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Proton completes Inmarsat-5 constellation

Anatoly Zak, Spaceflight Correspondent
Aug 29, 2015, 17:39 UTC

Sen—Russia's workhorse rocket is back on the commercial market more than three months after its latest failure

Performing a daylight launch, the 705-ton Proton-M vehicle lifted off into cloudless sky over the Baikonur Cosmodrome in Kazakhstan at 14:44 Moscow Time (11:44 UTC). During a nine-minute ascent, all the rocket's booster stages preformed flawlessly, including the third stage, whose failure doomed the previous mission.

After the separation of the third stage, the Briz-M space tug conducted its first engine firing delivering itself and its payload into an initial parking orbit around 180 kilometers above Earth's surface. Four subsequent pre-programmed maneuvers by the Briz-M were spread over a 15-hour period culminating with the release of the Inmarsat-5 F3 satellite, known in the industry as I5F3, into a 4,341 by 65,000-kilometer orbit tilted almost 27 degrees toward the Equator. At such a great distance from Earth, the satellite will need less of its own propellant to counteract the planet's gravitational field, while matching its orbital plane with that of the Equator. Inmarsat-5 F3 will also use its own ion thrusters to slowly descend to a 36,000-kilometer circular orbit, where it will appear hanging over the Pacific Ocean to an observer on Earth. In reality, the spacecraft will me making a single revolution in 24 hours, thus matching the Earth's own rotation. Like all its siblings, the satellite is expected to operate in its orbital position for at least a decade and a half.

The arrival of Inmarsat-5 F3 will complete the original Global Xpress constellation for the London-based Inmarsat. Conceived as a three-satellite fleet, which was launched entirely on the Proton, Global Xpress, or GX for short, promises to provide seamless voice and data communications around the world. GX has ability to deliver unprecedented mobile broadband speeds of up to 50 megabytes per second and its customers would include the government, maritime, enterprise, energy and aeronautical sectors, the service provider said.

Weighing slightly more than six tons, the satellites for the Inmarsat-5 series were developed by the Boeing company, which based them on its standard platform designated 702HP. In October 2013, Inmarsat decided to procure the fourth such satellite from Boeing with a completion date in the middle of 2016.

The successful delivery of Inmarsat-5 F3 opens a floodgate of Proton missions, which were squeezed into the reminder of 2015 by the rocket's accident on May 16. The Russian investigation into the failure was completed on May 29, however only by the end of July had all the international underwriters and customers agreed that a combination of relaxed quality control procedures in Russia with an old design flaw caused excessive vibration and failure of the engine turbine, leading to the May 16 crash, as well as to a similar failure exactly a year earlier. The new sensors installed on the engine in the wake of the 2014 accident enabled operators to pinpoint finally the real culprit, Russian space officials said.

The next Proton liftoff is now expected in slightly more than two weeks, carrying a Russian communications satellite. That mission will also be an important test of the upgraded Block DM-03 upper stage, which the Russian space agency (Roscosmos) plans to use for its own Proton missions and later integrate the same space tug into the new-generation Angara rocket.

The first attempt to employ Block DM-03 for the launch of Russia's GLONASS navigation satellite trio at the end of 2010 ended in a destructive plunge of all three satellites into the Pacific Ocean. The investigation then revealed that a fueling team in Baikonur working with outdated specifications had loaded too much propellant into the stage.

Following the September test flight with the Block DM-03 stage, the Proton is expected to fly roughly once a month not only to fulfill its busy commercial manifest, but also as a confidence-building measure before the historic launch of the Russian-European ExoMars mission early next year. It will be the first attempt to use the rocket for a trip to the Red Planet since a disastrous launch of the Mars-96 mission almost two decades ago.