1. The ship that could not afford a dead bus

James S. A. Corey’s Leviathan Wakes returns as the recalled work because the ships of The Expanse run on reactor power, and a bus fault or a blown converter can kill communications, life support, or maneuvering just as surely as enemy fire. A LEO desktop is smaller but has the same shape of risk: if the electrical bus fails, every subsystem downstream fails with it.

This entry reads about energy budgets and fault protection.

2. The energy budget as a ledger

The energy budget is an accounting of energy in and energy out over one orbit and over the mission life. It must cover:

  • Sunlit generation: solar array output at the expected Sun angle and temperature;
  • Eclipse draw: load power multiplied by eclipse duration, taken from the battery;
  • Recharge: the energy needed to restore the battery after eclipse;
  • Margin: design margin for cell degradation, unexpected loads, and cold batteries.

AAC Clyde Space notes that the EPS must manage this continuously, maximizing power from the panels during sunlight and providing an efficient discharge path during eclipse. The budget is not a one-time calculation; it changes as the array degrades and as the battery capacity fades.

3. Depth of discharge and battery life

A battery’s cycle life depends strongly on how deeply it is discharged. Shallow discharges allow thousands of cycles; deep discharges shorten life. A LEO spacecraft with a 90-minute orbit and a five-year mission faces roughly 30,000 cycles. Limiting depth of discharge is therefore essential, even though it means carrying more battery capacity than a single eclipse strictly requires.

The energy budget must also account for the fact that battery capacity drops at low temperatures. Heaters add load, which reduces margin, which may require more battery, which requires more heater power. This loop is one reason power and thermal cannot be designed separately.

4. Fault protection

NASA’s power chapter emphasizes that PMAD systems must protect against harmful transient disturbances and fault conditions. A SciELO paper on small-spacecraft EPS architectures adds that the EPS is one of the main causes of CubeSat failures, especially in the first three months. Common protections include:

  • undervoltage lockout, which prevents over-discharge;
  • overvoltage protection, which prevents over-charge;
  • overcurrent protection, which isolates shorted loads;
  • thermal protection, which disables charging or discharging outside safe temperatures;
  • latch-up protection, which recovers from single-event effects;
  • redundant strings or dual-bus architectures, which limit the impact of a single failure.

The SciELO paper compares single-bus, dual-bus, and cross-strapped architectures. Redundancy reduces theoretical failure probability, but added complexity can reintroduce risk through design errors. For the desktop, a dual-bus or string-redundant approach may be warranted because attachments can be swapped and the load profile is not fixed.

5. What this changes

  • The desktop’s power system must be sized for end-of-life, worst-case eclipse, and margin, not for nominal beginning-of-life conditions.
  • Battery depth of discharge is a life-limiting choice and must be chosen early.
  • Fault protection is not an afterthought; the EPS is a common cause of small-spacecraft failure.
  • A modular, redundant architecture may be appropriate for a platform whose loads change over time.
  • The next entry will close the power reading arc and prepare for the wondering arc.