Rocket fuel oxidizers

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Rocket fuel oxidizers

Rocket propellants are substances that produce the necessary reaction mass to generate thrust when expelled from a rocket engine. The energy required can either come from the propellants themselves, as with a chemical rocket, or from an external source, as with ion engines. Most commonly, propellants are chemical mixtures of a fuel and an oxidizer. 

An oxidizer is a chemical that provides the oxygen or the chemical equivalent needed for fuel to burn. Rockets must carry their oxidizer because space doesn't have atmospheric oxygen. That is a difference between a car engine and a rocket engine.

A Delta IV Heavy during liftoff. The rocket is launched using liquid hydrogen and liquid oxygen cryogenic propellants. Source: Wiki.


A car engine burns fuel with atmospheric oxygen and converts that energy into rotating power to turn the wheels, while a rocket engine carries its own fuel and oxidizer to produce hot exhaust gases, which are expelled at high speed to create forward thrust according to Newton's Third Law, allowing it to work in space and achieve incredible speeds. 

Car engine vs. Rocket Engine

The oxidizer's job in simple words is:
1. Supply oxygen for combustion.
2. Control combustion temperature and reaction speed.
3. Influence ignition method; some oxidizer/fuel pairs ignite on contact - hypergolic, others need an igniter. 

Rocket fuel oxidizers can be divided into liquid, solid, or hybrid. The most common ones are liquid oxidizers including liquid oxygen (LOX), hydrogen peroxide, and nitrogen tetroxide. Common solid oxidizers are ammonium perchlorate and potassium nitrate, while hybrid systems can use combinations of both. The choice of oxidizer depends on factors such as desired performance, storability, and the specific application of the rocket. 

Common liquid oxidizers for rockets include Liquid Oxygen (LOX), Nitrogen Tetroxide (NTO), Hydrogen Peroxide, and Nitrous Oxide (N2O). 

Liquid oxygen (LOX) is crucial to rocket propulsion as an oxidizer, providing the necessary oxygen for a fuel to combust even in the vacuum of space, creating the immense thrust for space travel. When combined with fuels like liquid hydrogen (LH2), RP-1 (a highly refined kerosene), or liquid methane, LOX enables rockets to generate the powerful, explosive force needed to lift off and travel into orbit. 

Not all rocket missions rely on LOX-based propellants. Some specialized applications require different type of combinations.

While liquid oxygen is a cryogenic oxidizer used with fuels like kerosene and hydrogen for high performance, for example for lift off from the ground; nitrogen tetroxide and nitric acid are hypergolic and storable, making them suitable for long-term readiness in missiles. Hydrogen peroxide can also serve as a monopropellant, providing an alternative method for vaporization and heating. 

Example of Liquid Oxidizers:

  • Liquid Oxygen (LOX): A widely used and effective oxidizer, especially when paired with fuels like liquid hydrogen or kerosene. 
  • Hydrogen Peroxide (H₂O₂): A powerful oxidizer that is relatively safe to handle compared to some other options. 
  • Nitrogen Tetroxide (N₂O₄): Used in storable liquid-propellant rockets, often mixed with nitric oxide to form mixed oxides of nitrogen (MON), which lowers its freezing point. 
  • Other Nitric Acid Compounds: Such as red fuming nitric acid, which contains N₂O₄ and a corrosion inhibitor. 

Common solid rocket oxidizers include Ammonium Perchlorate (AP), which is the standard for high-performance rockets, and Ammonium Nitrate (AN), preferred for gas generators and slower-burning propellants. Potassium Nitrate is used in simpler "gunpowder" based systems and amateur rocketry. These solid oxidizers are mixed with fuels, such as aluminum and binders like PBAN, in a composite propellant to create a solid fuel-oxidizer grain for solid rocket motors. 

Example of Solid Oxidizers:

  • Ammonium Perchlorate (AP): The most common oxidizer for solid rocket propellants, often used in composite propellants. 
  • Ammonium Nitrate: Used in special cases, often in smokeless-exhausting propellants, though its performance is typically lower than AP. 
  • Potassium Nitrate: Found in gunpowder-based rockets and some sugar-based amateur rocket propellants. 
  • Nitramines: Higher-energy oxidizers like RDX and HMX are used for maximum density and energy, producing smokeless exhaust. 

Hybrid Systems 

These systems combine liquid or gaseous oxidizers with solid fuels, or vice-versa. For instance, a reverse hybrid could use frozen oxygen as the oxidizer and a liquid fuel.

There are key factors that might be considered to choose the correct type of oxidizer:

  • Performance, such as Specific Impulse Isp: LOX generally gives the highest Isp for common fuels. 
  • Storability: Cryogens boil off, so they are bad for long missions. Storable oxidizers, such as Nitrogen Tetroxide and Hydrogen Peroxide, can sit in tanks at normal temperatures. 
  • Toxicity and safety: Hydrazines and Nitrogen Tetroxide are toxic and require special handling. Nitrous is relatively benign for comparison. 
  • Ignition behavior: Hypergolic pairs simplify ignition, which means that no igniter is needed, but often cost safety and environmental handling. Non-hypergolic pairs need reliable igniters. 
  • Density and tankage mass: Denser oxidizers allow smaller tanks and sometimes simpler structure as well. 
Examples of usage:

  • Examples of rockets that use the LOX (Liquid Oxygen) + RP-1 (Refined Kerosene) propellant combination include the historical Saturn V (first stage), Atlas, and Titan rockets, as well as modern vehicles like the SpaceX Falcon 9 and Falcon Heavy, Soyuz, Delta, and various Long March and Angara rockets.
  • Rockets that utilize Liquid Oxygen (LOX) and Liquid Hydrogen (LH2) include the Space Launch System (SLS), the Delta IV, the Ariane 5, the H-IIA, the Space Shuttle's main engines, and the RL-10 and RS-25 engines. 
  • Rockets and spacecraft frequently use nitrogen tetroxide (N₂O₄) as an oxidizer with hydrazine-based fuels like monomethylhydrazine (MMH) or unsymmetrical dimethylhydrazine (UDMH), and also with the mixture Aerozine 50 (a 50% UDMH/50% hydrazine mix). Examples include the Apollo Command and Service Module's reaction control system (MMH/N₂O₄), the Space Shuttle's Orbital Maneuvering System (MMH/N₂O₄), and historical American rockets like the Titan II (Aerozine 50/N₂O₄). Many Russian, Chinese, and some European rockets also utilize N₂O₄ with UDMH. 
  • N₂O + solid fuel (hybrid rocket) is used by some hybrid rockets because N₂O self-pressurizes and is easier to handle than LOX. 
Hybrid rocket motor detail. Source: Wiki.


Conclusion

If you need maximum performance and you don't mind cryogenics, then LOX is usually preferred. 
If you need a long term storability and on demand starts in space, then storable N₂O₄ etc. are common despite their toxicity. 
If you want easy handling and simple systems N₂O could be the answer. 

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