Rocket Engine/Power Cycle

**Power Cycles** determine the method used for injecting propellant into rocket engines. This greatly impacts performance, most noticeable with thrust.

### Pressure-Fed

This cycle offers very low thrust and terrible sea-level efficiency in exchange for increased vacuum efficiency. This cycle is usually used for deep space maneuvers where thrust is not a priority.

### Electric

This cycle offers low thrust, and low sea-level efficiency in exchange for high vacuum efficiency. This cycle is used for deep space maneuvers, as well as landing on non-atmosphere, low gravity bodies. However, this engine also consumes electricity, which means heavy batteries are needed in the rocket.

### Gas Generator I

This cycle offers medium thrust, low/medium sea level efficiency, and medium vacuum efficiency. This cycle is used for less powerful engines, and other purposes. This cycle also allows the Bravo nozzle type, which increases thrust and efficiency.

### Gas Generator II

This cycle offers high thrust, medium sea level, and vacuum efficiency. This cycle is used for powerful engines and more. This cycle also allows the Omega nozzle, which greatly increases thrust and efficiency.

### Staged

This cycle offers high thrust and high efficiency. This cycle is used for powerful, high efficiency engines.

### Full Flow Staged

This cycle offers very high thrust and efficiency. This cycle is used for very powerful and efficient engines.

The minimum chamber pressure for each power cycle is half of the value displayed on this table.

## Real Life

Below explains what actually happens in each power cycle, in real life

### Pressure-Fed

Pressurant tanks are used to pressurize the fuel and oxidizer tanks to high pressure, higher than the engine's main combustion chamber. The pressure in the tanks will feed propellant into the engine when the valves are opened, where the propellants mix and burn. This cycle results in low power engines. The pressure in the tanks limit the engine's combustion chamber pressure (chamber pressure), which limits thrust greatly. However, this makes very simple engines, because of the elimination of the need for turbopumps.

### Electric

An electric motor is utilized to power turbopumps that pump fuel and oxidizer into the engine's main combustion chamber. This makes higher thrust engines, while allowing low pressure propellant tanks relative to pressure-fed engines. The disadvantage is increased dry mass due to added batteries.

### Gas Generator I and II

A small mix of propellants, usually fuel-rich, is burned in a separate burner, which then uses the exhaust to spin a turbine, which powers the turbopumps. The hot exhaust is then usually exhausted, but few engines redirect the exhaust back into the engine. This allows for high thrust engines, at the cost of lower efficiency due to lost propellant, and makes the engine harder to start. In the game, Gas Generator II simply feeds more propellant into the separate burner.

### Staged

Fuel and a small portion of the oxidizer (sometimes the opposite) are fed into a separate burner, called the preburner. The hot gas is then used to spin a turbine, which powers the turbopumps. The hot gas is then injected into the main combustion chamber and burned with the remaining fuel or oxidizer. This is comparable to gas generator, but with increased efficiency due to the re-direction of preburner exhaust. The main disadvantage is engineering complexity, partially due to preburner exhaust producing extremely harsh conditions for the turbine. Also, carbon-based fuels (like kerosene) cannot be used in fuel-rich preburners, because when burned fuel-rich, produce soot, which will clog the pipes. To use those fuels, the preburner must be oxidizer-rich, which is even harsher for the turbine.

### Full Flow Staged

Two preburners are used, each rich in one propellant. Both propellants are burned, and each preburner's exhaust is used to power each turbine, which drives each turbopump. The hot gases are then fed into the main combustion chamber. The injection of both hot fuels improves combustion and efficiency, and produces milder conditions for the turbines.

Furthermore, there are two additional power cycles not introduced in the game.

### Expander

All fuel or oxidizer is routed through the walls of the nozzle, making it expand and then spin a turbine, powering the turbopumps. This makes very efficient engines, since no fuel is burned, but as the engine gets bigger, the fuel passing through the engine increases more. In engines designed to produce more than approximately 70,000 lbf (300kN), heat generated by the engine is insufficient to expand the fuel, unless a bypass cycle is used. However, this cycle is far easier on the turbines, because of the lack of hot gases passing through the turbine.

### Combustion tap-off

Products from the main combustion chamber are taken to spin turbines, which power the turbopumps. The gas is then exhausted. This makes an efficient engine, but the gas from the engine is very hot, and the engine is even harder to start.