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National Advisory Committee for Aeronautics, Technical Notes - Shape of Initial Portion of Boundary of Supersonic Axisymmetric Free Jets at Large Jet Pressure Ratios

Calculations have been made of the initial portion of the boundary
of axisymmetric free Jets exhausting at large ratios of Jet static pres-
sure to stream static pressure from a sonically divergent nozzle having
a Jet exit Mach number of 2.5 and a semidivergence angle of 15°. The
results of the calculations indicate the size and shape of the Jet to
be expected at large pressure ratios, the effects of the ratio of spe-
cific heats, and the large initial inclinations of the boundary that
are likely to be encountered by hypersonic vehicles at high altitude.

In the proposed trajectories of most rocket—propelled hypersonic
vehicles, the rocket propulsion unit will remain in operation long
enough for the exhausting Jet to encounter the very low pressures of
high altitudes. When this condition occurs, the ratio of the Jet static
pressure to stream static pressure becomes very large at the nozzle exit,
and as a result the free Jet expands greatly. The problems created by
the presence of this large, bulbous, free Jet of gases immediately down—
stream of the nozzle exit are, in the main, twofold. First, the large
deflections of the free-stream flow caused by the Jet flow may result
in maJor aerodynamic interference upon nearby surfaces; for example,
large regions of separated flow may be created on the vehicle surface
ahead of the rocket exit. Second, the heat existing in the core of
this large mass of gases may at hypersonic speeds introduce direct heating
problems through radiation by causing the temperatures of the nearby
surfaces which are already experiencing high aerodynamic heating to
exceed the critical values. None of the aerodynamic interference effects
can be estimated with any reliability without some idea of the magnitude
of the region encompassed by the free Jet and the shape of its boundary.