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 surface. • 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 Celsius). • 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 chamber. • 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.