Resistojet rocket

Many satellite missions necessitate an ability for minor alterations in trajectory even after the craft has been inserted into orbit. Most satellites use monopropellant rocket motors or cold gas thrusters for such orbital adjustments. Both methods, however, suffer from some limiting drawbacks: Hydrazine, the most commonly used monopropellant, is highly expensive and due to its volatile nature unsuitable for smaller satellites that are sent to space as secondary cargo. Cold gas thrusters, while utilizing relatively cheap, inert and therefore "safe" gasses like nitrogen, suffer from low specific impulse in comparison to monopropellant motors. Resistojets are designed to bridge the gap between these two methods of propulsion, offering the safety of an inert propellant coupled with specific impulse nearing that of hydrazine. == Disadvantages ==

The main disadvantage of a resistojet design in comparison to simpler cold gas thrusters is the need for a power supply, which takes up space and is therefore sometimes a prohibitive factor for Microsat missions. In addition, the increased technical complexity of a resistojet relative to simpler solutions results in a greater risk of technical failure. Since they do not take advantage of chemical combustion, resistojets (and similar designs) have a lower thrust that is orders of magnitude lower than those of more conventional solid fuel and liquid-propellant rockets. As a result, they are unsuitable for orbital maneuvers that require high delta-V over shorter periods. == Physical principles ==

Resistojets can be considered an evolution of traditional cold gas thrusters, which are the simplest form of rocket engine available. Their fuel tank holds the propellant, which is then led into the nozzle where it decompresses, propelling the craft forward. In a resistojet, a resistor is used to heat the fluid before it enters the nozzle, making it expand more forcefully, resulting in higher specific impulse. A resistor is an electrical component which converts electrical energy into heat. Therefore, the thrust of a resistojet engine can be regulated by simply altering the wattage flowing through the resistor. Heating a room-temperature (27 °C) fluid by 300 °C in such a way results in a 41% increase in specific impulse. If heated by 900 °C, specific impulse could be doubled in comparison to a cold gas thruster using the same propellant. In other words, a quadrupling of the temperature yields a doubling in specific impulse. ==See also ==