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Differential Drag

Differential drag changes a spacecraft's trajectory by varying its drag profile rather than by burning propellant, using attitude adjustments or deployable components such as solar panels. The effect is an in-track delta-V, so it can substitute for an in-track burn on a spacecraft with limited or no propulsion. It only works at low altitude, below 1200 km, where atmospheric drag is strong enough to move the spacecraft relative to common conjunction geometries.

Maneuver Designer models the maneuver as a single off-nominal drag period with a drag scale factor. That covers any mechanism which changes the drag profile: attitude adjustments that make the effective drag area larger or smaller than nominal, or deploying and retracting components.

Because differential drag maneuvers are typically many hours long, differential drag tradespaces do not align off-nominal drag periods relative to the antipode of the TCA, unlike the other maneuver types. Instead, the tradespace can be generated with a configurable number of maneuver start times. Tradespaces with more maneuver start times will have a finer resolution of maneuver start time, but will take much longer to generate.

Differential drag is inefficient at mitigating collision risk even at low altitude, so a successful maneuver usually needs a long lead time: the off-nominal drag period typically begins several days before TCA and lasts six hours or more, depending on the conjunction geometry and the spacecraft's effective drag area.

Modes​

Duration mode​

Duration Mode varies the start time and duration of a period in which the spacecraft is placed in an off-nominal drag configuration to influence the spacecraft's trajectory with a fixed off-nominal drag scale factor. This mode is most useful when the spacecraft has a known off-nominal drag configuration that can be maintained for a specific duration, such as when the spacecraft is capable of maintaining a specific attitude or deploying/retracting components for a known period of time.

Scale mode​

Scale Mode varies the start time and off-nominal drag scale factor of a period in which the spacecraft is placed in an off-nominal drag configuration to influence the spacecraft's trajectory with a fixed off-nominal drag duration. This mode is most useful when the spacecraft has more flexible off-nominal drag capabilities, such as precise and adjustable attitude control or variable deployment/retraction of components with high surface area.

Settings​

Differential drag does not use dV Direction or dV magnitude. Alongside the shared tradespace settings, it exposes:

  • Differential drag mode: which of the two modes to evaluate.
  • Off-nominal duration and Off-nominal drag scale factor min / max: used in Scale mode.
  • Off-nominal drag scale factor and Duration min / Duration max: used in Duration mode.