WASHINGTON — tensions with Russia, the U.S. Air Force is examining what it would take to accelerate production of United Launch Alliance’s Delta 4 rocket to ensure continued access to space for military satellites.

Gen. William Shelton, commander of Air Force Space Command, told reporters here July 22 that he asked the service’s acquisition arm, the Space and Missile Systems Center at Los Angeles Air Force Base, to look at what types of contract modifications are necessary to ramp up production of Delta 4 rockets.

“It may be time to start thinking seriously about this,” Shelton said, emphasizing that no contract action has been taken to date.

The Delta 4 is one of two rockets the U.S. Defense Department relies on to launch national security payloads. The other is ULA’s Atlas 5, which is powered by the Russian-built RD-180 engine whose future availability has come into question amid the rhetorical exchanges between Moscow and Washington about Russia’s annexation of Crimea and continued meddling in Ukraine.

Shelton, who oversees Air Force space operations and procurement, and ULA have both said they see no signs of a slowdown in shipments of the RD-180, produced by NPO Energomash and imported to the United States by RD-Amross, a joint venture of Energomash and United Technologies Corp. of Hartford, Connecticut. But that has not stopped what Shelton described as “prudent planning” against the possibility that the pipeline dries up.

Michael Gass, ULA’s president and chief executive, said in June that the company has already begun the preliminary work necessary to accelerate the Delta 4 production at the company’s main rocket plant in Decatur, Alabama.

Shelton said the situation in Ukraine, underscored by the recent shootdown of a Malaysian commercial jetliner by what U.S. government officials say were Russian-equipped Ukrainian separatists, has diminished his confidence that RD-180 deliveries will continue unabated.

“The events of the last couple of weeks give us even more pause,” Shelton said. “They certainly call into question the continued supply of RD-180 engines.”

Already, ULA has accelerated the delivery of five engines it was expected to receive in November. Two of those engines are now expected to arrive in Alabama in August with three more to follow in September or October.

Currently, ULA has 15 RD-180 engines in its possession, enough for about two years’ worth of Atlas 5 missions. The new engines arriving this fall would give the Air Force another nine months of cushion, Shelton said.

Without the Atlas 5, the Air Force would rely on the Delta 4 and, eventually, Space Exploration Technologies Corp.’s Falcon 9, once that vehicle is certified to carry military payloads. The Air Force certification process is expected to wrap up in December at the earliest.

But the disruption to the Air Force’s launch manifest resulting from an RD-180 cutoff could cost the service $1.5 billion to $5 billion, Shelton said. This funding would be needed to cover the cost of storing delayed satellites and the higher prices associated with the Delta 4 rocket.

“Here we are in what appears to be a rainy day potentially,” Shelton said.

Meanwhile, Shelton — during the briefing and in a speech at the Atlantic Council here earlier in the day — declined to say which U.S. satellites, if any, provided intelligence that helped U.S. authorities to determine that a missile launched from rebel-held Ukrainian territory shot down the Malaysian airliner. The Air Force operates missile warning satellites that can pick up the infrared-heat signature of a large surface-to-air missile, while classified signals intelligence satellites are able to determine whether radars used to guide such weapons have been activated.

Shelton said the Air Force only acknowledges the ability of its Space Based Infrared System missile warning satellites to detect strategic-class missiles and space launches, even though they are widely known to be able to detect shorter-range missiles. Shelton did say, however, that “they are very good satellites. They are very sensitive satellites.”

Shelton also said that despite reports to the contrary, the Air Force operates a network of missile warning satellites with full global coverage 24 hours a day, seven days a week.

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.