8.3 Design Constraints and Analysis
275
(2) The propulsion subsystem could not only be firing continuously with constant
thrust for a long time but also be throttled to provide continuously variable
thrust to satisfy the requirements of descending, hovering and obstacle avoiding
during soft-landing.
(3) The allocation and grouping of attitude control engines should satisfy the
requirements of GNC control, configuration, failure isolation/regrouping and
redundancy for the orbit control engine.
(4) The propulsion subsystem should have self-management capability to some
extent, especially in a long time without telecommunication to ensure the
operation safety.
8.3.3 Technical Specification Requirements [4]
1. Structural Mass Requirement
The total mass of the lunar lander was limited by the carrying capacity of launch
vehicle. Generally, more than 60% of the total the lunar lander mass was the propulsion subsystem. So there was strict constraint on propulsion subsystem mass. On the
one hand, propellant mass could be reduced as result of improving engine specific
impulse. On the other hand, the structural mass of the propulsion could be reduced by
introduction of new technology, new material and system optimization. For example,
the constraint of propulsion subsystem of the Chang’E-3 lunar lander was that the
total mass of propulsion subsystem should be no more than 295 kg with propellant
load of 2600 kg.
2. Configuration Constraint
Because of large-scale components with special shape (sphere, spherocylinder, bell
shape and etc.) in propulsion subsystem like propellant tanks, gas tanks, orbit control
engines and etc., most of available space in the lunar lander was used for installation
of propulsion subsystem components, which had impact on utilization efficiency of
the lunar lander configuration. For example, the diameter of the propellant tank of the
Chang’E-3 lunar lander was 1050 mm, the length of the gas tank was 1400 mm, and
the maximal radial envelope of propulsion subsystem was no more than 3650 mm.
3. Requirement for the Specific Impulse of Orbit Control Engine
The specific impulse of orbit control engine had direct impact on propellant load
as well as total the lunar lander mass. Improving engine specific impulse could
effectively reduce propellant load as well as volume of propellant tank and gas tank,
so as to reduce propulsion subsystem structural mass. When the propellant option was
determined, specific impulse of the engine could be improved by means of increasing
combustion efficiency and enlarging nozzle expansion ratio. However, increasing
combustion efficiency usually caused high working temperature of the engine and
affected duration of anti-oxidization coating. Enlarging nozzle expansion ratio also
led to increasing of engine volume, mass, and decreasing of basic structural frequency
275
(2) The propulsion subsystem could not only be firing continuously with constant
thrust for a long time but also be throttled to provide continuously variable
thrust to satisfy the requirements of descending, hovering and obstacle avoiding
during soft-landing.
(3) The allocation and grouping of attitude control engines should satisfy the
requirements of GNC control, configuration, failure isolation/regrouping and
redundancy for the orbit control engine.
(4) The propulsion subsystem should have self-management capability to some
extent, especially in a long time without telecommunication to ensure the
operation safety.
8.3.3 Technical Specification Requirements [4]
1. Structural Mass Requirement
The total mass of the lunar lander was limited by the carrying capacity of launch
vehicle. Generally, more than 60% of the total the lunar lander mass was the propulsion subsystem. So there was strict constraint on propulsion subsystem mass. On the
one hand, propellant mass could be reduced as result of improving engine specific
impulse. On the other hand, the structural mass of the propulsion could be reduced by
introduction of new technology, new material and system optimization. For example,
the constraint of propulsion subsystem of the Chang’E-3 lunar lander was that the
total mass of propulsion subsystem should be no more than 295 kg with propellant
load of 2600 kg.
2. Configuration Constraint
Because of large-scale components with special shape (sphere, spherocylinder, bell
shape and etc.) in propulsion subsystem like propellant tanks, gas tanks, orbit control
engines and etc., most of available space in the lunar lander was used for installation
of propulsion subsystem components, which had impact on utilization efficiency of
the lunar lander configuration. For example, the diameter of the propellant tank of the
Chang’E-3 lunar lander was 1050 mm, the length of the gas tank was 1400 mm, and
the maximal radial envelope of propulsion subsystem was no more than 3650 mm.
3. Requirement for the Specific Impulse of Orbit Control Engine
The specific impulse of orbit control engine had direct impact on propellant load
as well as total the lunar lander mass. Improving engine specific impulse could
effectively reduce propellant load as well as volume of propellant tank and gas tank,
so as to reduce propulsion subsystem structural mass. When the propellant option was
determined, specific impulse of the engine could be improved by means of increasing
combustion efficiency and enlarging nozzle expansion ratio. However, increasing
combustion efficiency usually caused high working temperature of the engine and
affected duration of anti-oxidization coating. Enlarging nozzle expansion ratio also
led to increasing of engine volume, mass, and decreasing of basic structural frequency