Minimum thrust of engines at lift-off

[zebra1707]

Homework Statement

Hi there

Rocket has a total mass of 3.4 x 10^6 kg at lift-off. The first stage contains 2.0 x 10^6 kg of propellants which is consumed at a rate of 1.4 x 10^4 kg s

Find the minimum thrust of the engines at lift-off?

I would have thought that you needed one more variable to be able to solve this?

Homework Equations

The Attempt at a Solution

I used the mass flow x escape velocity and ended up with 15.7 x 10^6 N. It takes 142 sec for the first stage to finish its burn.

I just don't think that this is right - as the next part states that if this thrust is sustained - find the time over which the first stage operates and the acceleration of the vehicle at that time. (Neglect air friction - and suume whole flight is vertical.

Help needed please. Cheers

 

[zebra1707]

Any thoughts?

Cheers

 

[thehacker3]

Wouldn't the minimum thrust just be m*g? In order to get the rocket moving, you would need at least 3.3*10^7 N of force.