[ethbro]

The US looked at this in the 50s and the 60s as Pluto & SLAM.

The consensus was that it was a dumb idea, especially after ICBMs were deployed. Their only benefit vs alternatives -- incredibly long flight time (on the order of months / circling the globe 4.5 times) -- doesn't allow you to do anything you couldn't already do.

This was pre-stealth, but I can't imagine it's easy to stealth something with a red-hot exhaust (remember, to have endurance, it's superheating air rather than combusting limited fuel).

Hypersonic re-entry vehicles are far more useful, but also harder to develop.

Honestly, this seems a pretty blatant attempt to trot out Cold War era technology for a ra-ra "look, we're still relevant" showing than new R&D.

https://en.m.wikipedia.org/wiki/Project_Pluto

https://en.m.wikipedia.org/wiki/Supersonic_Low_Altitude_Miss...

[varenc]

For those curious on how a nuclear-powered missle could work...

On the Pluto project a ramjet scoops up air, and then the decay heat from a radioisotope is used to heat that air and expel it to generate thrust.

The range really is practically unlimited. The Pluto missle could have flown for months. Nuclear fuel just has over 1000x the energy density of fossil fuels (but lots of complexity around using that energy)

Hypersonic re-entry vehicles seem effective at beating missle defense systems, but a nuclear ramjet missle always in the air seems like it'd have other advantages. (First strike? Persistent threat even after all land/water based missle systems are destroyed?)

[danbruc]

You can, at the very least, probably aim them a lot better or change the target altogether, for example for misdirection. An ICBM mostly goes up and falls down again on a ballistic trajectory, at least in a first approximation, they can of course correct the trajectory to some extend. On the other hand ICBMs are a lot faster which makes them harder targets for interceptors even if the trajectory is reasonably predictable.

[ethbro]

Fair enough, but what's the use case for this?

I think the reason the project was shelved is that it did provide different capabilities, but none of those capabilities were actually useful in a military context.

That said, after a bit more review, there is one scenario where they would be useful -- launched en masse as a first strike weapon.

While easy to detect at radar range, the smaller boost signature does make launch detection more difficult.

Assuming a launch from the west coast, and design speeds comparable to Pluto (mach 4.2), you'd have about 9 min before they hit Midwest US silos.

Which is one of the other reasons we never developed them. Announcing loudly that you're developing something that's primarily a first strike weapon tends to make the other person jumpy...

[imglorp]

One Cold War era tactic comes to mind, probably void now with modern interception.

A "failsafe point" is where you can stage bombers (or flying bombs in this case) in neutral airspace near your enemy, and have them orbit there for long periods. If things get more tense, you can stage more there without committing to a strike. With tankers, they can park there for days before being swapped out. Then if things turn sour, they're not as far from the target; they can drop to terrain following and go downtown for business. See the awesome Henry Fonda 1964 movie Failsafe for a period story about how talking to your bombers can go wrong.

https://en.wikipedia.org/wiki/Fail-Safe_%28novel%29