Space Shuttle

Vehicle Configurations

Vehicle Components

NASA's Space Shuttle has been the U.S. human crewed spaceflight flagship for two and a half decades.  Although shuttle has recorded many achievements, history may recall its tragic failures more than its successes.

Development

After years of study, authorized development of a reusable winged orbiter-based Space Transportation System (STS) began in 1972. Post-Apollo funding was tight, so NASA agreed to a number of compromises in order to build STS. Foremost among the compromises was a plan to use shuttle to launch all U.S. civil and defense payloads, replacing all exisiting U.S. expendable launch vehicles. Early plans called for two orbiters to be dedicated to U.S. Air Force launches from Vandenberg AFB, California. NASA would use three additional orbiters to launch scientific and commercial satellites from Kennedy Space Center, Florida. Mission models called for as many as 50 shuttle flights per year.

The compromises allowed the Pentagon to support development of STS, but also drove NASA toward unwanted design elements. These included a shuttle that was larger than desired, an orbiter that was made out of aluminum rather than titanium (requiring use of a labor intensive and delicate thermal protection system), and a high-cross range orbiter that required heavy delta wings and a long reentry phase.

But even before these compromises, tight funding had forced the use of an expendable external propellant tank instead of orbiter-integral tanks, the use of partially-reusable solid rocket boosters (SRBs) instead of a fully-reusable fly-back booster, and the elimination of jet engines for landing. Cost and mass considerations had also driven NASA to drop crew launch escape systems. The final design featured a reusable orbiter powered by LOX/LH2 engines fed from an external tank that burned in parallel with two SRBs during the first two minutes of flight.

Orbiter

Early orbiter concepts projected use of upgraded Saturn J-2 LOX/LH2 engines that were given the J-2S designation. This would have allowed NASA to benefit from its massive Apollo/Saturn investments, but heavier payload requirements forced costly development of a new high-pressure, staged-combustion Space Shuttle Main Engine (SSME). SSME development would be a shuttle program pacing item.

Each SSME - there are three on each orbiter - weighs only 3.4 metric tons, but can produce up to 232 tons of thrust in vacuum. The engines have a 452 second specific impulse and can be throttled in a range from 67 to 109 percent of rated thrust. Initial requirements called for engines to be rated for 55 starts, but turbopump problems reduced that to 30 starts.

SSME development was troublesome.  A complete static firing with three non-flight engines mounted to Main Propulsion Test Article (MPTA)-098 on a Mississippi test stand was not performed until December 17, 1979. A successful certification test of three flight-rated engines did not take place until January 17, 1981.

The orbiter thermal protection system also presented a development challenge.  Thousands of lightweight thermal protection (TPS) tiles had to be bonded to the orbiter - actually to a felt strain isolation pad that was bonded to the orbiter skin - with adhesives in a labor-intensive process.  The adhesive process had to be modified when engineers discovered that the TPS bonding system did not provide enough strength. Columbia was delivered to KSC with many tiles yet to be installed, and some tiles were damaged during transit atop the 747 Shuttle Carrier Aircraft (SCA).   TPS post-flight maintenance subsequently proved to be a costly labor effort when it was discovered that many tiles were typically damaged during ascent by particles shed from the external tank.      

North American Rockwell built two test articles and six orbiters in Palmdale, California. Original plans had called for Approach and Landing Test (ALT) article OV-101 Enterprise to be converted into a space orbiter, but NASA decided to convert Structural Test Article STA-099 instead. It became OV-099 Challenger.

Enterprise was used in the Approach and Landing Test program at Dryden Flight Research Facility in 1977, where it demonstrated that an orbiter could be carried on the modified 747 shuttle carrier aircraft and that it could glide to a runway landing.  In 1978,Enterprise was stacked for dynamic testing with an external tank and solid rocket boosters at Marshall Space Flight Center.  In 1979, Enterprise was stacked on a mobile launch platform at Kennedy Space Center and rolled out to Pad 39A for facility testing.  In 1984-85, it was stacked on the shuttle launch pad at Vandenberg Air Force Base, California.

Funding issues forced the planned five-orbiter fleet to be scaled back to four, but large "structural spare" fuselage pieces were completed and stored.  These spares were used to build Endeavour as a replacement for Challenger after the latter was destroyed in a launch failure on January 28, 1986. 

Columbia's reentry destruction in 2003 did not result in a replacement orbiter.  Instead, the failure inititated plans to end the shuttle program as early as 2010, at least 10 years earlier than prior plans.

The first Space Shuttle orbiter, Columbia, weighed 82.4 metric tons empty. Subsequent orbiters weighed about 80 tons.

Marshall Space Flight Center built an additional orbiter mass simulator named "Pathfinder" that was used for facility fit checks. Orbiters included the following.

External Tank

The external tank (ET) consists of a lower liquid hydrogen (LH2) tank, an intertank section, and an upper liquid oxygen (LOX) tank.  The orbiter is side-mounted to the ET.  Liquid hydrogen fuel and liquid oxygen oxidizer feed from the tank through disconnects, located on the aft bottom side of the orbiter, to the SSMEs in the orbiter's propulsion section.  The ET is expendable.  After the main ascent burn, it is jettisoned to fall into the Indian or Pacific Ocean.

Martin Marietta (later Lockheed Martin) built external tanks in Michoud, Louisiana. The external tank design progressed from the 35 ton (empty weight) Standard Weight original (1981) to a 29.5 ton Light-Weight tank (1983) to the final 26.1 ton Super Light Weight tank (1998) made out of Aluminum Lithium alloy.  An external spray-on foam insulation was applied to minimize cryogenic propellant boil-off and to prevent ice buildup.

The first external tank was the Main Propulsion Test Article External Tank (MPTA-ET).  It was mated to MPTA-098 for static engine tests at what is now Stennis Space Center.  MPTA-ET is now located at the Marshall Space Flight Center (MSFC) Alabama Space and Rocket Center where it is mated with the Pathfinder orbiter simulator.

Solid Rocket Booster

Two solid rocket boosters (SRBs), mounted on each side of the ET, provide the primary thrust during the first two minutes of flight.Morton Thiokol builds and casts SRB segments in Utah. 

The SRBs are the largest solid-propellant motors ever flown.  They are also the first designed to be recovered and reused.  Each SRB consists of four motor segments topped by a frustrum/cone section with recovery equipment and an aft skirt with a movable nozzle that provides thrust vector control.  A drogue and three 41 meter diameter main parachutes lower each SRB to the ocean about 225 km downrange for recovery by specially designed ships.

SRB propellants include a high density ammonium perchlorate (AP) oxidizer that is embedded in a rubber fuel binder (PBAN or HTPB). Aluminum (Al) fuel particles are also in the binder.  The solid fuel motor typically consists, by weight, of 70% AP, 16% Al, and 14% binder. The solid fuel is shaped to provide a varying thrust profile.  The SRBs "throttle down" during the period of maximum dynamic pressure (Max-Q) on the vehicle.

Upper Stages

Original plans called for development of a "space tug" upper stage to deploy satellites from the orbiter payload bay.  When funding problems delayed those plans, NASA and the Air Force initiated development of an "Interim Upper Stage" (IUS).  The stage was later renamed "Inertial Upper Stage" when funding for space tug development failed to appear. 

IUS was a two-stage solid motor vehicle capable of boosting 2.27 metric ton payloads into geosynchronous orbit from a space shuttle. IUS was also designed to be compatible with Titan 34D, and later Titan IV, expendable launch vehicles.  After the Challenger failure, use of IUS aboard shuttle was gradually phased out.

The solid fuel Payload Assist Module (PAM-D) and its more power PAM-D2 cousin were additional upper stages adapted for use by STS.  They were used to boost Delta-class payloads from the Space Shuttle.  PAM-D also served as an upper stage on Delta expendable launch vehicles. PAM-D could handle 1.27 metric ton GEO payloads.  PAM stopped flying on shuttle after the Challenger accident.

A powerful wide-body (4.3 meter diameter) LOX/LH2 cryogenic "Centaur G-prime" upper stage and a smaller "Centaur-G" variant were also developed for shuttle, to handle heavy payloads and higher energy deep space missions such as the Galileo probe to Jupiter and the European Solar-Polar mission.  Two Centaur G-prime flight stages had begun mission integration testing at KSC and Cape Canaveral when the Challenger accident forced reconsideration of the idea.   Ultimately, Shuttle Centaur was cancelled, but development of the longer "G-prime" version (sometimes confusingly called Centaur G but more correctly identified as Centaur T or "Titan/Centaur") was continued by the Air Force for use on Titan IV.

Launch Sites 

Kennedy Space Center Complex 39, NASA's Saturn-Apollo launch site, was modified to launch space shuttle.  Two of the Vertical Assembly Building (VAB) high bays, both launch pads, and all three mobile launch platforms were converted for use. Portions of two of the original three Saturn V mobile launch towers were used to build launch umbilical towers at Pads 39A and 39B.  New rotating service structures were added to replace the massive Saturn V mobile service tower, which was disassembled.  The rotating structure allowed payloads to be inserted into the orbiter payload bay at the pad, rather than in the VAB.

A west coast space shuttle launch site was completed at Vandenberg AFB Space Launch Complex 6 (SLC-6, or "Slick Six").  SLC-6 had originally been built to handle Air Force Titan 3M launches for the Manned Orbiting Laboratory program that was cancelled during the 1960s.  The site consisted of a fixed pad encased by three large mobile structures that opened clamshell-like for launches.  Enterprise was stacked on SLC-6 in 1984-5 for facility testing, but the site was mothballed after the 1986 Challenger failure.

Flight History

NASA astronauts John Young and Bob Crippen were aboard Columbia for the first space shuttle launch on April 12, 1981. It was the first of four two-man research and development flights. Columbia also performed the first operational mission (STS-5) in 1982 with four crewmembers.  During that mission, two commercial communications satellites, ANIK C-3 for TELESAT Canada and SBS-C for Satellite Business Systems, were deployed to be boosted into geosynchrounous transfer orbit by their solid propellant Payload Assist Module-D (PAM-D) motors. 

Challenger flew for the first time in 1983 on the STS-6 mission, which deployed Tracking and Data Relay Satellite-1 (TDRS-1).  The payload's IUS stage failed, leaving TDRS-1 in an improper elliptical orbit.  TDRS-1 subsequently used station-keeping propellant to reach GEO, albeit with a shortened lifetime. 

Upper stage problems troubled the early shuttle program.  Four of the thirty-seven satellites deployed by STS by early 1986 were left in improper orbits. Two commercial satellites, Palapa-B2 and Westar-VI, were stranded in low earth orbit during shuttle Challenger's STS-41B mission in 1984 when both satellite's PAM-D motors failed to ignite.  The satellites were recovered during a later shuttle mission (STS-51A in November 1984) and returned to earth for reuse.  Leasat-2 suffered a similar problem with its built-in propulsion system during the STS-41D mission in June 1984. NASA astronauts performed a remarkable on-orbit repair of Leasat-2 about one year later.

During the first 24 missions, Discovery and Atlantis joined the four orbiter fleet, but technical problems limited the launch rate to a maximum of nine in 1985. Two pad aborts and one abort to orbit were caused by SSME problems. There were two DoD and four SpaceLab missions. A half-dozen women astronauts flew aboard shuttle, but so did two U.S. members of Congress.

On January 28, 1986, during the 25th launch, shuttle Challenger and crew were lost in a failure caused by a failed SRB segment joint. Analysis determined that the joint design, which engineers already thought needed improvement, was susceptible to failure at low temperatures. Challenger was launched on an unusually cold Florida day over the objections of some Thiokol engineers.

After Challenger, President Reagan instructed NASA to stop launching commercial satellites on STS.

For a number of years, shuttle was kept busy launching civil space satellites like TDRS, Magellan, Galileo, Ulysses, and the Hubble Space Telescope, along with SpaceLab and DoD missions. Endeavour joined the fleet in 1992.

In 1995, the first of nine Shuttle/Mir missions took place, as the U.S. and Russia initiated cooperative space station efforts. Space shuttle missions to the new International Space Station began in 1998. Gradually, shuttle became almost totally dedicated to ISS. As the annual flight rate declined, the per-mission costs soared to one-half billion dollars or more.

Columbia, too heavy for high inclination ISS missions, continued SpaceLab missions in non-ISS orbits. The first reusable spaceplane was completing one such mission, on the 113th shuttle flight, when it disintegrated during reentry on February 2, 2003, resulting in seven more astronaut deaths. The failure was the result of ET foam insulation ripping away from the tank, striking and fracturing reinforced carbon composite pieces of the thermal protection system on the left wing's leading edge. The possibility that this particular failure mode could occur was not understood by NASA during the first two decades of the shuttle program.

No new orbiters were built to replace Columbia. Instead, the disaster motivated decision makers to plan for an early end to the shuttle program. As of early 2005, planning called for shuttle to fly about 20 more missions to complete assembly of ISS by about 2010. After that, shuttle would be retired while NASA's human spaceflight program transitioned to a new ballistic entry-type spacecraft.

Vehicle Configurations

Vehicle Components

Vehicle Components Cont'd

STS Launch Sequence (Typical)

References

Shuttle Reference Manual, NASA, 1988

Space Shuttle, Dennis R. Jenkins, 2001

 Last Update:  October 30, 2005