LH₂ and LOX Cryogenic Fuel: The Cleanest Source Of Propulsion
The combination of liquid hydrogen and liquid oxygen has been widely regarded as the cleanest rocket propellant in the modern era.
How It Works
The principle of this propulsion system lies in the following chemical reaction:
The reactants of every combustion reaction must contain a type of fuel (something that burns) and a type of oxidizer (something that makes the fuel burn). Since every combustion reaction is exothermic, every combustion reaction releases energy and byproducts. In this case, hydrogen is the fuel, oxygen is the oxidizer as there is no external air intake, and steam (gaseous water) is the byproduct.
Storage of Reactants
Since hydrogen and oxygen exist in gaseous form at room temperature, storing enough molecules to create a large combustion reaction to power a rocket would need an enormous gas tank. To get around this problem, these gases are condensed into their liquid states and stored at extremely cold temperatures: hydrogen at -423°F (-253°C, or 20K) and oxygen at -297°F (-183°C, or 90K)
Combustion
From their respective storage tanks, liquid hydrogen and liquid oxygen are pumped into the combustion chamber seconds before liftoff. There, the gases are mixed and ignited using a spark or a hypergolic initiator, which produces a massive amount of energy packed in highly pressurized steam (according to the equation above). The steam then expands rapidly and leaves the nozzle at a high velocity (around 10,000mph), propelling the rocket upward.
It’s pretty clear that this process is environmentally friendly: heat and water are the only two products of this reaction. However, that’s not all. LH₂ and LOX propulsion generate the highest specific impulse of any commonly used rocket fuel, which translates to higher efficiency as more thrust can be generated with the same amount of fuel.
Specific impulse measures how efficiently a reaction mass engine generates thrust. In most cases, the specific impulse of an engine is proportional or directly related with the velocity of gas exiting the nozzle.
Conclusion
Many modern-day rockets use LH₂ and LOX as propellants, such as Delta IV, Ariane 5, and the Space Launch System (SLS) that’s currently used by the Artemis Program. Although next-generation rockets are considering methane (CH₄) as a potential propellant for its higher thrust and its relatively smaller footprint than RP-1 (kerosene propellant), LH₂ and LOX will remain as the most environmentally friendly source of propulsion.
