Overview
Selecting the fuel and oxidizer combination is one of the most important parameters in bi-propellant liquid rocket engine development. This combination is oftentimes referred to as the propellant combination. This selection will be the most important parameter for engine sizing, engine performance, and a multitude of other design, engineering, and operations considerations.
Caveats
This document will focus exclusively on bi-propellant systems. Many groups, including amateurs, will use some of the propellants discussed here for other propulsion systems such as hybrids, tri-brids, tri-propellant liquids, etc. The technical considerations for these engines differ from those discussed here. Moreover, the decision to use a bi-propellant architecture is a complex one that will not be discussed here, it will be assumed that a bi-propellant system is being used when discussing engineering or other considerations. Lastly, some engines will use fuel additives, such as PDMS, for thermal insulation. This is not a propellant and will not be discussed here.
Oxidizer
Oxidizers make up the most exciting (and dangerous) of the three sides of the fire triangle. Table 1 lists most of the oxidizers used by both amateurs and industry. Note that many of these were not seriously considered but are listed for the sake of completeness.
Oxidizer | Pros | Cons | Notes |
|---|---|---|---|
Liquid Oxygen (LOX) | Highest performance of any oxidizer Extensive use by amateur groups Used extensively in industry | Cryogenic fluid adds significant operations and engineering complexity | Our school will not allow us to use LOX so it has not been considered. It is listed here as it is wildly common. |
Gaseous Oxygen (GOX) | Easier to acquire and store than LOX Non cryogenic | Much lower density, far less ideal for flight engines More dangerous than LOX | Much of the argument for GOX is in terms of LOX. It often sees use in small engines on the ground or igniters (see Ursa Major’s engines) as it doesn’t require cryo compatibility. |
Nitrous Oxide (N2O) | |||
High-test peroxide (HTP) | |||
Dinitrogen tetroxide (N2O4) |