Hybrid Rocket Engines: Bridging the Gap Between Solid and Liquid Propulsion Systems

What is a Hybrid Rocket Engine?

Hybrid rocket engines (HREs) are gaining attention as a promising propulsion technology due to their ability to combine the advantages of both solid and liquid propulsion systems, offering unique benefits in terms of performance, cost-efficiency, and sustainability. Unlike traditional rocket engines, where the fuel and oxidizer exist in the same state, HREs utilize different states of aggregation. For example, in a direct scheme, the fuel is solid, and the oxidizer is liquid, while in a reverse scheme, the fuel is liquid, and the oxidizer is solid. This design allows for greater safety, flexibility, and control compared to solid-propellant rocket engines, while avoiding the complexity of liquid-fueled systems.

While HREs are becoming more prominent today, their roots trace back to early research in the 20th century. Pioneers like Sergei Korolev, Friedrich Tsander, and Mikhail Tikhonravov explored hybrid propulsion nearly a century ago. In the 1930s, these Soviet scientists developed and launched the GIRD-09, an early hybrid rocket engine that paired solid fuel made from rosin and kerosene with liquid oxygen (LOX) as the oxidizer. This work was part of the Soviet “Group for the Study of Reactive Propulsion” (GIRD), a group that humorously called themselves “A Group of Engineers Working for Free.” While early experiments showed promise, hybrid engines were soon overshadowed by liquid propulsion systems due to performance limitations. GIRD’s later project, the GIRD-X, marked a shift to a fully liquid-fueled design. However, advancements in materials, simulation tools, and manufacturing techniques have revived interest in hybrid propulsion, making it a competitive option for modern aerospace applications.

Hybrid propulsion systems can operate at either normal atmospheric temperatures or at cryogenic temperatures. However, cryogenic temperatures present significant challenges in storage, transportation, and operation. For instance, Liquid Oxygen (LOX), a commonly used oxidizer, requires storage at extremely low temperatures, typically between 50 to 90 K under standard atmospheric pressure. Other oxidizers, such as high-concentration hydrogen peroxide (H₂O₂), nitrogen tetroxide (N₂O₄), and nitrous oxide (NO), can be stored at normal temperatures and are also used in hybrid propulsion systems [6].

Fuel and Oxidizer Composition in Hybrid Rocket Engines

HRE fuels are typically based on polymer hydrocarbon compounds, such as polyethylene (C2H4)n, polybutadiene with terminal hydroxyl groups xxx, synthetic resins, and rubbers. Cryogenic fuels, like methane solid propellant charges cooled below 90.5 K before launch, are also viable options.  [1].

The key difference between HREs and solid-propellant rocket engines (SPREs) lies in the fuel composition. In HREs, the oxidizer and fuel are separate (single-base fuel), whereas in SPREs, they are combined into a single fuel grain (dual-base fuel).  (Fig. 3).

In short, if, during the manufacturing of a solid propellant charge, we combine the liquid fuel HTPB with the oxidizer—ammonium perchlorate (AP) crystals (NH4ClO4) into a single, unitary two-component fuel grain, we create fuel for an SPRE that no longer requires external oxygen for combustion. At first glance, this seems like an excellent solution, as the mass and size characteristics of SPREs are advantageous, reducing both the engine’s size and weight. However, as the saying goes, the disadvantages are simply the flip side of the advantages. Let’s illustrate this in Table 1.

Comparison of Hybrid and Solid Rocket Engines

It should be noted that both HREs and SPREs feature highly variable fuel compositions, typically consisting of 4-6 ingredients, including the fuel itself, a hardener, a plasticizer, a curing catalyst, a combustion inhibitor or catalyst, and energy additives such as metal powders.

While early experiments demonstrated the feasibility of hybrid propulsion, modern applications still face significant challenges that must be addressed.

Disadvantages of Hybrid Rocket Engines

1. Hard start – A short surge in thrust at ignition, caused by a large amount of pure oxidizer accumulating in the combustion chamber without combustion products. This phenomenon has been confirmed through calculations.
2. Thrust drop during fuel burn – As fuel burns, thrust gradually decreases, with the rate of decrease accelerating towards the end of the burn.
3. Inferior performance at low thrust: HREs perform worse in the low-thrust range compared to SPREs, and at high thrust, they become less cost-effective than liquid rocket engines (LREs) due to the larger size of the combustion chamber.’

Specific requirements for solid fuel include low thermal conductivity and viscosity at elevated temperatures. During operation, the oxidizer flow should blow the fuel away from the surface of the grain, preventing it from liquefying and flowing away.

Comparison of Hybrid and Liquid Rocket Engines

The specific impulse of the HRD thrust lies in the range of 310-390 s

Applications of Hybrid Rocket Engines

In recent decades, interest in hybrid rocket engines has grown after years of obscurity. While hybrid and liquid engines evolved alongside each other during the early days of jet propulsion, hybrid engines were eventually overshadowed by liquid engines with superior performance. Nonetheless, hybrid engines have found specific applications, such as:

1. Starting boosters and booster units: These are used for tasks like placing spacecraft with radioactive waste into safe orbits around the Sun.
2. Light-class mobile rocket and space complexes.
3. Transport and technical servicing of space objects: This includes using reusable interorbital tugs for space operations.

Cost and Manufacturing Benefits of Hybrid Rocket Engines

HREs offer several advantages, but one of the most significant is their lower manufacturing cost, as they do not require expensive high-tech equipment. Fuel grains can be produced semi-artisanally, allowing for flexible formulations. Additionally, advanced manufacturing techniques like 3D printing can be used for engine housings, while nozzle walls can incorporate perforations or cooling microchannels that utilize the oxidizer for thermal regulation. Design elements such as graphite inserts in nozzle throats or ablative coatings on nozzle walls can also be adapted for thermal protection in liquid-propellant rocket engines. Moreover, full-scale testing of hybrid rocket engines is significantly more cost-effective—at least an order of magnitude cheaper—than testing liquid rocket engines.

Closing Thoughts

Hybrid rocket engines strike a unique balance between the strengths of solid and liquid propulsion systems. To optimize these designs, engineers can leverage advanced tools like AxSTREAM System Simulation software, which allows for detailed thermodynamic calculations and combustion process analysis within the HRE chamber. By utilizing such software, engineers can make more informed decisions during the design process, ensuring better performance and efficiency. In our next blog, we will take a closer look at how these capabilities enhance hybrid rocket engine design.

SOURCES

1. Investigation Of Thermal Protection Systems For Hybrid Rocket Motors https://cisas.unipd.it/sites/cisas.unipd.it/files/rp_34_Franco_Massimo_ad2.pdf
2. Ukraine: К. Н. Орехов. Повышение Эффективности Метеорологических Ракет При Применении Гибридных Двигателей KNIT#22_2.indd
3. Hybrid Experimental Rocket Stuttgart: A Low-Cost Technology Demonstrator https://www.researchgate.net/publication/321494881
4. Source
5. Maximilian Bambauer and Markus Brandl. Development of a 10 kN LOX/HTPB Hybrid Rocket Engine through Successive Development and Testing of Scaled Prototypes EUCASS2017-334.pdf
6. Calabro Overview On Hybrid Propulsion https://www.researchgate.net/publication/258491207