|Artist's impression of Syncom 2. Image credit: NASA.|
Clarke then proposed a network of three such Geosynchronous Earth Orbit (GEO) satellites spaced equidistantly along the equator. These would, he wrote, be well placed to relay radio signals all over the world.
By most accounts, his proposal was not taken seriously. German V-2 missiles had demonstrated that large rockets needed to launch satellites were possible; however, most people with an interest in radio communication saw his GEO radio-relay network as a project for the far future. Everyone else, of course, paid no attention whatsoever.
Less than 20 years later (26 July 1963), NASA launched drum-shaped Syncom 2 (image at top of post). Through a series of careful maneuvers, the Hughes Aircraft Company-built satellite reached a 35,786-kilometer-high orbit inclined 33° relative to Earth's equator on 16 August 1963.
In its inclined synchronous orbit, 68-kilogram Syncom 2 oscillated daily along a 66°-long path centered over a spot on the equator at 55° west longitude. This took it over the North Atlantic and Brazil, enabling test transmissions between North America, Europe, South America, and Africa.
A year later (19 August 1964), NASA launched Syncom 3 to a point directly over the equator, making it the world's first geostationary comsat. From its mid-Pacific position at the intersection of the equator and the International Date Line, Syncom 3 was well placed to relay TV signals from the 1964 Tokyo Olympic Games to North America.
On 6 April 1965, NASA launched Intelsat I, the first commercial GEO comsat. Nicknamed "Early Bird" by the international consortium that funded it, Syncom-derived Intelsat I remained operational until January 1969. It was turned on again briefly in July 1969 to relay signals from Apollo 11, the first piloted lunar landing mission.
A year after the Intelsat I launch, Samuel Fordyce of the NASA Headquarters Office of Manned Space Flight (OMSF) circulated a memorandum in which he proposed that an Apollo Lunar Module (LM) be outfitted as a radio communications "space lab," and launched to GEO altitude. He called the modified LM the LM Relay Experiment Laboratory (LM REL).
|A 1966 Lockheed proposal for a Lunar Module-derived radio experiment laboratory in the Apollo Applications Program.|
The LM REL would be "visited periodically by crews to replenish, repair, install, initiate[,] and operate a variety of experiments," Fordyce wrote. Some of these experiments would "test the capability of a [GEO] relay to replace the aircraft, ships, and certain of the 30[-foot-diameter] antenna ground stations of the Manned Space Flight Network (MSFN)," he added.
Fordyce explained that, during Apollo missions, eight specially instrumented KC-135 aircraft, five tracking ships, and eleven 30-foot-diameter dish antennas would be needed to continuously link the Apollo spacecraft and the Mission Control Center in Houston, Texas. If a GEO communication satellite network replaced much of the MSFN, he wrote, then the result could be a "significant [cost] savings for NASA."
The "continuous contact capability" the GEO satellite network would provide would be especially valuable for missions in low-Earth orbit, Fordyce continued. Such missions spend only a few minutes above the horizon as viewed from any one Earth-surface antenna. Relaying transmissions from LEO spacecraft via GEO satellites would, he wrote, "permit greater flexibility in [spaceflight] operations by relaxing requirements to conduct difficult maneuvers [such as dockings and mission aborts] over instrumented sites."
Fordyce proposed two methods for placing the LM REL into its operational orbit (a Syncom 2-type synchronous orbit inclined 13.2° relative to Earth's equator). Both would see the LM REL launched with an Apollo Command and Service Module (CSM) spacecraft bearing a crew of three.
The most capable LM REL would rely on a three-stage Apollo Saturn V to reach GEO. The first two Saturn V stages would burn to depletion and fall away, then the S-IVB third stage would fire briefly to place itself, the CSM, and the LM REL into 100-nautical-mile-high low-Earth orbit (LEO). The S-IVB's J-2 engine would then be used to conduct three additional maneuvers over six hours to change the spacecraft's orbital inclination relative to the equator and increase its altitude.
Following the fourth S-IVB burn, the piloted CSM would separate, turn end for end, dock with a drogue docking unit on top of the LM REL, and withdraw it from the spent S-IVB stage. A CSM Service Propulsion System (SPS) main engine burn at GEO altitude would then nudge the LM REL into its operational orbit. The crew would enter the LM REL and conduct experiments for an unspecified period of time. After completing their mission, the astronauts would undock their CSM from the LM REL and ignite the SPS to return to Earth.
Alternately, the LM REL could climb from LEO to its GEO operational orbit on its own, though at the cost of reduced capabilities. Fordyce did not mention it — no doubt he expected that his audience would need no explanation — but the alternate approach would eliminate the need to develop an S-IVB J-2 engine capable of starting four times (the S-IVB J-2 for Apollo lunar missions was designed to ignite only twice).
The LM REL and a piloted CSM would reach 100-nautical-mile LEO together on a Saturn V or separately on a pair of two-stage Saturn IB rockets. In both cases, the CSM would dock with the LM REL and extract it from the S-IVB stage that boosted it into LEO. The crew on board the CSM would then ready the LM REL for operations. Their work completed, the crew would undock in the CSM; the LM descent stage engine would then ignite to begin the LM REL's 5.25-hour climb to geosynchronous orbit.
As the LM REL reached GEO altitude, the spent descent stage would separate, then the LM REL ascent stage engine would ignite to complete insertion into operational orbit. Fordyce called the ascent-stage-only LM REL a "prototype" lab.
|Artist's impression of first-generation TDRS satellite. Image credit: NASA.|
The first satellite in the TDRSS network, the 2268-kilogram Tracking and Data Relay Satellite (TDRS)-1, reached LEO on 4 April 1983 on board the Shuttle Orbiter Challenger. A malfunctioning solid-propellant rocket stage failed to boost TDRS-1 all the way to GEO; controllers were able, however, to use the satellite's small attitude-control thrusters to nudge it into GEO over a period of about three months.
At launch, TDRS-1 was expected to operate for seven years, but it continued as part of the TDRSS network until October 2009, when its last remaining amplifier malfunctioned. In 2010, controllers used its attitude-control thrusters to boost it to a graveyard orbit about 483 kilometers (300 miles) above GEO altitude.
The second TDRS satellite was lost with the Orbiter Challenger and its seven-person crew during Space Shuttle mission STS 51-L (28 January 1986). The Space Shuttle launched five more first-generation TDRS satellites in 1988, 1989, 1991, 1993, and 1995. TDRS-3, the second to reach GEO, was repositioned to take over from TDRS-1. It is the oldest operational TDRS satellite. TDRS-4 was retired in December 2011 and placed in graveyard orbit in April 2012.
Three second-generation TDRS satellites, launched on Atlas IIA expendable rockets, reached GEO in 2000 and 2002. All remain operational.
NASA launched the third-generation TDRS-11, TDRS-12, and TDRS-13 satellites on Atlas V 401 rockets in 2013, 2014, and 2017, respectively. They also remain operational.
|Launch of the 3454-kilogram (7615-pound) TDRS-13 satellite on 18 August 2017. Image credit: NASA.|
Memorandum with attachment, MLO/Samuel Fordyce, SAA Flight Operations, to MLD/Deputy Director, Saturn/Apollo Applications and MLA/Director, Apollo Applications, AAP Synchronous Mission, April 29, 1966.
Starfish and Apollo (1962)
A Forgotten Rocket: The Saturn IB
"Without Hiatus": The Apollo Applications Program in June 1966
Electricity from Space: The 1970s DOE/NASA Solar Power Satellite Studies
What Shuttle Should Have Been: NASA's October 1977 Space Shuttle Flight Manifest