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Information about Bipropellant

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Schematic of a pumped bipropellant rocket
A bipropellant rocket engine is a rocket engine that uses two fluid propellants (very often liquid propellants) which are stored in separate tanks prior to injection into, and undergo a strong exothermic reaction within, a rocket's combustion chamber. In contrast, solid rockets have single solid propellant, and hybrid rockets use a solid propellant lining the combustion chamber that reacts with an injected fluid. Because bipropellant systems permit precise mixture control, they are often more efficient than solid or hybrid rockets, but are normally more complex and expensive, particularly when turbopumps are used to pump the propellants into the chamber to save weight.

Properties of bipropellant rockets

Bipropellant rocket engines are extremely powerful rockets- they can provide the highest specific impulse (ISP) of all current Earth launchable rocket engines whilst at the same time as providing thrust to weight ratios of 70-100+, and permitting extraordinarily lightweight tankage and vehicle structure.

The highest ISP bipropellant rocket engine in existence is the hydrogen/oxygen fuelled SSME which gives very high performance; but in terms of overall performance the dense-fuelled NK-33 is comparable due to better mass ratios; in spite of lower specific impulse.

Principle of operation

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Bipropellant rockets are simple in concept but due to high temperatures and high speed moving parts, very complex in practice.
Bipropellant rockets have to introduce the propellants into the chamber at high pressure, mixing them well to give stable and thorough combustion and stop the chamber from melting.

As propellants need to leave the tanks at sufficiently high rate they are stored under pressure, normally as liquids for maximum density. Gaseous storage can be used but is rarely employed as the tanks are inevitably heavy. Liquid propellants are pressurised by a pressurant gas, either an inert one, often helium, or in some cases the vapourised propellant itself is used. Early experiments by Goddard of directly pressurising the fuel with oxidiser vapour led to frequent in-tank explosions, and this is no longer done; although sometimes a common tank is used with flexible membrane or piston to avoid mixing.

The propellants must be introduced into the combustion chamber at high pressure (typically 2 to 20 MPa (20–200 atm) and reasonably high flow (0.1-1000+ liters per second). This is achieved either via high pressure (heavy) tankage, or from lightweight, low pressure tankage through suitable pumps. The pumps used are typically turbopumps, often powered by tapping off 1-2% of the propellants or using a separate system, such as decomposed hydrogen peroxide and powering the pump via a gas turbine. The exhaust from the gas turbine is either dumped over the side, used to cool the nozzle, or placed into the combustion chamber. These turbopumps are the most complex aspect of the bipropellant system. The Space Shuttle Main Engine's turbopumps spin at over 30,000 rpm, delivering 150 lb of liquid hydrogen and 896 lb of liquid oxygen to the engine per second.[1]

Propellants are introduced to the combustion chamber through injectors. Injectors can be as simple as drilled holes with sharp edges which aim jets of liquid propellants to collide with the optimum mixture ratios. However, liquid fuels are not precisely flammable- the liquids must be first turned to gas before combustion can take place. This readily occurs within the engine, but takes longer, uses up volume in the chamber and can cause combustion instabilities. High performance rocket engines such as the Space Shuttle Main Engines take great pains to gasify the propellants before injection into the chamber. This gives more thorough, quicker and much more stable combustion; and permits the combustion chamber to be smaller and hence lighter.

The injectors' job is also to drop the pressure slightly from the propellant line feeds. This decouples the flow through the injectors from the natural variations in chamber pressure that occur during the combustion process. Failure to drop sufficient pressure in the injectors can cause oscillations in pressure in the chamber that can badly damage the engine and cause 'hard-starts' or even self disassembly of the engine during the ignition process.

The high temperature combustion products accelerate along the chamber from the injectors and then pass through the throat; and then expand out the nozzle, pressing on the inside of the nozzle, accelerating and generating an equal and opposite thrust on the rocket.

Cooling

Bipropellant rockets can use any of the standard cooling systems used by rockets. See Rocket engine cooling.

Ignition

Prompt ignition of bipropellant rocket engines at start-up to avoid hard starts is critical, particularly on manned rockets. XCOR Aerospace recommend using a choked igniter (essentially an overengineered mini rocket engine in its own right) with a pressure sensor interlock to detect the presence of an steady ignition source before introducing the propellants into the combustion chamber, together with an oxidiser lead on startup and an oxidiser lag on shutdown to empty the chamber of fuel.[2]

However the presence of these extra interlocks can reduce the reliability of achieving the mission objectives, and simply using a well-tested powerful igniter has been shown to be more effective for unmanned missions, at the cost of increased risk of catastrophic failure.[3]

Propellants

Main article: Liquid rocket propellants

Thousands of combinations of fuels and oxidizers have been tried over the years. Some of the more common and practical ones are:
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Robert Goddard and his rocket
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 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.

Small scale rocket engines

XCOR Aerospace, a California based company, is developing small scale rocket engines to power small airplanes for suborbital flights. They have tested various combination of propellants including nitrous oxide/propane, nitrous oxide/alcohol, LOX/alcohol, LOX/kerosene with success.

See also

External links

References

1. ^ Hill, P & Peterson, C.(1992) Mechanics and Thermodynamics of Propulsion. New York: Addison-Wesley ISBN 0-201-14659-2
2. ^ [1]
3. ^ Design of Liquid Propellant Rocket Engines- Huzel and Huang
rocket engine is a reaction engine that takes all its reaction mass from within tankage and forms it into a high speed jet, thereby obtaining thrust in accordance with Newton's third law.
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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.
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In thermodynamics, the word exothermic describes a process or reaction that releases energy in the form of heat. Its etymology stems from the Greek prefix ex-, meaning “outside” and the Greek word thermein, meaning “to heat”.
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solid rocket or a solid-fuel rocket is a rocket with a motor that uses solid propellants (fuel/oxidizer). The earliest rockets were solid fueled, powered by gunpowder, used by the Chinese in warfare as early as the 13th century.
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hybrid rocket propulsion system comprises propellants of two different states of matter, the most common configuration being a rocket engine composed of a solid propellant lining a combustion chamber into which a liquid or gaseous propellant is injected so as to undergo a strong
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As the name suggests, a turbopump comprises basically two main components: a pump and a driving turbine, both mounted on the same shaft.

A turbopump can refer to either of two types of pumps.
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Mass is a fundamental concept in physics, roughly corresponding to the intuitive idea of "how much matter there is in an object". Mass is a central concept of classical mechanics and related subjects, and there are several definitions of mass within the framework of relativistic
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Specific impulse (usually abbreviated Isp) is a way to describe the efficiency of rocket and jet engines. It represents the impulse (change in momentum) per unit of propellant.
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NK-33 and NK-43 were rocket engines designed and built in the 1960s for the ill-fated Russian N-1 rocket moon shot. The NK-33 engine achieves the highest thrust to weight ratio of any Earth-launchable rocket engine, whilst achieving a very high specific impulse.
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A propellant is a material that is used to move an object by applying a motive force. This may or may not involve a chemical reaction. It may be a gas, liquid, plasma, or, before the chemical reaction, a solid.
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Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.
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In English, to be inert is to be in a state of doing little or nothing.

In chemistry, the term inert is used to describe something that is not chemically active. The noble gases were described as being inert because they did not react with the other elements or themselves.
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Helium (He) is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2.
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oxidizing agent (also called an oxidant or oxidizer) is
  1. A chemical compound that readily transfers oxygen atoms or
  2. A substance that gains electrons in a redox chemical reaction.
The former definition is not applicable to what most people read about.
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In medicine, microbiology, cellular physiology and biochemistry a membrane is a thin layer that separates various cellular structures or organs. It usually includes lipid bilayer reinforced by proteins and other macromolecules, and can refer to:

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The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Young's modulus and tensile strength). It is a measure of perpendicular force per unit area i.e. equivalent to one newton per square meter or one Joule per cubic meter.
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As the name suggests, a turbopump comprises basically two main components: a pump and a driving turbine, both mounted on the same shaft.

A turbopump can refer to either of two types of pumps.
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Hydrogen peroxide (H2O2) is a very pale blue liquid which appears colourless in a dilute solution, slightly more viscous than water. It is a weak acid.
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turbine is a rotary engine that extracts energy from a fluid flow. Claude Burdin (1788-1873) coined the term from the Latin turbinis, or vortex during an 1828 engineering competition.
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A nozzle is a mechanical device designed to control the characteristics of a fluid flow as it exits (or enters) an enclosed chamber or pipe.

A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid
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As the name suggests, a turbopump comprises basically two main components: a pump and a driving turbine, both mounted on the same shaft.

A turbopump can refer to either of two types of pumps.
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injector or ejector is a pump-like device that uses the Venturi effect of a converging-diverging nozzle to convert the pressure energy of a motive fluid to velocity energy which creates a low pressure zone that draws in and entrains a suction fluid and then recompresses the
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Space Shuttle Main Engines (SSMEs) are the three main engines on the Space Shuttle orbiter. They are constructed by Pratt & Whitney's Rocketdyne Division. The SSME is also designated as the RS-24 for engineering purposes.
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explosion is a sudden increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases.

The most common artificial explosives are chemical explosives, usually involving a rapid and violent oxidation
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Newton's laws of motion are three physical laws which provide relationships between the forces acting on a body and the motion of the body, first compiled by Sir Isaac Newton.
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A hard start is a rocketry term referring to an explosion of propellant at ignition.

Rocket fuels, hypergolic or otherwise, must be mixed in the right quantities to have a controlled rate of production of hot gas.
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XCOR Aerospace is a private rocket engine and spaceflight development company based at the Mojave Spaceport in Mojave, California. XCOR was formed by former members of the Rotary Rocket rocket engine development team in September, 1999. XCOR is headed by Jeff Greason.
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The highest specific impulse chemical rockets use liquid propellants. This type of propellent has a long history going back to the first rockets and is still in use in for example the Space Shuttle and Ariane 5.
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