What is the fuel used in modern rockets?

[SilasLee]

Rockets are jet propulsion devices that move forward by expelling high-speed hot gas streams backward, utilizing the generated reaction force. They carry their own fuel and oxidizer, not relying on atmospheric oxygen for combustion, enabling operation both within the atmosphere and in outer space. During flight, rockets become variable-mass vehicles as their propellants are consumed. Relevant information about rockets is as follows: Introduction 1: Modern rockets serve as rapid long-distance transport tools, such as sounding rockets, launch vehicles for artificial satellites, manned spacecraft, space stations, and boosters for other aircraft. Introduction 2: When equipped with warheads for combat delivery, they form rocket weapons. Guided versions are called missiles, while unguided ones are termed rocket projectiles. Common liquid oxidizers include liquid oxygen and nitrogen tetroxide, with fuels comprising liquid hydrogen, unsymmetrical dimethylhydrazine, and kerosene.

[DeLila]

The combination of liquid hydrogen and liquid oxygen is actually quite interesting. When I was researching energy systems, I found it to be one of the mainstream solutions. Liquid hydrogen has an extremely high calorific value, and when paired with liquid oxygen, it can generate tremendous thrust—like the main engines of the Space Shuttle, which relied on it. However, storage is particularly troublesome, requiring temperatures as low as -253°C to maintain the liquid state, so the outer layers of rockets have to be wrapped in insulation. On the other hand, the kerosene + liquid oxygen solution is more cost-effective, used by SpaceX's Falcon rockets, with lower propellant temperature requirements. The future trend is toward liquid methane fuel, which produces less carbon buildup after combustion, making it easier to reuse rockets during recovery.

[Sawyer]

Current rocket fuels are divided into two main categories: liquid and solid. My child studied this for a science competition. Common liquid fuels include liquid oxygen paired with kerosene or liquid hydrogen. The kerosene option is cheaper and more stable, and Russia's Soyuz rockets have been using it for over 60 years. Solid fuels are like the propellant in firecrackers, directly cast into the rocket. Military missiles prefer this type because they can launch immediately upon ignition. However, solid fuels can't adjust thrust in real-time like liquid fuels can.

[Maeve]

Worked with rocket fuel safety standards, highly toxic fuels like unsymmetrical dimethylhydrazine are being phased out now, previously used in the Long March series. The current mainstream is environmentally friendly combinations like liquid oxygen kerosene and liquid oxygen liquid hydrogen. Liquid hydrogen combustion only produces water vapor, which is eco-friendly but has extremely high production costs, requiring 12 kWh of electricity to cool per kilogram. SpaceX's methane fuel is a compromise solution, offering high specific impulse and the convenience of being sourced locally on Mars.

[JosephFitz]

Simply put, fuel selection depends on mission requirements: Near-Earth orbit missions often use liquid oxygen and kerosene (like the Falcon 9) for cost-effectiveness and durability; high-energy missions to the Moon or Mars use liquid oxygen and liquid hydrogen (like the SLS core stage); military or solid boosters use solid fuels mixed with aluminum powder and ammonium perchlorate. Interestingly, the Progress spacecraft supplying the International Space Station still uses unsymmetrical dimethylhydrazine due to its simplicity in storage.

[Morales]

During a visit to the aerospace exhibition, I overheard engineers comparing modern rocket fuels to mixing cocktails. Kerosene paired with liquid oxygen offers great cost-performance, but with a specific impulse of just over 300 seconds. Liquid hydrogen and liquid oxygen can push the specific impulse up to 450 seconds, though the storage tanks end up costing more than the fuel itself. The new favorite, methane fuel, strikes a balance between storage difficulty and environmental friendliness—Blue Origin's BE-4 engine relies on it. Additionally, hybrid solid-liquid rockets have emerged, using a solid fuel core at the front and spraying liquid oxidizer at the rear, currently undergoing lab testing.