Propellants
Dr. Andrew Ketsdever
Propellants
• Several Factors Must Be Addressed When
Deciding on a Propellant
– Performance
• Chemical energy content, Achievable Isp
– Economics
• Availability, logistics of production and supply
– Physical Hazards
• Stability, corrosiveness, carcinogens
Liquid Propellants
• Desirable Properties
– Low Freezing Point
– High Specific Gravity
– Good Stability
– High Specific Heat
– Low Vapor Pressure (better pumping)
– Easy Ignition (hypergolic?)
Liquid Propellants
• Highest Potential Specific Impulse
– Fluorine Oxidizer
– Hydrogen Fuel with suspended Beryllium
– Isp=480 sec at Po = 1000 psia (sea level)
– Oxidizer highly corrosive (not storable)
– Beryllium suspension can not be maintained
uniformly
Liquid Propellants
• Oxidizers
– Liquid Oxygen
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Boils at 90K
Heat of Vaporization 213 kJ/kg
High attainable performance
Does not burn spontaneously with hydrocarbons at
ambient pressures and temperatures
• Noncorrosive and nontoxic
• Cryogenic temperature poses handling issues
Liquid Propellants
– Liquid Fluorine
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Boils at 53.7K
Highest values of performance (typically)
High specific gravity
Extremely toxic and corrosive
Poisonous exhaust gases
Low commercial consumption
Expensive
Liquid Propellants
– Hydrogen Peroxide (H2O2)
• Requires high concentration (70-99%)
• Storage concerns
• Decomposition with certain metal catalysts
– Nitric Acid (HNO3)
• Highly corrosive
• Certain stainless steels and gold containers
required
• Skin burns
Liquid Propellants
– Nitrogen Tetroxide (N2O4)
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Most common storable oxidizer used in the US
High density
Relatively high freezing point
Mildly corrosive
Hypergolic with many fuels
High vapor pressure
Used with hydrazine (UDMH, MMH) propellants
Liquid Propellants
• Fuels
– Hydrocarbon Fuels
• RP-1 (Kerosene-like)
– Easy to handle
– Cost effective
• Methane (CH4)
– Cryogenic (denser than liquid hydrogen)
Liquid Propellants
– Liquid Hydrogen
• Boils at 20K
• Specific Gravity 0.07
• Bulky fuel tanks, large volumes
– Increased drag
• Feed system must be cryogenic compatable
• Increased insulation
• Nontoxic products (typically)
Liquid Propellants
– Hydrazine
• Monomehtylhydrazine (MMH)
• Unsymmetrical dimethylhydrazine (UDMH)
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Toxic
High freezing point 274.3K (MMH) / 216K (UDMH)
Hypergolic with some oxidizers
Spontaneous ignition with air can occur
Positive heat of formation (good)
Good monopropellants with the right catalyst
Reasonable stability
Solid Propellants
• Classes
– Double Base Propellant: forms a
homogeneous propellant grain of a
nitrocellulose and a solid ingredient dissolved
in nitroglycerin plus minor additives
– Composite Propellant: form a heterogeneous
propellant grain with the oxidizer crystals and
a powdered fuel held together in a matrix of
synthetic rubber binders.
• Less hazardous than double base
Solid Propellants
• Oxidizers
– Ammonium Perchlorate (NH4ClO4)
• Most widely used crystalline oxidizer
• Good performance
• Good availability
– Potassium Perchlorate (KClO4)
• Medium performance
• Higher density than AP
• Low burning rate
Solid Propellants
• Fuels
– Powdered Aluminum
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5-60 micron diameter
14-18% of propellant by weight (typical)
Small particles can burn spontaneously in air
Oxide particles can agglomerate and form larger
particles (two phase issues)
– Other metallic fuels
• Boron, Beryllium
Solid Propellants
• Binders
– Hydroxy Terminated Polybutadiene (HTPB)
• Binder provides the structure or matrix in which
solid ingredients are held together (composite
propellant)
• Polymer, synthetic rubber
• Primary effect on motor reliability, storability and
cost
• Other Additives can be included
– Improve burn rate, storability, curing
Hybrid Propellants
• Fuel and Oxidizer mixtures of liquid
oxidizer and solid propellants (typically)
– Combinations of propellants already
discussed
– Fuel: HTPB, PMMA, HS
– Oxidizer: LOx, H2O2, N2O
Nuclear Propellants
• Fuels
– Low molecular weights
• Hydrogen, Water, Methane
– No chemical reactions necessary