Orbital ATK’s Taurus-XL rocket – now renamed Minotaur-C – returned to flight on Tuesday with a commercial mission deploying ten satellites for Planet Labs, six and a half years after its last launch ended in failure. Liftoff took place from Vandenberg Air Force Base’s Space Launch Complex 576E (SLC-576E) at 14:37 local time (21:37 UTC).
Tuesday’s launch was the first time that Orbital ATK has flown their Minotaur-C rocket since it was renamed in 2014. Formerly known as Taurus, the rocket last flew in March 2011 when it suffered the second of two consecutive – and near-identical – launch failures.
Minotaur-C is based on Orbital ATK’s air-launched Pegasus rocket, substituting the carrier aircraft for a powerful solid rocket motor to carry the vehicle away from the ground.
Early launches used SR118 solid rocket motors from decommissioned Peacekeeper missiles. However, Orbital later switched to Castor 120 motors in order to open up the rocket up to commercial customers.
Beginning with a March 1994 maiden flight, Minotaur-C made nine launches under the Taurus name: six successful and three failures.
The rocket’s first failure occurred during the sixth launch in September 2001 when control of the vehicle was briefly lost after first stage separation. Although the rocket resumed its planned course after about five seconds, it was unable to place the commercial OrbView-4 imaging satellite into orbit.
The second failure occurred in February 2009, when a Taurus-XL lifted off with NASA’s Orbiting Carbon Observatory (OCO) aboard.
The payload fairing, designed to protect the spacecraft as the rocket ascends through the atmosphere, failed to separate after it was no longer needed. Carrying the additional mass of the fairing for longer than expected, the rocket was too heavy to achieve orbit and reentered over the Antarctic.
After the OCO failure, Orbital conducted an investigation and made changes to the rocket that were expected to ensure that the failure would not happen again on a future launch.
The repeated failure shook confidence in the rocket and led to NASA canceling the planned launch of its OCO-2 satellite – a replacement for the original OCO – and transferring the satellite to United Launch Alliance’s Delta II vehicle instead.
An investigation into the Glory failure was unable to identify what had caused the fairing not to separate. Earlier this year, Portland newspaper The Oregonian reported that NASA was investigating a link with a local company that supplied aluminum components – and who had allegedly falsified testing results to indicate that their components had passed strength tests which they had not.
Since the two launch failures, Orbital has sought to distance itself from the Taurus brand. The Taurus II rocket, then under development alongside the company’s Cygnus spacecraft as part of NASA’s Commercial Orbital Transportation Services (COTS) program, was renamed Antares in late 2011.
In early 2014 Taurus itself was rebranded as Minotaur-C, adopting the name of Orbital’s family of missile-derived rockets used for military satellite and suborbital launches.
The ten satellites aboard the Minotaur-C for Tuesday’s launch will all be operated by Planet Labs.
The primary payload consists of six SkySat high-resolution imaging spacecraft for Planet’s subsidiary Terra Bella – formerly known as Skybox Imaging – which was acquired from technology giant Google in April.
The other four spacecraft are Dove CubeSats, the Flock-3m mission, which will form part of Planet’s large constellation of Earth-observing CubeSats. It is the first dedicated launch for Planet Labs or Terra Bella – as previously both companies have launched their satellites as secondary payloads on other companies launches,
Based in the United States, SkyBox Imaging was formed in 2009 and bought by Google in 2014 at a cost of half a billion dollars. The company’s first prototype satellite, SkySat-1, was launched by a Dnepr rocket in November 2013. A second prototype, SkySat-2, flew in July 2014 aboard a Soyuz-2-1b/Fregat rocket: a secondary payload to the launch of a Russian Meteor-M weather satellite.
The first operational SkySat spacecraft were launched in 2016: SkySat-3 was carried to orbit by India’s PSLV rocket in June, while SkySat-4, 5, 6 and 7 flew aboard a European Vega rocket in September.
Planet Labs announced in February 2017 that it had entered an agreement with Google to acquire Terra Bella, with the purchase being completed on 14 April. As part of the deal, Google bought a stake in Planet Labs and agreed to purchase images back from the company. Tuesday’s launch is the first for Terra Bella since its change of ownership.
The six satellites aboard Tuesday’s launch are SkySat-8, 9, 10, 11, 12 and 13 – which have also been known as SkySat-C6 to C11.
Each SkySat spacecraft has a mass of approximately 120 kilograms (260 lb). The prototype satellites were built by SkyBox themselves, who then licensed the design to Space Systems/Loral and contracted them to produce the operational spacecraft.
The prototypes were slightly smaller than the operational satellites, and are not equipped with propulsion systems. The operational satellites use one-newton (0.22-pounds-force) High-Performance Green Propulsion (HPGP) thrusters developed by ECAPS, a subsidiary of the Swedish Space Corporation.
SkyBox initially ordered thirteen satellites from Space Systems/Loral, in addition to the two prototypes it had built itself. In January 2016, six further satellites were ordered, while Terra Bella holds options that could be exercised to procure further spacecraft.
The imaging system aboard each SkySat consists of a Ritchey-Chretien Cassegrain telescope, which has a focal length of 3.6 meters (11.8 feet). Incoming light is collected by complementary metal-oxide-semiconductor (CMOS) sensors, with half of each satellite’s detector allocated to panchromatic imaging and the other half divided into four bands – near-infrared, red, green and blue – for multispectral imagery.
SkySat can produce panchromatic images at resolutions of up to 86 centimeters (2.8 feet) and multispectral images at resolutions of one meter (3.3 feet). As well as taking still images, the SkySat spacecraft can also record short high-definition videos from orbit.
For Tuesday’s launch, two of the SkySat spacecraft were attached directly to Minotaur-C’s payload adaptor, with the remaining four attached to an “upper bulkhead”, a multi-satellite adaptor allowing two levels of payloads to be carried aboard the rocket. As well as two SkySat satellites, the Flock-3m CubeSats were located underneath this adaptor.
Flock-3m is the latest batch of satellites to be launched for Planet Labs’ constellation of medium-resolution satellites. Each Flock mission consists of a number of three-unit CubeSats, which can image the Earth at resolutions of up to three meters (9.8 feet), depending on their orbit.
By using a large fleet – or flock – of satellites, frequent passes can be made over the same points on the ground. This allows for near-real-time monitoring of changes on the surface and can provide up-to-date images to customers.
Planet Labs itself was founded in December 2010. Originally named Cosmogia Incorporated, the company saw its first prototype satellite – Dove-2 – deployed by a Soyuz-2-1a rocket in April 2013.
Dove-1 was launched two days later aboard the maiden flight of Orbital Sciences Corporation’s Antares rocket. Two further prototypes were launched by a Dnepr in November 2013: one was deployed from the rocket itself and the other was to have been released from another satellite, UniSat-5 but this failed to deploy it.
The first operational Dove satellites, Flock-1, were deployed from the International Space Station following launch as cargo aboard the OA-1 Cygnus mission, carried to orbit by an Antares rocket in January 2014.
Over three hundred Doves have been launched since, using Antares, Atlas V, Dnepr, Falcon 9, H-IIB, PSLV and Soyuz rockets.
Two Flocks were lost in separate launch failures: the twenty-six satellites of Flock-1d were destroyed in the October 2014 Antares launch failure, shortly after the rocket lifted off with the OA-3 Cygnus mission aboard, while the eight-satellite Flock-1f was lost with SpaceX’s CRS-7 Dragon mission when its Falcon 9 rocket failed in June 2015.
Minotaur-C is a four-stage rocket, using solid propellant in all four stages. The numbering of the rocket’s stages reflects its heritage as a ground-launched version of Orbital’s Pegasus: the Castor 120 that will carry it away from the launch pad and into the upper reaches of the stratosphere is designated Stage Zero; the Pegasus stages stacked atop it are stages one, two and three.
The Planet Labs launch used the Minotaur-C’s 3210 configuration. Each digit of this designation indicates a different aspect of the vehicle which is designed to be interchangeable.
The first digit indicates the type of stage zero motor and the configuration of the rocket as a whole. The three indicates that it is a Minotaur-C-XL vehicle (formerly Taurus-XL), with a Castor-120 stage zero and stretched “XL” versions of the first and second stages.
If the value of this digit were a one, it would indicate the “SSLV” configuration – formerly known as the ARPA Taurus – with a Peacekeeper first stage and standard first and second stages. A two would indicate the “standard” Minotaur-C (or “Commercial Taurus”) which also used standard first and second stages but with the Castor-120 stage zero.
The second digit of the rocket’s configuration denotes the type of payload fairing being used. A one indicates the smaller of the vehicle’s two fairings, which has a diameter of 1.6 meters (63 inches), while a two – as in the case of Tuesday’s launch – represents the larger 2.3-metre (92-inch) fairing.
The third and fourth digits are always a one and a zero respectively, indicating that the rocket is using an Orion-38 third stage and no fourth stage. A Star-37FM motor could be used as either a third or a fourth stage – in which case the corresponding digit would be a three – however this has never been flown.
Tuesday’s launch was the second to use the 3210 configuration, which was last used in May 2004 to deploy Taiwan’s ROCSAT-2 satellite – later renamed Formosat-2. The ROCSAT-2 mission was the last successful Taurus launch. Across all Minotaur-C-XL configurations, the SkySat launch is the fourth flight: the failed OCO and Glory launches both used the Taurus-XL 3110 configuration.
Launches of the Minotaur-C take place from Vandenberg Air Force Base in California, making use of Space Launch Complex 576E (SLC-576E).
Originally built as an Atlas missile silo as part of the 576th Strategic Missile Squadron’s test complex at Vandenberg, along with two other silos, five hardened “coffin” launchers and three surface launch pads, 576E was used for four launches of Atlas-F missiles between August 1962 and December 1964.
After this, the complex would not see another launch until Taurus made its maiden flight in 1994. In addition to the nine Taurus launches to date, the complex was also used for the suborbital “Taurus-Lite” launch in February 2003, which tested another ground-launched version of Pegasus – without an additional stage to boost it – that would be used to support the Ground Based Interceptor (GBI) missile defense system.
Tuesday’s launch began with ignition of the Castor 120 motor, at the zero mark in the countdown. This stage burned for 85.5 seconds before burning out and separating. What is designated the first stage – an Orion-50XLST motor – then ignited to power the rocket out of the atmosphere.
The Orion-50XLST is the same motor that Pegasus-XL rocket uses as its first stage, but when flown on Taurus or Minotaur-C, it is not fitted with the wings or tail planes that the Pegasus requires for its horizontal launch profile. Its burn lasted 78.7 seconds, with the spent stage being jettisoned 5.1 seconds after burnout. Stage two – an Orion-50XLT motor – ignited 2.2 seconds after separation.
Seven and a half seconds after stage two ignited, Tuesday’s mission reached the point at which Taurus’ two most recent launches both failed. The payload fairing, which protects the satellite from damage as the rocket passes through the atmosphere, was jettisoned from the nose of the vehicle. This occurred successfully.
This event was critical, not just because the payloads will not be able to separate cleanly with the fairing still attached, but also because its additional weight would prevent Minotaur-C reaching its planned orbit if it was not discarded successfully.
At the point of fairing separation, Minotaur-C was at 356 kilometers (221 miles, 192 nautical miles) downrange, at an altitude of 160 kilometers (99 miles, 86 nautical miles) and traveling at a velocity of 4.27 kilometers per second (9,550 mph). Tuesday’s mission was using a different payload fairing to the OCO and Glory launches – the larger 2.3 meter (92-inch) fairing instead of the 1.6-metre (63-inch) fairing used on the failed launches.
The second stage burned for 77.9 seconds. After it burns out, the mission entered a coast phase as the rocket ascended towards the apogee of its trajectory. Sixty-four seconds after burnout the second stage was jettisoned, with the Orion-38 third stage igniting after another four minutes and 2.6 seconds of coasting. The third stage burned for 71.2 seconds.
Two minutes after the third stage burned out – thirteen minutes and 22.2 seconds after lifting off – the first SkySat spacecraft separated from the top of the upper bulkhead payload adaptor. The other three satellites attached to this adaptor separated at twenty-second intervals. The upper bulkhead itself was jettisoned one minute after the fourth SkySat separates.
Spacecraft separation events resumed 80.5 seconds after the upper bulkhead was released, with the fifth SkySat being released and the sixth following twenty seconds later. This concluded separation of the primary payloads.
The first two Flock-3m satellites were deployed seventy seconds after the last SkySat, with the remaining two being released 24.5 seconds afterwards. With all payloads separated, Minotaur-C’s upper stage was secured, and the mission concluded 21 minutes and 25.9 seconds after liftoff.
The planned orbit at spacecraft separation will be a circular sun-synchronous orbit at an altitude of 500 kilometers (310 miles, 270 nautical miles).
Tuesday’s launch was currently the only Minotaur-C mission on Orbital ATK’s books. Orbital ATK has given no indication that it plans to retire the Minotaur-C, so the rocket will likely remain available should its services be called upon in the future. That said, if future launches do not emerge, Tuesday’s could be the rocket’s final flight.
Orbital’s next launch is scheduled for 11 November, with an Antares rocket flying out of Virginia’s Mid-Atlantic Regional Spaceport (MARS) on Wallops Island with the OA-8 Cygnus mission bound for the International Space Station. Another launch, scheduled for 8 December, will see the air-launched Pegasus-XL rocket deploy NASA’s Ionospheric Connection Explorer (ICON) satellite from Kwajalein Atoll in the Marshall Islands.
(Images via NSF’s Jay DeShelter, Orbital ATK, SkySat, PlanetLabs and NSF’s Derrick Stamos/L2 – Click here to join L2).