After months of speculation into changes relating to the OML (Outer Mold Line) for the Atlas V and Starliner design, United Launch Alliance (ULA) and Boeing finally revealed an updated aerodynamic configuration that provides an aeroskirt aft of the spacecraft to counter loads margins and return the stresses on the stack into acceptable flight levels.
Issues with the Atlas V/Starliner configuration were noted at the start of the year (L2), with models of the configuration put through a battery of wind tunnel and CFD (Computational Fluid Dynamics) testing.
The tests are understood to have included experts at the Marshall Space Flight Center (MSFC), adding their years of expertise to find solutions to an issue relating to aerodynamic stability and loads during ascent.
These issues occurred during first stage flight, specifically with the loads flowing from Starliner on to the Centaur upper stage on the Atlas V.
Unlike Atlas V’s regular passengers, Starliner is without a fairing, thus resulting in a different aerodynamic regime for the rocket that has proved to be challenging.
To counter this issue, engineers provided and tested a new configuration that incorporates an aeroskirt aft of the spacecraft, extending the Starliner Service Module cylindrical surface to improve the aerodynamic characteristics of the integrated launch configuration and bring loads margins back to acceptable flight levels.
“Through incredible coordination and continued innovative thinking, the collective team of NASA, Boeing and United Launch Alliance completed three wind tunnel tests in six months to investigate the aerodynamic stability of various configurations and to anchor our analytical predictions,” noted Gary Wentz, ULA vice president of Human and Commercial Services.
“Based on that information, we updated the configuration for the Atlas V Starliner integrated vehicle stack. This configuration is unique because it combines the Atlas V launch vehicle without a payload fairing with Boeing’s Starliner capsule, resulting in different aerodynamic interactions.”
The changes won’t impact on Atlas V’s role with the Dream Chaser spacecraft, not least because the version that is continuing through a NASA contract is the cargo variant that will be launched whilst inside a payload fairing.
Although the new Atlas V/Starliner stack design has been met with some negative comments on the aesthetics of the vehicle, the addition of an aeroskirt will provide a much smoother ride uphill for the Atlas V and Starliner – and importantly for the crew that will be launched via NASA’s Commercial Crew Program (CCP).
Also, the aeroskirt – a metallic orthogrid structure – is designed to be jettisoned during flight for improved performance. It also has inbuilt safety margins in the event of an off-nominal ascent.
“In the unlikely event that an emergency occurs during boost phase of flight, the aeroskirt has venting provisions to control over-pressurization if the Starliner’s abort engines are fired,” added a release from ULA.
“Our testing indicates the solution we chose will sufficiently smooth the air flow around the vehicle during ascent, ensuring crew safety and mission success,” added John Mulholland, vice president and program manager of Boeing’s Commercial Crew Program.
The ULA team completed the aeroskirt Preliminary Design Review (PDR) earlier this month. Fabrication of the aeroskirt is scheduled to begin this month at ULA’s factory in Decatur, Alabama, following completion of a Production Readiness Review (PRR).
The Atlas V with Starliner has a planned uncrewed flight test in 2018 with operational missions to follow. The vehicle has been suffering from schedule slips, with the original plan closely aligned with its CCP partner, Dragon 2.
That plan for Starliner was for the first uncrewed flight via an Atlas V 422 variant in April 2017. That mission has also been slated to utilize the dual engine Centaur upper stage.
Had all gone well, the first crewed flight of the CST-100 Starliner was to take place in 2017. NASA also signed the first crew launch services contract with Starliner, before Dragon 2.
The uncrewed test flight to the Station via the “Boe-OFT” mission was originally scheduled for April 2017 – on a 30 days mission, ending with a parachute assisted return.
The second mission, involving a crew is designated “Boe-CFT”, and was to launch in July, 2017, on a 14 day mission to the ISS.
With Starliner not set to begin uncrewed test flights until 2018, SpaceX is in first place on their schedule, despite also slipping from the original plan.
The first Dragon 2 mission, designated “SpX-DM1”, had an originally manifested December, 2016 launch date, ahead of a 30 day mission – most of which would be docked to the ISS – ending with a parachute assisted landing in the Pacific ocean.
That would be followed by “SpX-DM2”, a crewed flight, originally classed as launching in April of 2017, on a 14 day mission.
SpaceX – despite its recent standdown due to the loss of the Amos-6 rocket and payload during a static fire test – is still expecting to launch the Dragon 2 on a test flight in 2017.
This will mark the first time astronauts have launched from American soil on a US built spacecraft since Atlantis’ STS-135 mission in 2011.
Regardless of which vehicle launches NASA astronauts first, both vehicles will be required to provide domestic crew launch capability.
“We look forward to our continued partnership with Boeing and NASA to ensure mission success and safety for American astronauts flying from U.S. soil on the Atlas V Starliner,” added Mr. Wentz.
(Images: ULA, Boeing and L2 – including renders from L2 artist Nathan Koga – The full gallery of Nathan’s (SpaceX Dragon to MCT, SLS, Commercial Crew and more) L2 images can be *found here*)
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