Solid rocket engine

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Launch of a Scout solid rocket

A solid rocket engine or solid rocket drive is a rocket engine with a drive set made of solid material. Both reducing and oxidizing components are bound as solid substances. In contrast, there is the liquid rocket , in which both the oxidizer and the reducing fuel are carried along in liquid form, mixed and burned. In so-called hybrid rockets , solid fuel and liquid oxidizing agent are combined with one another.

The oldest rocket types are solid-based. For a long time only solid rockets were built.


Early use

Apollo rescue missile test

The oldest rockets were solid fuel rockets. They were probably built in the Byzantine Empire in the 7th century and consisted of bamboo as a rocket body and a mixture of saltpeter and sulfur as fuel.

Presumably independently of this, rockets propelled with black powder were developed in China in the 13th century . There they were used, among other things, for military purposes. They became known later in Europe, but only had their main meaning here as fireworks .

The British officer William Congreve developed a missile for military use in the early 19th century . It was used, for example, in the bombing of Copenhagen (1807) . Due to the progress in artillery , the missile experienced a rather shadowy existence. It was not until the end of the 19th century that research and development began to take place again in this area.

A rocket test (1931) by Karl Poggensee and Reinhold Tiling is named as the first successful launch of a solid rocket in Europe .

Modern use

Solid rocket are used differently today, both military and civilian purposes, such as aviation and aerospace . They are due to their low price to help launch missiles ( " booster ") and aircraft ( RATO used) as well as in small school drives, as well as intercontinental ballistic missiles like the Trident be performed as a solid rockets. In addition, because of their high maximum acceleration, they are used as rescue missiles to quickly bring manned spaceships out of the danger zone of a failing launch vehicle.

In APCP , for example, ammonium perchlorate (NH 4 ClO 4 ) is used as an oxidizer, which results in, for example, 4 H 2 O + N 2  + 2 O 2  + Cl 2 when two molecules break down (in practice, HCl is also formed). Oxygen and chlorine react with aluminum to form Al 2 O 3 and AlCl 3 as well as a polymer binder to form H 2 O and CO 2 , which releases more energy. The mass fraction of aluminum is up to 30 percent.

Due to their simple structure, solid rockets can also be built in very small sizes, for example as small rockets for fireworks, signaling or as special rocket projectiles for the use of handguns . Such missiles have rather simple propellants like black powder .


In the future there are plans to reduce the heavy weight of large solid rocket rockets by replacing steel with carbon fiber reinforced plastic. This could drastically reduce the empty mass of large solid rocket rockets. Calculations showed that this improvement in the full to empty mass ratio alone could increase the payload of the Ariane 5 with the aim of geostationary transfer orbit by 2  t .

These lightweight materials could also make pure solid rockets possible, which can transport large satellites economically into near-earth orbits.



With the exception of applications with swiveling nozzles, solid fuel rockets come from completely without moving parts or additional devices for fuel pumps or lines, which would specify a technical minimum mass. They also contain their fuel at all times, so that storage and refueling facilities are not required. This makes it easier to use, for example, for meteorological or ground-based scientific purposes; For military use, they can sometimes be stored in large numbers in magazines or carried and are ready for use in a short time.

The fuel itself is solid and therefore much easier to handle than liquid or gaseous fuels: it cannot escape in this form and can then be harmful to health or the environment. There are also no instabilities caused by sloshing liquid fuel with solid fuel. Due to the shape of the propellant, the so-called burn-off characteristics, i.e. the development of thrust over the burning time, can be seen and the burning time itself can be influenced very easily. In this way, thrust forces that are greater than those of liquid engines can also be achieved. In addition, most types of propellants mean that the center of gravity of the rocket changes relatively little during the burn, which is important for flight stability.

These advantages make solid rockets reliable in use, powerful and inexpensive to develop, manufacture, maintain and use.


Since solid rockets always contain their explosive fuel, they also pose a permanent increased risk. This also makes them heavier than comparably large liquid fuel rockets, which can be transported empty and only refueled when needed.

The products of combustion from solid fuel rockets are mostly ejected at a slower rate than the products of combustion from liquid propellant rockets. Because the thrust is according to the formula

calculated, their advantage of high thrust has to be bought at the cost of high fuel consumption. This results in the short burning time compared to liquid rockets. A thrust control during the burn is not possible; and in the event of an incident, a solid fuel rocket cannot be switched off. Only during the manufacture of the booster can the thrust profile be influenced over the burning time, for example by filling different segments with differently reactive fuel mixtures or by shaping the fuel (see below).

The entire interior of a solid rocket is also its combustion chamber . When the fuel is burned, high pressures and temperatures occur. Therefore, the walls must be designed for relatively high loads. As the size of the rocket increases, the load on the combustion chamber wall increases with the same internal pressure, the walls have to become thicker and therefore heavier. For example, the empty mass of a solid rocket increases in comparison to the total mass with increasing size, while it continues to decrease in the case of liquid fuel rockets. Their maximum technical mass is therefore below that of other missile types.

Solid fuel rockets are often more polluting than other types of construction. When the fuel is burned, chlorine, hydrogen chloride, sulfur compounds or other toxic substances are produced, for example, depending on the filling. In the case of an Ariane 5 start with EAP P238 solid propellant engines, the total solid propellant mass is 476 tons. With a solids content of 86% by weight or 68% by weight of ammonium perchlorate (see Ariane 5 technology) in the solid propellant, the total mass of ammonium perchlorate is 324 tons. When an Ariane 5 is started, 100 tons of hydrogen chloride are produced . Hydrogen chloride then reacts with water to form hydrochloric acid (270 tons with 37 percent acidity).

Fuel geometry and thrust curve

Simplified solid rocket.
1. Propellant with a cylindrical recess in the middle.
2. Detonator sets the surface of the propellant on fire.
3. Cylindrical recess acts as a combustion chamber.
4. Exhaust gas is throttled through a constriction to regulate thrust.
5. Exhaust gas emerges from the rocket.

In the simplest case, such as fireworks rockets, the entire interior of the solid rocket is filled with the fuel. This burns evenly from back to front. If the burning time is short, this is not a problem; However, if it is burned for a longer period of time, it leads to a very high thermal load on the rear, already “empty” part of the rocket through which the hot combustion gases flow.

To prevent this from happening, the fuel is shaped into a hollow tube that burns from the inside out. The rocket fuel that is still present acts as a heat insulator and protects the rocket hull from overheating. However, with increasing expansion of the cavity, the thrust of the engine also increases, since this is approximately proportional to the surface area of ​​the burning fuel. In the case of rockets, on the other hand, the thrust requirement is usually highest in the launch phase, since the missile is the heaviest here and has to be accelerated first.

By appropriately shaping the cross section of the fuel pipe, however, the thrust course can be influenced in such a way that it corresponds to the requirements. Thus, the cavity can be made approximately star-shaped. When igniting, the surface of the burning fuel is then the largest. After the fuel spikes have burned away, the cross-section is approximately circular and the fuel surface and the thrust are therefore lower.

See also


  • George Paul Sutton: Rocket Propulsion Elements . Wiley-Interscience, New York 2000. ISBN 0-471-32642-9
  • Willfried Ley, Klaus Wittmann, Willi Hallmann (eds.): Manual of space technology . Carl Hanser, Munich 2008. ISBN 3-446-41185-2

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