used for the inner covers are Dacron and Nomex netting. Mylar is not used because of flammability concerns. MLI blankets are an important element of the thermal control system. Louvers Louvers are active thermal control elements that are used in many different forms. Most commonly they are placed over external radiators, louvers can also be used to control heat transfer between internal spacecraft surfaces or be placed on openings on the spacecraft walls. A louver in its fully open state can reject six times as much heat as it does in its fully closed state, with no power required to operate it. The most commonly used louver is the bimetallic, spring-actuated, rectangular blade louver also known as venetian-blind louver. Louver radiator assemblies consist of five main elements: baseplate, blades, actuators, sensing elements, and structural elements. Heaters Heaters are used in thermal control design to protect components under cold-case environmental conditions or to make up for heat that is not dissipated. Heaters are used with thermostats or solid-state controllers to provide exact temperature control of a particular component. Another common use for heaters is to warm up components to their minimal operating temperatures before the components are turned on. The most common type of heater used on spacecraft is the patch heater, which consists of an electrical-resistance element sandwiched between two sheets of flexible electrically insulating material, such as Kapton. The patch heater may contain either a single circuit or multiple circuits, depending on whether or not redundancy is required within it. Another type of heater, the cartridge heater, is often used to heat blocks of material or high-temperature components such as propellants. This heater consists of a coiled resistor enclosed in a cylindrical metallic case. Typically a hole is drilled in the component to be heated, and the cartridge is potted into the hole. Cartridge heaters are usually a quarter-inch or less in diameter and up to a few inches long. Another type of heater used on spacecraft is the radioisotope heater units also known as RHUs. RHUs are used for travelling to outer planets past Jupiter due to very low solar radiance, which greatly reduces the power generated from solar panels. These heaters do not require any electrical power from the spacecraft and provide direct heat where it is needed. At the center of each RHU is a radioactive material, which decays to provide heat. The most commonly used