Payload Assist Module (PAM)

The Payload Assist Module (formerly called the Spinning Solid Upper Stage - SSUS) is designed as a higher altitude booster of satellites deployed in near Earth orbit but operationally destined for higher altitudes. The PAM is used to boost various satellites to geosynchronous transfer orbit (22,300 miles) or other higher energy orbits after deployment from the space shuttle vehicle. There are three versions of the PAM that can be used aboard the space shuttle. The PAM-D is capable of launching satellites weighing up to 2,750 pounds. The PAM-DII is for satellites weighing up to 4,150 pounds. The PAM-A is for satellites weighing up to 4,400 pounds. The PAMs are also used for non-geosynchronous transfer orbits.

The PAM's deployable (expendable) stage consists of a spin-stabilized, solid-fueled rocket motor; a payload attach fitting to mate with the unmanned spacecraft; and the necessary timing, sequencing, power and control assemblies.

The reusable airborne support equipment consists of the cradle structure for mounting the deployable system in the space shuttle orbiter payload bay; a spin system to provide the stabilizing rotation; a separation system to release and deploy the stage and unmanned spacecraft; and the necessary avionics to control, monitor and power the system.

The PAM stages are supported through the spin table at the base of the motor and through restraints at the PAF. The forward restraints are retracted before deployment.

The PAM-Ds and DIIs have a sunshield that provides thermal protection of the PAM/unmanned spacecraft when the space shuttle orbiter payload bay doors are open on orbit.

The PAM's ASE consists of all the reusable hardware elements that are required to mount, support, control, monitor, protect and operate the PAM's expendable hardware and unmanned spacecraft from lift-off to deployment from the space shuttle. It will also provide the same functions for the safing and return of the stage and spacecraft in case of an aborted mission. The ASE is designed to be as self-contained as possible, thereby minimizing dependence on orbiter or flight crew functions for its operation. The major ASE elements include the cradle for structural mounting and support, the spin table and drive system, the avionics system to control and monitor the ASE and the PAM vehicle, and the thermal control system.

The cradle assembly provides the vertical structural mounting support for the PAM/unmanned spacecraft assembly in the orbiter payload bay. The cradle is 15 feet wide. The length of the cradle is 93 inches static and 96 inches dynamic. The open-truss-structure cradle is constructed of machined aluminum frame sections and chrome-plated steel longeron and keel trunnions.

The spacecraft-to-cradle lateral loads are reacted by forward retractable reaction fittings between the PAF and cradle, which are driven by redundant dc electrical motors. After the reaction fittings are retracted, the spin table is free to spin the PAM and unmanned spacecraft when commanded.

The spin table consists of three subsystems-spin, separation and electrical interface. The spin subsystem consists of the spin table, the spin bearing, the rotating portion of the spin table, a gear and gear support ring, two redundant drive motors, a despin braking device, and a rotational index and locking mechanism. The separation subsystem includes four compression springs mounted on the outside of the rotating spin table, each with an installed preload of 1,400 pounds and a Marman-type clamp band assembly.

The electrical interface subsystem consists of a slip ring assembly to carry electrical circuits for the PAM and spacecraft across the rotating spin bearing. The electrical wiring from the slip ring terminates at electrical disconnects at the spin table separation point. The slip ring assembly is used to carry safety-critical command and monitor functions and those commands required before separation from the spin table.

The system provides a capability for spin rates between 45 and 100 rpm. Upon command, the spin table will be spun up to the nominal rpm by two electric motors, either of which can produce the required torque. When the spin table rpm has been verified and the proper point in the parking orbit has been reached, redundant, debris-free, explosive bolt cutters are fired upon command from the electrical ASE to separate the band clamp (which is mechanically retained on the spin table), and the springs provide the thrust to attain a separation velocity of approximately 3 feet per second.

In case of an abort mode after spinup, the multiple-disc stack friction-type braking device will despin the PAM and unmanned spacecraft assembly, and the spin drive motor will slowly rotate the assembly until the solenoid-operated indexing and locking device is engaged. Upon confirmation by the ASE that the spin table is properly aligned and locked, the restraint arms will be re-engaged.

The PAM-D PAF structure is a machined forging and provides the subsystem mounting installations and mounts on the forward ring of the motor case. PAM-DII offers two structural designs for the PAF, one of aluminum monocoque and the other of graphite epoxy. The two cradle reaction fittings provide structural support to the forward end of the PAM stage and unmanned spacecraft and transmit loads to the ASE cradle structure. The forward interface of the PAF provides the spacecraft mounting and separation system. One Marman-type clamp band is preloaded as required to properly support the spacecraft, and separation is achieved by redundant bolt cutters. Four separation springs, mounted inside the PAF, provide the impetus for clear separation. For PAM-D, installed preload for each spring is approximately 200 pounds with a spring stroke of 5.25 inches, providing a spacecraft separation velocity of about 3 feet per second. The electrical interface connectors between the PAM and the spacecraft are mounted on brackets on opposite sides of the PAF. Other subsystems mounted on the PAF include the redundant safe and arm device for motor ignition and telemetry components (if desired) and the optional S-band transmitter.

The electrical ASE performs sunshield opening and closing, control and monitoring of restraint withdrawal, spin table spin and deployment functions; arms (and disarms, if necessary) the SRM; controls and monitors the PAM vehicle's electrical sequencing system (and telemetry system, when used); and provides wiring to carry required spacecraft functions. And, as a mission option, it provides control and monitoring of spacecraft systems.

The PAM thermal control system alleviates severe thermal stresses on both the unmanned spacecraft and the PAM system. The system consists of thermal blankets mounted on the cradle to provide thermal protection and a sunshield mounted on the cradle to control the solar input to and heat loss from the payload when the orbiter payload bay doors are open.

Thermal blankets consisting of multilayered insulation are mounted to the forward and aft sides of the cradle. The orbiter payload bay liner provides thermal protection on the sides and the bottom.

The sunshield consists of multilayered Mylar lightweight insulation supported on a tubular frame. The sunshield panels on the sides are stationary. The portion of the shield covering the top of the PAM/unmanned spacecraft is a clamshell structure that remains closed to protect against thermal extremes when the orbiter payload bay doors are open. The sunshield resembles a two-piece baby buggy canopy. The clamshell is opened by redundant electric rotary actuators operating a control-cable system.

The PAM-D/unmanned spacecraft sunshield accommodates spacecraft up to 86 inches in diameter. The PAM-DII/unmanned spacecraft sunshield accommodates spacecraft up to 115 inches in diameter.

The PAM expendable hardware consists of the Thiokol solid-fueled rocket motor, the payload attach fitting, and its functional system.

The avionics and other systems are common to both the PAM-D and PAM-DII. The PAM-DII, however, required revision or new design of the cradle and structural parts due to the larger volume required and heavier weight than the PAM-D. The PAM-DII utilizes a Thiokol Star 63D motor for greater performance.

PAM-DII functions in the same manner as PAM-D and utilizes all existing PAM-D parameters of preparation, interface, sequence and operation. PAM-DII's thermal, mechanical, and electrical interface and environment are comparable to the PAM-D's.

The payload assist modules are designed and built by McDonnell Douglas Astronautics Co., Huntington Beach, Calif.