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## Question

Hello! Could somebody please explain what Delta-V is, but simply? Currently I think it is how much total thrust my craft can produce with the fuel it has. Now, I do not want a paragraph of mathematical and scientific stuff, I would just like a simple explanation.

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2 hours ago, The_Cat_In_Space said:

So if my craft has like 5000m/s of Delta-V, that means that it can produce 5000m/s of thrust before running out of fuel?

No.  If your craft has 5000 m/s of delta-v, that means it can change its velocity by 5000 m/s before running out of fuel.  Thrust has nothing to do with delta-v.

Thrust is the amount of force that's pushing the craft.  If the craft has a high thrust, it will produce the 5000 m/s change in velocity more quickly than if it has a low thrust.  But in either case, high thrust or low, you're still going to get a change in velocity of 5000 m/s.

Delta-v is kind of like how far your automobile can go an a tank of gas.  The distance your car can travel depends on the capacity of the fuel tank, and how many miles per gallon is gets.  Similarly, how much change in velocity a rocket can produce depends how much fuel it holds (measured as a ratio of fully fueled mass to empty mass) and how efficient the propulsion system is (measured by the velocity of the exhaust gas.)

Thrust, on the other hand, is more like the horsepower of the car's engine.  Horsepower has nothing to do with how far the car can travel on a tank of gas, but it does determine has fast the car can accelerate.  The same is true of thrust.

Edited by OhioBob
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Delta: Change.

V: Velocity.

Delta-V: Change in velocity.

Literally, that's it. If your craft was sitting alone in space going 0m/s, and you burned all your fuel, you'd now be going a certain velocity. That velocity is (well, was before you burned all your fuel) your Delta V.

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Note that the correct writing of the term is Δv, where Δ is the uppercase Greek letter delta.  In mathematics the letter Δ is often used to signify a difference, or change, in a certain quantity.

While Δv means "change in velocity", it has a couple different uses.  It can refer to how much of a change in velocity that your rocket or spacecraft is capable of producing.  It can also refer to how much of a change in velocity is required to perform certain maneuvers or tasks.  For example, it might take 3400 m/s Δv to reach Kerbin orbit.  It might take another 860 m/s Δv to leave Kerbin orbit and to place the spacecraft on a trajectory that will intercept Mun.  And it might take another 260 m/s Δv to insert the spacecraft into orbit around Mun.

In this regard you can think of Δv as the currency of space travel.  To complete a particular mission, you have make sure you spacecraft is carrying enough currency to buy the specific set of maneuvers that it must perform.  For example, suppose you want to place your spacecraft into an orbit around Mun.  The cost of this mission in Δv is, 3400 + 860 + 260 = 4520 m/s.  So to perform this mission, your vehicle must be capable of producing ≥4520 m/s Δv, or else it will fail to complete the needed maneuvers.  It's just like running our of money if the middle of a trip and becoming stranded, unable to either get to your destination or return home.

Expanding on what @Scarecrow said, the Δv of a rocket is the hypothetical change in velocity that it is capable of producing if it where in gravity free space.  For instance, it is generally accepted in KSP that it takes 3400 m/s Δv to attain Kerbin orbit.  But once in low orbit a spacecraft is traveling only about 2300 m/s.  So what happened to the other 1100 m/s?  It was lost in overcoming gravity and atmospheric drag.

In an extreme case, we could have a rocket on a launch pad burning its engines, but if the engines aren't powerful enough to lift the weight of the rocket, it can burn through all its fuel without going anywhere.  In that case our actual change in velocity is zero, but the rocket sill has a hypothetical Δv.  If this rocket burned its engines while in deep space somewhere, it's velocity would change because it wouldn't have to flight Kerbin's strong gravity.

It is general practice to include gravity and drag losses in the stated cost of any particular maneuver.

Edited by OhioBob
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So if I have like 5

31 minutes ago, 5thHorseman said:

Delta: Change.

V: Velocity.

Delta-V: Change in velocity.

Literally, that's it. If your craft was sitting alone in space going 0m/s, and you burned all your fuel, you'd now be going a certain velocity. That velocity is (well, was before you burned all your fuel) your Delta V.

So if my craft has like 5000m/s of Delta-V, that means that it can produce 5000m/s of thrust before running out of fuel?

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33 minutes ago, The_Cat_In_Space said:

So if my craft has like 5000m/s of Delta-V, that means that it can produce 5000m/s of thrust before running out of fuel?

Exactly.

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1 hour ago, The_Cat_In_Space said:

So if my craft has like 5000m/s of Delta-V, that means that it can produce 5000m/s of thrust before running out of fuel?

And your craft would now be travelling at 5000 m/s.

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In a single word, delta-V is the rocket equivalent to range.  The comparison isn't perfect because a rocket in orbit is always moving, unlike an automobile that can park, but that's the closest concept.

The main difference between delta-V and range is that range depends on points of reference that don't move.  You can drive a car from your home to the next city, and figuring whether you have enough fuel to do that depends on knowing how far apart your home and the city are.  If the distance to the next city is somehow variable--think of cars on conveyor belts and skyscrapers on wheels!--then knowing how far your car can go is not enough.  In fact, it's almost meaningless:  since you're already moving, you can cover great distances while doing nothing, but there's no guarantee that this will get you where you want to go.

To extend the example, you could try to go north on a south-moving conveyor, in which case you can expend all of your fuel and never reach your destination, or you can get onto a north-bound conveyor and go where you wish while using only a tiny amount of fuel.  Delta-V, in this example, measures your ability to choose different conveyors.  If you have more of it, then you have more choices, which includes more paths that go where you want to go.  If you have less, then you have fewer choices.  If you have none, then you're along for the ride, wherever the conveyor you're already on takes you.

Edited by Zhetaan
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It's also not quite so straight forward as thinking that if your craft has 5000 m/s of dV, it can increase the velocity of your craft by that much.  If you are burning in atmosphere or fighting gravity, then both of those will have a negative effect on the final increase that you get out of the resources you have available.  So whilst burning all your fuel may impart 5000 m/s of dV to your craft, gravity and atmospheric drag may account for 1000 m/s (just as an example) so your craft actually ends up going 4000 m/s faster.

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Some great explanations here. Note that if you change nothing aside from adding dry mass (structural or payload mass) your available dV will be less.

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15 hours ago, The_Cat_In_Space said:

So if my craft has like 5000m/s of Delta-V, that means that it can produce 5000m/s of thrust before running out of fuel?

Yes, except that you need to change the word "thrust" to "velocity", because "thrust" is something else-- thrust is force, i.e. how hard your engine can push.

Delta-V means "if you turn on your engine and thrust until you run out of fuel, how fast are you going".

Thrust means "how hard does the engine push" and has nothing to do with delta-V.

For example, you could have two spaceships, out in space.  Suppose that each of them has 5000 m/s of delta-v.  One of them might have a teeny little engine, with very low thrust, which can only accelerate the ship very feebly.  The other one might have a big powerful engine with a lot of thrust that can accelerate it really really hard.

The delta-V of both ships are the same as each other.  For each one, if you start the engine and burn until you run out of fuel, you'll be going 5000 m/s.  But the one with the teeny-weeny engine will take a really long time to burn all the fuel (because it's using the fuel slowly), whereas the more-powerful engine will burn through the fuel much quicker.  They have the same max speed, it's just that one of them reaches that speed quicker than the other.

In short:  How powerful the engine is has nothing to do with what speed the rocket can reach.

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Reading the posts, I came to an analogy:

Do you remember Potential Energy? The energy a body has, given a situation? For example, a 1Kg mass rock hung at 10 meters high has way less Potential Energy than the same rock hung at 100 meters.

Think on Delta-V as "Potential Velocity". Given my mass and the fuel I have on the tanks, how much velocity I can get from where I am?

Edited by Lisias
typos
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16 hours ago, 5thHorseman said:

Delta: Change.

V: Velocity.

Delta-V: Change in velocity.

Literally, that's it. If your craft was sitting alone in space going 0m/s, and you burned all your fuel, you'd now be going a certain velocity. That velocity is (well, was before you burned all your fuel) your Delta V.

This ^

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Rocket Science is 'Rocket Science' for a reason.

As others have pointed out, Delta-V stands for Change in velocity.  The 'Delta' part of it is actually a term used in Calculus.

The reason calculus comes into play here is because you are trying to determine the behavior of things that are always in motion.  Moreso, you are trying to do so to an object that changes as it performs an action.  Calculus is thus known as the 'mathematics of change'.

But we're going to ignore that.  You don't need to know calculus to know Delta-V.  It's just useful to know that Calculus is not as intimidating as it may look.  In fact, it's a shortcut.

For Delta-V, we're going to imagine we're in a car.  But we're going to make some assumptions.  For example, the car never slows down when you coast, and the brakes don't work.

When you put your foot on the gas, you accellerate from zero to say, 100 miles per hour.  Then you take your foot off the gas, and the car remains at 100 MPH.  If I phrase this in plain english, you have had a 'change in velocity' of 100 MPH.  Or, you have executed a Delta-V of 100 MPH.

Now, let's say you only have enough gas in the tank to accellerate from zero, to 70 MPH.  (Not a lot of fuel, but work with me here).    The Delta-V available to you is 70 MPH.  Once you accellerate your car to 70, it stays at 70, and you can't do anything else but coast at 70, forever.  Your Delta-V 'Budget' was 70 MPH.  If you wanted to say, accellerate to speed, then slow to a stop, you need to find how much you can put your foot on the gas, and then pop the car into reverse and put the foot on the gas again.  In this scenario, you can speed up to 35 MPH, then you have to put it in reverse and slow down 35 MPH back to zero.

Simple enough, right?  Let's move to rockets.

First and foremost, the notion of 'fixed speed' like in a car on the highway is almost pointless to use as a metric once you're in orbit.  That's because an orbit is simply you falling AROUND an object as gravity pulls on you.  At any given moment, your speed changes depending on where you are.  A low orbit around earth, for example, tends to be faster than a high orbit.  Or, in an elongated orbit, you're REALLY fast at the lowest point, and slow as molasses up at the highest point.  You still need to know your speed relative to the object your orbiting, but that's only so you can do the math needed to figure out what your orbit is supposed to look like.  We have computers doing that for us here, so we don't care beyond understanding that it is happening.

Let's say we want to execute a change in orbit, going from a low orbit, to a higher orbit.  We need to perform several 'burns' of specific length and power.  The lower the power, the longer the burn, and the higher the power, the shorter the burn.  Either way, in order to get into the higher orbit, you need to think about what you're actually doing.  Since an orbit is simply glorified falling, you need to 'fling' yourself faster so that you'll 'fall' farther up to the higher orbit.  Flinging yourself is a change in velocity.  No matter how big your engine is, or how long you have to burn, the change in velocity is your fixed value.

Once you get up to the higher orbit, you have to burn again, or speed yourself up more, so that your new velocity keeps you up high as you fall around the parent body.  Once you have done so, the only way to come back down is to slow yourself down in that high orbit with another burn, usually, a change in velocity equal to the burn that stabilized your high orbit.

Every move you do from one orbit to another in this manner requires this to happen, and thus you have to burn fuel for any change you make.  Now, the reason you use 'Delta-V' instead of say, 'gallons' or 'liters' when talking about rockets is simple.  No two burns use the same amount of fuel.  If a rocket weighs 10 tons, you need to burn MORE fuel to change its velocity than a rocket that weighs 5 tons.

But wait!  There's more.  As you burn fuel to change velocity, your rocket becomes significantly lighter, really fast.  That 10 ton rocket BECOMES the 5 ton rocket in a hurry.  That means the math you need to do to find how long and how powerful a burn you need changes by the second.  This is where the 'calculus' part actually comes into the picture.  Regular algebraic math cannot easily solve this problem without making you rewrite the problem over and over again dozens of times a second.  A computer can do that, but a rocket scientist with a slide rule back in the apollo days can't.  Calculus, while more abstract a math system, has the tools needed to solve these 'changing' equations in real time.  Again, we're going to skip over it.

Anyway, because you can't base your ability to go places based on blind fuel capacity and a 'gas mileage' like a car, you have to determine your range through a different method.  Luckily, rocket science does allow you to find out how much total change in velocity your rocket will have.  As long as you know the total mass of your rocket, and the mass of your fuel, you can determine how much it can change in velocity.  Likewise, orbital changes are all based on velocity changes.

Thus, the Delta-V budget can almost instantly tell you where you can go, and whether or not you can come back.  It is the single most important number out of all the statistics on your spacecraft.  Even above the Thrust to Weight Ratio of launch.  If you pull up one of the 'Delta V Maps of the Kerbol System' that are on the internet, you can chain the paths to determine how much Delta you need to get to say, low Duna orbit from Low Kerbin orbit.  Then, you can look at your rocket and go 'I don't have enough, need a bigger rocket'.

This isn't as 'simple' an explanation as others might be putting out.  But I feel that making the explanation TOO simple may become a bit misinforming, and understanding the background usually makes the knowledge more solid.  But amazingly enough, the end point of all this is pretty simple, even if the journey to get there is filled with complexity.  Then again, the point of these kinds of systems is to simplify the complicated into something nice and bite-size.

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@AdmiralTigerclaw How many paragraphs is this??? How long did it take you to write that?!

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5 hours ago, The_Cat_In_Space said:

@AdmiralTigerclaw How many paragraphs is this??? How long did it take you to write that?!

9-ish paragraphs, 20 minutes.  (Some of those lines I don't call a 'paragraph' because they're one-sentence points.)

I'm a writer.  I can put out a LOT of information in a very short amount of time.  If I know what I'm talking about, I can put out up to 4,200 words an hour.  Or around 8 to 9 pages.  I don't normally go that high because I need to think about what I'm saying and/or give my eyes a break.  But I've spent entire days just typing.

The Delta-V explanation can be simple, but I want you to understand what it's doing under the hood.  You don't need to know all the math, but as my calculus instructor in college emphasized, if you can RECOGNIZE what's going on, you can make the computer do the work.  (Which is especially important in Calculus because there are equations that you actually CAN'T put in a computer or calculator because they'll just get stuck in a recursive loop trying to solve them.  Learning to recognize when an equation was going to get stuck looping was a big point there.  And not knowing that could happen would have been... BAD.)

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On 8/30/2018 at 5:02 PM, AdmiralTigerclaw said:

(SNIP) *SLICE* *SHRED* CUT* (SNIP)

Not really that "simple," per OPs request, but some good points.

Edited by Lo Var Lachland
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Now, @The_Cat_In_Space, we know you wanted a short, simple answer, but the reality, like everything else in rocket science, there's actually a lot going on.

The previous posters have already given good descriptions of what it means in terms of describing what a rocket can do, but I'm going to go at it from the other direction - describing what your rocket is attempting to do.

Orbits are paths of constant total energy within a gravity field. We know that energy is a function of mass and position (potential energy) or mass and velocity (kinetic energy). However, because the mass of an object doesn't change throughout an orbit, and because of how gravity works, the mass terms actually "fall out" of the orbital math and we're left with position and velocity being the only important variables.

When we describe orbits in KSP, we usually use orbial elements to do that job, since they're easier to digest. But an orbit is equally a set of positions and the velocity vectors at each point. And we don't need the whole set - it is sufficient to use only a single location and the associated velocity vector to uniquely describe an orbit. KSP, in fact, does have to use this formulation - in order to do patched conics, you find your xyz position and xyz velocity vectors at the SOI boundary, transform them into the new reference frame, and then derive the new orbital elements from that information. And if you're doing N-body physics, you don't have true orbits, you only have position and velocity.

All right, this is getting long, but stay with me here.

So when we compare two different orbits, we need to compare both the position vector and the velocity vector for each. However, when we're trying to describe a maneuver, we're not describing any two old orbits - we're describing intersecting ones. We're moving from one orbit to another where the position is the same in each orbit. Since the positions are the same, the only difference between the orbits is their respective velocity vectors. And the math you need to do those comparisons isn't calculus, you only need arithmetic.

In terms of describing these maneuvers, delta-V (change in velocity) is exactly that - the vector difference between the two velocity vectors. Note that I didn't say speed, but I emphasized vector and velocity, because that's important. Most maneuvers are of the increase/decrease speed variety, but if you've ever done a pure plane change, you'll notice your starting and ending speeds are the same. The velocity change is due entirely to direction change. We normally assume that pointing your vessel in the correct direction is a free action.

And to make things more confusing, this has absolutely nothing to do with the fact that you'll be moving at different speeds at different times during an orbit (fast at periapsis, slow at apoapsis). That's a whole different thing.

So the delta-V of a manuever is the cost of doing that maneuver - how much do I need to be able to change my speed and direction to get to that new path. The delta-V of your rocket (described by the other posters) is a measure of how-much-maneuver-can-my-rocket-do-before-I'm-out-of-gas.

Edited by pincushionman

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