Next mission covering delivering to orbit two Geosynchronous Space Situational Awareness Program (GSSAP, also known as GEO-SSAP) satellites is getting close. Rocket will start at 04:47 GMT on 19th August 2016.
United Launch Alliance as launch service provider decided to use Space Launch Complex 37 (at Cape Canaveral AFS) and their reliable Delta IV rocket in this mission. Rocket in the (4,2) configuration will deliver to orbit two satellites designed and manufactured by Orbital Sciences Corp. (OSC) for U.S. Air Force. These are third and fourth satellites from constellation of four spacecrafts under GSSAP program. First pair was delivered by Delta IV (4,2) rocket on July 28, 2014. Whole system is kind of support for Space Based Space Surveillance (SBSS) and it will help in detecting threats for USA (like approaching ICBM missiles) and also help in tracking objects in space and preventing collisions between spacecrafts and pieces of the space debris. Due the participation of the military in whole project, technical specification of the GSSAP is classified; according to few sources, satellites are weighing around 700 kg and due the extended amount of hydrazine monopropellant are able to perform orbit change maneuvers frequently and perform inspection of different spacecrafts. Onboard computers, instruments and secure downlink transponders are powered by two deployable solar arrays and onboard batteries. Main equipment of the satellites is electro optical sensors for tracking other spacecrafts from orbits below and over the GEO belt. Again, it was not unveiled what designated orbit is for GSSAP-3 and GSSAP-4, but previous two satellites from this constellation are remaining on altitude of around 35970 km.
During this mission Delta IV will also deliver to space two small satellites designed by Air Force Research Laboratory and contracted for Orbital Sciences in 2007 (with contract worth $29.5 million). These light satellites (with mass at around 90 kg) are Automated Navigation and Guidance Experiment for Local Space (ANGELS) satellites and will be deployed by upper stage of the Delta IV above the GEO orbit. After separation satellites will reach distance of 50 km from Delta upper stage and start to perform rendez vous maneuver with number of hold points around upper stage. ANGELS satellites will gradually reduce distance during next months to few kilometers around Delta upper stage. Experiment will help in developing technologies useful for Space Situational Awareness purposes.
Delta IV rocket in (4,2) configuration is known also as Delta IV Medium+. It is able to deliver 6150 kg of payload to GTO orbit under fairing with diameter of 4 m and length at 11.75 m. Second parameter stands for number of boosters. This time Delta IV will be equipped with two solid fueled boosters (GEM-60) built by Alliant (based on graphite-epoxy solid fuel). Rocket is long for 62.5 m with diameter of the core of 5 m. Launch mass of the rocket is 292 t. First stage of the rocket is Common Booster Core with length of 40.8 m and single RS-68A engine as propulsion. RS-68A is fueled with 202 t of LOX and LH2 (with fuel/oxidizer ratio at 1/6) and provides 3137 kN of thrust. CBC is controlled by avionics installed inside second stage. Two GEM-60 boosters provide 879 kN of thrust each for 91 seconds after start. They are long for 13.2 m with diameter at 1.5 m and with 29.6 t of HTPB (single booster weighs 33.6 t). Second stage of the Delta IV is called Delta Cryogenic Second Stage and is powered by single restartable RL-10B-2 engine (110 kN of thrust) fueled with LOX/LH2 (21 t of propellant with total weight of the second stage at 24 t) with additional hydrazine fueled thrusters for attitude control. Stage is long for 12.2 m with diameter at 4 m. Attitude control system and rest of the avionics (Redundant Inertial Flight Control Assembly-RIFCA) is placed between the LH2 and LOX tanks.