The SR-72 will travel at six times the speed of sound—the fastest military jet ever made—and fly as high as 80,000 feet. The SR-72,will evade assault, take spy photos, and attack targets at speeds of up to Mach 6. That’s twice as fast as its predecessor.
Aeronautical engineers at Lockheed Martin and Aerojet Rocketdyne have been designing the SR-72 at their Skunk Works black site in California for the past several years. It will require a hybrid propulsion system: a conventional, off-the-shelf turbo jet that can take the plane from runway to Mach 3, and a hypersonic ramjet/scramjet that will push it the rest of the way. Its body will have to withstand the extreme heat of hypersonic flight, when air friction alone could melt steel. Its bombs will have to hit targets from possibly 80,000 feet. Lockheed says the craft could be deployed by 2030. Once it is, the plane’s ability to cover one mile per second means it could reach any location on any continent in an hour—not that you’ll see it coming.
The aircraft will accelerate to about Mach 3 under turbojet power, switch to ramjet power to take it to about Mach 5, and then switch again to scramjet mode, which uses supersonic air for combustion.
The SR-72 may face significant challenges to being accepted by the Air Force, as they are opting to develop the Northrop Grumman RQ-180 stealth UAV to perform the task of conducting ISR missions in contested airspace. Compared to the SR-72, the RQ-180 is less complex to design and manufacture, less prone to problems with acquisition, and can enter service as soon as 2015.
In December 2014 NASA awarded Lockheed Martin a contract to study the feasibility of building the SR-72's propulsion system using existing turbine engine technologies. The $892,292 contract funds a design study to determine the viability of a TBCC propulsion system by combining one of several current turbine engines, with a very low Mach ignition Dual Mode Ramjet (DMRJ). NASA previously funded a Lockheed Martin study that found speeds up to Mach 7 could be achieved with a dual-mode engine combining turbine and ramjet technologies. The problem with hypersonic propulsion has always been the gap between the highest speed capabilities of a turbojet, from around Mach 2.2 to the lowest speed of a ramjet at Mach 4. Typical turbine engines cannot achieve high enough speeds for a ramjet to take over and continue accelerating. The NASA-Lockheed study is looking at the possibility of a higher-speed turbine engine or a ramjet that can function in a turbine engine's slower flight envelope; the DARPA HTV-3X had demonstrated a low-speed ramjet that could operate below Mach 3. Existing turbofan engines powering jet fighters and other experimental designs are being considered for modification. If the study is successful, NASA will fund a demonstrator to test the DMRJ in a flight research vehicle
Aerodynamic friction at speeds exceeding Mach 5 will heat an aircraft’s exterior to 2,000 degrees. At that point, conventional steel airframes will melt. So engineers are looking at composites—the same kinds of high-performance carbon, ceramic, and metal mixes used for the noses of intercontinental ballistic missiles and space shuttles.
SOURCES - Popular Science, Wikipedia