SpaceX launches OneWeb 17 mission and returns booster to LZ-1

by Trevor Sesnic

SpaceX launched 40 high-speed internet communication satellites to a polar low Earth orbit on the OneWeb 17 mission on Thursday. Launching from Space Launch Complex 40 (SLC-40) at the Cape Canaveral Space Force Station in Florida, the Falcon 9 lifted off at 2:13 PM EST (19:13 UTC) and marked SpaceX’s 16th orbital launch of 2023–a launch every 4.25 days.

Following the launch, just under 90% (578 of the planned 648) of operational satellites have been launched. The satellites were placed into a ~580 by 600 km orbit, inclined at 86.51 degrees. The satellites will now spend the coming months raising their orbits to the operational 1,200 km circular polar orbit at 86.4 degrees.

On March 7, Space Launch Delta 45 released a launch mission execution forecast giving the weather a 5% chance of violating the weather constraints. The only concern was the cumulus cloud rule. In the event of a delay, a backup launch opportunity existed 24 hours later, with the probability of violating weather constraints increasing to 15%.

The Falcon 9 booster supporting this mission was B1062-13; as the name implies, the booster has supported 12 previous missions: seven Starlink missions, two GPS missions, Inspiration4, Axiom-1, and Nilesat-301.

Following the launch, the stage performed a return to launch site (RTLS) landing, landing on SpaceX’s Landing Zone 1 (LZ-1). This was SpaceX’s fifth RTLS landing this year, 177th booster landing, 103rd consecutive successful landing, and 27th landing on LZ-1. 

Once jettisoned from the second stage, the payload fairings softly splash down in the ocean under a parachute. SpaceX’s multi-purpose recovery vessel Bob will then recover them from the water.

OneWeb’s constellation is a satellite internet constellation with the goal of providing internet coverage to the entire globe. Once complete, the constellation will be made up of 36 satellites in each of the 18 orbital planes. 600 of these satellites are needed for global coverage, with an additional 48 on-orbit spares. Based on demand, the constellation could later be expanded to more than 900 satellites.

Each OneWeb satellite has a compact design and masses ~150 kg. They are equipped with a Ku-band antenna, which operates between 12 and 18 GHz, allowing for each satellite to have 8 gigabits per second of throughput. Initially, the satellites had a Russian Fakel SPT-50 ion thruster; however, due to the conflict in Ukraine, this was changed to the Busek BHT-350 Hall effect thrusters on the newer satellites.

The satellites have two solar panels for power and are built by Airbus Defence and Space; the first 10 were built in Toulouse, France, and the remainder have been made in OneWeb’s US-based factory.

Originally, OneWeb’s satellites were being launched by Arianespace and Roscosmos atop the Soyuz-2.1b/Fregat-M. In 2022, also due to the conflict in Ukraine, OneWeb moved these launches over to SpaceX’s Falcon 9 and ISRO’s GSLV Mk. III.

OneWeb satellites being prepared for integration ahead of the next ISRO OneWeb launch. (Credit: OneWeb)

The now routine Falcon 9 launch sequence started at T-38 minutes, where the launch director verified that all systems are go for propellant load. Fueling of both the first and second stages starts at T-35 minutes, alongside first stage liquid oxygen (LOX) load.

The Falcon 9 utilizes super-chilled LOX and sub-cooled rocket propellant-1 (RP-1). This increases the density of the propellants, increasing the amount that can be stored on the vehicle and the Merlin engine’s performance.

At T-20 minutes, fueling of the second stage was completed and the transporter erector began purging its lines, preparing the lines for stage two LOX load, which started at T-16 minutes. This purging is what creates the Falcon 9’s classic T-20-minute vent and is done to thermally condition and clean the lines ahead of LOX load.

At T-1 minute, Falcon 9 entered start-up and the tanks began to pressurize to flight pressures. 15 seconds later, the launch director verified that the vehicle was go for launch.

At T-3 seconds, the engine controller commanded engine ignition of all nine Merlin 1D engines on the first stage. This is done by flowing trimethylaluminum and triethyl-borane (TEA-TEB) through the engines. TEA-TEB is pyrophoric, meaning it ignites when in contact with oxygen.

Falcon 9’s nine Merlin 1D engines shortly after liftoff. (Credit: SpaceX)

Just before launch, the flight computer verified that all nine engines and the vehicle were operating nominally and commanded the launch clamps to release the vehicle.

The first stage burned for 137 seconds before shutting down and the stages separating. At this point, the second stage’s single Merlin vacuum engine ignited, and the first stage began a flip and ignited three engines for its boostback burn, which lasted 48 seconds.

Three and a half minutes into the flight, the second stage jettisoned the fairings. The first stage performed two more burns (the reentry and landing burns), before softly touching down on LZ-1 just under eight minutes after launch. Upon successful landing, the booster’s designation changed to B1062-14.

Second stage engine cutoff 1 (SECO-1) occurred at T+8:34 before the stage entered into a ~47-minute long coast phase. The second stage then performed a three-second long burn, with SECO-2 occurring at T+55:20, before starting to deploy OneWeb satellites two at a time at T+58:50. This process lasted a little under 37 minutes, with all satellites being deployed by T+1:35:18.

This launch marked SpaceX’s third launch of March, with CRS-27, SES-18 & SES-19, and Starlink missions scheduled for later in the month.

(Lead image: B1062 launches with OneWeb 17. Credit: NASASpaceflight)

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