New to the 'delta-V' concept

[cephalo]

I'm seeing this all over the internet now. Can someone point me to guide that explains its relationship to the game? Does it allow me to predict how big of a rocket/fueltank I need for a mission? Guessing and trial and error has gotten me to orbit, but in a looping the loop, crazy unreliable sort of way.

Edited May 7, 2015 by cephalo

[Reddeyfish]

have you used maneuver nodes? you know the green bar on the right side that shows the velocity change?

That's the amount of 'delta-v' (change in velocity) required for that maneuver. the total delta-v of a craft is the total amount of velocity change that the spacecraft can do.

It's not unique to KSP, so wikipedia is a decent guide. http://en.wikipedia.org/wiki/Delta-v.

delta-v is a measurement. Most people don't use it to plan out their rockets, and instead just use it to tell if they need a bigger rocket (or more boosters). For example, if your rocket has 1000 m/s of delta-v, and you need about 5000 m/s of delta v to go where you want to go, you need a bigger rocket.

Delta-v is also rather imprecise for a few specific things, such as docking, landing, very long burns, or moving through an atmosphere, as it is very hard to calculate out all the little errors and adjustments.

Edited May 6, 2015 by Reddeyfish

[cephalo]

I just got those after upgrading my facilities but I haven't used them yet. Can I plan whole routs that way?

[3_bit]

For example, let's say you're in orbit traveling at 2,000 m/s. If you have 500 m/s of "delta-V", that means that there's enough propellant in your tanks that, with your current ship's mass and engines, you can accelerate up to 2,500 m/s or down to 1500 m/s before you completely run out of fuel.

Just my quick 2 kredits.

[cephalo]

How do you know if you have 500 m/s of deltaV?

[Reddeyfish]

How do you know if you have 500 m/s of deltaV?

usually you use a mod, such as kerbal engineer.

you try to do it by hand, but it involves calculus.

[Norpo]

How do you know if you have 500 m/s of deltaV?

Math, you can do the math by hand, or there are mods that can do the math for you.

In a future release i've heard they're going to put in a Delta-V Calculator in the default game, until then there are mods for that.

[youkofoxy]

what you need to know:

more fuel = more Delta V

more ISP = more Delta V

less dry mass = more Delta V

Delta V = acceleration.

basically, the more Delta V, the highest maximum theoretical speed you can achieve.

Bytheway, is possible to know your Delta V in stock KSP, without the mat, but it use a loot of tricks.

Edited May 6, 2015 by youkofoxy

[cephalo]

Can you think of 'delta-V' as an expendable resource? Let's say you're on the launch pad with x amount of mass, fuel and thrust. Can you say, ok I have 10000 deltaV to use on this mission, or does it not work like that at all?

[Trenchfoot]

I just got those after upgrading my facilities but I haven't used them yet. Can I plan whole routs that way?

You can, but I would perform maneuvers one by one because you won't be accurate 100% with your burns and the difference will really add up.

[cephalo]

You can, but I would perform maneuvers one by one because you won't be accurate 100% with your burns and the difference will really add up.

Does doing so help you decide how much fuel to take along or how to build the craft? That's usually the part I get catastrophically wrong.

[adinfinitum]

usually you use a mod, such as kerbal engineer.

you try to do it by hand, but it involves calculus.

Wat... there's no calculus involved. The equation for deltaV is g*Isp*ln(m1/m2), where g is 9.81 m/s, Isp is the given value in the VAB, m1 is the mass with fuel, and m2 is the mass without fuel.

[Jovus]

Can you think of 'delta-V' as an expendable resource? Let's say you're on the launch pad with x amount of mass, fuel and thrust. Can you say, ok I have 10000 deltaV to use on this mission, or does it not work like that at all?

Yes, it does work like that.

However, in order to be able to calculate delta-V by eyeball based on your own rocket design you'd have to be some kind of savant. At best you're going to get a gut feeling 'this needs more' or 'this has enough' and that will probably be wrong.

Trust me, use a mod that displays it. The game's purely better that way.

[Legendary Emu]

This may be totally incorrect, but I usually think of Delta V as something similar to miles per gallon. How far you can go depends on the engine, or the fuel tank, or the weight of the car.

Also the equation for calculating delta V is pretty fun to do by hand. First you find the mass of your rocket, write it down, then you take the propellant (liquid fuel) out of your first stage and write it down, then find the effective exhaust velocity (9.81 squared * the isp of the engine) and write it down. You divide the mass of your whole rocket, m0 by the mass of your rocket without the propellant (m1) and it would take too long to explain so just look up tsiolkovsky rocket equation.

[cephalo]

This may be totally incorrect, but I usually think of Delta V as something similar to miles per gallon. How far you can go depends on the engine, or the fuel tank, or the weight of the car.

Also the equation for calculating delta V is pretty fun to do by hand. First you find the mass of your rocket, write it down, then you take the propellant (liquid fuel) out of your first stage and write it down, then find the effective exhaust velocity (9.81 squared * the isp of the engine) and write it down. You divide the mass of your whole rocket, m0 by the mass of your rocket without the propellant (m1) and it would take too long to explain so just look up tsiolkovsky rocket equation.

That doesn't sound too hard, and potentially quite useful.

[Rickenbacker]

Does doing so help you decide how much fuel to take along or how to build the craft? That's usually the part I get catastrophically wrong.

Yes. I use the Engineer mod to show me how much Delta-V I have available, which saves lots of time. For example, if a rocket has less than 3000 m/s Delta-V, I know it most likely won't make orbit. Or if it does, it won't be going anywhere useful from there, as it takes around 3000 to get to low Kerbin orbit currently (and another 1000 to get to the Mun and so on). There's a Delta-V chart that helps you figure out how much you need to get anywhere in the Kerbol system, but I don't think it's updated for 1.0.

[Garius]

You need calculus to derive the Tsilkovsky rocket equation (though nothing more than some simple integration by substitution), but don't need any to actually use it. It's just Isp * 9.81 * ln(Wet mass/Dry mass). You don't need any knowledge of what ln is, you just press the button on your calculator (but a very very general idea is that ln tells you how many digits a number has, except it's smoothed out so that it doesn't jump, and it's multiplied by a useful constant that makes it work in things like this). It adds up between stages, and it's not so much a miles per gallon (that's closer to Isp) as it is a miles before empty (though not a perfect analogy)

Having a mod is definitely helpful, I recommend KER, but it's not at all hard to do it with a pocket calculator, just not as instant. It can get a bit complicated when you have different types of engines firing at the same time, but still doesn't need any calculus.

Googling KSP Delta V map gives a lot of good pictures showing how much DV is needed for various trips. Most will have too high values for achieving orbit from atmospheric bodies (eg Kerbin) but will be accurate enough for anything without atmosphere, but you'll get some variance based on piloting (just like if you plan a 100mi trip, you might not make it on 100mi of gas if you take detours)

[cerberusti]

This may be totally incorrect, but I usually think of Delta V as something similar to miles per gallon. How far you can go depends on the engine, or the fuel tank, or the weight of the car.

Also the equation for calculating delta V is pretty fun to do by hand. First you find the mass of your rocket, write it down, then you take the propellant (liquid fuel) out of your first stage and write it down, then find the effective exhaust velocity (9.81 squared * the isp of the engine) and write it down. You divide the mass of your whole rocket, m0 by the mass of your rocket without the propellant (m1) and it would take too long to explain so just look up tsiolkovsky rocket equation.

MPG would be more similar to ISP (or really MPG would be like the combination of ISP and vehicle weight.) Delta V would be closer to the range of the car on a full tank of gas.

Edited May 7, 2015 by cerberusti

[mrclucks]

Delta v literally difference of v(velocity). I've explained it to some friends as the change of speed. Its easiest to see it when using it in prograde(go faster direction) and retrograde(go slower direction.

Assuming youre in 2D with no other forces and you're traveling in a straight line going 100m/s and you burn 10 m/s delta v prograde, then you'll be traveling at 110m/s. If then you burn 220 m/s delta v in retrograde, then you'll be traveling 110m/s in the the opposite direction. Another 110 m/s delta v in the new retrograde direction (old prograde), then you completely stop.

Burning normal, anti normal, radial, anti radial have the same effects, but are a bit harder to notice in relationship to speed since you are burning between prograde and retrograde

Hope this helps

[RainDreamer]

Can you think of 'delta-V' as an expendable resource? Let's say you're on the launch pad with x amount of mass, fuel and thrust. Can you say, ok I have 10000 deltaV to use on this mission, or does it not work like that at all?

Close, but remember, delta-V can change over time due to several factors, such as change in mass. I remember my amateur mistake of hauling a light rocket to intercept a class E asteroid, then realized that my 1000 delta V left on that light rocket only amount to single digit delta V when that asteroid mass is added. So plan accordingly!

This also works the other way around. If you have a big craft built from connecting multiple component together using docking ports (like an interplanetary craft), and you are in a pinch for more delta V, you can try ditching some non-essential component to increase it (like the science lab), as your mass is now smaller, the amount of dead mass that your fuel have to haul is lessen, and so you can get more change in velocity.

Edited May 7, 2015 by RainDreamer

[Awaras]

Close, but remember, delta-V can change over time due to several factors, such as change in mass. I remember my amateur mistake of hauling a light rocket to intercept a class E asteroid, then realized that my 1000 delta V left on that light rocket only amount to single digit delta V when that asteroid mass is added. So plan accordingly!

And the opposite: If you are (for example) carrying a lander docked to your main craft, once that lander is separated, the main craft will suddenly have a lot more delta-V for the return trip. Yay!

[RSwordsman]

delta-V can change over time due to several factors, such as change in mass.

That's without accounting for the Oberth Effect, which can be pretty flippin' important in some cases. Gotta love astrophysics.

[Xannari Ferrows]

I will point you here. Surely something you need can be found lurking.

[KerikBalm]

That's without accounting for the Oberth Effect, which can be pretty flippin' important in some cases. Gotta love astrophysics.

Incorrect.

The oberth effect affects how much dV you need to get somewhere.

It has no effect on how much dV you have.

[Red Iron Crown]

Delta-V is essentially a measure of a spacecraft's ability to change the size and shape of its trajectory. The more delta-V, the more distant destinations can be reached because greater changes in its trajectory are possible. It's important to understand that delta-V ignores the size of the vessel; a 2-ton probe and a 2,000-ton mothership with the same amount of delta-V can reach the exact same destinations.

To use a car analogy (which has some flaws but gives the general idea): Delta-V is like the "distance until empty" readout that some cars have. It doesn't matter if the vehicle is a fuel sipping compact car or a huge fuel-guzzling semi truck, if they both show the same distance to empty they can reach the same destinations.

Someone already mentioned the Rocket Equation used for calculating delta-V, so I'll just note the consequences of it:

There are four strategies to increase delta-V:

1. More fuel mass.

2. Less non-fuel mass.

3. Engines with higher Isp.

4. Staging away unneeded parts (ties in with 2).

Edited May 7, 2015 by Red Iron Crown