What is that little number on certain stages?

[thundershield]

Pretty much title. I've been searching all around, but have yet to find an answer, not helped by the fact that I don't actually know what to search. It's a velocity, but doesn't match with any of the delta v calculators I've used, so I really have no idea what it's measuring.

[HebaruSan]

Welcome to the forum!

Press F1 to take a screenshot of whatever it is you're talking about, then find it in the KSP/Screenshots folder. Upload to https://imgur.com/ and post the URL of the image so we can see it.

[jimmymcgoochie]

It’s probably the stock delta-V display, and the reason it doesn’t match what you see from mods like KER is probably because it’s set to show the delta-V on the surface of Kerbin, which is lower (in some cases much, MUCH lower) than in space. This value is shown in a blue background in the editors and in flight

There’s a button in the bottom right of the editor that lets you change the reference body and altitude to get a better idea of delta-V and thrust to weight ratio (TWR) for where you’re going. It’s best to check your surface TWR when launching to make sure the rocket can actually get off the ground, then switch to vacuum mode to get a proper idea of your total range with space-optimised engines. When in flight, the readout automatically adjusts to the conditions the craft is in.

[Snark]

5 hours ago, thundershield said:

Pretty much title. I've been searching all around, but have yet to find an answer, not helped by the fact that I don't actually know what to search. It's a velocity, but doesn't match with any of the delta v calculators I've used, so I really have no idea what it's measuring.

As @HebaruSan said, it would be helpful to see a screenshot of what you're talking about.

However, I'll hazard a guess as to what you're referring to:

In recent versions, KSP has a dV display (little numbers in blue) that shows how many m/s you'll get out of various stages. It can be super handy, but it's important to understand how it works, because there's some subtlety there. In particular, there can be a substantial difference between the vacuum and atmospheric Isp of engines, which has a major effect on dV calculations.

How the game handles that is different between the VAB versus in flight.

In the VAB, those numbers will assume Kerbin sea-level pressure by default. This is good for calculating the dV of your first booster stage off the pad... but if you have an upper stage with a vacuum engine that has very low atmospheric Isp, the number displayed will vastly underestimate how much dV that stage will give you, because it's not smart enough to realize that you plan to use it when you're in vacuum.

You can control this behavior. One of the option buttons (in gray, at lower right) in the VAB allows you to control what pressure the dV calculations should assume. You can, for example, tell it to assume a vacuum... which will give you accurate dV numbers for your orbital stages, but now it's too optimistic about the dV for your launch-from-the-pad booster. Or, for example, if you're designing a crewed Eve lander, you could tell it to assume Eve sea level pressure.

The important thing is that you can only pick one pressure for it to assume. If you have a rocket that will be operating in different pressure domains over its path (such as a rocket that goes to orbit from Kerbin's surface), then it's impossible to get numbers that are simultaneously accurate for all stages, because there's no way for you to tell the UI "assume this pressure for stage 1, assume that pressure for stage 2", etc. Basically you just have to try flipping it back and forth to display reasonably accurate numbers for the different stages.

That's in the VAB.

In flight, it's a bit different (simpler, actually). There's no option for choosing what pressure to assume. Instead, it bases all its calculations, for all stages, on the current pressure at the rocket's current location.

Thus, if you have an orbital rocket sitting on the launch pad, and it has a vacuum engine (like a Terrier) on an upper stage... well, it will show very inaccurately low numbers for the Terrier stage's dV as it sits on the pad, because it'll be using current atmospheric pressure for the calculations. But then, after you launch: if you watch the number as your rocket climbs, it will show that stage's dV getting bigger and bigger (because the pressure is dropping as the rocket gains altitude). By the time you're out of the atmosphere (or nearly so) and actually are ready to activate the Terrier, the number will be showing the correct (vacuum) dV, because that's what the current pressure is.

Does this help?

[thundershield]

Thanks you so much for the help! Sorry about not uploading a screenshot, but you guys got it anyway. I haven't played KSP in years, and just picked the game up on the recent sale, so I had no idea what the number was. This will definitely help with landing on the mun. Once again, thank you all so much!

[Kerbal Productions]

Probably how much Delta V will you gain in a full or a (whatever you call it, but not full maybe partial)  burn.