We are just after third in this year launching of secret payload y United Launch Alliance for National Reconnaissance Office. Again NRO left much space for assumptions regarding characteristic of the payload covered with 4 meter fairing of Atlas V.
Atlas V was launched from Cape Canaveral SLC-41; rocket was rolled out after assembling and payload integration process on July 26, 2016 in the morning local time. According to previous weather forecasts, sky was clean over Florida and it seemed that we have perfect conditions for rocket launch. At 11:20 GMT two boosters (Atlas V in 421 configuration bases on two SRBs, 4 meter payload fairing long for 12.9 m and single RL-10A engine installed in Centaur upper stage) were already filled with LOX in 50%. According to NOTAM announced on July 25, 2016, hazardous zones were set on Western North Atlantic - according different sources it is confirmation that Atlas V lifted next Quasar data relay satellite to GEO orbit (39000 km x 300 km) to retransmit data from reconnaissance satellites to help in contact with polar ground stations.
According to short briefing before launch, weather was described as "100% for go"; Rocket was prepared to sixth launch in 421 configuration and was remaining on SLC-41 supported by strongback tower. At 12:37 GMT every subsystem were checked and confirmed as ready to launch-polling was completed almost at the same time as fueling procedure. At 12:34 GMT rocket was already switched into internal power. At T-1' fuel tanks were checked and rocket was armed. At T-20" final check was finished and last sequence of countdown was started. Punctually at 12:37 GMT rocket ignited engines and started to rise over SLC-41. Speed of 1 Ma was reached after one minute of flight; maxQ point, where rocket is put on maximum stress during flight, was reached 30 seconds later on altitude of 16 km during flight with speed at 3194 km/h. At T+2' two Solid Rocket Boosters were jettisoned after using all their propellant. At T+3' rocket was on altitude of 67 km and was continuing flight with speed at 9800 km/h. At T+4'10" main engine of the core was shut down. Few seconds later upper stage was separated and started its RL-10A engine. At T+4'33" payload fairing was separated. According to United Launch Alliance all parameters were still correct, but live broadcast due the confidential payload was stopped before separation of Centaur and payload.
Atlas V in 421 version is long for 58.3 m with core diameter at 3.81 m. Total weight is 428 t with maximum payload for LEO at 13600 kg. Rocket is supported with two SRB boosters long for 17 m (with diameter at 1.58 m) providing thrust at 1688.4kN. Each booster weighs around 46 t. Main core of the rocket is powered with two chambered Energomash RD-180 engine fueled with RP1/LOX propellant with thrust at 4152kN. Upper stage is equipped with single engine Centaur-long for 12 m with diameter at 3.05 m. It is fueled with 20 t of LOX/Liquid Hydrogen burned by RL-10A engine providing thrust at 99.2 kN.
On picture above: Today's launch was pretty awesome due the clear sky: two boosters could be easily spotted along with 4 m wide fairing.