KSP Atmospheric Entry Timing Calculator

Enter your current altitude, orbital velocity, and periapsis distance into the calculator at right. The calculator will give the time in (d, h, m, s) till atmospheric entry, max drag, v = 250 m/s, and landing, or atmospheric exit (where applicable).

The calculator assumes that your ship is headed down toward periapsis. To use this calculator when your ship is headed up toward apoapsis, add twice the 'time to apoapsis' from the game to the results.

How to Use the Solution

This calculator is primarily meant for use with mods like RemoteTech where craft cannot be controlled instantly; rather, burns and actions must be pre-programmed.

Program the ship it to retract fragile solar panels, antennas, etc before atmospheric entry, and to deploy parachutes after slowing to ~250 m/s. Remember to wait longer before opening solar panels, else they may break when the chutes fully open.

If your goal is an aerocapture, program the ship to circularize its orbit at the appropriate time.

Suggestions for Best Results

The calculator will probably not work for ships with lift surfaces, because the lift force depends on the craft's orientation in the atmosphere. Lift forces are not considered.

Sometimes, when coming out of timewarp, the periapsis will jump around a bit, potentially messing up the re-entry maneuver...

How it Works

Given a current altitude, orbital velocity, and periapsis, we have enough information to resolve the orbit of the ship (there are some ambiguities in orientation, but they do not matter for this problem). The point of intersection of this orbit with the atmosphere and the velocity of the ship at that point are calculated. Using these data, the trajectory in the atmosphere is determined using numerical methods. If the ship reaches the ground, the time and altitude of maximum drag and of reaching 250 m/s (in the Orbital frame, not the Surface frame) will be given, and the time of landing/impact (assuming no parachutes) will be given. If the ship escapes, the velocity at which the ship escapes the atmosphere is found, and used to compute the new orbit.

Q&A

Why isn't the mass of the ship needed?
This is an artifact of the (unrealistic) drag model in KSP at the moment. Both the gravitational and drag forces are proportional to mass. As a result, the acceleration (and therefore the path of the ship, neglecting lift, in the atmosphere) is independent of mass. Note: In the real world, aerodynamic drag is not proportional to mass.
When can I use 0.2 as the drag coefficient?
For most ships, the drag coefficient is very close to 0.2. This is because most parts in the game have a drag coefficient of 0.2. Those that do not (for instance, docking ports), tend to have a small mass and therefore do not significantly alter the drag coefficient. However, if using modded parts (or if your ship's mass is largely made up of docking ports), you'll want to calculate the drag coefficient for your ship. This can be done following the methods here.
Current Orbit Details

Supported units: m, km or Mm

Select the direction of your orbit for more accurate results.

Ship Details (Advanced)
For almost any ship, this will be approx. 0.2.
Results
Landing results
Aerobrake results