Tutorial: Basic Orbiting (Math)/fr

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Revision as of 16:05, 23 April 2014 by Mixoupe (talk | contribs) (Vitesse orbitale)
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KSP apparently greatly increases the understanding of orbital mechanics

Dans le tutoriel de mécanique orbitale de base à été introduit le concept d'orbite, de stabilisation d'orbite, d'orbite de transfert ainsi qu'une table des vitesses orbitales. Mais que faire si vous désirez suivre une orbite qui n'est pas dans cette table ? Ou si vous voulez orbiter autour d'un autre corps que Kerbin ? Et avec une période spécifique ? C'est là que vient le moment de faire un peu de maths !

Vitesse orbitale

La relation entre vitesse orbitale et accélération est donnée par la formule

est l'accélération centripète due à la gravité, est la vitesse horizontale (orbitale) et est le rayon de l'orbite (distance au centre du corps orbité).

Bien sûr, la gravité dépend de la distance au centre du corps orbité et se calcule comme suit :

  • est un paramètre gravitationnel du corps orbité (3530,461 km³/s² pour Kerbin).
  • est la constante gravitationnelle ().
  • est la masse du corps orbité.
  • est le rayon du corps orbité (600 km pour Kerbin).
  • est l'altitude de l'orbite.

Si nous orbitons autour de Kerbin, on peut obtenir cette valeur de à partir de la valeur de , qui est l'accélération de la pesanteur à la surface de Kerbin, par la formule


Now we have two expressions for a, so we set them on opposite sides of an equation and simplify:

Finally, substituting known values for g and R,

Orbital Period

From the basic mechanics formula:

We know v from the above, and d is simply the circumference of a circle with a radius equal to your orbital altitude plus the radius of Kerbin:

Orbital Equations

For this section it is assumed that the craft in question is orbiting a particular body and is a lot smaller than the body that it is orbiting. Other then in the general orbital equation and the energy equation, the orbits are assumed to be bounded orbits completely within the body's sphere of influence and no others. This section also assumes that classical physics applies in the game. All distances and radii are from the center of the body being orbited and the sea level radius of the body needs to be added to the altitude given in game to recover the values here. The section uses the standard gravitational parameter which is Newton's Gravitational Constant and is the mass of the body that is being orbited. From here on out will refer to the mass of the craft.

When the craft is orbiting a body, the distance between the craft and the body is given by the equation

where is the angular momentum of the craft, is the eccentricity of the orbit, and is an angle parameter for the orbit. runs from to , at apoapsis or periapsis, and . When the orbit is bounded and is an ellipse and when the orbit is unbounded. At the craft is at the periapsis and at the craft is at the apoapsis. At periapsis,

and at apoapsis,

.

The semimajor axis is

and the semiminor axis is

.

Given the results for and above,

and

thus

The velocity for an object in elliptical orbit about a much larger object is

.

The energy for an object in orbit is

.

One should note that for bounded orbits, the energy is negative and for escape orbits, energy is non-negative.

The orbital period is

.

Let and be the two apsides of a given orbit and let a craft execute a burn imparting a specific all at the single point in time the craft is at . For this problem can be either positive or negative. If it is positive then the thrust is to speed up the craft but if it is negative the ture delta v is the oppositie of that in the problem but the thrust is to slow down the craft. This problem assumes that the thust is in the same or opposite direction of travel and is not enough to reverse the craft. The new orbit will have apsides of and with .

The speed of the craft before the burn is

.

The speed of the craft after the burn is

which yields

and