MAE 4262: ROCKETS AND MISSION ANALYSIS

Rocket Cycle Analysis

Mechanical and Aerospace Engineering DepartmentFlorida Institute of Technology

D. R. Kirk

CONTENTS• Overview

• Propellant Feed Systems / Cycle Examples1. Gas Feed System2. 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

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

GOAL: Understand and describe propellant feed system / rocket cycle

OVERVIEW

How can we represent this complex system in a simplified way?

• 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?

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-68Delta IV

STAGED-COMBUSTION / PREBURNER (CLOSED)

SSME RD-180

• 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

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

• 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

Gas pressurization

Turbopumps and Valves

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

• 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

Starter Motor

PRESSURIZED PROPELLANT FEED LINES

• 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

Solenoid Valve

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

• 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

Small combustion chamber

OIL PRESSURE

• 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 Supply

OIL COOLANT

• 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

Heat Exchanger

FUEL TANK PRESSURE

• 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

Fuel TankPressurization andHeat Exchanger

OXIDIZER TANK PRESSURE

• 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

Oxidizer PressurizationHeat Exchanger

ATTITUDE CONTROL

• Remaining exhaust gasses from motor driving turbopump:– Dumped overboard– Roll attitude control

Attitude Control Thruster

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

EXAMPLES: RD-170

EXAMPLE: H-1 (SATURN C-1 BOOSTER)

EXAMPLE: SHUTTLE OMS

EXAMPLE: ARIANE 5

EXAMPLE: VIKING

EXAMPLE: ARIANE HM7B

EXAMPLE: SSME

EXAMPLE: ARIANE VULCAN

EXAMPLE: TURBOPUMP (HPFTP)

EXAMPLE: TURBOPUMP (RS-27 DELTA)

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

• References– http://www.pratt-whitney.com/how.htm– http://woodmansee.com/science/rocket/r-liquid/index-liquid.html