So Never Get Into The Space War..

With all this frightfulness flying at your ship, you’d want some kind of defense, besides just hoping they’ll miss. 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 [a] the location of the target when the weapons fire arrives, [b] the flight path of the weapons fire given characteristic of the shot and the environment though which the shot passes, and [c] the weapon’s aiming precision. Volume of fire is governed by [d] the weapon’s rate of fire and [e] 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. 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.

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, point defense, and science-fictional force fields. 

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. 

Recently a discussion about “armor belts” and the durability of space warships has cropped up by me. This got me thinking about compartments and how they’d be an integral part of a ships survival. Modern naval vessels are divided up into compartments to make them more survivable. Compare a naval frigate to a main battle tank. A tank is basically one compartment. Breach its (very thick) armor and you wreck the tank, since the hit will usually kill the entire crew and/or destroy the internal systems. A frigate however has multiple compartments. Breach the hull of the frigate and while you might wreck one compartment, the entire ship will still float and will often still be able to fight. You have to wreck many compartments, or very specific compartments, in order to mission-kill the ship. It seems to me this vital part of naval design would not be overlooked in space warship design. Beyond the obvious benefits of making it easier to control atmospheric leaks, a space warship built with many compartments that can be isolated would gains a structural benefit in combat. 

Now, compartments would be worthless if one hit could completely disable vital systems like life support or command-and-control. Thus all these systems would be distributed all across the ship, with multiple redundancies. Thus if you lose a compartment with life support systems, you have others to fall back on. Having the main CiC compartment destroyed will not totally eliminate your ability to control the ship. This is standard for real world navy ships. Engine systems, command rooms (bridges, CiC’s, etc.) would have secondary locations kept manned in battle in case the main compartments for them are destroyed. 

  This is also why those compartments would be buried as deep inside the ship as possible. No sense in making things easy for your enemy. True, on modern wet navey warships bridges are still mainly at the highest point of the ship, but that’s mainly to facilitate visual tracking and identification. In space, you cannot see the enemy with the naked eye anyway, so you might as well put your command centers where the enemy has to destroy the entire ship to get at it.  

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).    

  

ThereforeTherefore, 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.    

In science fiction movies and television, we have never really seen all of these features at once. Ironically Star Trek managed to get the distributed systems part correct, we eventually even saw that Federation starships had “battle bridges” to provide emergency control should the main bridge be damaged. But Star Trek has utterly failed to put the bridge in defended positions, or show proper compartments in their designs (As David Gerrold noted, that silly bridge perched on the saucer top of the Starship Enterprise would have been shot off a long time ago). Apparently they rely on their handwavium deflector shields to do the job, which is great until you run out of power. Battlestar Galactica came pretty close, though. The ships systems are distributed, the ship itself compartmentalized, and it has a bridge buried deep in the hull. We just never see redundant engine rooms or command centers, which is probably more of a failing of the script writers than of design. In novels we see this idea used properly, though. The Honorverse novels showcase the benefits of compartmentalization in a very obvious and graphic form, in nearly every novel.

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About bruceleeeowe
An engineering student and independent researcher. I'm researching and studying quantum physics(field theories). Also searching for alien life.

5 Responses to So Never Get Into The Space War..

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  3. Machinus says:

    One thingabout Anti-laser armor; How about a “simple” 100% (or as close as possible) reflection mirror. All that bulky and massive vaporising armor is fine, however why take the hit when you can deflect/scatter it (or most of it)??

    • bruceleeeowe says:

      Interesting point you have raised, Machinus! A simple 100% or close to that would reflect laser beam but main problem is that even if .01% energy of laser beam is absorbed , it would be enough to melt down that reflecting device.

  4. Pingback: Why Self Destruction? « Bruceleeeowe's Blog

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