Just over two days after its successful launch of BulgariaSat-1, SpaceX’s Falcon 9 rocket conducted another launch on Sunday, carrying ten Iridium-NeXT communications satellites into orbit. Falcon lifted off from a foggy Vandenberg Air Force Base at 13:25 local time (20:25 UTC) and again saw the first stage land on the drone ship.
Sunday’s launch took place forty-nine hours and fifteen minutes after that of the previous Falcon 9 mission, which successfully deployed the BulgariaSat-1 satellite into geostationary orbit on Friday.
Friday’s launch was made from the Kennedy Space Center in Florida, while Sunday’s was from Falcon’s West Coast home: Space Launch Complex 4E at the Vandenberg Air Force Base in California.
SpaceX has separate launch teams for the two sites, allowing the campaigns to progress in parallel with their launches in quick succession.
The BulgariaSat-1 mission was SpaceX’s second launch to re-use a “flight proven” first stage – one recovered following a previous launch.
The booster used for the BulgariaSat mission had first been flown in January, as part of a launch which deployed ten communications satellites for Iridium Communications as part of a multi-launch deal.
Sunday’s launch, which used a previously-unflown first stage, was the second under the Iridium contract with a further ten satellites aboard.
The timespan between the BulgariaSat and Iridium launches is the shortest turnaround between two orbital launches of a family of American rockets since a pair of Titan launches in early September 1989.
The 1989 launches – of a Titan III(34)D/Transtage from Cape Canaveral, followed forty-four hours later by a Titan II(23)G from Vandenberg Air Force Base – used related, but very different rockets. The Titan II(23)G was a converted two-stage Titan II missile while the Titan III(34)D was a heavy-lift vehicle with two large solid rocket motors either side of a stretched core topped with a Transtage upper stage.
For a turnaround between two launches of the same type of rocket flying in a similar configuration, it is the fastest the United States has achieved since December 1966.
Then, Atlas SLV-3 rockets made two launches, forty-one hours apart, with Agena-D upper stages. The first of these Atlas-Agena launches was made from Vandenberg on 5 December to deploy a KH-7 GAMBIT reconnaissance satellite – departing from the same launch pad that Falcon will fly Sunday’s mission from. The second Atlas-Agena was a launch from Cape Canaveral Air Force Station, carrying NASA’s first Applications Technology Satellite (ATS) towards geostationary orbit.
Although such a short time between launches is unusual for US rockets, Russia’s Soyuz and China’s Chang Zheng – or Long March – regularly make multiple flights in a short space of time. Twice in recent years – in 2013 and 2015 – pairs of Soyuz launches have been made just two hours apart.
The ten Iridium-NeXT satellites that were aboard Falcon 9 were the second batch of spacecraft to be launched towards a constellation which, when complete, will consist of sixty-six satellites plus on-orbit spares. The Iridium-NeXT constellation will replace Iridium Communications’ current fleet of satellites which were launched between 1997 and 2002.
Iridium relies on a large fleet of satellites in low Earth orbit to provide worldwide mobile communications. The company’s name comes from the metal iridium, which has atomic number 77 – the number of satellites that were originally anticipated to achieve global coverage. This was later revised down to sixty-six, but the name was retained.
The total setup cost of Iridium’s first satellite constellation was estimated at around five billion dollars, and none of the spacecraft could be put into service until enough had reached orbit to enable global coverage.
Combined with a slower-than-expected uptake among customers, this led to Iridium SSC filing for bankruptcy in 1999. Motorola, who had financed the project, came close to ordering that the entire fleet be deorbited, but in 2001 a new company – Iridium Satellite LLC – purchased the entire constellation for about 25 million dollars. Iridium Satellite LLC would later be renamed Iridium Communications.
The first-generation Iridium constellation consisted of 689-kilogram (1,520-lb) LM-700A satellites manufactured by Lockheed Martin. Ninety-eight satellites were built, of which ninety-five were launched. The satellites were designed to provide eight years of service, however all but seven of the satellites currently in operation are at least eighteen years old with some having been in orbit for over two decades. Aside from the second-generation satellites launched in January, the youngest satellites were deployed by a June 2002 Rokot launch.
Iridium has ordered eighty-one second-generation, or Iridium-NeXT, satellites to replace its entire fleet at a cost of around three-billion dollars. This includes spacecraft that will be used as on-orbit and ground spares in addition to the 66 required to deliver service. In orbit, the constellation will have six planes of eleven satellites, orbiting at an altitude of 778 kilometers (483 miles, 420 nautical miles) and a near-polar inclination of 86.4 degrees.
The spacecraft are being constructed by Thales Alenia Space and Orbital ATK. Orbital is undertaking final assembly of the spacecraft in Gilbert, Arizona, while Thales is the prime contractor and has provided the ELiTeBus-1000 platform upon which they are based. Also known as the Extended Lifetime Bus, this is a derivative of the company’s Proteus bus with an increased lifespan of at least ten years. Iridium intend to operate their new constellation for fifteen years.
Iridium spacecraft carry an L-band communications payload to transmit and receive data from handsets, while Ka-band transponders are used both to transmit data to and from ground stations and to establish crosslinks between satellites. The crosslink capability allows the spacecraft to route calls anywhere in the world without needing to use intermediary ground stations to downlink the data and uplink it to the next satellite.
SpaceX has been contracted to launch most of the Iridium satellites, in seven groups of ten satellites and one group of five. The five satellites will be carried along with the GRACE Follow-On geodesy mission, which is to be launched next year for NASA and Germany’s space agency DLR.
The launch was Falcon 9’s return to flight following an explosion on the launch pad in September 2016 of a rocket that was being fuelled ahead of a static fire test. It also marked the first time that SpaceX successfully recovered Falcon 9’s first stage on a West Coast launch – allowing the booster, Core 1029, to be re-used as part of the rocket that successfully launched BulgariaSat-1 on Friday. Following the BulgariaSat launch, the core was successfully landed once again.
Falcon 9 is the only rocket currently flying that is designed to be partially reusable, with the first stage capable of either returning to the launch site or landing downrange aboard a floating platform after it separates from the second stage and payload.
For Sunday’s launch SpaceX positioned an Autonomous Spaceport Drone Ship (ASDS), named Just Read the Instructions, downrange from Vandenberg to receive the first stage. Despite poor weather, requiring a relocation of the ship, the booster once again made a successful landing on its deck.
SpaceX operates Space Launch Complex 4 at Vandenberg Air Force Base for its west-coast launches. A former Atlas-Agena and Titan launch pad at Space Launch Complex 4E is used to launch Falcon, while the former Space Launch Complex 4W is being converted to serve as a landing zone.
Falcon 9 first flew in June 2010, with Sunday’s launch its thirty-seventh flight. Three major revisions of the rocket have flown: the original Falcon 9, which is now known as the Falcon 9 v1.0, made five launches before giving way to the stretched and enhanced Falcon 9 v1.1 in 2013. A further-enhanced Falcon 9 v1.2 – also known as the Falcon 9 Full Thrust – first flew in December 2015.
Sunday’s launch was the first from Vandenberg to use SpaceX’s new fast fuelling process.
After the launch conductor polled flight controllers to proceed with the operation, loading of RP-1 propellant into the rocket’s first and second stages began an hour before liftoff. Oxidiser tanking – the loading of supercold liquid oxygen – started twenty-five minutes later and continued until just before liftoff.
Three seconds before the countdown reached zero, the nine Merlin-1D engines at the base of Falcon’s first stage came to life. The rocket climbed away from the foggy Vandenberg on a southerly trajectory, passing through the area of maximum dynamic pressure – or Max-Q – sixty-nine seconds after liftoff.
The first stage, Core 1036, burned for the first two minutes and twenty-four seconds of the flight before reaching main engine cutoff, or MECO. Four seconds after cutoff the stage separated and began its descent towards Earth. Ignition of the second stage’s Merlin-1D vacuum engine came seven seconds after stage separation, beginning a six-minute, 29-second burn.
Falcon’s payload fairing separated forty-one seconds into second stage flight. On some recent launches SpaceX has conducted experiments to try and recover the fairing for potential reuse after separation. Elon Musk noted that Sunday’s launch included an attempt, but was unsuccessful due to the steering parachute technology requiring more work.
One thing that SpaceX was definitely attempting again was to recover the first stage. This is equipped with legs that are deployed shortly before landing, and grid fins to provide control as it flies back through the atmosphere. The grid finds will be deployed shortly after stage separation.
On previous launches, SpaceX has used grid fins manufactured out of aluminum, which have frequently been burned and damaged during entry. Sunday’s launch was the first to use larger grid fins, made of titanium. These are expected to be more resistant to reentry heating, so they will not need to be replaced for each flight.
The larger size of the fins, along with other improvements to their aerodynamics, increase the force they can apply to control the rocket’s course and improve the amount of lift they provide relative to drag. This improves the gliding ability of the booster, reducing the amount of fuel that needs to be reserved for bringing the core home. The new fins will also aid Falcon Heavy’s side boosters for their return.
Three minutes and thirteen seconds after separating, the first stage began a short engine burn as it returned through the atmosphere, slowing its descent to protect itself from heating as it descends. The landing burn began a little under two minutes later, guiding the booster onto the deck of Just Read the Instructions.
About eighty seconds after the first stage landed, at nine minutes and four seconds mission elapsed time the second stage shut down at the end of its first burn. Following a 43-minute, two-second coast phase in a parking orbit, the second stage restarted for its second burn. This burn – lasting three seconds – circularized the orbit.
Deployment of the ten Iridium spacecraft began at 57 minutes, ten seconds mission elapsed time, with the final satellite separating from Falcon 9 four minutes and 50 seconds later. The deployment orbit is aimed to be a little below the satellites’ operational orbit: the satellites will be released into a circular orbit at 625 kilometers (388 miles, 337 nautical miles) in altitude and an inclination of 86.66 degrees.
Iridium confirmed a good deployment and early phase to the checkout of the satellites.
Sunday’s launch was the ninth of the year for SpaceX and the Falcon 9, as well as the rocket’s third launch of June. Falcon 9’s next launch is due no earlier than (NET) 4 July, with the Intelsat 35e communications satellite from the Kennedy Space Center.
UPDATED: The next launch from Vandenberg is currently targeting August 24 with the Republic of China’s FORMOSAT-5 spacecraft, after which the next Iridium launch is scheduled in September.
(Images: SpaceX, Iridium and L2 SpaceX – including a large collection of photos from this launch campaign via Sam Sun and Chris Gebhardt and Philip Sloss for NASASpaceFlight.com – BulgariaSat-1 launch photo via Brady Kennison for NASASpaceFlight.com)
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