WASHINGTON — International Launch Services is adding a larger payload fairing for its Proton rocket, but deferring development of one of two smaller versions of the vehicle announced last year.

The company, which markets the Proton to commercial customers, announced March 7 that Proton manufacturer Khrunichev was developing a payload fairing five meters in diameter that will be available for launches starting in the first quarter of 2020. The vehicle’s current payload fairing is four meters in diameter.

“It will enable us to fit the big high-throughput satellites into the payload fairing, stack configurations easier and also enables us to multi-manifest in support of the large LEO constellations that are anticipated,” Kirk Pysher, president of ILS, said in a briefing to reporters here during the Satellite 2017 conference.

Development of the fairing, company officials said, was driven by market demand from operators planning more large geostationary satellites as well as proposed low Earth orbit systems. Other major launch providers, including Arianespace, SpaceX and United Launch Alliance, already offer payload fairings at least five meters in diameter for their vehicles.

The wider payload fairing will also be one meter longer. “That will give plenty of margin for future growth,” said Jim Kramer, vice president of engineering and mission assurance. In many cases now, he said, “You have more [launch vehicle] performance than what you can fit into the fairing.”

The larger fairing will slightly reduce vehicle’s payload performance. Kramer said that earlier designs for the larger fairing, stretched by several meters in length, reduced the vehicle’s payload capacity by up to 500 kilograms. “By the time we finalize this design, it will be somewhat less than that,” he said.

ILS is also making changes to two new, smaller versions of the Proton announced last September, called Proton Medium and Proton Light. Those vehicles, when originally announced, would not use the second stage currently flown on the Proton M, but retain the first and upper stages. The Proton Medium would be able to place 5,000 kilograms into geostationary transfer orbit while the Proton Light, which would also eliminate two of six engines on the first stage, could carry 3,600 kilograms to GTO.

The new design of the Proton Medium calls for retaining the second stage and instead deleting the third stage currently used by the Proton M. “What that allows us to do is significantly reduce the development program for this vehicle,” Kramer said, focusing primarily on modifications of the interstage adapter between the second and upper stage. It would keep the first two stages the same between the Proton M and Proton Medium.

The new Proton Medium will still have a payload capacity of 5,000 kilograms to GTO. It will be ready for launches in the fourth quarter of 2018.

ILS, though, is deferring development of the Proton Light. “What we’re seeing is that the demand on the customer side is looking stronger in the 5,000-kilogram range,” Kramer said. “The market demand at the lower mass is less than what we were originally projecting.” He said ILS would reassess the Proton Light after it introduces both the Proton Medium and the five-meter payload fairing.

In the near term, ILS is focused on resuming Proton launches as soon as late April. Khrunichev postponed launches of the Proton and recalled a number of second- and third-stage engines after testing discovered a defect in one such engine linked to the use of the wrong solder.

“This is what I would say is a quality success story,” Pysher said of the engine recall. “The quality system within Russia did its job.” A batch of six engines is built for the four-engine second stage of the Proton, with the other two subjected to extended testing and then analysis. It was that testing, he said, that discovered the defect caused by the use of the wrong solder.

While Russian media reports claimed that the solder used in the defective engine was a less expensive one, the opposite was the case, Kramer said. A technician working on another program at the factory, he said, violated procedures and returned excess solder, which got mixed up with the solder used for the Proton engines. “The bad solder was the solder with the precious metals,” he said.

Assuming the Proton does return to flight by late April, Pysher said he expects seven Proton missions this year, three commercial launches and four Russian government ones. The three commercial missions are EchoStar 21, AsiaSat 9, and either Amazonas 5 or Hispasat 1F.

Pysher said that, with the new Medium version and a larger payload fairing, Proton should remain in service well into the 2020s. While the vehicle was intended to be eventually phased out in favor of the Angara 5 rocket, he said that new vehicle will not be ready to handle commercial missions until the mid-2020s, when a new launch facility for the vehicle is operational at the Vostochny Cosmodrome in Russia’s Far East.

“I believe these vehicles will be there past the 2025 timeframe,” he said of Proton. “It depends on where the market goes.”