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**Engines** are vital parts in any successful rocket. They provide continuous propulsion to drive rockets to their destination.

In Spaceflight Simulator, there are seven liquid-fueled engines, a reaction control system and two solid rocket boosters. Currently, all of the engines consume fuel to run. The ion engine is formerly required to consume both fuel and electricity, but the electricity rework disabled this feature. Solid rocket boosters have their own integrated solid fuel tanks.

## Types of engines

There are seven liquid fueled engines in the game. They are named Titan, Hawk, Peregrine, Frontier, Valiant, Kolibri and Ion. The odd one is the RCS Thruster; its name doesn't end with "Engine", but "Thruster", unlike the other six do. The engine thrusts on 4 vectors, unlike other engines, which expel fuel out of the nozzle, and the ion engine, that do some process to expel the fuel out.

Here are the types of engines:

## Vitalness

They are very important parts of the rocket. Engines are used to launch rockets, correct trajectories, transfer burns and many more.

## How engines work?

There are mainly two types of engines, liquid fuel engines and solid fuel engines and they work differently.

### Solid fuel engine

There are two types of solid fuel engines in game, but as of 1.5.5.2 beta they are exactly like Peregrine Engine available only in Career Mode. As of January 29, 2022, they both have the same in-game name, but in blueprints they are named `Booster Tut 1` and `Booster Tut 3`. They have lowest efficiency of all the engines with Isp of only 60 seconds.

> Solid rocket engines are used on air-to-air and air-to-ground missiles, on model rockets, and as boosters for satellite launchers. In a solid rocket, the fuel and oxidizer are mixed together into a solid propellant which is packed into a solid cylinder. A hole through the cylinder serves as a combustion chamber. When the mixture is ignited, combustion takes place on the surface of the propellant. A flame front is generated which burns into the mixture. The combustion produces great amounts of exhaust gas at high temperature and pressure. The amount of exhaust gas that is produced depends on the area of the flame front and engine designers use a variety of hole shapes to control the change in thrust for a particular engine. The hot exhaust gas is passed through a nozzle which accelerates the flow. Thrust is then produced according to Newton's third law of motion.
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*Source from NASA*

### Liquid fuel engine

There are, of course liquid fuel engines in the game. Four out of six engines are liquid fuel engines. They are:

* Titan Engine
* Hawk Engine
* Frontier Engine
* Valiant Engine
* Kolibri Engine

This is how it works:

> Liquid rocket engines are used on the Space Shuttle to place humans in orbit, on many un-manned missiles to place satellites in orbit, and on several high speed research aircraft following World War II. In a liquid rocket, stored fuel and stored oxidizer are pumped into a combustion chamber where they are mixed and burned. The combustion produces great amounts of exhaust gas at high temperature and pressure. The hot exhaust is compressed at the smallest point of the nozzle, making it reach supersonic speed. To accelerate further, the exhaust rushes into a widening nozzle. **Thrust** is produced according to Newton's third law of motion. This is very similar to when a balloon filled with air is let go.
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*Source from NASA*

### Odd engines

The odd engines, RCS and Ion Engine work differently than solid or liquid fuel engines. They have some process of pumping fuel out of the tanks. Here is how they work:

#### RCS thrusters

> Reaction control systems often use combinations of large and small (vernier) thrusters, to allow different levels of response. Spacecraft reaction control systems are used for:
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> * attitude control during re-entry;
> * stationkeeping in orbit;
> * close maneuvering during docking procedures;
> * control of orientation, or 'pointing the nose' of the craft;
> * a backup means of deorbiting;
> * ullage motors to prime the fuel system for a main engine burn.
>

*Source from Wikipedia*

#### Ion engines

> An electrostatic ion engine works by ionizing a fuel (often xenon or argon gas) by knocking off an electron to make a positive ion. The positive ions then diffuse into a region between two charged grids that contain an electrostatic field. This accelerates the positive ions out of the engine and away from the spacecraft, thereby generating thrust. Finally, a neutralizer sprays electrons into the exhaust plume at a rate that keeps the spacecraft electrically neutral.
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*Source from New Atlas*

## Optimality

When aerodynamic drag isn't accounted for (ie. in the sphere of influence of a body with no atmosphere) and assuming they are being fired opposite to gravity, the non-engine (payload+fuel) mass per engine can be adjusted to maximise efficiency, measured in delta-v (from thrust minus that from gravity throughout their operation) divided by fuel consumption. Being too high will lead to accelerating too slowly and operating for too long in the gravity well, but being too low will lead to the engines comprising too high a proportion of the rocket's mass and spend unnecessary fuel lifting themselves. An optimal ratio exists between either extreme, the non-engine mass per engine to achieve it is plotted with respect to gravity for each type in the first image, and the efficiency resultant of this in the second.

Optimal engine proportions at given gravities.pngOptimal non-engine mass per engine for different types with respect to gravity undergone, to maximise delta-v per fuel consumptionOptimal engine efficiencies at given gravities.pngMaximum delta-v per fuel consumption, assuming optimal proportions from the first image
## Trivia

* In 1.5.4, you cannot clip engines together. Stef said it's too unchallenging if you do this and he received tons of hate for removing that feature. However, he re-enabled it in 1.5.4.1, but engines now heat each other, causing them to explode.