Does dV calculation use standard g or altitude-dependent gravity?

[Dinoduck94]

TL;DR

When calculating the dV available from a rocket booster, is the ISP always multiplied by 9.81 to get the exhaust velocity?

When calculating the dV available from a rocket booster, the below calculation is used:

(ISP . g) . ln(Mass when full/Mass when empty)

Is 'g' always equal to 9.81 in this equation, or do you use the actual gravitational acceleration that the booster will experience, at it's given altitude, to calculate the exhaust velocity?

 

[trurle]

Summary:: When calculating the dV available from a rocket booster, is the ISP always multiplied by 9.81 to get the exhaust velocity?

When calculating the dV available from a rocket booster, the below calculation is used:

(ISP . g) . ln(Mass when full/Mass when empty)

Is 'g' always equal to 9.81 in this equation, or do you use the actual gravitational acceleration that the booster will experience, at it's given altitude, to calculate the exhaust velocity?

Constant value of 1kgf (9.806650) is used. The question about ISP is the classical example of the troubles associated with the non-metric units. Historically, ISP measured in seconds was useful because it can be derived from thrust and fuel flow rates expressed in both metric and imperial units, without changing the formula.

 

[cjl]

As trurle said, it's just a unit conversion, so you just use 32.2 if you want your result in ft/s or 9.8 if you want your result in m/s. It doesn't matter at all what conditions the rocket is experiencing.