WASHINGTON — The U.S. military is exploring hypersonic technology that would propel missiles or aircraft at up to six times the speed of sound — nearly 4,000 mph.

Known by code names such as Falcon, High Fire and Blackswift, the experiments and tests are being kept closely guarded as the Air Force looks toward a future generation of airpower and weaponry midway into the 21st century — or possibly sooner.

One possibility is an ultra-fast long-range bomber that the Air Force wants to field within 30 years. Air Force officials hope to deploy a new interim bomber by 2018, followed by a more advanced, and possibly unmanned, bomber in 2035 that would incorporate many of the concepts emerging from current research.

Dr. Mark Lewis, the chief scientist for the Air Force, said that a hypersonic cruise missile may be the first operational product to emerge from the research. Government teams, working with private contractors, also hope to develop long-range hypersonic aircraft that would take off from conventional runways, travel more than 10,000 miles in two hours and land on runways.

"My goal is to make sure that the United States is the first country that ever brings this technology to the fight," Lewis said.

Military analyst Loren Thompson, an executive at the Lexington Institute, a defense policy organization in Arlington, Va., said the Air Force "has great interest in long-range hypersonic vehicles that can do two things: collect intelligence and target time-sensitive assets."

Thompson defined "time-sensitive assets" as "something that if you don't hit right now it will be gone if you come back later." He cited, as one example, a ballistic missile being prepared for launch against the United States.

The development of hypersonic technology has taken on new urgency after China destroyed one of its satellites 530 miles above Earth in a January 2007 test. The test raised fears within the U.S. government that a foreign power is capable of destroying military satellites in low Earth orbit.

"The Chinese and Russians have learned how to disable our spy satellites, so we need some way to avoid being blinded in a war," said Thompson. "A really fast aircraft that could get over those countries right away would be a good backup to losing our spy satellites."

The best known military plane that approached hypersonic speeds was the now-retired Air Force SR-71, which flew at 3.2 times the speed of sound. In 2004, an experimental hypersonic craft known as the X-43A tripled that speed, flying at Mach 9.6, or nearly 7,000 mph.

The X-43 flights, however, were only seconds long and scientists are trying to find ways to keep hypersonic craft airborne for long distances.