Rocket engine

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RS-68 being tested at NASA's Stennis Space Center. The nearly transparent exhaust is due to this engine's exhaust being mostly superheated steam (water vapour from its propellants, hydrogen and oxygen), plus some unburned hydrogen.

 

Viking 5C rocket engine

 

A rocket engine uses stored rocket propellant mass for forming its high-speed propulsive jet. Rocket engines are reaction engines, obtaining thrust in accordance with Newton's third law. Most rocket engines use combustion, although non-combusting forms (such as cold gas thrusters and nuclear thermal rockets) also exist. Vehicles propelled by rocket engines are commonly called rockets. Since they need no external material to form their jet, rocket engines can perform in a vacuum and thus can be used to propel spacecraft and ballistic missiles. 

Compared to other types of jet engines, rocket engines are by far the lightest, and have the highest thrust, but are the least propellant-efficient (they have the lowest specific impulse). The ideal exhaust is hydrogen, the lightest of all gases, but chemical rockets produce a mix of heavier species, reducing the exhaust velocity. Rocket engines become more efficient at high velocities, due to greater propulsive efficiency and the Oberth effect. Since they do not require an atmosphere, they are well suited for uses at very high altitudes and in space.

Terminology

Here, "rocket" is used as an abbreviation for "rocket engine". 

Thermal rockets use an inert propellant, heated by a power source such as electric or nuclear power.

Chemical rockets are powered by exothermic chemical reactions of the propellant:

• Solid-fuel rockets (or solid-propellant rockets or motors) are chemical rockets which use propellant in a solid state.
• Liquid-propellant rockets use one or more liquid propellants fed from tanks.
• Hybrid rockets use a solid propellant in the combustion chamber, to which a second liquid or gas oxidiser or propellant is added to permit combustion.
• Monopropellant rockets use a single propellant decomposed by a catalyst. The most common monopropellants are hydrazine and hydrogen peroxide.

Principle of operation

A simplified diagram of a liquid-fuel rocket.

1. Liquid rocket fuel.
2. Oxidizer.
3. Pumps carry the fuel and oxidizer.
4. The combustion chamber mixes and burns the two liquids.
5. The hot exhaust is choked at the throat, which, among other things, dictates the amount of thrust produced.
6. Exhaust exits the rocket.

A simplified diagram of a solid-fuel rocket.
1. A solid fuel-oxidizer mixture (propellant) is packed into the rocket, with a cylindrical hole in the middle.
2. An igniter combusts the surface of the propellant.
3. The cylindrical hole in the propellant acts as a combustion chamber.
4. The hot exhaust is choked at the throat, which, among other things, dictates the amount of thrust produced.
5. Exhaust exits the rocket.

 

Rocket engines produce thrust by the expulsion of an exhaust fluid which has been accelerated to a high speed through a propelling nozzle. The fluid is usually a gas created by high pressure (150-to-2,900-pound-per-square-inch (10 to 200 bar)) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber. The nozzle uses the heat energy released by expansion of the gas to accelerate the exhaust to very high (supersonic) speed, and the reaction to this pushes the engine in the opposite direction. Combustion is most frequently used for practical rockets, as high temperatures and pressures are desirable for the best performance, permitting a longer nozzle, giving higher exhaust speeds and better thermodynamic efficiency. 

An alternative to combustion is the water rocket, which uses water pressurised by compressed air, carbon dioxide, nitrogen, or manual pumping, for model rocketry.

Propellant

Rocket propellant is mass that is stored, usually in some form of propellant tank, or within the combustion chamber itself, prior to being ejected from a rocket engine in the form of a fluid jet to produce thrust. 

Chemical rocket propellants are most commonly used, which undergo exothermic chemical reactions which produce hot gas which is used by a rocket for propulsive purposes. Alternatively, a chemically inert reaction mass can be heated using a high-energy power source via a heat exchanger, and then no combustion chamber is used. 

Solid rocket propellants are prepared as a mixture of fuel and oxidising components called 'grain' and the propellant storage casing effectively becomes the combustion chamber.

Injection

Liquid-fuelled rockets force separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants. Both liquid and hybrid rockets use injectors to introduce the propellant into the chamber. These are often an array of simple jets – holes through which the propellant escapes under pressure; but sometimes may be more complex spray nozzles. When two or more propellants are injected, the jets usually deliberately cause the propellants to collide as this breaks up the flow into smaller droplets that burn more easily.

Combustion chamber

For chemical rockets the combustion chamber is typically just a cylinder, and flame holders are rarely used. The dimensions of the cylinder are such that the propellant is able to combust thoroughly; different rocket propellants require different combustion chamber sizes for this to occur. 

This leads to a number called ${\displaystyle L^{*}}$:

${\displaystyle L^{*}={\frac {V_{c}}{A_{t}}}}$

where:

• ${\displaystyle V_{c}}$ is the volume of the chamber
• ${\displaystyle A_{t}}$ is the area of the throat

L* is typically in the range of 25–60 inches (0.64–1.52 m). 

The combination of temperatures and pressures typically reached in a combustion chamber is usually extreme by any standard. Unlike in airbreathing jet engines, no atmospheric nitrogen is present to dilute and cool the combustion, and the propellant mixture can reach true stoichiometric ratios. This, in combination with the high pressures, means that the rate of heat conduction through the walls is very high.

In order for fuel and oxidizer to flow into the chamber, the pressure of the propellant fluids entering the combustion chamber must exceed the pressure inside the combustion chamber itself. This may be accomplished by a variety of design approaches including turbopumps or, in simpler engines, via sufficient tank pressure to advance fluid flow. Tank pressure may be maintained by several means, including a high-pressure helium pressurization system common to many large rocket engines or, in some newer rocket systems, by a bleed-off of high-pressure gas from the engine cycle to autogenously pressurize the propellant tanks For example, the self-pressurization gas system of the BFR is a critical part of SpaceX strategy to reduce launch vehicle fluids from five in their legacy Falcon 9 vehicle family to just two in BFR, eliminating not only the helium tank pressurant but all hypergolic propellants as well as nitrogen for cold-gas reaction-control thrusters.

Nozzle

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

 

The hot gas produced in the combustion chamber is permitted to escape through an opening (the "throat"), and then through a diverging expansion section. When sufficient pressure is provided to the nozzle (about 2.5-3 times ambient pressure), the nozzle chokes and a supersonic jet is formed, dramatically accelerating the gas, converting most of the thermal energy into kinetic energy. Exhaust speeds vary, depending on the expansion ratio the nozzle is designed for, but exhaust speeds as high as ten times the speed of sound in air at sea level are not uncommon. About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber, and the rest comes from the pressures acting 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. 

The four expansion regimes of a de Laval nozzle:
• under-expanded
• perfectly expanded
• over-expanded
• grossly over-expanded

 

The most commonly used nozzle is the de Laval nozzle, a fixed geometry nozzle with a high expansion-ratio. The large bell- or cone-shaped nozzle extension beyond the throat gives the rocket engine its characteristic shape. 

The exit static pressure of the exhaust jet depends on the chamber pressure and the ratio of exit to throat area of the nozzle. As exit pressure varies from the ambient (atmospheric) pressure, a choked nozzle is said to be

• under-expanded (exit pressure greater than ambient),
• perfectly expanded (exit pressure equals ambient),
• over-expanded (exit pressure less than ambient; shock diamonds form outside the nozzle), or
• grossly over-expanded (a shock wave forms inside the nozzle extension).

In practice, perfect expansion is only achievable with a variable-exit area nozzle (since ambient pressure decreases as altitude increases), and is not possible above a certain altitude as ambient pressure approaches zero. If the nozzle is not perfectly expanded, then loss of efficiency occurs. Grossly over-expanded nozzles lose less efficiency, but can cause mechanical problems with the nozzle. Fixed-area nozzles become progressively more under-expanded as they gain altitude. Almost all de Laval nozzles will be momentarily grossly over-expanded during startup in an atmosphere.

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 at all altitudes.

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 the exhaust's pressure is lower than the ambient pressure, then the vehicle will be slowed by the difference in pressure between the top of the engine and the exit; on the other hand, if the exhaust's pressure is higher, then exhaust pressure that could have been converted into thrust is not converted, and energy is wasted. 

To maintain this ideal of equality between the exhaust's exit pressure and the ambient pressure, 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 in a lightweight fashion, although is routinely done with other forms of jet engines. In rocketry a lightweight compromise nozzle is generally used and some reduction in atmospheric performance occurs when used at other than the 'design altitude' or when throttled. 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 providing 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 altitudes.

When exhausting into a sufficiently low ambient pressure (vacuum) several issues arise. One is the sheer weight of the nozzle—beyond a certain point, for a particular vehicle, the extra weight of the nozzle outweighs any performance gained. Secondly, as the exhaust gases adiabatically expand within the nozzle they cool, and eventually some of the chemicals can freeze, producing 'snow' within the jet. This causes instabilities in the jet and must be avoided. 

On a de Laval nozzle, exhaust gas flow detachment will occur in a grossly over-expanded nozzle. As the detachment point will not be uniform around the axis of the engine, a side force may be imparted to the engine. This side force may change over time and result in control problems with the launch vehicle. 

Advanced altitude-compensating designs, such as the aerospike or plug nozzle, attempt to minimize performance losses by adjusting to varying expansion ratio caused by changing altitude.

Propellant efficiency

Typical temperature (T), pressure (p), and velocity (v) profiles in a de Laval Nozzle

 

For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. 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 to maximise translational velocity

Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass 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 for any given engine, 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. This is termed exhaust velocity, and after allowance is made for factors that can reduce it, the effective exhaust velocity is one of the most important parameters of a rocket engine (although weight, cost, ease of manufacture etc. are usually also very important). 

For aerodynamic reasons the flow goes sonic ("chokes") at the narrowest part of the nozzle, the 'throat'. Since the speed of sound in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at room temperature the speed of sound in air is about 340 m/s while the speed of sound in the hot gas of a rocket engine can be over 1700 m/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. 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.

Thrust vectoring

Vehicles typically require the overall thrust to change direction over the length of the burn. A number of different ways to achieve this have been flown:

• The entire engine is mounted on a hinge or gimbal and any propellant feeds reach the engine via low pressure flexible pipes or rotary couplings.
• Just the combustion chamber and nozzle is gimballed, the pumps are fixed, and high pressure feeds attach to the engine.
• Multiple engines (often canted at slight angles) are deployed but throttled to give the overall vector that is required, giving only a very small penalty.
• High-temperature vanes protrude into the exhaust and can be tilted to deflect the jet.

Overall performance

Rocket technology can combine very high thrust (meganewtons), very high exhaust speeds (around 10 times the speed of sound in air at sea level) and very high thrust/weight ratios (>100) simultaneously as well as being able to operate outside the atmosphere, and while permitting the use of low pressure and hence lightweight tanks and structure. 

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

The most important metric for the efficiency of a rocket engine is impulse per unit of propellant, this is called specific impulse (usually written ${\displaystyle I_{sp}}$). This is either measured as a speed (the effective exhaust velocity ${\displaystyle v_{e}}$ in metres/second or ft/s) or as a time (seconds). An engine that gives a large specific impulse is normally highly desirable. 

The specific impulse that can be achieved is primarily a function of the propellant mix (and ultimately would limit the specific impulse), but practical limits on chamber pressures and the nozzle expansion ratios reduce the performance that can be achieved.

Net thrust

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

${\displaystyle F_{n}={\dot {m}}\;v_{e}={\dot {m}}\;v_{e-opt}+A_{e}(p_{e}-p_{amb})}$

where:
${\displaystyle {\dot {m}}}$	=  exhaust gas mass flow
${\displaystyle v_{e}}$	=  effective exhaust velocity
${\displaystyle v_{e-opt}}$	=  effective jet velocity when Pa = Pe
${\displaystyle A_{e}}$	=  flow area at nozzle exit plane (or the plane where the jet leaves the nozzle if separated flow)
${\displaystyle p_{e}}$	=  static pressure at nozzle exit plane
${\displaystyle p_{amb}}$	=  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 ${\displaystyle {\dot {m}}\;v_{e-opt}\,}$ term represents the momentum thrust, which remains constant at a given throttle setting, whereas the ${\displaystyle A_{e}(p_{e}-p_{amb})\,}$ term represents the pressure thrust term. At full throttle, the net thrust of a rocket motor improves slightly with increasing altitude, because as atmospheric pressure decreases with altitude, the pressure thrust term increases. At the surface of the Earth the pressure thrust may be reduced by up to 30%,depending on the engine design. This reduction drops roughly exponentially to zero with increasing altitude. 

Maximum efficiency for a rocket engine is achieved by maximising the momentum contribution of the equation without incurring penalties from over expanding the exhaust. This occurs when ${\displaystyle p_{e}=p_{amb}}$. Since ambient pressure changes with altitude, most rocket engines spend very little time operating at peak efficiency.

Vacuum specific impulse, I_sp

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 ${\displaystyle {\dot {m}}}$, provided the mixture ratios and combustion efficiencies are maintained. It is thus quite usual to rearrange the above equation slightly:

${\displaystyle F_{vac}=C_{f}\,{\dot {m}}\,c^{*}}$

and so define the vacuum Isp to be:

${\displaystyle v_{evac}=C_{f}\,c^{*}\,}$

where:

  ${\displaystyle c^{*}}$  =  the speed of sound constant at the throat

${\displaystyle C_{f}}$  =  the thrust coefficient constant of the nozzle (typically about 2)

And hence:

${\displaystyle F_{n}={\dot {m}}\,v_{evac}-A_{e}\,p_{amb}}$

Throttling

Rockets can be throttled by controlling the propellant combustion rate ${\displaystyle {\dot {m}}}$ (usually measured in kg/s or lb/s). In liquid and hybrid rockets, the propellant flow entering the chamber is controlled using valves, in solid rockets it is controlled by changing the area of propellant that is burning and this can be designed into the propellant grain (and hence cannot be controlled in real-time). 

Rockets can usually be throttled down to an exit pressure of about one-third of ambient pressure (often limited by flow separation in nozzles) and up to a maximum limit determined only by the mechanical strength of the engine. 

In practice, the degree to which rockets can be throttled varies greatly, but most rockets can 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 be optimised and tested for wider ranges. For example, some more recent liquid-propellant engine designs that have been optimised for greater throttling capability (BE-3, Raptor) can be throttled to as low as 18–20 percent of rated thrust. Solid rockets can be throttled by using shaped grains that will vary their surface area over the course of the burn.

Energy efficiency

Rocket vehicle mechanical efficiency as a function of vehicle instantaneous speed divided by effective exhaust speed. These percentages need to be multiplied by internal engine efficiency to get overall efficiency.

 

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. Rocket nozzles give an excellent approximation to adiabatic expansion which is a reversible process, and hence they give efficiencies which are very close to that of the Carnot cycle. Given the temperatures reached, over 60% efficiency can be achieved with chemical rockets. 

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 energy efficiency goes to 0% at zero speed (as with all jet propulsion.) See Rocket energy efficiency for more details.

Thrust-to-weight ratio

Rockets, of all the jet engines, indeed of essentially all engines, have the highest thrust to weight ratio. This is especially true for liquid rocket engines.

This high performance is due to the small volume of pressure vessels that make up the engine—the pumps, pipes and combustion chambers involved. The lack of inlet duct and the use of dense liquid propellant allows the pressurisation system to be small and lightweight, whereas duct engines have to deal with air which has a density about one thousand times lower. 

Of the liquid propellants used, density is worst for liquid hydrogen. Although this propellant is marvellous in many ways, it has a very low density, about one fourteenth that of water. This makes the turbopumps and pipework larger and heavier, and this is reflected in the thrust-to-weight ratio of engines that use it (for example the SSME) compared to those that do not (NK-33).

Cooling

For efficiency reasons, higher temperatures are desirable, but materials lose their strength if the temperature becomes too high. Rockets run with combustion temperatures that can reach ~3,500 K (~3,200 °C or ~5,800 °F).

Most other jet engines have gas turbines in the hot exhaust. Due to their larger surface area, they are harder to cool and hence there is a need to run the combustion processes at much lower temperatures, losing efficiency. In addition, duct engines use air as an oxidant, which contains 78% largely unreactive nitrogen, which dilutes the reaction and lowers the temperatures. Rockets have none of these inherent combustion temperature limiters. 

The temperatures reached by rocket exhaust often substantially exceed the melting points of the nozzle and combustion chamber materials (~1,200 K for copper). Most construction materials will also combust if exposed to high temperature oxidizer, which leads to a number of design challenges. The nozzle and combustion temperature walls must not be allowed to combust, melt, or vaporize (sometimes facetiously termed an "engine-rich exhaust"). 

Rockets that use the common construction materials such as aluminium, steel, nickel or copper alloys must employ cooling systems to limit the temperatures that engine structures experience. 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. 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. 

Two material exceptions that can directly sustain rocket exhaust temperatures are graphite and tungsten, although both are subject to oxidation if not protected. Materials technology, combined with the engine design, is a limiting factor of the exhaust temperature of chemical rockets. 

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/m². The strongest heat fluxes are found at the throat, which often sees twice that found in the associated chamber and nozzle. This is due to the combination of high speeds (which gives a very thin boundary layer), and although lower than the chamber, the high temperatures seen there.

In rockets 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 vaporised and carried away).
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 (liquid rockets use the fuel, or occasionally the oxidiser, 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. 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 practice, regenerative cooling is nearly always used in conjunction with curtain cooling and/or film cooling. 

Liquid-fuelled engines are often run fuel-rich, which lowers combustion temperatures. This reduces heat loads on the engine and allows lower cost materials and a simplified cooling system. This can also increase performance by lowering the average molecular weight of the exhaust and increasing the efficiency with which combustion heat is converted to kinetic exhaust energy.

Mechanical issues

Rocket combustion chambers are normally operated at fairly high pressure, typically 10–200 bar (1–20 MPa, 150–3,000 psi). When operated within significant atmospheric pressure, higher combustion chamber pressures give better performance by permitting a larger and more efficient nozzle to be fitted without it being grossly overexpanded. 

However, these high pressures cause the outermost part of the chamber to be under very large hoop stresses – rocket engines are pressure vessels.

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

In addition, significant temperature gradients are set up in the walls of the chamber and nozzle, these cause differential expansion of the inner liner that create internal stresses.

Acoustic issues

The extreme vibration and acoustic environment inside a rocket motor commonly result in peak stresses well above mean values, especially in the presence of organ pipe-like resonances and gas turbulence.

Combustion instabilities

The combustion may display undesired instabilities, of sudden or periodic nature. The pressure in the injection chamber may increase until the propellant flow through the injector plate decreases; a moment later the pressure drops and the flow increases, injecting more propellant in the combustion chamber which burns a moment later, and again increases the chamber pressure, repeating the cycle. This may lead to high-amplitude pressure oscillations, often in ultrasonic range, which may damage the motor. Oscillations of ±200 psi at 25 kHz were the cause of failures of early versions of the Titan II missile second stage engines. The other failure mode is a deflagration to detonation transition; the supersonic pressure wave formed in the combustion chamber may destroy the engine.

Combustion instability was also a problem during Atlas development. The Rocketdyne engines used in the Atlas family were found to suffer from this effect in several static firing tests, and three missile launches exploded on the pad due to rough combustion in the booster engines. In most cases, it occurred while attempting to start the engines with a "dry start" method whereby the igniter mechanism would be activated prior to propellant injection. During the process of man-rating Atlas for Project Mercury, solving combustion instability was a high priority, and the final two Mercury flights sported an upgraded propulsion system with baffled injectors and a hypergolic igniter. 

The problem affecting Atlas vehicles was mainly the so-called "racetrack" phenomenon, where burning propellant would swirl around in a circle at faster and faster speeds, eventually producing vibration strong enough to rupture the engine, leading to complete destruction of the rocket. It was eventually solved by adding several baffles around the injector face to break up swirling propellant. 

More significantly, combustion instability was a problem with the Saturn F-1 engines. Some of the early units tested exploded during static firing, which led to the addition of injector baffles. 

In the Soviet space program, combustion instability also proved a problem on some rocket engines, including the RD-107 engine used in the R-7 family and the RD-216 used in the R-14 family, and several failures of these vehicles occurred before the problem was solved. Soviet engineering and manufacturing processes never satisfactorily resolved combustion instability in larger RP-1/LOX engines, so the RD-171 engine used to power the Zenit family still used four smaller thrust chambers fed by a common engine mechanism.

The combustion instabilities can be provoked by remains of cleaning solvents in the engine (e.g. the first attempted launch of a Titan II in 1962), reflected shock wave, initial instability after ignition, explosion near the nozzle that reflects into the combustion chamber, and many more factors. In stable engine designs the oscillations are quickly suppressed; in unstable designs they persist for prolonged periods. Oscillation suppressors are commonly used. 

Periodic variations of thrust, caused by combustion instability or longitudinal vibrations of structures between the tanks and the engines which modulate the propellant flow, are known as "pogo oscillations" or "pogo", named after the pogo stick.

Chugging

This is a low frequency oscillation at a few Hertz in chamber pressure usually caused by pressure variations in feed lines due to variations in acceleration of the vehicle. This can cause 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 propellants.

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 is 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. Acoustic oscillations can be excited by thermal processes, such as the flow of hot air through a pipe or combustion in a chamber. Specifically, standing acoustic waves inside a chamber can be intensified if combustion occurs more intensely in regions where the pressure of the acoustic wave is maximal. Such effects are very difficult to predict analytically during the design process, and have usually been addressed by expensive, time consuming and 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 vaporising the propellant before injection, or use of Helmholtz dampers within the combustion chambers to change the resonant modes of the chamber.

Testing for the possibility of screeching is sometimes done by exploding small explosive charges outside the combustion chamber with a tube set tangentially to the combustion chamber near the injectors to determine the engine's impulse response and then evaluating the time response of the chamber pressure- a fast recovery indicates a stable system.

Exhaust noise

For all but the very smallest sizes, rocket exhaust compared to other engines is generally very noisy. As the hypersonic exhaust mixes with the ambient air, shock waves are formed. The Space Shuttle generates over 200 dB(A) of noise around its base. To reduce this, and the risk of payload damage or injury to the crew atop the stack, the Mobile Launcher Platform was fitted with a Sound Suppression System that sprayed 1,100,000 litres of water around the base of the rocket in 41 seconds at launch time. Using this system kept sound levels within the payload bay to 142 dB.

 

The sound intensity from the shock waves generated depends on the size of the rocket and on the exhaust velocity. 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 at close range, the acoustic effects could actually kill.

More worryingly for space agencies, such sound levels can also damage the launch structure, or worse, be reflected back at the comparatively delicate rocket above. This is why so much water is typically used at launches. The water spray changes the acoustic qualities of the air and reduces or deflects the sound energy away from the rocket. 

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 jet, 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 ${\displaystyle P=F*V}$ 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 jet and by deflecting the jet at an angle.

Testing

Rocket engines are usually statically tested at a test facility before being put into production. For high altitude engines, either a shorter nozzle must be used, or the rocket must be tested in a large vacuum chamber.

Safety

Rocket vehicles have a reputation for unreliability and danger; especially catastrophic failures. 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 has typically been placed 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.

Saturn family (1961–1975)

The Rocketdyne H-1 engine, used in a cluster of eight in the first stage of the Saturn I and Saturn IB launch vehicles, had no catastrophic failures in 152 engine-flights. The Pratt and Whitney RL10 engine, used in a cluster of six in the Saturn I second stage, had no catastrophic failures in 36 engine-flights. The Rocketdyne F-1 engine, used in a cluster of five in the first stage of the Saturn V, had no failures in 65 engine-flights. The Rocketdyne J-2 engine, used in a cluster of five in the Saturn V second stage, and singly in the Saturn IB second stage and Saturn V third stage, had no catastrophic failures in 86 engine-flights.

Space Shuttle (1981–2011)

The Space Shuttle Solid Rocket Booster, used in pairs, caused one notable catastrophic failure in 270 engine-flights. 

The Space Shuttle Main Engine, used in a cluster of three, flew in 46 refurbished engine units. These made a total of 405 engine-flights with no catastrophic in-flight failures. A single in-flight SSME failure occurred during Space Shuttle Challenger's STS-51-F mission.^[32] This failure had no effect on mission objectives or duration.

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. 

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 (actually, the vibration of the molecule should not be neglected). 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 favoured. 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.

It is important to note in computing the specific reaction energy, that the entire mass of the propellants, including both fuel and oxidiser, 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 car or turbojet engines, use atmospheric oxygen and so have a much better effective energy output per unit mass of propellant that must be carried, but are similar per unit mass of fuel. 

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

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 inside the chamber, and if/when ignition occurs the amount of hot gas created can exceed the maximum design pressure of the chamber, causing a catastrophic failure of the pressure vessel. This is sometimes called a hard start or a rapid unscheduled disassembly (RUD).

Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, a plasma torch can be used, or electric spark ignition may be employed. Some fuel/oxidiser combinations ignite on contact (hypergolic), and non-hypergolic fuels can be "chemically ignited" by priming the fuel lines with hypergolic propellants (popular in Russian engines). 

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.

Jet physics

Armadillo aerospace's quad vehicle showing visible banding (shock diamonds) in the exhaust jet

 

Rocket jets vary depending on the rocket engine, design altitude, altitude, thrust and other factors. 

Carbon rich exhausts from kerosene fuels are often orange in colour due to the black-body radiation of the unburnt particles, in addition to the blue Swan bands. Peroxide oxidizer-based rockets and hydrogen rocket jets contain largely steam and are nearly invisible to the naked eye but shine brightly in the ultraviolet and infrared. Jets from solid rockets can be highly visible as the propellant frequently contains metals such as elemental aluminium which burns with an orange-white flame and adds energy to the combustion process.

Some exhausts, notably alcohol fuelled rockets, can show visible shock diamonds. These are due to cyclic variations in the jet pressure relative to ambient creating shock waves that form 'Mach disks'.

The shape of the jet varies by the design altitude: at high altitude all rockets are grossly under-expanded, and a quite small percentage of exhaust gases actually end up expanding forwards.

Veneration of the dead

From Wikipedia, the free encyclopedia

 The veneration of the dead, including one's ancestors, is based on love and respect for the deceased. In some cultures, it is related to beliefs that the dead have a continued existence, and may possess the ability to influence the fortune of the living. Some groups venerate their direct, familial ancestors. Certain sects and religions, in particular the Roman Catholic Church, venerate saints as intercessors with God, as well as pray for departed souls in Purgatory.

In Europe, Asia, Oceania, African and Afro-diasporic cultures, the goal of ancestor veneration is to ensure the ancestors' continued well-being and positive disposition towards the living, and sometimes to ask for special favours or assistance. The social or non-religious function of ancestor veneration is to cultivate kinship values, such as filial piety, family loyalty, and continuity of the family lineage. Ancestor veneration occurs in societies with every degree of social, political, and technological complexity, and it remains an important component of various religious practices in modern times.

Overview

Ancestor reverence is not the same as the worship of a deity or deities. In some Afro-diasporic cultures, ancestors are seen as being able to intercede on behalf of the living, often as messengers between humans and the gods. As spirits who were once human themselves, they are seen as being better able to understand human needs than would a divine being. In other cultures, the purpose of ancestor veneration is not to ask for favors but to do one's filial duty. Some cultures believe that their ancestors actually need to be provided for by their descendants, and their practices include offerings of food and other provisions. Others do not believe that the ancestors are even aware of what their descendants do for them, but that the expression of filial piety is what is important. 

Most cultures who practice ancestor veneration do not call it "ancestor worship". In English, the word worship usually refers to the reverent love and devotion accorded a deity (god) or God. However, in other cultures, this act of worship does not confer any belief that the departed ancestors have become some kind of deity. Rather, the act is a way to respect, honor and look after ancestors in their afterlives as well as seek their guidance for their living descendants. In this regard, many cultures and religions have similar practices. Some may visit the graves of their parents or other ancestors, leave flowers and pray to them in order to honor and remember them, while also asking their ancestors to continue to look after them. However, this would not be considered as worshipping them since the term worship shows no such meaning.

In that sense the phrase ancestor veneration may convey a more accurate sense of what practitioners, such as the Chinese and other Buddhist-influenced and Confucian-influenced societies, as well as the African and European cultures see themselves as doing. This is consistent with the meaning of the word veneration in English, that is great respect or reverence caused by the dignity, wisdom, or dedication of a person.

Although there is no generally accepted theory concerning the origins of ancestor veneration, this social phenomenon appears in some form in all human cultures documented so far. David-Barrett and Carney claim that ancestor veneration might have served a group coordination role during human evolution, and thus it was the mechanism that led to religious representation fostering group cohesion.

West and Southeast African cultures

Ancestor veneration is prevalent throughout Africa, and serves as the basis of many religions. It is often augmented by a belief in a supreme being, but prayers and/or sacrifices are usually offered to the ancestors who may ascend to becoming a kind of minor deities themselves. Ancestor veneration remains among many Africans, sometimes practiced alongside the later adopted religions of Christianity (as in Nigeria among the Igbo people), and Islam (among the different Mandé peoples and the Bamum) in much of the continent. In orthodox Serer religion, the pangool is venerated by the Serer people.

Serer of Senegal & Gambia

The Seereer people of Senegal, The Gambia and Mauritania who adhere to the tenets of A ƭat Roog (Seereer religion) believe in the veneration of the pangool (ancient Seereer saints and/or ancestral spirits). There are various types of pangool (singular: fangol), each with its own means of veneration.

Madagascar

Famadihana reburial ceremony

 

Veneration of ancestors is prevalent throughout the island of Madagascar. Approximately half of the country's population of 20 million currently practice traditional religion, which tends to emphasize links between the living and the razana (ancestors). The veneration of ancestors has led to the widespread tradition of tomb building, as well as the highlands practice of the famadihana, whereby a deceased family member's remains may be exhumed to be periodically re-wrapped in fresh silk shrouds before being replaced in the tomb. The famadihana is an occasion to celebrate the beloved ancestor's memory, reunite with family and community, and enjoy a festive atmosphere. Residents of surrounding villages are often invited to attend the party, where food and rum are typically served and a hiragasy troupe or other musical entertainment is commonly present. Veneration of ancestors is also demonstrated through adherence to fady, taboos that are respected during and after the lifetime of the person who establishes them. It is widely believed that by showing respect for ancestors in these ways, they may intervene on behalf of the living. Conversely, misfortunes are often attributed to ancestors whose memory or wishes have been neglected. The sacrifice of zebu is a traditional method used to appease or honor the ancestors. Small, everyday gestures of respect include throwing the first capful of a newly opened bottle of rum into the northeast corner of the room to give the ancestors their due share.

Asian cultures

Cambodia

During Pchum Ben and the Cambodian New Year people make offerings to their ancestors. Pchum Ben is a time when many Cambodians pay their respects to deceased relatives of up to seven generations. Monks chant the suttas in Pali language overnight (continuously, without sleeping) in prelude to the gates of hell opening, an event that is presumed to occur once a year, and is linked to the cosmology of King Yama originating in the Pali Canon. During this period, the gates of hell are opened and ghosts of the dead (preta) are presumed to be especially active. In order to combat this, food-offerings are made to benefit them, some of these ghosts having the opportunity to end their period of purgation, whereas others are imagined to leave hell temporarily, to then return to endure more suffering; without much explanation, relatives who are not in hell (who are in heaven or otherwise reincarnated) are also generally imagined to benefit from the ceremonies.

China

In China, ancestor veneration (敬祖, pinyin: jìngzǔ), as well as ancestor worship (拜祖, pinyin: bàizǔ), seeks to honour and reminiscence the actions of the deceased; the ultimate homage to the dead. The importance of paying respect to parents (and elders) lies with the fact that all physical bodily aspects of one's being were created by one's parents, who continued to tend to one's well-being until one is on firm footings. The respect and the homage to parents, is to return this gracious deed to them in life and after, the ultimate homage. The shi (尸; "corpse, personator") was a Zhou dynasty (1045 BCE-256 BCE) sacrificial representative of a dead relative. During a shi ceremony, the ancestral spirit supposedly would enter the personator, who would eat and drink sacrificial offerings and convey spiritual messages.

India

Shraadha taking place at Jagannath Ghat in Calcutta, at end of Pitru Paksha.

 

Burning of incense during a veneration at Mengjia Longshan Temple, which is dedicated to Guan Yu, Mazu, and others

 

Ancestors are widely revered, honoured, and venerated in India and China. Amongst Hindus and Sikhs, ancestors may be worshiped as Gramadevata (village deity) or clan deity, such as Jathera (also called Dhok, from Sanskrit Dahak or fire). The spirit of a dead person is called Pitrs, which is venerated. When a person dies, the family observes a thirteen-day mourning period, generally called śrāddha. A year thence, they observe the ritual of Tarpan, in which the family makes offerings to the deceased. During these rituals, the family prepares the food items that the deceased liked and offers food to the deceased. They offer this food to crows as well on certain days as it is believed that the soul comes in the form of a bird to taste it. They are also obliged to offer śrāddha, a small feast of specific preparations, to eligible Bramhins. Only after these rituals are the family members allowed to eat. It is believed that this reminds the ancestor's spirits that they are not forgotten and are loved, so it brings them peace. On Shradh days, people pray that the souls of ancestors be appeased, forget any animosity and find peace. Each year, on the particular date (as per the Hindu calendar) when the person had died, the family members repeat this ritual. 

Indian and Chinese practices of ancestor-worship are prevalent throughout Asia as a result of the large Indian and Chinese populations in countries such as Singapore, Malaysia, Indonesia, and elsewhere across the continent. Furthermore, the large Indian population in places such as Fiji and Guyana has resulted in these practices spreading beyond their Asian homeland.

Assam

Mae Dam Mae Phi celebrations in Assam, India.

 

The Ahom religion is based on ancestor-worship. The Ahoms believe that a man after his death remains as ‘Dam’(ancestor) only for a few days and soon he becomes ‘Phi’ (God). They also believe that the soul of a man which is immortal unites with the supreme soul, possesses the qualities of a spiritual being and always blesses the family. So every Ahom family in order to worship the dead establish a pillar on the opposite side of the kitchen (Barghar) which is called ‘Damkhuta’ where they worship the dead with various offerings like homemade wine, mah-prasad, rice with various items of meat and fish. Me-Dam-Me-Phi, a ritual centred on commemorating the dead, is celebrated by the Ahom people on 31 January every year in memory of the departed. It is the manifestation of the concept of ancestor worship that the Ahoms share with other peoples originating from the Tai-Shan stock. It is a festival to show respect to the departed ancestors and remember their contribution to society. On the day of Me-Dam Me Phi worship is offered only to Chaufi and Dam Chaufi because they are regarded as gods of heaven.

Paliya

Four Paliyas, one dedicated to man and three to women at Chhatardi, Bhuj, Kutch, Gujarat, India

 

The Paliya memorial stones are associated with ancestral worship in western India. These memorials are worshiped by people of associated community or decedents of a person on special days such as death day of person, event anniversaries, festivals, auspicious days in Kartika, Shraavana or Bhadrapada months of Hindu calendar. These memorials are washed with milk and water on these days. They are smeared with sindoor or kumkum and flowers are scattered over it. The earthen lamp is lighted near it with sesame oil. Sometimes a flag is erected over it.

Pitru Paksha

Apart from this, there is also a fortnight-long duration each year called Pitru Paksha ("fortnight of ancestors"), when the family remembers all its ancestors and offers "Tarpan" to them. This period falls just before the Navratri or Durga Puja falling in the month of Ashwin. Mahalaya marks the end of the fortnight-long Tarpan to the ancestors.

Sacrifices

Burning offerings

In traditional Chinese culture, are sometimes made to altars as food for the deceased. This falls under the modes of communication with the Chinese spiritual world concepts. Some of the veneration includes visiting the deceased at their graves, and making or buying offerings for the deceased in the Spring, Autumn, and Ghost Festivals. Due to the hardships of the late 19th- and 20th-century China, when meat and poultry were difficult to come by, sumptuous feasts are still offered in some Asian countries as a practice to the spirits or ancestors. However, in the orthodox Taoist and Buddhist rituals, only vegetarian food would suffice. For those with deceased in the afterlife or hell, elaborate or even creative offerings, such as servants, refrigerators, houses, car, paper money and shoes are provided so that the deceased will be able to have these items after they have died. Often, paper versions of these objects are burned for the same purpose. Originally, real-life objects were buried with the dead. In time these goods were replaced by full size clay models which in turn were replaced by scale models, and in time today's paper offerings (including paper servants).

Indonesia

In Indonesia ancestor worship has been a tradition of some of the indigenous people. Podom of the Toba Batak, Waruga of the Minahasans and the coffins of the Karo people (Indonesia) are a few examples of the forms the veneration takes.

Korea

A Korean jesa altar for ancestors

 

In Korea, ancestor veneration is referred to by the generic term jerye (hangul: 제례; hanja: 祭禮) or jesa (hangul: 제사; hanja: 祭祀). Notable examples of jerye include Munmyo jerye and Jongmyo jerye, which are performed periodically each year for venerated Confucian scholars and kings of ancient times, respectively. The ceremony held on the anniversary of a family member's death is called charye (차례). It is still practiced today.

The majority of Catholics, Buddhists and nonbelievers practice ancestral rites, although Protestants do not. The Catholic ban on ancestral rituals was lifted in 1939, when the Catholic Church formally recognized ancestral rites as a civil practice.

Ancestral rites are typically divided into three categories:

1. Charye (차례, 茶禮) – tea rites held four times a year on major holidays (Korean New Year, Chuseok)
2. Kije (기제, 忌祭) – household rites held the night before an ancestor's death anniversary (기일, 忌日)
3. Sije (시제, 時祭; also called 사시제 or 四時祭) – seasonal rites held for ancestors who are five or more generations removed (typically performed annually on the tenth lunar month)

Myanmar

Ancestor worship in modern-day Myanmar is largely confined to some ethnic minority communities, but mainstream remnants of it still exist, such as worship of Bo Bo Gyi (literally "great grandfather"), as well as of other guardian spirits such as nats, all of which may be vestiges of historic ancestor worship.

Ancestor worship was present in the royal court in pre-colonial Burma. During the Konbaung dynasty, solid gold images of deceased kings and their consorts were worshiped three times a year by the royal family, during the Burmese New Year (Thingyan), at the beginning and at the end of Vassa. The images were stored in the treasury and worshiped at the Zetawunzaung (ဇေတဝန်ဆောင်, "Hall of Ancestors"), along with a book of odes.

Some scholars attribute the disappearance of ancestor worship to the influence of Buddhist doctrines of anicca and anatta, impermanence and rejection of a 'self'.

Philippines

Various Igorot bulul depicting anito or ancestor spirits (c. 1900)

 

In the animistic indigenous religions of the precolonial Philippines, ancestor spirits were one of the two major types of spirits (anito) with whom shamans communicate. Ancestor spirits were known as umalagad (lit. "guardian" or "caretaker"). They can be the spirits of actual ancestors or generalized guardian spirits of a family. Ancient Filipinos believed that upon death, the soul of a person travels (usually by boat) to a spirit world. There can be multiple locations in the spirit world, varying in different ethnic groups. Which place souls end up in depends on how they died, the age at death, or conduct of the person when they were alive. There was no concept of heaven or hell prior to the introduction of Christianity and Islam; rather, the spirit world is usually depicted as an underworld that is a mirror image of the material ("upper") world. Souls reunite with deceased relatives in the underworld and lead normal lives in the underworld as they did in the material world. In some cases, the souls of evil people undergo penance and cleansing before they are granted entrance into a particular spirit realm. Souls would eventually reincarnate after a period of time in the spirit world.

Souls in the spirit world still retain a degree of influence in the material world, and vice versa. Paganito rituals may be used to invoke good ancestor spirits for protection, intercession, or advice. Vengeful spirits of the dead can manifest as apparitions or ghosts (mantiw) and cause harm to living people. Paganito can be used to appease or banish them. Ancestor spirits also figured prominently during illness or death, as they were believed to be the ones who call the soul to the underworld, guide the soul (a psychopomp), or meet the soul upon arrival.

Ancestor spirits are also known as kalading among the Cordillerans; tonong among the Maguindanao and Maranao; umboh among the Sama-Bajau; ninunò among Tagalogs; and nono among Bicolanos. Ancestor spirits are usually represented by carved figures called taotao. These were carved by the community upon a person's death. Every household had a taotao stored in a shelf in the corner of the house.

The predominantly Roman Catholic Filipino people still hold ancestors in particular esteem—though without the formality common to their neighbours—despite having been Christianised since coming into contact with Spanish missionaries in 1521. In the present day, ancestor veneration is expressed in having photographs of the dead by the home altar, a common fixture in many Filipino Christian homes. Candles are often kept burning before the photographs, which are sometimes decorated with garlands of fresh sampaguita, the national flower. Ancestors, particularly dead parents, are still regarded as psychopomps, as a dying person is said to be brought to the afterlife (Tagalog: sundô, "fetch") by the spirits of dead relatives. It is said that when the dying call out the names of deceased loved ones, they can see the spirits of those particular people waiting at the foot of the deathbed.

Filipino Catholic and Aglipayan veneration of the dead finds its greatest expression in the Philippines is the Hallowmas season between 31 October and 2 November, variously called Undás (based on the word for "[the] first", the Spanish andas or possibly honra), Todos los Santos (literally "All Saints"), and sometimes Áraw ng mga Patáy (lit. "Day of the Dead"), which refers to the following solemnity of All Souls' Day. Filipinos traditionally observe this day by visiting the family dead, cleaning and repairing their tombs. Common offerings are prayers, flowers, candles, and even food, while many also spend the remainder of the day and ensuing night holding reunions at the graveyard, playing games and music or singing.

Chinese Filipinos, meanwhile, have the most apparent and distinct customs related to ancestor veneration, carried over from traditional Chinese religion and most often melded with their current Catholic faith. Many still burn incense and kim at family tombs and before photos at home, while they incorporate Chinese practises into Masses held during the All Souls' Day period.

Sri Lanka

In Sri Lanka, making offerings to one's ancestors is conducted on the sixth day after death as a part of traditional Sri Lankan funeral rites.

Thailand

In rural northern Thailand, a religious ceremony honoring ancestral spirits known as Faun Phii (Thai: ฟ้อนผี, lit. "spirit dance" or "ghost dance") takes place. It includes offerings for ancestors with spirit mediums sword fighting, spirit-possessed dancing, and spirit mediums cock fighting in a spiritual cockfight.

Vietnam

A Vietnamese altar for ancestors. Note smaller Buddhist altar set higher in the upper corner

 

An old man in traditional dress on the occasion of New Year offering

 

Ancestor veneration is one of the most unifying aspects of Vietnamese culture, as practically all Vietnamese, regardless of religious affiliation (Buddhist or Catholic) have an ancestor altar in their home or business. 

In Vietnam, traditionally people did not celebrate birthdays (before Western influence), but the death anniversary of one's loved one was always an important occasion. Besides an essential gathering of family members for a banquet in memory of the deceased, incense sticks are burned along with hell notes, and great platters of food are made as offerings on the ancestor altar, which usually has pictures or plaques with the names of the deceased. In the case of missing persons, believed to be dead by their family, a Wind tomb is made.

These offerings and practices are done frequently during important traditional or religious celebrations, the starting of a new business, or even when a family member needs guidance or counsel and is a hallmark of the emphasis Vietnamese culture places on filial duty.

A significant distinguishing feature of Vietnamese ancestor veneration is that women have traditionally been allowed to participate and co-officiate ancestral rites, unlike in Chinese Confucian doctrine, which allows only male descendants to perform such rites.

European cultures

A scenic cemetery in rural Spain.

 

In Catholic countries in Europe (continued later with the Anglican Church in England), November 1 (All Saints' Day), became known and is still known as the day to honor those who have died, and who have been deemed official saints by the Church. November 2, (All Souls Day), or "The Day of the Dead", is the day when all of the faithful dead are remembered. On that day, families go to cemeteries to light candles for their dead relatives, leave them flowers, and often to picnic. The evening before All Saints'—"All Hallows Eve" or "Hallowe'en"—is unofficially the Catholic day to remember the realities of Hell, to mourn the souls lost to evil, and to remember ways to avoid Hell. It is commonly celebrated in the United States and parts of the United Kingdom in a spirit of light-hearted horror and fear, which is marked by the recounting of ghost stories, bonfires, wearing costumes, carving jack-o'-lanterns, and "trick-or-treating" (going door to door and begging for candy).

Brythonic Celtic cultures

In Cornwall and Wales, the autumn ancestor festivals occur around Nov. 1. In Cornwall the festival is known as Kalan Gwav, and in Wales as Calan Gaeaf. The festivals bear some similarities to the better-known Gaelic festival of Samhain, from which modern Halloween is derived.

Gaelic Celtic cultures

During Samhain, November 1 in Ireland and Scotland, the dead are thought to return to the world of the living, and offerings of food and light are left for them. On the festival day, ancient people would extinguish the hearth fires in their homes, participate in a community bonfire festival, and then carry a flame home from the communal fire and use it light their home fires anew. This custom has continued to some extent into modern times, in both the Celtic nations and the diaspora. Lights in the window to guide the dead home are left burning all night. On the Isle of Man the festival is known as "old Sauin" or Hop-tu-Naa.

North America

Native cultures of the original inhabitants of North America honored the dead with various traditional ceremonies, including food offerings and prayers. 

In the United States and Canada, flowers, wreaths, grave decorations and sometimes candles or even small pebbles are put on graves year-round as a way to honor the dead. In the United States, many people honor deceased loved ones who were in the military on Decoration Day also known as Memorial Day. Times like Easter, Christmas, Candlemas, and All Souls' Day are also special days in which the relatives and friends of the deceased gather to honor them with flowers and candles. In the Catholic Church, one's local parish church often offers prayers for the dead on their death anniversary or on special days like All Souls' Day. 

Another important holiday in the United States, Memorial Day, is a Federal holiday for remembering the men and women who served in the nation's military that are now deceased; particularly those who died in war or during active service. In the 147 national cemeteries, like Arlington and Gettysburg, it is common for volunteers to place small American flags at each grave. Memorial Day is observed on the last Monday in May, allotting for a 3 day weekend in which many memorial services and parades take place not only across the country, but in 26 American cemeteries on foreign soil; in the countries of France, Belgium, United Kingdom. Philippines, Panama, Italy, Luxembourg, Mexico, Netherlands, and Tunisia. It is also common practice among veterans to memorialize fallen service members on the dates of their death. That practice is also common in other countries when remembering Americans who died in battles to liberate their towns in the World Wars. One example of this is on 16 August (1944) Colonel Griffith, died of wounds from enemy action sustained in Lèves, the same day he is credited with saving Chartres Cathedral from destruction. 

Many Mexicans celebrate Dia de los Muertos (Day of the Dead) on or around All Saints' Day (November 1), this being a mix of a native Mesoamerican celebration and an imported European holiday. Ofrendas (altars) are set up, with calaveras (sugar skulls), photographs of departed loved ones, marigold flowers, candles, and more. 

In Judaism, when a grave site is visited, a small pebble is placed on the headstone. While there is no clear answer as to why, this custom of leaving pebbles may date back to biblical days when individuals were buried under piles of stones. Today, they are left as tokens that people have been there to visit and to remember.

Americans of various religions and cultures may build a shrine in their home dedicated to loved ones who have died, with pictures of their ancestors, flowers and mementos. Increasingly, many roadside shrines may be seen for deceased relatives who died in car accidents or were killed on that spot, sometimes financed by the state or province as these markers serve as potent reminders to drive cautiously in hazardous areas. The Vietnam Veterans Memorial in Washington, D.C., is particularly known for the leaving of offerings to the deceased; items left are collected by the National Park Service and archived. Members of The Church of Jesus Christ of Latter-day Saints perform posthumous baptisms and other rituals for their dead ancestors, along with those of other families, with the permission of their descendants.

Islam

Islam has a complex and mixed view on the idea of grave shrines and ancestor worship. The graves of many early Islamic figures are holy sites for Muslims, including Ali, and a cemetery with many companions and early caliphs. Many other mausoleums are major architectural, political, and cultural sites, including the National Mausoleum in Pakistan and the Taj Mahal in India. However, the violent religious movement of Wahhabism views this respect for holy sites as a form of idolatry. Followers of this movement have destroyed many gravesite shrines, including in Saudi Arabia and in territory controlled by the Islamic State. 

Muhammad said: “All the earth is a mosque apart from the graveyards and bathrooms.” (Narrated by al-Tirmidhi, 317; Ibn Maajah, 745; classed as saheeh by al-Albaani in Saheeh Ibn Maajah, 606).

Ancient cultures

Ancestor worship was a prominent feature of many historical societies.

Ancient Egypt

Although some historians claim that ancient Egyptian society was a "death cult" because of its elaborate tombs and mummification rituals, it was the opposite. The philosophy that "this world is but a vale of tears" and that to die and be with God is a better existence than an earthly one was relatively unknown among the ancient Egyptians. This was not to say that they were unacquainted with the harshness of life; rather, their ethos included a sense of continuity between this life and the next. The Egyptian people loved the culture, customs and religion of their daily lives so much that they wanted to continue them in the next—although some might hope for a better station in the Beautiful West (Egyptian afterlife). 

Tombs were housing in the Hereafter and so they were carefully constructed and decorated, just as homes for the living were. Mummification was a way to preserve the corpse so the ka (soul) of the deceased could return to receive offerings of the things s/he enjoyed while alive. If mummification was not affordable, a "ka-statue" in the likeness of the deceased was carved for this purpose. The Blessed Dead were collectively called the akhu, or "shining ones" (singular: akh). They were described as "shining as gold in the belly of Nut" (Gr. Nuit) and were indeed depicted as golden stars on the roofs of many tombs and temples. 

The process by which a ka became an akh was not automatic upon death; it involved a 70-day journey through the duat, or Otherworld, which led to judgment before Wesir (Gr. Osiris), Lord of the Dead where the ka’s heart would be weighed on a scale against the Feather of Ma’at (representing Truth). However, if the ka was not properly prepared, this journey could be fraught with dangerous pitfalls and strange demons; hence some of the earliest religious texts discovered, such as the Papyrus of Ani (commonly known as The Book of the Dead) and the Pyramid Texts were actually written as guides to help the deceased successfully navigate the duat. 

If the heart was in balance with the Feather of Ma'at, the ka passed judgment and was granted access to the Beautiful West as an akh who was ma’a heru ("true of voice") to dwell among the gods and other akhu. At this point only was the ka deemed worthy to be venerated by the living through rites and offerings. Those who became lost in the duat or deliberately tried to avoid judgment became the unfortunate (and sometimes dangerous) mutu, the Restless Dead. For the few whose truly evil hearts outweighed the Feather, the goddess Ammit waited patiently behind Wesir’s judgment seat to consume them. She was a composite creature resembling three of the deadliest animals in Egypt: the crocodile, the hippopotamus and the lion. Being fed to Ammit was to be consigned to the Eternal Void, to be "unmade" as a ka. 

Besides being eaten by Ammit, the worst fate a ka could suffer after physical death was to be forgotten. For this reason, ancestor veneration in ancient Egypt was an important rite of remembrance in order to keep the ka "alive" in this life as well as in the next. Royals, nobles and the wealthy made contracts with their local priests to perform prayers and give offerings at their tombs. In return, the priests were allowed to keep a portion of the offerings as payment for services rendered. Some tomb inscriptions even invited passers-by to speak aloud the names of the deceased within (which also helped to perpetuate their memory), and to offer water, prayers or other things if they so desired. In the private homes of the less wealthy, niches were carved into the walls for the purpose of housing images of familial akhu and to serve as altars of veneration. 

Many of these same religious beliefs and ancestor veneration practices are still carried on today in the religion of Kemetic Orthodoxy.

Ancient Rome

Detail from an early 2nd-century Roman sarcophagus depicting the death of Meleager

The Romans, like many Mediterranean societies, regarded the bodies of the dead as polluting. During Rome's Classical period, the body was most often cremated, and the ashes placed in a tomb outside the city walls. Much of the month of February was devoted to purifications, propitiation, and veneration of the dead, especially at the nine-day festival of the Parentalia during which a family honored its ancestors. The family visited the cemetery and shared cake and wine, both in the form of offerings to the dead and as a meal among themselves. The Parentalia drew to a close on February 21 with the more somber Feralia, a public festival of sacrifices and offerings to the Manes, the potentially malevolent spirits of the dead who required propitiation. One of the most common inscriptional phrases on Latin epitaphs is Dis Manibus, abbreviated D.M, "for the Manes gods", which appears even on some Christian tombstones. The Caristia on February 22 was a celebration of the family line as it continued into the present.

A noble Roman family displayed ancestral images (imagines) in the tablinium of their home (domus). Some sources indicate these portraits were busts, while others suggest that funeral masks were also displayed. The masks, probably modeled of wax from the face of the deceased, were part of the funeral procession when an elite Roman died. Professional mourners wore the masks and regalia of the dead person's ancestors as the body was carried from the home, through the streets, and to its final resting place.