United Launch Alliance (ULA) has successfully deployed the US Navy’s fourth MUOS satellite on Wednesday, with an Atlas V carrying the communications spacecraft into a geostationary transfer orbit. Launch, from Space Launch Complex -41 (SLC-41) at Cape Canaveral Air Force Station, Florida, occurred at 10:18 UTC – inside a 44 minute window.
The US Navy’s Mobile User Objective System, or MUOS, is a constellation of five planned communications satellites designed to provide tactical mobile communications to the United States military.
Wednesday’s launch is carrying the fourth spacecraft of the series, with the final satellite expected to join it in orbit next July.
Although not yet complete, the introduction of MUOS-4 will give the MUOS system worldwide coverage for the first time, with the final satellite intended to serve as an on-orbit spare. The constellation is expected to be fully operational in 2017.
The MUOS constellation replaces the seven UHF Follow-On (UFO) satellites that were launched between 1993 and 2003.
These spacecraft, which themselves followed-on from the FLTSATCOM spacecraft deployed by Atlas-Centaur vehicles during the late 1970s and 1980s. UFO also served as a replacement for the five Leasat spacecraft operated by Hughes Communication Services on the Navy’s behalf.
The Leasat spacecraft, also known as Syncom IV, were deployed by the Space Shuttle – with Discovery carrying the first four during missions in 1984 and 1985. The fifth satellite, which had originally built to serve as a ground spare, was launched aboard Columbia in 1990 as part of STS-32.
Deployment of the MUOS constellation began with the launch of MUOS-1 in February 2012, with MUOS-2 following in July 2013. The most recent MUOS launch prior to Wednesday was that of MUOS-3 earlier this year, lifting off from Cape Canaveral on 21 January.
The satellite launched as MUOS-3 was the fourth production satellite, Space Vehicle 4 (SV-4).
This was launched ahead of Space Vehicle 3 after defective soldering was discovered on the SV-3 satellite necessitating that it undergo repairs. It is this SV-3 spacecraft that will be launched as MUOS-4.
Constructed by Lockheed Martin and based on the A2100M satellite bus, each MUOS spacecraft has a mass of approximately 6,740 kg (14,860 lb).
Powered by twin solar arrays, propulsion is provided by a BT-4 apogee motor manufactured by Japan’s IHI Corporation.
All MUOS launches to date have used United Launch Alliance’s Atlas V carrier rocket, flying in its heaviest-lift configuration.
This 551 configuration consists of a five-metre payload fairing, five Aerojet AJ-60A solid rocket motors augmenting first stage thrust. A single-engine Centaur upper stage will provide thrust to inject the MUOS spacecraft into orbit.
Wednesday’s mission used Atlas AV-056. The fifty-sixth Atlas V to fly, AV-056 was the sixth flight of the 551 configuration which was previously used to launch the New Horizons mission to Pluto and the Juno probe to Jupiter in addition to the three earlier MUOS flights.
Ahead of her mission the Atlas was assembled within ULA’s Vertical Integration Facility (VIF) to the south of the pad; although for the first time on the MUOS-4 mission the Centaur, payload and the lower section of the payload fairing were mated away from the VIF at ULA’s Delta Operations Center and stacked atop the first stage as an integrated unit.
Departing from an old Titan launch pad, Space Launch Complex 41 at the Cape Canaveral Air Force Station, AV-056 began her mission by igniting her RD-180 main engine around 2.7 seconds ahead of the planned liftoff time.
After ignition the RD-180, which powers the Atlas’ Common Core Booster first stage, built up to full thrust. Developed by Russia’s NPO Energomash and derived from the RD-170 series of motors developed for the Soviet Union’s Energia and Zenit rockets, the RD-180 is a twin-chamber engine that burns RP-1 propellant and liquid oxygen oxidiser.
Liftoff occurred 1.1 seconds after the countdown reaches zero when the solid rocket motors ignited and the rocket’s thrust exceeded its mass. Around 3.8 seconds into flight the rocket began a series of pitch and yaw manoeuvres to attain the correct orientation for its ascent to orbit.
Flying East South East over the Atlantic Ocean along an azimuth of 94.58 degrees, the rocket passed through the area of maximum dynamic pressure, or Max-Q, approximately 51.2 seconds into its flight.
Following Maximum Dynamic Pressure, the next major milestone for AV-056’s mission was the separation of the five solid rocket motors. The first pair of spent motors were jettisoned at 106.6 seconds mission elapsed time; the remaining three separated a second and a half later.
When the Atlas V flies with a five-metre payload fairing, the second stage is enclosed within the fairing as well as the payload.
As a result the fairing must separate during first stage flight so that the second stage is free to begin its burn as soon as the Common Core Booster shuts down at the end of its phase of the flight. MUOS-4 and the Centaur upper stage were encapsulated within the medium-length version of the Atlas V’s five-metre payload fairing. This separated at the three minutes and 22.4 second mark in the mission.
The Forward Load Reactor (FLR), used to dampen vibrations within the fairing by connecting the forward end of the Centaur with the structure of the fairing, separated from the Centaur five seconds after the fairing.
Four minutes and 23.7 seconds after lifting off, Booster Engine Cutoff (BECO) occurred, with the first stage engine shutting down and completing its burn. Six seconds after BECO the spent stage was jettisoned and ten seconds after staging the Centaur’s RL10 engine ignited for the first of three burns.
Powered by an RL10C-1 engine, in place of the RL10A-4-2 that was used for most of the first fifty Atlas V launches, the Centaur is a fully-cryogenic stage fuelled by liquid hydrogen and liquid oxygen. Its first burn lasted seven minutes, 44.5 seconds to establish an initial parking orbit.
Following an eight-minute, 5.6-second coast a second burn began, this time lasting five minutes and 47.1 seconds to raise the apogee of the orbit.
After the second burn, AV-056’s mission entered an extended, 142-minute 57.9-second, coast phase as the Centaur climbed towards the apogee of her orbit. The third and final burn, which occurred close to apogee, served predominantly to raise the perigee of MUOS-4’s deployment orbit.
The short, 58.3-second firing of the RL10 engine marked the last phase of powered flight before spacecraft separation, which took place about three minutes and thirty nine seconds after the end of the third burn, or at two hours, 53 minutes and 55.1 seconds elapsed time.
The target orbit for Wednesday’s launch is 3,819 by 35,784 kilometres (2,373 by 22,235 statute miles; 2,062 by 19,322 nautical miles), at an inclination of 19.11 degrees. From this initial transfer orbit MUOS-4 can manoeuvre into its operational geostationary orbit through a series of orbit raising burns.
Wednesday’s launch was the forty-sixth of 2015, based on the Soyuz TMA-18M mission departing from Baikonur earlier in the day. AV-056 was the fourteenth rocket launched towards orbit by the United States in 2015 and the fifth Atlas V.
The next Atlas launch is expected to occur in a month’s time when an Atlas V 421 will depart Cape Canaveral with the Morelos 3 commercial communications satellite.
Three further Atlas launches have been pencilled in by the end of the year – the NROL-55 mission from Vandenberg Air Force Base in early October, a launch with a GPS navigation satellite at the end of October and a mission in December to carry a Cygnus commercial resupply spacecraft to the International Space Station.
(Images via ULA and NASA).