SpaceX close out 2017 campaign with Iridium-4 launch

by William Graham

SpaceX conducted its final launch of 2017 Friday, with a Falcon 9 lifting off from Vandenberg Air Force Base to deploy ten more Iridium-NEXT communications satellites. The launch, which occurred at 17:27:23 Pacific Time (01:27 UTC on Saturday), took place 61 seconds after a Japanese H-IIA rocket is lifted off on the other side of the Pacific.

Friday’s launch carried the fourth group of ten satellites in Iridium Communications’ second-generation Iridium-NEXT constellation. Iridium provides global mobile satellite communications through a fleet of satellites in low Earth orbit. The company was formed in 2001 as Iridium Satellite LLC, taking over the assets of Motorola-backed Iridium SSC, which had deployed the first-generation Iridium constellation but bankrupted itself in the process.

To provide worldwide communications, the Iridium constellation uses six orbital planes with eleven satellites per plane. As well as sixty-six operational satellites, Iridium keeps several more spacecraft available as on-orbit spares. The first-generation constellation was deployed between 1997 and 2002, with satellites built by Lockheed Martin around the LM-700A platform, which had an expected design life of seven years. The satellites significantly out-performed this expectation, with some operating for over twenty years before replacements finally began to launch earlier this year.

Iridium-NEXT satellites – Credit Iridium

The Iridium-NEXT satellites, which are built by a partnership of Thales Alenia Space and Orbital ATK, will replace and modernize Iridium’s fleet. The first ten spacecraft were launched in January, with subsequent launches in June and October bringing the total number of new-generation satellites on orbit up to thirty. Iridium has begun decommissioning its older satellites: as of early November, the oldest spacecraft still in service were Iridium 10, 12 and 13, which were part of a seven-satellite launch atop a Proton-K rocket in June 1997.

The new satellites are based around Thales’ Extended Lifetime Bus 1000, or ELiTeBus-1000, platform. Also used by Iridium’s rivals Globalstar and O3b, the ELiTeBus platform provides the satellites with a ten-year design life, however Iridium is optimistic that the spacecraft will be able to operate for fifteen years. Each satellite has a mass of 860 kilograms (1,900 lb), and carries L- and Ka-band communications payloads.

Iridium uses L-band signals to communicate with users’ handsets, while Ka-band transponders are used to cross-link between satellites – allowing a call to be routed across the Iridium network without needing to transmit it via ground stations. Additional Ka-band transponders are used to provide a downlink where necessary.

Iridium CEO Matt Desch on an Iridium handset – Photo from Jay L. DeShetler‏ for NSF/L2

The fourth launch in SpaceX’s contract with Iridium, which calls for the launch of seventy-five satellites aboard eight Falcon 9 rockets, Saturday’s launch was the first Iridium launch to use a “flight-proven” – or previously-flown – first stage. The booster in question, Core 1036, is the same one that was used for SpaceX’s second Iridium launch which took place on 25 June.

SpaceX designed the Falcon 9 to be at least partially reusable. Where its primary mission allows, following separation the first-stage booster returns to Earth – landing either on a landing pad near to the launch site, or an Autonomous Spaceport Drone Ship (ASDS) at sea. Following June’s launch, Core 1036 landed aboard the West Coast ASDS, named Just Read the Instructions, in the Pacific Ocean.

Friday’s launch was the fifth time SpaceX have launched a flight-proven Falcon 9, and the first such mission to fly from Falcon’s west-coast launch pad at Vandenberg Air Force Base. Core 1036 is a Block 3 booster, part of an earlier production series that has now been superseded by a more powerful Block 4 version. As Block 3 stages are only flown twice – at most – before they are retired, SpaceX has opted to fly Friday’s mission in an expendable configuration.

Falcon 9 for Iridium-4 at Vandenberg. Photo by Philip Sloss for NSF/L2

No landing legs were fitted to the first stage. However, photos of the rocket being prepared for launch showed that the rocket was still sporting grid fins, which are not typically flown on expendable missions. Their presence suggested that SpaceX would attempt to guide the stage to a controlled landing in the ocean – a technique used on several early Falcon 9 missions to test landing procedures before the first recoveries were attempted – instead of simply letting it fall back to Earth.

Core 1036 was the first flight-proven booster not to be recovered a second time, and only the second expendable Falcon 9 to be flown from Vandenberg – the first being 2013’s deployment of CASSIOPE, which took place before SpaceX had begun attempting to recover its stages.

Friday’s launch was likely to see another attempt to recover the rocket’s payload fairing. A recovery ship, named Mr Steven, left the port of Los Angeles on Thursday bound for Guadalupe Island, off the coast of Mexico.

This will place the ship, which has been fitted with four large arms – possibly intended to hold a net for capturing the fairing, downrange at the time of launch. The fairing, which protects satellites from aerodynamic forces as the rocket ascends through the atmosphere, is the next part of the Falcon 9 that SpaceX aims to make reusable.

First flown in 2010, Falcon 9 is a two-stage rocket. Atop the reusable first stage, an expendable second stage is used to complete the ascent and inject payloads into their planned orbits. Both stages consume RP-1 kerosene propellant oxidized by subcooled liquid oxygen. Friday’s launch marked Falcon 9’s forty-sixth flight.

Falcon 9 for Iridium-4. Photo by Philip Sloss for NSF/L2

The first stage – Core 1036 – is powered by nine Merlin-1D engines ignited at the T-three-second mark in the countdown. At T-0, Falcon 9 lifted off began to climb away from the launch site. Seventy-four seconds into flight the rocket passed through the area of maximum dynamic pressure, or Max-Q.

The first stage provided thrust for the first two minutes and 33 seconds of the flight, at which point its nine engines shut down. This event, designated Main Engine Cutoff, or MECO, was followed three seconds later by the separation of the spent first stage.

Two seconds after stage separation, the second stage’s single Merlin Vacuum (MVac) engine ignited for the first of two planned burns. The MVac engine is a Merlin-1D that has been optimised for maximum efficiency in the vacuum of space, making it better suited to the rocket’s second stage.

Iridium-4’s payload fairing. Photo by Philip Sloss for NSF/L2

The payload fairing separated from the second stage about thirty-three seconds after ignition. The first burn lasted six minutes and 22 seconds, putting Falcon into an initial parking orbit. The mission then entered a planned 42-minute, 54-second coast phase before the second burn commenced at 51 minutes and 54 seconds mission elapsed time.

The upper stage’s second burn lasted just eleven seconds. This served to circularize the satellites’ deployment orbit ready for spacecraft separation. Five minutes after the burn was completed, the first satellite separated. It took fourteen minutes and 55 seconds to deploy all ten satellites – with the final separation event coming seventy-two minutes into the flight.

Friday’s launch took place from Space Launch Complex 4E (SLC-4E) at California’s Vandenberg Air Force Base. Originally constructed as part of the Point Arguello Naval Missile Facility, the launch complex became part of Vandenberg Air Force Base when the two adjacent launch sites were merged in 1964.

SpaceX facility at Vandenberg – Photo by Philip Sloss for NSF/L2

Originally named Launch Complex 2-4 (LC-2-4), SLC-4E was initially used by Atlas-Agena rockets, with the first launch in August 1964 occurring shortly after the complex’s transfer to Vandenberg. The pad was later used by Titan rockets – the Titan III(23)D, or Titan IIID, the Titan III(34)D (Titan 34D) and the Titan IV. The final flight of the Titan rocket – using a Titan IV(404)B vehicle – occurred from SLC-4E in October 2005.

SpaceX leased the former Titan launch pad from the US Air Force, demolishing the old service towers and clearing the pad in 2011 in preparation for Vandenberg’s first Falcon 9 launch, which took place in September 2013.

Space Launch Complex 4 originally consisted of two launch pads. SpaceX has converted the second pad, SLC-4W, into a landing pad for return-to-launch-site recovery of Falcon 9 boosters. Prior to the announcement that it would use a flight-proven Falcon, Friday’s launch was expected to be the first mission to use the new landing pad.

The Iridium launch took place just sixty-one seconds after the launch window for a Japanese H-IIA rocket opens. The H-IIA will carry Japan’s GCOM-C climate research satellite into orbit. If both launches occur on schedule this may be a new record for the shortest known time between two orbital launches worldwide – although this cannot be confirmed due to the margin of error of the previous record, which stands at 82±30 seconds.

That record, set by a pair of Thor rockets on 18 August 1960, is not known precisely because the exact second at which one of the launches occurred was not published. The first launch took place from Launch Complex 75-3-4 (later Space Launch Complex 1W) at Vandenberg, using a Thor DM-18A Agena-A rocket to orbit Discoverer 14.

Two Thor Rockets launching on the same day in 1960. Photos acquired by Ed Kyle, L2 Historical

An early Corona imaging satellite, Discoverer 14 would become the world’s first successful reconnaissance satellite, returning images of the Soviet Union to Earth via a film capsule the day after launch. The Discoverer 14 launch took place at 19:57:08 UTC.

The second launch of 18 August 1960 saw a Thor DM-21 Ablestar depart Launch Complex 17B at Cape Canaveral carrying an experimental communications satellite – Courier 1A – for the US Army. This launch failed to achieve orbit, with the Thor exploding about 150 seconds into its flight. The liftoff time for the Thor-Ablestar launch is recorded as 19:58 UTC. The uncertainty in its launch time means that the gap between the two launches could have been as low as 52 seconds, or as high as 112.

The shortest time between two successful orbital launches stands at 330±30 seconds – or between five and six minutes. The two launches that set this record took place on 16 September 1977, with a Tsyklon-2 lifting off from the Baikonur Cosmodrome at 14:25 UTC carrying Kosmos 952, a US-A radar imaging ocean reconnaissance satellite. This was followed at 14:30 by a Soyuz-U from the Plesetsk Cosmodrome carrying Kosmos 953, a Zenit-4MKM photoreconnaissance satellite.

A Notice to Airmen (NOTAM) indicates that China may launch a Chang Zheng 2D rocket from its Jiuquan Satellite Launch Centre only two-and-a-half hours after the SpaceX and Japanese launches. The NOTAM was, however, published unusually early for a Chinese launch and indicates the same launch window and hazard areas as a previous mission, suggesting that it may have been issued in error with no actual launch planned.

Friday’s launch was the last of 2017 for both SpaceX and for the United States. The Iridium mission was SpaceX’s eighteenth launch of the year – more than double the highest number of launches the company had previously conducted in a year: eight in 2016.

SpaceX Logo on the 39A HIF. Photo by Chris Gebhardt for NSF/L2

Friday’s mission was the thirtieth orbital launch of the year for the USA, if Electron – a rocket developed by US-New Zealand company Rocket Lab – is included in the total. This represents the most US launches conducted in a year since 1999.

The launch book-ends SpaceX’s year: the company’s first flight of the year also took place from Vandenberg carrying ten Iridium-NEXT satellites. That launch marked Falcon’s return to flight following an accident in September 2016 that destroyed a Falcon 9 rocket and its Amos 6 payload while the rocket was being fuelled for a static fire test, two days ahead of its planned launch.

These were two of four launches SpaceX have conducted for Iridium this year, along with four Dragon missions to resupply the International Space Station, seven launches with commercial geostationary communications satellites, a mission to deploy Taiwan’s Formosat-5 remote sensing satellite and the company’s first two major national security launches: deploying the NROL-76 payload for the National Reconnaissance Office and the fifth flight of the X-37B spaceplane for the US Air Force.

The secretive Zuma inside her payload fairing. Photo by Chris Gebhardt for NSF/L2

SpaceX is expected to begin their 2018 launch program on 4 January (Eastern Time – 5 January UTC) with the delayed launch of the Zuma mission for Northrup Grumman aboard a Falcon 9. Originally scheduled for November, Zuma slipped into 2018 following concerns about the rocket’s payload fairing. The maiden flight of the Falcon Heavy rocket is also planned for January, along with up to two more Falcon 9 launches – carrying the SES-16, or GovSat-1, communications satellite, and the Paz radar imaging spacecraft. SpaceX’s next Iridium launch is expected in February.

United Launch Alliance will make the next launch from Vandenberg on 10 January, opening their year with the launch of the NROL-47 national security mission – expected to carry a Topaz radar imaging satellite. This launch was also delayed from 2017, having previously been expected to take place in mid-December.

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