An in-depth review of the performance and technical levels of liquid-propellant rocket engines, looking at how they have developed over the years and affected the global powers’ space race
Among the various aerospace propulsion technologies, liquid-propellant rocket engines were the first to enter aerospace engineering applications due to their high performance and reliability, and good mission adaptability. They have always had a dominant position, fostering the first tests from 1926 onwards, as well as the birth and development of ballistic missiles (1944-1970) and carrier rockets (1957 to date), which opened the era of human space flight and supported the vigorous development of the related activities.
The main propulsion system, the auxiliary propulsion system of launch vehicles (with the exception of small solid launch vehicles), space shuttles, aerospace aircraft (such as the Space Shuttle), spacecraft, satellites, space stations, deep space probes and other means, currently use the liquid-propellant rocket.
Based on the different application needs, liquid-propellant rocket engines have developed various types and hundreds of engineered products with different levels of thrust, propellants, and power cycle feed methods.
Among them, the performance and technical level of the engines used for the main propulsion system of the ground and upper stages of the launch vehicle (referred to as the main engine) directly determine the efficacy of the launch vehicle and influence a country’s capability and level of access, exploration, utilisation, and development of space. Such systems are therefore considered the cornerstone of aerospace development, as well as an important strategic guarantee for national security and great power status.
At the same time, the main engine is technically complex and difficult, with a long development cycle and high costs. It belongs to the national strategic core industry and is a concentrated expression of the country’s industrial base, and of the scientific and technological level and overall national strength. In today’s world, only a few countries such as the United States of America, the People’s Republic of China, Russia, France and Japan are able to independently develop the main engine of the launch vehicle and create an industrial scale.
The launch vehicle requirements for the main engine include high thrust, specific impulse, thrust-to-weight ratio, reliability and low cost. These indicators make the engine operate with extreme parameters that exhaust the limit performance of materials and achieve the operating characteristics of high-level release and energy conversion in a small structural space.
These parameters of extremely high operating conditions and extremely short start-up times (generally less than 3 seconds) are not matched by all other thermodynamic machines.
Due to the above operating characteristics, combined with the environmental and mission profile, engines are becoming increasingly complex, and the main liquid-propellant rocket engine has unique technical characteristics, including the following ones:
1) the working process mechanism is complex and difficult to predict and control effectively;
2) problems such as system shock oscillation during the engine transition, multi-field coupling of components (such as combustion instability, flow-induced vibration, etc.), and sub-synchronous vibration of the flexible rotor, have caused engine failures many times in the history of space aeronautics, and it takes a lot of time and money to solve problems such as high-frequency unstable combustion, and sub-synchronous vibration of other hydrogen-oxygen engines.
The mechanism, however, has not been fully clarified yet and the project simulation method is not yet mature, thus resulting in high dependence on testing and difficulties in troubleshooting and improvement. For high-thrust engines, in particular, the issues relating to scale effects, such as combustion stability, turbo-pump axial force balance and rotor stability, will become increasingly important if seriously sidereal distances are to be achieved.
The load environment is complex and harsh, and structural strength and fatigue/stress issues are important, such as extreme operating load, including high speed, pressure, heat flow, temperature, thermal shock during start-up, etc.
The high thrust-to-weight ratio of the engine requires a light structure, and the complex and rigid load environment causes major problems such as low margin leading to high uncertainty and failure mode hazards in engine strength and fatigue/stress life cycle.
In terms of component processing, some special production technologies are difficult (such as moulding or precision machining on an extreme scale, efficient removal of difficult-to-process materials, special welding and coating preparation, etc.). Furthermore, the impact of the process on the performance of structural materials is difficult to test and evaluate.
In terms of general assembly and inspection, it is difficult to accurately connect components, ensure consistency of the assembly itself and detect the state of assembly (redundant items, errors, stress, etc.).
The main engine of liquid-propellant rockets originated from the application of strategic missiles, and was extensively developed under the drive of space transport systems based on launch vehicles.
The arms race between the United States of America and the Soviet Union – which started the struggle for the space race, and was certainly not the “man’s will to pure knowledge” – developed a series of ballistic missiles, and their derivative launch vehicles gave rise to the moon landing rockets. In such a context, the main engine of liquid-propellant rockets developed in an all-round manner, with a large number of research and production types and quantities, low performance and no emphasis on cost.
Propellants were mainly toxic and storable, namely liquid oxygen kerosene and later enriched liquid oxygen and liquid hydrogen. The method was mainly based on the gas generator cycle and later developed an additional high-performance combustion cycle and an expansion cycle.
Typical conventional propellant engines originated in the United States of America (Titan), and liquid oxygen, liquid hydrogen and expansion cycle kerosene engines include Thor, Delta and Saturn.
Typical conventional propellant engines developed by the Soviet Union included the Cosmos; oxygen-enriched supplementary combustion ones included the Proton, and the liquid oxygen kerosene ones included the Soyuz. France developed the Viking engine. China created the YF-20/24 engine to support the development of the CZ-2/3/4 series of conventional launch vehicles. From 1972 to 1993 high-performance engines for civil aerospace were developed.
The carrier rocket developed independently from the influence of ballistic missiles. The typical characteristics are the fact of making liquid oxygen kerosene and the liquid hydrogen of liquid oxygen non-toxic; and the fact of showing high performance relating to the additional combustion and expansion cycle to produce a high baseline thrust.
Civil space aircraft examples are the US Space Shuttle Main Engine (SSME); the former Soviet Energia and Zenit; the European Ariane and Vulcain; the Japanese LE-5 and Rich Afterburn, and the Chinese YF-75.
From 1994 to 2009 rocket-guided engines with high reliability, low cost and profiling were developed.
The international market for launch engines is booming, but the cost-effectiveness and safety of the Space Shuttle has not met expectations: high reliability, low cost and one-off modular rockets have become the focus of development.
The development of engines based on high reliability, low cost and modularisation of the propulsion system has become an important factor. Engine development and improvement in various countries are carried out based on this principle.
Giancarlo Elia Valori