Staff Writer (Updated: 3/20/2016):
The Falcon HTV-2 program (known as "Prompt Global Strike") was born in 2003 to help research a delivery vehicle for the United States military with the simple goal of reaching any part of the world in under an hour. Following the design phase, the HTV-2 was produced through computer simulations and wind tunnel testing to evaluated the validity of such a system. Both proved the concept sound though wind tunnel tests were limited to Mach 15 environments and this for only short periods of time. Nonetheless, with funding, DARPA proceeded to construct the real thing. The HTV-2 would be a rocket-launched system, unmanned at its core and controlled from the ground in the same way modern UAVs (Unmanned Aerial Vehicles) are - never putting the pilot in harm's way. The HTV-2 is a research vehicle intended to collect in-flight data for after-mission processing.
The concept of the Falcon HTV-2 is a rather basic one involving delivery of an unmanned aircraft into sub-earth orbit. The vehicle is released from its rocket "booster" system, achieves "self-control" and orientation and proceeds to dive back to earth, in the process reaching speeds of up to 13,000 miles per hour (Mach 20, twenty times the speed of sound). This allows the arrow-head design of the Falcon to literally "slice" through the upper atmosphere at unheard of speeds, essentially making it a hypersonic glider of sorts despite its Mach 20 speed. The aircraft then falls into the ocean in a controlled crash upon completion of its mission. Due to its sub-orbital flight, the body of the Falcon is able to shield itself from the extreme heat - as high as 3,500F degrees - during its high-speed flight at such high-altitudes. Additionally, its aerodynamic principles must be extremely stable and controllable at such speeds for the vehicle to be of any use.
The "piggyback" idea involving a primary "booster" aircraft is not wholly uncommon within United States Air Force lore for the USAF has been using such launch methods throughout a bevy of experimental systems in the 1950s and 1960s. One such method - this involving a modified Boeing B-29 Superfortress - was used to launch the Bell X-1 rocket plane that allowed the Americans to successfully break the sound barrier for the first time. Similarly, the North American X-15 rocket plane was launched from underwing of a Boeing B-52 Stratofortress. The Falcon HTV-2 is launched into sub-earth orbit by way of the Minotaur IV Lite rocket booster.