18
1 Introduction
1.4 Technical Challenges to Development of Lunar Lander
[5, 14, 16]
There are 6 major technical challenges from design to validation for the lunar lander.
The uncertainty of lunar terrain environment. The terrain of lunar surface is
complex including mare and upland. The mare is relatively smooth terrain covering
about 17 percentage of lunar surface, which is similar to the basin on the Earth.
The upland is the area higher than the mare covering about 83 percentage of lunar
surface, which is relatively rugged and undulatory. There are rock and craters in
different size and shape. Although there is some macro statistics information about
the distribution of craters, rock and slopes on lunar surface, it is still uncertain for
a particular landing site. Those environmental factors must be considered carefully
when a lunar lander is designed and simulation method for ground experiment and
test is chosen, which are also important for a successful soft-landing. If the terrain
of landing site is extremely rugged, the lunar lander might fail to have a stable land
on lunar surface.
Power descent without assistance of atmosphere. There is no atmosphere on lunar
surface. So powered descent of the lunar lander must be done by its own propulsion
system without assistance of air brake. The velocity of the lunar lander relative the
Moon must be decelerated from 1.7 km/s to several meters per second in order to
have a soft-landing. It was challenging for the propulsion system to provide enough
Delta-V and throttleable thrust at the same time. Because the Delta-V was big, a great
many propellant must be fueled in propulsion system. Sometimes the wet mass (mass
of the propellant) was bigger than 2/3 of the total launch mass of the lunar lander.
Thus a series of technical difficulties were induced in system design and structure
design of the lunar lander.
Autonomous navigation and control of landing process. The period of powered
descent was about 10 minutes, which was relatively short. The dynamics of landing
process varied fast so it was hard for ground operators to control the lander
during powered descent. It must be done autonomously in close loop control. To
complete autonomous control tasks including deceleration, descent along with attitude maneuver, hovering and obstacles avoidance, slow descent, autonomous navigation and control must be achieved by speedometer, ranging and terrain identification sensors onboard the lander. It was a big challenge because there was no such
technology in the past spacecraft including accurate terrain identification, control
of velocity and height, control of throttleable thrust, attitude control and obstacles
avoidance maneuvers.
Cushion of landing impact. When the lander touched lunar surface, the velocity
relative to the Moon was not zero. So the landing cushion system should absorb the
impact energy of the lander to avoid possible damage to devices onboard the lander.
At the same time, requirement of landing stability and envelop constraints of launch
vehicle fairing must be satisfied when the landing cushion system was designed.
1 Introduction
1.4 Technical Challenges to Development of Lunar Lander
[5, 14, 16]
There are 6 major technical challenges from design to validation for the lunar lander.
The uncertainty of lunar terrain environment. The terrain of lunar surface is
complex including mare and upland. The mare is relatively smooth terrain covering
about 17 percentage of lunar surface, which is similar to the basin on the Earth.
The upland is the area higher than the mare covering about 83 percentage of lunar
surface, which is relatively rugged and undulatory. There are rock and craters in
different size and shape. Although there is some macro statistics information about
the distribution of craters, rock and slopes on lunar surface, it is still uncertain for
a particular landing site. Those environmental factors must be considered carefully
when a lunar lander is designed and simulation method for ground experiment and
test is chosen, which are also important for a successful soft-landing. If the terrain
of landing site is extremely rugged, the lunar lander might fail to have a stable land
on lunar surface.
Power descent without assistance of atmosphere. There is no atmosphere on lunar
surface. So powered descent of the lunar lander must be done by its own propulsion
system without assistance of air brake. The velocity of the lunar lander relative the
Moon must be decelerated from 1.7 km/s to several meters per second in order to
have a soft-landing. It was challenging for the propulsion system to provide enough
Delta-V and throttleable thrust at the same time. Because the Delta-V was big, a great
many propellant must be fueled in propulsion system. Sometimes the wet mass (mass
of the propellant) was bigger than 2/3 of the total launch mass of the lunar lander.
Thus a series of technical difficulties were induced in system design and structure
design of the lunar lander.
Autonomous navigation and control of landing process. The period of powered
descent was about 10 minutes, which was relatively short. The dynamics of landing
process varied fast so it was hard for ground operators to control the lander
during powered descent. It must be done autonomously in close loop control. To
complete autonomous control tasks including deceleration, descent along with attitude maneuver, hovering and obstacles avoidance, slow descent, autonomous navigation and control must be achieved by speedometer, ranging and terrain identification sensors onboard the lander. It was a big challenge because there was no such
technology in the past spacecraft including accurate terrain identification, control
of velocity and height, control of throttleable thrust, attitude control and obstacles
avoidance maneuvers.
Cushion of landing impact. When the lander touched lunar surface, the velocity
relative to the Moon was not zero. So the landing cushion system should absorb the
impact energy of the lander to avoid possible damage to devices onboard the lander.
At the same time, requirement of landing stability and envelop constraints of launch
vehicle fairing must be satisfied when the landing cushion system was designed.