MAE 4262: ROCKETS AND MISSION ANALYSIS Rocket Cycle
MAE 4262: ROCKETS AND MISSION ANALYSIS Rocket Cycle Analysis Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk CONTENTS Overview
Propellant Feed Systems / Cycle Examples 1. Gas Feed System 2. Turbopump Systems Gas Generator Preburner Topping / Expander Cycle
Example: Step by Step Operation Process for Liquid Rocket Supplemental Rocket Flow Diagrams
Summary of Key Points OVERVIEW GOAL: Understand and describe propellant feed system / rocket cycle NOTE: Usually denser of two propellants is placed forward Shifts center of mass forward increases stability For STS, LOX is forward since it is denser than LH2 OVERVIEW
For liquid rockets: How do we feed propellants into combustion chamber? How do we select a pressurization cycle? For liquid and solid rockets: How do we ensure structural integrity and cool hot components?
How can we represent this complex system in a simplified way? SSME FLOW DIAGRAM GAS PRESSURIZATION Advantages Simplicity Reliability Disadvantages
Low chamber pressures Weight of both gas and propellant tanks Examples SSOMS, SSRCS GAS GENERATOR (OPEN) Advantages Simple start-up, even in space Straightforward development process Disadvantages
STAGED-COMBUSTION / PREBURNER (CLOSED) Advantages Ability to operate at very high chamber pressure, high Isp Flexibility of cycle design Disadvantages Complex design, cost, pump pressures Start-up issues Examples
SSME, RD-170, RD-180 SSME RD-180 EXPANDER / TOPPING CYCLE (CLOSED) Advantages Relatively high Isp simple relative to preburner
Disadvantages Complex start-up dependent on stored heat in system Limit on Pc, due to turbine drive gas limit Examples RL-10, Centaur CLASSIFICATION OF LIQUID FEED SYSTEMS EXAMPLE: LIQUID ROCKET OVERVIEW
FUEL: RED OXIDIZER: GREEN COMBUSTION GASES: YELLOW PROPELLANT STORAGE Gas pressurization Turbopumps and Valves
Fuel and oxidizer tanks with gas pressure systems Fuel and oxidizer stored in separate tanks Valve releases propellants into cycle Cryogenic propellants have to be carefully insulated Cryogenics re-circulated through umbilical to external cooler OPEN VALVES
Before operation valves are opened and propellant fills propellant feed lines Propellants flow past compressors in turbopump up to a second set of valves Compressors not pumping Downstream valves prevent propellant from oozing into combustion chamber This can cause problems, want fuel and oxidizer to flow into combustion chamber under high pressure and at high quantity STARTER MOTOR
Starter Motor Ready to start rocket engine Small solid rocket engine, called a starter motor, ignited by an electrical
charge This motor burns pushing turbine, which turns gearbox and starts compressor Exhaust from the starter motor will be discussed later Process can also be initiated by decomposition of monopropellant PRESSURIZED PROPELLANT FEED LINES Solenoid Valve
Compressor are pumping Fuel pressure rises rapidly to the operating pressure When this happens a solenoid detects pressure rise and opens downstream valves
allowing fuel to flow into combustion chamber COMBUSTION CHAMBER
High-pressure propellant flows into combustion chamber Fuel circulates around nozzle and combustion chamber for cooling Usually oxidizer flows into combustion chamber ahead of fuel for smoother start Ignition source in combustion chamber (electrical sparks, hot wire, small detonator, small flame) Hypergolic propellants will spontaneously combust when mixed SUSTAINING TURBOPUMP Small combustion chamber
Starter motor dies out very quickly Tap off some propellant to small combustion chamber to drive turbopump
Flow regulators are critical Too much propellant, push to turbopump too hard causing catastrophic failure Not enough propellant, turbopump moves too slowly and thrust is too low If adjustable throttle control of thrust accomplished by adjusting flow Small combustion chamber that drives turbine is run with a fuel rich mixture OIL PRESSURE Oil Supply
Turbopump and gearbox operate at extremely high speeds Oil is needed for them to function Oil is forced through system under pressure using exhaust from motor that sustains turbopump OIL COOLANT
Heat Exchanger Oil used to lubricate the turbopump and gear box must also be cooled Common to cool oil by running it through a heat exchanger with fuel
Fuel that goes through heat exchanger re-used But if connected back to main feed line, there would be no flow through heat exchanger Must be fed back into system at a low pressure area upstream of compressor Cooled oil then goes back into turbopump cooling gearbox and bearings FUEL TANK PRESSURE Fuel Tank Pressurization and Heat Exchanger
Two ways to provide pressurizing gas to a propellant tank Provide inert gas from separate tank Tap off excess gas from turbopump drive system (fuel rich) This gas is too hot and needs to be cooled, to cool this gas use a heat exchanger Some unused fuel is drawn from main fuel line to cool gas
Fuel sent back to fuel line upstream of the compressor in order to get a flow OXIDIZER TANK PRESSURE Oxidizer Pressurization Heat Exchanger Oxidizer tank pressurized in manner similar to fuel tank Cannot use exhaust gasses (fuel rich) Some oxidizer drawn from main oxidizer line and heated by exhaust gasses from engine used to drive turbopump
This vaporizes oxidizer inside a pressure line which is used to pressurize oxidizer tank ATTITUDE CONTROL Attitude Control Thruster Remaining exhaust gasses from motor driving turbopump: Dumped overboard
Roll attitude control SUMMARY Overview was one of many possible approaches Simpler engines possible (smaller thrusters) where turbopump not required
In these cases either a small electrical pump or pressure from tanks themselves provide enough propellant flow to provide design thrust. SHUTDOWN Running until fuel or oxidizer depletion Known as 'hard' shutdown As compressors ingest gas instead of liquid, resistance from pumps to turbine is reduced, and can quickly reach a point when turbine side goes too fast
Burns up bearings or turbine blades can break off Turbopump fails and locks up. Without a smooth flow of fuel to combustion chamber, combustion may be disrupted and 'cough'. Both of these conditions are destructive to engine and induce violent shaking of vehicle Controlled shutdown is more desirable Fuel and oxidizer left unused, inefficient Easier on vehicle and contents, reuse engine
To perform controlled shut down cut off propellant to motor driving turbopump Turbopump slows down and reduces pressure on propellant feed lines When this pressure gets below a minimum threshold solenoid controlling pressure valves downstream of compressors closes combustion chamber inlet valves The shut off pressure is same pressure at startup that solenoids had to detect before opening the valves EXAMPLE: RD-170
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