Liquid-Fuel Engines

## Liquid-Fuel Rocket Engines

This page contains information on all the modifiable parameters of liquid-fuel rocket engines and how each parameter affects engine performance. This page is for rocket engines only, for jet engines see .

*See also: Jet Engines*
### Introduction

Liquid-fuel rocket engines are the most customize-able, versatile, and simple to use of all rocket engines in the game. The "go-to" for almost any rocket, ship, or spaceplane, they allow the user great control over many aspects of their performance and can be tailored to meet almost any requirements that the player desires.

###### **How it works**

Liquid- and solid-fuel rocket engines allow a spacecraft to produce thrust in a total vacuum by containing all of the elements for combustion (fuel, oxygen, ignition) on-board, meaning it does not rely on external elements for combustion, such as a jet engine requiring oxygen from the atmosphere. Liquid-fuel is made up of two chemicals: the fuel, and the propellant. The fuel is the main combustor, a flammable substance (Kerosene, Hydrogen, Methane), and the propellant is oxygen. The fuel tank will contain the fuel and propellant pre-mixed to make up the liquid fuels known as Kerolox, Hydrolox, and Methalox, respectively. The engine ignites the fuel and oxygen mixture, producing thrust. A liquid-fuel engine benefits over a solid-fuel engine by being more efficient and vastly more controllable. A liquid engine can be throttled and switched on and off as needed, unlike solid-fuel. However, liquid engines have many more parts and are more expensive than solid, and are not practical for expendable, short duration use, and (in real life) have a larger chance of failure due to the extra number of parts. Therefore, liquid engines find use in main engines while solids are used as detachable boosters.

###### **A note on how to use this info**

Below is a table of every parameter of liquid-fuel engines and specifically what effect it has on rocket performance. This information was obtained by using the performance analyzer in the Part Properties menu, using a 5 square meter fuel tank with a single engine on the bottom. The chart contains the name of the modifiable parameter, then to the right is how the performance of the engine is affected by positioning the slider to either the minimum, middle, or max. Power Cycle, Fuel type, and nozzle type are simply modifyers and their alterations are listed below the chart. The measurements of each parameter are taken with all other parameters in their middle positions:

* Size: 125%
* Chamber Pressure: Middle (number depends on Power Cycle)
* Nozzle Throat size: 75%
* Nozzle Length: 100%
* Gimbal range: disabled
* Altitude: Vacuum
* Power Cycle: Gas Generator I (Power Cycle modifies all parameters on a percentage, a separate chart will be used for different power cycles. I am just using this one for reference).
* Fuel: Keralox (same idea as Power Cycle)
* Nozzle: Bell

The first number in each cell of the chart is the parameter setting, the second is the **percent** change in thrust in kN, the third is the **percent** change in burn time. Example: in the "minimum" cell for SIze, it will say 50% / -84 / +524. That is with the appropriate parameter (size in this case) set to 50% which is its minimum, produces 84% less thrust and 524% more burn time. All other parameters are set as stated above. Therefore, thrust and burn time under "middle" will be identical.

### Parameters

 Size
 Chamber Pressure
 Noz. Throat Size
 Noz. Length

### Fuels and Power Cycles

###### **Fuels**

The three liquid fuels in SimpleRockets 2 are Kerolox, Hydrolox, and Methalox. Keralox is a very dense fuel but produces very heavy molecular products, causing it to have a relatively low ISP. Methalox is between Hydrolox and Keralox in both density and molecular weight causing it to be sort of a halfway fuel. Hydrolox is a very low density fuel, but it also has one of the lowest molecular weights possible with conventional fuels, giving it an amazing ISP.

Popular Real Life Uses:

* **Kerolox**: Saturn-V 1st Stage, Soyuz, Titan, Atlas, Falcon 1 and 9
* **Hydrolox**: Space Shuttle main engines, Saturn-V 2nd and 3rd Stage, Delta IV 1st and 2nd Stage
* **Methalox**: SpaceX Raptor and BE-4 engines

###### **Power Cycles**

*Main article: [[Rocket Engine Power Cycles]]*Power cycle is the method of how the fuel is injected into the combustion chamber. High injection pressure is necessary to push the fuel against the pressure of the ongoing combustion. High pressure cycles are desired for atmospheric and launcher use, while low pressure is better used in orbital engines.
* **Gas Generator**: Some fuel is used in a pre-burner to power turbopumps to provide the necessary injection pressure. The fuel powering the turbopumps is exhausted through a separate nozzle and does not provide usable thrust, therefore this type is less efficient but provides very high pressure suitable for use as a launcher. (Saturn-V 1st and 2nd Stage, Delta IV 1st stage, SpaceX Merlin)
* **Staged**: Similar to Gas Generator, except the exhaust used to drive the turbopumps is sent to the combustion chamber and added to the rest of the fuel to power the engine. Also very high pressure, but more efficient than GG(Gas Generator). It is heavier and more expensive than GG due to the necessary plumbing and turbine design to allow the turbopump fuel to be injected into the combustion chamber. (Space Shuttle Main Engine)
* **Full-Flow Staged**: Uses two separate turbopumps for the fuel and oxidizer respectively. Eliminating the need for the two to share turbopumps, it essentially doubles the amount of available power versus Staged. Allows engine to run cooler, and is more reliable. (SpaceX Raptor)
* **Pressure-Fed**: Instead of turbopumps, Pressure-Fed uses a pressurized gas (usually helium) to drive the propellant to the combustion chamber. This is a low pressure system suitable for orbital use. (Space Shuttle OMS, Apollo Service Module, Delta II 2nd stage)
* **Electric**: A brushless DC motor drives the pumps. Simpler in design to all others but has extra mass of batteries. (Electron rocket)

Top row: Power Cycle | Left Row: Fuel type. Each cell is the **percent** change in thrust and burn time (from Gas Generator I and Keralox), typed as *thrust | time.* Example: +196 | -66 means 196% increase in trust, 66% decrease in burn time.

For clarification, this means that, for example, a Staged Methalox engine produces 253% more thrust and 75% less burn time than a Gas Generator I Keralox engine, all other parameters equal. A Staged Methalox produces 2% more thrust and 7% less burn time than a Staged Keralox.

### Nozzles

Nozzle type has a different effect depending on both the fuel type and the power cycle. To be continued...