Fourth shell of Starlink constellation reaches 1,000 satellites

SpaceX’s 31st flight of the year has launched another batch of Starlink satellites as part of the Starlink Group 4-22 mission onboard a Falcon 9 rocket. Liftoff occurred on Sunday, July 17 at 10:20 AM EDT (14:20 UTC) from Space Launch Complex 40 (SLC-40) at the Cape Canaveral Space Force Station in Florida.

Sunday’s launch saw another Falcon 9 booster reaching 13 flights, the highest number of flights currently in their fleet. Booster B1051 first flew supporting the Demo-1 mission back on March 2, 2019, and last flew on the Starlink Group 4-12 mission back on March 19, 2022.

This launch also involved the 1,000th satellite launched to the fourth shell of SpaceX’s Starlink constellation, approaching two-thirds of its completion.

The weather forecast released by Space Launch Delta 45 predicted only a 50% chance of favorable conditions for launch on Sunday. The main concerns were the violation of the thick cloud layer rule, cumulus cloud rule, and anvil cloud rules. These are related to the potential for the rocket to trigger lightning as it ascends through the lower parts of the atmosphere.

Falcon 9 rolls out to SLC-40 ahead of the first GPS-III mission. Credit: SpaceX/US Air Force

The forecast improved greatly for a backup launch opportunity on Monday with an 80% chance of favorable conditions – the only concern being a violation of the cumulus cloud rule. The weather forecast showed a low risk of other factors like upper-level wind shear, solar activity, or booster recovery weather to be an impediment for launch on either primary or backup launch opportunities.

Falcon 9 began its usual automated fueling sequence at the T-35-minute mark as the vehicle began to load RP-1 (a highly refined form of kerosene) on both stages and liquid oxygen (LOX) on the first stage.

RP-1 load on the second stage finished about 15 minutes later, followed by the now-famous “T-20 minute vent.” This vent is part of the purging procedures of the strongback ahead of LOX load on the second stage at the T-16 minute mark. 

Seven minutes before liftoff, the chilling sequence on the nine first stage main engines began, and a minute later, RP-1 load on the first stage was completed. Strongback retraction followed four minutes before launch, and at around T-three and two minutes, the LOX load sequence finished on the first and second stages, respectively. 

One minute before launch, the rocket’s onboard computers took control of the automated launch countdown sequence. The computers commanded ignition of the nine Merlin 1D engines on the first stage at the T-3 second mark and these ramped up to full power over the next 2.8 seconds. At T-0, the four hold-down clamps on the launch mount released the rocket.

The Falcon 9’s flight profile involved a two-minute and 28-second powered flight using the nine first-stage engines. After this, they shut down and the first and second stages separated.

Falcon 9 launches the Starlink v1.0 L14 mission in October 2020. (Credit: Stephen Marr for NSF/L2).

After separation, the Merlin 1D Vacuum (MVacD) engine on the second stage ignited for the six-minute ride to the 232-by-338 kilometer 53.22-degree inclination target orbit. 

Meanwhile, the first stage maneuvered engines first and executed its traditional entry and landing burns. Landing occurred on SpaceX’s Autonomous Spaceport Drone Ship Just Read The Instructions, which was located about 650 km downrange in the Atlantic Ocean.

The fairing halves separated from the second stage about 10 seconds after the first MVacD ignition. These will come down to a parachute-assisted splashdown in the Atlantic Ocean and be recovered by SpaceX’s multi-purpose recovery vessel Bob. 

After orbital insertion, the second stage initiated an end-over-end rotation maneuver to start the Starlink deployment sequence. The satellites separated about 15 minutes into the flight, after which they will deploy their solar panels and raise their orbits for checkouts and screening. If the satellites pass the checks, they will be allowed to raise their orbits to operational altitude. 

The second stage will execute a second and final burn of the MVacD engine after satellite separation to deorbit. It will burn up over the Indian Ocean.

Rendering of Starlink satellites deploying from the second stage. (Credit: Mack Crawford for NSF)

The Starlink constellation gained another 53 satellites after Sunday’s launch. A total of 2,805 satellites have been launched – of which 254 have reentered already. Of the 2,551 satellites currently in orbit, 44 of them have failed in orbit or are unresponsive and 2,052 are in operational orbits. 

Shell #1 Shell #2 Shell #3 Shell #4 Shell #5
Orbit 550km circular at 53º 570km circular at 70º 560km circular at 97.6º 540km circular at 53.2º 560km circular at 97.6º
# of orbital planes 72 36 6 72 4
Satellites per plane (target) 22 20 58 22 43
Total satellites (target) 1,584 720 348 1,584 172
Satellites launched 1,665 51 46 968 0
Satellites in operational orbit 1,464 19 0 569 0

(Status of Starlink constellation information from Jonathan McDowell as of July 14th)

Sunday’s launch marks SpaceX’s fourth launch of the month and the 31st of the year, matching their record for most launches in a given year – which was set in 2021. 

The company has at least two more launches scheduled for the rest of the month – both Starlink missions. The first will be Starlink Group 3-2 next Thursday from Vandenberg, while the second – Starlink Group 4-25 – is currently scheduled to lift off from Launch Complex 39A no earlier than July 24. 

(Lead image: Falcon 9 B1051 on SLC-40 ahead of the Starlink v1.0 L27 mission in May 2021. Credit: Stephen Marr for NSF)

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