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8 Propulsion Technology of Lunar Lander
which was harmful to mechanical endurance. So it should be considered including
propellant consumption, configuration, subsystem mass (wet and dry mass), coating
lifetime, mechanical condition and thermal environment to properly define specific
impulse of the engine. For example, specific impulse of the orbit control engine on
the Chang’E-3 lunar lander should be higher than 308 s.
4. Throttling Thrust Ratio Requirements of Orbit Control Engine
Wide range of continuous throttling thrust for the orbit control engine was necessary
for lunar soft-landing, which led to technical challenges for the design of propulsion
subsystem and orbit control engine including follows: (1) Wide range of throttling
thrust ratio led to large variation of flux conditions, which might cause variation
of pressure and pipeline resistance, as well as dynamic instability in rapid throttling thrust. (2) A large deviation of throttling condition from nominal design les to
decreasing of specific impulse, deviation of mixture ratio, which might reduce total
impulse of the subsystem, especially when deviation of mixture ratio led to excess
remaining of one type of propellant. For example, nominal thrust output of the orbit
control engine of the Chang’E-3 lunar lander was 7500 N, and throttling thrust was
5000~1500 N. When the engine output was the nominal thrust, its specific impulse
could be 308 s and mixture ratio was 1.64~1.65. However, when its output was lower
than 5000 N, the minimal specific impulse was only 285 s and the minimal mixture
ratio was 1.5. (3) In low thrust condition, because of low flux and small injector pressure drop, it is difficult to implement film cooling, spray, mixture and combustion.
The larger the throttling ratio was, it was more difficult. (4) Constrained by propellant tank pressure of pressurized-feed subsystem, flow regulator (cavitation tube)
for the orbit control engine with large throttling ratio was not capable of realizing
complete cavitation in full range of thrust variation. When the operation mode of the
engine changed from maximal thrust to throttling, flow regulator went through non
cavitation, transitional cavitation and complete cavitation. Due to larger uncertainty
and fluctuation of transitional cavitation, it was necessary for the engine to operate
rapidly from non-cavitation to complete cavitation to avoid instability of subsystem.
From application of throttling engine in past the lunar landers, except the throttling
ratio of the descent stage engine of Apollo lunar module was 10:1, the throttling ratio
was generally no more than 5:1 (4:1 for the Surveyor the lunar landers, 2.5~1 for the
Luna landers, 5:1 for the Chang’E-3 lunar lander). The larger throttling ratio was, it
was more difficult to design.
5. Expulsion Efficiency Requirement for Propellant Tank
Tank propellant expulsion efficiency was closely related with amount of available
propellant. Improving expulsion efficiency of propellant tank might increase propellant utilization efficiency. The tank expulsion efficiency was related with tank types.
There were different types of propellant tank including non-metallic bladder, surface
tension, metallic diaphragm (diaphragm or bellows) and piston. Except the surface
tension tank was of gas liquid mixed type, other all were of gas liquid physical
isolation type. Generally the expulsion efficiency for metallic diaphragm tank was
97%~98%, while for non-metallic bladder, surface tension tank, piston tank, the
8 Propulsion Technology of Lunar Lander
which was harmful to mechanical endurance. So it should be considered including
propellant consumption, configuration, subsystem mass (wet and dry mass), coating
lifetime, mechanical condition and thermal environment to properly define specific
impulse of the engine. For example, specific impulse of the orbit control engine on
the Chang’E-3 lunar lander should be higher than 308 s.
4. Throttling Thrust Ratio Requirements of Orbit Control Engine
Wide range of continuous throttling thrust for the orbit control engine was necessary
for lunar soft-landing, which led to technical challenges for the design of propulsion
subsystem and orbit control engine including follows: (1) Wide range of throttling
thrust ratio led to large variation of flux conditions, which might cause variation
of pressure and pipeline resistance, as well as dynamic instability in rapid throttling thrust. (2) A large deviation of throttling condition from nominal design les to
decreasing of specific impulse, deviation of mixture ratio, which might reduce total
impulse of the subsystem, especially when deviation of mixture ratio led to excess
remaining of one type of propellant. For example, nominal thrust output of the orbit
control engine of the Chang’E-3 lunar lander was 7500 N, and throttling thrust was
5000~1500 N. When the engine output was the nominal thrust, its specific impulse
could be 308 s and mixture ratio was 1.64~1.65. However, when its output was lower
than 5000 N, the minimal specific impulse was only 285 s and the minimal mixture
ratio was 1.5. (3) In low thrust condition, because of low flux and small injector pressure drop, it is difficult to implement film cooling, spray, mixture and combustion.
The larger the throttling ratio was, it was more difficult. (4) Constrained by propellant tank pressure of pressurized-feed subsystem, flow regulator (cavitation tube)
for the orbit control engine with large throttling ratio was not capable of realizing
complete cavitation in full range of thrust variation. When the operation mode of the
engine changed from maximal thrust to throttling, flow regulator went through non
cavitation, transitional cavitation and complete cavitation. Due to larger uncertainty
and fluctuation of transitional cavitation, it was necessary for the engine to operate
rapidly from non-cavitation to complete cavitation to avoid instability of subsystem.
From application of throttling engine in past the lunar landers, except the throttling
ratio of the descent stage engine of Apollo lunar module was 10:1, the throttling ratio
was generally no more than 5:1 (4:1 for the Surveyor the lunar landers, 2.5~1 for the
Luna landers, 5:1 for the Chang’E-3 lunar lander). The larger throttling ratio was, it
was more difficult to design.
5. Expulsion Efficiency Requirement for Propellant Tank
Tank propellant expulsion efficiency was closely related with amount of available
propellant. Improving expulsion efficiency of propellant tank might increase propellant utilization efficiency. The tank expulsion efficiency was related with tank types.
There were different types of propellant tank including non-metallic bladder, surface
tension, metallic diaphragm (diaphragm or bellows) and piston. Except the surface
tension tank was of gas liquid mixed type, other all were of gas liquid physical
isolation type. Generally the expulsion efficiency for metallic diaphragm tank was
97%~98%, while for non-metallic bladder, surface tension tank, piston tank, the