What is the difference between solid and liquid rockets

Still other igniters, especially those for large rockets, are rocket engines themselves. The small engine inside the hollow core blasts a stream of flames and hot gas down from the top of the core and ignites the entire surface area of the propellants in a fraction of a second.

The nozzle in a solid-propellant engine is an opening at the back of the rocket that permits the hot expanding gases to escape. The narrow part of the nozzle is the throat. Just beyond the throat is the exit cone. The purpose of the nozzle is to increase the acceleration of the gases as they leave the rocket and thereby maximize the thrust.

It does this by cutting down the opening through which the gases can escape. To see how this works, you can experiment with a garden hose that has a spray nozzle attachment. This kind of nozzle does not have an exit cone, but that does not matter in the experiment. The important point about the nozzle is that the size of the opening can be varied. Start with the opening at its widest point. Watch how far the water squirts and feel the thrust produced by the departing water.

Now reduce the diameter of the opening, and again note the distance the water squirts and feel the thrust. Rocket nozzles work the same way. As with the inside of the rocket case, insulation is needed to protect the nozzle from the hot gases.

The usual insulation is one that gradually erodes as the gas passes through. Small pieces of the insulation get very hot and break away from the nozzle. As they are blown away, heat is carried away with them. The other main kind of rocket engine is one that uses liquid propellants. This is a much more complicated engine, as is evidenced by the fact that solid rocket engines were used for at least seven hundred years before the first successful liquid engine was tested. Liquid propellants have separate storage tanks - one for the fuel and one for the oxidizer.

They also have pumps, a combustion chamber, and a nozzle. The fuel of a liquid-propellant rocket is usually kerosene or liquid hydrogen; the oxidizer is usually liquid oxygen.

They are combined inside a cavity called the combustion chamber. Here the propellants burn and build up high temperatures and pressures, and the expanding gas escapes through the nozzle at the lower end. To get the most power from the propellants, they must be mixed as completely as possible. Small injectors nozzles on the roof of the chamber spray and mix the propellants at the same time. Because the chamber operates under high pressures, the propellants need to be forced inside.

Powerful, lightweight turbine pumps between the propellant tanks and combustion chambers take care of this job. With any rocket, and especially with liquid-propellant rockets, weight is an important factor.

In general, the heavier the rocket, the more the thrust needed to get it off the ground. Because of the pumps and fuel lines, liquid engines are much heavier than solid engines. One especially good method of reducing the weight of liquid engines is to make the exit cone of the nozzle out of very lightweight metals. However, the extremely hot, fast-moving gases that pass through the cone would quickly melt thin metal.

Therefore, a cooling system is needed. A highly effective though complex cooling system that is used with some liquid engines takes advantage of the low temperature of liquid hydrogen. Hydrogen becomes a liquid when it is chilled to C. Before injecting the hydrogen into the combustion chamber, it is first circulated through small tubes that lace the walls of the exit cone.

In a cutaway view, the exit cone wall looks like the edge of corrugated cardboard. The hydrogen in the tubes absorbs the excess heat entering the cone walls and prevents it from melting the walls away. It also makes the hydrogen more energetic because of the heat it picks up. We call this kind of cooling system regenerative cooling. Engine Thrust Control Controlling the thrust of an engine is very important to launching payloads cargoes into orbit.

Too much thrust or thrust at the wrong time can cause a satellite to be placed in the wrong orbit or set too far out into space to be useful. Too little thrust can cause the satellite to fall back to Earth. Liquid-propellant engines control the thrust by varying the amount of propellant that enters the combustion chamber.

A computer in the rocket's guidance system determines the amount of thrust that is needed and controls the propellant flow rate. On more complicated flights, such as going to the Moon, the engines must be started and stopped several times. Liquid engines do this by simply starting or stopping the flow of propellants into the combustion chamber.

Solid-propellant rockets are not as easy to control as liquid rockets. Once started, the propellants burn until they are gone. They are very difficult to stop or slow down part way into the burn.

Sometimes fire extinguishers are built into the engine to stop the rocket in flight. But using them is a tricky procedure and doesn't always work.

Some solid-fuel engines have hatches on their sides that can be cut loose by remote control to release the chamber pressure and terminate thrust. The burn rate of solid propellants is carefully planned in advance.

The hollow core running the length of the propellants can be made into a star shape. At first, there is a very large surface available for burning, but as the points of the star burn away, the surface area is reduced. For a time, less of the propellant burns, and this reduces thrust. The Space Shuttle uses this technique to reduce vibrations early in its flight into orbit. NOTE: Although most rockets used by governments and research organizations are very reliable, there is still great danger associated with the building and firing of rocket engines.

Individuals interested in rocketry should never attempt to build their own engines. Even the simplest-looking rocket engines are very complex. Case-wall bursting strength, propellant packing density, nozzle design, and propellant chemistry are all design problems beyond the scope of most amateurs.

Many home-built rocket engines have exploded in the faces of their builders with tragic consequences. Stability and Control Systems Building an efficient rocket engine is only part of the problem in producing a successful rocket. The rocket must also be stable in flight. A stable rocket is one that flies in a smooth, uniform direction. An unstable rocket flies along an erratic path, sometimes tumbling or changing direction.

Unstable rockets are dangerous because it is not possible to predict where they will go. They may even turn upside down and suddenly head back directly to the launch pad. Making a rocket stable requires some form of control system.

Controls can be either active or passive. The difference between these and how they work will be explained later. It is first important to understand what makes a rocket stable or unstable.

All matter, regardless of size, mass, or shape, has a point inside called the center of mass CM. The center of mass is the exact spot where all of the mass of that object is perfectly balanced. You can easily find the center of mass of an object such as a ruler by balancing the object on your finger. A solid fuel rocket has its fuel and oxidant mixed together as fine powders and then pressed in to a solid 'cake'.

Once it has been lit it will carry on burning until it is used up. In a black powder rocket the fuel is carbon and the oxidant, potassium nitrate. Sulphur acts as a secondary fuel and also catalyses the reaction. In the Ariane 5 solid fuel boosters the fuel is aluminium powder, the oxidant, ammonium perchlorate and polybutadiene acts as a binder to hold the mixture together.

The pump is driven by burning small amounts of fuel and oxidizer usually with a large oxygen to fuel or fuel to oxygen ratio to keep the temperature relatively low inside the pump to drive a «water wheel» that spin a turbo fan around at extremely high speeds increasing the pressure of the fuel and oxygen.

For the most efficient motors, the fuel and oxygen used by the gas generator in the turbopump is again forced into the combustion chamber to use the reminding energy keep in mind that the gas generator is inefficient, due to the large oxygen to fuel or fuel to oxygen ratios to keep the temperature low.

This is called a staged combustion motor. The open cycle motor, on the other hand, discard the gas coming out of the turbopump. The open cycle is less efficient lower Isp , but is generally simpler to build. An example of an open cycle motor is the first stage core liquid motor of Ariane 5, the Vulcain 2, seen in figure 2.

The exhaust pipe seen on the side of the nozzle, which is attached to the turbopump partly visible embedded within the piping, is used to vent out the turbopump gas. The nozzle can become very hot, and some motors, usually large in size, need to cool the nozzle down.

This is done by letting some of the cold fuel pass around in tubes outside of the nozzle. When this is done, the fuel is working as a cooling agent. After passing through the nozzle piping, the fuel is forced into the combustion chamber. The other type of rocket motor most often used is the solid motor, where both the fuel and oxidizer are in solid state. The fuel and oxidizer is blended together and molded into a shape within a metal structure called the casing.

This casing is the mechanically bearing structure. Inside the propellant blend, there must be a volume where the fuel and oxidizer, which is released in gas form when heated up, are mixed and combusted before reaching the nozzle. See figure 3 for the parts making up a solid motor.

The propellant is usually ammonium perchlorate oxidizer and extremely fine aluminum powder fuel mixed with Hydroxyl-terminated polybutadiene HTPB is a translucent liquid with a color similar to wax paper and a viscosity similar to corn syrup as a binder, in addition to some catalysts.

The result is a black rubbery structure. Using a variation of catalysts and different mixture ratios of oxidizer, fuel and binder, the designers can vary the characteristics of the motor.