Hydrogen as rocket fuel

Hydrogen as rocket fuel

Hydrogen, especially in its liquid form, is an effective and widely used rocket fuel known for its high energy efficiency and clean combustion, which produces only water vapor. However, it requires cryogenic storage in extremely insulated vessels due to its very low boiling point, leading to large tank sizes and significant boil-off issues that increase costs and complexity. Despite these challenges, hydrogen's high specific impulse and environmental benefits continue to make it a key propellant, particularly for upper stages, with ongoing research into even more powerful forms like metallic hydrogen. 

Using liquid hydrogen as a rocket fuel!

Liquid hydrogen (LH2) is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form. Hydrogen is a colorless, odorless, non-toxic gas at standard temperature and pressure, while liquid hydrogen is its ultra-cold, condensed state, requiring temperatures of -253°C or lower.

The exhaust plumes of a hydrogen-fueled Delta IV Heavy rocket produce only water vapor as a byproduct. Source: Wiki

For H2 to exist as a liquid, it must be cooled below its critical point of 33 K. In order to be fully in liquid state at atmospheric pressure, it needs to be cooled to 20.28 K (−252.87 °C). Storing of hydrogen typically takes place in liquid form. Storing it as liquid takes up less space than storing it in gaseous state at normal temperature and pressure. However, the density of the liquid is very low compared to other common fuels. Once liquefied, it can be stored in a liquid state for a certain time in thermally insulated containers.

Liquid hydrogen (LH2) fuel has played an important role in space exploration since Apollo program. The Saturn rockets used it for the secondary stage engines, and the Space Shuttles used it to power three main rocket engines, and the current Space Launch System (SLS).

Let's talk about Advantages first!

The most of the rocket's mass covers the fuel, as much as 85 overall per cent in order to get through the Earth's gravity. So, the efficiency is clearly significant.  Efficiency means how efficiently a rocket can burn the fuel. The term describing such an efficiency is Specific Impulse, measured in seconds. 

The lowest molecular mass element is hydrogen, with an atomic mass of approximately 1.008 atomic mass units (u), and its most common liquid propellant form, liquid hydrogen (H2), has the lowest density of any liquid propellant at about 0.07 g/cm³. It is also the most energetic and burns at temperatures of up to 3 038° Celsius.

It burns cleanly to produce water vapor, but requires cryogenic temperatures for storage as a liquid. While hydrogen is the most energetic in terms of energy per kilogram, this refers to its high energy density, not the highest combustion temperature, which is closer to 3000°C. 

Comparison of Specific Impulse for different rocket engines, different fuel type:

1. LOX/LH2 (Liquid Oxygen/Liquid Hydrogen): High efficiency, used in upper stages.

• Space Shuttle Main Engines (SSME) / RS-25: ~366 to 453 s 
• Rocketdyne J-2 (Saturn V): ~421 s 

2. LOX/RP-1 (Liquid Oxygen/Kerosene or RP-1): Common for boosters, good density.

• Saturn V F-1: ~304 s
• RD-180: ~311 s 
• Space X Merlin Engine: 282 – 311 s

3. LOX/Methane (Methalox): Emerging high-efficiency propellant.

• SpaceX Raptor: ~330–350 s 
• BE-4 (Blue Origin): ~350 s 

However, there are also challenges. 

Liquid hydrogen still needs much larger fuel tanks than other propellants. Also, as it was already mentioned, it must be stored in low cryogenic temperatures, demanding insulations, and storage complexity. 

This type of fuel is clearly more complex, in producing, transporting, handling, storing, prevention of leaks and corrosion, which all leads to increase in the cost. 

One thing still needs to be point out. In case of direct exposure to hydrogen fuel, it makes it dangerous, even deadly because of the low temperatures. Compared to other rocket fuels, hydrogen fuel is non-toxic, environmentally friendly fuel, whose primary byproduct of combustion is water. While hydrogen is flammable and requires safety precautions, its ability to quickly rise and disperse makes leaks less likely to cause persistent contamination of water or soil. Which is in contrast to RP-1 propellant, which produces carbon dioxide, nitrogen oxides, sulfur compounds & carbon monoxide. 

Does LH2 provides less thrust than RP-1?

In case of comparing the fuels based on mass (specific impulse) it is not, but it is true when considering the volume of a rocket tank or a given tank size due to RP-1's much higher density. Liquid hydrogen has a far greater specific impulse and thus provides more thrust per unit mass than RP-1. However, RP-1's high density means it requires less tank volume for the same mass, leading to greater thrust from a smaller, more manageable tank or a given chamber volume. 

Some comparison in the end:

1. Thrust per Unit Mass (Specific Impulse):

• LH2/LOX: Offers a high specific impulse (up to 450 s), meaning it's very efficient and provides more thrust per unit mass of propellant.
• RP-1/LOX: Has a lower specific impulse (around 280-320 s) compared to LH2.

2. Thrust per Unit Volume (Density):

• LH2: Is extremely low-density, requiring very large tanks. 
• RP-1: Is much denser, allowing for smaller tanks. 

3. Different significance, different design:

• Upper Rocket Stages: The high specific impulse of LH2 makes it ideal for upper stages, where the volume disadvantage of large tanks can be managed. 
• Lower Rocket Stages: The need for high thrust to lift the rocket off the ground makes RP-1 more practical for boosters due to its high density and the resulting high thrust-to-volume ratio. 

Liquid hydrogen will continue to play crucial role in space propulsion, since it is highly efficient rocket fuel, especially when paired with liquid oxygen (LOX), due to its high energy density, great specific impulse, and clean combustion producing water as a byproduct. Despite the challenges related to its low density and extreme cryogenic temperatures, ongoing research  aims to improve performance, and mitigation of disadvantages. 

Future uses include powering next-generation reusable rockets for satellite launches and space tourism, enabling long-duration missions to the Moon and Mars, and supporting lunar bases with power and fuel. 

Source: NASA