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