8.3 Design Constraints and Analysis
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expulsion efficiency could be more than 99%. It was necessary to consider expulsion efficiency, flow rate, anti-vortexing, anti-sloshing, long term propellant compatibility, overload capacity, mechanical endurance, isolation complexity for two propellant components, structural mass and configuration, repeated venting times and etc.
when the type of propellant tank was determined. The surface tension or metallic
diaphragm tank was usually chosen for the lunar landers, for short term the lunar
lander the non-metallic bladder type might be the choice. For example, the composite
wrapped metallic diaphragm tank of expulsion efficiency more than 99% was chosen
for propulsion subsystem of the Chang’E-3 lunar lander.
6. Parallel Tank Propellant Equilibrium Venting Constraints
Generally there were four propellant tanks symmetrically distributed in propulsion
subsystem of the lunar lander (2 for oxidizer and 2 for fuel). Two symmetrical tanks
for same propellant component were parallel with small deviation from equilibrium
state in whole mission profile. Large deviation of equilibrium might cause large
deviation of mass center, which would increase propellant consumption for attitude
control and have impact on attitude control capability in severe case. In addition, for
gas liquid mixed tank, equilibrium venting also had impact on propellant utilization
efficiency (when all propellant in one tank was exhausted, remaining propellant in
other tank was not available). Due to small margin of propellant load for the lunar
lander, it was necessary to strictly control equilibrium propellant venting of parallel
tanks. For example, the equilibrium propellant venting should not be more than 3%
for the Chang’E-3 lunar lander. So it was necessary to diminish the impact of flow
resistance deviation of parallel tanks on equilibrium venting, for example, to reduce
the proportion of tank flow resistance in total subsystem flow resistance and adjust
subsystem consistency according to tank flow resistance test results. Moreover, strict
control of size, thickness, hardness of tank metallic diaphragm could improve flow
rate consistency.
8.3.4 Flight Environment
The lunar lander propulsion subsystem would experience mechanical environment
in launch phase, space environment in orbit, mechanical environment in descending
phase, impact environment in landing and survival environment after landing and etc.
So stringent requirements including mechanics, heat, radiation and EMC, especially
mechanics and heat environment, should be adapted to by propulsion subsystem.
Because most components of propulsion subsystem were an electromechanical
product with the feature of load bearing, pressure bearing, movement, complicated
pipelines and large loading mass, their function would be lost in case of failure.
Therefore the most critical requirement was to ensure structural reliability of propulsion subsystem throughout whole mission profile. When the structure of propulsion
subsystem was designed, the major concern was the mechanical environment in
launch phase (overload, vibration, shock), while the orbital mechanical environment
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expulsion efficiency could be more than 99%. It was necessary to consider expulsion efficiency, flow rate, anti-vortexing, anti-sloshing, long term propellant compatibility, overload capacity, mechanical endurance, isolation complexity for two propellant components, structural mass and configuration, repeated venting times and etc.
when the type of propellant tank was determined. The surface tension or metallic
diaphragm tank was usually chosen for the lunar landers, for short term the lunar
lander the non-metallic bladder type might be the choice. For example, the composite
wrapped metallic diaphragm tank of expulsion efficiency more than 99% was chosen
for propulsion subsystem of the Chang’E-3 lunar lander.
6. Parallel Tank Propellant Equilibrium Venting Constraints
Generally there were four propellant tanks symmetrically distributed in propulsion
subsystem of the lunar lander (2 for oxidizer and 2 for fuel). Two symmetrical tanks
for same propellant component were parallel with small deviation from equilibrium
state in whole mission profile. Large deviation of equilibrium might cause large
deviation of mass center, which would increase propellant consumption for attitude
control and have impact on attitude control capability in severe case. In addition, for
gas liquid mixed tank, equilibrium venting also had impact on propellant utilization
efficiency (when all propellant in one tank was exhausted, remaining propellant in
other tank was not available). Due to small margin of propellant load for the lunar
lander, it was necessary to strictly control equilibrium propellant venting of parallel
tanks. For example, the equilibrium propellant venting should not be more than 3%
for the Chang’E-3 lunar lander. So it was necessary to diminish the impact of flow
resistance deviation of parallel tanks on equilibrium venting, for example, to reduce
the proportion of tank flow resistance in total subsystem flow resistance and adjust
subsystem consistency according to tank flow resistance test results. Moreover, strict
control of size, thickness, hardness of tank metallic diaphragm could improve flow
rate consistency.
8.3.4 Flight Environment
The lunar lander propulsion subsystem would experience mechanical environment
in launch phase, space environment in orbit, mechanical environment in descending
phase, impact environment in landing and survival environment after landing and etc.
So stringent requirements including mechanics, heat, radiation and EMC, especially
mechanics and heat environment, should be adapted to by propulsion subsystem.
Because most components of propulsion subsystem were an electromechanical
product with the feature of load bearing, pressure bearing, movement, complicated
pipelines and large loading mass, their function would be lost in case of failure.
Therefore the most critical requirement was to ensure structural reliability of propulsion subsystem throughout whole mission profile. When the structure of propulsion
subsystem was designed, the major concern was the mechanical environment in
launch phase (overload, vibration, shock), while the orbital mechanical environment