The bed of nails defense and the related pebble fan defense are anti-ballistic missile concepts intended to defend missile silos against attack.
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. Warheads aimed at missile silos will hit at least one of the rebars before hitting the ground, potentially destroying the warhead before it can trigger. 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 neither system is wholly effective, compared with other missile defenses they cost very little to build, and are effective enough to increase the number of failed attacks and thus demand a larger number of warheads be expended in any attack on the silo – leaving the attacker fewer warheads to use on other targets. Moreover, the results of a counterforce attack would be much less predictable, making such an attack less valuable.
The question of the survivability of the Minuteman missile force was repeatedly raised by the Department of Defense (DOD) after 1969. 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. Much of this appears to be due to the desire to replace Minuteman with the MX missile.