KSC Release No. 252-85
Revised January 1992

Since the late 1960s, Pads A and B at Kennedy Space Center's Launch Complex 39 have served as backdrops for America's most significant manned space flight endeavors - Apollo, Skylab, Apollo-Soyuz and Space Shuttle.

Located on Merritt Island, Florida, just north of Cape Canaveral, the pads were originally built for the huge Apollo/Saturn V rockets that launched American astronauts on their historic journeys to the Moon and back. Following the joint U.S.-Soviet Apollo-Soyuz Test Project mission of July 1975, the pads were modified to support Space Shuttle operations.

Both pads were designed to support the concept of mobile launch operations, in which space vehicles are assembled and checked out in the protected environment of the Vehicle Assembly Building, then transported by large tracked vehicles to the launch pad for final processing and launch. During the Apollo era, key pad service structures were mobile. For the Space Shuttle, two permanent service structures were installed at each pad for the first time.

On April 12, 1981, Shuttle operations commenced at Pad A with the launch of Columbia on STS-1. After 23 more successful launches from A, the first Space Shuttle to lift off from Pad B was the ill-fated Challenger in January 1986. Pad B was designated for the resumption of Shuttle flights in September 1988, followed by the reactivation of Pad A in January 1990.

Major Features of Pads A and B

Both pads are octagonally shaped and share identical features. Pad A is located 18,159 feet from the Vehicle Assembly Building via the crawlerway, Pad B 22,400 feet. The pads are 8,716 feet apart. Each pad covers about a quarter-square-mile of land. Launches are conducted from atop a concrete hardstand 390 by 325 feet, located at the center of the pad area. The Pad A and Pad B hardstands are 48 feet and 55 feet above sea level, respectively.

Fixed Service Structure (FSS):

The FSS is the pad's most prominent feature, standing 347 feet from ground level to the tip of the lightning mast. The lightning mast itself, 80 feet tall and made of fiberglass, supports a one-inch stainless steel cable which starts from an anchor 1,100 feet south of the FSS, angles up and over the mast, and then extends back down to a second anchor the same distance to the north. Below the lightning mast is a hammerhead crane used for pad hoisting operations.

The FSS is equipped with three swing arms which provide services or access to a Shuttle on the pad. They are retracted when not in use. There are 12 floors on the FSS, positioned at 20-foot intervals. The first is located 27 feet above the pad surface.

The Orbiter Access Arm is the lowermost arm, located 147 feet above the pad surface. It allows personnel to enter the orbiter crew compartment. The outer end of the access arm features an environmental chamber or "white room" that mates with the orbiter and holds six persons. The arm remains in the extended position until seven minutes, 24 seconds before launch to serve as an emergency escape route for the flight crew. It is 65 feet long, 5 feet wide, and 8 feet high, and can be mechanically or manually repositioned in about 15 seconds in the event of a contingency.

At the 167-foot level, the 48-foot long External Tank Hydrogen Vent Umbilical and Intertank Access Arm (also called the External Tank Gaseous Hydrogen Vent Arm System) allows mating of the external tank umbilicals as well as contingency access to the external tank intertank compartment. The arm rotates 210 degrees to its extended position. The arm is retracted after umbilical/vent line mating, typically at about T minus five days, leaving the umbilical vent line connected to the external tank to support tanking and launch. The umbilical vent line provides continuous venting of the external tank during and after loading of the volatile liquid hydrogen. The vent line is disconnected from the vehicle at first motion and retracts vertically downward to a stored position.

The External Tank Gaseous Oxygen Vent Arm, attached between the 207- and 227-foot levels, is a retractable arm and vent hood assembly. The arm truss section measures 65 feet long from tower hinge to vent hood hinge. The 13-foot wide vent hood also is known as the "beanie cap." Heated gaseous nitrogen is pumped into the hood to warm the liquid oxygen vent system at the top of the external tank to prevent oxygen vapors exiting the vent louvers from condensing water vapor in the surrounding air into potentially damaging ice. About two and a half minutes before launch, the vent hood is raised to clear the external tank, a 25-second procedure. The arm is retracted against the Fixed Service Structure at about one minute, 45 seconds before liftoff. It is not latched in the event there is a hold, in which case the arm can be re-extended and the beanie cap again lowered onto the external tank. The arm is latched when the solid rocket booster ignition signal is given at T minus zero minutes.

Emergency Egress System

Located 195 feet above the ground, at the same level on the FSS as the Orbiter Access Arm, is the Emergency Exit System. It provides an escape route for personnel inside the orbiter or on the Orbiter Access Arm. The system includes seven baskets suspended from seven slidewires which extend from the Fixed Service Structure to a landing zone 1,200 feet to the west. Each basket can hold up to three people. A braking system catch net and drag chain slow and then halt the baskets sliding down the wire at about 55 miles per hour in about half a minute. Also located in the landing zone is a bunker, with an M113 armored personnel carrier stationed nearby.

Rotating Service Structure (RSS):

The RSS provides protected access to the orbiter for installation and servicing of payloads at the pad, as well as servicing access to certain systems on the orbiter. The majority of payloads are installed in the vertical position at the pad, partly because of their design and partly because it allows payload processing to take place further along in the launch processing schedule. Spacelab and other large horizontal payloads are loaded while the orbiter is in an Orbiter Processing Facility high bay.

The RSS is 102 feet long, 50 feet wide, and 130 feet high. It is supported by a rotating bridge that pivots about a vertical axis on the west side of the pad's flame trench. The RSS rotates through 120 degrees - one third of a circle - on a radius of 160 feet. Its hinged column rests on the pad surface and is braced against the Fixed Service Structure. The RSS is retracted for launch.

The major feature of the RSS is the Payload Changeout Room, an enclosed, environmentally-controlled area that supports payload delivery and servicing at the pad and mates to the orbiter cargo bay for vertical payload installation. Clean-air purges help insure that payloads being transferred from the payload canister into the Payload Changeout Room are not exposed to the open air. The payload is removed from the canister, and later installed inside the orbiter cargo bay using the Payload Ground Handling Mechanism (PGHM). Five platforms are positioned at five levels to provide access to the payload when it is installed on the PGHM. Each platform has extendible planks that can be configured to accommodate a particular payload.

Another feature of the RSS is the Orbiter Midbody Umbilical Unit, which provides access and services to the midfuselage portion of the orbiter. The unit is 22 feet long, 13 feet wide, and 20 feet high. It extends from the RSS at levels ranging from 158 to 176 feet above the pad surface, and includes a sliding extension platform and a horizontally moving line-handling mechanism. The unit provides access to the midbody umbilical door. It is used to supply fluids to the orbiter's power reactant storage and distribution system and payloads. Liquid oxygen and liquid hydrogen for the fuel cells are funnelled through here, as are gases such as nitrogen and helium.

Also found on the RSS is the Hypergolic Umbilical System. Hypergolic fuel and oxidizer, as well as helium and nitrogen service lines, are carried from the Fixed Service Structure to the Space Shuttle Orbital Maneuvering System (OMS) pods via this umbilical system. It includes six manually operated and locally controlled umbilical handling units which are structurally attached to the RSS. The Hypergolic Umbilical System lines can be mated and demated from the vehicle very rapidly.

Flame Trench and Deflector System:

The flame trench, built with concrete and refractory brick, bisects the pad at ground level. It is 490 feet long, 58 feet wide and 42 feet deep. The flame deflector system includes an inverted, V-shaped steel structure covered with a high temperature concrete material five inches thick that extends across the center of the flame trench. One side of the V receives and deflects the flames from the orbiter main engines, the opposite side the flames from the solid rocket boosters. There are also two movable deflectors at the top of the trench to provide additional protection to Shuttle hardware from the solid rocket booster flames.

Liquid Oxygen and Liquid Hydrogen Storage:

Liquid oxygen (LOX) used as an oxidizer by the orbiter main engines is stored in a 900,000-gallon tank on the pad's northwest corner, while the liquid hydrogen (LH2) used as a fuel is kept in an 850,000-gallon tank on the northeast corner. The propellants are transferred from the storage tanks in vacuum-jacketed lines that feed into the orbiter and external tank via the tail service masts on the Mobile Launcher Platform.

The liquid oxygen tank functions as a huge vacuum bottle designed to store the cryogenic fluid at a very low temperature - less than minus 297 degrees Fahrenheit. It is transferred to the pad by one of two main pumps capable of pumping 1,300 gallons per minute.

The lighter liquid hydrogen is stored in a vacuum bottle located at the northeast corner of each pad. It must be kept at an even lower temperature than the LOX: minus 423 degrees F. To move the LH2 to the pad, a small amount of the liquid hydrogen is allowed to vaporize, and the gas pressure exerted from the top of the tank pushes the LH2 into the transfer lines.

Hypergolic Storage:

The orbiter's Orbital Maneuvering System and Reaction Control System burn monomethyl hydrazine as a fuel and nitrogen tetroxide as an oxidizer. These hypergolic fluids are stored in well-separated areas on the southwest and southeast corners of the pads, respectively. Transfer lines convey the fluids through the Fixed Service Structure to the Hypergolic Umbilical System located on the RSS, with its three pairs of umbilicals attaching to the orbiter.

Pad Terminal Connection Room:

The Pad Terminal Connection Room is located on the west side of the flame trench, underneath the elevated hardstand. It is covered with as much as 20 feet of dirt fill. Housed here in a reinforced concrete room is the equipment which links elements of the Shuttle, Mobile Launcher Platform and the pad with the Launch Processing System headquartered in the Launch Control Center. Checkout, countdown and launch of the Shuttle are performed and controlled through the Launch Processing System.

Launch Pad/Mobile Launch Platform Interfaces:

The Space Shuttle is brought to the pad atop the Mobile Launcher Platform (MLP) and Crawler-Transporter. The MLP is parked on pedestals permanently located at the pad, and is the platform from which the Shuttle is launched. Several MLP systems interface with pad systems. These include the Sound Suppression System and the propellant transfer lines for the external tank. Helium and nitrogen, as well as ground electrical power and connections for vehicle data and communications, also are established through the tail service masts of the MLP.

The Space Shuttle Main Engine Hydrogen Burnoff System, located inside the tail service masts, eliminates free hydrogen present prior to main engine ignition. Hydrogen vapors are exhausted into the main engine nozzles during the start sequence; if ignited when the main engines ignite, a small explosion could ensue which might damage the engine bells. The six hydrogen burnoff pre-igniters are initiated just before main engine start. They throw off thousands of hot, luminescent balls into the area below the engine bells, igniting the free hydrogen and precluding a rough combustion when the main engines start.

Sound Suppression System:

A Sound Suppression System was installed on the pads and MLP to protect the orbiter and its payloads from being damaged by acoustical energy reflected from the Mobile Launcher Platform during liftoff. Water stored in a 290-foot high, 300,000-gallon tank on the northeast side of the pad is released just prior to main engine ignition and flows by gravity to special Mobile Launcher Platform outlets, including six 12-foot high quench nozzles, or "rainbirds. Peak flow rate is 900,000 gallons per minute nine seconds after liftoff. The system reduces acoustical levels within the orbiter payload bay to about 142 decibels, below the design requirement of 145 decibels.

Shuttle Era Pad Modifications

Since the startup of Space Shuttle operations, Pads A and B have undergone additional modification. Many of these changes were made during the standdown in launch operations after the Challenger accident in 1986. Others have been made since the resumption of flights in September 1988.

About $3.3 million in new weather protection structures were installed and tested at Pad B in 1986. The additions supplement the Rotating Service Structure, which shields most of the vehicle when it is on the pad. Without shielding, the orbiter's fragile heat protection tiles are subject to damage from hail and wind-blown debris, as well as from heavy rains which can erode the waterproofing on the tiles.

The protective hardware includes rolling doors which can be deployed between the orbiter's belly and the external tank to shield the lower portions of the orbiter. The doors are supported by a structural framework on both the Rotating Service Structure and the Fixed Service Structure. They replaced a fabric drape system. Other weather protection modifications were designed to provide a roof seal and to enclose the space between the RSS and the external tank.

A fully assembled Space Shuttle was rolled out to Pad B in the fall of 1986 to test the various additions and changes. The same weather protection modifications and additions were later made at Pad A.

While no facilities or ground support equipment (GSE) failures were identified in the Challenger accident investigation, NASA did conduct an extensive in-depth System Design Review which resulted in a number of modifications considered mandatory for return to flight. Launch pad changes were aimed at assuring processing timelines and improving margins of safety, and eliminating critical single failure points in the system wherever possible.

The agency also took advantage of the standdown in launch operations to implement other pad modifications planned prior to the STS 51-L accident. In addition, improvements were made to the pad as required by the redesigned solid rocket boosters. The changes were first made at Pad B:

*Solid Rocket Booster Joint Heater Umbilical. The report of the Presidential Commission on the Space Shuttle Challenger Accident mandated a redesign of the Shuttle solid rocket boosters (SRBs), which in turn resulted in a new requirement to keep the SRB field joints at a temperature on or about 75 degrees F. An umbilical was installed on the Mobile Launcher Platform to provide power, instrumentation and controls to the joint heaters. This is a T minus zero umbilical, meaning it is separated at the moment of vehicle liftoff from the pad.

*Payload Changeout Room. Major modifications were made at the Payload Changeout room of the Rotating Service Structure. Improvements in temperature and humidity controls were implemented, and the room is now a positive pressure environment - the pressure inside is higher than outside - to keep out possible contaminants.

*Freeze protection. Protection against freezing of the water services used at the pad was a concern of the Presidential Commission. The system was modified so the water system of the RSS and the FSS recirculates back to the main supply to maintain a minimum acceptable temperature.

*Emergency Exit System. Improvements to the system were extensive. The temperature and flame detector sensor network at the pad was upgraded. Solid panels were placed on the Orbiter Access Arm to provide fire protection and a water spray system was installed. On the 147-foot level - the crew escape route - the deck was made solid to keep flames from coming up from a lower level.

Two additional baskets were added to the original five slidewire baskets, and all were lined with a flame-retardant material. The slidewire system itself was modified to control the height of the wire and a "stop point" was installed on the baskets to keep them from rolling back. Also installed was a deceleration net, which catches "horns" on the basket front and activates the deceleration system. The bunker toward which a crew would exit after getting out of the baskets was relocated to a more accessible vantage point, and sidewalks were added to make the route easier. The bunker itself is now larger and equipped with air-conditioning, showers and bunk beds.

*External Tank Hydrogen Vent Umbilical and Intertank Access Arm. Along with the joint heater umbilical, the external tank hydrogen vent umbilical is a T minus zero umbilical. Pyrotechnics (explosives) release the umbilical and a cable acts as an automatic secondary release system should the pyrotechnics fail. The modifications were intended to improve clearance between the Shuttle - particularly the solid rocket booster aft skirts - as it lifts off from the pad and the vent umbilical as it is retracted.

*Liquid Oxygen/Liquid Hydrogen System Debris Trap. Special debris traps were installed in the ground interfaces between the orbiter and liquid oxygen/hydrogen servicing systems located in the tail service masts on the Mobile Launcher Platform. The debris trap, a circular plate with more than 1,000 tiny holes, prevents the entry of foreign material into the flight vehicle during propellant loading.

*Solid Rocket Booster Support/Holddown System. Studies are still being conducted of the stiffness and loads imposed on the solid rocket booster aft skirts by the holddown support system which bolts the booster to the Mobile Launcher Platform prior to launch. Strain gauge instrumentation will provide more quantifiable data.

Other upgrades included installation of new elevators on the RSS and improvements to the pad communications system. The modifications were implemented first at Pad B to prepare it for return to flight. Pad A, last used for a launch in January 1986, then underwent the same basic package of changes and began supporting liftoffs again in January 1990.

In June of 1991, Pad B was taken offline again to undergo needed repairs and refurbishment. It had served as the takeoff point for 12 of the 16 Shuttle launches conducted in the September 1988 to June 1991 timeframe. About 50 modifications and repairs were made to pad structures and processing equipment and the environmental control system. Work was performed on the hypergolic servicing system and in the Payload Changeout Room involving the platform system. Contamination control improvements were made to the payload handling areas.

The upgrades also included work on the Rotating Service Structure, hardware and electrical system improvements, and structural corrosion control. Improvements to the safety and overall efficiency of the pad also were made.

The upgrades cost about $3.5 million and took about six months to complete. Pad B was scheduled to again support launches starting with the maiden flight of the newest orbiter, Endeavour, set to liftoff on STS-49 in the spring of 1992.