290
8 Propulsion Technology of Lunar Lander
deviate from the geometric axis. Because of the significant impact of thrust vector
deviation to attitude control, the deviation was practically measured in engine hotfire test, and then the engine was installed and adjusted according to test results with
its force point located on the lunar lander longitudinal axis and coincidence between
thrust vector and the lunar lander longitudinal axis.
For orbit/attitude control engine, the accuracy of nozzle axis orientation relative
to head mounting surface was ensured by a combination manufacturing process,
wherein the precision of nozzle axis was ensured by high precision mold and the
precision of manufacturing process was ensured by processing equipment and device.
For orbit/attitude control engine, the accuracy of nozzle axis orientation was
measured by optical theodolite or 3-D Coordinate Measuring Machine. Generally
the optical method was used in the lunar lander test with cubic mirror installed on
the reference surface of the measured object. The measuring data as a reference of
the lunar lander was acquired from that relative to the cubic mirror through coordinate conversion. If the accuracy was not met with the lunar lander requirements, the
installation method of single engine or engine complex could be adjusted to satisfy
the accuracy requirements.
8.4.6 Major Components Design for Propulsion Subsystem
1. Orbit Control Engine [9]
Generally, continuously throttleable engine was used to adapt variable thrust requirements during soft-landing for orbit control in the lunar lander propulsion subsystem.
During LTO, LOI, circumlunar orbit de-orbit and main deceleration phase of powered
descent, fixed thrust operation mode was applied (according to the demand of
orbit control, constant thrust mode could be applied in any thrust level). During
powered descent process, real-time throttling thrust should be output by the demand
of autonomous control of the lunar lander.
The methods of continuously throttling thrust included three types: regulator
output pressure feed-back, injector throttling and “injector-cavitation tube” combined
throttling. The advantage of the first method was structural simplicity and possibly
to be designed by constant regime. The disadvantage was delay of thrust variation response inappropriate for rapid thrust throttling, especially because the ullage
volume of large propellant tank was large and the change of pressure was slow.
Meanwhile, it was difficult for the engine to be capable of outputting wide range
thrust. The advantage of the second method was rapid thrust variation response. But
for bi-propellant engine, the mixed ratio was difficult to be maintained precisely and
the propellant utilization efficiency was affected when the engine works in throttling
mode deviated from nominal design point. The third method could overcome the
disadvantages of above two methods and was widely applied. For example, orbit
control engine with thrust throttling ratio of 10:1 was applied in descent propulsion
8 Propulsion Technology of Lunar Lander
deviate from the geometric axis. Because of the significant impact of thrust vector
deviation to attitude control, the deviation was practically measured in engine hotfire test, and then the engine was installed and adjusted according to test results with
its force point located on the lunar lander longitudinal axis and coincidence between
thrust vector and the lunar lander longitudinal axis.
For orbit/attitude control engine, the accuracy of nozzle axis orientation relative
to head mounting surface was ensured by a combination manufacturing process,
wherein the precision of nozzle axis was ensured by high precision mold and the
precision of manufacturing process was ensured by processing equipment and device.
For orbit/attitude control engine, the accuracy of nozzle axis orientation was
measured by optical theodolite or 3-D Coordinate Measuring Machine. Generally
the optical method was used in the lunar lander test with cubic mirror installed on
the reference surface of the measured object. The measuring data as a reference of
the lunar lander was acquired from that relative to the cubic mirror through coordinate conversion. If the accuracy was not met with the lunar lander requirements, the
installation method of single engine or engine complex could be adjusted to satisfy
the accuracy requirements.
8.4.6 Major Components Design for Propulsion Subsystem
1. Orbit Control Engine [9]
Generally, continuously throttleable engine was used to adapt variable thrust requirements during soft-landing for orbit control in the lunar lander propulsion subsystem.
During LTO, LOI, circumlunar orbit de-orbit and main deceleration phase of powered
descent, fixed thrust operation mode was applied (according to the demand of
orbit control, constant thrust mode could be applied in any thrust level). During
powered descent process, real-time throttling thrust should be output by the demand
of autonomous control of the lunar lander.
The methods of continuously throttling thrust included three types: regulator
output pressure feed-back, injector throttling and “injector-cavitation tube” combined
throttling. The advantage of the first method was structural simplicity and possibly
to be designed by constant regime. The disadvantage was delay of thrust variation response inappropriate for rapid thrust throttling, especially because the ullage
volume of large propellant tank was large and the change of pressure was slow.
Meanwhile, it was difficult for the engine to be capable of outputting wide range
thrust. The advantage of the second method was rapid thrust variation response. But
for bi-propellant engine, the mixed ratio was difficult to be maintained precisely and
the propellant utilization efficiency was affected when the engine works in throttling
mode deviated from nominal design point. The third method could overcome the
disadvantages of above two methods and was widely applied. For example, orbit
control engine with thrust throttling ratio of 10:1 was applied in descent propulsion