A Global Guide to Liquid Propulsion Rocket Engines: Active and Planned

Understanding the Liquid Propellant Rocket Engine

At the heart of humanity’s access to space is the liquid-propellant rocket engine. It’s a machine that operates at the very edge of physics and material science, a controlled explosion continuously harnessed to produce immense power. Understanding these engines, their differences, and the choices that go into their design is to understand the forces shaping the twenty-first-century space race.

What Makes a Liquid Engine Different?

Launch systems can be broadly divided into two categories: solid-propellant motors and liquid-propellant engines. A solid-propellant motor, commonly seen in rocket boosters (like those on the Space Shuttle or NASA’s new Space Launch System) or missiles, is a marvel of simplicity and power. It consists of a case packed with a solid, rubbery propellant that includes both the fuel and the oxidizer. Once it’s lit, it’s like a firework: it burns with ferocious power until all the propellant is gone. It cannot be throttled, it cannot be steered (only its nozzle can), and it certainly can’t be shut down and restarted.

A liquid-propellant engine is an entirely different beast. It is a machine of high-speed plumbing, and its defining characteristic is control.

In a liquid engine, the fuel (like kerosene, hydrogen, or methane) and the oxidizer (almost always liquid oxygen) are stored in separate tanks. They are only mixed at the last possible moment, inside the engine’s combustion chamber, where they ignite and create a super-heated, high-pressure gas that expands and accelerates out of the engine’s nozzle. This high-speed exhaust provides the thrust.

Because the propellants are liquid, their flow can be controlled by valves. This is the fundamental difference. A liquid engine can be “throttled” up or down, just like a blowtorch. This allows a rocket to manage its acceleration, reducing stress on the vehicle as it gets lighter (from burning fuel) or “throttling down” to a gentle hover just before landing.

More importantly, a liquid engine can be shut off completely. And, if designed for it, it can be restarted later in flight. This ability to restart is what makes complex orbital mechanics possible. It’s how a spacecraft can first achieve a “parking orbit” and then, at the precise moment, fire its engine again to head for the Moon. It’s how SpaceX’s Falcon 9 boosters can shut down their main engines, flip around, and re-light a subset of them for a “boostback burn” to return to the launch site. This precise, on-demand control is why liquid engines are the technology of choice for almost every major orbital launcher and spacecraft.

The Propellant “Families”: A Simple Guide

The choice of propellant is the first and most defining decision in an engine’s design. It dictates the engine’s performance, its complexity, and its cost. There are four main “families” of liquid propellants.

How an Engine “Breathes”: Cycles and Performance

When discussing engines, two terms are unavoidable: Thrust and Specific Impulse (Isp).

• Thrust is simple: It’s the amount of “push” the engine generates, usually measured in pounds (lbf) or kilonewtons (kN).
• Specific Impulse (Isp) is more nuanced: It’s the engine’s efficiency – the rocket equivalent of “miles per gallon.” It measures how much “push” (thrust) the engine gets from a given amount of propellant over time. A high Isp means the engine is very efficient, which is what you want for an upper stage that needs to provide a small push for a long time to get to a distant planet.

To generate thrust, the engine must pump massive quantities of fuel and oxidizer into the combustion chamber. A single Falcon 9 first stage, for example, burns over 1,000 kilograms (2,200 pounds) of propellant every second. Moving this much liquid requires immensely powerful pumps. How those pumps are powered defines the engine’s “cycle.”

• Gas-Generator Cycle: This is the simplest and most common method. It’s like a car’s turbocharger. The engine “taps off” a small amount of fuel and oxidizer and burns them in a small, separate gas generator. This creates hot gas, which is used to spin a turbine. This turbine, in turn, is connected to the pumps that force the main flow of propellant into the combustion chamber. The (relatively) cool, sooty exhaust from this turbine is then “dumped” overboard, often through a separate, small exhaust pipe. It’s simple, reliable, and easier to build. The SpaceX Merlin, India’s CE-20, and Europe’s Vulcain are all gas-generator engines.
• Staged-Combustion Cycle: This is the high-performance method. It starts the same way, with propellants burned to spin a turbine. But here’s the difference: the exhaust from that turbine, instead of being dumped overboard, is re-routed into the main combustion chamber and burned again. This means all the propellant is used to create thrust, squeezing every last bit of energy out of it. This results in a much higher specific impulse (Isp) and overall performance. It’s also far, far more complex. The entire engine must operate at extreme pressures and temperatures, as the pumps are forcing propellant into a pre-burner that is already at high pressure. The RS-25 (Space Shuttle) and the Russian RD-180 are the most famous examples of this complex but powerful cycle.

The choice of cycle is a design trade-off. For a reusable, low-cost booster like the Falcon 9, a simple, reliable gas-generator (Merlin) is good enough. But when you need the absolute maximum performance, as NASA did for the Space Shuttle (RS-25), staged-combustion is the answer.

The Modern Revolution: Reusability and 3D Printing

Two trends are radically reshaping rocket propulsion today.

The first is 3D Printing, or additive manufacturing. This isn’t just a novelty; it fundamentally changes howengines are built. A rocket engine’s injector – the part that sprays and mixes the fuel and oxidizer – is one of its most complex components. A traditional injector might be made of hundreds of tiny, individual parts that must be painstakingly welded and brazed together by hand, a process that can take months. A 3D printer can create that same complex injector, with all its internal channels and pathways, as just one or two solid pieces of metal in a matter of days. This radically cuts manufacturing time, part count, and cost. It’s what allows companies like Rocket Lab and Relativity to build and test engines at a speed that was unthinkable a decade ago.

The second, and more important, trend is Reusability. For decades, rockets were an expendable good. The most expensive part of the launch vehicle – the engines – was thrown away on every flight. Reusability, as pioneered by SpaceX, changes the entire economic model of space access.

But reusability places extreme demands on the engines. They can’t just be designed to survive one flight. They must be robust enough to survive the violence of launch, the vacuum of space, and the fiery re-entry, and then do it all over again, quickly, with minimal refurbishment.

This is the unspoken driver for the global pivot to methane. Kerosene (kerolox) is a “dirty” fuel. It doesn’t burn perfectly and leaves behind a soot-like residue called “coking” inside the engine. SpaceX’s Merlin engines are kerolox and are reused, but they require a significant cleaning and refurbishment process between flights to deal with this. Liquid hydrogen is too finicky and its low density makes it a poor choice for a reusable booster.

Methane, by contrast, burns much cleaner. It’s a “clean” hydrocarbon, which means it leaves almost no soot behind. This opens the door for a truly “aircraft-like” operational model, where an engine can be flown, refueled, and flown again, with minimal post-flight work. The global pivot to methalox is not just because it’s a “good” fuel; it’s because it is seen as the only propellant that enables rapid and low-cost reusability. This is the central theme of the new space race, and its impact is visible in the development programs of every major spacefaring nation.

American Powerhouses: The United States

The American propulsion landscape is a study in contrasts, defined by a schism between legacy, government-funded hardware and a new, hyper-competitive private industry led by billionaires. This private rivalry has become the dominant engine-development driver in the world.

SpaceX: The Methane and Reusability Leader

SpaceX has fundamentally altered the global launch industry, and it did so by building its engines in-house.

Merlin (Active)

The Merlin engine is the engine that changed the industry. It’s a kerolox, gas-generator engine that powers the Falcon 9 and Falcon Heavy rockets. What makes the Merlin 1D so effective isn’t that it’s the most high-performance engine; it’s that it’s good enough, simple, and built for mass production and reuse.

SpaceX clusters nine Merlin 1D engines on the first stage of a Falcon 9. This “cluster” approach has several advantages. First, it provides “engine-out” capability: if one engine fails during ascent, the rocket can still complete its mission. Second, it’s a manufacturing dream. Instead of building two different-sized engines for its booster, SpaceX just builds nine identical ones. The Merlin has an extremely high thrust-to-weight ratio and was one of the first engines to make extensive use of 3D printing for key components. The Falcon 9 booster’s nine sea-level Merlin engines are designed for reuse and have now flown more than 20 times on a single booster. The upper stage is powered by a single, expendable variant called the Merlin Vacuum (MVac), which has a much larger nozzle optimized for the vacuum of space.

Raptor (Active/Under Development)

If Merlin changed the present, Raptor is aimed at the future. This is the engine for Starship, SpaceX’s next-generation, super-heavy-lift, fully reusable launch system. Raptor is a methalox engine, but its true innovation is its “Full-Flow Staged Combustion” (FFSC) cycle.

This is the “holy grail” of chemical propulsion. In a normal staged-combustion engine, only one of the propellants (usually the fuel) is used to spin the turbine. In FFSC, both the liquid methane and the liquid oxygen are “pre-burned” to spin separate turbines. This “full-flow” of both propellants through the turbines means the engine can operate at incredible pressures, generating massive thrust, while actually running coolerthan other staged-combustion cycles. Running cooler means less wear and tear on the engine, which is a key requirement for rapid reusability.

This cycle is legendarily difficult to build. Russia tried and failed to make a flight-ready FFSC engine in the past. SpaceX, by contrast, is not just flying them; it’s mass-producing them in a factory, constantly iterating the design. The Raptor 1, Raptor 2, and the planned Raptor 3 show a clear progression of simplifying the design, removing parts, and increasing both thrust and reliability.

Draco & SuperDraco (Active)

These are SpaceX’s small but mighty hypergolic thrusters. The tiny Draco thrusters are used for on-orbit maneuvering and attitude control for the Dragon capsule. The SuperDraco is a different class entirely. It’s a powerful thruster, 3D-printed from a superalloy, and eight of them are integrated into the side of the Dragon capsule. They are the capsule’s launch-abort system, capable of generating enough thrust to blast the crew to safety in the event of a rocket failure.

Kestrel (Retired)

A brief mention of Kestrel is a reminder of SpaceX’s rapid evolution. This was the simple, pressure-fed engine for the Falcon 1 rocket’s upper stage. Unlike modern engines, it used no turbopumps, instead relying on high-pressure helium tanks to “push” the propellants into the chamber. Its retirement for the far more powerful Merlin shows how fast SpaceX’s technology advanced.

Blue Origin: The Long-Term Vision

If SpaceX’s philosophy is “move fast and break things,” Blue Origin’s is “slow is smooth, smooth is fast.” Its engine development is deliberate, methodical, and just as ambitious.

BE-4 (Active)

This is the other great American methalox engine and the direct competitor to Raptor, though it uses a different cycle. The BE-4 is a high-performance “oxidizer-rich staged combustion” (ORSC) engine. This is a more “traditional” (though still extremely difficult) staged-combustion cycle, which Russia mastered with its kerolox engines.

The BE-4’s story is central to the entire US launch industry. It is slated to power two major rockets: Blue Origin’s own New Glenn super-heavy-lift rocket (which will use seven) and, critically, the Vulcan rocket from United Launch Alliance (ULA), which will use two. This makes Blue Origin a “merchant supplier” of engines, selling them to its direct competitor. This decision de-risked ULA’s new rocket but has also shackled its development. The BE-4’s development has been protracted, and its delivery delays have had a ripple effect, grounding the debut of both Vulcan and New Glenn for years.

BE-3 (Active)

This is the engine that powers Blue Origin’s New Shepard suborbital tourist rocket. It’s a hydrolox engine and is notable for its “tap-off” cycle. This is a simpler, highly reliable design that allows the engine to throttle down to a tiny fraction of its full power. This “deep-throttling” capability is the key technology that allows the New Shepard booster to slow down, hover, and land with pinpoint, gentle precision.

BE-7 (Under Development)

The BE-7 is a high-efficiency hydrolox upper-stage engine. It’s smaller than the BE-3 but is being developed specifically for lunar landers, including Blue Origin’s “Blue Moon” lander. This engine connects the company directly to NASA’s Artemis moon program.

Rocket Lab: Pioneers of the Small

Rocket Lab, a US company with New Zealand origins, proved that innovation isn’t just for heavy-lift rockets.

Rutherford (Active)

The Rutherford engine is one of the most innovative in the world. This small kerolox engine powers the Electron rocket, one of the most successful small satellite launchers. Its innovation is its “electric-pump-fed” cycle.

Instead of a complex and hot gas-generator turbine, the Rutherford’s pumps are powered by high-performance electric motors. Those motors, in turn, get their power from a set of high-discharge lithium-ion batteries. The Rutherford is, literally, a battery-powered rocket engine. This is a radical simplification of the engine’s plumbing, making it ideal for mass production. Most of its key components, including the entire engine chamber, are 3D-printed.

Archimedes (Under Development)

Rocket Lab is now scaling up, and this engine shows the company “graduating” to the next level. Archimedes is their new engine for the Neutron medium-lift rocket. It’s a reusable, methalox, gas-generator engine. This move is significant: it shows a successful small-launch company identifying the same trend as everyone else – that the future of medium-lift is reusable methalox.

United Launch Alliance (ULA) and its Legacy Partners

ULA is the joint venture of Boeing and Lockheed Martin, the “old guard” of US national security launch. Its rockets have relied on a global supply chain of legendary engines.

RL10 (Active)

The RL10 is a true legend of rocketry. This high-performance hydrolox upper-stage engine has been flying in various forms for six decades. It’s the “engine of choice” for high-energy missions that need to send priceless satellites to deep space or into precise orbits. It’s renowned for its incredible efficiency (Isp) and reliability, and features a famously complex “extensible nozzle” that unfurls in space to maximize performance. Variants of the RL10 are used on the Atlas V, the Delta IV, and will be the upper-stage engine for ULA’s new Vulcan and NASA’s SLS upper stage.

RD-180 (Active)

This is the engine that has been both a blessing and a curse for ULA. The RD-180 is a Russian-built, kerolox, staged-combustion engine that powers the first stage of the Atlas V. It is an absolute masterpiece of engineering – powerful, efficient, and reliable. However, the reliance of a primary US national-security launch vehicle on a Russian engine became a geopolitical nightmare. This strategic vulnerability was the primary driver behind the US government’s funding for a replacement, which ultimately led to the selection of Blue Origin’s BE-4 to power the new Vulcan rocket.

NASA: Charting the Course for Deep Space

NASA today is more of a customer than a developer of new engines, but it still owns and operates the most powerful hydrolox engine ever flown.

RS-25 (Active)

The RS-25, formerly known as the Space Shuttle Main Engine (SSME), is arguably the highest-performance liquid engine ever flown. It’s a hydrolox, staged-combustion engine that operated at the limits of material science in the 1970s. These engines are technological marvels.

For the new Space Launch System (SLS), NASA’s super-heavy-lift moon rocket, four RS-25s are used to power the core stage. For the first few SLS flights, NASA is using refurbished engines that previously flew on Space Shuttle missions. These are priceless, museum-quality artifacts being used for flight. Unlike on the Shuttle, where they were reused, the SLS core stage is expended on every flight, meaning four of these multi-million-dollar engines are dropped into the ocean every time. This is a source of major controversy. New, “expendable” versions are being built by Aerojet Rocketdyne for future flights, but at a very high cost.

Other US Innovators

The US propulsion landscape is defined by the tension between the “big two” private companies (SpaceX and Blue Origin) and a vibrant ecosystem of smaller startups. This small-launch sector has become a “propulsion proving ground.”

• Aerojet Rocketdyne (L3Harris): Beyond building the RS-25 and RL10, this legacy giant is the main supplier of small hypergolic thrusters for NASA’s deep-space missions and the Orion crew capsule.
• Northrop Grumman (NG): This company’s Antares rocket had the same problem as ULA’s Atlas V. It used the Russian-built RD-181 engine. With that supply chain cut off, NG has partnered with Firefly Aerospace to develop a new, kerolox, gas-generator engine called Miranda. This shows the scramble to “de-Russia-fy” the US supply chain.
• Firefly Aerospace: An emerging small-launch company, their Alpha rocket is powered by the Reaver engine, which uses a “tap-off” cycle similar to the BE-3. Their partnership with NG on the Miranda engine shows a new model of cooperation.
• Relativity Space: This company’s entire bet is on 3D printing. Their Aeon 1 engine, which flew on the Terran 1 rocket, was 3D-printed. Their Aeon R is the planned reusable, high-thrust methalox engine for their larger Terran R rocket. Their goal is to 3D-print the entire rocket, shifting the paradigm from assembly to manufacturing.
• Stoke Space: This ambitious startup is planning a fully reusable rocket with a unique second stage. They are developing a full-flow staged combustion hydrolox engine – an extremely difficult combination – where the engine’s plumbing and 30 nozzles are integrated into the spacecraft’s heat shield.

This vibrant, chaotic, and well-funded ecosystem shows two clear trends. First, the “Great Schism” between SpaceX’s rapid, vertically-integrated model (Raptor) and Blue Origin’s slow, methodical, merchant-supplier model (BE-4). This private rivalry, not a state-run program, is what’s pushing the boundaries of US engine development.

Second, the small-launch market is not the end-goal; it’s a filter. Companies like Rocket Lab and Relativity used their small, innovative engines (Rutherford, Aeon 1) as a “Series A” project to prove their technology. They are all now “graduating” to building larger, reusable, methalox engines (Archimedes, Aeon R). The small-launch market is essentially a high-stakes competition to find the next SpaceX, and the price of entry to the next round is a reusable methalox engine.

The Red Dragon: China’s Rapid Ascent

For decades, China’s space program advanced steadily but quietly. In the last ten years, its development in liquid propulsion has become arguably the most rapid and well-funded in the world. China is executing a “two-track” strategy, combining state-funded, iterative development with a new, state-sanctioned private sector that is moving at lightning speed.

The “Long March” Family: CASC’s State-Owned Dominance

The China Aerospace Science and Technology Corporation (CASC) is the state-owned behemoth that builds the “Long March” family of rockets.

• YF-20 Series (Active): This is the hypergolic backbone of China’s legacy fleet. This family of engines, which use a toxic but storable combination of unsymmetrical dimethylhydrazine (UDMH) and nitrogen tetroxide (NTO), powered the Long March 2, 3, and 4 rockets for decades. These are the engines that launched China’s first astronauts. China is now actively phasing them out for both environmental and national prestige reasons.
• YF-77 (Active): This is China’s first-generation high-thrust hydrolox engine. It powers the core stage of the Long March 5, China’s heaviest operational rocket (the one used for its space station and Mars missions). The YF-77’s development was a major national challenge for China, and it’s notably less powerful than its Western (RS-25) or Japanese (LE-9) counterparts.
• YF-100 Series (Active): This is the new state-owned workhorse and a massive technological leap. The YF-100 is a powerful, kerolox, staged-combustion engine. It is China’s equivalent to the Russian RD-180 and a design they mastered in-house. It’s a high-performance engine that powers the boosters for the Long March 5 and the core stages of the new-generation Long March 6, 7, and 8 rockets. This engine is the foundation of China’s modern space program.
• YF-130 (Under Development): This is the planned monster engine for the future. It’s a 400+ ton-thrust kerolox engine, even more powerful than the YF-100. It is intended to power the Long March 9, China’s answer to the Saturn V and SLS, for its future crewed moon missions.
• YF-90 (Under Development): This is the planned high-performance hydrolox upper-stage engine, also for the Long March 9, to handle the lunar-bound payloads.

China’s New Space: The Private Challengers

In the last decade, a vibrant private launch sector has emerged in China, clearly modeled on (and sanctioned by) the state. These companies are not iterating on old technology; they are jumping directly to the new global standard.

• LandSpace: This company is the “SpaceX” of China. They developed the TQ-12, a methalox, gas-generator engine. In July 2023, their Zhuque-2 rocket became the first methalox-fueled rocket in the world to reach orbit. This was a major technical and symbolic victory, beating SpaceX’s Starship, ULA’s Vulcan, and Blue Origin’s New Glenn to that milestone.
• i-Space (Interstellar Glory): Another leading private company, i-Space is also developing a family of methalox engines, including the JD-1, for its own planned reusable rockets.
• CAS Space: A “spin-off” from the state-owned Chinese Academy of Sciences (CAS), this company blends state and private models. It is also developing methalox engines, showing a clear national consensus on the new propellant.

China’s propulsion strategy is a brilliant “pincer movement.” The state-owned CASC follows a very traditional, iterative, “Roscosmos” model: methodically replace old hypergolics (YF-20) with modern, high-performance kerolox (YF-100) and hydrolox (YF-77/90) engines. This track is guaranteed to deliver on national prestige missions, like the space station and the Moon.

Simultaneously, the state has sanctioned a domestic “New Space” sector to pursue the high-risk, high-reward “SpaceX” model: reusable methalox. This allows companies like LandSpace to move fast, iterate quickly, and take risks that the state-owned giant cannot. This “two-track” system gives China the best of both worlds: a slow, steady, reliable program and a fast, innovative, disruptive one, both marching toward the same goals.

The Russian Legacy: Energomash and Roscosmos

No country has a deeper or more successful history with a single type of liquid engine than Russia. The engineers at NPO Energomash perfected the kerolox staged-combustion engine, creating designs so advanced they are still a global benchmark. But this legacy of perfection has become a strategic trap.

The Energomash Engines: A Powerful Heritage

• RD-180 (Active): This is the masterwork of the Russian engine-building school. It’s a powerful, high-performance kerolox, staged-combustion engine. It’s a “dual-chamber” design, meaning it has two combustion chambers and two nozzles fed by a single set of turbopumps. It’s a direct descendant of the RD-171. The RD-180 was so advanced and reliable that the United States bought it for decades to power the first stage of the Atlas V. The 2022 invasion of Ukraine and the subsequent sanctions have permanently ended this export, cutting off a major source of revenue for the Russian space program.
• RD-181 (Active): This is the “export-grade” single-chamber version of the RD-191. It was sold to Northrop Grumman to power the first stage of the Antares rocket, which launched cargo to the ISS. This supply chain is also now cut off, forcing Northrop Grumman to find a new engine.
• RD-171 / RD-191 (Active): The RD-171 was built for the Soviet Energia rocket (the booster for their “Buran” space shuttle) and was, for a time, the most powerful liquid-propellant engine ever flown. Its modern, single-chamber derivative is the RD-191. This is the engine Russia has chosen for its own future. It powers the new Angara rocket family, which is Russia’s intended replacement for its aging (and Ukrainian-built) Zenit and its toxic-propellant Proton rockets.

Other Roscosmos Engines

• RD-107 / RD-108 (Active): These are the original rocket engines. The direct descendants of the R-7 engine that launched Sputnik 1 in 1957, these kerolox, gas-generator engines are still in use today. They power the Soyuz rocket, the most-flown and one of the most reliable launch systems in history. Their distinctive “cross” pattern of four boosters surrounding a core stage is an icon of spaceflight.
• RD-0124 (Active): This is a more modern, efficient kerolox staged-combustion upper-stage engine used on the Soyuz-2 rocket, giving it a significant performance boost over older variants.
• RD-1025 (Planned): This is Russia’s future, or at least its planned one. The RD-1025 is Russia’s planned methalox engine. It’s intended to power the Amur, a planned reusable rocket that is a direct, if very belated, response to SpaceX’s Falcon 9.

Russia’s story is that of a “king in exile.” Energomash perfected the kerolox staged-combustion engine. Their designs are masterpieces. This success funded their industry for decades through exports to the US. This reliance on exporting 1990s-era perfection stifled innovation. They had no economic incentive to develop the next thing (reusable methalox).

Now, sanctions have cut off their main export revenue. The entire world, including their rival China, has moved on to the “methalox reusability” trend. Russia is left holding the “best buggy whip in the world” while everyone else is building cars. Their scramble to develop the Amur and RD-1025 is a direct, reactive response to SpaceX, but they are a decade behind and now financially and technologically isolated.

European Cooperation: ArianeGroup and ESA

Europe’s space program, a collaboration of member states under the European Space Agency (ESA), is a testament to political and industrial cooperation. Its engines, primarily built by ArianeGroup (a joint venture of Airbus and Safran), are reliable and high-performance. But this complex, multi-national structure also makes it slow-moving and risk-averse in an era of rapid disruption.

The Ariane Family Engines

• Vulcain 2 / 2.1 (Active/Under Development): This is the hydrolox, gas-generator core-stage engine for Europe’s heavy-lift launchers. The Vulcain 2 was the reliable workhorse for the Ariane 5, one of the most dependable rockets in history. The Vulcain 2.1 is the upgraded, lower-cost version for the new Ariane 6. It’s a solid, proven design.
• HM7B (Active): This is the veteran hydrolox upper-stage engine that powered the Ariane 5’s second stage. It’s reliable but represents older technology: it cannot be re-ignited in space. This limits a mission to a single “burn,” making it less flexible.
• Vinci (Active/Under Development): This is Europe’s new upper-stage engine for Ariane 6, and it’s a major upgrade. It’s a highly efficient hydrolox, expander-cycle engine (which uses the fuel’s expansion to drive the pumps). Most importantly, it is re-ignitable. This re-ignition capability gives Ariane 6 the flexibility to deploy multiple satellites into different orbits on the same mission, a key capability for the modern commercial market.

The Vega Engines

• M10 (Under Development): This is a small but significant engine. Developed by Avio in Italy, the M10 is Europe’s first methalox engine. It is a re-ignitable engine planned for the upper stage of the new Vega-Esmall launcher. This is a important, if small, first step for Europe to gain experience with the propellant that is defining the next generation of launchers.

Emerging European Startups

Like in the US, a new generation of startups is trying to fill the gaps left by the “legacy” provider.

• Orbex: A UK/Danish company developing a small satellite launcher from Scotland. Their engine is 3D-printed and uniquely runs on “bio-propane,” a “green” propellant choice.
• PLD Space: A Spanish company that developed the TEPREL, a kerolox, gas-generator engine for their Miura 1 suborbital and Miura 5 orbital rockets.

Europe’s propulsion strategy is one of “capability maintenance” and “risk-averse incrementalism.” The complex, multi-national political and industrial structure of its space program is excellent at producing reliable, high-performance, but expendable hydrolox engines. The new Ariane 6, their response to SpaceX, is essentially a more flexible and cheaper expendable rocket in a world that has pivoted to reusability.

Vinci is a high-tech marvel, but its development took decades. The M10 is their first methalox engine, and it’s a small upper-stage engine, not a booster engine. Europe is institutionally stuck in the expendable, hydrolox paradigm. The European startups are trying to fill the “reusable” gap, but they are years behind their American and Chinese counterparts.

India’s Ambition: The ISRO Engine Program

The story of the Indian Space Research Organisation (ISRO) and its engine program is one of fierce, methodical self-sufficiency, born from necessity.

• Vikas (Active): This is the workhorse of the Indian space program. The Vikas is a high-thrust, reliable hypergolic engine. It was originally based on the licensed French Viking engine from the 1970s, but India fully domesticated its production. This reliable engine is the foundation of India’s space program, powering the PSLV (Polar Satellite Launch Vehicle) and GSLV (Geosynchronous Satellite Launch Vehicle) rockets.

The Cryogenic Program (CE-series)

Mastering cryogenic (hydrolox) engines became a source of immense national pride for India, precisely because it was a technology they were denied. In the 1990s, a US-led technology-denial regime blocked Russia from transferring cryogenic engine technology to India. This act, intended to slow India’s program, instead forced it to become self-sufficient.

• CE-7.5 (Active): This was India’s first domestically-produced hydrolox (cryogenic) upper-stage engine. Mastering this incredibly complex technology was a major national achievement, breaking India’s reliance on Russian-supplied upper stages and making its GSLV rocket fully “home-grown.”
• CE-20 (Active): This is the big one. The CE-20 is India’s powerful, gas-generator hydrolox engine. It’s a new, modern design that powers the upper stage of the LVM3 (Launch Vehicle Mark 3), India’s most powerful rocket. It was the CE-20 that successfully propelled the Chandrayaan-3 mission to its historic landing on the Moon’s south pole.

Under Development & Planned

• SCE-200 (Under Development): This is India’s next big leap. The SCE-200 is a 200-ton thrust kerolox, staged-combustion engine. This is ISRO’s “YF-100” or “Merlin-class” engine. It’s designed to replace the aging hypergolic Vikas engine and power the core stage of India’s next-generation (and reusable) launch vehicles.
• Methalox (Planned): Like all other major players, ISRO is in the early stages of developing methalox engines, explicitly stating they are for its future reusable launch vehicle (RLV) plans.

India’s engine development provides a perfect case study in “strategic patience.” Theirs is a textbook example of a nation methodically climbing the technology ladder. They started with licensed hypergolics (Vikas). When they were denied the next step (cryogenics), they built it themselves, mastering the small hydrolox upper stage (CE-7.5) and then scaling it up to a large hydrolox main engine (CE-20). Now, having mastered hydrolox, they are circling back to master the powerful kerolox staged-combustion (SCE-200) that they will need for a reusable booster. They are slow compared to China’s “pincer” model, but their foundation is arguably more solid and 100% home-grown.

Japan’s Precision: JAXA’s Hydrolox Mastery

Japan’s space program (JAXA) has long reflected its national industrial philosophy: prioritize extreme reliability and advanced, efficient manufacturing over all else. This has led them to become masters of the most difficult propellant: liquid hydrogen.

• LE-9 (Active/Under Development): This is the new main engine for Japan’s new H3 rocket. It is a hydrolox engine that uses a unique “expander-bleed” staged-combustion cycle. This is a “gentler” form of staged combustion. It’s less complex and runs at lower pressures and temperatures than the RS-25, and it is designed from the ground up for low-cost manufacturing and reliability, even using 3D printing for some components. The H3’s first launch failure (which was not attributed to the LE-9) was a major blow, but the engine itself is a significant technical achievement.
• LE-5B-2 (Active): This is the veteran upper-stage engine, a hydrolox, expander-cycle engine. Used on the H-IIA and H-IIB rockets, it’s famed for its reliability.
• Methalox (Planned): Like every other nation, JAXA and its industrial partners are in the early stages of planning a reusable, methalox engine to stay competitive in the next decade.

Japan’s strategy is that of a “craftsman.” They have focused exclusively on hydrolox for their main launchers. The new LE-9 is a marvel not of raw power, but of elegance. Its “expander bleed” cycle is telling. A “full-flow” (Raptor) or “pre-burner” (RS-25) cycle is all about wringing out maximum performance. An “expander bleed” cycle is about achieving staged-combustion-level efficiency but with lower pressures, simpler manufacturing, and higher reliability. Japan isn’t trying to win the thrust-per-dollar race. They are trying to build the most reliable and elegant high-performance engine. This is both its greatest strength and its weakness in the new, cost-driven reusable world.

Other Global Players

While the “big six” (US, China, Russia, Europe, India, Japan) dominate, several other nations are now developing their own orbital-class liquid engines.

• South Korea (KARI): South Korea is a new, serious player. In 2022, their Nuri rocket successfully reached orbit, making them a spacefaring nation. The Nuri is powered by their first domestic liquid engine, the KRE-075. This is a 75-ton-thrust, kerolox, gas-generator engine. Their success in developing a modern, high-thrust engine from scratch is incredibly impressive.
• North Korea (NADA): North Korea’s program is less clear but has shown rapid progress. It is based on high-thrust, hypergolic engines, likely derived from modified Russian/Soviet designs like the RD-250, which they may have acquired through illicit channels.
• Iran (ISA): Iran’s satellite launchers (like the Simorgh) also use clusters of liquid-propellant engines, which are also believed to be derived from foreign (likely North Korean) hypergolic missile technology.

The “barrier to entry” for a new spacefaring nation has clearly shifted. For decades, the “starter” engine was a small, pressure-fed hypergolic or a licensed design (like India’s Vikas). Now, South Korea’s Nuri proves that the “price of admission” to the orbital club is a kerolox engine, not a hypergolic one. This is a much higher technology bar. It creates a clear divergence between nations pursuing space for commercial and scientific purposes (South Korea) and those pursuing it as a dual-use military program (North Korea, Iran), who are stuck on the older, toxic hypergolic path.

Summary

A survey of the world’s liquid propulsion programs reveals a clear and dramatic global shift. The old rules are gone, and a new paradigm is emerging, defined by three major trends.

The Global Pivot to Methane

The single clearest trend is the global “methane rush.” Nearly every single spacefaring entity – the US (SpaceX, Blue Origin, Rocket Lab, Relativity), China (state and private), Russia (Roscosmos), Europe (Avio), India (ISRO), and Japan (JAXA) – is actively developing or planning methalox engines. This isn’t a coincidence. It is the global consensus that methalox, with its “clean-burning” and easy-handling properties, is the propellant of choice for reusable rockets.

The New Arms Race: Cycles and Efficiency

The competition is no longer just about thrust. It’s about efficiency and the underlying engine cycle. Russia and China mastered the powerful kerolox staged-combustion engine (RD-180, YF-100) for their new expendable workhorses. NASA and Japan perfected the high-performance hydrolox engine (RS-25, LE-9), which are technical marvels but expensive. SpaceX has created a third path: mastering the “good enough” gas-generator kerolox engine (Merlin) for its reusable first stage, while simultaneously mastering the “holy grail” methalox staged-combustion (Raptor) for its next-generation rocket.

The State vs. The Startups

The “SpaceX effect” has redefined how propulsion is developed. In the US, private companies (SpaceX, Blue Origin) are now the primary innovators, with NASA acting as a customer. In China, the state is using private startups (LandSpace) as a high-speed “skunkworks” to develop methalox technology, running a parallel track to its own state-run program. In Europe, the state-backed incumbent (ArianeGroup) is struggling to adapt, while startups try to fill the reusability gap. And in Russia, the state-run model (Energomash) is falling behind, isolated and lacking the innovative pressure from a private sector.

Manufacturing is the New Frontier

The final, most important shift is that the battle is moving from the design to the factory. It’s no longer just about what you build, but how fast and how cheaply you can build it. Rocket Lab’s battery-powered pumps, Relativity’s 3D-printed factory, and SpaceX’s “assembly line” for mass-producing Raptor engines are the true indicators of future-tense. The rocket engine is no longer a bespoke jewel, hand-assembled over months. It’s becoming a mass-produced, reusable good. This is the paradigm shift that is reshaping humanity’s access to space.