Kinetic ICBMs?

Saphroneth

Banned
Would be interested to see your source for that. I could see the Mk2 RV as potentially being subsonic as it was a blunt design (I.e capsule-like). But the Mk3 and Mk6 RVs on the Atlas and Titan, respectively, were triconic designs and certainly hypersonic. As far as I can tell the Polaris did use the Mk2 RV even in later editions so maybe that's what you're thinking about re: chevaline?

I do agree with the majority here that kinetic payloads are generally inefficient. However, there certainly is a niche for something like it, hence why there were some efforts into modifying Trident to carry a kinetic penetrator. Using an SLBM gives the advantage of an endoatmospheric profile. One source states a 2000 lb penetrator at Mach 4 terminal velocity - they sacrifice a lot of velocity to obtain high crossrange capability (up to 3000 nmi - Russia or China would understandably be shitting bricks if you tried an overflight with a Trident :D). Not much of a hard target capability, hence why the Bush administration tried to bring back the idea of nuclear bunker busters.
My source is fellow poster pdf27, who cited that one of the Chevaline changes (not the most important one) was re-entry speed increase. There's several other sources from the discussion - it's in Blunted Sickle, search for Chevaline and you'll find the pages - which are on the lines of that it's not for some decades that hypersonic low atmosphere RVs come in - at least partly for material reasons, since a hypersonic RV has a lot of thermal stress on the tip since it has to be so aerodynamic.

One key point is that the force on an RV scales as the seventh power of the velocity.




That doesn't really tell us very much, unfortunately - according to NASA the space shuttle has a lift:drag ratio of about 1:1 (and hence a glide angle approaching 45 degrees in a steady state), and glides at about 250 mph. If we assume that it is still supersonic until 5 minutes before landing, then the supersonic:subsonic transition happens at an altitude of 21 miles (110,000 feet - well above the capability of any SAM system of the time). I've already dealt with the V-2 elsewhere in this thread - peak speed and altitude were low enough that re-entry thermal loads are not a problem so it isn't limited in the same way that other vehicles are.

My working assumption is that any warhead would be falling somewhere near vertically at approximately the speed of sound. Assuming any SAM system they have can engage up to 60,000 feet and that the warhead initiates at 5,000 feet, that gives them 50 seconds to engage. To get to 2 seconds they've either got to be maintaining a steady speed of Mach 25, in which case it's going to be immensely hot (a stagnation temperature of just under 40,000 Kelvin if I've calculated correctly - although it'll be behind a shock wave which will help a little, and I can't find the right book to confirm). Steady speeds are also simply not possible in atmosphere - the stagnation pressure goes with Mach number to the power of 7, so the pressure on the nose would be 22 million times atmospheric pressure. For a best-guess reentry vehicle (50cm diameter, 500 kg weight) that's a pressure of 2.24 TPa and hence a very approximate force of 440 x 10^9 N - so it would be slowing down at 89 million times the force of gravity.
All these numbers are out an out impossible (the g forces alone are enough to ensure no nuclear weapon would work afterwards). I'm being a little bit naughty by assuming Mach 25 at sea level when the real air pressure would be substantially lower at altitude (perhaps an order of magnitude - so still 9 million times the force of Gravity), but the problem is that these numbers are so far out of the possible that there is just no way that a reentry vehicle can be doing Mach 25 anywhere close to the ground.

There are ultimately only two ways this can be dealt with:
  1. By doing things very high up indeed, where the static temperatures are low and so are the pressures. This gets the forces and temperatures involved down to ones that engineering can deal with. That's what all spacecraft and ICBMs do, but doesn't affect early ABM systems as they don't have the range or sensors to hit anything up here.
  2. Slow the RV right down - the temperatures and pressures involved scale with Mach number to the power of 7. Halving the speed cuts the forces by a factor of 128. Assuming that the warhead can survive 50g, that gives a sea level velocity of somewhere around Mach 3. Problem is, the atmosphere gets thicker geometrically rather than linearly - so if you don't slow down at least a bit in the upper atmosphere (requiring a drag coefficient which will probably prevent you ever reaching Mach 3 down low) you'll hit the atmosphere far too fast and crush the warhead.




The warhead point is obviously invalid here, but even a dumb penetrator is going to be crushed by the kind of forces involved here. If it can survive 6400 g, that means it's limited to a mach 6 impact - for example.
 
So, if kinetic bombardment is a no go, chemical weapons are finicky, and only fools go biological, is there pretty much no use for ICBMs without nukes?
 
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