Solid-fuel rockets

A solid rocket or a solid fuel rocket is a rocket with ashaped grain, burning from one end to the other.
motor that uses solid propellants (fuel/oxidizer). TheMost grains, however, are cast with a hollow core,
earliest rockets were solid fueled, powered byburning from the inside out (and outside in, if not
gunpowder, used by the Chinese in warfare as earlycase bonded), as well as from the ends.
as the 13th century. All rockets used some form ofThe thrust profile over time can be controlled by
solid or powdered propellant up until the 20thgrain geometry. For example, a star shaped core will
century, when liquid rockets and hybrid rocketshave greater initial thrust because of the additional
offered more efficient and controllable alternatives.surface area. As the star points are burned up, the
Solid rockets are still used today in model rockets,surface area and thrust are reduced.
and on larger applications for their simplicity andCasing
reliability.The casing may be constructed from a range of
Basic conceptsmaterials. Cardboard is used for model engines. Steel
A simple solid rocket motor consists of a casing,is used for the space shuttle boosters. Filament
nozzle, grain (propellant charge), and igniter.wound graphite epoxy casings are used for high
The grain behaves like a solid mass, burning in aperformance motors.
predictable fashion and producing exhaust gases. TheNozzle
nozzle dimensions are calculated to maintain a designA Convergent Divergent design accelerates the
chamber pressure, while producing thrust from theexhaust gas out of the nozzle to produce thrust.
exhaust gases.Some designs include directional control of the
Once ignited, a simple solid rocket motor cannot beexhaust. This can be accomplished by gimballing the
shut off, because it contains all the ingredientsnozzle, as in the Space Shuttle SRBs, by the use of
necessary for combustion within the chamber thatjet vanes in the exhaust similar to those used in the
they are burned in. More advanced solid rocketV2 rocket, and by liquid injection thrust vectoring
motors can not only be throttled but can be(LITV).
extinguished and then re-ignited by controlling theAn early Minuteman first stage used a single motor
nozzle geometry or through the use of vent ports.with four gimballed nozzles to provide pitch, yaw, and
Modern designs may also include a steerable nozzleroll control.
for guidance, avionics, recovery hardwareLITV consists of injecting a liquid into the exhaust
(parachutes), self-destruct mechanisms, APUs,stream after the nozzle throat. The liquid then
controllable tactical motors, controllable divert andvaporizes, and in most cases chemically reacts, adding
attitude control motors and thermal managementmass flow to one side of the exhaust stream and
materials.thus providing a control moment. For example, the
DesignTitan IIIC solid boosters injected nitrogen tetroxide
Design begins with the total impulse required, thisfor LITV; the tanks can be seen on the sides of the
determines the fuel/oxidizer mass. Grain geometryrocket between the main center stage and the
and chemistry are then chosen to satisfy theboosters 1.
required motor characteristics.Performance
The following are chosen or solved simultaneously.Solid fuel rocket motors have a typical specific
The results are exact dimensions for grain, nozzle andimpulse of 285 seconds (2.6 kN·s/kg). This
case geometries;compares to ~330 seconds (3.2 kN·s/kg) for
The grain burns at a predictable rate, given itskerosene/Lox and ~450 seconds (4.4 kN·s/kg)
surface area and chamber pressure.for liquid hydrogen/Lox bipropellant engines 1.
The chamber pressure is determined by the nozzleSolid rockets have a long history as the final boost
orifice diameter and grain burn rate.stage for satellites. This is related to their simplicity,
Allowable chamber pressure is a function of casingreliability, compactness and reasonably high mass
design.fraction.
The length of burn time is determined by the grainSolids can also provide high thrust for relatively low
'web thickness'.cost. For this reason, solids have been used as initial
The grain may be bonded to the casing, or not. Casestages in rockets (the classic example being the
bonded motors are much more difficult to design,Space Shuttle), whilst reserving high specific impulse
since deformation of both the case and grain, underengines, especially less massive hydrogen fuelled
operating conditions, must be compatible.engines for higher stages.
Common modes of failure in solid rocket motorsBut the ability of solid rockets to remain in storage
include fracture of the grain, failure of case bonding,for long periods, and then reliably launch at a
and air pockets in the grain. All of these produce anmoments notice, makes them the design of choice
instantaneous increase in burn surface area and afor a very many military applications.
corresponding increase in exhaust gas and pressure,Amateur rocketry
which may potentially induce rupture of the casing.Solid fuel rockets can be bought for use in model
Another failure mode is casing seal design. Seals arerocketry; they are normally small cylinders of fuel
required in casings that have to be opened to loadwith an integral nozzle and a small charge that is set
the grain. Once a seal fails, hot gas will erode theoff when the fuel is exhausted. This charge can be
escape path and result in failure. This was the causeused to ignite a second stage, trigger a camera, or
of the Space Shuttle Challenger disaster.deploy a parachute.
GrainDesigning solid rocket motors is particularly interesting
Solid fuel grains are usually molded from a thermosetto amateur rocketry enthusiasts. The design is simple,
elastomer, fuel, oxidizer and catalyst. HTPB and PBANmaterials are inexpensive and constructions
are typical elastomers which double as fuel.techniques are safe.
Ammonium perchlorate is the most common oxidizer.Early amateur motors were gunpowder. Later, zinc
This fuel mixture is known as Ammonium perchloratesulfur formulations were popular.
composite propellant (APCP).Typical amateur formulations in use today are; sugar
The exhaust from a solid rocket motor contains(sucrose, dextrose, and sorbitol are all common)
hydrochloric acid and aluminium sulfate. These a havepotassium nitrate, HTPB (a rubber like epoxy)
negative effect on the environment. Furthermore, formagnesium/ammonium nitrate, and HTPB or PBAN
military use, the smoke trail and the infrared radiationaluminum/ammonium perchlorate (APCP). Most
from the hot particles make it possible to detect theformulations also include burn rate modifiers and other
launch from space. These problems lead to theadditives, and also possibly additives designed to
research in smokeless grain which containscreate special effects, such as colored flames, thick
nitrogen-containing organic molecules.smoke, or sparks.
The grain is cast in different forms for differentAmateur rocket builders are very active in hybrid
purposes. Slow, long burning rockets have a cylindermotor research.