The Engines That Power the Space Age: A Review of Operational Liquid Rocket Engines

A Primer on Liquid Rocket Propulsion

A liquid-propellant rocket engine is a machine that performs a controlled, continuous explosion to generate enormous power. Its operation, while appearing complex, is built on a straightforward principle. The engine’s purpose is to take two propellants, a fuel and an oxidizer, stored as liquids in a rocket’s tanks, and turn them into a high-speed jet of hot gas.

This process is a direct application of Newton’s third law of motion: for every action, there is an equal and opposite reaction. The engine violently throws a massive amount of gas downward, and in reaction, the rocket is pushed upward.

Every liquid rocket engine, regardless of its size or power, contains the same fundamental parts. It has a combustion chamber, which is the “bottle” where the fuel and oxidizer are mixed and burned, reaching thousands of degrees. It has an injector, which acts like a sophisticated showerhead, spraying the propellants into the chamber to ensure they mix perfectly. And it has a nozzle, the cone-shaped bell that takes the high-pressure, high-temperature gas from the chamber and converts its random energy into a focused, high-velocity exhaust jet that produces thrust.

The greatest challenge is pressure. The propellant tanks are at a relatively low pressure, but the combustion chamber operates at an extremely high pressure. To force the propellants into an already-burning chamber, the engine must use incredibly powerful pumps. The complex and ingenious methods engines use to power these pumps are what define their “engine cycle,” which is the main difference between various engine designs.

Unlike a solid-fuel rocket, which is like a firework that can’t be stopped once lit, a liquid-fueled engine can be controlled. Its pumps can be sped up or slowed down to “throttle” the engine, and the propellant valves can be closed to shut it down completely. This control is what allows a rocket like the Falcon 9 to not only launch a payload but also to re-ignite its engines and perform a gentle, powered landing.

The Power of Propellants

The “recipe” of fuel and oxidizer an engine uses dictates its performance, its complexity, and the very shape of the rocket it’s attached to. There are four main combinations in use today.

Kerolox (LOX/RP-1)

This is the workhorse propellant of the space industry, a combination of liquid oxygen (LOX) and RP-1, which is a highly refined form of kerosene. It’s the fuel used by engines like SpaceX’s Merlin, Rocket Lab’s Rutherford, and the powerful Russian RD-180.

The primary advantage of kerolox is its density. Kerosene is a dense liquid that can be stored at manageable, non-cryogenic temperatures. This means the rocket’s fuel tanks don’t have to be as large or as heavily insulated as those for other propellants, leading to a smaller, more compact rocket. It’s a well-understood, powerful, and relatively cost-effective propellant.

Its main drawback is that it’s “dirty.” Kerosene combustion produces soot, a process known as “coking.” This soot builds up in the engine’s turbines and combustion chamber, much like carbon deposits in a car engine. This isn’t a problem for an expendable engine that only fires once, but for a reusable engine, this residue must be extensively cleaned after every flight, which adds time and cost to operations.

Hydrolox (LOX/LH2)

This is the high-performance option, mixing liquid oxygen with liquid hydrogen (LH2). It’s the propellant that powered the Space Shuttle’s main engines (the RS-25) and is used today by the Space Launch System (SLS), the Ariane 6, and Japan’s H3 (rocket).

Its advantage is unmatched efficiency. Hydrogen is the lightest element in the universe. When it’s burned with oxygen, its exhaust particles are extremely light, allowing them to be accelerated out of the nozzle at enormous speeds. This gives hydrolox engines the highest “specific impulse,” or fuel efficiency, of any chemical rocket. It produces more “push” for every kilogram of propellant burned. Its only byproduct is water vapor.

The drawbacks are immense. Liquid hydrogen is one of the most difficult substances to handle. It must be kept cryogenically frozen at -251°C, just a few degrees above absolute zero. At these temperatures, it’s not just a liquid; it’s a “super-fluid.” Its atoms are so small that they can leak through solid metal, a problem called “hydrogen embrittlement.” It also has a very low density, meaning it takes up a lot of space. This is why hydrolox rockets, like the SLS, require massive, lightweight, and heavily insulated fuel tanks (the iconic orange color of the Shuttle and SLS tanks is the unpainted foam insulation).

Methalox (LOX/CH4)

This is the “new contender,” the propellant of choice for the newest generation of rockets. It combines liquid oxygen with liquid methane (CH4), the primary component of natural gas. It is the fuel for SpaceX’s Raptor and Blue Origin’s BE-4 engines.

Methalox is seen as the “best of both worlds.” Like kerosene, methane is relatively dense, so the tanks can be compact. Like hydrogen, it’s very efficient, offering a significant performance boost over kerosene.

Its main advantage, and the reason it was chosen for reusable rockets, is that it burns very cleanly. Unlike kerosene, methane combustion doesn’t produce soot or “coke.” This means an engine like Raptor can be flown many times with minimal cleaning, which is essential for the “fly, refuel, and fly again” model of an “airline-like” operation. As a bonus for long-term planning, methane can theoretically be manufactured on Mars using local carbon dioxide and water ice, which is a key part of SpaceX’s plan to make Starship a fully self-sustaining interplanetary vehicle.

Hypergolics (NTO/UDMH)

This is the “instant-on” propellant, a class of fuels that have a unique property. A hypergolic propellant consists of a fuel, such as unsymmetrical dimethylhydrazine (UDMH), and an oxidizer, like nitrogen tetroxide (NTO), that ignite spontaneously the moment they touch each other.

This chemical property makes them incredibly reliable. An engine using hypergolics doesn’t need a complex ignition system; the engineer simply needs to open the valves, and combustion is guaranteed. This is why they are the propellant of choice for ISRO’s workhorse Vikas engine and are almost universally used for the small maneuvering thrusters on spacecraft and satellites that must fire perfectly every time, often years after launch.

The trade-off is severe. Hypergolic propellants are extremely toxic, carcinogenic, and highly corrosive. They are difficult to handle safely on the ground and pose a significant environmental risk. This is why many space agencies are moving away from them for large launchers; for example, Roscosmos is phasing out its long-running Proton rocket family specifically because of its toxic hypergolic fuel.

Thrust vs. Efficiency (Specific Impulse)

To understand why engines are designed so differently, it’s helpful to separate two key metrics: thrust and efficiency.

Thrust is simple. It’s the raw power or “push” of the engine, usually measured in Newtons (N) or kilonewtons (kN). Thrust is what a rocket needs to overcome its own weight (mass) and Earth’s gravity to get off the launch pad. An engine designed for liftoff, a “main engine,” is all about maximizing thrust.

Efficiency is measured by a metric called specific impulse (Isp), often measured in seconds. It’s the “miles per gallon” of a rocket engine. It measures how much thrust an engine can produce for a given amount of propellant burned over a specific time. A high Isp means the engine is very “fuel-efficient,” squeezing every last bit of push out of every kilogram of propellant.

These two concepts come with a critical distinction: sea-level versus vacuum.

At sea level, the engine’s exhaust jet has to push its way through the thick, heavy blanket of Earth’s atmosphere. The air pressure “pinches” the exhaust, which keeps it from expanding and slows it down. This “back pressure” reduces the engine’s performance, lowering both its thrust and its specific impulse.

In the vacuum of space, there is no air and no pressure. An engine’s exhaust can expand freely. To take advantage of this, “vacuum-optimized” engines are equipped with a much larger, wider nozzle. This “expansion nozzle” allows the gas to expand and accelerate to much higher speeds than it ever could at sea level. This results in significantly higher thrust and, more importantly, a massive boost in efficiency (Isp).

This is why a rocket is built in “stages.” The first stage is powered by sea-level engines, which are designed for high thrust to get through the atmosphere. Once it’s in the thin upper atmosphere or in space, that stage falls away, and a second stage, powered by a high-efficiency vacuum engine, ignites to perform the final, precise push to orbit. This report is structured to reflect this fundamental difference, separating the high-thrust main engines from the high-efficiency vacuum engines.

The “Engine Cycle”: How it All Connects

The biggest engineering problem for a large liquid rocket is a plumbing problem. The propellants are in low-pressure tanks, but they need to be injected into a high-pressure, violently burning combustion chamber. The solution is a turbopump, an incredibly powerful device that spins tens of thousands of times per minute to boost the propellant pressure.

The real question is: what powers the turbopump? The answer to this defines the “engine cycle,” the internal plumbing that separates one engine family from another.

Gas-Generator Cycle

This is the classic, reliable “open” workhorse design. In a gas-generator cycle, the engine taps off a small amount of its main propellant and routes it to a small, separate combustion chamber called a gas generator. This “preburner” creates a flow of hot gas, which is then ducted to spin a turbine. This turbine is connected by a shaft to the main pumps, driving them like a windmill.

After the gas passes through the turbine, it has done its job, but it’s still hot and sooty. It is simply “dumped” overboard, exiting the engine through a separate, small exhaust pipe. This is why it’s called an “open cycle” – that exhaust gas is wasted and doesn’t contribute to the main thrust.

This design is mechanically simpler, more robust, and less stressed than other cycles, making it highly reliable. It’s the cycle of choice for SpaceX’s Merlin, ISRO’s Vikas, ArianeGroup’s Vulcain, and China’s new commercial methalox engines.

Staged Combustion Cycle

This is the “closed” powerhouse, the design for maximum power and efficiency. A staged combustion cycle also has a preburner that creates hot gas to spin the turbine. The difference is what happens next.

Instead of being dumped, that hot, high-pressure gas is piped directly into the main combustion chamber to be burned again with the rest of the propellant. It’s a “closed” cycle because no propellant is wasted. Every last drop is used to generate thrust. This makes staged combustion engines far more efficient and powerful than their gas-generator counterparts.

The trade-off is extreme engineering difficulty. The turbine must survive a constant blast of high-pressure, chemically-reactive gas that is often hot enough to melt most metals. The plumbing must handle these “unruly” gases without failing. It took Soviet engineers in the 1970s, with engines like the RD-170, to perfect this design, which at the time was considered nearly impossible by their American counterparts. It is the cycle used by the most powerful engines in operation today, including the RD-180, RS-25, BE-4, and YF-100.

Full-Flow Staged Combustion

This is a unique and even more complex variation of the staged combustion cycle, currently only operational on SpaceX’s Raptor engine.

A standard staged combustion engine has one preburner, which is typically “fuel-rich” or “oxidizer-rich.” A full-flow staged combustion engine has two separate preburners. One preburner gets all the fuel and a tiny bit of oxidizer to power the fuel-side turbine. The other preburner gets all the oxidizer and a tiny bit of fuel to power the oxidizer-side turbine.

This “split” design means that all the propellant passes through the turbines, turning completely to gas before even entering the main combustion chamber. The most significant benefit is that the turbines run at a much cooler temperature and lower pressure. This drastically reduces the stress on the engine’s components, which is a major advantage when designing an engine to be reused hundreds or thousands of times with minimal maintenance.

Expander Cycle

This is an elegant design used for high-efficiency, vacuum-only engines. An expander cycle engine has no preburner at all. It gets the power to spin its turbine by using its own waste heat.

This cycle only works with cryogenic fuels like liquid hydrogen. The engine takes the super-cold liquid hydrogen from the tank and pumps it through tiny channels and tubes built into the walls of the scorching-hot combustion chamber and nozzle. The fuel acts as a coolant, absorbing the engine’s heat.

This heat causes the liquid hydrogen to flash-boil into a high-pressure gas, “expanding” dramatically in volume. This high-pressure hydrogen gas is then routed to the turbine, spinning it, and is finally sent to the combustion chamber to be burned. It’s a “closed” and highly efficient cycle.

Its power is limited by the square–cube law. As an engine gets bigger, its volume (and the amount of fuel it needs to heat) increases faster than its surface area (the nozzle wall “skin” available to heat it). This means there is a physical limit to how big an expander engine can get, which is why it’s almost exclusively used for upper-stage engines like the RL10 and Vinci.

Electric-Pump-Fed Cycle

This is the most modern and, in some ways, simplest cycle. It completely does away with preburners, turbines, and hot-gas plumbing.

An electric-pump-fed engine uses high-performance electric motors to drive its propellant pumps. These motors are powered by a large, high-discharge battery pack.

This design, pioneered by Rocket Lab with its Rutherford engine, has huge advantages for small rockets. It makes the engine mechanically simple, easier to control, and much easier to 3D-print. It removes almost all the complex, high-temperature components that cause traditional engines to fail.

The drawback is the weight of the batteries. For a small rocket like the Electron, the trade-off is worth it for the cost and reliability gains. However, the energy density of current batteries makes this cycle impractical for large, heavy-lift rockets… for now.

Heavy-Lift Main Engines (Sea-Level Optimized)

These are the engines of liftoff. They are designed for one primary purpose: to generate the maximum possible thrust at sea level to push a rocket off the launch pad and through the thickest part of Earth’s atmosphere. Their designs are a fascinating mix of raw power, engineering trade-offs, and national strategy.

The thrust values presented here are the nominal figures for sea-level operation. Actual thrust can vary slightly between engine variants or be “throttled” up or down to meet mission requirements.

The engine’s design is unique. Instead of a single large combustion chamber, it features four separate chambers and nozzles. These four “mini-engines” are fed by a single, colossal turbopump, which itself is powered by two preburners. It runs on kerolox propellants and uses the extremely high-pressure, oxidizer-rich staged combustion cycle that its predecessors perfected.

The RD-171MV’s development was driven by geopolitical necessity. For years, Roscosmos used the Zenit rocket, which was built in Ukraine but used the Russian-made RD-171M engine. After 2014, access to this rocket was lost, leaving Russia with a gap in its launch capabilities, particularly at the Baikonur Cosmodrome.

The RD-171MV, built with all-Russian components, was developed to power the first stage of the new Irtysh rocket, which is the official, domestic replacement for the Zenit. After a successful series of ground-based “hot fire” tests, the engine is fully qualified. It is scheduled for its inaugural demonstration flight aboard the first Soyuz-5 in December 2025.

RD-180 (NPO Energomash)

For over two decades, the RD-180 has been a workhorse of the American space program, despite being manufactured in Russia by NPO Energomash. The engine is a direct descendant of the RD-170; it’s effectively an RD-170 “cut in half,” featuring two combustion chambers and nozzles instead of four, all fed by a single turbopump.

It uses kerolox propellants and the same powerful oxidizer-rich staged combustion cycle. A single RD-180, producing 3,830 kN of thrust, has powered the first stage of the United Launch Alliance (ULA) Atlas V rocket since 2000. It has been incredibly reliable, helping the Atlas V build a near-perfect launch record.

The engine’s story is now at its end. It became a symbol of a post-Cold War cooperation that no longer exists. Geopolitical tensions led the U.S. Congress to mandate an end to the use of Russian-made engines for national security launches. This decision forced ULA to find an American-made replacement, which spurred the development of Blue Origin’s BE-4 engine.

ULA announced the retirement of the Atlas V, and its production line was shut down in 2024. As of late 2025, the Atlas V is still operational, flying out its final manifest of sold-out missions. The RD-180 is now a living relic, a testament to a period of international collaboration that has definitively closed.

Raptor (SpaceX)

The Raptor engine, developed and manufactured by SpaceX, represents a fundamental shift in rocket engine design. It is the first operational engine to use the advanced full-flow staged combustion cycle, and the first operational methalox engine, having been beaten to orbit by a different cycle but now flying on the world’s most powerful rocket.

Raptor was designed from the ground up to achieve a single goal: make life multi-planetary. This required an engine that was not only powerful and efficient but also robustly and rapidly reusable. Every design choice flows from this.

Its methalox propellant was chosen because it burns cleanly, preventing the “coking” that plagues kerosene engines and simplifying reuse. Its full-flow staged combustion cycle, while incredibly complex, runs its turbines at much cooler temperatures, reducing wear and tear and enabling a long service life.

Raptor engines are used in massive clusters on SpaceX’s Starship launch system. The Super Heavy booster uses 33 Raptor engines on its first stage, and the Starship upper stage uses a combination of three sea-level Raptors and three vacuum-optimized Raptors. This reliance on a single engine type simplifies manufacturing, and SpaceX has already produced over 600 Raptor engines, signaling a move from bespoke engine building to true mass production. With its latest versions pushing thrust to 2,750 kN, Raptor is the engine intended to take humanity to the Moon and Mars.

BE-4 (Blue Origin)

The BE-4, or Blue Engine 4, is the other major American methalox engine, developed by Blue Origin. It is a large, powerful engine that, like Raptor, uses liquid methane and liquid oxygen. It employs an oxidizer-rich staged combustion cycle, making it a high-performance, high-pressure machine.

The BE-4 was developed to serve two critical roles, placing it at the center of the American launch industry. First, it is the engine United Launch Alliance selected to replace the Russian RD-180. A pair of BE-4 engines, producing a combined 4,800 kN of thrust, powers the first stage of ULA’s new Vulcan Centaur rocket. This rocket’s first flight in January 2024 was a success, making the BE-4 the engine that officially broke America’s reliance on Russian propulsion.

Second, the BE-4 is the engine for Blue Origin’s own heavy-lift rocket, New Glenn. The New Glenn’s reusable first stage is powered by a cluster of seven BE-4 engines, giving it a combined liftoff thrust of over 16,800 kN. After its own first launch in 2025, the BE-4 is now operational on two of the newest and most important rockets in the U.S. fleet, cementing methalox as the new propellant standard for heavy lift.

RS-25 (Aerojet Rocketdyne)

The RS-25 is an icon of spaceflight, one of the most-tested, highest-performing, and most complex rocket engines ever built. It is famous for its history as the reusable main engine of NASA’s Space Shuttle program, where it flew 135 missions.

It is a hydrolox engine, burning liquid hydrogen and liquid oxygen through a complex, fuel-rich staged combustion cycle. Its performance is so high that its exhaust is clean enough to be breathable water vapor.

After the Space Shuttle program was retired, NASA preserved its inventory of 16 flight-proven RS-25 engines for a new purpose: powering the core stage of the Space Launch System (SLS), the rocket for the Artemis program to return humans to the Moon. Four RS-25s are used on the SLS core stage. For this new role, the engines were fitted with new controllers and upgraded to operate at 111% of their original rated thrust.

The RS-25’s new life comes with a great irony. These engines, which were the very pioneers of reusability, are now treated as expendable. At the end of each SLS launch, the four engines and the entire core stage are dropped into the ocean. New-production, expendable-only versions of the RS-25 are now being built by L3Harris Technologies (which acquired Aerojet Rocketdyne) to continue the Artemis missions.

RD-191 (NPO Energomash)

The RD-191 is the modern, single-chamber workhorse of the Russian space program. Like its larger cousins, the RD-180 and RD-171, it is a kerolox-fueled engine using the high-pressure, oxidizer-rich staged combustion cycle. It was developed by NPO Energomash from the same RD-170 lineage, essentially representing one-quarter of the original four-chamber design.

The RD-191 was designed specifically to power Russia’s new-generation, modular Angara rocket family. The Angara rocket is built using a “Universal Rocket Module” (URM-1), and each URM-1 is powered by a single RD-191. This allows Roscosmos to build different rockets by clustering these modules. The light-lift Angara 1.2 rocket uses a single URM-1. The heavy-lift Angara A5, which is operational and flying in 2025, uses five URM-1s – one in the center and four as strap-on boosters.

Like the RD-180, the RD-191 family was also built for export. A variant known as the RD-181 was sold to Northrop Grumman to power its Antares rocket, which launches Cygnus cargo missions to the International Space Station. That supply chain is now also severed, and Northrop Grumman is in the process of re-engining the Antares with American-made engines from Firefly Aerospace.

LE-9 (JAXA/MHI)

The LE-9 is Japan’s new-generation main engine, developed by the Japan Aerospace Exploration Agency (JAXA) and Mitsubishi Heavy Industries to power the H3 (rocket) rocket. It is a hydrolox engine, burning liquid hydrogen and liquid oxygen.

The LE-9 is a fascinating piece of engineering due to its “engine cycle.” It is the first high-thrust, main-stage engine in the world to use an expander bleed cycle. This cycle, as explained in the primer, is extremely reliable and mechanically simpler than staged combustion, but it is notoriously difficult to scale up to large sizes. JAXA took on this immense engineering challenge, believing that the payoff in reliability and lower cost would be worth it.

After a challenging development, the engine is now operational. The H3 (rocket) rocket flies with either two or three LE-9 engines on its core stage, depending on the mission’s thrust requirements. This engine represents a strategic, long-term bet by Japan on a different, more robust path to engine development than its international competitors.

YF-100 (CASC)

The YF-100 is the high-performance backbone of China’s modern space program. It is a kerolox-fueled engine developed by the China Aerospace Science and Technology Corporation (CASC) and is the first Chinese engine to master the complex, high-pressure staged combustion cycle.

The YF-100 is a versatile engine. On China’s heaviest rocket, the Long March 5, it is not used on the core stage. Instead, eight YF-100 engines are clustered in pairs on the rocket’s four large strap-on boosters, providing the vast majority of liftoff thrust. It also serves as the main engine for the Long March 6, Long March 7, and Long March 8 rockets.

This engine represents the “state-funded” track of China’s space ambitions. It is a complex, high-performance machine designed for national-priority missions, such as launching modules for the Tiangong space station. Its development was a major technological leap for the China National Space Administration (CNSA), giving it a powerful engine on par with Russia’s RD-191. A new, upgraded YF-100K variant is also now qualified for flight.

Vulcain 2.1 (ArianeGroup)

The Vulcain 2.1 is the core-stage engine for Europe’s Ariane 6 rocket, developed by ArianeGroup on behalf of the European Space Agency (ESA). It is an evolution of the Vulcain 2 engine, which powered the Ariane 5 for two decades.

It is a hydrolox engine, burning liquid hydrogen and liquid oxygen, and it uses the reliable, well-understood gas-generator cycle. A single Vulcain 2.1 fires from the core stage of the Ariane 6.

It’s important to understand the Vulcain’s specific role. On the Ariane 6 (as on the Ariane 5), the liquid-fueled core engine does not provide the majority of liftoff thrust. That job belongs to the large solid rocket boosters (SRBs) strapped to the side. The Vulcain 2.1 is actually ignited on the launch pad before liftoff, but its main job is to act as a “sustainer” engine, firing for the first eight minutes of flight to carry the payload to altitude after the powerful SRBs have burned out and been jettisoned. This design philosophy is a hallmark of the Ariane family.

Merlin 1D (SpaceX)

The Merlin 1D is arguably the most disruptive rocket engine of the 21st century. It is the engine that powers SpaceX’s Falcon 9 and Falcon Heavy rockets, making it the most-flown and most-reused liquid rocket engine in history.

Developed in-house by SpaceX, the Merlin is a kerolox-fueled engine that uses the simple and reliable gas-generator cycle. On paper, its specifications are not record-breaking. Its thrust is modest, and its cycle is less efficient than the staged-combustion engines of its competitors.

SpaceX’s innovation was not in building the single “best” engine, but in building a “good enough” engine that was cheap, reliable, and designed for mass production and reusability. Instead of building one giant, complex engine, SpaceX clusters nine of them on the Falcon 9’s first stage. This redundancy also provides safety; the rocket can lose an engine and still complete its mission. It was this engine, combined with a business model focused on propulsive landing and reuse, that allowed SpaceX to fundamentally change the economics of space launch and dominate the global market.

YF-102 (CASC)

The YF-102 is a newer Chinese engine that highlights a major strategic shift. It is developed by the state-owned CASC, but it is aimed at the growing commercial launch market.

The engine’s design is telling. It is a kerolox-fueled engine that uses the gas-generator cycle. Its sea-level thrust of 835 kN is almost identical to the 845 kN of the SpaceX Merlin 1D. This is not a coincidence. It is a “Merlin-class” engine, designed to be a simple, cost-effective workhorse.

This engine represents the “commercial-track” of China’s dual-track strategy. While the YF-100 (a complex, staged-combustion engine) was built to emulate the high-performance “Russian-style” for national missions, the YF-102 is built to emulate the highly-successful “SpaceX-style” for the commercial market. It is currently operational, powering the Tianlong-2 rocket for the private Chinese launch company Space Pioneer.

Vikas (ISRO)

The Vikas is the venerable workhorse liquid engine of the Indian Space Research Organisation (ISRO). It has been the backbone of India’s space program for decades, reliably powering some of its most important launch vehicles.

Vikas is a hypergolic propellant engine, burning unsymmetrical dimethylhydrazine and nitrogen tetroxide. It operates on a standard gas-generator cycle. Its hypergolic nature makes it extremely reliable, as it requires no ignition system.

The engine is notable for its versatility. On India’s Polar Satellite Launch Vehicle (PSLV), a single Vikas engine powers the rocket’s second stage. On India’s most powerful rocket, the LVM3 (Geosynchronous Satellite Launch Vehicle Mk III), a cluster of two Vikas engines powers the L110 liquid core stage, which ignites in mid-air after the solid boosters. While the propellants are toxic and a “legacy” technology, ISRO has mastered this engine, using it to achieve a long and celebrated history of successful launches.

TQ-12 (LandSpace)

The TQ-12 is a historic engine, not because of its power, but because of what it represents. It was developed by LandSpace, a private Chinese launch company, and in July 2023, it became the first methalox-fueled engine in the world to successfully deliver a payload to orbit.

The TQ-12’s design is a brilliant example of pragmatic engineering. It is a methalox engine, using the same “fuel of the future” as Raptor and BE-4. But unlike those engines, which use complex and difficult-to-develop staged combustion cycles, the TQ-12 uses a much simpler and faster-to-develop gas-generator cycle.

LandSpace effectively took the “Merlin” philosophy (a simple, reliable cycle) and applied it to the “Raptor” propellant (clean-burning methalox). This “simple methalox” approach allowed them to beat their much larger, more heavily-funded American competitors to the historic milestone of the first orbital methalox launch. A cluster of four TQ-12 engines powers the first stage of the Zhuque-2 rocket.

YF-77 (CASC)

The YF-77 is China’s largest operational hydrolox engine, burning liquid hydrogen and liquid oxygen. It is a gas-generator cycle engine, and like the Vulcain, it serves as a “sustainer” engine.

A pair of YF-77 engines powers the 5-meter-diameter core stage of China’s heaviest rocket, the Long March 5. This rocket has a complex architecture: at liftoff, it fires both its eight kerolox-fueled YF-100 engines (on the boosters) and its two hydrolox-fueled YF-77 engines (on the core).

This mixed-propellant design gives the Long March 5 a powerful “one-two punch.” The kerolox boosters provide the dense, high-thrust “punch” needed to get the massive rocket off the ground, while the high-efficiency hydrolox YF-77s provide the long, “sustaining” burn needed to push the payload toward orbit. This architecture is what gives the Long March 5 the power to launch heavy space station modules and interplanetary missions.

BE-3PM (Blue Origin)

The BE-3PM (Propulsion Module) is the engine that powers Blue Origin’s reusable suborbital rocket, New Shepard, which is actively flying space tourism missions in 2025.

It is a hydrolox engine that uses a “combustion tap-off” cycle, a simple and reliable design where hot gas is “tapped” directly from the main chamber to power the turbine. Its maximum thrust of 490 kN is impressive, but its most important feature is its minimum thrust.

The BE-3PM was designed for reusability and can “throttle” down to as low as 90-110 kN. This deep-throttling capability is the key to New Shepard’s vertical landing. After reaching space, the booster falls back to Earth, re-ignites its BE-3PM, and throttles down to slow its descent, eventually coming to a gentle, controlled touchdown. This engine was the testbed that allowed Blue Origin to perfect its landing and reuse technologies, paving the way for the much larger New Glenn rocket.

Rutherford (Rocket Lab)

The Rutherford engine, developed by Rocket Lab, is one of the most innovative engines in operation, despite being one of the smallest. It is the engine that powers the Electron rocket, which serves the dedicated small-satellite market.

It is a kerolox-fueled engine, but its cycle is unique: it is the first and only operational electric-pump-fed engine. Instead of a complex and hot turbopump, the Rutherford’s pumps are spun by high-performance electric motors powered by a large lithium-polymer battery pack.

This design, combined with the fact that its main components are 3D-printed, makes the engine exceptionally simple, quick, and cheap to manufacture. Its thrust is tiny, at only 24.9 kN. To get to orbit, the Electron rocket clusters nine of them on its first stage. The Rutherford engine didn’t just create a new piece of hardware; its low-cost, mass-production-focused design created an entire new market for small-satellite launches.

Specialized Upper-Stage Engines (Vacuum-Optimized)

Once a rocket has cleared the atmosphere, the mission of the first stage is over. The “upper stage” takes over, and it operates in a completely different environment. In the vacuum of space, thrust is less important than efficiency.

These engines are designed to be “fuel-sippers,” using high-efficiency propellants and cycles to provide the precise, final push to place a satellite in a high-energy orbit, or send a probe on a path to another planet. They are characterized by their massive, wide-mouthed nozzles, designed to expand their exhaust plume as much as possible in the airless void.

Raptor 2 Vacuum (USA / SpaceX)

A vacuum-optimized variant of the Raptor 2 engine, this engine powers the second stage of SpaceX’s Starship vehicle. It is a groundbreaking engine that uses a full-flow staged combustion cycle, a highly complex and efficient design where both the liquid oxygen (LOX) and liquid methane (CH₄) propellants are fully gasified to drive their respective turbines before being injected into the combustion chamber. This design maximizes performance and allows for a very high specific impulse (Isp) for a non-hydrolox engine.

The Raptor Vacuum engine features a much larger, bell-shaped nozzle extension compared to its sea-level counterpart to expand the exhaust gases efficiently in the vacuum of space, generating maximum thrust from the propellant.

RL10B-2 (USA / Aerojet Rocketdyne)

A long-running and highly reliable engine, the RL10B-2 is a key component of the American space program, currently powering the Interim Cryogenic Propulsion Stage (ICPS) on NASA’s Space Launch System (SLS). It uses a highly efficient expander cycle with liquid hydrogen (LH₂) and LOX propellants. Its most prominent feature is a massive, lightweight extendible carbon-carbon nozzle, which deploys in space to achieve an extremely high specific impulse, making it ideal for high-energy lunar and interplanetary missions.

The expander cycle of the RL10 is a simple, closed-cycle design where the liquid hydrogen fuel is first used to cool the combustion chamber walls. In doing so, the hydrogen gasifies and expands, and this hot gas is then used to drive the engine’s turbines before being injected into the combustion chamber to burn with the oxygen. This cycle eliminates the need for a separate gas generator, improving efficiency and reliability. The RL10B-2’s ability to restart multiple times in space is critical for missions like Artemis, allowing the Orion spacecraft to perform a precise trans-lunar injection burn.

RL10C-1 / RL10C-1-1 (USA / Aerojet Rocketdyne)

These are modern variants of the venerable RL10 engine family, powering the Centaur upper stages for United Launch Alliance’s (ULA) Atlas V and Vulcan Centaur rockets. Like other RL10s, they use the hydrolox expander cycle. The RL10C-1-1 is a notable update that incorporates additively manufactured (3D-printed) components, which simplifies production, reduces parts count, and lowers costs while maintaining the engine’s high reliability and performance.

Unlike the RL10B-2, these variants typically use fixed, shorter nozzles that are optimized for a balance of performance and cost, particularly for missions to Earth orbit or for launch vehicles where the engine is not the final stage. The move to 3D printing for major components like the injector and thrust chamber is a significant step in modernizing the manufacturing process for an engine design that first flew in the 1960s. This adaptation ensures the RL10 family remains a cost-effective and reliable workhorse for U.S. national security and commercial spaceflight.

BE-3U (USA / Blue Origin)

This is the vacuum-optimized upper stage variant of Blue Origin’s BE-3 engine. A pair of BE-3U engines powers the second stage of the New Glenn heavy-lift launch vehicle. It is a high-performance expander cycle engine that runs on LOX and LH₂. It is designed for multiple restarts, giving the New Glenn’s upper stage the flexibility to deploy satellite constellations into different orbits or perform complex orbital maneuvers.

The BE-3U is an evolution of the BE-3PM (propulsive-module) engine that powers Blue Origin’s suborbital New Shepard vehicle. By adapting this proven, restartable engine for vacuum use, Blue Origin leverages its flight heritage. The high efficiency (Isp) of its hydrolox propellant combination makes the New Glenn’s second stage highly capable for delivering heavy payloads to geostationary transfer orbit (GTO) or for high-energy interplanetary trajectories, positioning it as a direct competitor in the heavy-lift market.

Rutherford (Vacuum) (USA & New Zealand / Rocket Lab)

This engine powers the second stage of Rocket Lab’s Electron rocket. The Rutherford is the first operational rocket engine to use an electric-pump-fed cycle. Instead of using complex turbopumps driven by propellant gas, it employs lightweight, high-power electric motors and batteries to drive its pumps. This design is simpler to build and test. The engine is also largely 3D-printed, including its thrust chamber, injectors, and pumps, which allows for rapid and low-cost manufacturing.

The “Curie” engine, which is often used as Electron’s third stage or “kick stage,” is a separate but related propulsion system designed for precise orbital insertion and is not to be confused with the main second-stage Rutherford. The vacuum Rutherford itself features a larger nozzle optimized for space, and its innovative electric-pump-fed cycle (named the “Rutherford cycle”) represents a significant departure from traditional engine designs. This architecture is particularly well-suited for small launch vehicles, where the mass of the batteries is less of a penalty compared to the complexity of a traditional turbopump.

Vinci (ESA / ArianeGroup)

The Vinci engine powers the upper stage of the Ariane 6 launch vehicle. It is a highly efficient LOX/LH₂ engine that uses an expander bleed cycle. A key feature is its large, lightweight composite extendible nozzle, which gives it a very high vacuum Isp. The Vinci is also designed to be restartable, allowing Ariane 6 to conduct complex missions, such as delivering multiple satellites to different orbits or sending payloads to deep space.

This engine represents a significant technological leap for Europe. It is the first European expander-cycle engine, and its multiple-restart capability is a critical feature for the modern commercial launch market, where ride-sharing and constellation deployment are common. The composite nozzle is also a key innovation, reducing mass compared to heavier metallic nozzles, which directly translates to increased payload capacity for the launch vehicle. Vinci’s high efficiency is essential for placing heavy telecommunications satellites into their target geostationary orbits.

RD-0124 (Russia / Khimavtomatiki)

This is a highly efficient engine that powers the upper stages for Russia’s Soyuz-2.1b/v and Angara rockets. It uses an oxygen-rich staged combustion cycle with LOX and kerosene (RG-1) propellants. A distinctive feature is its design, which uses a single turbopump to feed four separate, gimbaled combustion chambers (nozzles). This arrangement allows the engine to provide full thrust vector control without needing separate vernier engines.

The use of a staged-combustion cycle in a kerosene engine provides a much higher specific impulse than the simpler gas-generator cycle used on the older RD-0110. This higher efficiency gives the Soyuz-2.1b variant a significant performance boost over the 2.1a, allowing it to carry heavier payloads. The four-chamber design is a hallmark of many Soviet and Russian engines, providing robust steering capability by individually gimbaling each nozzle.

RD-0110 (Russia / Khimavtomatiki)

A workhorse of the Russian space program, the RD-0110 powers the third stage (Block I) of the Soyuz-2.1a rocket. It is a simpler, very reliable gas-generator cycle engine running on LOX and kerosene. Its design features four main fixed combustion chambers and four smaller, gimbaled vernier nozzles that sip exhaust from the gas generator. These vernier thrusters provide attitude control and steering for the rocket’s upper stage.

This engine is a direct descendant of the engine used on the Vostok rocket that carried Yuri Gagarin into space. Its design philosophy prioritizes extreme reliability and simplicity over raw performance, a trade-off that has made the Soyuz launch vehicle family famously dependable. While less efficient than the RD-0124, the RD-0110 remains in service on the Soyuz-2.1a, the workhorse for launching Progress cargo missions and Soyuz crew capsules to the International Space Station.

YF-75D (China / CALT)

This is China’s modern, high-performance cryogenic upper-stage engine, used in a dual-engine configuration on the Long March 5 rocket. It operates on a closed expander cycle using LOX and LH₂. The YF-75D is an advancement over its YF-75 predecessor and features multiple restart capabilities, which is important for complex orbital insertions, geostationary satellite deployments, and interplanetary missions like the Tianwen-1 Mars mission and Chang’e lunar missions.

The development of the YF-75D was a critical step for China’s space ambitions, enabling its heavy-lift Long March 5 rocket. This rocket is the cornerstone for building the Tiangong space station, launching large geostationary satellites, and conducting ambitious deep-space exploration. The engine’s restart capability allows the upper stage to “coast” in a parking orbit before performing a second burn to send its payload to a higher-energy trajectory, a necessary maneuver for lunar or Mars missions.

YF-75 (China / CALT)

The predecessor to the YF-75D, this LOX/LH₂ engine is used on the third stage of China’s Long March 3B/3C rockets, which are the primary vehicles for launching to geostationary transfer orbit (GTO). It uses a simpler gas-generator cycle and typically flies in a paired-engine configuration to provide the necessary thrust for GTO-bound payloads.

While the YF-75D has surpassed it in performance, the YF-75 has been the backbone of China’s commercial satellite launch industry for decades. The Long March 3B, powered by this engine on its third stage, has been responsible for deploying the vast majority of China’s Beidou navigation satellites and numerous international telecommunications satellites. Its reliability, though marred by some early failures, has established China as a major player in the global launch market.

YF-115 (China / AALPT)

This engine powers the upper stages of the Long March 6 and Long March 7 families of rockets. It is a high-performance engine that uses a staged-combustion cycle with LOX and kerosene propellants. Like the Russian RD-0124, a single engine often consists of multiple gimbaled thrust chambers, providing both propulsion and attitude control.

The YF-115 is part of China’s new generation of engines developed to move away from the toxic hypergolic propellants used on older Long March rockets. The switch to “kerolox” (LOX/kerosene) makes the launch vehicles more environmentally friendly, safer to handle, and less expensive to operate. This engine, paired with the YF-100 sea-level engine, powers the boosters and cores of the rockets that are essential for China’s space station cargo resupply missions (Long March 7) and for launching satellite constellations (Long March 6).

CE-20 (India / ISRO)

The CE-20 is India’s first indigenously developed cryogenic engine, powering the upper stage of the LVM3 (Launch Vehicle Mark 3) rocket. It is a gas-generator cycle engine that runs on LOX and LH₂. The successful development of this engine was a major milestone for India, making it self-sufficient in cryogenic technology, which is essential for launching heavy satellites to GTO and for future crewed (Gaganyaan) missions. It is also designed to be throttleable.

Developing cryogenic technology (using super-cooled liquid hydrogen) is extremely difficult, and ISRO’s success with the CE-20 placed India in an elite group of space-faring nations. This engine allows the LVM3 rocket to launch heavy 4-ton-class satellites to geostationary orbit, making India independent in this critical market and saving valuable foreign exchange. The engine’s throttleability and reliability were key factors in its selection for India’s upcoming Gaganyaan crewed spaceflight missions.

LE-5B-3 (Japan / Mitsubishi Heavy Industries)

This engine is the main powerplant for the upper stage of Japan’s H3 launch vehicle. It is an evolution of the highly reliable LE-5 engine family and uses an expander bleed cycle with LOX and LH₂. This cycle is very efficient, and the engine is known for its reliability and multiple restart capability, providing the H3 rocket with high flexibility for a variety of mission profiles.

The LE-5B-3 is a cost-optimized version of its predecessors (LE-5, 5A, and 5B), which had a perfect flight record on the H-II and H-IIA rockets. The “expander bleed” cycle is a Japanese innovation where some of the hot, turbine-driving gas is “bled” off and used for roll control or is simply dumped overboard, simplifying the engine’s plumbing. For the H3, engineers focused on drastically reducing the engine’s cost and production time, using fewer parts and simplified manufacturing to ensure the H3 can compete in the modern commercial launch market.

Summary

The landscape of liquid rocket propulsion is undergoing its most significant change in half a century. The engines currently in operation are a mix of Cold War-era legends, proven modern workhorses, and a new generation of disruptive powerplants that are rewriting the rules of access to space.

Several clear trends have emerged. The first is a major geopolitical pivot. The era of post-Cold War cooperation, best symbolized by the American use of the Russian-built RD-180, is over. This shift has directly fueled a new, domestic American engine industry, with Blue Origin’s BE-4 stepping in to fill the role once held by Russian hardware.

Second is the “Methane Revolution.” Once a theoretical concept, methalox is now a fully operational propellant. The arrival of SpaceX’s Raptor and Blue Origin’s BE-4 has established methane as the definitive choice for next-generation, reusable heavy-lift rockets. This decision was driven by methane’s unique combination of high performance and clean-burning properties, which are essential for engines designed to fly repeatedly with minimal refurbishment.

Third is the proven success of the “good enough” philosophy. SpaceX’s Merlin 1D engine demonstrated that a simpler, cheaper-to-build, mass-produced gas-generator engine, when combined with a reusable business model, could be more commercially successful than a more complex, high-performance, expendable engine. This lesson has not been lost on the global industry.

This highlights the fourth trend: China’s dual-track strategy. The Chinese government is simultaneously pursuing two paths. Its state-run programs are building complex, “Russian-style” staged-combustion engines like the YF-100 for high-performance national missions. At the same time, its new and aggressive private sector – along with CASC’s own commercial arm – is building simpler, “SpaceX-style” gas-generator engines like the YF-102 and TQ-12, which are designed for low-cost, commercial operations.

Finally, the technology of the “upper stage” has become a critical battleground. The mission is no longer just about reaching orbit; it’s about delivering payloads to precise, complex, and varied orbits. Highly efficient, reignitable engines like Europe’s Vinci and America’s RL10, as well as human-rated engines like India’s CE-20, are now just as vital to mission success as the high-thrust engines that get them off the ground.

Comparative Table of Rocket Engines