SpaceX launches 2nd mission in three days with SiriusXM-8

SpaceX launched its 18th mission of 2021 with the SiriusXM-8 (SXM-8) high-power broadcasting satellite.  A Falcon 9 rocket lofted the SXM-8 satellite to a sub-synchronous geostationary transfer orbit (GTO) after launching from Space Launch Complex 40 (SLC-40) on Sunday, June 6 at 00:26 EDT/04:26 UTC. 

The 45th Weather Squadron at Space Launch Delta 45 predicted a 70% chance of favorable weather with Debris Cloud and Anvil Cloud rules as the primary concerns.


SXM-8 is the second of two originally planned satellites to replace the aging XM-3 and XM-4 satellites launched in 2005 and 2006, respectively, by Zenit 3(SL) rockets.

SXM-8 is owned and operated by SiriusXM as a part of their high-power broadcasting satellite constellation.  The company ordered SXM-8 alongside its twin, SXM-7, in July 2016 to be built by Space System/Loral (SSL), now Maxar Technologies.  The two satellites are based on the Maxar 1300 satellite bus.

SXM-7 was launched on Falcon 9 booster B1051-7 in December 2020 and successfully reached geostationary orbit (GEO) and began in-orbit testing; however, six weeks after its launch, an anomaly occurred with its primary payload during its standard checkouts.

SiriusXM and Maxar attempted to recover the satellite; however, this was unsuccessful and the satellite was considered a total loss. 

While the satellite bus was understood to still be operating normally, losing the payload made the satellite useless.  Soon after, SXM-7 was likely placed into a graveyard orbit while the anomaly caused a delay with SXM-8’s launch from March to June 2021 due to the investigations and changes needed.

Given SXM-8 only suffered a three month delay, this likely points to a software issue on SXM-7 that was somewhat easy to resolve given a hardware failure would almost certainly have taken longer to fix as once the satellite is launched, it cannot be serviced in space and SiriusXM would not want to risk a repeat of the SXM-7 failure. 

SXM-7’s failure has since led to a “request for proposal” to replace the satellite with SXM-9.  XM-3 and XM-4 have enough fuel for several more years of operation, giving SiriusXM enough time for the SXM-9 replacement to launch.

For SXM-8, the satellite will operate in the S-band spectrum and will feature a large unfurling antenna reflector.  The antenna will enable broadcast to radios without the need for a sizable dish-type antenna on the ground.  To power the satellite, two large solar panels will generate more than 20-kilowatts of power.  

The satellite will have an expected lifetime of 15+ years, has a launch mass of approximately 7,000 kg, and was completed at Maxar’s Palo Alto production facility in Quarter 1 of 2021.  It was then delivered to Cape Canaveral Space Force Station to begin final launch preparations.

In Florida, SXM-8 was loaded with its service-life propellants — including the supply that will be needed to raise itself from its sub-synchronous GTO insertion orbit up to its specific GEO operational location.

Afterward, the satellite was encapsulated in the Falcon 9’s 5.2 meter diameter payload fairing.  Both fairing halves are brand new.

Before the satellite and fairing duo were integrated with the Falcon 9, the rocket conducted a static fire test at SLC-40 on Wednesday, June 2.  Once that was successfully completed, the satellite/fairing combo was moved to the pad and integrated with the Falcon 9 second stage before rolling out for launch. 

Booster and launch

The booster supporting this mission was B1061-3, the -3 indicating the booster’s third flight.  B1061-3 first flew on the Crew-1 mission which launched four astronauts to the International Space Station in November 2020.  After 158 days, the booster was used again to launch the Crew-2 astronauts to the ISS.

Now 44 days after its second flight, the booster launched again, this time to send SXM-8 on its way to a sub-synchronous transfer orbit, meaning the apogee of the orbit will be below the GEO belt location at 35,786 kilometers above Earth sealevel.  This is common for heavier spacecraft as it allows for cheaper, less-capable (from a payload mass to orbit standpoint) rockets to launch them. 

The Falcon 9 countdown’s major final milestones for this flight included:

T- time to launch Event
T-38 mins Launch Director confirms “go” for propellant loading
T-35 mins Fueling begins with RP-1 kerosene to both stages & liquid oxygen to Stage 1 only
T-22 mins RP-1 kerosene load to Stage 2 complete
T-16 mins Liquid oxygen load into Stage 2 begins
T-7 mins First stage Merlin engine chilldown begins
T-2 mins 30 secs Fueling of the Falcon 9 for launch complete
T-1min Falcon 9 takes control of countdown & pressurizes its propellant tanks for launch
T-45 secs Launch Director verifies “go” for launch
T-2.7 secs First stage Merlin 1D engine ignition sequence start
T0 Liftoff

At first stage engine ignition sequence start, each pair of engines were commanded to ignite 100 milliseconds apart (four pairs of two engines with Engine 9 lighting by itself last) to balance the startup acoustics and vibrational transients that work their way through the engine section of the Falcon 9.

Staggered engine start of multi-engine rockets to limit startup acoustics and potential damaging vibrations is a common practice seen most notably on the Saturn V, Space Shuttle, and SLS vehicles.  Delta IV Heavy from United Launch Alliance also employs a staggered start for its three RS-68A engines, albeit as a hydrogen concentration mitigation tool rather than a start-up acoustic concern.

For Falcon 9, the first major element of engine start is the flow of TEA-TEB (triethylaluminum-triethylboron) through the engines at T-2.7 seconds followed by a helium powered spin start.  Both the TEA-TEB and the helium are supplied by the ground service equipment (GSE) for the initial ignition of all nine Merlin 1D first stage engines.

Milliseconds later, densified liquid oxygen starts flowing through the engines followed in a precisely-timed sequence by the RP-1 propellant, which is the last to flow to avoid a “hard start” — which is where the engine can have unexpected pressure rise at startup that could damage or destroy the engines.

At T0, the launch clamps let go of the Falcon 9 and the strong back falls back to roughly a 45° angle to minimize damage and turnaround time of the pad between missions. 

Brady Kenniston, in a self-portrait, captures B1061-1 lifting off on the Crew-1 mission from LC-39A in November 2020. (Credit: Brady Kenniston for NSF)

At T+1 minute 12 seconds into flight, Falcon 9 reached the moment of peak mechanical stress, called Max Q.  As the rocket increases velocity, the stress on the vehicle increases; however, as the altitude of the rocket increases, the atmospheric density decreases.  Because of this, there is a time when the stress is at its highest, which usually happens on the Falcon 9 around the transonic region of flight.  

During this period, the Falcon 9 throttles down its engines to ensure the stress won’t damage the vehicle.

The 1st stage then shutdown all nine of its Merlin engines at T+2 minutes 33 seconds — with the engines shutting down in the same pairs as which they started.  This was followed quickly by stage separation, accomplished with four pneumatic separators, three in the interstage perimeter and one central pusher that goes in the MVacD engine.

At T+ 2 minutes 44 seconds into flight, the single MVacD engine on the second stage ignited, followed by fairing separation just over three and a half minutes into flight.

The fairings were separated by first opening a series of pneumatic latches along the line where the fairing halves meet; four pneumatic pushers then drove the fairing halves apart.  The active fairing half is home to the pneumatic pushers and helium COPV (composite overwrap pressure vessel).  Both fairing halves have additional COPVs to store propellant for their attitude control thrusters, which are used for fairing recovery.

At T+6 minutes 31 seconds, Booster 1063-3 ignited its central E9 engine followed shortly after by the E1 and E5 engines to perform the entry burn as it reentered Earth’s atmosphere. These engines are different from the other 6 engines as they have onboard TEA-TEB canisters to hold the lighter fluid required for igniting the engines.

After 8 minutes 12 seconds of flight, the second stage shutdown its MVacD engine upon reaching its initial parking orbit.  Seconds later, the first stage landed on Just Read the Instructions using its center E9 engine.

This was be the 87th landing of a Falcon 9 and the 62nd flight of a Falcon 9 with a flight-proven booster.

Just over 26 minutes into the mission, the second stage ignited its MVacD engine for a 44 second burn, which raised the satellite’s apogee to approximately 20,000 km.  SXM-8 then separated, completing the mission.

SpaceX has two more flights planned in June for a total of at least four flights this month.  On June 17 at 18:00 EDT/22:00 UTC, Falcon 9 (B1062-2) will launch the fifth GPS-III satellite into a medium Earth orbit (MEO).  Sometime thereafter, the Transporter-2 mission will launch 100+ rideshare payloads to Sun-synchronous orbit on B1060-8, the fourth booster to fly for an eighth time.

(Lead image: Falcon 9 launches on the SXM-8 mission. Credit: Stephen Marr)

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