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Liquid propellant rocket engine technology, trends and market

A rocket can be aircraft, spacecraft, missile, or a vehicle which generates thrust. Spacecraft themselves then use their own propulsion systems to adjust their orbits around Earth, travel through space, or make carefully controlled landings on the surfaces of other planets.


The standard means of propulsion for spacecraft uses chemical rockets. Thrust is generated by propulsion system of the rocket. In a normal rocket engine we use fuel and oxidizer in a chemical reaction to create hot combustion products. The combustion process involves the oxidation of constituents in the fuel that are capable of being oxidized, and can therefore be represented by a chemical equation.


A rocket engine is a reaction engine, an engine that expels mass to generate thrust. Thrust is the force that propels a rocket or spacecraft and is measured in pounds, kilograms or Newtons. Physically speaking, it is the result of pressure which is exerted on the wall of the combustion chamber. Rockets carry all of their own working fluid, called propellant, in contrast with air-breathing engines.


Rocket engines use three states of propellants: solid, liquid, and hybrid (combination of solid and liquid propellants). In all of these cases, a chemical reaction is employed to produce a very hot, highly pressurized gas inside a combustion chamber. Solid rocket engines use solid fuels, generate high thrust, and are more reliable than other propulsion systems. However, solid rocket engines do not have the capability of a restart.


While liquid rocket engine uses liquid fuel & oxygen (or other oxidizer). The fuel & oxidizer are mixed in the combustion chamber of the liquid propulsion system. Liquid-propellant systems carry the propellant in tanks external to the combustion chamber. Most of these engines use a liquid oxidizer and a liquid fuel, which are transferred from their respective tanks by pumps. The pumps raise the pressure above the operating pressure of the engine, and the propellants are then injected into the engine in a manner that assures atomization and rapid mixing.


There are many other variables that ultimately also determine the efficiency of a rocket engine, and scientists and engineers are always looking to get more thrust and fuel efficiency out of a given design. A good parameter for the effectiveness of a rocket is  called effective exhaust velocity which is the quotient of thrust (what we want) with propellant mass flowrate (what we have to pay). Specific Impulse,  is popularly spoken of as the “gas mileage” for a rocket cycle and it fundamentally indicates how much bang for the buck you get .For a rocket with Isp = 100s a unit mass, m of propellant can generate enough thrust to support its weight in Earth’s gravity for 100 seconds or 100 times its weight for one second.


Liquid-propellant engines have certain features that make them preferable to solid systems in many applications. These features include (1) higher attainable effective exhaust velocities (ve), (2) higher mass fractions (propellant mass divided by mass of inert components), and

(3) control of operating level in flight (throttleability), sometimes including stop-and-restart capability and emergency shutdown. In liquid propulsion rocket, the flow of fuel to the engine can be controlled. In addition, the amount of thrust generated can also be regulated in the liquid propulsion rockets.

The other advantage of rocket liquid propulsion is that engine can be turned off or on as needed. Also, in some applications it is an advantage that propellant loading is delayed until shortly before launch time, a measure that the use of a liquid propellant allows.


These features tend to promote the use of liquid systems in many upper-stage applications where high ve and high propellant mass fraction are particularly important.

Liquid rocket engines are used on the Space Shuttle to place humans in orbit, on many unmanned missiles to place satellites in orbit, and on several high speed research aircraft following World War II.

Liquid systems also have been used extensively as first-stage launch vehicles for space missions, as, for example, in the Saturn (U.S.), Ariane (European), and Energia (Soviet) launch systems.


liquid-rocket propulsion system

Most liquid-propellant rockets use bipropellant systems—i.e., those in which an oxidizer and a fuel are tanked separately and mixed in the combustion chamber. The typical components of a liquid-rocket propulsion system are the engine, fuel tanks, and vehicle structure with which to hold these parts in place and connect to payload and launch pad (or vehicle).

The fuel and oxidizer tanks are usually of very lightweight construction, as they operate at low pressure. In some applications, the propellants are cryogenic (i.e., they are substances like oxygen and hydrogen that are gaseous at ambient conditions and must be tanked at extremely low temperature to be in the liquid state).

The liquid-propellant engine itself consists of a main chamber for mixing and burning the fuel and oxidizer, with the fore end occupied by fuel and oxidizer manifolds and injectors and the aft end composed of the supersonic nozzle.


Computer drawing of a liquid rocket engine with the equation for thrust. Thrust equals the exit mass flow rate times exit velocity plus exit pressure minus free stream pressure times nozzle area.

The amount of thrust produced by the rocket depends on the mass flow rate through the engine, the exit velocity of the exhaust, and the pressure at the nozzle exit. All of these variables depend on the design of the nozzle. The smallest cross-sectional area of the nozzle is called the throat of the nozzle. The hot exhaust flow is choked at the throat, which means that the Mach number is equal to 1.0 in the throat and the mass flow rate m dot is determined by the throat area. The area ratio from the throat to the exit Ae sets the exit velocity Ve and the exit pressure pe.

Integral to the main chamber is a coolant jacket through which liquid propellant (usually fuel) is circulated at rates high enough to allow the engine to operate continuously without an excessive increase of temperature in the chamber. Engine operating pressures are usually in the range 1,000–10,000 kilopascals (10–100 atmospheres). The propellants are supplied to the injector manifold at a somewhat higher pressure, usually by high-capacity turbopumps (one for the fuel and another for the oxidizer).

From the outside, a liquid-propellant engine often looks like a maze of plumbing, which connects the tanks to the pumps, carries the coolant flow to and from the cooling jackets, and conveys the pumped fluids to the injector. In addition, engines are generally mounted on gimbals so that they can be rotated a few degrees for thrust direction control, and appropriate actuators are connected between the engine (or engines) and the vehicle structure to constrain and rotate the engine.

Desirable properties for propellant combinations are low molecular mass and high temperature of reaction products (for high exhaust velocity), high density (to minimize tank weight), low hazard factor (e.g., corrosivity and toxicity), low environmental impact, and low cost.

Choices are based on trade-offs according to the applications. For example, liquid oxygen is widely used because it is a good oxidizer for a number of fuels (giving high flame temperature and low molecular mass) and because it is reasonably dense and relatively inexpensive. It is liquid only below −183 °C (−297 °F), which somewhat limits its availability, but it can be loaded into insulated tanks shortly before launch (and replenished or drained in the event of launch delays). Liquid fluorine or ozone are better oxidizers in some respects but involve more hazard and higher cost. The low temperatures of all of these systems require special design of pumps and other components, and the corrosivity, toxicity, and hazardous characteristics of fluorine and ozone have prevented their use in operational systems. Other oxidizers that have seen operational use are nitric acid (HNO3), hydrogen peroxide (H2O2), and nitrogen tetroxide (N2O4), which are liquids under ambient conditions.


Technology trends

Recently, private company SpaceX has been conducting test flights of their Starship launcher prototype. This vehicle uses a “full-flow staged combustion (FFSC) engine,” the Raptor, which burns methane for fuel and oxygen for oxidizer. Such designs were tested by the Russians in the 1960s and the US government in the 2000s, but as yet none has flown in space. The engines are much more fuel-efficient and can generate a much higher thrust-to-weight ratio than traditional designs.


3D printing technology is a key trend gaining popularity in the rocket engines market. Space organizations and private companies are designing 3d printed rocket engines to reduce costs and speed up production. For instance, in September 2020, NASA’s Rapid Analysis and Manufacturing Propulsion Technology (RAMPT) project are developing the production of an additive manufacturing methodology utilizing metal powder and lasers to 3D print rocket engine components. This technology is expected to reduce the cost of manufacturing complex combustion parts and also reduce the lead time of production. In February 2020, Skyrora, a US-based rocket start-up company, had successfully tested its 3D-printed rocket engines.



Rocket Liquid Propulsion Market

The global rocket engines market is expected to grow from $2.73 billion in 2021 to $2.81 billion in 2022 at a compound annual growth rate (CAGR) of 2.9%. The growth is mainly due to the companies rearranging their operations and recovering from the COVID-19 impact, which had earlier led to restrictive containment measures involving social distancing, remote working, and the closure of commercial activities that resulted in operational challenges. The market is expected to reach $3.60 billion in 2026 at a CAGR of 6.4%.


Rise in demand for rocket propulsion owing to increase in number of space expeditions, rise in adoption of advanced liquid propulsion engines, and surge in commercial applications of the space industry are some of the major factors that drive the growth of the rocket liquid propulsion system market. However, political insurgencies between nations and lack of measures for disposal of orbital debris are the factors which are restraining the growth of the global rocket liquid propulsion system market. On the contrary, the introduction of space tourism and maturing technology of reusable rockets are expected to further contribute in the demand for rocket liquid propulsion in the future.


Major government bodies and private companies are investing heavily on research & development for the advancement of rocket liquid propulsion systems. Recently, few companies have started to demonstrate technology of reusable rockets. For instance, NASA has demonstrated their reusable rocket (Falcon 9) in July 2019. Further, SpaceX and Blue Origin have also recently demonstrated their reusable rockets. Use of such reusable rockets can reduce cost of every launch from 500 million USD to 50 million USD. Hence, such reduction in operational cost will boost the demand of reusable rockets and thereby will contribute in the growth of the global rocket liquid propulsion market.


North America is the largest region in the rocket engines market in 2021.North America is expected to be the fastest growing region in the forecast period. In 2019, there were 102 orbital launch attempts worldwide, with 97 of them reaching orbit. By 2025 the U.S. launch rate alone will double to around 200 launches each year, even if half of the strategies are successful. Therefore, the increasing rocket launches propel the growth of the rocket engine market.


in May 2020, NASA provided a contract to the U.S.-based rocket engine manufacturer, Aerojet Rocketdyne, to develop 18 Space Launch System (SLS) RS-25 rocket engines to support its Artemis Moon missions. This cost around $1.79 billion, including the amount required to develop and test these engines. Therefore, the high investment required for producing and procuring rocket engines restraints the growth of the rocket engines market.


The RS-25 is a staged-combustion engine cycle powered by liquid hydrogen and liquid oxygen, making it one of highest performing engines the nation has ever produced. The SSME engines on the shuttle typically operated at 491,000 pounds vacuum thrust (104.5-percent of rated power level). The required power level for the RS-25 engines that will fly on the SLS is 512,000 pounds vacuum thrust (109 percent of rated power level) to augment the vehicle’s heavy lift capability. Future evolutions will have even higher thrust capabilities.


Aerojet Rocketdyne has begun developing a new generation of RS-25 engines for when the 16 engines remaining from the space shuttle program are used. These engines are targeting a 30% cost reduction from the engines that flew on the space shuttle and will feature the latest in advanced manufacturing techniques, including 3-D printing.


Key Market Players are Antrix Corporation Limited, Mitsubishi Heavy Industries Ltd., Space Exploration Technologies Corp., Aerojet Rocketdyne., Safran, BLUE ORIGIN, Virgin Galactic,


Sierra Space Advances Storable Liquid Propulsion System, reported in June 2021

Sierra Space, the new commercial space subsidiary of global aerospace and national security leader Sierra Nevada Corporation (SNC), successfully completed testing of its hypergolic, or storable, liquid rocket propulsion system for orbit transfer, maneuvering and guidance control. Sierra Space’s patented VORTEX® engine cooling technology enables a compact and highly reliable propulsion system that can be stored for long periods of time on the ground and in space.


Sierra Space’s VORTEX engine design promotes efficient, stable combustion while maintaining cool combustion chamber walls, enabling a more compact engine chamber while sustaining high-performance, and allowing rapid adaptation to multiple propellants.


The use of hypergolic propellants makes the propulsion system low-risk, extremely reliable and storable. Hypergolic propellants automatically ignite upon mixing, removing the mechanical complexity of the traditional ignition system. The new propulsion system can be scaled to suit a wide range of applications including upper stage boosters, missile systems, in-space propulsion, guidance, reaction control, extraterrestrial ascent and descent.


Sierra Space performed the hypergolic testing over a broad range of pressures and thrust levels, from 1,500 to 6,000 pounds of thrust. Other additional engines above 6,000 lbf thrust and thrusters less than 100 lbf are viable next options.



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