| A rocket engine is a reaction engine that can | | | | material would fail. |
| be used for spacecraft propulsion as well as | | | | |
| terrestrial uses, such as missiles. Rocket | | | | Mechanical issues |
| engines take their reaction mass from within | | | | |
| the vehicle and form it into a high speed | | | | The combustion chamber is often under |
| jet, obtaining thrust in accordance with | | | | substantial pressure, typically 10-200 bar, |
| Newton's third law. Most rocket engines are | | | | higher pressures giving better performance. |
| internal combustion engines, although non | | | | This causes the outermost part of the chamber |
| combusting forms exist. | | | | to be under very large hoop stresses. |
| | | | |
| Classic rocket engines produce a high | | | | Worse, due to the high temperatures created |
| temperature, hypersonic gaseous exhaust. This | | | | in rocket engines the materials used tend to |
| is most often achieved by the combustion of | | | | have a significantly lowered working tensile |
| solid, liquid or gaseous propellant, | | | | strength. |
| containing oxidiser and a fuel, within a | | | | |
| combustion chamber at high pressure. | | | | Safety |
| Alternatively, a chemically inert reaction | | | | |
| mass can be heated to high temperature using | | | | They are tested at a rocket engine test |
| a high energy power source. | | | | facility before being put into production. |
| | | | |
| The hot gas produced is then allowed to | | | | Rockets have a reputation for unreliability |
| escape through a narrow hole (the 'throat'), | | | | and danger; particularly catastrophic |
| into a high-expansion ratio nozzle. The | | | | failures. |
| effect of the nozzle is to dramatically | | | | |
| accelerate the mass, converting most of the | | | | In fact, carefully designed rockets can |
| thermal energy into kinetic energy. The large | | | | probably be made arbitrarily reliable. In |
| bell or cone shaped expansion nozzle gives a | | | | military use, rockets are not unreliable. |
| rocket engine its characteristic shape. | | | | However one of the main uses of rockets is |
| Exhaust speeds as high as 10 times the speed | | | | for orbital launch. There the premium is on |
| of sound at sea level are not uncommon. | | | | minimum weight, and it is difficult to |
| | | | achieve high reliability and low weight |
| Part of the rocket engine's thrust comes from | | | | simultaneously. In addition the number of |
| the gas pressure inside the combustion | | | | flights launched is low, thus there is a very |
| chamber but the majority comes from the | | | | high chance of a design, operations or |
| pressure against the inside of the expansion | | | | manufacturing error causing destruction of |
| nozzle. Inside the combustion chamber the gas | | | | the vehicle. Essentially, as of 2006 all |
| produces a similar force against all the | | | | launch vehicles are test vehicles by normal |
| sides of the combustion chamber but the | | | | aerospace standards. |
| throat gives no force producing an unopposed | | | | |
| resultant force from the diametrically | | | | The X-15 rocket plane achieved a 0.5% failure |
| opposite end of the chamber. As the gases | | | | rate, with a single catastrophic failure |
| (adiabatically) expand inside the nozzle they | | | | during ground test, and the SSME has managed |
| press against the bell's walls forcing the | | | | to avoid catastrophic failures in over 300 |
| rocket engine in one direction, and | | | | engine-flights. |
| accelerating the gases in the opposite | | | | |
| direction. | | | | Noise |
| | | | |
| For optimum performance hot gas is used | | | | The Saturn V launch was detectable on |
| because it maximises the speed of sound at | | | | seismometers a considerable distance from the |
| the throat — for aerodynamic reasons | | | | launch site. As the hypersonic exhaust mixes |
| the flow goes sonic ("chokes") at the throat, | | | | with the ambient air, shock waves are formed. |
| so the highest speed there is desirable. By | | | | The sound intensity from these shock waves |
| comparison, at room temperature the speed of | | | | depends on the size of the rocket, and on |
| sound in air is about 340m/s, the speed of | | | | large rockets can actually kill. The Space |
| sound in the hot gas of a rocket engine can | | | | Shuttle generates over 200 dB(A) of noise |
| be over 1700m/s. | | | | around its base. |
| | | | |
| The expansion part of the rocket nozzle then | | | | Generally speaking noise is most intense when |
| multiplies the speed of the flow by a further | | | | a rocket is close to the ground, since the |
| factor, typically between 1.5 and 4 times, | | | | noise from the engines radiate up away from |
| giving a highly collimated exhaust jet. The | | | | the plume, as well as reflecting off the |
| speed ratio of a rocket nozzle is mostly | | | | ground. This noise can be reduced somewhat by |
| determined by its area expansion ratio | | | | flame trenches with roofs, by water injection |
| — the ratio of the area of the throat | | | | around the plume and by deflecting the plume |
| to the area at the exit, but details of the | | | | at an angle. |
| gas properties are also important. Larger | | | | |
| ratio nozzles are more massive and bulkier, | | | | Chemistry |
| but they are able to extract more heat from | | | | |
| the combustion gases, which become lower in | | | | Contrary to popular belief, while rocket |
| pressure and colder, but also faster. | | | | propellants require reasonably high energy |
| | | | per kilogram, many common materials are more |
| A significant complication arises when | | | | energetic; for example petrol/gasoline or |
| launching a vehicle from the Earth's surface | | | | paraffin has as much energy as a rocket fuel |
| as the ambient atmospheric pressure changes | | | | and far more than the fuel/oxidiser mix used |
| with altitude. For maximum performance it | | | | for rocket fuels. This is due to the |
| turns out that the pressure of the gas | | | | necessity of the propellant containing large |
| leaving a rocket nozzle should be the same as | | | | amounts of oxidiser, normal propellants used |
| ambient pressure; if lower the vehicle will | | | | on earth for say, Turbojet engines, are |
| be slowed by the difference in pressure | | | | reacted with the atmosphere and hence can |
| between the top of the engine and the exit, | | | | have several times higher energy density. |
| if higher then this represents pressure that | | | | |
| the bell has not turned into thrust. To | | | | Good rocket propellants require large |
| achieve this ideal, the diameter of the | | | | quantities of hydrogen in the propellant, as |
| nozzle would need to increase with altitude, | | | | this gives the highest exhaust speeds |
| which is difficult to arrange. A compromise | | | | primarily due to the low molecular mass; but |
| nozzle is generally used and some percentage | | | | this is not the whole story. |
| reduction in performance occurs. To improve | | | | |
| on this, various exotic nozzle designs such | | | | Programs exist to predict the performance of |
| as the plug nozzle, stepped nozzles, the | | | | propellants in rocket engines. |
| expanding nozzle and the aerospike have been | | | | |
| proposed, each having some way to adapt to | | | | Ignition |
| changing ambient air pressure and each | | | | |
| allowing the gas to expand further against | | | | With liquid propellants immediate ignition of |
| the nozzle giving extra thrust at higher | | | | the propellants as they first enter the |
| altitude. | | | | combustion chamber is essential. |
| | | | |
| Thermal issues | | | | Failure to ignite within milliseconds causes |
| | | | too much liquid propellant to be within the |
| The reaction mass's combustion temperatures | | | | chamber, and if/when ignition occurs the |
| can fairly typically reach ~3500 K (~5800 F) | | | | amount of hot gas created will often exceed |
| which is often far higher than the melting | | | | the maximum design pressure of the chamber. |
| point of the nozzle and combustion chamber | | | | The pressure vessel will often fail |
| materials (~1200K for copper). Indeed many | | | | catastrophically. This is sometimes called a |
| construction materials can make perfectly | | | | hard start. |
| acceptable propellants in their own right. It | | | | |
| is important that these materials be | | | | Ignition can be achieved by a number of |
| prevented from combusting, melting or | | | | different methods; a pyrotechnic charge can |
| vapourising to the point of failure. | | | | be used, the propellants can ignite |
| Materials technology could potentially place | | | | spontaneously on contact (hypergolic), a |
| an upper limit on the exhaust temperature of | | | | plasma torch can be used, or electric spark |
| chemical rockets. | | | | plugs may be employed. |
| | | | |
| To avoid this problem rockets can use | | | | Gaseous propellants generally will not cause |
| ablative materials that erode in a controlled | | | | hardstarts, with rockets the total injector |
| fashion, or very high temperature materials, | | | | area is less than the throat thus the chamber |
| such as graphite, ceramics or certain exotic | | | | pressure tends to ambient prior to ignition |
| metals. | | | | and high pressures cannot form even if the |
| | | | entire chamber is full of flammable gas at |
| Alternatively, rockets may use more common | | | | ignition. |
| construction materials such as aluminum, | | | | |
| steel, nickel or copper alloys and employ | | | | Solid propellants are usually ignited with |
| cooling systems that prevent the construction | | | | one-shot pyrotechnic devices. |
| material itself becoming too hot. | | | | |
| Regenerative cooling, where the propellant is | | | | Once ignited, rocket chambers are self |
| passed through tubes around the combustion | | | | sustaining and igniters are not needed, |
| chamber or nozzle, and other techniques such | | | | indeed chambers often spontaneously reignite |
| as curtain cooling or film cooling, may be | | | | if restarted after being shut down for a few |
| employed to give essentially unlimited nozzle | | | | seconds. However, when cooled, many rockets |
| and chamber life. These techniques ensure | | | | cannot be started more than once without |
| that the gas boundary layer touching the | | | | minor maintenance, such as replacement of the |
| material is kept below the point where the | | | | pyrotechnic igniter. |