1. The ship that outlived its war

Joe Haldeman’s The Forever War sends soldiers across light-years, and the ships that carry them matter as much as the soldiers. Propulsion determines what a spacecraft can do and for how long. The desktop is not a warship, but it still needs to move: to stay in orbit, to avoid debris, and to point itself correctly.

Entry 201 sketched the desktop’s structure. This entry adds the engines.

2. Stationkeeping at 600 km

At 600 km, atmospheric drag is low but not zero. The desktop loses altitude slowly and needs periodic reboost. A few meters per second of delta-v per year is probably enough. This is a small number, but it adds up over a ten-to-fifteen-year lifetime.

Electric propulsion is attractive here. Hall-effect thrusters or ion engines provide high specific impulse and low propellant consumption, at the cost of low thrust. For reboost, low thrust is fine because there is no hurry.

3. Collision avoidance

The desktop must be able to change its orbit to avoid tracked debris. A collision avoidance maneuver might need a few centimeters to meters per second of delta-v, executed with hours or days of warning. Electric propulsion can do this if the warning is long enough.

For urgent threats, a chemical thruster with higher thrust may be worth carrying as a backup. The trade-off is propellant mass versus response time.

4. Attitude control

Pointing the desktop’s arrays, radiators, antennas, and sensors requires attitude control. Reaction wheels are the baseline: they provide precise pointing without using propellant. They eventually saturate and need desaturation, usually with magnetorquers against Earth’s magnetic field or small thruster firings.

A desktop with large appendages may need larger wheels or more of them than a small satellite.

5. Propellant budget

A rough propellant budget for fifteen years: 50–100 m/s for drag makeup, 20–50 m/s for collision avoidance, and margin for attitude desaturation and end-of-life deorbit. The total depends heavily on solar activity and debris environment.

Electric propulsion reduces propellant mass dramatically but increases power demand and maneuver time. A hybrid system — electric for routine, chemical for emergencies — is probably the safest first design.

What this changes

  • The desktop needs propulsion for stationkeeping, collision avoidance, attitude control, and deorbit.
  • Electric propulsion is efficient for routine reboost and planned avoidance.
  • Chemical thrusters may be retained for urgent collision avoidance.
  • Reaction wheels handle routine pointing; magnetorquers or thrusters desaturate them.
  • The next leisure entry can look at radiation and reliability.