SpaceX’s 27th launch of the year was set for Wednesday, April 26, 2023, lofting another batch of 46 Starlink satellites into low Earth orbit. However, a scrub was called with just seconds to go, based on a high probability of a landing failure – likely related to weather conditions downrange in the recovery zone. The liftoff will now be scheduled to occur at 6:40 AM PDT (13:40 UTC) on Thursday from Space Launch Complex 4 East (SLC-4E) at Vandenberg Space Force Base in California. The flight was originally scheduled on Tuesday but was delayed before prop loading was set to take place on Tuesday morning.
The mission, Starlink Group 3-5, marks the resumption of Starlink Group 3 flights to sun-synchronous orbit, which started in the summer of 2022 and were paused after only four flights. This shell provides coverage to the polar regions. This launch also marks SpaceX’s 40th flight from SLC-4E.
The booster, B1061, will be flying for a 13th time, becoming the fifth booster to fly that many times. Its landing is set to occur on the deck of SpaceX’s Autonomous Spaceport Drone Ship Of Course I Still Love You (OCISLY), which will be stationed 634 km downrange in the Pacific Ocean.
Falcon 9 will begin its pre-launch propellant load sequence at the T-35 minute mark when the automated launch countdown sequence will start.
The launch autosequence is in charge of starting the load of RP-1 — a refined form of kerosene — on both stages, as well as starting the load of liquid oxygen (LOX) onto the first stage.
RP-1 load on the second stage will wrap up at around T-20 minutes, after which the ground systems will begin the purge and chill down of the liquid oxygen lines that connect to the second stage.
This is performed in order to properly cool the lines down to the right temperatures before liquid oxygen flows through them at the rates needed to fill the upper stage LOX tank. This creates the now famous “T-20 minute vent.” This vent will stop around four minutes later, signaling the start of LOX load onto the second stage.
Seven minutes before liftoff, the ground systems will initiate chill down of the nine Merlin 1D (M1D) engines on the first stage. Just like the chill down of the ground lines, this is also performed in order to thermally condition the oxygen turbopump before liquid oxygen flows at high rates during ignition.
About a minute later, RP-1 load will end on the first stage and the tanks will pressurize for the retraction of the strongback. The strongback is the vertical support structure of the transporter-erector (T/E) system that provides fluids and power to the second stage and purges the interstage and fairing of Falcon 9.
At SLC-4E, this structure retracts about 20 degrees away from vertical and stays that way through liftoff, instead of retracting 1.8 degrees and performing the “throwback” maneuver to 45 degrees at liftoff — the procedure done at Falcon 9’s Florida launch pads. This is a carryover from the old Falcon 9 launch pad design that was in use from 2010 to 2017 but still remains in use at Vandenberg to this day.
Liquid oxygen load on the first and second stages will wrap up at about T-3 and T-2 minutes, respectively. One minute prior to liftoff, Falcon 9’s onboard computers will take over control of the automated launch countdown and begin pre-flight checks.
Three seconds before liftoff, the engine computers initiate the engine ignition sequence of all nine M1D engines on the first stage. This ignition sequence is staggered and takes about two seconds to complete with the last second in the count used to perform engine health verifications prior to liftoff.
Targeting Tuesday, April 25 at 6:40 a.m. PT for a Falcon 9 launch of 46 Starlink satellites to low-Earth orbit from SLC-4E in California → https://t.co/bJFjLCiTbK
— SpaceX (@SpaceX) April 24, 2023
After liftoff, Falcon 9 will pitch downrange to the southwest from SLC-4E and should approach supersonic speeds at about 50 seconds into flight.
The two-and-a-half minute initial ascent will take the rocket and satellites up to around 65 km altitude and 2.3 km/s velocity before shutting down the engines and separating the first and second stages.
After separation, the first stage will orient itself into an engines-first attitude for entry back into the atmosphere. It’ll perform a three-engine entry burn to protect the rocket and engines from the heat of entry, and then — closer to the ground — it’ll perform a single-engine landing burn.
Once separated from the first stage, the second stage will ignite its single Merlin 1D Vacuum (MVacD) engine for about six minutes to insert the Starlink satellites into a low parking orbit. About 10 seconds after MVacD ignition, the fairing halves will separate for a parachute-assisted splashdown in the Pacific Ocean. These will then be retrieved by SpaceX’s west coast fairing recovery vessel NRC Quest.
About 45 minutes after orbital insertion, the MVacD engine will relight for circularization of the orbit and will prepare for satellite deployment. The upper stage will rotate end over end and then release four tension rods that hold the Starlink satellites together during launch.
Due to inertia, the satellites will fly away as they are released, while the second stage will stop its spin. The stage will then perform a third and final burn of its MVacD engine for deorbit and disposal over the southwest Pacific Ocean.
This launch will be carrying 46 Starlink v1.5 satellites into the Group 3 shell of Starlink’s first generation constellation. Satellites in this shell operate in a roughly circular orbit at 560 km in altitude and at 97.6 degrees orbital inclination.
|Starlink Gen 1||Starlink Gen 2|
|Missions||v1.0||Group 2||Group 3||Group 4||Group 5||Group 6|
|Orbit||550 km at 53º||570 km at 70º||560 km at 97.6º||540 km at 53.2º||530 km at 43º|
|Satellites in operational orbit||1426||82||187||1474||183||0|
This batch of satellites will bring the total number of Starlink satellites launched to 4,284, with 3,352 of them in operational orbits and 325 having reentered. Of these, 3,912 have been launched in support of Starlink’s first generation constellation and 372 have been launched in support of Starlink’s second generation constellation.
Starlink Group 3-5 will be SpaceX’s eighth launch of 2023 from SLC-4E, keeping it on track to launch about 25 times this year, five launches short of SpaceX’s goal of 30 launches from its Vandenberg launch site.
SpaceX is readying for a doubleheader later this week from Florida, featuring the launch of the sixth Falcon Heavy rocket putting the Viasat-3 Americas satellite into a near geostationary orbit. The other launch of the doubleheader will feature a Falcon 9 rocket lofting two SES internet satellites for the O3b mPOWER constellation.
(Lead image: Falcon 9 at SLC-4E ahead of the SDA Tranche 0 Flight 1 mission. Credit: Jack Beyer for NSF)