interstage

An interstage is the hollow structural framework that sits between two consecutive stages of a multistage rocket, connecting the engine bell of the lower stage to the base structure of the stage above. It contains no engines, fuel tanks, or main propellant lines, but carries all the mechanical, electrical, and instrumentation systems needed to manage the separation event and keep the stages structurally aligned during powered flight.

The interstage must be rigid enough to handle bending and compression loads from engine thrust and aerodynamic forces, yet light enough to not consume payload capacity. Most interstages are constructed from aluminum alloy shells, often with internal stringers or lattice reinforcement. The diameter typically matches the stage barrel diameter above and below, though some designs taper slightly. Thickness ranges from 2 to 6 mm depending on the rocket class and the loads expected at separation.

Mechanically, the interstage houses the separation system itself: explosive bolts or pyrotechnic charges that sever the connection between stages at the precise moment when the upper stage's engines ignite. It also contains hydraulic or pneumatic lines that route control fluid to the lower stage's thrust vector control actuators, electrical harnesses that fire explosive charges and ignition systems, and sensor attachment points for accelerometers and rate gyros. The structure must allow these systems to remain functional while upper stage engines fire in the gap between stages.

The interstage is also the mounting point for the lower stage's grid fins, canards, or other aerodynamic control surfaces on some designs. After separation, the interstage falls away with the lower stage and is not recovered unless the rocket is designed for booster reusability, in which case interstage design must accommodate landing loads and reuse cycles.

The term reflects its literal position: it exists in the space between stages rather than being part of a stage proper. In expendable launch vehicles, the interstage is often the last major structure engineered for minimum mass. In reusable systems like the Space Shuttle solid rocket booster, the interstage connection points must survive multiple flight cycles and thermal cycles from reentry heating.