why do space rockets take off from a upright position instead of taking off of a runway like a plane, reach 40,000 ft and entering space from there.
In: Engineering
Two complementary reasons:
1. Rocket engines are not air breathing. That is, they do not and cannot use the atmosphere to generate thrust. In fact, they generate more thrust and are more fuel efficient in a vacuum.
2. Rockets need to accelerate to a speed of ~8 km/s (~17,900 mph, ~Mach 23) to stay in orbit. Doing that inside any substantial amount of atmosphere results in incredible amounts of drag and heat, making it practically impossible to sustain.
Taking these facts together, the best launch trajectory from Earth is to head up, and only start motoring sideways in earnest once above 50-100km altitude.
However, if you can change one of these facts then you can change the launch profile. A spaceplane with air-breathing engines, such as the planned [Skylon](https://en.m.wikipedia.org/wiki/Skylon_(spacecraft)), could make a more lateral ascent through the atmosphere, using wings for lift.
On bodies without thick atmospheres, such as the Moon, rockets can turn sideways to gain speed as soon as sufficient height above the ground is reached (that is, enough height to avoid hitting mountains, etc). [This glorious chart](https://www.christies.com/en/lot/lot-6214825) shows the ascent profile for Apollo 11 from the moon’s surface. Nearby crater ridges are plotted, and it is seen that the ascent module is tilted 60° away from vertical by the time it is just 4 N.Mi. (4.6 mi, 7.4 km) from the ground; and 90° from vertical (i.e. fully horizonal) by the time it is 10 N.Mi (11.5 mi, 18.5 km) from the ground.
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