Liquid-propellant rocket

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Liquid rocket schematic.



Liquid rockets can be monopropellant rockets using a single type of propellant, bipropellant rockets using two types of propellant, or more exotic tripropellant rockets using three types of propellant. Some designs are throttleable for variable thrust operation and some may be restarted after a previous in-space shutdown. Liquid propellants are also used in hybrid rockets, in which a liquid oxidizer is generally combined with a solid fuel.






After Goddard's success, German engineers and scientists became enthralled with liquid-fuel rockets and designed and built rockets, testing them in the early 1930s in a field near Berlin.[5] This amateur rocket group, the VfR, included Wernher von Braun, who became the head of the army research station that secretly built the V-2 rocket weapon for the Nazis. The German-Romanian Hermann Oberth published a book in 1922 suggesting the use of liquid propellants.






Principle of operation[edit]















One of the most efficient mixtures, oxygen and hydrogen, suffers from the extremely low temperatures required for storing liquid hydrogen (around 20 K or −253 °C) and very low fuel density (70 kg/m³, compared to RP-1 at 820kg/m³), necessitating large tanks that must also be lightweight and insulating. Lightweight foam insulation on the Space Shuttle external tank led to the Space Shuttle Columbia's destruction, as a piece broke loose, damaged its wing and caused it to break up on atmospheric reentry.

For storable ICBMs and most spacecraft, including crewed vehicles, planetary probes, and satellites, storing cryogenic propellants over extended periods is unfeasible. Because of this, mixtures of hydrazine or its derivatives in combination with nitrogen oxides are generally used for such applications, but are toxic and carcinogenic. Consequently, to improve handling, some crew vehicles such as Dream Chaser and Space Ship Two plan to use hybrid rockets with non-toxic fuel and oxidizer combinations.




Types of injectors[edit]




The main type of injectors are

  • Shower Head type
  • Self Impinging doublet type
  • Cross impinging triplet type
  • Centripetal or Swirling type
  • Pintle injector type



Valentin Glushko invented the centripetal injector in the early 1930s, and it has been almost universally used in Russian engines. Rotational motion is applied to the liquid (and sometimes the two propellants are mixed), then it is expelled through a small hole, where it forms a cone-shaped sheet that rapidly atomizes. Goddard's first liquid-fuel engine used a single impinging injector. German scientists in WWII experimented with impinging injectors on flat plates, used successfully in the Wasserfall missile.

Combustion stability[edit]






Engine cycles[edit]

For liquid-propellant rockets four different ways of powering the injection of the propellant into the chamber are in common use.[9]

Fuel and oxidizer must be pumped into the combustion chamber against the pressure of the hot gasses being burned, and engine power is limited by the rate at which propellant that can be pumped into the combustion chamber. For atmospheric or launcher use, high pressure, and thus high power, engine cycles are desirable to minimize gravity drag. For orbital use, lower power cycles are usually fine.

  • pressure fed cycle – the propellants are forced in from pressurised (relatively heavy) tanks. The heavy tanks mean that a relatively low pressure is optimal, limiting engine power, but all the fuel is burned, allowing high efficiency. The pressurant used is frequently helium due to its lack of reactivity and low density. Examples: AJ-10, used in the Space Shuttle OMS, Apollo SPS, and the second stage of the Delta II.
  • gas generator cycle – a small percentage of the propellants are burnt in a preburner to power a turbopump and then exhausted through a separate nozzle, or low down on the main one. This results in a reduction in efficiency since the exhaust contributes little or no thrust, but the pump turbines can be very large, allowing for high power engines. Examples: Saturn V's F-1 and J-2, Delta IV's RS-68, Ariane 5's HM7B, Falcon 9's Merlin.
  • expander cycle – cryogenic fuel (hydrogen, or methane) is used to cool the walls of the combustion chamber and nozzle. Absorbed heat vaporizes and expands the fuel which is then used to drive the turbopumps before it enters the combustion chamber, allowing for high efficiency, or is bled overboard, allowing for higher power turbopumps. The limited heat available to vaporize the fuel constrains engine power. Examples: RL-10 for Atlas V and Delta IV second stages (closed cycle), H-II's LE-5 (bleed cycle).
  • staged combustion cycle – a fuel or oxidizer rich mixture is burned in a turbine to power the turbopumps, and this high pressure exhaust is fed directly into the main chamber where the remainder of the fuel or oxidizer undergoes combustion, permitting very high pressures and efficiency. Examples: SSME, RD-191, LE-7.

Engine cycle tradeoffs[edit]


Tradeoff comparison among popular engine cycles
Cycle type: Gas generator Expander cycle Staged-combustion Pressure-fed
Advantages simple; low dry mass; allows for high power turbopumps for high thrust high specific impulse; fairly low complexity high specific impulse; high combustion chamber pressures allowing for high thrust simple; no turbopumps; low dry mass; high specific impulse
Disadvantages lower specific impulse must use cryogenic fuel; heat transfer to the fuel limits available power to the turbine and thus engine thrust greatly increased complexity tank pressure limits combustion chamber pressure and thrust; heavy tanks and associated pressurization hardware


Main article: Rocket engine cooling

Injectors are commonly laid out so that a fuel-rich layer is created at the combustion chamber wall. This reduces the temperature there, and downstream to the throat and even into the nozzle and permits the combustion chamber to be run at higher pressure, which permits a higher expansion ratio nozzle to be used which gives a higher ISP and better system performance.[10] A liquid rocket engine often employs regenerative cooling, which uses the fuel or less commonly the oxidiser to cool the chamber and nozzle.







  4. ^ "Re-Creating History". NASA. 
  6. ^ NASA:Liquid rocket engines, 1998, Purdue University
  9. ^ "Sometimes, Smaller is Better". 
  10. ^ Rocket Propulsion elements - Sutton Biblarz, section 8.1

External links[edit]