Rocket Design
End Burning
This is the simplest, but also most inefficient form of combustion. As the name suggests, the propellant burns like a cigarette - from one end to the other. This means that only a very small surface area is available for combustion at any moment in time. This greatly constrains the amount of exhaust gas produced, resulting in very weak thrust. Some end burners employ a small conical combustion chamber in an attemt to increase the exposed surface area.
Cylinder
A circular cross-section hole running the length of the propellant increases the surface area exposed for combustion and insulates the motor case from the heat of combustion. The larger surface area increases thrust, and the propellant insulated motor case allows for a lighter construction. However, as the propellant burns, the diameter of the circular cross-section increases, resulting in steadlily increasing thrust.
Burning Star
The burning star combustion design was introduced at the same time as rubber binders. The use of synthetic rubber allowed the propellant to be moulded into complex shapes and bonded to the motor case. The star shaped cross-section of the combustion area allows the propellant to burn at an even rate, providing constant thrust for the duration of the burn. The burning star combustion chamber has become the standard design for ballistic missiles.
Rod And Tube
The rod and tube is usually used with ouble-base propellants, as it is easy the extrude this composition into rod shapes. The rod and tude design employs a circular cross-section case bonded propellant, plus a propeelant rod in the centre of the "tube". As the tube burns, its thrust increases. However, as the rod burns, its thrust decreases. The two thrust changes cancel out to provide a constant overall thrust. Rod and Tube designs are found in many battlefield missiles.
Multi-Rod
The multi-rod design tends to be used in boosters where a very rapid burn is require. Typical examples are man-portable weapons where the boost phase must terminate before the rocket leaves the hand held launch tube. As the name suggests, this design comprises a large number of small rods packed into the motor case. This creates a very large exposed surface area of propellant.
Dual-Thrust
Many rockets emply two separate motors to provide the different functions of boost thrust and sustain thrust for cruising to the target. This is usually attained by separate tandem stages, or strap-on boosters. However, some rockets combine both boost and sustain modes within the same motor case. The most important of these dual-thrust systems is the MIM-23 Hawk surface-to-air missile. This rocket uses a polyurethane rubber binder for both propellants. Within the motor, a case bonded tube provides the sustain thrust. Mounted within the circular cross-section tube is a three-point burning star boost motor. When the boost propellant burns to its outer layer, it ignites the sustain motor. This single stage design reduces cost and complexity when compared with multi-stage and strap-on arragements. However, it also increases the diameter of the case, and the sustain motor is subject to increasing thrust as a progressively larger surface area of propellant is exposed.
Hybrids
Hybrids combine a solid fuel, such as HTPB, with a separate oxidiser - usually N2O (nitrous oxide gas).
Solid Fuel Ramjet
The solid fuel ramjet has a tube of solid fuel cast into the combustion chamber. Atmospheric oxygen is passed the the radial hole in the centre of this fuel tube. Fuel ablates from the exposed surface causing a radial burn. In order to ensure sufficient air flow, the rocket must first be accelerated to an appropriate speed by an additional booster rocket. This approach is used in the Russian SA-6 Gainful SAM, the almost identical Indian Akash SAM, and the new MBDA Meteor air-to-air missile. The Akash rocket sustainer utilises a high energy composite solid propellant with a double base formulation. The metallic fuel used is finely powdered magnesium, rather than the more usual aluminium. The binder matrix is a nitrocellulose and nitroglycerine colloid. Despite being air-breathing, there is also some ammonium perchlorate present. The booster rocket accelerates the missile for about 4 seconds after launch, to a speed of about Mach 1.5. The booster is then jettisoned and the solid fuel ramjet ignited. Air is fed into the combustion chamber via four intake ducts mounted just in front of the centrebody wings. The sustainer burns for about 22.5 seconds, achieving a maximum speed of about Mach 2.8.
Ramrocket
The ramrocket operates in a slightly different way to the solid fuel ramjet. A fuel-rich solid propellant rocket motor is ignited, passing hot fuel-rich gas into a combustion chamber. Air is also passed into this chamber, via intake ducts mounted slightly forward. When the hot fuel-rich gas and air combine in the chamber, spontaneous ignition occurs.
