For over half a century, RP-1 propellant has been the fuel of choice to power orbital rocket’s first stages. For their upper stages, though, liquid hydrogen is primarily used. We take a closer look at this cryogenic fuel.

In rocket propulsion, liquid hydrogen is a cryogenic fuel used to power orbital rockets. It has the high Specific Impulse of all liquid propellants, making it the most fuel-efficient liquid propellant. It is a clean-burning fuel, resulting in no residue buildup in engines and no carbon emissions.

The Saturn V launch vehicles that first launched astronauts to the Moon during the Apollo Program of the 1960s and 70s used RP-1 propellant to provide the thrust to allow the giant rocket to push through Earth’s thick atmosphere and break free from its gravitational force.

To power both the second and third stages, though, liquid hydrogen was used. Both fuels were mixed and combusted with liquid oxygen inside their respective combustion chambers. This fuel configuration is still used today in a large number of orbital launch vehicles.

Examples of vehicles that are currently using liquid hydrogen in their upper stages include the Delta IV Heavy, Atlas V, Ariane 5, and Long March 5 (China) rockets.

(Both the Delta IV Heavy and Ariane 5 launch vehicles also use liquid hydrogen as the primary propellant for their first-stage boosters.)

Saturn V Second Stage
Both the second (pictured) and third stages of the Saturn V rocket used liquid hydrogen.

As the following sections will illustrate, there are several reasons why liquid hydrogen is still used today in the upper stages of the majority of orbital rockets.

What is Liquid Hydrogen?

In its normal state, hydrogen (H) is a naturally occurring gas and also the most abundant chemical element, estimated to make up 75% of the universe. In molecular form, it is present in all living beings, including plants, humans, and other animals.

However, as a gas, it is very scarce, especially in comparison to methane (natural gas) which can be found in abundance next to coal or oil reserves and deep below the Earth’s surface within soil and rock sediments.

(Liquid methane is currently being developed as an alternative to traditional rocket fuels. Learn more about what it is and what its advantages and drawbacks are in this article.)

In its liquid form, hydrogen is a cryogenic substance, which means the gas has to be cooled to temperatures below -253° Celsius (-423° Fahrenheit) to turn into liquid hydrogen (LH2).

Like all other fuels used in orbital rockets, liquid hydrogen needs an oxidizer to combust. This comes in the form of liquid oxygen (LOX). The fuel and oxidizer are mixed in the combustion chamber, where they combust to form the hot gases that propel the spacecraft.

Liquid Hydrogen
Hydrogen needs to be cooled to temperatures below -253° Celsius (-423° Fahrenheit) to turn into liquid.

As a result of its scarcity, hydrogen has to be specifically produced by a variety of different processes, as the following section will illustrate.

How Liquid Hydrogen Is Made

Unlike other fuel sources like crude oil and methane, which can be extracted in a raw form below the planet’s surface before being refined at processing plants, hydrogen needs to be produced from other sources like natural gas, nuclear power, and renewable power sources.

It can be produced in a variety of ways, but the two primary methods of producing hydrogen are steam-methane reforming and electrolysis.

Steam-methane reforming (the most widely used method of producing hydrogen in the United States) uses steam at extremely high temperatures, which reacts with methane in the presence of a catalyst to create hydrogen.

Electrolysis uses an electrical current, which is passed through water (H2O) to separate the hydrogen molecules from the oxygen. It is a cleaner process than steam-methane reforming since it doesn’t produce any byproducts such as carbon monoxide.

To produce liquid hydrogen, the gas is cooled to cryogenic temperatures of -253° Celsius (-423° Fahrenheit) or below before it can be used as fuel in an orbital launch vehicle.

Advantages Of Using Liquid Hydrogen For Orbital Rockets

Liquid hydrogen offers several advantages but also a few drawbacks compared to other liquid propellants. Some of the primary advantages of using liquid hydrogen include:

  1. Highest Specific Impulse Of All Liquid Fuels
  2. Little To No Coking And Other Forms Of Residue Buildup
  3. Environmentally Friendly
  4. Non-Toxic

1) Highest Specific Impulse Of All Liquid Fuels

Since more than 85% of a rocket’s mass consists of fuel to provide enough propellant for the thrust needed to allow a launch vehicle to push through Earth’s atmosphere and break free from its gravity to reach orbit, fuel efficiency is of the utmost importance.

As a result, one of the Holy Grails of rocket propulsion is how efficiently a rocket can burn its fuel. Specific Impulse is the term used to describe this efficiency and is typically measured in seconds. It is essentially the rocket equivalent of the automotive “miles per gallon.”

(A detailed discussion about Specific Impulse falls beyond the scope of this article, but you can learn more about it in the following article on nuclear propulsion.)

Hydrogen has the smallest molecular mass of all known elements and is the least dense of all liquid propellants. It is also the most energetic and burns at temperatures of up to 3 038° Celsius (5 500° Fahrenheit).

RL-10 Hydrogen Rocket Engine
Aerojet Rocketdyne’s RL-10 engine has been used in the upper stages of several orbital rockets for over half a century, thanks to the High Specific Impulse it achieves from burning liquid hydrogen.

Its light molecular weight and energetic nature allow the combusted hydrogen gas to be propelled through a rocket engine’s nozzle at a greater velocity, making it the most fuel-efficient rocket propellant with the highest Specific Impulse.

To illustrate this point, one can look at the Specific Impulse generated by modern examples of rocket engines running on each fuel type:

  1. Liquid Hydrogen: 366 – 452 seconds (Space Shuttle/SLS RS-25 engine)
  2. Liquid Methane: 330 – 350 seconds (SpaceX Raptor engine)
  3. RP-1 Propellant: 282 – 311 seconds (SpaceX Merlin engine)

(Learn more about the different types of fuel orbital rockets use and their various advantages and drawbacks in this article.)

2) Little To No Coking And Other Forms Of Residue Buildup

Unlike RP-1 and even the cleaner burning liquid methane, hydrogen burns practically completely when combusted, resulting in no coking, soot, or other types of residue buildup in rocket engines, which can clog up, decrease performance or even destroy an engine.

This not only helps the engines of orbital launch vehicles to perform optimally and more reliably, but with reusability becoming an increasingly important part of spaceflight, it makes the refurbishing of a reusable rocket much easier with a quicker turnaround time.

3) Environmentally Friendly

RP-1 propellant provides the most thrust of all the major liquid rocket fuels due to its high molecular mass, which is why it is used in the first stages of most launch vehicles, like the Atlas V, Falcon 9, Saturn V, and Soyuz rockets.

But RP-1 is a hydrocarbon with long chains of carbon and surrounding hydrogen molecules. These long chains of molecules mean RP-1 never burns completely. Apart from the resulting residue buildup in engines, its exhaust plumes also contain unwanted byproducts.

They contain carbon dioxide, soot, nitrogen oxides, sulfur compounds & carbon monoxide. All of which contribute to air pollution. The exhaust plumes of liquid methane, which is considered a clean burning fuel, also produce some carbon dioxide and nitrogen monoxide.

The exhaust plumes of hydrogen, on the other hand, contain only water as the byproduct, making it the cleanest burning and most environmentally friendly rocket propellant currently in use in orbital rockets.

(Learn more about RP-1 propellant, what it is, and its different advantages and drawback in this article.)

4) Non-Toxic

Liquid hydrogen is a cryogenic fuel that needs to be cooled to temperatures of -253° Celsius (-423° Fahrenheit) or below to remain a liquid. This makes it extremely dangerous to handle, and direct exposure to it can be deadly.

However, in case of an explosion or accidental spillage, it only produces water as a byproduct, making it non-toxic and safe to clean up with little to no danger for human crews and other lifeforms in the surrounding region.

This contrasts with RP-1 propellant, which produces carbon dioxide, nitrogen oxides, sulfur compounds & carbon monoxide. Some of these compounds are extremely hazardous to humans in close proximity, which also makes cleanup procedures more complex.

Delta IV Nozzles During Liftoff
The exhaust plumes of a hydrogen-fueled Delta IV Heavy rocket produce only water vapor as a byproduct.

Accidental spillage or the uncontrolled combustion of liquid methane, which is gaining popularity as rocket fuel, can lead to high concentrations of methane gas in the air, which can result in methane poisoning and affixation if too much oxygen is displaced by the gas.

Disadvantages Of Using Liquid Hydrogen For Orbital Rockets

Despite the several advantages of liquid hydrogen as rocket fuel, it also has some disadvantages. The most noteworthy drawbacks of using hydrogen include:

  1. Large Fuel Tanks Required
  2. Hydrogen Embrittlement
  3. Provides Less Thrust Than RP-1
  4. Extreme Cryogenic Temperatures
  5. Small Size Makes It Hard To Contain

1) Large Fuel Tanks Required

One of the main advantages of using hydrogen as rocket propellant is also one of its disadvantages. Their small molecular size allows hydrogen molecules to be accelerated through a rocket’s nozzle at high velocities, helping it to achieve its high Specific Impulse.

However, its extreme low density also means liquid hydrogen needs a lot more space than other liquid propellants with a similar mass, requiring much larger fuel tanks to house the large volume of propellant. This adds additional mass and size to an orbital rocket.

(Most of the space inside the large central cores of the Delta IV and Delta IV Heavy launch vehicles, as well as the giant external fuel tank used during the Space Shuttle Program, was used to store the liquid hydrogen needed to allow the vehicles to reach orbit.)

2) Hydrogen Embrittlement

Although hydrogen is a clean-burning fuel, leaves no residue buildup in rocket engines, and produces no byproducts apart from water vapor in its exhaust plumes, it can still negatively impact rocket engines.

Due to its small molecular size, hydrogen molecules can penetrate the metal of rocket engines, causing them to become rigid and lowering the amount of stress they can handle. This can cause cracks to appear over time, resulting in the metal becoming brittle.

3) Provides Less Thrust Than RP-1

As already stated throughout previous sections in this article, liquid hydrogen is the most fuel-efficient rocket propellant currently available, which is primarily thanks to its small molecular mass and the resulting high exhaust velocities achieved.

However, its small size & mass also mean that hydrogen cannot achieve the same amount of thrust as RP-1 propellant, which has much larger and heavier molecules, allowing the latter to provide more “raw power” to a launch vehicle when needed.

This is also why the first stages of most orbital rockets still use RP-1 propellant. The hardest part of an orbital launch is getting a spacecraft off the launchpad and providing enough force to power it through Earth’s thick atmosphere while also fighting its gravitational forces.

Falcon 9 First Stage
The thrust provided by the first stage boosters of an RP-1 propelled Falcon 9 rocket provides more lifting power than a hydrogen-fueled rocket engine can,

During this crucial period, fuel efficiency is not as important as pure thrust, and liquid hydrogen simply cannot match RP-1 propellant in this regard.

4) Extreme Cryogenic Temperatures

For hydrogen to remain in liquid form, it needs to be cooled and stored at temperatures of -253° Celsius (-423° Fahrenheit) and below. These extreme cryogenic temperatures require additional measures to be in place during transportation and storage.

Apart from safety measures to keep ground crews safe while handling the freezing cold fuel, additional insulation is also required on the outside of the fuel tanks to protect the cryogenic fuel from external heat sources.

Even with the extra insulation, liquid hydrogen still heats up and starts to boil while in the fuel tanks inside the launch vehicle on the launchpad. As a result, evaporated hydrogen gas needs to be vented to prevent excessive pressure buildup and possible tank rupture.

All these additional measures mean that handling and storing liquid hydrogen becomes a very complex, dangerous, and costly process. The extra insulation also adds to a launch vehicle’s mass.

5) Small Size Makes It Hard To Contain

The small size of hydrogen molecules helps them to achieve the high Specific Impulse that makes it so energy efficient. However, they also pose several problems, of which hydrogen embrittlement has already been highlighted. It has another disadvantage, though.

Hydrogen molecules are so small that it makes them extremely hard to contain. They are so small that they can literally penetrate solid metals and seep through cracks in the tiniest of openings in welded materials.

This not only poses problems for long-term storage, but the smallest leak in the hydrogen supply of a rocket engine can cause it to escape and react with liquid oxygen (LOX) in another part, leading to unintended combustion, which can destroy the entire engine.

Conclusion

Liquid hydrogen is without question the rocket fuel that provides the highest Specific Impulse and, as a result, is the most fuel-efficient rocket propellant currently available. This is mostly due to its small molecular size and highly energetic nature.

As this article illustrated, there are several disadvantages to using this cryogenic fuel, which makes it expensive to produce, difficult to handle and store, and need much larger fuel tanks than any other rocket propellant.

Despite all these drawbacks, its high Specific Impulse, complimented by several other advantages, including its clean burning nature and environmentally friendly exhaust plumes, still make it a highly sought-after fuel.

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