What is delta v

[darthvader15001]

So I'm a noob. And I have no idea what delta v is. So what is it? I need a simple explanation.

edit: Thanks for all the replies @Superfluous J, @jimmymcgoochie, @StrandedonEarth and @magnemoe. Now that I know what delta v is, there is no need for any more explanations, but please keep this thread open as a source for beginners so that they don’t have the troubles I did

again, thanks for the replies

Edited August 8, 2022 by darthvader15001

[Superfluous J]

V is velocity

Delta is change.

Delta-V is change in velocity.

If your ship has 5000 m/s of dV you can change its velocity 5000 m/s. So if it's going 5000 m/s you can stop it by firing backwards, make it go 10,000 m/s by firing forward, or make it go 5000m/s sideways in addition to the 5000m/s it's going "forward" (i.e. make it go diagonal).

Edited August 7, 2022 by Superfluous J

[darthvader15001]

Ok thanks @Superfluous J. I wasn't expecting such a fast reply!

[Superfluous J]

It was so fast I had to edit it several times for accuracy Be sure to reload before using it as a reference.

[darthvader15001]

These are the fastest posts I've ever seen

[jimmymcgoochie]

V is velocity, delta is a change in a parameter's value. Delta-V is a change in velocity.

On the ground, things like towns and cities are generally stationary. Driving from, say, Berlin to Madrid or Los Angeles to Dallas will always be the same distance if you drive on the same roads, so it makes sense to state the range of a car in distance- miles or kilometres. The same principle also applies to boats and aircraft, with the caveat that water and air can and usually do move so you naturally go further with the wind/current than against it.

In space, things are different: everything is constantly in motion and the distances between them change from one second to the next. The straight-line distance from Earth to Mars varies drastically as the two planets travel around the Sun in their different orbits, sometimes coming close to each other and sometimes ending up at opposite sides of the Sun. Just sitting in low Earth orbit, you're travelling close to eight kilometres every second, so the International Space Station covers a distance of almost a quarter of a billion kilometres every year without actually going anywhere relative to the Earth.

Orbital mechanics are all about relative velocities, not straight-line distances. The only way to change the shape of an orbit is by altering its relative velocity, which requires a change in velocity (delta-V) to make it happen. If you add to your orbital velocity you'll push your orbit higher on the other side, but once you get there you'll actually be going slower than you were before due to the conservation of momentum- you've traded velocity for altitude. Add enough velocity and you can escape from the gravity well entirely.

To get from A to B in space where they're both orbiting the same parent body (Earth and Mars, Minmus and the Mun, Phobos and Deimos, Laythe and Tylo...) you'll need to increase your velocity to escape the gravity of the origin body, fly an elliptical trajectory between the two bodies and then decrease your velocity upon arriving at the destination body to get captured by its gravity and stay in orbit. Ideally, your trajectory will be an ellipse where the periapsis just touches the inner body's orbit and the apoapsis just touches the outer body's orbit, and both bodies will arrive at those points at the same time as the spacecraft; this is called a Hohmann transfer and is the most efficient way of doing it, minimising the change in velocity (delta-V) required to make it work.

[StrandedonEarth]

Delta-v has now been defined. But it depends on a few things.

Delta-v is a function of mass, Isp and mass fraction (ratio of propellant mass to total mass) as per the Rocket Equation . So any increase in dry mass will reduce the available dV, which is why designers strive to minimize dry mass. Isp is generally fixed once the propellants are chosen and the engine is built, but can still vary depending on ambient pressure and engine performance (malfunctions, ;leaks, etc).

 Which leads to the "tyranny of the rocket equation:"  adding more propellants doesn't add much more dV, because those propellants also need to be accelerated. Which is why staging became a thing, to drop the extra mass of large tanks and the heavy, high-thrust engines required to accelerate enormous quantities of propellants..

[magnemoe]

4 hours ago, StrandedonEarth said:

Delta-v has now been defined. But it depends on a few things.

Delta-v is a function of mass, Isp and mass fraction (ratio of propellant mass to total mass) as per the Rocket Equation . So any increase in dry mass will reduce the available dV, which is why designers strive to minimize dry mass. Isp is generally fixed once the propellants are chosen and the engine is built, but can still vary depending on ambient pressure and engine performance (malfunctions, ;leaks, etc).

 Which leads to the "tyranny of the rocket equation:"  adding more propellants doesn't add much more dV, because those propellants also need to be accelerated. Which is why staging became a thing, to drop the extra mass of large tanks and the heavy, high-thrust engines required to accelerate enormous quantities of propellants..

This also using lower trust but lighter and more efficient engines for upper stages and interplanetary missions is smart. 
Also this is the map of the Kerbin system based on Dv cost. 
Its pretty useful as it show the cost of various missions, counter intuitive Minmus is cheaper to land on than the Mun even if the cost to reach it is more expensive because the much lower cost to land. 
It works both ways, the 280 dV cost from flyby to enter orbit is also the cost of going from low Mun orbit back to Kerbin but here you can use aerobraking to land. 
This is also for perfect burns so you will use more than 580 m/s dV to land on the Mun but to get back into orbit will be close to it. 
Add up the values so going to Duna is 3400 to get into orbit then 930+130, add up to 10 for inclination, you can then aerobrake into Duna orbit 

[darthvader15001]

Thanks for all the replies @Superfluous J, @jimmymcgoochie, @StrandedonEarth and @magnemoe. Now that I know what delta v is, there is no need for any more explanations, but please keep this thread open as a source for beginners so that they don’t have the troubles I did

again, thanks for the replies