[Llarry]

The most numerous U.S. military aircraft is the Sikorsky H-60 (manufacturer's designation S-70) which is used by all U.S. military services, as well as by several dozen other nations' militaries and some civil users as well.

The H-60 started life as a helicopter to replace the Army's UH-1 "Huey" of the Vietnam War era. The Army was looking for across-the-board improvements: performance in hot and high conditions, reliability, load-carrying capability and crashworthiness.

Sikorsky's design competed with a Boeing-Vertol design -- the YUH-61A -- for the contract and first flew in 1974. Sikorsky was selected as the winner in 1976 and the UH-60A Black Hawk (or Blackhawk) entered Army service in 1979. The Army ordered large numbers of the new aircraft, which soon dominated Army aviation. The A model was produced from 1977 to 1989. Twin General Electric T700 turboshaft engines of 1,500 hp each were developed at the same time and similarly became the gold standard of utility/troop carrier aviation in the Army.

By 1989, the UH-60A was replaced on the production line with the UH-60L model with more power and an improved gearbox. Many A models were modified to L standard as well.

In 2006, a further improved UH-60M began production. The M model featured 2,000 hp engines, improved rotor blades and electronic instrumentation. Many earlier models have been updated to UH-60M standards, but the M remains in production.

While the Blackhawk's primary mission is troop transport, other variants have been developed. Among these are UH-60Q and HH-60M aircraft for medical evacuation. Another is the VH-60M, which is used for VIP transportation in the Washington, D.C, area.

In a category all their own are the special operations variants of the UH-60. The first of these were 30 MH-60As modified with upgraded engines, forward-looking infrared (FLIR), night vision compatible cockpit, armament, aux fuel, etc. These were assigned to the Army's 160th SpecOps Aviation Regiment in the early 1980s.

Ten years later the MH-60As were replaced with MH-60Ls with general improvements. Some had inflight refueling probes for refueling from USAF tankers or Army H-47 Chinooks.

The MH-60L Direct Action Penetrator (DAP) is a heavily armed SpecOps gunship.

The next variant was the MH-60K which is configurable as either an assault helicopter or as a gunship. All include a refueling probe and terrain-following radar.

The most recent variant for the 160th is the MH-60M, which builds on the improvements of the previous models and includes 3,000 hp YT706 engines (developed from the T700). I believe that all previous MH-60s have been updated to this standard.

There is also a stealth variant of the MH-60 that was revealed in the aftermath of the raid that killed Osama Bin Laden in 2011. Details are still not available.

According to the latest data I've been able to find, the Army Blackhawk variants total some active 2,276 H-60s. This does not include foreign military sales, which are plentiful.