Atmospheric Flight

Propulsion

Propulsion systems make of Newton's 3rd Law: an action produces an equal and opposite reaction. Pushing (or accelerating) mass from behind the airplane generates a force. In reaction to this push, the airplane moves forward.

The mass that is pushed backwards is taken from the atmosphere or carried in the airplane. A wing produces lift by the way it disturbs the molecules in the atmosphere when moving through the air. A propeller can produce thrust by pushing the atmospheric gas from in front of the airplane back behind it. An inflated balloon that is released in a room pushes the gas from inside it out through a nozzle. This does not require mass from the atmosphere to generate thrust. The balloon carries its own "fuel" (the air). When the air runs out, the balloon will need to be filled up again before it can "jet" about the room.

two balloons with
air pressure arrows pointing all around the inside of the
balloons and out the bottom of the one which is open

Energy is needed to supply the force that accelerates the airplane. The energy must be stored until the force is needed. Chemical energy is often stored, and released in a controlled reaction at the appropriate time. This chemical energy is in the form of fuel. Storing fuel on an airplane adds weight. This added weight will require more lift. Aircraft designers need to design propulsion systems that give "the most bang for their buck." That means they need to use fuel that will use the least amount of fuel and generate the most amount of energy for thrust. The fuel should not take up a lot of fuel tank storage space, should be lightweight and should be economical to use.

On Earth, aircraft carry hydrocarbon fuels and gather oxygen from the atmosphere for combustion. Rockets that are launched into space cannot take enough oxygen from the atmosphere, so they carry the 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. That means that many batteries or incredibly large sized 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, placed on opposite sides and V-shaped. 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. A piston engine uses pistons to drive a spinning shaft. The propeller is attached to that shaft. At least 2 (but usually 3 or 4) blades make up the propeller. The more blades on the propeller, the more air being moved by the propeller. Each propeller blade has an airfoil shape that generates lift as the blade slices through the air. Because the propeller is pointed forward, the force generated is in the same direction.

Jet propulsion is a little like what happens when an inflated balloon is let go. The pressure inside the balloon is pushing in all directions equally until the mouth of the balloon is let go.

Releasing the mouth of the balloon creates an imbalance in the pressure. The pressure at the mouth of the balloon (where there is a fast moving stream of air) is now less than any place else on the balloon. This 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 out information we will look at a 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 its motion while raising the pressure inside the inlet. The air then moves through the compressor. Here sets of blades compress the air even more. This causes an increase in the pressure. The air then enters the combustion chamber. Here 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 at the back of the engine. This moves the engine forward.

a picture of an assembly line
of turboengines with large blades
Photo courtesy of Pratt&Whitney

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.

Graphic of the SR71 in flight
with full afterburners

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.

Photo courtesy of Pratt&Whitney

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 is the most efficient means of propulsion, however it is limited in 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:

Where will the mass (air molecules) that is pushed for thrust behind the airplane come from?
What method will be used to push the mass?
How will energy be stored so that it is available to power the engine at the right time?

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