After twenty-nine years of reliable service, United Launch Alliance’s veteran Delta II rocket made its final flight Saturday with the deployment of the ICESat-2 climate research satellite for NASA. Delta lifted off from Vandenberg Air Force Base at 06:02 Local Time, beginning a mission that also closed the final chapter in the 60-year-long history of the Thor missile, from which Delta II’s first stage evolved.



Thor, one of the oldest and most proven rockets in the US fleet, grew out of a requirement for a new missile that was issued to the Douglas Aircraft Company in 1955.

Ready for launch barely a year later, the missile was christened Thor after the Norse God of Thunder and designated SM-75 (later PGM-17). Thor saw service with the Royal Air Force from 1959, but quickly became obsolete and was retired by the end of 1963.

FEATURE ARTICLE: The evolution of Thor – Delta II prepares for swansong –https://t.co/BvU1eYdMae – 7,000 word feature article by William Graham ahead of the final Thor heritage rocket launch with the @ulalaunch Delta II this weekend. pic.twitter.com/CvTBE6MlBg — Chris B – NSF (@NASASpaceflight) September 13, 2018

Outfitted with an array of different upper stages, Thor found more success boosting satellites into orbit. One such concept was the Thor-Able, which mated the second stage of the Naval Research Laboratory’s Vanguard rocket with a Thor booster.

Thor-Able evolved into Thor-Delta, a NASA-sponsored program aimed at providing access to space for the agency’s scientific payloads before more capable rockets, based on the Atlas missile, became available.

Thor-Delta, later known as just Delta, endured and over time incremental upgrades to the design produced ever more powerful rockets. By the 1970s the number of different versions of Delta II had grown to the point that a system of numerical designations was introduced to help keep track of them.

By the mid-1980s Delta – along with many of the United States’ expendable launch systems – was being wound down as the Space Shuttle was expected to take over all of the nation’s satellite launches. Post-Challenger, the US Air Force commissioned new expendable launch vehicles, including a new version of Delta.

One of the primary roles for the new Delta II rocket was to deploy navigation satellites for the Air Force’s new Global Positioning System.

Delta II first flew in February 1989, carrying the GPS II-1 (USA-35) satellite, the first of forty-nine GPS spacecraft that it would launch over the next two decades. This included the entirety of the first operational GPS constellation and many replacement and replenishment satellites.

Delta II was developed by McDonnell Douglas, the successor to the Douglas Aircraft Company who produced the original Thor. McDonnell merged into Boeing in 1997, with the latter taking over the Delta program. In 2006 production and operations of the Delta II, along with the Delta IV and Lockheed Martin’s Atlas V, were transferred to United Launch Alliance (ULA), a new company formed as a joint venture between Boeing and Lockheed Martin.

A Delta II made ULA’s first launch on 16 December 2006, deploying the USA-193 – or NROL-21 – satellite for the National Reconnaissance Office. Although Delta performed perfectly, the payload failed immediately after separating from the rocket and in 2008 it was shot down by a US Navy destroyer using a RIM-161 Standard Missile 3 (SM-3), ostensibly to prevent the satellite causing any damage when it reentered the atmosphere. Including Saturday’s mission, ULA has launched thirty of the 155 Delta IIs to have flown.

United Launch Alliance began winding down the Delta II program after the US Air Force moved away from using the rocket in 2009. However, the company had parts available for an additional five rockets after production ended, enabling NASA to continue flying payloads on this reliable old workhorse. Saturday’s launch uses the fourth of the five additional vehicles – the fifth will not be built.

Delta II’s final mission deployed the Ice, Cloud and Elevation Satellite 2 (ICESat-2) spacecraft for NASA. ICESat-2 is a successor to the original ICESat spacecraft, which launched aboard a Delta II in January 2003. Part of NASA’s Earth Observation System (EOS), ICESat operated until mid-2010, when it was deorbited.

Following the end of the original ICESat mission, NASA began a series of aerial surveys to study the ice around Earth’s poles. The “Operation IceBridge” flights allowed NASA and the National Oceanic and Atmospheric Administration to maintain continuous measurements of the ice levels.

The ICESat-2 spacecraft was constructed by Orbital ATK, now part of Northrop Grumman. Based on the LEOStar-3 platform, the satellite has a mass of 1,580 kilograms (3,483 lb). ICESat-2 has a design life of three years but has been fuelled for a seven-year mission if it remains healthy.

ICESat-2 carries a single instrument: the Advanced Topographic Laser Altimeter System, or ATLAS. This will fire thousands of laser pulses per second at the surface of the Earth and, via a telescope, collect any that are reflected back to the satellite. The time taken for the photons from a laser pulse to return will be used to calculate the how far the light traveled – allowing ICESat-2 to measure the distance to the ground or other obstructions in Earth’s atmosphere.

Four principal scientific objectives have been defined for the ICESat-2 mission: to measure sea level changes as a result of ice melting in Greenland and Antarctica; to record the change in mass of glaciers and ice sheets; to produce an estimate of how thick layers of sea ice are and to collect data worldwide on the height of vegetation above the ground.

ICESat-2 will operate in a near-circular sun-synchronous low Earth orbit, at an altitude of 500 kilometers (310 miles, 270 nautical miles), inclined at 92 degrees to the Equator. This orbit will take it over Earth’s polar regions, allowing it to see the whole of the globe between 88 degrees North and 88 degrees South.

Educational Launch of Nanosatellites 18, or ELaNa XVIII, joined ICESat-2 for the journey into orbit. ELaNa is a series of launches that NASA has arranged for small satellites developed by universities and other research institutions. The ELaNa XVIII payload consists of four satellites built to the CubeSat standard – comprised of 10-centimeter cubic “units”. Saturday’s CubeSats consist of two three-unit spacecraft, a two-unit satellite and a single unit.

The two three-unit CubeSats are the Electron Losses and Fields Investigation (ELFIN), developed by the University of California at Los Angeles (UCLA). The two satellites are identical, with the overall objective of building a better understanding of what processes cause electrons to be lost from Earth’s radiation belts. The second satellite, Electron Losses and Fields Investigation’s Spatio-Temporal Ambiguity Resolver (ELFIN-STAR or ELFIN*), will complement the first satellite’s observations.

The University of Central Florida’s Surface Charging Satellite – or SurfSat – is a two-unit CubeSat which will test material samples in orbit to determine how they become charged in the space environment. The California Polytechnic University’s (CalPoly’s) CP-7 satellite – also known as Dave – is a single-unit CubeSat which will be used to determine how a spacecraft is affected by vibrations in microgravity.

A two-stage Delta II rocket, flying in the 7420-10C configuration, provided ICESat-2 and the CubeSats with their ride into orbit. The 7420-10C configuration consists of an Extra-Extended Long Tank Thor first stage augmented at liftoff by four GEM-40 solid rocket motors and a Delta-K second stage. ICESat-2 is encapsulated within a 10-foot (3-metre) wide composite payload fairing at the nose of the rocket.

Delta II’s first stage engine burns RP-1 propellant – a rocket-grade form of kerosene – and liquid oxygen. The Delta-K consumes storable propellant: Aerozine 50 – a mixture of equal parts hydrazine and unsymmetrical dimethylhydrazine – which is oxidized by dinitrogen tetroxide.

The rocket that performed Saturday’s launch was numbered Delta 381 – indicating that it is the 381st flight of a Delta rocket – however this figure includes launches of the largely unrelated Delta IV rocket and ignores versions of the rocket built under license in Japan and three flights of Thor rockets with Delta upper stages not part of NASA’s Delta program in the 1960s.

Delta II has flown reliably for almost three decades, achieving success in 142 of its 144 launches to date. The only blemishes on its record came in the mid-1990s: a partial failure when a solid rocket motor (SRM) failed to separate during the 1995 launch of Mugunghwa-1, and a failure in January 1997 where a crack in one of the SRM casings led to its structural failure, setting off the vehicle’s self-destruct.

The Delta II rocket stands 132 feet tall and weighs about 358,000 pounds fully fueled. It will launch on nearly 650,000 pounds of thrust from the RS-27A main engine and four side-mounted solid rocket motors. pic.twitter.com/ZM2c3Q35Nm — NASA Kennedy / KSC (@NASAKennedy) September 15, 2018

For the last decade, Delta II has been statistically the most reliable rocket in service worldwide – an accolade it acquired after Russia retired its Tsyklon-2 in 2006 and which will now pass to another of ULA’s rockets, Atlas V.

Since the 1997 failure – which resulted in the loss of a GPS navigation satellite – Delta II has performed 99 successful missions. A successful deployment of ICESat-2 on Saturday would see Delta II sign off with its one hundredth consecutive launch success.

Saturday’s final flight began from Space Launch Complex 2W (SLC-2W) at California’s Vandenberg Air Force Base. Delta II’s West Coast home, SLC-2W was originally part of Complex 75, a proving ground for Thor missiles in the late 1950s and early 1960s, whose numbering comes from the SM-75 missile designation of the original Thor. SLC-2W was originally Pad 75-1-2, while the nearby derelict SLC-2E was originally Pad 75-1-1.

Sun setting behind Space Complex-2 at Vandenberg AFB as we prepare to roll the tower back from the #DeltaII with #ICESat2 pic.twitter.com/DzFsjtYtGA — ULA (@ulalaunch) September 15, 2018

The long-inactive Space Launch Complexes 1 and 10 (SLC-1 and SLC-10), with two pads each, were also originally part of Complex 75, along with several other pads that were never redesignated Space Launch Complexes.

The launch pad that would become SLC-2W was first used for an operational test of a Thor DM-18A missile by the Royal Air Force. From 1962 until 1968, the pad was used by Thor-Agena rockets, before the first Delta launch from the facility took place in 1969.

What was planned to be the last Delta launch from SLC-2W was made in March 1984 with Landsat 5, however the pad returned to action – first with NASA’s Cosmic Background Explorer (COBE) on an interim Delta 5920 vehicle in 1989, followed by a more permanent return to service in 1995 with the Delta II.

While Saturday’s launch marked the end of SLC-2W’s long association with Thor and Delta, it is not the end of the pad’s story. In May it was announced that Firefly Aerospace had been authorized to take over the launch complex following the final Delta II launch, with a view to flying their Alpha rocket using much of the pad’s existing infrastructure.

Delta II’s final flight began with ignition of the rocket’s RS-27A main engine, along with the two vernier engines that provide roll control to the rocket during first stage flight.

This took place about 2.7 seconds before the planned liftoff, and then the rocket’s four GEM-40 boosters ignited, and Delta 381 climbed away from her launch pad. As Delta was flying with only four solid rocket motors, all of the boosters ignited at liftoff.

Flying South from Vandenberg Air Force Base, out over the Pacific Ocean, it will took Delta II about 30.7 seconds to reach Mach 1, the speed of sound, with the vehicle passing through maximum dynamic pressure 15.3 seconds later.

The GEM-40 boosters that gave Delta such a quick ascent burned out about 63.3 seconds after liftoff but remained attached for a while after burnout in order to ensure the spent cases are not dropped on oil rigs downrange. The cases were jettisoned about 82.5 seconds into the mission.

Delta II’s first stage fired its RS-27A engine for four minutes and 24.7 seconds before main engine cutoff (MECO) saw this shut down in preparation for stage separation. A few seconds later the two vernier engines also shut down – this milestone was vernier engine cutoff, or VECO. About 8.8 seconds after MECO the Delta-K second stage separated from the Thor first stage.

Five and a half seconds after staging, Delta-K’s AJ10-118K engine ignited to begin the first of four planned burns. This first burn established Delta 381 into an initial parking orbit and lasted for six minutes and 18.4 seconds.

Liftoff of the final @ULAlaunch Delta II flying NASA’s #IceSat2 into a polar orbit! This rocket is incredibly photogenic! Check out the full launch gallery later on https://t.co/bmjXsHet2J pic.twitter.com/tcQEE0hRZ4 — Brady Kenniston (@TheFavoritist) September 15, 2018

After its completion, Saturday’s mission entered a coast phase for the next 36 minutes and 39.1 seconds. Delta’s payload fairing separated 22.5 seconds into the second stage burn, exposing ICESat-2 to space for the first time.

The fairing protects the satellite from Earth’s atmosphere during the early stages of flight, but once the rocket reaches space it becomes dead weight and can be discarded to improve performance.

When the upper stage restarted to circularize its orbit for ICESat-2 separation, the burn lasted 5.8 seconds. Spacecraft separation took place five minutes and 1.2 seconds after the burn concluded – at fifty-two minutes, 43.5 seconds mission elapsed time.

Eight minutes and ten seconds after ICESat-2 separated, Delta restarted its AJ10 engine again for its third burn. This 8.1 second engine-firing put some distance between the upper stage and ICESat-2 prior to deployment of the CubeSats, which began separating five minutes and 0.4 seconds after the burn ends.

These CubeSats were the last satellites to be deployed in Delta II’s illustrious career.

With all payloads released, Delta 381’s upper stage restarted again at one hour, 50 minutes and four seconds mission elapsed time for its deorbit burn.

This propelled the stage out of orbit to a destructive reentry over the Pacific Ocean. Delta fired its engine for thirty-seven and a half seconds, with the upper stage burning up on entry into the atmosphere a little over two hours, 10 minutes after launch.

While Saturday’s launch marked the end of an era with Delta II’s retirement, the rocket’s legacy is secure.

1st clip was taken with my remote camera at SLC-2 pad, 2nd clip was shot from Santa Ynez, 4500' high, 40 miles away above the Vandenberg fog. NASA TV's video/audio was inserted. @NASASpaceflight @ulalaunch Delta II – NASA ICESat 2 – Highlightshttps://t.co/vJilFSLaC5 — Jay L. DeShetler (@jdeshetler) September 17, 2018

Delta II rockets have launched satellites that continue to play a role in everyday life – such as most of the US Air Force’s current Global Positioning System (GPS) as well as seventeen commercial geostationary communications satellites, much of the first-generation Iridium and Globalstar communications networks and weather satellites that provide accurate meteorological data to NOAA. Delta II has also made a huge contribution to science, deploying missions to study the Earth, visit the planets and observe the cosmos.

Delta II rockets were used to send eight missions to Mars: Mars Global Surveyor, Mars Pathfinder, Mars Climate Orbiter, Mars Polar Lander, Mars Odyssey, the two Mars Exploration Rovers: Spirit and Opportunity, and Phoenix.

Delta II also deployed the NEAR, Deep Space 1, Stardust, CONTOUR, Deep Impact and Dawn missions to explore asteroids and comets – with Dawn also visiting the dwarf planet Ceres – the MESSENGER probe to Mercury and the GRAIL probes – Ebb and Flow – to study the Moon’s gravitational field. Other significant missions that started atop a Delta II rocket included the Fermi, Kepler, Spitzer, Swift and WISE space telescopes and the Genesis spacecraft that returned a sample of solar wind to Earth.

Following this final mission, United Launch Alliance’s fleet now consists of the Atlas V and Delta IV rockets – although the Medium+ version of the Delta IV is being phased out which will soon leave only its Heavy version in operation.

The company is also developing a new rocket, Vulcan, which is currently targeting a maiden flight no earlier than mid-late 2020.

ULA’s next launch is expected no earlier than 17 October, with an Atlas V due to loft the Advance Extremely High Frequency 4 (AEHF-4) satellite for the US Air Force.

The next Delta IV flight will use a Delta IV Heavy to carry out the NROL-71 mission for the National Reconnaissance Office. While undisclosed, this payload is thought to be an optical imaging satellite and is slated to fly no earlier than 3 December.