1. The question left open

Entry 872 said the SEP backup is a 1-AU device. This entry asks a more domestic question: how does it plug into the rest of the tug? A hybrid tug already has solar arrays, batteries, avionics, sail-actuators, heaters, radios, and possibly a health-monitoring camera suite. Adding an electric thruster means adding a Power Processing Unit (PPU) that wants a particular voltage and a lot of current. Does the thruster get its own power train, or does it draw from the same bus that keeps the sail alive?

The answer is not purely electrical. It is architectural.

2. What the SEP side wants

A Hall or ion thruster needs its anode at hundreds of volts and its cathode at a controlled current. The PPU converts the spacecraft bus voltage into what the thruster needs. The Psyche mission overview notes that SSL’s SPT-140 PPU was qualified for a 100 V regulated bus; the Psyche array, however, had a peak-power voltage that varied from about 65 V to 100 V depending on temperature and solar distance. That mismatch meant the straightforward GEO bus architecture was not directly usable for deep space.

The EDDA H2020 direct-drive study lays out the standard SEP chain: solar array at 100–130 V, a Power Conditioning and Distribution Unit with Maximum Power Point Tracking, a PPU that boosts to 300 V or more for the thruster, and a secondary low-voltage bus for payload and avionics. The study also notes the drawback: a 20 kW SEP system can dissipate more than 1 kW in the PCDU and PPU, mass that does not go to thrust.

So the SEP side wants a high-voltage, high-current bus, stable enough for the PPU, and sized for the thruster’s peak power.

3. What the sail side wants

A solar sail is mostly passive. It does not need megawatts to create thrust. But it does need power to point itself: boom-mounted actuators, tip vanes or reflectivity-control devices, reaction wheels, star trackers, gyroscopes, radios, and computers. Lappas et al. give power budgets for sail attitude-control suites in the watts-to-hundreds-of-watts range, depending on size and complexity.

The sail’s power demand is modest and continuous, but it cannot tolerate a complete power loss during a manoeuvre. If the bus collapses while the sail is face-on to the Sun at an oblique angle, the sail becomes an uncontrolled weather vane. The avionics and actuators must stay alive.

4. Three architectures

Shared bus. The same solar array feeds a regulated main bus. The PPU, avionics, actuators, and communications all draw from it. This is the simplest and lightest arrangement. The arrays are sized for SEP peak power, so when the thruster is off there is plenty of margin for the sail and payload.

The risk is coupling. A PPU fault or a thruster arc can pull the bus down. If the sail’s actuators lose power at the wrong moment, the sail can torque the stack. A shared bus therefore needs good fault containment and a fast load-shedding policy: when SEP starts, non-essential loads get turned off.

Dedicated SEP bus. The thruster has its own array, PPU, and bus. The sail and avionics have a separate, smaller power system. This electrically isolates the two engines. A thruster fault cannot starve the sail, and a sail actuator fault cannot disturb the thruster.

The cost is mass. Two sets of arrays, two sets of regulators, and two battery packs add up. For an abort-sized SEP of a few kilowatts to a few tens of kilowatts, a dedicated array may be smaller than the avionics array, but the duplication of power-conditioning hardware is real.

Battery-buffered shared bus. A single array charges a battery, and the battery feeds the bus. SEP, avionics, and actuators all draw from the battery. The battery absorbs transients and decouples the loads. This is common on Earth-orbiting spacecraft and is the GOES-U approach: solar array power is regulated through shunts, batteries bridge eclipses and peak loads.

The catch is battery mass and cycle life. A SEP burn that lasts hours or days draws heavily on the battery if the array cannot keep up. For a large SEP backup, the battery would have to be enormous.

5. The mass and fault trade

For a minimoon tug with an abort-sized SEP backup, the shared bus is probably the honest starting point. The SEP is small enough that its PPU can share a 100 V-class bus with the avionics. The arrays are already large because the sail is large, although the sail does not itself consume much power. When the thruster fires, the avionics and sail actuators run from the same arrays; as long as the power budget is positive, the bus stays up.

The real design work is fault isolation. A PPU must not be allowed to collapse the main bus. A solid-state disconnect and a priority load-shedding table are minimum additions. Without them, sharing the bus is not mass savings; it is a single-point failure.

A dedicated SEP bus becomes attractive if the SEP grows large enough to compete with the avionics for array area, or if the mission profile requires the sail to remain controllable during long SEP burns. For a capture-alone SEP system — the hundreds-of-kilowatts case from Entry 871 — a dedicated high-voltage bus is almost mandatory.

6. The Anderson echo

Poul Anderson’s Tau Zero follows a starship whose Bussard ramjet cannot be shut off. The crew spends the novel managing the consequences: the engine that keeps them alive also threatens to outlive the universe. Their power, propulsion, and life support are all tied to one uncontrollable drive.

The hybrid tug is not that extreme, but the architectural anxiety is the same. Sharing the bus means the sail and the SEP are electrically married. A thruster problem becomes a sail problem unless the divorce papers — the fault isolation and load-shedding logic — are written in advance.

7. The Popperian note

The conjecture is that a shared bus is good enough for an abort-sized SEP backup. The refutations would be:

  • A PPU fault mode that collapses the main bus despite isolation, or
  • A mission profile where SEP must fire while the sail is under high control authority, making load shedding unacceptable, or
  • A solar-array voltage range, especially at varying Sun distance, that is incompatible with the avionics bus voltage.

If any of those is demonstrated, the shared-bus conjecture is falsified and a dedicated SEP power train is required.

8. What this changes

Entry 872 fixed the operating radius. Entry 873 fixes the electrical interface. For an abort-sized SEP backup on a minimoon tug, the SEP should draw from the same solar array and bus as the sail avionics, but only if the design includes fault isolation and a load-shedding hierarchy. A dedicated SEP power train is a heavier but more robust option; it becomes the default if the SEP is sized for capture alone.

The practical implication is that the tug’s power subsystem is not just an avionics power supply with a thruster attached. It is a resource allocator that must know, at every moment, which loads keep the sail under control and which can be shed to keep the thruster alive.

9. Next curiosity

If the shared bus is the answer, what is the minimum set of loads that must never be shed? And how long can the sail actuators and attitude sensors survive if the SEP PPU drags the bus down?