Specific Impulse Questions for Delta V

[srbbnd]

I am familar with how to calculate Delta V with the equation, .

However, what I am uncertain of is how to accurately decide what specific impulse to use regarding atmosphere and a vaccum. Once I leave the atmosphere is it an instant change to a vaccum, or is it a gradual change? For example, this formula: seems like it could work, but if it is a gradual change seems inaccurate.

Lastly, I am also uncertain what role thrust plays in Delta V for example:

Thanks:confused:

[FancyMouse]

>I leave the atmosphere is it an instant change to a vaccum, or is it a gradual change?

If you leave - that's vaccum. But if you stay in atmosphere, that is different depending on the air pressure. The number shown in parts description is the number on Kerbin surface. On different planet/altitude it will be different.

Usually I just use the atm number for first stage and vac number for all other stages.

>I am also uncertain what role thrust plays in Delta V for example

TWR<=1 means you can't leave the launchpad. Too low acceleration means it's harder to execute maneuver correctly. Other than that doesn't impact dV itself. Engines are balanced in such a way that usually if you tolerate a lower TWR/acceleration you can get more dV for the same mass, but that doesn't come from the formula itself.

[Empiro]

My practical advice is to ignore the atmospheric ISP. From a practical standpoint, you should be using engines with a high TWR and good sea level ISP when low in the atmosphere. By the time you switch to vacuum-only engines, the air will be thin enough that the ISP will be practically identical to the vacuum ISP.

[Hcube]

Yeah, since the change in Isp is gradual (right click an engine on ascent and you will see its thrust rising as you gain altitude, and for the same fuel consumption) you almost have a vacuum Isp @35km. For example, the 909 sucks on the ground but has a good Isp in vacuum : you will already be getting about 50.5 kN (vacuum is 60) of thrust at only 35-40km, although perfect vacuum is 35km higher.

As long as you use proper lifter engines for first stage (don't try to liftoff with a poodle or 909) , you shouldn't have to worry about that.