Go Taikonauts!

Inside Shenzhou Spacecraft

by Chen Lan


Spacecraft Overview

The Shenzhou spacecraft consists of four parts: the propulsion module, the reentry module, the orbital module, and the attachment package on top the orbital module. Its overall size can be found in following figures.

        

Actual length and mass of the spacecraft varies. For example, length of Shenzhou 1 and Shenzhou 2 is about 9.45m while length of Shenzhou 5 is about 9.2m. Launch mass of Shenzhou 4 is 7794kg while mass of Shenzhou 5 is about 7840kg.

The orbital module is a cylinder with truncated conical sections at both sides. Its diameter is 2.25m and length 2.8m. There is a hatch for ingress and EVA, and two portholes on the cylinder section. On the exterior of the module, there are a pair of solar panels with span of 10.4m, a propulsion package containing propellant tanks, Earth and Sun sensors, and a set of orbital maneuver and attitude control engines. Inside the module, there are three layers of racks to install equipments and payloads.

The reentry module is bell shaped. Its L/D (lift-drag ratio) is about 0.3 at attack angle of 20 degrees. Its maximum diameter is 2.517m and length is 2.5m. There is a 0.65m inner-diameter hatch at top, two portholes, an optical sight window, and the main and reserve parachute compartments. There are also six antenna mounting bases, a set of attitude control engines, the umbilical connector panel linking electrical/gas/liquid pipelines to the propulsion module, sensors and antennas, and the gamma ray altimeter and soft landing motors at bottom. The module is covered by ablative thermal protection materials. Inside the module, there are three seats, the control panel, the camera at sidewall, and gas bottles used during reentry. Most subsystem equipments are installed under the seats. Its interval volume is about 6m3.

The propulsion module is a non-pressurized cylinder with a truncated conical segment at bottom. Its length is 2.941m, maximum diameter is 2.8m (at bottom). A pair of solar panels with span of about 17m and a radiator is installed on exterior of the module. There are four main engines at bottom of the module and a set of attitude control engines at its side. Inside the module, there are multiple disk-shaped structures arranged along the module's axis, to install various subsystem equipments.

The attachment segment, normally non-pressurized, varies for individual missions. For example, the Shenzhou 5 attachment was a box with size of 0.9x1.268x0.86(m) in which a 1.6m resolution CCD camera was installed. It can be a docking mechanism in future.

Shenzhou spacecraft has 13 subsystems. Detailed descriptions of these subsystems are in sections below.

   

Structure and Mechanism Subsystem and Thermal Protection System

Shenzhou’s pressurized modules use semi-monocoque structure. For example, structure of the reentry module consists of titanium framework, aluminum alloy made sidewalls, the air-tight bottom and the heat shield.

Shenzhou’s reentry module uses ablative thermal protection technologies.

Maximum working temperature on sidewall of the reentry module, made by aluminum alloy, is about 200°C. It is covered by honeycomb-reinforced silicon based low-density ablative materials. Its ablative density varies from 0.72g/m3 at windward side to 0.56g/m3 at leeward side. There are more than 20 portholes, holes and covers at the module. Thermal protection rings made by medium-density (1.3g/m3-1.4g/m3) metlbond-glass composition material are used on these locations.

The heat shield at bottom of the module is the critical component with maximum heat load. It consists of the ablative layer, the base structural layer and the thermal protection ring at its edge. The ablative layer is 35mm-thick fiberglass honeycomb filled with silicon-based ablative material with density of 0.75g/m3. The base structural layer, used to withstand load and isolate heat, is 30mm-thick fiberglass sandwich structure. The thermal protection ring is made by short fiberglass and metlbond resin.

Total mass of the thermal protection layers covering about 22m2 is 450kg.

 

Environment Control and Life Support Subsystem

Shenzhou uses the non-recyclable environment control and life support system. It has 9 subsystems including gas supply and pressure control, ventilation and purification, temperature and humidity control, spacesuit loop, water management, food management, waste processing, smoke and fire monitoring and removal, measurement and control.

Both the orbital and the reentry module are pressurized. The cabin atmosphere is at 91±10kPa with oxygen pressure of 20-24kPa. It keeps temperature of 17-25°C (less than 40°C when reentry), humidity of 30%-70%, and maximum noise level of 125dB during launch and reentry, and 75dB during in-orbit operation. Maximum axial overload is 4g during normal lifting reentry, 11g during ballistic reentry, and 17g during emergency escape.

High-pressure nitrogen and oxygen gas bottles, at pressure of 21MPa, are made by high strength alloy steel. They are located in the propulsion module.

 

Thermal Control Subsystem

It is estimated that total heat inside the Shenzhou spacecraft is equivalent to about 1000W. There is a cooling system consisting of coldplates under equipments, condensers and desiccators inside the pressurized modules, sensors and controllers, pipelines linking to the heat exchanger, and the radiator installed on exterior of the propulsion module. On the orbital module, there are thermal control louvers. Shenzhou also uses other active or passive thermal control technologies, such as thermal control coating, multi-layer thermal isolations, fans and electrical heaters etc.

 

Guidance, Navigation and Control (GNC) Subsystem

Shenzhou GNC system includes digital and analogy Sun sensors, infrared Earth sensors, strapdown IMU, the hot-redundant GNC computer, attitude engine control system, and the manual control system.

The GNC computer is a three-unit hot-redundant fault tolerant computer using majority voting.

The manual control system includes optical observation windows, control panel, manual attitude control handles, the movement handle and their controller etc.

Shenzhou GNC system uses a series of new technologies including ascent control, in-orbit calibration of the strapdown IMU, fault diagnosis and system reconstruction, yaw maneuver control, L/D real-time prediction and self-adaptive reentry control etc.

 

Tracking, Control and Communication (TC&C) Subsystem

Shenzhou TC&C system consists of 6 subsystems: the in-orbit tracking and control subsystem, the data transmission subsystem, the communication subsystem, the rescue beacon subsystem, the mechanism parameter capturing and recording subsystem and the orbital module tracking and control system.

The tracking and control system includes the GPS receiver, the S-band transponders, the C-band single-pulse radar transponder, telemetry equipments and antennas. The orbital module tracking and control system is similar but without the GPS receiver and the C-band radar responder. The data transmission system includes data interfaces and the S-band transmitter. The Unified S-Band System (USB) is used to transmit combines telemetry, command, voice and video signals within one data stream. It is compliant to international standards.

The communication subsystem includes the voice processing unit, the communication headset, the video encoder, cameras, monitors, switches, the VHF communication system and antennas. Video signals are encoded in ITU-T H.261 format and transmitted in ITU-T H.221 format combined with voice signals. It then generates a data stream at rate of 768kb/s that is sent to the data transmission subsystem. Digital voice signals are encoded in 32KB/s, 24KB/s and 16KB/s, and are sent to the data transmission system, the S-band responders and the VHF system respectively. The VHF transceiver transmits the 24kB/s combined data stream of biological data and voice as backup of the USB system.

There are three kinds of rescue beacons at the reentry capsule. One is the 243MHz VHF beacon that sends orientation signals to the rescue helicopters. It starts transmission once the capsule leaves the radio blackout, at about 40km above the ground. The second is the HF beacon that sends position signals feed by the GPS receiver to the ground rescue vehicles. The HF system can also transmit two-way analogy voice signals during emergency. There are two erectable HF antennas at bottom and side of the capsule, one of which is deployed after landing depending on attitude of the capsule. The third beacon system is the 406MHz international Emergency Position Indicating Radio Beacon. The crew can also use a handheld emergency 406MHz beacon.

Shenzhou TC&C system uses a series of new technologies including the unified S-band system, digital video and voice processing, remote control and voice encryption, GPS based orbit determination, ablative antenna and microstrip antenna, high capacity solid-state storage technology etc. It utilizes C, S, L, UHF, VHF, HF bands and 17 antennas.

 

Data Management Subsystem

Shenzhou data management system is a distributed computer network system based on the 1553B bus.

There is a computer (CTU1 – Central Terminal Unit 1) on the reentry module that controls all tasks of the three modules during autonomous flight and reentry tasks after the orbital module and the propulsion module are separated. CTU1 is a three-unit hot-redundant fault tolerant computer using majority voting to achieve the Fail Operational – Fail Safe (FO-FS) requirement.

Another computer (CTU2) at the orbital module is a two-unit cold redundant computer. Control can be automatically switched to the backup unit if the main unit fails. It controls tasks of the orbital module during the 6-month extended mission.

There are also 6 single-board-computer based remote terminal units (RTU), two at each module, to collect data and transfer signals. There is a two-unit hot redundant data transmission and control unit (DTCU) at the propulsion module to combine 6 data streams into a single 768kb/s high-speed data stream for the downlink. There is an emergency data recorder (CRU – Clamant Record Unit), the so-called black box, to record various critical data from the reentry module. It has 10MB solid-state memory and is able to withstand overload of 10000g and temperature of 1200°C for 30 minutes.

All these devices, and other 11 terminals of other subsystems, form a local area network (LAN) by the standard serial 1553B bus at data rate of 1MB/s. The bus has dynamic configuration capability. Three modules share a redundant 1553B bus during flight.

 

Electrical Power Subsystem

The electrical power system consists of the primary power, the orbital module power, the reentry and landing power, the pyrotechnics power and the emergency power.

The primary power consists of two identical units, each of which has four solar panels, or a “single wing” at side of the propulsion module, a set of silver-zinc batteries and other equipments. Total area of solar array used by the primary power is 24.48m2. The orbital module power has totally four solar panels, two on each wing, at both side of the orbital module, a set of nickel–cadmium batteries and other equipments. Area of the orbital module solar panel is 12.24m2.

Both photovoltaic arrays use BSF (back surface filed) silicon solar cells with energy conversion efficiency of 14.8%. Frameworks of the solar panels are made by rigid aluminum honeycomb plates. There is a triangle frame at bottom to connect to the modules. There are sun sensors between panels to measure angle between the sunlight vector and the normal of the solar array so as to keep panel’s angle or rotate them by the attitude control system. There is a diverter at the triangle frame.

The emergency power system is inside the propulsion module. It uses silver-zinc batteries. It can provide six hours of power supply for the whole spacecraft in case of the primary power fails. Reentry and landing power and pyrotechnics power also use silver-zinc batteries.

All power units supply electricity to a single spacecraft-wide electric network. Voltage of the network’s bus is 28V. Output power of three solar power units is about 3.5kw.

It is the first time for China to use solar cells, nickel–cadmium battery and silver-zinc battery on a single spacecraft and an electric network supplied by three photovoltaic electricity generation units. It is the most complicated and comprehensive power system on Chinese spacecraft.

 

Propulsion Subsystem

The orbital module has sixteen 5N thrust monopropellant hydrazine engines used for maneuver, orbit keeping and attitude control, a 7-liter titanium gas tank working at pressure of 23Mpa, and a rubber propellant tank. The engines are divided into the main and backup groups.

The reentry module has eight 150N thrust hydrazine engines used for attitude control during reentry, two 4.5-liter titanium gas tanks working at pressure of 23Mpa, and two rubber tanks capable of storing 28kg of propellant. Similar to that of the orbital module, most equipment and the four roll thrusters near the bottom are divided into the main and backup groups.

The propulsion module is responsible for maneuver, orbit keeping, attitude control and retrofire, as well as high altitude escape. It uses a shared bipropellant supply system and has four 2500N thrust N2O4/MMH main engines with specific impulse of 290 seconds, eight 150N thrust and sixteen 5N thrust attitude control engines, six 20-liter titanium gas tank working at pressure of 23Mpa, and four 230-liter variable-thickness, variable-curvature metallic diaphragm-type propellant tanks working at pressure of 2Mpa. Except for tanks, all are redundantly configured and are divided into the main and backup groups. Launch mass of propellant stored in the propulsion module is about 1000kg.

 

Recovery and Landing Subsystem

Recovery and landing system consists of structure, parachutes, soft landing rockets, procedure control, pyrotechnics, airbag in the parachute compartment, and the position indicating subsystems.

The parachute system includes two pilot parachutes, the drogue parachute, the main parachute, and the reserve chute. The ring-sail type main chute has area of 1200m2 and weights about 90kg. The reserve chute has area of 760m2. The landing procedure is activated at altitude of 10km when the parachute compartment cover is jettisoned. The two pilot chutes are pulled out first and then the drogue chute. The main parachute is pulled out at last. After the heat shield is dropped at 5-6km altitude, single-point hanging of the main chute is changed to two-point hanging. The whole procedure reduces the capsule’s speed from 200m/s to about 8m/s.

There is a gamma ray altimeter at bottom of the reentry module. When the capsule is about 1m above the ground, it ignites four solid fuel soft landing rockets at bottom of the capsule. Final landing speed is around 3.5m/s. The shock-adsorbing seats will provide protection in case soft landing rockets fail.

To indicate location of the capsule, it has flashlight and dye marking materials to be released when splashed on the sea.

Landing precision is within an area of 30km by 15km.

 

Lifesaving Subsystem

There are two emergency scenarios. The launch escape tower on top of the spacecraft is used during ascent phase from ground to altitude of 110km. And the spacecraft itself can handle emergency during final ascent phase, orbital operation and reentry.

The launch escape system includes the escape tower and the upper part of the shroud. The tower is 8.35m in length and with six solid fuel motors. From up to down are four control motors, the low altitude separation motor (eight nozzles), and the low altitude escape motor(four nozzles). There are also six motors on the upper shroud among which are two high altitude separation motors and four high altitude escape motors. There are four aerodynamic flaps on the upper shroud. They are deployed to stabilize the escape vehicle during its high speed flight.

The low altitude escape mode is used below 39km, before jettison of the escape tower at about 120 seconds after the liftoff. In this case, the upper shroud, together with top two modules of the spacecraft will be separated from the lower shroud and the propulsion module. Motors on the escape tower will be ignited to pull the whole vehicle off the exploding launcher.

The high altitude escape mode works without the escape tower. In this case, once the upper shroud is separated from the launcher, it is the escape motors at side of the shroud that bring the vehicle to a safe distance from the launcher.

If emergency happens after jettison of the shroud, about 201 seconds after the liftoff, or above 110km, the whole spacecraft separates itself from the launcher, ignites its main engines at the propulsion module to keep it away from the launcher. Then it makes an emergency retrofire during the first or second orbit, or the non-standard 14th orbit. Emergency procedure during orbital operation is similar. If the GNC subsystem fails to take action, manual control can be applied to make a ballistic reentry. The emergency system can also abort a reentry at initial stage if the first and the second reentry attitude adjustment fails, to allows another try one day later.

Inside the reentry module, there are emergency food, water, GPS receiver and communication devices.

 

Instrumentation and Illumination Subsystem

The instrumentation and illumination Subsystem consists of the integrated digital display system, individual (mechanical-electrical or electrical-magnetic) instrument, manual handling and control system, and illumination equipments.

The integrated digital display system, the most important part of the control panel in the reentry module, includes two multi-functional color LCD displays. The control panel controller is hot redundant. The LCD panels are capable of displaying digits, text, icons, graphics and charts. It has a sophisticated user interface, for example, various data views, pagination, the zoom-able world map etc. Besides the LCD displays, there are various indicators, backup segmental digit displays and individual instruments, buzzers, switches, buttons and handles on the instrument panel. There is also a voice notification system to report important information to the crew by voice (in Mandarin).

The illumination system was designed ergonomically. All fluorescent lamps inside the spacecraft are non-flashing and brightness-adjustable.

 

Crew Subsystem

The crew system includes taikonauts, space suits, space food, medical monitoring and protection equipments, personal survival package, shock-absorbing cushion etc.

The crew is up to three members. From left to right are payload specialist, commander & pilot, copilot & flight engineer.

Space food includes recipe food for normal usage, reserved food for the possible extended mission, depressurization food for use inside the spacesuit, and emergency food for survival after landing. They all are stored in the personal survival package.

Space suits include the pressurized suit, the survival suit, underwear, working suits, headsets, and sleep bags etc.

The medical monitoring system can monitor heartbeat, respiration, body temperature and blood pressure of the crew. There are also medicines, health care stuff and exercise facilities.

The personal survival package, mainly used after landing, contains personal emergency radio beacon, smoke generator, signal rocket, flash light, dye marker, mirror, compass, life raft, pistol, knife, shark repellent, lighter, first aid kit, food and water etc.

 

Payload Subsystem

Payloads vary on specific missions.

 

References

  1. Zhu Zengyi, Fei Tian Meng Yuan, Huayi Publishing House, Beijing, 2003

  2. Qi Faren, Manned Spacecraft Technology (second edition), National Defense Industry Press, Beijing, 2003

  3. CAST, Shenzhou Spacecraft, Space International, November, 2003

Picture credit: Taikong Tansuo, Raumfahrt Concret

 (An edited version of this article was published on issue 4/5, 2004 of Raumfahrt Concret. Part of the content was published on the April, 2005 issue of  Spaceflight)

 

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