during release from the delivery aircraft. Only the Mod 0 to 10 versions of the B61 are equipped with a parachute retarder (currently a 24-ft (7.3 m) diameter nylon/Kevlar chute). This offers the aircraft a chance to escape the blast in its retarded delivery modes, or allows the weapon to survive impact with the ground in laydown delivery mode. Contact preclusion can also be selected by the pilot. The weapon can be released at speeds up to Mach 2 and altitudes as low as . In one of the weapon's laydown modes, it detonates 31 seconds after weapon release. The Mod 11 is a hardened penetration bomb with a reinforced casing and a delayed-action fuze; this allows the weapon to penetrate several metres into the ground before detonating, damaging fortified structures further underground. Developed from 1994, the Mod 11 went into service in 1997 replacing the older megaton-yield B53 bomb. About 50 Mod 11 bombs have been produced, their warheads converted from Mod 7 bombs. At present, the primary carrier for the Mod 11 is the B-2 Spirit. Mods The B61 has 13 variants, referred to as Mod 0 through Mod 12. B61 Mod 3 and 4 A Sandia document on the dates of the development phases of the US nuclear stockpile describes the B61-3 as an "improved Mod 2 w/IHE" and the B61-4 as "same as Mod 3 ex. Hi-Y". The B61 Mod 3 and 4 bombs do not contain beryllium. B61 Mod 5 A 1978 report on reducing or substituting beryllium in nuclear weapons noted that the B61-5 contained beryllium and that if the use of beryllium were to be restricted, the B61-5 could probably be replaced by the B61 Mod 3. B61 Mod 6 and 8 The B61 Mod 6 and Mod 8 bombs were developed for use by the US Navy beginning in March 1987 and cancelled at the end of the Cold War. The weapons weighed , had a length of and a diameter of , and were one-point safe, used insensitive high explosives (IHE) in their primary stages and utilized enhanced electrical safety (EES). The weapons were to be built by converting existing B61-0, −2 and −5 weapons into Mod 6 and 8 weapons and the weapons were to use B61-7 weapon parts where possible. By mass, the largest deviations from the B61-7 were the Acorn assembly, MC4137 TSSG (trajectory sensing signal generator) and