No, it was a matter of range.
That's not actually true...
The Army's air-defense role was essentially a continuation of ground-based artillery and coastal-defense: Previously part of this job was held by gunners on the ground and aircraft in the sky. When the USAF became an independent service, things sort of got balkanized.
The Army continued developing its AAA, which among other things included the Skysweeper. It had a listed range of 5-8 miles to an effective altitude of 30,000 feet, and a maximum effect altitude of 43,000 feet. Provided these numbers are remotely correct, it seems it would have only been of limited effectiveness against subsonic jet-bombers at altitude.
The Air to Air missiles were seen as a logical extension of this even before WWII ended and in the post-War period would grow to include the following
- MIM-3: Listed range of 25-30 miles, designed to kill jet-powered bombers like the B-47
- MIM-14: Listed range of 75-90 miles, and was designed to take out supersonic bombers like the B-58
- MIM-23: Listed range of around 1.2-16 miles at low altitudes it would appear, and later variants would extend high altitude range to around 50-62 miles
Problems occurred when it came to the USAF's role in national air-defense. They clearly had control of combat aircraft such as fighters and interceptors to kill bombers, and air-to-air and air-to-ground missiles were clearly their domain as well.
The problem was what to make of SAM's and SSM's: USAF probably felt that SAM's were a logical extension of air-defense, and SSM's a logical extension of bombardment; the US Army saw it as an extension of artillery (and both were technically right in their own way)
- Ballistic missiles had proven themselves to be quite a formidable threat in WWII as they were damned near impossible to intercept, and the Germans were already working on variants with intercontinental range: The limiting variable was accuracy and warhead size/yield; the nuclear warhead would greatly improve the yield of the warhead, but were quite massive in size at the time; the accuracy was tied to the gyro-stabilization system, and decreased over range: Both were improving with time though, and it wouldn't be long before nuclear warheads that were about the same as the V-2's would be available.
- Cruise missiles were also quite a threat, but were easier to shoot down: Supersonic performance would greatly change this, and accuracy issues would be fixed by a sufficiently powerful nuclear warhead. The missiles would be cheaper than a manned aircraft as they would not require life-support equipment, would only be used once, and if they were shot-down, no valuable aircrews would have to risk their necks (or the heads, and bodies attached to them).
- The US Army and US Navy were both concerned about the USAF acquiring a monopoly on nuclear-weapons, with the fear that they would be rendered obsolete if this occurred: The US Navy was concerned about being able to deliver a bomb with it's aircraft; the US Army wanted a missile of some sort that they could deliver.
- The US Army wanted to have a role in national defense other than artillery units (which were of limited effectiveness), and holding down the population in the event of nuclear war, which naturally lead them to want to develop SAM's into ABM's; and develop some kind of ballistic missile capability.
- The USAF was expressing a concern that their A/C should get shot down by SAM's without a unified command (which is of course theirs): I'm not sure if this was remotely valid at first (in politics, the argument matters more than the facts), but eventually things were streamlined and coordinated without removing the Army from the equation.
Ultimately the USAF was able to retain some SAM capability by labeling the BOMARC as the F-99 (if you call it a fighter, it's under AF control... names have power
), and DoD restrictions which limited the Army from developing missiles with ranges in excess of 200 miles (I'm not sure to what extent the Navy was affected as they were able to develop the Regulus I & II), but this was lifted with Sputnik).
The US Army developed the following anti-ballistic missiles
- Nike Zeus-A: Range of 200 miles with a 20 kiloton warhead detonated in the upper atmosphere to destroy single warheads. It didn't enter service and was mostly a testbed for the Zeus-B
- Nike Zeus-B: Range of 250 miles with a 400 kiloton warhead detonated either in the upper atmosphere or space to destroy single warheads. It was briefly used with modifications as an ASAT.
- Nike Spartan: Range of 460 miles with a 5 megaton warhead detonated in space to capitalize on X-ray kill so as to blow up a payload bus, and thus destroy multiple targets at once. A more advanced version was proposed with a more powerful rocket, a smaller warhead (more accuracy) and greatly increased range which either meant it would be command guided part of the way, or all of the way with TVM
- Sprint: Point-defense ABM shaped like a cone. It is a twin-staged rocket (the others were three-staged) with a 40 kiloton enhanced neutron warhead to maximize neutron kill; the warhead is designed to withstand being blasted out of the silo by a powder-charge (at which point, it might very well be supersonic) then subjected to maneuvering loads of several hundred g's, over 100g of continuous acceleration allowing it to accelerate to Mach 12 in 5 seconds, and the ability to take 'shocks' as much as 25,000g (flying through explosions of nearby blasts and also withstanding being shot out the silo). It basically would fly for all of 15-20 seconds with the booster being blown up 1.2 seconds after launch after the top-stage hot-fires.
The range of some of them clearly are quite massive and the Nike Spartan and Sprint were controlled by a series of PAR's (Perimeter Acquisition Radar) and MSR's (Missile Site Radar): The PAR was a blocky structure with a slanted wall on one side, which mounted the radar. The radar had a range of several hundred miles (not sure if that was just line of site and had OTH capability) and acquired the missiles in flight (I wouldn't be surprised if it was given data from the BMEWS as well), which then passed on the data to the MSR which launched and controlled the missiles in flight.
The MSR looked like a pyramid which could carry a phased-array antenna on each side that was capable of being serviced while online. Controlling the Sprint was a particular concern because of the fact that it's enormous temperatures would produce a sheath of plasma around the missile. The command signal was narrow and something like a megawatt in power so it was able to get through all of that anyway.
The computers used were remarkable for the time, though if I recall their processing power 10 MIPS (similar to a SEGA Game Gear), and utilized a multiple CPU core. It was able to rewire itself on the fly to handle the failure of one of the CPU's, and had self-correcting computer code.
As amazing as all this is: The most amazing thing was that it came in within cost, to specification, and on time.
It was part of a system called either Safeguard or Sentinel (I'm not sure which came first). The initial idea called for protecting 25 of the most economically prosperous cities with a limited missile-shield under the guise of countering China (not the USSR, though almost certainly it would have been used for this purpose). It was later reduced to 14 cities, which still worked out as the positioning of the missile sites and radars were still able to provide effective coverage of the United States (Far as I know the data from all this would be forwarded to NORAD, so they'd have a complete picture of what was happening)
The reason the system ironically wasn't fielded was basically political in nature: Admittedly it probably was a good one. Either as part of the SALT treaties, or as part of a specific treat on ABM's, there was an agreement to not develop a massive ABM system, but only two sites with 100 missiles each.
- One site would be to cover the National Command Authority
- The other site would be whatever each country decided
- The missile configurations could be whatever the nation decided
The US decided not to cover Washington D.C. for reasons that make little sense, but put one up in Grand Forks that would effectively cover most of the US from missiles coming from over the pole. Advanced proposals for the Spartan were developed which had superior acceleration, but the ability to throttle down, reducing acceleration during the climb allowing for greater target acquisition, or a coasting ability allowing it to acquire targets while in space: Supposedly this loiter mode would allow more accurate interceptions and greater ranges (2,000 nm) allowing much of CONUS to be effectively covered from Grand Forks.
The system would use an infrared seeker implicating either an active homing to target, or a TVM capability.
The system was defunded after it had been operational for all of a day, though it kept running for practical purposes several months after the fact: The Russians of course fielded the ABM-1 & ABM-3, and later the ABM-4. The number of sites, their placement and coverage I'm uncertain of, but the fact is that they would generally try and ensure that they would in practice develop SAM's that usually had some degree of inherent ABM functionality.
In the event of the abrogation of the treaty, there were plans to develop a more advanced system that was also more compact and was modular configuration (something that would have been amazing in itself even if it had the same capabilities of the less mobile MSR's), with greater capability to deal with the possibility of FOBS (Fractional Orbital Bombardment System), and MaRV's (Maneuvering Reentry Vehicle). There were at various times improved versions of the Sprint that were proposed with 2-4 times the acceleration and 3-4 times the maneuverability as insane as that sounds.