3.3 Mission Analysis
57
(1) Flight process before landing. There were two approaches to achieve soft
landing: one was direct landing and another was orbiting the Moon before
landing. For the direct landing, the trajectory of lunar transfer orbit (LTO) was
designed to intersect with lunar surface directly. When the probes approached
the Moon, powered descent was performed at the end of approach and the probe
was landed on the Moon at relative low speed, such as Soviet Luna 9 and Luna
11, the U.S. Surveyor landers. For the orbiting before landing, the trajectory of
LTO would not intersect with lunar surface. When the probe approached the
Moon, orbit insertion was performed for the probe to enter into lunar orbit. The
altitude would be adjusted at appropriate time and then the powered descent
was performed to land on the Moon, such as Soviet Luna 16, Luna 17, Luna
21 and Luna 24, the U.S. Apollo program.
(2) Design of lunar orbit. There were constraints including latitude of landing site,
Delta V of lunar orbit insertion (LOI), safety of LOI and long term safety of
lunar orbit in lunar orbit design. The altitude of perilune and semi-major axis
should be determined as well as inclination in lunar orbit design.
The capability of navigation and control, propellant consumption, thrust for
LOI, and safety of trajectory maneuver should be considered when the perilune
altitude before powered descent was determined. For example, if the altitude
of perilune was higher, the time for powered descent would be long so that the
propellant consumption and error of landing would increase. If the altitude of
perilune was lower, the accuracy of trajectory maneuver should be higher and
the risk of impacting lunar surface would increase. The inclination of the lunar
orbiter should be greater than the latitude of the landing site. Generally the
lander could be landed at anywhere on the Moon if the lunar orbit was polar
orbit.
(3) Selection of launch trajectory type. There were two types of launch trajectory
for lunar landing mission including phasing orbit and direct LTO. It would take
full advantage of the launch vehicle capability and decrease the requirement
of propellant consumption for lunar lander to be launched directly into LTO,
which was also common in deep space exploration.
2. Requirements of powered descent.
The powered descent process of soft landing was the key to the safe landing of
lunar lander. There were a series of new requirements different from conventional
spacecraft.
(1) Requirements for navigation and control. Guidance, navigation and control of
lunar lander during powered descent could not be provided by ground control
because of short time, great variation of velocity, unpredictable altitude and
velocity relative to lunar surface. Autonomous guidance, navigation and control
could be achieved by the speedometer and ranging sensors of GNC subsystem
and cooperation among GNC, propulsion and other subsystems. There might
be slopes, rock and pits in lunar terrain which could not be predicted, so lunar
lander should be capable of autonomous identifying lunar terrain and avoiding
obstacles to ensure not to turn over after soft landing.
57
(1) Flight process before landing. There were two approaches to achieve soft
landing: one was direct landing and another was orbiting the Moon before
landing. For the direct landing, the trajectory of lunar transfer orbit (LTO) was
designed to intersect with lunar surface directly. When the probes approached
the Moon, powered descent was performed at the end of approach and the probe
was landed on the Moon at relative low speed, such as Soviet Luna 9 and Luna
11, the U.S. Surveyor landers. For the orbiting before landing, the trajectory of
LTO would not intersect with lunar surface. When the probe approached the
Moon, orbit insertion was performed for the probe to enter into lunar orbit. The
altitude would be adjusted at appropriate time and then the powered descent
was performed to land on the Moon, such as Soviet Luna 16, Luna 17, Luna
21 and Luna 24, the U.S. Apollo program.
(2) Design of lunar orbit. There were constraints including latitude of landing site,
Delta V of lunar orbit insertion (LOI), safety of LOI and long term safety of
lunar orbit in lunar orbit design. The altitude of perilune and semi-major axis
should be determined as well as inclination in lunar orbit design.
The capability of navigation and control, propellant consumption, thrust for
LOI, and safety of trajectory maneuver should be considered when the perilune
altitude before powered descent was determined. For example, if the altitude
of perilune was higher, the time for powered descent would be long so that the
propellant consumption and error of landing would increase. If the altitude of
perilune was lower, the accuracy of trajectory maneuver should be higher and
the risk of impacting lunar surface would increase. The inclination of the lunar
orbiter should be greater than the latitude of the landing site. Generally the
lander could be landed at anywhere on the Moon if the lunar orbit was polar
orbit.
(3) Selection of launch trajectory type. There were two types of launch trajectory
for lunar landing mission including phasing orbit and direct LTO. It would take
full advantage of the launch vehicle capability and decrease the requirement
of propellant consumption for lunar lander to be launched directly into LTO,
which was also common in deep space exploration.
2. Requirements of powered descent.
The powered descent process of soft landing was the key to the safe landing of
lunar lander. There were a series of new requirements different from conventional
spacecraft.
(1) Requirements for navigation and control. Guidance, navigation and control of
lunar lander during powered descent could not be provided by ground control
because of short time, great variation of velocity, unpredictable altitude and
velocity relative to lunar surface. Autonomous guidance, navigation and control
could be achieved by the speedometer and ranging sensors of GNC subsystem
and cooperation among GNC, propulsion and other subsystems. There might
be slopes, rock and pits in lunar terrain which could not be predicted, so lunar
lander should be capable of autonomous identifying lunar terrain and avoiding
obstacles to ensure not to turn over after soft landing.