****Report of the PRESIDENTIAL COMMISSIONon the Space Shuttle Challenger Accident****
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****Chapter I: Introduction****
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[**2**] The Space Shuttleconcept had its genesis in the 1960s, when the Apollo lunar landingspacecraft was in full development but had not yet flown. From theearliest days of the space program, it seemed logical that the goalof frequent, economical access to space might best be served by areusable launch system. In February, 1967, the President's ScienceAdvisory Committee lent weight to the idea of a reusable spacecraftby recommending that studies be made "of more economical ferryingsystems, presumably involving partial or total recovery anduse."

In September, 1969, two months after theinitial lunar landing, a Space Task Group chaired by the VicePresident offered a choice of three long-range plans:

- A $8-$10 billion per year program   involving a manned Mars expedition, a space station in lunar orbit   and a 50-person Earth-orbiting station serviced by a reusable   ferry, or Space Shuttle. 
- An intermediate program, costing less than   $8 billion annually, that would include the Mars mission. 
- A relatively modest $4-$5.7 billion a year   program that would embrace an Earth-orbiting space station and the   Space Shuttle as its link to Earth. 1 

In March, 197O, President Nixon made it clearthat, while he favored a continuing active space program, funding onthe order of Apollo was not in the cards. He opted for theshuttle-tended space base as a long-range goal but deferred goingahead with the space station pending development of the shuttlevehicle. Thus the reusable Space Shuttle, earlier considered only thetransport element of a broad, multi-objective space plan, became thefocus of NASA's near-term future.

** **

**The Space Shuttle Design**

The embryo Shuttle program faced a number ofevolutionary design changes before it would become a system in being.The first design was based on a "fly back" concept in which twostages, each manned, would fly back to a horizontal, airplane-likelanding. The first stage was a huge, winged, rocket-powered vehiclethat would carry the smaller second stage piggyback; the carrierwould provide the thrust for liftoff and flight through theatmosphere, then release its passenger-the orbiting vehicle-andreturn to Earth. The Orbiter, containing the crew and payload, wouldcontinue into space under its own rocket power, complete its missionand then fly back to Earth.

The second-stage craft, conceived prior to1970 as a space station ferry, was a vehicle considerably larger thanthe later Space Shuttle Orbiter. It carried its rocket propellantsinternally, had a flight deck sufficiently large to seat 12 spacestation-bound passengers and a cargo bay big enough to accommodatespace station modules. The Orbiter's size put enormous weight-liftingand thrust-generating demands on the first-stage design.

This two-stage, fully reusable designrepresented the optimum Space Shuttle in terms of «routine,economical access to space," the catchphrase that was becoming theprimary guideline for development of Earth-to-orbit systems. It was,[**3**]however, less than optimum in terms of the development investmentrequired: an estimated $10-13 billion, a figure that met withdisfavor in both Congress and the Office of Management andBudget.

In 1971, NASA went back to the drawing board,aware that development cost rather than system capability wouldprobably be the determining factor in getting a green light forShuttle development. Government and industry studies soughtdevelopmental economies in the configuration. One proposal foundacceptance: eliminate the Orbiter's internal tanks and carry thepropellant in a single, disposable External Tank. It provided asmaller r, cheaper Orbiter r without substantial performanceloss.

For the launch system, NASA examined a numberof possibilities. One was a winged but unmanned recoverableliquid-fuel vehicle based on the eminently successful Saturn 5 rocketfrom the Apollo Program. Other plans envisioned simpler but alsorecoverable liquid-fuel systems, expendable solid rockets and thereusable Solid Rocket Booster. NASA had been using solid-fuelvehicles for launching some small unmanned spacecraft, but solids asboosters for manned flight was a technology new to the agency.Mercury, Gemini and Apollo astronauts had all been rocketed intospace by liquid-fuel systems. Nonetheless, the recoverable SolidRocket Booster won the nod, even though the liquid rocket offeredpotentially lower operating costs.

 

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**Artist's drawing depicts Space Shuttlestacked for launch in view from dorsal side of Orbiter (left) andfrom the left side of the stack.**

 

[**4**] The overriding reasonwas that pricing estimates indicated a lower cost of development forthe solid booster.

Emerging from this round of design decisionmaking was the Space Shuttle: a three-element system composed of theOrbiter, an expendable external fuel tank carrying liquid propellantsfor the Orbiter's engines, and two recoverable Solid Rocket Boosters.It would cost, NASA estimated early in 1972, $6.2 billion to developand test a five-Orbiter Space Shuttle system, about half what thetwo-stage "fly back" design would have cost. To achieve thatreduction, NASA had to accept somewhat higher system operating costsand sacrifice full reusability. The compromise design retainedrecoverability and reuse of two of the three elements and stillpromised to trim substantially the cost of delivering payloads toorbit.

The final configuration was selected in March,1972.

** **

**The Space Shuttle Development**

In August, 1972, NASA awarded a contract toRockwell International Corporation's Space Transportation SystemsDivision for design and development of the Space Shuttle Orbiter.Martin Marietta Denver Aerospace was assigned development andfabrication of the External Tank, Morton Thiokol Corporation wasawarded the contract for the Solid Rocket Boosters, and Rocketdyne, adivision of Rockwell, was selected to develop the Orbiter mainengines.

NASA divided managerial responsibility for theprogram among three of its field centers. Johnson Space Center,Houston, Texas, was assigned management of the Orbiter. MarshallSpace Flight Center, Huntsville, Alabama, was made responsible forthe Orbiter's main engines, the External Tank and the Solid RocketBoosters. Kennedy Space Center, Merritt Island, Florida, was giventhe job of assembling the Space Shuttle components, checking them outand conducting launches. Because these three centers will bementioned repeatedly in this report, they will hereafter beidentified simply as Johnson, Marshall and Kennedy.

It was in an increasingly austere fiscalenvironment that NASA struggled through the Shuttle development yearsof the 1970s. The planned five-Orbiter fleet was reduced to four.Budgetary difficulties were compounded by engineering problems and,inevitably in a major new system whose development pushes thefrontiers of technology, there was cost growth. This combination offactors induced schedule slippage. The initial orbital test flightswere delayed by more than two years.

The first Shuttle test flights were conductedat Dryden Flight Research Facility, California, in 1977. The testcraft was the Orbiter Enterprise, a full-size vehicle that lackedengines and other systems needed for orbital flight. The purpose ofthese tests was to check out the aerodynamic and flight controlcharacteristics of the Orbiter in atmospheric flight. Mountedpiggyback atop a modified Boeing 747, the Enterprise was carried toaltitude and released for a gliding approach and landing at theMojave Desert test center. Five such flights were made. They servedto validate the Orbiter's computers and other systems. They alsodemonstrated the craft's subsonic handling qualities, in particularits performance in the precise unpowered landings that would berequired on all Shuttle flights.

The Enterprise test flights were followed-in1977-80-by extensive ground tests of Shuttle systems, includingvibration tests of the entire assembly-Orbiter, External Tank andSolid Rocket Boosters-at Marshall. Main engine test firings wereconducted at National Space Technology Laboratories at Bay St. Louis,Mississippi, and on the launch pad at Kennedy.

By early 1981, the Space Shuttle was ready foran orbital flight test program. This was carefully crafted to includemore than 1,000 tests and data collection procedures. All flightswere to be launched from Kennedy and terminate at Edwards Air ForceBase, where the Dryden Flight Research Facility is located (actuallythe third flight landed at White Sands Test Facility, New Mexico,because the normally dry lakebed at Edwards was flooded). Originallyintended as a six-mission program, the orbital test series wasreduced to four flights:

 

- STS-1 (Space Transportation System-1),   April 12-14, 1981, Orbiter Columbia, was a two-day demonstration   of the Orbiter's ability to go into orbit and return safely. Its   [ 5 ]   main payload was a flight instrumentation pallet containing   equipment for recording temperatures, pressures and acceleration   levels at various points around the Orbiter. In addition, there   were checkouts of the cargo bay doors, attitude control system and   orbital maneuvering system. 

 

- STS-2, November 12-14, 1981, Orbiter   Columbia, marked the first test of the Remote Manipulator System   and carried a payload of Earth survey instruments. This was the   first time any spacecraft had flown twice. Failure of a fuel cell   shortened the flight by about three days. 

 

- STS-3, March 22-3O, 1982, Orbiter   Columbia, was the longest of the initial test series, staying   aloft eight days. Activities included a special test of the   manipulator in which the robot arm removed a package of   instruments from the payload bay but did not release it into   space. The flight included experiments in materials   processing. 

 

- STS-4, June 27-July 4, 1982, Orbiter   Columbia, featured another test of the robot arm, which extended a   scientific payload over the side of the payload bay, then   reberthed it. Materials processing experiments were conducted, as   were a number of scientific investigations. This flight carried   the first Department of Defense payload. 

 

With the landing of STS-4, the orbital flighttest program came to an end with 95 percent of its objectivesaccomplished. The interval between flights had been trimmed fromseven months to four, then three. NASA declared the Space Shuttle"operational," a term that has encountered some criticism because iterroneously suggests that the Shuttle had attained an airline-likedegree of routine operation. In any event, NASA regarded all flightsafter STS-4 operational in the sense that payload requirements wouldtake precedence over spacecraft testing, requiring largercrews.

After completing the orbital test in mid-1982, NASA began the "operational phase" of the Space Shuttleprogram, beginning with STS-5. The STS -for Space TransportationSystem- sequential numbering was still in effect at that time; afterSTS-9 NASA changed the method of numbering missions. Thereafter eachflight was designated by two numbers and a letter, such as 41-B. Thefirst digit indicates the fiscal year of the scheduled launch (4 for1984). The second digit identifies the launch site (1 is Kennedy, 2Vandenberg Air Force Base, California). The letter corresponds to thealphabetical sequence for the fiscal year, B being the second missionscheduled. Here is a brief summary of the 21 missions launched fromlate 1982 to January, 1986:

 

- STS-5, November 11-16, 1982, Orbiter   Columbia, launched two communications satellites, which later were   boosted to geosynchronous orbit by attached propulsion   systems. 

 

- STS-6, April 4-9, 1983, Orbiter   Challenger, was highlighted by the first Shuttle-based spacewalk,   or extravehicular activity. The crew successfully deployed the   5,000-pound Tracking and Data Relay Satellite, first of three   planned NASA communications satellites. 

 

- STS-7, June 18-24, 1983, Orbiter   Challenger, delivered a second pair of commercial communications   satellites. The mission also included additional payload release   and recapture tests using the Remote Manipulator System. This   flight marked the first retrieval of an object from orbit. 

 

- STS-8, August 30-September 6, 1983,   Orbiter Challenger, included more robot arm tests plus deployment   of a commercial/public service communications satellite. STS-9,   November 28-December 8, 1983, Orbiter Columbia, carried the first   Spacelab in the payload bay. The mission marked Columbia's return   to service after a year's hiatus, during which it had been   extensively modified. 

 

- Flight 10 (41-B), February 3-11, 1984,   Orbiter Challenger, was highlighted by the introduction of the   Manned Maneuvering Unit, a backpack propulsion unit that allows   astronauts to maneuver in space independent of the Orbiter. The   mission also launched two communications satellites, but their   boosters failed to put them into geosynchronous orbit. For the   first time, the Shuttle landed on the concrete runway at Kennedy   Space Center. 

 

- Flight 11 (41-C), April 6-13, 1984,   Orbiter Challenger, featured an important demonstration of Shuttle   ability: the retrieval, repair and redeployment of the   malfunctioning Solar Maximum Mission spacecraft with the help of a   Manned [ 6 ] Maneuvering Unit. Other activity included   deployment of the Long Duration Exposure Facility, a large   cylinder containing materials samples to be retrieved and examined   after long exposure to the space environment. 

 

- Flight 12 (41-D), August 30-September 5,   1984, Orbiter Discovery, was devoted primarily to launch of three   communications satellites. The mission demonstrated repeated   deployment and retraction of a large, foldable solar array to   investigate the practicability of using such solar wings as power   sources for extended Shuttle missions, space platforms or the   space station. 

 

- Flight 13 (41 -G), October 5- 13, 1984,   Orbiter Challenger, launched the NASA Earth Radiation Budget   Explorer. A cargo bay pallet carried instruments for Earth   observations, including an advanced imaging radar. 

 

- Flight 14 (51-A), November 8-16, 1984,   Orbiter Discovery, launched two communications satellites and   retrieved two others that had been sent into unusable orbits after   deployment on Flight 10. 

 

- Flight 15 (51-C),January 24-27, 1985,   Orbiter Discovery, carried a Department of Defense payload. 

 

- Flight 16 (51-D), April 12-19, 1985,   Orbiter Discovery, deployed two commercial satellites; one,   Leasat-3, remained in low orbit when the upper stage booster   failed to activate. 

 

- Flight 17 (51-B), April 29-May 6, 1985,   Orbiter Challenger, carried a second Spacelab mission and   materials processing experiments. 

 

- Flight 18 (51 -G), June 17 -24, 1985,   Orbiter Discovery, delivered three communications satellites,   deployed a low-cost Spartan scientific satellite and retrieved it   after a period of free flight. 

 

- Flight 19 (51-F), July 29-August 6, 1985,   Orbiter Challenger, carried the third Spacelab mission, which   covered a broad range of experiments in plasma physics,   astrophysics, solar astronomy and materials processing. 

 

- Flight 20 (51-I), August 27-September 3,   1985, Orbiter Discovery, deployed three communications satellites.   The Leasat-3 satellite which failed to activate after deployment   on Flight 16 was retrieved, repaired and successfully   redeployed. 

 

- Flight 21 (51-J), October 3-1O, 1985,   Orbiter Atlantis was devoted to another Department of Defense   mission. 

 

- Flight 22 (61-A), October 30-November 6,   1985, Orbiter Challenger, carried the fourth Spacelab mission,   devoted to materials processing experimentation. 

 

- Flight 23 (61-B), November 26-December 3,   1985, Orbiter Atlantis, was highlighted by an experiment in   astronaut assembly of structures in orbit and attendant study of   extravehicular dynamics and human factors. The mission also   deployed three communications satellites. 

 

- Flight 24 (61-C), January 12-18, 1986,   Orbiter Columbia, launched a commercial communications satellite,   deployed a Hitchhiker secondary payload, conducted experiments in   infrared imaging, acquired photos and spectral images of Comet   Halley. 

 

- Flight 25 (51-L), January 28, 1986,   Orbiter Challenger. The accident. 

 

Including the initial orbital tests, the SpaceShuttle flew 24 successful missions over a 57-month period. Columbiamade seven trips into space, Discovery six and Atlantis two.Challenger flew most frequently-nine times prior to its fateful lastflight.

In those 24 flights, the Shuttle demonstratedits ability to deliver a wide variety of payloads; its ability toserve as an orbital laboratory; its utility as a platform forerection of large structures; and its use for retrieval and repair oforbiting satellites.

** **

**Elements of the Space Shuttle**

The Space Shuttle is the principal componentof a national Space Transportation System designed to accommodate notonly NASA's predictable needs but also those of the Department ofDefense and commercial payload sponsors. Technically speaking,transportation system hardware embraces not only the Shuttle but itsSpacelab laboratory component, the upper stage propulsion units,contemplated heavy lift vehicles [**7**] and space tugs formoving payloads from one orbit to another. To provide for thebroadest possible spectrum of civil/military missions, the SpaceShuttle was designed to deliver 65,000 pounds of payload to aneasterly low Earth orbit or 32,000 pounds to polar orbit. Thefollowing sections describe the main elements of the Shuttlesystem.

** **

**The Orbiter**

The Orbiter is as large as a mid-size airlinetransport and has a structure like that of an aircraft: an aluminumalloy skin stiffened with stringers to form a shell over frames andbulkheads of aluminum or aluminum alloy. The major structuralsections of the Orbiter are the forward fuselage, which encompassesthe pressurized crew compartment; the mid fuselage, which containsthe payload bay; the payload bay doors; the aft fuselage, from whichthe main engine nozzles project; and the vertical tail, which splitsopen along the trailing edge to provide a speed brake used duringentry and landing.

The crew compartment is divided into twolevels-the flight deck on top and the middeck below. Besides workingspace, the crew compartment contains the systems needed to provide ahabitable environment (atmosphere, temperature, food, water, the crewsleep facilities and waste management). It also houses theelectronic, guidance and navigation systems.

The Orbiter crew may include as many as eightpeople, although generally the limit is seven. The crew consists ofthe commander, the captain of the ship; the pilot, second in command;and two or more mission specialists. One or more payload specialistscan also be accommodated. A mission specialist coordinates activitiesof the Orbiter and crew in support of a given payload objective. Apayload specialist may manage specific experiments. The commander,pilot and mission specialists are career astronauts assigned to themission by NASA. Payload specialists do not come from the AstronautOffice. They are assigned, by payload sponsors in coordination withNASA.

Cargoes up to 24 tons have been carried in thepayload bay. Clamshell doors on the top of the Orbiter meet along thecraft's spine to enclose the bay, which is 15 feet wide and 60 feetlong.

The payload bay is designed to hold securely awide range of objects. They may include one or more communicationssatellites to be launched from orbit, an autonomous Spacelab forexperiments in space, or cargo disposed on special pallets. To handlecargo in orbital flight, the payload bay has the 50-foot mechanicalarm that is controlled from within the crew compartment. A televisioncamera and lights mounted near the end of the arm enable the operatorto see what the "hand" is doing.

Just as important as delivering cargo to orbitis recovering a satellite and bringing it back to Earth-retrieving asatellite in need of refurbishment, for example. The Orbiter cancarry 16 tons of cargo back from space.

The feasibility of a reusable Space Shuttlehinges on a particularly vital requirement: protecting the Orbiterfrom the searing heat generated by friction with the atmosphere whenthe craft returns to Earth. Temperatures during entry may rise ashigh as 2,750 degrees Fahrenheit on the leading edge of the wing and600 degrees on the upper fuselage, the "coolest" area. The thermalprotection system devised for the Orbiter must prevent thetemperature of the aluminum skin from rising above 350 degrees duringeither ascent or entry.

The Orbiter has four kinds of externalinsulation that are applied to various parts of the structureaccording to the temperature each is likely to experience. Thecraft's nose cap and the leading edges of the wings are protectedwith an all-carbon composite consisting of layers of graphite clothin a carbon matrix. The outer layers are converted chemically tosilicon carbide, the same material that has long been used as anabrasive in grindstones. Areas subjected to the next greatest heatare shielded with high-temperature ceramic tiles about six inchessquare and varying in thickness from one to five inches, depending onthe protection needed. So-called "low temperature" tiles are of thesame material- nearly pure glass, of which 90 percent of the volumeis "air"-for use on areas requiring less protection. (Low-temperatureis relative; tiles so designated can withstand a temperature of 1,200degrees Fahrenheit.) About 3O,000 tiles, each different, areinstalled on each Orbiter.

** **

**Space Shuttle Main Engines**

The three high-performance rocket engines inthe aft section of the Orbiter fire for about the first 8 1/2 minutesof flight after liftoff. At sea level, each engine generates 375,000pounds of thrust at 100 percent throttle.

The propellants for the engines are the fuel[**8**](liquid hydrogen) and the oxidizer (liquid oxygen) carried in theExternal Tank. Combustion takes place in two stages. First, thepropellants are mixed and partly burned in pre-burners. Hot gasesfrom the pre-burners drive the high-pressure turbopumps which deliverpropellants to the main injector. Combustion, once initiated byelectrical igniters, is self-sustaining. Before firing, the very coldliquid propellant is allowed to flow into the system as far as thepre-burners and combustion chamber to cool the pumps and ducts sothat the hydrogen and oxygen in the system will remain liquid whenthe engine is started.

The main engines have been throttled over arange of 65 to 104 percent of the thrust at sea level. At liftoff,they are thrusting at 100 percent. Computers command engine thrust to104 percent as soon as the Shuttle clears the tower. They throttle to65 percent to reduce the maximum aerodynamic loads that occur at analtitude of about 34,000 feet. Thereafter, the thrust is againincreased to provide an acceleration of three times that of gravityin the last minute or so of powered flight.

** **

**External Tank**

The External Tank carries the propellants forthe Orbiter's main engines-143,000 gallons of liquid oxygen and383,000 gallons of liquid hydrogen, which is much lighter than acomparable volume of oxygen. Together, the propellants weigh a littlemore than 790 tons. Martin Marietta Denver Aerospace, Michoud,Louisiana, builds the tank, a welded aluminum alloy cylinder with anogive nose and a hemispherical tail. It is 154 feet long and 27 1/2feet in diameter.

Because the Orbiter and the two Solid RocketBoosters are attached to it at liftoff, the External Tank absorbs thethrust of the combined propulsion system. It withstands complex loadeffects and pressures from the propellants.

The liquid oxygen tank forms the nose of theExternal Tank. It contains oxidizer kept liquid at a temperature of -297 degrees Fahrenheit. A removable conical nose cap acts as anaerodynamic fairing. Inside the tank, baffles reduce sloshing and theassociated control problems. The liquid hydrogen tank does not needbaffles because the fuel is so light that sloshing does not inducesignificant forces. The liquid hydrogen tank accounts for the greaterpart of the External Tank. Its contents are even colder than the LOX:- 423 degrees Fahrenheit.

The intertank structure or "intertank"connects the two propellant tanks. It is a cylindrical structuralsection that houses instruments and receives and distributes most ofthe thrust load from the Solid Rocket Boosters. The front end of eachbooster is connected to the External Tank at the intertankmidsection.

A multi-layered thermal coating covers theoutside of the External Tank to protect it from extreme temperaturevariations during pre-launch, launch, and the first 8 1/2 minutes offlight. That insulation reduces the boil-off rate of the propellants,which must be kept at very low temperatures to remain liquid. It alsois meant to minimize ice that might form from condensation on theoutside of the propellant tanks.

In addition to the Solid Rocket Boosterforward attachment points on either side of the intertank, threeother attachment points link each booster to the aft major ring frameof the External Tank. The boosters are thus connected to the tank atfour points, one forward and three aft.

Three structural elements link the Orbiter tothe External Tank. A "wishbone" attachment beneath the crewcompartment connects the forward end of the Orbiter to the tank. Thetwo aft connections are tripods at the base of the ExternalTank.

A command from the Orbiter computer jettisonsthe External Tank 18 seconds after main engine cutoff, about 8 /2minutes after liftoff. To ensure that it will travel a predictablepath, a tumble system rotates the tank end-over-end at a minimum rateof two revolutions per minute. The tank breaks up upon atmosphericentry, falling into the planned area of the Indian or Pacific Oceanabout an hour after liftoff. The External Tank is the only maincomponent of the Space Shuttle that is not recovered andreused.

** **

**Solid Rocket Boosters**

The two solid-propellant rocket boosters arealmost as long as the External Tank and attached to each side of it.They contribute about 80 percent of the total thrust at liftoff; therest comes from the Orbiter's three main engines. Roughly two minutesafter liftoff and 24 miles down range, the solid rockets haveexhausted their fuel. Explosives separate the boosters from theExternal Tank. Small rocket motors move them away from the ExternalTank and the Orbiter, which continue toward orbit under thrust of theShuttle's main engines.

[**9**] The Solid RocketBooster is made up of several subassemblies: the nose cone, SolidRocket Motor and the nozzle assembly. Marshall is responsible for theSolid Rocket Booster; Morton Thiokol, Inc., Wasatch Division, BrighamCity, Utah, is the contractor for the Solid Rocket Motors. Each SolidRocket Motor case is made of 11 individual cylindrical weld freesteel sections about 12 feet in diameter. When assembled, they form atube almost 116 feet long. The 11 sections are the forward domesection, six cylindrical sections, the aft External Tank-attach ringsection, two stiffener sections, and the aft dome section.

The 11 sections of the motor case are joinedby tang-and-clevis joints held together by 177 steel pins around thecircumference of each joint.

After the sections have been machined to finetolerances and fitted, they are partly assembled at the factory intofour casting segments. Those four cylindrical segments are the partsof the motor case into which the propellant is poured (or cast). Theyare shipped by rail in separate pieces to Kennedy.

Joints assembled before the booster is shippedare known as factory joints. Joints between the four casting segmentsare called field joints; they are connected at Kennedy when thebooster segments are stacked for final assembly.

** **

**Orbital Maneuvering System**

The two engine pods on the aft fuselage of theOrbiter contain maneuvering engines and their propellant-monomethylhydrazine (the fuel) and nitrogen tetroxide (the oxidizer). Heliumpressurizes the propellant tanks, and the fuel and the oxidizerignite on contact.

Forty-four small rocket motors in theOrbiter's nose and aft section maneuvering system pods allowadjustments of the vehicle's attitude in pitch, yaw, and roll axes.They also may be used to make small changes of velocity along one ofthe Orbiter's three axes.

** **

**Flight of a Shuttle**

Except for ascent and entry, all of theShuttle's typical seven-day mission is in orbit. That is where thegoals of a given mission are accomplished: scientific experimentscarried out; satellites deployed into orbit, retrieved or repaired;observations made of the Earth and the solar system. The Shuttlemakes one revolution of the Earth approximately every 90 minutesduring the satellite mission.

When it comes out of orbit, the Shuttle ismoving at about 17, 500 miles an hour. Reaction engines position theOrbiter nose forward again for entry into the atmosphere. Thosethrusters continue to control the Orbiter's attitude until theatmosphere becomes dense enough for the aerodynamic surfaces to takeeffect.

The Shuttle enters the ever-thickening blanketof atmosphere at 400,000 feet of altitude and a speed of more than17,000 miles an hour (about Mach 25). The Orbiter's nose ispositioned 40 degrees above its flight path. That attitude increasesaerodynamic drag, thus helping to dissipate the tremendous amount ofenergy that the spacecraft has when it enters the atmosphere.Friction heats the surface of the Orbiter, which is protected bythermal tiles, and ionizes the surrounding air, preventing radiocommunication with Earth for the next 13 minutes.

The flight control system's computer programallows use of the reaction thrusters and aerodynamic surfaces incombination to control the spacecraft. At Mach 4.2, the rudder isactivated, and the last reaction thrusters are deactivated at Mach 1.Thereafter, the craft is entirely maneuvered like an airplane bymovement of the aerodynamic control surfaces: elevons, rudder, speedbrake, and body flap.

In the landing approach, the Orbiter has nopropulsion. It has only its velocity and altitude. Its energy must becarefully managed to maneuver the Shuttle aerodynamically to a safelanding. Beginning this terminal phase, the glide slope is steep-19degrees-as the Orbiter descends toward the runway. Half a minutebefore touchdown and two miles from the runway, the craft flares to ashallow, almost flat 1.5 degree glide slope. Touchdown occurs at 225miles per hour. On the runway, the Orbiter rolls to a stop, and themission is complete. .

** **

**References**

1. Space Task Group Reportto the President, "The Post-Apollo Space Program: Directions for theFuture," September, 1969, pages 20 and 21.

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