# DV Calculation clarification (rocket booster)

## 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?

## Answers and Replies

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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.

Dinoduck94
cjl