The Apollo Lunar Module was built for the US Apollo program to achieve the actual transit from Moon orbit to the surface and back. The module was also known as the LM from the manufacturer designation (yet pronounced "LEM" from NASA's early name for it, "Lunar Excursion Module").
The module was designed to carry two crew in a 6.5 m³ space. The total module was 6.4 m high and 4.3 m across, resting on four legs. It consisted of two stages - a descent stage and a module and ascent stage. The total mass of the module was 14,696 kg with the majority of that (10,149 kg) in the descent stage.
The Apollo program was developed over a number of key decisions. Once it had been decided to use a lunar orbit rendezvous (LOR) method to reach the Moon it became necessary to produce a separate unit capable of reaching the surface.
The LM contract was given to Grumman Aircraft Engineering and a number of subcontractors. Grumman had begun lunar orbit rendezvous studies in late 1960 and again in 1962. In July 1962 eleven firms were invited to submit proposals for the LM. Nine did so in September, and Grumman was awarded the contract that same month. The contract cost was expected to be around $350 million. There were initially four major subcontractors - Bell Aerosystems (ascent engine), Hamilton Standard (environmental control systems), Marquardt (reaction control system) and Rocketdyne (descent engine). The computer system on the LM was built by Raytheon, a similar design to that in the Command Module.
Configuration freeze did not start until April 1963 when the ascent and descent engine design was decided. In addition to Rocketdyne a parallel program for the descent engine was ordered from Space Technology Laboratories in July 1963, and by January 1965 the Rocketdyne contract was cancelled. As the program continued there were numerous redesigns to save weight (including 'Operation Scrape'), improve safety, and fix problems. For example initially the module was to be powered by fuel cells, built by Pratt and Whitney but in March 1965 they were paid off in favor of an all battery design.
The first LM flight was on January 22, 1968 when the unmanned LM-1 was launched on a Saturn IB for testing of propulsion systems in orbit. The next LM flight was aboard Apollo 9 using LM-3 on March 3, 1969 as a manned flight (McDivitt, Scott and Schweickart) to test a number of systems in Earth orbit including LM and CSM crew transit, LM propulsion, separation and docking. Apollo 10, which launched on May 18, 1969, was another series of tests, this time in lunar orbit with the LM separating and descending to within 10 km of the surface. From the successful tests the LM successfully descended and ascended from the lunar surface with Apollo 11.