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

Overboard dump of exhaust reduces effective Isp Examples V-2 (H2O2), Atlas, Delta, Saturn V, Titan, F-1 engine F-1 RS-68 Delta IV

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 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



SUMMARY OF KEY POINTS Rocket systems are complex, multi-purpose systems Choice of system, strongly related to: Combustion chamber pressure Size of engine Thrust requirement

Primary Propellant Feed System Types: Cold Flow / Pressurized Gas Turbopump Gas Generator Preburner Expander / Topping Understand Advantages / Disadvantages of each


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