The bed of nails consists of a field of 2-metre (7 ft) long steel rebars rising vertically out of the ground in a pattern in front of the silo to be defended.
The pebble fan is a similar concept consisting of a long strip of gunpowder topped with steel balls which would be triggered to fly into the air and produce a "curtain" of pellets.
Although these simple protective measures could be deployed rapidly and for almost no cost, there was practically no interest on the part of the DOD or US Air Force to do so.
In the early 1970s, Soviet RVs were thought to have accuracy on the order of 1 nautical mile (1.9 km; 1.2 mi), just enough to get one into the lethal radius if two were fired at every silo.
[1] After travelling in a ballistic path, enemy RVs would approach the silos from the north at an elevation angle of about 22 degrees above the horizon.
[4] Another possibility is to use a radio altimeter or similar device which directly measures the distance to the ground and thus can accurately trigger.
The problem with this approach is that the defenders could send out signals from ground-based radios that would either confuse the altimeter or cause it to trigger prematurely.
Inaccuracy is that of the RV's internal guidance, but when exploded on the surface it puts the maximum amount of energy into the ground and thus maximizes the lethal radius.
[3] Given the low approach angle, any warhead aimed at the silo that would fall close enough to be lethal would strike one of the rebars and be destroyed.
[3] If the balls are replaced by small darts, the amount of propellant is reduced and the vertical coverage increases, a concept known as a porcupine.
[6] From 1969, the US Department of Defense (DOD) began to raise the concern that future Soviet RVs, from the 1970s, would be powerful and accurate enough to attack the Minuteman missile force.
At the time the concept was proposed, it was noted that "Such systems, perhaps because of the old-fashioned technology employed or because they are incapable of defending a large spectrum of targets, arouse little interest within the Defense Department.
"[9] He later added that the lack of interest was likely due to such a system making the advancement of the MX more difficult to justify.
[10] The only comments about the systems were from the Ballistic Missile Defense Agency deputy program manager, who stated that the concepts were "difficult to synthesize and still meet the criteria of low cost, rapid deployability and adequate effectiveness.