1. The arithmetic of surviving the dark
Neal Stephenson’s Seveneves returns as the recalled work because the arklets and Endurance must balance generation, storage, and consumption through long periods when no sunlight is available. A LEO desktop does this many times per day: generate in sun, store enough for eclipse, and never let the battery depth of discharge exceed what the cells can survive for years.
This entry reads about power budgets and eclipse sizing.
2. The power budget as a mode table
A spacecraft power budget is not a single number. It is a table of operating modes, each with its own set of loads, duty cycles, and durations. An Atlas of Engineering eclipse power budget project emphasizes that a power subsystem is sized from orbit timing, eclipse duration, battery limits, array degradation, conversion efficiency, and operational modes—not from one peak load.
A typical power budget includes rows for:
- Payload instruments and processors.
- Communications transceivers and amplifiers.
- Attitude determination and control.
- Thermal heaters and louvers.
- Onboard computers and avionics.
- Power conversion and distribution losses.
Each row has power in watts, duty cycle as a fraction, and average power. The sum gives the mode average. Modes are then matched to orbit phases: sunlit, eclipse, safe mode, and peak-load events.
3. Eclipse duration and energy balance
Eclipse duration depends on orbit altitude, inclination, and beta angle. A SatBase power budget calculator computes energy balance from orbit period, eclipse duration, sunlit load, eclipse load, and allowed battery depth of discharge.
The basic energy balance question is: does the solar array generate enough energy during sunlit time to both run the spacecraft and recharge what was consumed during eclipse, plus losses and margins?
A University of Alabama K-band constellation design report gives the standard battery sizing equation: capacity equals eclipse energy divided by depth of discharge, number of battery strings, and efficiency. For LEO, DOD is often limited to around 30 percent to achieve the required cycle life.
4. Margins and degradation
Power budgets carry margins for the same reason mass budgets do: reality diverges from analysis. Typical margins include:
- Begin-of-life to end-of-life array degradation: radiation, UV, and atomic oxygen reduce output over time.
- Pointing and illumination losses: arrays are not always perfectly normal to the Sun.
- Conversion losses: DC-DC converters, battery charge-discharge efficiency, and distribution losses.
- Growth margin: payloads and subsystems often draw more power than predicted early in design.
- Safe-mode margin: the spacecraft must survive even when operating at minimum power.
A smallsat.market article on power budgeting describes these practical methods for closing a reliable power budget.
5. What this changes
- The desktop’s power budget must be built around operating modes, not a single worst-case load.
- Battery capacity is driven by eclipse energy and cycle-life-limited depth of discharge.
- Solar array area is driven by end-of-life energy balance, not beginning-of-life peak power.
- Margins and degradation must be included from the start.
- The next entry will read about power electronics and distribution.