An experimental scramjet-powered, ultrahigh speed strike vehicle is emerging as the Pentagon’s main choice for a new long-range, rapid attack weapon, a senior Pentagon official says.

Alan R. Shaffer, principal deputy assistant defense secretary for research and engineering, told a defense industry conference that prototypes and recent tests proved concepts for hypersonic arms, and several systems are part of a high-priority effort by Pentagon weapons developers, despite the era of sharply-diminished defense spending.

Hypersonic vehicles can deliver nuclear or conventional payloads in precision strikes against increasingly hard-to-penetrate air defenses of countries like ChinaRussia and Iran, he said.

“We, the U.S., do not want to be the second country to understand how to have controlled scramjet hypersonics,” Mr. Shaffer told the Precision Strike Association’s annual review on Tuesday.

The comments come 2 1/2 months after China’s surprise Jan. 9 test of a new hypersonic glide vehicle, dubbed the Wu-14. That ultrahigh speed maneuvering vehicle test represents a major challenge for current U.S. missile defenses, which are designed to counter non-maneuvering ballistic missile threats.

Lee Fuell, a technical intelligence specialist with the Air Force National Air and Space Intelligence Center, told a congressional China commission hearing Jan. 31 that China’s hypersonic glide vehicle is a ballistic missile-launched system that glides and maneuvers to its target at speeds up to Mach 10 (about 7,611 mph).

“At this point, we think that’s associated with their nuclear deterrent forces,” said Mr. Fuell, who noted the Chinese could use the system with conventional warheads for long-range precision strikes.

Mr. Shaffer declined to comment on how the Chinese hypersonic test has changed U.S. plans for hypersonic weapons.

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But the senior weapons research official said the Pentagon’s most promising hypersonic vehicle is the X-51, a cruise missile-sized weapon powered by an advanced engine called a scramjet. The X-51, developed by Boeing, flies at up to 3,882 mph, or Mach 5.1, and is launched from under the wing of a B-52 bomber.

The experimental aircraft is a good candidate to win this year’s Collier Trophy, the annual award recognizing the most significant recent achievement in air or space flight, Mr. Shaffer said.

Mr. Shaffer said hypersonic weapons, when fully developed, will be less expensive than current jets and cruise missiles powered by complex turbine engines with many parts. A scramjet, or supersonic combusting ramjet, hypersonic vehicle has few moving parts.

After three difficult tests, including one described awkwardly by testers as an “un-ignition event,” the X-51 scored a breakthrough last year. During a successful flight test, the vehicle flew for just 300 seconds but traveled several thousand miles and reached a height of 80,000 feet — considered near-space — at over Mach 5.

“It’s the second time we have shown a scramjet can ignite and give positive acceleration,” Mr. Shaffer said. “That is a huge deal. That means we are now starting to understand hypersonics.”

The next step is for weapons engineers to make the system affordable, and Mr. Shaffer urged engineers to tackle the problem.

Another hypersonic weapon on the Pentagon’s drawing board is the HTV-2, or Hypersonic Technology Vehicle, that is boosted by a missile — like the Chinese WU-14 — and then maneuvers and glides to its target at very high speeds.