L07 - Stage Combustion Fundamentals

Motivation and relevance

Stage combustion engines became a focus in recent years. Starship, Vulcan…

Motivation and relevance

Propellants, and why?

Most engine focus on the bio-methane/LOX.

• Eco-friendly

• Still high ISP

Motivation and relevance

Applications in terms of stages

Main stage in clustered config

Upper stages

Even kick stages

Engine cycles of liquid rocket engines

Staged combustion cycle

• Closed cycle

• One or several pre-burner to drive the turbine

• One of the two fuel mass flows is completely routed to pre-burner

• ROF in pre-burner such that max allowable turbine inlet temp. is not exceeded

• Exhaust gases from the turbine are fed to the MCC

Engine cycles of liquid rocket engines

Staged combustion cycle

Variants and variants have an influence on what?

• ROF of the pre-burner: FRSC vs ORSC

• Cooling circuit: Serial vs. parallel

• Turbine position: Serial vs. parallel

Variants have an influence on

• pressure loses and hence the required power of the turbopumps

• control mode

• start transients

Engine cycles of liquid rocket engines

Staged combustion cycle: Cooling circuit variants for FRSC

• Serial Cycle:

  • Pressure increase of H2-pump must account for losses through cooling ducts and turbines

  • More TP power required (Since the cooling ducts cause a pressure drop)

• Parallel Cycle:

  • Less mass flow avaliable for combustor cooling

  • Less TP power necessary

Engine cycles of liquid rocket engines

Staged combustion cycle: Turbine position variants in FRSC

• Serial Cycle:

  • Lower pressure drop across the turbines

  • More mass flow

  • Lower eff. of turbine

  • Higher thrust potential

• Parallel Cycle:

  • Higher pressure drop across the turbines

  • Smaller mass flow

  • Higher eff. of turbine

  • Lower thrust potential

Engine cycles of liquid rocket engines

Staged combustion cycle: FRSC vs. ORSC

ROF of the precombustion chamber

• ORSC: Challange of material compatibility with hot oxidizer

• FRSC: Risk of soot formation with fuel-rich combustion of hydrocarbons

  • LOX/LH2: FRSC better

  • LOX/RP-1: ORSC better

  • LOX/LCH4: in between

• ORSC’s turbine produce more power, higher CC pressure, higher thrust

Engine cycles of liquid rocket engines

Full Flow staged combustion cycle, ignition system?

• Closed cycle

• Turbines driven by two pre-burner

  • OPB: ox-rich pre-combustion

  • FPB: fuel-rich pre-combustion

• Oxidizer and fuel are fully fed to the pre-burner:

  • more mass flow

  • hot gas temp for turbines can be reduced

  • higher power output

• Ignition system

  • for each preburner needed

  • for MCC if T of hot ox and hot fuel not high enough for autoignition

Gas generators and pre-combustion chambers

Function, what is their challenge, their types, key factors

GG and Pre-CC provide hot combustion gases to drive the turbines of the turbopumps.

Challenge: Combustion far away from stoichiometric conditions

Types: Monergole, Diergole and Solid GG

Key Factor: Combustion T and gas composition are decisive for material compatibility with gg chamber wall and turbine blades.

Gas generators and pre-combustion chambers

Monergole gas generators

Used when simplicty is more important than performance and weight.

Catalyst bed reactors are a simple and reliable design.

Gas generators and pre-combustion chambers

Diergole gas generators and pre-combustion chambers

Basic design similar to the MCC

Fuel and oxidizer-rich combustion tend to be unstable

A high burn-out rate is less important than minimal thermal stratification at the outlet

Gas generators and pre-combustion chambers

Diergole gas generators and pre-combustion chambers

The Choice of Mixture Ratios for Pre-burner and Injector element(s)

1. Low temperature

2. Within flameability limit at injector-near region

3. Stable Combustion

Injector Elements:

1. With multistage propellant injection

   1. Initial stage for combustion

   2. Secondary injection

      1. to reduce/increase global ROF

Gas generators and pre-combustion chambers

Diergole pre-combustion chambers: Design criteria

1. Shape: Maximize mixing and minimize hot spots to avoid turbine blade damages

2. Flow path: shall not produce hot spots

3. Chamber-to-injector diameter ratio > 2 to enhance reverse flow within the pre-burner

4. Size: Large enough to ensure complete combustion

5. No nozzle is needed

6. Baffles to minimize combustion instabilities