Dazzling Proton-M Successfully Delivers Sirius FM-6 Digital Radio Satellite to Orbit

Characterized by its six outboard fuel tanks and central oxidiser tank, the Proton-M stands ready on Pad 39 at Baikonur's Site 200 for its launch with the Sirius FM-6 satellite. Photo Credit: ILS
Characterized by its six outboard fuel tanks and central oxidiser tank, the Proton-M stands ready on Pad 39 at Baikonur’s Site 200 for its launch with the Sirius FM-6 satellite. Photo Credit: ILS

A giant Proton-M booster lit up the darkened skies of Baikonur, Kazakhstan, tonight, successfully delivering the long-delayed Sirius FM-6 digital radio satellite toward orbit. Liftoff of the mission took place from Pad 39 at the cosmodrome’s Site 200 launch zone at 12:08 a.m. local time Saturday, 26 October (2:08 p.m. EDT Friday, 25 October). Destined to inject its satellite payload into geostationary transfer orbit—with an apogee, or “high point,” of some 22,000 miles (35,000 km)—the three stages of the Proton-M performed normally and the restartable Briz-M upper stage is currently in the process of delivering Sirius FM-6 into its correct orbital “slot.”    

This mission has been a long time coming, following problems with both the launch vehicle and the spacecraft itself. The Proton-M returned to flight on 29 September, delivering the Astra 2E communications satellite into orbit, following several months of corrective actions in the wake of a dramatic launch failure on 2 July. Alongside the Proton’s woes, Sirius FM-6 was originally scheduled to fly in March 2012, but was extensively delayed, due to concerns about a possible solar array defect. A short-lived glitch with the array deployment mechanism on the SES-4 satellite—a spacecraft of similar design and construction—prompted a decision to return Sirius FM-6 to prime contractor Space Systems/Loral’s (SS/L) facility in Palo Alto, Calif., for additional work. Launch was eventually rescheduled for 20 July 2013, but was further postponed following the 2 July explosion of a Proton-M seconds after liftoff. Preparations resumed in earnest in early September, when Sirius FM-6 was delivered to Baikonur for final pre-launch processing.

The Sirius FM-6 satellite undergoes pre-flight checkout. Photo Credit: ILS
The Sirius FM-6 satellite undergoes pre-flight checkout. Photo Credit: ILS

On 17 October, with the satellite stacked atop the booster, the vehicle was transported in a horizontal orientation from the assembly building to Pad 39 at Site 200 for final preparations. Liftoff was targeted for 12:12 a.m. local time Monday, 21 October (2:12 p.m. EDT Sunday, 20 October), but International Launch Services (ILS)—a joint U.S.-Russian company, headquartered in Reston, Va., which operates all Proton-M missions out of Baikonur—announced Sunday a further delay of at least 24 hours. Citing “issues with the satellite ground station network required for the launch and early orbital operations of the Sirius FM-6 spacecraft,” ILS explained that “as a precautionary measure” additional time was needed to rectify the problem.

Launch was initially rescheduled for 12:12 a.m. local time Tuesday, 22 October (2:12 p.m. EDT Monday, 21 October)—a postponement of exactly 24 hours—and the countdown clock on ILS’s website reflected this revised liftoff time until well into Monday afternoon. However, several media sources, including NASASpaceflight.com and Spaceflight Now, subsequently indicated that the launch had been further delayed “at the request of the American side” and that a “new launch date will be determined by the State Commission.” In a brief notice released on its website Monday afternoon, ILS formally revealed that the flight would occur no sooner than 12:08 a.m. local time Saturday, 26 October (2:08 p.m. EDT Friday, 25 October).

All components of the Proton-M, including its three main stages and the restartable Briz-M upper stage, are fueled by a mixture of nitrogen tetroxide and unsymmetrical dimethyl hydrazine. Loading of these propellants aboard the Briz-M—whose single engine has the capability to restart on multiple occasions and can support up to eight discrete “burns” to inject payloads into orbit—took place on Tuesday, 15 October. The State Commission formally granted its approval for the rollout of the Proton-M vehicle to the pad on Wednesday, 16 October, and by the afternoon of Thursday, 17 October the vehicle was “hard down” at the launch complex. At the time of the decision to effect a delay on Sunday, 20 October, ILS announced that both vehicle and payload remained in a safe configuration on the pad and were experiencing no other technical difficulties.

Fueling of the three main stages of the Proton-M got underway about nine hours ahead of tonight’s liftoff. The rocket’s first stage consists of a central oxidizer tank, surrounded by six outboard fuel tanks, each fed by an RD-276 engine, and these provided the initial impetus to boost the mission toward space. At T-5 minutes, the Briz-M was transferred to internal power and confirmed its status as healthy. Ignition of the six RD-276 engines commenced at T-2 seconds, ramping up to 100 percent of rated performance to produce 2.3 million pounds (1.04 million kg) of thrust at the instant of liftoff.

Sirius FM-6 undergoes Payload Fairing (PLF) encapsulation work, ahead of integration with the Proton-M vehicle. Photo Credit: ILS
Sirius FM-6 undergoes Payload Fairing (PLF) encapsulation work, ahead of integration with the Proton-M vehicle. Photo Credit: ILS

A little over a minute into the ascent, the Proton encountered a period of maximum aerodynamic turbulence (known as “Max Q”) on its flight surfaces, and the first stage separation occurred, as planned, at the two-minute point. The vehicle’s second stage then picked up the baton, with its four engines generating a combined 540,000 pounds (244,950 kg) of propulsive yield for a little over 3.5 minutes. Shortly after the second stage separated from the Proton-M, the Payload Fairing (PLF) was jettisoned and the single-engine third stage added a further 138,000 pounds (62,600 kg) of thrust for the next four minutes of flight. Shutdown of the third stage took place about 10 minutes after launch, by which time the vehicle had been placed onto a suborbital trajectory.

The turn then came for the single-engine Briz-M upper stage, which has exhibited a mixture of success and failure since its maiden voyage back in May 2000. In August 2012, a premature shutdown of the Briz-M left Indonesia’s Telkom-3 and Russia’s Ekspress-MD2 satellites in useless orbits, and in December 2012 another launch glitch impacted the Yamal-402 satellite. Last March, a Proton-M/Briz-M successfully injected Mexico’s Satmex-8 communications satellite into orbit, and on 29 September another vehicle transported Astra 2E perfectly into orbit. Capable of restarting up to eight times in flight, the Briz-M has a battery-powered lifetime of 24 hours and is scheduled to execute five “burns” to insert Sirius FM-6 into orbit. At the time of writing, those burns were underway.

The first burn lasted 4.5 minutes and commenced about 11 minutes into the mission, after which the Briz-M/Sirius FM-6 combo entered a circular “parking orbit” and coasted for almost an hour, preparatory to the second burn. This will be a much longer firing of almost 18 minutes, followed by a two-hour period of coasting, then a third burn of 11.5 minutes and a fourth of about six minutes in duration. After the fourth burn, the combo will coast for about five hours, ahead of the final scheduled Briz-M firing at T+8 hours and 52 minutes. This six-minute burn will position Sirius FM-6 into geostationary transfer orbit, and at T+9 hours and 12 minutes the satellite will separate from the upper stage. Sirius FM-6 (Radiosat-6) will enter an operational orbit at 115 degrees West longitude, covering the western half of the contiguous United States.

The satellite is assembled around SS/L’s LS-1300 “bus,” which was originally introduced in the 1980s and in its current incarnation can provide 5-25 kW of continuous electrical power throughout a project 15-year orbital lifespan. Lightweight and high-strength in construction, the LS-1300 can support as few as 12 and as many as 150 transponders. After insertion into orbit, Sirius satellites receive “Radiosat” designations to differentiate from a Swedish network of communications satellites, which are also known as Sirius.

The Proton-M is the descendant of a vehicle which made its first flight in 1965. Photo Credit: Roscosmos
The Proton-M is the descendant of a vehicle which made its first flight in 1965. Photo Credit: Roscosmos

In June 2000, ILS successfully lofted the 8,400-pound (3,800-kg) Sirius FM-6 (Radiosat-1) atop a Proton-K booster from Baikonur, injecting it into a highly elliptical, high-inclination geosynchronous orbit, known as a “tundra orbit,” from where it provided satellite radio communications services across North America. The FM-2 (Radiosat-2) mission was launched in September 2000, followed by FM-3 (Radiosat-3) in November. All three were placed into tundra orbits, completing a circuit of Earth in about 23 hours and 56 minutes, or one solar “day,” whose characteristics cause the satellites to spend most of their time over a specific point on Earth, known as “apogee dwelling.” The effect was that the Sirius/Radiosat satellites follow a predictable pattern in the sky at the same time, day after day, and the shape of their trajectories described a closed figure-eight.

Tundra orbits—with an apogee of about 29,200 miles (46,990 km) and a perigee of about 14,900 miles (23,980 km)—provide high-latitude users with higher elevation angles than can be offered by a geostationary orbit. The three Sirius/Radiosats spend about 16 hours of each solar day over the continental United States, with at least one of them over the country at all times. Sirius FM-4 (Radiosat-4) was built as a flight-ready spare and in October 2012 was donated to the National Air and Space Museum for display in the James S. McDonnell Space Hangar of the Steven F. Udvar-Hazy Center in Washington, D.C., to “tell the story of modern satellite communications and its powerful impact on everyday life.”

An uprated satellite, Sirius FM-5 (Radiosat-5), was launched in June 2009. Unlike its predecessors, which occupied tundra orbits, the 12,830-pound (5,800-kg) Sirius FM-5 was the first to be placed into geostationary orbit. Whereas its predecessors carried traditional parabolic reflecting antennas, the new satellite was equipped with a 30-foot (9.1-meter) unfurlable reflector, and its geostationary location was reportedly chosen because it allowed for more consistent reception for fixed-location users. By the time Sirius FM-5 began operations in September 2009, contracts for Sirius FM-6 had already been signed.

Tonight’s spectacular launch marks the 390th mission by a member of the Proton rocket family, whose impressive heritage stretches back to the mid-1960s. Built by Russia’s Khrunichev Research and State Production Centre, the vehicle has one of the most reliable track records of any launch vehicle in the world.

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