can someone explain delta-v to me?

[Super 6-1]

I've tried looking it up, but i can't find anywhere that explains it in simple terms

[RoboRay]

The key measure of a spacecraft's capabilities is how much it can change it's velocity. Every maneuver you make is altering your orbit by a specific velocity. If you're in low orbit around Kerbin and want to flyby the Mun, you need to change your velocity by about 860m/sec. To turn your flyby into an orbit around the Mun, you have to spend another 210m/sec. To land from orbit, it's 640m/sec.

Delta-v is literally "change in velocity" expressed mathematically, and is used to determine, more or less, just where you craft can go and where it can't. Using the example above, if your ship has only a total capability of 1200m/sec remaining when in Kerbin orbit, it can orbit the Mun but it can't land on it.

[EndlessWaves]

It's effectively range.

In space there's no friction so you keep on going forever so it doesn't make much sense to measure range in meters. Instead what you're interested in is how you can get to an orbit further out. Orbits further out have more energy so you add to your vessel's energy by increasing speed. DeltaV is the amount of energy you need to/can add to change your speed by the given amount at that point in space.

[rpayne88]

An even simpler way to look at it is to think of it as the fuel gauge in a car. 0Dv= no maneuvers. There are mods such as mech jeb and kerbal engineer that will compute your Dv for you. Dv takes into account all velocity changes. For example, to go from 0m/s to 100m/s takes 100dv, but to go from 0 to 100 back to 0m/s takes 200dv. If you want to calculate it manually, you can use Tsiolkovsky's (you'll probably butcher the pronunciation) rocket equation. The equation is as follows:

Dv=Ve*Ln(M0\M1)

Dv= delta v

Ve= effective exhaust velocity (see below)

Ln= natural logarithm

M0= initial total mass including propellent (in KSP, just its mass)

M1= empty or dry mass

Make sure you use this equation for each stage. As you jet stages, the mass decreases.

Note" Dv does not take into effect thrust to weight ratio (TWR> 1 needed to left off the pad for rockets) or aerodynamic drag

Effective Exhaust Velocity

Ve=Isp*Gn

Ve= effective exhaust velocity

Isp= initial specific impulse (the same Isp you see in the part descriptions)

Gn= Standard Gravity (on Earth as well as Kerbin, this is 9.8m/s^2 or 32ft/s^2) This does not effect your Dv when you are flying level with the ground

Natural Logarithm:

This is a mathematical constant so named because of its appearance in formulas used to predict the natural world. I can't fully explain it to you, but if you have a graphing calculator, you can hit the ln button.

If you like math, use the equations above. If you're like me, either use a mod or trial and error.

Sorry if this is redundant, I don't know your education level or how long you've been playing KSP and I do not wish to know.

[K^2]

DeltaV is the amount of energy you need to/can add to change your speed by the given amount at that point in space.

It is not, strictly speaking, energy. Energy is not a terribly convenient metric for a rocket. But if it makes it easier for you to think of it that way, then that's fine. Just keep in mind that there is a distinction.

[Super 6-1]

Thank you very much! I am surprised at how fast that someone responded, let alone 4 people.

[nyrath]

There is no friction in space. Here on Terra, if you are driving a car and take your foot off the accelerator, the car will coast to a stop due to the friction of the road. In space, if a spacecraft turns off its engines it will maintain its current velocity for the rest of eternity (unless is crashes into a planet or something). In the movie 2001 A Space Odyssey, you may have noticed that the spacecraft Discovery was traveling to Jupiter with nary a puff coming out of the rocket motors.

This is why it makes no sense to talk about the "range" of a rocket. Any rocket not in orbit around a planet or in the Sun's gravity well has a range of infinity. In theory it can do a burn and travel to, say, the Andromeda Galaxy, it is only that it will take millions of year to get there. Instead of a rocket's range, one should talk about a rocket's delta V capacity.

The important thing is that a mission can be rated according to how much delta V is required. For instance: lift off from Terra, Hohmann orbit to Mars, and Mars landing, is a mission which would take a deltaV of about 18,290 m/s. If the spacecraft has equal or more delta V capacity than the mission, it is capable of performing that mission. The sum of all the deltaV requirements in a mission is called the deltaV budget.