Looking much like a giant inverted surfboard, the Northrop design is classified as a "waverider" concept. A waverider does just what its name implies: It rides the shock wave of compressed air that is created by any vehicle moving faster than the speed of sound. The shape of Northrop’s vehicle is designed to produce a shock wave that emanates from the leading edge of the wing downward, creating high pressure only on the underside of the vehicle. By containing the shock wave below its wing, the aircraft can "ride" the wave, a trick that enables it to achieve a high lift-to-drag ratio.

Northrop Grumman’s hypersonic design is intended to confine most of the high-pressure airflow from a shock wave beneath the vehicle, so that the vehicle appears to be "riding" on top of the wave (top).

A ramjet or scramjet engine would power the waverider. A ramjet operates by burning fuel in air that is compressed by the forward speed of the aircraft itself. (In a normal jet engine, fan blades compress the air.) Ramjets operate from about Mach 2 to Mach 5. A scramjet (supersonic-combustion ramjet) is a ramjet engine in which the airflow through the whole engine is supersonic. Scramjets operate from about Mach 5 to Mach 10.

Before either type of engine will operate, the hypersonic bomber must already be flying at a high speed and altitude. Most likely, the bomber will use a conventional engine to reach the proper speed and altitude for the ramjet or scramjet to operate.

Like its high-speed progenitors, the SR-71 and the XB-70, the waverider bomber will present enormous challenges in thermal management. Flying at Mach 5 and above creates tremendous atmospheric friction, which in turn heats up the airframe.

Propulsion and guidance systems, weapons systems, hydraulics, fuel systems, and life support systems will all have to be capable of operating at temperatures in the thousands of degrees Fahrenheit.

Engineers who designed the SR-71 partially solved its heat problems with the invention of a high-flashpoint fuel that also served as a heat sink. Corrugated sections of the Blackbird’s skin allow the aircraft to expand and contract as much as a foot to deal with expansion caused by heat. The engineers also invented a lubricating oil that could be used effectively on parts with cruising speed temperatures of 600°F-plus, and could also remain fluid at temperatures as low as 40°F.

To solve heat-management problems in a hypersonic bomber, new exotic fuels that can be cooled to thousands of degrees below zero, such as liquid hydrogen or methane, will be required to power the craft. The hypersonic bomber will also have to be wrapped in a heat-resistant, flexible skin — possibly composed of exotic ceramics composites derived from the space shuttle and VentureStar programs. And since large cockpit windows would enable heat to radiate into the cockpit, the Northrop waverider concept features small porthole-type windows that would only be used on takeoffs and landings. The pilot of the hypersonic bomber would rely on video displays connected to external sensors.

This is not the first time a hydrogen-fueled aircraft has been on the drawing board. In the late 1950s, a top-secret study by Lockheed into the feasibility of creating a hydrogen-fueled spy plane (known as Suntan) was abandoned, chiefly because of the extreme difficulty of processing, transporting, and handling liquid hydrogen. These fundamental problems will have to be addressed before a hypersonic bomber is considered feasible.

Another major design hurdle to overcome is the creation of a weapons system that will enable bombs to be dropped at hypersonic speeds. Conventional bomb bays will not work, because the belly of the waverider aircraft serves as the lifting surface. Northrop Grumman designers envision a weapons release system in which bombs or missiles are ejected from long rails trailing behind the aircraft, much like depth charges behind a Navy ship.

The weapons themselves will have to be "brilliant," even smarter than today’s so-called smart bombs. "They will have to be capable of targeting themselves, and updating targeting, at hypervelocities," says Boccadoro. "New targeting systems will also have to be invented that can deal with moving targets while traveling at hypersonic speeds."

Can America afford a hypersonic bomber? Any future bomber program will undoubtedly be an expensive one. With Congress threatening to slash the F-22 Raptor program and B-2 bombers costing the taxpayers more than $1 billion each, a hypersonic bomber program may sound like a military planner’s pipe dream. But at some point in the not-so-distant future, the cost of upgrading the B-52 will approach the cost of building a new bomber. The B-1B, whose history has been plagued with mechanical problems, has never lived up to the Air Force’s expectations. And the B-2, easily the most advanced bomber in the world, is based on yesterday’s stealth technology.

"We will probably need a new bomber after the year 2030, when the existing bombers finally wear out," says John Pike of the Federation of American Scientists, "but there is no evident reason for assuming that the new bomber will need to be hypersonic. The feasibility of any hypersonic military aircraft remains unproven, and the need for a hypersonic bomber remains equally unproven," he says. "Efforts generations ago to develop high-speed bombers such as the B-58 and B-70 were disappointing, even if they did result in some of the best-looking aircraft of all time."

Low-cost stopgap measures are also being considered. For example, an old idea of turning a Boeing 747 into a cruise missile carrier has resurfaced as a viable alternative to building more bombers.

Pentagon planners hope that the Future Strike Aircraft study due this month will help them determine the overall shape of bombers to come. "The purpose of the study is to define the technologies we should invest in today to enable us to economically build the bombers of the future," says Boccadoro.

It’s possible that some of tomorrow’s bombers won’t even require pilots. One Northrop Grumman concept envisions four unmanned subsonic bombers, known as Unmanned Global Strike Aircraft, that are controlled by a crewman in a bomber mothership. In this scenario, waves of remotely controlled bombers could strike deep into well-defended enemy territory without risking pilots. The idea of unmanned bombers is particularly appealing to an Air Force that’s currently suffering from a shortage of pilots and an American public that’s increasingly sensitive to casualties.

Will the B-3 turn out to be a team of aircraft rather than a single bomber? That’s just one of the questions the Air Force hopes to answer with its ongoing studies. "Both manned and unmanned solutions will be investigated, and at this time there is no decision on which one will be used in the future," says Future Strike Aircraft program manager Laetta Gayheart. The designs proposed under the FSA study will be reviewed and updated annually to keep in step with rapidly changing technologies.