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Freon 12 will be used as the propellant for a small portable jet
rocket for maneuvering parts during assembly of a space station.
The device will consist of a small spherical tank containing
saturated Freon 12 at 80 oF, a
valve, and a nozzle. The user will attach the jet to the package,
open the valve momentarily, and let some gas escape through the
nozzle. Energy transfer as heat from the walls to the Freon 12
will keep the pressure constant in the cylinder during a short
burst, and the flow through the nozzle may be considered steady.
The valve will throttle the Freon 12 to 20 psia, and the nozzle
discharge state will be 2 psia and -80 oF.
The thrusting force is approximately given by F = MV/gc,
where M is the mass-flow rate and V the exit velocity. Use
requirements call for a thrust of 10 lbf. Determine the specific
impulse of this device, SI=F/M, in lbf/(lbm/s). By comparison, a
good chemical rocket has an SI of the order of 250 lbf/(lbm/s).
Specify the size of tamk required to hold enough Freon 12 for 1000s
of operation, assuming that the heat-transfer rate is sufficient to
keep the pressure constant for short-duration bursts. Compute the
total amount of energy which must be transferred from the walls to
the Freon 12 during this 1000 s of operation in order to maintain
the pressure (Fig. B.7, Table B.5). This requires a careful energy
analysis.
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