Molten Ferrometal as a Penetrator in Space Combat?

[Lord of the Abyss]

Hey all, was hoping I could get some input into the feasibility of this idea. I read in an Arthur C. Clarke book (I think) about a neat idea for a weapon. Some kind of coilgun would fire a thin jet of molten ferrometal at tens of km/s.

I recall that; the book is Earthlight. Clarke came up with the idea in part as a joke; the character who saw it was baffled because it looked just like a sci-fi energy weapon, a glowing beam. This made no sense since in the real world lasers and such aren't visible in space.

Wheeler saw it strike upward, a solid bar of light stabbing at the stars. He remembered the rumors that had gone around the Observatory. So this was what had been seen, flashing above the mountains. He did not have time to reflect on the staggering violation of the laws of optics which this phenomenon implied, for he was staring at the ruined ship above his head. The beam had gone through Lethe as if she did not exist; the fortress had speared her as an entomologist pierces a butterfly with a pin.

Clarke himself considered it impossible, since molten metals were thought to essentially lose their magnetism; however, science marches on and DARPA is presently working on a real world version. MAHEM, aka "Magneto Hydrodynamic Explosive Munition" - I'm sure they enjoyed coming up with that acronym. A mention here.

[Simon_Jester]

What happens if two relativistic kinetic kill vehicles collide with each other?


Like a 30 kg projectile traveling at 80% the speed of light hits another of the same size and speed headon?

Isnt that technically an FTL impact?

Relativity does not work that way. You can look up the equations; do the algebra yourself.

Is that the next study equivalent of the LHC once we can make RKKVs?

The LHC already collides protons moving at around, what is it... [maths] 7 TeV divided by roughly 1 GeV mass per proton, gamma equals seven thousand... what,

99.9999% of the speed of light?

Probably faster. Never mind.

[Guardsman Bass]

1) Ballistic weapons are short-ranged by nature. In HARD HARD SF they're probably limited to muzzle velocities in the single-digit km/s range; going higher as the SF gets softer won't change much because all the other weapons have their basic design limitations relaxed and (correspondingly) range increased.

The problem is that they can be dodged. Even if they're invisible it doesn't matter, because the enemy can just burn their engines at random to sidestep a shot they didn't see coming.

That makes detection even more important. Launching these kinds of kinetics at an enemy spacecraft can be nasty without even having to hit - just forcing the enemy ships to spend fuel maneuvering out of the way can eventually force the enemy ship (or fleet) to either absorb the impacts to get to their destination, or fire up what fuel they have left to go into a different trajectory (essentially conceding defeat).

2) 'Energy' weapons (lasers of varying frequency and charged-particle beams) are medium to long ranged. Unless you fight at light-second or longer ranges and have accelerations in the 10g-100g range, they're essentially undodgeable. Defense revolves around electronic warfare (trick the enemy into shooting where you aren't), passive defense (armor and magic-shields), and of course getting the other bastard before he gets you. Being dodgy only helps a tiny bit in that it keeps the enemy from focusing a beam on the same spot on your hull for very long- which makes a burnthrough less likely.

The limit on energy weapons' effective range comes from dispersion. Both lasers and particle beams are vulnerable to this, especially with realistic-sized hardware that can fit on a ship of hundreds of tons.

I figure it will be lasers, since particle beams are slower and require additional mass. The main limitations will probably be waste heat off the laser, plus damage to the laser lens on your ship from enemy lasers. Power limitations are going to be a big deal here, too - your ship will either be nuclear (with the resultant heat and need for massive radiator panels if it's a high-powered reactor, on top of radiating away the heat off the laser), or require huge solar arrays.

3) Guided missiles have nigh-unlimited range. With the right engine, you can even do something sadistic like:
-Launch missiles at Mars, through a fixed mass driver orbiting Earth
-Wait six months for them to coast to Mars, ballistically.
-Have them fire up their engines and independently seek out and destroy whatever targets you want exploded.

If the Martians are expecting this, then they could probably spot your missiles with a combination of telescopes and maybe even tight-beam radar on the major Earth-Mars trajectories. That said, missiles probably are the best weapons out of the bunch, because of their own (limited) engines and maneuverability. I'd expect most space warships not specifically designed as laser weapon platforms to carry them.

Conversely, lasers are easily redirected IF you can put a mirror in the path of the beam without frying it. This is easier said than done with a powerful weaponized laser- should work, but it's not something that can be totally ignored.

I doubt you could find a mirror reflective enough to do that. Even if all but a fraction of the light is reflected away, that fraction will heat up the mirror and cause it to melt/shatter at the energy levels we're talking about.

[Simon_Jester]

1) Ballistic weapons are short-ranged by nature. In HARD HARD SF they're probably limited to muzzle velocities in the single-digit km/s range; going higher as the SF gets softer won't change much because all the other weapons have their basic design limitations relaxed and (correspondingly) range increased.

The problem is that they can be dodged. Even if they're invisible it doesn't matter, because the enemy can just burn their engines at random to sidestep a shot they didn't see coming.

That makes detection even more important. Launching these kinds of kinetics at an enemy spacecraft can be nasty without even having to hit - just forcing the enemy ships to spend fuel maneuvering out of the way can eventually force the enemy ship (or fleet) to either absorb the impacts to get to their destination, or fire up what fuel they have left to go into a different trajectory (essentially conceding defeat).

True, but the enemy can do the same thing to you just as well. It doesn't so much 'allow you to win' as it does 'provide another victory condition.' And running out of fuel during combat has been a real concern for fighting units ever since World War One, if not earlier.

Long range detection of ballistic weapons and tracking them accurately enough to engage with point defense is really kind of problematic. It's essentially the same problem as ballistic missile defense, but the targets are smaller than ICBMs, and they aren't coming in from a single obvious direction that you can point huge fixed ten thousand ton radar installations at.

I figure it [energy weapons] will be lasers, since particle beams are slower and require additional mass.

How do you mean "require additional mass?"

Particle beams have a number of advantages- the big one being that they can accomplish something through an armor plate, even a plate that's too thick/tough/refrigerated to physically melt their way through. Spallation radiation, bremstrahllung, and just plain "not enough tenth-value-thicknesses in the way" can make particle beam weaponse effective in that respect.

If the Martians are expecting this, then they could probably spot your missiles with a combination of telescopes and maybe even tight-beam radar on the major Earth-Mars trajectories. That said, missiles probably are the best weapons out of the bunch, because of their own (limited) engines and maneuverability. I'd expect most space warships not specifically designed as laser weapon platforms to carry them.

Spotting a long range missile attack like that is certainly possible, but a lot easier said than done- could we reliably spot a black-painted object the size of a cruise missile as it coasted between planets?

I doubt you could find a mirror reflective enough to do that. Even if all but a fraction of the light is reflected away, that fraction will heat up the mirror and cause it to melt/shatter at the energy levels we're talking about.

In that case particle beams have a big advantage in steerability, at least assuming you can find a generation of superconductors that get you up to where refrigeration is enough and you no longer need active cryogenics like liquid nitrogen. Dry-ice temperature superconductors are not an unreasonable thing to posit in SF, if you ask me...

[mrtspence]

It's my understanding that mirror armour is entirely impossible. Aside from the logistical issues (keeping it clean/intact), the physics just don't work.

I think it would be pretty easy to spot a cruise missile. The thing being looked for is the heat and light emitted by its engine--which will stand out against the cold black of space no matter how the missile is painted!

[Sea Skimmer]

No reason the engine has to run the whole flight. Get up to cruise velocity, shut it down, discharge some coolant bottles to stabilize temperature and then coast between the planets. It wont be impossible to see but it wont be easy at distances.

The real problem with long range missile attacks is finding the target at the end. No actual reason exists why planetary defenses have to be static. You could build a giant radar array out of multistatic airships for example, fire interceptor missiles from ships ect. The faster the incoming missile travels the harder target acquisition and course adjustment becomes to the point it will be near impossible at fractions of light speed.

[mrtspence]

Missiles seem like amazing weapons as long as the target has no point defense. They seem to get prohibitively expensive and heavy as the target gets more point defense abilities (need either more missiles or to make your one missile more armoured). Using a few small, cheap kinetic-warhead missiles to scrap big enemy missiles seems like a good investment while just having armour would make smaller missiles less of a threat. A missile trying to hit your ship is going to be maneuvering (glowing like a sun-hot lightbulb) and would be pretty damned easy to detect with any future-tech infrared/light sensors.

Beam weapons seem incredibly deadly at close ranges (you could carve your name into their hull and dot the "i"s with little hearts if you wanted to--their sensors/gunports/squishy bits are as good as instantly gone). They get a lot worse over ranges where the beam spreads and loses its punch and where the target can maneuver to make it so that your beam cannot get through the armour quickly. Unless you are using an X-ray laser or something with a ludicrously short wavelength and a ludicrously big lens, there would be significant weakening over distance.

Lasers seem pretty inefficient. Coilguns and missiles are really power-efficient (coilguns have theoretical efficiencies of like 90+%) but lasers end up requiring a ton of juice (probably not an issue on a fusion-powered ship) and a ton of heat dissipation (definitely an issue on a fusion-powered ship). A powerful laser is going to have a gigantic lens and require radiators square kilometers in area. That lens is also going to be really, really expensive.

I don't see particle beams as a great weapon. A few feet of (relatively) light paraffin wax just over the crew compartments or sensitive electronics would, in combination with the armour that would likely already be present on a warship, make a particle beam pretty weak. They would also have insanely short ranges. Even neutral particle beams have trouble staying cohesive. They would also suffer from a lot of the inefficiency problems that a laser would. A laser seems like a way deadlier and more effective weapon.

Kinetic weapons don't have to worry about distance as far as hitting hard goes--just hitting in the first place. They would obviously get less accurate over distances. However, the target is probably moving really fast and would likely not be able to maneuver well, so it may not actually be that hard to hit them with a few shots covering its potential locations. It would entirely depend on the target.

As they are significantly smaller than missiles and, more importantly, don't have engines, it could be very hard to detect and successfully use point defense against kinetic projectiles. If shot from a really big coilgun, the projectiles could easily be moving faster than the most beefy of missiles. Coilguns are really goddamned heavy though, as they require a lot of bracing and huge superconducting electromagnets.

[Formless]

Lasers seem pretty inefficient. Coilguns and missiles are really power-efficient (coilguns have theoretical efficiencies of like 90+%) but lasers end up requiring a ton of juice (probably not an issue on a fusion-powered ship) and a ton of heat dissipation (definitely an issue on a fusion-powered ship). A powerful laser is going to have a gigantic lens and require radiators square kilometers in area. That lens is also going to be really, really expensive.

1) Fusion power? Ha! Not likely. Fission is the most energetic a power source you can realistically forsee, because of the conditions needed to undergo fusion makes it unlikely we will ever harness it as a fuel source (discounting solar energy, natch). Plus, we already know that it can be miniaturized-- the Soviets managed to create fission reactors as small as a kitchen table for powering submarines.

2) what makes you think that laser lenses are in any way expensive, on the scale of things? Ship to ship lasers aren't going to be spitting out X-rays, that kind of thing is only feasible for defending planets. The military can already mount (chemical) lasers on a Boeing 747 and shoot down freaking mortar shells with it. A laser satellite with a similar weapon could be as small as 20 tons-- that sounds large, but its no larger than a modern tank. It wouldn't get to shoot indefinitely, of course, but it wouldn't need to to preform its mission. And as a disposable drone, you wouldn't care about things like mission endurance from it.

3) how are you getting "square kilometer" large radiators? Care to show us your math?

[mrtspence]

I think we have totally different ideas of the scale of combat haha! We are currently making a fusion reactor that will get us 10 times the power out as we put in, and that efficiency will only go up as we figure things out. Should be done in like 2020. So that isn't even futuretech really. http://en.wikipedia.org/wiki/ITER

Fission engines do work of course! They are just less powerful and have the obvious risk of combat damage causing a runaway reaction. Powering your 20-ton satellite with one seems like a great idea and would probably work well.

As an important note, that boeing laser works well against missiles because there is atmosphere to wreck the chassis once it gets fouled. The laser barely has to do much more than warp a fin a little bit and the missile will fail. In space, at space-speeds, a laser like that would be much less effective.

A fusion reactor would obviously have some minimum size issues, so a ship using one would have to be really big, certainly on the hundreds-to-thousands-of-meters scale. Once we get that big, armour scales up in a wonderfully linear fashion.

I agree that X-ray lasers are impractical. Was using them as a statement of hyperbole to illustrate that there are range issues with lasers!

My math is based on this super-awesome online calculator: http://www.5596.org/cgi-bin/laser.php

The size of radiators I was referring to would be for a laser weapon designed to kill a big ship. It obviously slides around a lot.

Let's say you want a 10GW/s laser to really do some damage. A 20% efficiency would be pretty good, which means that you are putting 2GW/s of energy downrange at the target and putting 8GW/s of heat into your own ship. Assuming you are firing a laser in the middle of the visible light spectrum with a 10m radius lens, you could melt through an impressive 8m of carbon armour at 1 million meters in one freaking second! The effectiveness is about 1/8th of that at twice the distance and 1/16th of that at four times the distance. Your effectiveness diminishes really rapidly as the target gets further away. If the target is rotating or using armour with better thermal tolerance, it also gets WAY less effective.

To dissipate that heat using a really efficient droplet radiator (calculator here: http://www.5596.org/cgi-bin/dropletradiator.php) would take an area of around 310 square meters. A solid radiator would probably be less efficient. And this isn't even taking into account the heat generated by the ship's engine, which is going to be at the very least (1/(efficiency of the engine))*(10GW)! So the total area needed would likely be many times that, easily into the square km range.

As the mirror/lens has a radius of 10m, it probably won't suffer any damage from firing and you could use some tempered glass. As it gets smaller, the material you make the mirror out of needs to have better thermal tolerance or it will melt or crack when the laser is fired. Things like diamond. A few square meters of ultra-polished, super-transparent diamond is going to be expensive!

I'm currently on the train and don't have access to a good place to write my math. If you don't trust those guys who made those calculators (who are probably better at math than me), then I can get around to writing some equations later. Planck's law isn't too bad, but I'm rusty so the results might be mixed. I wish I could come up with a good rule of thumb, so I will maybe look into simplifying an equation that could get us a convenient way of expressing the area of radiator needed if it is a perfect blackbody at a set temperature with a given efficiency per unit of power for the laser.

[Sea Skimmer]

The Airborne Laser was a three megawatt weapon, it didn't need air pressure to finish off destroying the target. In space unless the enemy is under no thrust, and thus a sitting duck, his ship is still subject to tremendous loads that can cause structural breakup from structural damage. How important this is depends on how big, strong and fast of a space warship someone is going to build.

Your assumption that you must constantly radiate all waste heat is a bit false, any space going warship will have a huge tank of propellent it can use as a short term heat sink. This is also how hardened military facilities, including some rather large radar facilities, deal with waste heat when buttoned up for nuclear attack for example, though with a large underground water tank. I suspect a tank of hydrogen slush could soak up some really serious heat before you had to start venting any to bleed off pressure. But since you'll likely have some empty propellent tanks, from having expended propellent to get to the battle, you could just dump the expansion into the empty tanks and worry about cooling it back down later. No propellent lost until things get really really hot, and more or less turning your propellent storage into a form of giant heat pipe.

Cost for a space going warship is relative. The things are going to be absurdly expensive no matter what you do. I doubt even diamond laser optics would stop anyone.

Bomb pumped X-ray lasers make some real sense as missile warheads as a side note, if you could ever make one work. Good way to try to solve the problem of a missile being useful for firing at very long ranges, but kind of hopeless against terminal defenses. Being able to point the warhead before firing it also means your missile could be effective without actually being on a collision course with the target, it just has to get within a certain range.

[MrDakka]

Curses Sea Skimmer, you beat me to the punch. I was going to suggest bomb pumped lasers along with the Casaba Howitzer as missile warheads, although I'm not too sure about the Casaba's range, since from what I understand its essentially a nuclear shaped charge (is the resultant jet actual plasma? hyperplasticized metal? something in between?)

[mrtspence]

The Airborne Laser was a three megawatt weapon, it didn't need air pressure to finish off destroying the target. In space unless the enemy is under no thrust, and thus a sitting duck, his ship is still subject to tremendous loads that can cause structural breakup from structural damage. How important this is depends on how big, strong and fast of a space warship someone is going to build.

Your assumption that you must constantly radiate all waste heat is a bit false, any space going warship will have a huge tank of propellent it can use as a short term heat sink. This is also how hardened military facilities, including some rather large radar facilities, deal with waste heat when buttoned up for nuclear attack for example, though with a large underground water tank. I suspect a tank of hydrogen slush could soak up some really serious heat before you had to start venting any to bleed off pressure. But since you'll likely have some empty propellent tanks, from having expended propellent to get to the battle, you could just dump the expansion into the empty tanks and worry about cooling it back down later. No propellent lost until things get really really hot, and more or less turning your propellent storage into a form of giant heat pipe.

Cost for a space going warship is relative. The things are going to be absurdly expensive no matter what you do. I doubt even diamond laser optics would stop anyone.

Bomb pumped X-ray lasers make some real sense as missile warheads as a side note, if you could ever make one work. Good way to try to solve the problem of a missile being useful for firing at very long ranges, but kind of hopeless against terminal defenses. Being able to point the warhead before firing it also means your missile could be effective without actually being on a collision course with the target, it just has to get within a certain range.

3MW at a probably low-ish efficiency using an infrared frequency (it was infrared, IIRC) with a 1.5m-ish lens isn't going to do much to an armoured craft at distance--especially at space-fighting distances. I am not doubting that it could do more than melt a fin on a missile, but that point was to illustrate how much harder to kill a spacecraft is than one in an atmosphere. The boeing laser could easily take out a missile by warping a fin. The forces a spacecraft would be under (assuming it isn't in a gravity well in a serious way) would probably be between 0 and a few gees if it had a human crew (more if a drone). That isn't totally insignificant, but the air resistance is a huge factor. I think a laser meant to defeat armoured targets would need to be much more powerful. I totally agree that it is all dependent on the craft though!

You are right to assert that a craft doesn't have to be constantly radiating. Internal heatsinks would definitely be desirable in combat. Using propellant, although not optimal, would likely work just fine. Definitely a cool idea. The radiators would have to come out pretty quickly though if firing a big laser for long enough! It is worth noting that storing heat like this is way less effective for spacecraft than installations in an atmosphere (or an asteroid or something).

Costs absolutely are relative haha! I imagine the fusion engine on a large spacecraft would be astronomically expensive. A giant death ray would be well within the acceptable margins for a ship like that! I imagine all the superconductors for a km-long coilgun would be in a similar ballpark. As would the high-performance engines and warheads for missiles.

Bomb-pumped lasers are insanely cool. Could have the punch of a big laser without worrying about powerplants or heat. Their only downsides (other than the obvious 1-shot thing, which isn't so big a deal) would be that of any other big missile (vulnerable to point-defense). A cool way to get around that would be shooting one out of a railgun or coilgun. Would eliminate the need for tell-tale engine burns and make it more likely to get in range. Downside to that would be needing a gigantic mass driver.

Casaba howitzers would be roughly comparable to shaped-charged high explosives. More range and more focussed but not a totally different ballpark. I doubt there would be much of a jet of anything other than plasma after a thermonuclear explosion, so the range wouldn't be that big (as far as space is concerned).

[Simon_Jester]

Spence, bear in mind that there's a big difference between a missile that needs to get within 5 km to kill you (i.e., a largish nuclear warhead in space with a spherical burst against an armored target) and one that needs to get within, say, 500 km (because all the energy of the same warhead is being focused into a cone about 30 degrees wide and pointed straight down your throat).

Among other things, such a missile can do cunning tricks like "pretend to miss due to your BRILLIANT ELECTRONIC WARFARES, get ignored by your point defense, then pivot and shoot you in the flank." Or at least present point defense with a crossing target that is a whole new dimension in "hard to hit."


Hey Skimmer, is there a practical difference between "bomb pumped X-ray laser" and "nuclear shaped charge?" As far as I can tell they're different symbols with the same referent...

It's my understanding that mirror armour is entirely impossible. Aside from the logistical issues (keeping it clean/intact), the physics just don't work.

I think it would be pretty easy to spot a cruise missile. The thing being looked for is the heat and light emitted by its engine--which will stand out against the cold black of space no matter how the missile is painted!

You misunderstood. It is coasting; it's got no visible engine flare. The idea is to launch (using a first-stage rocket or a static mass driver) then let the missile coast a huge distance, then fire up its engine for maneuvers to steer it onto a target. It'll have to spot the target independently of course, which is an obstacle, but one we're already learning to deal with in the present day. Missile guidance with 2100-level hardware and software is going to be preeeetty fucking good, I suspect.

The real problem with long range missile attacks is finding the target at the end. No actual reason exists why planetary defenses have to be static. You could build a giant radar array out of multistatic airships for example, fire interceptor missiles from ships ect. The faster the incoming missile travels the harder target acquisition and course adjustment becomes to the point it will be near impossible at fractions of light speed.

I'm thinking in terms of closing speeds of ~3-30 km/s, plausible for 'hard SF' technology from a reasonable launcher (either a boost stage or an orbiting mass driver). And yes, planetary defenses can be made mobile. But there will be enough significant targets which are fixed (i.e. industrial complexes and big honking space stations) that this would be an effective attack under the right conditions.

And I'm not saying it's a beat-all tactic, just that it's conceivable and that thinking about it is good because it stretches our sense of the possible.

Missiles seem like amazing weapons as long as the target has no point defense. They seem to get prohibitively expensive and heavy as the target gets more point defense abilities...

Beam weapons seem incredibly deadly at close ranges... They get a lot worse over ranges where the beam spreads and loses its punch and where the target can maneuver to make it so that your beam cannot get through the armour quickly...

These are real limitations I consider in my own estimates. The main problem is that they can be eternally handwaved or argued back and forth- you can't say "missile beats laser," because which is better depends on the quality of undefined missile, laser, and defensive technologies.

I don't see particle beams as a great weapon. A few feet of (relatively) light paraffin wax just over the crew compartments or sensitive electronics would, in combination with the armour that would likely already be present on a warship, make a particle beam pretty weak.

A sufficiently powerful beam weapon could just melt off the wax. We're not talking 'glorified cosmic rays' here.

They would also have insanely short ranges. Even neutral particle beams have trouble staying cohesive. They would also suffer from a lot of the inefficiency problems that a laser would. A laser seems like a way deadlier and more effective weapon.

Depends. Particle beam cohesion is calculable- I'm too lazy to do the calculations but know the equations. A lot depends on the parameters of the weapon itself- you can MAKE things work if you make certain assumptions, but not without them.

Kinetic weapons don't have to worry about distance as far as hitting hard goes--just hitting in the first place. They would obviously get less accurate over distances. However, the target is probably moving really fast and would likely not be able to maneuver well, so it may not actually be that hard to hit them with a few shots covering its potential locations. It would entirely depend on the target.

Target velocity is irrelevant. Target acceleration matters. The real problem is... how do I explain?

Assume a ship with a given target profile- width X. Assume that it has 'lateral' acceleration A: that it can start accelerating at right angles to your line of fire at that acceleration. Now let it start jiggling around at random. How long will it take to sidestep by a distance equal to, say, four times its own width?

If the target can dodge by four times its own length in the time it takes your bullets to arrive, your odds of hitting with one shot are pretty slim. Because if it's dodging randomly, when your bullet arrives it could be ANYWHERE in a circle (or ellipse) that has an area sixteen times the size of the ship's own target area. Even with perfect information on its current position, and perfect aim you have a 1/16 chance of hitting.

Now, double the range. You've just doubled the time the target has to dodge, given fixed muzzle velocity. But since x=(1/2)*a*t^2 for constant acceleration, doubling the dodge time means the ship can travel four times as far on a fixed acceleration. The radius of that circle just got four times larger... so its area got 16 times larger. Now the target could be anywhere in a circle 256 times larger than its own area. Still assuming you have perfect vision and marksmanship, your chance of hitting just dropped to 1 in 256. Another doubling of the range and it drops to 1 in 4096- you're probably better off not even bothering to shoot.

The exact effective range obviously depends on the target's size, your muzzle velocity, and its acceleration. But you can see that this effect is very unfavorable. Basically, the number of shots you have to fire to score one hit increases with the fourth power of the range to the target, once you get far enough away that evasion is a real factor... however far that is.

[mrtspence]

Spence, bear in mind that there's a big difference between a missile that needs to get within 5 km to kill you (i.e., a largish nuclear warhead in space with a spherical burst against an armored target) and one that needs to get within, say, 500 km (because all the energy of the same warhead is being focused into a cone about 30 degrees wide and pointed straight down your throat).

Among other things, such a missile can do cunning tricks like "pretend to miss due to your BRILLIANT ELECTRONIC WARFARES, get ignored by your point defense, then pivot and shoot you in the flank." Or at least present point defense with a crossing target that is a whole new dimension in "hard to hit."


Hey Skimmer, is there a practical difference between "bomb pumped X-ray laser" and "nuclear shaped charge?" As far as I can tell they're different symbols with the same referent...

It's my understanding that mirror armour is entirely impossible. Aside from the logistical issues (keeping it clean/intact), the physics just don't work.

I think it would be pretty easy to spot a cruise missile. The thing being looked for is the heat and light emitted by its engine--which will stand out against the cold black of space no matter how the missile is painted!

You misunderstood. It is coasting; it's got no visible engine flare. The idea is to launch (using a first-stage rocket or a static mass driver) then let the missile coast a huge distance, then fire up its engine for maneuvers to steer it onto a target. It'll have to spot the target independently of course, which is an obstacle, but one we're already learning to deal with in the present day. Missile guidance with 2100-level hardware and software is going to be preeeetty fucking good, I suspect.

The real problem with long range missile attacks is finding the target at the end. No actual reason exists why planetary defenses have to be static. You could build a giant radar array out of multistatic airships for example, fire interceptor missiles from ships ect. The faster the incoming missile travels the harder target acquisition and course adjustment becomes to the point it will be near impossible at fractions of light speed.

I'm thinking in terms of closing speeds of ~3-30 km/s, plausible for 'hard SF' technology from a reasonable launcher (either a boost stage or an orbiting mass driver). And yes, planetary defenses can be made mobile. But there will be enough significant targets which are fixed (i.e. industrial complexes and big honking space stations) that this would be an effective attack under the right conditions.

And I'm not saying it's a beat-all tactic, just that it's conceivable and that thinking about it is good because it stretches our sense of the possible.

Missiles seem like amazing weapons as long as the target has no point defense. They seem to get prohibitively expensive and heavy as the target gets more point defense abilities...

Beam weapons seem incredibly deadly at close ranges... They get a lot worse over ranges where the beam spreads and loses its punch and where the target can maneuver to make it so that your beam cannot get through the armour quickly...

These are real limitations I consider in my own estimates. The main problem is that they can be eternally handwaved or argued back and forth- you can't say "missile beats laser," because which is better depends on the quality of undefined missile, laser, and defensive technologies.

I don't see particle beams as a great weapon. A few feet of (relatively) light paraffin wax just over the crew compartments or sensitive electronics would, in combination with the armour that would likely already be present on a warship, make a particle beam pretty weak.

A sufficiently powerful beam weapon could just melt off the wax. We're not talking 'glorified cosmic rays' here.

They would also have insanely short ranges. Even neutral particle beams have trouble staying cohesive. They would also suffer from a lot of the inefficiency problems that a laser would. A laser seems like a way deadlier and more effective weapon.

Depends. Particle beam cohesion is calculable- I'm too lazy to do the calculations but know the equations. A lot depends on the parameters of the weapon itself- you can MAKE things work if you make certain assumptions, but not without them.

Kinetic weapons don't have to worry about distance as far as hitting hard goes--just hitting in the first place. They would obviously get less accurate over distances. However, the target is probably moving really fast and would likely not be able to maneuver well, so it may not actually be that hard to hit them with a few shots covering its potential locations. It would entirely depend on the target.

Target velocity is irrelevant. Target acceleration matters. The real problem is... how do I explain?

Assume a ship with a given target profile- width X. Assume that it has 'lateral' acceleration A: that it can start accelerating at right angles to your line of fire at that acceleration. Now let it start jiggling around at random. How long will it take to sidestep by a distance equal to, say, four times its own width?

If the target can dodge by four times its own length in the time it takes your bullets to arrive, your odds of hitting with one shot are pretty slim. Because if it's dodging randomly, when your bullet arrives it could be ANYWHERE in a circle (or ellipse) that has an area sixteen times the size of the ship's own target area. Even with perfect information on its current position, and perfect aim you have a 1/16 chance of hitting.

Now, double the range. You've just doubled the time the target has to dodge, given fixed muzzle velocity. But since x=(1/2)*a*t^2 for constant acceleration, doubling the dodge time means the ship can travel four times as far on a fixed acceleration. The radius of that circle just got four times larger... so its area got 16 times larger. Now the target could be anywhere in a circle 256 times larger than its own area. Still assuming you have perfect vision and marksmanship, your chance of hitting just dropped to 1 in 256. Another doubling of the range and it drops to 1 in 4096- you're probably better off not even bothering to shoot.

The exact effective range obviously depends on the target's size, your muzzle velocity, and its acceleration. But you can see that this effect is very unfavorable. Basically, the number of shots you have to fire to score one hit increases with the fourth power of the range to the target, once you get far enough away that evasion is a real factor... however far that is.

Ya I think the idea of kinetic weaponry would be to totally saturate an area. Coilguns/railguns, if fed enough juice, could get some seriously ridiculous rates of fire.

And I do understand the distinction between acceleration and velocity in this context, my usage of the word velocity was incorrect, so I'm sorry I prompted you to write all of that xD. That was my bad. My point is that, relative to the main engine, I don't think maneuvering thrusters would do very much for a big ship. The main evasion would probably come from either accelerating more or cutting acceleration (giving more of a line-shaped potential profile as opposed to the circular one you would see from a craft with relatively stronger lateral thrusters). It would be a matter of hosing down an increasingly small area as you get closer until their craft is disabled. But I am in total agreement that kinetics are the most evade-able of probable weapons.

My point about missiles being visible is that any serious maneuvering thrust would be detectable from across our solar system. If we can check on and track asteroids in the asteroid belt that are a hell of a lot colder and darker than a missile right now, we could definitely track a missile--if the sensors were pointed the right way. Definitely wouldn't be fool proof though!

My envisioning of a bombardment would be ships billions of kilometers away accelerating and firing kinetics/missiles at stationary targets (they would have already plotted all of the relevant coordinates--orbital platforms/space stations/bunkers/whatever else that can't move). A few days/weeks later, the targets are annihilated from out of nowhere.

I wasn't meaning to say that a nuclear shaped charge didn't have relatively much more range than a conventional nuke, just that 500km is insanely ridiculously close by space-weapon standards. The difference in range is insignificant when compared to other types of weapons.

The idea behind a bomb-pumped laser is that, before the laser housing is annihilated, the device gives it enough juice for a powerful laser shot. So the bomb-pumped laser has way more range and is actually doing the damage with the laser beam itself. A shaped charge uses the shaped blast of the nuclear weapon.

[Formless]

I think we have totally different ideas of the scale of combat haha! We are currently making a fusion reactor that will get us 10 times the power out as we put in, and that efficiency will only go up as we figure things out. Should be done in like 2020. So that isn't even futuretech really. http://en.wikipedia.org/wiki/ITER

I was unaware of that project. Interesting, and I wish the people working on it the best of luck. We need better nuclear power technologies that haven't been politically poisoned. However, for the moment, lets remember that its still unproven technology and that it is quite possible it will still be something you won't want to throw up into space. Nor will it automatically lead to torchdrives needed to propel some of these thousand ton warships that have been proposed. I suspect that you can count on a few things. First that fusion reactors will probably be delicate and require lots of active maintenance to keep the reaction going. Putting it into a warship where it will get jostled around just from evasive maneuvers or even shots fired may not be good for it. Besides which, as you note, its likely that there is a minimum size for fusion reactors that would make such a ship a big and expensive target. Enemies love expensive when its your shit they are breaking. And I have no idea how much waste heat that it will create in the process of giving you useful energy, whereas fission is fairly well understood on this and many other technical aspects. So I see this as something far more influential to fixed or planet side installations, not warships.

I do think on different scales, in the sense that I put a range on the optimum size of ships between a minimum and a maximum that is practical for a warship. Some people put a minimum, but no maximum. Unless you have true sci-fi tech, like the Star Whatever franchises do, that is rather silly.

As an important note, that boeing laser works well against missiles because there is atmosphere to wreck the chassis once it gets fouled. The laser barely has to do much more than warp a fin a little bit and the missile will fail. In space, at space-speeds, a laser like that would be much less effective.

Adding onto what Skimmer pointed out, let me introduce you to the concept of "mission kill". Its already been talked about in this thread without being explicitly named, but let me spell it out. I said previously that primary targets on spacecraft are things like coolant lines, propellant tanks, electronics, and radiators. This is because if you can damage or destroy any one of these components, the ship is out of the fight until it is repaired. A terrestrial example is from WWII, where infantry discovered many tactics for taking out enemy tanks, frequently by destroying or jamming the tracks with explosives or other sabotage. An immobile tank is useless for warfighting.

You don't have to completely annihilate something for a weapon to do its job, you simply have to disable its functions. In the same way, you don't have to crush every bone in my body to kill me or stop me. Many people are stopped by tiny little hollow-point bullets, because the traumatic shock causes you to stop moving. You are actually quite likely to live despite this, if you get to a hospital and haven't been shot in the heart or the head. And you don't even need to kill me to take me out of action for the purpose of warfare-- you need merely give me no alternative but to surrender or die, and now I'm your prisoner without even firing a shot. These concepts aren't identical to mission kill, but the general idea is very useful to know.

A fusion reactor would obviously have some minimum size issues, so a ship using one would have to be really big, certainly on the hundreds-to-thousands-of-meters scale. Once we get that big, armour scales up in a wonderfully linear fashion.

Yes, but not necessarily its price tag. You have to remember that tactical considerations are often subject to strategic, economic, and political concerns. For more information, consult Sun Tzu's "The Art of War".

I agree that X-ray lasers are impractical. Was using them as a statement of hyperbole to illustrate that there are range issues with lasers!

Not really, or at least you are choosing very poor examples. Atomic Rockets showed the calculations for an X-Ray laser and the range of one. The conclusion he came to was that for 10 megawatts, you can hull enemy vessels from one light minute away, and rad kill targets from one light hour away, if not farther than that. Its a true space supremacy superweapon-- you can reach out and kill things as far out as Jupiter with one (provided no limits on targeting). So assuming you can make one, its quite practical and a good argument for never attacking a prepared and fortified planetary body.

[Formless]

Hey Skimmer, is there a practical difference between "bomb pumped X-ray laser" and "nuclear shaped charge?" As far as I can tell they're different symbols with the same referent...

Oh, they are different. Here's the AR article on bomb pumped lasers. They work by having actual laser optics that focus the gamma rays and x rays of a nuclear device a fraction of a second before being vaporized by the same weapon. These are more sub-munitions in that most of the bomb's energy is still thrown all over the place. You can put dozens or hundreds of them on each bomb, though, and they allow the bomb to hit targets that are hundreds of kilometers away with a concentrated dose of coherent gamma rays.

Casaba Howitzer, on the other hand, puts the bomb in a shell that shapes as much of the energy as possible into a cone. In that cone there is a filler material that absorbes gamma rays very well, such as powdered tungsten, which then vaporizes into a beam of superheated plasma. So one of these weapons is an array of laser cannons, while the other is a big ass particle cannon. But both are essentially modified warheads for a nuclear bomb, hence your confusion.

[mrtspence]

I am familiar with the mission kill idea. I don't think it would be easy to disable a spacecraft. It took absolutely ludicrous amounts of damage to take a turn-of-the-19th C battleship out of the fight and that was when said craft was surrounded by water to sink it and susceptible to raging fires. Any compartments pierced by weapons damage would likely be vacuumed, so I doubt fire would be a threat. You'd have to actually wreck those critical systems you mentioned and I think any military craft would make it their top priority to armour the crap out of those parts or, at the very least, put a lot of less-important crap in the way. With the ability to roll pretty easily to present un-injured aspects to the enemy, it could get pretty tough to kill a craft. All of this is, of course, depending on the level of futuretech we are talking about. Hell, even killing important things may not take one out of the fight. A ship with no propellant tank is a sitting duck for later, but until it streaks past you, it may have a bunch of nasty weapons that could kill you etc.

I love atomic rocket! Great website. To get an X-ray laser that can put 10MW downrange would be really tricky and impractical (again, depends on what futuretech we are talking about) but ya, they are insane deathrays. My understanding of their issues has to do with X-rays not being easy to wrangle (they get absorbed into their mirrors and can only be deflected at really shallow angles if at all). So X-ray laser ends up being insanely gigantic and inefficient. I think them on a space-station or orbital platform is, as you mentioned, probably the best place for them. The range issue I mentioned is that a missile or kinetic weapon could hit a stationary (or known-orbit) target from as far away as they want. IIRC, the rad-kill would only be against targets with no armour. Would be a really cool way to capture merchant ships with their cargo intact, but wouldn't work as well against armoured ships.

I'm not sure why you (Formless) think that there is a defined or plausible minimum or maximum size for space warships. If a bigger engine is more efficient, it makes sense to scale things up with it (ie. fusion engine). I do agree that it would be probably be expensive. Since we cannot actually do a proper cost-benefit analysis, it is possible that really big fusion ships (with lots of armour and guns) would be more cost-efficient and it is certainly possible that they wouldn't be.

Not sure if I agree that they would be fragile. Likely a big ring of electromagnets surrounded by tons and tons of bracing. Lots of solid metal. If it gets a bit damaged, the torch goes out and the crew gets to fixing it, if they can. If a fission reactor (tons of finnicky valves, under-pressure pipes, etc.) gets hit it would be possible for the reactor/engine to meltdown or worse and be really damned hard to fix, given the radiation that is being spewed everywhere.

You mention the socio-political factors as important and I agree. It could be that those socio-poltical factors would also align against building big "dreadnought" style ships. It could go the other way too. I'm not sure what part of the Art of War you are referring to, but a big ship that can accelerate harder than smaller ships seems like a pretty valuable strategic asset in any case.

[Formless]

I think them on a space-station or orbital platform is, as you mentioned, probably the best place for them.

Despite your protestations of my thinking at a different scale than you, I'd say you aren't thinking about this weapon's real strengths. You don't put them on a space station, because its a kilometer in diameter and has all sorts of heat dissipation issues to resolve. Instead... put it on a planet, and dug in like the LHC. Then you can use the entire crust of the planet as a practically limitless heat sink. Thus whoever controls Earth, Luna, Ceres, or Mars can project power across whole solar orbits without ever needing to send out a fleet of spacecraft at all. The planet is itself a gigantic naturally forming spacecraft-- the largest things on the battlefield that humans can live on and work with. Why lob missiles at people when you and the people of Mars have an understanding that you can start carving dicks in their cities with your doom laser if they piss you off?

Sci-fi writers treat planets like islands. I take the term "Spaceship Earth" in the literal sense it was intended to have. Its the outlier in my ideas about the plausible size of warships and with good reason.

IIRC, the rad-kill would only be against targets with no armour. Would be a really cool way to capture merchant ships with their cargo intact, but wouldn't work as well against armoured ships.

Radiation shielding is made of fucking lead, i.e. one of the heaviest damn elements in existence that isn't radioactive. Radiation shielding capable of protecting you against nuclear strikes, particle cannons, and X-Ray lasers will eat your mass allotment like Homer Simpson at a doughnut shop with a winning lotto ticket. Again, we aren't talking about background solar radiation or stray cosmic rays here, we are talking about hard radiation from weapons designed with the sole purpose of killing spacecraft. Its a whole different can of worms than you think it is. That's why I generally think weapons will outstrip the abilities of armor, certainly where warheads are concerned.

I'm not sure why you (Formless) think that there is a defined or plausible minimum or maximum size for space warships. If a bigger engine is more efficient, it makes sense to scale things up with it (ie. fusion engine).

Because you can't scale engine efficiency to infinity. Or any technology for that matter. Eventually the technological constraints will make it impossible barring an unforeseeable breakthrough. It could be limits in what technologies we can create-- I already said I think its a leap in logic to say fusion power therefore fusion engines, for instance. Or it might be a limit in the often overlooked technology known as materials science. Eventually, you get to a point where the structure is incapable of holding up to the stresses its own engines are putting on it just to change even a tiny amount of its momentum. And again there is economic concerns which flow directly into tactical concerns, exemplified in the destruction of expensive Russian tanks during the Winter War with Finland by Molotov cocktails that cost a dollar a dozen to manufacture. America may have fought the last several of its wars with million dollar per unit equipment, but I don't think that's an example every military should strive to be like.

Not sure if I agree that they would be fragile. Likely a big ring of electromagnets surrounded by tons and tons of bracing. Lots of solid metal. If it gets a bit damaged, the torch goes out and the crew gets to fixing it, if they can. If a fission reactor (tons of finnicky valves, under-pressure pipes, etc.) gets hit it would be possible for the reactor/engine to meltdown or worse and be really damned hard to fix, given the radiation that is being spewed everywhere.

Just listen to the things you are adding here. Every additional strip of metal is more mass you have to push around. Mass allotment is something that can easily spiral out of control even when you are paying close attention to the issue, let alone when you don't care. On the other hand... three megawatt fission reactor that fits in the same space as a dinner table. And you get to have wingmen that can fly in formations several kilometers wide and cover each other with PD. I'm not seeing the advantage of your design philosophy over my own.

Also, if you are under fire in a combat zone, the last place you want to be is outside the armored vehicle in a spacesuit with mere centimeters between you and suffocating, cold, radiation filled vacuum. At least, not most people.


Edit: to add;

With the ability to roll pretty easily to present un-injured aspects to the enemy, it could get pretty tough to kill a craft.

I don't think that's as good an idea as it sounds like. Rotating the craft means spinning your weapon's firing arcs. Sensors too, and those things really don't like being jostled. Passive sensors need a stable platform to function. Otherwise everything blurs into streaks during exposure.

[Simon_Jester]

I may look at some of the rest later, but for now...

Ya I think the idea of kinetic weaponry would be to totally saturate an area. Coilguns/railguns, if fed enough juice, could get some seriously ridiculous rates of fire.

In this case you hit diminishing returns really fast as a function of combat range. At X kilometers you're guaranteed a hit... but at 10X kilometers you have a 1 in 10000 chance of hitting anything. It's horrendous, and somewhere in there it stops being worth it to keep shooting. Suppose you're trying to score one hit with one 1 MJ impactor on their hull (probably not enough to actually stop anything that weighs over a hundred tons unless you get very lucky, but at least powerful enough to do some real damage...) Supposing you're firing at ten times your "can't miss" distance and your mass driver is 90% efficient, you dump a gigajoule of energy into your radiators before dumping a megajoule into your target. They may be able to kill you just by running you out of reactor fuel at rates like that, and it gets even worse as the range increases.

At some point you just say "fuck it" and figure out how to do this with guided munitions. Or use a beam weapon, which has a muzzle velocity of c and therefore a LONG effective range.

And I do understand the distinction between acceleration and velocity in this context, my usage of the word velocity was incorrect, so I'm sorry I prompted you to write all of that xD. That was my bad. My point is that, relative to the main engine, I don't think maneuvering thrusters would do very much for a big ship. The main evasion would probably come from either accelerating more or cutting acceleration (giving more of a line-shaped potential profile as opposed to the circular one you would see from a craft with relatively stronger lateral thrusters). It would be a matter of hosing down an increasingly small area as you get closer until their craft is disabled. But I am in total agreement that kinetics are the most evade-able of probable weapons.

THat just turns the target's maneuver envelope into an ellipse, as far as your gunsights are concerned- his forward-back maneuver options are wider than his left-right options.

I often visualize a fighting platform for space warfare as being long, narrow, and equipped with spinal weapons. That gives you minimum target profile, lets you concentrate passive defenses (i.e. armor) forward against incoming fire and get the most protection for the least mass, and makes it relatively easy for you to sidestep enemy fire because you don't need your maneuvering thrusters to MOVE you very far.

When times of flight are measured in minutes (and this is likely in space combat), you don't need a very impressive maneuvering thruster to push your ship several times its own length to one side.

My point about missiles being visible is that any serious maneuvering thrust would be detectable from across our solar system. If we can check on and track asteroids in the asteroid belt that are a hell of a lot colder and darker than a missile right now, we could definitely track a missile--if the sensors were pointed the right way. Definitely wouldn't be fool proof though!

Asteroids are orders of magnitude larger, have absolutely zero effort put into minimizing their radar cross-section, and are 100% ballistic so that spotting where they are NOW tells you exactly where they'll be five years from now. Detecting a coasting missile is a whole different order of problem.

My envisioning of a bombardment would be ships billions of kilometers away accelerating and firing kinetics/missiles at stationary targets (they would have already plotted all of the relevant coordinates--orbital platforms/space stations/bunkers/whatever else that can't move). A few days/weeks later, the targets are annihilated from out of nowhere.

This is practically equivalent to my proposal, except that mine is based on something more... hardish, in that we could physically build pretty much all the relevant equipment NOW given the budget. A large orbital mass driver is doable, so is an 'interplanetary missile' that consists of a booster stage (fired in Martian orbit or whatever) and a terminal seeker head (which activates and seeks out a target on approach to Earth or wherever)

I wasn't meaning to say that a nuclear shaped charge didn't have relatively much more range than a conventional nuke, just that 500km is insanely ridiculously close by space-weapon standards. The difference in range is insignificant when compared to other types of weapons.

500 km is insanely close as "distance from firing platform to target," but pretty damn long when measured in terms of "standoff attack range for munitions." Point defense will be most effective at very short range, because missiles do have huge lateral accelerations, or can, if you work to the limits of what we know to be possible (like the Sprint missile). A lot of defensive antimissile systems that excel at engaging a fixed, incoming target that has to fly in a straight line toward you will be less effective against 'crossing targets' that fly a hundred kilometers off your dorsal bow or whatever.

I mean fuck, just firing delay-fuzed shrapnel shell in the general direction of an approaching nuclear missile could help in space, as long as the shrapnel bursts happen several kilometers away from your ship (far enough out that the resulting debris cloud shreds the missile before it blows up your ship). Wouldn't do much good against a crossing target though, because there'd be no guarantee of the missile flying smack into the shrapnel.

The idea behind a bomb-pumped laser is that, before the laser housing is annihilated, the device gives it enough juice for a powerful laser shot. So the bomb-pumped laser has way more range and is actually doing the damage with the laser beam itself. A shaped charge uses the shaped blast of the nuclear weapon.

What I've read of the theory of both weapons suggests that they're... more similar than you might suspect in the underlying physics. Either way you're using a slab of selected material to focus the X-ray yield of the warhead. The only difference seems to be in the geometry- the lasing rod of a bomb-pumped laser and the focusing lens of a shaped charge work the same way as far as I know.

[mrtspence]

I think a few of us are trying to find disagreements where none exist.

A fusion engine is a leap. Absolutely. So is any kind of weaponized space warfare.

I think you misunderstand the concept of mass-ratio, Formless. If said fusion engine is big but also puts out a proportionately large amount of juice, the ship can be proportionately large. They hold a lot of promise as an engine that can give you a lot of thrust and a really good exhaust velocity (so good acceleration performance and a good mass ratio).

The structural forces acting on a spacecraft that isn't in a gravity well or atmosphere are not significant unless the ship is accelerating a lot, so it would be feasible to have ships much bigger and flimsier than any terrestrial equivalents (and things like aircraft carriers are pretty damned big). The forces at play are controlled by how much the ship wants to accelerate.

I think you misunderstand the concept of vacuum frequencies, Formless. An important reason human beings can survive is that atmosphere's absorb high frequency radiation (like x-rays). Putting a giant x-ray laser on a planetary surface (with atmosphere) would be a great way to irradiate passing birds and heat the air a bit, but not much else. A really large visible-frequency laser might be able to get results by just putting more power downrange.

Rotating a ship does have the downsides mentioned if done at a really fast rate. Weapon arcs would certainly suffer. Might still be worth while if the alternative is getting killed though.

I agree with everything you have said, Jester.

Long narrow ships built like skyscrapers make a ton of sense as you would want radial symmetry along the axis of your thrust to avoid hilarious (for an observer) tumbling and whatnot. Could also arrange decks so that your crew don't have to suffer from zero-g as long as you are accelerating. More room for long skinny weapons is also desirable. I am 100% with you on this point.

Kinetics are absolutely less efficient than lasers at long ranges due to inaccuracy. I wasn't ever trying to dispute that, just point out that, due to rates of fire and projectile speeds, that effective range may not so small as to make them ineffective weapons.

I agree that 500km would be a pretty long distance for slow-travelling cluster-munitions point defenses. It would take a really good laser to deal with missiles. I like the idea of missiles as a weapon in general. A simple chemical rocket would give them way more acceleration than any ship with most good mass ratio engines would have, so a bit of armour when coupled with rotation and those high speeds could make them tricky to deal with.

I think a shaped-charge nuclear missile would be a great way to deal with ships that have good close-in ballistic point defenses. I am with you on that one.

I think bomb-pumped lasers are, as you said, pretty similar to shaped charges, but a laser obviously has way more range than a jet of plasma. I have a feeling that a bomb-pumped laser wouldn't be able to hit nearly as hard though. It would be impossible for such a laser to convert a large amount of that energy into a useful laser beam whereas shaped-charge explosions would be using things like the thermal energy as part of the damaging mechanism. Much more efficient.

[Sea Skimmer]

I'm thinking in terms of closing speeds of ~3-30 km/s, plausible for 'hard SF' technology from a reasonable launcher (either a boost stage or an orbiting mass driver). And yes, planetary defenses can be made mobile. But there will be enough significant targets which are fixed (i.e. industrial complexes and big honking space stations) that this would be an effective attack under the right conditions.

Even 30km/s is not going to be a very difficult target for planetary defenses though, that's not even twice as fast as the peak closing speeds involved with modern day anti ballistic missile and anti satellite technology, which show little sign of being velocity limited anyway. For planetary defense with long reaction times the interceptor need not be anywhere near as fast as the incoming threat, this also makes it much cheaper and easier to deploy. Your going to need much higher speeds to be a plausible means of attack by anything but pure saturation or blind luck from not being seen coming. Neither is likely in the case of a well defended planet with reasonable future technology. 3km/s is not gonna even start to work, that's only the velocity of a ~1000nm range ballistic missile. A modern day SM-3 could engage it successfully, as could come lesser ABM weapons over a smaller footprint like Patriot with PAC-3 missiles.

Your going to need to go really, really fast to make this work, and that's why finding and guiding to hit a target becomes a problem. Even if the target is fixed very slight errors become massive over long distances, and yet speed makes the guidance and adjustment harder and harder.

[Simon_Jester]

I'm thinking in terms of closing speeds of ~3-30 km/s, plausible for 'hard SF' technology from a reasonable launcher (either a boost stage or an orbiting mass driver). And yes, planetary defenses can be made mobile. But there will be enough significant targets which are fixed (i.e. industrial complexes and big honking space stations) that this would be an effective attack under the right conditions.

Even 30km/s is not going to be a very difficult target for planetary defenses though, that's not even twice as fast as the peak closing speeds involved with modern day anti ballistic missile and anti satellite technology, which show little sign of being velocity limited anyway.

Credit where it's due, I said that not him. That said, it's not an unsolvable defense problem; what I was communicating was "on the close order of 10 km/s," because that is a minimum interplanetary speed. Kicking it up to 100 km/s is probably feasible IF you're determined enough, but it's nontrivial and makes the missiles more expensive to launch, or requires softer-SF technology.

One thing you might try to beat the target acquisition problem is to launch a wave of drones ahead that confirm the location of targets (again, this is more like "hit the anchorage and the steel mill" than "hit the defenses") and just omni-broadcast data back to the attack wave. This alerts the defenders, but at least your missiles know where they're going.

[Formless]

I think a few of us are trying to find disagreements where none exist.

A fusion engine is a leap. Absolutely. So is any kind of weaponized space warfare.

Stop here. Go and look up the term "leap in logic". It is NOT synonymous with an assumption made for the sake of argument. There are plenty of technologies which we know enough about to say they could work given the assumption that we have already colonized space and celestial bodies to a degree where interplanetary warfare is possible. Fusion engines are not one of those technologies, because fusion power in general is still in its infancy. That project you linked to is still seven years out from its target date. It may be that in those seven years they find they have to extend their projections another ten. Allowing them at all is a leap. Allowing you to jump from fusion power to fusion engines is simply idiotic. All claims about fusion rocketry is for the time being little more than handwaving a lot of non-trivial technical issues. Issues I doubt you are any more qualified to discuss than I am.

The structural forces acting on a spacecraft that isn't in a gravity well or atmosphere are not significant unless the ship is accelerating a lot, so it would be feasible to have ships much bigger and flimsier than any terrestrial equivalents (and things like aircraft carriers are pretty damned big). The forces at play are controlled by how much the ship wants to accelerate.

Isn't the whole point of a Torchdrive to give you acceleration comparable to chemical rockets but without the weight and inefficiency? I think you need to re-examine your own claims, because regardless of whether you agree with my skepticism I think you have some internal contradictions to resolve before presenting your ideas.

I think you misunderstand the concept of vacuum frequencies, Formless. An important reason human beings can survive is that atmosphere's absorb high frequency radiation (like x-rays). Putting a giant x-ray laser on a planetary surface (with atmosphere) would be a great way to irradiate passing birds and heat the air a bit, but not much else. A really large visible-frequency laser might be able to get results by just putting more power downrange.

True, but that's assuming that the laser energy is low enough it can't force the atmosphere aside. Pulse the laser beam, create a vacuum along the path of the beam. Problem solved. Roasting birds and heating the atmosphere? No more an issue than throwing up a rocket. Plus, this problem is only an issue for Earth. Luna, Mars, and Ceres don't have atmospheres, and terraforming is far less efficient a way of colonizing a planet than simply constructing an arcology of domed cities. B-)

And if that sounds waaaaaay too crazy inefficient and overkill to you, you can use slightly less powerful UV lasers instead. Earth's atmosphere doesn't absorb that.

Now, here's another interplanetary DOOOOOOM weapon for you if you like missiles. Solar Moth Rockets. You power them with solar power satellites, which in peacetime have obvious applications. Then in wartime, you launch the rockets with laser propulsion on a one way suicide run. Once they are up to speed, even after the solar power station laser has long since lost coherence needed to damage enemy targets, you attack a combat mirror to the missile so that it can focus death beams on secondary targets or defensive spacecraft. It has some of the fun of a missile, some of the fun of a planetary dooom laser!

[Terralthra]

Fusion engines are less of a leap than fusion reactors. We can create a fusion reaction now, and using it to propel something is trivial; it'd just be a net energy loss with present technology. A fission pile powering a fusion engine is possible with present tech.

[Sky Captain]

I think fusion rockets are fairly plausible especially given all other tech advancements that would be required for serious interplanetary warfare. Inertial confinemet fusion engine require powerful lasers for ignition so if you have tech to build worthwhile combat lasers you also should have capability to build fusion engine. Maybe a clever warship design could even combine both combat lasers and engine ignition lasers into one system.

Nuclear salt water rocket could make ideal missile engine with both high thrust and high fuel efficiency. Drawback would be even a tiny fuel leak in missile storage compartment aboard ship could easily end in nuclear disaster.