|Image credit: Martin Marietta|
Among the proposed improvements permanently grounded after the Challenger accident was Martin Marietta's Aft Cargo Carrier (ACC), a cargo canister meant to be bolted over the dome-shaped aft end of the Space Shuttle External Tank (ET). Martin Marietta, prime contractor for the 154-foot-long ET, had begun in-house studies of the ACC at about the time the first Shuttle launched into orbit (STS-1, 12-14 April 1981).
|Aft Cargo Carrier and Orbiter payload bay dimensions compared. The entire ACC is 31.9 feet long; the aft shroud is 20.8 feet long. Image credit: Martin Marietta|
The ACC's position adjacent to the Orbiter's three Space Shuttle Main Engines (SSMEs) and between the powerful twin Solid-Rocket Boosters (SRBs) meant that payloads it carried would be subjected to more heating and acoustic pounding than would those in the Orbiter payload bay. Martin Marietta proposed an ACC "environmental protection system" made up of 707 pounds of thermal insulation and a 2989-pound "acoustical barrier."
Adding these layers would make the ACC shell a little more than a foot thick, reducing the diameter of payloads it could carry to about 25 feet. Even so, this made the ACC payload volume about 10 feet wider than the 15-by-60-foot Orbiter payload bay.
Martin Marietta assumed that, with planned Shuttle performance upgrades, an Orbiter would be able to boost 36.9 tons of payload into a 160-nautical-mile-high orbit inclined 28.5° relative to Earth's equator. An empty ACC would add 8.3 tons to the Shuttle's mass at liftoff, potentially reducing the payload mass the Orbiter and ACC could inject into orbit. If the ACC remained attached until SSME cutoff, then the payload mass the Orbiter and ACC could place into orbit would total 28.7 tons.
|Left to right: ACC shroud; ring for mounting cargoes; and ACC skirt with twin solid-propellant rocket motors. Image credit: Martin Marietta|
The twin SRBs would burn out and fall away from the ET 120 seconds after liftoff at an altitude of about 146,000 feet. The ACC shroud would then detach from the skirt and fall away 35 seconds after SRB separation.
During Orbiter-only Shuttle missions, the Orbiter would shut down its SSMEs and discard the ET before attaining orbital velocity. The ET would reenter the atmosphere and be destroyed over the Indian Ocean. This would, of course, deprive the SSMEs of their source of liquid hydrogen/liquid oxygen propellants: hence, after ET separation, the three engines would amount to "dead weight." The astronauts would then ignite the Orbiter's twin Orbital Maneuvering System (OMS) engines for the first of two orbit-insertion burns.
Orbiter/ACC missions would see Orbiter, ET, ACC skirt, and payloads in a 57-by-160-nautical-mile orbit at SSME cutoff, so that the first orbit-insertion OMS burn would be unnecessary. When the assemblage attained apogee (the highest point in its orbit around the Earth), the astronauts would ignite the OMS engines, increasing its velocity by 183 feet per second. This would raise its perigee (the low point in its orbit around the Earth) and circularize its orbit at an altitude of 160 nautical miles.
Martin Marietta proposed a host of potential ACC payloads. Many would ride on a mounting ring attached to the ACC skirt. "Catch tanks" might collect liquid hydrogen/liquid oxygen propellants left in the ET at SSME shutdown for later use in orbit. A turbine generator might burn leftover propellants to augment the electricity the Orbiter fuel cells would provide.
The ACC skirt might carry a 25-foot-diameter, 20-foot-long space station module. The module might be designed to remain attached to the ET, so that the big tank could become a strong-back for mounting large payloads or, with the addition of an access hatch linking the ET's hydrogen tank with the module, a large enclosed volume for experiments or habitation. Large folded structures - for example, an umbrella-like radio dish antenna more than 50 feet across - might also be deployed from the skirt.
|Potential Aft Cargo Carrier payloads. Image credit: Martin Marietta|
Without the ACC, payload mass for Flight 1 would be limited to what could be carried in the Orbiter payload bay, or about a quarter of the 36.9-ton theoretical maximum for the flight. With the ACC, the Flight 1 payload could total 22.9 tons. Following deployment from the ACC skirt, the satellites would ride their PAM-D stages to their assigned slots in the geostationary orbit (GEO) belt, 22,236 miles above the equator.
The Orbiter crew would then cast off the ET and its attached ACC skirt. A two-ton pair of solid-propellant deorbit rocket motors on the ACC skirt would ignite over the western Pacific Ocean, causing the ET/ACC combination to tumble and reenter the atmosphere. Any parts that survived reentry would splash into the Pacific south of Hawaii.
The astronauts, meanwhile, would maneuver the Orbiter to a 190-nautical-mile-high orbit and deploy the large observatory from the payload bay. They would then ignite the OMS engines to slow the Orbiter and cause it to reenter Earth's atmosphere. The delta-winged space plane would glide to a runway landing.
Flight 2 would launch the 1.7-ton Tiros-N satellite inside the ACC and the 8.2-ton Atmosphere Monitor satellite at the aft end of the Orbiter payload bay. Because the Orbiter/ET/ACC skirt/payloads assemblage would be required to ascend to an energetically challenging 160-nautical-mile-high, 98.2° near-polar retrograde orbit, Flight 2's payload mass could total at most 11.8 tons.
The Flight 2 crew would first guide their spacecraft to a rendezvous with a two-ton Thermosat payload, which they would captured and stow at the front of the Orbiter payload bay for return to Earth. They would then fire the OMS engines to climb to a 380-nautical-mile orbit, where they would deploy the Atmosphere Monitor.
Next, they would ignite the OMS engines again to climb to a 448-nautical-mile orbit inclined 98.8° to Earth's equator. There they would deploy Tiros-N from the ACC skirt. After discarding the ET/ACC skirt, they would ignite the OMS engines to return Orbiter, crew, and Thermosat to Earth.
|Aft Cargo Carrier in flight. Image credit: Martin Marietta|
Martin Marietta noted that, by providing a second payload volume, the ACC could enable secret Department of Defense (DOD) payloads to be carried separate from but on the same flight as NASA civilian payloads. The Orbiter payload bay would thus on Flight 3 carry two Department of Defense payloads: the NATO IV/PAM-D satellite and the 35-foot-long, 10-foot-wide, 6.5-ton Synchronous Observation Satellite (SOS).
The OTV would scavenge residual ET propellants to fill its tanks, then would detach from the ACC skirt. The Orbiter crew, meanwhile, would raise the SOS on a tilt-table mounted in the payload bay. The OTV would dock with the SOS, detach it from the tilt-table, boost it to its assigned slot in GEO, and release it. Mission accomplished, the OTV would fire its engines to return to low-Earth orbit for a new mission.
The Orbiter crew, meanwhile, would cast off the ET/ACC skirt and maneuver to a 160-nautical-mile orbit, where they would deploy NATO-IV/PAM-D from the payload bay. The PAM-D stage would boost the satellite to GEO. The astronauts, meanwhile, would fire the Orbiter's OMS engines to reenter Earth's atmosphere.
Martin Marietta placed great emphasis on the cost savings that would accrue from making the ACC a Shuttle hardware element. First, however, it estimated the costs of developing and using the cargo canister. The company assumed that NASA would give a green light to begin ACC development in late 1983, and that the first ACC would lift off three years later.
The company calculated that ACC development would cost $113 million. Changes to the Shuttle design to accommodate ACCs would cost $78 million, and changes to Kennedy Space Center facilities would cost $35 million.
Martin Marietta quoted NASA when it placed the base cost of a Shuttle flight without an ACC at an optimistic $75 million. The base cost of a Shuttle flight would increase by about $5 million when it included an ACC, the company estimated.
For its cost-savings calculations, the company employed a Shuttle traffic model less optimistic than the one NASA touted. It assumed that 331 Shuttle flights would take place between 1988 and 2000, with 34 flights in 1988 and a steady decline to 20 flights per year in 2000. During the same 12-year period, NASA assumed 26 flights per year in 1988, an upward trend to nearly 60 flights per year by 2000, and a total of 581 flights.
Based on its "low" traffic model, Martin Marietta estimated that NASA might benefit from flying 71 civilian and 35 Department of Defense Shuttle/ACC missions. The company conservatively assumed, however, that NASA would be able to fund only a total of 75 civilian and Department of Defense Orbiter/ACC flights.
Martin Marietta determined that the added payload capacity the ACC could provide would permit the elimination of 40 Orbiter-only Shuttle missions. It placed the cost of 331 Orbiter-only missions at $24.8 billion and the cost of 216 Orbiter-only and 75 Orbiter/ACC missions at $22.2 billion. The ACC would thus save NASA $2.6 billion over 12 years.
Space Transportation System with Aft Cargo Carrier: A Natural Augmentation of System Capability, Martin Marietta, no date (late 1982)
"External Tank Aft Cargo Carrier," T. Mobley and J. Hughes; paper presented at the Twentieth Space Congress, Cocoa Beach, Florida, 26-28 April 1983
ACC External Tank Aft Cargo Carrier, Martin Marietta, no date (late 1985)
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