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Comment by sidewndr46 | Hacker News Reader

What I'm perpetually confused by is I am relatively certain we developed interceptors for these type of missions in the 1970s. The LIM-49 Spartan and the later "Sprint" missile were designed for exactly this kind of intercept. The Sprint missile was capable of moving so fast it was glowing white hot during its mission.

We elected not to deploy these weapons for whatever reason. So saying they don't exist at least in the case of the US is more like saying we threw them out because they were deemed useless. But the problem doesn't really seem unsolvable.

https://en.wikipedia.org/wiki/Sprint_(missile)

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Everyone keeps saying that Sprint was a nuke. That is true. However, it also scored some kinetic kills during testing at Kwajalein.

https://www.youtube.com/watch?v=YZZV464z9g8

I'm pretty sure that improvements in sensing, computation, actuation, and conventional warhead design over the past 50 years could have produced a reliable hit-to-kill Sprint. And in fact, some research was done on such a vehicle, the McDonnell-Douglas HEDI.

http://www.designation-systems.net/dusrm/app4/hedi.html

I think you are missing something;

The ABM systems we built in the early cold war worked by having nuclear payloads. We could absolutely not hit an incoming ICBM with the tech at the time, so we just slapped a nuke on it and hoped we could get within 1km at detonation.

Importantly, it was a completely dead end. They had no response to MIRVs and could not be built in sufficient numbers to deal with any actual launch. We threw them out because they were in fact useless.

Generally, we have moved away from Nuclear ABM systems because detonating a hundred warheads above a city is very unlikely to work out well.

Intercepting a cold war era ICBM turned out to be feasible with newer technology, and we currently have $2 billion missiles that can feasibly intercept ICBMs (at low quantity).

>No maneuvering boost-glide hypersonic vehicle has ever been fired in combat against a defended target

Nobody has fired one of those against a target because almost nobody has a functioning maneuvering hypersonic vehicle. Basically just China I think.

I would expect "real" hypersonic weapons like that are basically uncounterable. The physics just gets too obnoxious. Interceptors will struggle to get better than a coin flip, and they will be too expensive to use on anything else so they won't be general purpose, so equipping them will be full of tradeoffs.

That's the entire point of hypersonic weapons. $3 billion dollars to make that high value target go away, with extremely high probability. They replace 50 bombers launching still quite expensive anti-ship weapons at scale, which is the strategy it replaces.

This of course has rather negative implications for the concept of force projection in future wars. Which is why China made a hypersonic weapon.

I don't think you're wrong broadly, though I want to add that the particular interceptor warheads were relatively small and nukes detonated in the upper atmosphere generally don't do much to stuff on the ground. There's not enough material to create significant fallout so it's just a mild EMP and even milder pressure wave

I think the LIM-49 was 5 megatons or thereabout. So for varying definitions of 'small', this is true.

Interesting post. It made me think that a manned Navy of individual high value ships becomes essentially useless if you can take out a 13 billion dollar carrier with a 3 billion dollar missile.

It becomes a war of financial attrition at that point.

The weapon you linked to is an anti ballistic missile. The difficulty is not purely in how fast the target is going, but how much it maneuvers, the duration at which it can sustain those speeds, and the altitudes at which it operates. The article addresses this early on.

Aren't all hypersonic missiles based on re-entry vehicles? The altitudes they operate at are basically 'all of the above'. If someone can launch a low altitude missile that can sustain Mach 5+, I'm not sure what you are going to do against it.

So what are the hypersonic low altitude capabilities for maneuvers for these platforms? I don't deny the maneuver but I have this hard time believe someone just throws out some grid fins while doing Mach 24 at 10,000 ft. If the course changes are done at high altitude I can't see what it matters either.

TFA does not discuss the missiles that only reach a hypersonic speed, which is true for any ballistic missile, because such missiles have existed for a very long time.

It discusses the hypersonic missiles that are maneuverable, which for now have not been deployed in combat yet, by any of the 3 countries known to possess such missiles.

So what are the hypersonic low altitude capabilities for maneuvers of them missiles? I keep seeing it claimed but I don't see any numbers. Just hypersonic, maneuvering and glide vehicles doesn't really say much. That's the Space Shuttle. I've yet to hear anyone claim the space shuttle is a 21st century weapon.

The earlier interceptors were for ballistic missiles. They are traveling at hypersonic speeds but have high trajectories (so radar can see them earlier) and can't maneuver for significant parts of their flight (so they are easier to track and target).

FWIW they were cancelled because they didn't have a particularly good kill ratio and proliferation and MIRV meant you'd need a ton of them to prevent an attack landing (and doing so would involve a significant number of nuclear blasts pretty close to the targets anyway). Deterrence was more credible.

Wikipedia notes both the Spartan and Sprint missiles as having nuclear warheads. That was reasonable-ish, since Wikipedia also notes them being cold war-era anti-ICBM weapons. Less bad to have your own interceptor nukes going off "near" your city than to have enemy nukes scoring direct hit on it.

In contrast, modern hypersonic weapons have plenty of use cases where they'd be fitted with conventional warheads, and used against targets like US Navy ships.

There is plenty that could go wrong if USN ships mounted nuclear interceptor missiles, ready to launch on a moment's notice...

> We elected not to deploy these weapons for whatever reason.

Russia/China insisted that Americans perfecting the nuke interception game is an existential threat to them, and threatened to do something horrible like retaliating militarily if cities and nations were to be rendered immune to nukes, as if it would remotely harm their own through a magical power in doing so. The US/EU played along with that logic and limited deployments of interceptor techs.

Spirit is a nuke. Not really something we want to be detonating in the atmosphere.

I think you meant to say "Sprint". In any case, if you're being attacked I think the consequence of high altitude fallout is pretty small compared to dying.

Depends on what you're being attacked by. If it's just regular warheads, a nuclear interceptor is wildly inappropriate especially if the intercept happens over someone else's airspace.

Modern thermonuclear weapons don’t really have fallout.

That's better but people typically won't appreciate a thermonuclear explosion overhead in order to take out a missile armed with a few hundred kilos of explosives. That is doubly the case if the nuke is from a neighboring country intercepting a missile targeting them.

Weren’t those anti ballistic missile missiles all nuclear armed themselves?

And doesn’t the parent article to the Sprint article make it clear that they we didn’t deploy them because fall out shelters combined with building more nukes was deemed more cost effective at saving lives.

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