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".
As mentioned before, advances in effectiveness of weapon lethality and defensive protection are mainly focused on the targeting problem. That is, the weapons are generally already powerful enough for a one-hit kill. So the room for improvement lies in increasing the probability that the weapon actually hits the target.
And the room for improvement on the defensive side is to decrease the probability of a hit.
Weapons can be improved two ways: increase the precision of each shot (precision of fire), or keep the same precision but increase the number of shots fired (volume of fire).
Precision of fire is governed by
- the location of the target when the weapons fire arrives
- the flight path of the weapons fire given characteristic of the shot and the environment though which the shot passes
- the weapon's aiming precision
Volume of fire is governed by
- the weapon's rate of fire
- the lethality of a given shot
A defense can interfere with the [a] location of the target by evasive maneuvers.
There isn't really a way to interfere with [b] the characteristics of a shot, short of inserting a saboteur into the crew of the firing ship (in science fiction there are sometimes technobabble "nuclear damper fields" that prevent nuclear warheads from exploding). A defense can interfere with the environment through which the shot passes by such things as jamming the weapon's homing frequencies or clouds of anti-laser sand (which may work in the Traveller universe, but not in reality).
There isn't really a way to directly interfere with [c] the weapon's aiming precision (again short of a saboteur), though one can indirectly do so by decreasing the target's signature, increasing the range or jamming the firing ship's targeting sensors and degrade their targeting solution (in science fiction is the infamous "cloaking device").
Finally, while one cannot do much about the [d] weapon's rate of fire, the [e] lethality of a given shot can be effectively reduced by rendering harmless shots that actually hit. This is done by armor and point defense (in science fiction there are "force screens" and "deflector shields").
The first rule of fighting is: Don't get hit.
If you can complicate your opponent's firing solution enough (i.e., dodge enough so all the shots miss), you do not need all that heavy bulky armor. Of course, if a shot does hit, you are up doo-doo pulsar without a gravity generator.
With fighter aircraft: weapon speeds, aircraft speeds, and target ranges are such that the main targeting problem is the large angular changes the undertaken by the target (you cannot slew the gun around quick enough). With spacecraft, however, the problem is light-speed lag and weapon lag. Light-speed lag means if your target is at a range of one light-second, you are seeing it where it was located one second ago, not where it is now. Weapon lag means you have to lead your target so that your plodding weapon shot will intercept it (the technical term is "deflection").
The attacker can try to minimize this by reducing the range, or using homing weapons that guide themselves to the target. The defender will try to open the range, and use various counter-measures to confuse the weapon's guidance system.
Naturally, if the target moves at a constant velocity with no course changes, light-speed lag and weapon lag cease to be a problem. Attackers love a target wtih a perfectly predictable course. Therefore, a target that wishes to live had better dodge and jink as much as possible.
Light speed lag and weapon lag will put an upper limit on the maximum probablity that an unguided weapon will strike the target. If you dare, you can calculate it with an equation I cobbled together all by myself (which means you had better double-check it first as I have been known to make childish mistakes in algebra).
H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)
H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)
- H = maximum percent chance to hit target given light-speed lag (0.0 - 1.0 with 1.0 = 100%)
- Cm = target ship's mean cross section (m2, for a purely convex object this is approximately 1/4 of the surface area)
- a = target's acceleration (m/s, where 9.81 = 1 g)
- Dm = range to target (m)
- Wv = weapon velocity (m/s)
- 299,792,458 = meters in one light-second
- 0.7854 = π / 4
Cm is the average target cross section. The target will be trying to orient itself so it presents the minimum possible cross section to the attacker, but the requirements of its propulsion system and other factors will interfere.
Since laser weapons travel at lightspeed (Wv = 299,792,458), for them the formula simplifies to:
H = Cm / (0.7854 * a2 * ((Dm + Dm) / 299,792,458)4)
Please note that this equation does not work if the target's acceleration is zero (since dividing by zero is mathematically undefined). In that case the target's official status is Sitting Duck and H = 1.0 or 100%. Neither does the equation work if the range is zero, in which the target's official status is At Point Blank Range or Eating The Gun Muzzle, and again H = 1.0 (Thanks to Eric Henry for pointing this out). Just remember that H cannot go over 1.0 and you'll be fine.
How was this equation derived? (just wait until you get a load of my assumptions...) Well, if H is chance to hit, a is acceleration in m/s, Dm is range in meters, and Cm is target's mean cross section:
Scircle = π * Rcircle2
- Scircle = surface area of a circle
- Rcircle = radius of the circle
- π = Pi = 3.14159...
H = Cm / (π * displacement2)
- displacement = maximum distance perpendicular to line of fire that the target can move in time between a shot being fired and the shot arriving at target
In other words, take the cross section surface area of the target, divide it by the surface area of the circle the target can move to, and you have your maximum hit chance. e.g., if the target has a surface area of 1, and it can displace anywhere into a circle of surface area 3, then the maximum hit chance is 1/3.
d = 0.5 * a * t2
- d = distance (m)
- a = acceleration (m/s2)
- t = duration of acceleration (s)
which is the classic acceleration equation, assuming a starting velocity of zero. We can assume zero because all we care about is the change in the target's current velocity, that is, the jinking
Now, to use acceleration equation to calculate displacement:
t = (Dm / 299,792,458) + (Dm / Wv)
- t = time it takes light from target to travel to targeting sensors plus time it takes weapon to travel to target (s)
- t = time target has to jink before weapon arrives
- Dm = range to target (m)
- Wv = weapon velocity (m/s)
- 299,792,458 = meters in one light-second
- Dm / 299,792,458 = time it takes light from target to travel to targeting sensors (s)
- Dm / Wv = time it takes weapon to travel to target (s)
d = 0.5 * a * t2
replace t with jink time
displacement = 0.5 * a * ((Dm / 299,792,458) + (Dm / Wv))2
Inserting displacement equation into hit chance equation and simplifying:
H = Cm / (π * displacement2)
H = Cm / (π * (0.5 * a * ((Dm / 299,792,458) + (Dm / Wv))2)2)
H = Cm / (π * 0.25 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)
H = Cm / (0.7854 * a2 * ((Dm / 299,792,458) + (Dm / Wv))4)
Note this equation only calculates the percentage chance of missing due to light-speed lag. There are many other factors that can contribute to a miss. However, most of these are not under control of the target.
Roger M. Wilcox (creator of the indispensable Internet Stellar Database) spotted a major flaw with the above equation. In it, I assume that the target can instantly dodge in any desired direction. Bad assumption. Most rockets can only accelerate in the opposite direction of the main engine's thrust plume. True, a rocket can turn using its attitude control system so the plume is aimed in any desired direction, but this takes time. And it takes longer if the rocket is long and skinny like a pencil, instead of short and fat like an orange.
I too had no idea such huge amounts of torque were required (though I did know about the moment of inertia problem, having read Sir Arthur C. Clarke's short story "Hide and Seek"). Since torch drive attitude jets are highly unlikely, this drastically reduces the angle the ship's nose can be changed by, and thus drastically reduces the area the ship can dodge into. Instead of a sphere, it will be reduced to a cone sharing its long axis with the ship. The angular width of the cone will depend upon the target's moment of inertia, the torque produced by the target's attitude system, and the time it takes the weapon to fly to the target.
If your rocket exhaust is cool enough that material objects can actually survive being inserted into the exhaust plume, a possible solution to the dilemma is Cascade Vanes. So instead of making your attitude jets as big as your main engine, you actually use your main engine as an attitude jet. However, an exhaust that cool is probably way below torch rocket levels.
If the pressurized habitable section of your warship was one single area, a hull breech would depressurize the entire ship (I was going to recount the ancient joke about "why is a virgin like a balloon", but luckily good sense intervened). A prudent warship design would use air tight bulkheads to divide the interior of the pressurized section into separate areas. This comes under the heading of "not keeping all your eggs in one basket". The keyword is redundancy.
For the same reason, you'd want back-up life-support systems, power plants, control rooms, and other vital components. And these duplicate systems should be located in widely separated parts of the ship. Otherwise a single lucky enemy weapon shot could take both of them out.
Even in the non-pressurized section, bulkheads can help contain destructive effects of hostile weapons fire. So an explosive warhead, with any luck, will merely damage the interior of one compartment, instead of gutting the entire interior of the ship.
Mark Temple says:
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).
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 29.6 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 148 kilojoules and 20 milliseconds. An AV:T laser cannon with 50 megaJoules could burn through 330 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 148 kilojoules to 15 megaJoules, but now if we have an uncertainty in the target's velocity of up to 5 meters per second it doesn't matter.
Of course, if price is no object, you can do better than carbon. Boron has a vaporization energy of 45.3 kilojoules/gram and is only slightly denser than carbon. Expensive, though.
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. With kinetic weapons at closing velocities in excess of 10 km/sec, you're getting into the realm where armor is less important than blow-through. For armor, you want something that will resist being turned into a plasma for as long as is possible, followed by gaps made of vacuum to make it a Whipple shield.
Anti-radiation armor is discussed here.
This is a rather famous story from World War 2, but I'm going to re-tell it anyway because it is so cool.
During World War 2, the United States had several top-secret programs where mathematicians and statisticians helped with the war effort by fighting with math. Their analysis and optimizations were aimed at giving the Allies the edge in combat, their work saved lives. One of the more important programs was the Statistical Research Group, based in Manhattan.
And at the SRG, the smartest guy in the room was Abraham Wald.
Wald was a genius, Wald was brilliant, and Wald was yet another example of the Nazi racist policy towards those of the Hebraic persuasion turning around and savagely biting off both Nazi Gluteus Maximus cheeks (another example being Albert Einstein). Wald had a nice little job in Austria, but after the Nazi conquest it didn't take the genius of a Wald (or an Einstein) to see if you didn't get the heck out of Austria you'd soon be in a concentration camp. Wald moved to the United States where he was quickly offered a professorship of statistics at Columbia. From there he was recruited into the SRG.
At the time the Allies were sending near-constant bombing missions to blow up German factories and otherwise destroy the German ability to wage war. The German response was a near-saturation level of anti-aircraft barrage that made Allied bombing missions into suicide missions. The bombers had to fly way high in the sky where everybody could see you, and linger for hours while every ack-ack gun in Germany did its darndest to kill you. The bomber crews figured they had the same chance of surviving a given mission as winning a coin toss, you were quite likely to die long before you got your fifty-mission crush.
The Army Air Force knew that any improvement of the odds no matter how small would make a big difference over multiple missions. They came to the SRG. They already knew that additional armor would be a huge help. But you couldn't just armor plate the entire bomber or it couldn't get off the ground. The question was applying armor to which spots would get the maximum survival benefit for their armor poundage dollar.
The Army Air Force had already done some preliminary work. They did an analysis of bullet-holes in the bomber aircraft and made Chart 1 (see above) which they proudly displayed to Wald. They told Wald that the idea was to put the armor plate where the bullet holes where thickest, since obviously that was the places that needed it the most.
But lucky for them, Wald was a genius.
Wald shook his head and told them they were wrong, the place to put the armor was the spots with the least holes. Yes, this seems illogical, but as it turns out the Army Air Force had fallen into the trap of failing to consider data bias. Specifically Survivorship bias.
Wald pointed out that the data was not that of bullet holes in bomber aircraft. It was data of bullet holes in bomber aircraft that had survived long enough to make it back to base.
Since bullet holes are more or less randomly spread over the body of a bomber, the fact that there are bullet-free places on the surviving bombers implies that the chart for the bombers that died would be the exact opposite. It would look something like Chart 2 (see above). Chart 2 is the chart for bombers that did not return to base. Therefore a bullet in the bullet-zones of Chart 2 makes the bomber go down in flames. That's where you need to put the armor.
The bullet zones in Chart 1 were merely the spots you could shoot a hole in a bomber and not kill it. Those were the spots the bomber was strongest.
History doesn't record the reaction of the Army Air Force, but I'd hazard a guess they turned pale at the magnitude of the blunder that Wald had just rescued them from.
Wald refined the data and actually developed equations showing the relative vulnerability of each bomber part, and the probability of being shot in a given part depending upon the intensity of the anti-aircraft fire. Equations that are still in use to this day.
In this science fiction story, author Joseph Millard does what I have advocated: find some real scientific fact that is really weird, and use it as a springboard for your story plot. It gives you the author some inspiration. And if your readers check your facts, they will stub their toe on the reality of the matter and be duly impressed by your research.
Mr. Millard started with the fact that apparently Kansas is a meteorite magnet.
As of July 2009, there are 1,530 verified meteorites that have been discovered in the continental United States. There are 49 continental states so you'd figure each state would have 1/49 = 2% share of the 1,530 meteorites. But the state of Kansas has a whopping 9% share (137). Indeed it does seem to be a meteor-magnet.
Starting from that, Mr. Millard created a lurid tale of invaders from outer space. These initial meteorites were just advanced scouts. Then one fine day nine of the suckers landed simultaneously. A meteorite investigating team arrived to investigate the meteorites, and were immediately turned into alien-controlled automatons. They started building an infernal device for the invasion. About this time the deadly Crimson Plague struck, yet another fiendish part of the alien master plan.
As you can see, Mr. Millard sure got a lot of springboard action out of one little fact. So much action, in fact, that the novel was made into a movie called They Came From Beyond Space. Granted the movie was a big flop but I'm sure Mr. Millard was happy his little novel got that far.
On a tangent, I noticed that when the people in charge of the Superman franchise actually mentioned the location of Superman's home town of Smallville, a common choice was in Kansas.
As you impatiently tap your feet and pointedly look at your smart phone for the time, I'm sure you want to know why is Kansas a meteorite magnet. Well, actually it isn't. As mentioned above, the spectre of data bias raises its ugly head; this time in the form of Sampling Bias.
You see, there is an unspoken assumption that meteorites are equally easy to find in all of the continental states. But as it turns out, that just ain't so. For geological reasons Kansas has very few rocks of any kind close to the surface. In other states people spotting a meteorite would react "Oh, just another of the zillions of rocks lying around, how boring". In Kansas however, the reaction is "What the heck is that??!?". So Kansas meteors are immediately identified as something weird to take to your local geologist and find out if it is worth any money.
In addition most of the land in Kansas is heavily cultivated with crops. This means there is a distinct lack of [a] trees, [b] buildings, and [c] paved roads. All of which hinder the spotting of meteorites lying on the ground. Kansas is also relatively arid, so meteorites will disintegrate more slowly than in more damp states.
Therefore sampling bias does reveal that there is nothing special about Kansas, meteorite-wise. But unlike Abraham Wald's bullet-hole maps, it does not diminish Kansas' value as a springboard for authors. Many readers will only find the initial reports and still be impressed. And if the author wants to cover all the bases they only have to suggest that the sampling bias stories are part of a sinister government cover-up.
And you can forget about laser defenses like Traveller style Sandcasters. These fire clouds of magic "prismatic" dust that provide protection from hostile laser fire. In reality they would not work. There is no way that they can project a cloud dense enough to do any good.
However, the gang at rec.arts.sf.science are skeptical:
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 shields, deflectors, energy screens, they are all handwavium science fictional armor composed of some form of energy instead of dull boring matter.
Names include force fields, force shields, force screens, energy screens, energy shields, deflectors, deflector shields, ray-screens, repulsor screens, and many more.
Since they are imaginary their abilities and properties are only limited by the imagination of the science fiction author. Some stop bullets but let laser beams through, some stop lasers but are useless against bullets, some stop both. They can be invisible, mirrored like chrome, or glowing in various colors and levels of brilliance. They are occasionally used in a creative fashion, e.g., "structural integrity fields" are force fields interpenetrating a spaceship's metal frame in order to make it stronger than is possible with mere matter. Or gravity shields that allow one to ignore the gravity of a nearby planet.
In other words the author has to be real careful in order to avoid unintended consequences.
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". Researchers have been experimenting with using magnetic and electrostatic fields to ward off particle radiation. More on the fringe are plasma windows, which could defend against microwaves and particle radiation. But they have a long way to go before they can stop weapon-grade particle beam weapons.
Plasma windows can separate pressurized areas from unpressurized areas with a sort of force-shield "door". Air cannot pass through the plasma window into the unpressurized region, even under a pressure differential of up to nine atmospheres. So if a radiation bean generator requires vacuum but the item being irradiated is in an atmosphere, the plasma window works nicely.
It is currently used for electron beam welding. The beam generator is inside a vacuum chamber, the electron beam passes through the plasma window and welds the metal sheets on the workbench in the shirt-sleeve environment. In theory one could use this as a quick-pass airlock or hangar-bay door on a spacecraft.
Plasma windows have a bright glow, the color depends upon what gas the plasma is using. Argon is violet, nitrogen is orange.
The drawback of plasma windows is that they are power hogs. For a round window they need 8 kilowatts per centimeter of window diameter. Other than that there is no limit on diameter.
But there isn't anything in the real world like E.E."Doc" Smith's electromagnetic radiation stopping "ray-screens", nor his matter stopping "repellor screens."
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.
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.
What do they do?
Before embarking on any speculation of mechanism, we should first determine what a shield does. Shields in sci-fi generally serve 2 purposes:
Stop "energy weapons" (lasers, phasers, blasters, etc).
Stop physical objects (bullets, knives, people, vehicles).
Fair enough, but what does it mean to "stop" energy weapons and physical objects? Physical objects tend to stop abruptly when they hit a shield, but not always: sometimes the "slow blade passes" concept from Dune seems to be in effect, and objects can slip through (see the Gungan dome shields in the Battle of Naboo). Sometimes incoming objects bounce, and sometimes they explode on impact. And what about "energy weapons"? Some ships (such as the Trade Federation battleship in the end of TPM) look as if they're taking every hit right on the hull, but their shields are said to remain up, as if they are coincident with the hull surface. Others project their shields out into space, and these shields light up in a multi-coloured display when hit.
Are shields forcefields?
The most popular candidate for shields is forcefields. In fact, shields and forcefields are often treated as interchangeable terms in the literature and dialogue. This is encouraging, because the term "forcefield" comes from real-world science, not science fiction. Unfortunately, the resemblance between real forcefields and sci-fi forcefields ends at the name.
The two types of forcefield you are most familiar with are electromagnetic and gravitational. Sure enough, those are the forces routinely mentioned in sci-fi. In the 1950's classic "War of the Worlds", the Martian spacecraft were said to be using an "electromagnetic blister", which easily warded off artillery shells and all other methods of attack. In Star Trek, the writers gleefully steal terminology from particle physicists and say that they're based on "gravitons" (the theoretical carrier particle for gravitational forces).
But electromagnetic and gravitational forcefields share an interesting characteristic: they are both long-ranged, and their effects weaken with the square of distance. So if you double your distance from the centre of the Earth, the force of gravity drops to one quarter of its original value. Simple enough, correct? Unfortunately, this creates a problem for our shields: you see, they typically have no effect whatsoever until you reach some invisible point. When a man runs into a forcefield on Star Trek, he feels nothing until he touches the invisible wall, which produces a sparkly effect.
Now, if this were a gravitational forcefield, he should have felt its effects from anywhere in the room, gradually increasing in strength as he approaches the window. A forcefield has a volume effect, hence the name "force field", not "force wall". But this is obviously not what we saw. Have you ever tried to force the positive poles of two magnets together? You can't do it, can you? And you will notice that the forcefield effect is gradual, not abrupt. It gradually increases as they approach, until it eventually becomes so large that you cannot force them any closer together.
And what about the fact that they wear down? We've all seen the displays: DEFLECTOR POWER 70%
In Star Trek, rather than simply being up or down, shields have a strength property. It wears down after multiple hits, and when it goes to zero, the shields are nonexistent. But why would a forcefield weaken after use? Does the magnetic field of an electromagnet get weaker each time you use it to pick something up? No, so why would shields get weaker? Is there a "fatigue" property? None of this is consistent with a forcefield.
Are shields made of energy?
Rather than imagining shields as forcefields, some people imagine them as a "wall of energy". That seems like an improvement (after all, there are no real walls of energy which we can use for comparison, so it's not as easy to say that it's wrong), but even if we disregard the question of how you would go about constructing this beast, some obvious questions leap to mind:
What holds this energy in place? Pure energy is light, and moves at c. It does not sit in a particular spot, nor does it form walls of arbitrary shape. If you had some kind of mechanism which could control the energy and force it to move in a contour around the ship, why bother with the energy component? This mysterious energy-manipulating mechanism would obviously be capable of deflecting incoming energy weapons by itself if it can already manipulate a wall of energy to hold an arbitrary shape.
Why aren't incoming objects destroyed by this energy? Not only can Picard touch one of these shields with his hand, but incoming objects such as Roga Danar's flimsy escape pod in "The Hunted" have been observed to bounce off a starship shield with no ill effects.
Where does the energy go when they turn off the shields? If it's released, should it not be quite violent?
Why would the energy necessarily interact with other energy?
The problems with the "wall of energy" idea are extremely difficult to resolve for many reasons, not least of which is the fact that the mechanism for manipulating the energy into a shield would perform the function of a shield all by itself.
What about frequency?
Star Trek shields have a "frequency" characteristic, which implies that they oscillate. It should be noted that this behaviour is unusual to Star Trek, and there is no reason to assume it is universal to all shield concepts. Many natural phenomena are frequency-based, but even a device based on a frequency-based principle need not be phase-coherent, so it would not exhibit an aggregate "frequency" characteristic. There are some general advantages and disadvantages of frequency coherence in shields:
Against a frequency-based attack, a phase-coherent shield could be theoretically optimized to give greater protection than a flat shield with the same average amplitude, by synchronizing with the attack.
Against a non-coherent attack, a phase-coherent shield would allow partial penetration even if it's working perfectly.
It should be possible for a ship to fire outgoing weapons out through its own phase-coherent shield by matching frequencies but being 180 degrees out of phase. You would have to open a small hole in a flat shield in order to fire through it, which requires fine control over shield geometry.
The knife cuts both ways. An attacker could penetrate a phase-coherent shield by matching frequencies and being 180 degrees out of phase.
Note that it's possible to oscillate with respect to amplitude or vector, although we would expect a vector oscillation would cause significant scattering effects with outgoing beams (turning a tight beam into more of a spray), and we generally don't see that with Trek weapon/shield interactions, or those of any other sci-fi series for that matter. A square-wave (as opposed to sinusoidal wave) would allow perfect penetration with a synchronized weapon, thus eliminating the scattering effect of a vector oscillation, but it would also allow 50% penetration from an incoherent weapon.
There are interesting theoretical possibilities for a shield which oscillates, but the disadvantages outweigh the advantages, depending on what kinds of weapons the enemy is using, what kind of control you have over shield geometry, etc. Worse yet, if the enemy has sensors which can detect the activity of your shields, it should be trivially easy for him to match frequencies, synchronize phase, and shoot through your defenses. Ultimately, the idea of a phase-coherent frequency-based shield seems more attractive for its script-writing flexibility than its tactical attributes.
Sci-fi is diverse, and not all shield systems do the same thing. But there are a few questions you can ask to narrow down what basic phenomenon a shield represents (note that this is different from trying to invent some technobabble explanation for how it works).
Do energy bolts or beams "bounce off" the shields, still intact? If so, you are looking at reflection (for examples, see the trash compactor scene in ANH, or the battle droid shot which ricocheted off Anakin's Naboo starfighter in TPM).
Do energy bolts or beams splinter, or break apart into a shower of smaller bolts? If so, you are looking at scattering (for an example, see the ISD turbolaser bolt that struck the blockade runner's shields in the opening scene of ANH).
Do energy bolts or beams make a large area of the shield glow? If so, you are looking at absorption, conduction, and subsequent retransmission (for examples, see most TNG-era Trek shield incidents, as well as the incident in TESB where the Falcon was knocked about its longitudinal axis by a turbolaser hit).
Does shield geometry affect the shield/energy interaction? If so, it may be a vector effect, deflecting the bolt in specific directions based on geometry (phrases like "angle the deflector shield" in SW or "continuously vary the shield geometry" in ST hint at this possibility).
Does the shield completely block incoming energy, or does it allow a portion through even if it is still functional?
Ultimately, there are far more questions than answers when it comes to shielding, and one must be careful not to leap to facile conclusions.
Shielding is more complex than it may appear on first glance. There are more issues to consider, and more difficulties involved in evaluating strength than one may initially realize. However, this hardly means that the exercise is futile. On the contrary, a thorough and systematic examination of observed events can be used to determine realistic limits, given certain caveats:
The fact that they often call it a "forcefield&quoquot; does not mean it actually conforms to the description of one.
There is no intrinsic need for a great volume of energy in a shield, so it is wrong to assume that shields must consume large amounts of energy. Do not become attached to a particular model of shields simply because everyone else seems to accept it. A solid case can even be made for the idea that shields have mass.
Always remember to consider the weakest link in the chain, not the strongest link. This is an important lesson from real-life engineering which is often lost on sci-fi debaters, who tend to conceptualize sci-fi in a purely abstract theoretical sense in which they pick one particular phenomenon and concentrate on that phenomenon to the exclusion of all others (ie- focus on the non-physical shields and ignore physical constraints).
Do not assume that all energy shields employ the same mechanisms. We can view the behaviour of energy shields in action and see that there is significant diversity in their operation, and this must be considered when attempting to synthesize a consistent model of their operation.
At no point do any of these theories require that the shield must draw as much energy from the ship's systems as the incoming weapon carries with it. Yet I have often noted that virtually everyone assumes this energy equivalency to be the case, without making the slightest attempt to explain the logic. Why should a shield require energy equivalent to the weapon? Does a piece of armour on a tank consume energy when a shell bounces off its surface? Did you ever wonder why an air conditioner's rate of cooling can exceed its electrical power draw in watts?
Before making any leap in logic about how much power a shield must need or how it must work, just ask yourself whether your assumptions are coming from observation and logic, or from common practice. Because ultimately, common practice is a poor justification for anything.
It is widely assumed that if a sci-fi shield can withstand X joules of energy from a laser, it must be able to withstand X joules of energy from a physical impactor. However, this is not necessarily the case. As attractive as the simplistic numbers game is, if we apply a little bit of physics knowledge to the situation, we can see that if anyone were to build such a beast, the situation would be more complex than that.
So what would make physical impactors more dangerous? The answer to that question comes down to damage mechanisms. To put it simply, a physical impactor inflicts damage upon its target in a variety of ways. While an energy weapon will generally attempt to heat the target, thus permitting specialized one-dimensional defensive strategies, a large, fast-moving physical impactor presents a more complex threat:
Energy weapon (laser, phaser, turbolaser bolt, etc.)
Heats the target surface.
Physical impactor (asteroid, high-velocity ramming attack, hyper-velocity railgun, etc.)
Subjects the target to severe structural stresses, usually resulting in penetration. If it fails to penetrate, it pulverizes and/or vapourizes at the point of contact due to internal stresses and work-heating, thus producing a large cloud of high-temperature material at the target surface. This cloud heats the target surface through convection and radiation.
That is why an effective defense strategy would use guns to destroy large physical impactors, forcefields to deflect small physical impactors, and shields to reflect, absorb and retransmit, or scatter energy weapons rather than the "one size fits all" approach that seems to be popular among fanboys.
When a physical object strikes a shielded vessel, it must be decelerated by the vessel's defensive systems. Most people tend to assume that if the shield holds, the ship is undamaged. However, this is not necessarily the case. Consider the following image (and please, keep in mind that I am not a professional graphic artist):
Let's assume that the rectangular assembly at left is a shielded starship (yes, I know, it looks cheesy, but please bear with me). The big brown rock at right is hitting the ship's shields, and it is being decelerated (hence the rightward force F being applied to the rock by the forcefield). For every action, there is an equal and opposite reaction, so there must be a counter-balancing force for that forcefield. A forcefield must be coupled to something, and in this case, it would obviously be the shield generator. Therefore, there is a leftward force F being applied to the shield generator (the blue square) in the middle of the ship. But the shield generator cannot move relative to the ship or it would be torn loose from its moorings, so its mounting brackets (the four red blocks) must each apply a rightward force 0.25F in order to hold the shield generator in place. These four reaction forces, in turn, push the entire ship to the left with force F, so the net result is to stop the impactor while accelerating the ship.
Are we clear on that? Now here's where it gets interesting: what if the shield generator's projected forcefield is easily strong enough to decelerate the asteroid to zero before the moment of impact, but the four little red blocks aren't strong enough to hold the generator in place? Guess what: the shield generator will be torn from its moorings, and the rock will slam into the ship. This is where momentum can rule over energy; a low-momentum, high-energy weapon such as a laser might not be as dangerous to a shielded vessel as a high-momentum, low-energy physical impactor. In this scenario, the potential points of failure are the shield generator itself, the points where it is mounted to the vessel, and the structure of the vessel itself. In other words, the mounting brackets, bolts, welds, shield generator internal mechanisms, shield generator forcefield strength, and all other connecting bits are parts of a chain through which reaction forces must go in order to make the end-to-end connection between the ship and the impactor. It can be thought of as a chain, and as in any chain, it is the weakest link that will cause your downfall.
As you can see, even if it was possible to build a deflector shield generator of virtually infinite strength, the overall effectiveness of the system would still be limited by good old-fashioned structural limits. Ultimately, the survivability of a shielded spacecraft against physical impacts could (and would, given sufficient shield strength) conceivably come down to a set of bolts holding a shield generator onto the ship's spaceframe. This example highlights the severe problem with most attempts to rationalize sci-fi technologies, which is that people tend to look for the strongest link in the chain, not the weakest link in the chain.
Shield Collision Physics Summary
Physical impacts and energy weapons should not be treated as functionally identical, particularly in terms of the relationship of energy to structural stress in the target. Collision physics are still ruled by Newton, and all of the deflector shields and fancy tricks in sci-fi will not prevent reaction forces from acting upon the physical structure of a target spacecraft.
Ramming tactics are widely used in sci-fi (click here for an example analysis of a collision event). In Star Trek, Worf called for "ramming speed" in STFC, Jem'Hadar vessels rammed the USS Odyssey and destroyed it in DS9, and Commander Riker prepared to ram the Borg Cube in the TNG two-part episode "Best of Both Worlds". In Babylon 5, we saw a Starfury crash into and through a Minbari war cruiser's dorsal fin in the Battle of the Line as shown in the movie "In the Beginning", and Jeffrey Sinclair tried to ram another war cruiser later in that same battle. We also saw an Earth-force cruiser ramming a Minbari war cruiser in a brief flashback to the events leading up to that battle. Other examples include Battlestar Galactica, where Cylon raiders routinely crashed into the Galactica's flight decks (thus making the viewer wonder why there were no weapon emplacements near these flight deck entrances), Transformers (where the Autobots' stolen Quintesson corkscrew-ship rammed through Unicron's eye), and of course, ROTJ, where an A-wing crashed through the bridge windows of the Executor after its bridge shields were knocked out (which would imply that the Executor's unshielded bridge windows are similar in strength to the dorsal fin of a Minbari war cruiser).
The effectiveness of these popular ramming tactics has often been used as an excuse to downgrade shield estimates against energy weapons. But this implies an equivalency which does not exist. The "real-world" explanation for the effectiveness of ramming in sci-fi is that ramming is a very dramatic event, filled with imagery of martyrs and heroes. But the physics of collisions and reaction forces provide us with an "in-universe" explanation that works just as well.
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.
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.
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).
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.
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.
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.
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.
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.
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.
And no, you cannot solve the problem by using a thermocouple to convert the heat into electricity.
Having said all that, Isaac Kuo is having second thoughts about the impossibility of armoring radiators.
Jens Bartmann disagrees with Anthony Jackson:
Zane Mankowski (author of Children of a Dead Earth) makes a good case that heat radiators can indeed be armored. Mr. Mankowski says the thickness of the radiator material can be increased to provide armor-like protection for the working fluid tubes, with the price of reducing radiator efficiency.
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 vulnerable 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. This presumes that heat radiators are pathetically easy to destroy with hosile weapons fire.
Also note this is a very similar situation to the science-fictional Langson field. Specifically, if the enemy gives you enough time to cool down your heat sink, your ship becomes combat-ready again, and the battle starts anew. You will have to give the enemy a safe way to disable your ship, or they will be forced to destroy you. In case of the Langson field, the tradition is to allow the enemy to send a low-ranking ensign wearing a tactical nuclear weapon on a dead-man switch, who enters your ship, parks themself in the control room, and ensures you do not break your surrender.
Equations for heat sinks can be found here.