So What Exactly Is Delta-V?

[Kennedy988]

Hi everyone,

I'm more or less new to the game, (I've owned the game since last April and have played it for a few weeks at a time here and there,) and I understand a lot of the game concepts by now but one thing that i'm continually confused by is the use of the phrase "Delta-V."

You're probably thinking "How the HELL can you be confused about such a simple and important concept!?" but i'm ashamed to say that I've just never really had much experience with it, despite the fact that it's a concept that affects every single flight I take

For starters, I refuse to mod the game. A lot of the mods out there do look very shiny and attractive, but I'm a firm believer in using only what the game devs have provided, (Also hoping that flying old-school will make me a better pilot in the long run,) so I haven't installed MJ or any other mod that provides calculations or other such readouts of Delta-V. Basically the stock game has no way of calculating it whatsoever, so up until now it's been a 'Non-existent" part of my gameplay. If you know what I mean.

Soooo basically my understanding of Delta-V at this point in time is that the term "Delta-V" is almost synonymous with the term "Fuel efficiency." or "Effort." I see a lot of people throwing around the phrase "It takes x Delta-V to reach x destination," or to complete said maneuver, or to land on Eve, etc etc...

Can anybody clear this concept up for me in noob-friendly english? Furthermore, what are your opinions on flying stock? Do all the successful pilots out there use MJ and other such mods or do I have any hope at all of achieving anything noteworthy without modding the game?

So far I've made various trips to, from and around Kerbin's moons and a few trips across the system and back to Kerbin, but have not landed on any other planets. Everything so far has been extremely trial and error and without a system of calculating what kind of Delta-V I need to have in order to...land that rover on Eve, for example, before I actually get there and experience it, I feel like the only option is to go there and fail a dozen times until I figure out exactly what I need to build. It's a very time consuming process, which, in the end, has always led to me burning myself out.

Thanks

[Streetwind]

Delta-V is a term composed of the word "delta", meaning "difference", and the letter V, meaning "velocity". It's a shorthand for saying "difference in velocity" or "change in velocity", which is way too long to roll off the tongue if you have to repeat it constantly.

In physics, acceleration is measured in "meters per second per second", or m/s². If you have an acceleration of 10 m/s², that means that every second, you add 10 meters per second to your current velocity vector. (That vector can be retrograde, so you could actually be slowing down.)

This is equivalent to saying "every second, I have 10 m/s of change in velocity". Or, shorter: 10 m/s delta-V.

Because of how rockets work, it is impractical to try and specify fuel capacity in volume units or burn time. As you consume fuel, the rocket gets lighter, and thus you need to consume less fuel by volume (and need a shorter burn) to cause the same acceleration. You need a metric that doesn't change no matter how the rocket changes in order to reliably measure how much your rocket can still do before running dry. This is why the ability to accelerate was chosen as a metric.

Everything a rocket motor does is accelerating, and all you do in orbit is accelerating along different vectors. To reach the Mun, you must first accelerate prograde, to raise your orbit to the point of getting an encounter, and then accelerate retrograde to capture into orbit. This acceleration required, expressed in dV, is the cost of traveling to the Mun. Your rocket must be capable of this much change in velocity to go there. How the rocket actually looks in practice is not important, only the ability to change its velocity is.

I hope that helps

Edited November 22, 2014 by Streetwind
Tweaked for clarity

[Vectura]

'Also hoping that flying old-school will make me a better pilot in the long run' If you are flying planes, and rockets also, then using FAR or even NEAR will make you a more realistic game pilot, and allow you to be a better pilot in things like flight simulators. If you just want to be good at stock KSP, then sure, no mods needed. It's just that the stock aerodynamics barely deserve to be called aerodynamics (the lower atmosphere is often referred to as the souposphere).

[Redshift OTF]

Yeah, DeltaV is basically how much your ship can change speed which is roughly analogous to how much fuel you have. Whenever you are changing orbits or getting on or off bodies you are changing your speed which is determined by how much thrust you have, (which is determined by your engines, fuel and staging).

I use MechJeb a lot, mainly for calculating DeltaV when building crafts as I would hate to build something and have it fail to perform its mission because it didn't have enough DeltaV. Trial and error is OK for the beginning of the game but later on, especially for bigger projects, you want something more accurate. I honestly think there should be a stock DeltaV calculator in the game to help builders.

[MockKnizzle]

Streetwind pretty much covered it. Rocket engines throw propellant out the nozzle really fast in order to exert a force, which accelerates your rocket (thanks Newton!). This acceleration changes your velocity, generally increasing or decreasing it, but it can change the direction too. If you take your final velocity when you're done burning (i.e. accelerating) and subtract your initial velocity, you get the change in velocity, or dV. Just keep in mind that we're flying in three-dimensional space, so things like plane changes "cost" dV even though the number on your velocity indicator might not change.

A spacecraft has a limited reserve on propellant on board and thus can only run the engines for so long (and thus change your velocity) before it runs out of fuel, meaning that the ship has a maximum dV that it can exert. A smart Russian dude figured out that this total dV is proportional to both the exhaust velocity of the engine and the natural logarithm of the ratio of the initial "wet" mass divided by the final "dry" mass of the ship.

While KSP doesn't directly give you the exhaust velocity of any engines, you can use to substitute for it, where g₀ is a standard gravity of 9.81m/s². You can use these equations and the wet/dry masses given in KSP to calculate the dV of your craft

It's entirely possible to do all the math you need by hand, although it can get rather long and tedious so I would recommend setting up a spreadsheet. Alternatively, if you decide that hand-calculations are not your style, Kerbal Engineer is a great plugin that does a fantastic job of doing all the boring dV calculations for you without trying to fly your spaceships for you.

[RainDreamer]

Delta-V in spacecraft building, as I understand it, is the total amount of velocity that your vehicle is capable of achieving before it runs out of fuel. It varies based on mass of vehicle, thrust of engine, and fuel efficiency/specific impulse of engine. Note that some engine perform better in the vacuum of space, using less fuel to provide the same amount of thrust, and so will extend the delta V you have after you escape the atmosphere. Also note that mass of vehicle change through time as you use up fuel.

Delta-V in planning flight path in space, is the change of velocity that is needed to get you where you need to be. You can see this whenever you plot a maneuver node in the game map mode, which will give you a read out of how much delta V you need to complete the node.

You should at least get the Kerbal Engineer Redux mod. It does absolutely nothing to game play. But it gives you very important statistics like delta V, your current thrust, your horizontal/vertical velocities, etc. All of that is important for very precise maneuver. You can just fly without knowing all that, of course, but that would be like an airplane pilot flying ignoring 80% of their flight instruments. Most of the time it is fine. But sometimes when thing goes horribly wrong, Having those info let you know what happened and why it happened, so that you can design better vehicles in the future. There is a reason we have things like black boxes and stuff on airplanes.

[Red Iron Crown]

The above posters have covered the math and velocity change aspects of delta-V well, so I'll just add a way of thinking about it that helps with grasping the concept rather than calculating it:

Delta-V is a fuel gauge that is corrected for engine efficiency and ship size.

If I may used a dreaded car analogy, dV is roughly equivalent to a distance-to-empty function some vehicles have from their trip computers. It doesn't matter whether the car's engine is a miserly four cylinder or a thirsty V8, or whether the car is a 1000kg compact or a 5000kg commercial vehicle, the distance to empty is directly comparable even if the actual amount of fuel consumed is very different.

Similarly, delta-V can be directly compared between vessels no matter their mass or engine efficiency. A 2-ton ship with efficient ion propulsion can reach the same destinations as a 1000-ton ship powered by thirsty chemical rockets if their delta-V totals are the same.* This is incredibly useful because it allows analysis of the capabilities of craft while disregarding extraneous factors.

Delta-V is the currency of spaceflight, the thing we spend to change the size or shape of our orbits to reach our destinations.

* A small caveat here: Thrust-to-weight ratio also has an effect on reachable destinations, especially when landing on or ascending from celestial bodies.

[GoSlash27]

*D'OH!* Ninja'd by Red Iron Crown!

I have 1 more thing to add, so summarizing the above posts...

"DV" means "change in velocity".

We use it to describe 2 things:

1) The change in velocity a rocket stage is capable of imparting to a vehicle.

2) The change in velocity that is required to get our vehicle where we want it to go.

Best,

-Slashy

PS I have never used MechJeb and it's definitely not necessary to be successful in this game, but it *is* necessary to employ the rocket equation in your design process.

Not only in the classical "how much DV will this rocket generate" form, but also in the "which of these engines is best for this job" form and "how big a fuel tank do I need" form.

Edited November 22, 2014 by GoSlash27

[MockKnizzle]

Delta-V in spacecraft building, as I understand it, is the total amount of velocity that your vehicle is capable of achieving before it runs out of fuel. It varies based on mass of vehicle, thrust of engine, and fuel efficiency/specific impulse of engine. Note that some engine perform better in the vacuum of space, using less fuel to provide the same amount of thrust, and so will extend the delta V you have after you escape the atmosphere.

Interestingly, the dV of a rocket actually doesn't depend on thrust at all. The only engine-specific term in the rocket equation is exhaust velocity, which is not related to the thrust an engine can produce. It might initially seem that "more thrust lets me accelerate faster, right?" which leads many new players to add extra engines for more power. But adding extra engines adds extra mass to the spacecraft without adding more fuel - resulting in m₀/m₁ getting smaller and thus a decrease in the ship's dV.

[GoSlash27]

The only engine-specific term in the rocket equation is exhaust velocity, which is not related to the thrust an engine can produce. It might initially seem that "more thrust lets me accelerate faster, right?" which leads many new players to add extra engines for more power. But adding extra engines adds extra mass to the spacecraft without adding more fuel - resulting in m₀/m₁ getting smaller and thus a decrease in the ship's dV.

Minor correction which you have sorta- mentioned here:

The mass of the engine(s) is in the rocket equation, but it's hidden. (Mw/Md) is more usefully described as [(Mp+Me+Mt+Mf)/(Mp+Me+Mt)] where

Mp= payload mass

Me= engine mass

Mt= tank mass

and

Mf=fuel mass.

This is why adding more engine than you need will (as you stated) reduce your wet/dry ratio and DV.

Best,

-Slashy

[RainDreamer]

Interestingly, the dV of a rocket actually doesn't depend on thrust at all. The only engine-specific term in the rocket equation is exhaust velocity, which is not related to the thrust an engine can produce. It might initially seem that "more thrust lets me accelerate faster, right?" which leads many new players to add extra engines for more power. But adding extra engines adds extra mass to the spacecraft without adding more fuel - resulting in m₀/m₁ getting smaller and thus a decrease in the ship's dV.

I guess the reason sometimes adding more engine does increase my dV is because I have more fuel than my current engine can handle then?

[GoSlash27]

I guess the reason sometimes adding more engine does increase my dV is because I have more fuel than my current engine can handle then?

RainDreamer,

Adding more engine will never increase your DV, but it can (depending on the circumstances) reduce your losses due to drag and gravity, thus accomplishing more even though you have less DV. This also applies to the Oberth effect... and now we're getting a bit out into the weeds.

If your calculator says that adding engines is increasing your DV, it's misleading you.

HTHs!

-Slashy

[Red Iron Crown]

I guess the reason sometimes adding more engine does increase my dV is because I have more fuel than my current engine can handle then?

The only way more engine can increase the dV in a stage is if you're adding engines that are more efficient than the ones already on the ship (and their better efficiency outweighs the increased dry mass).

[Kerbart]

All of the above is correct.

In addition, delta-V allows you to express energy (both as requirement for an orbit change as for what's potentially available for a vessel) in a way that takes mass out of the equation. It takes about 4500 m/s dV to get into Kerbin Orbit. That number is useful because it tells you what your requirements are, regardless of the size of your spacecraft. It doesn't matter if it's a one-Kerbal capsule or a 500 ton space station, the amount of delta-V required to get into orbit is the same.

As such it's often used as it is such a convenient way to describe how much energy it takes to accomplish orbit changes.

[Red Iron Crown]

Careful Kerbart, delta-V is not an expression of energy. A m/s of delta-V can change the orbit by differing amounts of energy depending on the ship's situation.

[TinfoilChef]

Y'know, this deserves a sticky someplace where new players can easily find it. Some of the best explanations of Delta V I've seen are here, broken down so that new players can easily grasp what it means and it's importance.

[Farex]

All these posts are correct, but I will try to explain it in a simple way, one short sentence.

<<DeltaV is the distance your spacecraft can fly with it's fuel.>>

Think about it like this: Your car has 60 liter of gas in it's tank. Your car needs 10 liter every 100 km. This means your DeltaV / maximum distance with your car is 600 km.

Now this is not as physically correct as all the other posts before me, but it is way more simple to understand for a beginner.

Greetings!

[The_Rocketeer]

All these posts are correct, but I will try to explain it in a simple way, one short sentence.

<<DeltaV is the distance your spacecraft can fly with it's fuel.>>

Think about it like this: Your car has 60 liter of gas in it's tank. Your car needs 10 liter every 100 km. This means your DeltaV / maximum distance with your car is 600 km.

Now this is not as physically correct as all the other posts before me, but it is way more simple to understand for a beginner.

This is rather over-simplistic. Distance isn't a very good representation of delta-V, because it takes much less delta-V to go much further in lower gravity - notice how your Apogee moves faster as it gets further from Kerbin when you burn out to Mun?

I can offer an alternative "beginner's guide" way to think of it:

delta-V is a way to measure the difference in shape and size between two orbits.

The delta-V of a rocket is the MAXIMUM difference that that rocket can achieve.

The delta-V stated for reaching another planet is the MINIMUM difference between the orbit of Kerbin and the orbit of that planet.

Edited November 22, 2014 by The_Rocketeer

[r_rolo1]

Well, I just want to add a point that, while somewhat obvious for a lot of people, it definitely eludes a non trivial amount of newer players:

Velocity is a vectorial variable ... in other words, unlike other variables ( like time ), to define a velocity you need a amount and a direction. Basically, you're going a x value of speed in a certain direction. If you want to change direction your velocity will change, even if stay at x speed ( in other words, going 60 mph to the front is not the same that going 60 mph backwards or 60 mph to either left or right ) ... and, in more rocket related stuff, the equations other posters have put here need to be used with caution if you change directions during flight ( and OFC, if you change directions without a exterior force acting on you, your dV will be diferent of zero )

[The_Rocketeer]

Well, I just want to add a point that, while somewhat obvious for a lot of people, it definitely eludes a non trivial amount of newer players:

Velocity is a vectorial variable ... in other words, unlike other variables ( like time ), to define a velocity you need a amount and a direction. Basically, you're going a x value of speed in a certain direction. If you want to change direction your velocity will change, even if stay at x speed ( in other words, going 60 mph to the front is not the same that going 60 mph backwards or 60 mph to either left or right ) ... and, in more rocket related stuff, the equations other posters have put here need to be used with caution if you change directions during flight ( and OFC, if you change directions without a exterior force acting on you, your dV will be diferent of zero )

Yes, this is a point I've had to wave the flag for in the past.

So when I say the rocket's delta-V is the maximum difference it is capable of, that means the maximum amount that you can change the orbit in any respect, be that just make it bigger, just make it smaller, or make it a bit bigger, then a bit smaller, then a bit bigger again. You could potentially use up all your delta-V without making your final orbit very different from your initial orbit at all if you just alternated between a little prograde thrust and a little retrograde thrust.

[GoSlash27]

All,

Let's not get so bogged down in the details that we confuse the beginners.

It's sufficient to say that

1) "Delta V" means "change in velocity"

2) It is the measure of what it takes to get you where you're going and can be found in various Delta V maps

3) It is the measure of what your stage can accomplish and can be calculated using the rocket equation

You could fill out entire chapters on the ins and outs of it, but this is all a beginner really needs to know.

Best,

-Slashy

[mhoram]

This question pops up time and agian.

Best answers from different point of views so far was in this thread:

http://forum.kerbalspaceprogram.com/threads/60189-What-Is-Delta-V

[The_Rocketeer]

All,

Let's not get so bogged down in the details that we confuse the beginners.

It's sufficient to say that

1) "Delta V" means "change in velocity"

2) It is the measure of what it takes to get you where you're going and can be found in various Delta V maps

3) It is the measure of what your stage can accomplish and can be calculated using the rocket equation

You could fill out entire chapters on the ins and outs of it, but this is all a beginner really needs to know.

Best,

-Slashy

I'm not sure that that helps a beginner any more than anything else that's been suggested. For a newcomer who thinks velocity is just a scientific word for speed, everything you've said just means 'it's how fast you need to go to get where you're going'. The rocket equation doesn't mean anything to anyone who doesn't understand algebra. In my experience, delta-V maps are also pretty non-specific about the kind of intercept manoeuvre they're based on, which can also make a world of difference.

In any case, there's no 'right' answer to the OP's question, because it depends completely how much a beginner already understands. Therefore a lot of different answers at different depths of understanding, and using different imagery, have a much better chance of helping people understand what we're on about when we talk about delta-V.

[Red Iron Crown]

Yes, this is a point I've had to wave the flag for in the past.

So when I say the rocket's delta-V is the maximum difference it is capable of, that means the maximum amount that you can change the orbit in any respect, be that just make it bigger, just make it smaller, or make it a bit bigger, then a bit smaller, then a bit bigger again. You could potentially use up all your delta-V without making your final orbit very different from your initial orbit at all if you just alternated between a little prograde thrust and a little retrograde thrust.

It's even odder than that, a ship can spend every bit of its delta-V without ever changing Ap or Pe by a single meter, just by doing inclination changes.

[The_Rocketeer]

It's even odder than that, a ship can spend every bit of its delta-V without ever changing Ap or Pe by a single meter, just by doing inclination changes.

That would take some extraordinarily accurate manoeuvering... but theoretically, yes :-)