I've noticed something that seems like an arbitrary limit on supercruise speeds. Sometimes when I close on a spaceport my ship slows down involuntarily and I can't speed it up. It is extremely annoying when I'm stuck with a 100km/s limit like now (I'm heading towards Birkeland City in Ross 154). What could be the problem? Why do I get these speed limits? I think it has nothing to do with flight assist since it does not seem to work in supercruise.
You are caught in the gravity well of a nearby object. Your maximum Supercruise speed will be lower whenever this occurs.
Your supercruise top speed is based on your distance from large gravitational bodies (stars, planets, moons, rings, etc). If you get too close to a planet, the speed reduction is huge. If you get so close that you're forced out of supercruise, you'll have to point at an "escape vector" to get back into supercruise.
If you're very close to a planet/moon/etc and your speed drops, the best thing to do is head straight away from it until your speed picks up, before heading towards your target.
Because your speed is reduced by being close to planets/moons/etc, and most bodies in a system orbit near a single plane ("ecliptic"), a straight line is rarely the fastest way to get somewhere. Straight line travel will often lead to "arbitrary" slowdowns as you pass planets.
The fastest way to travel from your jump-in point to a typical station in a typical system is to arc above or below the plane of the ecliptic, and drop in on the station from above. That way you stay well away from any planets/moons/stars along the path to the body your station is orbiting, and you won't have any trouble avoiding the body your station is orbiting, either.
The routine looks something like this:
- Head directly away from the star, full throttle, until your speed increases to something measured in "c" instead of "km/s".
- Point towards the station.
- Look for the orbit lines (yellow lines) to figure out where the ecliptic plane is.
- Point 20-45 degrees above or below the orbit lines. (keep the station at the top of your screen instead of center)
- Keep pointed at that angle, so that you arc towards it, instead of going in a straight line.
- When you start being able to see the station's orbit lines around its planet/moon, the orbit lines of the moon around the planet or other clear orbit lines, get lined up so that those are all circles, like an archery target.
- When the reported time to arrive at the station decreases to 6-10 seconds, set your throttle to 75% (center of blue range, but having a 75% throttle button really helps). If you overshoot, either start spiraling (pitch and roll), or aim nearer the planet so it slows you down.
- If the station is a rotating station (not an outpost), aim between the planet and the station, then arc towards the station, so that the planet is behind you when you drop out of supercruise. Rotating stations usually have their entrance pointed more-or-less towards the body they orbit, so this helps ensure you can see the entrance and can dock faster.
- Once your supercruise speed drops below 1000km/s (the blue area on the target info panel that shows approach speed and distance targets as you get closer), you can throttle back up to 100%, but keep the speed in that blue region on the target info panel.
Instead of the arc I describe, you could try just shooting straight up from the ecliptic plane, making a right angle turn to parallel the ecliptic plane, then another right angle turn towards the station when it moves to the edge of your compass.
Another advantage of arcing away from the ecliptic is that it makes it a bit less likely another player will interdict you, since you spend less time with the rear of your ship towards where player ships are likely to be, especially during the deceleration phase when it's easiest to interdict you.