RS-68 being tested at NASA's Stennis Space Center. The Rocketdyne RS-68 (Rocket System 68 is a Liquid hydrogen / Liquid oxygen engine developed starting in the 1990s with the goal of producing a simpler The nearly transparent exhaust is due to this engine's exhaust being mostly superheated steam (water vapor from its propellants, hydrogen and oxygen)

A rocket engine is a jet engine[1] that takes all its reaction mass ("propellant") from within tankage or the body of a vehicle and forms it into a high speed jet, thereby obtaining thrust in accordance with Newton's third law. specific --->A jet engine is a Reaction engine that discharges a fast moving jet of Fluid to A propellant is a material that is used to move ("propel" an object A rocket or rocket vehicle is a Missile, Aircraft or other Vehicle which obtains Thrust by the reaction of the See also Jet (nozzle A jet is a coherent stream of fluid that is projected into a surrounding medium usually from some kind of a nozzle or aperture Newton's laws of motion are three Physical laws which provide relationships between the Forces acting on a body and the motion of the Rocket engines can be used for spacecraft propulsion as well as terrestrial uses, such as missiles. A rocket or rocket vehicle is a Missile, Aircraft or other Vehicle which obtains Thrust by the reaction of the Spacecraft propulsion is any method used to change the velocity of Spacecraft and artificial Satellites There are many different methods A missile (see also pronunciation differences) is a self-propelled explosive Projectile used as a weapon towards a target Most rocket engines are internal combustion engines, although non combusting forms also exist. The internal combustion engine is an engine in which the Combustion of Fuel and an Oxidizer (typically air occurs in a confined space called a

## Principle of operation

Rocket engines give part of their thrust due to unopposed pressure on the combustion chamber

Most rocket engines produce thrust by the expulsion of a high-temperature, high-speed gaseous exhaust. This is typically created by high pressure (10-200 bar) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber. Rocket propellant is mass that is stored usually in some form of Propellant tank prior to being used as the propulsive mass that is ejected from a rocket engine in the form Fuel is any material that is burned or altered in order to obtain energy An oxidizing agent or oxidising agent (also called an oxidant, oxidizer or oxidiser) can be defined as either a Chemical compound A combustion chamber is the part of an Engine in which Fuel is burned

### Introducing propellant into the combustion chamber

Liquid-fueled rockets typically pump separate fuel and oxidiser components into the combustion chamber, where they mix and burn. A liquid rocket is a Rocket with an engine that uses Propellants in Liquid form Solid rocket propellants are prepared as a mixture of fuel and oxidizing components and the propellant storage chamber becomes the combustion chamber. A solid rocket or a solid-fuel rocket is a Rocket with a motor that uses solid propellants ( Fuel / Oxidizer) Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. A 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 Alternatively, a chemically inert reaction mass can be heated using a high-energy power source. Working mass is a mass against which a system operates in order to produce Acceleration.

### Combustion chamber

For chemical rockets the combustion chamber is typically just a cylinder. The dimensions of the cylinder is such that the propellant is able to combust thoroughly; different propellants require different combustion chamber sizes for this to occur. This leads to a number called L * :

$L* = \frac {V_c} {A_t}$

where:

Vc is the Volume of the chamber
At is the area of the throat

L* is typically in the range of 25-60 inches (0. 6 - 1. 5 m)

### Rocket nozzles

Main article: Rocket engine nozzle

The hot gas produced is permitted to escape from the combustion chamber through an opening (the "throat"), in a high expansion-ratio 'de Laval nozzle'. A rocket engine nozzle is a Propelling nozzle usually of the de Laval type used in a Rocket engine to expand and accelerate the Combustion A rocket engine nozzle is a Propelling nozzle usually of the de Laval type used in a Rocket engine to expand and accelerate the Combustion The nozzle dramatically accelerates the gas, converting most of the thermal energy into kinetic energy. The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape. Exhaust speeds as high as ten times the speed of sound at sea level are not uncommon. Sound is a vibration that travels through an elastic medium as a Wave.

Rocket thrust is caused by pressures acting in the combustion chamber and nozzle. From Newtons third law, equal and opposite pressures act on the exhaust, and this accelerates it to high speeds.

A portion of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber but the majority comes from the pressures against the inside of the nozzle (see diagram). As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other. This article covers adiabatic processes in Thermodynamics. For adiabatic processes in Quantum mechanics, see Adiabatic process (quantum mechanics

### Propellant efficiency

For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created by a specific amount of propellant acting on the chamber and nozzle. This can be achieved by all of:

• heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and sometimes metals such as aluminium, or even using nuclear energy)
• using a low specific density gas (as hydrogen rich as possible)
• using propellants which are, or decompose to, simple molecules with few degrees of freedom so as to maximise molecular speeds

Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the amount of propellant present to be accelerated as it pushes on the engine, and since from Newton's third law the pressure that acts on the engine also reciprocally acts on the propellant, it turns out that the speed that the propellant leaves the chamber is unaffected by the chamber pressure (although the thrust is proportional). However, speed is significantly affected by all three of the above factors and the exhaust speed is an excellent measure of the engine propellant efficiency.

For aerodynamic reasons the flow goes sonic ("chokes") at the narrowest part of the nozzle, the 'throat'. Choked flow of a fluid is a fluid dynamic condition caused by the Venturi effect. Since the speed of sound in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. Sound is a vibration that travels through an elastic medium as a Wave. By comparison, at room temperature the speed of sound in air is about 340m/s while the speed of sound in the hot gas of a rocket engine can be over 1700m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air.

Expansion in the rocket nozzle then further multiplies the speed, typically between 1. 5 and 2 times, giving a highly collimated hypersonic exhaust jet. Collimated light is Light whose rays are nearly parallel and therefore will spread slowly as it propagates The speed increase of a rocket nozzle is mostly determined by its area expansion ratio—the ratio of the area of the throat to the area at the exit, but detailed properties of the gas are also important. Larger ratio nozzles are more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity.

Nozzle efficiency is affected by operation in the atmosphere because atmospheric pressure changes with altitude; but due to the supersonic speeds of the gas exiting from a rocket engine, the pressure of the jet may be either below or above ambient, and equilibrium between the two is not reached.

### Back pressure and optimal expansion

For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure; if lower the vehicle will be slowed by the difference in pressure between the top of the engine and the exit, if higher then this represents pressure that the bell has not turned into thrust.

To maintain this ideal the diameter of the nozzle would need to increase with altitude, giving the pressure a longer nozzle to act on (and reducing the exit pressure and temperature). This increase is difficult to arrange. A compromise nozzle is generally used and some reduction in performance occurs. To improve on this, various exotic nozzle designs such as the plug nozzle, stepped nozzles, the expanding nozzle and the aerospike have been proposed, each having some way to adapt to changing ambient air pressure and each allowing the gas to expand further against the nozzle, giving extra thrust at higher altitude. The plug nozzle is a type of Rocket engine nozzle that unlike traditional designs maintains its efficiency at a wide range of altitudes The expanding nozzle is a type of Rocket nozzle that unlike traditional designs maintains its efficiency at a wide range of altitudes 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

## Overall rocket engine performance

Rocket technology can combine very high thrust (meganewtons), very high exhaust speeds (around 10 times the speed of sound at sea level) and very high thrust/weight ratios (>100) simultaneously as well as being able to operate outside the atmosphere. The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical

Rockets can be further optimised to even more extreme performance along one or more of these axes at the expense of the others.

### Specific impulse

Main article: Specific impulse

The most important metric for the efficiency of a rocket engine is impulse per unit of propellant, this is called specific impulse (usually written Isp). Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines In Classical mechanics, an impulse is defined as the Integral of a Force with respect to Time: \mathbf{I} = \int \mathbf{F}\ A propellant is a material that is used to move ("propel" an object Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines This is either measured as a speed (Ve in metres/second or ft/s) or as a time (seconds). An engine that gives a large specific impulse is normally highly desirable.

### Net thrust

Below is an approximate equation for calculating the net thrust of a rocket engine:

$F_n = \dot{m}\;V_{e} = \dot{m}\;V_{e-act} + A_{e}(P_{e} - P_{amb})$[2]

where:

$\dot{m} = \,$exhaust gas mass flow
$V_{e} =\,$effective exhaust velocity
$V_{e-act} =\,$actual jet velocity at nozzle exit plane
$A_{e} =\,$flow area at nozzle exit plane
$P_{e} =\,$static pressure at nozzle exit plane
$P_{amb} =\,$ambient (or atmospheric) pressure

Since, unlike a jet engine, a conventional rocket motor lacks an air intake, there is no 'ram drag' to deduct from the gross thrust. Consequently the net thrust of a rocket motor is equal to the gross thrust (apart from static back pressure).

The $\dot{m}\;V_{e-act}\,$ term represents the momentum thrust, which remains constant at a given throttle setting, whereas the $A_{e}(P_{e} - P_{amb})\,$ term represents the pressure thrust term. At full throttle, the net thrust of a rocket motor improves slightly with increasing altitude, because the reducing atmospheric pressure increases the pressure thrust term.

If the pressure of the exhaust jet varies from atmospheric pressure, nozzles can be said to be underexpanded, ambient or overexpanded. If under or overexpanded then loss of efficiency occurs, grossly overexpanded nozzles lose less efficiency, but the exhaust jet is usually unstable. Rockets become progressively more underexpanded as they gain altitude. Note that almost all rocket engines will be momentarily grossly overexpanded during startup in an atmosphere. [3]

### Vacuum Isp

Due to the specific impulse varying with pressure, a quantity that is easy to compare and calculate with is useful. Because rockets choke at the throat, and because the supersonic exhaust prevents external pressure influences travelling upstream, it turns out that the pressure at the exit is ideally exactly proportional to the propellant flow $\dot{m}$, provided the mixture ratios and combustion efficiencies are maintained. Choked flow of a fluid is a fluid dynamic condition caused by the Venturi effect. It is thus quite usual to rearrange the above equation slightly:

$Fvac = C_f \dot{m} c^*$[4]

and so define the vacuum Isp to be:

Vevac = Cfc *

Where:

$c^* =\,$ the speed of sound constant at the throat
$C_f =\,$ the thrust coefficient constant of the nozzle (typically between 0. 8 and 1. 9)

And hence:

$F_n = \dot{m} V_{evac} - A_{e} P_{amb}$

### Throttling

Rockets can be throttled by controlling the propellant rate $\dot{m}$ (usually measured in kg/s or lb/s).

In principle rockets can be throttled down to an exit pressure of about one-third of ambient pressure (due to flow separation in nozzles) and up to a maximum limit determined only be the mechanical strength of the engine.

In practice, the degree to which rockets can be throttled varies greatly, but most rockets may be throttled by a factor of 2 without great difficulty; the typical limitation is combustion stability, as for example, injectors need a minimum pressure to avoid triggering damaging oscillations (chugging or combustion instabilities); but injectors can often be optimised and tested for wider ranges. Additionally, it is important that the exit pressure not be too far below ambient to avoid flow separation in the nozzle.

### Energy efficiency

Rocket energy efficiency as a function of vehicle speed divided by effective exhaust speed

Rocket engine nozzles are surprisingly efficient heat engines for generating a high speed jet, as a consequence of the high combustion temperature and high compression ratio in accordance with the carnot cycle. A heat engine is a physical or theoretical device that converts Thermal energy to mechanical output The compression ratio is a single number that can be used to predict the performance of any engine particularly piston engines (but can be used on essentially any Internal-combustion The Carnot cycle is a particular Thermodynamic cycle, modeled on the hypothetical Carnot heat engine, proposed by Nicolas Léonard Sadi Carnot in 1824 and For a vehicle employing a rocket engine the energetic efficiency is very good if the vehicle speed approaches or somewhat exceeds the exhaust velocity (relative to launch); but at low speeds the efficiency asymptotically approaches 0% at zero speed (as with all jet propulsion. specific --->A jet engine is a Reaction engine that discharges a fast moving jet of Fluid to ) See Rocket energy efficiency for more details. A rocket or rocket vehicle is a Missile, Aircraft or other Vehicle which obtains Thrust by the reaction of the

## Cooling

The reaction mass's combustion temperatures can typically reach ~3500 K (~5800 °F) which is often far higher than the melting point of the nozzle and combustion chamber materials, two exceptions are graphite and tungsten (~1200 K for copper). The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 Indeed many construction materials can make perfectly acceptable propellants in their own right. It is important that these materials be prevented from combusting, melting or vapourising to the point of failure. Materials technology could potentially place an upper limit on the exhaust temperature of chemical rockets.

Alternatively, rockets may use more common construction materials such as aluminum, steel, nickel or copper alloys and employ cooling systems that prevent the construction material itself becoming too hot. Regenerative cooling, where the propellant is passed through tubes around the combustion chamber or nozzle, and other techniques, such as curtain cooling or film cooling, are employed to give longer nozzle and chamber life. Regenerative cooling is a method of Cooling Gases in which compressed gas is cooled by allowing it to expand and and thereby taking heat from the surroundings the These techniques ensure that a gaseous thermal boundary layer touching the material is kept below the temperature which would cause the material to catastrophically fail. In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface

In rockets, the heat fluxes that can pass through the wall are among the highest in engineering, fluxes are generally in the range of 1-200 MW/m^2. The strongest heat fluxes are found at the throat, which often sees twice that found in the the associated chamber and nozzle. This is due to the combination of high speeds (which gives a very thin boundary layer), and the high temperatures seen there.

The coolant methods include:

1. uncooled (used for short runs mainly during testing)
2. ablative walls (walls are lined with a material that is continuously vapourised and carried away). Ablation is defined as the removal of material from the surface of an object by Vaporization, Chipping, or other erosive processes
3. radiative cooling (the chamber becomes almost white hot and radiates the heat away)
4. dump cooling (a propellant, usually hydrogen, is passed around the chamber and dumped)
5. regenerative cooling (uses the propellant to cool the chamber via a cooling jacket before being injected)
6. curtain cooling (propellant injection is arranged so the temperature of the gases is cooler at the walls)
7. film cooling (surfaces are wetted with liquid propellant, which cools as it evaporates)

In all cases the cooling effect that prevents the wall from being destroyed is caused by a thin layer of insulating fluid (a boundary layer) that is in contact with the walls that is far cooler than the combustion temperature. Radiative cooling is the condition in which a body loses more energy by radiation than it gains from its surroundings Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Regenerative cooling is a method of Cooling Gases in which compressed gas is cooled by allowing it to expand and and thereby taking heat from the surroundings the In Physics and Fluid mechanics, a boundary layer is that layer of Fluid in the immediate vicinity of a bounding surface Provided this boundary layer is intact the wall will not be damaged.

Disruption of the boundary layer may occur during cooling failures or combustion instabilities, and wall failure typically occurs soon after.

With regenerative cooling a second boundary layer is found in the coolant channels around the chamber. This boundary layer thickness needs to be as small as possible, since the boundary layer acts as an insulator between the wall and the coolant. This may be achieved by making the coolant velocity in the channels as high as possible. In Physics, velocity is defined as the rate of change of Position.

## Mechanical issues

Rocket combustion chambers are normally operated at fairly high pressure, typically 10-200 bar (1 to 20 MPa), higher pressures often give better performance (by permitting a more efficient nozzle to be fitted). This causes the outermost part of the chamber to be under very large hoop stresses. Hoop stress is mechanical stress defined for rotationally-symmetric objects being the result of forces acting circumferentially (perpendicular both to the axis and to the radius

Worse, due to the high temperatures created in rocket engines the materials used tend to have a significantly lowered working tensile strength.

## Acoustic issues

In addition, the extreme vibration and acoustic environment inside a rocket motor commonly results in peak stresses well above mean values, especially in the presence of organ pipe-like resonances and gas turbulence. An organ pipe is a sound-producing element of the Pipe organ that resonates at a specific pitch when pressurized air (commonly referred to as wind

### Combustion

Three different types of combustion instabilities occur

Chugging

This is a low frequency oscillation at a few Hertz in chamber pressure usually caused by interaction between acceleration and chamber pressure. This causes cyclic variation in thrust, and the effects can vary from merely annoying to actually damaging the payload or vehicle. Chugging can be minimised by using gas filled damping tubes on feed lines of high density propellant's.

Buzzing

This can be caused due to insufficient pressure drop across the injectors. It generally is mostly annoying, rather than being damaging. However, in extreme cases combustion can end up being forced backwards through the injectors- this can cause explosions with monopropellants.

Screeching

This the most immediately damaging, and the hardest to control. It is due to acoustics within the combustion chamber that often couples to the chemical combustion processes that are the primary drivers of the energy release, and can lead to unstable resonant "screeching" that commonly leads to catastrophic failure due to thinning of the insulating thermal boundary layer[5]. Such effects are very difficult to predict analytically during the design process, and have usually been addressed by expensive and time consuming extensive testing, combined with trial and error remedial correction measures.

Screeching is often dealt with by detailed changes to injectors, or changes in the propellant chemistry, or vaporizing the propellant before injection of the use of Helmholtz dampers within the combustion chambers to change the resonant modes of the chamber. Helmholtz resonance is the phenomenon of air Resonance in a cavity

Testing for the possibility of screeching is sometimes done by exploding small explosive charges in the combustion chamber to determine the engine's impulse response and then evaluating the time response of the chamber pressure- a fast recovery indicates a stable system. The impulse response of a system is its output when presented with a very brief input signal an impulse

### Exhaust noise

For all but the very smallest sizes, rocket exhaust compared to other engines is generally very noisy indeed. As the hypersonic exhaust mixes with the ambient air, shock waves are formed. In Aerodynamics, hypersonic speeds are speeds that are highly Supersonic. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called The sound intensity from these shock waves depends on the size of the rocket, and on large rockets could potentially kill at close range. The sound intensity, I, (acoustic intensity is defined as the Sound power Pac per unit area A. [6]

The Saturn V launch was detectable on seismometers a considerable distance from the launch site. The Saturn V (pronounced 'Saturn Five' popularly known as the Moon Rocket was a multistage liquid-fuel expendable Rocket used by NASA 's Seismometers (from Greek Seism - "the shakes" - and Metro - "I measure" are instruments that measure and record motions of the ground including The sound intensity from the shock waves generated depends on the size of the rocket and on the exhaust velocity. The sound intensity, I, (acoustic intensity is defined as the Sound power Pac per unit area A. Such shock waves seem to account for the characteristic crackling and popping sounds produced by large rocket engines when heard live. These noise peaks typically overload microphones and audio electronics, and so are generally weakened or entirely absent in recorded or broadcast audio reproductions. For large rockets the acoustic effects could actually kill. The Space Shuttle generates over 200 dB(A) of noise around its base. NASA 's Space Shuttle, officially called the Space Transportation System ( STS) is the Spacecraft currently used by the United States

Generally speaking noise is most intense when a rocket is close to the ground, since the noise from the engines radiates up away from the plume, as well as reflecting off the ground. Also, when the vehicle is moving slowly, little of the chemical energy input to the engine can go into increasing the kinetic energy of the rocket (since useful power P transmitted to the vehicle is P = F * V for thrust F and speed V). Then the largest portion of the energy is dissipated in the exhaust's interaction with the ambient air, producing noise. This noise can be reduced somewhat by flame trenches with roofs, by water injection around the plume and by deflecting the plume at an angle.

## Testing

Rocket engines are usually statically tested at a test facility before being put into production. Rocket Engine Test Facility was the name of a facility at the NASA Glenn Research Center, formerly known as the Lewis Research Center, in Ohio For high altitude engines, either a shorter nozzle must be used, or the rocket must be tested in a large vacuum chamber.

## Safety

Rockets have a reputation for unreliability and danger; especially catastrophic failures. A rocket or rocket vehicle is a Missile, Aircraft or other Vehicle which obtains Thrust by the reaction of the Contrary to this reputation, carefully designed rockets can be made arbitrarily reliable. In military use, rockets are not unreliable. However, one of the main non-military uses of rockets is for orbital launch. In this application, the premium is on minimum weight, and it is difficult to achieve high reliability and low weight simultaneously. In addition, if the number of flights launched is low, there is a very high chance of a design, operations or manufacturing error causing destruction of the vehicle. Essentially all launch vehicles are test vehicles by normal aerospace standards (as of 2006). Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar.

The X-15 rocket plane achieved a 0.5% failure rate, with a single catastrophic failure during ground test, and the SSME has managed to avoid catastrophic failures in over 350 engine-flights. Albert Scott Crossfield (October 2 1921 – April 19 2006 normally known as Scott Crossfield, was an American naval officer and Test pilot SSME redirect here For the services field see Service Science Management and Engineering The Space Shuttle Main Engines ( SSMEs

## Chemistry

Rocket propellants require a high specific energy (energy per unit mass), because ideally all the reaction energy appears as kinetic energy of the exhaust gases, and exhaust velocity is the single most important performance parameter of an engine, on which vehicle performance depends. Rocket propellant is mass that is stored usually in some form of Propellant tank prior to being used as the propulsive mass that is ejected from a rocket engine in the form

Aside from inevitable losses and imperfections in the engine, incomplete combustion, etc. , after specific reaction energy, the main theoretical limit reducing the exhaust velocity obtained is that, according to the laws of thermodynamics, a fraction of the chemical energy may go into rotation of the exhaust molecules, where it is unavailable for producing thrust. Monatomic gases like helium have only three degrees of freedom, corresponding to the three dimensions of space, {x,y,z}, and only such spherically symmetric molecules escape this kind of loss. A diatomic molecule like H2 can rotate about either of the two axes perpendicular to the one joining the two atoms, and as the equipartition law of statistical mechanics demands that the available thermal energy be divided equally among the degrees of freedom, for such a gas in thermal equilibrium 3/5 of the energy can go into unidirectional motion, and 2/5 into rotation. In classical Statistical mechanics, the equipartition theorem is a general formula that relates the Temperature of a system with its average energies A triatomic molecule like water has six degrees of freedom, so the energy is divided equally among rotational and translational degrees of freedom. For most chemical reactions the latter situation is the case. This issue is traditionally described in terms of the ratio, gamma, of the specific heat of the gas at constant volume to that at constant pressure. The rotational energy loss is largely recovered in practice if the expansion nozzle is large enough to allow the gases to expand and cool sufficiently, the function of the nozzle being to convert the random thermal motions of the molecules in the combustion chamber into the unidirectional translation that produces thrust. As long as the exhaust gas remains in equilibrium as it expands, the initial rotational energy will be largely returned to translation in the nozzle.

Although the specific reaction energy per unit mass of reactants is key, low mean molecular weight in the reaction products is also important in practice in determining exhaust velocity. This is because the high gas temperatures in rocket engines pose serious problems for the engineering of survivable motors. Because temperature is proportional to the mean energy per molecule, a given amount of energy distributed among more molecules of lower mass permits a higher exhaust velocity at a given temperature. This means low atomic mass elements are favored. Liquid hydrogen (LH2) and oxygen (LOX, or LO2), are the most effective propellants in terms of exhaust velocity that have been widely used to date, though a few exotic combinations involving boron or liquid ozone are potentially somewhat better in theory if various practical problems could be solved[7].

It is important to note in computing the specific reaction energy, that the entire mass of the propellants, including both fuel and oxidizer, must be included. The fact that air-breathing engines are typically able to obtain oxygen "for free" without having to carry it along, accounts for one factor of why air-breathing engines are very much more propellant-mass efficient, and one reason that rocket engines are far less suitable for most ordinary terrestrial applications. Fuels for automobile or turbojet engines, utilize atmospheric oxygen and and so have a much better effective energy output per unit mass of fuel that must be carried. TurboJET (噴射飛航 is the brand name for the operations of the Hong Kong -based Shun Tak-China Travel Ship Management Limited (信德中旅船務管理有限公司

Computer programs that predict the performance of propellants in rocket engines are available. [8].

## Ignition

With liquid and hybrid rockets, immediate ignition of the propellant(s) as they first enter the combustion chamber is essential.

With liquid propellants (but not gaseous), failure to ignite within milliseconds usually causes too much liquid propellant to be within the chamber, and if/when ignition occurs the amount of hot gas created will often exceed the maximum design pressure of the chamber. The pressure vessel will often fail catastrophically. This is sometimes called a Hard start. A hard start is a Rocketry term referring to an explosion of a rocket engine at ignition

Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, the propellants can ignite spontaneously on contact (hypergolic), a plasma torch can be used, or electric spark plugs may be employed.

Gaseous propellants generally will not cause hard starts, with rockets the total injector area is less than the throat thus the chamber pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at ignition.

Solid propellants are usually ignited with one-shot pyrotechnic devices.

Once ignited, rocket chambers are self sustaining and igniters are not needed. Indeed chambers often spontaneously reignite if they are restarted after being shut down for a few seconds. However, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the pyrotechnic igniter.

## Types of rocket engines

### Physically powered

water rocketPartially filled pressurised carbonated drinks container with tail and nose weightingVery simple to buildAltitude typically limited to a few hundred feet or so (world record is 623 meters/2044 feet)
cold gas thrusterA non combusting form, used for attitude jetsNon contaminating exhaustExtremely low performance
hot water rocketHot water is stored in a tank at high temperature/pressure and turns to steam in exhaustSimple, fairly safeLow performance due to heavy tank

### Chemically powered

Solid rocketIgnitable, self sustaining solid fuel/oxidiser mixture ("grain") with central hole and nozzleSimple, often no moving parts, reasonably good mass fraction, reasonable Isp. A water rocket is a type of Model rocket using Water as its Reaction mass. A water rocket is a type of Model rocket using Water as its Reaction mass. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International A foot (plural feet or foot; symbol or abbreviation ft or sometimes &prime – the prime symbol) is a non-SI unit A cold gas thruster is a rocket engine/thruster that uses a (typically inert gas as the reaction mass A vernier thruster is a Thruster used on a Spacecraft for Attitude control. A hot water rocket, or steam rocket uses water held in a pressure vessel at a high temperature such that its Saturated vapor pressure is significantly greater than The highest Specific impulse chemical Rockets use liquid propellants. A solid rocket or a solid-fuel rocket is a Rocket with a motor that uses solid propellants ( Fuel / Oxidizer) The Moving Parts was a late 1970s Boston-based Rock music band Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines A thrust schedule can be designed into the grain. Once lit, extinguishing it is difficult although often possible, cannot be throttled in real time; handling issues from ignitable mixture, lower performance than liquid rockets, if grain cracks it can block nozzle with disastrous results, cracks burn and widen during burn. Refuelling grain harder than simply filling tanks, Lower specific Impulse than Liquid Rockets.
Hybrid rocketSeparate oxidiser/fuel, typically oxidiser is liquid and kept in a tank, the other solid with central holeQuite simple, solid fuel is essentially inert without oxidiser, safer; cracks do not escalate, throttleable and easy to switch off. A 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 Some oxidisers are monopropellants, can explode in own right; mechanical failure of solid propellant can block nozzle, central hole widens over burn and negatively affects mixture ratio.
Monopropellant rocketPropellant such as Hydrazine, Hydrogen Peroxide or Nitrous Oxide, flows over catalyst and exothermically decomposes and hot gases are emitted through nozzleSimple in concept, throttleable, low temperatures in combustion chambercatalysts can be easily contaminated, monopropellants can detonate if contaminated or provoked, Isp is perhaps 1/3 of best liquids
Bipropellant rocketTwo fluid (typically liquid) propellants are introduced through injectors into combustion chamber and burntUp to ~99% efficient combustion with excellent mixture control, throttleable, can be used with turbopumps which permits incredibly lightweight tanks, can be safe with extreme carePumps needed for high performance are expensive to design, huge thermal fluxes across combustion chamber wall can impact reuse, failure modes include major explosions, a lot of plumbing is needed. A Monopropellant rocket (or " monoprop rocket " is a Rocket that uses a single chemical as its power source and propellant Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines A bipropellant rocket engine is a Rocket engine that uses two propellants (very often liquid propellants) which are kept separately prior to reacting to form a hot
Dual mode propulsion rocketRocket takes off as a bipropellant rocket, then turns to using just one propellant as a monopropellantSimplicity and ease of controlLower performance than bipropellants
Tripropellant rocketThree different propellants (usually hydrogen, hydrocarbon and liquid oxygen) are introduced into a combustion chamber in variable mixture ratios, or multiple engines are used with fixed propellant mixture ratios and throttled or shut downReduces take-off weight, since hydrogen is lighter; combines good thrust to weight with high average Isp, improves payload for launching from Earth by a sizeable percentageSimilar issues to bipropellant, but with more plumbing, more R&D
Air-augmented rocketEssentially a ramjet where intake air is compressed and burnt with the exhaust from a rocketMach 0 to Mach 4. Dual mode propulsion systems combine the high efficiency of Bipropellant rockets with the reliability and simplicity of Monopropellant rockets Dual mode A tripropellant rocket is a Rocket that uses three Propellants, as opposed to the more common Bipropellant rocket or Monopropellant rocket designs Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines Air-augmented rockets (also known as rocket-ejector ramrocket ducted rocket integral rocket/ramjets or ejector ramjets use the supersonic exhaust of some kind of rocket engine to further 5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties, very noisy, thrust/weight ratio is similar to ramjets.
TurborocketA combined cycle turbojet/rocket where an additional oxidizer such as oxygen is added to the airstream to increase maximum altitudeVery close to existing designs, operates in very high altitude, wide range of altitude and airspeedAtmospheric airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous. A turborocket is a type of Aircraft engine combining elements of a Jet engine and a Rocket. An oxidizing agent or oxidising agent (also called an oxidant, oxidizer or oxidiser) can be defined as either a Chemical compound Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Much heavier than simple rockets.
Precooled jet engine / LACE (combined cycle with rocket)Intake air is chilled to very low temperatures at inlet before passing through a ramjet or turbojet engine. A precooled jet engine is a concept for high speed Jet engines that features a cryogenic fuel-cooled heat exchanger between the air intake and the (LP compressor to precool A liquid air cycle engine (LACE is a Spacecraft propulsion engine that attempts to gain efficiency by gathering part of its oxidizer from the atmosphere. Can be combined with a rocket engine for orbital insertion. Easily tested on ground. High thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, mach 0-5. 5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid intercontinental travel. Exists only at the lab prototyping stage. Examples include RB545, SABRE, ATREX

### Electrically powered

Resistojet rocket (electric heating)A monopropellant is electrically heated by a filament for extra performanceHigher Isp than monopropellant alone, about 40% higher. The RB545 was an air-breathing rocket engine that was proposed to propel a British space shuttle (see HOTOL) to orbit using a single stage SABRE (Synergic Air Breathing Engine is a design for a Hypersonic hydrogen-fueled air breathing combined cycle Rocket engine / Turbojet engine The ATREX engine (Air Turbo Ramjet Engine with eXpander cycle developed in Japan is an experimental precooled Jet engine that works as a Turbojet at low A resistojet is a way of Spacecraft propulsion that provides thrust by Heating a (typically non-reactive Fluid. Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines Uses a lot of power and hence gives typically low thrust
Arcjet rocket (chemical burning aided by electrical discharge)Similar to resistojet in concept but with inert propellant, except an arc is used which allows higher temperatures1600 seconds IspVery low thrust and high power, performance is similar to Ion drive. Arcjets are a form of Electric propulsion for Spacecraft, whereby an electrical discharge (arc is created in a flow of propellant (typically Hydrazine Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines An ion thruster is a form of Electric propulsion used for Spacecraft propulsion that creates thrust by accelerating Ions Ion thrusters are characterized
Pulsed plasma thruster (electric arc heating; emits plasma)Plasma is used to erode a solid propellantHigh Isp , can be pulsed on and off for attitude controlLow energetic efficiency
Variable specific impulse magnetoplasma rocketMicrowave heated plasma with magnetic throat/nozzleVariable Isp from 1000 seconds to 10,000 secondssimilar thrust/weight ratio with ion drives (worse), thermal issues, as with ion drives very high power requirements for significant thrust, really needs advanced nuclear reactors, never flown, requires low temperatures for superconductors to work

### Solar powered

The Solar thermal rocket would make use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. Pulsed plasma thrusters are a method of Spacecraft propulsion which use an arc of electric current adjacent to a solid propellant to produce a quick and repeatable burst of Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines The Variable Specific Impulse Magnetoplasma Rocket ( VASIMR) is an electro-thermal thruster for Spacecraft propulsion. Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines An ion thruster is a form of Electric propulsion used for Spacecraft propulsion that creates thrust by accelerating Ions Ion thrusters are characterized Solar thermal propulsion is a form of Spacecraft propulsion that makes use of solar power to directly heat Reaction mass, and therefore does not require an electrical Working mass is a mass against which a system operates in order to produce Acceleration. A solar thermal rocket only has to carry the means of capturing solar energy, such as concentrators and mirrors. Solar energy is the Light and radiant heat from the Sun that powers Earth 's Climate and Weather and sustains Life A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. The heated propellant is fed through a conventional rocket nozzle to produce thrust. The engine thrust is directly related to the surface area of the solar collector and to the local intensity of the solar radiation.

Solar thermal rocketPropellant is heated by solar collectorReasonably simple, good performance with liquid hydrogen propellant, adequate performance with in-situ water for short-range interplanetary flightonly useful once in space, as thrust is fairly low, but hydrogen is not easily stored in space, otherwise moderate/low Isp if higher molecular mass propellants are used

### Beam powered

light beam powered rocketPropellant is heated by light beam (often laser) aimed at vehicle from a distance, either directly or indirectly via heat exchangersimple in principle, in principle very high exhaust speeds can be achieved~1 MW of power per kg of payload is needed to achieve orbit, relatively high accelerations, lasers are blocked by clouds, fog, reflected laser light may be dangerous, pretty much needs hydrogen monopropellant for good performance which needs heavy tankage, some designs are limited to ~600 seconds due to reemission of light since propellant/heat exchanger gets white hot
microwave beam powered rocketPropellant is heated by microwave beam aimed at vehicle from a distancemicrowaves avoid reemission of energy, so ~900 seconds exhaust speeds might be achieveable~1 MW of power per kg of payload is needed to achieve orbit, relatively high accelerations, microwaves are absorbed to a degree by rain, reflected microwaves may be dangerous, pretty much needs hydrogen monopropellant for good performance which needs heavy tankage, transmitter diameter is measured in kilometres to achieve a fine enough beam to hit a vehicle at up to 100km. Solar thermal propulsion is a form of Spacecraft propulsion that makes use of solar power to directly heat Reaction mass, and therefore does not require an electrical Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines An ion thruster is a form of Electric propulsion used for Spacecraft propulsion that creates thrust by accelerating Ions Ion thrusters are characterized Beam-powered propulsion is a class of Spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant Beam-powered propulsion is a class of Spacecraft propulsion mechanisms that use energy beamed to the spacecraft from a remote power plant

### Nuclear powered

Nuclear propulsion includes a wide variety of propulsion methods that use some form of nuclear reaction as their primary power source. Nuclear propulsion includes a wide variety of propulsion methods that use some form of Nuclear reaction as their primary power source Spacecraft propulsion is any method used to change the velocity of Spacecraft and artificial Satellites There are many different methods In Nuclear physics, a nuclear reaction is the process in which two nuclei or nuclear particles collide to produce products different from the initial particles Various types of nuclear propulsion have been proposed, and some of them tested, for spacecraft applications:

Radioisotope rocket/"Poodle thruster" (radioactive decay energy)Heat from radioactive decay is used to heat hydrogenabout 700-800 seconds, almost no moving partslow thrust/weight ratio. The radioisotope rocket is a type of Rocket engine that uses the heat generated by the decay of Radioactive elements to heat a Working fluid, which is then
Nuclear thermal rocket (nuclear fission energy)propellant (typ. In a nuclear thermal rocket a working fluid usually Hydrogen, is heated to a high temperature in a Nuclear reactor, and then expands through a rocket nozzle hydrogen) is passed through a nuclear reactor to heat to high temperatureIsp can be high, perhaps 900 seconds or more, above unity thrust/weight ratio with some designsMaximum temperature is limited by materials technology, some radioactive particles can be present in exhaust in some designs, nuclear reactor shielding is heavy, unlikely to be permitted from surface of the Earth, thrust/weight ratio is not high. Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines
Gas core reactor rocket (nuclear fission energy)Nuclear reaction using a gaseous state fission reactor in intimate contact with propellantVery hot propellant, not limited by keeping reactor solid, Isp between 1500 and 3000 seconds but with very high thrustdifficulties in heating propellant without losing fissionables in exhaust, exhaust inherently highly radioactive, massive thermal issues particularly for nozzle/throat region. Gas core reactor rockets are a conceptual type of rocket that is propelled by the exhausted coolant of a Gaseous fission reactor. Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines
Fission-fragment rocket (nuclear fission energy)Fission products are directly exhausted to give thrustTheoretical only at this point. The fission-fragment rocket is a Rocket engine design that directly harnesses hot nuclear Fission products for Thrust, as opposed to using a separate
Fission sail (nuclear fission energy)A sail material is coated with fissionable material on one sideNo moving parts, works in deep spaceTheoretical only at this point. The fission sail is a type of Spacecraft propulsion proposed by Robert Forward that uses fission fragments to propel a large Solar sail -like
Nuclear salt-water rocket (nuclear fission energy)Nuclear salts are held in solution, caused to react at nozzleVery high Isp, very high thrustThermal issues in nozzle, propellant could be unstable, highly radioactive exhaust. A nuclear salt-water rocket (or NSWR is a proposed type of Nuclear thermal rocket designed by Robert Zubrin that would be fueled by Water bearing Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines Theoretical only at this point.
Nuclear pulse propulsion (exploding fission/fusion bombs)Shaped nuclear bombs are detonated behind vehicle and blast is caught by a 'pusher plate'Very high Isp, very high thrust/weight ratio, no show stoppers are known for this technologyNever been tested, pusher plate may throw off fragments due to shock, minimum size for nuclear bombs is still pretty big, expensive at small scales, nuclear treaty issues, fallout when used below Earth's magnetosphere. Specific impulse (usually abbreviated I sp is a way to describe the efficiency of rocket and jet engines Spall are flakes of a material that are broken off a larger solid body and can be produced by a variety of mechanisms including as a result of Projectile impact Corrosion
Antimatter catalyzed nuclear pulse propulsion (fission and/or fusion energy)Nuclear pulse propulsion with antimatter assist for smaller bombsSmaller sized vehicle might be possibleContainment of antimatter, production of antimatter in macroscopic quantities isn't currently feasible. Antimatter catalyzed nuclear pulse propulsion is a variation of Nuclear pulse propulsion based upon the injection of Antimatter into a mass of nuclear fuel which normally Theoretical only at this point.
Fusion rocket (nuclear fusion energy)Fusion is used to heat propellantVery high exhaust velocityLargely beyond current state of the art. A fusion rocket is a Rocket that is driven by Fusion power. The process of Nuclear fusion is well understood and recent developments indicate this
Antimatter rocket (annihilation energy)Antimatter annihilation heats propellantExtremely energetic, very high theoretical exhaust velocityProblems with antimatter production and handling; energy losses in neutrinos, gamma rays, muons; thermal issues. An antimatter rocket is a proposed class of Rockets that use Antimatter as their power source Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Gamma rays (denoted as &gamma) are a form of Electromagnetic radiation or light emission of frequencies produced by sub-atomic particle interactions The muon (from the letter mu (μ--used to represent it is an Elementary particle with negative Electric charge and a spin of 1/2 Theoretical only at this point

## History of rocket engines

According to the writings of the Roman Aulus Gellius, in c. Aulus Gellius (ca 125 AD—after 180 AD Latin author and grammarian possibly of African origin probably born and certainly brought up at Rome. 400 BC, a Greek Pythagorean named Archytas, propelled a wooden bird along wires using steam. Events By place Persian Empire Artaxerxes II King of Persia appoints Tissaphernes to take over all the districts in The Greeks ( Greek: Έλληνες) are a Nation and Ethnic group native to Greece, Cyprus and neighbouring regions Archytas (Ἀρχύτας 428 BC – 347 BC was an Ancient Greek Philosopher, Mathematician, Astronomer, Statesman, and strategist [9] However, it would not appear to have been powerful enough to take off under its own thrust.

The aeolipile invented in the first century (known as Hero's engine) essentially consists of a hot water rocket on a bearing. An aeolipile, a rocket -like Jet engine invented in the first century by Hero of Alexandria, is considered to be the first recorded Steam engine The 1st century was the Century that lasted from 1 to 100 according the Julian calendar. An aeolipile, a rocket -like Jet engine invented in the first century by Hero of Alexandria, is considered to be the first recorded Steam engine A hot water rocket, or steam rocket uses water held in a pressure vessel at a high temperature such that its Saturated vapor pressure is significantly greater than It was created almost two millennia before the industrial revolution. Apparently Hero's steam engine was taken to be little more than a toy, the principles behind it were not well understood, and its full potential not realized for a millennium.

The availability of black powder to propel projectiles was a precursor to the development of the first solid rocket. Gunpowder is a an explosive mixture of Sulfur, Charcoal and Potassium nitrate (also known as saltpetre/saltpeter that burns rapidly producing volumes Ninth Century Chinese Taoist alchemists discovered black powder in a search for the Elixir of life; this accidental discovery led to fire arrows which were the first rocket engines to leave the ground. The 9th century is the period from 801 to 900 in accordance with the Julian calendar in the Christian / Common Era. The term Chinese people may refer to any of the following A person who resides in and holds citizenship of the People's Republic of China (including Hong Taoism (pronounced /ˈdaʊɪzəm/ or /ˈtaʊɪzəm/ also spelled '''Daoism''') refers to a variety of related Philosophical and Religious traditions Alchemy a part of the Occult Tradition is both a philosophy and a practice with an ultimately unknown aim involving the improvement of the alchemist as well as the making of Gunpowder is a an explosive mixture of Sulfur, Charcoal and Potassium nitrate (also known as saltpetre/saltpeter that burns rapidly producing volumes The elixir of life, from Arabic الإكسير also known as the elixir of immortality or Dancing Water or Persian: Aab-e-Hayaat آب حیات The Fire Arrow is a projectile weapon that uses Black powder.

Slow development of this technology continued up to the later 20th Century, when the writings of Konstantin Tsiolkovsky first talked about liquid fuelled rocket engines. A liquid rocket is a Rocket with an engine that uses Propellants in Liquid form

These independently became a reality thanks to Robert Goddard. Robert Hutchings Goddard, PhD ( October 5, 1882 &ndash August 10 1945 U

## References

1. ^ Rocket Propulsion Elements; 7th edition- chapter 1
2. ^ Rocket Propulsion Elements seventh edition eq-2-14
3. ^ Huzel, D. K. and Huang, D. H. (1971). NASA SP-125, Design of Liquid Propellant Rocket Engines, 2nd Edition, NASA.
4. ^ Rocket Propulsion Elements seventh edition eq-3-33
5. ^ G. P. Sutton & D. M. Ross, Rocket Propulsion Elements, 4th Ed. 1975 (WileyInterscience New York), Ch 8, Sec 6 & especially Sec. 7, re combustion instability.
6. ^ NASA CR-566
7. ^ Newsgroup correspondence, 1998-99
8. ^ Complex chemical equilibrium and rocket performance calculations, Cpropep-Web
9. ^ Leofranc Holford-Strevens, Aulus Gellius: An Antonine Author and his Achievement (Oxford University Press; revised paperback edn. 2005)
• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain. The Encyclopædia Britannica Eleventh Edition (1910–1911 is a 29-volume reference work that marked the beginning of the Encyclopædia Britannica The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone