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Atmospheric Flight
PropulsionPropulsion systems rely on Newton's 3rd Law: an action produces an equal and opposite reaction. Pushing (or accelerating) mass behind the vehicle produces a force, so the vehicle mass moves forward in reaction. The mass that is pushed backwards may be taken from the atmosphere or carried in the vehicle. Just as the wing produces lift by affecting the gas in the atmosphere, a propeller can produce thrust by pushing atmospheric gas backwards. On the other other hand, an inflated balloon that is released in a room expels gas from inside it through a nozzle, and does not require mass from the atmosphere. Energy is required to supply the force that accelerates the aircraft. The energy must be stored until the propulsive force is required. Chemical energy is often stored, and released in a controlled reaction at the appropriate time. Since mass is important for an aircraft (the lift requirement is dictated by the vehicle mass) aircraft designers want to minimize the mass of fuel required to produce the necessary energy that generates the propulsive force. On Earth, aircraft carry hydrocarbon fuel and gather oxygen from the atmosphere for combustion. Rockets that are launched into space cannot extract enough energy from the atmosphere, so they carry oxygen with them. They have both a fuel tank and an oxygen tank. Electric batteries are also commonly used for energy storage, but their energy density is low, so many batteries or incredibly large size batteries would be needed to generate enough propulsion for continuous flight. The sole purpose of the propulsion system is to provide thrust for the airplane. Aircraft engines must be as lightweight and compact as possible. There are many variations of piston engines with propellers and jet engines that are used in propulsion systems. Within a piston engine, the pistons can be arranged in 4 ways: radial, in-line, oppositional and "V". The radial engine has pistons arranged in a circle with the spinning shaft in the middle. These engines were once the most widely used aircraft engine. They never found much favor outside of aviation and are not used in modern aviation. A piston engine uses pistons to drive a spinning shaft. The propeller is attached to the shaft. At least 2 (but naturally 3 or 4) blades make up the propeller. The more blades on the propeller, the more air that can be moved by the propeller. Each propeller blade has an airfoil shape which generates lift as the blade slices through the air. Because the propeller is pointed forward, the force generated is in a forward direction.
Jet propulsion is similar to the release of an inflated balloon. The pressure inside the balloon is pushing in all directions while the mouth of the balloon is held tightly closed.
Releasing the mouth of the balloon creates an imbalance and causes the balloon to move in the direction opposite to the stream of air "jetting" out of the balloon's mouth. Jet engines work in a similar manner. There are several types of jet engines: ramjet, turbojet, and turbofan. For our discussion here we will consider only the turbojet. The turbojet was the first really useful jet engine to be built. The air flows into the engine through the inlet. The design of the inlet makes the air slow while raising the pressure inside the inlet. The air then moves through the compressor where sets of blades compress the air even more, greatly raising the pressure. The air then enters the combustion chamber where the fuel is added and ignited. The extremely hot, high pressure air rushes by the turbine blades making them spin at a high rate of speed. The turbine blades are connected back to the compressor blades by a shaft. The turbine blades take some of the energy from the air and returns the energy to the compressor. The hot, high pressure air that gets through the turbine, "jets" out the exhaust nozzle thrusting the engine forward.
The most efficient way to propel is to add a little bit of energy to a lot of mass. A propeller works well for an airplane that flies slowly in a dense atmosphere, where a large mass of gas is available to be accelerated a little bit. At higher altitudes, where airplanes must fly fast to develop adequate lift, jet engines gather less atmospheric gas, but accelerate it much more. When the atmosphere is very thin, only the on board propellant is available to be accelerated, and the exhaust velocity is very high indeed. To increase the thrust, a device called an afterburner is sometimes built into the engine. Fuel is dumped into the hot exhaust gas coming out of the nozzle. This causes another controlled explosion. This makes the air even hotter which adds more energy to it. This increases the thrust. This is not an energy efficient method. So it is used only for brief periods when extra thrust is needed. It is typically used on takeoff or when a burst of speed is needed.
The turbofan is a refinement to the turbojet. The turbofan is a more efficient engine. A large set of fan blades is set directly in front of the inlet. The fan works much like a propeller. It thrusts the engine forward while pushing a large amount of air backwards in the process. As the air is pushed back by the fan, some of the air goes into the engine and some of the air bypasses the engine. The engine that sits behind the fan is basically a turbojet. The air that goes into this engine gets the same treatment as air that goes through the turbojet. The turbine in the turbofan drives the fan as well as the compressor. The air that "jets" out the back of this engine has less thrust than air that exits a turbojet, but this is made uyp for by the added thrust from the fan. A turbofan engine actually is more efficient than a turbojet and runs quieter than most other jet engines.
The turboprop engine is basically a turbofan engine with the fan being replaced by a propeller. The propeller is placed outside of the inlet. A gearbox is introduced which controls the spinning of the shaft. This allows the pilot to control the speed of the propeller's rotation. This forward speed. Because the propeller is out in the free stream air, not mounted in the inlet (where the airspeed is reduced) the propeller has to rotate at faster speeds. The speed of the propeller approaches the speed of sound a lot more quickly than the airplane does itself. As the speed of the rotating propellers approaches the speed of sound, drag greatly increases. This means that the speed of the airplane must remain well below the speed of sound to prevent the tips of the propeller from going too fast. Engines, like wings, are designed for specific types of flight. The turboprop is an engine that flies at medium speed efficiently. The trubofan engins is a good choice for flight at top subsonic speeds. The turbofan with an afterburner would be an efficient engine use to fly at supersonic speeds. For slow speeds with a small airplane, then a piston engine would be best. The most efficient way to give thrust is to add a little bit of energy to a lot of mass. A propeller works well for an airplane that flies slowly in a dense atmosphere, where a large mass of gas is available. At higher altitudes, where airplanes must fly fast to develop adequate lift, jet engines gather less atmospheric gas, but accelerate it much more. When the atmosphere is very thin, only the onboard fuel is available to be accelerated, and the exhaust velocity is very high. There are three main propulsion questions that need to be considered for the Mars airplane:
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