WASHINGTON — United Launch Alliance intends to phase out all but the heavy-lift version of its Delta 4 rocket as early as 2018 as it seeks to sharpen its competitiveness in the face of a challenge by SpaceX.

Denver-based ULA will continue building the Delta 4 Heavy as long as its Air Force customer desires, said Tory Bruno, the company’s president chief executive. The vehicle, whose first stage consists of three Delta 4 cores in a side-by-side configuration, is used to launch classified national security payloads but flies infrequently — roughly once every few years.

The Delta 4 is the latest in a line of rockets that has been serving U.S. government and commercial customers for more than 50 years. Though reliable, the rocket is far more expensive than ULA’s other workhorse, the Atlas 5, which has come under fire because its core stage is powered by a Russian-built engine dubbed RD-180.

In a March 2 interview, Bruno, said both rockets ultimately will be replaced by a new launch vehicle currently known as the Next Generation Launch System, or NGLS. The NGLS will be powered by a new main engine now under development.

“We’re going to take [out] the redundant, more expensive Delta single-stick-line and fly only Atlas until we have NGLS available and until the government decides they’re done with [Delta 4] Heavy,” Bruno said.

The Delta 4’s versatility — it can launch all of the missions handled by the Atlas 5 — assured that the Defense Department would have access to space if something were to go awry with the Atlas 5.

But with the emergence of Hawthorne, California-based SpaceX, which has a proven, low-cost vehicle in its Falcon 9 and plans to demonstrate a heavy-lift rocket this year, the case for keeping the Delta 4 has disintegrated.

“Great rocket,” Bruno said of the Delta 4. “But it’s more expensive than the equivalent Atlas rocket.”

The last of the single-stick, or intermediate-class, Delta 4 launches would take place around 2018-2019, Bruno said.

While pledging to produce Delta 4 Heavy rockets as long as the government wants, Bruno said he expects the Defense Department to eventually adopt a comparable variant of the NGLS. “I’ve promised to the government we will retire the Delta Heavy only when they are ready because satellites are configured specifically for that launch vehicle,” he said.

Bruno said he would announce further details about the NGLS system, including the provider of the upper-stage engine, in April. ULA is working with XCOR Aerospace of Mojave, California, Aerojet Rocketdyne of Sacramento, California, and others on upper-stage engine possibilities, he said.

ULA has been working on two options for the first-stage engine: the BE-4, which is being developed by Blue Origin of Kent, Washington, in partnership with ULA, and the AR-1 proposed by Aerojet Rocketdyne. ULA, which has characterized the BE-4 as its first choice, expects to decide which to pursue in 2016 or 2017, Bruno said.

Either option is expected to cost about $1 billion to develop. Bruno estimated that less than half of that cost will come from a government development program. He also said ULA, Blue Origin and Aerojet Rocketdyne have “identified” a combined $1 billion of private investment in the BE-4 and AR-1.

Currently ULA operates a combined five Atlas 5 and Delta 4 launch pads at Cape Canaveral Air Force Station, Florida, and Vandenberg Air Force Base, California. Bruno said that while the company is still completing studies on the infrastructure needed for the NGLS, he has told the Air Force ULA plans to reduce that number to two.

Meanwhile, Bruno hopes to get some legislative relief from a congressional ban on the use of the RD-180 for national security missions that takes effect in 2019. The ban, imposed in the National Defense Authorization Act of 2015, is in response to escalating U.S.-Russian tensions over Ukraine.

Congress has directed the Air Force to begin work on an RD-180 replacement and allocated $220 million for that purpose in 2015, but Air Force officials doubt that a new engine would be ready to fly by 2019.

Bruno has said the BE-4 could debut on the NGLS by 2019 but that the vehicle would not be certified to carry national security payloads until 2022 or 2023.

In the interview, Bruno said he hoped Congress would change the language to allow continued use of the RD-180 until a replacement is ready.