Spaceflight Insider

Delta IV Medium to launch situational awareness satellites

AFSPC-4 flight Delta IV Medium

Archive photo of a Delta IV Medium+ launching the AFSPC-4 mission. Photo Credit: Mike Howard / SpaceFlight Insider

CAPE CANAVERAL, Fla. — United Launch Alliance (ULA) is expecting an 80 percent chance of favorable weather conditions when it attempts to launch two Orbital ATK satellites for the U.S. Air Force’s Space Command’s (AFSPC) Geosynchronous Space Situational Awareness Program (GSSAP). Liftoff is scheduled for 12:47 a.m. EDT (04:47 GMT) Aug. 19 – the beginning of a 65-minute launch window.

Wednesday morning, ULA completed the Launch Readiness Review for the Delta IV Medium+ (4,2) rocket that will carry the satellites and was cleared to proceed with the countdown – slated to begin Thursday evening. Liftoff will take place at Cape Canaveral Air Force Station’s Space Launch Complex 37 (SLC-37).

The GSSAP satellites will collect data on space situational awareness, allowing for more accurate tracking and characterizing of human-made objects in orbit.

GSSAP Delta IV Medium

An artist’s rendering of the GSSAP satellites in space. Image Credit: U.S. Air Force

The two satellites will operate in near-geosynchronous orbit where they will monitor other spacecraft as well as orbital debris. They are designed to perform Rendezvous and Proximity Operations (RPO), which will allow them to maneuver near space objects  for a closer look while still maintaining a safe distance.

GSSAP’s goal is to provide up-to-date and accurate tracking of space traffic and debris, enhancing the USAF’s knowledge of the geosynchronous orbital environment, and reducing potential collisions in orbit through early detection of potentially hazardous objects. Currently, the USAF tracks over 23,000 pieces of orbiting debris. The AFSPC-6 mission will complete the four-satellite constellation. The first two GSSAP satellites were declared operational in September 2015.

GSSAP satellites will communicate through Air Force Satellite Control Network (AFSCN) ground stations. The 50th Space Wing’s satellite operators of the 1st Space Operations Squadron (1 SOPS) out of Schriever Air Force Base, Colorado, are responsible for controlling day-to-day operations.

The Delta IV Medium+ (4,2) is a two-stage, 207-foot (63-meter) tall rocket. This version – the 4,2 – sports a 4 meters fairing and two solid rocket motors.

The rocket’s common booster core (CBC), which is its first stage, is 133.9 feet (40.8 meters) tall and 16.7 feet (5.1 meters) in diameter. The CBC has a single RS-68A engine on the bottom. It burns liquid hydrogen and oxygen to produce 702,000 pounds (3,123 kilonewtons) of thrust at sea level.

Each of the Orbital ATK-built strap-on GEM 60 solid rocket boosters are 60 inches (152 centimeters) in diameter and 53 feet (12.2 meters) tall. They burn for about a minute and a half and are detached from the stack about 100 seconds into flight. They each produce 280,000 pounds (925 kilonewtons) of thrust.

The Delta IV Medium + 4.2 is protected and worked on underneath what is known as the Mobile Service Tower. The day of launch, the building is moved back away from the rocket - the distance of a football field. Photo Credit: Jason Rhian / SpaceFlight Insider

An archive photo of a previous Delta IV Medium+ (4,2) prior to launch. Photo Credit: Jason Rhian / SpaceFlight Insider

The second stage of the vehicle is the Delta Cryogenic Second Stage (DCSS). It utilizes a single RL10B-2 engine. According to ULA’s product sheet, the engine is the world’s largest carbon-carbon extendable nozzle.

The DCSS produces 24,750 pounds (110 kilonewtons) of thrust. It burns liquid hydrogen and oxygen for up to 465 seconds. The second stage is 13.1 feet (4 meters) wide and 39.5 feet (12 meters) long.

A Delta IV Medium+ (4,2) is capable of sending 28,440 pounds (12,900 kilograms) into low-Earth orbit and 13,580 pounds (6,160 kilograms) into geostationary orbit.

At launch, the RS-68A will ignite at five seconds prior to lifting off to throttle to full power while the flight computer verifies all is well with the system.

When the clock reaches zero, the two strap-on boosters will ignite and the four hold-down bolts will be released, allowing the stack to begin to rise. Less than a minute later, at around 44 seconds, the vehicle will reach Mach 1, and then, at around 47 seconds, Max-Q – the region of maximum pressure on the rocket.

The strap-on boosters will separate at about 100 seconds into the flight and will crash into the ocean.
Almost 4 minutes into the flight, the CBC will have depleted its fuel; the RS-68A will then shut down and, five seconds later, the whole stage will separate from the DCSS. Some 15 seconds after that, it too will ignite to begin its push toward geostationary transfer orbit. Some 10 seconds later, the fairing will separate, revealing the payload to space.

At this point, an expected news blackout will occur. RL10B-2 engine cutoff and subsequent firings to place it into near-geostationary orbit will be done in secret.

AFSPC-6 will be ULA’s 110th mission overall and the seventh ULA launch of 2016. Additionally, it will be the 14th time this variant has been called upon to send a payload spaceward. ULA previously launched two GSSAP satellites on July 28, 2014, aboard a Delta IV Medium+ (4,2) rocket from CCAFS.

The next Delta IV launch of any kind will occur on Oct. 20 when a Medium+ (5,4) – a five-meter payload fairing and four strap-on boosters – launches the eighth Wideband Global SATCOM spacecraft into geostationary orbit.


Bart Leahy is a freelance technical writer living in Orlando, Florida. Leahy's diverse career has included work for The Walt Disney Company, NASA, the Department of Defense, Nissan, a number of commercial space companies, small businesses, nonprofits, as well as the Science Cheerleaders.

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