Physically plausible explanation for missile based space combat?

[Artribution]

An Ohio-class submarine has 24 ICBM tubes, each of which can launch a Trident missile that splits into 12 separate, independently targeted missiles, for a total of 288 missiles in one barrage. If it were launching old Peacekeeper missiles from the 1960s, there would be 336 missiles per barrage. Just seems like a point-defense laser might have some trouble coping is all.

And those are ICBMs. If we were just talking about tactical nuclear warheads, it could be an entire new definition of spam.

Yeah, it's expensive, all those missiles, but victory is the only thing that counts. If you don't win, your entire spacecraft is a waste of money.

And if you launch from the other side of the planetoid (I'm guessing most space combat happens in orbit), the enemy wouldn't have any chance of detecting their engine heat. After the initial burn, they could just coast the rest of the way to intercept.

 

[GTOM]

In case of convoy attack, fight happens in deep space. Otherwise, most likely they fight around an asteroid.

 

[Ryan_m_b]

Gain it depends on how good your point defence is. If you can deliver multi-mega joule particle beam shots then it's a question how how rapid is your rate of fire (for orbital distances the chance of missing is likely to be nil). As a quick exercise:

Let's say missiles and target have a closing speed of 50kmps and they're detected 1000km out. That's 20 seconds before being hit. If there are 500 missiles incoming then the rate of fire to guarantee safety is 25 shots per second. Now it comes down to how many beam emitters you have and how rapidly they can fire. If you have many...it starts seeming reasonable within the confines of the setting.

As for where battles occur I don't see why it would be primarily in orbit. If you're attacking a planet then you have to travel from somewhere else. Given that's the case battle would likely start once the chances of hitting someone with a long range weapon within a reasonable time converge. I expect the shooting would start at distances of light seconds, if you had beam technology good enough. Even if not the attacker in this scenario would be better off firing a whole bunch of beam drones ahead of them to engage the enemy.

In an effort to jump ahead (we seem to be circling a bit) I'd like to pose this question: why would a barrage of missiles be better than a barrage of beam-drones? The latter seem the obvious choice to me (assuming equivalently effective tech) because as the two barrages close the drones begin to fire, picking off the missiles. For a missile to kill a drone it has to be very close, likely much closer than the effective range of a beam weapon.

 

[GTOM]

My problem with beam drones, that they don't have the power source and waste heat treatment of the ship.
(Although the ship could give them power with lasers.)
If one attacks an asteroid, he will come with superior firepower, cover in this situation could help the defenders.
I rather care about total power output than number of lasers.
In case of particle waepons also appropriate, i wonder, what if the defender had a really strong magnetic field, to bring back the metallic particles and reattach it to the hull?

 

[Ryan_m_b]

Likely a drone would have to be fairly large to carry a power source and radiators. Still even if they were 10 tonnes a squadron of 10 would mass just 20% of the ISS. The advantage to beam drones is that they could fire at distance whilst approaching the enemy allowing you to keep your craft further away. Missiles might be good if you fire enough at close distance but if you have to fire from long range they'll be picked off.

I don't know what you mean by a magnetic field to draw in metal. If the attacked was shooting metal at you then maybe (but it would have to be insanely powerful to deflect anything) but that won't help against a neutral beam of non metal particles.

In your setting I presume the asteroids are habitats? If so a beamed weapon would be quite appropriate. You could fit quite a large reactor and radiators on an asteroid. Having a continual gigawatt weapon would seem possible in that situation.

 

[Artribution]

I'd like to pose this question: why would a barrage of missiles be better than a barrage of beam-drones? The latter seem the obvious choice to me (assuming equivalently effective tech) because as the two barrages close the drones begin to fire, picking off the missiles. For a missile to kill a drone it has to be very close, likely much closer than the effective range of a beam weapon.

Beam drones seem a bit more high-tech. They need more power, more accurate targeting, more efficient propulsion/fuel (missiles aren't supposed to return when their assignment is completed), etc.

Against a larger spacecraft , they could partly make up for the lower energy of their individual beams by focus-firing, though.

And your prediction rests on the assumption that the missiles' stealth and ECM is ineffective.

 

[GTOM]

I thought about use the magnetic field to reattach the metallic particles torn from the armor by the beam. Or make them super hard to evaporate.
(So a kind of energy shield...)

Yes it is a problem, that planetary defence has quite an amount of resources, it is hard to overwhelm.

I also give a fairly low chance to that stealth thing, if they really try to discover them with both active and sensitive IR sensors (and they can really detect very small intensity of IR radiation), i focus on other solutions.

I think on the equip the fighters with lasers thing, after all, they don't need all the power output of ship, just produce a big pulse to produce a beam capable to damage sensors, focusing equipment etc.

 

[Ryan_m_b]

Stealth just doesn't work in space, for reasons discussed (you can find long essays/articles on this online that go over this issue). Drones would require more mass and technology, they'd essentially have to be small unmanned nuclear space craft. But even if you carry less of them it's a question of how much more effective they are. IIRC project NERVA consisted of a small nuclear reactor capable of in excess of a hundred megawatts of power. If you take just 10% you have a potential ten megawatt beamed weapon.

 

[Ryan_m_b]

Interesting article relevant to beam weapon accuracy:

http://m.iopscience.iop.org/article...sessionid=CAFE9D2D2646BBEAF09F5A871E47EE34.c1

It reports an X-Ray free election laser with an accuracy of 1 nanoradian. Over a light second such a beam would only spread to a couple of feet in diameter if starting from a small point.

 

[Artribution]

I'm just not convinced of the "no stealth in space" claims. It seems not to account for likely developments in technology. Plasma stealth and infrared invisibility are both current areas of active research and new systems are being worked on all the time.

 

[GTOM]

I'm just not convinced of the "no stealth in space" claims. It seems not to account for likely developments in technology. Plasma stealth and infrared invisibility are both current areas of active research and new systems are being worked on all the time.

If thrusters operate it would require Mass Effect magitech to make it IR invisible. Also there is a finite limit of containing waste heat, plus heat comes from swallowing beams of active sensors.
Maybe in orbital combat, but i would exclude it in deep space.

 

[Artribution]

Thrusters don't necessarily have to operate at all after the initial launch. It depends what you're trying to do.

And besides, stealth can take other forms besides just rendering something undetectable. Stealth also encompasses decoys, sensor ghosts, ECM, etc.

 

[GTOM]

Thrusters don't necessarily have to operate at all after the initial launch. It depends what you're trying to do.

And besides, stealth can take other forms besides just rendering something undetectable. Stealth also encompasses decoys, sensor ghosts, ECM, etc.

ECM, well that is my original idea of jamming is easier than burn through armor with beam weapons, no matter how sensors are hardened. but focus X-ray this punctual...
Maybe i should really forget the epic space battle thing in this setting, and save it for FTL setting...
Laser drone fighters still rather unepic, no dodging shrapnels, no missiles closing in etc...

 

[Ryan_m_b]

I've read books that take the drone approach that do have some pretty epic space battles. The nights dawn trilogy I'd highly recommend if you're interested in that (a fantastic trilogy overall as well). It is an FTL setting so the battles do tend to be closer. The tension comes from when enemy drones burst through your own (through luck or greater numbers) so now they're careering towards the ship intent on destroying it.

 

[Artribution]

Also, the earlier estimates for missile barrage (336 independent missiles, etc.) are based on Trident missiles, the MIRVs of which are all 100-kiloton warheads (Hiroshima was 16 kilotons). If you're firing smaller, 1-kiloton tactical nuclear weapons, you wouldn't be talking about hundreds of missiles, but several thousands per barrage. I think even a network of advanced beam drones could have difficulty stopping that.

 

[GTOM]

Also, the earlier estimates for missile barrage (336 independent missiles, etc.) are based on Trident missiles, the MIRVs of which are all 100-kiloton warheads (Hiroshima was 16 kilotons). If you're firing smaller, 1-kiloton tactical nuclear weapons, you wouldn't be talking about hundreds of missiles, but thousands or tens of thousands per barrage. I think even a network of advanced beam drones could have difficulty stopping that.

If the goal is to take out the ship, with only 25km/s closing speed, one don't need any nuclear warhead. The blind sensors part is the though thing, but directed radiation is more efficient.
Ok, i will read Night's dawn, and digest ideas, thanks.
(I have already wrote a part with captain is sweating as fighter approaches and try to take it out with laser... but i sure haven't calculated with nanorad xaser...
Plan B would be stick to the idea, that no advanced high-sensitivity sensor unharmed if that x burst focused on entire ship, and continue that semi stealth with only sensors robust and sensitive as an axe from a light second...)

 

[Ryan_m_b]

Again it all depends on how far away the attackers are and how many shots off the defenders can get. The type of missiles you're talking about use chemical rocket propulsion and tend to be quite small. So thrust time is in the low minutes at best. Even if we generously assume 10gs for 10 minutes that's a top speed of 60kmps. It would take 5000 seconds to cross a light second distance. Even if you had a thousand the enemy would only have to fire once every five seconds to be safe.

The limits to missiles thrust time are very important. Small missiles means less sophisticated propulsion and less fuel so a lot less delta V. Longer range missiles are going to resemble small space ships, which brings us back to the laser drone idea.

 

[GTOM]

Otherwise the good thing about that xaser, that if they have such stuff, i could pretty much rule out using lasers against infantry, atmosphere swallows and scatters them so much (at least Earth's i don't know what is the transparency of Mars atmosphere) that it is a waste to use it, even if reactor don't run out of power, i don't think such a high energy application don't need regular, expensive maintenance.

 

[Ryan_m_b]

This might help you work it out:
http://web-docs.gsi.de/~stoe_exp/web_programs/x_ray_absorption/index.php

I'm not sure how much the atmosphere absorbs X-Ray's, if you can generate powerful focused ones I don't think it would stop it much. Using the link and assuming a 10 MJ X-Ray and 10km of air (given that the vast majority of atmospheric mass is within the first 10km) it seems like virtually all of it would hit the target.

Then again if you're in the situation of needing boots on the ground you probably care about not destroying all the infrastructure. If the fighting is inside (or underground in burried habitats) orbital support is useless.

 

[GTOM]

This might help you work it out:
http://web-docs.gsi.de/~stoe_exp/web_programs/x_ray_absorption/index.php

I'm not sure how much the atmosphere absorbs X-Ray's, if you can generate powerful focused ones I don't think it would stop it much. Using the link and assuming a 10 MJ X-Ray and 10km of air (given that the vast majority of atmospheric mass is within the first 10km) it seems like virtually all of it would hit the target.

Then again if you're in the situation of needing boots on the ground you probably care about not destroying all the infrastructure. If the fighting is inside (or underground in burried habitats) orbital support is useless.

It said 100% swallow with 100km air. Otherwise x-ray is ionizing radiation cause cancer, it is pretty sure a war crime to use it against a settlement.

 

[Ryan_m_b]

Oh yeah so it is, there was an error when I first used it. I thought the length was in meters. Yeah doesn't seem like an XFEL is much use in firing through atmosphere. For that sort of thing kinetic bombardment would be more effective.

At least it works both ways, a ground based XFEL wouldn't be able to shoot an orbiting ship.

 

[GTOM]

Ok, thank you really much, whatever solution i will take from the two i have mentioned earlier, i surely have to rewrite things.

 

[Artribution]

The limits to missiles thrust time are very important. Small missiles means less sophisticated propulsion and less fuel so a lot less delta V.

In long-range missiles (ICBMs), most of the thrust is supplied by the main rocket before the MIRVs separate, so the propulsion can actually be quite sophisticated and with high delta V.

The type of missiles you're talking about use chemical rocket propulsion and tend to be quite small. So thrust time is in the low minutes at best. Even if we generously assume 10gs for 10 minutes that's a top speed of 60kmps. It would take 5000 seconds to cross a light second distance. Even if you had a thousand the enemy would only have to fire once every five seconds to be safe.

Does a missile always use chemical rockets, and does it have to be small? Do we have to assume that every time a drone fires, it's a hit, and it hits a real MIRV and not a decoy or sensor ghost, and it's an instant burn-through and a kill?

Also, you could program the missiles to detonate when critically damaged, so that their emissions would help obscure the rest from targeting scanners.

Also, the missiles might actually be passing through a region of space not occupied by laser drones. I mean, that's possible, right? The laser drones could be elsewhere, maybe intercepting other missiles?

Maybe i should really forget the epic space battle thing in this setting, and save it for FTL setting...
Laser drone fighters still rather unepic, no dodging shrapnels, no missiles closing in etc...

If you don't want super-effective laser drones, can't you just write that they don't exist because no one's made them yet? They are kinda high-tech.

Closing argument:

 

[Ryan_m_b]

It's late here so just a few quick comments:

1) If the missiles are using more than chemical rockets (e.g a nuclear thermal drive) then they automatically become a lot bigger as they need a nuclear reactor, heat radiators etc. Suddenly you're talking about a small spaceship rather than a small missile. That knocks down the number a ship can carry by quite a bit.

1.5) You mentioned MIRVs, using that system might give you lots of missiles but it means that in the boost phase the defender can knock out 12 missiles in one shot. So even if you gave your first stage a nuclear rocket you've traded slow speed for low number of targets.

1.75) Even if you could make nuclear rockets small enough that every missile could be one the enemy could do the same but swap the warhead out for an X-Ray laser. Powered by the on-board reactor they'd be able to each shoot one enemy missile and destroy the entire threat.

2) I don't see how a decoy could work. With a reasonably good sensor and software you could work out the mass of a ship by the exhaust velocity, mass flow and the overall acceleration of the ship. To be exactly the same as a real missile a decoy would need to mass the same. Unless uranium was rare I don't see the point.

3) I don't know what a sensor ghost is (googling it brings up something from Star Trek and crackpot ghost detectors) but I don't see how any sort of ECM could make it look like a missile existed a light second away when it doesn't.

4) As for being an instant kill we're assuming a high level of accuracy based on the nanoradian XFEL linked above. Speculating that this space age civilisation could produce one capable of mega joule emission (seems reasonable given that this is a world where fusion propulsion technology exists) that's equivalent to a tank shell. It's hard to see a missile surviving that.

5) I don't see how a drone could be elsewhere to the extent it wasn't useful. Given that beam weapons effective to up to a light second are compatible with GTOMs setting you'd have to really screw up as a tactician to not have one around to act on point defence.

6) Even ignoring drones a 1-light second MJ X-Ray laser would be a potent point defence. If you could fire once every few seconds you could pick off thousands of missiles crossing a light second distance at a few tens of kmps

 

[Artribution]

1.75) Even if you could make nuclear rockets small enough that every missile could be one the enemy could do the same but swap the warhead out for an X-Ray laser. Powered by the on-board reactor they'd be able to each shoot one enemy missile and destroy the entire threat.

This is how small actual MIRVs are:

Those are W87 warheads - 475-kiloton city-destroyers, well beyond our needs.

If the missiles are using more than chemical rockets (e.g a nuclear thermal drive) then they automatically become a lot bigger as they need a nuclear reactor, heat radiators etc. Suddenly you're talking about a small spaceship massing a few tonnes rather than a small missile. That knocks down the number a ship can carry by quite a bit.

I don't know what the tonnage of interplanetary spacecraft in this story is like, but actual ICBMs weigh a lot more than that already.

2) I don't see how a decoy could work. With a reasonably good sensor and software you could work out the mass of a ship by the exhaust velocity, mass flow and the overall acceleration of the ship. To be exactly the same as a real missile a decoy would need to mass the same. Unless uranium was rare I don't see the point.

3) I don't know what a sensor ghost is (googling it brings up something from Star Trek and crackpot ghost detectors) but I don't see how any sort of ECM could make it look like a missile existed a light second away when it doesn't.

Well, using plasma to absorb and distort EM profile, varying electric and magnetic fields, just plain old-fashioned invisibility via active camouflage, clouds of chaff like real ICBMs use... It just seems that there are various possible ways of messing with sensors, and then you can just detonate some of the swarm to mask the rest in radiation, heat and light, or have them detonate automatically when damaged.

Also, I've mentioned this before, but after MIRV separation, there might not be any exhaust velocity or acceleration to measure in order to work out mass; assuming you can actually see and measure the exhaust in the first place, you would really only be measuring the mass of the discarded boost stage.

1.5) You mentioned MIRVs, using that system might give you lots of missiles but it means that in the boost phase the defender can knock out 12 missiles in one shot. So even if you gave your first stage a nuclear rocket you've traded slow speed for low number of targets.

If they're close enough to blast your missiles with lasers as they launch, doesn't that imply you're close enough to blast them back and possibly jam them?

 

[GTOM]

Ok i decided the following: at first laser fighters. If they maintain 1g acceleration they arent trivial to hit precisely from a lightsec. Maybe they arent really good against a battleship with much bigger armor and firepower ( although if they can get into right angle to attack radiators ) but good against a convoy.

Then with help of alien metal missiles will be introduced as new fearful weapon. They will be lidar invisible able to contain heat for hours in inertial phase while modify their position with thethers ( connect a squadron with cables ) and near unkillable with lasers.

 

[Ryan_m_b]

This is how small actual MIRVs are:

Those are W87 warheads - 475-kiloton city-destroyers, well beyond our needs.

I don't know what the tonnage of interplanetary spacecraft in this story is like, but actual ICBMs weigh a lot more than that already.

But on top of that you need to add a nuclear reactor, radiators and reaction mass. That requires each MIRV to be more akin in mass to Project Pluto or a Saturn C-5N.

I can't find much information on how much those would have massed (pictures of NERVA and PLUTO look a fair size bigger than those warheads) but we can use the rocket equation to figure out how good such an engine would be. A bit of googling suggests an exhaust velocity of 9kmps. Mass ratio for different delta Vs:

10kmps - 3.03
20kmps - 9.22
50kmps - 256.67
100kmps - 66910.49

So if you have a nuclear thermal MIRV massing X tonnes it will need that ratio of fuel above for those speeds. Not going anywhere fast it doesn't seem, unless GTOM is allowing really small fusion drives in his setting. Even if you could accelerate one up to 100kmps it would take 50 minutes to cross a light second.

Well, using plasma to absorb and distort EM profile, varying electric and magnetic fields, just plain old-fashioned invisibility via active camouflage, clouds of chaff like real ICBMs use... It just seems that there are various possible ways of messing with sensors, and then you can just detonate some of the swarm to mask the rest in radiation, heat and light, or have them detonate automatically when damaged.

All of those are going to make your missiles shine like beacons in infra red. If a bomb detonates and over-exposes a sensor then presumably it's going to be back to working fine a second later when the explosion is over.

Also, I've mentioned this before, but after MIRV separation, there might not be any exhaust velocity or acceleration to measure in order to work out mass; assuming you can actually see and measure the exhaust in the first place, you would really only be measuring the mass of the discarded boost stage.

As I've mentioned before even if you shut down everything and run very cold your previous speed and heading is likely to be known (unless we're assuming that military space ships won't have simple technology like IR sensors and a tracking computer). So even if you were completely invisible you'd be as easy to shoot as a fish in a barrel.

If they're close enough to blast your missiles with lasers as they launch, doesn't that imply you're close enough to blast them back and possibly jam them?

That was part of my point but even if they weren't close enough unless they have very good engines they're going to crawl across interplanetary space. I'm not sure why the defender (presumably with a fusion drive as GTOM has indicated) wouldn't just out maneuver them and pick them off from a distance with beam weapons.

 

[Ryan_m_b]

Ok i decided the following: at first laser fighters. If they maintain 1g acceleration they arent trivial to hit precisely from a lightsec. Maybe they arent really good against a battleship with much bigger armor and firepower ( although if they can get into right angle to attack radiators ) but good against a convoy.

Depends how many there are. If you assume a squadron of ten each capable of putting out a MJ beam then every salvo from them is the equivalent of 5-10 tank shells. Even with a slow rate of fire it seems like it could add up to a lot of damage, especially if they hit thrusters to limit the battleships ability to dodge.

Then with help of alien metal missiles will be introduced as new fearful weapon. They will be lidar invisible able to contain heat for hours in inertial phase while modify their position with thethers ( connect a squadron with cables ) and near unkillable with lasers.

Well fair enough. It's your story if you want to take it down that route. The Polity novels had a similar technology, an alien race could produce this exotic metal armour that would absorb massive amounts of energy and re-radiate it out into space. Don't think such a thing is physically plausible based on what we know now but it is SF after all.

 

[Artribution]

I can't find much information on how much those would have massed (pictures of NERVA and PLUTO look a fair size bigger than those warheads) but we can use the rocket equation to figure out how good such an engine would be. A bit of googling suggests an exhaust velocity of 9kmps. Mass ratio for different delta Vs:

10kmps - 3.03
20kmps - 9.22
50kmps - 256.67
100kmps - 66910.49

So if you have a nuclear thermal MIRV massing X tonnes it will need that ratio of fuel above for those speeds. Not going anywhere fast it doesn't seem, unless GTOM is allowing really small fusion drives in his setting. Even if you could accelerate one up to 100kmps it would take 50 minutes to cross a light second.

I wonder if smaller rocket-propelled missiles could actually be launched from a railgun for higher acceleration and to save on fuel and mass. But we're kind of talking in circles here—speed only matters if you don't think stealth or countermeasures could protect them.

If a bomb detonates and over-exposes a sensor then presumably it's going to be back to working fine a second later when the explosion is over.

Can we be sure of that? Thermonuclear weapons can be very versatile. The blast characteristics and emissions can be modified in all sorts of ways. They can be "enhanced radiation weapons" like neutron bombs, throwing out specific kinds of emissions and particles. They can be nuclear shaped charges, which (instead of a spherical blast) direct a jet or cone-shaped blast in a desired direction. If designed for it, they could even channel their blasts into a tight beam like a kind of x-ray or gamma-ray laser. I don't even think it would be very difficult to design a missile with an outer casing that could adjust itself in-flight to alter the shape and emission profile of its blast.

As I've mentioned before even if you shut down everything and run very cold your previous speed and heading is likely to be known (unless we're assuming that military space ships won't have simple technology like IR sensors and a tracking computer). So even if you were completely invisible you'd be as easy to shoot as a fish in a barrel.

If your missile launch is observed by an enemy capable of attacking it with lasers, then you're also close enough to attack or jam that enemy with your own lasers, and therefore, you can't assume the enemy would be aware of the speed and heading of your missiles. Also, once the MIRVs separate from the launch stage, that data is no longer applicable. The MIRVs could have rockets of their own, or just RCS thrusters. And if they're only eight inches long (tactical nuclear warheads are) and traveling at relatively high velocity, and making random minor course adjustments, hitting them with a laser at a distance of a light-second would seem to pose a challenge even for an advanced computer, and that's all assuming that none of their stealth and countermeasures work at all.

 

[Ryan_m_b]

I wonder if smaller rocket-propelled missiles could actually be launched from a railgun for higher acceleration and to save on fuel and mass. But we're kind of talking in circles here—speed only matters if you don't think stealth or countermeasures could protect them.

Quite.

Can we be sure of that? Thermonuclear weapons can be very versatile. The blast characteristics and emissions can be modified in all sorts of ways. They can be "enhanced radiation weapons" like neutron bombs, throwing out specific kinds of emissions and particles. They can be nuclear shaped charges, which (instead of a spherical blast) direct a jet or cone-shaped blast in a desired direction. If designed for it, they could even channel their blasts into a tight beam like a kind of x-ray or gamma-ray laser. I don't even think it would be very difficult to design a missile with an outer casing that could adjust itself in-flight to alter the shape and emission profile of its blast.

Unless you're pretty close to the enemy I don't see how any of that matters.

If your missile launch is observed by an enemy capable of attacking it with lasers, then you're also close enough to attack or jam that enemy with your own lasers, and therefore, you can't assume the enemy would be aware of the speed and heading of your missiles. Also, once the MIRVs separate from the launch stage, that data is no longer applicable. The MIRVs could have rockets of their own, or just RCS thrusters. And if they're only eight inches long (tactical nuclear warheads are) and traveling at relatively high velocity, and making random minor course adjustments, hitting them with a laser at a distance of a light-second would seem to pose a challenge even for an advanced computer, and that's all assuming that none of their stealth and countermeasures work at all.

An enemy could be extremely far away and observe your missiles. There's no line of sight in space, detection depends on the size of your sensors and the wavelength being observed. Not only that but sensors across a ship and between different ships can act as interferometers forming extremely capable long range sensors. If we're positing a world where space combat is a part of life then you can bet that sensors will constantly be scanning the whole sky, tracking as many ships in the system as possible and looking for things like a MIRV launch.

MIRVs separating presumably requires some form of power, that means there's a source of heat on the missile that can be detected. If their making random course corrections there's also a source of power and an exhaust emission. You mention traveling fast but unless they're traveling extremely fast crossing a distance like a light second is going to take on the order of hours. Plenty of time to shoot down with long range beam weapons or simply fly away if you're in a ship.