Effect of Nozzle Expansion

AP is a valuable oxidizer for formulating smokeless propellants or smokeless gas generators. However, since the combustion products of AP composite propellants contain a relatively high concentration of hydrogen chloride (HCI), white smoke is generated when they are expelled from an exhaust nozzle into a humid atmosphere. When the HCI molecules diffuse into the air and collide with H2O molecules therein, an acid mist is formed which gives rise to visible white smoke. Typical examples are AP composite propellants used in rocket motors. Based on experimental observations, white smoke is formed when the relative humidity exceeds about 40 %. Thus, AP composite propellants without any metal particles are termed reduced-smoke propellants. On the other hand, a white smoke trail is always seen from the exhaust of a rocket projectile assisted by an aluminized AP composite propellant under any atmospheric conditions. Thus, aluminized AP composite propellants are termed smoke propellants. 

Space Shuttle, for example, has two large solid-rocket boosters for its launch stage. The booster propellant has the following composition  

the two boosters generate large amounts of gaseous and condensed products, including HCI, which are expelled into the atmosphere. When the exhaust HCI gas 

Propellant mass of the two booster propellants Mass fraction of HQ contained within the two booster propellants  

Total mass of HQ within the combustion products Hydrochloric add  

---