How to calculate how much DV you need?

[DuartePires]

Hi there guys!

My main problem in KSP is the fuel, how do you calculate how much DV do you need to go where you want and back?

Edited February 24, 2015 by DuartePires

[ShadowZone]

Hi there! I think this might be of service:

http://i.imgur.com/NKZhU57.png

[EdFred]

Something is odd with that dV chart. How do you aerobrake going to Moho, Dres, and Eeloo?

[eddiew]

Old chart? Moho used to have an atmosphere... The chart from the KSP wiki may be better:

[ShadowZone]

Something is odd with that dV chart. How do you aerobrake going to Moho, Dres, and Eeloo?

I think since the arrow points down it's supposed to be an aerobraking manouver towards Kerbin.

[Superfluous J]

Something is odd with that dV chart. How do you aerobrake going to Moho, Dres, and Eeloo?

YOu're reading them wrong. The arrow above "Moho Tranfer" does not mean you can aerobrake to Moho coming from Low Sun Orbit. It's saying that you COULD eliminate all of that fuel need by aerobraking if you were coming back from Low Sun Orbit to KERBIN.

So if you were in low Sun orbit, it takes 26,820 to raise your apoapsis to Kerbin's orbit. Then it takes 5150 to raise your periapsis to Moho's orbit, 670 more to raise it to Eve's orbit, and 90 more to raise it to Kerbin's orbit at which point you'd be at the same orbit as Kerbin. However, once you get your apoapsis up to Kerbin's orbit, a smart and skillful player can actually hit Kerbin and erase that nearly 6,000 dV in a harrowing but completely safe aerobraking trajectory.

Preferably straight down onto the Helipad on top of the VAB

[UmbralRaptor]

My main problem in KSP is the fuel, how do you calculate how much DV do you need to go where you want and back?

The simplest (if somewhat inaccurate, depending on how you fly) method is using one of the many ÃŽâ€V maps out there. A more thorough approach involves digging into Hohmann transfers, the vis-viva equation, and when to patch conics. Technically inclination changes also.

[Jovus]

If, in addition to needing to know how much dV you need to get somewhere, you want to know 'how much dV this much fuel is', I recommend downloading and installing Kerbal Engineer Redux, which, among the many other potential functions it has, will give you a dV readout for the craft both in the VAB and in flight.

[Pecan]

Squad has always been passionately oppossed to having the information you need available in the game because "it wouldn't be fun" to design as if you were using a spreadsheet. Solution - design with an actual spreadsheet *sigh*. It's just one of those things ... if you think it's fun to throw things at the sky to see what explodes, you use stock. If you think it's fun to have some idea of whether your mission stands a chance of succeeding before you launch, you use mods. (KER/MJ/VOIDin this case)

Edited February 21, 2015 by Pecan

[Kelderek]

Also, if you use KER or MechJeb or something similar to show you DV values, make sure you pay attention to whether the number is for atmosphere or a vacuum. Most engines have different ISP values for these and will result in different amounts of DV depending on where you are. So for example, make sure that for the stages of your rocket that take you from the launch pad to low orbit you look at the atmosphere numbers and then use the vacuum numbers for any orbital maneuvers or transfers.

I have made the mistake a few times of seeing 4500 m/s DV and thinking I was fine to get to Kerbin orbit only to realize that I was looking at vacuum numbers and did not have enough atmospheric DV to get to orbit.

[UmbralRaptor]

If we're going for the how much ÃŽâ€V a rocket has...

4.5 km/s (really 4.4 - 4.7 km/s for typical mostly liquid fueled designs and orbits) is about the actually expended ÃŽâ€V. This amount is generally closer to the vacuum amount for ascent in general, though you'll have to make some guesses about intermediate I_sp values in multistage rockets. (or run simulations or due tests and take notes)

That said, the equation itself is simple enough. Just keep in mind that for a given stage in KSP, g0 is 9.82 while g is 9.81.

ÃŽâ€V == I_sp * 9.82 * ln(wet_mass/dry_mass)

[rkman]

Squad has always been passionately oppossed to having the information you need available in the game because "it wouldn't be fun" to design as if you were using a spreadsheet.

Hasn't it recently been confirmed that engineers will be able to calculate available dV?

[Sidereus]

I have made the mistake a few times of seeing 4500 m/s DV and thinking I was fine to get to Kerbin orbit only to realize that I was looking at vacuum numbers and did not have enough atmospheric DV to get to orbit.

I just make sure my boosters have an extra 500-1000 dV. It's particularly nice when I've over-budgeted and my next stage consists of LV-N's...

Hasn't it recently been confirmed that engineers will be able to calculate available dV?

Yes, though it's unknown (AFAIK) as to whether or not you'll have to level them up before they'll provide it...

[Superfluous J]

Hasn't it recently been confirmed that engineers will be able to calculate available dV?

Well, it's been TEASED as such but you could interpret it that the engineers at KSC have found a way to add it to all command modules.