Lunar-orbit rendezvous, the mission plan selected to land the first human on the Moon, involved launching a rocket comprised of a command module, a landing module, and a service module carrying fuel and the main propulsion system. Once in Earth orbit, the last stage of the rocket would send the remaining components and crew to the Moon. Once in lunar orbit, the command module would deploy the smaller landing module to the Moon's surface. The smaller module would both save on fuel and execute a more agile landing under the conditions of the Moon's surface, which are very different from those on Earth.

Because the lunar lander would be jettisoned once it re-docked with the orbiting command module, design engineers could tailor it exclusively for space flight. Unlike Earth, the Moon has no significant atmosphere. Its dusty, cratered surface is subjected to enormous temperature swings (up to 280 degrees Celsius). Most notable is its weaker gravitational force. Overcoming gravity on Earth requires a great deal of fuel and good aerodynamic design, but on the Moon there is little gravity to overcome, so the requirements are different.

Thus, engineers involved in the Apollo 11 mission could ignore aerodynamic considerations for the lander. However, they could not ignore weight. To fuel both the descent and the ascent of the lander, three pounds of fuel for every pound of weight would be needed. Throughout the design process, then, engineers considered the weight of every component and seized upon every weight-reduction opportunity that would not compromise crew safety.

Since the flight would be brief and the impact of landing moderate, engineers designed the lander's interior without seats. Because the lack of atmosphere ruled out frictional drag, the lander's outer "skin" could be thin and lightweight and its shape practical, with no need for streamlining. In place of rigid heat shields, a wrapping of thin, lightweight aluminum-mylar foil would protect the craft from intense solar radiation, saving about 110 pounds. Although on Earth the lander would be fragile, its design would be strong enough for the low-gravity, no-atmosphere conditions at the surface of the Moon.