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Background on Propulsion

Propulsion is the act of pushing or driving an object forward, often through a force called thrust. Machines like airplanes and rockets take advantage of Newton's 3^rd law of action and reaction to produce thrust. Thrust can be expressed with the following equation.

If P_e doesn't equal P_o:

F thrust = m_ev_e - m_ov_o + A_e(p_e - p_o)

The subscript e denotes trait at the exit of propulsion device, and o denotes a trait prior to the entrance of propulsion device (unaffected fluid). v = velocity, p = pressure, A = area, and m = mass flow rate (mass / time or density x velocity x area).

This shows that the amount of thrust created is directly related to the mass flow through the engine and the exit velocity of the gas. The amount of thrust must be greater than countering forces like weight and/or drag, in order for the object to accelerate forward. There are four main propulsion systems used by aircraft today:

• propeller
• turbine (jet engine)
• ramjet
• rocket

Propeller

Many airplanes use internal combustion engines to turn propellers to generate thrust. Propellers are usually comprised of 2 to 6 blades, and each blade acts like a rotating wing to produce thrust. Blades are usually twisted, changing the "angle of attack" of the blade relative to the air to take into account the fact that the propeller tips move faster than the ends at the hub. Air changes pressure from low to high as it moves through the propeller.

After World War II, some airplanes like the C-130 transport plane used turboprops, or jet engines to turn propellers. Here, propellers still provided the main thrust force.

Human-powered aircraft have also been designed that rely on propellers for thrust. The turning of the propeller is often linked to the spinning of bicycle gears that the pilot must move with his/her feet.

NASA has developed its Pathfinder airplane that converts solar energy to electrical energy, then from electrical to mechanical energy, through use of solar panels, an electric engine, and propellers. Other NASA experimental aircraft have combined the use of propellers (handy for high-thrust vertical takeoff without a runway) and wings (handy for high-speed horizontal movement). These include the XV-15 Tilt Rotor and X-Wing Research Vehicle, shown below.

Turbine (Gas Turbine or Jet Engine)

Turbines were developed during World War II in England and Germany. Turbines pressurize air and gasoline using a compressor, then combust it, producing a gas that passes at a high velocity through a nozzle to produce thrust.
A general turbine schematic with color-coding and animation can be found at:
http://www.grc.nasa.gov/WWW/K-12/airplane/Animation/turbtyp/etcs.html

Most military and passenger airplanes use turbines for propulsion today, although their size and number per airplane vary a great deal. As an example, compare the turbines of the DC-8 airliner, F-14 fighter, C-130 cargo, and T-38 trainer airplanes, to see examples of four high-bypass turbofan engines, two afterburning low-bypass turbofans, four turboprop engines, and two turbojet engines, respectively. Use this link to find further details about each of these four types of turbine engines: http://www.grc.nasa.gov/WWW/K-12/airplane/trbtyp.html, shown below.

To better understand jet engines, you can play with EngineSim and RangeGames, both interactive Java applets. Use the links below to locate and download the software. http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsimr.html

Ramjet

Ramjets burn gasoline like turbines do, however, compression of air and fuel is not produced by a mechanical compressor, but rather by "ramming" air into the engine using great forward speeds of the vehicle. The faster the air "ramming," the better a ramjet engine functions.

Without compressors, ramjets are lighter and simpler than turbines but cannot produce thrust unless they are already moving or high-velocity air is provided. Therefore, vehicles using ramjet engines must have another means of propulsion for commencing movement. As an example, NASA has developed an X-15 rocket-powered airplane with a ramjet slung underneath the body. A rocket initiates movement of the airplane until the ramjet can successfully produce thrust, after which the ramjet works on its own, enabling the airplane to move at very high speeds.

You can design and test ramjet engines using EngineSim at this link: http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html or experiment with ramjet nozzles at http://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.html

Rocket

During and after World War II, rockets were developed for high-speed and space research. NASA's X-1 broke the sound barrier, and subsequent X-models like the X-15 also use rockets for some or all of their propulsion. The X-15 is shown at left. Like other engines, rockets rely on combustion to produce a gas and hence thrust. However, unlike other engines, a rocket carries its own supply of oxidizer (oxygen) and does not use the external atmosphere as its "working fluid." As it has to carry its own oxidizer and hence much weight, it is the least efficient means of propulsion on Earth, where oxygen is readily available.

There are two types of rockets: solid and liquid.	In a solid rocket, fuel and oxidizer are mixed together, but remain in a solid, stable form, without combusting until activated by heat. These rockets are lighter and simpler than liquid rockets, but combustion is extremely difficult to stop, so generally this means of propulsion is only used at one time.
	Liquid rockets have fuel and oxidizers stored in separate tanks. To begin combustion, fuel and oxidizer is pumped from storage to a mixing tank, where burning takes place. Liquid rocket combustion is easier to control, as pumping can be regulated, but its components are relatively unstable, unlike the solid forms of fuels, which leads liquid rocket developers to attach fuel tanks only immediately prior to take-off. Conversely, solid rockets can sit in storage, fully assembled, prior to use. Model rockets are generally solid rockets.

Propulsion Research

Propulsion is usually dependent on an engine converting between chemical energy (fuel) and mechanical energy. Much research has been done to improve:

• efficiency of the engine so less fuel is used
• cheaper, more available fuel sources
• cheaper cost to launch (it costs about $30,000 per kilogram)

Propulsion Research

         Since the 1950s, an emerging field of research has been using solar energy to fuel electric generators, which rely on exhaust gases comprised of ions for propulsion. Mercury, xenon, or cesium would be turned into gases, bombarded with electrons in order to create ions, then these electrostatically charged particles would be accelerated out of the rear of the engine. Xenon is a preferred element because it does not have to be heated (like mercury and cesium do) in order to make it into a gas.
Propulsion systems like this have been used in satellites like the SCATHA and PAS-5 satellites. It has been shown to be durable (one has been running continuously since 1998) and about 10 times more efficient than chemical engines. Xenon is naturally found in the Earth's atmosphere and is environmentally safe.
         Thrust is very low (approximately equal to that provided by an 8.5 x 11 sheet of paper), but over time, acceleration could bring a spacecraft relying on this form of propulsion to high speeds.

         The most famous system of this type is called Deep Space 1, rendered by an artist above. Deep Space 1 burns 83 kg of xenon in one year at 2.5 kW or 3.4 horsepower.
Further information: http://www.grc.nasa.gov/WWW/RT2000/5000/5430lapointe.html , http://science.nasa.gov/headlines/y2000/ast15jun_1.htm and http://www.highway2space.com/news/going.html.

Laser/Solar Sails:

         Using massive sails for travel in space has been a popular notion for decades, but issues arise about how to generate wind on demand. Recent studies have focused on light as a source, which could be harnessed from the sun or created with lasers. When light hits an object, it applies a slight force upon it, so if enough light is focussed over a large area, force could cause significant movement.

         Scientists have suggested using lasers as light sources and lenses to concentrate the light onto sails. This would be a relatively quick means of travel (a thousand-ton vehicle could reach our nearest star in about ten years), but the energy required to fuel the 10,000,000-gigawatt laser required would require 10,000 times more power than is used on Earth in one day. The original model has since been revised to more reasonable, but still somewhat obscene power requirements of a 10-gigawatt microwave laser for a 16-gram vehicle.

         An artist's sketch of this technology is shown at left. See this link for more information: http://www.howstuffworks.com/light-propulsion.htm

Space Tethers:

Tethers are simply rope-like links between two objects. One object is typically fixed, while the second object moves relative to the other. While air-breathing propulsion could be used on Earth, tethers would be used to propel objects from place to place in space, or space to Earth. Permanent tethers could be established to satellites and other places of interest, with grappling systems at their ends. A vehicle coming from Earth could control its propulsion by conventional means until bound to a tether, then the vehicle would be passive while the tether swung it around a satellite and released it at an appropriate time, to send it on its journey. In addition, if the tether was of a conductive material, as it passed through different magnetic fields, electrical potential would be created, thus electricity that could be harnessed for other uses.
As John Grant explains in a summary of his research on the Hypersonic Airplane Space Tether Orbital Launch System, "The system is revolutionary in that it minimizes, and perhaps even eliminates, the use of rockets for satellite launch, while limiting the design requirements for a reusable air-breathing hypersonic vehicle to Mach 10. The benefits which accrue from the eventual development of this system are a reusable 'pipeline' from runways near the equator to Medium Earth Orbit."
A drawing of space tethers in action is shown above.

See John Grant's paper and links at:
http://www.niac.usra.edu/studies/study_master.jsp?action=Advanced_Propulsion&rsnum=null&lastDisp=null for more information.
A nice article about tether applications can be found at: http://www.techtv.com/news/scitech/story/0,24195,3377408,00.html.
Using a tether to generate electricity is explained well at: http://www-istp.gsfc.nasa.gov/Education/wtether.html.

Space Elevator:

With inspiration of buildings like the Eiffel Tower or Space Needle, scientists have considered developing a "tower" that could "float" (no foundation) above the same point of Earth, and span from Earth's atmosphere to space. It would move with the Earth in geosynchronous orbit. Once it was built, items and people could move from Earth to space via elevators.

The concept is physically possible, but extremely expensive. Expense relates to the amount of material required to span such an immense distance, as well as the requirement of using the space shuttle to establish the space end of the tower, before its elevator could be used to ship more building materials into space. Current research explores new procedures for constructing the elevator and new lightweight, strong materials that could be used.
A drawing of the potential elevator is shown at left. At the bottom right is Earth.

See the following links for more information:
http://liftoff.msfc.nasa.gov/academy/TETHER/spacetowers.html and http://science.nasa.gov/headlines/y2000/ast07sep_1.htm.

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The concept is physically possible, but extremely expensive. Expense relates to the amount of material required to span such an immense distance, as well as the requirement of using the space shuttle to establish the space end of the tower, before its elevator could be used to ship more building materials into space. Current research explores new procedures for constructing the elevator and new lightweight, strong materials that could be used.
A drawing of the potential elevator is shown at left. At the bottom right is Earth.

See the following links for more information:
http://liftoff.msfc.nasa.gov/academy/TETHER/spacetowers.html and science.nasa.gov/headlines/ y2000/ast07sep_1.htm

You Decide Intro
You Decide Scenario
You Decide Decision Making Process