MAE 4262: ROCKETS AND MISSION ANALYSIS

Rocket Cycle Analysis

November 27, 2012

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

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-1RS-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