Mercury-Atlas LV3B

On April 12th, 1961, the American's worst fears became true: the Soviets managed to safely put a human in space. Not only did they do that first, but they also had Gagarin sent to orbit, while the Mercury-Redstone stack, then in final stages of development, could only send humans on a suborbital trajectory. When Alan Shepard flew four weeks later, America was still lagging seriously behind because they still couldn't go orbital. That's when an order was issued to accelerate the human rating program of the SM-65 Atlas ballistic missile. With a Mercury capsule on top, it put John Glenn into Low Earth Orbit on Feb 20th, 1962, making him the fifth human in space, and the third human in orbit, finally closing the technology gap between the US and USSR.

To operate this rocket, you will need to follow a shallow ascent profile, until your apoapsis goes above the Karaman line. Then, press AG2 twice, to cut off the booster engines. Your orbit should be no higher than 300km, or you will have troubles with reentry.

Should anything go wrong, there are two abort modes. First, activated by AG1, involves the use of LES. It drives your capsule away from the launcher so you can deploy parachutes safely. It can only be used until skirt separation, after that the tower is jettisoned.

Another option, when LES is not available, involves the use of retrorockets. Use AG3 to activate them, and it will produce a similar effect to LES, but it is available at any altitude.

P.S. Many years after Freedom 7, the first American crewed orbital flight, the Atlas missile is still used to launch humans to orbit. Atlas 5 is the rocket of choice for CST-100 and Dream Chaser spacecraft.