Nibbles in Space: Orbit Transfer

A spacecraft can change its orbit by changing its velocity.

And, perhaps surprisingly, to go higher you need to go slower.

The main way of changing the velocity of a spacecraft is to take part of the mass of the spacecraft, and eject it. The force required to do so creates an equal and opposite force on the spacecraft, meaning the spacecraft changes velocity. This is the basic principal of function of a rocket, and is what happens when you let the air escape from a balloon, and the balloon flies across the room.

Pad abort technology tested by Boeing Rocketdyne at a test stand at NASA’s Marshall Space Flight Center in Huntsville, Alabama, in 2003.

The simplest and most common approximation to changing orbit is termed a Hohmann transfer, after Walter Hohmann. Typically, this provides the minimum required change in velocity between two orbits. A Hohmann transfer is an arc, or intermediate orbit, connecting the two orbits with the start and end of the arc tangential to each orbit. It requires the spacecraft’s rockets to fire twice, once at either end of the arc to enter the intermediate orbit, and to leave it and enter the target orbit.

This tool calculates how much velocity change is required to move between two orbits, it will also warn if a Hohmann transfer may not be the most mass efficient option. It will calculate the propellant mass required to move between the orbits, and provides a simple 2D visualisation of the orbit transfer. To determine the propellant mass, the specific impulse, I_sp, of the rocket is required. This is a measure of the rockets efficiently and, curiously, has units ‘seconds’. A typical rocket may vary from around 200 – 450 seconds.

You can read more Nibbles in Space at SpaceProf.xyz , with 000A₁₆ on ‘How do you move around in space?’. And, if you want to go into more depth at The International Handbook of Space Technology which is a definitive modern text on space technology with contributions from globally leading agency experts from NASA, ESA, JAXA, and CNES, as well as European and North American academics and industrialists. This comprehensive handbook provides an overview and a holistic understanding of the system-of-systems that is a modern spacecraft.