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Capsule Aerodynamics

ESA and NASA are launching three planetary mis-
sions involving entries with very different atmo-
spheric conditions: HUYGENS, to Saturn’s satellite
Titan, ROSETTA, to a comet and back to earth, and
MARSNET, to the planet Mars. Below these three
missions will be presented, as well as their aerody-
namic characteristics, and some of the aerothermo—
chemical data related to them. Typical entry trajec-
tories are presented. Table 1 shows the Titan, Mars
and Earth properties. (reference 17).
The impact of aerothermodynamics on the design
and the feasibility of these vehicles is briefly acknowl-
edged. The aerodynamic issues identified for these
projects are listed.
The European Space Agency (ESA) is currently
studying three scientific probes for planetary mis-
sions, reference 4, in conjunction with the National
Aeronautic and Space Administration (NASA). For
the three missions, NASA will provide the European
probes with a launcher, a carrier and communica-
tions through their Deep Space Network (DSN). The
aerodynamic shape of the three probes is very simi-
lar, based on a 60° sphere-cone for their front heat—
shield. This choice represents a trade-off between
the drag and the stability of the vehicle, but is also
supported by the availability of a large aerodynamic
database. These three probes involve ballistic atmo-
spheric entries with very different conditions. For
manned missions, conservative designs are adopted.
For robotic missions, the low cost — low mass ap—
proach requires an accurate design, with small mar-
gins and a somewhat higher risk. A very good knowl-
edge of the entry environment is needed to fullfill
these requirements.
During the hypersonic phase of its atmospheric en-
try, the probe is protected by a sphere-cone shaped
front heat shield, and a back-cover. When a low su-
personic Mach number is reached, the back part of
the heat-shield is jettisoned, a pilot chute is opened
and finally a large parachute is deployed, to stabilize
the vehicle during the transonic phase and to brake
more efficiently as seen in figure 8. The front part
of the heat shield is then jettisoned, to allow the de-
ployment of the scientific instruments analysing the
atmosphere and the planet. The power supply is only
sufficient for a descent time of less than three hours.
Besides, the orbiter is also available for communi-
cations only during a limited time. The minimum
descent time required to perform the scientific mea-
surements is two and a half hours. In order to achieve
this, the parachute is jettisoned at an altitude of the
order of 50 km, and the last part of the descent is
free-fall.