Construction of Solid-Propellant Rockets

    After World War II solid-propellant rockets were developed for many purposes, mostly as boosters for guided missiles. The principal parts of the solid-propellant rocket are the payload, consisting of the warhead or scientific instruments, and the combustion chamber, or motor, containing the fuel charge and nozzles to expel the combustion gases. Fins may be added to stabilize its flight.

    Solid-propellant rockets today are divided into two categories, those with unrestricted burning charges and those with restricted burning charges, known also as wall-fitting charges. A typical example of the unrestricted-burning charge rocket is the HVAR utilized in World War II, which was charged with a single stick of powder having a cruciform cross section and suspended in the centre of the rocket motor. This charge burned at all its surfaces, except at the two ends, which were covered by non-flammable plastics. An unrestricted burning charge also may be shaped in the form of a thick-walled hollow tube, which burns at both its inside and outside surfaces. No matter what its size or shape, the charge is called a grain and the devices that hold it in place are known as traps. Unrestricted burning charges have burning times of less than one sec.

    For longer burning times a wall-fitting charge is used. This type of charge either burns across its cross section, or it may be hollowed out at the centre so that it burns from the inside towards the rocket wall. The latter method permits a reduction in the thickness of the wall of the outer metal tube of the rocket, because for virtually all of the burning time, the metal tube is reinforced by what is left of the charge.

    Modern solid-propellant charges are of very large size. For example, the take-off weight of the solid-propellant submarine-launched Trident-II D5 missile is about 59,000 kg (about 130,000 lb). The two solid rocket boosters (SRBs) on the space shuttle weigh more than half a million kg (more than 1 million lb) each. Made of 11 steel segments, the SRB is the largest solid-propellant rocket ever built in the United States. As a result of the Challenger space shuttle disaster, the seals between the segments were redesigned to prevent a recurrence of the problem that caused the destruction of the spacecraft.

    The problem of developing a defensive antiballistic missile (ABM) to intercept approaching ICBMs requires very quick reaction times and high acceleration. The solid-propellant rocket meets these requirements best, and, therefore, the Safeguard ABM system used solid propellants, which included the Sprint, a low-altitude 24-40 km (15-25 mi) intercept missile, and the Spartan, a high-altitude (over 160 km/100 mi) antiballistic missile.

    Modern types of solid propellants are synthetic rubbers with an oxidizer such as ammonium perchlorate mixed in during the manufacturing process. Synthetic rubbers are good fuels, and they also have the advantage of staying somewhat flexible so that they do not develop cracks when handled roughly. The synthetic rubber and ammonium perchlorate mixture can be made more powerful by the addition of powdered metals such as aluminium.