One was a spy plane, designed to fly fast and stealthy into enemy territory; on its last flight, it cruised from Los Angeles to Washington D.C. in an hour and four minutes, hitting 2,124 mph, a speed record.

The other was a sleek commercial jet for wealthy, time-is-money travelers willing to plunk down $10,000 for a ticket-or more-for the luxury of zipping across the Atlantic at Mach 2, or twice the speed of sound.

Today, the SR-71 Blackbird and the Concorde have been retired, sitting motionless as showcase exhibits at Smithsonian’s Air and Space Museum by Washington Dulles International Airport. But now in conjunction with the Pentagon and NASA, Lockheed Martin is trying to develop new technologies that would build on their legacies, designing aircraft designed to go fast-really, really fast.

Worried that adversaries are limiting the U.S. military’s ability to strike when and where it wants, the Pentagon is pushing an effort to develop new hypersonic weapons, capable of flying at Mach 5 and above. There are also concerns over reports that China is also developing hypersonic technology, which could give it a tactical advantage.

In a recent state-of-the-company speech Marillyn Hewson, Lockheed Martin’s chief executive, highlighted the company’s work in hypersonics, saying the world’s largest defense contractor is developing technologies that would “allow quicker response times to increasingly mobile threats, and the ability to project strength more rapidly around the globe.”

Bethesda, Maryland-based Lockheed is working on two programs with DARPA, the Pentagon’s research agency, she said. While she did not mention it by name, the company is currently developing the SR-72, a follow-on to the Blackbird, at its notoriously secretive Skunk Works division. The unmanned aircraft would fly at Mach 6, or “so fast, an adversary would have no time to react or hide,” Lockheed says on its website.

While typical passenger planes can fly at about 550 mph, or about eight miles per minute, Lockheed has said it is developing missiles that would fly at a speed of 1 mile per second, and other vehicles that would travel four miles per second, the company says.

“Hypersonic aircraft, coupled with hypersonic missiles, could penetrate denied airspace and strike at nearly any location across a continent in less than an hour,” Brad Leland, Lockheed Martin’s hypersonics, said in a press release. “Speed is the next aviation advancement to counter emerging threats in the next several decades.”

Last year, Raytheon received a $20 million contract from DARPA to develop maneuverable missiles than can travel at Mach 5, or from New York to Los Angeles in 39 minutes.

“Hypersonic weapons can be more survivable because of the extreme speed and high altitude,” J.R. Smith, Raytheon’s director of Advanced Land Warfare Systems, said in a release last year. “They would be hard to stop.”

That could be a problem, said Richard Aboulafia, an aerospace analyst with the research firm Teal Group. The weapons could be moving so fast, foreign governments could overreact with so little time to respond.

“The real problem with hypersonic weaponry is it increases the risk of-how to put this gently-global annihilation,” he said.

But he also said that engineers have been talking about the technology for years, with little to show for it. “Eventually they will finally get there,” he said. “Then again, it’s been just over the horizon for half a century now.”

During her speech, Hewson said that the efforts could also “enable hypersonic passenger flights, and even easier access to space.”

In addition to its work with DARPA, the company also recently won a $20 million NASA contract to help design a passenger jet that could travel faster than the speed of sound. But unlike the Concorde, which unleashed a sonic boom every time it crested the sound barrier, the new plane would be much quieter, emitting what NASA calls “a supersonic ‘heartbeat’-a soft thump.”

If successful, that would be a significant step in taking care of the noise problem. But what also killed the Concorde was its exorbitant price. Mike Buonanno, who is managing the program for Lockheed, said market studies show there is a demand for commercial supersonic flight . And while it “will always be more expensive” than traditional air travel, new technologies and fuel efficiencies, “could be impactful in reducing the overall cost.”

Aboulafia isn’t buying it.

“Mach 2, taxpayer zero,” he said, citing an old joke about past efforts to develop super-fast passenger aircraft.

“If you ever find an aerospace company spending their own cash on this, dump their shares,” he said.