In spaceflight, distance is a poor measure of how hard it is to get somewhere. What counts is delta-v, the change in velocity a vehicle must achieve. Delta-v is the common currency of mission planning: it links rockets, propulsion systems and orbits, and it explains why some destinations are far more expensive to reach than others.
What delta-v means
Delta-v is the total change in speed that a spacecraft must produce through its engines to perform a manoeuvre or a whole mission. Each manoeuvre, such as raising an orbit, changing inclination or leaving orbit at the end of a mission, has a delta-v cost. Adding up these costs gives the delta-v budget of the mission.
The cost of reaching orbit
A satellite in low Earth orbit travels at roughly 7.8 kilometres per second. Because the rocket also has to overcome gravity and air resistance during the ascent, the total delta-v needed to reach low Earth orbit is typically around 9 to 10 kilometres per second. This is why most of a rocket's mass at lift off is propellant.
The rocket equation
The Tsiolkovsky rocket equation connects delta-v with the exhaust velocity of the engine and the ratio between the fully fuelled and the empty mass of the vehicle. The key insight is that the required propellant grows exponentially with the delta-v. Small additional manoeuvres can therefore demand a surprisingly large amount of extra propellant, and efficient engines with high exhaust velocity save a lot of mass.
Typical costs in orbit
- Plane changes: changing the inclination of an orbit is among the most expensive manoeuvres, which is why satellites should be launched as close as possible to their target plane.
- Orbit raising: moving from a low orbit to a higher one requires two burns, one to enter a transfer ellipse and one to circularise.
- Deorbiting: lowering the orbit so that the satellite reenters the atmosphere must be included in the budget to meet space debris rules.
Why it matters when choosing a launch
A launch that places a satellite exactly in its target orbit saves delta-v, and therefore propellant, on the satellite. A cheaper rideshare to a nearby orbit may shift that cost onto the satellite or onto an orbital transfer vehicle. Comparing launch offers means comparing where the delta-v is spent.
Frequently asked questions
What does delta-v mean?
Delta-v is the total change in velocity a spacecraft must produce with its engines to perform a manoeuvre or a whole mission.
Why does delta-v matter for launch planning?
A launch that places a satellite close to its target orbit saves delta-v, and therefore propellant, on the satellite. A cheaper rideshare may shift that cost to the satellite or a transfer vehicle.
Which manoeuvres cost the most delta-v?
Plane changes, meaning changes of orbital inclination, are among the most expensive manoeuvres in orbit.
Related reading
- Geostationary Transfer Orbit (GTO) Explained: How Satellites Reach GEO
- Electric vs Chemical Propulsion: How Satellites Move in Orbit
- Rocket Staging Explained: Why Rockets Drop Parts on the Way to Orbit
Sources
Cover image: BIRDS-2 CubeSats deployed from the International Space Station, 2018. Credit: NASA (iss056e130478), public domain.