Advantages and disadvantages of solid and liquid propellants
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Advantages of solid propellants
• Solid propellant rockets are much easier to store and handle than liquid propellant rockets.
• High propellant density makes for compact size as well.
• These features plus simplicity and low cost make solid propellant rockets ideal for military
applications.
• Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are
needed and cost is an issue.
Disadvantages of solid propellants
• Relative to liquid fuel rockets, solid fuel rockets have lower specific impulse.
• The propellant mass ratios of solid propellant upper stages is usually in the .91 to .93 range
which is as good as or better than that of most liquid propellant upper stages but overall
performance is less than for liquid stages because of the solids' lower exhaust velocities.
• The high mass ratios possible with (unsegmented) solids are a result of high propellant density
and very high strength-to-weight ratio filament-wound motor casings.
• A drawback to solid rockets is that they cannot be throttled in real time.
• Solid rockets can be vented to extinguish combustion or reverse thrust as a means of
controlling range or accommodating warhead separation.
• Casting large amounts of propellant requires consistency and repeatability which is assured by
computer control.
Advantages of liquid propellant
There's several ways to judge rocket performance. Solids are best for giving maximal kick (impulse) near the ground. This is why you see them often used as strap-on boosters that are dropped early in flight.
But they are not as efficient as liquids, by and large, where it's not just thrust-to-weight but specific impulse that matters, a measure related to thrust per unit of propellant mass. The higher you go on a rocket stack, the more this matters, so you'll tend to see increasingly high-Isp engines in upper stages. Among the highest is the RL-10, which is designed to operate in a vacuum.
Liquids provide other important advantages, including throttle-ability, shutdown capability (important for aborts) and re-light (important for upper stages). Some propellant types, known as 'storable' propellants (generally hypergolics) remain usable for years in space. Liquids also offer much simpler ground handling, when tanks are not yet filled.
• Solid propellant rockets are much easier to store and handle than liquid propellant rockets.
• High propellant density makes for compact size as well.
• These features plus simplicity and low cost make solid propellant rockets ideal for military
applications.
• Their simplicity also makes solid rockets a good choice whenever large amounts of thrust are
needed and cost is an issue.
Disadvantages of solid propellants
• Relative to liquid fuel rockets, solid fuel rockets have lower specific impulse.
• The propellant mass ratios of solid propellant upper stages is usually in the .91 to .93 range
which is as good as or better than that of most liquid propellant upper stages but overall
performance is less than for liquid stages because of the solids' lower exhaust velocities.
• The high mass ratios possible with (unsegmented) solids are a result of high propellant density
and very high strength-to-weight ratio filament-wound motor casings.
• A drawback to solid rockets is that they cannot be throttled in real time.
• Solid rockets can be vented to extinguish combustion or reverse thrust as a means of
controlling range or accommodating warhead separation.
• Casting large amounts of propellant requires consistency and repeatability which is assured by
computer control.
Advantages of liquid propellant
There's several ways to judge rocket performance. Solids are best for giving maximal kick (impulse) near the ground. This is why you see them often used as strap-on boosters that are dropped early in flight.
But they are not as efficient as liquids, by and large, where it's not just thrust-to-weight but specific impulse that matters, a measure related to thrust per unit of propellant mass. The higher you go on a rocket stack, the more this matters, so you'll tend to see increasingly high-Isp engines in upper stages. Among the highest is the RL-10, which is designed to operate in a vacuum.
Liquids provide other important advantages, including throttle-ability, shutdown capability (important for aborts) and re-light (important for upper stages). Some propellant types, known as 'storable' propellants (generally hypergolics) remain usable for years in space. Liquids also offer much simpler ground handling, when tanks are not yet filled.
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