Designing the Space Power Supply

In the space environment there is no atmosphere and heat may be removed only through conduction to an ultimate radiator. This requires thermal testing to be performed under high vacuum conditions.

Also, because of the lack of an atmosphere, the nuclear radiation environment is severe. Natural radiation effects may include total ionizing dose, single event effects of energetic particles, proton and neutron fluence. Man made nuclear effects such as prompt dose and SGEMP may also be present. The orbit and the amount of shielding provided by the spacecraft are additional variables to be considered.

Spacecraft and satellites have self contained electrical systems where the power source is usually solar cell arrays, sometimes radioisotope thermal generators. Often, a battery or fuel cell is incorporated. These elements are regulated to form a relatively regulated electrical bus of limited power capacity. The equipment on the spacecraft will then use power supplies, DC/DC converters or EPCs (electronic power converters) to provide power for the many types of equipment on the satellite.

The important issue facing space power supply applications is that extremely high reliability must be achieved in a harsh environment with quantities of power supplies that are relatively low. The low quantities discourage learning curve and experience improvements.

The methods used today for designing and producing space power supplies are intended to capture past lessons learned to overcome the burden of small production quantities.

One method of capturing past lessons learned is the use of “heritage” designs. These are designs and processes that have been successfully used on other spacecraft. To the extent that the new application provides the same type of environment as the heritage application, there is benefit from this approach. As time passes between the heritage design and the present design, the benefit decreases. This is because the heritage parts become more difficult to get, heritage processes may have changed, and finally, heritage employees throughout the supply chain are no longer involved.

Piece parts on preferred parts lists become harder and harder to get as years pass. Many of the components may be commercially obsolete, leading to so called “high rel” parts actually coming from the scrap barrel. Equipment designed with obsolete, but preferred parts becomes heavier and larger than equipment designed with up to date parts.

A successful design takes what’s best from heritage combined with the careful use of what’s best from current practice.

Space power supplies are unique because:

• Quantities are low, but reliability must be high
• The unique space environment is harsher than terrestrial environment