Main Battery as PD?

[Whiskey144]

Okay, so I've got this setting I'm working on, and this time the question is about main battery and point defense armament.

Simply put: could a main battery of laser turrets be designed to pull double-duty as a point defense array?

To answer questions about combat in-setting: it occurs at multiple thousands of kilometers, typically around 10,000-100,000+. Main armaments are various types of usually multi-stage nuke thermal propelled missiles, laser cannon and particle beams (obvious particle beams are shorter-ranged weapons). Missiles typically mount various warheads; from single-shot railguns, bomb-pumped lasers, and plain old "missile=giant guided bullet", to nuclear weapons (omni-directional and shaped charge variant) and sometimes even antimatter bombs.

Missiles typically have a 10-60 second burn times, with 50-100 Gs of acceleration. Usually 10-20 second is average; long-range specialist missiles have 60-second burn times, but lower acceleration.

[Brother-Captain Gaius]

Well... can your main guns fire very rapidly and/or very accurately at close quarters? Can their turrets or mountings traverse rapidly, and can the guns depress and elevate quickly? Can the battery as a whole put out a very large volume of fire?

Those are really the qualities of a point defense weapon. You can call it whatever you want, it's just a matter of whether it can feasibly and practically perform in the desired role.

[GrandMasterTerwynn]

Okay, so I've got this setting I'm working on, and this time the question is about main battery and point defense armament.

Simply put: could a main battery of laser turrets be designed to pull double-duty as a point defense array?

To answer questions about combat in-setting: it occurs at multiple thousands of kilometers, typically around 10,000-100,000+. Main armaments are various types of usually multi-stage nuke thermal propelled missiles, laser cannon and particle beams (obvious particle beams are shorter-ranged weapons). Missiles typically mount various warheads; from single-shot railguns, bomb-pumped lasers, and plain old "missile=giant guided bullet", to nuclear weapons (omni-directional and shaped charge variant) and sometimes even antimatter bombs.

Missiles typically have a 10-60 second burn times, with 50-100 Gs of acceleration. Usually 10-20 second is average; long-range specialist missiles have 60-second burn times, but lower acceleration.

First, you might want to actually run the numbers you've produced. A missile with 100 G of acceleration over twenty seconds will have covered 196 kilometers in that time, and attain a velocity of 19.6 km/sec. It'll take about eight and a half minutes to cross 10,000 kilometers, at which point it will be easy pickings for point-defense, thanks to having expended all its delta-vee at the start. Even if I take the second part of your statement to mean that a long range missile has 50 gravities of acceleration and sustains it for 60 seconds, we find that it covers just 882 kilometers while its motor is burning. It still takes over five and a half minutes of coasting to travel 10,000 kilometers . . . at which point, it's still easy pickings for PD.

But to answer your original question, the answer is "it depends." Can the main lasers be tuned to work at low enough power to allow for fast follow-up shots? Can their emplacements be slewed quickly enough to enable them to track incoming projectiles? Can they even be pointed accurately enough to hit a missile-sized object? If we were to assume the smallest target a ship could hit at 10,000 kilometers was 100 meters wide, the main lasers would need to be pointed accurately within 5.73E-4 degrees. That'd equate to hitting a meter-wide missile at up to 100 kilometers out. Which is actually really good, since there's no feasible nuclear weapon that could possibly seriously damage a ship made of anything sturdier than tissue paper at that distance.

[Whiskey144]

First, you might want to actually run the numbers you've produced. A missile with 100 G of acceleration over twenty seconds will have covered 196 kilometers in that time, and attain a velocity of 19.6 km/sec. It'll take about eight and a half minutes to cross 10,000 kilometers, at which point it will be easy pickings for point-defense, thanks to having expended all its delta-vee at the start. Even if I take the second part of your statement to mean that a long range missile has 50 gravities of acceleration and sustains it for 60 seconds, we find that it covers just 882 kilometers while its motor is burning. It still takes over five and a half minutes of coasting to travel 10,000 kilometers . . . at which point, it's still easy pickings for PD.

*sigh* I think I've got another case of thinking people can read my mind and instantly determine what I mean when I post something.

Okay, since I rather did poorly clarifying missile armament design:

Missiles are built with a minimum of two stages, and more often five or more. These stages are typically designated by flight 'phase', and are as follows for a totally powered flightplan:

1) Ejector. This is the stage that kicks the missile out of the launch tube and then orients it towards the target. Delta-v of 25 m/s, acceleration is a meager 0.5 Gs.
2) Boost Phase. The stage that gets the missile going towards a target. For the rare PD missiles that used, this is where it the stage ends, as the majority of the missile is a blast-frag or nuclear warhead designed to get close, and explode, before using a close-range blast or a shower of fragments to mission-kill the enemy warhead. Acceleration 10 Gs, stage operates for 10 seconds, allowing it to cover about 481 kilometers and have a final velocity of around 981 m/s, though, if the missile tube were to be pointing at the enemy, it would have a terminal velocity of 1006 m/s.
3) Cruise Phase 1. Main stage, provides the majority of propulsion towards the target. 50 Gs acceleration, 15 second boost time. Covers about 27,100 kilometers, terminal velocity is now 8,338.5 m/s.
4) Cruise Phase 2. Main stage, pretty much identical to above; 50 Gs for 15 seconds. Terminal becomes 15,696 m/s, displacement is about 27,200 kilometers towards the target. Total displacement so far is 54,781 kilometers.
6) Terminal Phase. Terminal stage, carrying whatever is used to make life unpleasant for the enemy. This one does a 100 G sprint for 5 seconds. Terminal velocity is a blistering 20,601 m/s, with distance covered being about 12,100 kilometers. Total displacement is now 66,881 kilometers for 6-stage, powered flight missile.

Add additional Cruise Phase stages (which would have identical performance) for additional range. High-performance 70 G cruise stages are available, as are longer-burn terminal stages. Ejector and Boost stages remain the same.

Side question: could you use an NTR solid engine as the bomb-part of a bomb-pumped laser?

But to answer your original question, the answer is "it depends." Can the main lasers be tuned to work at low enough power to allow for fast follow-up shots? Can their emplacements be slewed quickly enough to enable them to track incoming projectiles? Can they even be pointed accurately enough to hit a missile-sized object? If we were to assume the smallest target a ship could hit at 10,000 kilometers was 100 meters wide, the main lasers would need to be pointed accurately within 5.73E-4 degrees. That'd equate to hitting a meter-wide missile at up to 100 kilometers out. Which is actually really good, since there's no feasible nuclear weapon that could possibly seriously damage a ship made of anything sturdier than tissue paper at that distance.

Do you have mind-reading powers?

Seriously though, this is freaky-scary close to the kind of things I've got flying around; the smallest proper capital ships are corvettes that are 100-120 meters long. The main lasers are designed as "dial-a-yield", ball-turret mounts; they have fast tracking and targeting times. Rate of fire varies; high-powered, long-range shots can get off a shot every couple of seconds, mostly due to capacitance and thermal reasons rather than optical limitations. PD-yield shots can be fired at around 10/second, though the way I envision a ship's main laser batteries to function in a PD role is by using multiple turrets to bracket each incoming missile, reducing the chances of a large number of hits.

Well... can your main guns fire very rapidly and/or very accurately at close quarters? Can their turrets or mountings traverse rapidly, and can the guns depress and elevate quickly? Can the battery as a whole put out a very large volume of fire?

The battery as a whole can spit out a pretty high volume of fire, especially on larger vessels who's mounts aren't very much larger (though with much higher grade optics and heatsinks).

[Lord of the Abyss]

Simply put: could a main battery of laser turrets be designed to pull double-duty as a point defense array?

One way I saw it done was in the novel The Farside Cannon. The laser "Cannon" in question was actually composed of thousands of smaller lasers, and point defense was implemented by assigning a percentage of the component lasers to the task. So it could function either as one big weapon or a multitude of smaller ones as necessary.

[Whiskey144]

One way I saw it done was in the novel The Farside Cannon. The laser "Cannon" in question was actually composed of thousands of smaller lasers, and point defense was implemented by assigning a percentage of the component lasers to the task. So it could function either as one big weapon or a multitude of smaller ones as necessary.

Interesting idea. Sounds pretty neat, actually, and with a sufficiently large ship and laser mount would possibly be quite easy to rig up.

OTOH, I'm much more partial to multiple laser mounts distributed over the surface of a ship, especially as I'm thinking part of combat will be sniping off the enemy's turrets once you break through the shields.

[avatarxprime]

One way I saw it done was in the novel The Farside Cannon. The laser "Cannon" in question was actually composed of thousands of smaller lasers, and point defense was implemented by assigning a percentage of the component lasers to the task. So it could function either as one big weapon or a multitude of smaller ones as necessary.

Interesting idea. Sounds pretty neat, actually, and with a sufficiently large ship and laser mount would possibly be quite easy to rig up.

OTOH, I'm much more partial to multiple laser mounts distributed over the surface of a ship, especially as I'm thinking part of combat will be sniping off the enemy's turrets once you break through the shields.

Another idea, depending on how "hard" of a universe you're going for is to borrow an idea from Halo and have beam splitters and steerers integrated into the design of your main laser battery. This way you can split the beam of the main weapons into tons of smaller beam that shoot out all over the surface of the ship and act as point defense. IIRC the Covenat were supposed to use this method so their giant DOOM cannon also doubled as point defense. Now, admittedly any laser weapon that qualifies as a main weapon will likely possess far too much energy to actually do this since it would melt the devices meant to split and steer the beam, but it's a fun idea.

[Whiskey144]

Another idea, depending on how "hard" of a universe you're going for is to borrow an idea from Halo and have beam splitters and steerers integrated into the design of your main laser battery. This way you can split the beam of the main weapons into tons of smaller beam that shoot out all over the surface of the ship and act as point defense. IIRC the Covenat were supposed to use this method so their giant DOOM cannon also doubled as point defense. Now, admittedly any laser weapon that qualifies as a main weapon will likely possess far too much energy to actually do this since it would melt the devices meant to split and steer the beam, but it's a fun idea.

Aside from my general dislike of Halo, "beam splitter" can cover a few possibilities, like the one you mention, and one in which the beam generator has a "splitter" that allows a single laser engine to power multiple laser mounts.

In this case, I'm opting for a single laser engine per mount, as this increases reliability, though also increases maintenance requirements and mass..........OTOH, as I'm finding out from my plasma drive thread, I'm either terribad at doing the math pertaining to rocketry, or I have crazy-efficient engines that let me literally have thousands of tonnes of ship mass to play with.

[Dwelf]

I would suspect that missiles would be much lighter armoured than even the smallest ship. This might make it reasonable to increase the divergance on the beam. This would result in a larger area covered by less energy. Harmless to ships but enough to detonate a missle even if you don't have the accuracy to pinpoint a beam on it.

You could possibly combine this with a lower powered continous operation mode and just wave the beam over the incomming missiles.

[Whiskey144]

I would suspect that missiles would be much lighter armoured than even the smallest ship. This might make it reasonable to increase the divergance on the beam. This would result in a larger area covered by less energy. Harmless to ships but enough to detonate a missle even if you don't have the accuracy to pinpoint a beam on it.

Well, yes. The problem is, however, that missiles typically have around two to three orders of magnitude acceleration advantage; the terminal phase engine gets a whopping four OOM acceleration advantage over typical ship accelerations.

Hence why having the rapid-firing main battery be used as PD is quite useful; the mounts have more than enough power to zap a ship at long range, and since these missiles are on trajectories designed to get relatively close to their targets, they can be (hopefully) bracketed at several hundred kilometers, allowing time for the ship to get out of the way of any debris. The rapid-firerate coupled with the high power allows much longer zap ranges........though it's still not enough, as a sufficiently large missile salvo will do Bad Things to the target.

Though I'm still curious as to whether or not a solid-core nuke thermal rocket reactor could be used as the "bomb" part of a bomb-pumped x-ray or gamma ray laser.

[Dwelf]

I would suspect that missiles would be much lighter armoured than even the smallest ship. This might make it reasonable to increase the divergance on the beam. This would result in a larger area covered by less energy. Harmless to ships but enough to detonate a missle even if you don't have the accuracy to pinpoint a beam on it.

Well, yes. The problem is, however, that missiles typically have around two to three orders of magnitude acceleration advantage; the terminal phase engine gets a whopping four OOM acceleration advantage over typical ship accelerations.

Hence why having the rapid-firing main battery be used as PD is quite useful; the mounts have more than enough power to zap a ship at long range, and since these missiles are on trajectories designed to get relatively close to their targets, they can be (hopefully) bracketed at several hundred kilometers, allowing time for the ship to get out of the way of any debris. The rapid-firerate coupled with the high power allows much longer zap ranges........though it's still not enough, as a sufficiently large missile salvo will do Bad Things to the target.

Though I'm still curious as to whether or not a solid-core nuke thermal rocket reactor could be used as the "bomb" part of a bomb-pumped x-ray or gamma ray laser.

At the distances involved optical sensors are basically providing real time telemetry and lasers are so close to instant hit that it doesnt matter. The only reason to bracket a missile would be weapon accuracy issues which would impact the effective range of the main batteries on other ships.

For other forms for PD weapon that are not light speed only the lateral acceleration of the missile is relevant to your chance of hitting it. The latteral acceleration is limited by your requirement to get close to the target and the increased costs of adding more engines to a missile. The vast majority of any missile's acceleration is going to go towards getting to the target quicker.

[Whiskey144]

At the distances involved optical sensors are basically providing real time telemetry and lasers are so close to instant hit that it doesnt matter. The only reason to bracket a missile would be weapon accuracy issues which would impact the effective range of the main batteries on other ships.

Hmm. Hadn't thought about that. OTOH, it just occurred to me, that, with the majority of anti-missile PD concentrated in the main laser battery, in the average fight, a ship will be using its main battery as just that- a main battery for damaging an enemy vessel.

So secondary PD weapons, like kinetics or missiles or perhaps even particle beams, which would likely have fewer numbers or lack the range and/or punch of the primary laser mounts, would have to be used. This also allows me to include a few dedicated-PD mounts.

[Beowulf]

Though I'm still curious as to whether or not a solid-core nuke thermal rocket reactor could be used as the "bomb" part of a bomb-pumped x-ray or gamma ray laser.

Unlikely in the extreme. The requirements for a reactor and the requirements for a bomb in terms of fissile material shaping are so dramatically different as to be infeasible for one component to do double duty. It's potentially feasible that the materials NTR core would sympathetically fission from the sleet of neutron radiation from the bomb, raising the yield a bit, but you still need the bomb.

[Whiskey144]

Unlikely in the extreme. The requirements for a reactor and the requirements for a bomb in terms of fissile material shaping are so dramatically different as to be infeasible for one component to do double duty. It's potentially feasible that the materials NTR core would sympathetically fission from the sleet of neutron radiation from the bomb, raising the yield a bit, but you still need the bomb.

I suppose it's like a lot of ideas; sounds good on paper, but is simply not feasible. Ah well.

[someone_else]

The answer should be more or less yes.

The point is that the difference between hitting a craft 100 or more meter wide (a ship) and hitting a missile (10? 5? 1 meter wide?) is relatively irrelevant if it can aim at stuff at multiple thousands of Km with ease. To switch targets at that range you just have to make tiny adjustments to the optics, which will be instantaneous. I hope your sensors can track more than one target at once.

If necessary, the laser can be kept with a wider beam, a beam 10 or even 100 meter wide will compensate the difference in target size difference. That is assuming your missiles aren't heavily armored. For the same reason you can dial down the power level and keep the laser as a hose.

The longer range also means you have much more time to down the missiles. Although depends from their fire rate, that's the only way to keep them limited as antiship weapons.

At the speeds they are going, particle cannons are crap (too short range), but kinetics get awesome. Not projectiles, but small-ish seekers. Too fast for slugs.

But anyway, with the engines you have, there is no need for PD. The engine's exhaust will torch away anything that gets too close.

Rate of fire varies; high-powered, long-range shots can get off a shot every couple of seconds, mostly due to capacitance and thermal reasons rather than optical limitations. PD-yield shots can be fired at around 10/second

You can easily engineer a laser to dial down the output and have the main optics shoot PD-yeld shots at the same identical range of the high-powered ones.

in the average fight, a ship will be using its main battery as just that- a main battery for damaging an enemy vessel.

Unless the enemy is completely dumb, missiles won't be launched from your own fire range. That's one of the main reason to use (fighters with bombs in the past and now) missiles over direct-fire weapons.

it's still not enough, as a sufficiently large missile salvo will do Bad Things to the target.

It depends from how powerful the laser is. But unless it's a world-busting beast, it's likely the case. Have a pack of "missile destroyers" guard them, in fact glorified missile launch platforms, firing two-stage chemical rockets against them. Such antimissiles are vastly cheaper than your torpedoes and will destroy them just fine exploiting their own kinetic energy.

Though I'm still curious as to whether or not a solid-core nuke thermal rocket reactor could be used as the "bomb" part of a bomb-pumped x-ray or gamma ray laser.

No. You need a few dozen digits much more output, and also a different shape.

The only nuclear drive you can use for that is the Orion drive (that uses nukes). (ninja'd by Beowulf, nevermind)

[Whiskey144]

The point is that the difference between hitting a craft 100 or more meter wide (a ship) and hitting a missile (10? 5? 1 meter wide?) is relatively irrelevant if it can aim at stuff at multiple thousands of Km with ease. To switch targets at that range you just have to make tiny adjustments to the optics, which will be instantaneous. I hope your sensors can track more than one target at once.

The average anti-ship missile is a big beast; 2-5 meters in diameter, with most erring on the side of 2-3. Mostly this is to carry the numerous drive assemblies that are needed (as noted above in the 5-stage missile concept), but also to carry submunitions, where applicable.

Sensor systems are......complicated, I guess. Mostly because there's a somewhat bewildering variety of sensor systems, and usually they have redundancies. So I'm going to simply say that it's very likely that sensor systems have multi-tracking capability, if only because there are multiple sensors that are mounted, with each being able to be tasked a separate target.

If necessary, the laser can be kept with a wider beam, a beam 10 or even 100 meter wide will compensate the difference in target size difference. That is assuming your missiles aren't heavily armored. For the same reason you can dial down the power level and keep the laser as a hose.

Well, missiles are armored, though to varying degrees. Generally the missiles have a great advantage in that secondary PD systems haven't got the range to snipe the incoming ordnance before said ordnance is nail-bitingly-close to kill-range, which for most is allowing for the standoff munitions, like 1-shot railguns, bomb-pumped lasers, or kinetic-kill submunition rods. The former two have the obvious standoff range, while the latter has the advantage in that you now have ten or twenty (or more) times the amount of stuff to shoot down.....all within a couple hundred kilometers, when said secondary PD systems have a range of maybe 200 klicks.

At the speeds they are going, particle cannons are crap (too short range), but kinetics get awesome. Not projectiles, but small-ish seekers. Too fast for slugs.

Could you elaborate on the particle beam crap-itude? At present, my PB-ideas orient around multi-thousand km/s beam velocities, with a usually 1 shot/second rate of fire at lower yields.

But anyway, with the engines you have, there is no need for PD. The engine's exhaust will torch away anything that gets too close.

Funny thing about that, I've been trying to not have such OMGWTFPWN engines, as I found out in aforementioned engine thread that the drive I spec'd out had a power requirement in the petawatt region and a thrust plume that spat out megaton/second energies.........which would make the sub-kiloton/second main battery a pretty useless weapon, if you could simply hose the enemy down with your exhaust plume. ATM, I'm busily running the numbers to try to get the main propulsion down to something a lot more reasonable and less Plume of Doom-ey.

I'll also note that, within this particular thread, the only engines I've noted have been the missile engines........which tend to not scale well to full-on spacecraft engines, as the average frigate is in the region of 250-400 meters in length, usually tending towards 300 meters.

You can easily engineer a laser to dial down the output and have the main optics shoot PD-yeld shots at the same identical range of the high-powered ones.

.......Which is what I said. I noted that the main limitation to main battery PD ops was thermal and power related; i.e. they can't dispose of waste heat fast enough and the capacitors can't recharge fast enough to send out a laser-storm.

Unless the enemy is completely dumb, missiles won't be launched from your own fire range. That's one of the main reason to use (fighters with bombs in the past and now) missiles over direct-fire weapons.

The thing is, though, that combat typically occurs inside laser-range........hence why missile launch is similarly within laser range.

It depends from how powerful the laser is. But unless it's a world-busting beast, it's likely the case. Have a pack of "missile destroyers" guard them, in fact glorified missile launch platforms, firing two-stage chemical rockets against them. Such antimissiles are vastly cheaper than your torpedoes and will destroy them just fine exploiting their own kinetic energy.

The one problem I can see is that the anti-ship missiles are going to have vastly superior accelerations; as spec'd above, I'm giving the 'average' anti-ship torpedo a set of stages that give a 15-second, 50G acceleration. I'm pretty sure that that's a whole lot better than what a 2-stage chemfuel can manage........though said 2-stage chemfuels are somewhat similar to the anti-missile missile system I envisioned, except I tended towards proximity kill rather than impact kill.

The only nuclear drive you can use for that is the Orion drive (that uses nukes). (ninja'd by Beowulf, nevermind)

Which would rather defeat the purpose of such a weapon, wouldn't you say?

[someone_else]

Well, missiles are armored, though to varying degrees.

Let me clarify what I meant.

What's the difference in power level between a antiship shot and an antimissile shot (that is how much energy you need to deal significant damage to ship or to missiles)?

For example, if your ships have enough armor that you need power output X to deal damage to a ship, but missiles only need power output X/1000, then the shot aimed at a missile can be 1000 times as large, so if the missile is even 1000 times smaller than the ship, hitting it or the ship has the same level of difficulty.

Anyway, you can have all the armor you want, but your optics and sensors will have to be exposed to work (also your engines). That just screams MISSION KILL FROM STUPENDOUS RANGEEEEE!!!
That's why lots of authors like force shields.

Could you elaborate on the particle beam crap-itude? At present, my PB-ideas orient around multi-thousand km/s beam velocities, with a usually 1 shot/second rate of fire at lower yields.

Optimal particle beam designs work more or less like Ion cannon from SW. Saved that they don't "ionize" and stun robots and electronics, but *kill* stuff AND people with ionizing radiation. That's because in this case you count on relatively little amount of stuff going at crazy speeds (more than 30-40% light-speed) and causing braking radiation (Bremsstrahlung) from the enemy's own hull/machinery (shower of x-rays for everyone! yay!). Those do have a good range, but aren't exactly PD weapons in either size and power consumption (accelerating stuff to so high speeds requires powa!!!), although I don't know the power level of the stuff you are placing as PD, so they may still qualify. This was the kind of particle beam design supposed to go in the Star Wars Program, and kill enemy ICBMs with *massive* radiation doses on their electronics. Without leaving a single scar. Scary shit huh?

If you are using them to melt and burn stuff like more conventional weapons, then they start to look far dumber than a laser for a myriad of reasons.

look here for a dissertation about particle cannons. (btw, have a read at the whole site if you have some spare time, it's worth it).

And the lower power particle cannons can be affected by electric fields and/or magnetic fields. Which means that if you are close to a planet they will work funny. Or won't be able to hit properly shielded vessels.

.......Which is what I said. I noted that the main limitation to main battery PD ops was thermal and power related; i.e. they can't dispose of waste heat fast enough and the capacitors can't recharge fast enough to send out a laser-storm.

See the first answer, it depends from the difference in power level between an antiship shot and an antimissile shot. You may be able to fire a shot that for the Uber Laser is low-power (so you can keep it up constantly), but for a missile is still damn deadly.

The thing is, though, that combat typically occurs inside laser-range........hence why missile launch is similarly within laser range.

Can you explain why? Did fair fight return popular lately?

I'm not picking on you, it's something that needs some kind of reason to happen other than "they do like this".

For example, in my own setting there are missile transport ships (called "Arsenals" to remember a ship that never was), which bring missiles across interplanetary distances with their engines. When sensor and command crafts of the same team spot an enemy far before it gets into laser range (which isn't exactly difficult unless you have wildly overpowered lasers), they give them targeting data, and the missiles are fired off.

Why none does this in your setting?

The one problem I can see is that the anti-ship missiles are going to have vastly superior accelerations

EEEK!!! I read only now your missile's performance up in the first posts of the thread. Scratch the above. The impact speed is well into the manageable for the targeting system, but you are right. Chemical rockets cannot reach the performance to be a threat to them.

Although this sentence:
Add additional Cruise Phase stages (which would have identical performance) for additional range. High-performance 70 G cruise stages are available, as are longer-burn terminal stages. Ejector and Boost stages remain the same.
makes me wonder if you understood what staging is about.

Staging is a desperate measure to trick the rocket equation when your engines suck.

Assuming that the Cruise 1 stage is the most efficient performance of the missile engines (the max delta-v you can get out of a given engine tech), that's something with a Ve in the nuclear-thermal engines range (exhaust velocity of 5.4 km/s), and that means a missile massing 1 ton needs around 746 kg of propellant, and has a delta-v of 7.4 km/s. It doesn't have a decent payload but I'll disregard that for a moment (i'm not gonna care about acceleration because that's a factor of the engine number/size and it's not relevant for the example).
Now, second stage, same engine tech. To have a suitable payload (1 ton) it must weight around 60 tons. Third stage, to carry them all must weight around 2500 tons.

You see how weight is getting out of control fast.

If you have better engines that can carry higher delta-v reserves than around 7 km/s before hitting their upper delta-v limit, why are you wasting mass and engines (= money) with staging?

The only nuclear drive you can use for that is the Orion drive (that uses nukes). (ninja'd by Beowulf, nevermind)

Which would rather defeat the purpose of such a weapon, wouldn't you say?

If you were thinking of using it both for missile engine and missile warhead... yeah, you're right. In other cases it should work fine.

[Whiskey144]

What's the difference in power level between a antiship shot and an antimissile shot (that is how much energy you need to deal significant damage to ship or to missiles)?

Ships carry triple-digit gigawatt to single-digit terawatt scale fusion reactors, which can then be used to charge up capacitors that can then dump the energy into the shields.

So a shielded ship requires sub-kiloton/second firepower, though this is never used on an unshielded vessel unless you have a situation where the ship and every single thing, both living and artificial, on it, must die. Missiles, OTOH, probably require a single-digit gigajoule/second firepower......at most.

Anyway, you can have all the armor you want, but your optics and sensors will have to be exposed to work (also your engines). That just screams MISSION KILL FROM STUPENDOUS RANGEEEEE!!!
That's why lots of authors like force shields.

Funny enough, mission-killing is what tends to occur most often; in fact, an integral part of a laser duel between two ships that are unshielded is shooting off the enemy's laser turrets faster than he can do the same to you. OTOH, it's also not uncommon for a ship to be 'hulked', with the crew preoccupied with keeping the vessel in one piece, or being dead/dying.

Optimal particle beam designs work more or less like Ion cannon from SW. Saved that they don't "ionize" and stun robots and electronics, but *kill* stuff AND people with ionizing radiation. That's because in this case you count on relatively little amount of stuff going at crazy speeds (more than 30-40% light-speed) and causing braking radiation (Bremsstrahlung) from the enemy's own hull/machinery (shower of x-rays for everyone! yay!). Those do have a good range, but aren't exactly PD weapons in either size and power consumption (accelerating stuff to so high speeds requires powa!!!), although I don't know the power level of the stuff you are placing as PD, so they may still qualify. This was the kind of particle beam design supposed to go in the Star Wars Program, and kill enemy ICBMs with *massive* radiation doses on their electronics. Without leaving a single scar. Scary shit huh?

Ah, I was under the impression initially that you were saying that particle beams suck in general, rather than as PD ordnance.

look here for a dissertation about particle cannons. (btw, have a read at the whole site if you have some spare time, it's worth it).

Heh, I use that site for lots of stuff. Like the ever-useful boom table.

Can you explain why? Did fair fight return popular lately?

It's mainly to do with targeting sensor limitations. They can see ships from fairly far away, but it's the kind of "there's this dot, and with a little more time we can figure out if it's a rocket or something boring". At sub-lightminute ranges, sensors are able to adequately resolve returns as whatever, like say an enemy ship, but the targeting systems simply don't have the precision to effectively target something at that kind of range, unless it's a super-awesome dreadnought that's more intended as a strategic weapons platform than a fleet combatant- though it does fairly well in the latter role.

It's also a bit of a missile limitation; the missiles can coast for a long time, but a military vessel can use bursts of high-G acceleration to rapidly put on additional velocity and get out of the missile's tracking envelope.

EEEK!!! I read only now your missile's performance up in the first posts of the thread. Scratch the above. The impact speed is well into the manageable for the targeting system, but you are right. Chemical rockets cannot reach the performance to be a threat to them.

Yeah, I've rather quickly found that such POWA is required for these missiles if I'm going to have reasonable lasers for the techlevel I'm intending to set up.

Now, second stage, same engine tech. To have a suitable payload (1 ton) it must weight around 60 tons. Third stage, to carry them all must weight around 2500 tons.

WOW. That's a heavy missile.

If you have better engines that can carry higher delta-v reserves than around 7 km/s before hitting their upper delta-v limit, why are you wasting mass and engines (= money) with staging?

The main reason I considered staging is that, well, at first it was more-or-less "hey, this sounds kinda cool and like it could work". At this point, it's mainly because I figured that staging would boost the range of the missiles by having multiple engines that fire, allowing them to get going as fast possible and as far as possible.

If you were thinking of using it both for missile engine and missile warhead... yeah, you're right. In other cases it should work fine.

For the most part, I'm thinking the missiles are carrying small, sub-kiloton fusion bombs.....though it's also likely that they'd be carrying a couple that fly ballistic just before impact. Other than fusion bombs are standoff rounds like single-shot railguns and bomb-pumped lasers. In both cases, they need to get to around 100 kilometers away from the target to ensure accurate targeting....after all, the missile sensor packages are a lot worse than those used by a ship.

[Alyrium Denryle]

Ok. There are a few other things to consider.

First, frankly, even at those close ranges, optical sensors are NOT in fact providing real-time telemetry, and lasers are NOT instant hit. Fractions of a second do actually matter when you have to be accurate to miniscule fractions of a degree, and these discussions tend to ignore target selection, which takes time, either for a human gunner, or a targeting algorithm. I mention this, because using minimum amounts of fuel, a computer aboard a missile with maneuvering thrusters can vary its acceleration along several secondary vectors, and provided its net displacement along those vectors is zero between launch and impact, they will in effect be able to dodge lasers by increasing the size of the confidence interval of the missile's location in the defending ships targeting computers. Moreover, they will be able to do this without meaningfully impacting their accuracy. This makes it necessary to weaponize bayesian statistics, and resample their evasive maneuvering. The targeting computer can then direct a volley of rapid fire to the regions in the resulting probability density cloud (think of it like electron orbitals, but with missiles) where the missile is most likely to be.

If they have any volume of fire at all, you wont be able to use your main battery. It would be best to have a series of smaller guns dedicated to point defense, that continually send out a stream of fire intersecting the path that any missiles would have to take, maximizing their efficiency through bayesian targeting algorithms.

[Simon_Jester]

If they have any volume of fire at all, you wont be able to use your main battery. It would be best to have a series of smaller guns dedicated to point defense, that continually send out a stream of fire intersecting the path that any missiles would have to take, maximizing their efficiency through bayesian targeting algorithms.

That depends on the way your main battery works- a continuous beam weapon (or something that fires a very large number of rapid pulses functionally equivalent to a continuous beam) can be panned through a large volume. At which point you recover most of the advantages of quick-firing secondary weapons.

[Alyrium Denryle]

If they have any volume of fire at all, you wont be able to use your main battery. It would be best to have a series of smaller guns dedicated to point defense, that continually send out a stream of fire intersecting the path that any missiles would have to take, maximizing their efficiency through bayesian targeting algorithms.

That depends on the way your main battery works- a continuous beam weapon (or something that fires a very large number of rapid pulses functionally equivalent to a continuous beam) can be panned through a large volume. At which point you recover most of the advantages of quick-firing secondary weapons.

It still does not solve the problem of not being able to bring your main battery to bear on your actual enemy. All the enemy ship would have to do, is fire off a salvo and accelerate along a vector which takes it out of the line of fire for your beam weapons--if said beam weapons are used for PD--so said beams cannot pull simultaneous double duty. Of course, you can match their course, but then you just end up playing a complicated geometry game. Suddenly, euclid rather than newton (or Bayes) becomes the deadliest son of a bitch in space, and I would prefer that a greek platonist not have that particular honor.

[Simon_Jester]

It still does not solve the problem of not being able to bring your main battery to bear on your actual enemy. All the enemy ship would have to do, is fire off a salvo and accelerate along a vector which takes it out of the line of fire for your beam weapons--if said beam weapons are used for PD--so said beams cannot pull simultaneous double duty. Of course, you can match their course, but then you just end up playing a complicated geometry game. Suddenly, euclid rather than newton (or Bayes) becomes the deadliest son of a bitch in space, and I would prefer that a greek platonist not have that particular honor.

As opposed to an English alchemist, or (gasp!) an English minister? Heh.

In any case, this depends on main battery gun traverse rates. For turreted armament, it's not unreasonable to assume the ability to shift fire in a matter of seconds. And time spent shooting down missiles attacking your ship is probably better spent than the marginal value of more time spent shooting the enemy's ship. Especially since if the combat is fought at ranges where light speed weapons have significant accuracy problems against a maneuvering target, there is a good chance your main battery weapons wouldn't be hitting the target anyway.

If the action is prolonged- i.e. your ships don't just exchange one or two salvoes of beam fire and have whichever side got a larger percentage of its ships to survive the first minute of combat declare victory- taking your main guns out of action to remove an urgent, potentially crippling threat to your ship is a good trade-off.

[Dwelf]

Ok. There are a few other things to consider.

First, frankly, even at those close ranges, optical sensors are NOT in fact providing real-time telemetry, and lasers are NOT instant hit. Fractions of a second do actually matter when you have to be accurate to miniscule fractions of a degree, and these discussions tend to ignore target selection, which takes time, either for a human gunner, or a targeting algorithm.

I have to disagree with you on this Alyrium the comment was in relation to point defence not the anti ship ranges. If we assume a point defence evelope of around 10 000 km you are looking at ~33ms light second delay a value I would consider close to real time. The almost real time comment I made was not in relation to the 100 000km anti ship ranges but the point defence ranges. At anti ship beam ranges you will be looking at ~1/3 of a second + processing time delay on your sensors probably not a significant issue given the relatively low acceleration of the ships.

I mention this, because using minimum amounts of fuel, a computer aboard a missile with maneuvering thrusters can vary its acceleration along several secondary vectors, and provided its net displacement along those vectors is zero between launch and impact, they will in effect be able to dodge lasers by increasing the size of the confidence interval of the missile's location in the defending ships targeting computers. Moreover, they will be able to do this without meaningfully impacting their accuracy. This makes it necessary to weaponize bayesian statistics, and resample their evasive maneuvering. The targeting computer can then direct a volley of rapid fire to the regions in the resulting probability density cloud (think of it like electron orbitals, but with missiles) where the missile is most likely to be.

Unless you are going to install full scale boosters laterally on the missile I would expect it's lateral accelration to be significantly lower than the missile's linear acceleration. Adding these manouvering thrusters is going to add mass and cost to your missiles. Even if we allow for maximum acceleration in any direction the missile can't evade very far.

As stated the average anti ship missile is 2-3 metres in diameter so lets take 3
The maximum acceleration of any stage of a missile is listed at 100g ~981m/s^2
I'll pick 10 000km as the range.
~33ms light speed lag either way.
To round it out I'll pick 34ms processing time 0.100s lag.

1/2*981*(0.1)^2 = 4.905m

This would be a best case evasion for the missile and unless you plan on putting several terminal phase boosters on the missile mounted sideways is simply not going to happen without rotating the missile. Rotating the missile would give away which was it was planning to evade and increase it's target profile so I'll assume that is not going to happen.

[Simon_Jester]

A missile that can dodge by 1.5 times its own diameter will evade a significant fraction of fire aimed straight at it in this way- you start needing two shots to kill the bastard instead of one, or three, or four. Which makes a difference.

If, however, inherent limits in sensor resolution or gunlaying prevent you from aiming to a precision of a meter, then your defense fire will be 'sprayed' into the volume around the missile anyway, which defeats the purpose of evasion.

Basically, dodging enemy fire becomes far more appealing as range and time of flight increases. The area of space a ship can potentially reach by evasive maneuvers scales with the fourth power of the time it has to dodge, and the time it has to dodge scales linearly with increasing range and inversely with decreasing weapon speed.

Railgun shells that take two minutes to cross the gap between two ships, at soft-SF accelerations, aren't going to hit a damn thing. Lasers that can cross the same gap in a second or less probably will.

As you've just shown, it takes really really high accelerations to make evasion all that helpful in and of itself, at thousand-kilometer ranges against light speed weapons. Much more useful at longer distances.

[Dwelf]

Can you elaborate why it is the fourth power?

Assuming you know their maximum acceleration.
Distance to their last know position( How old the image is)
Their last known velocity.
How long your processing and targeting takes.

the area of uncertainty should just be a circle with a radius of
1/2at^2

where t is (age of data) + (processing time) + (weapon travel time)

At least that is my understanding of the maths involved.