Combustion rocket

A **combustion rocket**, also known as a **chemical rocket**, is a type of spacecraft propulsion system that uses explosive chemical reactions to generate thrust, using the superheated products of a propellant as reaction mass.

Combustion rockets are unique in that they allow you to propel your spacecraft without using any nuclear material and are therefore very cheap. Nuclear thermal rockets (NTR's) require fissile material in their reactors, and MPDs and Resistojets require fissile materials in the reactors that power them. This, coupled with their high thrust makes them ideal for use on small drones and missiles, or anything in where low cost is a priority.

## Design

### Propellant overview

* The most optimal propellant for maximizing raw exhaust velocity is Fluorine-Hydrogen.
* Oxygen can be burned with a choice of hydrocarbon options: Methane, Ethane, Propane, Pentane, Butane, or RP-1 (Dodecane).
* Each hydrocarbon choice offers increased density, allowing for smaller propellant tanks, but results in slightly lower exhaust velocity.
* Realism-oriented players may consider the following propellants:
	+ LOX/LH2, LOX/LCH4, LOX/RP-1 (LOX/kerosene) as cryogenic propellants.
	+ NTO/MMH or NTO/UDMH as hypergolic propellants (hydrazine presents an explosion hazard in the game).
* RP-1 is not considered a cryogenic propellant, but LOX, LH2, and LCH4 are.
* Cryogenic propellants require a heat pump to keep them in their liquid state for extended periods.
* Hypergolic propellants, on the other hand, do not require cryogenic cooling but are toxic.

### Propellant characteristics

* LOX/LH2 offers the highest exhaust velocity among the realistic propellants.
* Hydrogen storage poses challenges due to its small molecule size, causing leakage, embrittlement, and requiring extremely low temperatures.
* LOX/LCH4 offers the second-best exhaust velocity and is easier to store compared to LOX/LH2.
* LOX combined with other alkanes has intermediate properties but hasn't been used in real-life applications.
* LOX/RP-1 utilizes the densest fuel among cryogenic propellants, but it yields the lowest exhaust velocity due to increased combustion products and potential stoichiometric ratio adjustments.
* NTO/MMH provides the highest exhaust velocity and density among hypergolic propellants.
* NTO/UDMH has slightly lower exhaust velocity and density compared to NTO/MMH.

### Propellant tank structure

* LOX and hypergolic propellants should be stored inside ferritic stainless steel tanks for sufficient corrosion resistance.
* Tanks made of chromium stainless steel or austenitic stainless steel offer increased corrosion resistance but have lower mass ratios.
* Alkanes can be stored in martensitic stainless steel tanks as they are non-corrosive and provide a slightly higher mass ratio.

### Stoichiometric ratio

Ideally, maintaining a stoichiometric ratio of 1:1 between the propellants is desirable for achieving the highest exhaust velocity.

## Thermal rocket

   A high chamber contraction ratio can increase pressure and therefore thrust. Chambers are best made from strong, heat resistant materials. Diamond is a solid choice for all-around performance. Vanadium Chromium Steel is even stronger, though heavier and with a lower melting point. Tungsten and various ceramics can offer extremely high heat tolerances at the cost of higher cost and more mass. A wide large nozzle expansion ratio is suggested to achieve high exhaust velocity. The best nozzle angle is roughly 8-12 degrees.

   Keep the propellant mixture ratio in mind when you build your spacecraft and choose the fuel tanks, as spacecraft with incorrect propellant ratios tend to display incorrect Delta-V values and ships or drones can run out of Delta-V unexpectedly.

### Dealing with errors

* If the chamber walls are melting or cracking due to thermal expansion stress, use a material with a higher melting point. You can also set regenerative cooling to 100% with no penalty to mass or cost as of version 1.11. You can also try making chamber walls thinner.
* If the chamber walls crack due to pressure, choose a stronger material, reduce the chamber contraction ratio, make the walls thicker, or reduce the fuel flow.