A Degree Makes a Difference (Stock Edition)

[foamyesque]

2 hours ago, LN400 said:

One question: When there is talk about dv, is there a standard as reference? I mean, is it all amt, or all vac, or a specific combination? This question really goes out to all who use dv benchmarks of any kind but it would be relevant to know what is being used here..

I use vacuum dV in all cases.

[numerobis]

When MechJeb reports dV spent, that's based on the instantaneous Isp.

When you look at dV available it's usually vacuum Isp that you use, because you get out of the atmosphere so quickly on Kerbin.

[Norcalplanner]

Just added a link in the RSS version pointing to this thread, since some people were wondering about the numbers for stock Kerbin.

[KerikBalm]

On 8/20/2016 at 5:13 AM, Norcalplanner said:

6. Getting a rocket to orbit in stock for less than 3,200 m/s of delta V is very doable.  Using less than 3,100 m/s is a harder but still achievable goal.

Indeed, this got into orbit with less than 3,200 vac dV, but just barely with a full payload:

Spoiler

Yep, that's right, I burned through monoprop to get the PE over 70km... but there was enough left to deorbit. This was pre-vector.

But keep in mind, not all vacuum dV is equal... 3,200 vacuum dV with a Rhino is not the same as 3200 vacuum dV with a vector or a mammoth... because the difference in atmospheric dV and vacuum dV is less.

In almost any reasonable scenario, the majority of your dV is expened in near vacuum conditions, but how much and how "near" to vacuum that is for Isp purposes varies.

While it matters less for a Mammoth, for a Rhino getting higher in the atmosphere is as important for increasing Isp as it is for decreasing drag.

If you have 3200 vacuum dV with a rhino, but burn all your fuel at 1 atm (even assuming a tiny drag coefficient and super heat resistant parts), you only get a change in velocity of 2,400 m/s.. which is only enough for orbital velocity if you head east... heading west - no orbit

In contrast, a mammoth with 3,200 vacuum dV, if it expended all its fuel at 1 atm (I guess we're assuming infinite TWR and zero drag here) it would have a change in velocity of 2,996 m/s, which easily exceed orbital velocity even if you went against Kerbin's spin