On 29 June 1979, Martin Marietta rolled out the first flight ET. NASA loaded ET-1 onto a barge and shipped it across the Gulf of Mexico, around the southern tip of Florida, and up the Atlantic coast to NASA Kennedy Space Center (KSC). There the tank was moved to the Vehicle Assembly Building (VAB) and mated to a pair of Solid Rocket Boosters (SRBs) and the Orbiter Columbia in preparation for the first mission of the Space Transportation System (STS), which was aptly designated STS-1.
NASA rolled the STS-1 stack out of the VAB on 29 December 1980. Four months later (12 April 1981), it lifted off from Launch Complex 39-A. On board Columbia for her maiden flight were astronauts John Young and Robert Crippen. Shortly after the first Orbiter's triumphant return to Earth, NASA reduced the number of flight tests to four, freeing two of the flight test ETs for operational flights.
The ET performed two critical functions during every Shuttle flight. It carried about 800 U.S. tons (725 metric tonnes) of LH2 fuel and LOX oxidizer for the three SSMEs in the Orbiter's tail; in addition, it bound together and provided thrust load paths for the 120-U.S.-ton (109-metric-tonne) Orbiter and twin 650-U.S.-ton (590-metric-tonne) SRBs. Together the three SSMEs on the Orbiter and the SRBs generated about seven million pounds (31,100,000 newtons) of thrust at liftoff.
The SRBs expended their propellants and separated from attachment fixtures on either side of the ET about two minutes after liftoff. They fell into the ocean and were recovered for reuse. The ET supplied propellants to the SSMEs for a further six and a half minutes; then, shortly after SSME shutdown, it was cast off and made to tumble to hasten its fall into Earth's atmosphere. When the ET separated from the Orbiter, it typically contained about 15 tons of leftover propellants (weight is approximate, so U.S. and metric units both apply). Reentry destroyed the ET; surviving pieces fell in remote ocean areas.
Orbiter and ET attained about 98% of orbital velocity before the latter was discarded. Two small Orbital Maneuvering System (OMS) engines in the Orbiter's tail then supplied the remaining 2% of the velocity needed to boost it, its crew, and its payload into a stable circular orbit about the Earth.
The process by which NASA arrived at the Shuttle design was complex. Until mid-1971, most designs paired a reusable, winged, piloted Orbiter with a reusable, winged, piloted Booster. The latter would have released the former just short of orbit. In most designs, the Booster would then have performed a wide 180° turn, deployed jet engines, and flown to a runway landing near its launch site. The semi-reusable Orbiter/ET/SRB stack, forced on NASA by funding limits imposed by President Richard Nixon, was, by comparison, a kludge — but in the minds of some spaceflight planners, it created an opportunity.
Beginning about the time the MPTA ET rolled out at Michoud, planners proposed that NASA boost ETs into orbit and put them to use. Some assumed that the ET would supply the SSMEs with LOX and LH2 until orbit was attained. Others assumed that the SSMEs would shut down just short of orbital velocity as during a normal flight, but that the Orbiter would retain the ET; then, when the twin OMS engines ignited to complete injection into orbit, it would bring the ET along for the ride.
When one reads of plans to exploit the ET in space, it is important to recall the giddy optimism many felt during Shuttle development in the 1970s. It started early — for example, the aerospace industry publication Aviation Week & Space Technology reported at the time Martin Marietta won its initial ET contract that NASA anticipated that 439 flight ETs would be manufactured through 1984. Assuming a first launch at the start of 1977, this implied a Shuttle launch every six days.
The Shuttle, it was expected, would fly so cheaply that NASA would be able to spend the lion's share of its human spaceflight budget on payloads the Orbiter could carry to orbit in its 15-by-60-foot (4.6-by-18.3-meter) payload bay, not on transportation costs. At a bare minimum, such payloads would include government and commercial satellites and components and supplies for an expansive Space Station that Orbiter crews would assemble in orbit.
Proposed ET uses fell into three categories: propellant scavenging, exploitation of ET aluminum, and conversion of ET structures. LOX and LH2 scavenged from the ET could, some estimated, economically supply Space Tugs based at the Space Station; they would transport astronauts and cargo throughout cislunar space. Ground up or melted down, ETs could become propellant for aluminum-burning rocket engines, aluminum girders and trusses for large space structures, and reaction mass for electromagnetic mass drivers. Partially disassembled or clustered, ETs might be converted into space habitats, telescopes, propellant depots, warehouses, greenhouses, space warfare decoys, and platforms for instruments and weapons.
|Brown tank: liftoff of Columbia at the start of STS-4, the final Orbital Flight Test mission (27 June-4 July 1982). Only STS-1 and STS-2 flew with white tanks; starting with STS-3, NASA opted not to paint the ETs. Image credit: NASA.|
As its name implies, the ACC, which would include two sections, was intended chiefly to augment Shuttle payload capacity. Use of the 27.5-foot-diameter (8.4-meter-diameter), 31.9-foot-long (9.7-meter-long) ACC with the Shuttle Orbiter payload bay would nearly double maximum Shuttle payload diameter and volume. Other ACC applications were possible, however; its lower section might, for example, serve as a protective shroud covering a "Space Facility Module" bolted to the ET LH2 tank aft dome. The ACC shroud would shield the drum-shaped pressurized module from the harsh thermal and acoustic environment the SRBs would create at the aft end of the ET during Shuttle ascent.
Space Facility Modules would thus resemble the Spent Stage Experiment Support Module (SSESM) proposed in the early 1960s for use with Apollo Saturn S-IVB rocket stages. The S-IVB, the second stage of the two-stage Saturn IB rocket and the third stage of the three-stage Saturn V, included in its upper two-thirds an LH2 tank. The drum-shaped SSESM, launched attached to the top of a Saturn IB S-IVB, would have enabled astronauts to enter the empty LH2 tank to outfit it in orbit as an Earth-orbiting space station. A 1966 plan proposed landing a Saturn V-launched SSESM/S-IVB combination on the Moon (see "More Information" below).
The Orbiter crew would vent residual ET propellants through the SSMEs and would hand off ET stabilization to an attitude control/orbit-maintenance propulsion system in the Service Module, then would separate their spacecraft from the ET/Service Module combination and perform station-keeping with it. The Service Module would deploy a pair of electricity-producing solar arrays and orient them toward the Sun.
|The Space Facility would include three Docking/Service Tunnels. Image credit: Martin Marietta.|
In addition to its propulsion system, power system, and airlock linking it to the ET LH2 tank, the Service Module would contain life support systems and living and working space for several astronauts. Its single pressurized volume would, however, only be occupied if an Orbiter were docked to it; this was a safety measure meant to ensure that the crew could reach a safe haven in the event of Space Facility depressurization, fire, or atmospheric contamination.
|Space Facility Module: the Habitat Module. Image credit: Martin Marietta.|
The astronauts would next use the RMS to hoist a Logistics Module out of the payload bay. They would attach the small module, which would contain supplies and small experiment apparatus, to one of the four Habitat Module radial ports. With that task completed, they would dock with and enter the Space Facility. With the addition of the Habitat Module, astronauts could remain on board after the Orbiter departed.The third Space Facility assembly flight would see a Shuttle Orbiter arrive with a full payload bay and no ET or Space Facility Module. A third Docking/Service Tunnel would be hoisted from the payload bay and linked to a Service Module radial port, then a small piloted space tug designed for satellite deployment, retrieval, and repair would be docked to the new tunnel.
Finally, an experiment pallet based on the Spacelab pallet designed originally for operation in the Orbiter payload bay would be attached to the exterior of one of the ETs. It would be the first of many experiment payloads that would employ the ETs as stable space platforms.
Martin Marietta saw no reason to stop there. It proposed that astronauts would eventually outfit the interiors of the Space Facility's ET LH2 tanks with decks and furnishings. NASA might also expand the Space Facility by adding new ETs. These could be converted in orbit into hangars for storing and servicing satellites. The 27.5-foot-diameter (8.4-meter-diameter) LH2 tank would, the company noted, provide ample room for satellites sized for launch in the Orbiter payload bay.
|Space Facility expansion: a scheme for outfitting the interior of an ET liquid hydrogen tank as a comfortable habitat housing 16 astronauts. Image credit: Martin Marietta.|
"News Digest," Aviation Week & Space Technology, 20 August 1973, p. 25.
"Shuttle Tanks Undergo Tests at Michoud," Aviation Week & Space Technology, 23 May 1977, p. 49.
"The Low (Profile) Road to Space Manufacturing," G. O'Neill, Astronautics & Aeronautics, Vol. 16, No. 3, March 1978, pp. 24-32.
"NASA Studying Shuttle-Derived Launch Vehicles," Aviation Week & Space Technology, 8 March 1982, p. 81.
"NASA Seeks Shuttle Capability Growth," C. Covault, Aviation Week & Space Technology, 23 April 1982, pp. 42-43, 45, 47, 51-52.
"Martin Studies Shuttle Aft Cargo Unit," E. Kolcum, Aviation Week & Space Technology, 12 July 1982, p. 65-66.
"External Tank Applications in Space," K. Timmons, A. Norton, and F. Williams, Martin Marietta; paper presented at the Unispace Conference in Vienna, Austria, 9-17 August 1982.
"External Tank Depicted as Space Station Element," Aviation Week & Space Technology, 6 September 1982, p. 246.
External Tank ACC Aft Cargo Carrier, Martin Marietta, no date (late 1982).