The destruction of the Orbiter Challenger at the start of the Space Shuttle Program's 25th mission on 28 January 1986, put an end to many proposals and plans for Shuttle augmentation. The Manned Maneuvering Unit, the powerful liquid hydrogen-liquid oxygen Centaur-G' upper stage, routine satellite servicing, launches from the U.S. West Coast, polar and retrograde orbits, frequent non-astronaut passengers, long-duration missions relying on solar panels, on-orbit satellite refueling, and a flight rate upwards of 50 per year - all of these were abandoned as NASA acknowledged the Shuttle's frailties and foibles.
Among the proposals abandoned after *Challenger *was Martin Marietta's Aft Cargo Carrier (ACC), a 27.5-foot-diameter, 31.9-foot-long cargo canister that would ride into space bolted over the dome-shaped aft end of the Shuttle External Tank (ET). Martin Marietta, prime contractor for the 27.5-foot-diameter, 154-foot-long ET, had begun in-house studies of the 13,000-cubic-foot ACC at about the time of the first Space Shuttle mission (STS-1, 12-14 April 1981). It began to aggressively pitch the concept at conferences by mid-1982. NASA's Marshall Space Flight Center in Huntsville, Alabama, soon contracted with the company for ACC engineering and economic feasibility studies.
The ACC was a response to the realization that, while in NASA's traffic model more than 90% of Shuttle Orbiter volume capacity was spoken for, on average the Orbiters would carry into space only 66% of the mass they were theoretically capable of delivering to any given orbit. The mass shortfall occurred in part because the Orbiter's payload bay measured 15 feet wide by 60 feet long. While well-suited to the U.S. Air Force spy satellites that had dictated its size, the narrow, 10,600-cubic-foot volume placed restrictions on other payloads. Center-of-gravity and launch-and-ascent abort considerations also limited what an Orbiter could carry. In general, heavy payloads could ride only in the aft half of the payload bay, where they would be centered over the Orbiter's main landing gear.
Martin Marietta described an ACC Shuttle flight to a 160-nautical-mile circular orbit inclined 28.5° to Earth's equator. As in flights lacking an ACC, the Orbiter's three Space Shuttle Main Engines (SSMEs) would ignite, then twin Solid Rocket Boosters (SRBs) would kick the Shuttle stack off the launch pad. The SSMEs would draw liquid hydrogen fuel and liquid oxygen oxidizer from the ET.
The ACC's position adjacent to the SSMEs and between the powerful SRBs meant that payloads it carried would be subjected to more heating and acoustic pounding than would those in the Orbiter's payload bay. Martin Marietta proposed an ACC "environmental protection system" comprising 707 pounds of thermal insulation and a 2989-pound "acoustical barrier." These protective layers would thicken the ACC's walls, limiting the maximum diameter of the payload it could carry to about 25 feet.
The SRBs would burn out and separate 120 seconds after liftoff at an altitude of about 146,000 feet. The ACC would have two main parts: the aft shroud and the forward skirt. The 7429-pound shroud would detach from the skirt and fall away 35 seconds after SRB separation.
Martin Marietta assumed that, with planned Shuttle performance upgrades and mass reductions, an Orbiter could put payloads with a combined mass of 73,800 pounds into a 160-nautical-mile orbit inclined 28.5° to Earth's equator. An empty ACC would add 16,508 pounds to the Shuttle's mass at liftoff, This would reduce by a corresponding amount the payload mass the Orbiter and ACC could deliver to orbit. If the entire ACC remained with the Shuttle until SSME cutoff, then the payload mass the Orbiter and ACC could place into orbit would total 57,300 pounds. Discarding the ACC shroud as early as possible in the Shuttle's eight-minute climb to orbit, on the other hand, would mean a payload mass loss of only about 7900 pounds. The Orbiter payload bay and ACC skirt could thus together deliver payloads totaling 65,900 pounds.
On non-ACC Shuttle missions, the Orbiter would shut down its SSMEs and discard the ET before it had attained orbital velocity so that the tank would reenter the atmosphere and be destroyed over the Indian Ocean. This would, of course, deprive the SSMEs of their source of propellants. The astronauts would then ignite the Orbiter's twin Orbital Maneuvering System (OMS) engines for the first of two orbit insertion burns.
On ACC missions, SSME cutoff would see Orbiter, ET, ACC skirt, and payloads in a 57-by-160-nautical-mile orbit, so the first orbital insertion OMS burn would not be necessary. 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, raising its perigee (the low point in its orbit around the Earth), and circularizing its orbit at an altitude of 160 nautical miles.
Martin Marietta proposed a host of potential ACC payloads. "Catch tanks" might collect residual ET propellants for later use in orbit, or a turbine generator might burn leftover propellants to make more electricity than the Orbiter's fuel cells could provide. The ACC skirt might also carry a 25-foot-diameter, 20-foot-long space station module, a space tug almost as large, or deploy a large structure such as an umbrella-like radio dish antenna more than 50 feet across. A space station module might be designed to remain attached to the ET on which it launched, enabling the big tank to serve as a strong-back for mounting payloads or a large enclosed volume for experiments or habitation. By providing a second payload volume, the ACC could also enable secret Department of Defense (DOD) payloads to be carried separate from but on the same flight as NASA civilian payloads.
Martin Marietta described three example Orbiter/ACC payload manifests and deployment scenarios. Flight 1, a mission with an initial 160-nautical-mile orbit at 28.5° of inclination, would see three satellites with identical solid-propellant upper stages launched in the ACC: the 8848-pound Brazilsat/Payload Assist Module (PAM)-D, the 8848-pound GOES/PAM-D, and the 9399-pound Telsat/PAM-D. The Orbiter, meanwhile, would carry a 58-foot-long, 14-foot-diameter "large observatory" with a mass of 18,700 pounds.
Without the ACC, payload mass for Flight 1 would be limited to the 18,700 pounds carried in the Orbiter payload bay, or about a quarter of the 73,800-pound theoretical maximum for the flight; with the ACC, the payload could total 45,800 pounds. Following deployment from the ACC skirt, the satellites would ride their PAM-D stages to their assigned slots in the geostationary orbit (GEO) belt.
The Orbiter crew would then cast off the ET and ACC skirt. A 4100-pound pair of solid-propellant deorbit rocket motors on the ACC skirt would ignite over the western Pacific Ocean, causing the ET/ACC skirt to tumble and reenter the atmosphere. Any parts that survived reentry would splash harmlessly into the Pacific south of Hawaii.
The astronauts, meanwhile, would maneuver the Orbiter to a 190-nautical-mile orbit and deploy the large observatory. 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 3343-pound Tiros-N satellite inside the ACC and the 16,300-pound Atmosphere Monitor satellite at the aft end of the payload bay. Because the Orbiter/ET/ACC skirt/payloads assemblage would ascend to an energetically challenging 160-nautical-mile, 98.2° near-polar retrograde orbit, Flight 2's payload mass could total only 23,640 pounds.
The crew would first retrieve from orbit the 4000-pound Thermosat and stow it at the front of the payload bay. They would then fire the OMS engines to climb to a 380-nautical-mile orbit, into which they would deploy the Atmosphere Monitor.
Next, they would ignite the OMS engines 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.
Flight 3, with an initial 100-nautical-mile orbit at 28.5° of inclination, would see introduction of a new reusable hardware element made possible by the ACC's large payload envelope: the 15-foot-long, 25-foot-diameter, 34,100-pound Orbital Transfer Vehicle (OTV). The payload bay would carry the NATO IV/PAM-D DOD satellite and the 35-foot-long, 10-foot-wide, 13,000-pound Synchronous Observation Satellite (SOS), bringing the total payload mass for Flight 3 to 52,950 pounds.
The OTV would fill its tanks with residual ET propellants, 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 and extract it from the bay, then boost it to its assigned slot in GEO. The OTV would subsequently return to low-Earth orbit for refueling and 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. The PAM-D stage would boost the satellite to GEO and the astronauts would fire the Orbiter's OMS engines to return to Earth.
Martin Marietta placed great emphasis on the cost savings it said would accrue from adding the ACC to the Shuttle system. 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 reach space three years later. They calculated that ACC development would cost $113 million, changes to the Shuttle system to accommodate it would total $78 million, and changes to Kennedy Space Center facilities would cost $35 million. Use of the ACC would also add about $5 million in recurring costs to the $75-million base cost of a Shuttle flight.
For its cost-savings calculations, the company bravely employed a Shuttle traffic model less optimistic than the "official" NASA model. It assumed that 331 Shuttle flights would occur between 1988 and 2000, with the number of flights per year starting at 34 and trending downward to 20 by the end of the 12-year period. For the same period, NASA assumed 26 flights per year to start, an upward trend to nearly 60 flights per year, and a total over 12 years of 581 flights. Based on its "low" model, the company estimated that NASA might benefit from flying 71 civilian and 35 DOD Orbiter/ACC missions. In a further effort at conservativism, however, it assumed that NASA would fund only 75 Orbiter/ACC missions.
Martin Marietta determined that the increased payload capacity that the ACC would provide would permit elimination of 40 non-ACC 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 program that included the ACC would thus save NASA $2.6 billion.
References:
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).
