Page 73 - Intro to Space Sciences Spacecraft Applications

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Introduction to Space Sciences and Spacecraft Applications
tubes surrounding the combustion chambers and nozzles to keep their
temperatures within acceptable limits.
Inside the brackets of equation 3-5 is a ratio of exhaust pressure to com-
bustion chamber pressure. Minimizing this ratio contributes to higher
exhaust velocities which calls for as large a combustion chamber pressure
as possible. This pressure is produced by the combustion of the propel-
lants; however, as was mentioned earlier, the pumps in a liquid-fueled sys-
tem affect the combustion chamber pressure as well as the mass flow rate
in contributing to rocket thrust. Again, limitations to achievable combus-
tion chamber pressures are structural in nature.

Specific Impulse. We can define the term specific impulse (IFp! as the
thrust produced per time rate of change of weight. Remembermg that
thrust can be related to exhaust velocity, we can write:

The specific impulse changes for different propellants and different
rocket designs and their computation is quite complicated. However, spe-
cific impulse is the most commonly used criteria for comparing rocket
systems since the resulting units are merely seconds. Table 3-1 shows
some of the average properties of three propellant combinations.

Table 3-1
FueVOxidizer Properties

FueVoxidizer To (OK) M (glmole) I,, (sea
Keroseneloxygen 3,144 22 240
Hydrogenlox ygen 3,517 16 360
H ydrogedfluorine 4,756 10 390

The Russians use the kerosene/oxygen combination of fuels exclusive-
ly in their launch boosters. These propellants were also used on the first
stage of the Saturn V rocket used in the Apollo missions to the moon, but
the United States regularly uses liquid hydrogen and oxygen for propel-
lants in a majority of their launch vehicles including the main engines of
the Space Shuttle. The difference in the use of these fuels is mostly due to
their ease of handling; liquid kerosene (also known as RP-1) is much eas-