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: