Soyuz launches seventh OneWeb cluster
A Soyuz-2-1b/Fregat rocket successfully delivered a fresh batch of 36 satellites for the UK-based OneWeb company after lifting off on May 28, 2021, from Vostochny Cosmodrome. Delayed for 24 hours by a technical problem, it was the third launch of the year into the OneWeb constellation, boosting it from 182 to 218 spacecraft.
The seventh Soyuz mission with OneWeb satellites at a glance:
Preparing the seventh OneWeb mission
The seventh OneWeb launch had the designation ST32 in the naming system used by Arianespace and its affiliate Starsem, which managed the campaign.
On April 29, 2021, Roskosmos said that the Fregat upper stage for the May mission had been undergoing propellant loading operations at the fueling station. On May 6, Roskosmos announced that specialists at Vostochny branch of the TsENKI infrastructure center installed the integrated stack of the first and second stages of the Soyuz-2-1b rocket on its testing work site inside the vehicle processing building, MIK RN. On the same day, the two parts of the payload fairing for the mission were transferred from the MIK RN building to the spacecraft processing building, MIK KA, in preparation for the encapsulation of the payload section.
On May 7, Roskosmos announced that an An-124-100 transport plane with the fresh batch of OneWeb satellites had arrived at the Ignatievo airport near Vostochny.
The specialists then performed electrical and pneumatic tests of the rocket, while the fueling operations with the Fregat stage were to be completed on May 13, 2021. The following day, Roskosmos confirmed that the fueling had been completed and that Fregat had been transferred to the spacecraft processing building for the assembly of the payload section, which was finished by May 18 with the encapsulation of the satellites under their payload fairing. On May 20, the payload section was transferred to the vehicle assembly building, MIK RN, where it was integrated with the third stage of the Soyuz-2 launch vehicle. In parallel, the lower composite of the rocket, consisting of the first and second stages, was lifted from its work site and loaded on the mobile transporter-erector in preparation for the final assembly of the vehicle, which was completed on May 21. The State Commission overseeing the preparations met the following day and cleared the vehicle for the rollout to the pad which took place on the morning of May 24, local time.
In the course of the day on May 24, 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 May 25, at 08:00 local time (02:00 Moscow Time), launch pad personnel resumed checks of the rocket and of the spacecraft batteries, as well as conducting 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 May 26, starting at 09:00 local time (03:00 Moscow) launch personnel re-confirmed the battery charge on the satellites and conducted thermal conditioning of the payload section. The flushing of the fueling system with high-concentration hydrogen peroxide was scheduled to start at 16:15 Moscow Time. It was followed by the attachment of fueling lines to the rocket and final operations.
The State Commission was scheduled to convene for the final time before launch on the evening of May 27 to clear the rocket for fueling and liftoff.
On May 27, Roskosmos confirmed that the rocket was cleared for fueling according to the following schedule:
The fueling was to be followed by final operations: the automated pre-launch process, withdrawl of the service platform and the mobile service tower and the setup of the payload separation system, PRSS, in flight mode.
Less than an hour before the scheduled liftoff on May 27, 2021, at 20:43:29 Moscow Time, Roskosmos announced that the mission had been postponed for a backup window. According to the State Corporation, the rocket remained in launch readiness and the time of the new launch attempt would be announced separately.
Arianespace then said that due to the replacement of one item of electrical equipment on the rocket, the launch had to be postponed for 24 hours. Roskosmos the confirmed that the new attempt had been scheduled for 20:38:39 Moscow Time (1:38 p.m. EDT) on May 28 and said that cryogenic oxidizer would be drained from the rocket during the delay.
According to one source, the technical problem which forced the delay was similar to a failure of the inertial measurement unit during the Sentinel-1B launch attempt on April 24, 2016. According to another source, a three-axis gyro stabilization instrument was replaced ahead of the new launch attempt on May 28. The meeting of the State Commission on May 28 then cleared the vehicle for flight.
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 May 28, 2021, at 20:38:39.549 Moscow Time (17:38 UTC, 1:38 p.m. EDT on May 28). It was 02:38 local time on May 28 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 were 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 was taking place out of contact with mission control. The restart of the Fregat's engine for a 31-second firing then happened near the apogee of the transfer orbit making it circular at an altitude of around 450 kilometers.
The first quartet of OneWeb satellites was 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 had 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.
Timeline of the OneWeb mission on May 28, 2021:
The official logo of the seventh OneWeb mission, ST32, dedicated to the constellation's production in Florida. Credit: OneWeb
Payload arrangement for the OneWeb launch on the Soyuz rocket with 36 satellites. The satellites are attached to a 5.5-meter-tall, 1.7-meter-in-diameter adapter built by RUAG Space. Credit: Arianespace
In early May, integrated booster stages of the Soyuz rocket for the seventh OneWeb mission were placed on the work site for pneumatic and electric tests. Click to enlarge. Credit: Roskosmos
Click to enlarge. Credit: Arianespace
Click to enlarge. Credit: Arianespace
Click to enlarge. Credit: Arianespace
Close-up view of OneWeb satellites being prepared for the seventh launch. Click to enlarge. Credit: Roskosmos
Payload section with 36 OneWeb satellites and the Fregat upper stage was integrated with the payload fairing by May 18, 2021. Click to enlarge. Credit: Roskosmos
Soyuz rocket for the 7th OneWeb mission rolls out from the vehicle assembly building in Vostochny on May 24, 2021. Click to enlarge. Credit: Roskosmos
Four boosters of the first stage separate from the Soyuz rocket on May 28, 2021. Credit: Roskosmos
Fregat upper stage separates from the third stage of the launch vehicle during the seventh OneWeb mission. Click to enlarge. Credit: Roskosmos