An entry vehicle whose shape resembles that of a loaf of bread has been
proposed for missions to be flown as secondary payloads on the Ariane V
launch vehicle. This shape conforms well within the volume available
inside the Ariane structure for auxiliary payloads, and provides a much
more efficient and larger internal packaging capability for the given
launch-vehicle constraints than the conventional sphere-cone geometry
(such as that of the Mars-Pathfinder entry vehicle). Because
aerodynamic stability for this new class of vehicles must be evaluated,
initial ballistic range testing was conducted to assess the supersonic
behavior of loaf-shape vehicles. Figure 1 shows a shadowgraph image of
the ballistic range model flying at supersonic speeds. The loaf-shaped
model for the ballistic range tests is sized to be launched from a
1.75-inch-bore gun.
The model has geometry and mass properties that are similar to those being considered for the mission. Model dimensions are 1.15 x 0.78 x 0.66 inches and the center of gravity is at 39% of the length (measured from the nose). An ambient density of 0.18 kilogram per cubic meter of carbon dioxide (CO2 ) is chosen so that the Reynolds number will be consistent with that for the full-scale entry body at Mars. A nominal launch velocity of 680 meters per second ensures Mach numbers ranging from 2.5 down to 2 as the model
decelerates along the length of the test section.
Four tests were conducted at consistent conditions in order to reduce uncertainty in
the estimated aerodynamic parameters. The results summarized in Table 1
confirm the stable behavior of the loaf-shaped vehicle. The
experimental value of drag coefficient is lower than the computational
fluid dynamic (CFD) estimate because it includes wake effects. Pitch
and yaw coefficients also indicate a discrepancy that is up to 20% of
the CFD estimate, which confirms that analysis of the forebody alone
does not adequately predict aerodynamic behavior at low supersonic
speeds. Uncertainty in lift coefficient is relatively high because the
angle-of-attack variation was small in some tests.
| Table 1. Aerodynamic parameters of bread-loaf geometry in supersonic speed range. |
| Parameter | Forebody CFD estimate | Ballistic range estimate |
| CD_0 | 1.6 | 1.381 ± 0.002 |
| Cm_α | -0.208 | -0.18 ± 0.01 |
| Cn_β | -0.235 | -0.28 ± 0.02 |
| CL_α | -1.07 | -0.80 ± 0.19 |
The results of this study indicate the feasibility of
using a loaf-shaped entry vehicle which maximizes
packaging capability for the Ariane V launch vehicle.
If this shape is selected for future missions, additional
tests and full-body CFD analysis should be performed
to model the detailed flow more accurately and to
reliably define the vehicle aerodynamic coefficients.
Point of Contact: P. Gage
(650) 604-0193
pgage@mail.arc.nasa.gov
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