Assisted take off is any system for helping aircraft into the air (as opposed to strictly under its own power). The reason it might be needed is due to the aircraft's weight exceeding the normal maximum take off weight, insufficient power, or the available runway length may be insufficient, or a combination of all three factors. Assisted take off is also required for gliders, which do not have an engine and are unable to take off by themselves.
A well-known type of assisted take off is that using the aircraft catapult. In modern systems fitted on aircraft carriers, a piston, known as a shuttle, is propelled down a long cylinder under steam pressure. The aircraft is attached to the shuttle using a tow bar or launch bar mounted to the nose landing gear (an older system used a steel cable called a catapult bridle; the forward ramps on older carrier bows were used to catch these cables), and is flung off the deck at about 15 knots above minimum flying speed, achieved by the catapult in a 4 second run.
The United States is replacing carrier steam catapults with linear induction motors. The system is called the Electromagnetic Aircraft Launch System (EMALS). An electromagnetic wave travelling through the motor propels the armature along its length, pulling the plane with it. With this system, it will be possible to match launch power and aircraft weight more closely than with the steam system, causing less wear on the aircraft.
The catapult approach is also used for towing gliders into the air. This can be accomplished using an elastic bungee arrangement, or more commonly using a cable wound onto a winch, powered by a large diesel engine. The bungee approach is rarely used for man-carrying gliders, as the acceleration is uncontrolled and can yield very high G-forces. It is commonly used to launch model gliders however. Manned gliders are commonly launched simply by towing them aloft behind a powered aircraft.
During WW2 the German Arado 234 and the Messerschmitt 323 "Gigant" used rocket units beneath the wings for assisted take off. Such systems were popular during the 1950s, when heavy bombers started to require two or more miles of runways to take off fully laden. This was exacerbated by the relatively low power available from jet engines at the time - for example the B-52 Stratofortress required 8 turbojet engines to yield the required performance, and still needed RATO for very heavy payloads (a proposed update of the B-52 replaces these with half the number of much more powerful engines).
In a Cold War context, RATO and JATO bottles were seen as a way for fighter aircraft to utilize the undamaged sections of runways of airfields which had been attacked.
In the interwar years in order to achieve long ranges with the technology of the time, trials were undertaken with float planes piggy backed atop flying boats. With the float plane carried part of the way to its destination and freed from having to use any of its own fuel in the initial climb these combinations could deliver light but time critical cargos faster and farther than a single individual aircraft.
An unusual assisted take off scheme was partially developed during the 1950s which consisted of a jet powered truck or dolly which ran either on rubber tires or rails, used to push a heavy aircraft into the air. Once airborne, the dolly would instantly detach. Because the dolly did not need to fly itself, it was not constrained by the need for low weight, and so could be fitted with very large and powerful engines. The system only reached the early stages of development. The same company was also drawing up plans for a flyable version of the dolly launch system, which it called "jet donkeys". The idea was that a small powerful secondary aircraft could push the heavy main aircraft into the air, detach in flight and return to the airfield to be reused. Sketches of the proposed system show a strange canard-layout aircraft with its cockpit in the tail, pushing the main aircraft via a long extended nose. The system was never developed, it was not long before the further development of the jet engine meant that most of these assisted take off schemes became unnecessary.
WIPO ASSIGNS PATENT FOR "INFRASTRUCTURE FOR DRIVING AND ASSISTED TAKE-OFF OF AIRFOILS FOR TROPOSPHERIC AEOLIAN GENERATOR" (ITALIAN INVENTOR)
Dec 05, 2010; GENEVA, Dec. 5 -- Publication No. WO/2009/147692 was published on Dec. 10. Title of the invention: "INFRASTRUCTURE FOR DRIVING...
Publication No. WO/2009/147692 Published on Dec. 10, Italian Inventor Develops Driving, Assisted Take-Off Infrastructure
Dec 11, 2009; GENEVA, Dec. 11 -- Massimo Ippolito, Italy, has developed an infrastructure for driving and assisted take-off of airfoils for...