Space Fleet Lasers

Steven den Beste, always interesting, was posting on some theoretical and practical aspects of space fleet combat. Let me mention one little trick that might help defend against laser weapons, because the overall subject of fleet tactics is very broad. However to get a deeper understanding of some important aspects of the tactics of any type of fleet I'd heartily recommend "Fleet Tactics" which develops a methodology for approaching tactical problems so naval officers can correctly judge how to deploy their fleets, no matter what types of ships and weapons you drop into their laps. Much of the mathematics and concepts would apply equally to space fleets. Anyway, let me start out with a bit of background on laser weapons, then move on to some ways their effects can be ameliorated, if not largely nullified, at least in regards to penetration of a warship's pressure hull.

Here's an article talking about a solid state laser system developed at Lawrence Livermore, where they separated out the output pulse phase from the cooling period, allowing them vastly reduced beam spread. They're talking about putting a 100,000 watt laser with a 10 second burst mode on a Humvee by 2007.

An update is here where they're working it up to 40 kW output power and in a much smaller package. I don't doubt that they'll soon reach their goal of 100 kW on a Humvee. Interestingly, one of the suppliers, Decade Optical Systems, makes a 72 kW diode array, mentioned here. Some more high power diode lasers are listed here at Northrop Grumman's space technology site. The scary thing is that you can easily afford to buy these.

The DARPA SHEDS (Super High Efficiency Diode Sources) program aims to up laser diode efficiency to 80%, from its current level of about 50%. If they accomplish this then in future, even absent a very tight focus, just by keeping your entire beam on the target it would be receiving 4 times more energy than you're dissipating as heat. However, if they're surface is more than 75% reflective, which is almost certain given aluminum or white hulls, then the firing ship still has to cope with more heat than the target. Of course, if the firing ship is significantly larger than the target this may not even be an issue. The key would remain a tight focus so you can burn into a spot on the target, not just heat overload.

For background on the history and capability of our laser weapon programs I recommend "Airborne Laser – Bullets of Light", which is a bit dry, since it goes deep into program details, but overall pretty fascinating.

Single Rotating Skins

However, there are some nifty tricks to defend against a laser attack, and one I'd like to mention is having a ship with an exterior skin that rotates at high speeds. Suppose you've got a laser focused on the side of a cylindrical ship, in essence trying to puncture a beer can. If the can is spinning at low speed you might burn through directly. However, you still have to maintain your focus on the same spot of the can's exterior, say the top of the 'B' in Budweiser. Just standard aiming systems would probably be sufficient for this. However, at higher rotation speeds it becomes a little harder to keep your beam concentrated on the same spot. At long range this requires your aiming system to introduce a tiny sinusoidal warble that's phase locked with the rotation of the target. By phase lock, I'm not talking Star Trek. I'm merely using the engineering term for matching the target's rotation period, along with an angular offset from some arbitrary angle that represents "0 degrees" of target rotation.

If you don't do this then your shot must be able to burn through the outer skin while your beam is traveling across it at whatever the tangential speed of the skin is. If you can already do this then you'll saw through the entire hull in just one rotation anyway, so the point is moot. With lasers that powerful nobody would rotate their hulls in the first place, since it would just spread out the damage. Of course also if your laser could do this you'd just burn a smiley face through the enemy ship, and obviously nobody would be hanging around trying to play space combat if they were that vulnerable. So lets assume our hulls are a bit tougher than that, with moderately thick titanium and other measures taken to render them worth of being called warships. So what I'm confining myself to are hulls and lasers where the laser burn through time is reasonable measured in seconds, or maybe a large fraction of a second.

So to penetrate these rotating hulls your "gun barrel" has to have a built in vibration that's adjustable in frequency and amplitude. Finding out the rotation rate of the target for the phase lock is rather trivial, since you can just flash the target with a millisecond burst and then time the reappearance of the resultant hot spot or burn mark, using your infrared sensors or at closer ranges your optical systems. Of course, if your target is close enough to where you can directly and closely observe it, you wouldn't need the initial flash, since you'd just shoot at the "Bud".

So you sweep down the rotating beer can, staying focused on the same spot, until that spot disappears underneath and comes up the other side. After your target vanishes around the other side you sweep back up, your beam traveling the wrong way and merely raking across the hull, until you once again meet the target spot coming up and around the top. You're only hitting the target spot 50% of the time, but you're still working through the target's hull. So your aiming system needs to be able to carefully adjust the amplitude of the aiming vibration to match the angular size of the target hull, which varies with target diameter and range, and of course you'll already have a phase lock on a rotating point.

Of course you're unlikely to be exactly perpendicular to the target ships axis of rotation, so the target spot will not be traveling straight up and down, but will appear to move in an ellipse. Thus your aiming system will have X and Y axis vibrations, which will be driven off the same signal, but have independently adjustable amplitudes. This is a little bit more sophisticated, but not awfully much. The final adjustment is that the guns X-Y vibration axes can rotate to align with the axis of rotation of the target, which basically rotates the major axis of your gun's elliptical warble into alignment with the major axis of the target spot's elliptical path. As long ago as the way to the 1930's we'd have done all this mechanically, with a pair of electric motors providing the mechanical vibration, a tunable sine wave generator to drive them, a pair of amplitude pots to adjust an amplitude linkage, and a greasy haired tech watching an oscilloscope with his hand on a couple of X and Y adjustment knobs. Nowadays we'd just get rid of the rotation for target axis alignment and use a computer to generate the required control signals. It'll all be automatic. In fact, our systems are already far, far more sophisticated than this, tracking not just a missile like a sidewinder, but a spot on that missile, as it continues on its high speed and slightly erratic flight path. Against aircraft we use a telescopic aiming system that tracks a point on the aircraft. We're already pretty far along the path that leads to the situation I'm discussing, aside from the whole space fleet thing.

Thick Rotating Skins

So now our defensive designers throw in another refinement. They make that exterior skin into something more like a single deck on a rotating space colony, with an inner and outer surface separated by some distance, and some bulkheads in between for structural strength. Now our targeting system will succeed in making a hole in the outer surface, which I'll call the floor, but in trying to penetrate deeper past that hole will merely rake back and forth across the ceiling, because the whole target is spinning around and around. It's trying to punch a pair of collinear holes but the holes only line up exactly once per target rotation. This means that the inner hole gets focused on only a tiny fraction of the hull's rotation period, instead of the full 50% of the outer hole. This forces the attacking ship to modify the outer burn into a long line in order to expose a single point on the inner surface, or ceiling, to sustained illumination.

So to prevent this second penetration the defending ship will maneuver. The attacking ship has a laser that it's using much like a drill, trying to burn through two surfaces. Anyone who's ever used a drill bit knows that you have to keep the bit aligned with the hole. If the defending ship changes its heading, or more particularly the direction of its spin axis, then the inner and outer hole no longer line up with the beam. Going back to that floor to ceiling analogy, the opponent has a vertical beam that's burned up through your toilet and into the ceiling fan. Tilt the house and the two holes don't line up vertically anymore. If your attacker has made two holes like this just turn toward him ten degrees and his second hole becomes useless to him, forcing him to burn a new one. Keep up your slow rotation and you can stop his progress, forcing him to spend time "hogging out" the hole.

Multiple Rotating Skins

There's still a remaining problem, even after burning through the second surface, which is that both these outer skins aren't part of the pressure hull, but merely the ship's armored exterior. Once you've got a whole burned all the way through this rotating exterior, it still only exposes the inner pressure hull to one brief pulse per rotation. On a 50 foot diameter hull, for example, if the laser burned a one foot diameter hole, the outer hull would have to travel the 157 foot circumference of the ship before re-exposing the same spot. So in attacking the inner pressure hull the laser can only achieve a 0.63% effective duty cycle on a single spot, and is effectively back to raking back and forth over half of the inner pressure hull, which isn't a very efficient penetration strategy, as outlined above in the section on targeting a rotating hull in the first place.

Now let's add an intermediate skin, rotating opposite the outer one. Once you burn a spot through the outer hull, possibly having to rake your way through one of the layers, as in the case of a thick floor/ceiling outer hull, you're faced with a new hull moving opposite the direction of the first one. This really kills your effective duty cycle, and if you can burn through it at the fraction of a percent duty cycle that you have left, then you'd probably be able to carve smiley faces in the outer hull anyway. But let's suppose that you somehow accomplish single-point penetration of both hulls. Then you've got two contra-rotating cylinders, each with a tiny hole, and you're trying to manage to shoot your beam through both holes in the brief instant that they're aligned. If the attacker can't achieve this then he's just wasting his time, because all he's doing is running up the target's eventual repair bill for a couple holes in some plate.

But this gets us back to the two-hole alignment problem. Those holes, traveling around in opposite directions, meet each other at some particular angle relative to the inner hull, say top dead center for a vertical attack. By briefly adjusting the rotation rate of one of those two skins you can easily change the angle at which they meet. So you get a double penetration and immediately vary the rotation speed of the two cylinders, till the two holes start lining up underneath instead of on top. In fact you could have the two rotation rates oscillate a bit so that no pair of points ever stays lined up relative to a fixed point in space. Maybe on Star Trek they'd call that "nutating your shield frequencies", who knows. But it does making the solution to the multi-hole beam alignment problem nearly insoluble, while the second hull also allows you to maintain a total rotational inertia of zero, so you're not cruising around in a giant gyroscope.

If you wanted to take the concept further, you'd be in a cylindrical pressure hull with multiple outer skins, mounted on gimbals, so the different skins wouldn't even rotate coaxially. At that point the only way to have even a small chance of penetration is to possess a beam with enough energy to penetrate through all the layers in a single, extremely brief burst. It would act more like an ultra-high velocity depleted uranium penetrator than a continuous wave laser. But the whole point of this post was to consider defenses against lasers that take a significant fraction of a second or longer to burn through a single point. One other advantage of the gimballed spheres is that they also don't leave the single axis as a weak point that's not protected by all the rotations. In the cylindrical schemes that area would have to be protected by thicker armor, or independentally moveable plates. It still would harken back to the days of sailing ships when having your bow or stern raked was a very bad situation, and to reduce the exposure of these areas it might be wise to have the bow and stern of the space vessel recessed, much like the Bud can I'm so fond of.

But this gets us to a final point. A laser that could penetrate all these layers, even with all the difficulties the rotation provides, would have to be able to penetrate multiple thick layers in one shot. In vaporizing the outer layers it would throw superheated particles and smoke into its own path. It's trying to burn through the thick cloud of vaporized hull it creates, and it would be difficult to maintain beam integrity and energy density it does so, especially as it gets into deeper areas of the ship. With air force weapons all the targets are washed in an airstream traveling at hundreds of knots, which automatically prevents this buildup. They also only have to puncture through thin aluminum skin to reach a fuel tank. But in space there's nothing to clear the smoke and debris, aside from centrifugal forces, so the vaporized material might cause problems. A follow-up shot would give time for this debris to dissipate, but by rotating the hulls we've made sure a sustained burst or follow up shot is ineffective. We also don't have any air pressure inside these outer skins, so there's no massive depressurization that would blow out the debris as the laser is burning through. After all, why would you fill them with air if it's just going to leak out with the first shot anyway? So considering all this, it seems we've made the job of the offensive laser harder by what look like orders of magnitude.

However, all this may protect the defending ship's pressure hull, but does nothing to protect its weapons, sensors, and assundry other items that by their nature simply have to be exposed on the outside. These systems will either need to be hardened and distributed across the surface, such as phased array radar or arrays of thousands of cameras and sensors, or follow a duck and cover approach, whereby the lasers are hidden internally and firing through multiple ports, mirrors, or windows. Similar distribution, disguise, and obscuration can help protect missile tubes or other equipment, while other systems like antennas and telescopes could be retracted or kept on the "lee" side of the hull.

Where lasers will certainly prove useful in any combat role is to serve as blinders, to make sure a target ship is swamped with bright laser light, unable to make out any details of the attacking ship, and thus rendering specific system targeting unlikely. This blinding would just as easily be accomplished with small remotes, stationed close enough to the firing ship so that the target can't easily resolve the attacker with any optics that are shadowed from the blinding, yet far enough away that a homing missile or counterfire targeting the blinder remote wouldn't threaten the attacking ship. Of course this all begs the question as to how the two ships got close enough that they hadn't already alternately blinded and fried each other, but I guess Newtonian flight paths play a role. Sometimes fleets are going to coast into each other, since each side would be thinking they would win the battle or they wouldn't be engaging in it at all, unless of course one side is forced to protect something vital, such as a planet or space colony. But that's getting back into fleet tactics, which is too broad a subject for this post.

So now wasn't that all just simple? No wonder they keep me stationed at tactical.