What Is Delta V?

[Blank]

Can someone please explain to me what Delta V is? Everyone talks about it, but I have no clue what it is, it's so frustrating.

Thanks, Blank.

[vexx32]

Moved to Gameplay Questions and Tutorials.

Delta-V, or dV as it's sometimes abbreviated, is a measure of the total amount of acceleration your ship can output. There're certain amounts required to get into orbit, to do certain interplanetary transfer burns, etc. Delta is most commonly (at least on paper) represented by the Greek letter Delta, meaning change. Literally, it's the total change in velocity your ship can possibly produce. Someone else'll have to school you in the exact uses of and calculations to determine it, as I am myself not a very math-heavy user of KSP. I tend to avoid the mathematical side of things wherever I can.

I usually see it as a sort of qualifier, determining whether or not a ship is capable of reaching certain places and so forth.

[lajoswinkler]

Let's say you have a spaceship in the middle of nowhere, standing motionless next to an object. Everything is in vacuum, there's no gravity, but nothing is orbiting. Everything is in an endless void. It's the simplest scenario.

The ship contains a known amount of propellant. When you fire your engines and let all of the propellant to be spent, your ship will be moving away from the object at a certain speed. That speed is delta v. The change in speed your ship can provide. From zero to n m/s.

If the ship is already moving away from the object at a speed of n m/s, and you fire all propellant with the nozzle pointed towards the object, the relative speed will be n + delta v. If you fire in the opposite direction, it will be n - delta v.

The scenario gets more complicated when things are orbiting, which happens in real world.

[Geschosskopf]

I usually see it as a sort of qualifier, determining whether or not a ship is capable of reaching certain places and so forth.

Very good explanation, Vexx.

I'll add a little more context. Basically, spaceflight is all about velocity because velocity is what determines which orbit you're in, or whether you're in an orbit at all. Spaceflight boils down simply to moving from 1 orbit to another, which is accomplished by changing velocity. Because each orbit has a particular velocity associated with it, you know how much you need to change your velocity (which as Vexx explained is what "delta-V" means) to move to any given orbit. So, the amount of delta-V in a ship is a good way to measure where it can go.

One of the most amazing things about KSP is that it provides no way in-game to calculate a ship's delta-V, despite this being essential information. Thus, you need to get a mod like Kerbal Engineer Redux or Mechjeb that can do this calculation during ship construction. Or you can do the math to calculate it yourself on paper, although this is a real pain. But without knowing how much delta-V you have, you have no idea whether or not your ship will actually reach Kerbin orbit, much less go anywhere else or come back (there are 3rd-party charts showing how much it takes to get somewhere).

[mhoram]

When you want an answer to the question "how much fuel do I need to get from the surface of Kerbin to the surface of the Mun", then the answer is: it depends on the composition of the rocket (the heavier the payload the more fuel you need).

However to make this answer independent of the rocket, then Delta-V is your friend.

You will ask: "how much Delta-V do I need to get from the surface of Kerbin to the surface of the Mun".

And the answer is: about 6260 m/s according to http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet

(Plan to have some reserve on board) (there are also some other dV-Maps available)

Stock KSP does not display the dV of your rocket, but there are some AddOns like Kerbal Engineer Redux that display it.

Edited December 1, 2013 by mhoram

[Specialist290]

Looks like I'm a little late to the party today, but all of the above users have posted some very good answers, so there isn't much I can add. Still, might as well go ahead and throw in a little more info:

A rocket works essentially by exploiting Newton's Third Law of Motion: "Every action has an equal and opposite reaction." To move a rocket, you have to shoot propellant out the other end. The propellant accelerates in one direction with a certain amount of energy; as a reaction, the rocket is pushed with an equivalent amount of energy in the opposite direction. However, because the rocket is shedding large amounts of mass, the effect of this energy per unit of propellant expended on the rocket's velocity (i.e. its speed in a given direction) isn't constant, because the mass this energy is acting on is changing as the rocket's propellant burns.

Over a century ago, a smart man by the name of Konstantin Tsiolkovsky came up with a formula by which you can calculate the amount of change in velocity using a measure of engine efficiency known as exhaust velocity, as well as the ratio of the rocket's full mass to final, dry mass. Essentially, what all the math boils down to is this: To increase the delta-v of a given rocket, you want either a lighter dry mass (which can be accomplished by either adding more fuel or subtracting mass from the payload) or a more efficient engine.

If you want more detail than that, I'd highly recommend you read this article at MyKSPCareer.com, as well as these three pages on the Atomic Rockets site.

Hope this helps

[capi3101]

http://en.wikipedia.org/wiki/Delta-v

Mathematically, delta-V is calculated with the Rocket Equation Specialist290 mentioned - it takes the form:

delta-V = ln(M/M_o)* I_sp *g_o

where delta-V is the change in velocity (a vector that's a dot-product of the scalar component of speed with the vector component of direction), ln is the natural logarithm function (look for it on a scientific calculator, or use =LN() in MS Excel), M is the full mass of the rocket stage, M_o is the dry mass of the rocket stage (i.e. what it weighs when all its fuel tanks are empty), g_o is standard gravity (9.81 m/s²regardless of what body you're orbiting/launching from) and I_sp is the specific impulse of the engine (a way of measuring the engine's efficiency). It's importance, as has been mentioned, is in determining the total magnitude of the changes the rocket may make to its velocity before it runs out of fuel; in the process it determines where a rocket may go given a certain mission profile.

There are three main ways of increasing a rocket's delta-V:

1) improving propellant mass fraction (i.e. moar fuel)

2) increasing specific impulse (by selecting an engine combination that increases this value - the main reason nuclear engines are recommended for interplanetary flight)

3) staging (shedding mass that's no longer needed, which has the effect of improving the propellant mass fraction)

[GreenLight]

So from what i can see its how much speed your ship can go i don't know.

[Streetwind]

5 hours ago, GreenLight said:

So from what i can see its how much speed your ship can go i don't know.

Not exactly, no. It's how much your ship can change ("delta") its speed ("V").

If you start at zero and accelerate linearly in one direction until you're dry, then yes, this is equal to the maximum speed you can achieve (barring gravity and such). But you can also spend dV to slow down. Or to deflect your trajectory into a curve. Or hover in place against the pull of gravity. And so on. That's why we talk about "velocity", which is the combination of a speed and a direction.

The thing you need to understand is that in space, there is no such thing as not moving. It is physically impossible to not move in space. Everything always moves. Therefore it makes no sense to talk about going from one place to another in terms of distance - it just keeps changing constantly. You cannot measure it in terms of time either, because if the distance keeps changing, the time required to travel that distance changes along with it. This of course begs the question: how do you describe going from one place to another then?

Think about it logically. If everything is always moving, then the only way to stay at a certain place is to also move in exactly the same manner (the same speed in the same direction) as that place. But that only works if you are already where you want to stay. If you are physically elsewhere, you first need to change your velocity so that you cross paths with the place where you want to be. Then, once your paths cross, you need to again change your velocity to make it match the velocity of the place where you want to stay.

So the way you measure going from one place to another in space is simple: measuring the effort of changing your velocity to cross paths with your target, plus the effort of then changing your velocity to then match that of the target. As long as you can do these two things, it literally does not matter how far away your destination is or how long it takes you to get there. You will be able to reach it.

This is why everyone talks about dV, the measure of your ability to change your velocity. The more you can change it, the more places you are able to go.  

 

Edited October 24, 2017 by Streetwind

[DrLicor]

As explainded best by many above. Delta-V is the change of velocity your ship is capable of. There a also very usefull charts how much delta-v is required.

Here you can see how much delta-v your ship must have to go to a place. For instance, when you want to go from kerbin to a munar orbit, you need 3400m/s (to kerbin orbit) + 860m/s (tranfer phase to mun) + 310 (capture + orbit at mun) = 4570 m/s of delta-v. 
This way you can design your ships carefully and more practicall. Your rocket/lifter will have 3400m/s your tranferstage 860m/s and your probe only has to have 310m/s. 

Delta-v readouts can be done with a few mods, like KER and Mechjeb:

 

 

[GreenLight]

17 hours ago, Streetwind said:

Not exactly, no. It's how much your ship can change ("delta") its speed ("V").

If you start at zero and accelerate linearly in one direction until you're dry, then yes, this is equal to the maximum speed you can achieve (barring gravity and such). But you can also spend dV to slow down. Or to deflect your trajectory into a curve. Or hover in place against the pull of gravity. And so on. That's why we talk about "velocity", which is the combination of a speed and a direction.

The thing you need to understand is that in space, there is no such thing as not moving. It is physically impossible to not move in space. Everything always moves. Therefore it makes no sense to talk about going from one place to another in terms of distance - it just keeps changing constantly. You cannot measure it in terms of time either, because if the distance keeps changing, the time required to travel that distance changes along with it. This of course begs the question: how do you describe going from one place to another then?

Think about it logically. If everything is always moving, then the only way to stay at a certain place is to also move in exactly the same manner (the same speed in the same direction) as that place. But that only works if you are already where you want to stay. If you are physically elsewhere, you first need to change your velocity so that you cross paths with the place where you want to be. Then, once your paths cross, you need to again change your velocity to make it match the velocity of the place where you want to stay.

So the way you measure going from one place to another in space is simple: measuring the effort of changing your velocity to cross paths with your target, plus the effort of then changing your velocity to then match that of the target. As long as you can do these two things, it literally does not matter how far away your destination is or how long it takes you to get there. You will be able to reach it.

This is why everyone talks about dV, the measure of your ability to change your velocity. The more you can change it, the more places you are able to go.  

 

Ah ok thanks for telling me that.