Download Cryogenic Engine in Rocket Propulsion PDF

TitleCryogenic Engine in Rocket Propulsion
TagsRocket Propellant Rocket Engine Cryogenics Space Shuttle Main Engine Rocket
File Size615.7 KB
Total Pages29
Document Text Contents
Page 14

rate of both oxidizer and fuel to generate a sufficient thrust. At that time
oxygen and low molecular weight hydrocarbons were used as oxidizer
and fuel pair. At room temperature and pressure, both are in gaseous
state. Hypothetically, if propellants had been stored as pressurized
gases, the size and mass of fuel tanks themselves would severely
decrease rocket efficiency. Therefore, to get the required mass flow
rate, the only option was to cool the propellants down to cryogenic
temperatures (below −150 °C, −238 °F), converting them to liquid form.
Hence, all cryogenic rocket engines are also, by definition, either liquid-
propellant rocket engines or hybrid rocket engines.

 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. Both components are easily and
cheaply available, and when burned have one of the highest entropy
releases by combustion, producing specific impulse up to 450 s
(effective exhaust velocity 4.4 km/s).


The major components of a cryogenic rocket engine are: combustion
chamber (thrust chamber), pyrotechnic igniter, fuel injector, fuel
cryopumps, oxidizer cryopumps, gas turbine, cryo valves, regulators, the
fuel tanks, and rocket engine nozzle. In terms of feeding propellants to
combustion chamber, cryogenic rocket engines (or, generally, all liquid-
propellant engines) work in either an expander cycle, a gas-generator
cycle, a staged combustion cycle, or the simplest pressure-fed cycle.

The cryopumps are always turbopumps powered by a flow of fuel through
gas turbines. Looking at this aspect, engines can be differentiated into a main
flow or a bypass flow configuration. In the main flow design, all the pumped
fuel is fed through the gas turbines, and in the end injected to the combustion
chamber. In the bypass configuration, the fuel flow is split; the main part goes
directly to the combustion chamber to generate thrust, while only a small
amount of the fuel goes to the turbine.

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Liquid-fuelled engine

Propellant: LOX / Liquid hydrogen

The engines burn liquid hydrogen and liquid oxygen from the Space Shuttle
external tank. They are used for propulsion during its ascent, in addition to the
two more powerful solid rocket boosters and partly the Orbital Maneuvering
System. Each engine can generate almost 1.8 meganewtons (MN) or 400,000
lbf of thrust at liftoff. The engines are capable of generating a specific impulse
(Isp) of 453 seconds in a vacuum, or 363 seconds at sea level (exhaust
velocities of 4440 m/s and 3560 m/s respectively). Overall, a space shuttle
main engine weighs approximately 3.2 t (7,000 lb). The engines are removed
after every flight and taken to the Space Shuttle Main Engine Processing
Facility (SSMEPF) for inspection and replacement of any necessary

The Space Shuttle's rocket engines are capable of operating at extreme
temperatures. The liquid hydrogen fuel is stored at −253 degrees Celsius (−423
degrees Fahrenheit). However, when burned with liquid oxygen, the
temperature in the combustion chamber reaches 3,300 °C (6,000 °F), higher
than the boiling point of iron. Each engine consumes 1,340 liters (340 gallons)
of propellant per second. If the engine pumped water instead of liquid oxygen
and liquid hydrogen, an average-sized swimming pool could be drained in 75
seconds - or 25 seconds for the sum of the three used for the space shuttle

The engines perform as follows: Fuel and oxidizer from the external tank
enters the orbiter at the orbiter/external tank umbilical disconnect and then
the orbiter's main propulsion system feed lines. There the fuel and oxidizer
each branch out into three parallel paths, to each engine. In each branch,
prevalves must be opened to permit flow to the low-pressure fuel or oxidizer

Page 28

This advanced propulsion technology is now available only with Russia and
USA. India capability to meet existing mission requirements. The semi cryogenic
engine will facilitate applications for future space missions such as the Reusable
Launch Vehicle, Unified Launch Vehicle and vehicle for interplanetary missions.

 High Specific Impulse
 Non-toxic and non-corrosive propellants
 Non-hypergolic, improved ground safety

 Low density of liquid Hydrogen –more structural mass
 Low temperature of propellants -Complex storage
 Transfer systems and operations
 Hazards related to cryogens
 Overall cost of propellants relatively high
 Need for ignition system

Drawbacks of Cryogenic Propellants

 Highly reactive gases

Cryogens are highly concentrated gases and have a very high reactivity. Liquid
oxygen, which is used as an oxidizer, combines with most of the organic
materials to form explosive compounds. So lots of care must be taken to
ensure safety.

 Leakage

One of the most major concerns is leakage. At cryogenic temperatures, which
are roughly below 150 degrees Kelvin or equivalently (-190) degrees
Fahrenheit, the seals of the container used for storing the propellants lose the
ability to maintain a seal properly. Hydrogen, being the smallest element, has
a tendency to leak past seals or materials.

 Hydrogen can burst into flames whenever its concentration is approximately
4% to 96%. It is hence necessary to ensure that hydrogen leak rate is minimal


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