Disclaimer. The Strangelovian nature of what follows is disturbing. I can’t change that.

It takes a nuke  to kill a nuke. It did, before the advent of hit-to-kill, and it may again, if BB’s, gumballs, and miniature projectiles don’t work. A nuke only has to get near.

In the 1960’s the technology for a hit-to-kill EKV did not exist. Computers of our experience were the stuff of sci-fi. Solid state cameras did not exist; imaging relied on large, bulky special purpose vacuum tubes that you cradled in your arms, and which shattered if dropped.

The leap to PESA scanned radars had been made, but in the form of installations of massive size and expense. Since PESA radars rely on intensive computing to form the beam and analyze the echo. tracking simultaneous targets was limited to small numbers. The primitiveness of control theory was an obstacle to guiding the interceptor in flight.

So post war, defense against strategic strategic bombers and missiles centered on detonating a small nuke in the vicinity of the incoming. This included actual operational systems. The Sprint ABM used the W66 nuke.  The Air-2 Genie, deployed until 1985, required no guidance at all; against a manned bomber; a thousand feet was close enough. Collateral damage to the defended region was a concern, so a group of volunteers stood hatless beneath the only air-burst test of Genie, of  a 1.5 kiloton W66 warhead. Film here, with on-site live commentary resembling a Rose Bowl parade: Genie Test.

How could a nuclear explosion be so  harmless to the observers, located 3 – 4 miles directly underneath?

•  In a ground burst or near-ground burst, the highly radioactive fission products mix with soil, building materials, and everything else, to produce a radioactive mix of heavy particles that falls to earth with heavy local concentration. This is lethal.
• In a high air burst, if the blast radius does not touch ground,  there is no crater.  The fission products do not mix with heavy particles, so instead of concentrating locally, they diffuse through the atmosphere. It adds to the global burden of radioactive fallout, the lesser of two evils.
  
• The 3 -4 miles of atmosphere provided effective gamma and neutron shielding. Since they were never  in close proximity to the fission products,  doses to the observers were described as negligible, at least according to the standards of the time.
• It was a small warhead, a W66. Directly beneath an air burst, there is a zone in which the blast wave is attenuated. This combination could have resulted in further moderation of the blast effect experienced by the observers.

So if a 1.5 kiloton warhead is such a nothing-burger, how does it destroy an incoming round? There are several ways.

A “chain-reaction” is central to the A-bomb. A lump of plutonium or uranium is imploded (squashed) into a compact form.  A neutron splits an atom (fission),  yielding two or three more neutrons, which split more atoms. Provided there is  a big enough lump of plutonium or uranium (critical mass), the numbers look good. Heat is liberated. As long as the numbers look good, this continues. It continues until all that heat makes an explosion.

The air burst of a small anti-missile warhead has small blast effects. But it produces neutrons. If the nearby aggressor warhead absorbs some of those neutrons, its own  lump heats up. If it heats up enough, it melts, at least a little. This deprives it of precision, so it cannot “assemble” to produce a high-order detonation. The effective radius for the Genie was thought to be about 900 feet.

As with the ideas of making holes in a hypersonic warhead with a pack of BB’s or gumballs, the challenge is destroying a device without knowing exactly how it is made. Neutrons are thought to be universal and inescapable. This was the original motivation for the neutron bomb. But for the sake of completeness, consider a countermeasure.

Neutrons, unlike gamma rays, can be shielded by light elements. Borated polyethlene is sold commercially for this purpose.  The fission end of the aggressor nuke could be enclosed in a bucket of this stuff.  The bucket has to be open-ended so it does not interfere with the fusion secondary. At the cost of some weight, a bucket 11 centimeters  (4.3 inches) thick  would halve the effective distance of the Genie’s neutron burst. A future Avangard could be constructed partly of boron nanotubes, a material providing both structural strength and neutron shielding.

In the early ABM era, the  destructive pure energy pulse gained traction as a compliment to neutrons. A nuke produces both. In our universe, photons are the carriers of both information and radiant heat. X-rays, gamma-rays, visible light, and the warmth of an infrared lamp are all the same to physics. Although the W66 warhead of the Genie and Sprint does not make a big bang at high altitudes, it  releases a lot of energy. Different wavelengths (“colors” of light) in the pulse have different penetrating power. The visible part of the pulse heats up the aggressor surface. Perhaps this can be withstood, but a significant fraction of the energy is in the form of X-rays, penetrating the structure and melting the insides.

A caveat. Outside the atmosphere, the pulse travels long distances, attenuated only by the inverse square law. The range sharply decreases inside the atmosphere.  We could use charts to study how far each range of radiation travels, but it gets complicated. We know from  the Genie air burst experiment that 3 -4 miles of air rendered the observers safe from harm.

A good rule of thumb is that a physical gadget, with notable solid state exceptions, resists damage at 10,000 times the dose of the human observers. The inverse square law says 150 feet for 10,000X the dose. But the inverse square law does not directly apply, because atmospheric absorption is the stronger effect.  For instance, soft X-rays attenuate with only a few feet of air at sea level.

A wild-ass guess of the effective radius of hard X-rays, at 80,000 feet, for a Genie-style W66 warhead, is some number under 100 meters. The exact distance is hard to pin down, because the effects increase  rapidly as the distance decreases.  The different “colors” of the pulse are advertised to have these effects:

• Hard X-rays, to which Avangard is partially transparent, would heat the interior beyond tolerable levels, possibly melting the structural plastic.
• Infrared and visible light create thermal effects that would destabilize the craft, and shock waves that might destroy it.
• X-rays and gamma rays damage semiconductors, rendering the circuits that follow inoperable.
• The warhead has an electrical power supply, without which it cannot detonate.
• Without working guidance and control systems, the vehicle tumbles out of control, with destruction from aerodynamic forces and heating in the wrong places.

Besides hard X-rays, Avangard must also deal with EMP, neutrons, other “colors” of energy such as infrared, and even some blast. Enough brickbats to kill any gadget – we hope.

We still haven’t gotten to directed energy weapons. To be continued shortly.