Soyuz launches sixth OneWeb cluster
The deployment of the OneWeb Internet constellation continued from Vostochny Cosmodrome with the second launch in 2021. A Soyuz-2-1b rocket carrying a fresh batch of 36 satellites for the UK-based company lifted off on April 26. The mission increased the number of spacecraft in the network from 146 to 182.
The sixth Soyuz mission with OneWeb satellites at a glance:
Preparing the sixth OneWeb mission
The sixth OneWeb launch had the designation ST31 in the naming system used by Arianespace and its affiliate Starsem, which managed the campaign.
On March 29, 2021, a booster cluster consisting of the four modules of the first stage and the second stage of the Soyuz-2-1b rocket for the mission was moved from storage to a work site for processing and testing inside the launch vehicle processing building in Vostochny. In parallel, the specialists began working with the segments of the payload fairing for the same vehicle, Roskosmos said on March 30. According to the State Corporation, the Fregat stage for the mission was at the fueling station being loaded with propellant components and pressurized gases, a process scheduled to continue until April 11, 2021.
The transport aircraft carrying the 36 satellites for the mission departed Florida, where OneWeb assembles its spacecraft, on April 3, 2021. After landing at the Ignatievo airfield near Blagoveshensk and a ground trip to Vostochny, the processing of the satellites for launch officially started on April 6, 2021.
On April 14, Roskosmos announced that the fueling operations with the Fregat stage for the mission had been completed and that it had been transferred from the ZNS fueling station to the spacecraft processing building. On the same day, OneWeb announced that all 36 satellites for the mission had been integrated with their payload dispenser.
The final assembly of the launch vehicle was was completed on April 20 and the next day, the rocket was cleared for rollout to the launch pad, which started at 07:00 local time (01:00 Moscow Time) on April 22, 2021.
Booster stages of the Soyuz rocket for the sixth OneWeb mission are being transferred to a test worksite on March 29, 2021.
Operations on the pad
In the course of the day on April 22, the rocket was installed on the pad, the mobile service tower was moved into position around the vehicle and it was connected to various systems of ground equipment. The autonomous checks of the rocket's systems were also scheduled to begin before the end of the day.
On April 23, at 08:00 local time (02:00 Moscow Time), launch pad personnel resumed checks of the rocket and the spacecraft batteries, as well as conducted the launch readiness simulation of the Fregat upper stage. In the second half of the day, the rocket's systems were taken through the simulation of the initial active phase of the orbital insertion up to the moment of separation between the payload section and the third stage. All the telemetry received during the tests was then analyzed.
On April 24, launch personnel re-confirmed the battery charge on the satellites and conducted thermal conditioning of the payload section.
On April 25, the launch personnel started operations on the pad at 08:50 local (02:50 Moscow Time). At 03:00 Moscow Time, the cooling of the fuel started, followed by the conditioning of the hydrogen peroxide system. Another spacecraft battery charge check took place at 04:00 Moscow Time, as well as the purging of the fueling system with hydrogen peroxide. Next, the propellant loading lines were connected to the rocket and final operations were conducted on the launch pad. The State Commission was scheduled to convene before the end of the day on April 25, to clear the rocket for fueling and liftoff.
Planned countdown milestones for a typical OneWeb mission, according to Arianespace:
Launch profile of the OneWeb mission originating from Vostochny
Approximate ground track of the OneWeb mission.
A Soyuz-2-1b rocket carrying a cluster of 36 OneWeb satellites lifted off from 1S pad in Vostochny on April 26, 2021, at 01:14:08.194 Moscow Time (22:14 UTC, 6:14 p.m. EDT on April 25). It was 07:14 local time in Vostochny.
The ascent profile of the mission had a timeline and flight parameters similar to those employed in the previous OneWeb launches from Vostochny. After a few seconds of vertical ascent, the launch vehicle headed northward to align its ascent trajectory with a near-polar orbit inclined 87.4 degrees toward the plane of the Equator. The particular ground track employed during the ascent to orbit from Vostochny had so far been unique to OneWeb missions.
The four boosters of the first stage separated 1 minute and 58 seconds after liftoff (L+117.8 sec.), but the core booster of the second stage continued firing until 4 minutes and 48 seconds into the flight (L+287.5 sec.). In the midst of its operation, the payload fairing protecting the payload in the dense atmosphere split into two halves and dropped off at T+3 minutes 35 seconds (L+214.8 sec.). Immediately after the separation of the second stage, the aft section of the third stage split into three fragments and separated at L+292.1 seconds.
The fragments of the rocket are expected to fall at Drop Zones No. 873 and 875 in the Aldan and Kobyask Districts of the Sakha (Yakut) Republic in the Russian Far East.
The third stage continued firing until 9 minutes and 22 seconds into the flight, releasing the Fregat upper stage and its cargo on a ballistic trajectory with an apogee of 249 kilometers, just short of orbital velocity at L+561.8 seconds. This allowed the third stage to reenter and fall back to the ground at a predicted remote area of the ocean instead of reaching orbit.
Upper stage maneuvers
One minute after separation from the third stage, Fregat fired its main engine for 307 seconds to enter an elliptical (egg-shaped) 150 by 427-kilometer transfer orbit with the highest point (apogee) near the target altitude for the release of OneWeb satellites.
After its first maneuver, Fregat climbed passively for nearly an hour. Soon after an orbital insertion, the space tug and its passengers left the communications range of ground stations for 1 hours 18 minutes and 42 seconds, therefore Fregat's second maneuver and the separation of the first batch of satellites took place out of contact with mission control. The restart of the Fregat's engine for a 31-second firing took place near the apogee of the transfer orbit making it circular at an altitude of around 450 kilometers.
The first quartet of OneWeb satellites was then released in opposite directions from their dispenser 1 hour 18 minutes and 20 seconds after launch. It was followed nearly 16 minutes later by a 15-second firing of the small attitude control thrusters, SOZ, aboard Fregat to get the vehicle in position for the second quartet drop around three minutes later.
Fregat was programmed to repeat its thruster firing and the four-satellite release routine seven more times, evenly distributing quartets of satellites along their orbit.
When the Fregat reentered the communications range, mission control confirmed that the 3rd, 4th, 5th and 6th batch of OneWeb satellites indeed separated from their carrier. Then, still in direct view of ground stations, the 7th quartet also disembarked from the space tug. However, the vehicle then again went out of communications range for another 1 hour 18 minutes and 55 seconds.
The final four of the 36 passengers separated from their space tug 3 hours 51 minutes and 40 seconds after their liftoff from Vostochny. But the empty Fregat reappeared in the view of ground stations later, making it possible to confirm the separation of the 8th and 9th OneWeb quartets.
Around an hour after the release of its final passengers, Fregat was programmed to initiate a braking maneuver with its main engine designed to push the stage on a disposal orbit, resulting in its quick destruction in the upper atmosphere nearly six hours after launch over a remote area of the Pacific Ocean. In total, Fregat was to perform 11 active maneuvers: three with its main engine and eight firings of the SOZ attitude control thrusters.
As in all previous missions, OneWeb satellites had to use their own electric propulsion systems to climb to an operational orbit of around 1,200 kilometers.
Wuthin 24 hours after the launch, the OneWeb company confirmed signal acquisition for all 36 satellites delivered into orbit during the 6th mission.
Timeline of the OneWeb mission on April 26, 2021:
The official logo of the sixth OneWeb mission dedicated to the 60th anniversary of Gagarin's mission. Credit: OneWeb
Payload arrangement for the OneWeb launch on the Soyuz rocket with 36 satellites. Credit: Arianespace
Sixth OneWeb cluster during assembly in Vostochny in mid-April 2021. Credit: Arianespace
Final assembly of the launch vehicle for the sixth OneWeb mission. Click to enlarge. Credit: Roskosmos
The launch vehicle for the 6th OneWeb mission arrives at the launch pad on April 22, 2021. Click to enlarge. Credit: Roskosmos
Soyuz lifts off with 36 OneWeb satellites on April 26, 2021. Click to enlarge. Credit: Roskosmos
Onboard view of the second stage separation during the launch on April 26, 2021. Credit: Roskosmos
Fregat upper stage separates from the third stage of the Soyuz launch vehicle on April 26, 2021. Credit: Roskosmos