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Gemini provided a training program for the maneuvers which had to be performed in order to execute the lunar orbit rendezvous method of landing on the Moon, a concept attributed to NASA researcher John Houbolt. This method was adopted by NASA in July 1962 after two others - direct ascent from Earth and Earth orbit rendezvous - were rejected. In terms of energy expenditure, lunar orbit rendezvous (LOR) was the most economical system. It called for ascent via Earth parking orbit to lunar orbit. From there, two astronauts would land in a lunar module (LM) while a third remained in orbit aboard the Apollo command and service modules (CSM). At the conclusion of surface activity, the explorers would return to lunar orbit in the LM ascent stage. They would make rendezvous and dock with the CSM for the return to Earth. A calculation of the weight to be lifted to the Moon, about 50,000 kg, showed that LOR was within the lifting capability of the Saturn V launcher.

Like Gemini, Apollo consisted of two sections: a conical command module where the crew rode, and a cylindrical service module containing the main engine, fuel-cells, and the environmental control system. A docking tunnel and probe at the apex of the command module provided the means of docking with the lunar module, which ferried two of the crew between lunar orbit and the surface. The service module was jettisoned before reentry into Earth's atmosphere. An ablative heat shield protected the command module from the heat of reentry at lunar return speed (about 39,000 km/h).

NASA launched the first manned flight of the spacecraft, Apollo 7, on 11-Oct-1968. The test flight, lasting 11 days in Earth orbit, was a success. Fearful that the Soviet Union was prepared to launch one or two cosmonauts on a circumlunar flight, NASA intended to send the next Apollo test flight - Apollo 8 - to the Moon. The mission was launched on 21-Dec-1968. Falling into the gravitational field of the Moon on 24-Dec, the spacecraft's engine was fired to insert it into a 111 km orbit. The crew flew ten revolutions of the Moon on Christmas Eve before again firing its engine to place Apollo 8 on a return trajectory to Earth. It splashed down in the Pacific on 27-Dec. Apollo 9 was flown to rehearse rendezvous and docking maneuvers with the lunar module in Earth orbit. The LM was then flown in lunar orbit on Apollo 10 for the dress rehearsal of a Moon landing. With the conclusion of that mission, NASA announced that it was ready to attempt to land men on the Moon.

Apollo 11 launched on 16-Jul-1969 with its crew of Neil Armstrong, Eugene "Buzz" Aldrin, and Michael Collins. The landing site was designated on the surface of Mare Tranquillitatis (Sea of Tranquility). Armstrong and Aldrin landed there in the lunar module Eagle on 20-Jul at 4:17:43 pm EDT. They erected the United States flag, set up a laser reflector, a seismometer to detect moonquakes, and a sheet of aluminum foil to trap particles of the solar wind. They took photographs of the rocks and landscape, and they collected 21 kg of soil and rock samples. First out of the LM and last in, Armstrong spent 2 hr 31 min on the Moon.

From 1969 to 1972, NASA launched six more missions to the Moon, of which, five successfully landed and carried out they lunar explorations. The other, Apollo 13, aborted its mission when an oxygen tank exploded. The crippled vessel flew around the Moon and returned safely. These later missions became progressively more ambitious with longer stay times and extended excursions; including the use of a lunar rover. By the end of the Apollo lunar program, 14 lunar EVA's had been conducted, totalling 81 hr 5 min, and 379 kg of soil and rock samples were collected.


First flight: 28-May-1964; first manned flight 11-Oct-1968 (Apollo 7)
Last flight: 15-Jul-1975 (ASTP)
Number of flights: 23 total; 15 manned
Principal uses: manned earth and lunar orbiter
Unit cost: $77.00 million
Crew size: 3
Overall length: 11.0 m (without escape tower)
Maximum diameter: 3.9 m
Habitable volume: 6.17 m3
Launch mass: about 34,502 kg (including escape tower)
Propellant mass: 18,488 kg total
RCS total impulse: 3,774 kNs
Primary engine thrust: 97.86 kN
Main engine propellant: NTO/Aerozine-50
Total spacecraft delta v: 2,804 m/s
Power: fuel cells/batteries; ≈915 kWh total (J-series)
Contractor: Rockwell International

(Masses are not typical for every mission, figures are approximate.)

Overall length: 10.2 m
Diameter: 0.66 m
Total mass: 4,173 kg

Crew size: 3
Length: 3.5 m
Maximum diameter: 3.9 m
Habitable volume: 6.17 m3
Total mass: about 5,806 kg
(structure 1,567 kg; heat shield 848 kg; reaction control system 400 kg; recovery equipment 245 kg; navigation equipment 505 kg; telemetry equipment 200 kg; electrical equipment 700 kg; communications systems 100 kg; crew seats & provisions 550 kg; crew mass 216 kg; misc contingency 200 kg; environmental control system 200 kg; propellant 75 kg)
Reaction control system
    thrusters: 12 x 410 N
    propellant: NTO/MMH
    specific impulse: 290 s
    total impulse: 257 kNs
L/D hypersonic: 0.3
Power: Ag-Zn batteries; 3 × 40 Ah each, 28 V DC; 3.4 kWh; inverters produced 115 V AC
Environment: pure oxygen at 340 mbar
Landing system: service module is jettisoned before entering Earth's atmosphere. Pitch control jets turn command module so heat shield orientates to withstand heating up to 2,742oC. Skipping by module's aerodynamic lift, commander maneuvers within 42 km wide reentry corridor. Drag-braking reduces speed to around sonic velocity at 9.14 km. Drogue parachutes deploy at 7.62 km. From 4.57 km, three 24.4 m diameter ringsail parachutes lower module gently into sea. Atmosphere entry angle 5.3o-7.7o at 121.92 km. Maximum deceleration ~6 g

Length: 7.5 m
Principal diameter: 3.9 m
Total mass: 24,523 kg maximum (about 23,250 kg for H-series)
(structure 1,910 kg; electrical equipment 1,200 kg; maneuvering system 3,000 kg)
Propellant mass: 18,413 kg max
Reaction control system
    thrusters: 16 x 445 N; four clusters, or quads, equally spaced around the perimeter
    propellant: NTO/MMH
    specific impulse: 290 s
    total impulse: 3,517 kNs
Service propulsion system
    thrust: 91.19 kN
    propellant: NTO/Aerozine-50
    specific impulse: 314 s
    delta v: 2,804 m/s
Power: H2/O2 alkaline fuel cells; 3 × 400-1420 W each, 28 V DC; 2.0 kW average (Apollo 11); reactants for 600 kWh total H-series (900 kWh J-series); one 415 Ah auxillary battery added after Apollo 13
Guidance: to effect control of the combined command/service modules, the Honeywell Stabilization and Control Subsystem (SCS) directed the firing of RCS thrusters, and positioned the SPS engine (by gimballing) before firing. Some of the key functions performed by the Honeywell SCS and lunar equipment during Apollo missions were link-up with the LM (after trans-lunar injection), mid-course corrections en-route to the Moon; lunar-orbit insertion; descent of the LM and subsequent ascent and re-docking with the CSM; trans-Earth injection; execution of mid-course corrections during the return flight; and orientation of the command module for reentry

(Apollo 15, 16 and 17) Oxygen and hydrogen for electrical and environmental control systems raised to 502 kg, an increase over Apollo 13 supplies of 176.4 kg. Scientific Instrument Module (SIM) bay introduced.

The SLA structure houses the LM during launch and the translunar injection maneuver until CSM separation, transposition and LM extraction. The SLA quarter panels are jettisoned as CSM separation.
Diameter: truncated cone tapering from 3.9 m to 6.6 m
Length: 8.5 m
Mass: 1,800 kg

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