Rocket Engine Cycles (Pump Cycles): Unveiling How Rocket Engines Soar to the Stars

At the heart of every space mission lies a marvel of engineering – the rocket engine. A complex yet fascinating technology, it propels spacecraft beyond Earth’s atmosphere and into the cosmos.

Rocket engines utilize the principles of momentum, energy, and force; they burn rocket propellants to produce high-pressure and high-temperature gasses.

These gasses are then expelled through a nozzle, causing the rocket to move in the opposite direction with tremendous velocity.

A typical rocket engine comprises several crucial components:

Each component plays a pivotal role in ensuring a working rocket engine.

In this article, we will be diving into the science behind the pump system and its importance to the entire rocket.

Types of Pump Cycles

The pump is responsible for supplying the propellant (fuel and oxidizer) from the rocket’s storage tanks to the combustion chamber, where they are mixed and ignited to produce thrust.

There are multiple ways for the pumps to be designed to efficiently transfer the propellant into the combustion chamber, which are called pump cycles or engine cycles.

The types of pump cycles range from very simple cycle types like cold gas thrusters to more and more complicated ones like the famous full-flow staged combustion. There are four main types of pump designs used in various rocket engines.

1. Pressure-Fed Engine Cycles

The pressure-fed engine cycle is a relatively straightforward pump design with minimal moving parts. Despite the simplicity, pressure-fed engines offer significantly higher performance compared to cold gas thrusters.

These engines can be categorized into two types: monopropellant pressure-fed engines, which utilize a single propellant, and bipropellant pressure-fed engines, which employ two different propellants as the names imply.

2. Monopropellant Pressure-Fed Engines

A monopropellant pressure-fed engine, also known as a monoprop pressure-fed engine, bears resemblance to a cold gas thruster in design.

It consists of two tanks: one containing high-pressure inert gas and the other holding the propellant, usually hydrazine, at lower pressure.

During operation, monoprop engines open the valve from the propellant tank to the engine while maintaining pressure within the propellant tank.

Additionally, they regulate another valve between the high-pressure tank and the propellant tank, with the high-pressure tank typically containing inert gasses like nitrogen or helium.

One downside is the power of the engine is capped by the pressure of the system, which is why we don’t see monoprop engines powering rockets completely.

3. Bipropellant Pressure-Fed Engines

Bipropellant pressure-fed engines, also known as biprop pressure-fed engines, share a fundamental similarity with monoprop engines. However, the key distinction lies in the presence of both fuel and oxidizer tanks, as the name implies.

Similarly to monoprop engines and cold gas thrusters, these engines operate with minimal moving parts, primarily simple valves.

The advantage of bipropellant engines is their ability to utilize more energetic and efficient propellants, such as RP-1 and LOx, or even CH4 and LOx.

Many bipropellant systems opt for hypergolic propellants due to their simplicity and reliability. Hypergolic propellants spontaneously ignite upon contact, eliminating the need for an ignition source.

Despite the straightforward design, these systems still offer respectable performance.

The challenge lies in the overall pressure of the system, with the pressurant tanks serving as the limiting factor, much like what we observed with the monopropellant pressure-fed engines.

Increasing system pressure enhances performance but also adds weight, leading to a trade-off where the added weight reduces payload capacity more than the performance gain.

This explains why we have not witnessed a fully pressure-fed orbital rocket, where all stages rely on pressure-fed engines.

Achieving orbit solely with pressure-fed engines is virtually impossible due to their limited overall performance, even with the latest advancements in technology, including the use of carbon composite tanks.

4. Electric Pump-Fed Engine Cycles

The previous engine cycles use natural pressure in the propellant tanks to drive the propellants into the combustion chamber, setting a limit on the chamber pressure since it has to be lower than the tank pressure.

As we aim to achieve higher chamber pressure without raising the propellant tank pressures, we need an active mechanism to force the propellants against their natural flow.

This is where pumps play a crucial role. By using pumps, we can increase the pressure after the pump without affecting the pressure before it, allowing us to reduce the pressure of tanks.

This saves a ton of mass and weight that could go into the propellant. However, the electric pump demands substantial power, often in the order of thousands of horsepower, which requires big, heavy motors and power sources.

This is where the Gas Generator Engine Cycle comes in.

5. Gas Generator (Open) Engine Cycles

gas generator engine cycle diagram
Credit: Everyday Astronaut

As previously mentioned, pumps in rocket engines require a significant amount of energy to push propellants into the combustion chamber at the desired pressure.

To address this, the open cycle, also known as the gas generator cycle, was developed.

In this cycle, a small amount of propellant is taken from the main tanks and sent to the gas generator, where it undergoes combustion to generate high-pressure exhaust gasses.

These gasses then drive the turbine to power the pumps, ensuring a self-sustaining process.

An early example of this cycle was seen in the German-designed V-2 rocket’s A4 engine, where a high concentration of hydrogen peroxide (H2O2) was passed over a catalyst to produce steam, spinning the turbine that powered the pumps.

The open cycle has the advantage of simplicity, but it also comes with inefficiencies. It leaves unburnt fuel in the gas generator’s exhaust plume, visible on rockets like the Falcon 9.

However, engineers often consider this waste acceptable, as the simplicity of the open cycle outweighs the small amount of fuel lost in the process.

The open cycle provides an effective solution for powering the pumps while maintaining overall engine efficiency. With the drawback of a more complex design, we can do better.

6. Staged Combustion (Closed) Engine Cycles

The closed, or staged-combustion, cycle is the more advanced approach that seeks to utilize the combustion products more efficiently compared to the open cycle.

In the closed cycle, the exhaust from the gas generator is not simply directed to the main combustion chamber, as doing so would cause several issues during flight.

The pressure driving the turbine is usually kept as low as possible, resulting in the pressure downstream from the turbine being lower than upstream. This pressure difference would cause combustion chamber gasses to flow back up the exhaust pipe, which is undesirable.

To address these challenges, modifications are made to the propellant routing. Instead of using a small amount of fuel and oxidizer to power the gas generator, the engine will pump either all of the fuel or all of the oxidizer through the gas generator and over the turbine.

The term “gas generator” is then replaced with “burner” since now, only a small portion of the propellant flowing past the turbine gets combusted in the burner.

Whether the engine is considered fuel-rich or oxidizer-rich depends on which propellant entirely passes through the burner and turbine.

7. Oxygen-Rich Staged Combustion Engine Cycles

The oxidizer-rich closed cycle was the first to be developed historically, and Soviet rocket designers and engineers successfully tackled its challenges as early as the 1950s with the S1.5400 engine used in the upper stage of the R7 rocket—a significant achievement.

Oxidizer rich staged combustion engine cycle diagram
Credit: Marc Schaefer

The Soviet engineers opted for the oxygen-rich route due to potential issues with coking and soot build-up when using hydrocarbon-based fuels like RG-1 or RP-1 in a fuel-rich cycle.

In the oxygen-rich staged combustion cycle, all the oxygen passes through the turbine and into the main combustion chamber, with only the minimum amount of propellant entering the burner to spin the pumps effectively.

Since only a small amount of fuel goes into the burner, only a small amount of oxidizer reacts with the fuel.

Any remaining oxidizer leaves the burner unburned and heads to the combustion chamber, where the main combustion occurs, releasing the energy stored in the remaining unreacted propellants.

One challenge of oxidizer-rich staged combustion is the high reactivity of hot gaseous oxygen and the need for specific metal alloys to withstand the hostile environment, which is why the United States steered towards a fuel-rich staged combustion system.

8. Fuel-Rich Staged Combustion Engine Cycles

fuel rich staged combustion engine cycle diagram
Credit: Marc Schaefer

Now, let’s delve into the alternative to the oxygen-rich cycle—the fuel-rich cycle. In this configuration, the relationship between the oxidizer and fuel is reversed, where all of the fuel passes through the burner, while only a minimal amount of oxygen does.

If attempting this cycle with long-chain hydrocarbons like RP-1, issues with soot build-up and coking would quickly arise, as mentioned earlier.

However, the United States took a different approach, opting for liquid hydrogen as the fuel for the Space Shuttle’s main propulsion system.

Hydrogen, being light in carbon content, avoids soot build-up and is suitable for running as hot gaseous hydrogen in the engine.

While this may seem like a straightforward solution, fuel-rich staged combustion presents its own challenges, particularly when using hydrogen as fuel.

The extreme lightness and volatility of hydrogen necessitate large pumps with multiple stages to achieve the high pressures required.

The RS-25 engine, used in the Space Shuttle, employed dual pre-burners, each with its own shaft and being fuel-rich. One burner powered the fuel pump stages, while the other powered the oxygen pump.

This arrangement, though effective, introduced the challenge of preventing high-pressure hot gaseous hydrogen from seeping through the shaft seals and encountering liquid oxygen, which would lead to catastrophic failure.

To address this issue, the engineers developed an intricate purge seal. This seal employs an even higher pressure inert gas, helium, in the middle to prevent propellant leakage and maintain separation between the fuels.

There is an engine cycle that merges the advantages of both the fuel-rich and oxygen-rich cycles, while also incorporating certain drawbacks from each of them.

Nevertheless, a specific upside makes it a compelling pursuit, even though only a few organizations have explored it with only one creating a successful engine that flew.

9. Full-Flow Staged Combustion Engine Cycles

diagram of full flow staged combustion engine cycles
Credit: Marc Schaefer

The full-flow staged combustion cycle derives its name from the way propellants are routed through the burners.

Both fuel and oxidizer pass through separate pre-burners and turbines, resulting in a fuel-rich burner and an oxidizer-rich burner.

The majority of oxidizer flows through the oxidizer-rich burner and turbine, with only a small amount passing through the fuel-rich burner.

Conversely, most of the fuel passes through the fuel-rich burner and turbine, with a minimal amount flowing through the oxidizer-rich burner.

This arrangement ensures that both propellants reach the combustion chamber in gaseous form, offering significant advantages over other cycles.

Gas-gas interaction is highly efficient, leading to improved mixing of gas products before combustion, resulting in faster combustion and fewer unburnt residuals compared to liquid-liquid or liquid-gas interactions.

While managing hot gaseous oxygen remains a challenge, coupling the oxidizer-rich turbine and shaft to the oxidizer pump, and likewise coupling the fuel-rich turbine and shaft to the fuel pump, simplifies the system and eliminates the need for complex seal mechanisms.

The most significant advantage of the full-flow staged combustion cycle lies in the temperatures at which the burners operate.

Higher enthalpy yields more work, but it also means higher temperatures. In the case of burners, the amount of enthalpy required to power the pumps can be calculated based on fixed thrust and chamber pressure values.

Since both full-flow and other closed-cycle engines require roughly the same enthalpy for pump operation, the main variable that changes between them is the mass of propellant flowing through the burners.

With both fuel and oxidizer passing through, the mass flows approximately double, resulting in significantly lower preburner temperature, which is a dream come true for an aerospace engineer.

Despite its advantages, the full-flow staged combustion cycle has been considered complex and challenging due to the interconnected nature of its components.

Small changes in one part can lead to ripple effects throughout the engine, making valve timing, startup, and throttling difficult to master.

The Soviet Union developed the RD-270 as the first full-flow staged combustion cycle engine, but it never flew due to the cancellation of the UR-700 and UR-900 rockets it was designed for.

The United States also made progress with the Integrated Powerhead Demonstrator, but it was never fully developed into an engine.

Today, SpaceX employs the full-flow staged combustion cycle in its Raptor engines for the Starship upper stage and SuperHeavy booster.

Conclusion

Among the various rocket engine cycles available, no single cycle type can be deemed the “best.” Each system employs a specific approach to power a rocket engine, and with each comes inevitable trade-offs and compromises to consider.

In the current landscape, the electric pump-fed cycle is gaining popularity as lithium-based battery energy density advances through materials science innovations.

The gas generator cycle remains one of the most prevalent types due to its effective compromise between performance and relative simplicity.

Closed-cycle engines have long been coveted for their performance benefits over open-cycle engines, despite the added complexity they entail.

Full-flow staged combustion represents the most intricate system in this context, offering the potential for cooler turbines and hotter combustion chambers, resulting in tremendous pressure and thrust capabilities, all while ensuring safety.

Surprisingly, the tap-off cycle has not seen more extensive development, considering its relative simplicity and reliability, which make it capable of achieving high performance.

The expander cycle, as demonstrated by the RL-10 engine, stands as a viable option, but it has limitations on thrust output levels, making it less suitable for use in sea-level engines.

Ultimately, the quest for the perfect rocket engine cycle remains ongoing, driven by the pursuit of enhanced performance, reliability, and efficiency in the pursuit of reaching new celestial horizons.

Check out the other articles in this series!

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