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[Ethanadams]

What in the world is ÃŽâ€V?

[Th3F3aR]

Wow

That is the amount of fuel/power you have to change your velocity (Basically)

For a moar detailed insight -> http://en.wikipedia.org/wiki/Delta-v

[RainDreamer]

It is delta V, the amount of velocity change you can perform with your current set up. May increase and decrease for a lot of reasons relating to ISP, mass, and current fuel.

[UmbralRaptor]

ÃŽâ€V is change in velocity, and it acts as a size-independent measure of maneuver difficulty and craft performance. The rocket equation gives you how much a given craft has. For the amount required with various maneuvers, I'd suggest looking at the Vis-viva equation or various ÃŽâ€V'>http://bit.ly/1qAmN1K]ÃŽâ€V maps

[Pecan]

There are two figures you need to work-out for your rockets in order to know how they'll perform.

TWR - Thrust to Weight Ratio = How much thrust your engines can provide 'up' versus how much gravity is pulling you 'down'. Less than 1 and gravity wins, you aren't going to space today. Exactly 1 and a vehicle can hover or maintain its current vertical speed but not accelerate upwards (if the current vertical speed is 0, on the pad, you aren't going to space today). A launch TWR of 1.2 - 1.6 is generally considered best. More engines (thrust) or less vehicle (mass) increases TWR so keep your vehicles light. Not so important once in orbit, but a low TWR means your vehicle can't accelerate as quickly (isn't as agile).

DeltaV - potential change in velocity vector = To change direction or speed in space you need to fire your engines in the opposite direction. How much a vehicle can change its speed or directon (the velocity vector) depends on i) the engines' Isp (shown in the VAB), ii) how much fuel the engines have, iii) how much the vehicle masses. Use more efficient engines (higher Isp), add extra fuel or reduce the mass of the vehicle (eg; by staging to jettison empty tanks and other used components) to increase the deltaV.

The tricky bit for launch-vehicles is that getting a high TWR usually means big, heavy and inefficient engines ... which then can't lift the amount of fuel they need to get to orbit. Using more efficient engines often means they don't have enough thrust to get off the pad in the first place. It's rocket science. (See 'Exploring The System' linked below).

[OhioBob]

There are actually two common usages of the term ÃŽâ€V. There is the one that everyone here has given, i.e. the potential change in velocity that your propulsion system can provide. But it is also the change in velocity needed to produce a change in orbit, or to complete a particular maneuver. For example, let's say you are in a 100 km circular orbit around Kerbin and you want to raise your apoapsis to 1000 km. That maneuver is going to require a change in velocity, or a ÃŽâ€V, of 403 m/s. To complete a mission you need to identify all the maneuvers you'll have to perform and add up the ÃŽâ€V for all those maneuvers to produce a ÃŽâ€V budget. You then have to design your spacecraft to make certain it can produce a ÃŽâ€V equal to or greater than your ÃŽâ€V budget (likely a little bit more to play it safe).

[LethalDose]

There are actually two common usages of the term ÃŽâ€V. There is the one that everyone here has given, i.e. the potential change in velocity that your propulsion system can provide. But it is also the change in velocity needed to produce a change in orbit, or to complete a particular maneuver. For example, let's say you are in a 100 km circular orbit around Kerbin and you want to raise your apoapsis to 1000 km. That maneuver is going to require a change in velocity, or a ÃŽâ€V, of 403 m/s. To complete a mission you need to identify all the maneuvers you'll have to perform and add up the ÃŽâ€V for all those maneuvers to produce a ÃŽâ€V budget. You then have to design your spacecraft to make certain it can produce a ÃŽâ€V equal to or greater than your ÃŽâ€V budget (likely a little bit more to play it safe).

This highlights a good point. I think of dV like a currency. The dV remaining in your vessel is what's in your "wallet" or "purse", and the dV needed to make a maneuver is the "cost". In what Bob has described, "what everyone here has given" is the former, and "change in velocity needed etc" is the latter. Note both "kinds" of dV are in m/s, the same way both costs in a store and money in your wallet is represented in dollars or pesos or pounds or whatever.

Trips from A to B, like from LKO to the surface of the Mun, can be done via multiple different paths with differing dV costs. The trick is to find a low dV cost path, and then design a vessel with sufficient dV (and TWR, but that's beyond this discussion) to fly that path.

One final note: The dV costs of the trips are independent of vessel mass, which makes it a convenient measure for discussing and comparing orbital flight paths to reach various locations.

[Red Iron Crown]

ÃŽâ€V is change in velocity, and it acts as a size-independent measure of maneuver difficulty and craft performance. The rocket equation gives you how much a given craft has. For the amount required with various maneuvers, I'd suggest looking at the Vis-viva equation or various ÃŽâ€V maps

So much this. With particular emphasis on the "size-independent" part, that's a big reason why dV is so tremendously useful. It doesn't matter whether a spacecraft is a tiny probe or a 1000-ton monstrosity, it is the vessel's dV that determines what destinations it can reach.

[Norpo]

With particular emphasis on the "size-independent" part...

A visual representation of the "size-independent" part.

Small probe, weighs 1.3 tons. 4,150 m/s launchpad delta-v. (The isp of an engine can change based on atmosphere levels, and engines are typically most efficient in space.)

(In case you didn't know, I'm using Kerbal Engineer Redux for the delta-v data.)

Large ship, weighs a whopping 46.1 tons (That's 35.4 times heavier than the small probe.): Launchpad delta-v: 3,727 m/s. (That's less than the probe.)

The reason you'd still want to use a bigger ship is simple: staging. If you have a big stage, you can put a smaller stage on top of it, with little delta-v change to the larger stage. And so on, and so on. (Practical example: if I put on the small probe onto the large ship, and then put a decoupler between them, once the large ship had no more fuel, I could decouple the large ship entirely, and have the fuel of the probe, still. In essence, that'd be a ship of 7000-8000 m/s delta-v.

It's simple, it's only rocket science, after all.

[quietsamurai98]

ELI5 Explanation:

ÃŽâ€v is change in velocity.

The symbol Άdenotes the average change in a variable per one unit, and normally a lowercase v being used as a variable is velocity.

In the context of KSP, a craft with 1 ÃŽâ€v can change its velocity by 1 meter per second.

In order to change your orbit, you have to change your craft's velocity, and this is where knowing your craft's total ÃŽâ€v is useful.

Does that help you out?

[Alias72]

generalizing a bit. V is a letter that we use to represent the physical quantity velocity. Άis a mathematical symbol that means "the change in/of". so Άanything is the change in that thing. ÃŽâ€V is the change in velocity.

[cicatrix]

You drive a car along the road at 80 km/h, you need to stop. The delta (means difference) between your current speed and zero speed is 80 km/h. You need to go back at 40 km/h - the delta is 80 + 40 = 120 km/s.

Same thing but in three dimensions. Delta-V is very convenient for deciding where you can go and where you cannot.

[Zargg]

Some suggested reading:

http://mykspcareer.com/faqs/starting/

[Xannari Ferrows]

http://forum.kerbalspaceprogram.com/threads/107932-Planning-your-rockets-What-is-Delta-V

[EtherDragon]

What in the world is ÃŽâ€V?

Literal Translation:

Ά= Delta, meaning "a change to"

V = Velocity, usually measured in Meters per Second

So, ÃŽâ€V translates to "a change to meters per second."

ÃŽâ€V is used in a few places in rocket science.

First, ÃŽâ€V is used to denote the change in velocity required to perform a specific maneuver, such as circularizing an orbit. During ascent, the rocket performs a circularization burn at the Apopsis to add enough velocity to circularize the orbit. Doing so is a change in velocity, thus referred to as ÃŽâ€V.

Second, ÃŽâ€V is often short-hand for a rocket's Total ÃŽâ€V, which is the sum total that the rocket could change it's velocity in a single continuous burn assuming no outside factors.

Since a mission is a series of maneuvers, each with their required ÃŽâ€V, one of two main factors that determines the kinds of missions a rocket can complete is its total ÃŽâ€V budget.

[Red Iron Crown]

It's also important to note that velocity is a vector, with both magnitude and direction. Expending dV can result in a change in direction, change in speed, or both.

[Alias72]

a perfect example being thrusting towards kerbin. you spend a lot of delta V and don't change your velocity. Look at the map and you will find the shape of your orbit has changed.

[Acet]

As others pointed out, in purely mathematical terms ÃŽâ€v literally means "change in velocity".

In ship terms, a ship having a delta-v of X m/s means that, in a flat gravitational field (no gravity or a constant gravity curve which is so large that it can in practice be treat as a flat surface - for example high Kerbol orbit), then if you point the ship in any direction and use that ship's entire delta-v, then at the end of the acceleration that ship will be travelling X m/s faster in that direction than it did before you started that acceleration. Note that it doesn't mater what the ship's original speed was, nor what the ship's mass is

Some confusion might arise because in a highly curved gravitational field, such as Kerbin's low orbit, the values in the speed indicator before and after you have done a burn do not exactly match the amount of delta-v spent, simply because the gravitational pull over the period of the acceleration will have affected the actual velocity that the burn has imparted to the ship. Delta-v only represents a precise amount of speed change in the absence of any external forces, which is why one can spend an infinite amount of delta-v just hovering above the surface of a planet (since the speed imparted upwards by spending that delta-v exactly matches the speed taken by the gravitational pull downwards).

[Red Iron Crown]

Delta-v only represents a precise amount of speed change in the absence of any external forces,

Careful there, it's change in velocity rather than change in speed, they are not the same thing.

[mrfire45]

Delta-V is how much acceleration your ship could achieve in a vacuum.

[Red Iron Crown]

Delta-V is how much acceleration your ship could achieve in a vacuum.

Acceleration is the rate of change of velocity, not the amount of velocity change. A ship can have a pile of delta-V and very little acceleration.

[worir4]

Acceleration is the rate of change of velocity, not the amount of velocity change. A ship can have a pile of delta-V and very little acceleration.

Darn ion drives....

[Starwhip]

You drive a car along the road at 80 km/h, you need to stop. The delta (means difference) between your current speed and zero speed is 80 km/h. You need to go back at 40 km/h - the delta is 80 + 40 = 120 km/s.

Same thing but in three dimensions. Delta-V is very convenient for deciding where you can go and where you cannot.

Um, I think you're units are off by a *wee* bit. (Km/s instead of Km/h)

Edited March 5, 2015 by Starwhip