United Launch Alliance is planning to launch Cygnus OA-4 spacecraft with 3500 kg of payload utilizing ATLAS V rocket.
Atlas V with Orbital’s Cygnus spaceship providing supplies for ISS since 2013. Generally Cygnus construction is based on two modules: Pressurize Cargo module and Service Module (SM). Cargo module with capacity of 18 cubic meters (there is planned upgrade of PCM to 27 cubic meters) is manufactured and designed by Thales-Alenia Space. Service module with propulsion and navigation components was designed by Orbital. During engineering Service Module, Orbital used experience from their other constructions : STAR spacecraft bus and DAWN spacecraft. Service Module with weight of 1800 kg is powered by thrusters fueled with hydrazine and nitrogen tetroxide. 4 kW of electric energy for spacecraft is provided by to gallium arsenide solar arrays.
Supply missions for ISS are critical for crew members and station. As far as ISS kitchen equipment is highly reduced to refrigerator and two warmers, most of food is prepared and vacuum packed on Earth; especially vegetables and fresh fruits are highly demanded to preserve crew members on good health. Another reason is fact that there is no place to keep large amounts of food. Water is critical for ISS for the obvious (men needs about 3.7 liters of water daily) and less obvious reason which is OGS. This acronym stands for Oxygen Generating System (installed on ISS in 2007), designed by NASA to provide oxygen for pressurized modules. It is constantly recovering oxygen from Water Recovery System. It is worth mention that it was first time in the history of American space conquest to create atmosphere on a spacecraft; before that oxygen was provided from pressurized tanks. Station also requires spare parts and for working properly and for repairing damage caused by space debris. This mission is the last one before planned for 15th December 2015 launch of Soyuz-FG with Tim Kopra (USA) and Timothy Peake (GB) and Yuri Malenchenko (RUS) onboard.