OrbitalHub

After a short introduction to the power systems requirements and design factors, we will continue by covering the first solutions adopted by spacecraft designers: the batteries and the solar arrays (aka solar cells).

Credits: NASA

Batteries were used as a primary source of power onboard early spacecrafts. The obvious limitation is that batteries have limited energy storage capabilities and could not keep spacecrafts operational for more then a few days. Most space missions require a reliable power source running for a longer period of time.

Batteries remain the primary means of energy storage onboard spacecrafts. Batteries are divided into two major categories: primary batteries and secondary batteries.

Primary batteries offer higher energy and power densities but are not rechargeable. They are useful for one-time events such as expendable launch vehicle stages. Secondary batteries are rechargeable batteries.

Solar arrays are very well suited for long missions in space. The life expectancy of a solar cell power system is limited only by the degradation of its components. Spacecrafts operating for extended periods of time become feasible with the development of solar arrays. However, if only solar cells are used for generating power, spacecrafts that enter eclipse periods cannot employ only solar cells for power generation.

Credits: NASA

The first low-powered spacecraft designs were using the spacecraft skin for the solar cell deployment. In the case of drum-shaped spacecrafts, only about 40% of the arrays were illuminated by the Sun at any time. Because most of the time the available area on the fixed spacecraft structure is not enough from the standpoint of power requirements, deployable solar arrays are now used. The solar arrays of this type are deployed from the main structure after the spacecraft is injected into orbit.

The deployable panels are designed as extremely lightweight structures due to the fact that they are firmly locked to the spacecraft during the launch. In order to optimize the generation of power, these panels are designed to allow sun tracking.

Credits: NASA

Considering the limitations of the solar arrays, a reliable solution can be reached by employing solar cells and batteries at the same time. Solar arrays can generate power when direct sunlight is available in orbit, while rechargeable batteries can handle peak loads and provide power during eclipse periods. Solar panels and batteries in combination are a common solution used for the unmanned spacecrafts launched to date. The most notable exception is the deep space mission probes using radioisotope thermoelectric generators (we will cover them in a future post).

The early manned spacecrafts, including Mercury, some of the Gemini, and the Russian Vostok /Voshkod vehicles, used batteries. The Russian Soyuz employs solar cells and batteries similar to a typical unmanned spacecraft. The space stations built so far, Salyut, Skylab, Mir, and the International Space Station, have all used solar cells as the primary power source, having secondary batteries for load leveling and eclipse periods.

In the following posts we will see what solutions are available for missions that cannot rely on solar power as a primary source of energy.