Applications of modeFRONTIER in liquid propellant rocket engine design

The experience at DCTA - Brazil Department of Aerospace Science and Technology

Background • The L75 Engine Project

– Objective • Develop human resources and technologies in the

liquid propulsion field – Main engine requirements

• 75 kN of thrust in vacuum • Specific impulse greater than 300 seconds

– Thrust / (Mass Flow * Grav.)

• Operation in vacuum (upper stage engine) – Started in 2008

• Propellant pair was Liquid Oxygen/Kerosene • Cycle designed using traditional methods • Estimated specific impulse of 324 s

L75 LOx/Kerosene version

~380KW

L75 Engine cycle

• Gas generator cycle

Main Combustion Cycle

Auxiliar Combustion Cycle

Pump power: ~400KW

~450KW

Pumps take only 5% of fuel

L75 Engine cycle

• Gas generator cycle

The L75 Kerosene version

• Engine was undergoing detailed design in 2013 • Fuel is changed to Ethanol

– Back to cycle parametric definition

Kerosene ~45MJ/Kg

Ethanol ~30MJ/Kg

30% less powerfull

Challenge: get the same specific impulse 324s

Design Variables

• Input variables: 1. Water fraction in Ethanol;

• Improves combustion chamber cooling • Reduces combustion instability

2. Pump efficiencies; • To understand wheter if they are important to the final thrust or not;

3. Thrust Chamber parameters:

• Pressure; • Mixture ratio; • Throat diameter; • Area expansion ratio;

Affects combustion

Design Variables

• Combustion analyses: – Reaction parameters are changed, but combustion needs

to occur at specific project conditions; • Reverse calculation of chemical equilibrium models

– This would normally be solved with a Nested Optimization approach

Combustion Analyses

• An alternative for Nested Optimization:

Best Fit

Variables from external workflow

Variable calibrated to meet the target k k

Combustion Analyses

• An alternative for Nested Optimization: C=F(A,B)

Workflows for reverse combustion calculation DoE of a nested optimization

Optimization workflow finds mixture ratio for target temperature, given water fraction and pressure

DoE points used to generate the reverse combustion RSMs

Cycle design workflow

RSM for the Main Combustion

Chamber

RSMs for the Gas Generator Combustion

In-house TCA analysis and design code

Cycle design

CASE

1° STUDY: ENGINE DESIGN

Cycle design process

• The cycle design started with design space exploration, no objective functions – Design variables

• TCA combustion chamber pressure • TCA mixture ratio • TCA throat diameter • TCA nozzle area expansion ratio • TCA film cooling flow • Water fraction in fuel • Pumps and turbine efficiencies

– Constraints • Height • Diameter • Fuel pump outlet pressure – avoid the need for a two stage pump • Thrust

Cycle design process

• Recommended relations between engine internal paramenters are taken from russian rocket literature – Eliminates several design variables for pumps, feed lines, gas

generator and valves

• Turbine inlet temperature, a critical parameter, was fixed on

900 K – Reduce risk of failure on our first turbopump – Provide a margin to increase this temperature

• Off-design operation with increased thrust • In case the turbine efficiency is below expected level

Exploration and statistical analysis

• Quick optimizations could be performed to gain the confidence of more experienced designers, showing that the method “makes sense”

• Student charts helped us to focus on the main variables

Spec

ific

Impu

lse

Film cooling

Nozzle exhaust diameter

Pump efficiency

Engine design point choice

• Similar methods were applied to other responses – Design variables were eliminated

• The amount of insight gained by design space exploration and statistical analysis allowed the design point choice to be made by navigating through the RSM charts – Neural networks were used for

Specific Impulse and Thrust

Kerosene vs. Ethanol

• Comparison of engines – Same analysis tools – Same combustion code

Parameter Kerosene version Ethanol version

Thrust 75 kN 75 kN

Total mass flow 23,5 kg/s 23,5 kg/s

Liquid oxygen flow 16,2 kg/s 14,0 kg/s

Fuel flow 7,3 kg/s 9,5 kg/s

Combustion chamber pressure

70 bar 60 bar

Gas generator temperature

1073 K 900 K

Turbopump speed 30.000 rpm 24.000 rpm

Pressure expansion ratio

1400 2248

Same specific impulse

Better TCA cooling

Less complexity

Uses all available space to expand the gases

CASE

2° STUDY: THRUST CHAMBER OPERATIONAL ENVELOP

The TCA’s operational envelope

• The TCA’s operational envelope defines the set of operating conditions where its operation complies with performance and safety constraints

• Envelope limits are used for: – Interface specification – Structural dimensioning – Engine envelope composition – TCA test planning

• The considered envelope is constrained by: – Minimum and maximum thrust – Minimum specific impulse – Maximum allowed metal temperature – Maximum mixture ratio

Finding the TCA’s operational envelope • Different constraint levels exist for the

nominal and extreme envelopes • Finding all the operating conditions that

lie on the edges of the envelope can be very time consuming if done manually

• An optimization problem was created to find these points – Operating conditions are combinatorially

extremized by a multiobjective optimization • MOGT performed very well

– Operational constraints are modeled as optimization constraints

• One set of constraints for the nominal envelope • Another for the extreme envelope

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70

75

80

85

90

1.0 1.2 1.4 1.6 1.8 2.0

Nominal Envelope

Extreme Envelope

Design Point

Thrust Chamber Envelope

Thru

st [k

N]

Mixture ratio

TCA envelope workflow

• For each set of constraints, all combinations of maximization and minization of operating conditions are run

Obtaining the envelope

• The envelope search uses RSMs of the in-house code for TCA analysis to speed up the search – 15,000 plus evaluations

• After all sets of constraints and extremizations are combined, the operational space is filled with points, concentrated around the edges of the envelope

• The envelope points can be “mined” • Just one day of work!

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65

70

75

80

85

90

1.0 1.2 1.4 1.6 1.8 2.0

Nominal Envelope

Extreme Envelope

Design Point

Mixture ratio

Thru

st [k

N]

Acknowledgements

• ESTECO • ESSS • Brazilian Space Agency

Questions?

Bernardo Reis Dreyer de Souza bernardobrds@iae.cta.br