Figure 4.1. Staging of the Pioneer Venus large-probe entry shell and parachute. Entry interface was defined at 200 km altitude, with the main deceleration lasting about 38 s. At about 70 km altitude, a mortar deployed a pilot chute, which removed the aft cover and deployed the main chute. The entry shield is then allowed to fall away, and after 16.5 minutes (at 48 km) the main parachute was jettisoned, allowing the probe to fall faster. Free descent took another 39 minutes. This descent sequence was also used (with some modifications in timing and the addition of a small stabilizing parachute in the last segment) on the Huygens probe.
The term Vterm100 is the terminal velocity for surface atmospheric conditions (pressure, temperature, density and gravity Psurf, Tsurf, psurf and gsurf) for a spacecraft with a ballistic coefficient fl of 100kgm~ 2. The term iscale is the time taken to fall through one scale height H (the e-folding distance for pressure).
It can be seen from the table that Venus and Titan are rather similar in having exceptional descent times - both the Pioneer Venus large probe (Figure 4.1) and Huygens on Titan jettisoned their main parachutes to increase the ballistic coefficient and descend more rapidly.
In some applications, the ballistic coefficient employed at entry may be adequate for descent. In this case, the entry shield may remain attached, although care must be taken that the subsonic stability is adequate. The Pioneer Venus small probes and DS-2 Mars microprobes used this approach.
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