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Cryogenic propellants

Rocket engines need high mass flow rates of both oxidizer and fuel to generate useful thrust. Oxygen, the simplest and most common oxidizer, is in the gas phase at standard temperature and pressure, as is hydrogen, the simplest fuel. While it is possible to store propellants as pressurized gases, this would require large, heavy tanks that would make achieving orbital spaceflight difficult if not impossible. On the other hand, if the propellants are cooled sufficiently, they exist in the liquid phase at higher density and lower pressure, simplifying tankage. These cryogenic temperatures vary depending on the propellant, with liquid oxygen existing below −183 °C (−297.4 °F; 90.1 K) and liquid hydrogen below −253 °C (−423.4 °F; 20.1 K). Since one or more of the propellants is in the liquid phase, all cryogenic rocket engines are by definition liquid-propellant rocket engines.³

Various cryogenic fuel-oxidizer combinations have been tried, but the combination of liquid hydrogen (LH2) fuel and the liquid oxygen (LOX) oxidizer is one of the most widely used.⁴⁵ Both components are easily and cheaply available, and when burned have one of the highest enthalpy releases in combustion,⁶ producing a specific impulse of up to 450 s at an effective exhaust velocity of 4.4 kilometres per second (2.7 mi/s; Mach 13).

Components and combustion cycles

The major components of a cryogenic rocket engine are the combustion chamber, pyrotechnic initiator, fuel injector, fuel and oxidizer turbopumps, cryo valves, regulators, the fuel tanks, and rocket engine nozzle. In terms of feeding propellants to the combustion chamber, cryogenic rocket engines are almost exclusively pump-fed. Pump-fed engines work in a gas-generator cycle, a staged-combustion cycle, or an expander cycle. Gas-generator engines tend to be used on booster engines due to their lower efficiency, staged-combustion engines can fill both roles at the cost of greater complexity, and expander engines are exclusively used on upper stages due to their low thrust.

LOX+LH2 rocket engines by country

Currently, six countries have successfully developed and deployed cryogenic rocket engines:

Comparison of first stage cryogenic rocket engines

Comparison of upper stage cryogenic rocket engines

External links

• USA's Cryogenic Rocket engine RL10B-2
• Russian Cryogenic Rocket Engines
• [1]

References

1. Bilstein, Roger E. (1995). Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles (NASA SP-4206) (The NASA History Series). NASA History Office. pp. 89–91. ISBN 0-7881-8186-6. 0-7881-8186-6 ↩

2. Bilstein, Roger E. (1995). Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles (NASA SP-4206) (The NASA History Series). NASA History Office. pp. 89–91. ISBN 0-7881-8186-6. 0-7881-8186-6 ↩

3. Biblarz, Oscar; Sutton, George H. (2009). Rocket Propulsion Elements. New York: Wiley. p. 597. ISBN 978-0-470-08024-5. 978-0-470-08024-5 ↩

4. Bilstein, Roger E. (1995). Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles (NASA SP-4206) (The NASA History Series). NASA History Office. pp. 89–91. ISBN 0-7881-8186-6. 0-7881-8186-6 ↩

5. The liquefaction temperature of oxygen is 89 kelvins, and at this temperature it has a density of 1.14 kg/L. For hydrogen it is 20 K, just above absolute zero, and has a density of 0.07 kg/L. /wiki/Kelvin ↩

6. Biswas, S. (2000). Cosmic perspectives in space physics. Bruxelles: Kluwer. p. 23. ISBN 0-7923-5813-9. "... [LH2+LOX] has almost the highest specific impulse." 0-7923-5813-9 ↩

7. "Le-7A｜エンジン｜H-Iiaロケット｜ロケット｜Jaxa 宇宙輸送技術部門". https://www.rocket.jaxa.jp/rocket/engine/le7/ ↩

8. "Le-5B｜エンジン｜H-Iiaロケット｜ロケット｜Jaxa 宇宙輸送技術部門". https://www.rocket.jaxa.jp/rocket/engine/le5b/ ↩

9. "Le-9｜エンジン｜H3ロケット｜ロケット｜Jaxa 宇宙輸送技術部門". https://www.rocket.jaxa.jp/rocket/engine/le9/ ↩

10. without nozzle 48.52kN (4.9 tf) ↩

11. without nozzle 66.64kN (6.8 tf) ↩

12. without nozzle 286.8 ↩

13. without nozzle 291.6 ↩