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
