Replacing PDCs

[Barkhorn]

Ofcourse there is...

What you need to understand is that inaccuracy depends on unplanned deviation during projectile travel which scales linearly with travel time. ( deviations in course both from a moving target and on the projectile itself ).

If a railgun projectile travels at 5km/s compared to a battleship shell at 500m/s this means the projectile can travel 10 times as long distance before the inaccuracy from outside influence becomes identical.

This is further helped by the railgun shell spending less time at lower altitude in "thicker" atmosphere where the deviations are higher then at higher altitudes ( for example wind or air/particle resistance ).

If you fire in space and your projectile travel at 50000km/s then the projectile spends less then a millisecond traveling through the thicker atmosphere compared to a battleship shell spending up to 30 seconds or more traveling meaning it's over 30,000 times more accurate...

The limitations on accuracy instead depends on the quality of the weapons which we can also assume are thousands of times better then today.

Further orbit isn't as far away as you would think... The ISS orbits at 300-400km up compared to Battleship guns max range of 30-40km, that's just 10 times as far which compared to the other numbers involved here is nothing.

An Aurora 4x tech level railgun should technically have no problems at all hitting an ant from orbit assuming it knew exactly where to aim.

On Earth though, there will be a giant deadzone where the ground is in the way, because a railgun round has a much flatter trajectory than a traditional round.  So for instance, you could direct fire on any target between you and the horizon, then there's a region between the horizon and some more distant point where your target is in the "shadow" of the horizon, then after that a region where indirect fire is possible out to the maximum range of the gun.

[alex_brunius]

On Earth though, there will be a giant deadzone where the ground is in the way, because a railgun round has a much flatter trajectory than a traditional round.  So for instance, you could direct fire on any target between you and the horizon, then there's a region between the horizon and some more distant point where your target is in the "shadow" of the horizon, then after that a region where indirect fire is possible out to the maximum range of the gun.

This is true, which is why you probably would use lower shell velocities as needed ( even if accuracy suffers a bit it should be better then Battleship bombardment accuracy ).

But for Aurora purposes of fire to and from orbit there will be no deadzones.

[bean]

Ofcourse there is...

What you need to understand is that inaccuracy depends on unplanned deviation during projectile travel which scales linearly with travel time. ( deviations in course both from a moving target and on the projectile itself ).

Battleship gunnery is a major interest of mine, to the point I've read several books mostly or wholly devoted to the subject, and written a couple of essays on it.  So yes, I know that's one cause of inaccuracy.  But only one.  If it was the major one, then I'd expect that it would affect all shells in a salvo equally, and that the primary cause of misses would be tight patterns landing off-target.  This is not really the case.  Pattern size was often greater than Mean Point of Impact error.  Yes, I'd expect a railgun to be better at consistency than a WWII-era 16" gun.  If nothing else, you have a lot lower variation in muzzle velocity.  But not enough to make unguided KE rounds work at >10 km. 

If a railgun projectile travels at 5km/s compared to a battleship shell at 500m/s this means the projectile can travel 10 times as long distance before the inaccuracy from outside influence becomes identical.

That assumes the railgun projectile is the same size as the battleship shell.  This is not true.  It's considerably smaller, which means it's much more affected by wind.  Look at a 5" vs 16" range table if you don't believe me.
That aside, let's assume you're right.  Typical battleship pattern size for a 3-gun salvo was about 1% of range.  We'll take 10% of that.  At 36 km (outer limits of battleship range) you're still missing by an average of 18m (1% is diameter, not radius).  An M1 tank has a hull that's 8m long and 3.66 m wide.  It covers about 3% of the area we expect our projectile to land in.  I'll take my guided projectiles, thank you very much.

This is further helped by the railgun shell spending less time at lower altitude in "thicker" atmosphere where the deviations are higher then at higher altitudes ( for example wind or air/particle resistance ).

Flatter trajectory means this isn't true, either.

If you fire in space and your projectile travel at 50000km/s then the projectile spends less then a millisecond traveling through the thicker atmosphere compared to a battleship shell spending up to 30 seconds or more traveling meaning it's over 30,000 times more accurate...

You're moving the goalposts.  This particular discussion started with someone bringing up the Navy's railgun program.  Orbital fire support with that kind of velocity of weapons is a very different thing, and a somewhat better case for unguided projectiles.

Further orbit isn't as far away as you would think... The ISS orbits at 300-400km up compared to Battleship guns max range of 30-40km, that's just 10 times as far which compared to the other numbers involved here is nothing.

Can you please stop assuming things about what I think?

[Barkhorn]

It's considerably smaller, which means it's much more affected by wind.

This isn't exactly true.  A railgun round is denser than a traditional battleship round.  Aerodynamics, including wind, correlate with the density of the round, not just the surface area or volume.  A stone ball with a diameter of 1m, thrown at 20m/s will go further than an inflated rubber ball of the same diameter at the same speed.  Because the rubber ball is less massive, the force of drag affects it more.

[bean]

This isn't exactly true.  A railgun round is denser than a traditional battleship round.  Aerodynamics, including wind, correlate with the density of the round, not just the surface area or volume.  A stone ball with a diameter of 1m, thrown at 20m/s will go further than an inflated rubber ball of the same diameter at the same speed.  Because the rubber ball is less massive, the force of drag affects it more.

If we're going to be really pedantic, the actual controlling factor is sectional density, mass/unit area.  Tungsten is ~2.5x as dense as steel.  (HE content of US AP shells was very low, and can be ignored at this scale.)  Mass scales with the cube of size, area with the square.  So a tungsten shell with the same sectional density will be 40% of the size of an equivalent battleship shell.  For a US 16", that means 6.4" diameter and about 2.5' long.  This is considerably bigger than a railgun shell being hypothesized.  Playing with the shape of the railgun round doesn't help, because what you gain in one dimension you lose in the others.
(OK, the actual controlling number is ballistic coefficient, but assuming similar shapes that drops out to sectional density.  And a long rod is more vulnerable to crosswinds than a normal shell, so that doesn't help the railgun, either.)

[alex_brunius]

So yes, I know that's one cause of inaccuracy.  But only one.  If it was the major one, then I'd expect that it would affect all shells in a salvo equally, and that the primary cause of misses would be tight patterns landing off-target.  This is not really the case.  Pattern size was often greater than Mean Point of Impact error.  Yes, I'd expect a railgun to be better at consistency than a WWII-era 16" gun.  If nothing else, you have a lot lower variation in muzzle velocity.  But not enough to make unguided KE rounds work at >10 km. 

My point here is that outside influence and moving target is the ONLY cause of inaccuracy that can't ( and will in an Auorora 4x tech level gun ) be engineered away thanks to smaller tolerances and near molecular level perfect gun assembly.

You're moving the goalposts.  This particular discussion started with someone bringing up the Navy's railgun program.  Orbital fire support with that kind of velocity of weapons is a very different thing, and a somewhat better case for unguided projectiles.

I'm not moving goalposts. You need to take a step back and realize that your trying to have a discussion about the Navy's railgun program on a forum about a Sci-Fi game. The ultimate purpose of any discussion in this thread as far as I am concerned is to promote a better and more realistic game, not to debate the viability of the Navy's railgun program. If you want to argue about the Navy's railguns I'm sure there are plenty of other forums better suited to that purpose!

That aside, let's assume you're right.  Typical battleship pattern size for a 3-gun salvo was about 1% of range.  We'll take 10% of that.  At 36 km (outer limits of battleship range) you're still missing by an average of 18m (1% is diameter, not radius).  An M1 tank has a hull that's 8m long and 3.66 m wide.  It covers about 3% of the area we expect our projectile to land in.  I'll take my guided projectiles, thank you very much.

Now let's run those math again with Aurora 4x railgun speeds of say 50000000m/s instead of 500m/s as well as orbital range (x10). This mean it's not 10 times accurate, it's 10000 times more accurate. Your average miss turns from 18m to 18cm.

That's not a "somewhat better case"... That's a totally different case.

Something else to consider is that the destructiveness of KE shells scale with speed as well, and not linear but exponential (Ek = ½mv^2). A shell going 100000 times faster contains 100000^2 as much kinetic energy. A 1kg shell travelling at 50000km/s contains about 10 million times as much kinetic energy as a 1000 kg Battleship shell at 500m/s, and about 1 million times as much total energy including HE ( considering about 10% of the energy in a Battleship shell for firesupport would be Kinetic and 90% HE ).

I'm not 100% sure exactly what would happen when shells impact the ground at those kind of speeds, and it's possible the atmosphere would slow them down considerably too, but considering the numbers involved I wouldn't feel safe even 18m away regardless of if I was in a tank or not...

A third consideration ( which Aurora models pretty well already on the scale of space combat at least ) is the cost of the shells vs guided missiles. KE shells are basically free while missiles will cost about 20% as much as the tank it's destroying cost. Using current costs it's $1.87M for a tomahawk vs $8.92M for a M1 tank or 21%. And that's assuming no missiles get intercepted on the way.

Can you please stop assuming things about what I think?

I am not assuming what you (person) think, but what you ( general audience reading ) normally would think. You are all welcome to join us in discussions improving the game.

[swarm_sadist]

Also, a steel round hitting dirt will only penetrate about 6 times it's length. All the extra kinetic energy will go outwards and make a crater.

[83athom]

If we're going to be really pedantic, the actual controlling factor is sectional density, mass/unit area.  Tungsten is ~2.5x as dense as steel.  (HE content of US AP shells was very low, and can be ignored at this scale.)  Mass scales with the cube of size, area with the square.  So a tungsten shell with the same sectional density will be 40% of the size of an equivalent battleship shell.  For a US 16", that means 6.4" diameter and about 2.5' long.  This is considerably bigger than a railgun shell being hypothesized.  Playing with the shape of the railgun round doesn't help, because what you gain in one dimension you lose in the others.
(OK, the actual controlling number is ballistic coefficient, but assuming similar shapes that drops out to sectional density.  And a long rod is more vulnerable to crosswinds than a normal shell, so that doesn't help the railgun, either.)

Just for information, the design in current testing (and most likely for final product) is a long cone with 4 fins.

[Felixg]

I am not sure I like the idea of Static defenses losing their fortification if they open up on orbital targets. Makes them virtually useless after the first salvo (Which likely won't do any good because of how hardy the ships are designed unless they have incredible anti armor/shield properties) since the enemy will pinpoints and obliterate them.

If they spend months(years?) with combat engineers digging in, and digging in, and so on then they should get to keep that. Maybe have a staged fortification loss? Each progressive attack makes it easier to pinpoint the exact spot to hit to silence the gun, rather than immediately knowing.

Could have an interesting branch of Stealth tech (When improved by Combat Engineers rather than the unit itself) as well to improve the fortification decay as well, so rather than dropping with every shot it drops with every other shot, or every third shot and so on.

[Steve Walmsley (OP)]

They don't lose their fortification.

[Felixg]

They don't lose their fortification.

Ahh whew, thats good to know. I saw this post

If a unit chooses to attack, it will lose any fortification bonus. However, you will be able to attack with selected units and leave others in place.

More details later on how long it takes to fortify.

and started to worry, glad it has been changed!

[Hazard]

It hasn't been changed; it just refers to ground to ground combat rules instead of orbital combat rules.

[bean]

Also, a steel round hitting dirt will only penetrate about 6 times it's length. All the extra kinetic energy will go outwards and make a crater.

That's going to be dominant at really high velocities.  Long rods are still useful for armor penetration at tens of km/s.  In Aurora, you probably couldn't tell the difference.  The near future is a rather different matter.

[Hazard]

If a steel round only penetrates up to 6 times its length in ground once sufficiently large speeds are achieved and you can form needles of arbitrary lengths on demand you've basically got the perfect fortress breaking round right there. Only thing you need to do is relative densities and how far down you need to go and the round stops right as it has passed the reinforced concrete. And if it doesn't, it hits with enough force to make it moot anyway.

[bean]

If a steel round only penetrates up to 6 times its length in ground once sufficiently large speeds are achieved and you can form needles of arbitrary lengths on demand you've basically got the perfect fortress breaking round right there. Only thing you need to do is relative densities and how far down you need to go and the round stops right as it has passed the reinforced concrete. And if it doesn't, it hits with enough force to make it moot anyway.

That's not quite how it works.  6x depth is based on the Newtonian/hydrodynamic penetration approximation.  Basically, at some point your ability to penetrate is limited by your ability to push things out of the way, regardless of how fast you're going.  However, at higher speeds you start making craters, which can penetrate deeper.  Even if you're not quite to proper cratering, all of that energy has to go somewhere, which is likely to be unpleasant to those nearby.