Why is delta v for each stage sometimes shown inconsistently

[jnbspace]

Since I have started using delta v maps to try to help plan my missions I have noticed that the delta v shown while in the VAB and while in flight are inconsistent.

For example I have created a mission to carry a satellite into orbit around the Mun. According to the delta v maps I require about 860 - 1170 m/s of delta v for transfer to the Mun and orbit insertion. So I create a craft which has a stage with a delta V of 1251 m/s

However when I get into orbit around Kerbin and have just that stage it shows a delta v of 2962 m/s

Why is there this discrepancy? and how when building in the VAB do I know what my craft's delta v really is?

 

 

[HebaruSan]

You're probably in atmospheric mode in the VAB. Hover the Δv icon in the toolbar and switch to vacuum.

[Snark]

54 minutes ago, jnbspace said:

Why is there this discrepancy?

Just to add on to what @HebaruSan said:

The issue is that engines have different Isp at different atmospheric pressures.  For some engines, the Isp doesn't change much; for others (the so-called "vacuum engines", like the Terrier), it changes a lot.  This makes a big difference to dV numbers.

• When you're in flight, it always calculates dV based on the atmospheric pressure at the ship's current location.  So, for example, if you watch that number during ascent to orbit, you'll see the numbers change gradually as the atmospheric pressure gets lower with altitude.
• When you're in the editor, there is no "current location", so it just has to pick one.  By default, it's set to atmospheric, but there's an option toggle that lets you switch it to vacuum to see what the numbers look like there.

[Zhetaan]

3 hours ago, jnbspace said:

According to the delta v maps [...]

To add to what others have said, you should also know that delta-V maps generally give the vacuum values because those are the most consistent.  Necessary delta-V to reach orbit from a surface with an atmosphere depends not only on atmospheric pressure (which itself varies with location and altitude), but also on drag (which varies with location, pressure, altitude, speed, orientation, and craft design) and gravity (which varies with your location and your ascent profile).  Vacuum ascent depends only on the gravity that you need to fight in order to reach orbit, which is usually not much when you don't also have drag and atmospheric pressure working against you.  Note that I include pressure separately even though it is a component of drag (higher pressure means more air to do the dragging) because pressure directly affects your engine efficiency.

In other words, while you may want to build a rocket with an eye to the vacuum delta-V values in the VAB, you may want to consider building a margin for error into the lifter design for an atmospheric ascent.

[jnbspace]

Merci. I had assumed that the dv values would default to vacuum and built in a margin of error on that assumption. On the plus side it did mean that my craft had huge margins of error!

Edited May 23, 2019 by jnbspace

[cantab]

To add to the previous, I've found other oddities with the stock delta-V readout.

With a series staged rocket, if I put the decoupler and engine in the same stage, the VAB display seems fine but in-flight I get an incorrectly low reading until I stage. My guess (without having calculated) is KSP calculates the delta-V with the empty mass of the previous stage still attached.

I'm currently designing a rocket with drop tanks, and in the VAB KSP seems hopeless at that, it doesn't even display the core delta-V at all.

So yeah, it's not perfect.