A rocket with a total mass of 330,000 kg when fully loaded burns all 280,000 kg of fuel in 250 s. The engines generate 4.1 MN of thrust. What is this rocket's speed at the instant all the fuel has been burned if it is launched in deep space?

6900 m/s

Explanation:

The mass of the rocket is:

m = 330000 - 280000 (t / 250)

m = 330000 - 1120 t

Force is mass times acceleration:

F = ma

a = F / m

a = F / (330000 - 1120 t)

Acceleration is the derivative of velocity:

dv/dt = F / (330000 - 1120 t)

dv = F dt / (330000 - 1120 t)

Multiply both sides by - 1120:

-1120 dv = - 1120 F dt / (330000 - 1120 t)

Integrate both sides. Assuming the rocket starts at rest:

-1120 (v - 0) = F [ ln (330000 - 1120 t) - ln (330000 - 0) ]

-1120 v = F [ ln (330000 - 1120 t) - ln (330000) ]

1120 v = F [ ln (330000) - ln (330000 - 1120 t) ]

1120 v = F ln (330000 / (330000 - 1120 t))

v = (F / 1120) ln (330000 / (330000 - 1120 t))

Given t = 250 s and F = 4.1*10⁶ N:

v = (4.1*10⁶ / 1120) ln (330000 / (330000 - 1120*250))

v = 6900 m/s