THE INERTIAL UPPER STAGE (IUS)/PAYLOAD ASSIST MODULE (PAM-S)
FOR STS-41

The Inertial Upper Stage (IUS) will be used with a
Payload Assist Module (PAM-S) and the Space Shuttle during
the STS-41 mission to transport NASA and ESA's Ulysses
spacecraft out of Earth's orbital plane to explore the polar
regions of the sun.

The IUS was originally designed as a temporary stand-in
for a reusable space tug and the vehicle was named the
Interim Upper Stage. The word "Inertial" (signifying the
guidance technique) later replaced "Interim" when it was seen
that the IUS would be needed through the mid-1990's.

In addition to the Ulysses mission, the IUS has been
used to place three Tracking and Data Relay Satellites
(TDRS's) into orbit, inject the Magellan spacecraft into its
interplanetary trajectory to Venus, and send the Galileo
Spacecraft to orbit Jupiter. The IUS, with a third stage
PAM-S, is now being used by the agency for the Ulysses
solar-polar mission.

The IUS was developed jointly with and built under
contract to the Air Force Systems Command's Space Systems
Division. The Space Systems Division is executive agent for
all Department of Defense activities pertaining to the Space
Shuttle system and provides the IUS to NASA for Space Shuttle
use. After 2-1/2 years of competition, Boeing Aerospace,
Seattle, Washington, was selected in August 1976 to begin
preliminary design of the IUS. Since then, 23 IUS vehicles
have been built and six have been flown on NASA missions.

Specifications

IUS-17, the IUS to be used on Mission STS-41, is a two
stage vehicle weighing approximately 32,500 pounds.

The generic IUS is 5.18 meters (17 feet) long and 2.8
meters (9.25 feet) in diameter. It consists of an aft skirt;
an aft stage solid rocket motor containing 21,400 pounds of
propellant and generating approximately 42,000 pounds of
thrust; an interstage; a forward stage solid rocket motor
with 6,000 pounds of propellant generating approximately
18,000 pounds of thrust; and an equipment support section.

The equipment support section contains the avionics
which provide guidance, navigation, control, telemetry,
command and data management, reaction control and electrical
power.

The IUS vehicle provides redundant operation for
mission-critical components in the avionics system, the
thrust vector actuator system, the reaction control thruster
system, and the solid rocket motor ignition and pyrotechnic
stage separation systems to assure reliability of better than
98 percent. The IUS utilizes a specially developed third
stage called the PAM-S to provide the energy required for the
Ulysses mission.

Payload Assist Module (PAM-S)

The Payload Assist Module (PAM-S) is the third stage of
the upper stage system used to transfer the Ulysses
spacecraft from the Orbiter's low earth orbit into a
solar-polar trajectory via Jupiter. The PAM-S utilizes a
PAM-D Star-48B solid rocket motor with 4437 pounds of
propellant. It is spin stabilized after separation from the
IUS using a gas generator powered free spin system. A
nutation control system provides coning control of the
PAM-S/ULS spacecraft during the spinning operation period
following motor burn. The PAM-S utilizes an ordnance
sequencing system (OSS) to control its event sequencing which
includes an ordnance timer. The timer is activated by IUS
command prior to its separation from the IUS. Once separated
from the IUS, all commands to the PAM-S are internally
generated.

Airborne Support Equipment (ASE)

The IUS Airborne Support Equipment (ASE) is the
mechanical, avionics, and structural equipment located in the
Orbiter. The ASE supports the IUS/PAM-S and the Ulysses
spacecraft in the Orbiter payload bay and elevates the
combination for final checkout and deployment from the
Orbiter.

The IUS ASE consists of the structure,
electromechanical devices, batteries, electronics, and
cabling to support the Ulysses/IUS/PAM-S combination. These
ASE subsystems enable the deployment of the combined vehicle;
provide, distribute and/or control electrical power to the IUS;
provide plumbing to cool the radioisotope thermoelectric
generator (RTG) aboard Ulysses; and serve as communication
paths between the IUS/PAM-S and spacecraft and the Orbiter.

IUS/PAM-S Structure

The IUS/PAM-S structure physically unites the
components of the IUS/PAM-S and attaches them to the
spacecraft and the Orbiter. The structure is capable of
withstanding all the loads generated during operation
including those imposed by the cantilevered spacecraft. In
addition, the structure physically supports all the equipment
and solid rocket motors within the IUS/PAM-S, and provides
the mechanisms for IUS/PAM-S stage separation. The major
structural assemblies of the three-stage IUS/PAM-S are the
adapters, the IUS equipment support section, and interstage.

The adapters attach the IUS to the Orbiter, the IUS to
the PAM-S, and the PAM-S to the spacecraft.

The Equipment Support Section houses the majority of
the avionics of the IUS. The forward end of the equipment
support section contains the PAM-S interface mounting adapter
and electrical interface connector segment for mating and
integrating the PAM-S with the IUS. Thermal protection for
the ESS is provided by a multilayer insulation blanket across
the interface between the IUS and PAM-S.

The interstage joins the two stages of the IUS.

IUS Avionics Subsystems

The avionics subsystems consist of the telemetry,
tracking, and command subsystems; guidance and navigation
subsystem; data management; thrust vector control; and
electrical power subsystems. These subsystems include all
the electronic and electrical hardware used to perform all
computations, signal conditioning, data processing, and
formatting associated with navigation, guidance, control,
data and redundancy management. The IUS avionics subsystems
also provide the equipment for communications between the
Orbiter and ground stations, as well as electrical power
provision, control and distribution.

IUS Solid Rocket Motors (SRM's)

The IUS/PAM-S three-stage vehicle uses a large IUS
solid rocket motor, a small IUS solid rocket motor and a
PAM-S solid rocket motor. The IUS motors employ movable
nozzles for thrust vector control or steering. These nozzles
provide up to four degrees of steering on the large motor and
seven degrees on the small motor. The PAM-S solid rocket
motor employs no active steering.

The IUS-17 first stage motor will carry 21,638 pounds
of propellant; the second stage 6,057 pounds; the PAM-S has
4,437 pounds of propellant and expendable inerts.

Reaction Control System (RCS)
and Nutation Control System (NCS)

The IUS reaction control system controls the IUS and
payload's attitude during coasting, roll control during SRM
thrusting, and a final collision avoidance maneuver after
separation from the PAM-S/Ulysses spacecraft.

The reaction control system consists of hydrazine fuel
tankage and reaction control thrusters. IUS-17 will carry
one tank, with 50 pounds of fuel, for the IUS RCS.

To minimize spacecraft contamination, the IUS has no
forward facing thrusters.

The PAM-S has no RCS but uses spin stabilization and
nutation suppression to control vehicle pointing during its
operation. PAM-S carries propellant (six pounds) for its
nutation control system.

Following separation of the PAM-S/Ulysses spacecraft
from the IUS/PAM-S aft adapter, a gas generator system
mounted on the motor support section spins the
PAM-S/spacecraft to approximately 70 RPM. After spin-up the
motor support section is separated by release of a clamp band
assembly. The PAM-S and Ulysses spacecraft continue to spin
throughout PAM-S burn. At PAM-S burnout, the PAM-S/Ulysses
are despun using the despin assembly.

Despin Assembly

The despin assembly is used to reduce the spin rate of
the PAM-S/SC from approximately 70 RPM down to approximately
7 RPM after SRM burnout and prior to spacecraft separation.
The despin assembly consists of two weights (approximately 2
pounds each) attached to a 477-inch flat kevlar cable which
dissipate energy to slow the PAM-S during deployment. These
weights are released by cable cutters initiated by the
ordnance sequencing system and are detached automatically
from the PAM-S after the deployment.

Spacecraft Separation

Separation of the PAM-S from the spacecraft is
accomplished with four redundantly actuated pyrotechnic
devices at the spacecraft-to-PAM-S interface and two
redundantly actuated pyrotechnic disconnects at the
spacecraft RTG interface, and two electrical disconnects
between PAM-S and the spacecraft.

Flight Sequence

After the Orbiter payload bay doors are opened in
orbit, the Orbiter will maintain a preselected attitude to
keep the payload within thermal requirements and constraints.

On-orbit predeployment checkout begins, followed by an
IUS command link check and spacecraft communications command
check. Orbiter trim maneuver(s) are normally performed at
this time.

Forward payload restraints will be released and the aft
frame of the airborne support equipment will tilt the
Ulysses/IUS/PAM-S to 29 degrees. This will elevate the
spacecraft just outside the Orbiter payload bay, allowing
direct communication with the ground during systems checkout.
The Orbiter will then be maneuvered to the deployment
attitude. If a problem develops within the spacecraft or
upper stage, the upper stage and its payload can be restowed
from this position.

Prior to deployment, the payload electrical power
source will be switched from Orbiter power to IUS internal
power by the Orbiter flight crew. After verifying that the
payload is on IUS internal power and that all
Ulysses/IUS/PAM-S predeployment operations have been
successfully completed, a GO/NO-GO decision for
Ulysses/IUS/PAM-S deployment will be sent to the crew.

When the Orbiter flight crew is given a GO decision,
they will activate the ordnance that separates the IUS
umbilical cables. The crew will then command the
electromechanical tilt actuator to raise the tilt table to a
58-degree deployment position. The Orbiter's RCS thrusters
will be inhibited and an ordnance separation device initiated
to physically separate the Ulysses/IUS/PAM-S combination from
the tilt table. Compressed springs provide the force to
separate the Ulysses/IUS/PAM-S from the Orbiter payload bay
at approximately 0.14 meters (5.7 inches) per second. The
deployment is normally performed in the shadow or the Orbiter
or in Earth eclipse.

Following deployment, the tilt table will then be
lowered into the payload bay. A small Orbiter maneuver will
be made to back away from the Ulysses/IUS/PAM-S.
Approximately 19 minutes after deployment, the Orbiter's
engines will be ignited to move the Orbiter farther away from
the IUS/spacecraft.

At this point, the IUS/PAM-S is controlled by the IUS
onboard computers. Approximately 10 minutes after deployment
from the Orbiter, the IUS primary onboard computer will send
out signals used by the IUS to begin mission sequence
events. This signal will also enable the reaction control
system and the Ulysses RTG pressure release device. All
subsequent operations from transfer orbit injection through
PAM-S separation and IUS deactivation will be sequenced by
the IUS computer.

When the IUS RCS has been activated, the IUS will
maneuver to the required thermal attitude and perform any
required spacecraft thermal control maneuvers.

At approximately 45 minutes after deployment from the
Orbiter, the ordnance inhibits for the first solid rocket
motor will be removed. The belly of the Orbiter will have
already been oriented towards the Ulysses/IUS/PAM-S
combination to protect the Orbiter windows from the IUS's
plume. The IUS will recompute the first ignition time and
maneuvers necessary to attain the proper attitude for the
first thrusting period. When the proper transfer orbit
opportunity is reached, the IUS will orient to the proper
burn attitude and the IUS computer will send the signal to
ignite the first-stage motor. This is expected at
approximately 65 minutes after deployment. After firing
approximately 150 seconds, the IUS first stage will have
expended its fuel and then be separated from the second IUS
stage.

Approximately 125 seconds after burnout of the first
stage, the second-stage motor will be ignited, thrusting
about 108 seconds. The IUS second stage will then be
separated from the PAM-S and perform a final
collision/contamination avoidance maneuver before
deactivating. The PAM-S will ignite at approximately 83
seconds after IUS SRM-2 burnout. PAM-S burns approximately
88 seconds. Following a 10-minute coast and despin to
approximately 7 RPM, the Ulysses spacecraft will separate,
leaving the Ulysses spacecraft on its trajectory to Jupiter
and its final mission orbiting the poles of the Sun.

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