Only days after the spectacular 29 September return to flight of the troubled Proton-M booster, International Launch Services (ILS) is gearing up for its next mission in two weeks’ time. The three-stage Proton and its Briz-M upper stage is reportedly scheduled to liftoff from Pad 39 at Site 200 at the Baikonur Cosmodrome in Kazakhstan at 12:13 a.m. local time on 21 October (2:13 p.m. EDT on the 20th), carrying the Sirius FM-6 digital radio satellite into orbit. Already delayed since early 2012, due to technical difficulties with the satellite, and again following the dramatic failure of a Proton-M shortly after launch in July 2013, Sirius FM-6 was delivered to Baikonur on 6 September and is presently undergoing pre-flight checkout, ahead of integration with the launch vehicle. The mission will see the restartable Briz-M perform five “burns” to inject the payload into a 22,300-mile geostationary transfer orbit.
Built by Space Systems/Loral (SS/L) in Palo Alto, Calif., Sirius FM-6 is assembled around the LS-1300 satellite “bus.” Originally introduced in the 1980s, the current configuration of the craft is lightweight and high-strength in construction and can provide 5-25 kW of continuous electrical power throughout a projected 15-year orbital lifetime. It can support as few as 12 and as many as 150 transponders. After launch, the Sirius satellites receive “Radiosat” designations, to differentiate from a Swedish network of telecommunications satellites, also known as Sirius.
In June 2000, ILS successfully lofted the 8,400-pound 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 and a perigee of about 14,900 miles—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 Sirius FM-5 was the first to be placed into geostationary orbit. Unlike its predecessors, which carried traditional parabolic reflecting antennas, the new satellite was equipped with a 30-foot-diameter unfurlable reflector, and its geostationary location was reportedly chosen because it allowed for more consistent reception for fixed-location users. By the time that Sirius FM-5 began operations in September 2009, contracts for the construction of Sirius FM-6 had already been signed.
Originally scheduled for launch in early March 2012, the satellite has spent almost two years on the ground. According to a Space News article, concerns were raised about a possible solar array defect, following a short-lived problem experienced with the deployment mechanism aboard the SES-4 satellite. Since Sirius FM-6 was of similar design, it was considered prudent to return it to SS/L’s facility for additional work and launch was rescheduled for the summer of 2013. The loss of a Proton-M a few seconds after liftoff on 2 July placed Sirius FM-6 on hold yet again, but preparatory work picked up in early September when the satellite arrived at Baikonur.
The launch on 20/21 October will be conducted under the auspices of International Launch Services (ILS), a joint U.S.-Russian company, headquartered in Reston, Va., with exclusive rights to all Proton flights from Baikonur. Flown 71 times since April 2001—with six failures and two partial failures—the current Proton-M variant of the rocket comprises three main stages, and on the Sirius FM-6 launch it will also utilize the restartable Briz-M upper stage. The vehicle is built by the Khrunichev Research and State Production Centre, and its heritage extends back to the mid-1960s, with a cumulative launch history of almost 400 flights and one of the most reliable track records in the world. All of its stages, including the Briz-M, utilize nitrogen tetroxide and unsymmetrical dimethyl hydrazine propellants. Its first stage consists of a central oxidizer tank, surrounded by six outboard fuel tanks, each fed by an RD-276 engine, and these will provide the initial impetus to boost Sirius FM-6 toward orbit.
Fueling of the main three stages is expected to begin about nine hours ahead of liftoff. Ignition of the six RD-276 engines will commence at T-2 seconds, ramping up to 100 percent to produce 2.3 million pounds of thrust at liftoff. A little over a minute into the ascent, the Proton will encounter the period of maximum aerodynamic turbulence (known as “Max Q”) on its flight surfaces, with the separation of the first stage scheduled at the two-minute point. The second stage will then pick up the baton, with its four engines generating a combined 540,000 pounds of impulse for a little over 3.5 minutes. Shortly after the second stage separates from the vehicle, the Payload Fairing (PLF) will be jettisoned and the single-engine third stage will add an extra 138,000 pounds of thrust for the next four minutes of flight. Shutdown of the third stage is expected about 10 minutes after launch, by which time the vehicle will have been placed onto a suborbital trajectory.
The turn will then come 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 Astra 2E into orbit.
Its first burn will last 4.5 minutes and is due to commence about 11 minutes into the mission, after which the Briz-M/Astra 2E combo will enter a circular “parking orbit” and coast 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.