Propulsion Model for Chandrayaan-3 Lander Propulsion System

Chandrayaan-3 Lander propulsion system designed and developed by Liquid Propulsion Systems Center (LPSC) played a vital role in achieving successful soft landing near the south pole of moon on 23rd August 2023. Four number of 800 N lander engines reduced the lander orbital velocity of 1683 m/s to 0 m/s at the designated landing site within a span of 1131s through a series of complex thrust control maneuvers while 8 Nos. of 58 N attitude control thrusters oriented the lander along the desired flight path. The throttleable propulsion system consisting of lander engines, control thrusters, flow control valves, flow control electronics package, propellant supply and regulation system along with numerous flow control elements is a sophisticated bi-propellant propulsion system which has undergone extensive development, testing and qualification process over a span of 6 years. The lander engines alone have undergone hot firings for a cumulative duration exceeding 21720 s over 107 hot tests on three different hardware as part of the qualification program. A unique element of the propulsion system involves a real-time throttling algorithm which regulates the engine thrust to meet the guidance and navigation thrust requirements every 16 ms in open-loop. The onboard algorithm is validated extensively through 23 Nos. of system level engine hot tests in addition to numerous computer simulations. A detailed analysis of the thrust regulation algorithm indicated an additive 3σ dispersion of ±5% in thrust setting accuracy and ±3% mixture ratio regulation in open-loop. In order to accurately regulate the total thrust, the onboard throttling algorithm is augmented with a novel closed loop thrust control logic based on real-time lander acceleration feedback with an regulation accuracy < 1%. In Chandrayaan-3 mission, the lander propulsion system performance is demonstrated through a series of planned deboosting maneuvers on lunar orbit 2 days prior to the actual powered descent of the Chandrayaan-3 lander. In flight experience of the performance of lander propulsion system and throttling algorithms are found to be within the acceptable dispersion. The closed loop thrust regulation algorithm has accurately maintained the lander guidance acceleration requirements throughout the powered descent duration of ~19 min resulting in precise nominal successful soft-landing of Chandrayaan-3 lander near the designated landing zone.