CHEMISTRY: PROPELLANTS

CHEMISTRY: PROPELLANTS

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Overview

• A propellant is a material that is used to propel an object
• The object is usually expelled by the pressure created by a gas
• This pressure may be created by a compressed gas or by a gas produced by a chemical reaction
• Propellants may be solids, liquids, gases or plasmas
• Common chemical propellants consist of a fuel and an oxidiser

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Types of propellants

• Aerosol sprays
  • Aerosol spray is a dispensing system that creates an aerosol (fine) mist of liquid particles
  • In aerosol sprays, the propellant is simply a pressurised gas in equilibrium with its liquid form
  • As some gas escapes to expel the payload, more liquid evaporates thereby maintaining an even pressure
  • The aerosol spray can was invented by Erik Rotheim (Norway) in 1927
  • Aerosol sprays are typically used to dispense insecticides, deodorants and paints
• Propellants used for propulsion
  • Rockets typically use bipropellants, which contain a combination of a fuel and an oxidiser. Tripropellants, which are not used commonly, use liquid hydrogen as a third component to provide additional efficiency
  • Propellants are usually made from low explosives, which deflagrate (burn) rather than detonate (explode)
  • The controlled burning of the propellants produces thrust by gas pressure which is then used to accelerate a rocket, projectile or other vehicles
  • Propellants are commonly used in rockets, firearms and artillery

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Solid propellants

• Solid propellants are used for rockets, firearms and artillery
  
• Examples of solid propellants include gunpowder (sulphur + charcoal + potassium nitrate), nitrocellulose and cordite 
  
• Single based propellants: They have nitrocellulose as its chief ingredient. Stabilizers and other chemicals may be added for chemical stability
• Double based propellants: they contain nitrocellulose with nitroglycerin or other liquid nitrate explosives added. Nitroglycerin reduces smoke and increases energy output. Used in small arms, cannons, mortars and rockets
• Triple based propellants: consist of nitrocellulose, nitroquanidine, and nitroglycerin or other nitrate explosives. Used in cannons
• Composite propellants: consist of a fuel such as metallic aluminium, a binder such as synthetic rubber and an oxidiser such as ammonium perchlorate. Used in large rocket motors such as spacecraft
• Solid propellants have been used since the 11^th century to power rockets based on gunpowder
• Solid fuel rockets offer ease of handling, reliability and long storage periods
• Solid fuel rockets are used for missiles due to their long storage periods and reliability of launch on short notice
• Currently, solid fuel rockets are not used for space explorations, but are commonly used as booster rockets to launch spacecraft

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Liquid propellants

• Liquid propellants are usually used in combinations of fuel and oxidiser
• Common liquid propellant combinations include
  • Liquid oxygen and liquid hydrogen
  • Liquid oxygen and kerosene
  • Nitrogen tetraoxide and kerosene
• Liquid fuel rockets are desirable because they offer higher energy output, they can be throttled and shut down and can be reused
• Liquid fuel rockets are used to power space shuttles
• A variant of liquid fuel engine is cryogenic fuel engine – these are engines that use gases which are super-cooled into their liquid forms

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Propellants used in the PSLV

• The Polar Satellite Launch Vehicle (PSLV) has a four stage propulsion system, using solid and liquid propellants alternately
• First stage: solid – Hydroxyl terminated polybutadiene (HTPB)
• Second stage: liquid – unsymmetrical di-methyl hydrazine (UDMH) as fuel and nitrogen tetraoxide as oxidiser
• Third stage: solid – HTPB
• Fourth stage: solid – mono methyl hydrazine as fuel and mixed oxides of nitrogen as oxidiser

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Propellants used in the GSLV

• The Geosynchronous Satellite Launch Vehicle (GSLV) is a three stage launch vehicle using solid, liquid and cryogenic propellants
• First stage – solid – HTPB
• Second stage – liquid – UDMH as fuel and nitrogen tetraoxide as oxidiser
• Third stage – cryogenic – liquid hydrogen and liquid oxygen

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JOHAR