Cryogenic rocket engines use cryogenic, or very cold liquefied gases, as propellants. They are typically fueled by liquid hydrogen and liquid oxygen which must be stored at temperatures below -423°F and -297°F respectively to remain in liquid form. Cryogenic engines offer advantages like being able to control and restart the fuel flow, but require complex turbopumps and thermal insulation to handle the extreme cold temperatures. Major components include thrust chambers, fuel injectors, turbopumps, igniters, valves, tanks, and de Laval nozzles. The Space Shuttle's main engines were cryogenic, using this technology to produce high exhaust velocities and thrust.

1. Cryogenic rocket engine and propellents
Rocket engine
A rocket engine, or simply "rocket", is a jet engine[1] that uses only propellant mass for forming
its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in
accordance with Newton's third law. Since they need no external material to form their
jet, rocket engines can be used for spacecraft propulsion as well as terrestrial uses, such
as missiles. Most rocket engines are internal combustion engines, although non combusting
forms also exist.
Rocket engines as a group have the highest exhaust velocities, are by far the lightest, but are the
least propellant efficient of all types of jet engines.
Types of rocket engines
Rocket motor (or solid-propellant rocket motor) is a synonymous term with rocket engine that
usually refers to solid rocket engines.
Liquid rockets (or liquid-propellant rocket engine) use one or more liquid propellants that are
held in tanks prior to burning.
Hybrid rockets have a solid propellant in the combustion chamber and a second liquid or gas
propellant is added to permit it to burn.
Thermal rockets are rockets where the propellant is inert, but is heated by a power source such
as solar or nuclear power or beamed energy.
Monopropellant rockets are rockets where the propellant is one chemical, typically hi-test
(85%+) hydrogen peroxide, which is decomposed by a catalyst producing steam and oxygen.
There is no flame.
Liquid-propellant rocket
A liquid-propellant rocket or a liquid rocket is a rocket engine that
uses propellants in liquid form. Liquids are desirable because their reasonably high density allows
the volume of the propellant tanks to be relatively low, and it is possible to use lightweight pumps
to pump the propellant from the tanks into the engines, which means that the propellants can be
kept under low pressure. This permits the use of low mass propellant tanks, permitting a
high mass ratio for the rocket.

3. Liquid Propellants
In a liquid propellant rocket, the fuel and oxidizer are stored in separate tanks, and are
fed through a system of pipes, valves, and turbopumps to a combustion chamber where
they are combined and burned to produce thrust. Liquid propellant engines are more
complex than their solid propellant counterparts, however, they offer several
advantages. By controlling the flow of propellant to the combustion chamber, the
engine can be throttled, stopped, or restarted.
A good liquid propellant is one with a high specific impulse or, stated another way, one
with a high speed of exhaust gas ejection. This implies a high combustion temperature
and exhaust gases with small molecular weights. However, there is another important
factor which must be taken into consideration: the density of the propellant. Using low
density propellants means that larger storage tanks will be required, thus increasing
the mass of the launch vehicle. Storage temperature is also important. A propellant
with a low storage temperature, i.e. a cryogenic, will require thermal insulation, thus
further increasing the mass of the launcher. The toxicity of the propellant is likewise
important. Safety hazards exist when handling, transporting, and storing highly toxic
compounds. Also, some propellants are very corrosive, however, materials that are
resistant to certain propellants have been identified for use in rocket construction.
Liquid propellants used in rocketry can be classified into three types: petroleum,
cryogens, and hypergols.
Cryogenic rocket engine
A cryogenic rocket engine is a rocket engine that uses a cryogenic fuel or oxidizer, that is, its
fuel or oxidizer (or both) are gases liquefied and stored at very low temperatures.
It is a type of liquid propellent rocket engine.
Cryogenic Propellants
In a cryogenic propellant the fuel and the oxidizer are in the form of very
cold, liquefied gases. These liquefied gases are referred to as super
cooled as they stay in liquid form even though they are at a temperature
lower than the freezing point. Thus we can say that super cooled gases
used as liquid fuels are called cryogenic fuels.
Cryogenic Engines are rocket motors designed for liquid fuels that have to be held at very low "cryogenic"
temperatures to be liquid - they would otherwise be gas at normal temperatures. Typically Hydrogen and
Oxygen are used which need to be held below 20°K (-423°F) and 90°K (-297°F) to remain liquid.
The Space Shuttle's main engines used for liftoff are cryogenic engines. The Shuttle's smaller thrusters for
orbital manuvering use non-cyogenic hypergolic fuels, which are compact and are stored at warm

4. temperatures. Currently, only the United States, Russia, China, France, Japan and India have mastered
cryogenic rocket technology.
The use of liquid fuel rocket engines was first considered by the German, American and Soviet engineers
independently, and all discovered that rocket engines needed high mass flow rate for both liquid oxidizer
and fuel, for generating the necessary thrust. Higher thrust levels were achieved when liquid oxygen (LOX)
and liquid hydrocarbon were used as fuel.
At atmospheric conditions, liquid oxygen and low molecular weight hydrocarbons are in gaseous state and
to get required mass flow rate, the only option is to feed them to the engine in liquid form. For that, these
are stored in liquid form by cooling them down and hence the name cryogenic rocket engines. Various
cryogenic fuels combination were tried and the liquid oxygen oxidizer and the liquid hydrogen (LH2) fuel
combination caught special attention of engineers as: both components are easily and cheaply available,
bio-friendly, non-corrosive and have the highest entropy release by combustion, among all non-toxic pairs.
Liquefaction temperature of oxygen is 89 kelvins and at this temperature liquid oxygen achieves a density of
1,140 kg/m3 (1.14 g/cm3). And, for hydrogen it is 20 kelvins, a few kelvins above absolute zero, and gains
a density of 70 kg/ m3 (70 mg/cm3). All cryogenic rocket engines work on expander cycle or gas-generator
cycle or staged combustion cycle depending on thrust requirement, since the oxidizer and fuel are at sub -
zero temperatures. LOX LH2 cryogenic rocket engines produce specific impulse up to 450 s (4.4 kN·s/kg).
Construction
The major components of a cryogenic rocket engine are:
 The thrust chamber or combustion chamber,
 Gas turbine,
 The combustion chamber in gas turbines and jet
engines (including ramjets and scramjets) is called the combustor.
 The combustor is fed high pressure air by the compression system, adds fuel and burns
the mix and feeds the hot, high pressure exhaust into the turbine components of the
engine or out the exhaust nozzle.


 Fuel injector,

5. The injector implementation in liquid rockets determines the percentage of
the theoretical performance of the nozzle that can be achieved. A poor injector
performance causes unburnt propellant to leave the engine, giving extremely
poor efficiency. The injector, as the name implies, injects the propellants into
the combustion chamber in the right proportions and the right conditions to
yield an efficient, stable combustion process. Placed at the forward, or upper,
end of the combustor, the injector also performs the structural task of closing
off the top of the combustion chamber against the high pressure and
temperature it contains. The injector has been compared to the carburetor of
an automobile engine, since it provides the fuel and oxidizer at the proper
rates and in the correct proportions, this may be an appropriate comparison.
However, the injector, located directly over the high-pressure combustion,
performs many other functions related to the combustion and cooling
processes and is much more important to the function of the rocket engine
than the carburetor is for an automobile engine.

 Additionally, injectors are also usually key in reducing thermal loads on the nozzle; by
increasing the proportion of fuel around the edge of the chamber, this gives much lower
temperatures on the walls of the nozzle.

 Fuel turbopumps,
 A turbopump is a gas turbine that comprises basically two main components: a
rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or
sometimes geared together. The purpose of a turbopump is to produce a high pressure
fluid for feeding a combustion chamber or other use.
 A turbopump can comprise one of two types of pumps: centrifugal pump, where the
pumping is done by throwing fluid outward at high speed; or axial flow pump, where
alternating rotating and static blades progressively raise the pressure of a fluid.

Pyrotechnic igniter,
A pyrotechnic initiator (also initiator or igniter) is a device containing a pyrotechnic
composition used primarily to ignite other, more difficult-to-ignite materials, e.g. thermites, gas
generators, and solid-fuel rockets. The name is often used also for the compositions themselves.

 Cryo valves,
 Regulators,
 The fuel tanks and
 Rocket engine nozzle.
 A rocket engine nozzle is a propelling nozzle (usually of the de Laval type) used in
a rocket engine to expand and accelerate the combustion gases produced by
burning propellants so that the exhaust gases exit the nozzle at hypersonic velocities.


6. The fuel flow can be differentiated into a main flow or a bypass flow configuration. In the main flow design,
the entire fuel is fed through the gas turbines, which intern drive the cryopump for fuel and oxidizer, and
then injected to the combustion chamber. In the bypass configuration, the fuel flow is split, the main part is
goes to the combustion chamber to generate thrust, while a small amount of the fuel goes to the turbine, to
drive the cryopumps for fuel and oxidizer and is subsequently injected to combustion chamber.
Rocket engine can generates 5,000 degree steam and 13,800 pounds of thrust form icicles at the rim of its
nozzle. It's cryogenic. The Common Extensible Cryogenic Engine (CECE) has completed its third round of
intensive testing. This technology development engine is fueled by a mixture of -297°F liquid oxygen and -
423°F liquid hydrogen.
The engine components are super-cooled to similar low temperatures. As CECE burns its frigid fuels, gas
composed of hot steam is produced and propelled out the nozzle creating thrust. The steam is cooled by the
cold engine nozzle, condensing and eventually freezing at the nozzle exit to form icicles. Using liquid
hydrogen and oxygen in rockets will provide major advantages for landing astronauts on the moon.
Hydrogen is very light but enables about 40% greater performance (force on the rocket per pound of
propellant) than other rocket fuels. Therefore, NASA can use this weight savings to bring a bigger spacecraft
with a greater payload to the moon than with the same amount of conventional propellants. CECE is a step
forward in NASA's efforts to develop reliable, robust technologies to return to the moon, and a winter
wonder.
Working
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.
Pressure-fed cycle (rocket)

7. Pressure-fed rocket cycle. Propellant tanks are pressurized to supply fuel and oxidizer to the engine, eliminating
the need for turbopumps.
The pressure-fed cycle is a class of rocket engine designs. A separate gas supply, usually
helium, pressurizes the propellant tanks to force fuel and oxidizer to the combustion chamber. To
maintain adequate flow, the tank pressures must exceed the combustion chamber pressure.
Pressure fed engines have simple plumbing and lack complex and often unreliable turbopumps.
A typical startup procedure begins with opening a valve, often a one-shot pyrotechnic device, to
allow the pressurizing gas to flow through check valves into the propellant tanks. Then the
propellant valves in the engine itself are opened. If the fuel and oxidizer are hypergolic, they burn
on contact; non-hypergolic fuels require an igniter. Multiple burns can be conducted by merely
opening and closing the propellant valves as needed. They can be operated electrically, or by gas
pressure controlled by smaller electrically operated valves.
Care must be taken, especially during long burns, to avoid excessive cooling of the pressurizing
gas due to adiabatic expansion. Cold helium won't liquify, but it could freeze a propellant,
decrease tank pressures, or damage components not designed for low temperatures. The Apollo
Lunar Module descent propulsion system was unusual in storing its helium in a supercritical but
very cold state. It was warmed as it was withdrawn through a heat exchanger from the ambient
temperature fuel.

8. Pressure-fed engines have practical limits on propellant pressure, which in turn limits combustion
chamber pressure. High pressure propellant tanks require thicker walls and stronger alloys which
make the vehicle tanks heavier. Thereby reducing performance and payload capacity. The lower
stages of launch vehicles often use solid fuel and pump-fed liquid fuel engines instead, where
high pressure ratio nozzles are considered desirable.[citation needed]
Expander cycle (rocket)
From Wikipedia,the free encyclopedia
(Redirected from Expander cycle)
Expander rocket cycle. Expander rocket engine (closed cycle). Heat from the nozzle and combustion chamber
pow ers the fuel and oxidizer pumps.
The expander cycle is a pow er cycle of a bipropellant rocket engine meant to improve the efficiency of fueldelivery.
In an expander cycle, the fuelis heated before it is combusted, usually w ith w aste heat fromthe main combustion
chamber. As the liquid fuelpasses through coolant passages in the w alls of the combustion chamber, it undergoes
a phase change into a gaseous state. The fuelin the gaseous state expands through a turbine using the pressure

9. differentialfromthe supply pressure to the ambient exhaust pressure to initiate turbopump rotation. This can provide a
bootstrap starting capability as is used on the Pratt & Whitney RL10 engine. This bootstrap pow er is used to
drive turbines that drive the fueland oxidizer pumps increasing the propellant pressuresand flows to the rocket
engine thrust chamber. After leaving the turbine(s), the fuelis then injected w ith the oxidizer into the combustion chamber
and burned to produce thrust for the vehicle.
Because of the necessary phase change, the expander cycle is thrust limited by the square-cube rule. As the size of a
bell-shaped nozzle increases with increasing thrust, the nozzle surface area (fromwhich heat can be extracted to expand
the fuel) increases as the square of the radius. How ever, the volume of fuelthat must be heated increases as the cube of
the radius. Thus there exists a maximum engine size of approximately 300 kN of thrust beyond w hich there is no longer
enough nozzle area to heat enough fuelto drive the turbines and hence the fuel pumps. Higher thrust levels can be
achieved using a bypass expander cycle where a portion of the fuelbypasses the turbine and or thrust chamber cooling
passages and goes directly to the main chamber injector. Aerospike engines do not suffer fromthe same limitations
because the linear shape of the engine is not subject to the square-cube law. As the width of the engine increases, both
the volume of fuelto be heated and the available thermal energy increase linearly, allow ing arbitrarily wide engines to be
constructed. Allexpander cycle engines need to use a cryogenic fuelsuch as hydrogen, methane, or propane that easily
reach their boiling points.
Some expander cycle engine may use a gas generator of some kind to start the turbine and run the engine until the heat
input fromthe thrust chamber and nozzle skirt increases as the chamber pressure builds up.
In an open cycle, or "bleed" expander cycle, only some of the fuelis heated to drive the turbines, w hich is then vented to
atmosphere to increase turbine efficiency. While this increases poweroutput, the dumped fuelleads to a decrease in
propellant efficiency(lowerengine specific impulse). A closed cycle expander engine sends the turbine exhaust to the
combustion chamber (see image at right.)
Advantages
The expander cycle has a number of advantages over other designs:[citation needed]
 Low temperature. The advantage is that after they have turned gaseous, the
fuels are usually near room temperature, and do very little or no damage to
the turbine, allowing the engine to be reusable. In contrast Gas-
generator or Staged combustion engines operate their turbines at high
temperature.
 Tolerance. During the development of the RL10 engineers were worried that
insulation foam mounted on the inside of the tank might break off and
damage the engine. They tested this by putting loose foam in a fuel tank and
running it through the engine. The RL10 chewed it up without problems or
noticeable degradation in performance. Conventional gas-generators are in

10. practice miniature rocket engines, with all the complexity that implies.
Blocking even a small part of a gas generator can lead to a hot spot, which
can cause violent loss of the engine. Using the engine bell as a 'gas
generator' also makes it very tolerant of fuel contamination because of the
wider fuel flow channels used.
 Inherent safety. Because a bell-type expander-cycle engine is thrust limited, it
can easily be designed to withstand its maximum thrust conditions. In other
engine types, a stuck fuel valve or similar problem can lead to engine thrust
spiraling out of control due to unintended feedback systems. Other engine
types require complex mechanical or electronic controllers to ensure this does
not happen. Expander cycles are by design incapable of malfunctioning that
way.
Gas-generator cycle (rocket)
From Wikipedia, the free encyclopedia
Gas generator rocket cycle. Some of the fuel and oxidizer is burned separately to power the pumps and then discarded.
Most Gas-generator engines use the fuel for nozzle cooling.

11. The gas generator cycleis a power cycle of a bipropellant rocket engine.Some of the propellant is burnedin a gas-generator andthe
resultinghot gas is usedto power the engine's pumps. The gas is then exhausted. Because somethingis "thrown away"this type of
engine is also known as open cycle.
There are several advantages to the gas generatorcycleover its counterpart, the stagedcombustion cycle. The gas generator turbine
does not needto deal with thecounterpressure of injectingthe exhaust intothe combustionchamber.This simplifies plumbingand
turbine design, andresults in a less expensiveandlighter engine.
The main disadvantage is lost efficiencydue to discardedpropellant.Gas generator cycles tendtohave lower specific impulse than
stagedcombustion cycles.[citation needed]
As in most cryogenic rocket engines, someof the fuel in a gas-generatorcycleis usedto cool the nozzle andcombustionchamber.
Current constructionmaterials cannot standextreme temperatures ofrocket combustionprocesses by themselves. Coolingpermits the
use of rocket engines forrelativelylongerperiods of time withtoday’s material technology. Without rocket combustionchamber and
nozzle cooling, the engine wouldfail catastrophically.[1]
Staged combustion cycle (rocket)

12. Staged combustion rocket cycle. Usually, all of the fuel and a portion of the oxidizer are fed through the pre-burner
(fuel rich) to pow er the pumps. An oxygen rich circuit is possible also, but less common because of the metallurgy
required.
The staged combustioncycle, also called toppingcycle or pre-burner cycle,[1]
is a thermodynamic cycle
of bipropellant rocket engines. Some of the propellant is burned in a pre-burner and the resulting hot gas is used to pow er
the engine's turbines and pumps. The exhausted gas is then injected into the main combustion chamber, along w ith the
rest of the propellant, and combustion is completed.
The advantage of the staged combustion cycle is that all of the engine cycles' gases and heat go through the combustion
chamber, and overall efficiencyessentially suffersno pumping losses at all. Thus this combustion cycle is often called
closed cycle since the cycle is closed as all propellant products go through the chamber; as opposed to open cycle w hich
dumps the turbopump driving gases, representing a few percent of loss.
Another very significant advantage that staged combustion gives is an abundance of pow er which permits very high
chamber pressures. Veryhigh chamber pressures mean high expansion ratio nozzles can be used, w hilst stillgiving
ambient pressures at takeoff. These nozzlesgive far better efficienciesat low altitude.
The disadvantages of this cycle are harsh turbine conditions, that more exotic plumbing is required to carry the hot gases,
and that a very complicated feedbackand controldesign is necessary. In particular, running the full oxidizer stream
through both a pre-combustor and main-combustor chamber (oxidizer-rich staged combustion) producesextremely
corrosive gases. Thus most staged-combustion engines are fuel-rich, as in the schematic.
Staged combustion engines are the most difficult types of rocket engines to design. A simplified version is called the gas-
generator cycle.
Power Cycles
Liquid bipropellant rocket engines can be categorized according to their power cycles,
that is, how power is derived to feed propellants to the main combustion chamber.
Described below are some of the more common types.
Gas-generator cycle: The gas-generator cycle, also called open cycle, taps off a small
amount of fuel and oxidizer from the main flow (typically 3 to 7 percent) to feed a
burner called a gas generator. The hot gas from this generator passes through a
turbine to generate power for the pumps that send propellants to the combustion
chamber. The hot gas is then either dumped overboard or sent into the main nozzle
downstream. Increasing the flow of propellants into the gas generator increases the
speed of the turbine, which increases the flow of propellants into the main combustion
chamber, and hence, the amount of thrust produced. The gas generator must burn
propellants at a less-than-optimal mixture ratio to keep the temperature low for the
turbine blades. Thus, the cycle is appropriate for moderate power requirements but

13. not high-power systems, which would have to divert a large portion of the main flow to
the less efficient gas-generator flow.
As in most rocket engines, some of the propellant in a gas generator cycle is used to
cool the nozzle and combustion chamber, increasing efficiency and allowing higher
engine temperature.

15. Staged combustion cycle: In a staged combustion cycle, also called closed cycle, the
propellants are burned in stages. Like the gas-generator cycle, this cycle also has a
burner, called a preburner, to generate gas for a turbine. The preburner taps off and
burns a small amount of one propellant and a large amount of the other, producing an
oxidizer-rich or fuel-rich hot gas mixture that is mostly unburned vaporized
propellant. This hot gas is then passed through the turbine, injected into the main
chamber, and burned again with the remaining propellants. The advantage over the
gas-generator cycle is that all of the propellants are burned at the optimal mixture
ratio in the main chamber and no flow is dumped overboard. The staged combustion
cycle is often used for high-power applications. The higher the chamber pressure, the
smaller and lighter the engine can be to produce the same thrust. Development cost for
this cycle is higher because the high pressures complicate the development process.
Further disadvantages are harsh turbine conditions, high temperature piping required
to carry hot gases, and a very complicated feedback and control design.
Staged combustion was invented by Soviet engineers and first appeared in 1960. In the
West, the first laboratory staged combustion test engine was built in Germany in 1963.
Expander cycle: The expander cycle is similar to the staged combustion cycle but has
no preburner. Heat in the cooling jacket of the main combustion chamber serves to
vaporize the fuel. The fuel vapor is then passed through the turbine and injected into
the main chamber to burn with the oxidizer. This cycle works with fuels such as

16. hydrogen or methane, which have a low boiling point and can be vaporized easily. As
with the staged combustion cycle, all of the propellants are burned at the optimal
mixture ratio in the main chamber, and typically no flow is dumped overboard;
however, the heat transfer to the fuel limits the power available to the turbine, making
this cycle appropriate for small to midsize engines. A variation of the system is the
open, or bleed, expander cycle, which uses only a portion of the fuel to drive the
turbine. In this variation, the turbine exhaust is dumped overboard to ambient
pressure to increase the turbine pressure ratio and power output. This can achieve
higher chamber pressures than the closed expander cycle although at lower efficiency
because of the overboard flow.

18. Pressure-fed cycle: The simplest system, the pressure-fed cycle, does not have pumps or
turbines but instead relies on tank pressure to feed the propellants into the main
chamber. In practice, the cycle is limited to relatively low chamber pressures because
higher pressures make the vehicle tanks too heavy. The cycle can be reliable, given its
reduced part count and complexity compared with other systems.
Advantages
The cryogenic engines uses liquid nitrogen as the fuel and the exhaust is also nitrogen. Since
nitrogen is present nearly 78% in our atmosphere, the engine is non pollutant. whereas in the
case of normal IC engine, the exhaust is carbon monoxide, CO2 and other harmful gases.
Efficiency is higher in the case of cryogenic engine than that of petrol engines.
High Energy per unit mass
Propellants like oxygen and hydrogenin liquid form give very high amounts
of energy per unit mass due to which the amount of fuel to be carried
aboard the rockets decreases.

19. Clean Fuels
Hydrogen and oxygen are extremely clean fuels. When they combine, they
give out only water. This water is thrown out of the nozzle in form of very
hot vapor. Thus the rocket is nothing but a high burning steam engine.
Economical
Use of oxygen and hydrogen as fuels is very economical, as liquid oxygen
costs less than gasoline.
Cryogenic propellants suffer from certain drawbacks. Let us see what
these drawbacks are and how they can be overcomed.
Drawbacks of Cryogenic Propellants
Boil off Rate
Since these propellants are in extremely low temperature conditions they
are very hard to handle. They must be protected from heat so as to prevent
boiling of gases. When liquid propellants are stored at temperatures above
their boiling point they vaporize. If these vapors are contained in a tank,
then the pressure increases with temperature.
Since the tank weight increases with design pressure, a pressure
relief valve is generally provided to prevent the tank from over
pressurizing and exploding. When the relief valvereleases the pressure,
some of the propellant escapes from the tank. This lost propellant is
referred to as boil-off loss. and proves of no use for propulsion.
For liquid hydrogen the boil off rate per day is 0.127% while that of liquid
oxygen is 0.016%. Thus the boil of rates of liquid hydrogen per month is
3.81% while that of liquid oxygen is 0.49%. Boil off rate is governed by
heat leakage and by the amount of propellant in the tanks. With partly
filled tanks, the percentage loss is higher. Heat leakage depends on surface
area, while the original mass of propellant in the tanks depends on volume.
So by square/cube law smallerthe tank, the faster the liquids will boil off.
To lower the boil off rates, we must protect the propellant tank from the
heat of the sun. Tanks at more distance from the sun have lowerboil off
rates. Lowerboil off rates can also be ensured if tanks are maintained at an
angle by which their cross sectional area exposed to sun is minimum or
tanks are protected by a sunshade.
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

20. organic materials to form explosive compounds. So lots of care must be
taken to ensure safety.
Leakage
One of the most majorconcerns is leakage. At cryogenic temperatures,
which are roughly below 150 degrees Kelvin or equivalently (-190oF), the
seals of the containerused 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 wheneverits concentration is
approximately 4% to 96%. It is hence necessary to ensure
that hydrogen leak rate is minimal and does not present a hazard. Also
there must be some way of determining the rates of leakage and checking
whether a fire hazard exists or not. The compartments where hydrogen gas
may exist in case of a leak must be made safe, so that the hydrogen buildup
does not cause a hazardous condition.
Hydrogen Embrittlement
Due to cryogenic propellants, various significant thermal stresses are
introduced into the launch vehicle. These stresses can damage the
structural integrity of the vehicle. Hydrogenreacts with certain materials to
alter its grain structure causing it to become brittle, in a process known
as hydrogen embrittlement
Zero Gravity conditions
It is difficult to store cryogenic propellants in zero gravity conditions.
Generally, the propellants are cooled continuously, allowing excess heat to
be carried away as the gasses boil off. The vapors of the gas are vented
away. But in zero gravity conditions, the liquid may flash into vapor, and
go away from the nozzle that dumps the gas overboard. This pushes the
propellant in liquid form, out of the nozzle resulting in wastage of large
amounts of fuel into space.
The cryogenic propellants certainly have their own disadvantages. But
their advantages outweigh the disadvantages by far. Thus they are
preferred for use in rockets.
The next liquid propellant is hypergolic propellant.