Introduction - FDS

FDS (Flight Dynamics System) is the popular name used across the industry. I think it’s because it’s a legacy term coming from the flight dynamics of aircraft and missiles, rockets, etc. I’m not especially a big fan of this naming because it fails to capture the essence of the complex challenge of dealing with chaotic conic sections **Orbital Mechanics** is the specialized term used for space context, describing the motion of objects that are orbiting another object, usually with a much bigger mass. All thanks to Newton and Kepler, we are able to very accurately calculate how the Celestial bodies move in space, and can apply the very same equations to find out the trajectory of the spacecraft we send outside Earth. If we disregard all the minor influences, the actual driver of this motion is none other than gravity. In high-school physics class we learn Newton’s second law as the equation `F = m⋅a` , force is equal to mass times acceleration. Now force is an abstract concept that we need to hold the universe in place, but mass and acceleration are very visible. In terms of Earth, we have the gravitational acceleration, known as `g` , which is around `$9.80655 m/s^2$` on the surface (6378.137 km away from the center of Earth around equator, and 6357 km around the poles – on average we can say 6371 km. Some values every astrodynamics fellow know by heart, add onto that the gravitational parameter of Earth `μ = 398600 km3/s2` , the distance between Earth and Moon 384400 km, and the Astronomical Unit – distance between Earth and Sun – AU = 1.496x10 8 km). So, the equation becomes which gives us the weight of an object on Earth. For simplicity, you can treat `g = 10` , so if your body mass is 80 kg, then your weight is 800 N on Earth. Now Newton's law of universal gravitation tells us that each particle attracts each other particle in the universe, and the force of that attraction is given by the following equation