A propellant is any substance that produces force by expanding rapidly, pushing something else in the desired direction. That “something else” might be a rocket, a bullet, medicine from an inhaler, or hairspray from a can. The underlying principle is always the same: the propellant generates high-pressure gas, and that gas does the work.
How Propellants Actually Work
Every propellant operates on Newton’s third law: for every action, there is an equal and opposite reaction. When a propellant burns or rapidly expands, it releases gas in one direction, which pushes the object in the other direction. A rocket engine shoots hot exhaust downward, and the rocket goes up. A gun’s propellant ignites inside the chamber, and the expanding gas drives the bullet forward through the barrel.
Not all propellants involve combustion, though. In an aerosol can, a liquefied gas simply boils into vapor at room pressure, and that expanding vapor pushes the product out of the nozzle. The core idea is pressure differential: create high-pressure gas in a confined space, give it only one way out, and it will move whatever is in its path.
Rocket Propellants
Rocket propellants come in two broad categories: liquid and solid. Both need a fuel (the stuff that burns) and an oxidizer (the stuff that supplies oxygen so burning can happen in the vacuum of space).
Liquid Propellants
The two most common liquid fuel-oxidizer pairings are kerosene with liquid oxygen and liquid hydrogen with liquid oxygen. Kerosene-based propellant (called RP-1) paired with liquid oxygen is what powers SpaceX’s Falcon 9. It’s relatively cheap and easy to handle, but it’s less efficient than the alternative. Liquid hydrogen paired with liquid oxygen delivers higher thrust and better energy efficiency. When hydrogen and oxygen combine in the combustion chamber, they produce steam, and that superheated steam is what generates thrust. NASA and other major aerospace organizations have increasingly shifted toward this combination for primary launch vehicles.
More exotic pairings exist but see limited use. Hydrogen and fluorine react with extreme energy but are dangerously volatile, so most companies avoid them. Hydrazine paired with nitrogen tetroxide is common in satellite thrusters, where small, precise adjustments matter more than raw power. Ammonia with nitrogen tetroxide serves a similar role for spacecraft that need gentle maneuvering rather than launch-level thrust.
Solid Propellants
Solid rocket propellants pack the fuel, oxidizer, and a rubber-like binder into a single pre-mixed block or grain. The most widely used oxidizer in solid rockets is ammonium perchlorate, found in everything from the Space Shuttle’s solid rocket boosters to tactical missiles. Once ignited, a solid propellant burns at a predictable rate and can’t be shut off, which makes solid motors simpler but less controllable than liquid engines. Engineers are now developing “green” solid propellants that reduce the hydrochloric acid gas released during combustion.
Firearm Propellants
Modern firearms use smokeless powder, which replaced black powder in the late 1800s. Despite the name, it isn’t really a “powder” in the dusty sense. It’s a carefully manufactured material whose two main ingredients are nitrocellulose and nitroglycerin. Stabilizers and other additives control how fast it burns.
When smokeless powder ignites inside a cartridge, it deflagrates (burns very fast without detonating) and produces carbon dioxide, water vapor, and nitrogen gas. Small amounts of carbon monoxide and nitrogen oxides also form. The rapid gas expansion behind the bullet is what accelerates it down the barrel. Because it burns rather than explodes, smokeless powder is classified as a low explosive, falling under hazard Division 1.3, meaning it poses a fire risk but won’t produce a mass explosion.
Aerosol Propellants
The propellant in a spray can serves a simpler purpose: it pressurizes the container so the product comes out as a fine mist. There are two types used in consumer products.
- Liquefied gas propellants are substances like propane, butane, and isobutane that exist as liquid under pressure inside the can but instantly vaporize when you press the nozzle. Their expansion atomizes the product into tiny droplets.
- Compressed gas propellants are gases like nitrogen, carbon dioxide, or nitrous oxide pumped into the can at high pressure. They push the product out but don’t vaporize the same way, which is why compressed-gas cans often produce a coarser spray or foam.
For decades, aerosol cans used chlorofluorocarbons (CFCs) as propellants. CFCs turned out to destroy the ozone layer, and the Montreal Protocol phased them out globally. The replacements, hydrofluorocarbons (HFCs), don’t harm the ozone but are potent greenhouse gases, with warming effects hundreds to thousands of times greater than carbon dioxide. The EPA has moved to restrict HFCs in aerosol products to those with a global warming potential of 150 or less, with manufacturing restrictions taking effect in 2025 and sales restrictions in 2026. Hydrocarbon propellants like butane and propane, which have much lower climate impact, are now the standard in most consumer sprays.
Propellants in Medicine
If you use an asthma inhaler, a propellant is what turns your medication into a breathable mist. Metered-dose inhalers originally used CFC propellants, but after December 31, 2008, CFC inhalers were pulled from the U.S. market. They were replaced by inhalers using hydrofluoroalkane (HFA), a propellant that doesn’t deplete the ozone layer. The medication itself, typically albuterol sulfate, stayed the same. The propellant simply acts as the delivery mechanism, pushing a precise dose of medicine out of the canister so you can inhale it directly into your lungs.
How Propellants Are Stored and Handled
Industrial and military propellants carry real safety requirements. Propellants classified as explosives must be stored in specially constructed magazines that are fire-resistant and theft-resistant. Indoor storage magazines have to be painted red on the exterior, marked with “EXPLOSIVES – KEEP FIRE AWAY” in white lettering at least three inches tall, and fitted with wheels so they can be moved quickly in an emergency. They must sit within 10 feet of a building entrance, and no more than two magazines are allowed per building. A designated person at least 21 years old must supervise the storage at all times, and physical inspections are required at least every seven days to account for all materials.
Consumer propellants like the butane in a spray can carry far less regulatory burden, but the flammability warnings on those cans exist for good reason. Hydrocarbon propellants are combustible, and puncturing or heating a pressurized can creates a genuine explosion risk.