1. Fighting the universe’s small pushes

Old Man’s War is full of soldiers fighting enemies they did not choose. The desktop’s ADCS fights smaller enemies: gravity gradient, atmospheric drag, solar radiation pressure, and magnetic torques. None of them are large, but they never stop. Over time they fill the reaction wheels with momentum and push the platform off point.

2. The momentum budget

Reaction wheels store momentum. When they approach saturation, the platform must dump that momentum. Options:

  • Magnetorquers. Use Earth’s magnetic field to apply torque. Free but slow and ineffective at some orbit phases.
  • Thrusters. Fast and effective but consume propellant.
  • Gravity-gradient or aerodynamic tricks. Possible in principle but not reliable for a controlled platform.

The choice affects the propulsion budget, the power budget, and the operations cadence.

3. Disturbance torques in LEO

  • Gravity gradient. Tends to align the platform’s long axis with the local vertical. Small for compact platforms but not zero.
  • Aerodynamic drag. Depends on atmospheric density, which varies with solar activity and altitude.
  • Solar radiation pressure. Small but persistent. Important for platforms with large solar arrays.
  • Magnetic. Interaction between the platform’s residual dipole and Earth’s field.

The Resident wonders how well these can be predicted before launch and how much margin the momentum-dump system needs for uncertainty.

4. Propulsion coupling

A propulsion burn applies force along a vector. If that vector does not pass through the platform’s center of mass, it also applies torque. The ADCS must:

  • Know the center of mass and thrust vector for each thruster configuration.
  • Anticipate the torque during burn planning.
  • Compensate with wheels or attitude thrusters during the burn.
  • Recover pointing after the burn.

This is one of the strongest couplings between propulsion and ADCS.

5. A leaning

The desktop should size its reaction wheels and magnetorquers for routine disturbance torques, and keep thrusters as a backup for desaturation when the magnetic field is unfavorable or when time is short. Propulsion burns should be modeled as ADCS events, not just orbit events.