Fuel

**Fuel**, also called propellant, is an expendable resource used by reaction engines to generate thrust. There are many different types of fuel in KSP2, each used for different purposes and each acquired through different means.

## Methalox

Methalox is by far the most common fuel type and has the most engines that utilize it. It consists of a 4:1 ratio of oxygen to methane, which means that it has onboard oxidiser and can burn anywhere. It has a wide array of fuel tanks with different sizes and shapes, which, in combination with the fact that it can be used virtually anywhere, makes it an all—around fuel type that can be used for almost all purposes. But in comparison to other fuel types, it has a rather low Isp.

## Solid Fuel

Solid Fuel is a fuel type only found in boosters. It is a mixture of solid oxidizing agents and solid fuel, which makes it usable everywhere. It is fairly cheap, has a high thrust but terrible Isp. However, since solid fuel is only used for boosters, which require high amounts of thrust, low Isp isn't that big of a deal. Any booster powered by solid fuel cannot be throttled or turned off: as soon as the booster has been ignited, it will burn until it either runs out of fuel or gets destroyed. The amount of solid fuel a booster has and the thrust can be adjusted in the VAB; thrust is also adjustible before activating a rocket on the launch pad.

## Methane

Methane is the fuel type most commonly used for planes. Consisting of only methane, Jet Engines need an atmosphere with oxygen and air intakes for intake air to burn fuel. For each unit of Methane, a craft needs 8 units of intake air. As jet engines don't need any oxidizer onboard, the Isp of jet fuel is astronomically high, exceeding even that of ion engines. As they do not work in space or on celestial bodies without oxygen, their usage is fairly limited. When making SSTOs (**S**ingle **S**tage **T**o **O**rbit vehicles), it is heavily recommended to use a combination of Methane and Methalox engines for spaceplane designs.

## Monopropellant

Monpropellant is an inefficient and low-thrust fuel type, but necessary for space stations, satellites and other bodies which need to fine-tune their orbits. In real life, this monopropellant could be nitrogen used for cold gas thrusters, or a hypergolic propellant mixture. It is mainly used by RCS thrusters, as the only Engine using monopropellant - the O-10 "Puff" - is easily outdone by methalox and ion engines both in terms of Isp and thrust, and is only applicable in niche situations. For precise maneuvering, low-thrust thruster blocks which can vector in pretty much any direction when placed right, monopropellant has no replacement and is a vital part of every somewhat complex vehicle or everything that needs to do docking maneuvers.

## Xenon

Xenon is a highly efficient and low-thrust fuel type used mainly for interplanetary probes and satellites. Used by Xenon engines, it utilizes the Hall Effect to accelerate xenon ions to high speeds into space while using Electric Charge. This leads to high exhaust velocities, meaning incredibly high Isp. As it has incredibly low thrust, using it in an atmosphere or on large spacecraft vehicles is not advised. There is currently only one engine that uses Xenon: the IX-6315 "Dawn".

## Hydrogen

Hydrogen is an efficient fuel type with a decent amount of thrust. It has about the same thrust many methalox engines have, while being a lot more efficient as their Isp in vacuum is a lot higher. Hydrogen has a variety of possible uses: it can be used to propell SSTOs out of an atmosphere or do circularization burns to orbit, propell large space stations efficiently yet fast or function as a fuel for motherships for interplanetary trips. Since hydrogen has a very small density, hydrogen tanks are very large, as even the biggest hydrogen tank - the biggest part in the game so far - only holds 50t of hydrogen, making it hard to efficiently bring hydrogen into space. For atmospheric launches, methalox engines are generally more suitable.