Bipropellant rocket
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Replacing NASA bipropellant Lrockth.gif with PNG version
[[Image:NASA bipropellant Lrockth.png|right|thumb|350px|Schematic of a pumped bipropellant rocket]]
A '''bipropellant rocket engine''' is a [[rocket engine]] that uses two propellants (very often [[Liquid rocket|liquid propellants]]) which are kept separately prior to reacting to form a hot gas to be used for propulsion.
In contrast, most [[solid rocket]]s have single solid propellant, and [[hybrid rocket]]s use a solid propellant lining the combustion chamber that reacts with an injected fluid. Because liquid 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 [[turbopump]]s are used to pump the propellants into the chamber to save [[mass|weight]].
==Properties of bipropellant rockets==
Bipropellant rocket engines are extremely powerful rockets- they can provide the highest [[specific impulse]] (ISP) of all [[as of 2006|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==
[[Image:NASA bipropellant GPN-2000-000548.jpg|right|thumb|350px|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 [[propellant]]s 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 [[pascals|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 [[turbopump]]s, 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 [[turbopump]]s 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.<ref>Hill, P & Peterson, C.(1992) Mechanics and Thermodynamics of Propulsion. New York: Addison-Wesley ISBN 0-201-14659-2</ref>
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 [[explosion|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 [[Newton's Third Law|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|Rocket engine cooling]].
==Ignition==
Prompt ignition of bipropellant rocket engines at start-up to avoid [[hard start]]s 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 a 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.<ref>[http://www.xcor.com/products/vehicles/ez-rocket_faq.html XCOR Aerospace: EZ-Rocket FAQ<!-- Bot generated title -->]</ref>
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.<ref>Design of Liquid Propellant Rocket Engines- Huzel and Huang</ref>
==Propellants==
[[Liquid rocket 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:
* liquid oxygen ([[LOX]], O<sub>2</sub>) and liquid [[hydrogen]] ([[LH2]], H<sub>2</sub>) - [[Space Shuttle]] main engines, [[Ariane 5]] main stage and the Ariane 5 ECA second stage, the first stage of the [[Delta IV rocket | Delta IV]], the upper stages of the [[Saturn V (rocket)|Saturn V]], [[Saturn IB (rocket)|Saturn IB]], and [[Saturn I (rocket)|Saturn I]] as well as [[Centaur (rocket stage)|Centaur]] rocket stage
* liquid oxygen (LOX) and [[kerosene]] or [[RP-1]] - [[Saturn V (rocket)|Saturn V]], [[Zenit rocket]], [[R-7 Semyorka]] family of [[Soviet Union|Soviet]] boosters which includes [[Soyuz launch vehicle|Soyuz]], [[Delta rocket|Delta]], [[Saturn I (rocket)|Saturn I]], and [[Saturn IB (rocket)|Saturn IB]] first stages, [[Titan (rocket family)|Titan I]] and [[Atlas rocket]]s
* liquid oxygen (LOX) and alcohol ([[ethanol]], C<sub>2</sub>H<sub>5</sub>OH) - early liquid fueled rockets, like [[Germany|German]] ([[World War II|WW2]]) [[A4 (rocket)|A-4]], aka V-2, and [[Redstone (rocket)|Redstone]]
* liquid oxygen (LOX) and [[gasoline]] - [[Robert Goddard (scientist)|Robert Goddard]]'s first liquid-fuel rocket
* [[T-Stoff]] (80% hydrogen peroxide, H<sub>2</sub>O<sub>2</sub> as the oxidizer) and [[C-Stoff]] (methanol, CH<sub>3</sub>OH, and hydrazine hydrate, N<sub>2</sub>H<sub>4</sub>•''n''(H<sub>2</sub>O as the fuel) - Walter Werke HWK 109-509 engine used on [[Messerschmitt Me 163]]B Komet a rocket fighterplane of ([[World War II|WW2]])
* [[nitric acid]] (HNO<sub>3</sub>) and kerosene - [[Soviet Union|Soviet]] [[Scud]]-A, aka [[SS-1]]
* inhibited red fuming nitric acid (I[[RFNA]], HNO<sub>3</sub> + N<sub>2</sub>O<sub>4</sub>) and unsymmetric dimethyl hydrazine ([[UDMH]], (CH<sub>3</sub>)<sub>2</sub>N<sub>2</sub>H<sub>2</sub>) Soviet [[Scud]]-B,-C,-D, aka [[SS-1]]-c,-d,-e
* nitric acid 73% with [[dinitrogen tetroxide]] 27% (=AK27) and kerosene/gasoline mixture - various Russian (USSR) cold-war ballistic missiles, [[Iran]]: [[Shahab]]-5, [[North Korea]]: [[Taepodong-2]]
* hydrogen peroxide and kerosene - [[United Kingdom|UK]] (1970s) [[Black Arrow]], [[United States|USA]] Development (or study): [[BA-3200]]
* [[hydrazine]] (N<sub>2</sub>H<sub>4</sub>) and red fuming nitric acid - Nike Ajax Antiaircraft Rocket
* [[Aerozine 50]] and dinitrogen tetroxide - [[Titan (rocket family)|Titans 2–4]], Apollo [[lunar module]], Apollo [[service module]], interplanatary probes (Such as [[Voyager 1]] and [[Voyager 2]])
* Unsymmetric dimethylhydrazine (UDMH) and dinitrogen tetroxide - [[Proton rocket]] and various Soviet rockets
* [[monomethylhydrazine]] ([[MMH]], (CH<sub>3</sub>)HN<sub>2</sub>H<sub>2</sub>) and dinitrogen tetroxide - [[Space Shuttle]] [[Orbital maneuvering system]] (OMS) engines
[[Image:Goddard and Rocket.jpg|right|thumb|150px|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 [[ICBM]]s 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 rocket]]s have recently been the vehicle of choice for low-budget private and academic developments in aerospace technology.
==Small scale rocket engines==
[http://www.xcor.com XCOR Aerospace], a California based company, is developing small scale rocket engines to power small airplanes for [[suborbital flight]]s.
They have tested various combination of propellants including nitrous oxide/propane, nitrous oxide/alcohol, LOX/alcohol, LOX/kerosene with success.
On April 12, 2008, California State University, Long Beach and industry partner Garvey Spacecraft Corporation jointly conducted a flight test of the [http://www.csulb.edu/colleges/coe/mae/views/projects/rocket/news_2008/flight_4_12_2008.shtml Prospector 14] launch vehicle that featured a 1,000 lbf-thrust LOX/methane rocket engine developed by CSULB students. It is believed that this is the first such powered flight of this cryogenic propellant combination.
==See also==
* [[spacecraft propulsion]]
* [[Liquid rocket]]
* [[tripropellant rocket]]
* [[hypergolic]] rocket fuels
* [[Rocket engine nozzles]]
==External links==
*[http://www.permanent.com/t-mikesc.htm Fuel Propellants - Storable, and Hypergolic vs. Ignitable]
==References==
{{reflist}}
[[Category:Industrial design]]
[[Category:Spacecraft propulsion]]