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3 System Design of Lunar Lander
3.4.4 Analysis of Circumlunar Orbit
Major concerns of determination of circumlunar orbit before powered descent were
including:
(1) Inclination of circumlunar orbit. It was better for the powered descent in orbit
plane of circumlunar orbit. If it was not, a massive amount of propellant would
be requirement to implement orbit maneuver outside the orbit plane. Therefore,
the landing site should be on track of subsatellite point of circumlunar orbit
before powered descent. It would be helpful to increase landing opportunities
for the polar orbit with 90° inclination to cover global lunar surface.
(2) Altitude of perilune. The altitude of perilune of circumlunar orbit was defined
as 100 km after synthesis of circumlunar orbit safety and Delta V requirement
for LOI.
(3) Altitude of start point of powered descent. It should be considered for orbital
safety and adaptability to lunar terrain undulatory along powered descent path
when the altitude of start point of powered descent was determined. The altitude
of the highest mountain is 10 km. So the range of altitude of start point of
powered descent was usually set to 12–18 km after trade-off.
(4) Time on circumlunar orbit. The time of circumlunar orbit was up to the relative
position of landing site to the orbit plane of circumlunar orbit after LOI. The
orbit shadow and power balance of lunar lander should be considered when
the time on circumlunar orbit was determined.
(5) Illumination condition of circumlunar orbit. The illumination condition on
lunar surface had direct relation to the β angle of circumlunar orbit (the angle
of orbit plane to solar vector) before powered descent for lunar lander. If the
solar elevation on landing site was 15° when lunar lander was landed, the β
angle of circumlunar orbit was 68.9°. If the solar elevation on landing site was
40° when lunar lander was landed, the β angle of circumlunar orbit was 26.5°.
3.4.5 Determination of LTO
The aim of LTO determination is to decrease Delta V requirement of LOI for lunar
lander. The LTO with 110–140 h transfer time is optimum in energy for the lunar
polar orbit after analysis. At the same time, the constraints should be satisfied such as
argument of perigee for launch trajectory, illumination condition of LTO, telecommunication access of LOI, launch window requirements, and etc. The major parameters
of LTO for the CE-3 lunar lander are defined after analysis as following.
(1) Altitude of perigee for LTO: 200 km.
(2) Altitude of perilune: 100 km.
(3) Inclination of LTO: 28.5°.
(4) Argument of perigee for LTO: 143.579°–225.530°
(5) Flight time of LTO: about 112 h or 114 h, 136 h or 138 h.
3 System Design of Lunar Lander
3.4.4 Analysis of Circumlunar Orbit
Major concerns of determination of circumlunar orbit before powered descent were
including:
(1) Inclination of circumlunar orbit. It was better for the powered descent in orbit
plane of circumlunar orbit. If it was not, a massive amount of propellant would
be requirement to implement orbit maneuver outside the orbit plane. Therefore,
the landing site should be on track of subsatellite point of circumlunar orbit
before powered descent. It would be helpful to increase landing opportunities
for the polar orbit with 90° inclination to cover global lunar surface.
(2) Altitude of perilune. The altitude of perilune of circumlunar orbit was defined
as 100 km after synthesis of circumlunar orbit safety and Delta V requirement
for LOI.
(3) Altitude of start point of powered descent. It should be considered for orbital
safety and adaptability to lunar terrain undulatory along powered descent path
when the altitude of start point of powered descent was determined. The altitude
of the highest mountain is 10 km. So the range of altitude of start point of
powered descent was usually set to 12–18 km after trade-off.
(4) Time on circumlunar orbit. The time of circumlunar orbit was up to the relative
position of landing site to the orbit plane of circumlunar orbit after LOI. The
orbit shadow and power balance of lunar lander should be considered when
the time on circumlunar orbit was determined.
(5) Illumination condition of circumlunar orbit. The illumination condition on
lunar surface had direct relation to the β angle of circumlunar orbit (the angle
of orbit plane to solar vector) before powered descent for lunar lander. If the
solar elevation on landing site was 15° when lunar lander was landed, the β
angle of circumlunar orbit was 68.9°. If the solar elevation on landing site was
40° when lunar lander was landed, the β angle of circumlunar orbit was 26.5°.
3.4.5 Determination of LTO
The aim of LTO determination is to decrease Delta V requirement of LOI for lunar
lander. The LTO with 110–140 h transfer time is optimum in energy for the lunar
polar orbit after analysis. At the same time, the constraints should be satisfied such as
argument of perigee for launch trajectory, illumination condition of LTO, telecommunication access of LOI, launch window requirements, and etc. The major parameters
of LTO for the CE-3 lunar lander are defined after analysis as following.
(1) Altitude of perigee for LTO: 200 km.
(2) Altitude of perilune: 100 km.
(3) Inclination of LTO: 28.5°.
(4) Argument of perigee for LTO: 143.579°–225.530°
(5) Flight time of LTO: about 112 h or 114 h, 136 h or 138 h.