Delta-V (again)

[qazsedcft]

In most of the delta-v maps and discussions on this forum it seems that the generally accepted surface to Kerbin orbit delta-v is 4.5 km/s, but I can consistently send probes into low orbit (100 km) with only about 3800 m/s delta-v. I just use the Engineer plugin for stats but besides that I play fully manual. I'm quite surprised because I'm very new to this game so I thought I would worse results. Where does the 4500 m/s value come from?

Edited July 10, 2014 by qazsedcft

[Laie]

Hmmmm. (scratches head) You know that most engines work better in thin air/vacuum, do you? There's usually two delta-v figures, one for atmosphere and one for vacuum. Can it be that you're looking at the atmosphere figure? Because actually, most of the ascent happens in such thin air that the vacuum figure is much (much!) closer to the truth.

Edited July 9, 2014 by Laie

[Alshain]

Hmmmm. (scratches head) You know that most engines work better in thin air/vacuum, do you? There's usually two delta-v figures, one for atmosphere and one for vacuum. Can it be that you're looking at the atmosphere figure? Because actually, most of the ascent happens in such thin air that the vacuum figure is much (much!) closer to the truth.

I agree with this. I've found KER numbers are a bit low if you base it off of atmosphere, not that KER is wrong mind you, it's 100% dead on, it's just static. Your either in space or on the ground and there is no in between.

[Taki117]

Generally speaking once you hit about 20km the Isp values of most engines are near their vacuum Isp values anyway. It is entirely possible to spend about 3800 m/s of dV to get into orbit if you are flying correctly.

[Padishar]

I agree with this. I've found KER numbers are a bit low if you base it off of atmosphere, not that KER is wrong mind you, it's 100% dead on, it's just static. Your either in space or on the ground and there is no in between.

The 4500m/s quoted is for vacuum ISP values. KER uses the current atmospheric pressure when calculating during flight so, when sat on the pad which is only a little above sea level, the values are generally significantly lower because most engines have a lower ISP in atmosphere. In the VAB the "atmospheric stats" button switches between vacuum and atmospheric ISP and you can also adjust the actual pressure the "atmospheric stats" setting uses in the engineer tweakable (right-click the engineer part should give a slider that sets a percentage of sea level pressure on the selected reference body).

Edit: to find how much your launch took subtract the remaining, in-orbit, deltaV total of your ship given by the flight engineer from the in-vacuum total given by the build engineer in the VAB before launch.

Edited July 9, 2014 by Padishar

[Alshain]

The 4500m/s quoted is for vacuum ISP values. KER uses the current atmospheric pressure when calculating during flight so, when sat on the pad which is only a little above sea level, the values are generally significantly lower because most engines have a lower ISP in atmosphere. In the VAB the "atmospheric stats" button switches between vacuum and atmospheric ISP and you can also adjust the actual pressure the "atmospheric stats" setting uses in the engineer tweakable (right-click the engineer part should give a slider that sets a percentage of sea level pressure on the selected reference body).

Edit: to find how much your launch took subtract the remaining, in-orbit, deltaV total of your ship given by the flight engineer from the in-vacuum total given by the build engineer in the VAB before launch.

We aren't referring to "during flight", he's talking specifically about the SPH or VAB Delta V read outs.

[qazsedcft]

Actually, I'm using both the VAB and in-flight values. In the VAB I selected the Kerbin button and compared that to the delta-v remaining at the end of the flight. I had assumed that KER would automatically calculate the correct value. Like I said I'm new so probably it's just my mistake and I'm trying to figure it out.

[LethalDose]

Are you using FAR? Because that will reduce your dV to about 3800 m/s.

[Superfluous J]

I would like to see a video of how you launch, because if I could consistently get 3800dV to Kerbin orbit that'd shave about 1000dV off my ships.

[mhoram]

@qazsedcft: Do you use jet engines on your ship? Calculation of their fuel usage is even more complicated than for rocket engines.

[qazsedcft]

OK, I just checked again comparing the KER vacuum value from the VAB with the value remaining at the end of the flight and it's indeed about 4600 m/s. Thanks for the info.