Force on Rocket: Find Force & Time to Zero

[songoku]

Homework Statement

A rocket with mass 60 x 10³ kg (25 % of it is "empty mass"). The rocket burns the fuel at rate 250 kg/s and expel the gas at relative speed 1.6 km/s. Find the force experienced by the rocket and the time until the force equals to zero

Homework Equations

Δp = F.Δt
p = mv
W = mg

The Attempt at a Solution

I am not sure what is the meaning of "empty mass". Is it the mass of rocket without fuel?

This is my attempt:
Force experienced by rocket
= Δp / Δt
= Δ(mv) / Δt
= v . Δm / Δt
= 1.6 x 10³ x 250
= 4 x 10⁵ N

Is that correct?

For the time until the force ceases:
Assume empty mass means the mass of rocket without fuel, so the mass of fuel = 75 % of 60 x 10³ kg = 45,000 kg

Time = 45,000 / 250 = 180 s ---> this means that the rocket accelerates for only 3 minutes?

Thanks

 

[haruspex]

That all looks right to me, except that it's not clear whether "the force experienced by the rocket" is supposed to be just that due to the engine or is to include gravity.

 

[songoku]

That all looks right to me, except that it's not clear whether "the force experienced by the rocket" is supposed to be just that due to the engine or is to include gravity.

If includes weight, it should be 4 x 10⁵ N - weight, correct?

 

[haruspex]

If includes weight, it should be 4 x 10⁵ N - weight, correct?

Yes.

 

[songoku]

OK thanks for your help :)