• Armor
• Force Fields
• Point Defense
• Radiators

With all this frightfulness flying at your ship, you'd want some kind of defense, besides just hoping they'll miss. Go to The Tough Guide to the Known Galaxy and read the entry "SHIELD"

Artwork by Jack Gaughan for "None But Man" (1969)

Armor Armor is a shell of strong material encasing and protecting your tinfoil spacecraft. Unfortunately as a general rule, armor is quite massive, so it really cuts into your payload allowance. Basically, the energy requirement to damage a surface is measured in joules/cm2. If you exceed that value, you do damage, otherwise you fail. Keep in mind that a Joule is the same thing as a watt-second. There are three ways that weapon energy damages a surface: thermal kill, impulse kill, and drilling.

Thermal kill destroys a surface by superheating it. Impulse kill destroys a surface by thermal shock. In the calculations for the SDI, the amount to thermal kill a flimsy Soviet missile is about 1 to 10 kilojoules/cm² (100 MJ/m²) deposited over a period of a second. The same energy deposited over a millionth of a second is required for an impulse kill. Since the laser beam tends to be meters wide, the beam energy is in the hundreds of megaJoules.

However, neither thermal kill nor impulse kill works very well with armor. So we use the third method: drilling. The amount of energy required to drill through an object is within a factor of 2 or so of the heat of vaporization of that object. There are also two other limits: the maximum aspect ratio of the hole is usually less than 50:1, and the actual drilling speed, for efficient drilling, is limited to about 1 meter per second (depending on the material).

Star Trek: Wrath of Khan (1982)

Therefore, the best anti-laser armor will be that material with the highest vaporization energy for its mass. The best candidate is some form of carbon, at 40 kilojoules/gram. You do not want a form that is soft or easily powdered, or the vapor action under laser impact will blow out flakes of armor, allowing the laser to penetrate much faster. Steel has a higher vaporization energy, but it masses more as well.

Under laboratory conditions, if an armor layer was 5 g/cm² of carbon, burning through a 1 cm² (1.12 cm diameter) spot of armor would take about 200 kilojoules and 20 milliseconds. An AV:T laser cannon with 50 megaJoules could burn through 250 such armor layers in a few seconds, under laboratory conditions (i.e., enough layers to burn through the entire ship the long way).

However, under combat conditions there is no way one could focus the laser down that tiny and keep it on the same spot on the target ship for multiple seconds.

It would be better to use a beam focused down to a larger 10 cm² spot (11.2 cm diameter). Granted the beam power required to penetrate jumps from 200 kilojoules to 20 megaJoules, but now if we have an uncertainty in the target's velocity of up to 5 meters per second it doesn't matter.

In a 1984 paper on strategic missile defense, it suggested that your average ICBM would require about 10 kilojoules/cm² to kill it. This would rise to 20 to 30 kilojoules/cm² with ablative armor, and it would be tripled if the ICBM was spinning on its long axis since the laser couldn't dwell on the same spot 100% of the time.

As a side note, a Whipple shield is very effective at stopping hypervelocity weapons.

Anti-radiation armor is discussed here.

And you can forget about laser defenses like Traveller style Sandcasters. There is no way that they can project a cloud dense enough to do any good.

In Frank Chadwick's starship combat game Star Cruiser, there are anti-laser fields.

From Star Cruiser by Frank Chadwick

However, the gang at rec.arts.sf.science are skeptical:

Michael Grosberg:

Erik Max Francis:

Wayne Throop:

: Seems to me it might even be worse.

Well... yes, there is that. Much easier to vaporize each particle, though it might be quite hard to get a particle to actually recoil and hit the ship. Hmmm. Anyways... yes, I suspect it wouldn't really work well, and you'd have to levitation a large, large: mass of particles.

Erik Max Francis:

Seems like most of the particles would hit the ship. To serve their purpose, after all, they are have to be between the beam and the ship. Whether that would be really dangerous to the ship depends on how thick the "shield" is and how big each particle is.

In his novel The Wellstone, Wil McCarthy proposes a unit called the TW or "train wreck". It is measure of impulsive acceleration (i.e., from a crash or explosion) equal to an inertial acceleration of 40 g. A human being can survive a 1 TW impulse lasting no more than a couple of seconds, while a 2 TW impulse of longer than a second is typically fatal. In The Hitch-Hiker's Guide To The Galaxy, Douglas Adams creates a tongue-in-cheek unit called the "hurt", with spacecraft weapons rated in "mega-hurts". Har-har.

Force Fields In the real world, defensive force fields do not exist. But if they did it would make things so much easier. There are a couple of remotely possible real-world "force fields". Dr. Geoffrey Landis speaks of magnetic fields to ward off positively charged particle radiation. More on the fringe are cold plasmas, which could ward off microwaves and particle radiation. But they have a long way to go before they can stop weapon-grade particle beam weapons.

Cold Plasma

But there isn't anything like E.E."Doc" Smith's electromagnetic radiation stopping "ray-screens", nor his matter stopping "repellor screens." There is a very good in-depth analysis of the science and issues of force fields at the alway authoritative Stardestroyer.net website here and here. As always when dealing with rubber science, the smart move is to nail down the ground rules for the item in question, think out all the logical consequences and implications, and stick to them.

If the force field blocks incoming laser fire, will it block your outgoing fire as well? In Isaac Asimov's "Black Friar of the Flame", a ship has to drop its field entirely in order to fire its weapons. This lead to chain reactions, ship A drops and fires, then it is hit by ship B who drops and fires, who is hit by ship C who drops and fires... In Larry Niven and Jerry Pournelle's The Mote in God's Eye, the Langston Field can have temporary holes opened to allow egress of your laser fire. In other novels, the field is on stroboscopically, that is, it flickers. It will be on, say, 80% of the time, and off for 20%. If your lasers flicker in synch with your field, 100% of their energy will penetrate. But since your opponent's lasers will probably not be in synch, only 20% of their energy will penetrate. However, if your opponent manages to match your synch rate, you'll be clobbered.

Artwork by Russ Mannings for "Captain Johner and the Aliens", 1963 Based on "First Contact" by Murray Leinster, 1945

Artwork by Leo Morey (1941). Only a ray-screen, whotta pity.

Does the force field block matter only (e.g., kinetic weapons), energy only (e.g., lasers), or both? Doc Smith had separate types of force fields for each ("repellors" and "ray-screens"), while the Langston Field would absorb both the kinetic energy of projectiles as well as the electromagnetic energy of lasers. The fields in "Black Friar of the Flame" only block energy, so the good guys get a bright idea from the Battle of Salamis. Is the field a bubble around the ship, or flat planes that can be positioned? There was that throwaway line in the movie Star Wars, where Red Leader tells the Red Squadron X-Wing pilots to angle their deflector shields "double-front". Presumably this means rotating the rear shields to face forwards, so there is double the protection forwards and zero protection aft.

How fast can the field be charged up? The usual model is that energy is fed into the field, and each incoming shot reduces the energy in the field ("Deflector shields are down to 40%, Captain"). When the field energy reaches zero, the field goes down and the incoming weapons fire impacts directly on the ship. For dramatic reasons, it is desirable to have the rate of shield charging to be a fraction of the rate of shield reduction. Otherwise ship's shields will never go down. Does the field obey the law of Conservation of Momentum? Say your force field generator is located in the Engineering deck. You put the force field around the ship, then quite by accident the ship crashes into an asteroid. One would expect that as the field hit the asteroid, the shock of impact would be transmitted to the field generator. You might wind up with the generator plowing through the hull and out the rear of the ship. In Poul Anderson's novel Shield, the field has a sharp gradient on the outside, and a more gradual one on the inside. This means if you were running and collided with the shield it would feel like hitting a brick wall. But if you were inside the shield it would feel like hitting a mound of feather pillows.

Artwork by David Mattingly for The Armageddon Inheritance.

In space combat it pretty much looks like the first to get a hit wins. This isn't really surprising; it's true of most combat these days (air combat, submarine combat, etc.). The weapons will be devastating enough that one hit will put a ship out of combat, if not vaporize it outright (i.e., they will have a very high Single Shot Kill Probability).

Larry Niven and Jerry Pournelle knew this, but wanted to write about dramatic extended space combat anyway. They contracted physicist Dr. Dan Alderson to design a self-consistent science-fictional gadget to allow this. He created the Langston Field.

In the SF trade, the Langston field is a "capacitor" or "tank" field. The field drinks up energy. It will absorb a laser beam, a nuclear blast or the kinetic energy in a coilgun shot. It then tries to radiate the energy away. However, the field cannot radiate away the energy as fast as the enemy can load the field with weapons fire. The field can only hold so much, and when the limit is reached, the field explodes, vaporizing the ship.

Also, the more destructive energy currently being held in the field, the more of the ship's own power that will be required to keep the field from exploding. If the field gets too full, the ship will not have energy to spare for movement or its own weapons.

From "Reflex" by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye, collected in There Will Be War I)

A torpedo had penetrated her defensive fire to explode somewhere near the hull. The Langston Field, opaque to radiant energy, was able to absorb and redistribute the energy evenly throughout the field; but at cost. There had been been an overload at the place nearest the bomb: energy flaring inward...

All through Defiant nonessential systems died. It took power to maintain the Langston Field, and the more energy the Field had to contain the more internal power was needed to keep the Field from radiating inward. Local overloads produced burnthroughs, partial collapses sending bursts of energetic photons to punch holes in the hull. The Field moved toward full collapse, and when that happened, the energies it contained would vaporize Defiant. Total defeat in space is a clean death.

Temporary "portals" or "holes" can be opened in the field to allow the ship's laser fire to hit enemy starships. Otherwise the laser beams will hit the underside of their own field. Of course the more energy being held in the field, the more difficult it is to open a hole.

Sensors are on booms so they can be extended outside of the field, otherwise the ship is blind. As the exposed sensors are blown away, the booms are retracted and fresh sensors are mounted. If the attack is ferocious enough, a ship can become blinded (i.e., all exposed sensors destroyed before any new ones can be deployed), and the enemy will quickly move out of the path of the ship's weapon fire while still pouring death and destruction into the blind ship's field. Then the blind ship frantically tries to deploy enough sensors so that at least one will last long enough to plot the position of the attacker.

Unfortunately, if the field becomes too full of energy, sensors or any other item being extended through the field will be fried or vaporized by the contained energy.

A hot field will also fry any object attempting to pass through the field en route to the ship inside (such as a shuttle containing a boarding party). Any object would also become embedded in the field, since the field also absorbs kinetic energy, unless is was moving really fast.

In a nod to E.E."Doc" Smith, when radiating, the field starts glowing red, then moves its way up the spectrum. The only thing a blinded ship can see is the color of the inside of its field. Note the implication. When a ship's field is ten seconds from detonation, the ship is near death. But nothing has been physically damaged. If the ship is left alone long enough the field will cool off and the ship is as good as new. This made surrender a tricky proposition. If you gave too much mercy to the surrendering ship, it would recover and you'd be right back where you started.

Artwork by Jack Gaughan for "Skylark Three" (1930)

The solution was interesting. If a ship with hot fields surrendered to you, the captain asks for a volunteer from the midshipmen. If nobody volunteers, the captain shrugs and signals to destroy the enemy ship anyway. But if there is a volunteer, they get to strap on their chest a tactical nuclear weapon with a hand detonator (dead-man switch or other fail-deadly type). Under pain of destruction, the surrendering ship has to allow the midshipmen to board, and let the midshipmen go to the control room or other vulnerable spot. You can now allow the surrendering ship's field to cool off. If it doesn't do exactly what you say, the midshipmen will detonate the bomb (you hope).

From "Reflex" by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye, collected in There Will Be War I)

A ship in Defiant's situation, her screens overloaded, bombarded by torpedoes and fired on by an enemy she cannot locate, is utterly helpless; but she has been damaged hardly at all. Given time she can radiate the screen energies to space. She can erect antennas to find her enemy. When the screens cool, she can move and she can shoot. Even when she has been damaged by partial collapses, her enemy cannot know that.

Thus, surrender is difficult and requires a precise ritual...

...Weapons in the hand of a defeated enemy are still dangerous. Indeed, the Scottish skean dhu is said to be carried in the stocking so that it may be reached as its owner kneels in supplication...

Defiant erected a simple antenna suitable only for radio signals. Any other form of sensor would have been a hostile act and would earn instant destruction. The Imperial captain observed and sent instructions.

Meanwhile, torpedoes were being maneuvered alongside Defiant. (Captain) Colvin couldn't see them. He knew they must be in place when the next signal came through. The Imperial ship was sending an officer to take command.

Colvin felt some of the tension go out of him. If no one had volunteered for the job, Defiant would have been destroyed.

Something massive thumped against the hull. A port had already been opened for the Imperial. He entered carrying a bulky object: a bomb.

"Midshipman Horst Staley, Imperial Battlecruiser MacArthur," the officer announced as he was conducted to the bridge. ... "I am to take command of this ship, sir."

Captain Colvin nodded. "I give her to you. You'll want this," he added, handing the boy the microphone. "Thank you for coming."

..."Midshipman Staley reporting, sir. I am on the bridge and the enemy has surrendered." He listened for a few seconds, then turned to Colvin. "I am to ask you to leave me alone on the bridge except for yourself, sir. And to tell you that if anyone else comes on the bridge before our Marines have secured the ship, I will detonate the bomb I carry. Will you comply?"

For dramatic purposes, Dr. Alderson decreed that the Langston field was subject to "local burn-throughs". That is, a given weapon strike might be too intense to be absorbed all at once, so a fraction of the damage pokes through the field into the ship. This gives enough damage to the ship to be cinematically interesting, but not enough to vaporize the ship outright or something boring like that.

This had the intended side-effect of ensuring that the ship with the best damage control crew would win the battle.

The Langston field may be science fiction, but at least it is internally self-consistent. Niven and Pournelle used it in their novel "The Mote in God's Eye", which Heinlein said was "possibly the finest science fiction novel I have ever read." High praise indeed.

From "Reflex" by Larry Niven and Jerry Pournelle (the deleted first chapter of The Mote in God's Eye, collected in There Will Be War I)

In principle Defiant was a better ship than she'd been when she left New Chicago. The engineers had automated all routine spacekeeping tasks, and no United Republic spacer needed to do a job that a robot could perform. Like all of New Chicago's ships, and like few of the Imperial Navy's, Defiant was as automated as a merchantman.

No robot could cope with the complexity of decisions damage control could generate, and if there were such a robot it might easily be the first item destroyed in battle. Colvin had been a merchant captain and had seen no reason to object to the Republic's naval policies, but now that he had experience in warship command, he understood why the Imperials automated as little as possible and kept the crew in working routine tasks: washing down corridors and changing air filters, scrubbing pots and inspecting the hull. Imperial crews might grumble about the work, but they were never idle. After six months, Defiant was a better ship, but...

And now for something totally different. Leonard Erickson came up with an interesting model for force fields: use the equation for gas pressure.

Yet another possibility is the system described in Poul Anderson's novel Shield (1963).

Defense Grid, Babylon-5 (1995)

Point Defense Point Defense is a fancy name for all the short ranged weapons and anti-missile missiles used to shoot at incoming enemy missiles. They are analogous to anti-aircraft guns. A low powered weapon would do for defense against nuclear warheads. John Schilling says that nuclear weapons are rather complex and fragile devices, and it doesn't take much to put them out of action. And they do not undergo sympathetic detonation, i.e., they don't go boom just because you hit them real hard. So if your point-defense system can score a solid hit, the nuke is effectively useless.

Eric Rozier has an on-line calculator here that does calculation of Kinetic point defense hit probabilities (i.e., a point defence using bullets). Eric Rozier: Modeling kinetic point defense is no easy task, the simulation I've created is a discrete event simulator which simulates individual bullets fired from a Phalanx style weapons system. The initial parameters for the CIWS are equivalent to a Phalanx with a perfect targeting computer. You can increase the number of CIWS firing at an incoming missle by increasing the number of linked CIWS, it is not as simple as multiplying the probability.

A Close-In Weapon System (CIWS) fires during a systems test aboard the amphibious assault ship USS Kearsarge (LHD 3). U.S. Navy photo by Photographer's Mate Airman Kenny Swartout.

RIM-116 being fired. United States Navy photograph.

The indefatigable Eric Rozier has an on-line calculator here that does calculation of Missile point defense hit probabilities (i.e., a point defence using anti-missile missiles). Ok, I think I've got a pretty well justified CIWS missile system. It models anti-missile missile point defense, similar to the RAM system in development by the military.

Cm = (Rb + Rt)² * π

Hp = Cm / (π * d²)

d = 0.5 * (9.8 * (At - (Ac * e)) * t²

When it comes to laser point defense vs incoming missiles, there is some controversy. This is the subject of a long-running "Purple/Green" debate on SFConSim-L (The term "Purple/Green" comes from an episode of Babylon-5 called "The Geometry of Shadows". The episode involving the ritual Drazi civil war, where the sides are chosen by randomly choosing colored sashes from a barrel. It is a science-fictional version of Miller Lite partisans shouting "Tastes Great!" and "Less Filling!". More specifcally, as Christopher Weuve explains: "It's the SFConsim-L brevity phrase meaning 'an argument in which no actual agreement can be reached, usually (but not always) because it is dependent on going-in assumptions.'").

Green Drazi leader. Note green sash.

Anyway the argument is about what happens in the last hundred kilometers to the target ship.

The laser gang asserts that they can zap a missile before it ever gets to kill range, even for a nuclear warhead. And do it every time, at least so much of the time that missiles aren't worth firing. Even if the missile fragments into 10,000 pieces of shrapnel (each with substantial killing power), tracking gear can determine the fragments that will hit, and zap them before they reach target.

The laser gang's theory is that lasers never miss. If you can paint the target with photons to see it, you can hit it with a laser. In addition: missiles, by definition, need to close on the target, which means there are some trigonometry tricks that will allow you to lock them up hard with lasers - they can't laterally juke in space without missiing the target, for example.

The missile gang contends that laser point defense can always be saturated. Fire a big enough missile, or a salvo of missiles, coming in fast enough, and there will just be more mosquitoes than the bug zappers can zap in the short time till impact.

The missile gang's theory is that you can derive the number of missiles needed to overwhelm a given number of lasers by inputting some variables, like amount of energy per square cm needed to guarentee a kill on a missile, the wattage of output of the lasers, and the cycle/recharge time of the lasers. Lasers do require some time to recharge, and need some time to cool off.

The laser gang reply that lasers have the advantage in that they are reusable, unlike missiles. If lasers are dominant, it's also an offensive weapon to zap enemy ships, not a purely defensive one.

The missile gang retorts that the missile can be fired outside of laser range, and if it does penetrate point defense and smoke your ship, your laser is no longer reusable, now is it?

There is the cost effectiveness argument. Can you afford to carry point-defense lasers that can stop my missiles? Can I afford to carry missiles that can penetrate your point defense? Which is cheaper?

Can there be any tactics in a long-range duel between two missile armed ships? It comes down to whether you can afford to fire a missile on anything but a certain intercept, this is also ultimately a matter of cost.

Can there be any tactics in a long-range duel between two laser armed ships? It can be argued that it is the equivalent of two crack marksmen at opposite ends of a football field, shooting at each other with scope-equipped, tripod-mounted sniper rifles.

Given equal quality lasers, if I can zap you, you can zap me. Given laser ranges of at least a few hundred km, maybe a few thousand how can ships maneuver? If they are slow, it will take minutes to change position, meanwhile zapping away with multimegajoule lasers. If they are fast, they'll hurtle past each other in a drive-by, then take hours to swing around for another pass, unless they have science-fictional levels of acceleration. Possible solutions include long recharge and/or cooling-off times between laser volleys, and restricted firing arcs on the laser turrets.

The argument rages on, which probably means you can just pick which side appeals to you and be able to justify it. By carefully selecting, say, the proper minimum laser recycle time one can decide whether missiles are a viable weapon or not.

The Attack Vector: Tactical wargame adds an additional wrinkle. The laser recycle time is set such that missiles are viable. However, laser cannons have a limited number of "flash cooler" loads which can drastically cut the recycle time. But once you've used up your flash cooler loads, the laser is stuck at the standard recycle time.

Radiators The "Achilles Heel" of combat spacecraft are the heat radiators. Drives, power plants, and most weapons generate incredible amounts of waste heat. For unlimited operations, the heat has to be disposed of with radiators. However, since by their nature radiators are difficult or impossible to armor, radiators will probably be the first thing shot off by hostile weapons fire. Then you have about thirty seconds to scram the ship's reactor before the engineering section turns into a sea of molten metal. This is because shooting a hole in a spacecraft's radiator will have the same effect as shooting a hole in your automobile's radiator, except at a much higher temperature.

Droplet style heat radiators cannot be armored, but they are relatively immune to hostile weapons fire, since they are basically liquid sprays of coolant instead of physical panels. There are some notes on weapon radiators here. And before somebody mentions the "refrigerator laser" from David Brin's novel SUNDIVER, there appears to be certain theoretical reasons why it would not work. For one it probably violates the second law of thermodynamics.

In AV:T, ships going into battle retract their radiators into armored cubbies. They then rely upon internal heat sinks to dispose of waste heat. The good thing is that the heat sinks are armored. The bad news is that they can only store a few minutes worth of heat. This puts a severe time limit on the length of combat. Naturally a battleship will have a larger heat sink than a destroyer, but it will also have a higher waste heat level to dissipate. If one's heat sink fills up too soon, the only option is to "strike the colors" and signal surrender to the enemy by extending the heat sinks (sort of like a dog in a dogfight surrendering by lying on its back and baring its throat). The alternative is being roasted alive as your ship melts.

Retractable thermo dump panels from Peter F. Hamilton's Night's Dawn trilogy. Read more at Scott Halls' Night's Dawn web site.

One concept for an orbital laser fort was to use a largish asteroid nudged into an appropriate orbit. The idea was to use the immense mass of the asteroid as a heat sink. During those long uneventful months the fort would use its radiators to cool down the asteroid as much as possible. When an attack occurs, all the fort's radiators are of course immediately retracted or are shot off by hostile fire. Both the fort and the hostiles will then commence lobbing laser beams at each other, and filling up their heat sinks with waste heat from laser cannon. However, when it comes to heat sinks, the internal sink of a warship is miniscule compared to the millions of tons of cold rock in an asteroid. It will require a fleet or two in order to even those odds.