Overview

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Liquid rocket engines burn some combination of fuel and oxidizer to achieve combustion, create pressure in the chamber, and then thrust. Selecting which fuel and oxidizer to use for your engine/program is critical and oftentimes a non-trivial task. Note that this page focuses only on bi-propellant systems, not mono-prop, hybrid, tribrid, or any other shenanigans.

The propellant combination is likely the most important decision to make when sizing an engine. It defines nearly all aspects of your propulsion system and will drive significant aspects of your system architecture.

Some “common” fuel choices for both industry and/or amateur use include Liquid Hydrogen (LH2), Liquid Methane (LCH4), Hydrazine (N2H4), various alcohols, various kerosene-based fuels such as RP1 or jet fuel, Propane (C3H8), and Butane (C4H10). Image Added

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 or other benefits. This is not a propellant and will not be discussed here.

Derived Requirements for Propellant Combination

This engine is subject to numerous requirements. Some of these come from our “governing bodies” i.e. the CPLC rules and Rose-Hulman restrictions. Both of these groups allow for leniency but they must allow us to use the propellant combination we select.

CPLC Requirements:

The following are the requirements for propellant combination outlined by the CPLC Challenge Rules document as of February 2024.

• “By default, teams are banned from using toxic propellants or hypergols.”

• We allow liquid oxygen (LOX), nitrous oxide, white fuming nitric acid, hydrogen
  peroxide and other non-toxic propellants like commercial ammonium perchlorate
  composite propellant (APCP), potassium nitrate, and sugar (also known as
  “rocket candy”), kerosene, propane, alcohol, and similar substances considered
  non-toxic.

• Toxic propellants are defined as those requiring breathing apparatus, extensive
  personal protective equipment (PPE) such as chemical suits, breathing filters, or
  Self Contained Atmospheric Protective Ensemble (SCAPE) suits and posing
  significant public and/or environmental hazards in the event of a spill.
  Hydrazines, dinitrogen tetroxide (NTO), and red fuming nitric acid (RFNA) are
  examples.

Rose-Hulman Requirements:

We are a student group and as such must adhere to the rules and requirements put in place by the school. These are not explicitly laid out in the student handbook but the result of discussion and compromise with various stakeholders at the Institute. The school has firmly disallowed the use of liquid or gaseous oxygen by student groups such as ours. While there isn’t an explicit rule or decision against other propellants, such as using Hydrazine as a fuel, we can be very confident that it would not be allowed. Moreover, asking to use certain propellants such as hypergols would undermine our credibility. As such the requirements, outside of banning LOX and GOX, are qualitative.

Our stance thus far has been to select propellants that are:

1. Common in the amateur community.

2. Used by other teams/groups with proven track records.

3. Easily commercially available (i.e. from Airgas or weld shops for oxidizers)

While these criteria are vague our relationship with risk management has matured to the point where we can fairly accurately predict what propellents would and would not be allowed before we ask for explicit permission.

Internal Engineering Requirements:

WIP

Internal Operations/Logistic Requirements:

WIP

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.