default [Ultra-Tech] Building a 25mm Gyroc

Hey guys,

Pretty much what it says in the title, what would the stats of a 25mm Gyroc be? Damage, range, speed, weight, etc. And roughly how much would it increase the ST requirement of the corresponding weapon by, if at all?
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Old 11-20-2016, 03:14 PM   #2
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

Assuming equal proportions, the new gyroc would weigh around 0.45 lb and cost around $22.50. Using the same equation as Douglas Cole used for his spreadsheet, x4.5 to weight (and thus KE) and x1.67 to caliber (and thus cross-section) works out to right around x2 to damage, for 12d pi++. Velocity, range, Acc, etc are all the same. The launcher would need to be a bit larger, but not by much - considering the holdout gyroc is basically just the launch tube, and that's the only thing that really needs to be increased in weight, you're probably looking at roughly +1 lb, which in general isn't going to boost MinST requirements.

This is all very back-of-the-envelope type stuff, but may be close enough to work. With the exception of using that spreadsheet, this was basically "Imagine a gyroc were a spaceship built using the rules from the GURPS Spaceships line; what effects would simply scaling it up to 25mm have?"

Last edited by Varyon; 11-20-2016 at 03:17 PM.
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Old 11-20-2016, 05:56 PM   #3
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

Re Gyroc size - I once made the acquaintance of a man who had done testing for new munitions for tanks at Ft Knox. He had the opportunity to test a rocket round some time before the US Army settled on the upgrade to 120mm for the M-1 tanks. It was 75mm in diameter and about 4 feet long. He said that it would reach sufficient velocity within 10 meters of being fired to breach armors and that it kept accelerating out to about 2000 meters. The propellant was smokeless so no trails pointing back to the firing tank. Very accurate, I don't believe any guidance was needed but that could certainly be added. I have no reason to doubt him but I also have no means to verify testing done 30+ years ago. I like the idea of having Gyroc-like rounds radiate across the usual gun niches in space settings.
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Old 11-21-2016, 06:10 AM   #4
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

Quote:
Originally Posted by Varyon View Post
Assuming equal proportions, the new gyroc would weigh around 0.45 lb and cost around $22.50. Using the same equation as Douglas Cole used for his spreadsheet, x4.5 to weight (and thus KE) and x1.67 to caliber (and thus cross-section) works out to right around x2 to damage, for 12d pi++. Velocity, range, Acc, etc are all the same. The launcher would need to be a bit larger, but not by much - considering the holdout gyroc is basically just the launch tube, and that's the only thing that really needs to be increased in weight, you're probably looking at roughly +1 lb, which in general isn't going to boost MinST requirements.
These were about the same back of the envelope calculations I did, but I'm not sure about their validity. Mainly because a quick internet search suggests the weight of a rocket increases according to it's volume, while thrust increases according to it's cross-sectional area.

A 25mm round is 5/3x bigger in linear scale than a 15mm round. So it's going to be 125/27 or roughly 4.5x heavier as you said, but it's only going to generate 25/9, or about 2.8x as much thrust. I don't really know much about ballistics, but a quick equation setting Drag Force equal to Thrust when velocity is at it's maximum suggests that these projectiles would in fact have the same maximum speed, but acceleration would be roughly 0.6x that of the standard gyroc, so the 25mm gyroc would take about 16-17 yards to reach full speed, not 10 yards.

Again, that's from my limited understanding of ballistics.

As for ST, looking back over it, it probably depends on the weapon. For something like a launch pistol that carries 6 rounds you're going to have that +1 lb. for the overall increase in weapon size, plus about 2 lbs. for the increase in ammunition weight. It's also going to be much bulkier and more unwieldy. Probably good for at least a +1 to Min ST.
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Old 11-21-2016, 07:08 AM   #5
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

I thought I'd show my workings, so people can see where I went wrong, if I did in fact go wrong.

Drag Force = 0.5 x P x Cd x A x V^2

Where P is the air density, Cd is the coefficient of drag, A is the cross-sectional area of the projectile, a V is its velocity.

At it's maximum speed, by definition the particle has stopped accelerating, which means that there's no net force acting on it. So the drag force and the thrust, T, must be equal. For the 15mm gyroc we have:

0.5 x P x Cd x A1 x Vmax, 1^2 = T1

And for the 25mm we have:

0.5 x P x Cd x A2 x Vmax, 2^2 = T2

But since area is 25/9x that of the 15mm round, and thrust scales with area, we can put:

0.5 x P x Cd x 25/9 x A1 x Vmax, 2^2 = 25/9 x T1

From this we see that Vmax, 2 = Vmax, 1. We can find the acceleration of the missile from Force = Mass x Acceleration. Here

T1 = m1 x a1 and
T2 = m2 x a2

Substituting in T2 = 25/9 x T1, and m2 = 125/27 x m1 we get

25/9 x T1 = 125/27 x m1 x a2

Which we can solve to find a2 = 3/5 x a1. Therefore the projectile will travel 5/3 times as far before it reaches max speed, 5/3 x 10 yards = 16.66 yards.
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Old 11-21-2016, 08:05 AM   #6
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

How does use in a vacuum change all of that? Or more to the point are the changes worth noting and making two stat lines?

ETA Because it is not unheard of to have to fight over a broken orbital habitat or assault the installations on asteroid 253 Mathilde.
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Last edited by Joseph Paul; 11-21-2016 at 08:43 AM.
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Old 11-21-2016, 08:06 AM   #7
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

Quote:
Originally Posted by Wavefunction View Post
These were about the same back of the envelope calculations I did, but I'm not sure about their validity. Mainly because a quick internet search suggests the weight of a rocket increases according to it's volume, while thrust increases according to it's cross-sectional area.
For solid fuel rockets it's going be a square factor though it will be surface area exposed to combustion rather than simple cross-section. However, burn time is definitely going to go up with mass so all other factors being equal you're going to get greater endurance.

Also, all other factors are not always equal. The larger rocket sin UT (especially the 100mm) assume _very_ high accelerations and has those increase by 50% between TL9 and 10. So there are things that can offset simple square/cube factors.

There are even Real world rockets that used very high accel. The TL7 "Sprint" ABM prototype accelerated at 100 Gs for approx. 4 seconds.
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Old 11-21-2016, 09:05 AM   #8
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

Quote:
Originally Posted by Wavefunction View Post
A 25mm round is 5/3x bigger in linear scale than a 15mm round. So it's going to be 125/27 or roughly 4.5x heavier as you said, but it's only going to generate 25/9, or about 2.8x as much thrust. I don't really know much about ballistics, but a quick equation setting Drag Force equal to Thrust when velocity is at it's maximum suggests that these projectiles would in fact have the same maximum speed, but acceleration would be roughly 0.6x that of the standard gyroc, so the 25mm gyroc would take about 16-17 yards to reach full speed, not 10 yards.
Sounds alright, but two things to consider. First, as Fred notes, the 25mm gyroc holds more fuel, so would have greater endurance; SS has a variety of ways to increase acceleration by decreasing delta-v, so our 25mm gyroc might be able to make use of those to get roughly the same acceleration and endurance. Secondly, as things get larger you can make fiddly mechanisms (like rockets) more efficient, up to a point, and I wouldn't be shocked to find out that the 15mm gyroc has to use a less efficient rocket to fit it in its frame than a 25mm one. Personally, I'd just handwave the two away and give it the stats I mentioned upthread.

Quote:
Originally Posted by Wavefunction View Post
As for ST, looking back over it, it probably depends on the weapon. For something like a launch pistol that carries 6 rounds you're going to have that +1 lb. for the overall increase in weapon size, plus about 2 lbs. for the increase in ammunition weight. It's also going to be much bulkier and more unwieldy. Probably good for at least a +1 to Min ST.
Fair enough.

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How does use in a vacuum change all of that? Or more to the point are the changes worth noting and making two stat lines?
Gyrocs in general would behave quite differently in vacuum than in atmosphere. If they use air-breathing rockets, you'd have to use special space gyrocs or they wouldn't function at all. Aside from that, instead of just having a range they'd have an acceleration and top speed, then once they were out of fuel they'd just drift at top speed until they hit something.
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Old 11-21-2016, 09:41 AM   #9
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

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Originally Posted by Varyon View Post
Gyrocs in general would behave quite differently in vacuum than in atmosphere. If they use air-breathing rockets, you'd have to use special space gyrocs or they wouldn't function at all. Aside from that, instead of just having a range they'd have an acceleration and top speed, then once they were out of fuel they'd just drift at top speed until they hit something.
Pretty much what I thought but I may just have to break out my Handbook of Model Rocketry and see if no drag conditions will give enough extra acceleration to warrant more dice of damage etc.

The book Body Armor:2000 mentions a ram-jet round for use in atmosphere. Anything like that show up in GURPS?
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Old 11-21-2016, 09:55 AM   #10
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Default Re: [Ultra-Tech] Building a 25mm Gyroc

Quote:
Originally Posted by Joseph Paul View Post
Pretty much what I thought but I may just have to break out my Handbook of Model Rocketry and see if no drag conditions will give enough extra acceleration to warrant more dice of damage etc.

The book Body Armor:2000 mentions a ram-jet round for use in atmosphere. Anything like that show up in GURPS?
By RAW for other missiles 1/2D range in atmosphere converts to acceleration in vacuum. This won't actually matter much with 1 second of endurance. You will note that if you convert yards/second of 1/2D to acceleration we're talking huge numbers. 277 Gs for 5 seconds for the TL 10 100mm.

With those numbers you won't hit max speed in atmosphere until you get to very high velocities and as long as you keep the one second burn for the Gyroc it won't be a big difference.

However, if the "Gyroc" actually does use fold-out micro-aerodynamic control surfaces for stabilization and guidance it will have neither in vacuum. A vacuum-ready missile needs a reaction control system. You can just assume those as standard or have special vacuum-ready missiles.
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