Question

Why rocket is proposed in upward direction when launched

Answer 1

To launch an object into a stable low-earth orbit requires accelerating the object to orbital velocity (approximately 5 miles per second) tangential to the Earth's surface.

Intuitively, getting out of (the majority of) the atmosphere and then accelerating more or less tangential to the surface seems like a good plan. This implies a nearly vertical ascent early and then a gradual gravity turn to a nearly tangential trajectory.

A hybrid combination of a large airplane-like vehicle that raises the rocket high into the atmosphere and then launching it from there is another approach which is used for smaller launch vehicles, e.g., the Pegasus launch system.


At least one company, Stratolaunch Systems, is taking this approach seriously for a larger launch vehicle.

A rocket's goal is to get out of the Earth's gravitational field as quickly as possible because this is the way that requires the minimum amount of fuel.

A helicopter that is just "sitting" in the air has to waste fuel (because it has to fight against the Earth's attractive gravitational force) but it is making no progress in escaping the Earth's gravitational field. In the case of rockets, one wants to be as "dissimilar" to the helicopter as possible. If the goal is for the rocket to get from h=0h=0 to h=10,000kmh=10,000km (a reference level at which the Earth's gravitational acceleration is already very low) using the minimum amount of fuel, it's optimum to connect these two potential levels (concentric spheres) by the shortest possible path, and such a shortest possible path is clearly vertical (or "radial", relatively to the center of the Earth). Its length is 10,000km10,000km in my case while all other trajectories connecting the two spheres are longer.

One can construct rigorous proofs that the vertical trajectory is the most efficient one (it consumes less fuel than every other path, and doesn't just have the shortest length) but the heuristic argument above should suffice. It's not just a good idea to start in the perfectly vertical direction. To save fuel, one also wants to achieve the greatest possible/allowed acceleration as quickly after the launch as possible. Any delay in the acceleration would make the rocket resemble the helicopter that just wastes fuel by sitting in the air.

Airplanes aren't launched vertically because their goal isn't really to efficiently escape from the Earth's gravitational field (or at least from Earth's atmosphere) but to move at a different location in the horizontal direction, to a different place on the Earth's surface. An approximately horizontal takeoff (with some tilt, to achieve the height where the atmosphere is less dense and the air resistance is lower) is better for that purpose.