LIQUID PROPELLANT ROCKET ENGINE CONTROL SYSTEMS V Gnanagandhi Programme Director CSP /LPSC/ISRO TRIVANDRUM WORKSHOP ON ENGINE CONTROL SYSTEM TECHNOLOGY IIT – MUMBAI 19TH NOVEMBER 2004
LIQUID ROCKET ENGINES DEVELOPED IN ISRO BI-PROPELLANT PUMP FED CRYOGENIC PUMP FED BI-PROPELLANT PRESSURE FED MONO-PROPELLANT 6.4 KN 75KN 11N 50N 7.35 KN 22N 1N 440N 735KN
GAS GENERATOR • MAJOR SUB-ASSEMBLIES ARE: • GAS GENERATOR • TURBO-PUMP • THRUST CHAMBER • INJECTOR HEAD • IGNITER • THRUST FRAME • COMMAND BLOCK • 16 FLUID COMPONENTS AND • SENSORS TURBO-PUMP THRUST CHAMBER CUS MAIN ENGINE
Functions of LPE Control System The engine control system interconnects the components and logics of the engine and ensure proper functioning of the engine with the desired performance.
Basic LPE Control Systems • Engine start/cutoff sequence control • Engine duration control • Engine safety control • Propellant Mixture ratio control • Engine Thrust control • Propellant tank pressurization control • Thrust vector control by gimballing • Engine system Checkout and test control
Engine Start/Cut off Sequence control • Start sequence control brings the engine systems safely from start signal to nominal operation. • Cut off sequence control ensures rapid and safe shut down during normal operation as well as in an emergency with minimum and repeatable cut off impulse.
Design requirements of Start Sequence Control System • Engine conditioning • Safe ignition • Safe Thrust and Mixture ratio profile • Adequate Thrust chamber cooling • Lead time of fuel admission with respect to Oxidizer admission for safe engine operation. • Adequate pump inlet pressures to avoid Cavitation of pumps • Build-up characteristics of pumps & turbines • Proper Valve response characteristics • Purging the oxidizer circuits with inert gas to avoid the entry of fuel/hot gases.
GHe purge valve Ignitor Thrust Chamber Cavitating Venturi Main Valve H2 Injection Valve Selector Valve By pass Orifice H2 Vent Valve (HVV) LOX Tank GH2 Source Vent Line Vent Line
Schematic diagram of a Typical LOX/LH2 Pump fed engine
To – Engine start Tf – Engine shutoff Operating sequence of a LOX/LH2 Pump fed engine
Chamber pressure build up in start transient of a Cryogenic engine
Cryogenic engine hot test Mixture ratio bulild up
Thrust/MR control regulator movement in start transient of a Cryogenic engine
Engine Duration Control • Engine shut down is either by guided cutoff or by propellant depletion cutoff. • In guided cutoff, integrating accelerometer will give the required signal to cutoff, when the required vehicle velocity is achieved. • In propellant depletion cutoff scheme, engine is stopped either by the signal from vehicle accelerometer or engine parameters like chamber pressure, injection pressure etc, indicating depletion of any one of the propellants.
Engine Shut down transient Guidance based cutoff
Engine Shut down transient Depletion based cutoff
Engine System Safety Control • Engine safety control system monitors major engine parameters during engine operation and safely aborts the operation in case of malfunctioning of any system. • The upper and lower abort limits are fixed based on the safe operation limits of the engine.
Typical Engine parameters to be monitored for assessing the health • Chamber pressure • Coolant channel outlet temperature • Turbo pump speed • Pump inlet & outlet pressures • Gas generator pressure & temperature
Lower abort limit Cryogenic engine test Engine safety control by lower abort
Thrust & Mixture ratio Control Systems • Thrust & Mixture ratio control systems are necessary to achieve safe engine operation, required vehicle performance and minimum propellant outage. Thrust & Mixture Ratio Control Schemes • Open Loop mode • Closed Loop mode
Thrust Control Schemes in Pressure Fed Engines • Open loop mode – Pre calibrated flow control devices (orifices, venturies etc) are used in the propellant feed circuits to maintain the thrust within specified limits. • Closed loop mode – Variable area flow control valves in the feed circuits or propellant tank pressure variation is used for controlling the thrust, based on the feed back signal.
Thrust Control Schemes in Pump Fed Engines • Thrust is controlled by controlling the throughput to the turbine. • Open loop mode – Propellant flow to Gas generator is controlled using fixed area orifices or venturies. • Closed loop mode – Propellant flow to GG or hot gas flow from GG to turbines is controlled by variable area flow control valves, based on the feed back signal.
Feed back signals for closed loop Thrust control systems • Engine parameters like chamber pressure, thrust chamber injection pressures etc. • Vehicle acceleration
Mixture Ratio Control Schemes • Open loop mode – Pre-calibrated flow control elements are used in the propellant feed circuits to attain the required mixture ratio within the specified limits. • Closed loop mode – Variable area flow control valves are used in the propellant feed circuits to control the mixture ratio, based on the feed back signal.
Feed back signals for closed loop Mixture Ratio control systems • Onboard computer estimates the mixture ratio using the flow meter and temperature data, which is compared with the desired value and corrected. • In propellant utilization control system, the available propellants in the tanks are estimated using level sensors. Modified mixture ratio based on the available propellant is arrived at for the optimum utilization of the propellants and the control valves are adjusted to deplete the propellants simultaneously.
Schematic diagram of GG cycle engine with open loop Thrust & MR control system
Typical Open Loop Thrust/MR Control System for a Cryogenic Rocket Engine • Engine Thrust and Mixture ratio is set to required level by properly sizing the orifices and venturies employed in the propellant feed lines of GG and main Combustion chamber. • Control accuracy : + 3 %
Schematic of SCC engine with closed Loop Thrust and MR control system
CUS THRUST CONTROLLER MAJOR SPECIFICATIONS : FLUID MEDIUM = LOX, FLOW RATE 1.4 KG/SEC, OPERATING PRESSURE = 130 BAR NORMAL OPERATING RANGE = -125 TO +125, MAX: RANGE OF MOVEMENT = -140 TO +140 MAX RESISTANCE TORQUE AT 130 BAR,(NO FLOW CONDITION) = 20 KG CM LEAK RATE OF SHAFT PRIMARY SEL= 50 SCC/MIN,GN2 AT 150 BAR, SECONDARY SEAL=10SCC/MIN,1 BAR FLOW CHARACTERISTICS POSITION(DEGRS) -125 -80 -50 -25 0 25 50 75 100 125 WATER FLW RATE,KG/SEC 0.46 1.26 1.26 1.26 1.26 1.26 1.26 1.26 1.26 1.26 PR : DROP(BAR) 39.8-46.8 59.6-79.9 42.9-58.1 32.3-43.2 27.1-27.9 15.6-23 9.1-15.6 5.7-9.1 3.8-5.3 2.7-4.1 TEST RESULT, A0 (BAR) A1 35.4 38.4 59.3 77.5 24.6 26.3 5.85 6.91 43.0 45.5 33.0 35.5 17.4 18.9 10.7 12.0 3.23 3.58 2.36 1.84
Typical Closed Loop Thrust Control System for a Cryogenic Rocket Engine • Thrust is regulated by controlling the LOX flow to Pre-combustion chamber by a variable area flow control valve, operated by a stepper motor. • Using the feed back signal, the thrust control electronics estimates the engine thrust, its deviation from the requirements and command pulses required to nullify the deviation using the thrust control algorithm and actuates the thrust regulator.
Thrust control algorithm For a Cryogenic engine
Test result Permissible limits Engine chamber pressure with closed loop control system in a Cryogenic engine hot test
CUS MIXTURE RATIO CONTROLLER MAJOR SPECIFICATIONS FLUID MEDIUM = LOX, FLOW RATE = 12.8 KG/SEC, OPERATING PR := 130 BAR. NORMAL RANGE OF OPERATION = -125 TO +125,MAX: RANGE = -140 TO +140 LEAK RT OF PINTLE FORE END SEAL= 2000SCC/MIN AIR AT 100 BAR LEAK RT OF PINTLE REAR END SEAL=1500 SCC/MIN AIR AT 65 BAR MAX REST TORQUE AT NO FLOW, = 20 KGCM FLOW CHARACTERISTICS POSITION -125 -80 -50 -25 0 25 50 75 100 125 WATER,FLOW RATE,KG/SEC 4.97 12.35 12.35 12.35 12.35 12.35 12.35 12.35 12.35 12.35 PR.DROP BAR 46.5-52.4 82-93.9 73.7-83.7 67.2-76.7 61.2-69.9 55.8-63.7 50.1-58 44.9-52.4 42.7-43.6 34.2-41.5 A0 A1 47.2 45.5 87.3 88.1 74.5 79.1 68.0 72.2 64.1 67.2 59.1 60.0 38.7 38.8 49.1 49.6 TEST RES (BAR) 52.9 54.9 44.2 44.2
Typical Closed Loop MR Control System for a Cryogenic Rocket Engine • Mixture ratio is regulated by controlling the LOX flow to the thrust chamber by a variable area flow control valve (MRC regulator), operated by a stepper motor. • The MR control electronics estimates the MR and the command pulses required to correct the deviation using the signals from flow meters and temperature sensor. MRC regulator is actuated by a stepper motor to achieve the required mixture ratio.
Mixture ratio control algorithm For a Cryogenic engine
Mixture ratio control algorithm For a Cryogenic engine-contd
Test result Permissible limits Engine mixture ratio with closed loop control system in a Cryogenic engine hot test
Design Criteria of Thrust/MR Control Regulators • The regulator flow area profile is designed based on the following conditions • Rate of change of thrust/MR w.r.t pintle movement should be constant dF/db : Constant dk/da : Constant • Cross coupling between thrust and MRC systems should be minimum dk/db~ 0 dF/da ~ 0 K : MR F : Thrust a : MRC regulator angle b : Thrust regulator angle