Conventional Guns, Turrets and Ranges
Oct 4, 2016 23:13:26 GMT
argonbalt, leerooooooy, and 1 more like this
Post by tophattingson on Oct 4, 2016 23:13:26 GMT
Conventional Guns
Railguns and Coilguns seem to be the obvious choice for Main Guns, but history has far more examples of conventional gun usage for drawing inspiration. I took some early/mid 1900s Battleship guns and fed their specs into the game. It actually worked ok, yielding roughly correct projectile velocity. Here's an example.
BL 15 inch Mk I naval gun
Length: 17 m (actually 16.52)
Bore Radius: 38cm (actually 381mm)
Projectile: APC Mark Ia, 870kg (actually 871kg)
Projectile Material: Austenitic stainless steel (Information suggests it's a Chromium/Nickel stainless steel)
Propellant Charge: 190kg Nitroglycerine (Information suggests Cordite MD, 194kg)
Barrel Material: Austenitic stainless steel (Couldn't find information beyond "Steel")
I couldn't find data on Grain and Thickness (and thickness cannot be properly emulated here for reasons I will explain), so I just went with high efficiency values the barrel could handle. That would be 6.7cm and 24.0cm respectively.
The Muzzle Velocity of this ended up being 679m/s, reasonably close to the real gun's 752m/s. I suspect Cordite MD is slightly better than plain Nitroglycerine. The gun came to 192t in weight, way above the real weight of the gun. The working pressure of the real gun is 308 MPa, while the module instead peaked at 129 MPa.
That's not how Barrels work
The barrel model used by Children of a Dead Earth, if the schematic in the bottom view is to be believed, is a simple cylinder. For small arms, this is ok. However, no practical gun being used on these ships are small arms scale. Very small artillery has also sometimes been manufactured with simple cylindrical barrels, but this is definitely the exception, not the rule.
Wikipedia actually provides a diagram of the cross section of the BL 15 inch Mk 1: upload.wikimedia.org/wikipedia/commons/5/5c/BL_15_inch_Mk_I_naval_gun_in_section.svg
Real guns have two things that massively optimise conventional guns.
1. The barrel is just plain thicker where the highest working pressure is located, and narrower where the least pressure is located. This makes a substantial difference to the final gun mass vs a hollow cylinder. This is the likely main reason my gun clocked in at 192t to the real gun's 100t.
2. Very large guns are "Built-up", meaning the additional outer layers are kept under tension, which improves their ability to resist pressure. The main barrel is essentially prevented from rupturing because it's being compressed by the outer barrel.
My work on this also revealed a secondary problem with building very large guns, but this applies to more than just conventional guns.
Turrets
All turrets currently work by using reaction wheels to rotate the gun. This is fine for small weapons, but if you want to rotate a 20 meter long 200t barrel, any solution with a speed greater than 1 degree/s is going to blow your mass budget. Simply put, real turrets don't work via reaction wheels (as far as I know for any existing design) but instead by mechanically operating on whatever it is mounted to. Given the mass of the turret vs that of the rest of the ship, this should be an acceptable solution even in space. A slight rotation across the whole vessel in response to turret rotation is vastly preferable to the turret taking 30 seconds to aim at a target.
For the record, the real turret for the BL 15 inch Mk I gets 2 degrees/s train and 5 degrees/s elevation, and that's with two guns mounted on the same turret.
Ranges
In July 1940, HMS Warspite struck Guilio Cesare at a distance of 23.8km with the BL 15 Mk 1. Using an arbitrary gun with a muzzle velocity of 752m/s (I made the BL 15 Mk 1 replica shoot faster) and a cross section of 1,600 m^2 (in practice this would have been lower), gives a range of ~6km. And this is without the naval gunnery effects of gravity, air resistance and various other complexities, such as fire control relying on analogue computers, which WWII battleships had to deal with and we in space don't.
However the effective ranges are being determined by the game is a massive underestimate. I can confirm this is the case both in theory and in practice, as enabling fire from beyond the effective range during combat still can give a solid number of hits. With powerful guns, when one hit can do the job, even a 1 in 1000 hit chance is good enough.
I assume the internal calculation for ranges is, in pseudo-math, something like "Find the maximum range at which this gun will hit a disk of cross sectional area X with 1% of fired shots".
The user needs to be able to adjust the "Desired hit chance" for their gun modules. The graph should probably display both the user-set desired hit chance effective ranges and the current default effective ranges, to allow for easier comparison of weapon systems.
Railguns and Coilguns seem to be the obvious choice for Main Guns, but history has far more examples of conventional gun usage for drawing inspiration. I took some early/mid 1900s Battleship guns and fed their specs into the game. It actually worked ok, yielding roughly correct projectile velocity. Here's an example.
BL 15 inch Mk I naval gun
Length: 17 m (actually 16.52)
Bore Radius: 38cm (actually 381mm)
Projectile: APC Mark Ia, 870kg (actually 871kg)
Projectile Material: Austenitic stainless steel (Information suggests it's a Chromium/Nickel stainless steel)
Propellant Charge: 190kg Nitroglycerine (Information suggests Cordite MD, 194kg)
Barrel Material: Austenitic stainless steel (Couldn't find information beyond "Steel")
I couldn't find data on Grain and Thickness (and thickness cannot be properly emulated here for reasons I will explain), so I just went with high efficiency values the barrel could handle. That would be 6.7cm and 24.0cm respectively.
The Muzzle Velocity of this ended up being 679m/s, reasonably close to the real gun's 752m/s. I suspect Cordite MD is slightly better than plain Nitroglycerine. The gun came to 192t in weight, way above the real weight of the gun. The working pressure of the real gun is 308 MPa, while the module instead peaked at 129 MPa.
That's not how Barrels work
The barrel model used by Children of a Dead Earth, if the schematic in the bottom view is to be believed, is a simple cylinder. For small arms, this is ok. However, no practical gun being used on these ships are small arms scale. Very small artillery has also sometimes been manufactured with simple cylindrical barrels, but this is definitely the exception, not the rule.
Wikipedia actually provides a diagram of the cross section of the BL 15 inch Mk 1: upload.wikimedia.org/wikipedia/commons/5/5c/BL_15_inch_Mk_I_naval_gun_in_section.svg
Real guns have two things that massively optimise conventional guns.
1. The barrel is just plain thicker where the highest working pressure is located, and narrower where the least pressure is located. This makes a substantial difference to the final gun mass vs a hollow cylinder. This is the likely main reason my gun clocked in at 192t to the real gun's 100t.
2. Very large guns are "Built-up", meaning the additional outer layers are kept under tension, which improves their ability to resist pressure. The main barrel is essentially prevented from rupturing because it's being compressed by the outer barrel.
My work on this also revealed a secondary problem with building very large guns, but this applies to more than just conventional guns.
Turrets
All turrets currently work by using reaction wheels to rotate the gun. This is fine for small weapons, but if you want to rotate a 20 meter long 200t barrel, any solution with a speed greater than 1 degree/s is going to blow your mass budget. Simply put, real turrets don't work via reaction wheels (as far as I know for any existing design) but instead by mechanically operating on whatever it is mounted to. Given the mass of the turret vs that of the rest of the ship, this should be an acceptable solution even in space. A slight rotation across the whole vessel in response to turret rotation is vastly preferable to the turret taking 30 seconds to aim at a target.
For the record, the real turret for the BL 15 inch Mk I gets 2 degrees/s train and 5 degrees/s elevation, and that's with two guns mounted on the same turret.
Ranges
In July 1940, HMS Warspite struck Guilio Cesare at a distance of 23.8km with the BL 15 Mk 1. Using an arbitrary gun with a muzzle velocity of 752m/s (I made the BL 15 Mk 1 replica shoot faster) and a cross section of 1,600 m^2 (in practice this would have been lower), gives a range of ~6km. And this is without the naval gunnery effects of gravity, air resistance and various other complexities, such as fire control relying on analogue computers, which WWII battleships had to deal with and we in space don't.
However the effective ranges are being determined by the game is a massive underestimate. I can confirm this is the case both in theory and in practice, as enabling fire from beyond the effective range during combat still can give a solid number of hits. With powerful guns, when one hit can do the job, even a 1 in 1000 hit chance is good enough.
I assume the internal calculation for ranges is, in pseudo-math, something like "Find the maximum range at which this gun will hit a disk of cross sectional area X with 1% of fired shots".
The user needs to be able to adjust the "Desired hit chance" for their gun modules. The graph should probably display both the user-set desired hit chance effective ranges and the current default effective ranges, to allow for easier comparison of weapon systems.
