|The Space Shuttle Orbiter as conceived in July 1972. The following month, NASA would make Rockwell International the Space Shuttle Orbiter prime contractor. Image credit: NASA|
Aerodynamic heating creates challenges during reentry and, to a lesser degree, during ascent to orbit. Before the Space Shuttle, almost all piloted spacecraft designed to operate for some portion of their mission in an atmosphere withstood such heating by employing single-use ablative heat shields. (The only exception was the X-15A-2 rocket plane, which, for part of its career, included a replaceable ablative heat shield — see "More Information" at the end of this post.) During reentry, ablative heat shields char and break away, carrying away heat.
The Space Shuttle, approved for development by President Richard Nixon on 5 January 1972, marked a dramatic departure in heat shield technology. Originally conceived as a fully reusable, economical Space Station resupply and crew rotation vehicle, Nixon's partially reusable Shuttle had as its only approved goal a dramatic reduction in the cost of launching things into space. A reusable heat shield was believed to be essential for achieving that objective.
Over the decades, engineers have considered many reusable heat shield concepts, typically in combination. High on the list was a layer of overlapping "shingles" made of exotic metal alloys. Other approaches included liquid or solid heat sinks, thick metal or composite adjoining plates, or even an "active" system with cooling fluid circulating through a network of tubes behind a metal-alloy hull.
Unfortunately, all of these concepts would be heavy. To compensate for a heavy heat shield, engineers could design a more powerful booster system or could cut back on payload capacity (or both). Both approaches would boost development and operations costs. The Nixon White House had made clear that the Shuttle development budget of $5.15 billion was carved in stone, leaving NASA with little choice but to find new approaches — including some that accepted a significant increase in eventual operations cost.
|Image credit: NASA|
Though RSI was meant to block almost all heat, enough would get through that, combined with aerodynamic buffeting, the Orbiter's mostly aluminum skin would tend to warp and flex ("flutter"). This meant that large ceramic panels affixed to the skin would crack, leaving it vulnerable to reentry heating.
Shuttle engineers sought to avoid damage by gluing RSI ceramics to a flexible fabric "strain isolator" layer glued to the Orbiter's skin and by making individual ceramic elements small in size. By resorting to many small "tiles" in place of a relatively few large panels, engineers designed an RSI heat shield that was in effect "pre-cracked."
The tiles, each milled to conform to its place on the Orbiter's complexly curvaceous hull, would number in the tens of thousands. By late in the 1970s decade, when their number hovered around 31,000, the tiles earned the Orbiter the nickname "The Flying Brickyard."
Some engineers harbored doubts about RSI; enough that NASA Langley Research Center in Hampton, Virginia, paid the Denver Division of Martin Marietta Corporation (MMC) to examine an alternative. Between May 1972 and August 1973, MMC engineers sought to determine whether Space Shuttle Orbiters could employ an ablative heat shield.
The ablative shield was seen as a stand-in system meant to provide NASA with more time for RSI development should problems arise. In his October 1975 report on the ablative heat shield study, Rolf Seiferth, who managed the MMC study between 5 September 1972 and its conclusion on 31 August 1973, envisioned that the ablative shield might fill in for RSI for five years. Based on a November 1972 NASA-generated Space Shuttle traffic model, this meant that 151 flights between 1979 and the end of 1983 would rely on the stand-in ablative system.
Seiferth noted that, in past programs, ablative heat shield materials had been glued directly to the spacecraft hull. This was, he explained, a cost-saving, weight-saving approach; scraping away a used directly applied ablative shield would, however, add time to Orbiter refurbishment between flights and generate considerable debris, including invasive dust.
In addition to the directly applied heat shield, MMC examined three types of "mechanically attached" ablative panels. These had ablative material glued to panels made of aluminum, magnesium, graphite composite, or beryllium/aluminum "Lockalloy" sheet or honeycomb.
The panels would be joined to oversized holes in the Orbiter's skin using nut-and-bolt fasteners, enabling entire panels to be replaced as necessary. The oversized holes would allow for thermal expansion of the heat shield components.
The simplest mechanically attached ablative panel would see ablative material glued to a metal or composite sheet. Adhesive and sheet would together measure only about 0.06 inches thick. Attachment points for the sheet panel design would typically occur five inches apart over much of the Orbiter, though larger spacings (up to 20 inches) were also possible.
The two more complex mechanically attached ablative panels substituted metal or composite "honeycomb" for the metal or composite sheet. One had ablative material glued to the honeycomb, which was then bolted to oversized holes in the Orbiter's skin.
The other — to which MMC gave considerably less attention — added rib-like standoffs to the Orbiter's skin. The honeycomb was then mechanically attached to oversized holes in the standoffs, leaving a gap between the underside of the honeycomb and the Orbiter skin.
Honeycomb panel attachment points would typically occur 10 inches apart over much of the Orbiter. Larger (up to 20 inches) and smaller (down to five inches) spacings were possible.
Seiferth's team used computer models to determine required ablator thickness, which would vary depending on its location on the Orbiter. All models assumed a maximum reentry deceleration equal to 2.5 times Earth's surface gravity (that is, 2.5 G) and a maximum allowable Orbiter aluminum skin temperature of 350° Fahrenheit, variables which indicated a relatively benign reentry environment (as compared to an Apollo lunar-return reentry, for example).
MMC used for its calculations properties of several types of ablative material it had developed for other missile and space projects (notably, the Titan missile family and the Viking Mars lander). It found that, for most locations on the Orbiter, its least robust ablator would be sufficient.
The ablative layer for most locations could be surprisingly thin. For the simplest mechanically attached panel design, for example, the MMC computer models indicated that a point on the Orbiter's underside on the fuselage centerline 50 feet aft of its nose would need a layer of ablative material only 1.7 inches thick.
Assessing the cost of the ablative designs relative to RSI was difficult in part because Space Shuttle Program cost estimation was, for want of a better term, eccentric. Seiferth supplied no development or operations cost estimate for RSI in his report, though he did provide estimates for several of MMC's ablative designs.
A system with an ablator glued directly to the Orbiter's aluminum skin would, Seiferth estimated, cost a total of $164.8 million for 151 flights over five years. Of this, installation and removal would account for $27.9 million.
A mechanically attached system comprising an aluminum sheet, adhesive, and an ablator (that is, the simplest mechanically attached ablative system) with attachment points five inches apart would cost $168.3 million with an installation and removal cost of $21.9 million. The aluminum honeycomb system with no standoffs and attachment points five inches apart came in at $187.1 million with $25.7 million for installation and removal.
NASA provided MMC with an RSI weight estimate of 30,240 pounds, enabling an RSI/ablative system weight comparison. The MMC study determined that an ablator directly attached to the Orbiter's skin would weigh 27,199 pounds, while the sheet and honeycomb (no standoffs) mechanically attached systems would weigh 32,577 pounds and 32,158 pounds, respectively.
Seiferth noted that modifications to the Orbiter's aluminum skin design would need to be put in place soon if mechanically attached ablative panels were used. Delaying until after the Orbiter's skin was in place would make prohibitive the cost and difficulty of adopting the ablative Space Shuttle heat shield. By the time Seiferth's report saw print in October 1975 — more than two years after the MMC study concluded — a stand-in ablative heat shield, never high on NASA's list of Space Shuttle priorities, was in fact no longer an option.
Late in the 1970s decade, problems with the Space Shuttle Main Engine, RSI, computers, and systems contributed to delays in STS-1, the Space Shuttle's orbital maiden flight. RSI problems in particular became very public in March 1979, when the Space Shuttle Orbiter Columbia flew from California to NASA Kennedy Space Center (KSC), Florida, atop its 747 carrier aircraft. It was the first Orbiter's first visit to its home base. At the time, Columbia was scheduled to carry out STS-1 in November 1979.
|Columbia rolls into the Orbiter Processing Facility at Kennedy Space Center, Florida, on 25 March 1979. Though the image displays only the area around the front of the fuselage, many RSI gaps are evident. Image credit: NASA|
Some permanent RSI tiles had, however, fallen off Columbia for other reasons. Close examination revealed tile manufacturing flaws, installation errors, and an overall unexpected degree of fragility. Even as Columbia entered the processing flow for STS-1, NASA conceded that the flight might be delayed until 1980.
Much was made of the "zipper effect," a hypothetical catastrophic failure mode that would begin with the loss of a single tile during reentry. The Orbiter was believed likely to survive loss of a single tile unless it occurred in an especially critical area. Loss of a single tile anywhere would, however, weaken surrounding tiles, potentially leading to a cascading loss of thermal protection. In fact, few tiles fell off Orbiters during the series of 135 Shuttle missions that began with Columbia's first launch on 12 April 1981.
The RSI system did, however, prove prone to impact damage during processing, launch, landing, and transport. The most extreme example before January 2003 occurred during STS-27 (2-6 December 1988), a classified Department of Defense mission. Eighty-five seconds after liftoff, debris broke free from the right Solid Rocket Booster, battering the right wing of Orbiter Atlantis. More than 700 RSI tiles were damaged and one was lost. Because the mission was classified, the near-disaster was not widely known for nearly 20 years.
The debris strike was captured on video and immediately became the subject of urgent debate within the Shuttle Program. Knowledge of the strike was not shared widely. The viewing angle meant that the strike area was not visible in launch video recorded from the ground and its location meant that the STS-107 crew could not see it. Managers decided that Columbia's wing leading edge was probably intact.
The hole admitted hot gas as Columbia reentered on 1 February 2003. Its internal structure compromised, NASA's oldest Orbiter broke up over east Texas and western Louisiana, killing its seven-person crew and grounding the Space Shuttle fleet for 30 months.
The following January, President George W. Bush declared that the Space Shuttle would be retired after it performed its last International Space Station (ISS) assembly mission. The final Shuttle flight, STS-135 (8-21 July 2011), saw Atlantis, veteran of the STS-27 near miss, deliver supplies to ISS ahead of an anticipated gap in U.S. piloted space flights of indefinite duration.
"Space Shuttle Orbiter and Subsystems," D. Whitman, Rockwell International Corporation; paper presented at the 11th Space Congress in Cocoa Beach, Florida, 17-19 April 1974.
Ablative Heat Shield Design for Space Shuttle, NASA CR-2579, R. Seiferth, Denver Division, Martin Marietta Corporation, October 1975.
"Space Shuttle Orbiter Status April 1980," S. Jones, NASA Johnson Space Center; paper presented at the 17th Space Congress in Cocoa Beach, Florida, 30 April-2 May 1980.
STS-27R OV-104 Orbiter TPS Damage Review Team, Volume I, Summary Report, NASA TM-100355, February 1989.
X-15: Lessons for Reusable Winged Spaceflight (1966)
Where to Launch and Land the Space Shuttle? (1971-1972)
What If a Space Shuttle Orbiter Had to Ditch? (1975)
What If a Space Shuttle Orbiter Struck a Bird? (1988)