L04 - Engine cycles

Engine cycles of liquid rocket engines

• Pressure fed engines

• Pump fed engines: Gas generator, Expander & Expander-Bleed, Tap-Off, Staged Combustion

Pressure Fed Cycle – Basics

• Uses a high-pressure gas (e.g., He, N₂) to push propellant into the combustion chamber.

• Simplest and most reliable system.

• Used in reaction control systems and upper stages due to:

  • Low combustion chamber pressure,

  • Low thrust.

• Needs reheat to prevent fuel freezing due to adiabatic expansion.

Why must the pressurizing gas be reheated?

• Short: To avoid freezing the fuel.

• Long: Adiabatic expansion cools the gas significantly, potentially freezing cryogenic fuel; reheating prevents this.

What are the main advantages of the pressure-fed cycle?

• Short: Simplicity and reliability.

• Long: It has few moving parts, making it robust and easy to implement, particularly in RCS or upper stages.

Pressure Fed Cycle – Increase of Thrust

• Thrust increases with higher mass flow.

• Two options:

  • Option 1: Keep chamber size same, increase chamber pressure (pc′ > pc).

  • Option 2: Keep pressure the same (pc′ = pc), increase combustor/throat size.

How can thrust be increased in a pressure-fed cycle?

• Short: By increasing mass flow.

• Long: Thrust is proportional to mass flow rate. You can increase it by either raising chamber pressure or enlarging combustion chamber/nozzle throat.

What is the downside of increasing chamber pressure in a pressure-fed cycle?

• Short: Increases tank mass.

• Long: Higher chamber pressure demands thicker, heavier tanks to withstand it, which adds structural mass.

What changes when you increase throat diameter instead of pressure?

• Short: Engine gets bigger.

• Long: A larger engine allows more flow at the same pressure, but this increases engine mass and complexity.

Pump Fed Cycle – Overview

• Pressurization via pumps driven by turbines.

• Turbines powered by:

  • Gas generator,

  • Preburner,

  • Heat exchanger (expander),

  • Main chamber (tap-off).

• Enables high pressure, high thrust.

• Used in main and upper stages.

How is pressurization achieved in pump-fed cycles?

• Short: Using pumps driven by turbines.

• Long: Turbopumps move propellant into the chamber. Turbines powering these pumps derive energy from various thermal sources (gas gen, preburner, etc.).

Why are pump-fed cycles more complex than pressure-fed?

• Short: They use turbines and pumps.

• Long: While offering higher thrust and pressure, they introduce mechanical complexity due to the turbomachinery involved.

Pump Fed Cycle – Types

• Open Cycles: Gas generator, tap-off, expander-bleed.

• Closed Cycles: Expander, staged combustion.

• Turbines powered by:

  • Gas generator → gas generator cycle.

  • Preburner → staged combustion.

  • Regenerative cooling → expander cycles.

  • Main chamber → tap-off.

What defines an open vs closed pump-fed cycle?

• Short: Whether turbine exhaust goes to the combustion chamber.

• Long: In open cycles, exhaust is dumped; in closed cycles, it's fed into the combustion chamber to improve efficiency

How is the turbine powered in an expander cycle?

• Short: Via regenerative cooling.

• Long: The propellant is heated in the cooling channels around the combustion chamber, vaporizes, and powers the turbine.

Gas Generator Cycle

• Open cycle.

• Only 3–7% of propellant is used to power the turbine.

• Exhaust is either dumped or used for cooling (not returned to combustion).

• Simple, but sacrifices Isp.

What fraction of propellant goes to the gas generator?

• Short: 3–7%.

Why does gas generator cycle have lower efficiency than staged combustion?

• Short: Exhaust is not reused.

• Long: The exhaust gases are not fed back into the combustion chamber, so some propellant energy is wasted.

What happens to turbine exhaust in a gas generator cycle?

• Short: Dumped or used for cooling.

• Long: It’s either vented to the atmosphere or routed to cool the nozzle, but not used for thrust generation.

Expander Cycle

• Closed cycle.

• Entire fuel flow is heated (regenerative cooling), vaporized, and drives turbines.

• Exhaust gas is fed to main chamber.

• Lower chamber pressures (≤ 60 bar).

• Long combustion chamber needed for enough heat transfer (Δh).

What is the working fluid source in an expander cycle?

• Short: Regeneratively cooled fuel.

• Long: The fuel absorbs heat from the combustion chamber walls, vaporizes, and powers the turbine.

Why is a long combustion chamber used in an expander cycle?

• Short: To provide more heat to the fuel.

• Long: A longer chamber increases surface area and allows more time for the fuel to absorb sufficient heat to vaporize and spin the turbine effectively.

What is a key limitation of the expander cycle?

• Short: Limited chamber pressure.

• Long: As combustion pressure increases, more energy is needed to pump fuel, which may exceed the thermal energy recoverable from regenerative cooling.

Expander Cycle – Increase of Thrust (Options 1 & 2)

Option 1: Increase total mass flow

• Chamber pressure p_c is constant.

• Increase m˙→ higher throat area At

• Requires:

  • More turbine power → larger Δh in cooling loop

  • Longer chamber wall (bigger heat exchange surface)

  • Higher cooling channel mass flow → more pressure losses

  • Increase in engine mass

Option 2: Increase chamber pressure p_c, constant m˙

• Smaller throat area and chamber diameter → larger expansion ratio

• Heat transfer scales as Q˙∝p_c^0.8 not linear

• Requires:

  • More Δh for turbine

  • Longer chamber to achieve necessary heat absorption

  • Higher engine mass and cooling losses

How can thrust be increased in an expander cycle?

• Short: By increasing mass flow or chamber pressure.

• Long: You can increase thrust by either raising total mass flow (option 1) or increasing chamber pressure (option 2), each with trade-offs in heat transfer and engine mass.

Why does increasing thrust in an expander cycle increase engine mass?

• Short: More cooling and larger structures.

• Long: Higher mass flow or pressure requires more turbine power, longer chambers, and bigger cooling channels, increasing total engine mass.

CECE Engine (Common Extensible Cryogenic Engine)

• LOX/LH2 expander cycle engine by Pratt & Whitney Rocketdyne

• Used in Atlas V / Delta IV

• Deep throttling (down to 10%)

• Icicles form at nozzle → Why?

  • Cold hydrogen used for cooling → condensation/freezing of ambient humidity

Why do icicles form at the nozzle in CECE engine tests?

• Short: Hydrogen cooling causes freezing.

• Long: Cold hydrogen fuel cools the nozzle, freezing ambient moisture and forming icicles during test firing.

Expander-Bleed Cycle

• Hybrid between expander and open cycle

• Only part of the fuel is used in regenerative cooling

• Remaining fuel bypasses turbine and goes directly to combustion

• Exhaust is not reused (like gas generator)

• No gas generator/preburner needed

• Slight Isp loss due to open exhaust

• Lower pressures (~50 bar)

What makes the expander-bleed cycle different from the expander cycle?

• Short: Only part of fuel goes through turbine.

• Long: In expander-bleed, only a portion of the fuel is used for turbine operation, the rest goes directly to combustion, causing slight Isp losses.

Tap-Off Cycle

• Open cycle

• Hot gases tapped directly from main combustion chamber

• Mixed with fuel and expanded through turbine

• No gas generator/preburner

• Reason for mixing: reduce temp for turbine blades

• Medium chamber pressures (~100 bar)

• Slight Isp loss

What is the purpose of mixing hot gas with fuel in the tap-off cycle?

• Short: Lower turbine temperature.

• Long: The hot gas from the chamber is too hot for turbines, so it’s mixed with cooler fuel to reduce temperature and protect turbine blades.

Staged Combustion Cycle

• Closed cycle

• Uses preburners to burn one of the propellants (fuel-rich or oxidizer-rich)

• Exhaust used to power turbines, then injected into main combustion chamber

• High efficiency (no losses)

• Limited by turbine temperature (~900 K)

• Variants:

  • FRSC (Fuel-Rich)

  • ORSC (Oxidizer-Rich)

What is a key advantage of the staged combustion cycle?

• Short: High efficiency, no propellant loss.

• Long: All propellants are used efficiently—preburner exhaust is injected into the combustion chamber, leading to high specific impulse.