Spacecraft bus (James Webb Space Telescope)

The spacecraft bus is a carbon fibre box that houses systems of the telescope and so is the primary support element of the James Webb Space Telescope, launched on 25 December 2021. It hosts a multitude of computing, communication, propulsion, and structural components. The other three elements of the JWST are the Optical Telescope Element (OTE), the Integrated Science Instrument Module (ISIM), and the sunshield. Region 3 of ISIM is also inside the spacecraft bus. Region 3 includes the ISIM Command and Data Handling subsystem and the Mid-Infrared Instrument (MIRI) cryocooler. The spacecraft bus must structurally support the 6.5 ton space telescope, while weighing only 350 kg (770 lb). It is made primarily of graphite composite material. It was assembled by Northrop Grumman in Redondo Beach, California by 2015, and then it had to be integrated with the rest of the space telescope leading up to its planned 2018 launch. The bus can provide pointing precision of one arcsecond (1⁄3600°) and isolates vibration down to two milliarcseconds. The fine pointing is done by the JWST fine guidance mirror, obviating the need to physically move the whole mirror or bus. The spacecraft bus is on the Sun-facing "warm" side and operates at a temperature of about 300 kelvins (80 °F, 27 °C). Everything on the Sun-facing side must be able to handle the thermal conditions of JWST's halo orbit, which has one side of continuous sunlight and the other shaded by the spacecraft sunshield. Another important aspect of the spacecraft bus is the central computing, memory storage, and communications equipment. The processor and software direct data to and from the instruments, to the solid-state memory core, and to the radio system which can send data back to Earth and receive commands. The computer also controls the pointing and movement of the spacecraft, taking in sensor data from the gyroscopes and star tracker, and sending the necessary commands to the reaction wheels or thrusters.

Overview

The bus is a carbon fibre box that houses a large number of major systems that keep the telescope functioning, such as the solar panels and computers. It also contains the MIRI cooler and some ISIM electronics. There are six major subsystems in the spacecraft bus:

Electrical Power Subsystem Attitude Control Subsystem Communication Subsystem Command and Data Handling Subsystem (C&DH) Command Telemetry Processor Solid State Recorder (SSR) Propulsion Subsystem Thermal Control Subsystem The spacecraft bus has three star trackers (plus one for redundancy), six reaction wheels, and the propulsion systems (fuel tank and thrusters). Two major tasks are pointing the telescope and performing station keeping for its metastable L2 halo orbit.

Command and Data Handling (C&DH) The Command and Data Handling system includes a computer, the Command Telemetry Processor (CTP), and a data storage unit, the Solid State Recorder (SSR), with a capacity of 58.9 GB.

Communications The communications dish which can point at Earth is attached to the bus. There is Ka-band and S-band radio communication. The Common Command and Telemetry System is based on Raytheon ECLIPSE system. The system is designed to communicate with NASA's Deep Space Communication Network. The main Science and Operations Center is the Space Telescope Science Institute in the U.S. state of Maryland.

Rocket engines, attitude control The JWST uses two types of thrusters. The Secondary Combustion Augmented Thrusters (SCAT) use hydrazine (N2H4) and the oxidizer dinitrogen tetroxide (N2O4) as propellants. There are four SCATs in two pairs. One pair is used to propel the JWST into orbit, and the other performs station keeping in orbit. There are also eight Monopropellant Rocket Engines (MRE-1), so called because they use only hydrazine as fuel. They are used for attitude control and momentum unloading of the reaction wheels. JWST has six reaction wheels for attitude control, spinning wheels that allow the orientation to be changed without using propellant to change momentum. Finally, there are two titanium helium tanks to provide unregulated pressurant for all propellants. To detect changes in direction JWST uses hemispherical resonator gyroscopes (HRG). HRGs are expected to be more reliable than the gas-bearing gyroscopes that were a reliability issue on Hubble Space Telescope. They cannot point as finely, however, which is overcome by the JWST fine guidance mirror.

Thermal Thermal systems on the bus include the Deployable Radiator Shade Assemblies. There are two, one vertical (DRSA-V) and one horizontal (DRSA-H), for vertical and horizontal respectively (with respect to the coordinate system of the spacecraft bus). The membrane that makes up the DRSA is a coated Kapton membrane. Other thermal elements on the outside include a small radiator for the battery. There is also a narrow lower-fixed radiator shade, also made of coated Kapton membrane. The coating of the membrane is silicon and VPA. Other areas of the outside are covered with JWST multi-layer insulation (MLI).

… excerpt ends here. Continue reading the full article.