sketch of the Atlas V 421 is shown to the right. You can see the core stage with a single RD- 180 engine, the Centaur upper stage, and the payload fairing. Also shown are two solid rocket boosters used to provide additional thrust necessary at launch. The total mass of the rocket at takeoff including payload is 435,000 kg. Performance specifications of the core stage and solid rocket booster at sea level are as follows: Core stage: F = 3800 kN, Isp = 311 s; Solid rocket booster: F = 1600 kN (per booster), Isp = 245 s. Neglect drag and any effects of misalignment between the thrust and velocity vectors. Make any other assumptions that are necessary to solve the problem. Calculate the velocity of the rocket 15 seconds after takeoff