# Rocket propulsion

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1. Oct 18, 2014

### praveenpandiyan

1. well. I have this question. That my rocket is launched from mar in an return mission to earth. To calculate specific impulse of the rocket. Should i consider mar or earth gravity and if why!? Help me with this

2. Oct 18, 2014

### SteamKing

Staff Emeritus
If your rocket is being launched from Mars, why would you use earth's gravity in your calculations?

3. Oct 18, 2014

### praveenpandiyan

problem Is we have formula for specific impulse for earth standard..That Isp=Cj/Go
Cj-effective jet velocity
Go-gravity
..and thats the reason i asked..why its not mars gravity!!! Im not sure of it.

4. Oct 18, 2014

### SteamKing

Staff Emeritus
That's because until now, all rockets have been launched from earth, in earth's gravity. If ever a probe lands on Mars and returns to earth from there, obviously, Mars gravity will determine how much impulse will be required for the rocket to return to earth. It's like when Apollo went to the moon: did the lunar module need to exceed the gravitational pull of the moon or the earth?

If you had used the HW template per the rules of this forum, and provided the entire question as worded originally, perhaps this confusion of yours could have been resolved by now.