The Andromeda Engine and Stoke Space’s Vision for Reusability

The Novel Idea

In the dynamic and crowded field of commercial space launch, new companies must present a truly novel idea to stand out. Stoke Space, a company based in Kent, Washington, has done just that. Founded by a team of engineers with experience from industry giants like Blue Origin and SpaceX, Stoke is tackling the most difficult remaining challenge in orbital rocketry: making the entire system, not just the first stage, rapidly and fully reusable.

At the heart of this ambitious endeavor are two distinct and innovative propulsion systems. While Stoke’s unique reusable second stage has captured significant attention, its equally advanced first stage is powered by a new engine known as Andromeda.

The Andromeda engine is Stoke’s answer to the need for a powerful, reliable, and highly reusable booster engine. It’s a liquid-propellant rocket engine designed from the ground up to power the first stage of the company’s Nova launch vehicle. It represents a major engineering undertaking, placing Stoke in a small, elite group of companies developing one of the most complex types of rocket engines ever conceived. To understand Andromeda is to understand the technological currents shaping the future of spaceflight, from its advanced combustion cycle to the next-generation fuel it burns.

A New Contender in the Rocket Engine Arena

The rocket engine business is famously difficult. The barriers to entry are immense, requiring staggering capital investment, deep expertise, and a tolerance for explosive, high-stakes testing. For decades, the field was dominated by a handful of national space agencies and their large aerospace contractors. The arrival of NewSpace companies has changed this, but even within this new generation, building a new engine from scratch is a monumental task.

Stoke Space was founded on the principle that the current models for rocket reusability, while successful, are incomplete. The industry has largely solved first-stage reusability, as demonstrated by the Falcon 9 rocket. The first stage booster flies to the edge of space, separates, and then flies itself back to a landing pad or droneship. The second stage, which carries the payload into orbit, has historically been expendable, burning up on reentry.

Stoke’s primary goal is to solve this second-stage problem with a radical new design. But to make the economics work, their first stage must also be a model of efficiency and robustness. This is Andromeda’s job.

Andromeda is a booster engine. It’s not a small, high-precision orbital maneuvering thruster. It is a workhorse, designed to produce immense thrust to lift the entire Nova rocket and its payload off the launch pad and through the thickest part of theatmosphere. In the Nova design, a cluster of Andromeda engines would propel the first stage, which would then perform a propulsive landing, just like other modern boosters.

What sets Andromeda apart isn’t just its role, but its internal architecture. It’s a methalox engine, meaning it uses liquid methane and liquid oxygen as its propellants. More specifically, it is a full-flow staged combustion engine, a design that is notoriously difficult to engineer but offers powerful advantages for performance and reusability.

The Full-Flow Staged Combustion Cycle

To appreciate what makes the Andromeda engine special, it’s helpful to understand how rocket engines create thrust. In simple terms, a liquid-propellant engine is a controlled explosion. It mixes a fuel and an oxidizer in a combustion chamber, where they ignite, creating incredibly hot, high-pressure gas. This gas is then accelerated out of a nozzle at supersonic speeds, and this acceleration pushes the rocket in the opposite direction.

The challenge is getting the propellants into that high-pressure combustion chamber. The chamber’s pressure is so high that you can’t just “pipe” the propellants in. They must be injected at an even higher pressure. This requires powerful pumps, called turbopumps.

This is where engine designs diverge. A turbopump is like a jet engine’s turbine, and it needs power to spin. The central question of rocket engine design is: How do you power the turbopump? The answer defines the engine’s “cycle.”

How a Rocket Engine Works: The Basics

Almost all large liquid-fueled engines have a few key parts. They have propellant tanks, which hold the fuel and oxidizer as super-cooled liquids. They have turbopumps, one for the fuel and one for the oxidizer. They have a combustion chamber where the propellants meet and burn. And they have a nozzle to direct the exhaust.

To spin the turbopumps, the engine must divert a small amount of fuel and oxidizer, burn them in a small, separate chamber called a preburner, and use the resulting hot gas to spin a turbine, which is connected by a shaft to the pump.

The different engine cycles are all about how this process is handled, and what happens to the exhaust gas from the preburner.

The Problem with Traditional Cycles

There are two common traditional cycles, each with its own compromises.

The simpler method is the “gas generator” cycle. In this design, the preburner’s exhaust gas is simply dumped overboard through a separate, small nozzle. It’s like a tiny, separate rocket engine attached to the side. This is simple, reliable, and relatively easy to build. The Rocket Lab Rutherford engine and the SpaceX Merlin engine use this cycle. Its main drawback is that it’s inefficient. The propellant used to drive the turbopump is “wasted” and doesn’t contribute to the main thrust.

The more powerful method is the “staged combustion” cycle. In this design, the hot exhaust from the preburner isn’t dumped. It’s channeled directly into the main combustion chamber to be burned again. This is much more efficient because no propellant is wasted. However, it’s a far more difficult engineering problem. The preburner’s exhaust is extremely hot and, depending on the mixture, chemically reactive. Forcing this aggressive gas through the turbine and into the main chamber puts enormous stress on the engine’s plumbing and materials. Engines like the Space Shuttle’s main engines and the Blue Origin BE-4 use this cycle.

The Full-Flow Solution

The full-flow staged combustion (FFSC) cycle, which Andromeda uses, is the most advanced and complex of these designs. It’s a special type of staged combustion that solves many of the problems of traditional staged combustion.

In a traditional staged-combustion engine, the preburner only burns a small portion of one propellant. For example, in an “oxidizer-rich” cycle, the preburner mixes all the oxidizer with a tiny bit of fuel. This creates hot, pure oxygen gas that spins the turbine. This is very harsh on the metal parts.

In a full-flow cycle, the engine gasifies all the propellant before it even gets to the main chamber. It does this by having two preburners instead of one.

1. Fuel-Rich Preburner: One preburner mixes all the fuel (methane) with a little bit of oxidizer. This creates a hot, fuel-rich gas that powers the turbine for the fuel pump.
2. Oxidizer-Rich Preburner: The second preburner mixes all the oxidizer (liquid oxygen) with a little bit of fuel. This creates a hot, oxidizer-rich gas that powers the turbine for the oxidizer pump.

These two streams of hot gas, one full of methane and the other full of oxygen, are then piped to the main combustion chamber. They mix and ignite, creating the main thrust.

Benefits of Full-Flow

This “full-flow” architecture is incredibly complicated, but it provides several key benefits, especially for a reusable engine like Andromeda.

First, it offers higher performance. Because all the propellant is “pre-burned” and enters the chamber as a gas, it mixes and combusts more efficiently. This can lead to a higher specific impulse, which is the rocket engine’s equivalent of fuel economy.

Second, and perhaps most important for reusability, is lower turbine temperatures. Because the preburners are gasifying the entire flow of both propellants, the turbines can spin at lower temperatures. In a traditional staged-combustion engine, a small amount of propellant is heated to extreme temperatures. In a full-flow engine, a large amount of propellant is heated to a more moderate temperature.

This “cooler” operation is much gentler on the turbine blades and the engine’s internal plumbing. Parts don’t wear out as quickly, they experience less stress, and they are less likely to crack or fail. This is a massive advantage when the goal is to refly the engine many times with minimal inspection and refurbishment.

A Rarefied Club

The engineering challenges of FFSC are so great that for decades, it was a theoretical ideal. Only two organizations in history have successfully built and flown an FFSC engine. The first was SpaceX with its Raptor engine, which powers the Starship vehicle.

By choosing to develop Andromeda as a full-flow engine, Stoke Space is not taking an easy path. It is placing itself in the company of the most advanced propulsion teams in the world and betting that it can master this complex technology. The success of this bet is central to the viability of the entire Nova rocket.

Methalox: The Fuel of the Future

Andromeda’s design is defined by more than just its cycle. Its fuel choice, a combination of liquid oxygen(LOX) and liquid methane (LNG), is also a key part of its identity. This combination, often called “methalox,” has become the propellant of choice for most next-generation launch vehicles.

Why Not Kerosene or Hydrogen?

For decades, rocket designers had two main choices for high-performance engines.

The first was RP-1, a highly refined form of kerosene. It’s the fuel used in the Falcon 9’s Merlin engine and the Saturn V’s F-1 engine. RP-1 is stable, dense (so it doesn’t need huge tanks), and well-understood. Its big drawback is that it’s a “dirty” fuel. When it burns, it leaves behind a sooty residue called “coking.” In a reusable engine, this coke builds up in the plumbing, turbopumps, and injectors, requiring a long and costly cleaning process between flights.

The second choice was liquid hydrogen (LH2). Hydrogen is the most efficient rocket fuel available, offering a very high specific impulse. The Space Shuttle’s main engines and the upper stage of many rockets use it. But hydrogen has two major problems. First, it’s extremely difficult to work with. It must be kept at incredibly cold temperatures (close to absolute zero). Second, it has a very low density. As a liquid, it’s almost fluffy, like styrofoam. This means it requires massive, bulky, and heavily insulated tanks, which makes the whole rocket larger and heavier.

The Advantages of Methane

Methane (the primary component of natural gas) hits a sweet spot right between kerosene and hydrogen.

First, it’s a clean-burning fuel. When it combusts with oxygen, it produces carbon dioxide and water vapor. It doesn’t leave behind the sooty coke residue of kerosene. This is a massive advantage for reusability. An engine like Andromeda can be run, inspected, and run again without needing to be disassembled and scrubbed.

Second, it has good performance. While not as efficient as hydrogen, it’s significantly more efficient than kerosene.

Third, it’s much easier to handle than hydrogen. It’s a “cryogen,” meaning it must be kept cold to stay liquid, but its boiling point is much higher (and more manageable) than hydrogen’s. It’s also much denser than hydrogen, meaning the rocket’s tanks can be smaller and lighter.

Finally, methane has a long-term, forward-looking advantage. It’s believed that methane can be manufactured on Mars using local carbon dioxide (from the atmosphere) and water (from ice). This process, known as the Sabatier reaction, means that a methalox-powered vehicle could, in theory, be refueled on Mars for a return trip. This makes it the fuel of choice for companies with interplanetary ambitions.

An Industry-Wide Shift

Stoke’s choice of methalox for Andromeda is part of a clear industry trend. Nearly every major new rocket in development has adopted this propellant combination. SpaceX’s Starship is powered by methalox Raptorengines. Blue Origin’s New Glenn and United Launch Alliance’s Vulcan Centaur are both powered by the BE-4methalox engine. Relativity Space’s Terran R and Rocket Lab’s Neutron will also be methalox vehicles.

By building Andromeda as a methalox engine, Stoke Space is ensuring its technology is on the main branch of rocket development, benefiting from a supply chain and knowledge base that is growing every day.

Designing for Manufacturability and Reusability

Developing a full-flow staged combustion engine is only half the battle. The engine must also be manufacturable at a reasonable cost and built to withstand the rigors of repeated flights. This is where modern manufacturing techniques, particularly additive manufacturing, become essential.

The Power of Additive Manufacturing

Traditional rocket engines are a nightmare of complexity. They are made of thousands of individual parts – pipes, valves, welds, brackets, and bolts – that must be painstakingly assembled by hand. Every weld and every joint is a potential point of failure.

3D printing changes this equation. It allows engineers to design complex components as a single, unified piece. For a rocket engine, this is revolutionary.

• Injectors: The injector plate, which mixes the fuel and oxidizer at the top of the combustion chamber, is one of the most complex parts of an engine. It can contain hundreds of tiny, intricate channels. Traditionally, this part was made by precisely drilling, brazing, and welding hundreds of small pieces together. With 3D printing, it can be printed as one solid part, complete with all its internal “plumbing.”
• Cooling Channels: The combustion chamber and nozzle must withstand temperatures of thousands of degrees. They do this through regenerative cooling, where the super-cold fuel is piped through tiny channels in the walls of the chamber before it’s burned. Printing these microscopic, winding channels is far simpler and more reliable than traditional manufacturing methods.
• Turbopumps: The impellers and housings for the turbopumps, with their complex curved blades, can also be printed as single, optimized parts.

Reducing Part Count

Stoke Space has embraced this philosophy for Andromeda. The goal is to design an engine with the lowest possible part count. Every part that is eliminated is a part that doesn’t need to be sourced, inspected, tested, or joined to another part.

Fewer parts means fewer potential failure points. This simplifies assembly, speeds up production, and lowers cost. It also makes the engine more reliable, which is the single most important quality in a launch vehicle.

Designing for a Tough Life

The design of Andromeda is wholly focused on a long, low-maintenance life. This is a significant shift from the philosophy that governed old-world, expendable engines.

The choice of the FFSC cycle, with its cooler turbine temperatures, is the first part of this. The engine is designed to operate with lower internal stress.

The choice of methalox fuel is the second part. The clean-burning fuel prevents the buildup of residue that would foul the engine’s internal workings.

The use of 3D printing is the third part. By printing complex parts from advanced metal alloys, engineers can create components that are stronger and more resistant to fatigue than their assembled-from-pieces counterparts.

All of this is intended to support Stoke’s goal of rapid reusability. The plan for the Nova booster, powered by Andromeda, is not to land it and then spend six months taking it apart. The goal is to land it, inspect it, refuel it, and fly it again, perhaps in as little as 24 hours. This kind of “airline-like” operation is only possible if the engines are designed for it from the beginning.

Andromeda’s Role in the Nova Rocket

The Andromeda engine, for all its complexity, is a single component in a much larger system: the Nova launch vehicle. Its specific job is to make the first stage of that vehicle a reusable workhorse.

The Nova First Stage

The Nova first stage is a large booster designed to provide the main thrust for the first few minutes of flight. It will be powered by a cluster of Andromeda engines, working in concert. During ascent, these engines will push the vehicle to the edge of space before the second stage separates and continues to orbit.

After separation, the first stage will begin its return to Earth.

The Re-entry and Landing Profile

The Andromeda engines will be used multiple times in a single flight. They will fire at full power for the initial launch. Then, after separation, some of the engines will likely need to relight for a “boostback” burn to aim the rocket back toward the landing site.

As the booster falls back to Earth, it will perform a re-entry burn, again using its Andromeda engines to slow itself down as it hits the atmosphere. Finally, as it approaches the landing pad, it will perform a final “landing burn,” with one or more Andromeda engines firing to bring the vehicle to a gentle, propulsive landing, ready to be refurbished for its next flight.

This “propulsive landing” profile requires an engine that is throttleable (can be powered up and down) and can be relit multiple times in flight, sometimes in rapid succession. These capabilities are being designed into Andromeda from its inception.

A System of Systems

Andromeda is the solution for the first stage. But it’s only half of Stoke’s reusability puzzle. The company’s vision for full reusability rests on an even more unconventional design for the second stage, a system that is entirely distinct from Andromeda.

The Other Half: Stoke’s Groundbreaking Upper Stage

It’s important to differentiate between Stoke’s two propulsion projects. Andromeda is the powerful booster engine for the first stage. The upper stage uses a completely different and revolutionary approach that has become the company’s signature innovation.

The Challenge of Second Stage Reusability

Reusing a second stage is an order of magnitude harder than reusing a first stage. A first stage only goes to the edge of space and doesn’t reach orbital velocity. A second stage travels at over 17,000 miles per hour.

At that speed, re-entering the atmosphere generates unimaginable heat. SpaceX’s Starship plans to solve this with a belly-flop maneuver and a shield of thousands of delicate heat shield tiles. The Space Shuttle used similar tiles. These tiles are effective, but they are fragile, time-consuming to inspect, and difficult to maintain.

Stoke’s Radical Solution

Stoke Space has designed an upper stage that has no heat shield. Instead, the entire base of the upper stage is the heat shield. And it’s a “regeneratively cooled” metallic shield.

This design is a ring of thrusters integrated into a high-temperature metal base. During ascent, these thrusters act as the second stage’s main engine, pushing the payload into orbit.

For re-entry, the vehicle comes in “engine first.” The super-cold propellants are pumped through a network of channels in the metallic base, absorbing the intense heat of re-entry. This process keeps the shield from melting and also pre-heats the propellant for the landing burn. The hot gases are then vented out of the thrusters, which also helps to steer the vehicle.

This means the upper stage uses the same system for its ascent engine, its orbital maneuvering, its re-entry heat shield, and its landing engines. It’s an incredibly integrated and elegant, though unproven, solution.

Clarifying the Naming

This upper stage system is the “brains” of the operation, while Andromeda is the “brawn.” Andromeda provides the raw power to get the stack off the ground. The sophisticated upper stage propulsion system finishes the job of getting to orbit and, more important, handles the much more difficult task of coming home from orbit safely and quickly.

The Competitive Landscape

Andromeda and the Nova rocket do not exist in a vacuum. They are entering one of the most competitive markets in technology, with multiple well-funded companies all chasing the same prize: a dominant, low-cost, and reliable launch vehicle.

The Reign of Raptor

The most direct competitor to Andromeda is the SpaceX Raptor engine. As the only other operational FFSC methalox engine, it’s the standard by which all others are measured. SpaceX has been building, testing, and flying hundreds of Raptor engines on its Starship prototypes. They have a massive lead in operational experience and manufacturing.

BE-4 and the Legacy Players

The Blue Origin BE-4 is another major competitor. While it is also a methalox engine, it uses a more traditional (though still very advanced) oxygen-rich staged combustion cycle. The BE-4 is a powerhouse, selected to power the Vulcan Centaur rocket as well as Blue Origin’s own New Glenn booster. It represents a different, and perhaps more conservative, design philosophy than Andromeda’s full-flow cycle.

Other NewSpace Engines

Other startups are also building their own engines. Relativity Space is developing its Aeon engines, which are heavily 3D-printed. Rocket Lab is developing the Archimedes engine for its Neutron rocket. Interestingly, Archimedes is a gas-generator cycle, a deliberate choice by Rocket Lab to trade a small amount of performance for a much simpler, more reliable, and easier-to-reuse engine.

This diverse landscape shows that there isn’t one “right” way to build a reusable methalox engine. Each company is making a different set of engineering bets.

Stoke’s bet with Andromeda is that the complexity of full-flow staged combustion is a worthwhile investment. They believe the long-term reusability and performance benefits will outweigh the difficult development path.

Development, Testing, and the Path to Orbit

Stoke Space has been executing a rapid development and testing program. The company has focused heavily on “hardware-rich” development, a philosophy of building, testing, breaking, and iterating on real hardware as quickly as possible.

A History of Progress

The company first gained public attention by testing prototypes of its unique upper stage. This included a “Hopper” test vehicle, which performed several low-altitude “hops,” similar to the early tests of the SpaceXStarhopper. These tests, while focused on the second stage, provided invaluable data on building and operating a liquid-propellant rocket vehicle.

In parallel, development of the Andromeda booster engine has been underway. This has involved scaling up its 3D printing capabilities and conducting tests on individual components, such as the preburners, turbopumps, and injectors, before integrating them into a full engine.

Securing Support

This iterative progress has helped Stoke secure significant funding from private investors as well as validation from government agencies. The company has been awarded contracts from NASA and the U.S. Space Force, including under NASA’s Tipping Point program. These contracts not only provide capital but also serve as a strong endorsement of Stoke’s technology and approach. They are often focused on specific development goals, such as maturing the metallic heat shield technology or testing new manufacturing processes for engine components.

The Road Ahead

The path from a test stand to orbit is long and difficult. The next major steps for the Andromeda program involve a full-scale static fire campaign. This is where the fully assembled engine is bolted to a concrete test stand and fired at full power for extended durations. These tests will prove whether the engine’s design, from its full-flow cycle to its 3D-printed parts, can withstand the incredible temperatures and pressures of operation.

After a successful static fire campaign, the engines will be integrated into a first-stage prototype for hop tests, demonstrating the booster’s ability to launch and land. Following that, the company will face the ultimate test: integrating the Andromeda-powered first stage with its novel upper stage for the first orbital launch attempt of the Nova rocket.

Summary

The Andromeda engine is far more than just another piece of aerospace hardware. It is a physical manifestation of Stoke Space’s ambitious bet on the future of rocket propulsion.

It is a bet on the superiority of the methane fuel cycle for reusability. It is a bet on the power of additive manufacturing to simplify complex machines. And, most significantly, it is a bet on the full-flow staged combustion cycle, a technology so difficult that only one other company has mastered it.

Andromeda is the workhorse designed to make the first stage of the Nova rocket as reliable and routine as an airliner’s engine. Its development is a high-stakes endeavor, but its success is a necessary step for Stoke Space to realize its full vision: a complete, end-to-end, rapidly reusable launch vehicle that could fundamentally change the economics of access to space.