A liquid-fuel rocket or 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 tanks 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 liquid 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.
The idea of liquid rocket as understood in the modern context first appears in the book Исследование мировых пространств реактивными приборами (Transliteration: Issledovaniye mirovykh prostranstv reaktivnymi priborami) (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 a Peruvian scientist named Pedro Paulet. 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. 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 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 much lower density), while requiring only relatively modest pressure 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 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 well under 1% of the thrust. Indeed, overall rocket engine thrust to weight ratios including a 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 often much lower due to the extra mass of the tankage reducing performance; but for high altitude or vacuum use the tankage mass can be acceptable.
Liquid propellant rockets can be throttled in realtime, and have good control of mixture ratio; they can also be shutdown without any great drama, and, with a suitable ignition system, restarted. They can also employ regenerative cooling which uses the fuel (or occasionally the oxidiser) to cool the chamber prior to injection.
Disadvantages of liquid rockets
Use of liquid propellants can be associated with a number of issues:
Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:
One of the most efficient mixtures, oxygen and hydrogen, suffers from the extremely low temperatures required for storing hydrogen and oxygen as liquids (around 20 K or −253 °C)) and low fuel density (70 kg/m³), necessitating large and heavy tanks. The use of lightweight foam to insulate the cryogenic tanks caused problems for the Space Shuttle Columbia's STS-107 mission, as a piece broke loose, damaged its wing and caused it to break up and be destroyed on atmospheric reentry.
For storable ICBMs and interplanetary spacecraft, storing cryogenic propellants over extended periods is awkward and expensive. Because of this, mixtures of hydrazine and its derivatives in combination with nitrogen oxides are generally used for such rockets. Hydrazine has its own disadvantages, being a very caustic and volatile chemical as well as being toxic. Consequently, hybrid rockets have recently been the vehicle of choice for low-budget private and academic developments in aerospace technology. Also the RP-1/LOX combination has become a popular choice for reliable and cost-sensitive commercial spaceflight applications.
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.
The main type of injectors are
Other injector types include the pintle injector, which potentially permits good mixture control over a wide range of flow rates. The pintle injector was used on the Apollo Lunar Module engines and the Merlin and Kestrel engines designed by SpaceX.
The Space Shuttle Main Engine uses a system of fluted posts, which use the relative 'warmth' of the liquid oxygen to vapourise the liquid hydrogen and this improves the rate and stability of the combustion process; previous engines such as the F-1 used for the Apollo program had significant issues with oscillations that led to destruction of the engines, but this was not a problem in the SSME due to this design detail.
Valentin Glushko invented the centrifugal injector in the early 1930s, and it has been almost universally used in Russian engines. Rotational motion is applied to the liquid (and sometimes the two propellants are mixed), then it is expelled through a small hole, where it forms a cone-shaped sheet that rapidly atomizes.
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.
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.
Injectors are commonly laid out so that a fuel rich layer is created at the combustion chamber wall. This reduces the temperature there, and downstream to the throat and even into the nozzle 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. Regenerative cooling is also often used in liquid rocket engines.
Ignition can be performed in many ways, but perhaps more so with liquid propellants than other rockets a consistent and significant ignitions source is required; failure to ignite within (in some cases as small as) a few tens of milliseconds can cause overpressure of the chamber due to excess propellant. A hard start can even cause an engine to explode.
Generally, ignition systems try to apply flames across the injector surface, with a mass flow of approximately 1% of the full mass flow of the chamber.
Safety interlocks are sometimes used to ensure the presence of an ignition source before the main valves open; however reliability of the interlocks can in some cases be lower than the ignition system. Thus it depends on whether the system must fail safe, or whether overall mission success is more important. Interlocks are rarely used for upper, unmanned stages where failure of the interlock would cause loss of mission, but are present on the SSME, to shut the engines down prior to liftoff of the Space Shuttle. In addition, detection of successful ignition of the igniter is surprisingly difficult, some systems use thin wires that are cut by the flames, pressure sensors have also seen some use.
Methods of ignition include pyrotechnic, electrical (spark or hot wire), and chemical. Hypergolic propellants have the advantage of self igniting, reliably and with less chance of hard starts. In the 1940s, the Russians began to start engines with hypergolic fuel, then switch over to the primary propellants after ignition.
Published - July 2009
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