1. The question left open

Entry 874 said the SEP PPU is sheddable during a bus sag, and that the sail’s attitude-control loads are essential. This entry asks a finer-grained operations question: can the tug ever run the sail actuators and the SEP thruster at full power at the same time? Or are they like two heavy appliances on a household circuit that must take turns?

The answer depends on what “full power” means for each subsystem, and on how patient the trajectory is.

2. Power magnitudes

A solar sail’s actuators are not, in steady state, large power consumers. Reflectivity control devices switch pixels between reflective and absorptive states; the power is in the switching, not the holding. Solar Cruiser’s ADCS combines small FEEP micro-thrusters and RCDs with conventional sensors and a flight computer. The total continuous ADCS power for a large sailcraft is typically in the tens to hundreds of watts, with transient peaks when many RCDs change state or when a motor drives a gimbal.

An abort-sized SEP backup, by contrast, is kilowatts to tens of kilowatts continuous while firing. The actuator steady-state load is therefore a small addition to the SEP load. If the solar array was sized for SEP, it can almost certainly run the actuators at the same time.

The problem is not average power. It is transient power.

3. The transient problem

Electric motors and RCD driver circuits draw inrush current when they start. A commanded step in sail attitude can momentarily demand several times the steady-state actuator power. If that transient coincides with the SEP PPU ramping up, the combined bus current can trip a fuse or trigger the load-shedding logic from Entry 874.

The EDDA direct-drive study, cited in Entry 873, notes that a 20 kW SEP system can dissipate more than 1 kW in the power-conditioning chain. That dissipation is continuous, but the PPU startup and throttling transients are worse. Adding an actuator inrush on top is exactly the kind of event that makes the bus sag before the FDIR switches can react.

So the constraint is not “SEP or actuators.” It is “SEP plus actuator inrush must fit under the bus protection threshold.”

4. Operational scheduling

One way to stay under the threshold is to schedule the loads. Command large sail attitude steps only when the SEP is off or throttled down. Command SEP ramp-ups only after the actuators have reached their new steady state. This is the “conflicting train tracks” model from the queue: two heavy loads never start at the same instant.

This works when the mission is not in a hurry. During a long heliocentric spiral, pausing SEP for a few minutes while the sail reorients costs almost nothing. During an abort burn, when minutes matter, the schedule becomes tighter. But even then, the SEP can usually be throttled rather than shut off, and the actuator move can be broken into smaller steps.

5. The Ceriotti hint

Ceriotti’s pole-sitter design offers a useful pattern. In that concept, the SEP thruster is used as the fine actuator while the sail attitude is held nearly fixed. Tilting a large sail is slow because of its inertia, whereas varying SEP thrust is fast. The control law therefore leaves the sail in a favourable attitude and trims the trajectory with the thruster.

This maps cleanly onto a minimoon tug. For small thrust-vector corrections during an SEP burn, keep the sail steady and modulate the thruster. For large changes in thrust direction — the kind that require re-pointing the sail — accept an interruption or throttle-down of the SEP burn. The sail provides the coarse, slow authority; the SEP provides the fine, fast authority.

6. The Clarke echo

Arthur C. Clarke’s The Sands of Mars is a quiet novel about the first permanent Mars settlement. The colonists spend a lot of time managing life-support cycles, power budgets, and the fact that every watt has to be earned. There is no single dramatic failure, just the continuous arithmetic of living inside a machine.

A hybrid tug feels the same. The array is big enough for the mission, but not big enough for carelessness. Running the SEP and the sail actuators together is not forbidden; it just has to be accounted for in the daily power arithmetic, like adding another heater to a dome that is already close to its limit.

7. The Popperian note

The conjecture is that simultaneous operation is normal, with transient scheduling as the only constraint. The refutations would be:

  • A sail actuator design whose inrush is comparable to the SEP power, making the shared bus untenable, or
  • A mission phase where SEP must fire continuously and the sail must also slew continuously, so no scheduling window exists, or
  • A bus protection threshold so conservative that every actuator move triggers shedding.

If any of those is true, the architecture must add either a larger array, a dedicated actuator bus, or a throttling protocol that is stricter than simple scheduling.

8. What this changes

Entry 874 kept the SEP PPU on the sheddable list. Entry 875 says that, in normal operations, the SEP and the sail actuators will run together. The shared bus is viable because the sail actuators’ steady-state power is small compared with SEP. The real protection is against overlapping transients, not against simultaneous operation.

For the keeper arc, this means the flight software needs a power-scheduling rule: no large actuator steps during SEP ramp-up, and no SEP ramp-up during actuator transients. It is a sequencing problem, not an either-or decision.

9. Next curiosity

If the SEP is throttled down while the sail reorients, how does the trajectory control loop handle the interruption? Does it coast through the manoeuvre, or does it need a predictive model that accounts for the scheduled start and stop of thrust?