n order for a satellite to go into orbit it must accomplish two major tasks. First, the satellite must rise above the atmosphere which surrounds the Earth's surface. The atmosphere contains enough particles which slow the spacecraft preventing it from orbiting the planet. A propulsion device must strain against gravity to rise above the atmosphere. Second, the satellite must also be provided with enough horizontal velocity above the atmosphere to at least equal the local circular speed upon orbital injection otherwise it will reenter the atmosphere and burn due to friction. Both of these jobs are done by rockets.
A simple rocket is usually a tall cylinder containing propellant. Propellant always contains two items: fuel and oxidizer. Fuel is the item which b urns to provide rocket thrust. In a simple liquid rocket it is stored in its own separate fuel supply tank. To support fuel combustion the rocket also contains a source of oxygen needed after the spacecraft passes above the atmosphere and cannot collect oxygen in any form. This oxygen is in the form of an oxidizer to aid in combustion; it is stored in a container which resembles the fuel supply tank.
The contents from fuel tank and the oxidizer tank flow from their respective tanks via plumbing; valves, pipes and pumps; into an area called the combustion chamber where the oxidizer joins with the fuel to burn. This combustion causes pressure to build up within the chamber's walls; the resultant pressure, called exhaust gas, is forced through a bell-shaped nozzle at the rocket's base. The nozzle is tapered in the middle in an area called the throat to allow the exhaust gas to build up even more pressure and to increase its flow rate out into the wider portion of the nozzle. The gas goes into the wide nozzle portion very fast and produces a force called thrust.
If the thrust is greater than the rocket's weight, the craft will lift off. This principle is called the thrust-to-weight ratio which must be greater than one or the vehicle will not lift off its pad. This thrust not only overcomes the payload's mass, but also its gravitational attraction to Earth. Any additional thrust above the thrust-to-weight ratio of one causes the rocket to accelerate. The greater excess thrust means greater rocket acceleration in a unit known as g's or numbers of times the norm al acceleration due to gravity at 9.8 meters/second2. In other words, one g equals 9.8 m/s2 , two g's equal an acceleration of 19.6 m/s2, three g's equal 29.4 m/s2, etc. Weight is also measured in g's because a natural part of weight is the acceleration due to gravity. Therefore at one g a 100 pound woman weighs 100 pounds; at 2 g's she weighs 200 pounds; 3 g's she weighs 300 pounds; etc. As more fuel and oxidizer are used the rocket's weight (mass) decreases and its thrust to weight ratio increases. To maintain the same thrust with a mass reduction, the spacecraft's acceleration must increase in order to obey Newton's second law, F = ma.
The first rockets needed wings to guide or steer them into space. Engineers soon found that these wings could be ripped off the rocket's body as it approached the sound barrier. The wings were then downsized into small little winglets called guide vanes . These guide vanes steered the rocket on to its appropriate trajectory to gain altitude and to increase the vehicle's horizontal velocity. As rockets matured, the engineers found that steering could be accomplished more efficiently by including small jets instead of vanes around the base of the craft. The original Atlas rocket employed this capability. As the engineers grew cleverer they found that the same steering could be done by moving the spacecraft engines and diverting the thrust into a different direction. This was called gimbaling and is used exclusively to launch today's modern rockets.
At the top of the rocket is its business end, a hollow cone containing the spacecraft's payload. The upper stage is shaped like a cone to minimize the rocket's cross section which has to penetrate the atmosphere. This reduces the amount of energy required to push the rocket through the atmosphere into space. The nose cone protects the payload against aerodynamic wind blast which is very prevalent when a vehicle speeds through the Earth's atmosphere.
As previously stated, rocket fuel is also called propellant. Propellant includes not only a fuel which is actually burned, but also an oxidizer which supplies oxygen for the combustion process. Propellant efficiencies are measured by a term called specific impulse, Isp. This measurement determines how much thrust a propellant produces; it is a similar gauge rating such as octane is for gasoline. Isp is measured in seconds; it is the amount of time one pound of propellant produces one pound of thrust. There are two classes of propellant mixtures: liquid and solid.
Liquid propellants develop the most efficient thrusts for rocket power. There are many liquid propellant combinations which are used for rocket flight such as kerosene/liquid oxygen (LOX), alcohol/LOX, gasoline/LOX, and liquid hydrogen (LH2)/LOX. Such a propellant with the highest Isp is LH2/LOX with a 400 second rating for operation in the atmosphere and a 453 second rating in a vacuum. Another liquid fuel with a medium efficiency is Aerozine 50 (kerosene) with Nitro Tetroxide (N2O4) for an oxidizer. This propellant has an Isp of 254 seconds at sea level and a 302 second rating in a vacuum. These highly efficient fuels and oxidizers still need an ignition spark to start the combustion process.
A special liquid propellant which does not need ignition to start combustion is a fuel which ignites spontaneously when it comes into contact with its oxidizer. This type of fuel is called hypergolic. A typical hypergolic propellant combination is hydrazine (N2H4) and nitrogen tetroxide (N2O4). Most spacecraft use a monopropellant for operation; the most popular of these fuels is hydrazine which is easily stored and used on orbit for many years.
There are two important advantages of using a liquid fuel. The first is the capability to throttle the thrust. A liquid engine can start, stop, restart, or be reduced in thrust as the rocket flies. The second big advantage of liquid fuel is its increased efficiency, Isp. LH2 has a typical Isp of about 496 seconds whereas the solid fuel has a typical Isp of about 250 seconds.
The disadvantages of using liquid fuel are included in three areas. The first is the cryogenic nature of the fuel meaning it is difficult to store because of the required cold temperatures. A second disadvantage is the handling of this fuel by workers and the special precautions such as wearing heavy, insulated gloves and eye protection must be taken. A third disadvantage is that liquid fuel rocket engines are extremely complex with many moving parts including pumps, valves, lines, and chambers. Every one of these parts must work perfectly or the engine fails.
Solid propellant consists of a flammable putty or rubber which contains both the fuel and the oxidizer within this mixture. For example, the mixture in the space shuttle's solid rocket booster is a typical solid rocket fuel. The ingredients include 16% atomized aluminum powder as the fuel, 70% ammonium per chlorate as the oxidizer, a 12% polybutadiene acrylic acid acrylonitril as a binding agent, 2% epoxy for curing and extremely small traces of iron oxide to control the burn rates during flight.
The solid rocket fuel fills the inside of the rocket from its top to the bottom. In the middle of the rocket is a shaped clearing that provides combustion area and allows the mixture to burn evenly. This shape may be a circle or a star depending upon the type of thrust desired for launch. The rocket's ignition commences by shooting flames down its entire length to initiate the combustion evenly throughout the grain, another term for solid rocket propellant. As the fuel burns and its waste parts are ejected out the nozzle the propellant area grows. As the propellant area grows there is more propellant to burn which means that the rocket's thrust increases.
The advantages of the solid rocket boosters include easy handling. Once the propellant is manufactured and shipped, the technicians need no extensive protective clothing or procedures. The solid rocket fuel can be stored indefinitely in its solid state with only random inspections to insure that its seals are still functioning. In a solid rocket motor there are no moving parts which can fail just by mechanical movement. Despite of the numerous advantages of solid rocket propellant there are a number of disadvantages as well.
The biggest disadvantage of a solid rocket booster is that once it starts it is not going to be stopped. Therefore, it has no control functions such as throttles to control the burn. If thrust is to be either reduced or increased it must be done in the design of the grain. For example, the space shuttle SRBs are designed so that the burn reduces during transonic region passage also known as the maximum dynamic pressure. A second disadvantage is the low Isp rating. Solid propellant is just not efficient because it burns so quickly. A third disadvantage is that the rocket emits solid particles not only polluting the atmosphere, but once the vehicle gets into space, these particles also become solid debris, a hazard for satellites and other launchers.
Which type of rocket propellant does one choose? It depends on the mission and the type of energy required for it. If the engineer needs fast and responsive energy, a solid would probably work best, but if the scientist needs a slow, but steady launch capability then perhaps a liquid would be better. The choice depends upon the mission.
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