The June 28 explosion of a Falcon 9 rocket carrying supplies to the International Space Station has members of Congress asking NASA and the U.S. Air Force for assurance that SpaceX is qualified to carry military payloads to space.

A bipartisan group of 14 U.S. representatives sent a letter saying they have “serious reservations” about SpaceX’s internal investigation process and question whether the “engineering rigor applied will be sufficient to prevent future military launch mishaps.”

“We are committed to our nation’s leadership in space, but equally believe we must be responsible stewards of taxpayer dollars when it comes to achieving our priorities and goals for spaceflight,” reads the letter to NASA administrator Charles Bolden and Air Force secretary Deborah James.

The panel asked Bolden and James to outline the oversight responsibilities of NASA and the Air Force, however, some questions raised in the letter have already been addressed in other arenas.

On May 28, SpaceX was certified by the Air Force to carry military payloads to space, offering competition to Centennial-based United Launch Alliance for the first time in more than a decade.

The letter’s signators include Colorado’s four Republican congressmen — Ken Buck, Mike Coffman, Doug Lamborn and Scott Tipton— and five members from Alabama, where ULA’s rockets are built.

The representatives want to know if the Falcon 9 launch system will be decertified for military launches. If it is not, they want details about the process that will be required to return it to service.

SpaceX officials said they had not seen the letter and so could not comment.

In a July 2 interview with Spaceflight Now, Air Force’s Space and Missile Systems Center commander Lt. Gen. Samuel Greaves, who oversees military certification, said SpaceX’s certification will remain intact leaving it free to compete with ULA for upcoming launches of the government’s GPS III satellites.

“The Air Force will continue to objectively review all proposal submissions for launch services,” he said.

As part of their federal license applications, launch providers must file an accident investigation plan with the Federal Aviation Administration.

The FAA classifies incidents such as the Falcon 9 loss — where the rocket did not veer off course and there was no loss of life — as “mishaps.” More serious events, where deaths occur, are classified as “accidents” and are investigated by the National Transportation Safety Board.

Under its approved plan, SpaceX is responsible for investigating the mishap, with FAA oversight and input from NASA. The Air Force has also been at the table, though in an unofficial capacity, a military spokesman said.

SpaceX is following the same Congressionally approved investigation process followed by Orbital ATK after its Antares rocket exploded in October. The Antares rocket also was taking supplies to ISS.

However, Coffman’s deputy chief of staff Tyler Sandberg said SpaceX’s launch system requires special scrutiny.

“The Falcon 9 is certified for military payloads, so there should be confidence from the Air Force and NASA that this rocket system is safe from their perspective,” he said in an e-mail.

Coffman, whose district includes ULA’s headquarters, is traveling and could not be reached for comment.

Bolden issued a statement shortly after the mishap expressing confidence in SpaceX: “We will work with and support SpaceX to assess what happened, understand the specifics of the failure and correct it to move forward.”

Preliminary results of the investigation point blame at a 2-foot-long strut that snapped inside the Falcon 9, SpaceX founder and CEO Elon Musk said July 20.

The letter also calls into question the process and oversight for certification of changes and upgrades to the Falcon 9 design — a point Lamborn questioned Air Force officials about during a June 26 House Armed Services Subcommittee hearing.

During that hearing, Greaves said he meets with SpaceX executives biweekly to discuss any Falcon 9 design changes and ensure they meet certification standards.

“This is no different, sir, than we have done with ULA in the past. In fact, last December, when we flew the RL10C, which is an upgraded second stage engine, we went through a significant effort with ULA ahead of time to understand the changes of that system,” Greaves testified. “So, today, as we speak, SpaceX has provided what changes they envision for the upgraded Falcon 9. We are daily in an intense effort with them to understand and hopefully certify that system.”

Laura Keeney: 303-954-1337, lkeeney@denverpost.com or twitter.com/LauraKeeney