What is delta v?

[Prat4545]

I'v heard of delta v in many videos but i dont know what it is? Thanks for the explaining!(sorry for my english)

[HebaruSan]

The forum FAQ has an entry for this:

On 2/19/2018 at 8:57 AM, Deddly said:

Q: What is Delta-V / dV / ΔV?
A₁: dV literally means 'change in velocity'. Basically what you can do depending on the amount of fuel you have, the total weight of your craft and the efficiency of your engines.
A₂: Mathematically, delta-V is calculated with the Rocket Equation - 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)

 

[Prat4545]

okay i'll check it out!

 

 

 

[Streetwind]

Conceptually, delta-v is for rockets roughly what range is for cars. Your shiny new Tesla Model S might say: this goes 400 miles on a charge. So you know that if your destination is 300 miles away, you can reach it without recharging.

In space, however, this doesn't work, because there is no such thing as a fixed distance in space. In fact, there is no such thing as "being in a location" at all. Everything moves, all the time. It is physically impossible for an object in space to not move. So, if you are moving all the time, and your destination moves all the time, how do you know that you have enough "range" to get there? Even if right now you are 5 million kilometers apart, next week it might be 6 million, and a month later, 4 million. You just cannot plan any trips this way!

...Except, you can. All it takes is reading what I wrote up above again and really realizing what it means. "In space, everything moves, all the time". What this truly means is that range doesn't matter. You are moving, and your destination is moving. If you simply happen to move in such a way that you cross the path your destination moves along, at exactly the same time that the destination also moves through that point in space, then you will encounter each other. It does not matter how far away you were when you started.

So how do you make sure you move in the specific way you need to be moving in order to cross the path of your destination at the right time? Why, you fire your engines, and change the way you are currently moving. You change your speed, and you change the direction of your speed. In physics, a speed with a directional component has a name: velocity. So in order to encounter the destination, you must change your velocity in a certain, very specific way.

And once you have crossed paths with your destination, you obviously want to stay there. In space, where everything moves all the time, the only way to stay somewhere is to move at the same speed, and in the same direction. So once more, you fire your engines and change your velocity until it matches that of your destination.

Delta-v is the numerical measure of how much you can change your velocity until your fuel tanks are dry and your engine can no longer fire. Therefore, it is the go-to number for measuring how far - to which destinations - your rocket can go (without refuelling).

[king of nowhere]

to make it shorter: deltaV represents how much you can change your velocity.

a rocket has a total deltaV available which depends on its fuel, and it spends some of that deltaV every time it makes a manuever to change its trajectory.

for example, to move from a low orbit to a higher orbit you need to accelerate, to escape the planet's gravity. that acceleration is measured in deltaV, how much your rocket accelerated. and you expended fuel, so your available deltaV for future manuevers will be lower.

there are many different ways to get the same result in space, but some are more expensive than others. finding ways to get to the same place with less deltaV is of primary importance in a space program.

[Vanamonde]

Since everything in space travel is moving, your ability to reach places is determined by your ability to change the velocity of your craft. The V in dV stands for velocity and the "d" part is short for the Greek letter delta, used in mathematics to represent change. So the dV rating of your craft is a measure of its ability to travel and determines the places it can reach. 

[jimmymcgoochie]

In the simplest terms possible, delta-V is a measure of how much your rocket can accelerate. If you take a rocket with 1000m/s of delta-V and put it in orbit of Kerbin so that the orbital velocity is 2400m/s, then fire the engines to accelerate in the same direction that you’re orbiting (prograde) until the fuel runs out, you’ll end up going at 3400m/s; conversely if you fire the engines to slow down your orbit (retrograde) you’ll slow down to 1400m/s- and then fall out of orbit.

Gravity is constantly trying to drag your spaceship back down to the ground, but when it’s in a circular orbit the acceleration by gravity (9.81metres per second, per second or 9.81m/s²) is cancelled out by the fact that the object you’re orbiting is spherical (more or less) so as you ‘fall’ towards it, it’s constantly curving away from you.
Decrease your speed and the acceleration from gravity (m/s²) has more seconds to pull the rocket down meaning that it gains speed (m/s) towards the surface, losing altitude but gaining orbital velocity; increase orbital speed and the opposite happens- gravity has less time to drag you down compared to the surface’s curve so you gain altitude but lose velocity.
A rocket in orbit is going fastest at periapsis and slowest at apoapsis so while it’s best to make plane change burns at apoapsis- because the velocity you’re putting in is a bigger fraction of what’s already there- it’s better to do transfer burns at periapsis because the rocket has maximum velocity already and so you need to add less to reach the required speed to get where you’re going.

Don’t worry too much if you don’t understand everything, this is rocket science after all!

[Prat4545]

So the difference is 1000z delta v?