The kinetic energy of a mass m = 43000 kg at velcocity v=2400 m/s is evidently 43 000*2400²/2= 123.84 GJ

At v=1500 m/s the kinetic energy is 48.375 GJ.

The difference in kinetic energy of a mass of 43000 kg at 2400 and 1500 m/s is therefore 123.84-48.375=75.465 GJ.

In order to reduce the velocity from 2400 to 1500 m/s, which takes a certain time t (seconds) you must apply a force F (Newton), while the space ship displaces a distance d (meter).

Say that the time t is 600 seconds? What is the force F? And the distance d? Show me that you can calculate.

The acceleration would be 1.5 m/s

^{2} so the force is 64,500 N.

The distance would be the average velocity (assuming constant acceleration, which would not be the case) = 1,170,000 m.

The kinetic energy in joules would be 43,000 kg · 1,170,000 m · 1.5 m/s

^{2} = 75.465 GJ.

Doing the same calculations with different velocities that differ by 900 m/s, say from 10,000 m/s to 9100 m/s, we get:

The acceleration would be the same: 900 m/s ÷ 600 s = 1.5 m/s

^{2} so the force is the same.

The distance 5,730,000 m.

Kinetic energy = 43,000 kg · 5,730,000 m · 1.5 m/s

^{2} = 369.585 GJ.

Using your equation kinetic energy is 43,000 kg · (10,000

^{2} - 9100

^{2}) ÷ 2 = 369.585 GJ, the exact same value.

But notice the acceleration remains the same, 1.5 m/s

^{2}, regardless of the initial velocity. It is force that accelerates a spacecraft, not energy. Force = mass · acceleration which means acceleration = force ÷ mass. Nowhere in this acceleration equation is there a place for energy.