Expanding nozzle
Encyclopedia
The expanding nozzle is a type of rocket nozzle that, unlike traditional designs, maintains its efficiency at a wide range of altitudes. It is a member of the class of altitude compensating nozzle
s, a class that also includes the plug nozzle
and aerospike
. While the expanding nozzle is the least technically advanced and simplest to understand from a modeling point of view, it also appears to be the most difficult design to build.
In the traditional bell nozzle
the engine skirt is shaped to gradually flare out from the small-diameter exit from the combustion chamber, growing larger further from the chamber. The basic idea is to lower the pressure of the exhaust by expanding it in the nozzle, until it reaches ambient air pressure at the exit. For operations at sea level
the skirt is generally short and highly angled, at least in comparison to a skirt designed for operations in space, which are longer and more gradually shaped. This means that a rocket engine that spends any significant amount of time climbing through the atmosphere cannot be optimally shaped; as it climbs the ambient pressure changes, so the exact shape and length of the skirt would have to change in order to maintain the proper pressure. Rocket designers have to select the sweet spot
that is most appropriate to their needs, realizing that this will reduce thrust by as much as 30% at other altitudes.
The expanding nozzle addresses this to a degree by including two skirts on a single engine, one inside the other. The first skirt, attached directly to the combustion chamber
, is designed for use at lower altitudes and is short and squat. The second, sitting outside the first, fits over the lower altitude bell to extend it into a longer and narrower (measured in terms of length) bell used for higher altitudes. At liftoff the outer bell is pulled up from the inner bell, out of the way of the exhaust. As the spacecraft climbs, the outer bell is pushed back down over the inner bell to increase the thrust efficiency. Thus an expanding nozzle can have two sweet spots, which can lead to a major improvement in overall performance.
Generally simple in concept, the expanding nozzle is considerably more complex to build than it might seem. Engine bells must be cooled to avoid damage from the hot rocket exhaust, and this has presented problems in expanding nozzle designs. The cooling is normally accomplished by running either the oxidizer or fuel (in the case of LH2 fueled engines) through tubing in the bell. With the bell moving, plumbing carrying the coolant to the bell has to be flexible and this increases complexity to the extent that the advantages of the design are often considered too costly. In the case of liquid hydrogen, the fluid also has the disadvantage of being highly reactive chemically, making a variety of common flexible materials unsuitable for use in this role. It is also possible to use ablative materials in place of the active cooling systems, thereby reducing the complexity, but it appears this relatively modern technique has not been applied to the expanding nozzle technology in known systems.
The first engine to include an expanding nozzle appears to be the Rocketdyne RS-129. The RS-129 was developed in order to power a McDonnell Aircraft
boost-glide aircraft design that was entered as part of the Project ISINGLASS
(or RHEINBERRY) study looking at follow-on designs to replace the Lockheed A-12 that was just entering service. It was a liquid oxygen
/liquid hydrogen
design that used staged combustion and generated about 250,000 lbf
(1.1 MN) thrust. A related design appears to be the "HG-1", which was described as being a direct predecessor of the SSME
, although the exact relationship between the RS-129 and HG-1 is unclear. In any event, the work on the RS-129 was later enlarged to produce the SSME, which was also originally designed to use an expanding nozzle. Since these engines are fired from the point of liftoff into extra-atmospheric space flight, any sort of altitude compensation could dramatically improve their overall performance. The expanding nozzle was later abandoned in a cost-cutting phase, and today the SSME suffers a 25% loss of performance at low altitude as a result.
Glushko
has used an expanding nozzle on one design, the RD-701
tripropellant rocket
. Funding ran out with the fall of the Soviet state
, but the designers are convinced the engine has potential and have approached several parties for additional funding.
Altitude compensating nozzle
An altitude compensating nozzle is a class of rocket engine nozzles that are designed to operate efficiently across a wide range of altitudes.- Conventional designs :...
s, a class that also includes the plug nozzle
Plug nozzle
The plug nozzle is a type of nozzle which includes a centerbody or plug around which the working fluid flows. Plug nozzles have applications in aircraft, rockets, and numerous other fluid flows.-In Rockets:...
and aerospike
Aerospike engine
The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. It is a member of the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low...
. While the expanding nozzle is the least technically advanced and simplest to understand from a modeling point of view, it also appears to be the most difficult design to build.
In the traditional bell nozzle
Bell nozzle
The Bell-shaped or Contour nozzle is probably the most commonly used shaped nozzle. It has a high angle expansion section right behind the nozzle throat; this is followed by a gradual reversal of nozzle contour slope so that the nozzle exit the divergence angle is small, usually less than a 10...
the engine skirt is shaped to gradually flare out from the small-diameter exit from the combustion chamber, growing larger further from the chamber. The basic idea is to lower the pressure of the exhaust by expanding it in the nozzle, until it reaches ambient air pressure at the exit. For operations at sea level
the skirt is generally short and highly angled, at least in comparison to a skirt designed for operations in space, which are longer and more gradually shaped. This means that a rocket engine that spends any significant amount of time climbing through the atmosphere cannot be optimally shaped; as it climbs the ambient pressure changes, so the exact shape and length of the skirt would have to change in order to maintain the proper pressure. Rocket designers have to select the sweet spot
Sweet spot
Sweet spot may refer to:*Sweet spot *Sweet spot *Sweet spot...
that is most appropriate to their needs, realizing that this will reduce thrust by as much as 30% at other altitudes.
The expanding nozzle addresses this to a degree by including two skirts on a single engine, one inside the other. The first skirt, attached directly to the combustion chamber
Combustion chamber
A combustion chamber is the part of an engine in which fuel is burned.-Internal combustion engine:The hot gases produced by the combustion occupy a far greater volume than the original fuel, thus creating an increase in pressure within the limited volume of the chamber...
, is designed for use at lower altitudes and is short and squat. The second, sitting outside the first, fits over the lower altitude bell to extend it into a longer and narrower (measured in terms of length) bell used for higher altitudes. At liftoff the outer bell is pulled up from the inner bell, out of the way of the exhaust. As the spacecraft climbs, the outer bell is pushed back down over the inner bell to increase the thrust efficiency. Thus an expanding nozzle can have two sweet spots, which can lead to a major improvement in overall performance.
Generally simple in concept, the expanding nozzle is considerably more complex to build than it might seem. Engine bells must be cooled to avoid damage from the hot rocket exhaust, and this has presented problems in expanding nozzle designs. The cooling is normally accomplished by running either the oxidizer or fuel (in the case of LH2 fueled engines) through tubing in the bell. With the bell moving, plumbing carrying the coolant to the bell has to be flexible and this increases complexity to the extent that the advantages of the design are often considered too costly. In the case of liquid hydrogen, the fluid also has the disadvantage of being highly reactive chemically, making a variety of common flexible materials unsuitable for use in this role. It is also possible to use ablative materials in place of the active cooling systems, thereby reducing the complexity, but it appears this relatively modern technique has not been applied to the expanding nozzle technology in known systems.
The first engine to include an expanding nozzle appears to be the Rocketdyne RS-129. The RS-129 was developed in order to power a McDonnell Aircraft
McDonnell Aircraft
The McDonnell Aircraft Corporation was an American aerospace manufacturer based in St. Louis, Missouri. The company was founded on July 16, 1939 by James Smith McDonnell, and was best known for its military fighters, including the F-4 Phantom II, and manned spacecraft including the Mercury capsule...
boost-glide aircraft design that was entered as part of the Project ISINGLASS
Project Isinglass
|-References:NotesCitationsBibliographyFurther reading-External links :* * *...
(or RHEINBERRY) study looking at follow-on designs to replace the Lockheed A-12 that was just entering service. It was a liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...
/liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...
design that used staged combustion and generated about 250,000 lbf
Pound-force
The pound force is a unit of force in some systems of measurement including English engineering units and British gravitational units.- Definitions :...
(1.1 MN) thrust. A related design appears to be the "HG-1", which was described as being a direct predecessor of the SSME
Space Shuttle main engine
The RS-25, otherwise known as the Space Shuttle Main Engine , is a reusable liquid-fuel rocket engine built by Pratt & Whitney Rocketdyne for the Space Shuttle, running on liquid hydrogen and oxygen. Each Space Shuttle was propelled by three SSMEs mated to one powerhead...
, although the exact relationship between the RS-129 and HG-1 is unclear. In any event, the work on the RS-129 was later enlarged to produce the SSME, which was also originally designed to use an expanding nozzle. Since these engines are fired from the point of liftoff into extra-atmospheric space flight, any sort of altitude compensation could dramatically improve their overall performance. The expanding nozzle was later abandoned in a cost-cutting phase, and today the SSME suffers a 25% loss of performance at low altitude as a result.
Glushko
Valentin Glushko
Valentin Petrovich Glushko or Valentyn Petrovych Hlushko was a Soviet engineer, and the principal Soviet designer of rocket engines during the Soviet/American Space Race.-Biography:...
has used an expanding nozzle on one design, the RD-701
RD-701
RD-701 - liquid-fuel rocket engine developed by Energomash, Russia. It was proposed to propel the reusable MAKS space plane before cancellation of this project. The RD-701 is a tripropellant engine that uses staged combustion cycle afterburning of oxidizer-rich hot turbine gas...
tripropellant rocket
Tripropellant rocket
A tripropellant rocket is a rocket that uses three propellants, as opposed to the more common bipropellant rocket or monopropellant rocket designs, which use two or one fuels, respectively...
. Funding ran out with the fall of the Soviet state
History of post-Soviet Russia
With the dissolution of the Soviet Union on 29 May 1991, the Russian Federation became an independent country.Russia was the largest of the fifteen republics that made up the Soviet Union, accounting for over 60% of the gross domestic product and over 50% of the Soviet population. Russians also...
, but the designers are convinced the engine has potential and have approached several parties for additional funding.