The guidance computer
One of the most important things if you have the ambition of going to space is the guidance computer. This incredible machine will ensure that your rocket stays on its calculated trajectory, performs roll maneuver when required, and initiates the gravity turn at the correct time.
The guidance computer monitors everything during flight and make tiny adjustments when needed. Without it, we wont go anywhere but up in smoke.
A rocket cannot be controlled like you would control a plane, therefore the guidance computer must assist.
- Ascent trajectory guidance computer control
While the initial ascent parameters will be calculated for us, the guidance computer will listen to the inputs you set. But adjusting here can have very big consequences, so be careful…
Loss of a vehicle is a very expensive experience…
- Space shuttle launch
For the space shuttle, the roll program is very important. Quickly after launch it would roll to a belly up position. This was done for a few reasons, most importantly to keep structural and control loads within safe limits.
Because the Space Shuttle was mounted on the side of the external tank, its engines did not point through the center of mass by default. The roll reoriented the vehicle so the engines could gimbal in a desirable direction, keeping thrust aligned and minimizing stress loads on the tank and boosters.
Additionally the belly up roll had other advantages such as better antenna ranges.
The roll program was a consequence of bringing a winged aircraft into space.
- Completed roll program, and active gravity turn
The gravity turn
While the roll program is important, so is the gravity turn. To reach orbit efficiently, building horizontal velocity is key. The gravity turn gradually pitches the vehicle from vertical to a near horizontal attitude, allowing thrust to be used mainly to increase orbital speed while gravity curves our trajectory.
Orbit does not require escaping Earths gravity, but achieving a specific horizontal velocity that depends on the chosen orbital altitude.
Orbital velocity is calculated using:
v = sqrt(mu / r)
v = sqrt(mu / (R + h))
Where:
mu = Planet gravitational parameter
R = Planet radius
h = Target orbit altitude
Example orbital speeds around Earth:
200 km: 7,790 m/s
400 km: 7,670 m/s
800 km: 7,460 m/s
2,000 km: 6,900 m/s
35,786 km (Earth Geostationary): 3,070 m/s
Logically it would make sense that a higher velocity was required at a higher orbit altitude, but this is actually opposite because gravity gets weaker the further we get from a planet and therefore less centripetal force is required to stay in orbit.