Mechanical Devices Subsystem (DEV)

The Mechanical Devices Subsystem (DEV) typically supplies equipment to the spacecraft that provides non-feedback controlled motion. It also typically supplies pyrotechnically initiated mechanisms. For the Cassini spacecraft, the DEV supplies a number of spacecraft/launch vehicle separation mechanisms, as well as all of the required pyrotechnic devices and initiators. It also provides a deployable magnetometer science boom; an articulated platform for a redundant reaction wheel (separately supplied by the AACS); passive (non-electrical), thermostatically controlled variable heat transfer mechanisms called thermal louvers; and variable radioisotope heater units.

The DEV is also responsible for the mechanical and thermal integration of the boom-mounted magnetometer science instrument sensors and the Sun sensor heads, which are components of the AACS.

Structural separation of the Cassini orbiter from the lower adapter/Centaur launch vehicle is accomplished by the linear separation assembly (LSA). The cylindrical shell of the lower adapter is structurally attached to the shell of the lower equipment module with two frangible (i.e., easily broken) plates, rolled and fastened as doublers between the two structures. A soft metal tube between the two plates is forcefully expanded by a pyrotechnic detonation cord installed within it, which causes the frangible plates to deform and break at circumferential notches, resulting in structural separation. The products of combustion are contained by the metal tube, and after separation all material is retained either on the spacecraft side or the ejected lower adapter/Centaur side. The LSA structural elements, the soft metal tube, and associated components are provided by the Structure Subsystem, while the DEV provides the pyrotechnic elements.

The primary components of the DEV are the detonation cord assembly with detonators, detonator blocks, separation ring assemblies, the magnetometer boom assembly, the articulated reaction wheel mechanism (ARWM) with a dual-drive actuator (DDA) and a pyrotechnic pinpuller, variable radioisotope heater units (VHRUs), thermal louver assemblies, and Sun sensor head integration structures. For more information on these components, click on their names.

The linear separation assembly (LSA) is activated by a high-explosive cord installed in a silicone rubber matrix. This detonation cord assembly is installed after spacecraft assembly but prior to integration to the launch vehicle. The detonation cord is activated by redundant detonators at the ends of the cord, which are initiated by electroexplosive cartridges.

Detonator blocks bolted to flanges on each end of the soft metal detonation cord assembly tube function to integrate the ends of the detonator cord into a chamber containing a detonator port. After separation, the detonator blocks seal the ends of the metal tube, thus containing the products of combustion within the tube.

Three separation ring assemblies, mounted equally spaced on the lower adapter structure, are used to accelerate the spacecraft away from the Centaur/adapter structure during the separation event. Each assembly consists of a compression spring controlled by a piston rod, which is guided by a pair of linear rolling bearings.

After separation, the spacecraft side of the Cassini/Centaur umbilical electrical connectors need to be protected against electrostatic discharge (ESD) or "static electricity." This is accomplished by separation connector covers--spring-loaded metal doors, electrically grounded to the spacecraft, that enclose the connector cavities on the spacecraft.

The magnetometer boom assembly consists of a number of mechanical elements. The magnetometer boom mast assembly includes two coilable, continuous mast structures that are self-erecting in microgravity. After deployment, the triangular cross section of the magnetometer boom mast is oriented in such a way that one flat surface is nominally sun-facing during cruise, allowing the thermal "sock" (discussed below) to sunshade the mast components.

The magnetometer boom canister holds the stowed magnetometer boom assembly prior to deployment, and, with the support truss, provides a structural mount between the spacecraft and the magnetometer boom. A "Mag Boom Released" telemetry signal microswitch is incorporated on the canister. A controlled rate of deployment of is provided by the use of a magnetometer boom deployment rate limiter mechanism. The rate limiter consists of a silicone fluid-filled shear damper that plays out a lanyard from a spool. The lanyard is tensioned between the deploying mast and the restraining damper. A "Mag Boom Deployed" telemetry signal microswitch is incorporated into the rate limiter.

The magnetometer boom midsection provides a structural link between the inboard and outboard coilable mast sections, a mounting interface for the inboard magnetometer thermal mount, a latching interface for the mast release mechanism, and provisions for any required midsection-mounted sunshades. The deployable mast sections are covered with a single layer of thermal blanketing, called the magnetometer thermal sock. The thermal sock is spaced a sufficient distance from the mast elements to provide micrometeoroid and ESD protection and thermal control while allowing unhindered deployment. The magnetometer boom pyrotechnic pinpuller latches the magnetometer boom canister launch restraint system. Upon command, the pinpuller actuates the magnetometer boom release mechanism, allowing the boom to deploy.

There are two types of mounts on the magnetometer boom midsection. The inboard magnetometer thermal mount attaches the flux gate magnetometer (FMG) to the boom midsection and provides a space-facing thermal radiator, integral radioisotope heater unit (RHU) containers, and thermal blanket attachment points. The outboard magnetometer mounting plate attaches the scalar/vector helium magnetometer (S/VHM) to the boom mast structure and contains provisions for attaching sunshades and other thermal control hardware as necessary.

The spare reaction wheel (remember, the reaction wheels are four-for-three redundant) is mechanically and electrically integrated to the Cassini orbiter by the articulated reaction wheel mechanism (ARWM). The reaction wheel is bolted to the movable ARWM platform and is supported through the launch environment by a large, preloaded pair of angular contact bearings. Launch position, which is the center null position of the movable platform, is maintained by a latch mechanism operated by a pyrotechnic pinpuller. The movable platform can be rotated upon command to one of two positions, plus or minus 2.09 radians (120 degrees) from null, by a d.c. motor/gearhead. To enable the reaction wheel to transmit torque effectively to the spacecraft, zero backlash is maintained between the movable platform and the spacecraft by the use of a friction drag brake.

All of the electrical interfaces for the reaction wheel pass through the ARWM by the use of a flex capsule. The flex capsule provides micrometeoroid protection and thermal control to the reaction wheel cabling across the rotating joint, allowing minimum torque variations and the highest reliability. Four microswitches are incorporated in the ARWM to provide bi-level voltage position telemetry indications. Each rotation end-position is indicated within 90 milliradians by bi-level voltage telemetry. The ARWM also contains two removable subassemblies, the dual-drive actuator and the pyrotechnic pinpuller.

The movable platform of the ARWM is rotated by the dual-drive actuator (DDA). The DDA is a fully redundant rotary actuator that uses independent motors and gear trains to drive a common output plate. The motors are a rare-earth permanent-magnet type, utilizing a 30-volt d.c. power supply. The gearbox consists of two independent 20:1 spur gear input stages driving two coaxially mounted, independent 110:1 harmonic gearsets that drive the common output plate. The DDA accelerates the inertia of the reaction wheel, the movable platform, and associated masses and drives against the torque losses of the flex capsule, the bearings, the drag brake, and the microswitch actuators.

The second removable subassembly of the ARWM is the pyrotechnic pinpuller. This unit provides rotational restraint of the movable platform during launch. The pyrotechnic pinpuller used on Cassini consists of a piston with an integral latch pin housed in an enclosed chamber. The pinpuller is driven by pressure from the gas generation of dual simultaneously fired NASA standard initiators (NSIs).

The variable radioisotope heater units (VRHU) accomplish variable heat transfer from a fixed heat source (an RHU) through the mechanical rotation of the RHU housing. The Cassini VRHUs consist of a cylindrical housing, designed to contain up to three RHUs, which is mounted on rolling element bearings to allow rotation about the long axis of the cylinder. The housing is insulated lengthwise over half of its surface and painted with a high-emissivity thermal coating along the other half. The rotation position that places the insulated surface towards space and the high-emissivity surface toward the mounting surface radiatively couples the RHU housing to the VRHU mounting surface (i.e., maximizes the heat transfer from the housing to the mounting surface). conversely, heat transfer to the VRHU mounting surface is minimized when the high-emissivity surface of the RHU housing has been rotated space-facing. The rotation of the housing is accomplished thermostatically (i.e., automatically, by thermostat) using a bimetallic spring actuator thermally coupled to the mounting surface.

Radiative heat rejection (i.e., heat dissipation) is necessary for some Cassini spacecraft components, and this is accomplished by thermal louvers. The thermal louver assemblies provide thermostatically positioned, low-emissivity blades located over a high-emissivity radiative mounting surface. The blades are mounted in such a way as to minimize their thermal coupling to the mounting surface. When the blades are opened, the mounting surface is exposed to space, thereby providing heat rejection from that surface. Like the VRHU housing, the louver blades are positioned using bimetallic spring actuators thermally coupled to the mounting surface.

The two identical Sun sensor heads (SSHs), which are part of the AACS, are mechanically and thermally integrated to the Cassini spacecraft on the high-gain antenna (HGA) by box-like structures mounted on bipod struts. These assemblies are called the Sun sensor head integration structures. The electrical cabling for the SSHs passes through the integration structures and is hardmounted to them in accessible locations. The bipod mounting struts provide thermal isolation from the HGA, and the SSH allowable flight temperature range is maintained by using RHUs, thermal blanketing, and a space-facing thermal radiator.