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3 System Design of Lunar Lander
3.8.2 Process of Powered Descent and Soft Landing
There were seven phases in powered descent of Chang’E-3 lunar lander including
preparation of ignition, main deceleration, rapid attitude adjustment, approaching,
hovering, obstacle avoiding and slow descent.
(1) Preparation of ignition. The major tasks of preparation of ignition were to
correct the time and orbit of ignition, calculate the attitude for ignition and
adjust attitude as calculated before ignition. The attitude was determined by
gyroscopes together with star sensors and controlled with 10 N thrusters during
preparation of ignition.
(2) Main deceleration. The altitude relative to the Moon was from 15 km to 3 km
during main deceleration. The major tasks were to decelerate velocity to the
Moon from about 1.7 km/s to a predefined value and lower the altitude to
about 3 km. The trajectory was a natural gentle descent trajectory caused by
deceleration with orbit control engine according to the requirements of main
deceleration. The drop of altitude was about 12 km and the range was about
430 km.
The initial pitch angle relative to lunar surface was 90° at the start point of
main deceleration. The final pitch angle relative to lunar surface was about 60°
at the end of main deceleration.
(3) Rapid attitude adjustment. The altitude relative to the Moon was from 3 km
to 2.7 km during rapid attitude adjustment. The major tasks were to connect
main deceleration with approaching, shift attitude to entry of approach rapidly
and lower the thrust of main engine at the same time. The trajectory was
a natural gentle descent trajectory caused by deceleration with orbit control
engine according to the requirements of rapid attitude adjustment. The drop of
altitude was about 300 m and the range was about 1 km.
The initial pitch angle relative to lunar surface was 60° at the start point of
rapid attitude adjustment. The final pitch angle relative to lunar surface was
about 9° at the end of rapid attitude adjustment.
(4) Approaching. The altitude relative to the Moon was from 2.7 km to 100 m
during approaching. The major task was to avoid obstacle roughly. The trajectory of approaching target landing site was designed to satisfy the requirements
of attitude and descent trajectory in order to align field of view of optical sensor
with landing site according to requirement of rough obstacle avoiding. Lunar
lander was approaching landing site gradually at 45° pitch angle in a nearly
straight line. The large obstacles such as rock or pits whose diameter was
greater than 1 m were identified by optical sensors to determine safe landing
site and avoid obstacles. At last lunar lander approached the position above
the landing site at 100 m height and near 0 relative velocity. The range of
approaching along the flight path was about 3 km.
The initial pitch angle relative to lunar surface was 9° at the start point of
approaching. The process of rough obstacle avoiding was implemented by
adjusting attitude angle of pitch and yaw of lunar lander. When the necessary
3 System Design of Lunar Lander
3.8.2 Process of Powered Descent and Soft Landing
There were seven phases in powered descent of Chang’E-3 lunar lander including
preparation of ignition, main deceleration, rapid attitude adjustment, approaching,
hovering, obstacle avoiding and slow descent.
(1) Preparation of ignition. The major tasks of preparation of ignition were to
correct the time and orbit of ignition, calculate the attitude for ignition and
adjust attitude as calculated before ignition. The attitude was determined by
gyroscopes together with star sensors and controlled with 10 N thrusters during
preparation of ignition.
(2) Main deceleration. The altitude relative to the Moon was from 15 km to 3 km
during main deceleration. The major tasks were to decelerate velocity to the
Moon from about 1.7 km/s to a predefined value and lower the altitude to
about 3 km. The trajectory was a natural gentle descent trajectory caused by
deceleration with orbit control engine according to the requirements of main
deceleration. The drop of altitude was about 12 km and the range was about
430 km.
The initial pitch angle relative to lunar surface was 90° at the start point of
main deceleration. The final pitch angle relative to lunar surface was about 60°
at the end of main deceleration.
(3) Rapid attitude adjustment. The altitude relative to the Moon was from 3 km
to 2.7 km during rapid attitude adjustment. The major tasks were to connect
main deceleration with approaching, shift attitude to entry of approach rapidly
and lower the thrust of main engine at the same time. The trajectory was
a natural gentle descent trajectory caused by deceleration with orbit control
engine according to the requirements of rapid attitude adjustment. The drop of
altitude was about 300 m and the range was about 1 km.
The initial pitch angle relative to lunar surface was 60° at the start point of
rapid attitude adjustment. The final pitch angle relative to lunar surface was
about 9° at the end of rapid attitude adjustment.
(4) Approaching. The altitude relative to the Moon was from 2.7 km to 100 m
during approaching. The major task was to avoid obstacle roughly. The trajectory of approaching target landing site was designed to satisfy the requirements
of attitude and descent trajectory in order to align field of view of optical sensor
with landing site according to requirement of rough obstacle avoiding. Lunar
lander was approaching landing site gradually at 45° pitch angle in a nearly
straight line. The large obstacles such as rock or pits whose diameter was
greater than 1 m were identified by optical sensors to determine safe landing
site and avoid obstacles. At last lunar lander approached the position above
the landing site at 100 m height and near 0 relative velocity. The range of
approaching along the flight path was about 3 km.
The initial pitch angle relative to lunar surface was 9° at the start point of
approaching. The process of rough obstacle avoiding was implemented by
adjusting attitude angle of pitch and yaw of lunar lander. When the necessary