Let’s learn about Rocket propellants today.
Rocket Engines produce thrust by expelling a reaction mass. All the substances that create this reaction mass directly (for example, water rockets) or indirectly by a chemical reaction (for example, Falcon 9) constitute Rocket Propellants.
So, by definition Rocket propellants includes both the oxidizer and the fuel. In this article, we will understand the major types of rocket propellants.
Chemical Propellants: All chemical propellants are in a way a combination of fuel and oxidizer. The method in which these two are brought together leads to further classification in 5 types: solid, storable liquid, cryogenic liquid, a liquid monopropellant, and hybrid solid/liquid bi-propellant. We will look into solid and liquid types in this article.
Solid propellants: These are composites with 3 major constituents – Oxidizer granules (like ammonium nitrate, ammonium dinitramide, ammonium perchlorate, potassium nitrate) in a polymer binder (binding agent) with powders of compounds acting as fuel (examples: RDX, HMX). Many additives are added which include plasticizers, stabilizers, and burn rate modifiers.
Major Advantages: Easy to store/handle. Compact.
Major Disadvantages: Of all the disadvantages like a low specific impulse, temperature, and pressure sensitivity, the most important disadvantage is of lack of real-time throttling.
Used in: Space Shuttle booster stages.
Liquid Propellants: These are the most commonly used propellants today. The common subtypes are: –
a. Liquid oxygen (LOX) and highly refined kerosene (RP-1). Used for the first stages of the Saturn V, Atlas V, and Falcon 9. The equation for the corresponding chemical reaction will be as follows –
2C12H26 + 37O2 → 24CO2 + 26H2O
Note: Kerosene is represented as C-12 since a majority of the polymeric units are in this form. Further, the above equation represents a complete ideal case reaction in which 100% combustion efficiency is achieved. This is not the real case with many incomplete combustion happening and a lot of carbon left as the output.
b. LOX and liquid hydrogen, used in the Space Shuttle orbiter and Saturn V upper stages. The chemical reaction is straightforward –
2H2 + O2 = 2H2O
c. Dinitrogen tetroxide (N2O4) and hydrazine (N2H4), MMH, or UDMH. Used in military, orbital, and deep space rockets because both liquids are storable for long periods at reasonable temperatures and pressures.
d. Monopropellants such as hydrogen peroxide, hydrazine, and nitrous oxide are primarily used for attitude control and spacecraft station-keeping where their long-term storability, simplicity of use, and ability to provide the tiny impulses needed, outweighs their lower specific impulse as compared to bipropellants.
Advantages: Liquid-fueled rockets have higher specific impulse than solid rockets and are capable of being throttled, shut down, and restarted.
Disadvantages: Storage, the addition of lots of valves, and control systems increase the overall weight and complexity.
Special Note on Hydrogen: Although liquid hydrogen gives a high Isp, its low density is a significant disadvantage: hydrogen occupies about 7x more volume per kilogram than dense fuels such as kerosene. This not only penalizes the tankage, but also the pipes and fuel pumps leading from the tank, which need to be 7x bigger and heavier. (The oxidizer side of the engine and tankage is of course unaffected.) This makes the vehicle’s dry mass much higher, so the use of liquid hydrogen is not as advantageous as might be expected. Liquid hydrogen is quite an expensive fuel to produce and store, and causes many practical difficulties with the design and manufacture of the vehicle.
