Q.Write notes on chemical propellants used in propulsion system of rockets.

Chemical propellants combine a fuel and an oxidiser so that combustion can occur without atmospheric oxygen (essential in space); the hot, high-pressure gases produced are expelled through a nozzle to generate thrust by Newton's third law.

Requirements of a good propellant

• High energy content / high specific impulse (thrust per unit propellant consumed)
• Controlled and steady (not explosive) rate of combustion
• Chemical and physical stability during storage and handling
• Reasonably high density (more propellant mass per unit tank volume)
• Combustion products should ideally be of low molecular mass and, where possible, non-toxic/non-corrosive

Types of propellants

1. Solid propellants — the fuel, oxidiser and a polymeric binder are cast together into a single solid "grain" that is stored ready-to-use inside the rocket motor casing.

• Composite propellant example: powdered aluminium (fuel) + ammonium perchlorate, $NH_4ClO_4$ (oxidiser), held together in a synthetic rubber-like polymer binder (e.g. hydroxyl-terminated polybutadiene, HTPB).

• Advantages: simple, robust, long storage life, high thrust — used in booster/first stages (e.g. solid strap-on boosters).

• Disadvantage: once ignited, burning cannot easily be stopped, throttled or restarted.

2. Liquid propellants — a liquid fuel and a liquid oxidiser are stored in separate tanks and pumped into a combustion chamber where they mix and burn.

• Monopropellant: a single liquid (e.g. hydrazine, $N_2H_4$) that decomposes exothermically over a catalyst.
• Bipropellant: separate fuel and oxidiser, e.g. liquid hydrogen ($H_2$) or kerosene or UDMH (unsymmetrical dimethylhydrazine) as fuel, with liquid oxygen ($O_2$), nitrogen tetroxide ($N_2O_4$), or red fuming nitric acid as the oxidiser. …