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How Do Rocket Enginers Work?

Newton’s Third Law: Every action has an equal and opposite reaction

Rocket Engines require propellent or exhaust that is forced out of the nozzle

  • The equal and opposite force pushing back is called Thrust
  • Momentum depends on the size and speed of an object
  • Conservation of Momentum ensures that the engine moves forward

Types of Rocket Engines

  • Physical, Electrical, Chemical, Thermal, Nuclear
  • Different types of ways to store and release energy to create thrust
  • Chemical Engines are the most common

Chemical Engines

  • Gasoline in the fuel tank → fuel tank pumps gasoline into the engine → tiny explosions convert chemical energy into kinetic energy
  • Types of Chemical Engines
    • Solid Rocket Motors: burn solid rocket fuel
    • Liquid Rocket Motors: burn some combination of propellents
    • Bipropellent Engine: burn two types of propellents (often Kerosene and Oxidizer)

Rocket Nozzles

  • The size and design of the nozzle determines how much force can be outputted
  • SpaceX Merlin Engine: has different designs for atmospheric and space levels

SpaceX Raptor Engine

  • Methane-Powered Full-Flow Staged-Combustion-Cycle Engine
  • Using liquid methane – never been done before on an orbital class rocket
  • Not the most powerful, not the highest thrust to weight ratio, not the most efficient

Pressure Fed Rocket Engine

  • Pressurizes the fuel tank and open the nozzle to let propellent out
  • Cold gas, Monoprop, Biprop
  • Used in reaction control systems
  • Limiting factors: tanks must be strong → thicker skin → more weight

Enthalpy: increase this to decrease engine weight

  • The relationship between volume, pressure, and temperature
  • Higher volume and pressure inside the chamber = higher efficiency

Turbo Pumps and Staged Combustion Cycle

  • Use a small pressure-fed rocket engine to spin a pump that shoots propellent out
  • Limiting factors: high pressure always wants to goto low pressure, heat can melt stuff etc.
  • Solutions: gas generator cycle, partial flow staged combustion cycle, full-flow staged combustion cycle

Gas Generator Cycle (Open Cycle)

  • Most commonly used
  • Pumping fuel and oxidizer into the combustion chamber with turbopump
  • Turbopump has components: preburner, turbine, shaft, pumps
  • Preburner just throws away exhaust, does not contribute thrust
  • Also called powerpack
  • Hard to startup: preburner requires turbopump to spin, but turbopump requires preburner to fire
  • To keep the temperature low, we run the preburner at a less than optimal ratio by being fuel-rich or oxygen-rich → running fuel-rich creates soot which coats the inside of the chamber and provides insulation
  • Coking problem: when running fuel-rich the extra fuel undergoes chemical reactions and sticks to everything blocking the pumps/chambers/nozzles
  • This is not efficient since extra fuel are not being used/wasted

Staged Combustion Cycle (Closed Cycle)

  • Reuse lost exhausts and funnel them to the combustion chamber
  • Oxygen-Rich approach: all the oxygen goes to the preburner and only the optimal amount of fuel goes to the preburner
  • Chamber pressure cannot be higher than the pump pressure
  • Hydrogen approach: fuel-rich but change fuel from RP-1 to hydrogen to avoid coking
    • RS-25: use two preburners, one for hydrogen and one for oxygen. Hydrogen needs to be sealed tightly, so uses helium to pressurize it

Full-Flow Staged Combustion Cycle

  • Two preburners: one fuel-rich, one oxygen-rich
  • SpaceX’s metal alloy: SX500
  • Better combustion since both fuel and oxygen arrive at the combustion chamber at high temperatures. Also no need for elaborate helium seals
  • RD-270, Integrated Powerhead Demonstrator, Raptor Engine
Koko Xu

Lover of international cuisines and Class B chess player.