Rocket engine

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RS-68 being tested.

A rocket engine is a device that produces a force by pushing gases at high speed out of a nozzle. Rocket engines burn chemicals such as petroleum and liquid oxygen at very high pressures and temperatures to turn the chemical energy into motion. In some cases (such as NASA rockets), the force created can be over 1,000,000 pounds-force (4,400,000 newtons).

A garden hose shows how moving fluid can create a force. When a hose is turned up the hose will snake around unless it is held still. The exiting water is creating a force on the hose just like how the rocket engine gases push on the rocket engine.

Liquids, solids and hybrids[change | change source]

Some rocket engines burn liquid fuels while some burn solid fuels. Solid fuel rocket engines are sometimes called "rocket motors".

Liquid fuel rocket engines often require complex pumps and valves to properly move (and pressurize) the liquids from the fuel tank to the actual engine. These machines must work in extreme temperatures and pressures. Liquid oxygen is very cold (-223˚C) while the engine is very hot (3000˚C), and the pressure is oftentimes hundreds of times higher than normal air pressure. Because of these conditions, liquid fuel rocket engines are often very complex and require very specialised materials (metals, ceramics, etc.).

Solid fuel rocket motors have the fuel (called propellant) as a solid mixture of an oxidizer and fuel. An oxidiser supports the burning of fuel much like oxygen supports burning. The common oxidiser is powdered Ammonium Perchlorate, while the common fuel is powdered aluminum metal. The two powders are stuck together with a third component known as the binder. The binder is a rubbery solid that also burns as a fuel. The simple idea makes solid rocket engines cheaper, but they cannot be turned off or controlled, and are more likely to explode than liquid rocket engines. Solid rockets also provide a smaller specific impulse, hence must be heavier to launch the same payload.

Military missiles commonly use solid rockets because they can be kept ready for many years. Many satellite launchers use solid rocket boosters when they start, but liquid rockets for the majority of the flight.

Hybrid rocket engines combine the two ideas. The two propellants are different states of matter, often with liquid oxidisers and solid fuels. They are not used much, but might be safer than solid rocket motors or liquid rocket motors

  RL-10 HM7B Vinci KVD-1 CE-7.5 CE-20 YF-75 YF-75D RD-0146 ES-702 ES-1001 LE-5 LE-5A LE-5B
Country of origin  United States  France  France  Soviet Union  India  India  China  China  Russia  Japan  Japan  Japan  Japan  Japan
Cycle Expander Gas-generator Expander Staged combustion Staged combustion Gas-generator Gas-generator Expander Expander Gas-generator Gas-generator Gas-generator Expander bleed cycle
(Nozzle Expander)
Expander bleed cycle
(Chamber Expander)
Thrust (vac.) 66.7 kN (15,000 lbf) 62.7 kN 180 kN 69.6 kN 73 kN 200 kN 78.45 kN 88.26 kN 98.1 kN (22,054 lbf) 68.6kN (7.0 tf)[1] 98kN (10.0 tf)[2] 102.9kN (10.5 tf) r121.5kN (12.4 tf) 137.2kN (14 tf)
Mixture ratio 5.2 6.0 5.2 6.0 5.5 5 5
Nozzle ratio 40 100 80 80 40 40 140 130 110
Isp (vac.) 433 444.2 465 462 454 443 438 442 463 425[3] 425[4] 450 452 447
Chamber pressure :MPa 2.35 3.5 6.1 5.6 5.8 6.0 3.68 7.74 2.45 3.51 3.65 3.98 3.58
LH2 TP rpm 125,000 41,000 46,310 50,000 51,000 52,000
LOX TP rpm 16,680 21,080 16,000 17,000 18,000
Length m 1.73 1.8 2.2~4.2 2.14 2.14 2.8 2.2 2.68 2.69 2.79
Dry weight kg 135 165 280 282 435 558 550 242 255.8 259.4 255 248 285
  1. without nozzle 48.52kN (4.9 tf)
  2. without nozzle 66.64kN (6.8 tf)
  3. without nozzle 286.8
  4. without nozzle 291.6