For a ballistic or hypersonic missile you need it anyway, to radiate away atmospheric heating.
The challenge with an ABM laser weapon system is generating enough power on-target that it's not just a rounding error for the energy that needs to be dissipated anyway. A quick back-of-the-envelope physics calculation indicates that'll be hard. An 80kg warhead is moving at about 5000 m/s upon re-entry and has a kinetic energy of about 1 GJ. I dunno what fraction of that kinetic energy is converted into thermal energy upon re-entry; for manned spacecraft it's "almost all of it" because they need to go subsonic for parachutes to deploy, but for ballistic missiles, let's guess 1/1000th for ease of math, or 1 MJ. Current laser weapons systems have a range of about 1mi (and the range of lasers is heavily limited by atmospheric considerations - atmospheric refraction will reduce your accuracy, while heating of water vapor, dust, and air can sap the energy). That's about 1/3 of a second on target, so the power output needed is about 3 MW. Current laser weapon systems like the AN/SEQ-3 have a power output of about 30 kW. This needs to scale up by a factor of 100 under conservative assumptions.
Like the article said, the solution to many of these issues is to put the lasers in space and intercept missiles in the boost phase. Then they're moving slower, you're targeting volatile fuel & oxidizer instead of heat shields, and you don't have atmospheric losses to worry about. You do have the issue of how to loft a giant laser, capacitor, and energy source into orbit, though, and how to protect them up there when a conflict starts.
This is a good point, the weak spot is that the missile designer "knows" where the extra energy is coming from (friction with the air). Once you add a laser, the energy can come from "any" angle (within some limits of course).
Think of it this way, the shuttle could not dissipate massive energy on the 'top' because re-entry involved air over the bottom.
As for power rating, the Boeing High Energy Lasers were demonstrating effective missile defense in 2017[1], they were ramping up their power levels pretty quickly.
I see the biggest impact early on being fleet defense against hyper-sonic anti-ship missiles rather than ABM defense. Basically a CWIS replacement since it doesn't help to lose your $10B aircraft carrier to a $100M missile.
Warheads spin on re-entry anyway, so laser energy directed at any point other than the centerline (which is pretty hard to hit) will get dissipated and spread across the whole warhead.
I think the biggest impact is actually against low-tech threats: swarms of fast attack suicide boats, or 1960s-era cruise missiles that have ended up on the black market. There's a big cost advantage to being able to take these out with a quick 30kW pulse rather than a million-dollar missile, particularly since the threat itself probably cost less than a million dollars.
In a great-power conflict (where "great-power" is rapidly expanding to include private multinational corporations) the U.S. military is fucked anyway, but then, so is the opposing power. Perhaps that's the best we can hope for, because it's a pretty strong incentive not to start great-power conflicts in the first place.
> In a great-power conflict [...] the U.S. military is fucked anyway, but then, so is the opposing power. Perhaps that's the best we can hope for, because it's a pretty strong incentive not to start great-power conflicts in the first place.
Yes. That is indeed the concept of a (nuclear) deterrent.
Let's suppose the threat to such a laser weapons platform in orbit during conflict which you touch on in your last paragraph consists of 1+ missiles.
Could the targeting system of the platform not consist of some kind of movable/adjustable prism that could acquire a target not just straight down on Earth's surface but also in other directions?
Such a system (if feasible) could probably also cut down hugely on time needed for and the amount of repositioning the weapons platform has to do during operation (thinking about and writing this sentence made the image of Tony Starks basement particle accelerator appear in my head again).
Would probably need a good bit of extra fuel for course corrections too. I'd imagine that a lot of ablative material gassing off on the side close to the earth would have course impact. And influence aerodynamics.
The challenge with an ABM laser weapon system is generating enough power on-target that it's not just a rounding error for the energy that needs to be dissipated anyway. A quick back-of-the-envelope physics calculation indicates that'll be hard. An 80kg warhead is moving at about 5000 m/s upon re-entry and has a kinetic energy of about 1 GJ. I dunno what fraction of that kinetic energy is converted into thermal energy upon re-entry; for manned spacecraft it's "almost all of it" because they need to go subsonic for parachutes to deploy, but for ballistic missiles, let's guess 1/1000th for ease of math, or 1 MJ. Current laser weapons systems have a range of about 1mi (and the range of lasers is heavily limited by atmospheric considerations - atmospheric refraction will reduce your accuracy, while heating of water vapor, dust, and air can sap the energy). That's about 1/3 of a second on target, so the power output needed is about 3 MW. Current laser weapon systems like the AN/SEQ-3 have a power output of about 30 kW. This needs to scale up by a factor of 100 under conservative assumptions.
Like the article said, the solution to many of these issues is to put the lasers in space and intercept missiles in the boost phase. Then they're moving slower, you're targeting volatile fuel & oxidizer instead of heat shields, and you don't have atmospheric losses to worry about. You do have the issue of how to loft a giant laser, capacitor, and energy source into orbit, though, and how to protect them up there when a conflict starts.