Setesh's physics question thread

[Setesh]

I've started this thread to ask various questions I can't seem to find an answer to via google, ect. Some of these will be for my own edification others for writing purposes. More questiions will be added over time. (this is mostly to avoid flooding the front page with a new thread every time a new question pops up.) If anyone wants to add their own questions go ahead.


Is there a limit on how much energy a laser beam can carry beyond the material limitations of the laser emitter itself?

[GrandMasterTerwynn]

I've started this thread to ask various questions I can't seem to find an answer to via google, ect. Some of these will be for my own edification others for writing purposes. More questiions will be added over time. (this is mostly to avoid flooding the front page with a new thread every time a new question pops up.) If anyone wants to add their own questions go ahead.


Is there a limit on how much energy a laser beam can carry beyond the material limitations of the laser emitter itself?

No, there's no limit to the energy carried by a laser beam. You could have a 1 W laser deliver the total energy produced by the Sun in a given year, provided you were willing to wait for 1.26144e+34 seconds for it to do so (this is approximately thirty million billion times the present age of the universe.)

However, if you were to attempt to deliver this power on a much smaller time-scale, you start to run into problems, as you have to conserve energy and momentum when you're generating photons. Longer wavelength photons carry less energy, and shorter-wavelength photons are harder to generate. So, the more energy you're trying to deliver for a given unit of time, the more photons your laser has to produce. That will limit the amount of energy your laser can carry.

Does this help?

[Setesh]

That's more what I need, is if there is a realistic limit I to lasers for weapon purposes, especially for space combat.

[lPeregrine]

For an infinitely durable laser system (even 99.9999% efficiency will melt internal parts at high power levels) with no size limit, and an infinite power supply, no, there's no limit to how much energy the laser can have. If you want a better answer, you'd have to give more detail on the technology and size limits for this laser.

[Kuroneko]

Since you explicitly ignore material limitations, the theoretical energy limit for the laser device is simply the maximum amount of energy it can contain. Given a mass m, it is mc²; given a size a (along some axis), it is ac^4/(4G). As for limits on energy delivery, the limit for any device is c^5/(4G) = 9.07e51W, far outstripping the energy generating capabilities of even the most uber of sci-fi universes. While the beam will not self-gravitate radially, it will do so axially. Fortunately, there are static arrangements possible at least in principle for arbitrary per-length energy of the beam. (Points of interest: Melvin magnetic universe and the Bonnor beam.)

[Setesh]

For an infinitely durable laser system (even 99.9999% efficiency will melt internal parts at high power levels) with no size limit, and an infinite power supply, no, there's no limit to how much energy the laser can have. If you want a better answer, you'd have to give more detail on the technology and size limits for this laser.

Assuming the tech level is not much beyond our own in materials science, but power generation is very advanced. Viable tokamak fusion reactors are the norm and can be made small enough to fit inside a main battle tank. Space battles work much like submarine battles. Stealth and detection are the key elements of battle. He who fires first and hits gets the victory. Laser weapons are common but unpopular because if you miss or get a non-lethal hit you are real easy to find.

Using the THEL as a basis I'm trying to find a realistic compramise between a 'one hit kill' on an armored ship and the risk of detection from thermal effects of firing the weapon.

Standard armoring is lightweight but impact resistant titanium armor layers under a ceramic 'scale' layer to resist thermal effects.

(sorry if this all seems a little jumbled I havn't gotten much sleep lately)

[lPeregrine]

Assuming the tech level is not much beyond our own in materials science, but power generation is very advanced.

That's a problem then. Lasers aren't too efficient, and generate a lot of waste heat. Someone else will have to give exact numbers, but you're going to have a strict limit on power levels based on your efficiency numbers and how much heat your components can absorb without damage.


And what I meant by size was how much space is available for the laser and its power source. At the same tech level, a battleship-scale weapon is going to be capable of much greater firepower than a tank-scale one. Or, comparing a fast-tracking turret mount to a massive fixed mount that forms the core of the ship.

[Setesh]

That's a problem then. Lasers aren't too efficient, and generate a lot of waste heat. Someone else will have to give exact numbers, but you're going to have a strict limit on power levels based on your efficiency numbers and how much heat your components can absorb without damage.


And what I meant by size was how much space is available for the laser and its power source. At the same tech level, a battleship-scale weapon is going to be capable of much greater firepower than a tank-scale one. Or, comparing a fast-tracking turret mount to a massive fixed mount that forms the core of the ship.

I gave the tank as a scale for reactor size. Average output for that size reactor is 400-600 MWe and one would be installed just to power the weapons.

The laser itself I'm still debating it depends on how much output would be needed to kill the average target. An average ship is roughly the size of a ballistic missile sub.

[Wyrm]

How good is the refrigeration unit? After powering the damn thing, this is going to be your big problem. You're laser is no good if it melts itself, after all.

[Setesh]

How good is the refrigeration unit? After powering the damn thing, this is going to be your big problem. You're laser is no good if it melts itself, after all.

I'm going with a liquid nitrogen flow system where the LN runs through several seperate pipelines, each with its own pump system. They pass around the weapon and through passive heat sinks attached to it, then back to a cryocooler deeper in the ship. In an emergency the passive heat sinks can be exposed to space, though this will make the ship easier to spot.

[Kuroneko]

Assuming that the density of liquid nitrogen is 808kg/m³ and a specific heat capacity of 1.4kJ/(kgK), the maximum amount of energy the liquid nitrogen at initial temperature T kelvin will absorb before it starts to boil is about [1130kJ/m³][77-T]. This is an upper limit, since specific heat capacity goes down as one approaches absolute zero. If T = 7, neglecting the difficulty in storing large amounts of something that cold, then this is about 7.92e7J/m³. To compensate for inefficiencies and the overestimate of the specific heat, let's just say 1e7J/m³. Given a specific laser efficiency and power output, this should give you an idea of how large a cooling system to have for some requirement of continuous firing time.

[Setesh]

Assuming that the density of liquid nitrogen is 808kg/m³ and a specific heat capacity of 1.4kJ/(kgK), the maximum amount of energy the liquid nitrogen at initial temperature T kelvin will absorb before it starts to boil is about [1130kJ/m³][77-T]. This is an upper limit, since specific heat capacity goes down as one approaches absolute zero. If T = 7, neglecting the difficulty in storing large amounts of something that cold, then this is about 7.92e7J/m³. To compensate for inefficiencies and the overestimate of the specific heat, let's just say 1e7J/m³. Given a specific laser efficiency and power output, this should give you an idea of how large a cooling system to have for some requirement of continuous firing time.

That does make for a problem. Though I can cut down the requirment by having short firing times.

Okay assuming the 600MWe generator has to power the whole weapon system. The lasers draw power off batteries the generator charges

Each laser expends 200mw firing a 1 second burst, assuming the 50% efficiency limit of lasers remains unsolved the actual beam is 100MW with 100MW wasted as heat, 1e7j of heat. So with the cooling system I'm imagining a 1 second shot every 2 seconds is feasable for stasis.

That increases if I assume that DARPA's ongoing project to get that up to 80% efficiency succeeds.

Does any of this make sense or is the sleep deprivation getting to me?

[Wyrm]

You missed a decimal place. 100MW * 1 s = 100 MJ = 1e8 J.