3.1 Concepts of Space Flights
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In order to carry out various space activities, a large-scale space engineering
system needs to be built, which is usually composed of five major systems: transportation system, launch site system, spacecraft system, telemetry, tracking and control
(TT&C) system, as well as application system [1]. Besides the five major systems,
the manned space engineering system should also particularly be included other two
systems, the astronaut system and the recovery and landing system. The spacecraft
system is a core segment in the whole space engineering system and accomplishes
specific space missions together by cooperating and coordinating together with other
systems. According to whether there are crews involved in space missions, the spacecrafts can usually be classified into two categories: unmanned spacecraft and manned
spacecraft, as shown in Fig. 3.1, where the former is also be divided into artificial
earth satellites and space probes, and the latter into space stations, manned vehicles
and space shuttles.
Fig. 3.1 Classification of the spacecrafts