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SpaceX to launch 2,500th Starlink satellite in third back-to-back mission

SpaceX is preparing for its third back-to-back Starlink mission in barely a fortnight Friday. Falcon 9 is due to lift off from Vandenberg Space Force Base at 3:07 PM PDT (22:07 UTC) to add another 53 satellites to SpaceX’s low Earth orbit communications constellation, bringing the total launched to over 2,500.

The Starlink Group 4-13 launch continues a run of Starlink launches that has seen three missions in just over 14 days, using all of SpaceX’s Falcon 9 launch pads at Cape Canaveral, the Kennedy Space Center, and now Vandenberg. Two further launches, using both of the East coast pads again, are also expected in the coming week.

Friday’s launch will take place from Space Launch Complex 4E (SLC-4E) at the west-coast launch site, targeting a 53.2-degree orbit in the fourth shell of the Starlink constellation.

An individual Starlink satellite masses a little under 300 kilograms, although SpaceX has not published a specific mass. They are typically launched in large batches – 53 are aboard Falcon 9 for the Group 4-13 launch – using a novel deployment mechanism that allows all spacecraft to be released from the rocket at once when they have reached their planned orbit.

As currently designed, the first-generation Starlink constellation will consist of 4,408 satellites. Before Friday’s launch, 2,494 satellites had been launched, however, this number includes over 240 that have already been decommissioned, failed after deployment, or rejected during on-orbit testing. The addition of another 53 satellites brings the total launched up to 2,547.

The first-generation Starlink constellation is divided into five shells, with most recent launches focusing on filling the fourth shell. This resides in a circular orbit at an altitude of 540 kilometers and an inclination of 53.2 degrees. The shell is divided into 72 planes with 22 satellites per plane, for a total of 1,584 satellites once all have been deployed and are operational.

A big factor in SpaceX’s ability to deploy the Starlink constellation so rapidly has been its Falcon 9 rocket. Not only does SpaceX have more control over launch manifests and operations by flying on its own rocket, but Falcon 9’s reusability also allows it to support a high flight rate. SpaceX has also been more willing to take risks with Starlink missions than they can with customer payloads, allowing them to push the envelope in terms of the number of times boosters can be re-flown and how quickly they can be turned around between launches.

Nowhere has this been clearer than with the two previous launches. The Starlink 4-16 mission, which lifted off from the Cape Canaveral Space Force Station on 29 April, set several new records for Falcon 9 boosters, most notably the fastest turnaround time for a booster: B1062 had previously supported the Axiom-1 launch earlier the same month. Starlink 4-17 launched from the Kennedy Space Center on 6 May, marking the twelfth flight for booster B1058 — bringing it level with B1051 and B1060 as SpaceX’s most-flown cores.

By rapidly and repeatedly turning around Falcon 9 boosters, SpaceX can maintain its impressive launch cadence, allowing Starlink launches to be slotted into the manifest where rockets are not required to carry out missions for SpaceX’s customers. Falcon’s payload fairing can also be recovered and re-used, although the rocket’s second stage is expendable and a new one must be manufactured for each flight.

B1063 waits to be unloaded from the drone ship after completing its previous mission. (Credit: Pauline Acalin for NSF)

Following a Falcon 9 launch, the Starlink satellites are deployed into an initial orbit below their operational altitude, ensuring any satellites that fail to activate will quickly re-enter the atmosphere. Functioning spacecraft will first raise themselves into a more stable orbit to undergo checkouts before taking up their stations in the operational constellation.

In total, Thursday’s launch will be the 153rd flight of the single-core Falcon 9 rocket, and the type’s 152nd orbital launch — a single suborbital flight having been conducted to test the Crew Dragon spacecraft’s launch abort capabilities. To date, Falcon 9 has only suffered one in-flight loss of mission, which occurred on the rocket’s 19th flight back in 2015 with the CRS-7 Dragon ISS resupply mission. A second rocket was lost, along with the Amos-6 satellite it was to have carried into orbit, during fueling for a static fire ground test in September 2016.

With a long run of successes since these two incidents, Falcon 9 has become a highly reliable workhorse serving the US and international space industry. SpaceX has also demonstrated impressive consistency in returning and recovering the first-stage boosters. Since achieving its first successful recovery in December 2015 – on Falcon 9’s return to flight after the CRS-7 launch failure – SpaceX has quickly turned this once-experimental aspect of its missions into something that almost looks routine.

OCISLY with a recovered booster (B1058 following the DM-2 mission) while operating out of Port Canaveral in 2020 (Credit: SpaceX)

With Friday’s launch, SpaceX will be hoping to add to a run of 44 consecutive successful booster recoveries since B1049 was lost during a Starlink mission early last year. Launches of “flight-proven” Falcon 9’s have gone from being a noteworthy occurrence to the norm — indeed a completely new Falcon 9 is now a very rare sight. Despite the long string of successes and the importance of booster reuse to maintaining SpaceX’s launch cadence, recovering the booster remains secondary to the rocket’s primary task of delivering its payload into orbit.

The booster that will be used for Friday’s Starlink mission is B1063-5. A veteran of four previous missions, B1063 first flew in November 2020 to deploy the Sentinel-6 Michael Freilich satellite for NASA, the National Oceanic and Atmospheric Administration (NOAA), and the European Space Agency (ESA). This launch took place from Vandenberg and included a return-to-launch-site landing at Landing Zone 4 (LZ-4), located near the launch pad.

For its second launch, B1063 was transferred to the East coast, where it flew with sixty Starlink satellites in May 2021 and landed aboard the Autonomous Spaceport Drone Ship (ASDS) Just Read the Instructions in the Atlantic Ocean. Returning to Vandenberg, the booster was next used to launch NASA’s Double Asteroid Redirection Test (DART) mission in November 2021 and then the Starlink 4-11 mission with fifty satellites in February 2022.

Both the DART and Starlink 4-11 launches saw B1063 recovered downrange using the West-coast drone ship, Of Course I Still Love You (OCISLY). Friday’s launch will again be targeting a landing aboard OCISLY, which is stationed in the Pacific Ocean to receive the stage. Landing aboard the drone ship instead of returning to the launch site eliminates the need for the rocket to make a boostback burn, saving fuel and therefore performance that can instead be contributed to the primary mission.

B1063 lifts off on its prevision mission, Starlink 4-11, in February 2022. (Credit: SpaceX)

West coast Falcon 9 launches take place from Space Launch Complex 4E (SLC-4E) at the Vandenberg Space Force Base. Used by Titan rockets up to 2005, the complex was completely renovated by SpaceX ahead of the first Falcon launch from Vandenberg, which was the sixth flight of the Falcon 9 overall in September 2013. SpaceX also secured the lease to SLC-4E’s sister pad, SLC-4W, which now serves as Landing Zone 4 for boosters returning to the launch site.

Following integration in SpaceX’s hangar close to the launch pad, Falcon 9 was moved into position and raised to the vertical using the transporter-erector (T/E), also known as the Strongback. The Strongback supports the rocket while it is at the pad and houses umbilical connections to the upper stage and payload fairing. During the late stages of the countdown, it rotates away from the rocket to its launch position.

Before the strongback is retracted, however, the rocket must be fueled. This process begins about 38 minutes before launch when the Launch Director gives the go-ahead for propellant loading to commence. At T-35 minutes, RP-1 propellant – rocket-grade kerosene – begins flowing into tanks on both stages of the rocket, while loading of liquid oxygen (LOX) into the first stage tanks also begins. Second stage LOX loading does not start until later in the countdown, at the T-16-minute mark.

The supercooled liquid oxygen used by Falcon 9 boils off from the oxidizer tanks as the countdown proceeds, and must continually be topped up until loading terminates and the tanks are pressurized with about one minute to go until liftoff. The final minute of the countdown will also see the onboard computer perform final pre-launch checks on the vehicle and the Launch Director give a final “go” for launch.

B1063 lifts off with NASA’s DART mission in 2021 (Credit: NASA/Bill Ingalls)

At T-3 seconds, the ignition command is sent to the nine Merlin-1D engines that power the first stage. These ignite before the rocket lifts off at T-0 and begins to climb skyward. Pitching downrange, Falcon 9 maneuvers to its proper trajectory, offshore but following the coast as it heads southeast from Vandenberg. Falcon passes through the area of maximum dynamic pressure, or Max-Q, about 72 seconds after launch.

The first stage, B1063-5, will power Falcon 9 for the first two and a half minutes of flight before shutting down its engines, an event known as main engine cutoff, or MECO. Four seconds after MECO, the first and second stages will separate, with second stage ignition expected about six seconds later.

Falcon’s second stage is powered by a single Merlin Vacuum (MVac) engine, a version of the Merlin-1D optimized to operate most efficiently in the vacuum of space. This will make two burns during Friday’s mission to deliver the Starlink satellites into their deployment orbit, with an additional deorbit burn after spacecraft separation to ensure the safe disposal of the upper stage.

B1063 comes in to land during the Sentinel-6 mission (Credit: SpaceX)

The first burn of the second stage will last six minutes and six seconds. About five seconds into the burn, Falcon’s payload fairing that protects the Starlink satellites during ascent will separate from the nose of the rocket and fall back to Earth. The end of the upper stage burn, second stage engine cutoff 1 (SECO-1), will mark the start of a coast phase for Friday’s mission.

The second stage will coast for 44 minutes and 54 seconds before restarting for a brief second burn, lasting just one second, to circularize its orbit. Nine minutes and one second after the end of this burn – SECO-2 – the Starlink satellites will be deployed from the upper stage.

While the second stage carries out Falcon’s primary mission of delivering the Starlink satellites into orbit, B1063-5 will be returning to Earth. Shortly after separating it will carry out a flip maneuver to point its engines in the direction of flight. After deploying its grid fins to help control the stage during descent, it will pass back into the atmosphere, firing three engines in an entry burn to help slow and protect the booster as it re-enters.

As it approaches the drone ship, Of Course I Still Love You, B1063-5 will restart its center engine and deploy its landing legs. All being well, the booster will touch down on the deck of OCISLY about eight minutes and 33 seconds after liftoff. The drone ship will then bring the booster back to the Port of Long Beach to begin preparations for its next mission.

The rapid pace of SpaceX’s Starlink launches shows no sign of letting up. On Saturday, just 22 and a half hours after the Group 4-13 launch, another Falcon 9 will lift off from the Cape Canaveral Space Force Station with Group 4-15. Starlink Group 4-18 is due to lift off from the Kennedy Space Center next Wednesday.

The next Falcon 9 launch from Vandenberg is currently expected to be that of the German SARah-1 radar-imaging satellite, slated for no earlier than June.

(Lead image: A Falcon 9 sits at SLC-4E before the RADARSAT mission in June 2019. Credit: Jack Beyer for NSF)

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