Information about Liquid Fuel Rocket

A liquid rocket is a rocket with an engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume and hence the mass of the tankage to be relatively low, resulting in a high mass ratio. Liquid rockets have been built as monopropellant rockets using a single type of propellant, bipropellant rockets using two types of propellant, or more exotic tripropellant rockets using three types of propellant. Bipropellant liquid rockets generally use one liquid fuel and one oxidizer, such as liquid hydrogen and liquid oxygen. Liquid propellants are also sometimes used in hybrid rockets, in which they are combined with a solid or gaseous propellant.

History

The idea of liquid fuel rocket as understood in the modern context first appears in the book Исследование мировых пространств реактивными приборами (Romanisation: Issledovanie mirovh prostranst[o]v) (The Exploration of Cosmic Space by Means of Reaction Devices), by Konstantin Eduardovitch Tsiolkovsky. This seminal treatise on astronautics was published in 1903.

The only known claim to liquid propellant rocket engine experiments in the nineteenth century was made by Pedro Paulet, a Peruvian scientist.^[1] However, he did not immediately publish his work. In 1927 he wrote a letter to a newspaper in Lima, claiming he had experimented with a liquid rocket engine while he was a student in Paris three decades earlier. Historians of early rocketry experiments, among them Max Valier and Willy Ley, have given differing amounts of credence to Paulet's report. Paulet described laboratory tests of liquid rocket engines, but did not claim to have flown a liquid rocket.

The first flight of a vehicle powered by a liquid-rocket took place on March 16, 1926 at Auburn, Massachusetts, when American professor Robert H. Goddard launched a rocket which used liquid oxygen and gasoline as propellants.^[2] The rocket, which was dubbed "Nell", rose just 41 feet during a 2.5-second flight that ended in a cabbage field, but it was an important demonstration that liquid-fueled rockets were possible.

Advantages of liquid rockets

Unlike gases, a typical liquid propellant has a density similar to water, approximately 0.7-1.4g/cm³ (except liquid hydrogen which has a very much lower density), while requiring only relatively modest pressuristation to prevent vapourisation. This combination of density and low pressure permits very lightweight tankage; approximately 1% of the contents for dense propellants and around 10% for liquid hydrogen (due to its low density and the mass of the required insulation).

For injection into the combustion chamber the propellant pressure needs to be greater than the chamber pressure at the injectors; this is can be achieved with a pump. Suitable pumps usually use turbopumps due to their high power and lightweight, although reciprocating pumps have been employed in the past. Turbopumps are usually extremely lightweight and can give excellent performance; with an on-Earth weight under 1% of the thrust. Overall rocket engine thrust to weight ratios including turbopump have been as high as 133:1 with the Soviet NK-33 rocket engine.

Alternatively, a heavy tank can be used, and the pump foregone; but the delta-v that the stage can achieve is much lower due to the extra mass of the tankage reducing performance.

Liquid propellant rockets can be throttled in realtime, and have good control of mixture ratio.

Disadvantages of liquid rockets

Liquid propellants can cause a number of issues:

• because the propellant is a very large proportion of the mass of the vehicle, the center of mass shifts significantly rearward as the propellant is used; it is important that the center of mass not get too close to the center of drag of the vehicle, otherwise loss of control normally occurs.
• liquid propellants are subject to slosh, which has frequently lead to loss of control of the vehicle
• liquid propellants can leak, possibly leading to an explosive mixture forming
• turbopumps to pump liquid propellants are complex to design, and can suffer serious failure modes, such as overspeeding if they run dry or shedding fragments at high speed if metal particles from the manufacturing process enter the pump
• propellants are subject to vortexing within the tank, particularly towards the end of the burn, which can result in gas being sucked into the engine or pump
• cryogenic propellants, such as liquid oxygen freezes atmospheric water vapour into very hard crystals. This can damage or block seals and valves and can cause leaks and other failures. Avoidance of this problem often requires lengthy chilldown procedures which attempts to remove as much of the vapour from the system as possible.
• cryogenic propellants can cause ice to form on the outside of the tank, this can fall and damage the vehicle itself.

Injectors

Types of injectors

Injectors can be as simple as a number of small diameter holes arranged in carefully constructed patterns through which the fuel and oxidiser travel. The speed of the flow is determined by the square root of the pressure drop across the injectors, the shape of the hole and other details such as the density of the propellant.

The first injectors used on the V-2 created parallel jets of fuel and oxidiser which then combusted in the chamber. This gave quite poor efficiency.

Injectors today classically consist of a number of small holes which aim jets of fuel and oxidiser so that they collide at a point in space a short distance away from the injector plate. This helps to break the flow up into small droplets that burn more easily.

Other injector types include the pintle injector, which potentially permits good mixture control over a wide range of flow rates.

Combustion stability

To avoid instabilities such as chugging which is a relatively low speed oscillation the engine must be designed with enough pressure drop across the injectors to render the flow largely independent of the chamber pressure. This is normally achieved by using at least 20% of the chamber pressure across the injectors.

Nevertheless, particularly in larger engines, a high speed combustion oscillation is easily triggered, and these are not well understood. These high speed oscillations tend to disrupt the gas side boundary layer of the engine, and this can cause the cooling system to rapidly fail, destroying the engine. These kinds of oscillations are much more common on large engines, and plagued the development of the Saturn V, but were finally overcome.

Some combustion chambers, such as the SSME uses Helmholtz resonators as damping mechanisms to stop particular resonant frequencies from growing.

To prevent these issues the SSME injector design instead went to a lot of effort to vapourise the propellant prior to injection into the combustion chamber. Although many other features were used to ensure that instabilities could not occur, later research showed that these other features were unnecessary, and the gas phase combustion worked reliably.

Testing for stability often involves the use of small explosives. These are detonated within the chamber during operation, and causes an impulsive excitation. By examining the pressure trace of the chamber to determine how quickly the effects of the disturbance die away, it is possible to estimate the stability and redesign features of the chamber if required.

Curtain cooling

Injectors are also commonly laid out so that a fuel rich layer is created at the wall. This reduces the temperature there, and permits the combustion chamber to be run at higher pressure, which permits a higher expansion ratio nozzle to be used which gives a higher Isp and better system performance.

Ref^[3]

References

See also

• Solid rocket
• Liquid rocket propellants

External links

• An online book entitled ”How to Design, Build, and Test Small Liquid-Fuel Rocket Engines”