Orbits and Propagators - AGI

Diseño de Vehículos Espaciales 1DVE1‐T07‐39Atmospheric Drag• For low‐orbit spacecraft, the very thin edge of the atmosphere creates dragand lift forces, as it would for any flying object. We are going to ignore lift.FD1 SC2 DV2r VVrr• C D is hard to estimate; there is a large mean free path, so free molecular flowis most suitable model (no shock wave, no particle interactions). Typicalvalues are around C D = 2,5.• S is usually taken as the cross‐sectionalsectional area projected in the direction of thevelocity vector.• By inspection, drag will reach a peak at perigee, because both velocity anddensity are at their maximum values. Intuitively, the drag force can beconsidered a negative, impulsive velocity increment at perigee.• For an ellipse, this reduces the semi‐major axis; for a circular orbit, dragoccurs continuously and reduces the radius.