There was a thread where someone computed that UFOs need about 1 month to reach Earth from Mars. I can't find it. I tried to calculate the time and got much faster trip.

Does the original game mention the maximum accelerations of UFOs anywhere? I don't think it does.

Let's assume top speed of UFOs is given for lower atmosphere when air friction equals the force of their drives, shall we? In the vanilla a Battleship's speed is 5000 km/h. I tried running a CFD simulation, but ran into problems with ParMGridGen. Let's simply assume that it is a cylinder 25 m in diameter and 9 m tall, about as aerodynamic at supersonic speeds as a sphere (drag coefficient Cd ~1). Drag force at top speed will be 1/2 * 1.3 kg/m^3 * (1400 m/s)^2 * 1 * 225 m^2 ~ 3*10^8 N That's the force its engines can give. If the speed is capped by some other factor, like heat, this means even more powerful engines.

How heavy are battleships? Let's assume about as heavy as Earth aluminium military hovercraft of equal size. I found only craft twice smaller at 100 metric tons and twice larger at 500-600 tons. Let's say 200-300 tons.

This gives maximum acceleration in space up to 300 000 000 / 200 000 = 1500 m/s^2 or 153 g.

Here's a good question: how will the crew and fragile machinery survive this acceleration? I suppose that aliens can maintain comfortable artificial gravitation of ~0.5 g inside their vessels. Either that, or gravity engines make the ship fall free at 150 g, thus nobody inside is hurt.

What is the travel time from Mars to Earth? Minimum distance from Mars to Earth is about 230 - 150 = 80 million km. Maximum distance is 230 + 150 = 380 million km plus some unspecified amount to avoid passing too close to Sun. The shortest trip would take sqrt( 2 * 8*10^10 m / 1500 m/s^2 ) = 10328 s or less than 3 hours.

It is safe to ignore relativistic effects since the speed would not exceed 10 000 s / 2 * 1500 m/s^2 = 7 500 000 m/s or 0.025c.

For longer distance the time and top speed will increase as square root of the distance — still hours and several percent of c.

Any mistakes?