Comments on Rocketpunk Manifesto: Space Warfare V: Laser Weapons

Welcome to another thread! I tend to think that b...

2012-01-12T18:46:39.049-08:00

Welcome to another thread!

I tend to think that by the time we can build lasers in this class at all, we will be able to adequately solve the fire control / precision aiming problem.

But this is by no means a given, and anyone who wants to avoid uber-lasers in a midfuture setting can reasonably do so by invoking devils in the details.

"Our laser discussions (here and elsewhere) d...

2012-01-10T15:33:32.543-08:00

"Our laser discussions (here and elsewhere) do tend to leave out the niggly little details of maintaining sub-microradian alignments in an environment that is a LOT less placid than the Hubble."

I've browsing the comments for just this. It may or may not be a problem when technology has also progressed enough to deliver the kind of lasers discussed here, but it does seem to be a problem: you'd better have pretty good stabilisation systems on your laser mount, otherwise the merest vibration in your ship will play hell with your fire control.

fire control systems are a critical aspect of modern warfare that is even more overlooked than logistics - this goes as far back as WWI where you can't really understand the maneuveurs of both sides during Jutland if you don't know about the differences between German and British FCS.

(SA Phil) So the more-Dakka approach. --- On th...

2011-04-05T11:10:43.628-07:00

(SA Phil)

So the more-Dakka approach.

---
On the other hand-

How fast could you realistically get a missile in a Mid-future scenario?

What if you gave it a really long intial "push" with a high-power particle beam?

Essentially if you had a relatavistic missile/smart bullet With a final stage that allowed it to "steer" it would be in the Laser's threat window for a very short time.

I am not sure which is purple and which is green! ...

2011-04-05T10:08:57.666-07:00

I am not sure which is purple and which is green!

Here is my most recent take on kinetics v lasers. Short form, kinetics are most effective when deployed as a massive swarm of individually small target seekers. None too flexible, but could be cost effective as a defense.

A laserstar does indeed have honking big radiators, but so does any spacecraft with a high performance drive engine. But lens/mirror drones might still work out if they don't need a 'main drive,' but are simply deployed from the laserstar as it closes for action.

(SA Phil) Thanks for the welcome. --- Purple vs G...

2011-04-05T09:24:54.540-07:00

(SA Phil)

Thanks for the welcome.
---
Purple vs Green:

So the problem from how I can see it is given lasers that are effctive out to 1/2 a light second or more, the fact they have "limited range" compared to kinetics is a non-starter given the actual likely relative closing speeds of any weapon that has a longer range than they do (such as a missile or other kinetic weapon)

Making the Laser the winner in the purple vs green (which one is which?)debate.

Unless you can make a suitably faster kinetic.

--------
Remote Mirrors and lenses

I imagined the laserstar was some massive Heat Radiator with a spacecraft strapped to it.

Thus the a flying lens/mirror would have higher acceleration and delta-vee. They could just let it melt after/as it did its duty.

Byron - Really the answer is above my pay grade, b...

2011-04-04T16:33:46.170-07:00

Byron - Really the answer is above my pay grade, but my guess/understanding is that the pulse rate would be high, in the kHz or higher, so that the energy per pulse would not be that enormous.

SA Phil - Welcome to the comment threads, by the way! I don't know of an inherent reason why you couldn't mount a refocusing lens/mirror on a drone. But depending on propulsion tech (etc.), it might end up more cost effective and flexible to simply mount a laser on it as well.

(SA Phil) So what are the practical limitation o...

2011-04-04T15:18:25.141-07:00

(SA Phil)

So what are the practical limitation on these Mirrors and Lenses?

Could your drones be Lenses or mirrors? Allowing your big GW Laserstar to effectively fire farther (smaller spot)?

I have one more question about this topic, even th...

2011-04-04T14:27:13.567-07:00

I have one more question about this topic, even though it's been dead for a while.
Why do we assume that the generators for CW and pulsed lasers are the same? Yes, it may be 1.5x as energy-efficient to use a given pulsed laser, but what if the pulsed generator is twice the mass and cost?
I would think that pulsed lasers would be bigger, simply because they have to handle a lot more energy at once. I'm not sure about this, and Luke may shoot me down, but I thought it needed to be said.

I thought it did check for that, but you caught me...

2010-12-03T08:49:32.094-08:00

I thought it did check for that, but you caught me in a bad time (holidaze, etc.) for playing with computations and sim results.

Luuuuuuke!

Incidentally, I encourage posters logged in as 'anon' to sign a name to their posts. (It doesn't need to be real, just consistent!)

His sim for pulse lasers does have an error; it fa...

2010-12-02T21:59:02.570-08:00

His sim for pulse lasers does have an error; it fails to check for peak intensity. If that falls below 10^13 watts, your pulse doesn't do much of anything, so I had to figure those numbers manually assuming a nanosecond pulse duration over the spot size.

The comment flood on a couple of recent posts mean...

2010-12-02T21:34:40.648-08:00

The comment flood on a couple of recent posts means I don't even really have time to analyze this scenario. But I probably wrote this post before seeing Luke's sim, so you can pretty safely trust that the sim trumps anything I wrote here.

And just as a followup, my pulse lasers have a gre...

2010-12-02T08:01:58.262-08:00

And just as a followup, my pulse lasers have a greater range than your cw laser.

Scenario: You have a 5 GW cw keel gun laser. I have 5 500 MW pulse lasers. We both have identical mirror sizes. As we close, I begin firing on you when my spot size is 10 meters. Each hit, I'm doing damage over ten meters to a depth of 54 cm (the hole depth is minuscule, the damage is all shock damage - think shrapnel). Meanwhile, in desperation, you fire you 5 GW CW keel gun at me. At a spot size of 10 meters, you burn off 4.5 millimeters of my armor every second. If I give my ship a slight spin, you'll burn off even less. If I have ten centimeters of armor, you need to hold that beam on one spot for 20 seconds to get a burn through. Meanwhile I've blown through five 10-meter chunks of your armor and sprayed shrapnel through your ship five times in one microsecond.

Yep, that's the same site that I've used, ...

2010-12-02T07:41:24.788-08:00

Yep, that's the same site that I've used, but the results aren't similar at all, so I don't know what numbers you were playing with. Blasting is far superior to burning, especially at distance and with lower power. Consider the results against titanium alloy for the following scenario:

One meter spot size.
5 gigajoules of energy.
For CW, 300 K initial temperature (very hot for space), 0 atmosphere (vacuum).

CW results: a 1 meter wide, .5 meter deep hole after one second. Jet speed is subsonic in the material, so no shock damage. Beam has to be maintained for one second.

Pulse delivered over 0.1 microseconds:1.59 meter diameter hole. Damaged diamater 2.46 meters. Hole depth is only 26 cm, but damage depth is 1.43 meters. Beam only needs to be maintained for 100 nanoseconds.

So, with a pulse, you get damage over 6 times the area and three times as deep. The pulse time is so short spinning won't help spread the energy out.

The results are even more dramatic with lower energies. With a 500 MW laser, a CW beam gives you a burn depth of 5.34 cm across 1 meter. The pulse gives you a hole depth of 14 centimeters over one meter, with damage over 1.4 meters to a depth of 68.4 cm.

Pulse lasers allow you to do more damage with lower energies. So, while you play your 5 GW laser on me for seconds at a time, I'll pound away at you with 5 500 MW lasers, doing 50 times the damage for half the power.

Welcome to the discussion threads! I've gener...

2010-12-01T20:52:59.953-08:00

Welcome to the discussion threads!

I've generally thought of blasting, as you describe, as more efficient than burning, but any real evaluation is above my pay grade.

Commenter Luke has a section of his website devoted to laser weapons, complete with sims of both pulsed and CW lasers. My impression from playing with them is that either approach is roughly comparable in all round zapping power, though some threshold conditions (and obviously details of laser and armor tech) might favor one or the other.

Way late to the party, but had to offer a correcti...

2010-12-01T16:48:47.333-08:00

Way late to the party, but had to offer a correction on pulsed vs CW lasers. Pulsed lasers are destructive at much, much greater ranges than CW lasers. If your pulses are short enough, you can easily deliver peak power in excess of 10^14 watts/m^2 across very large spot sizes. With this intensity, your laser's joules aren't heating the target, they're blowing it apart. Think of it as instead of putting a blowtorch to a steel plate, you stick a pack of C4 on it and hit the detonator. Midfuture tech isn't going to worry about terawatt CW lasers when you can use a pulse laser to deposit 5 gigajoules of energy in 100 nanosecond pulses spread out over a microsecond, which is the equivalent of putting a one-ton shaped charge on the hull of your ship and detonating it.

A 5 gigajoule capacitor can be charged in less than a minute from a 100 MW power plant. One ton of shaped charge per minute is pretty darned deadly.

Think of the difference between good quality, low ...

2010-10-05T06:09:45.521-07:00

Think of the difference between good quality, low scatter (or the other term for spreading out) but low power for communication lasers, and the punch-through your armor weaponized lasers....

Bookmarking.

There are 4 characteristics of a laser that must b...

2010-08-24T08:33:59.860-07:00

There are 4 characteristics of a laser that must be considered in determine whether/how a laser will damage a target (I'll not discuss type [CW/RP] and frequency).

Intensity
Fluence
Power
Energy

Intensity is a measure of the instantaneous brightness the laser is at its brightest point. (e.g. E/cm^2/s). Low intensity means that the ship is too far away for the given laser weapon

Fluence is a measure of the accumulated energy at its brightest point (e.g. E/cm^2). A low fluence (with high intensity) may mean the laser can't fire or the beam director can't keep the beam on target long enough to deposit enough energy to damage the ship.

Power is a measure of the instantaneous beam energy integrated over the whole beam area (e.g. E/s). A low power beam with high intensity really should never happen. Focusing your com laser into a 1 nm spot on the enemy ship parked 2 meters away is likely to annoy the heck out them ;)

Energy is the energy deposited by the beam.

If the first three are high enough the damage inflicted by the beam ~ f(E). If the previous values are too low, than a very high energy beam may not be sufficient to do any damage (think of laser that couldn't do more than mimic the sun's illumination of the Space Station).

If one or more of the previous three values is too low, you get secondary effects which can help ameliorate the effects of the laser damage (conductance, reflectance, etc.).

Near term lasers may not have sufficiently high values in the first three values to get to the damage ~ f(E) realm. In the long-term I'm certain they will. So that leaves only the mid-term, which I suppose means it's up to the author :)

Jim

Another quick thought: "Telescope active opti...

2010-08-24T07:07:54.129-07:00

Another quick thought: "Telescope active optics."

Your laser weapon beam makes the ultimate "guide star" for active optics. You might ask, "why do we need active optics when we're not firing through an atmosphere?"

The answer is that your laser weapon and optics are going to heat - a lot. Active optics will enable your weapon to adjust its beam on the fly to keep the beam as concentrated as possible. Of course at very long ranges, I'm not sure how well the adaptive optics would work since the lag time is so great.

Anybody here work with adaptive optics? Is a 1 (or 2 or 60) second lag too great to be of any use?

Jim

As in the "stealth reconsidered thread" ...

2010-08-24T06:38:28.986-07:00

As in the "stealth reconsidered thread" most of the comments here quote theoretical laser performance and leave off a lot of the engineering troubles.

Some of the issues which were mentioned but need to be reiterated:
jitter
heat rejection (dumping waste heat)
combined arms (your opponent won't just be shooting lasers at you)
smaller ships are harder to hit than big ship and not just based on size but also maneuverability
finally lasers as PD would be great for killing the sensors on a KEW *BUT* do nothing about the mass behind that sensor!

If your opponent seeds your trajectory with grapeshot with no seeker heads, your 100 TW graser isn't going to do much for you (well a 100 TW laser fired in along your expected trajectory ought to vaporize everything in its path and cause it to dissipate quite quickly but you get my drift.)!

In an amusing way, this could recycle the whole concept of loading a black powder cannon with chains and grapeshot (only your cannon would be replaced by a coilgun or similar and grapeshot would be carefully calculated to be big enough to do severe damage.

Also I'd expect laser damage to be inflicted at a much lower rate than shown by the theoretical calculations shown here.

Consider reflectance (melted aluminum is VERY shining in most frequencies around optical), conductance, obscuration (by the heated plasma coming off of the target as long as the ship isn't under heave acceleration), jitter moving the beam around, etc.

Some of these effects make lasers better in space (no blooming and heat transfer to the atmosphere) and others hurt (no atmosphere to clear away the heated plasma of the target structure).

Jim

Welcome to the discussion threads! I'll quibb...

2010-08-01T16:58:47.056-07:00

Welcome to the discussion threads!

I'll quibble that if you have ship speeds measurable as a percentage of c, you probably have uber powerful lasers that can deliver awesome zaps at stupendous ranges.

But I agree with your broader point, except to say that battles probably won't be fought in the middle of nowhere anyway, just as even 'blue water' naval battles were generally fairly close to land.

See the current discussion thread about Running on Rails, which touches on the threshold of local space in military terms.

I planned a longer comment, but I think I'll put further discussion on the front page instead - look for it there soon.

About Interstellar/Deep interplanetary combat: To...

2010-08-01T12:07:13.972-07:00

About Interstellar/Deep interplanetary combat:

To travel between stars, or even between the inner and outer solar system, a Constellation would have to accelerate for months, even years.
I read a comment earlier on saying that the only situation where lasers can be used freely, in 'Blue water', would be in the case where constellation X intercepts constellation Y heading from A to B. The space-going equivalent of a clear horizon.

I do however have objections. Because on the long time spent accelerating on the part of X, constellation could only close relative velocities if:
-It accelerated at 1000g
-It is travelling at the same/similar speeds but at a different vector.

Option 1 is out, option 2 is unlikely.

My point is, there is no way lasers would be used in that level of clarity. 1 light second damage range, including lag? How much time would X need to cross that distance going at a nice 12%c?

Otherwise the interceptors would try to line up with constellation X to reduce arc distance, or just use kinetics (can accelerate much faster) or even better, mines (Kirklin or any large area, dumb KE mine bus). Lasers at perpendicular vectors would have engagement, damage dealing times of milliseconds! 1MW, 1 micron UV lasers with midfuture tech mirrors wouldn't even shave a millimiter off the other ships' armor!!!

PS: Can be summed up to lasers being interplanetary and 'brown water' weapons.

In regard to the dust comments... If you make the...

2010-06-27T13:15:33.041-07:00

In regard to the dust comments...

If you make the dust out of some substance with a high refractive index and low transparency, such as Titanium Oxide, it's going to totally scatter the beam without being fried. The individual particles are so small, their surface area to volume ratios are so high that all the heat energy is simply radiated away very rapidly. If you make your particle a dielectric, and its diameter one quarter the wavelength of the beam, it will reflect it pretty neatly (~99.99+% efficiency). Of course, this applies to mirrors, I'm not sure if they apply to individual particles. But you can bet this is what they'll use on the main laser mirror, because that's what people use today.

I forgot to say welcome to the comment boards!

2010-03-22T09:24:54.587-07:00

I forgot to say welcome to the comment boards!

Our laser discussions (here and elsewhere) do tend...

2010-03-22T09:24:10.231-07:00

Our laser discussions (here and elsewhere) do tend to leave out the niggly little details of maintaining sub-microradian alignments in an environment that is a LOT less placid than the Hubble.

Also, in the near future you probably won't have the Hollywood six-shooter endless ammo supply in space, since it requires being plugged in to a multimegawatt power supply.

I see, based on what is said here, that combat bet...

2010-03-21T14:48:55.820-07:00

I see, based on what is said here, that combat between laser-armed ships to be akin to that between Langston Field-equipped ships, in that in both cases seeing comes down to damage control. The sensors are easily burned out and have to be replaced regularly.
Also, I don't honestly believe that early lasers will achieve nearly the ranges predicted. The following equation from Space Mission Analysis and Design is a better equation for flux, at least in my opinion:
F=PD2/(piR2[(1.22LQ)2+(JD)2]
F is flux, P is output power, D is mirror diameter, pi is pi, R is range, L is wavelength (lambda, but I don't have that key), Q is quality, J is jitter (in radians). SMAD says that typical quality for 1999 is between 1.5 and 3.
Personally, I think that we won't approach 1 (diffraction limited) quality or zero jitter for a long time. A laser may be a jet engine in a telescope eyepiece, but the telescope has to survive quite a lot including shocks and heat that would destroy an observatory mirror.
And on the tumbling pigeon idea, it makes sense if you're facing a laser-armed enemy only, but it would be a bad idea in a combined-arms situation.