Block IV Tomahawk is the current generation of the Tomahawk family of cruise missiles. The BGM-109 Tomahawk family began life in the 1980s as sub-sonic, low-flying nuclear strike weapons, before being developed into long-range RGM/UGM-109 conventional attack missiles. They’re most frequently launched from submarines and surface ships, and have been the US Navy’s preferred option for initial air strikes in Iraq, Libya, et. al. Britain has also bought Tomahawk missiles, and launches them exclusively from submarines.
Block IV is the latest variant, adding innovative technologies that improve combat flexibility, while dramatically reducing the costs to buy, operate, and support these missiles. That program, under PMA-280, has been one of the USA’s acquisition success stories over the last decade. Now a June 2012 order will add another 361 missiles to the fleet.
Submarine-launched UGM-109 missiles are more expensive than their RGM-109 counterparts, because their launch mechanism is more involved and more strenuous. Their “all-up-round” storage and interface canisters come in 2 types: CLS and TTL. CLS canisters launch UGM-109s from vertical launch tubes installed on many of America’s Los Angeles Class (SSN 719 on), all Virginia Class, and all SSGN Ohio Class submarines. TTL canisters are used to launch Tomahawk missiles from a submarine’s torpedo tubes, which is Britain’s preferred method.
In both cases, a Tomahawk launches “wet”, unlike most anti-ship missiles. The canister remains in the vertical-launch or torpedo tube, while the missile is ejected. Once the UGM-109 has reached a safe distance from the submarine, its rocket booster ignites underwater to power it airborne. That booster falls away just before the missile ignites its jet engine. If the submarine needs to “clear the tube” for torpedoes, anti-ships missiles, mines, UUVs, etc., TTL canisters can be ejected into the sea after launch, as a separate evolution. CLS canisters are removed portside, when the submarine comes into base for servicing and reloading.