# Can someone ezplain what DeltaV is?

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I always here people talking about it around here, and now I feel like an idiot for asking what it is, but what is DeltaV?

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To put is simply it is the potential change in your velocity. It is effected by the power of your engines, the weight of the ship, and changes in these as fuel is burnt as you fly.

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Delta, being the fourth letter in the Greek alphabet, is often used in Physics and Mathematics to refer to change, or difference.

V, is short for Velocity. So - together this term describes the Change in Velocity. But that only goes so far, the rest depends on the context the term is being used.

If you are looking at Delta-V as a property of a Rocket, it refers to the total change in velocity available to the rocket. This value is governed by Tsiolkovsky's rocket equation.

If you are looking at Delta-V as the result of a maneuver (a plane change, or circularization burn, for example), it describes how much your velocity needs to change to complete the maneuver. This kind of Delta-V is governed by the equations describing orbital mechanics.

Basically, your rocket ship "has so much Delta-V" meaning it can change it's velocity only so much before running out of propellant. Each maneuver requires a certain amount of Delta-V to complete. Every mission, being a series of maneuvers, requires a certain total Delta-V to be able to accomplish safely.

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... now I feel like an idiot for asking ...

Don't.

Ever.

The only stupid question is the one you don't bother to ask.

The only idiots are the ones who don't bother to ask what they don't know.

[Now the other answer is - deltaV is this bit of jargon that means 'acceleration'. Trick there is you might 'accelerate' backwards (burn postgrade) or sideways]

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Don't.

Ever.

The only stupid question is the one you don't bother to ask.

The only idiots are the ones who don't bother to ask what they don't know.

[Now the other answer is - deltaV is this bit of jargon that means 'acceleration'. Trick there is you might 'accelerate' backwards (burn postgrade) or sideways]

I whole heartedly agree! After-all we're talking about Rocket Science, so the unasked question can literally be the difference between landing on Duna and orbiting aimlessly in space for eternity with a dead craft - no fuel, no power, little chance of rescue. =p

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Thanks I think I am slightly closer to understanding it... Ah whatever, I will probably learn it in high school

Don't.

Ever.

The only stupid question is the one you don't bother to ask.

The only idiots are the ones who don't bother to ask what they don't know.

[Now the other answer is - deltaV is this bit of jargon that means 'acceleration'. Trick there is you might 'accelerate' backwards (burn postgrade) or sideways]

Nice quote

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Thanks I think I am slightly closer to understanding it... Ah whatever, I will probably learn it in high school

Probably not. I didn't even see it in college and I was there to learn chemistry and had to take the "hard" physics classes using calculus.

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Now the other answer is - deltaV is this bit of jargon that means 'acceleration'.

Not exactly. Acceleration is how fast you can change your speed, delta-v is by how much you can change your speed.

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As I understand it, and I am only a layman, Delta V is the change of speed (or more correctly, change of velocity) that your rocket is capable of. If you had a fully fueled rocket far out in space with the engine off you can consider it, for the purpose of this example, to be stationary. If the rocket had a Delta V of 2000 metres per second, that means that if you fired up the engine and ran it until you were out of fuel you would be traveling at 2000 M/S at the end. Also, disregarding mass changes due to fuel use, if you accelerated from stop to 1000 M/S, turned retrograde and fired until your tank ran dry you would finish up stationary again. But your speed would still have changed by 2000M/S, 1000 up and 1000 back. So the Delta V figure gives you the total of all the changes of speed that the rocket is capable of. The importance of the Delta V figure is that regardless of what you want to do in space, go from Kerbin orbit to Mun orbit, Go from Minmus orbit to surface, change an orbit, anything at all, there will be a required Delta V figure that your rocket will need.

If this explanation is wrong I am sure that one of our genuine rocket scientists will correct me.

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It isn't the sort of thing you'll be taught about unless you specifically get involved in the field. There aren't many places you'd use it outside astrophysics/spacecraft just because drag/friction features elsewhere meaning it loses its worth a little.

But yes, change in velocity is dV. Rate of change of dV is acceleration.

E.g. I have a constantly accelerating object with an initial velocity 20m/s. Its velocity after 5 seconds is 40m/s.

So dV is 40-20 = 20m/s

Acceleration is (40-20)/5 = 4m/s^2

Course it is more complicated in rocketry 'cause a significant mass of the rocket is fuel, so acceleration changes rapidly (F/m = a).

@benzman dV takes in to account the mass change so the second bit is fine

Edited by Supernovy
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As I understand it, and I am only a layman, Delta V is the change of speed (or more correctly, change of velocity) that your rocket is capable of.

Technically, it's speed not velocity. Velocity is a vector, and hence, has a direction. Delta-V is directionless, so it's actually change in speed. I suspect rocket scientists chose to use "velocity" because it sounds more sciency.

To the OP, think of delta-V as how much gas you have in your car, or better yet, how many miles you can go until your gas runs out. With a rocket, you expend fuel to get places by changing your orbit. Knowing how much delta-V you have lets you know how much you can change your orbit, and hence how far you can go.

Why not just say how much fuel? You could, but the amount of fuel needed to make the same change in speed (delta-V) is different for a big ship versus a small ship. It's different with an efficient engine compared to a less efficient engine. It's different when you start with a full tank, and when you've lost most of your initial mass by burning fuel. So instead of dealing with that confusion, it's easier to deal with the final result - change in speed or delta-V.

Edited by Soda Popinski
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Basically, when in orbit, the path of your orbit can be be defined by your periapsis and apoapsis, along with the mass of the object you are orbiting around (there are other constants involved but these three are the only variables).

This means that if you are at a 100km periapsis, your velocity at periapsis will determine the size of your apoapsis (at any point in your orbit the speed and direction are enough to calculate the trajectory of your entire orbit. It is simplest at apoapsis or periapsis as you will be travelling perpendicular to the object you are orbiting).

It means that you can calculate how fast you would need to be travelling at periapsis to achieve any given apoapsis, so you can work out the amount of velocity that you need to "add" to your current velocity. This is delta-V.

In an example where I have plucked numbers out of thin air and no maths is involved because I need to leave for work in 15 minutes and I don't have time.

Suppose you are in a 100km circular orbit around a planet and your orbital velocity is 4000 m/s.

There is a moon, in a 100,000km circular orbit around the planet, that you want to encounter.

You calculate that a velocity at a 100km periapsis of 9000 m/s will give you an apoapsis of 100,000km. This means you need a delta-V of 5000 m/s to encounter the moon.

In KSP all the delta-V requirements can be obtained from delta-V maps, which people have made, so you don't need to do calculations on this front if you don't want to. It helps to have some appreciation of it, though.

In addition to knowing the delta-V requirements, you also need to know the delta-V that your ship has. This is generally calculated for each separate stage and is given by the equation... dv = Isp * g0 * ln(m0/m1). Unless you have certain mods installed, you will need to calculate this yourself.

Isp is the specific impulse of your engine (can be seen in the VAB when you select it), g0 is always 9.82 m/s^2, m0 is the total mass of your rocket in at the beginning of a particular stage and m1 is the mass of your rocket at the end of that stage. Isp and mass are in seconds and tonnes respectively (as they are given in KSP).

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The best thing about using delta-V to analyse craft is that it is allows comparison between ships of greatly differing masses. If you only looked at the amount of fuel in a craft, it would not be directly comparable to another craft if the masses were different, because the lighter craft could accelerate more for the same amount of fuel.

It also makes mission planning much easier. All craft require the same amount of dV to reach a given destination, but the fuel requirements vary greatly depending on mass. So as long as your craft has enough dV to reach its destination it doesn't matter whether it's a 300 kg probe or a 1000 ton space station.

There are four basic techniques to increase dV:

1. Add more fuel (the most obvious)

2. Use more efficient engines (higher Isp, converts fuel into acceleration more efficiently)

3. Reduce dry mass (the mass of the ship without fuel in it)

4. Staging (in effect reducing dry mass mid-flight)

Kerbal Engineer Redux and MechJeb are two mods that do the delta-V calculations for you, they take much of the guesswork out of craft design.

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While there have been good explanations above, I'll just make a few comments. dV (delta Velocity), as someone wrote, means change of velocity. The other thing I want to comment on is: think of things this way: Rather than thinking of yourself or your craft in stationary, non moving points, think of being in an always moving situation. On Kerbin's surface, while sitting on the launch pad, your ship rides the surface around the center of mass of the planet, "orbiting" as the day and night progress. In order to move your orbit higher, and to be able to more easily escape from Kerbin's sphere of gravitic influence, you need to change your dV by about +4500 meters per second. That's true for a rocket, regardless of how big it is or how many stages you have. When you launch and fly, you are not just flying straight up or horizontally; you are flying in a new orbit, which always changes dimensions as your dV changes. It might seem to be an arc in map view, but the hidden part of the orbit is now under the surface of Kerbin. When you achieve orbit, it is a new orbit which is completely visible in map view. Not only do you have the apoapsis (high point) displayed, but also the periapsis (low point as well). In this case, KLO (Kerbin Low Orbit) is in space, a little over 70,000 meters altitude above sea level.

The same principle applies when you change your dV to move from Kerbin orbit to another planet or moon; or coming back to Kerbin. Changing dV is how you change your orbit from one sphere of influence to another. During an interplanetary flight you may leave the SOI of planets completely, and find that your craft is orbiting Kerbol (Kerbin's sun) during part of the flight. So you are always in an orbit, and you get from one place to another by changing your orbit; done by changing your velocity.

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Not exactly. Acceleration is how fast you can change your speed, delta-v is by how much you can change your speed.

(facepalm to myself for messing that up!)

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Technically, it's speed not velocity. Velocity is a vector, and hence, has a direction. Delta-V is directionless, so it's actually change in speed. I suspect rocket scientists chose to use "velocity" because it sounds more sciency.

Actually it's velocity, not speed. Delta-v is a scalar quantity that measures the magnitude of difference between two vector quantities. A 100 m/s burn always changes the velocity by 100 m/s to the direction of the burn. The change in speed is usually less, unless the direction happens to be prograde or retrograde.

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Actually it's velocity, not speed. Delta-v is a scalar quantity that measures the magnitude of difference between two vector quantities. A 100 m/s burn always changes the velocity by 100 m/s to the direction of the burn. The change in speed is usually less, unless the direction happens to be prograde or retrograde.

Yes, what Jouni said. It is vector math. Speed is directionless, vectors are not. When you "use" dV, it is not directionless. If you do a theoretically perfect inclination change, your speed didn't change but your vector sure did, and it took some dV to do that. dV is a measure of how much you can move that vector around (make it longer, shorter, or "turn it").

Yeah, it's more complicated than that, but I guess I think of dV as a measure of "how much can I move this particular ship around."

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