Post by peasant on Nov 7, 2023 16:23:40 GMT
We're used to think that in the future laser weapons will be employed in combat at ranges of millions of km, but recently I've learned that, for the foreseeable future (a century of two), their effective range might by orders of magnitude lower than was previously thought.
Source: www.projectrho.com/public_html/rocket/spacegunconvent2.php
So, if you plot the graph of maximum distance at which a laser weapon will be able to deliver an average of ~1MW/m^2 on target you get this:
if you are thinking: "Well, what's the problem? Just use a bigger mirror, that's all." your mind is about to be blown, just like mine was when I realized that there is pretty much a hard limit on laser's range in space. You can play with this calculator yourselves and see, that , all other thing equal, even increasing the mirror size from 3m to 30m will raise its range by a mere 1%. In fact, you cant appreciably raise its range by tinkering with its beam quality, wavelength or mirror size! The only way is to reduce the jitter or by brute-forcing the problem and increasing its raw power.
No matter how well the energy is focused along the beam, if you cant point it accurately enough on target, after a certain distance, you wont be able to use it as a weapon (or as means of communication for that matter).
Using a realistic value for jitter of 1micro-radian for cutting edge modern tech, suggests that mirror size of about 3m will be maximum useful size to be used on a spaceship.
It's actually quite hard to get rid of all vibrations on a spaceship, as it has many system that have to be functioning 24/7, like the reactor's cooling system, or life support and spaceships are usually designed to be as light as possible which doesnt help them absorb vibrations well. Perhaps lasers on airless celestial bodies, like asteroids or small moons, will have considerably greater range due to them being a much more stable firing platform.
All of the above analysis assumes an ideal, diffraction-limited beam. This is obviously not the case in reality, and a more realistic equation is:
where BD, R, L, and D mean the same as in the equation above, Q is beam quality, a dimensionless measurement of the actual beam diameter to the theoretical beam diameter, and J is jitter in radians. Typical values for Q for modern lasers are generally less than 3, and likely below 1.5. Numbers for jitter, which is caused by vibration of the platform, are harder to come by, but on the Airborne Laser Laboratory in the 1980's, jitter numbers of around 25 microradians (25×10-6radians) were achieved. It is not unreasonable to assume that a two order of magnitude reduction in this number could be achieved between technological development and the fact that the ALL was mounted on an aircraft in the atmosphere, and it is entirely possible that significant farther reductions are possible. As a word of caution, this equation only holds true for Continuous-wave (CW) lasers, and the author is unsure of the impact of jitter on pulsed lasers. It is possible that some or all of the jitter will instead become pointing error for a pulsed laser, significantly increasing the efficiency of such vis à vis CW lasers.
The potential issues caused by vibration are such that it is likely that a laserstar's designer will pay as much attention to them as the designer of a ballistic missile submarine does to noise. Current texts on space optical communications systems (Deep Space Optical Communications, JPL) indicate that there is potential for sub-microradian pointing accuracy and active jitter control. Passive vibration damping can remove the low-frequency components of the vibration, and severely attenuate the high-frequency ones. However, these are for low-powered communication laser systems with small optics operating in the relatively benign environment of a satellite, not large mirrors and high-power laser systems on a thrusting spacecraft with active cooling systems. The exact impact of these factors is unknown at the present.
where BD, R, L, and D mean the same as in the equation above, Q is beam quality, a dimensionless measurement of the actual beam diameter to the theoretical beam diameter, and J is jitter in radians. Typical values for Q for modern lasers are generally less than 3, and likely below 1.5. Numbers for jitter, which is caused by vibration of the platform, are harder to come by, but on the Airborne Laser Laboratory in the 1980's, jitter numbers of around 25 microradians (25×10-6radians) were achieved. It is not unreasonable to assume that a two order of magnitude reduction in this number could be achieved between technological development and the fact that the ALL was mounted on an aircraft in the atmosphere, and it is entirely possible that significant farther reductions are possible. As a word of caution, this equation only holds true for Continuous-wave (CW) lasers, and the author is unsure of the impact of jitter on pulsed lasers. It is possible that some or all of the jitter will instead become pointing error for a pulsed laser, significantly increasing the efficiency of such vis à vis CW lasers.
The potential issues caused by vibration are such that it is likely that a laserstar's designer will pay as much attention to them as the designer of a ballistic missile submarine does to noise. Current texts on space optical communications systems (Deep Space Optical Communications, JPL) indicate that there is potential for sub-microradian pointing accuracy and active jitter control. Passive vibration damping can remove the low-frequency components of the vibration, and severely attenuate the high-frequency ones. However, these are for low-powered communication laser systems with small optics operating in the relatively benign environment of a satellite, not large mirrors and high-power laser systems on a thrusting spacecraft with active cooling systems. The exact impact of these factors is unknown at the present.
Source: www.projectrho.com/public_html/rocket/spacegunconvent2.php
So, if you plot the graph of maximum distance at which a laser weapon will be able to deliver an average of ~1MW/m^2 on target you get this:
if you are thinking: "Well, what's the problem? Just use a bigger mirror, that's all." your mind is about to be blown, just like mine was when I realized that there is pretty much a hard limit on laser's range in space. You can play with this calculator yourselves and see, that , all other thing equal, even increasing the mirror size from 3m to 30m will raise its range by a mere 1%. In fact, you cant appreciably raise its range by tinkering with its beam quality, wavelength or mirror size! The only way is to reduce the jitter or by brute-forcing the problem and increasing its raw power.
No matter how well the energy is focused along the beam, if you cant point it accurately enough on target, after a certain distance, you wont be able to use it as a weapon (or as means of communication for that matter).
Using a realistic value for jitter of 1micro-radian for cutting edge modern tech, suggests that mirror size of about 3m will be maximum useful size to be used on a spaceship.
It's actually quite hard to get rid of all vibrations on a spaceship, as it has many system that have to be functioning 24/7, like the reactor's cooling system, or life support and spaceships are usually designed to be as light as possible which doesnt help them absorb vibrations well. Perhaps lasers on airless celestial bodies, like asteroids or small moons, will have considerably greater range due to them being a much more stable firing platform.