Determining flight time

[Nepos]

I was wondering if there was an easy way to estimate flight time to an orbital object? I suspect I would have to know something about differential calculus to determine it being newton discovered this to describe the motion of the planets.

Right now if I wanted to know what the flight time it is to the Mun in its 12 000 000 apoapsis (per the wiki), I will go to my space center to the dead satellite orbit Kerbin and make a navpoint that get into the SOI Mun the checking the time.

Or it there a map that show the time like there are maps the show the delta-V? I

Thanks

Edited March 21, 2014 by Nepos

[Pecan]

For interplanetary travel use http://alexmoon.github.io/ksp/ - not only will it tell you when the transfer-windows are available but also exactly how much deltaV you'll need and what your transfer time will be.

[Red Iron Crown]

The delta-V map on this Wiki page has transit times on it, if that's what you mean.

[Nepos]

The delta-V map on this Wiki page has transit times on it, if that's what you mean.

DOH I have that map printed out in color and I never notice the transit times.

Thanks

[cantab]

Kepler's Third Law: For all orbits around a given primary, the period (P) squared is proportional to the semi-major axis (a) cubed. So if you know P and a for one thing orbiting, such as your current orbit or that of a moon, you can find the period for any other orbit.

To give an example, let's say we're in an 80km orbit above Kerbin and we want to go to to the Mun. A transfer orbit with a periapsis of 80km and an aopapsis of 10,000 km will do us.

For our known orbit, we can use the Mun. Its orbit is circular, so a_m is just the orbital radius, 12000 km. P_m is 138984 s. Both values from the Wiki.

P_m²/a_m³ = 0.011

To get the semi-major axis we add our periapsis altitude, apoapsis altitude, and the diameter of the primary (Kerbin), then divide by two.

a_t = (80 + 10000 + 1200)/2 = 5640 km

We simply need a value of P_t such that P_t²/a_t³ = 0.011, same as the Mun. You can do this algebraically, or just using trial and error.

With trial and error, P_t of 45000 seconds does the job. Divide by 3600 to get hours, 12.5, then divide by half since our transfer is half an orbit. (Technically, a bit less, since we hit the Mun's SOI before reaching apoapsis).

Our transfer will take about 6 1/4 hours.

Somewhat lower than the map says, but more in accord with my experience.

You can also use this method to get a time estimate for, eg, a Duna-Dres transfer, something that's not on the map. Or you can just use a launch window planner.

Edited March 21, 2014 by cantab