Tuesday, 22 May 2018

Lasers, Mirrors and Star Pyramids

Lasers can hit targets at extreme ranges, at the fastest speed possible. They are ideal weapons for space warfare. 
However, everyone knows that lasers bounce off mirrors... does this make lasers useless?
The post is inspired by the discussion that arose from the conclusions stated by Kyle Hill (@sci_phile) in 'The Truth about Space Warfare' video for Because Science.
Many commenters asked me about how mirrors could be used to counter lasers. This is the response I promised. 


Common assumptions and common answers
An electrically pumped VECSEL, which promises high efficiency and powerful beams. 
The usual understanding of laser weapons is that they produce a bright light that slices through armor. Mirrors can bounce away this beam, rendering it harmless.
A laser defeated by a mirror?
The realistic science fiction community, centered around the efforts of the Atomic Rockets website, did their research with regards to the effectiveness of mirrors against lasers, and found that no mirror could be 100% reflective. It would always absorb some of the laser's energy, which would result in it heating up, melting and therefore losing its reflective properties. Because laser weaponry produce several kilowatts to hundreds of megawatts of power, the transition from 'cold and reflecting' to 'molten and ineffective' would happen very quickly.

As a result, the common answer to 'what about mirrors?' is that they are ineffective.

ToughSF will revisit the concept, as it has done with other 'established' truths of science fiction.
How do lasers deal damage?
A laser guide star for a telescope's adaptive optics. 
To first understand the effectiveness of a defense, we must first understand the type of damage it is trying to mitigate.

Lasers produce an intense spot of light on the surface of the target. As a directed energy weapon, its primary purpose is to heat up that surface until it deals damage. 

Continuous lasers produce a constant stream of energy. Pulsed lasers produce short bursts of much higher power, but much shorter duration, spaced by intervals as small as a microsecond.


Pulsed laser effects at different pulse lengths. 
One measure of a laser's destructive capability is its intensity at the target, in watts per square meter (W/m^2).

Sunlight has an intensity of 1kW/m^2 on Earth's surface, lightbulb filaments produce an intensity of 1MW/m^2 at their surface and lightning strikes flash at an impressive 10GW/m^2 in some cases. 

When a laser strikes a surface, it raises the temperature of that surface's material. At a certain temperature, it melts and moves out of the way. Other materials vaporize, turning into a gas that expands rapidly. At very high intensities, such as those produced by a pulsed laser, the vaporization gases expand so quickly that they overcome the mechanical strength of the surrounding material and shear off chunks or drag along debris out into space.
The best armor materials are those that take the most energy to heat up (high heat capacity), the most energy to destroy (high melting or vaporization energy) and have the highest mechanical strength (tensile strength). In space, every kilogram is important, so we also want the armor to have good characteristics for its weight.

For these reasons, steel is a bad armor material while graphite is an excellent armor material.
Steel cutting.
Steel has a low heat capacity (0.5kJ/kg/K), a low melting energy (240kJ/kg at 1640K) and good strength (600MPa). 
Vaporizing graphite to produce carbon nanotubes require extreme temperatures. 
Graphite has a great heat capacity (0.72kJ/kg/K), an extreme vaporization energy (59.5MJ/kg at 4000K) and poor strength (30MPa). 

Steel can only absorb 923kJ/kg before it is destroyed. Graphite absorbs up to 62MJ/kg, meaning each kilogram of graphite is worth 67kg of steel armor. 

What does this all mean in practice?
Laser heating up and vaporizing the surface of an asteroid. 
Well, when no mirrors are being used, lasers can burn through significant depths of armor from long distances, and can keep scratching away at armor from even farther away. To find the penetration rate of a laser, we divide the laser intensity at the target by the armor's ability to absorb energy before being destroyed. 

Here's a worked example:
100kW MBDA laser.
Imagine a laser producing 10 MegaWatts of power. It has a wavelength of 450 nanometers, which is great at travelling through our atmosphere. The focusing mirror is 10 meters wide, about half as wide as the one the James Webb telescope uses. 
At 100 kilometers, this laser produces a beam 11mm wide with an intensity of 105.6GW/m^2 at the target. It can melt away 10.32 kg of steel per second, or vaporize 154 grams of graphite. This translates into a penetration rate of 13.5m/s and 0.7m/s respectively.
We'll assume a perfect weapon that produces diffraction-limited beams (as focused as possible).
At 1000km, the beam spreads to 110mm wide intensity drops to 1.05GW/m^2. The penetration rate falls to 7mm/s in graphite.

At 10,000km, the penetration rate falls to 0.07mm/s. At 20,000km, it is 0.017mm/s, and so on.

With each increase in distance, the penetration rate falls by the square of that increase. These numbers might not seem to be impressive at the distances usually discussed when talking about space travel (millimeters?!) but they do add up over time. If the distances are great, they take a long time to cross. During that time, a huge amount of armor can be removed. 
When talking lasers, we are handling distances long enough to make the Earth look small. 
For example, a spaceship travelling from the Moon (400,000km away) in a straight line towards the Earth at a rapid rate (10km/s) while facing the 10MW laser described above would lose a full 3358 meters of graphite armor before it even reaches Low Earth orbits (200km)! It would be very impractical if all spaceships had to cover themselves in several kilometers of armor to survive crossing the relatively short Earth-Moon distance!

Hot Surfaces

At high temperatures, surfaces emit light and heat. 
Is the scenario described above actually realistic? Can lasers scrape away meters of armor if left unchecked?

Like many SF writers and thinkers so far, we ignored the fact that as the armor heats up, it radiates away energy like any hot surface does. Blackbody radiation is proportionate to the 4th power of temperature and can be measured in Watts of heat radiated per square meter (W/m^2).

Thermal radiators on all spacecraft are needed to radiate away waste heat.
This blackbody radiation is directly comparable to the laser intensity that reaches the armor.

If the blackbody radiation is equal to the laser intensity, then the armor is dissipating as much heat as it is absorbing. The temperature therefore cannot increase to the point where melting or vaporization happens!



Planets achieve thermal equilibrium with their star.
So, at what distance does the laser beam become weak enough that the armor's heat dissipation properties prevent any damage from occurring? Let's work this out for steel and graphite, using the same laser as before.

Steel melts at 1640K. The blackbody radiation rate at that temperature is 420.2kW/m^2. For the laser intensity to match this value, it needs its 10MW of power to be spread out over a 23.8m^2 surface area. This happens at a distance of... 50,000km. 


Graphite vaporizes at 4000K. It can dissipate up to 14.5MW/m^2, meaning it can withstand the laser beam from a distance of 8506km. 


What does this mean in practice?


Well, for steel, no damage occurs at distances further than 50,000km. For graphite, this distance is as little as 8506km. The armor only accumulates damage at distances shorter than that number of kilometers.


Furthermore, as the spaceship approaches the laser, you get to subtract the radiated energy from the laser energy to get the amount of heat absorbed and used to vaporize the armor. 


So, for example, if the spaceship closed in to 8000km, the laser intensity reaches 16.5MW/m^2. We subtract the heat radiated (14.5MW/m^2) and obtain a 'net' intensity of 2MW/m^2. This 'net' amount only vaporizes about 20 grams of graphite per second. 


The Mirrors

What if we used mirrors?

A mirror with 90% reflectivity at the wavelength the laser uses would reduce the heat absorbed by a factor 10. 

However, they are not good blackbody surfaces. A 90% reflective mirror would radiate 10 times less heat through blackbody radiation than a black surface such a graphite. Alone, if would not provide any benefit and it would just melt or burn up.

Low emissivity (reflective) surfaces don't lose a lot of heat through blackbody radiation.
Mirrors must be paired with active cooling.

Mechanisms such as those used to keep rocket nozzles cool or jet turbines functional can be applied to mirror armor to remove massive amounts of heat in a short amount of time. It involves moving a large amount of coolant at high pressure and velocity through the narrow channels of a heat exchanger to transfer heat from the component into the coolant. 


Again, we can describe the effectiveness of active cooling in terms directly comparable to laser intensity: W/m^2. 


If the laser intensity equals the active cooling rate, the mirror surface will not heat up.

Thermal receivers for concentrated solar power face many of the same challenges.
Let's continue the previous example and equip a spaceship with actively cooled mirrors that reflect 90% of the laser energy away. To use a real world example, we will use the numbers from the cooling system that absorbs intense, concentrated sunlight in solar thermal receivers. This example of a volumetric gas tube absorbs over 2MW/m^2. Another example from the solar energy industry is the High-Flux Solar Furnace that handles up to 11MW/m^2.  


Liquid salts coolants allow for higher heat fluxes to be absorbed.
We work out that the actively cooled mirror can survive a laser intensity of 11*10: 110MW/m^2. 

The laser would have to be firing from a distance of 3500km to overwhelm the set-up so that it can start damaging the mirror and then the armor underneath. 


3500km is of course, much shorter than the distances cited so far, and no damage takes place at longer distances.  


Active cooling systems that can handle even more impressive heat fluxes, and mirrors that reflect a greater percentage of the laser energy, will shorten the lasers' effective range even further. 


Sloped surfaces and Fresnel Reflection


There is more that can be done to reduce the combat distances imposed by powerful space lasers. 
Sloped surfaces are able to spread a laser beam's energy over a larger surface area. This decreases its intensity and the damage that it can do. We can work it out using cos(slope angle). 
At 45 degrees, the surface area is increased by 41% and the laser intensity decreases by 29%.

At 60 degrees, the surface area is increased by 100% and the laser intensity decreases by 50%.

At 80 degrees, the the surface area is increased by 476% and the laser intensity decreases by 83%.

If graphite armor is sloped at 80 degrees, it can withstand a laser of 5.76 times the intensity it could have without sloping. This reduces the 'no damage' distance from the laser by a factor 2.4.

This suggests that sharp cones are very effective at reducing laser effectiveness, and would be the optimal shape for armor on a spaceship. 
Space warship with conical frontal section, by Grokodaemon.
Polygonal shapes such as pyramids with triangular, square, hexagonal or other bases could be more effective than a rounded cone by creating a compound angle (vertical and horizontal sloping) against the laser.
The 75.04 degree vertical angle compounds with the 45 degree horizontal angle to create a 10.55 compound angle.
An even better arrangement would be a pyramid with a star-shaped base, or 'star pyramid'. 
Drawn in Geogebra.
The vertical angle can be compounded by a very sharp horizontal angle. For example, an octagram 'star' can form the base of a pyramid 10 meters wide and 57 meters long, and achieve an 80 degree vertical angle and a horizontal slope of 22.5 degrees. The compounded angle becomes just 3.81 degrees, allowing for a 15.05x decrease in laser intensity. 

Another benefit from extreme sloping is that Fresnel Reflection becomes significant. 
At low angles, even dark surfaces become shiny. 
We first have to find the refractive index of the armor material with regards to the wavelength the laser is using. A database such as this one is excellent for this purpose.
We find that graphite has an refractive index n = 1.5179 in 450nm light. 

That index, along with the slope angle, can be used to calculate how much of the laser's energy is reflected away. At 45 degrees, it is an insignificant 5.45%. An extreme compounded slope such as the one produced by the star pyramid as described above would allow for 68.9% of the laser energy to bounce off harmlessly. 

A graphite octagram star pyramid with 80 degrees of vertical slope and 67.5 degrees of horizontal slope would survive the 10MW laser at a distance of 2010km. 

Dielectric mirrors, when sloped, lose effectiveness against lasers of certain polarizations. Omnidirectional mirrors are needed. 

If we used an extremely sloped pyramid (80 degrees) in addition to a star-base with a very large number of sides (18 sides for 80 degrees), omnidirectional dielectric mirrors (99% reflectivity) and active cooling (11MW/m^2), we could expect to increase the damaging intensity threshold to 33.16*100*11: 36.5GW/m^2. 

The effective range of the 10MW laser falls to a mere 1700km.

Further damage reduction techniques and conclusion


With a bit of imagination, we can come up with even more ways to reduce laser damage.

By rotating the armor, we would force the laser beam to wander over fresh, unheated surface area. The maximum effectiveness of spinning is to divide the circumference of the armor by the width of the laser beam.
For example, a 4 meter wide cylinder would have a circumference of 12.56 meters. At 2000km, the laser example we have used so far would produce a beam 0.195 meters wide. Spinning the armor would therefore spread the laser by a maximum of 12.56/0.195: 64.4 times. 

The effectiveness of armor rotation increases as distances become shorter. The laser beam becomes narrower, so we divide the circumference by a smaller number to get a bigger reduction in intensity. At 1000km, the beam thins to 0.0976m, increasing the spread to a factor 128.7.

Or, instead of just spinning in circles, armor can be split into segmented bands (like tank tracks) that move diagonally up a spaceship's hull. This would spread the laser both along the circumference of the spaceship and along the band's length. 

Another option would be to stop thinking of armor as solid blocks of material. A forest of metal wires could intercept a laser and absorb its energy, but it would have a much better surface area to mass ratio - like a hot surface of graphite, it could passively radiate away a lot of energy.

If we using several of these techniques together, we can reasonably conclude that lasers will always be powerful, long-ranged weapons, but they actually have an effective range that can be reduced to a few hundreds kilometers, instead of the tens of thousands of kilometers assumed so far.

Why bother with all this?
Well, at shorter distances, other weapons can come into play. Railguns would be able to hit targets before they can dodge out of the way, and missiles don't have to grow to obscene sizes so that they may pack enough propellant to drive themselves up to immense speeds. 
Particle beam weapons, generally shunned for their short effective range when compared to lasers, can bypass reflective surfaces and deal damage directly. You'd even want different types of laser, to shoot at wavelengths that mirrors struggle to handle, or in pulses intense enough to overwhelm active cooling. 

More weapons means more options in combat, and sci-fi writers have more tools to shake up their action scenes and make them more interesting.

Maneuvering can become important again too. At extreme distances, a spaceship accelerating as hard as it can will only shift its position in its enemy's sky by a few degrees, and it cannot close or open the distance meaningfully before the battle is over.

At shorter ranges, flanking maneuvers become possible. Tactics such as rushing the target or accelerating away are suddenly more effective. The importance of pre-positioning before a battle become less important than the maneuvers taken during a battle. 
"Very, very far away..."
In other words, shorter ranges allow for more dynamic battles where tactical decisions matter and human actors would have a role to play.

Without all this, space warfare would solely be determined by which side brought the most spaceships with the biggest lasers into the fight. Fleets would know whether to engage or not deterministically, and even if they do lock themselves into battle, it will be a drawn-out sequence of spaceships' armor being burned away over the course of days to weeks. That's boring.

60 comments:

  1. I'd like to give special thanks to Imallett (https://plus.google.com/+IanMallett) for his help figuring out Fresnel reflection and dielectric mirrors.

    ReplyDelete
  2. Clever. But you have ignored an important factor. Pulsed lasers can concentrate energy not only in space (any laser can do this) but in time. As this picture (you yourself had posted it) https://1.bp.blogspot.com/-j5pYFe-4zBI/WwQJsPRSAII/AAAAAAAADcw/ESgPJQH2-_EU2z8cemmifHsb7MbDeMaBQCLcBGAs/s1600/c3ja50200g-f1.gif shows femtosecond pulses can deliver energy so fast that no transport mechanism can carry it away, and the extreme electric field tears electrons out, leaving the rest to a coulomb-explosion. Mirrors and active cooling are useless against this. Even if pointing accuracy limits or the effects of the ejecta from the previous shot prevent high rate of fire pulsed lasers from punching through thick armor, they could - even at extreme distances - 'loosen up' mirrors for more conventional lasers or tear subsurface coolant chanels.

    ReplyDelete
    Replies
    1. Hello Anonymous.

      I did mention that pulsed lasers would become attractive tools for overcoming the increased defenses that reflective surfaces pose against continuous lasers.

      It should be noted that pulsed lasers can ignore reflectivity only at extremely high intensities, reaching 3J/cm^2 in mirrors designed to have a high damage threshold.

      https://www.osapublishing.org/DirectPDFAccess/6022EF9E-0075-B914-EE07B15466A16EF8_385826/oe-26-9-11764.pdf?da=1&id=385826&seq=0&mobile=no

      If that amount of energy is delivered in a picosecond, then the mirror can survive an intensity of up to 30 PetaWatts per square meter. At lower intensities, it retains its reflectivity and bounces away 90 to 99.9% of the laser energy.

      Even so, I do agree that pulsed lasers can overwhelm the active cooling system's ability to prevent destructive temperature increases in a dielectric mirror. That is what makes pulsed lasers attractive.

      Despite this, they are still subject to sloping, rotation and active cooling lowering average temperatures in between pulses, so there still some benefits to be gained.

      Delete
    2. Mirror armor only works if the intensity of the laser at the armor is the same as the intensity of the laser at the aperture. A laser can focus a beam to a intensity that would destroy the material that its own mirror is made out of. This is why the idea of mirror armor is routinely discredited. Once the laser damage threshold is crossed, mirror armor is no more effective than non-mirror armor.

      Delete
    3. Anonymous,
      As distance increases, the laser spreads out and its intensity drops quadratically (to the square of the increase in distance).

      Therefore, there will always be a distance at which the laser beam is not intense enough to ignore reflectivity, and an even greater distance at which is won't reach the damage threshold of mirror armor.

      It is these distances that matter, because they are much shorter than the distance at which a laser beam can deal damage to unprotected armor, such as a simple layer of graphite.

      If these distances are shortened by mirror armor, then space combat becomes more interesting.

      Delete
  3. How difficult is it to keep a laser trained precisely on a single spot on a spaceship, particularly if the spaceship is maneuvering?

    ReplyDelete
    Replies
    1. We don't exactly know for sure!
      If we compare it directly to the ability of modern telescopes to hold a tiny image of the sky inside their narrow field of view for hours on end, day per day, it will be easy. Pointing accuracy for adaptive optics is cited in micro-arc seconds, which is 0.0000000002778 degrees! At a distance of 100,000km, it is only an error of 0.5 millimeters.

      However, telescopes on the ground are undisturbed, stabilized and un-moving, facing very predictable targets. A laser warship might wobble, vibrate, be jolted around by impacts and propellant sloshing. Mirrors can get heated up and warp. The actuators that control the adaptive optics can make mistakes or stop working. You might only have enough resolution to target objects with a meter of accuracy anyway.

      The Hubble Space Telescope only achieves 0.05 arc-second pointing accuracy after switching between multiple guide stars (http://www.stsci.edu/hst/HST_overview/documents/multidrizzle/ch42.html).

      That's an accuracy of 24 meters at 100,000km.

      Delete
  4. Great article as always, you gave me a lot to think about
    I've a lot of questions for you, hope you can answer them all

    First question, the silly one: you say that a way to mitigate the damage of a laser is to give the ship some angles, like sharp cones or pyramid like structures; the thing I don't get is, would it mean that the whole ship is going to be shaped like a cone or pyramid or that it would have some portions of the hull shaped like weird structure to bounce off the laser?

    Now the serious questions
    2) With your various methods to mitigate the damage done by laser guns, you've pretty much reduced the engagment distance to under ten thousand km. If I don't remember bad, in your article on particle beam weapons you estimated 10.000 km as the maximum range of some particle cannons. As particle beam weapons lack the big mirror of laser guns and are somewhat easy (or easier) to aim, could it be possible that they would be a preferable choice for weapons mountend on moving spaceships and relegate lasers to "fixed" space stations or, in some cases, ground installations?

    3)You mentioned railguns, so that made me wonder: what exactly would be the range of a railgun? The shot is unguided, just a dumb slug. But in space, with no atmosphere to mess up everything a, hopefully, powerfull rails that don't break apart under their own magnetic field, that slug could fly pretty fast. Their range would be in hundreds of km or thousands?

    4) Would the weird shape like pyramids and cones be an inteligent solution to increase the resistance of missile against point defence lasers? Maybe to increase the amount of time they can fly under laser fire before being destroyed

    5) Speaking of missiles, what would be a credible and interesting propulsion system for a ship-to-ship missile in space? In the expanse they use torch missile, which some on Atomic Rockets think was a bad idea, do to economy/feasibility. How could one justify the presence of missiles in space if a spaceship can simply dodge it?

    ReplyDelete
    Replies
    1. Thanks, Frank.

      The questions, in order:

      1) You want the laser to only touch surfaces which are heavily sloped and with coolant channels running underneath their surface.

      This means that you want some portion of the spaceship's frontal arc to be capped by a sharp cone or star pyramid. You can also fill the cone with propellant tanks or weapons if you don't want to waste that empty space within.

      The base of the cone will cover the thickest part of your spaceship, and you now have three options:
      -Put all your delicate equipment and engines in the cone's 'shadow'.
      -Extend the shadow using a cylinder of armor.
      -Add a second cone to the rear of the ship to protect against rear attacks.

      2) You are exactly right. I mentioned particle beam weapons as being a possible counter or compliment to the laser vs mirror set-up. PBeams are less intense than lasers can be. At very short distances, lasers are intense enough to overwhelm the reflective armor. At very long distances, lasers are the only useful weapon as PBeams spread out too much. There is a sweet spot in the middle where lasers are unable to go through mirror armor but particle beams, which cannot be reflected, can burn through the surface.

      The burnt surfaces can then be focused by a laser to dig through the armor underneath.

      Lasers prefer large space stations and ground installations, while particle beams are suited to heavily armored spaceships that want to have a narrow armor cone and no vulnerable optics.

      3) A railgun slug is too slow to catch anything beyond a few tens of kilometers, even if it explodes into shrapnel. You need kinetics to be massively faster (like the pellet gun described here: http://toughsf.blogspot.com/2016/10/the-solution-to-long-range-space-combat.html) or to add a maneuvering capability to the railgun rounds.

      They then turn into fast, small guided missiles.

      If you want to go faster than the ~10 to 30km/s limit on railguns, you want coilguns.

      4) Yes, cooling, sloping and rotating work at all scales.

      5) Something interesting that isn't chemical rockets or nuclear engines that might be too expensive to throw away? How about beamed propulsion concepts? You have a big laser on your warship sitting at an ineffective distance from your enemy. You can use the megawatts it puts out to drive a laser thermal rocket, or a pulsed ablative rocket and more. Alternatively, you can create a kinetic impactor propulsion set-up, and so on.

      Delete
    2. The pellet gun! I remember that well, it was very interesting. My bad in naming the damn thing "railgun" when I should have said "EM cannon". I was thinking of EM weapons to shoot very light projectiles, in the order of a kilo or so of tungsten carbide to maximize penetration, even if I don't know if it is a good strategy.

      For the missiles I was thinking to use beamed propulsion (like a laser sail with a warhead of sort), but then I stumbled upon the same problem: a giant laser gun. In my world I want to get rid of any laser gun mounted on spaceships (except point defence lasers and even them are not going to stay alive for long); mounting a giant laser to propel missiles give me the same liability of the giant laser: it's big, lot of waste heat, poor mobility and extremely fragile (unless someone finds a way to "fold" the mirror and store it away during travel/manouvers).
      I was wondering the idea of a multi stage kind of propulsion for my missiles, but I don't know if they might work: get launched by the firing ship (with some kind of acceleration, maybe electromagnetic) and mounting a number of discardable of engines that, when detached, bring some of the waste heat with them, so the warhead isn't too much affected. Do you think it could work somehow?

      Anyway, thank you a lot for your time!

      Delete
    3. Don't worry, pellet gun is just what I like to call a (usually electromagnetic) launcher than reduces the size of the projectile to be able to shoot much faster.

      A laser sail would be a terrible option for propelling a missile, as it would be an easy target and accelerate much more slowly than any other option.

      A propulsion laser won't have to reach out as far as an offensive laser, so you can afford to use small mirrors and longer wavelengths that are more easily generated by compact, efficient diodes.

      You might even use microwaves!

      Conventional thermal engines, such as a laser-powered thermal rocket, a nuclear thermal rocket or chemical rocket, carry away all of their heat in the exhaust they shoot out of the back. They do not have to worry about waste heat.

      Delete
    4. Microwaves-beamed propulsion, that's something I've never believed was possible... That's how ignorant am I. I read that there was a recent attempt at lifting a rocket using what I believe is called "maser". A beamed propulsion would require line-of-sight between the shooting ship, the "missile" (which in this case I would call a beamed warhead) and the target, that would impose some restrictions on maneuvers, but is definitely worth looking more in dept... Time to go to the drive table and see how much of a thrust/velocity I can crank out of something like this.

      As for the nuclear-thermal idea, I read on Atomic Rockets that there are design (even recent ones) that are very close to achieve torch-like performances without going full-fusion. Aside from the economic consideration of putting "expendables" atomic engines on missiles, my real concern is what kind of radiation I would have to deal with while storing the missile.
      I'm prone to set aside the economic factor, because I'm imagining a 'verse where people are ok with the fact of spending untold amount of money to build a torchship, equip it with a limited FTL drive, staff it with some hundreds of soldiers who would likely die while attempting a orbital drop o some rogue colony; I'm more concerned about the technical aspects of things, the portion where I'm real weak.

      Delete
    5. Something that popped up in my mind while I was sending the previous answer is that I might have to embrace the idea of having two different types of "missiles", the beamed and the nuclear ones, designed to handle different scenarios... Maybe not the best option in the world, but if it works I'm fine with that.

      Delete
    6. Frank,
      The initial boost phase can start at the spaceship and end a relatively short distance away. Let's call this the 'boost track', which is the distance covered by the missile as it is boosted by a laser or maser.

      Imagine we want to boost a 1 ton payload of kinetic impactors to a velocity of 20km/s. A chemical booster with hydrogen and oxygen propellants would gives us an Isp of 450s (4.44km/s exhaust velocity) and would be very vulnerable to a laser strike while it is accelerating. A mass ratio of 92.8 would be needed, leading to a ridiculous booster stage massing 91.8 tons.

      Instead, we use a maser thermal booster. A maser beam is focused onto a reaction chamber containing water. Water absorbs microwaves pretty well, and with no physical heating element involved, you can turn up the heat and reach extreme temperatures. At 20,000K, water breaks down into a hydrogen/oxygen plasma and reaches an exhaust velocity of 11.6km/s (1188s).

      Isp Calculator: https://space.geometrian.com/calcs/isp.php

      This reduces the mass ratio required down to 5.6, meaning the booster only needs 4.6 tons of water propellant. We'll use an average mass of 3.3 tons for the following calculations.

      A short acceleration track is preferable. At 200km, you'd need an average acceleration of 102g and a maser power input of 19GW. At 500km, you need 40.8g and 7.6GW. At 1000km, it is 3.8GW and at 2000km it is 1.9GW. These are short distances if your gigawatt lasers are keeping the enemy at bay at 10,000km+

      The radiation from an inactive missile, where the uranium is just decaying naturally, is quite low. Only when the control rods are removed does the neutron flux increase to dangerous levels.

      I can help with technical aspects!

      Three types of missiles are likely:
      -Chemical missiles that are launched off a warship that is already approaching the enemy at high velocity.
      -Nuclear missiles that can be launched from very far away, accelerate gradually up to a high velocity, then detach from their payloads and come back.
      -Beam missiles that accelerate hard over short distances.

      Delete
    7. Sorry for the late reply, notifications have been a little erratic

      Thank you a lot for all the efforts you've put in this last answer.
      If my conversion is correct, a maser thermal booster will require arround 4.6 cubic meters of pure water to propel a ton of warhead. if I put that in perspective, the missile will be realative small compared to the ship launching it, but still has the potential of wrecking hostile warships if armed correctly. A ton of warhead is, if i remember correctly, the same weight of a large casaba howitzer bolt study that I found back on Atomick Rockets. I don't know if a Casaba Howitzer would increase the lethal range of such a missile by much, but we may spare a few Km of flight and, thus, achieve a better chance of successfully hit the tyarget. Now, on the same study, a small casaba howitzer bolt would be 50 Kg or so; if we maintain the mass ratio, it would require only 230 Kg of water (my calculations could be off, however), which transaltes in some 0.23 cubic meters. This is a very small missile, but may be able to deliver one hell of a punch to anything. The smaller the missile, the more a ship could be able to carry; and if we add the ability to such missiles to be "fired" in volleys, maybe 10 to 20 missiles per volley, it could make a very interesting weapon system.
      Yet, I must point out, my reasoning and calculations may be flawed.

      The three missile types you propose do open up a series of scenario that may be plausible (in my mind). For example, the chemical missile could be used as a torpedo of sorts (slower acceleration than a warship, but capable of crippling damage) which could even be the final stage of another weapon; it could also work as a guided bomb of sorts, "dropped" by a warship orbiting a planet, deorbited with the chemical engine and pointend against a surface target, guided by something like a MARV. The Nuclear missile is something akin to an anti-ship cruise missile, gets fired from a very long distance, but is still very fast; the fact that the drive module could be detached and reused adds benefits to the cost of such a weapon (and de fact that it would be carried in minimal numbers). The Beam-propelled missiles could be used in various scenarios, maybe even self defence; I would probably equip them with Casaba Howitzer bolts, to maximize damage potential.
      This are some of the idea that I have based upon what you said are likely.

      Sorry for the long answer and, again, thanks a lot!

      Delete
    8. Edit: I just noticed that the article I was referring to was actually written by you. How embarrassing for me to not spot that earlier.

      Delete
    9. I have had problems with notifications recently too - the solution, it seems, is to click on the 'notify me' box at the bottom of the reply section.

      A Casaba Howitzer would make a missile massively more effective. It doesn't have to directly hit the target, so tracking its movements and matching them becomes much easier. Even more importantly, the target cannot afford to rely on Whipple shields and interceptor drones. They will be forced to launch anti-missile missiles and use powerful lasers, which is much more mass and energy intensive.

      Overall, Casaba Howitzers level the playing field between attacker and defender when using projectiles with a high relative velocity.

      Note that the temperature and Isp I used for the Maser-Water rocket is just an example. The actual temperatures you can reach depend mostly on how tightly you can focus the beam into the reaction chamber and/or how much energy each laser pulse contains. Laboratory tests regularly push metals and plastics to temperatures as high as 1,000,000K. That would translate into an Isp increase of 10x over the 10,000K example. However, there are diminishing returns. You'd end up reducing the projectile mass by a few kg while increasing the 'burn track' by thousands of kilometers...

      A chemical missile shouldn't be used as a 'torpedo'. You should capitalize on the fact that it is the cheapest propulsion option and you can fire you entire ammunition load at once. The best way to use them is therefore in massive, single waves of hundreds if not thousands of projectiles. They become even better if you build up a bit of relative velocity with the launching spaceship.

      De-orbiting projectiles for orbital bombardment is a bit of a special case. You only really need to deliver about 100m/s in LEO to lower the projectile's periapsis enough for it to get caught in the atmosphere and dragged down. On an airless body, you'd need to spend more deltaV but it is still just a small fraction of the orbital velocity.

      I agree with the rest of your points.

      Delete
  5. Lasers are excellent space weapons presuming a few things:
    1) Lots and lots of power. However, this is pretty much requisite for space travel, since without it you wouldn't be able to get anywhere with any speed or power all the hungry sub-systems that spacecraft pretty much require to be useful at all.

    2) High-frequency lasers with high efficiency. This is a serious problem and may not be possible to solve with real-world engineering. If you can't build a high-frequency, high-efficiency laser then you will not have enough power output (for size) to make them worth using over conventional guns, missiles or electro-magnetic cannons. Issues such as diffraction, wobble, thermal lensing, etc. can seriously impair the effective range of a weapon. If you need to stay focused on a target to do significant damage, and that target has a relative velocity of 150kps, and you need a gigantic mounted laser to be able to achieve significant output and thermal diffusion (thus making it slow to aim) lasers might actually be pretty piss poor weapons.

    For the speed of lasers to make a big difference ships also need to be very fast. Otherwise a coilgun or missile ought to be plenty fast enough, and a lot more efficient.

    The title might be more accurate if it was 'possibly unrealistic lasers that no one can actually build, power or aim in a reasonable amount of time IRL are the ultimate space weapons'.

    ReplyDelete
    Replies
    1. In 'The Laser Problem' series of posts, I described how lasers can get their 'lots of power' even if it is extracted as a fraction of a fraction of the power that is used to propel rockets through space. If you have a multi-megawatt nuclear reactor feeding a set of electric rockets, then there is no reason why that power cannot be diverted to feeding weapons.

      'High frequency' is relative. We are already producing high efficiency (60-80%) diodes that can produce near infrared to deep red beams. The use of frequency doubling converts these beams into rather useful green or blue wavelengths, which mirrors can handle rather nicely.

      The direct comparison of the output of a laser to the output of a kinetic weapon isn't quite possible. A laser does damage over time, but it is very likely that it hits 100% of the time. A 10MW laser delivers 10MW to the target. A kinetic weapon can be very efficient or not need any power! But, the majority of its projectile won't hit the target. 10MW of railgun isn't equal to 10MW at the target!

      'Fast enough' is also a relative term. A 7km/s railgun sounds fast to us, when we compare it to bullets at 1.5km/s. But, it it would cross the thousands of kilometers that would separate spaceships during a battle at a snail's pace. The target only has to nudge their ship gently in any direction to cause the bullet to miss.

      The only way to hit targets using kinetic weapons is to shoot an enormous amount of them, shoot them extremely quickly or to give them onboard propulsion to match the movements of the target...

      Delete
    2. This all depends on relative technologies. For most engines we have any idea on how to build their acceleration and even delta-V is pretty poor. So they might not be moving all that quickly, relatively speaking.

      To figure out how useful lasers are you have to figure out your general technology. If you have high-efficiency fusion power plants then you can build torch fusion drives and power super-lasers and presumably deal with the heat from all 3. The higher the energy output the greater the encounter velocity and the greater the advantage of lasers (though this also transfers partly to kinetic weapons: super-high velocity magnetic cannons and missiles). The more contemporary (and possibly 'realistic') the technological paradigm the more kinetic weapons dominate (as, obviously, they do at present).

      Given the right assumptions you can justify laser beams and particle guns being the dominant weapon. If you can build a Graser then that gives you a freakish range advantage, rendering conventional guns utterly useless because (by the same assumptions) a ship that can shoot a Graser can pull 2Gs and have a delta-V of 10mps, rendering kinetic attacks laughably slow. On the other hand if you can't build useful-mass X-Ray lasers or fusion torch rockets then an ordinary HEDM chemical rocket can easily chase down and destroy the slow, pathetic tin can doing .0005Gs because it has to carry bags-of-mostly-water and conserve its fuel.

      Overall I would guess that kinetic weapons dominate the near future of space combat (if there is space combat) but that the more exotic your science the more beam weapons have an edge.

      Delete
    3. I 100% agree with all of the points you've made, Rikhard.

      Delete
  6. Free-Electron Laser should be an effective weapon against mirror. Although the enormous size and mass of FEL limit its practicability, the overwhelming efficiency and flexible wavelength make it an attractable candidate.
    http://www2.lbl.gov/Science-Articles/Archive/sb/July-2004/Intense-Attosecond-Radiation.pdf
    Also, this paper proposed a (theoretical) approach to create an *attosecond* pulsed FEL, further enhance its damaging ability.

    ReplyDelete
    Replies
    1. A Free Electron Laser's greatest strength is that it can shoot X-ray wavelengths. These cannot be reflected by any known material, and a grazing incidence is hard to produce in a chaotic battle.

      Without reflectivity in play, the 'star pyramid' armor would be a hundred times less effective.

      Delete
    2. "cannot be reflected by any known material"
      Does it mean that the X-Ray mirror at here something impossible, or at least irrelevant to reflect anything?

      https://imagine.gsfc.nasa.gov/news/xray_mirror.html

      Delete
    3. Felix, there is no known material with a useful refractive index in the X-ray wavelengths. You must rely on grazing incidence mirrors, which don't 'reflect' the beam so much as deflect it using diffraction.

      Delete
    4. So does that mean that the X-Ray Mirror from NASA is not a "real mirror", only something else that behaves more or less similar to a mirror?

      Delete
    5. The longest X-ray wavelengths, called 'soft X-rays' can be reflected with difficulty by some very large, heavy and specialized mirrors.

      The X-rays that nuclear warheads and XFELS produce are hard X-rays, and pass through everything or are absorbed, with no reflection.

      Delete
  7. Interesting how you have worked to turn many of the existing tropes from "cannon" sources like Atomic Rockets and Rocketpunk Manifesto on their heads.

    I might suggest that these specialty spacecraft you are designing might actually be rather limited, having to devote a large fraction of their mass or systems to provide stealth or reflect away RBoDs (Ravening Beams of Death). Are you thinking of these as specialty ships moving far ahead of the constellation to scout the enemy and clear a path, or as penetrating strike craft like the F-117, B-2 and now F-35? The long, narrow shapes do lend themselves to "spinal mounts" of various weapons, rail/coil/pellet guns, particle beam accelerators and of course lasers (either large lasing cavities or electron beam accelerators for FEL's). One thing which I had read about but not longer seem able to find the source for is the idea of using a column of plasma on top of a nuclear reactor and energizing it with the neutron radiation in order to drive a laser cascade in the plasma. Of course opening the shutter on the reactor shield isn't going to be healthy for the crew, so this would make for a remote control laser gunship at best.

    My prediction is space warfare isn't going to be much different than Rick Robinson suggested in Atomic Rockets. the deterministic nature of orbital mechanics still reigns (even admirals with ORION nuclear pulse drives need to budget their expenditures of thrust units), and if there are "specialty" spacecraft in the enemy order of battle, multiple weapons systems will be employed to overcome their defences. In this case, ironically, the best defence against an anti laser ship is the ultimate anti laser weapon: filling the sky with SCoD's (Soda Cans of Death). These small weapons are launched by pellet guns or carried on rocket busses and dispersed in their thousands. The main purpose is to overwhelm the ability of RBoD's to track and burn them out of the sky through sheer numbers, but the kinetic energy of SCoDs slamming into anti laser ships will damage the mirrors and cooling systems, resulting in burn throughs when the RBoD focuses on the ships (that is assuming the ship survives an encounter with a flock of SCoD's). And you yourself have posted on the sovereign recipe for dealing with threats of all kinds, nuclear driven weapons (using the effects of nuclear explosions to drive effects like blasts of pellets, HEAT and EFP charges or star hot spindles of plasma).

    The AO of a space battle will be a very impressive (and dangerous) sight, filled with energy, fast moving objects and the spinning hulks of platforms which failed to get out of the way.

    ReplyDelete
    Replies
    1. Basically the Atomic Rockets formula is consistent with our current understanding of physics and engineering and is probably to some extent correct because of that.

      Most exotic physics and engineering are unrealistic, and not because better engineering isn't possible but because they're an unrealistic "wish-I-may" version of present physics. A lot of exotic physics sci-fi amounts to tying 10,000 horses to a carriage and hoping it goes Mach 1 because of it. If there are breakthroughs that violate the current tyranny of gravity, Relativity, and energy density they most likely will not be one of these gonzo applications of theoretical physics but something completely unforseen. For these reasons I consider something like a Graviton beam more plausible than a Graser death cannon. The more people have forseen but failed to produce it the more likely it is just wrong.

      Delete
    2. @Thucydides:
      Sorry for the late reply. The recent comments did not appear in my email for some reason.

      The techniques I described can be applied in degrees to match the intensity of the lasers the spaceship's role asks that it be able to handle.

      For example, a spaceship that is supposed to sit very far from battle might simply use a reflective coating and nothing else.

      A spaceship that intends to make high speed passes straight at the enemy, perhaps to discharge missiles at close range, would want the full suite of multi-layered reflective surfaces, redundant active cooling, extreme sloping and rapid rotation.

      Something in between might be a layer of graphite supplemented with active cooling, but does not bother with sloping or reflective surfaces.

      I have given thought to the synergy between spinal mounts and sharp star pyramid armor, but I think the initial concept is flawed. A barrel sticking out of the most critical point, which is the tip of the pyramid, would make it significantly weaker. Instead, I think the spinal weapon will fire off-center. Even if it is by just 5 degrees, it leaves the entire top of the pyramid free to dedicate itself to protecting against laser fire aimed at the center of the spaceship.

      Going further, I think the rotation should be along two axis: the armor should wobble as it spins and spread the incoming beam both horizontally and vertically. This gives you a free bonus to intensity the hull can handle, but also puts your off-center spinal weapon straight into the line of fire once per rotation.

      Nuclear-pumped lasers are a topic I am researching. Details are scarce, for good reason, but I think I can scrounge together enough data to write something like the Casaba Howitzer post. The initial assessment is: goodbye every other weapon.

      The effectiveness of SCoDs is being shaken on the ToughSF discord by the findings that user OMGitsWTF on Phased Array Lasers. They are actually pretty powerful, and they can switch targets or split their beam on the order of milliseconds. It would mean that splitting the throw weight of a missile bus into a thousand sub-munitions would only gain you a second of switching time...

      A well placed Casaba Howitzer shot, even at far beyond its effective range, could burn away the reflective surfaces of anti-laser armor and suddenly increase the laser intensity by 10 to 100-fold!

      Delete
    3. @Rikhard von Katzen:
      I think the previous discussions started with some abstractions and simplifications to ease the conversation, but never re-visited or question them afterwards.

      Ignoring physical realities for the sake of convenience is the domain of Soft Scifi.

      Delete
    4. An off centre spinal mount would be interesting, although one of the complications would be dealing with the off axis recoil forces in a rai/coil/pellet gun mounting. For a laser weapon it might not be so bad, the beam expander mirror is covering the "point", and "eyeball frying contests" could be decided by simply tipping the mirror slightly (a more agressive laserstar commander might actually reorient the mirror to attack a different target altogether). When not in use, the beam expander could simply fold up something like an umbrella to protect the point.

      The issue of these being very "single purpose" may not have been clear in my question. Since the vehicle needs to be both "smooth" and likely spinning rapidly to deflect enemy lasers, there won't be much area devoted to things like AESA radar panels or optical sensors. These would be weak spots in the protective finish, and rapidly absorb laser energy whenever the beam intersects that part of the spaceship. Even suggesting the vehicle is getting third party information still means there is at least one high bandwidth antenna, and one to 100 off board drones feeding sensor information to the vehicle. Once the antenna is targeted via ECM, or sufficient drones are shot down, the vehicle is far less able to achieve a firing solution. So to me there seem to be pretty severe limits on how you would actually use these.

      OMGits WTF, apologies for not joining you guys on Discord, but I didn't find it a satisfying experience. Probably aging out and will have to be confined to blogs ;-)

      I'm not convinced about phased array lasers, mostly since the principle has not been demonstrated anywhere to my knowledge. I do know and acknowledge that AESA radar panels provide both the inspiration and you are extrapolating from that, but the actual hardware implementation is missing (or am I confusing what your talking about with something else?).

      As an interesting aside, I've been watching Issac Arthur's channel again after a long pause, and he made an interesting point in his "Interstellar Warfare" video. The amount of energy needed for interstellar flight is staggering, and measurements should be realistically compared to the outputs of stars (doing "x" results in the use of energy equal to "y" seconds of stellar output). Even in interplanetary space, you are talking about the sorts of energies that are found in small to medium nuclear devices. Control over this much energy makes me believe we are vastly underestimating the size and economic outputs of Interplanetary societies. While I doubt there would be direct analogues the way pre WWII economies were usually compared in terms of steel production, for example, we might have to think a lot harder about just what these societies could actually do, and how fast they could possibly do them. Just scaling up current nation states might not really be a proper way of understanding; that much energy and output would need far different organization and institutions to function.

      Delete
    5. I am not too worried about recoil.
      Useful velocities from a kinetic weapon implies very little momentum per shot. A 100g projectile at 100km/s has the same momentum as a 10kg shot at 1km/s but with a hundred times more kinetic energy.

      If it is a pellet gun, then the projectiles are properly miniscule!

      You touch on a point I was trying to develop into a blog post:
      -Anti Laser Armor is not likely to leave any gaps for optical sensors to look through, and periscopes will be shot off quickly.
      -Remote sensors can be jammed with intense lasers
      -Radio waves can travel through armor layers
      -Therefore, the primary fire control sensors will be RADARs.

      Even if detection can be done at very long distances using optical and infrared sensors, it will be RADAR that dominantes for tracking and aiming purposes, as it is unimpeded by an armor cone and unaffected by the megawatts of IR/Vis/UV light being thrown around. It opens up a very familiar and complex Electronic Warfare battleground to fight on, on top of the actual physical battle.

      Blogs are fine with me!

      Phased Arrays can be applied broadly to any electromagnetic emitter that uses beam combination techniques. Multiple fiber lasers shining in sync, or a meter by meter grid of diode lasers with a single coherent wavefront, can act as a single huge emitter in a manner similar to AESA radars, albeit with lower focusing efficiency.

      Nation states are severely limited by communications limits, as it determines how
      political decisions take effect and how power can be held, applied and redistributed. At interstellar distances, these communications delays make nation states impossible to exist as they do today.

      Delete
    6. I would have thought that there would be both some degradation of the radar, unless it is covered by a proper dielectric panel which allows the beam to pass though unimpeded (much like the nose cones and radomes of ships and aircraft). These panels would then be weak points in the laser armour. Even if the emitter is covered by the dielectric mirror, that fraction of a percent of laser energy that penetrates the mirror will still heat the radar emitter, with the potential to do things ranging from flexing due to differential heating to actually being damaged by the intense energy pulse coming through.

      I am interested in seeing examples of these Phased Array lasers, since I wasn't aware that the state of the art has advanced that much (or is it still all theory?).

      While Issac Arthur was mostly speaking about the sheer scale and scope of any civilization capable of mounting Interstellar battles, consider that if "they" can build a Dyson Swarm and generate a Nichol-Dyson beam to drive a laser ablative rocket to relativistic speeds (a rather low tech RKKV), they would realistically be able to launch *billions* of the things at a target.

      Even an interplanetary civilization such as what we are talking about would be working on much larger scales than we might expect. Robert L Forward spoke of driving a laser lightsail on an interstellar mission, with the beam emitter in the orbit of Mercury generating 70+ Terawatts of laser energy. That sort of machinery could be used for all kinds of purposes, such as delivering high quality energy to the Oort cloud, moving asteroids or supporting open air potato farming on Triton. The example seems both trivial and ridiculous simply because the vast majority of people simply are not thinking at that scale (I'm obviously not). Which was really my point about having to think differently about economics, politics, social structures and so on.

      Delete
    7. Impedance might be a issue, but it beats being completely blind.
      If the layer of laser armor is graphite with water cooling and maybe a reflective layer of aluminium on top, then it will not prove a problem for meter-long wavelengths that Radar uses.

      Lasers would reflect off the aluminium layer or ablate the graphite layer, but would not reach the radio emitter until all the armor has been excavated.

      Interstellar warfare is a whole other ball game, that's for sure. Once you have terawatt lasers sitting around, warfare would be transformed, but it is a big assumption to add those sorts of energies to any setting without proper justification and an extended timeline.

      Delete
  8. Hello Matter Beam, I am sorry if this is bothersome, but I had a question regarding anti-laser armor. How practical would it be to have the set up you described with the addition of light bending metamaterials? I know optical invisibility is redundant for stealth in space, but I was thinking it could be used to provide some degree of protection against lasers. My idea is that the metamaterials could bend some of the laser wavelengths away from the target; I don't think that this alone can protect a ship, I'm pretty sure some wavelengths will be able to get through due to whatever the limitations to metamaterials are, but I was thinking that it might add an increased chance of survivability if added to the system described in this post in the scenario if the ship was fired on by purely lasers.

    ReplyDelete
    Replies
    1. Anonymous:
      Metamaterials are interesting.
      Bragg mirrors, also known as dielectric mirrors, are built by layering a high-refractive index material on top of a low refractive index material extremely thinly, over a thickness less than the wavelength of light (so a few nanometers).

      When that light (such as a laser) passes from one layer to another, a large portion of its energy is reflected. After multiple layers, 99.9%+ of it is reflected.

      Metamaterials can create high refractive index layers out of materials that are not as vulnerable to heating and damage as the current materials. I should look into this further, as it opens the door to dielectric mirrors that can handle high temperatures.

      However, even metamaterials have to be made out of something solid, and this makes them unable to reflect extremely short wavelengths, such as X-rays.

      Delete
    2. Thanks, this justifies the uses of short wavelength lasers.

      Delete
    3. Modern dielectric mirrors have been proven to reflect wavelengths as short as 193nm very effectively (98% https://www.newport.com/f/long-lived-deep-uv-excimer-mirrors), so by 'short', you must mean soft X-rays and shorter (10nm and below).

      Delete
  9. Great article! It's interesting that graphite and mirrors have comparable effects in the end.

    I think there's a typo in "Graphite has a great heat capacity (0.72J/kg/K)". Shouldn't that be kJ/kg/K ?

    ReplyDelete
    Replies
    1. Thanks for spotting that. I'll fix it.

      Delete
  10. Is REFRACTION a viable means of defending against a laser? Say, if you could put a prism between yourself and the laser source, diverting the beam.

    ReplyDelete
    Replies
    1. Hi RavingManiac.
      Refraction is indeed how dielectric mirrors gain their incredible reflectivity.

      Just putting a prism in between yourself and the laser source could work in some cases, but it can be beaten by powerful lasers, as the prism would absorb some heat, and by some wavelengths that the prism's material isn't good at refracting.

      Delete
  11. Hey Matter Beam. Srry if this is off-topic, but I have been looking at your revised "space submarine" thing using latent heat storage (using hydrogen, and helium), and I have been wondering if there is room for further improvement by using thermochemical heat storage into the construction of the "space submarine" instead of latent heat storage...because apparently thermochemical storage can store 10 times more heat than latent heat storage :/

    https://www.youtube.com/watch?v=_2go6aIS5rQ skip to 10:12
    Any thoughts???

    ReplyDelete
    Replies
    1. The video cites energy absorbed per cubic meter, which is not as important in space as energy absorbed per kilogram.
      Using the numbers provided, and taking 2660kg/m^3 as the density of magnesium sulphate, I work out a thermochemical energy storage density of 1.05MJ/kg. That's good, but far below the 60MJ/kg of hydrogen.

      And, that storage happens at a transition temperature of 120°C, which is not very stealthy!

      Delete
  12. I think thermochemical storage stores more heat than latent storage, because thermochemical storage relies on using endothermic/exothermic chemical reactions to store, or release heat. Whereas latent storage is just phase changes

    ReplyDelete
  13. Also, about this laser thingy. Could a laser be used as an Active Protection System by vaporizing a part of an incoming projectile, so that this vaporized mass becomes reaction mass (momentum exchange), and causes the projectile to move away from the target, or even miss entirely??? And would that be practical?? Just curious :D

    ReplyDelete
    Replies
    1. Since you can use a laser to drive an ablative rocket, then in principle you can use the energy of the laser weapon to knock an incoming round off course. This principle is behind proposals like a "Laser Broom", a powerful ground mounted laser used to de-orbit small pieces of debris which can threaten active spacecraft and satellites. Scaled up much, much larger, the X-ray pulse from a nuclear weapon detonated alongside an asteroid could vapourize enough material on the asteroid's surface to shit its orbit (this is different from the way intrepid astronauts are depicted saving the Earth from an asteroid in most movies, where they use the nuke to blow it to pieces).

      Delete
    2. That's cool...but I was hoping to get some calculations that explains how effective such a laser defense system is for space. Anyways thanks for replying to my comment, I just curious about a laser defense system for space, because I read something about a laser-based anti-rpg system. Apparently, the thing works by ionizing the air in front of the RPG. This ionized air can cause changes to the airflow around an object, and decrease air drag. So, theoretically if one side of the projectile is subject to normal air drag, and if the other side is subjected to reduced air drag (caused from the ionized air emitted from the laser), then that means the projectile can be steered (or flown) by the laser. Which is pretty cool, since you don't have to completely vaporize a projectile in order to nullify It. It said nothing about vaporizing material off the projectile to create reaction mass....but I guess that can still be applied

      Delete
    3. ...but I guess a missile could counter this laser system by adding on extra fuel, and sensors to apply a counter-thrust to deal with "steering" issues the laser is causing in order to hit the vehicle, or whatever...but I guess that's what hard-kill APS systems, and physical armour are for I guess :/

      Delete
    4. Well, we can ignore ionized air for space projectile, but here's a working out for ablative lasers:

      Take the laser output in watts, and divide it by the vaporization energy of the missile's armor.

      For example, if you have a 100MW laser and the missile is covered by graphite that takes 62MJ/kg to vaporize, then the laser is vaporizing 1.62kg/s.

      Graphite vaporizes at 4000K. Using a Root Mean Square gas velocity calculator (http://calistry.org/calculate/kineticTheoryVelocityCalculator), we can work out that vaporized carbon expands at a velocity of 2883m/s.

      With a mass rate and a velocity, we can calculate a thrust. 1.62*2883: 4650N.

      A laser shining on a flat surface can create a flat nozzle with an efficiency of 50%. That means that 4650/2: 2325N of force is applied to the missile.

      If the laser spot is wider than the entire missile, then it will only be slowed down. If the laser spot is smaller than the missile, it will create more thrust on one side than the other, and start to push the missile sideways.

      It can be countered by rolling the missile and jinking to spread the beam.

      Delete
    5. And here we see the process of diminishing returns. The missile either runs out of reaction mass to jink or roll, and drifts harmlessly away, or it must be built larger to hold more fuel, making it a larger target, meaning it needs to do more evasive manoeuvres, etc.

      In this sort of scenario, I find the Soda Cans of Death (SCoD) idea more convincing, since once they are on the way, they are almost sacrificial. All they need is a very small motor to do terminal manoeuvres if they are close enough, and if one is shot down, there are still several thousand to go. Even at close range, the molten pieces of the SCoD barrage is still full of kinetic energy, and likely to cause damage to you or some following ship in the constellation (or maybe rain down on the planet or asteroid "below").

      I suspect this sort of calculus is already being worked on right here and now, as the US military begins to formalize the "Third offset" and introduces swarms of cooperative robotic vehicles on land, sea and air to overwhelm increasingly effective defences and AA/AD (Anti-Access/Area Denial) systems opposing the entry of American and Coalition forces.

      Delete
    6. Rolling doesn't consume reaction mass, and I have come to believe that the optimal motion of a cone of armor is a wild wobble: both vertical and horizontal rotation! This can spread the beam over a maximum surface area.

      The jinking only has to cover the distance of the beam width or the projectile, whichever is smaller. If it is burning on the left (creating a force towards the right), you spend a tiny amount of maneuvering propellant to move the beam onto the right, to create a force towards the left that cancels out the previous force.

      If the projectile is, say, 10cm wide and 10kg, you'd only need 1.36kg of 350s Isp propellant accelerating at 0.1m/s^2 to maintain a perfect dodge for an hour.

      I think this is proposing that wobbling needles are better than SCoDs.

      For one, the Electronic Warfare aspect was never considered when developing SCoDs. Small sensors with very narrow openings to look through are easily confused, and if they are guided from the launcher by a larger sensor and computer module, then you have a predictable signal to start jamming or interfering with...

      Delete
  14. Excellent post. JWST is huge for sure, but also look up ATLAST and RAMST.

    ReplyDelete
    Replies
    1. Huge mirrors in space have always fascinated me. Thanks for pointing those ones out!

      If anything, they can give us an idea of how much huge optics in space are supposed to weigh.

      Delete
  15. You should probably give more consideration to thermal stress. Powerful pulsed lasers can blow out chunks of solid material along with the vapor.

    ReplyDelete
    Replies
    1. Agreed, pulsed lasers can be very damaging. Mirror armor only works at ranges where the laser is not intense enough to heat up the armor to the point where it melts or vaporizes. All the tricks and techniques mentioned here, from mirrors to active cooling and sloping, are meant to extend that range.

      Pulsed lasers, with their ability to generate high intensities, will force the armored target to stay a long distance away to remain safe.

      If you take the heat resistance of the armor, and divide it by the peak intensity of a laser, you can work out the distance needed to stay safe.

      Delete
  16. This comment has been removed by a blog administrator.

    ReplyDelete