Sunday, April 17, 2016

Sci-fi time, part 2: Really big lasers

I had a whole post written about laser weapons, and then the announcement came out about trying to build laser-launched interstellar probes, so I figured I should revise and talk about that as well.

Now that the future is here, and space-faring rockets can land upright on autonomous ships, it's clearly time to look at other formerly science fiction technologies.  Last August I wrote a post looking at whether laser pistols really make practical physics sense as weapons.  The short answer:  Not really, at least not with present power densities.

What about laser cannons?  The US military has been looking at bigger, high power lasers for things like anti-aircraft and ship defense applications.  Given that Navy ships would not have to worry so much about portability and size, and that in principle nuclear-powered ships should have plenty of electrical generating capacity, do big lasers make more sense here?  It's not entirely clear.  Supposedly the operating costs of the laser systems are less than $1/shot, though that's also not a transparent analysis.

Let's look first at the competition.  The US Navy has been using the Phalanx gun system for ship defense, a high speed 20mm cannon that can spew out 75 rounds per second, each about 100 g and traveling at around 1100 m/s.   That's an effective output power, in kinetic energy alone, of 4.5 MW (!).  Even ignoring explosive munitions, each projectile carries 60 kJ of kinetic energy.  The laser weapons being tested are typically 150 kW.  To transfer the same amount of energy to the target as a single kinetic slug from the Phalanx would require keeping the beam focused on the target (assuming complete absorption) for about 0.4 sec, which is a pretty long time if the target is an inbound antiship missile traveling at supersonic speeds.   Clearly, as with hand-held weapons, kinetic projectiles are pretty serious in terms of power and delivered energy on target, and beating that with lasers is not simple.

The other big news story recently about big lasers was the announcement by Yuri Milner and Stephen Hawking of the Starshot project, an attempt to launch many extremely small and light probes toward Alpha Centauri using ground-based lasers for propulsion.  One striking feature of the plan is the idea of using a ground-based optical phased array laser system with about 100 GW of power (!) to boost the probes up to about 0.2 c in a few minutes.  As far as I can tell, the reason for the very high power and quick boost is to avoid problems with pointing the lasers for long periods of time as the earth rotates and the probes become increasingly distant.  Needless to say, pulling this off is an enormous technical challenge.  That power would be about equivalent to 50 large city-serving powerplants.   I really wonder if it would be easier to drop the power by a factor of 1000, increase the boost time by a factor of 1000, and use a 100 MW nuclear reactor in solar orbit (i.e. at the earth-sun L1 or L2 point) to avoid the earth rotation or earth orbital velocity constraint.  That level of reactor power is comparable to what is used in naval ships, and I have a feeling like the pain of working out in space may be easier to overcome than the challenge of building a 100 GW laser array.  Still, exciting times that anyone is even entertaining the idea of trying this.


DanM said...

There was a story at one point that the Army had concluded that the best way to hurt somebody with a laser was to throw it at them. It wouldn't shock me if that was still the best use of a laser as a weapon. Although that was some time ago and it may no longer be the case.

It's also worth pointing out that using a laser as a blinding weapon would be a violation of the Geneva Convention. Surely your government would never contemplate such a thing.

Ted said...

This may be naive, but wouldn't the orbital reactor be pushed out of its target position over the course of the boost due to conservation of momentum?

Douglas Natelson said...

Ted, sure. Options include ion propulsion or even photon propulsion (oppositely directed bonus 100 MW laser array :-) ) for station-keeping. I have not really thought this through very carefully, but I'm pretty sure that no matter what, 100 MW is easier to do than 100 GW.

Anonymous said...

Out of curiosity, one of the problem with laser boosting is that ship has to be very reflective, not to be melted by the optical radiation. Is it possible to create a mirror of free electrons, confined in some sort of the trap, trailing the ship? The reflectivity of free electron gas is pretty high below its plasma frequency.