Orbit is a miss, repeated indefinitely
Throw a stone horizontally and gravity bends its path towards the ground. Throw it faster and it lands farther away. In Newton's famous thought experiment, keep increasing the horizontal speed and eventually the surface of the Earth curves away as quickly as the projectile falls. The projectile is still falling. It simply keeps missing.
At about 200 km above the Earth, circular orbital speed in this simplified model is approximately 7.79 km/s. A spacecraft moving at that speed horizontally can circle the Earth. The same speed directed straight upwards has essentially no angular momentum and produces a very different trajectory.
Why launch vertically at all?
A launch vehicle cannot begin by accelerating horizontally through the lower atmosphere. Near the ground, the atmosphere is dense, aerodynamic loads are severe, there are towers and landscape to avoid, and the vehicle still needs enough vertical motion to gain altitude. So the first part of flight is close to vertical.
Very soon after clearing the pad, however, a conventional orbital launcher begins to pitch. The flight path bends progressively towards the local horizontal. In a well-designed gravity turn, the vehicle does not fight continuously to trace an arbitrary curve. A small initial pitch lets gravity help rotate the velocity vector while thrust continues to add speed.
By the time the upper stage approaches orbital insertion, most of the useful velocity is tangential rather than vertical. The objective is not "up forever". The objective is a velocity vector that is nearly parallel to the local horizon.
Why a purely vertical launch is wasteful
A straight-up vehicle spends its powered flight increasing altitude while gravity subtracts velocity from it every second. If the engine eventually stops below escape speed, the vehicle coasts to an apogee and comes back. To enter a normal Earth orbit it must eventually acquire sideways velocity anyway, so postponing that work merely increases gravity losses.
The second equation is the crucial one. A radial velocity component, straight away from the Earth, contributes no angular momentum about the Earth's centre. The tangential component does.
Same speed, radically different result
| At about 200 km | Velocity | What happens in the idealised model? |
|---|---|---|
| Straight up | 7.79 km/s radial | Large ballistic arc. It rises thousands of kilometres, stops, and falls back because the path has essentially zero angular momentum. |
| Sideways | 7.79 km/s tangential | Approximately circular low Earth orbit. Gravity continuously bends the trajectory around the planet. |
| Sideways, faster | Between circular and escape speed | Elliptical orbit, provided the ellipse does not intersect the Earth. |
| Any direction at escape energy | About 11.0 km/s at 200 km | Unbound in the two-body model, although real mission design must also consider the Moon, Sun and other perturbations. |
Configure the velocity vector
The calculator below treats the spacecraft as an instantaneous point mass at the chosen altitude. The angle is measured from local vertical: 0° is straight up, 90° is horizontal. It deliberately ignores atmosphere, thrust after the initial state, oblateness and the Moon and Sun. That is useful here because it isolates the one thing we are trying to see: direction matters as much as speed.
PHP velocity decomposition
What a real launch adds
A launch vehicle does not receive its final velocity instantaneously. Thrust acts over minutes, mass falls as propellant is consumed and stages are discarded, aerodynamic pressure changes rapidly, and guidance must satisfy structural, range-safety and mission constraints. A real ascent therefore has a continuously changing position, velocity, acceleration and attitude.
Nevertheless the central lesson survives all the engineering detail: by orbital insertion, the rocket has turned most of its hard-won speed sideways. The next page lets you give a spacecraft arbitrary radial and tangential velocity components and watch the resulting path.
Loading this essay’s discussion…