X-15: Lessons for Reusable Winged Spaceflight (1966)

An X-15 rocket plane separates from its B-52 carrier aircraft. During this 9 November 1961 flight, the 45th in the X-15 series, U.S. Air Force Major Robert White piloted X-15 No. 2 to a world-record speed of Mach 6.04 (4093 miles per hour). It was the first time a piloted aircraft exceeded Mach 6. Image credit: NASA.

The X-15 is a strong contender for the title of "Everyone's Favorite X-plane." Conceived in the 1952-1954 period, before Sputnik (4 October 1957) and the birth of NASA (1 October 1958), the North American Aviation-built rocket plane was intended to pioneer the technologies and techniques of piloted hypersonic flight — that is, of flight faster than Mach 5 (five times the speed of sound).

Between 1959 and 1968, three X-15 rocket planes, two modified B-52 bombers, and a dozen pilots took part in joint U.S. Air Force/NASA X-15 research missions. Before the start of each mission, an X-15 was mounted on a pylon attached to the underside of a wing of a B-52 carrier aircraft at Edwards Air Force Base, California. Wearing a silver pressure suit, a single pilot boarded the 50-foot-long X-15 as it hung from the pylon, then the B-52 taxied and took off from a runway.

Early X-15 missions were "captive" flights, meaning that the rocket plane stayed attached to the B-52, or gliding flights, meaning that it carried no propellants and relied on its wings, which spanned only 22 feet, to make a controlled — though fast and steep — descent to a landing. Early powered flights used stand-in rocket engines taken from earlier X-planes. By late 1960, however, the X-15's throttleable 600,000-horsepower XLR99 rocket engine was ready. The engine was designed to burn the nine tons of anhydrous ammonia fuel and liquid oxygen oxidizer in the X-15's tanks in about 90 seconds at full throttle.

During high-speed flight and Earth atmosphere reentry, the X-15 compressed the air in front of it, generating temperatures as high as 1300° Fahrenheit on its nose and wing leading edges. The rocket plane's designers opted for a "hot structure" approach to protecting it from aerodynamic heating through most of its career. An outer skin made of Inconel X, a heat-resistant nickel-chromium alloy, covered an inner skin of aluminum and spun glass, which in turn covered a titanium structure with a few Inconel X parts. Heat caused the skin and structure to expand, warp, and flex, but they would return to their original shapes as they cooled. The X-15's cockpit temperature could reach 150° Fahrenheit, but the pilot usually remained cool in his pressure suit.

Most missions followed two basic profiles. "Speed" missions saw the rocket plane level off at about 101,000 feet and push for ever-higher Mach numbers. The X-15 reached its top speed — Mach 6.72, or about 4520 miles per hour — during the 188th flight of the series on 3 October 1967 with Air Force Major William "Pete" Knight at the controls.

Knight flew X-15A-2, the former X-15 No. 2, which had rolled over during an abort landing on 9 November 1962, seriously injuring its pilot, John McKay. When NASA and the Air Force rebuilt X-15 No. 2, they modified its design to enable faster flights. One modification was the addition of a replaceable ablative heat shield so that it could withstand the higher temperatures that came with faster speeds. Ablative heat shields are designed to char and break away, carrying away heat.

For "altitude" missions, the X-15 climbed steeply until it exhausted its propellants, then arced upward, unpowered. X-15 reached its peak altitude — 354,200 feet (almost 67 miles) above the Earth's surface — on 22 August 1963, with NASA pilot Joseph Walker in the cockpit.

During altitude missions, the pilot experienced several minutes of weightlessness as the X-15 climbed toward the high point of its trajectory, above 99% of the atmosphere, then fell back toward Earth. Aerodynamic control surfaces (for example, ailerons) could not work while the X-15 soared in near-vacuum, so the space plane included hydrogen peroxide-fueled attitude-control thrusters so that the pilot could orient it for reentry.

It was during an altitude mission that the X-15 program suffered its only pilot fatality. On 15 November 1967, Major Michael Adams piloted X-15 No. 3 to 266,000 feet despite an electrical problem that made control difficult. During descent, Adams lost control of the space plane, which went into a flat spin at Mach 5, then an upside-down dive at Mach 4.7. Adams might have recovered control at that point, but then an "adaptive" flight control system malfunctioned, thwarting maneuvers that might have damped out excessive pitch oscillations and compensated for increasing atmospheric density. The X-15 broke apart at about 65,000 feet.

Flights of early rocket-powered X planes, such as the first aircraft to break the sound barrier, the Bell X-1, took place over Edwards Air Force Base, but the X-15 needed more room for its speed and altitude flights. In both powered X-15 mission profiles, the B-52 released the X-15 about 45,000 feet above northern Nevada with its nose pointed southwest toward its landing site on Edwards dry lake bed. Two radio relay stations and six emergency landing sites on dry lake beds were established along the X-15 flight path. Adams might have landed on Cuddeback dry lake bed, 37 miles northeast of Edwards, had he regained control of X-15 No. 3.

This NASA cutaway of the X-15 displays the aircraft's XLR99 engine, weight-saving aft skids, propellant tanks, wing, fin, and fuselage structure, cockpit, and forward landing gear. The lower tail fin was necessary for flight stability, but got in the way during landing, so was designed to drop away during approach.

NASA's Project Mercury, which began officially on 6 October 1958, opted for a different approach to aerodynamic heat management: a bowl-shaped single-use ablative heat shield. As piloted Mercury capsule flights commenced (5 May 1961) and President John F. Kennedy put NASA on course for the Moon (25 May 1961), public attention shifted away from the X-15 and Edwards Air Force Base and toward Mercury, Apollo, and Cape Canaveral, Florida. X-15 research planes continued to fly, however, pushing the hypersonic flight envelope well past their original design limits.

In the same period, some within NASA planned Earth-orbiting space stations. Before Kennedy's Moon speech, a space station was seen as the necessary first step toward more advanced space activities. It would serve as a laboratory for exploring the effects of space conditions on astronauts and equipment and as a jumping-off place for lunar and interplanetary voyages. 

Station supporters often envisioned that it would reach orbit atop a two-stage Saturn V rocket, and that reusable spacecraft for logistics resupply and crew rotation would make operating it affordable. After the Moon speech, station proponents hoped that, once Kennedy's politically motivated Moon goal was reached, piloted spaceflight could resume its "proper" course by shifting back to space station development.

In November 1966, James Love and William Young, engineers at the NASA Flight Research Center at Edwards Air Force Base, completed a brief report in which they noted that the reusable suborbital booster for a reusable orbital spacecraft would undergo pressures, heating rates, and accelerations very similar to those the X-15 experienced. They acknowledged that the X-15, with a fully fueled mass of just 17 tons, might weigh just one-fiftieth as much as a typical reusable booster. They nevertheless maintained that X-15 experience contained lessons applicable to reusable booster planning.

Love and Young wrote that some space station planners expected that a reusable booster could be launched, recovered, refurbished, and launched again in from three to seven days. The X-15, they argued, had shown that such estimates were wildly optimistic. The average X-15 refurbishment time was 30 days, a period which had, they noted, hardly changed in four years. Even with identifiable procedural and technological improvements, they doubted that an X-15 could be refurbished in fewer than 20 days.

At the same time, Love and Young argued that the X-15 program had demonstrated the benefits of reusability. They estimated that refurbishing an X-15 in 1964 had cost about $270,000 per mission. NASA and the Air Force had accomplished 27 successful X-15 flights in 1964. The cost of refurbishing the three X-15s had thus totaled $7.3 million.

Love and Young cited North American Aviation estimates when they placed the cost of a new X-15 at about $9 million. They then calculated that 27 missions using expendable X-15s would have cost a total of $243 million. This meant, they wrote, that the cost of the reusable X-15 program in 1964 had amounted to just 3% of the cost of building 27 X-15s and throwing each one away after a single flight.

NASA test pilot Neil Armstrong flew the X-15 seven times in 1960-1962. Armstrong became a member of NASA Astronaut Group 2 ("The New Nine") in September 1962. He orbited the Earth as commander of Gemini 8 (March 1966) and became the first man to set foot on the Moon during Apollo 11 (July 1969). Another X-15 pilot, Joseph Engle, became a member of NASA Astronaut Group 5 in April 1966. Engle flew the Orbiter Enterprise during Space Shuttle Approach and Landing Test (ALT) flights in 1977, commanded Columbia for mission STS-2 in November 1981, and commanded Discovery for mission STS 51-I in August-September 1985. Image credit: NASA.

The last X-15 flight, the 199th in the series, took place on 24 October 1968. Flight experience gained and hypersonic flight data collected during the nine-year program contributed to the development of the U.S. Space Shuttle, though not exactly as Love and Young had envisioned.

When, in 1968, NASA Headquarters management first floated Space Station/Space Shuttle as the space agency's main post-Apollo piloted program, the Shuttle was conceived as a reusable piloted orbiter vehicle with a reusable piloted suborbital booster — that is, the design that Love and Young had assumed. By late 1971, however, funding limitations forced NASA to opt instead for a semi-reusable booster stack comprising an expendable External Tank and twin reusable solid-propellant Solid Rocket Boosters.

The space agency was also obliged to postpone its Space Station plans at least until after the Space Shuttle became operational. Saturn V was on the chopping block, so the semi-reusable Shuttle would be used to launch the Station as well as to resupply it and rotate its crews.

Shuttle Orbiter Columbia first reached Earth orbit on 12 April 1981, but no Orbiter visited a space station until Discovery rendezvoused with the Russian Mir station on 6 February 1995 during mission STS-63. The first Shuttle Orbiter to dock with a station — again, Russia's Mir — was Atlantis during mission STS-71 (27 June-7 July 1995).

Sources

Survey of Operation and Cost Experience of the X-15 Airplane as a Reusable Space Vehicle, NASA Technical Note D-3732, James Love and William Young, November 1966.

"I Fly the X-15," Joseph Walker and Dean Conger, National Geographic, Volume 122, Number 3, September 1962, pp. 428-450.

Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane, Monographs in Aerospace History No. 18, Dennis R. Jenkins, NASA, June 2000.

More Information

Space Station Resupply: The 1963 Plan to Turn the Apollo Spacecraft Into a Space Freighter

McDonnell Douglas Phase B Space Station (1970)

From Monolithic to Modular: NASA Establishes a Baseline Configuration for the Shuttle-Launched Space Station (1970)

Where to Launch and Land the Space Shuttle? (1971-1972)

NASA Johnson's Plan to PEP Up Shuttle/Spacelab (1981)

Early artist concept of a Space Shuttle Orbiter with a "Sortie Lab" at the front of its Payload Bay. The Sortie Lab pressurized module is shown as a cutaway illustration. At this point in its history, the Sortie Lab was expected to be manufactured by a U.S. aerospace contractor. The Sortie Lab depicted is dedicated at least partly to astronomy, as evidenced by the large telescope attached to the aft end of its pressurized module. Image credit: NASA.

On 29 November 1972, NASA Administrator James Fletcher abolished the Space Station Task Force formed in early 1969 by his predecessor, Thomas Paine, and formed the Sortie Lab Task Force. The "Sortie Lab," a concept that emerged during Phase B Space Station planning in 1970, was envisioned as a pressurized laboratory module which would be carried in the Shuttle Orbiter's Payload Bay.

Fletcher's move acknowledged that the Space Shuttle, conceived originally as a vehicle for transporting crews and cargoes between Earth and an Earth-orbiting Space Station at low cost, would need to become a Space Station — or, at least, an interim space laboratory that could demonstrate that a Space Station would be a desirable new NASA goal after the Space Shuttle became operational.

Strapped for funds and encouraged by President Richard Nixon to use spaceflight as a vehicle for international cooperation, NASA asked the European Space Research Organization (ESRO), a predecessor of the European Space Agency (ESA), to provide the Sortie Lab in exchange for European astronaut flights on board the Shuttle. In August 1973, ESRO and European aerospace contractors agreed to build the Sortie Lab, which became known as Spacelab.

Cutaway illustration of a drum-shaped, ESA-built Spacelab module (center) with a pair of U-shaped Spacelab pallets (left). A bent tunnel with an airlock on top for spacewalks (note space-suited astronaut atop pallet at left) links Spacelab with the Shuttle Orbiter Mid-Deck, the main living space for the crew. Above that is the Flight Deck, the Orbiter cockpit. Image credit: NASA.

Spacelab would provide scientists with ample pressurized volume in which to conduct research, but it would rely on limited resources — for example, electricity — provided by the Shuttle Orbiter. Orbiter electricity came from a trio of liquid oxygen/liquid hydrogen fuel cells that in early 1981 were expected to generate 21 kilowatts continuously for just seven days. Of this, 14 kilowatts were required for Orbiter systems. The Orbiter could thus supply only seven kilowatts to Spacelab. Of those seven kilowatts, between two and five kilowatts would be needed for basic Spacelab systems, leaving a paltry two to five kilowatts for Spacelab experiments.

In 1978, NASA Johnson Space Center (JSC) in Houston, Texas, launched the Orbital Service Module Systems Analysis Study, which looked into ways that the Space Shuttle Orbiter could be augmented to enable it to better support Spacelab research. An early product of the study was the Power Extension Package (PEP) concept.

Stowed PEP components in the Space Shuttle Orbiter Payload Bay, between the front of a Spacelab module (right) and the rear of the Orbiter crew cabin. Image credit: NASA.

The PEP deployed in orbit. PEP displays and controls were meant to be located on the Shuttle Orbiter Flight Deck. Image credit: NASA.

The PEP concept was linked with NASA's extensive efforts in cooperation with the U.S. Department of Energy to justify the construction of enormous Earth-orbiting Solar Power Satellites (SPSs). It was portrayed as an experience-building experimental test-bed for SPS technology in the Von Karman Lecture JSC director Christopher Kraft presented to the 15th meeting of the American Institute of Aeronautics and Astronautics in July 1979. The PEP may also have been conceived as a rival for NASA Marshall Space Flight Center's Power Module (see "More Information" below).

The PEP Project Office (PEPPO) at JSC pitched the PEP in a brief report published one month before the first Space Shuttle flight (STS-1, 12-14 April 1981). The PEPPO envisioned the PEP as a "kit" that could be installed easily in the Shuttle Orbiter Payload Bay over the tunnel that would link the Orbiter Mid-Deck with the Spacelab pressurized module.

One hour after launch from Earth, an astronaut on the Orbiter Flight Deck would use the Canada-built Remote Manipulator System (RMS) robot arm to grapple the PEP's Array Deployment Assembly (ADA) and extend it out over the Orbiter's side. The ADA would then unroll a pair of lightweight solar array wings that together would measure more than 100 feet wide. PEP deployment would require about 30 minutes.

The PEP arrays would track the Sun automatically no matter how the Orbiter became oriented, so almost no astronaut intervention would be needed after they were deployed. The RMS and arrays would be sufficiently sturdy to remain deployed during Orbiter attitude-control maneuvers, but the crew would need to stow them before Orbital Maneuvering System burns lest the acceleration cause damage.

The twin arrays would generate a total of 26 kilowatts of electricity. A cable built into the RMS would carry the electricity from the ADA to the PEP's Power Regulation and Control Assembly (PRCA) in the Payload Bay. The PRCA would then distribute it to the Orbiter's electrical system.

The three Orbiter fuel cells would "idle" while the PEP arrays were in sunlight. Each would generate one kilowatt of electricity, bringing the total available on board to 29 kilowatts. Fifteen kilowatts would be available for Spacelab, of which between 10 and 13 kilowatts could be devoted to experiments.

Keeping the Spacelab electricity supply constant throughout each 90-minute orbit of the Earth would require that Orbiter fuel cell output ramp up rapidly from three to 29 kilowatts as the PEP arrays passed into darkness over Earth's night side. To achieve this output, each fuel cell would need to exceed its normal maximum by nearly three kilowatts. The fuel cells would then return to their idle state as the PEP arrays passed again into sunlight. Although it would almost certainly place unusual demands on the Orbiter fuel cells, the PEPPO judged this approach to be "feasible."

The PEPPO estimated that a PEP could extend Shuttle/Spacelab endurance in Earth orbit by four days (that is, to a total of 11 days). If other Orbiter resources (for example, life support consumables) could be augmented, then mission duration might be stretched to 45 days.

The PEPPO explained that it jointly managed PEP solar cell development with NASA's Lewis Research Center. Industry involvement in the PEP project was, it added, already "extensive," with several companies working on small NASA contracts or funding PEP-related work themselves. It estimated that the PEP could power a Spacelab module in Earth orbit as early as 1985 for a total development cost of only $150 million.

Spacelab 1 in Columbia's Payload Bay during STS-9 as viewed from the Flight Deck windows. Cables linking the Orbiter to Spacelab 1 are visible at lower right. Image credit: NASA.

The first Spacelab, appropriately designated Spacelab 1, reached orbit in the Payload Bay of the Orbiter Columbia on 28 November 1983, as part of the ninth Shuttle mission. Columbia's crew for mission STS-9 included ESA's Ulf Merbold, the first non-U.S. astronaut to reach space on board a U.S. spacecraft. Merbold was part of a six-man crew that also included Gemini, Apollo, and Shuttle veteran John Young, Skylab 3 veteran Owen Garriott, and spaceflight rookies Brewster Shaw, Robert Parker, and Byron Lichtenberg. Columbia landed at Edwards Air Force Base, California, on 8 December, ending a busy 10-day mission.

Columbia's fuel cells powered Spacelab 1, and all of the 27 Spacelab missions that followed relied on Orbiter fuel cells for their electricity. PEP work had ended in late 1981 as NASA Headquarters took charge of and terminated Shuttle augmentation and Space Station development efforts across the agency. It did this in part to clear the decks as it began formally to seek approval for a Space Station, which it billed as the "next logical step" after the Space Shuttle. President Ronald Reagan called on Congress to approve new-start funding for a Space Station during his annual State of the Union address in January 1984, less than two months after STS-9. 

Sources

Power Extension Package (PEP) Concept Summary, JSC-AT4-81-081, NASA Johnson Space Center, PEP Project Office, March 1981.

The Solar Power Satellite Concept, NASA JSC 14898, Christopher C. Kraft; Von Karman Lecture, 15th Annual Meeting of the American Institute of Astronautics and Aeronautics, July 1979.

"Spacelab joined diverse scientists and disciplines on 28 Shuttle missions," Science@NASA, 15 March 1999 (https://science.nasa.gov/science-news/science-at-nasa/1999/msad15mar99_1/ - accessed 25 March 2017).

More Information

Electricity from Space: The 1970s DOE/NASA Solar Power Satellite Studies

Evolution vs. Revolution: The 1970s Battle for NASA's Future

Lunar GAS (1987)

During the STS-91 (2-12 June 1998) mission to the Russian Mir space station, the Space Shuttle Orbiter Discovery carried four pairs of GAS canisters along its Payload Bay walls. The red arrow points to one pair. Image credit: NASA.

NASA's Get Away Special (GAS) Program (officially the Small Self-Contained Payloads Program) was conceived in 1976 as a way of providing researchers with low-cost opportunities to fly experiments in the Space Shuttle Orbiter's 15-foot-by-60-foot payload bay. The first operational GAS canister, with a suite of 10 experiments developed by students at Utah State University, Weber State University, and the University of California at Davis, reached low-Earth orbit (LEO) during mission STS-4 (27 June-4 July 1982) on board the Orbiter Columbia. By 17 March 2005, when NASA terminated the GAS Program in the aftermath of the 1 February 2003 Columbia disaster, nearly 170 GAS canisters had flown in low-Earth orbit (LEO).

If four engineers at the Jet Propulsion Laboratory (JPL) in Pasadena, California, had had their way, a GAS payload might have traveled far beyond LEO. In May 1987, the team proposed that an advanced-design small spacecraft be launched on board a Space Shuttle inside an Extended GAS canister and ejected into Earth orbit. The spacecraft, called Lunar GAS (LGAS), would then use electric-propulsion thrusters to spiral outward to the moon.

Close-up of two of the STS-91 GAS canisters in Discovery's Payload Bay. Image credit: NASA.

LGAS anticipated the small, relatively cheap spacecraft of NASA's 1990s Discovery Program, the first mission of which, Near Earth Asteroid Rendezvous (NEAR), departed Earth in 1995. The Discovery Program, a significant break from the large-spacecraft paradigm that characterized much U.S. planetary mission development in the 1970s and 1980s, got its start in 1991-1992 as Defense Department space technology developed for President Ronald Reagan's Strategic Defense Initiative "missile shield" trickled into the civilian space sector. The Discovery Program would become an intermediate evolutionary step leading toward the present-day Cubesat revolution.

The LGAS mission would begin up to three months before planned Space Shuttle launch with the insertion of the 149-kilogram spacecraft into its Extended GAS canister. The spacecraft would at that point enter the routine GAS payload processing flow and no one would see it again until it left its canister in LEO.

The Shuttle Orbiter bearing the LGAS spacecraft would lift off from Kennedy Space Center in Florida and enter an orbit inclined 28.5° relative to Earth's equator. The astronauts would then open its payload bay doors, exposing the closed Extended GAS canister bearing LGAS to space.

NASA required that GAS experiments place minimal demands on Shuttle expendables and astronaut time. The JPL team insisted that, despite its complexity, the LGAS mission could meet this requirement. A few hours after launch, one astronaut would flip a single switch on the Shuttle flight deck to open the motorized Extended GAS canister lid, then would flip two more to release a latch and activate a spring ejection mechanism.

Simplified schematic of the LGAS spacecraft following deployment from its GAS canister. Image credit: JPL/NASA.

The barrel-shaped LGAS spacecraft would leave the Extended GAS canister moving at one meter per second; then, as it moved away from the Shuttle Orbiter, it would automatically extend its twin accordion-fold solar-array wings and its science boom. The slender advanced-design rectangular solar arrays would each have a mass of about 15 kilograms. Their combined 7.25 square meters of collecting area would generate 1.467 kilowatts of electricity at mission start.

Two small chemical-propellant thrusters would turn the spacecraft to point its solar arrays and spin axis toward the Sun, then would spin its barrel-shaped body end over end at up to five revolutions per minute to create gyroscopic stability. After it had moved a safe distance away from the Shuttle, the LGAS spacecraft would switch on one of its twin electric thrusters. Mounted on opposite sides of the spacecraft body, these would take turns thrusting parallel to its spin axis. Fueled from a round tank containing 36 kilograms of compressed xenon gas, the thrusters would each be designed to withstand 3500 start/stop cycles and to operate for a total of 4500 hours (187.5 days).

LGAS spacecraft electric-propulsion thrust and coast arcs during escape from Earth orbit. Image: JPL/NASA.

The LGAS spacecraft's orbit about the Earth would for mission operations purposes be divided into four 90° arcs, the JPL engineers explained. In the first arc, one thruster would point opposite the LGAS spacecraft's direction of motion so that when it operated it would accelerate the spacecraft. In the second arc, which would occur in Earth's shadow, both thrusters would point perpendicular to the spacecraft's direction of motion; this would mean that they could not contribute to accelerating the spacecraft, so they would not operate.

In the third arc, the second thruster would point opposite the LGAS spacecraft's direction of motion, so it would switch on to take its turn accelerating the spacecraft. In the fourth arc, which would see the spacecraft pass between the Earth and the Sun, the thrusters would again point perpendicular to its direction of motion, so would not operate.

Overcoming drag from Earth's atmosphere would require about one-third of the LGAS spacecraft's thrust early in the departure spiral, the team calculated, but drag would taper off quickly as the spacecraft raised its orbital altitude by up to 20 kilometers per day. Starting about three months after launch from the Shuttle, the LGAS spacecraft would spend between 100 and 150 days inside the Earth-girdling Van Allen Belts. High-energy particles in the Belts would gradually degrade the twin wing arrays, reducing their electricity output.

Image credit: JPL/NASA.

About 600 days after launch, the LGAS spacecraft would reach a point about 130,000 kilometers away from the Earth. It would then turn off its thrusters and coast in a lazy 15-day "linking orbit" that would deposit it into a loosely bound 40,000-kilometer circular lunar polar orbit.

The xenon-fueled thrusters would then resume alternating operation with their 90° thrust arcs centered over the moon's polar regions; this time, however, the thrusters would point in the spacecraft’s direction of motion when they operated, gradually slowing the LGAS spacecraft so that it would spiral in toward the moon.

The spacecraft would achieve a 100-kilometer-high, two-hour lunar polar orbit about two years after it departed its Extended GAS canister. In its orbit over the moon's poles, the moon would rotate beneath it about once per month, enabling it to eventually overfly the entire lunar surface. Irregularities in the moon's gravity field would mean that the electric thrusters would need to adjust the spacecraft's orbit about every 60 days.

The LGAS spacecraft would have room for only one science instrument: a 15-kilogram gamma-ray spectrometer (GRS) for charting the composition of the moon's crust. The JPL engineers proposed that the unflown Apollo 18 GRS be mounted on the LGAS science boom. Lunar-orbital science operations would continue for about one year.

Sources

"Lunar Get Away Special (GAS) Spacecraft," AIAA-87-1051, K. T. Nock, G. Aston, R. P. Salazar, and P. M. Stella; paper presented at the 19th AIAA/DGLR/JSASS International Electric Propulsion Conference in Colorado Springs, Colorado, 11-13 May 1987.

"Getaway Special," Wikipedia (https://en.wikipedia.org/wiki/Getaway_Special — accessed 18 March 2017).

More Information

On the Moons of Mighty Jupiter (1970)

Cometary Explorer (1973)

Catching Some Comet Dust: Giotto II (1985)

NASA Johnson Space Center's Shuttle II (1988)

Image credit: NASA.

Although the fact is mostly forgotten today, NASA launched plans to augment or replace the Space Shuttle even before the first Shuttle reached orbit on 12 April 1981. Much — though by no means all — of this planning occurred as part of joint Department of Energy/NASA Solar Power Satellite studies.

In 1985, U.S. President Ronald Reagan signed a directive ordering the U.S. civilian space agency to develop a Space Shuttle successor. Notably, this occurred before the 28 January 1986 Challenger accident laid bare the Shuttle system's many frailties.

One proposed Shuttle successor was called Shuttle II. Most Shuttle II design work took place at NASA Langley Research Center (LaRC) in Hampton, Virginia. Shuttle II first achieved prominence in 1986 in the high-level National Commission On Space report Pioneering the Space Frontier.

LaRC's Shuttle II design evolved — for a time it was to have been a single-stage-to-orbit vehicle. The favored design included a winged manned Orbiter and a winged unmanned Booster, both of which would take off vertically and land horizontally on runways. Both the Booster and the Orbiter would be entirely reusable. LaRC's Shuttle II Orbiter fuselage was meant to be crammed full of propellant tanks, so would tote cargo in a sizable hump on its back.

NASA Langley Research Center's dumpy Shuttle II, 1987. Image credit: NASA.

Shuttle II was intended mainly as a crew transport complementing a "mixed fleet" of launchers that would have included unmanned heavy-lift rockets capable of placing from 50 to 100 tons into space. LaRC envisioned that its Shuttle II would transport a small amount of cargo — perhaps 10 tons — and up to 25 astronauts, of whom only three would be considered Shuttle II crew members. The remainder would be passengers bound for a large advanced Space Station or a Transportation Node station. There they would board Moon or Mars spacecraft.

Although a good case can be made for calling LaRC's Shuttle II the Shuttle II, it was in fact not the only proposed Shuttle II design. The Advanced Programs Office at NASA Johnson Space Center (JSC) in Houston, Texas, put forward the sleek Shuttle II design depicted in the last seven images of this post. They portray JSC's Shuttle II as it would appear over the course of a typical mission.

The LaRC design was favored by NASA Headquarters and is relatively well documented. Neither can be said for JSC's design.

In flight: the Evolved Shuttle climbs toward space, probably sometime in the 1990s. Image credit: Eagle Engineering/NASA.

Model of proposed Evolved Shuttle showing major components. Image credit: NASA.

Engineers in Houston envisioned that their Shuttle II might develop from an Evolved Space Shuttle. In the Evolved Shuttle, Liquid Replacement Boosters stood in for the Shuttle's twin Solid Rocket Boosters. The Evolved Shuttle would retain the Shuttle's expendable External Tank and, with minor modifications, the Shuttle Orbiter's Space Shuttle Main Engines (SSMEs). Like the Space Shuttle, the Evolved Shuttle stack would ride to its Launch Complex 39 pad atop a creeping crawler-transporter with its nose aimed at the sky.

Winglets on the tips of the Evolved Shuttle's modified delta wings would replace the Shuttle's single vertical tail fin. Redesigned Orbital Maneuvering System (OMS) engines based on the venerable RL-10 engine would draw liquid hydrogen/liquid oxygen propellants from insulated tanks built into the Evolved Shuttle Orbiter wings.

The most dramatic changes would, however, be reserved for the Evolved Shuttle crew compartment. JSC engineers designed it so that it could separate from the Evolved Shuttle in the event of catastrophic failure and operate as an independent spacecraft. Canard winglets meant to improve the Evolved Shuttle's aerodynamic characteristics would separate with the crew compartment and become its wings.

JSC gave no timeline for the evolution of Shuttle to Evolved Shuttle. If, however, JSC's Shuttle II was to become operational in the same timeframe as LaRC's Shuttle II (the early 21st century), then one may assume that the Evolved Shuttle would have made its debut in the 1990s.

Shuttle II ready for a tow to its launch pad. A round panel covering an extendable docking adapter is visible just above the American flag on the fuselage. Image credit: NASA.

The JSC Shuttle II was meant to be towed horizontally on its tricycle landing gear from a hangar to its launch pad just four hours before planned launch. Unlike the Space Shuttle and Evolved Shuttle, JSC's Shuttle II would have no need of the Vehicle Assembly Building, the massive cuboid structure built at Kennedy Space Center in the 1960s for the assembly of Apollo Saturn V heavy-lift rockets.

Nor would it use the twin Launch Complex 39 pads, which were built in the 1960s to launch Saturn V rockets and rebuilt in the 1970s to launch the Space Shuttle. Shuttle II would instead lift off from a new-design pad, and Complex 39 would be given over once again to heavy-lift rocket launches. In fact, the JSC Shuttle II would make a complete break from the massive-scale Apollo-era infrastructure upon which the Space Shuttle relied.

JSC's Shuttle II in launch configuration. The round panel covering the extendible docking adapter is again visible; it leads to a crew access tunnel that runs the length of the spacecraft. Image credit: NASA.

At the launch pad, crew and passengers would board JSC's Shuttle II, then it would be tipped up to point its nose at the sky. Its landing gear doors would be closed, then its ground crew — small compared with the army of personnel that serviced the Space Shuttle — would load it with three kinds of propellants: liquid hydrogen fuel, liquid hydrocarbon (kerosene or propane) fuel, and liquid oxygen oxidizer.

For safety, most of the volatile fuels would be pumped into Shuttle II's four expendable over-wing tanks, while an integral, reusable tank within the spacecraft would carry most of the dense liquid oxygen. Fully loaded with propellants and payload, Shuttle II would weigh about 550 tons, or a little more than a quarter of the Shuttle's weight at SSME ignition.

JSC designers hoped to minimize Shuttle II weight in part by building it from advanced materials. The Space Shuttle Orbiter, with an empty mass of about 85 tons, had a more-or-less conventional load-bearing aluminum-titanium airframe clad in aluminum and lightweight thermal-protection materials. These included thousands of uniquely shaped ceramic tiles and Reinforced Carbon-Carbon (RCC) wing leading edges. Shuttle II, with an empty mass of 50 to 75 tons, would also rely on RCC, "but in larger, load-bearing, monolithic panels." The over-wing tanks would be made from lightweight welded aluminum-lithium alloy.

At launch, Shuttle II's single Space Transportation Main Engine (STME) and twin Space Transportation Boost Engines (STBEs) would ignite simultaneously. The former, designed to burn liquid hydrogen and liquid oxygen, was envisioned as a second-generation SSME. The latter, located between the STME and the Shuttle II body flap, would burn hydrocarbon fuel and liquid oxygen and employ liquid hydrogen as engine coolant. The STME and STBEs would together generate about 30% more thrust than the Space Shuttle's three SSMEs — between 1.3 and 1.6 million pounds.

Climb to orbit: JSC's Shuttle II following detachment of its outboard tanks and its twin STBEs. Image credit: NASA.

When it reached a velocity of between two and three kilometers per second, JSC's Shuttle II would shed its depleted outboard over-wing tanks and the STBEs. Dropping the STBEs would improve Shuttle II's flight performance by shifting its center of gravity forward. The tanks would break up and fall into the sea, but NASA would recover the twin engines for reuse. JSC engineers envisioned that they would descend in reentry shells, deploy maneuvering parachutes, and land in arresting nets aboard recovery ships.

The STME, meanwhile, would extend its telescoping exhaust nozzle to its full length and diameter to improve its performance in vacuum. Following separation of the outboard tanks and STBEs, the spacecraft would burn only liquid hydrogen/liquid oxygen propellants.

Immediately following STME cutoff, the engine's nozzle would retract and the inboard over-wing tanks would be cast off. Upon reaching apogee (the highest point in its orbit about the Earth), Shuttle II's twin OMS engines would ignite to raise its perigee (the lowest point in its orbit) out of the atmosphere. This would place it into a circular "Space Station rendezvous orbit" 485 kilometers high and inclined 28.5° relative to Earth's equator. The inboard tanks, meanwhile, would intersect Earth's atmosphere as they reached perigee and be destroyed.

The Shuttle II OMS would comprise a pair of new-design Advanced Space Engines or RL-10-derived engines. RL-10 had the advantage of a long flight history; derivatives of that engine have propelled upper stages and spacecraft since the 1960s. Liquid hydrogen and liquid oxygen for Shuttle II's OMS and the Reaction Control System (RCS) thrusters would be stored in double-walled, heavily insulated tanks in its tail section. Some propellants from the tail section would be combined in next-generation fuel cells to generate electricity and water for the spacecraft.

A crew access tunnel would run aft from the forward crew compartment for most of the length of the fuselage. Midway along the tunnel, on its left side, Shuttle II's docking adapter for linking up with the Space Station would be stowed behind a streamlined panel. The round panel is visible near the American flag in images that display the left side of the Shuttle II model. Prior to rendezvous with the Space Station, the panel would hinge out of the way, then the crew would extend the cylindrical docking adapter.

The image above shows Shuttle II in its orbital configuration with inboard tanks in place; this is apparently a photographer's error, since image captions make plain that the inboard tanks would separate immediately after STME cutoff, before the crew opened the payload bay. Image credit: NASA.

JSC engineers chose a novel method for exposing Shuttle II payloads to space: the crew would disable the OMS engines, vent and disconnect hoses that had linked the over-wing tanks to the STME, disengage locks, and hinge the tail section downward using electric motors. RCS thrusters in the tail would continue to operate; to minimize flexible wiring links between the main fuselage and the tail section, engineers proposed that the astronauts control the RCS thrusters via a short-range radio link.

Hinging the tail section down would expose a large round window and the open aft end of the 15-foot-wide-by-30-foot-long cylindrical payload bay. Astronauts at an aft workstation would look out through the window as they extended the cradle bearing their mission's payload. The photo captions do not name specific Shuttle II payloads, but it is logical to assume that these would include experiment packages for mounting on the Space Station and reusable Station logistics modules packed full of supplies and equipment. The payload bay would include an airlock for spacewalks and a pair of robot arms.

Unlike the Space Shuttle and Evolved Shuttle payload bays, the Shuttle II bay would normally not include radiators for dissipating heat generated by onboard equipment and astronaut exertions. Instead, Shuttle II's radiators would be built into the top surface of its wings. Supplemental radiators would be mounted on the payload cradle before flight only if "special purpose, high heat load conditions" were expected.

Before return to Earth, the astronauts would retract the payload cradle, then hinge shut the tail section. Shuttle II would include triple-redundant electric motors and a mechanical backup system for closing the payload bay "to assure that the vehicle configuration for entry [would] not have paths for hot plasma to enter the vehicle interior." During the first few Shuttle II flights, an astronaut would exit through the docking adapter and clamber over the fuselage to inspect the hinge area and seam between the tail section and the rest of the spacecraft. He or she might carry a repair kit "to fill any voids."

Reentry would occur as in the Space Shuttle Program; that is, Shuttle II would turn so that its aft end pointed in its direction of flight, then its OMS engines would ignite to reduce its orbital velocity. The spacecraft would then flip to point its nose forward as it fell toward the atmosphere. Following reentry, Shuttle II would glide to a runway landing.

JSC's Shuttle II in landing configuration. Image credit: NASA.

Unlike the Space Shuttle, which even after the Challenger accident included few realistic options for crew escape in the event of catastrophic failure, Shuttle II could in theory protect its crew through all phases of its mission. Like the Evolved Shuttle, Shuttle II would include a separable crew compartment; after separation, Shuttle II's canard fins — proportionately larger than those of the Evolved Shuttle — would become the crew compartment's wings.

The crew compartment aft end would include launch escape/deorbit rocket engines, a crew hatch, and a deployable aerodynamic flap. Following separation in orbit, the crew compartment could support 11 astronauts for up to 24 hours. This endurance was meant to ensure that Earth's rotation could bring into range a suitable landing site on U.S. soil. The crew compartment would touch down and slide to a halt on extendable skids.

Crew cabin separation on the launch pad or during ascent. Image credit: NASA.

Crew cabin separation in orbit or during reentry. Image credit: NASA.

JSC engineers acknowledged that wind-tunnel testing might show that the Shuttle II crew compartment shape was not flight-worthy in all abort situations. They proposed that inflatable or extendable structures "be employed to obtain an acceptable configuration for hypersonic, supersonic, and subsonic controlled flight."

They also proposed that the Shuttle II crew compartment become the Space Station's Crew Emergency Rescue Vehicle (CERV). The CERV was conceived as a "lifeboat" for use if the Space Station had to be evacuated rapidly, if a crew member became seriously ill or injured and needed hospital treatment on Earth, or if Shuttle II became grounded due to malfunction or accident and could not retrieve a Space Station crew.

The JSC engineers noted that the Shuttle II crew compartment/CERV, like Shuttle II itself, would subject its occupants to no more than three gravities of acceleration or deceleration. This would help to ensure that, during return to Earth, it would not inflict additional harm on a sick or injured Space Station crewmember.

NASA continued to attempt to develop a Shuttle successor — a winged spacecraft that would enable it to apply the lessons learned from the Shuttle Program. Some proposed complex new vehicles employing scramjets; others, vehicles smaller and less capable than the Shuttle tailored mainly for Space Station crew rotation and crew escape. Unfortunately, the space agency's budget was not expanded to permit simultaneous ongoing Shuttle operations, Space Station development and assembly, and development of a Shuttle successor.

By the mid-1990s, many in the Shuttle Program had changed their tactics; they declared that the Shuttle should continue to fly at least until 2010. In 2001, Boeing proposed that the Shuttle should fly until 2030.

The 2003 Columbia accident ended such plans. When the Shuttle was retired in 2011, a new NASA Shuttle design was as far away as it had been during Shuttle II planning in the late 1980s.

Sources

Caption Sheet, NASA Photo S88 29029, Shuttle II Candidate Configuration, 1988.

Caption Sheet, NASA Photo S88 29035, Shuttle II Launch Configuration, 1988.

Caption Sheet, NASA Photo S88 29032, Shuttle II Post-Boost Flight Configuration, 1988.

Caption Sheet, NASA Photo S88 29028, Shuttle II Orbital Flight Configuration, 1988.

Caption Sheet, NASA Photo S88 29026, Shuttle II Entry and Landing Configuration, 1988.

Caption Sheet, NASA Photo S88 29024, Shuttle II Pad Abort Crew Escape, 1988.

Caption Sheet, NASA Photo S88 29030, Shuttle II Crew Escape System, 1988.

Caption Sheet, NASA Photo S89 34837, Evolved Shuttle, 1989.

"Shuttle II Progress Report," T. Talay, NASA Langley Research Center; paper presented at the 24th Space Congress, 21-24 April 1987, Cocoa Beach, Florida.

Pioneering the Space Frontier: the Report of the National Commission on Space, Bantam Books, 1986.

"At 15, A Safer, Cheaper Shuttle," J. Asker, Aviation Week & Space Technology, 8 April 1996, pp. 48-51.

"Boeing upgrade would keep Space Shuttle flying to 2030," G. Warwick, Flight International, 8-14 May 2001, p. 37.

More Information

Electricity from Space: The 1970s DOE/NASA Solar Power Satellite Studies

What If a Space Shuttle Orbiter Had to Ditch? (1975)

One Space Shuttle, Two Cargo Volumes: Martin Marietta's Aft Cargo Carrier (1982)

Rube Goldberg's Space Shuttle

By mid-1971, this was one of the two leading Space Shuttle design configurations. The first stage, bearing the letters "USA" and a single stabilizing oversized tail fin, might have been derived from the Saturn V S-IC first stage. Image credit: NASA.

For Americans above a certain age, the phrase "Rube Goldberg Machine" elicits a chuckle or perhaps a sneer, depending on the context of its use. Rube Goldberg (1883-1970) was an award-winning cartoonist. His most famous drawings were of whimsical machines that accomplished a simple task in the most complex way possible.

It is not too unkind, given that most of the factors that led to its complexity were outside of NASA's control, to place the Space Shuttle in the category of a Rube Goldberg Machine. It began as a simple idea — economically deliver crews, supplies, and equipment to an Earth-orbiting Space Station — and, through conflicting, expanding demands placed on it, unwise cuts in funding for its development, and deferral of the Space Station it was meant to serve, grew into something large, complex, and costly.

Throughout the Space Shuttle design process, NASA fought a rearguard action to preserve reusability. In 1969, the U.S. civilian space agency sought a fully reusable Shuttle design with a piloted Booster and a piloted Orbiter, each carrying liquid propellants for placing the Orbiter into Earth orbit. Inadequate funding support from the Nixon White House and Congress coupled with a U.S. Air Force requirement that the Orbiter include a payload bay at least 60 feet long and 15 feet wide soon made that design untenable, however.

NASA and its contractor teams took a rapid series of cost-cutting steps during 1970-1972. The design process became messy and almost untrackable, with concepts proposed, abandoned, and proposed again in rapid succession or even simultaneously by different contractor and NASA teams.

The piloted Booster shrank after engineers tacked a pair of reusable solid-propellant rocket motors onto its tail. Then it ceased to be piloted, becoming part of what amounted to a three-stage rocket. Riding bolted to the top or side of the Booster's expendable second stage, the piloted Orbiter became in effect a reusable third stage that would complete its climb to Earth orbit by burning liquid hydrogen (LH2) fuel and liquid oxygen (LOX) oxidizer carried in tanks inside its streamlined fuselage.

In part to prevent the Orbiter from growing out of all proportion as its payload bay grew, NASA moved low-density LH2 out of the Orbiter fuselage into cheap expendable drop tanks. The move also ended worries about safe containment within the Orbiter of volatile LH2, which is prone to slow seepage even through solid metal.

The Orbiter carried LOX for its ascent to orbit inside its fuselage for a little while longer. By August 1971, however, the delta-winged Orbiter contained only enough propellants to maneuver in orbit and to slow itself so that it could deorbit and reenter Earth's atmosphere. At first, its orbital maneuvering engines were expected to burn LH2/LOX, but then NASA substituted hypergolic (ignite-on-contact) propellants.

During the same period, the preferred Shuttle stack design flip-flopped between two candidates. One (image at top of post) had two LH2/LOX stages stacked one atop the other. The first-stage engines were mounted directly beneath their stage, as on a conventional rocket. The engines for the second stage were built into the tail of the Orbiter mounted on its side. They would ignite at altitude after the first stage separated and, owing to their position on the side of the second stage, would thrust off center.

The first stage would be reusable; after depleting its propellants and separating from the second stage, it would deploy parachutes and lower to a gentle landing at sea, where it would bob with its engines pointed at the sky. A specially designed ship would then recover it and tow it to port for refurbishment. The second stage would reach orbit attached to the Orbiter, then would separate, reenter, and break up over the ocean.

The other candidate design (image below) featured a reusable Orbiter and a pair of reusable LH2/LOX boosters mounted on the sides of a single large expendable External Tank (ET). The lightweight ET's interior would be split between a small tank for LOX and a large one for LH2. Both the twin boosters and the tail-mounted Orbiter engines would ignite on the launch pad. The side-mounted boosters would expend their propellants and fall away about two minutes after liftoff. They would each deploy parachutes and descend to a gentle ocean landing to await recovery. Pipes leading from the ET tanks would feed propellants to the Orbiter's engine cluster throughout ascent to orbit.

That looks familiar: the other Space Shuttle stack design leading the pack by mid-1971. Note off-center thrust plumes from the delta-winged Orbiter's tail-mounted engines (lower left). Image credit: NASA.

In a final cost-cutting move, NASA replaced the reusable liquid-propellant boosters with reusable solid-propellant boosters. The liquid-propellant boosters could be turned off in the event of a major malfunction; the solid-propellant boosters could not.

Mounting engines on the reusable Orbiter meant that they would be returned to Earth for refurbishment and reuse. The resulting off-center thrust troubled many engineers, however, because it meant that thrust forces would be transmitted through the Orbiter to the second stage (in the case of the first Shuttle design alternative) or the ET (in the case of the second). This would place added stress on the Orbiter, its links to the second stage or ET, and the second stage or ET. Links between the second stage/ET and the Orbiter would include propellant pipe connections, which engineers expected would be prone to leaks even without the added stress of off-center thrust.

Off-center thrust also meant that the short LOX tank, when full the heaviest part of the second stage or ET, had to be situated atop the long LH2 tank, the lightest part of the second stage or ET. Putting the dense LOX on top helped the Shuttle stack to remain stable in flight as the Orbiter's engines rapidly emptied the second stage or ET and the stack's center of gravity shifted, but it also placed added stress on the second stage or ET structure. Because the LOX at the top of the second stage/ET needed a long pipe to reach the engines on the Orbiter's tail, the arrangement also increased the risk of propellant pipe rupture.

During the 1970-1972 Shuttle design evolution, several engineers proposed and re-proposed a novel alternative to off-center thrust: a cluster of reusable engines that would operate attached to the bottom of the expendable second stage or ET. After the Orbiter reached Earth orbit and its main engines shut down, the engine cluster would be detached from the second stage or ET and, using an armature system of booms or struts, be swung into a storage compartment inside the aft end of the Orbiter fuselage.

The second stage or ET would then be cast off. In the case of the ET, vented residual propellants would cause it to tumble, rapidly reenter the atmosphere, and break up. When the astronauts on board the Orbiter completed their mission in Earth orbit, the engine cluster would return to Earth with them, where it would be removed from the compartment, refurbished, and mounted on a new second stage or ET.

The NASA Manned Spacecraft Center — renamed the Lyndon B. Johnson Space Center (JSC) in February 1973 — managed Space Shuttle development. Shuttle engineers were quick to reject the swing-engine design. They did this mainly because its armature system seemed overly complex and thus prone to malfunctions.

The Rube Goldbergian swing-engine concept would not die, however. In March 1974, in fact, JSC chief of engineering Maxime Faget (co-designer of the Mercury capsule and a 1969 all-reusable Shuttle) and JSC engineers William Petynia and Willard Taub filed an application to patent the swing-engine design. By then, the decision to settle on the second stack configuration described above was two years old (NASA Administrator James Fletcher announced the choice on 16 March 1972).

The JSC engineers proposed three swing-engine design approaches. The U.S. Patent Office granted their patent on 30 December 1975.

All of their design approaches would, they argued, eliminate stress on the Shuttle stack caused by off-center thrust, enable transposition of the ET LOX and LH2 tanks, and improve stack flight characteristics during ascent through Earth's atmosphere. The results would, they explained, include a lighter Orbiter and ET, more payload, and greater safety.

As a bonus, the swing-engine system would enable the Orbiter to adjust its center of gravity after it released or took on an orbital payload, thus improving its reentry and atmospheric gliding flight characteristics. It would do this by shifting the engine cluster forward toward the back of the Orbiter payload bay using the same mechanical armature system that would swing the engines away from the bottom of the ET. The armature system would also serve to gimbal (swivel) the engines to steer the Orbiter/ET stack during ascent to orbit.

Other benefits would spring from the swing-engine design. The ET and engine cluster could be tested together without an Orbiter attached. All piping linking the Orbiter and the ET would be eliminated. Separable links between the ET and the engine cluster would be required, of course. The engine cluster would, however, be quite small and light compared to the Orbiter; this meant that it could be easily mounted on the ET, tested for leaks, and (if necessary) removed and repaired before flight.

First method for transferring engine cluster from aft end of the ET to storage in the Orbiter aft fuselage. 1 = ET; 2A = mounting ring for four engines (in thrust position on ET); 2B = mounting ring for four engines (in stored position in Orbiter aft fuselage); 3 = joint linking lower armature to engine ring (1 of 2); 4 = lower armature strut (1 of 2); 5 = upper armature strut (1 of 2); 6 = joint linking upper armature to Orbiter aft fuselage (1 of 2); 7 = trailing edge of wing (1 of 2); 8 = opening in aft fuselage for engine cluster storage; 9 = solid-propellant ascent abort rocket (1 of 2); 10 = vertical stabilizer. Image credit: NASA/U.S. Patent Office.

The JSC engineers' first swing-engine design, illustrated above, assumed a quartet of Shuttle engines, a single vertical stabilizer, and an aft-pointing fuselage opening. The armature system would swing the engines into the fuselage so that their engine bells pointed aft.

The second design, illustrated below, assumed three Space Shuttle engines in a vertical row and an Orbiter with twin out-splayed vertical stabilizer fins. The armature system would swing the engines up and over the aft end of the Orbiter fuselage and lower them into a rectangular slot between the fins. After a horizontal landing on Earth, their engine bells would point skyward.

Second method for transferring the Space Shuttle engine cluster from the aft end of the ET to the storage space in the Orbiter aft fuselage. 1 = Orbiter payload bay; 2 = LOX tank in aft end of ET; 3 = ET; 4 = vertical stabilizer (1 of 2); 5A = engine cluster in thrust position on aft end of External Tank; 5B = engine cluster in stowed position in Orbiter aft fuselage; 6A = centerline of engine cluster in thrust position; 6B = centerline of engine cluster in stowed position; 7A = armature strut for transferring engine cluster (thrust position) (1 of 2); 7B = armature strut for transferring engine cluster (stowed position) (1 of 2); 8 = center armature joint (1 of 2); 9 = path of center armature joint (8) during engine cluster transfer to stowed position. Image credit: NASA/U.S. Patent Office.

The JSC engineers' third swing-engine design also assumed three engines arranged in a vertical row, but could be used with either single or double vertical stabilizer Orbiter configurations. The armature system would tilt the engine cluster 45° and slide it on rails into a rear-facing opening in the aft fuselage. As with their second design, the engine bells would point upward after the Orbiter glided to a landing.

Orbital Flight Test-1 (OFT-1), also known as Space Transportation System-1 (STS-1), the first flight of the Space Shuttle. Columbia lifted off from Launch Complex 39A at Kennedy Space Center, Florida, on 12 April 1981, and landed at Edwards Air Force Base, California, two days later. Veteran astronaut John Young was Commander and rookie Robert Crippen was Pilot. Image credit: NASA.

The swing-engine concept had, of course, become a mere curiosity well before the U.S. Patent Office granted Faget, Petynia, and Taub their December 1975 patent. Following the March 1972 selection of the Shuttle stack configuration, NASA awarded Rockwell International the contract to build Space Shuttle Orbiters on 26 July 1972. The company built a total of five space-worthy Orbiters, each with three Space Shuttle Main Engines mounted in a triangle on their aft fuselages, over a span of more than 20 years.

The Orbiters functioned admirably, though they needed far more costly refurbishment and maintenance than NASA envisioned when it proposed its all-reusable Space Shuttle design in 1968-1969. Booster system malfunctions claimed two Orbiters and their seven-person crews, however. Challenger was destroyed on 28 January 1986 when a solid-propellant booster field joint burned through, leading to ET structural failure and Orbiter break-up 73 seconds after launch. Columbia, the first Orbiter to orbit Earth (12-14 April 1981), was lost after foam insulation on the ET it rode broke loose during ascent and struck and damaged its wing leading edge. This led to wing structural failure and Orbiter breakup during reentry on 1 February 2003, at the end of a 16-day mission.

Sources

Patent No. 3,929,306. Space Vehicle System, Maxime A. Faget, William W. Petynia, and Willard M. Taub, NASA Johnson Space Center, 5 March 1974 (filed), 30 December 1975 (granted).

Space Shuttle: The History of the National Space Transportation System, the First 100 Missions, Dennis R. Jenkins, 3rd Edition, 2008.

Wikipedia: Rube Goldberg Machine (https://en.wikipedia.org/wiki/Rube_Goldberg_machine — accessed 28 November 2016)

More Information

George Landwehr von Pragenau's Quest for a Stronger, Safer, Better Space Shuttle

Series Development: A 1969 Plan to Merge Saturn V and Shuttle to Spread Out Space Program Cost

One Space Shuttle, Two Cargo Volumes: Martin Marietta's Aft Cargo Carrier (1982)

George Landwehr von Pragenau's Quest for a Stronger, Safer Space Shuttle

The Space Shuttle Challenger and its booster system moments before they were destroyed. The plume of flame emerging from the side of its malfunctioning SRB is clearly visible. Image credit: NASA.

The Space Shuttle Orbiter Challenger was minding its own business on 28 January 1986, working hard to get its seven-member crew and its large satellite payload to low-Earth orbit, when its booster stack betrayed it and everything began to go badly wrong. First, hot gas within its right Solid Rocket Booster (SRB) began to burn through a seal meant to contain it. Soon, a fiery plume gushed from the side of the SRB, robbing it of thrust, and reached out menacingly toward the side of the brown External Tank (ET) and the strut linking the lower end of the SRB to the ET (image at top of post). The plume broke though the ET's foam insulation and aluminum skin, then the strut pulled free of the weakened ET.

Challenger fought back as the ET began to leak liquid hydrogen fuel. It swiveled (the aerospace term is "gimballed") the three Space Shuttle Main Engines (SSMEs) in its tail as it struggled to stay on course. The plume from the SRB, meanwhile, glowed brighter as it began to burn hydrogen leaking from the ET. At the same time, the SRB began to rotate around the single strut left holding it to the ET. That strut was located not far from the Orbiter's gray nose, near the conical top of the errant SRB.

Throughout these events, Challenger's last crew remained oblivious to the technological drama taking place around them. This was just as well, since they had no way to escape what was about to happen to them.

When Challenger at last lost its struggle against its own booster stack, significant events were separated by tenths or hundredths of seconds. Immediately after the right SRB's lower strut came free, the entire Shuttle stack lurched right. Mike Smith, in Challenger's pilot seat, had time for a startled "Uh-oh" less than a second after the lurch. The ET's dome-shaped bottom then fell away, freeing all the hydrogen fuel it contained. The right SRB's pointed nose slammed into and crushed the top of the ET, freeing liquid oxygen oxidizer. The escaped hydrogen blossomed into a fireball that encompassed Orbiter, rapidly disintegrating ET, and SRBs.

Yet the Orbiter Challenger did not explode. Instead, it broke free of what was left of the ET and began a tumble. The aerodynamic pressures the Orbiter experienced as its nose pointed away from its direction of flight were more than sufficient to snap it into several large pieces: the crew cabin, the satellite payload, the wings, and the SSME cluster emerged from the fireball more or less intact. The SRBs, still firing, flew out of the fireball, tracing random trails across the blue Florida sky until a range safety officer commanded them to self-destruct. The Orbiter's wreckage, meanwhile, plummeted into the Atlantic within sight of the Florida coast.

NASA recovered the bodies of the crew and portions of the wreckage, including the section of the right SRB that had leaked hot gas. The wreckage was turned over to accident investigators.

This 1975 NASA illustration depicts the basic components of the Space Shuttle system. The Orbiter includes three Space Shuttle Main Engines (left). Two Solid Rocket Boosters, one of which is mostly hidden behind the External Tank, provide thrust during liftoff and the early part of ascent. The tank includes (from right to left) a small tank for dense liquid oxygen, a drum-shaped structural support ring/tank separator below the Orbiter's nose, and a large tank for low-density liquid hydrogen.

During a Shuttle launch, the three SSMEs ignited first. This caused the twin SRBs, the bases of which were mounted to the launch pad by explosive bolts, to flex along their entire length away from the SSMEs, then straighten out again just as they ignited. O-ring seals between the cylindrical segments making up the SRBs often became unseated during flexure, then had to reseat to contain hot gases after SRB ignition. Accident investigators concluded that failure of one of those seals doomed Challenger. Even more damning, they found that partial seal failures followed by hot exhaust leaks had occurred on pre-Challenger flights — and had been disregarded by NASA managers.

After Challenger, NASA and its contractors redesigned the SRB joints and seals, added crew pressure suits and a limited crew escape capability, and banned potentially unsafe practices and payloads from Shuttle missions. Yet the U.S. civilian space agency might have gone much farther when it sought to enhance Space Shuttle safety after Challenger.

Even before the accident, NASA had at its disposal redesign proposals that could have made the Shuttle stack stronger and safer. In 1982, for example, George Landwehr von Pragenau, a veteran engineer at NASA's Marshall Space Flight Center, filed a patent application — granted in 1984 — for a Shuttle stack design that would have made the Challenger accident impossible.

Born and educated in Austria, von Pragenau joined the von Braun rocket team in Huntsville, Alabama, in 1957. He became a U.S. citizen in 1963. He specialized in rocket stability and flight effects on rocket behavior. He had, for example, been part of the team that found the cause of the "pogo" oscillations that crippled Apollo 6, the second unmanned Saturn V-launched Apollo test mission (4 April 1968).

In the conventional Shuttle stack, von Pragenau explained, SRB thrust was transmitted through the forward SRB attachment points to a reinforced intertank ring between the ET's top-mounted liquid oxygen tank and its liquid hydrogen tank.  He considered this "indirect routing" of thrust loads to be perilously complex. SSME thrust loads, for their part, passed through the Orbiter to its twin aft ET attachment points on the large, fragile liquid hydrogen tank.

By the time he filed his 1982 patent application, von Pragenau had spent almost a decade thinking about how the Shuttle stack might be rearranged to reduce weight and aerodynamic drag, increase stability, simplify thrust paths, and provide greater structural strength. His 1984 patent was, in fact, not his first aimed at Shuttle improvement.

Von Pragenau's 1974 alternative Shuttle stack. Image credit: U.S. Patent Office.

In 1974, von Pragenau had filed a patent — granted the following year — in which he proposed a more slender, more vertically oriented Shuttle stack; that is, one that would mimic conventional rocket designs in which stages are stacked one atop the other. He linked the twin SRBs side by side. Moving the tank for dense liquid oxygen from the ET's nose to its tail placed its concentrated mass nearer the base of the stack, improving in-flight stability. He then mounted the SRBs to the Orbiter's belly and perched the ET atop the SRB/Orbiter combination. SRB and SSME thrust loads were conveyed through struts to meet at the ET's flat, reinforced base.

Von Pragenau's 1982 Shuttle stack design was in some ways a less radical departure from the existing Shuttle design than was his 1974 design. He left the SRBs, ET, and Orbiter in their normal positions relative to each other, but made other significant changes. As in his 1975 patent, he moved the liquid oxygen tank from the ET's nose to its tail and brought the SRBs closer together to improve stability. The liquid oxygen tank became skinny, cylindrical, and almost as long as the Orbiter and SRBs attached to it. The liquid hydrogen tank, fat with low-density fuel, von Pragenau mounted atop the oxygen tank, partially overhanging the Orbiter and SRBs.

Von Pragenau's 1982 Shuttle stack redesign. The numeral "15" points to the rigid thrust structure framework. "34," "35," "36" are Solid Rocket Booster attachment fixtures. These would link to slide rails ("31" and "32") that would run the length of the liquid oxygen tank ("20"). "19" is the liquid hydrogen tank. Image credit: U.S. Patent Office. 

Von Pragenau could not tolerate flexing SRBs. He proposed to mount a slide rail on either side of the liquid oxygen tank. Three attachment fixtures on each SRB would link to the slide rails, helping to ensure rigidity. When the SRBs depleted their propellant, pyrotechnic bolts would fire, freeing them to slide backwards down the rails and fall neatly away from the Orbiter/ET stack.

The most important feature of von Pragenau's redesign was a rigid framework – a thrust structure – that would link the bottom of the SRBs just above their rocket nozzles. In addition to holding the SRBs rigidly in place, the thrust structure would transmit SRB thrust loads to the bottom of the ET oxygen tank, which would rest atop the center of the thrust structure. When the spent SRBs slid away from the Orbiter/ET stack, they would take the thrust structure with them.

Side view of Von Pragenau's 1982 Shuttle stack concept. Image credit: U.S. Patent Office.

Von Pragenau's concepts apparently exerted little influence on NASA's post-Challenger recovery effort. A likely explanation is that neither of his proposals — if they were known to decision-makers at all — was deemed affordable. In addition to extensive changes in manufacturing tooling, both proposals would have required modifications to the Vehicle Assembly Building, the twin Complex 39 Shuttle pads at Kennedy Space Center (KSC), and even the barge that delivers ETs to KSC. Instead of beefing up the existing Shuttle, NASA studied designs for new shuttles which, for lack of funding, remained firmly in the low-cost realm of CAD drawings, conference papers, and conceptual artwork.

On 1 February 2003, the Space Shuttle claimed another crew. The oldest Orbiter, Columbia, was heavier than her sisters Atlantis, Discovery, and Endeavour, which limited the amount of cargo she could deliver to the International Space Station (ISS). For this reason, NASA largely relegated to Columbia the few remaining non-ISS missions — for example, Hubble Space Telescope servicing.

As they began Earth-atmosphere reentry at 8:44 a.m. Eastern Standard Time after a nearly 16-day life sciences mission, the seven STS-107 astronauts on board Columbia were unaware that, during ascent, a piece of ice-impregnated insulating foam nearly a meter wide had broken free from the ET and impacted their spacecraft's left wing. Ice and foam had broken free from ETs before, but the damage they caused was, after cursory examination, deemed acceptable by Shuttle Program managers. This time, however, the impact opened a hole up to 10 inches wide in the Orbiter's left wing leading edge.

Hot plasma generated during reentry entered the hole and began to destroy Columbia's left wing from the inside out. Observers along the Orbiter's flight path, which cut across the southern tier of U.S. states, reported unusual flashes. Meanwhile, members of the STS-107 crew on Columbia's Flight Deck observed and recorded on video flashes visible outside their windows. In the recovered video, the astronauts appear to realize that the flashes were unusual but show no signs of panic.

Much as Challenger had before it, Columbia fought bravely against the forces destroying it. Onboard computers took account of increased drag on the left side of the Orbiter and sought to compensate to keep it on the flight path. At 8:59 a.m. Eastern Standard Time, however, Columbia tumbled and disintegrated over northeast Texas.

Both of von Pragenau's design concepts placed all or part of the ET above the Orbiter, so one might argue that they would not have prevented a failure resembling that which killed the STS-107 crew. On the other hand, one can be forgiven for speculating that a U.S. civilian space agency provided with the means after Challenger to rebuild the Shuttle system to make it safer might also have evolved an organizational culture more prone to investigating and less prone to tolerating recurring flight anomalies.

Von Pragenau retired from NASA in 1991 after more than 30 years of service. He remained involved in engineering efforts at NASA Marshall Space Flight Center during his retirement. He died two years after the Space Shuttle's final flight (STS-135, 8-24 July 2011), on 11 July 2013, at the age of 86.

Sources

Patent No. 4,452,412, Space Shuttle with Rail System and Aft Thrust Structure Securing Solid Rocket Boosters to External Tank, George L. von Pragenau, NASA Marshall Space Flight Center, 15 September 1982 (filed), 5 June 1984 (granted).

Patent No. 3,866,863, Space Vehicle, George L. von Pragenau, NASA Marshall Space Flight Center, 21 March 1974 (filed), 18 February 1975 (granted).

Hampton Cove Funeral Home Obituaries: George Landwehr von Pragenau (http://www.hamptoncovefuneralhome.com/fh/obituaries/obituary.cfm?o_id=2150841&fh_id=13813 — accessed 27 October 2016).

NASA History: Columbia Accident Investigation Board (http://history.nasa.gov/columbia/CAIB.html — accessed 29 October 2016).

NASA History: Challenger STS 51-L Accident (https://www.hq.nasa.gov/office/pao/History/sts51l.html — accessed 29 October 2016).

More Information

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What If Galileo Had Fallen to Earth? (1988)