Subido por Daniel Rios

Rocket Engines

Chemical Rocket Thrust Chambers
• For the liquid rocket the fuel and oxidizer are stored in
separate thin-walled tanks at low pressure.
• Before combustion they pass through turbine-driven
pumps and are injected into the combustion chamber,
where they burn at high pressure.
• In contrast, solid propellants consist of a premixed
combination of fuel and oxidant that burns when it is
sufficiently heated.
• Combustion proceeds from the surface of the
propellant at a rate that depends on the
temperature and pressure in the combustion
chamber and on the geometry of the burning
• Besides specific impulse, the masses of the fuel
and oxidant tanks, turbomachinery, and engines
must be considered.
• The relatively low density of liquid hydrogen
gives it a strong disadvantage in tank size.
• Hydrogen stays in liquid form at low pressure
only if its temperature is less than 77 K(-196.15
• Thus liquid hydrogen is not considered a
storable propellant for space missions.
• Thus, depending on the mission, there are a variety of fuel-oxidant
combinations with lower specific impulse than that of the H2-02 combination
that are widely used.
• There are fuel-oxidant combinations that can have even higher specific
impulse than H2 and 02, but they are not widely used because of factors such
as toxicity, instability, or unavailability at reasonable cost.
Chemical Rocket Propellant
• Another important incentive for studying
combustion in rockets is that the intensity of
combustion (energy transformed per unit time
per unit volume) directly determines the size,
and therefore the mass, of the combustion
• The performance of the rocket vehicle
depends to some extent on the mass of the
engine as well as on its specific impulse.
• The products of combustion in a well-designed
rocket combustion chamber are very nearly in
thermodynamic equilibrium.
• At typical combustion chamber temperatures and
pressures, the time required for mixing of fuel
and oxidant and for chemical reactions is small
compared with the average residence time
(several milliseconds) of the propellant within the
combustion chamber.
• During expansion in the nozzle, the temperature
falls so rapidly that there may be insufficient time
for the dissociation products to recombine.
• Under certain conditions the rates of
recombination are so slow that the gas
composition effectively "freezes" (surely an odd
word for gases as hot as 3000 K), and the gas
expands at constant composition.
• A special case arises with those solid rocket propellants that use powdered
metals-for example, aluminum-as a fuel additive.
• The aluminum particles burn with oxygen to form Al203 particles that are
initially liquid and solidify during expansion.
• They tend also to agglomerate to become large particles that don’t accelerate
as quickly as the gas around them.