How to enter another planetary system with going 5000ms

[Cameron Brown]

Hi all,

I'm starting to explore moons around Jool, the only issue i'm having is my speed as I enter the system. I'll encounter jool and then adjust my manouver until I encounter one of the moons. The problem is my speed is like 5000ms and I end up having to use about 3000 delta V to slow down and orbit. How can i encounter another planet or moon without coming in so quickly? I feel like i'm doing something wrong.

[Starhawk]

Hello @Cameron Brown and welcome to the KSP Forum!

Since this is a question about how to play the game, your thread has been moved to the Gameplay Questions section of the forum.

Essentially, what you want to do is use a 'gravity assist' from one of Jool's heavier moon's (usually Tylo) which changes the direction of your motion in Jool's SOI.  If done properly a craft can enter Jool orbit without any braking burn at all.

Glad to have you aboard.

Happy landings!

[Stamp20]

You could also adjust your path to go by Laythe or towards Jool itself, and aerobrake in the upper atmosphere if your craft can handle it.

[Superfluous J]

It sounds like you're meeting the moon head on or at an angle. You want to aim yourself to meet the moon's orbit (Tylo and Laythe are the best here) on the "right" side, both correct and on the right side looking in the default (North up) map view, from your ship's perspective.

Edited April 22, 2020 by Superfluous J

[jimmymcgoochie]

Jool has a huge gravity well so you'll pick up a lot of speed on the way 'down' towards it. You can try aerobraking through the atmosphere of Jool itself, but you'll need to bring heat shields as that speed results in very high heating even though the atmosphere is so thin a barometer can't actually detect it. You can also try braking in Laythe's atmosphere, but that's a smaller, moving target and the atmosphere isn't as thick either. Aiming is critical- too low and you burn up or have to expend more fuel to regain the lost speed, too high and you'll overshoot and if you miss the atmosphere entirely you'll just end up with a huge gravity assist that will hurl you out of Jool and the entire solar system.

With some very careful planning it's possible to use the gravity of the moons combined with some well-timed burns to slow yourself down without using too much fuel, although I haven't ever done it myself and trying to judge the timing by eye is all but impossible.

[Vezbot]

On 4/22/2020 at 3:21 PM, jimmymcgoochie said:

Jool has a huge gravity well so you'll pick up a lot of speed on the way 'down' towards it. You can try aerobraking through the atmosphere of Jool itself, but you'll need to bring heat shields as that speed results in very high heating even though the atmosphere is so thin a barometer can't actually detect it. You can also try braking in Laythe's atmosphere, but that's a smaller, moving target and the atmosphere isn't as thick either. Aiming is critical- too low and you burn up or have to expend more fuel to regain the lost speed, too high and you'll overshoot and if you miss the atmosphere entirely you'll just end up with a huge gravity assist that will hurl you out of Jool and the entire solar system.

With some very careful planning it's possible to use the gravity of the moons combined with some well-timed burns to slow yourself down without using too much fuel, although I haven't ever done it myself and trying to judge the timing by eye is all but impossible.

Yeah, assuming you enter the Jool system at 5000 m/s, and your trajectory is extremely close to Jool, you will have 5000 m/s of excess speed to shed as well as the speed you also picked up from Jool's gravity well. You will need to shed all that speed in the form of aerobraking or an engine burn. A gravity assist with Jool's moons is easier but more difficult to plan, with several moon orbits being made before you get close to Jool. I have never really tried gravity assists with Jool's moons but they are probably super useful.

So you would probably need to aim for a gravity assist, with a trajectory that can be corrected with a burn to then go to Jool's atmosphere and aerobrake until your trajectory isn't a straight line but curves around like a proper orbit, or aerobrake until you have a capture entirely. And then finally prograde at apoapsis to take your perapsis out of the atmosphere.

[Empiro]

The key is getting your orbit to match as closely as possible to your target before your encounter. If you're coming in directly from a Jool escape trajectory, then naturally your orbit won't look anything like your target's. Instead, try to use gravity assists to capture into a Jool orbit, and then use more assists to adjust your orbit so that it closely matches the moon you want to get to

Generally, it's OK if your AP or PE is some ways off, but you want the other side to be just barely touching your target's orbit and have the encounter happen there (so it looks like a Hohmann transfer). Having a big inclination difference is also bad. Finally, get your encounter as close to your target as possible, which will also cut down on the delta-V you need to capture (maximizing Oberth effect).

Note that it IS possible to efficiently capture directly into a moon's orbit, but you'll need to follow the advice above (no inclination difference, having the orbit around Jool just touching your target's, and getting the encounter there). However, it might be hard to time the encounter just right.

Edited April 29, 2020 by Empiro