Hacker News Reader

Comment by jltsiren | Hacker News Reader

Boost-phase interceptors would be big expensive weapons systems. Because they have only a few minutes to hit before the target deploys the payload, they would have to expend lots of delta-v to guarantee intercept. Deploying such large-scale missile defense systems would be obvious to everyone. By the usual cold war calculus, such attempts would likely trigger a nuclear war. If you attempt to neutralize enemy nuclear deterrent, you are announcing your first strike intentions to everyone.

As for why, maybe the reason wouldn't even involve America that much. Maybe China decides to take Taiwan, and to prevent America from interfering, they choose to destroy LEO for everyone. Maybe it would be an acceptable sacrifice for them, as they are more interested in controlling their neighborhood than projecting power to faraway places.

View on HN

> Boost-phase interceptors would be big expensive weapons systems

Not very big actually, not if the interceptors are placed in LEO. They're a hell of a lot smaller and cheaper than the missiles they're meant to intercept. The kill vehicles themselves are only a few kilograms, very small and light weight. For such light-weight interceptors positioned in LEO, you don't need much booster to get the job done. You can get away with a booster much smaller than an SM-3.

> Because they have only a few minutes to hit before the target deploys the payload, they would have to expend lots of delta-v to guarantee intercept

The more you have, the less delta-v they need. That's why Brilliant Pebbles proposed thousands if not tens of thousands of them. Launching such a huge number of interceptors would be possible with Falcon 9, but Starship would be much better at it.

> Maybe China decides to take Taiwan

Maybe they would, but they would have to be insane to start a symmetric war with America itself. And that's the whole point.

> Deploying such large-scale missile defense systems would be obvious to everyone.

If you've been paying attention, you'll have noticed that China and Russia haven't been very secret about their interest in countering Starlink. It's not because they're afraid the US might give uncensorable internet to Chinese and Russian people, as I've seen some on HN speculate. They obviously perceive the strategic threat already. Besides starting the war first, there's not a whole lot they can do about it. If they start the war first, start it now, they'll probably kill a lot more Americans than if they wait and do it later. But that doesn't matter because they'd still all die, so they aren't going to do it. Neither China nor Russia has a BMD system that could prevent America from killing them all.

I think as far as operational capabilities go, all BPI stuff is just not there (at least from public info) and at best, starlink would only be good a large scale detection, target aquisition and tracking:

[0]"While boost-phase defense has been advocated as the most efficient way to deal with fractionated payloads and exoatmospheric (midcourse) penetration aids, it is extremely sensitive to assumptions about threat booster characteristics. Over time, boost-phase defense tends to be renamed ascent-phase defense when the kinematic realities set in. In fact, ascent-phase defense is code for engagement in the postboost or early midcourse phases of flight.

The limitations and complications of a surface-based boost-phase defense lie primarily in the concepts of operations (CONOPS), policy, time, and geography. Since the timelines for engagement in the boost-phase are extremely short, the on-site commander must have authorization from the National Command Authority to launch an interceptor immediately after a threat missile has been detected.1 Also, access must be gained to countries adjacent to the threat country in order to position a boost-phase system close enough, and at the correct geometries, to successfully engage the threat missile. Finally, boost-phase systems are only effective against countries that do not have large enough landmasses to allow them to launch missiles from deep within their territory.

The airborne laser is designed to deliver energy at the speed of light to perform the boost-phase intercept mission. Space-based lasers were also pursued in the past. Virtually no fly-out time is involved, and the beam agility is a function only of how fast the pointing optics can be repositioned. While laser weapons sound like the obvious answer, the energy that a laser can deliver on a target is limited by the power available and the aperture of the device. Atmospheric effects disturb the beam. Much has been accomplished in advancing the pointing and tracking capabilities and the adaptive optics to maintain beam quality, but some fundamental limitations remain."

[0] https://nap.nationalacademies.org/read/13189/chapter/4

The part of this document that you're quoting is talking about the limitations of land/sea based boost-phase interceptors. Scroll down to the Space-Based Interceptors section. I don't think there's much substance here.. It talks about kill vehicle size (see: LEAP), and mentions that coverage depends on the number of interceptors and the inclination of their orbits (no kidding.)

The section i quote referrers to space based lasers too. I also haven't see anywhere publicly on any actual space based kinetic system even in testing for BPI usuage (which would still face the same kinematic issues with land and sea based system).

I read in the Scientific American that a defense against lasers would be simply spinning the target, which would require the attack laser to have much more power. The target can be made shiny, and can be coated with an ablative material.

Yeah, I'd factor that into major limitations with the space based laser systems. Only think I can think of that could work around that is if one used an array of lasers that could constructively interfere during boost phase that could cause some kind of major turbulence at the point of intersection (maybe along the lines of the laser collisions here[0]:

"Fülöp and colleagues argue that the energy problem could be overcome by splitting the laser pulse into two equal-energy pulses, which then hit the target simultaneously at different, precisely calculated angles of incidence (see figure). Through simulations, the team showed that when the laser pulses collide, a standing wave forms inside the solid target that increases the peak electric field. With the same pulse intensity used in previous experiments, their calculations suggest that the technique can double the energy of the proton beam and produce five times as many protons — all with far more flexible control parameters than required before.").

[0] https://physicsworld.com/a/laser-collisions-could-boost-prot...

Reply on news.ycombinator.com