Apollo 15 Lunar Module / ALSEP

Description

The Apollo 15 lunar module (LM) "Falcon" was the fourth crewed vehicle to land on the Moon. It carried two astronauts, Commander David R. Scott and LM pilot James B. Irwin, the seventh and eighth men to walk on the Moon. The LM also carried a Lunar Roving Vehicle (LRV), an Apollo Lunar Surface Experiments Package (ALSEP) that contained scientific experiments to be deployed and left on the lunar surface, and other scientific and sample collection apparatus. The experiments performed on the Moon, in addition to the ALSEP suite, were geologic sample collection, surface photography, soil mechanics investigations to study physical properties of the lunar regolith, and the solar wind composition experiment which collected samples of solar wind particles for return to Earth.

Mission Profile

The LM separated from the Command/Service Module (CSM) at 18:13:30 UT and landed at 22:16:29 UT (6:16:29 p.m. EDT) on 30 July 1971 in the Mare Imbrium region at the foot of the Apennine mountain range at 26.1322 N latitude, 3.6339 E longitude (IAU Mean Earth Polar Axis coordinate system). Scott and Irwin made three moonwalk extra-vehicular activities (EVAs) totaling 18 hours, 35 minutes. During this time they covered 27.9 km and collected 77.31 kg of rock and soil samples. The LRV was used to explore regions within 5 km of the LM landing site. This was the first time a vehicle of this type had been used, and its performance on the lunar terrain was very successful. Two hours after landing the cabin was depressurized and Scott performed a standup EVA for 33 minutes, describing and photographing the surrounding terrain from the LM upper hatch. The first moonwalk EVA was on 31 July from 13:13:10 UT to 19:45:59 UT, during which time the LRV was unloaded, deployed, and driven. Photographs of the lunar surface were taken and geologic samples were collected from the LM site and during the three geological traverses. The traverse on the first EVA covered 10.3 km to the edge of Hadley Rille to Elbow Crater and near St. George Crater and back to the LM, where a core sample was taken from three meters below the surface. The ALSEP was deployed at the end of the traverse. On the second EVA, on 1 August from 11:48:48 UT to 19:01:02 UT, the LRV was driven on a 12.5 km traverse southeast along the base of the Appenine Mountains near Index, Arbeit, Crescent, Dune, and Spur craters and back to the ALSEP site. On the third EVA on 2 August from 08:52:14 UT to 13:42:04 UT the LRV was driven a total of 5.1 km west to Scarp Crater and northwest along the edge of Hadley Rille and back east across the mare. After the final EVA Scott performed a televised demonstration of a hammer and feather falling at the same rate in the lunar vacuum. The astronauts also left a plaque and small figure on the surface in memory of all fourteen American and Soviet space explorers who had died during the two nation's space programs. The LM lifted off the Moon on 2 August at 17:11:22 UT after 66 hours, 55 minutes on the lunar surface. After docking with the CSM (piloted by Alfred M. Worden) at 19:09:47 UT the LM was jettisoned on 3 August at 01:04:14 UT and impacted on the Moon 2 hours later (03:03:37.0 UT) at 26.36 N, 0.25 E.

Lunar Module Spacecraft and Subsystems

The lunar module was a two-stage vehicle designed for space operations near and on the Moon. The spacecraft mass of 16,434 kg was the mass of the LM including astronauts, expendables, and approximately 12,000 kg of propellants. The fully fueled mass of the ascent stage was about 4971 kg and the descent stage 11,463 kg. The ascent and descent stages of the LM operated as a unit until staging, when the ascent stage functioned as a single spacecraft for rendezvous and docking with the command and service module (CSM). The descent stage comprised the lower part of the spacecraft and was an octagonal prism 4.2 meters across and 1.7 m thick. Four landing legs with round footpads were mounted on the sides of the descent stage and held the bottom of the stage 1.5 m above the surface. The distance between the ends of the footpads on opposite landing legs was 9.4 m. One of the legs had a small astronaut egress platform and ladder. A one meter long conical descent engine skirt protruded from the bottom of the stage. The descent stage contained the landing rocket, two tanks of aerozine 50 fuel, two tanks of nitrogen tetroxide oxidizer, water, oxygen and helium tanks and storage space for the lunar equipment and experiments, and in the case of Apollo 15, 16, and 17, the lunar rover. The descent stage served as a platform for launching the ascent stage and was left behind on the Moon.

The ascent stage was an irregularly shaped unit approximately 2.8 m high and 4.0 by 4.3 meters in width mounted on top of the descent stage. The ascent stage housed the astronauts in a pressurized crew compartment with a volume of 6.65 cubic meters. There was an ingress-egress hatch in one side and a docking hatch for connecting to the CSM on top. Also mounted along the top were a parabolic rendezvous radar antenna, a steerable parabolic S-band antenna, and 2 in-flight VHF antennas. Two triangular windows were above and to either side of the egress hatch and four thrust chamber assemblies were mounted around the sides. At the base of the assembly was the ascent engine. The stage also contained an aerozine 50 fuel and an oxidizer tank, and helium, liquid oxygen, gaseous oxygen, and reaction control fuel tanks. There were no seats in the LM. A control console was mounted in the front of the crew compartment above the ingress-egress hatch and between the windows and two more control panels mounted on the side walls. The ascent stage was launched from the Moon at the end of lunar surface operations and returned the astronauts to the CSM.

The descent engine was a deep-throttling ablative rocket with a maximum thrust of about 45,000 N mounted on a gimbal ring in the center of the descent stage. The ascent engine was a fixed, constant-thrust rocket with a thrust of about 15,000 N. Maneuvering was achieved via the reaction control system, which consisted of the four thrust modules, each one composed of four 450 N thrust chambers and nozzles pointing in different directions. Telemetry, TV, voice, and range communications with Earth were all via the S-band antenna. VHF was used for communications between the astronauts and the LM, and the LM and orbiting CSM. There were redundant tranceivers and equipment for both S-band and VHF. An environmental control system recycled oxygen and maintained temperature in the electronics and cabin. Power was provided by 6 silver-zinc batteries. Guidance and navigation control were provided by a radar ranging system, an inertial measurement unit consisting of gyroscopes and accelerometers, and the Apollo guidance computer.

Apollo Lunar Surface Experiments Package (ALSEP)

The Apollo Lunar Surface Experiments Package (ALSEP) consisted of a set of scientific instruments emplaced at the landing site by the astronauts. The instruments were arrayed around a central station which supplied power to run the instruments and communications so data collected by the experiments could be relayed to Earth. The central station was a 25 kg box with a stowed volume of 34,800 cubic cm. Thermal control was achieved by passive elements (insulation, reflectors, thermal coatings) as well as power dissipation resistors and heaters. Communications with Earth were achieved through a 58 cm long, 3.8 cm diameter modified axial-helical antenna mounted on top of the central station and pointed towards Earth by the astronauts. Transmitters, receivers, data processors and multiplexers were housed within the central station. Data collected from the instruments were converted into a telemetry format and transmitted to Earth. The ALSEP system and instruments were controlled by commands from Earth. The uplink frequency for all Apollo mission ALSEP's was 2119 MHz, the downlink frequency for the Apollo 15 ALSEP was 2278.0 MHz.

Radioisotope Thermoelectric Generator (RTG)

The SNAP-27 model RTG produced the power to run the ALSEP operations. The generator consisted of a 46 cm high central cylinder and eight radiating rectangular fins with a total tip-to-tip diameter of 40 cm. The central cylinder had a thinner concentric inner cylinder inside, and the two cylinders were attached along their surfaces by 442 spring-loaded lead-telluride thermoelectric couples mounted radially along the length of the cylinders. The generator assembly had a total mass of 17 kg. The power source was an approximately 4 kg fuel capsule in the shape of a long rod which contained plutonium-238 and was placed in the inner cylinder of the RTG by the astronauts on deployment. Plutonium-238 decays with a half-life of 89.6 years and produces heat. This heat would conduct from the inner cylinder to the outer via the thermocouples which would convert the heat directly to electrical power. Excess heat on the outer cylinder would be radiated to space by the fins. The RTG produced approximately 70 W DC at 16 V. (63.5 W after one year.) The electricity was routed through a cable to a power conditioning unit and a power distribution unit in the central station to supply the correct voltage and power to each instrument.

ALSEP Scientific Instruments

All ALSEP instruments were deployed on the surface by the astronauts and attached to the central station by cables. The Apollo 15 ALSEP instruments consisted of: (1) a passive seismometer, designed to measure seismic activity and physical properties of the lunar crust and interior; (2) a lunar surface magnetometer (LSM), designed to measure the magnetic field at the lunar surface; (3) a solar wind spectrometer, which measured the fluxes and spectra of the electrons and protons that emanate from the Sun and reach the lunar surface; (4) a suprathermal ion detector, designed to measure the flux, composition, energy, and velocity of low-energy positive ions; (5) a cold cathode ion gauge, designed to measure the atmosphere and any variations with time or solar activity such atmosphere may have; (6) a lunar dust detector, to measure dust accumulation, radiation damage to solar cells, and reflected infrared energy and temperatures; and (7) a heat flow experiment, designed to measure the rate of heat loss from the lunar interior and the thermal properties of lunar material. The central station, located at 26.1341 N latitude, 3.6298 E longitude, was turned on at 18:37 UT on 31 July 1971 and shut down along with the other ALSEP stations on 30 September 1977.