Rocket Engine Cycles

Introduction

• Tim Dodd, the Everyday Astronaut, discusses the complexity of rocket engines and their various power cycles.
• A wide range of methods exist to power rocket engines, from simple to highly complex designs.
• Understanding the engine cycle type is crucial for grasping how rocket engines function.

Overview of Engine Cycles

• Different engine cycles are akin to the variations in piston engines (naturally aspirated, turbocharged, etc.).
• This presentation will cover several engine types: cold gas, pressure fed, electric pump fed, open cycle, closed cycle, full flow staged combustion cycle, tap-off, and expander cycles.
• Pros and cons will be discussed, with examples for each cycle type.

Key Concepts

• Rocket engines expel mass (exhaust) at high velocities to generate thrust (Newton's third law).
• Key terms:
  • Exhaust Gas Velocity: Directly related to thrust and efficiency.
  • Enthalpy: The total energy within a system, important for understanding engine performance.
  • Pressure Flow: Pressure moves from high to low.

Cold Gas Rocket Engine

• Basic Mechanism: Uses stored gas at high pressure; no combustion occurs.
• Utilizes the Joule-Thomson effect; as gas expands, it cools down.
• Limitations:
  • Low thrust and efficiency (around 60 seconds of specific impulse).
  • Requires high-pressure tanks which can be heavy.
• Applications: Suitable for small spacecraft and maneuvering thrusters (e.g., Falcon 9's maneuvering thrusters).

Pressure Fed Engines

• Types: Monopropellant and bipropellant engines.
• Monopropellant Engines: Use a single propellant and a high-pressure oxidizer to create thrust; often utilizes catalysts to release energy.
  • Examples: Satellites, Soyuz spacecraft thrusters.
• Bipropellant Engines: Use two propellants (fuel + oxidizer); generally more efficient.
  • Examples: Space Shuttle's OMS pods.
• Limitations: All pressure-fed rockets have not reached orbit due to performance constraints.

Electric Pump Fed Engines

• Use electric motors to drive pumps that feed fuel into the combustion chamber.
• Advantages: Lower pressure tanks (lightweight).
• Challenges: Requires substantial power; may not scale well.
• Example: Rocket Lab's Electron rocket.

Open Cycle Engines

• Utilize a gas generator to produce hot gas that powers the pumps.
• Exhaust gases are expelled and not used in the combustion process.
• Examples: Merlin engines (Falcon 9), F-1 engines (Saturn V).
• Drawbacks: Wastes some propellant as it is expelled with the exhaust.

Closed Cycle Engines

• Involve using a preburner where all or most of the propellant is processed before reaching the combustion chamber.
• Can be oxidizer-rich or fuel-rich:
  • Oxidizer-rich: Developed by Soviets; avoids coking issues.
  • Fuel-rich: Developed by the U.S.; uses liquid hydrogen to minimize soot.
• Challenges: More complex designs and materials needed to handle high temperatures and pressures.

Full Flow Staged Combustion Engines

• Both fuel and oxidizer pass through separate preburners and turbines before entering the combustion chamber.
• Advantages: Highly efficient due to gas-gas interactions.
• Challenges: Extremely complex design and engineering.
• Examples: SpaceX's Raptor engines.

Tap-Off Cycle Engines

• A simpler version uses combustion gases to drive the engines directly.
• Benefits: Reduces complexity by removing preburners.
• Example: Blue Origin's BE-3 engine.

Expander Cycle Engines

• Utilize heat from the combustion chamber to help power the pumps.
• Challenges: Limited thrust output due to cooling capacity; needs high-pressure conditions.
• Examples: RL-10 engine, Vinci engine.

Conclusion

• Each rocket engine cycle has unique advantages and trade-offs.
• The field continues to evolve with new technologies and hybrid approaches.
• Future videos will explore exotic options like ion propulsion and nuclear rocket engines.

Resources

• For further reading, visit everydayastronaut.com.
• Visit the shop for merchandise related to rocket science.

Acknowledgements

• Special thanks to Patreon supporters for making this educational content possible.