What is Delta V?

[hughesj]

The way I understand it the Delta V is the maximum possible change in velocity in metres/second. So if you wanted to speed up or slow down it would use up your Delta V?

Is this right?

Jason

[Nasakev]

Yes. Basically, dV is a measure of a ship's ability to change its speed. A ship with 1000 m/s dV could change its velocity by 1000 m/s faster or slower. This assumes you're in space- thrusting in an atmosphere or against gravity will sap your dV.

[Stratoroc]

Delta v is change in velocity. Hence why maneuver nodes show how much dv you need to accelerate to your new orbit. Ship information refers to delta v as how much change in velocity of the dry ship mass you can get by burning all the fuel. Since change in velocity is the part you care about fuel range is measured in delta v in the same sense as a car's range is measured in miles per tank or such

[rottielover]

Delta represents change. V represents velocity. Simply, "Delta V" = "change in velocity" it's quicker to say "deltaV" than it is to say "change in velocity" , plus "deltaV" makes you sound awesome.

There are many ways to look at it. The terms your using, your representing a particular fuel/engine combo as the total DeltaV available with that combo combines with the mass of your ship. In other words "this stage will let me change my velocity by xxxx/ms "

Hope that helps

[hughesj]

Thanks everyone, I think I've got it. So to get into an orbit of Kerbin I'd need a Delta V of around 4500 m/s from what I've read. Anything from that is for manoeuvring in space?

Another question is when I put solid fuel rocket boosters on my rocket it didnt change my Delta V. I'm guessing this is because it doesn't have a fuel mass and an empty mass so no delta v could be calculated. So in theory solid fuel boosters should give you some velocity without using your delta v?

[Bacterius]

Thanks everyone, I think I've got it. So to get into an orbit of Kerbin I'd need a Delta V of around 4500 m/s from what I've read. Anything from that is for manoeuvring in space?

A lot of that delta-v is going to be spent thrusting against the atmosphere to get into a suborbital trajectory. Once you finally leave the atmosphere it takes maybe 200-1500 dV to circularize depending on your apoapsis (the lower the more dV you have to spend to reach orbital velocity at your altitude) and then most orbital maneuvers don't take that much dV e.g. going to the Mun is 800 or so (to get there and smash into it, that is - realistically you will want to get captured into a stable orbit and maybe land, oh and maybe return as well). Interplanetary isn't actually that much more expensive, and if you are going to a planet with an atmosphere you can use that to slow you down upon capture instead of thrusting retrograde at the periapsis, it saves a ton of fuel (see aerobraking).

Edited July 19, 2013 by Bacterius

[capi3101]

Another question is when I put solid fuel rocket boosters on my rocket it didnt change my Delta V. I'm guessing this is because it doesn't have a fuel mass and an empty mass so no delta v could be calculated. So in theory solid fuel boosters should give you some velocity without using your delta v?

No, that's not the case. The way delta-V is calculated, it's dependent on two things: the potential change of mass and the specific impulse of the engine involved. Any time you burn a rocket, you're throwing mass out the fiery end; SRBs are no different. They're just pretty lousy about it, to be honest.

I don't think I'm explaining this very well...okay, so let's say you've got a two tonne payload and you want to throw that into space with a couple of the big BACC SRBs (you can't actually do this, but let's just go through the math real quick). When an BACC is fully fueled, it weighs 7.875 tonnes; when it's empty, it weighs 1.5 tonnes. You've got two of them and a two tonne payload, so fully fueled your rocket weighs 17.75 tonnes, and empty it weighs 5 tonnes on the nose. With me so far? SRBs have a specific impulse of 230s in atmosphere.

Now we do the hard math: the Tsiolkovsky Rocket Equation - deltaV = ln (M-full / M-empty) * 9.81 * Isp, where ln is a natural logarithm (find the button on your calculator), M-full is the full mass, M-empty is the empty mass, and Isp is the specific impulse. So we have deltaV = ln (17.75 /5) * 9.81 * 230 = 2858.61 m/s.

Long and the short of it: any fueled rocket gives you delta-V. Provided you light it of course.

[balu0]

Thanks everyone, I think I've got it. So to get into an orbit of Kerbin I'd need a Delta V of around 4500 m/s from what I've read. Anything from that is for manoeuvring in space?

Another question is when I put solid fuel rocket boosters on my rocket it didnt change my Delta V. I'm guessing this is because it doesn't have a fuel mass and an empty mass so no delta v could be calculated. So in theory solid fuel boosters should give you some velocity without using your delta v?

You can get to orbit for around 3600 ms if you optimize your ascent. But you are basically right.

When you build a rocket it have a dV it can spend on maneuvering. Every maneuver cost some dV.

Adding mass to your rocket will decrease your total dV, adding more fuel and/or extra engines will increase your dV. You have to balance it. (adding fuel will add mass too)

When you put on the SRB-s were they in the right stage ? If they are not , the dV calculation was messed up probably. They should add dV.

[zerotwo]

I was once told a good analogy for delta-v was imagining a penny on a string spinning around a pencil.

In order to make the penny spin faster in it's orbit, you have to apply extra energy to it. If the penny was something heavier like gold it would take more energy, or if aluminium, less.

Also, the method by which you deliver the energy has a huge effect. Imagine you can only increase the speed of this penny by blowing on it, it would take forever! Breath is a low thrust, efficient engine if you think of it as a long slow exhale (you can breathe for hours and hours a day!) While something like flicking the penny is going to deliver massive thrust much quicker, but the flick is over in a moment.

Delta-v is more complex, but essentially it measuring the amount of speed you can add or subtract from that penny based on how efficient the engine is, effects of gravity, and the % of fuel that your craft is made up of.

Hope that among all math guy answers this helps.

Regards,

[Skorpychan]

Delta represents change. V represents velocity. Simply, "Delta V" = "change in velocity" it's quicker to say "deltaV" than it is to say "change in velocity" , plus "deltaV" makes you sound awesome.

There are many ways to look at it. The terms your using, your representing a particular fuel/engine combo as the total DeltaV available with that combo combines with the mass of your ship. In other words "this stage will let me change my velocity by xxxx/ms "

Hope that helps

<nitpicky physicist> Technically, delta is the symbol for 'change in', rather than 'change'. V is the abbreviation for velocity. So, ÃŽâ€V = Delta V = change in velocity. The Άsymbol is the greek letter 'delta', which someone used on here a while ago and I saved. </nitpicky physicist>

Potential Delta V (or dV as I tend to use) is basically the fuel in your tanks, and the ratio of fuel remaining to specific impulse of the engine to the engine's thrust. Speeding up or slowing down uses that potential up, by turning it into actual dV.

http://en.wikipedia.org/wiki/Delta_v is a pretty good explanation of it, but there's a LOT of maths there that I don't understand any more.

[Mr Shifty]

You can get to orbit for around 3600 ms if you optimize your ascent. But you are basically right.

I would really like to see this ascent profile. I've never heard of anyone getting to a stable LKO for this little.

[Jason Patterson]

You can get to orbit for around 3600 ms if you optimize your ascent. But you are basically right.

Unless you're playing with mods that change air resistance, I think you're off by about 800 m/s. A very very clean launch to a very low orbit costs about 4400 m/s (it can be cheated somewhat by using parts with 0.1 resistance.) Anything from 4500-4800 is a reasonably clean launch.

[PPR]

Delta V is a great way to measure how far your ship can go. If you want to understand it better, I'd suggest you use Kerbal Engineer, a mod which calculates the Delta V for you (along with other things). It will give you a better idea of how big your spaceship needs to be.

http://kerbalspaceprogram.com/0-18-1-kerbal-engineer-redux-v0-5/

[CCKinnison]

FAR, I believe, is supposed to make it easier to get to orbit, that may be the issue involved. I'd like it for the Aviation-level changes, but I don't want to make it easier on my rockets.

[Tank Buddy]

Yes, Delta V is the measure of how much change in velocity your ship can make. Burning fuel in any way "uses up" your Delta V. Delta V is best used to determine if a ship can reach a certain destination, like another planet of a moon in the Kerbin system. Now that you can see your total mass, you can calculate your Delta V in flight.

[arq]

Another question is when I put solid fuel rocket boosters on my rocket it didnt change my Delta V. I'm guessing this is because it doesn't have a fuel mass and an empty mass so no delta v could be calculated. So in theory solid fuel boosters should give you some velocity without using your delta v?

I assume you're referring to the readouts of Kerbal Engineer? It is a known bug that Engineer doesn't handle SRBs correctly (it generally likes to ignore them). I don't know how much the SRBs added to your dV, but yes they certainly added some.