Space Simulator  1.0.9
space flight simulator
Orbital Attitude

In Space, the standard concepts of orientation ( North , Up , down,etc ) are meaningless. The only absolute reference is the spacecraft's current orbit.So , when defining the orientation of a spacecraft , we refer it as its attitude respect its current orbit OR Orbital Attitude.

A spacecraft can orient towards some special axis.

  • Prograde : The spacecraft is oriented "nose" first , and the engine pointing "backwards" relative to its orbit.
  • Retrograde : The spacecraft travels 'engine first'. It is the opposite to prograde.
  • Normal Plus : (or normal + ) , The spacecraft points perpendicular 'up' to the plane that lies in the orbit
  • Normal Minus : ( also normal - ) the spacecraft points opposite Normal+ , that is perpendicular 'down' to the plane of the orbit
  • Radial Plus : (Or Radial+ ) the Spacecraft lies perpendicular to the velocityVector and in the same plane as the orbit.
  • Radial Minus : (Or Radial- ) the Spacecraft lies perpendicular to the velocityVector and in the same plane as the orbit ( opposite )

Each of those orbital attitudes are relevant for certain orbital maneuvers. For example:

  • Using the propulsion engine when the spacecraft is Prograded or retrograded, increases or decreases its orbital velocity , without altering the orbital plane. These are the only practical orientations, in which all the energy used in the engine, is transformed to an increase/decrease of orbital velocity. UNLESS there is a good reason for not doing so , the engine should only be fired in the Prograded ( or retrograded ) orientation.
  • Alternatively , firing the engine when the spacecraft is in its normal_plus or normal_minus orientations, will change its orbital inclination.the orbital plane will rotate -very slowly- but the final orbital velocity will be identical. We have used fuel , but the final kinetic (orbital ) energy of the spacecraft is still the same ( althought at a different orbital angle ). This maneuver is intrinscly wasteful , and so its -most of the time- avoided , other than a very fine correction of orbital plane when performing orbital rendezvous.

SpaceCraft Attitude Control

Spacecrafts in orbit arent affected by anything else than the gravity of the object they are orbiting around.In the vacuum of space , the spacecraft will keep its rotational velocity ( whatever it is ) for ever, only affected by its own momentum. In other words, a steady spacecraft will keep its orientation until a rotation force is applied. A rotating/ tumbling spacecraft will keep its rotation or tumbling until a new rotational force is exerted. As the spacecrafts are in the vacuum of space, normal aeronautic attitude controls (ailerons, elevators, rudders ) wouldnt work in space. To solve this problem , there exist just two ways to apply rotational forces to an spacecraft:

A rotational force ,is a force that doesnt cross the center of gravity , and as such ,makes the object change its rotational speed. In more precise terms is named TORQUE. From now onwards, we will refer to TORQUE as the forces that make a spacecraft accelerate its rotation . Please note that -unlike aircrafts - spacecrafts will keep their rotation for ever, so in order to -say, a change in roll- we need to accelerate the roll velocity , and -once nearing the desired roll- decelerate the roll velocity to an standstill.

  • Reaction Control System (RCS ) . A set of -ussually paired- small rockets, that fire in pairs, to apply some Torque.
  • Reaction Wheels. A set of 3 or more heavy electrically controlled flywheels that -by spinning them up - increase the rotation around the desired axis.

Which way is Up,anyway?

Well ,in ausence of fixed coordinates, we can only refer to rotations as viewd from within the rocket.

  • Roll is the angle that varies when the rocket rotates along its longer (UP) axis . For a forward(up) facing observer , a roll will be seen as a rotation of the whole view . This could be similar to the movement side-to side of an aircraft aileron control.
  • Pitch is the angle that varies when the rocket rotates around its side . For a forward(up) facing observer , a pitch movement will make its horizon move up/down. This could be similar to moving an aircraf's yoke forward or pulling backwards.
  • Yaw is the angle that varies then the rocket rotates around its up/down side. Think of this as an aircraft rudder ( foot-rudder ).

Some spacecrafts (notabbly the Grumman LM Moon Lander ) have the main windows not facing UP , but forward. But the main engine still fires traditionally "up". The controls in the forward facing landers, are modified accordingly ( roll is pitch , yaw is roll ,etc ) for simplicity.

Little rocket engines ?

Well , it sounds simple but is not that simple at all. RCS is one of the most prone to malfunction elements in story of spaceflight. The dangers of a non functional RCS are dwarfed by the dangers of a non-stoppable RCS (as it happened to N.Amstrong in Gemini X mission ).They need to work, and most impotantly they need to stop when commanded so. Given the non-zero chance of a valve sticking open ( RCS could not stop , waste of fuel ) , or stickin closed , most critical RCS fuel valves are designed on 'Valve Quads'. A valve Quad is a combiation in paralell of two combination of valves in series. It can be seen that not a single valve malfunction would prevent normal operation. The rockets themselves tend to be simple. Normal RCS are monopropellant , just a highly active ( and toxic! ) fluid that -in contact with some exotic catalists on the rocket's bell surface- will ignite spontaneusly. Basicly as simple as a rocket engine could be. For the Apollo program , a diferent kind of RCS were used, mainly for the Moon lander. As the final lunar orbit rendezvous maneuvers would take an unknown ammount of fuel for final corrections, and given the extremely serious consecuences of running out of RCS fuel , the moon lander's RCS used the same fuel as the Ascencion Stage main engine. This way, the (hopefully lots of ) unused fuel and oxidizer from the main ascent engine could be used to trim the final docking velocities via RCS. Also , in case of a non catastrofic Ascent Engine malfunction , the fuel could be used by the four RCS in translation mode to -with a bit of luck- limp into orbit.