How do you properly visit a planet for aerobrake and DV

[Guest]

So I recently tried twice to land on Laythe but both times failed because It took too much delta-V (4000-10000), or aerobraking speeds were too powerful 8000 meters per second at some times. I know there is an easier way to circulate moons of Jool and Dres. Please tell me what I'm doing wrong

 

[Kryxal]

Try to make your Laythe rendezvous at your periapsis, and heading in the same direction around Jool.  If you're already in the Jool system, fine-tune your Laythe pass to capture via gravity assist, preferably with a high apoapsis, and fine-tune the next encounter from there.

[Snark]

On 4/30/2019 at 9:11 AM, The Doodling Astronaut said:

So I recently tried twice to land on Laythe but both times failed because It took too much delta-V (4000-10000), or aerobraking speeds were too powerful 8000 meters per second at some times. I know there is an easier way to circulate moons of Jool and Dres. Please tell me what I'm doing wrong

First, make sure that you're following a good transfer window.  This minimizes your arrival velocity at the Jool system.  If your transfer window to Jool isn't great, you'll be coming in a lot hotter.

Second, make sure that you arrive at Laythe with proper timing.  Specifically, you want to arrive when your craft is going in the same direction as Laythe in its orbit around Jool.  This lowers your Laythe-relative speed on arrival, because Laythe's orbital speed around Jool will be subtracted from your own.

Third, it's worth doing a reverse gravity assist upon Jool arrival.  Either Tylo or Laythe can work well for this purpose.  It's a standard gravity-assist maneuver, but done with the objective of slowing down rather than speeding up.  In this fashion, you don't directly hit Laythe's atmosphere upon arrival at Jool.  You first do a close (vacuum) pass by Tylo or Laythe to bleed off many hundreds of m/s off your arrival speed, and then you go for your Laythe aerobraking on your second pass.

[Guest]

1 hour ago, Snark said:

First, make sure that you're following a good transfer window.  This minimizes your arrival velocity at the Jool system.  If your transfer window to Jool isn't great, you'll be coming in a lot hotter.

...

Third, it's worth doing a reverse gravity assist upon Jool arrival.  Either Tylo or Laythe can work well for this purpose.

So first, I am following what it said from http://ksp.olex.biz/

Third, can you explain to me a little more of what a mission like this would look like?

[Snark]

1 hour ago, The Doodling Astronaut said:

So first, I am following what it said from http://ksp.olex.biz/

Great, in that case you're doing exactly the right thing and you already have a good transfer window there.

What about my second point, the one you didn't quote?

1 hour ago, Snark said:

Second, make sure that you arrive at Laythe with proper timing.  Specifically, you want to arrive when your craft is going in the same direction as Laythe in its orbit around Jool.  This lowers your Laythe-relative speed on arrival, because Laythe's orbital speed around Jool will be subtracted from your own.

^ That's really important.  Laythe orbits Jool at over 3200 m/s, so depending on what direction you're approaching Laythe from, this can make up to 6400 m/s of difference in your relative velocity when you hit atmosphere.  This is huge, and very important to be aware of.

1 hour ago, The Doodling Astronaut said:

Third, can you explain to me a little more of what a mission like this would look like?

Sure.  Basically, it's a reverse slingshot maneuver.

Let's imagine the reverse situation.  Suppose you're in low circular orbit around Jool, and you want to eject from the Jool system with as much velocity as you can, without using too much of your own fuel for dV.

Well, one way would be to just do a great big burn to eject.  But there's a way you can gain several hundred m/s of dV for free, using a gravity assist maneuver, or "slingshot:  you do a burn that's enough to raise your Ap a bit past Tylo, such that you're crossing right behind Tylo (deep into its SOI, passing as close to Tylo as possible) when you're crossing its orbit.  Tylo's gravity and orbital velocity will give you a "helping hand" and fling you out of Jool's orbit much harder than your own small dV burn provided.

So, basically, do that in reverse.

1. Set up your Jool arrival so that your target Jool Pe is really low down (i.e. lower than Laythe's orbit).
2. Have it timed such that as you're falling closer and closer to Jool, at the point where you cross Tylo's orbit, you're doing it just in front of Tylo in its orbit (i.e. you cross Tylo's SoI on your inbound trajectory towards Jool, passing very close to Tylo as you do so).
3. When you fly past Tylo in this fashion, its gravity and orbital velocity will bend your trajectory in a way that removes a lot of your velocity:  done well, it can bleed off so much speed that it actually captures you to Jool orbit without needing to burn your engine at all!
4. This puts you in a very eccentric Jool orbit, with a Pe down below Laythe and an Ap way up high, maybe out past Pol.
5. As you exit Tylo's SOI, continue falling in towards Jool.
6. When you arrive at Jool Pe, do a moderate burn (your first since entering Jool's SoI) to lower your Ap further, to something manageable, e.g. around Pol's orbit or below.
7. Coast up to Ap, then do a small burn to raise your Pe up to match Laythe's orbit.
8. Coast back down to Pe (now at Laythe's orbital height), and do a burn to adjust the timing of your orbit such that you'll meet Laythe the next time you reach Pe.
9. Swing 'round by an orbit, then do your aerobraking at Laythe as you like.

 

Incidentally, if getting it timed for a Tylo gravity assist in this fashion seems too complicated and difficult for you... the above strategy still works pretty well, just skip steps 2 and 3.  Set up your Jool approach to aim for a very low Pe, i.e. as low as you can go without hitting atmosphere, then do a burn at Pe to capture and lower your Ap to Pol's orbit or below.  Then just do the remaining steps 7-9 to arrive at Laythe.  Even without the reverse gravity assist from Tylo, this is an efficient way to arrive because it takes advantage of Jool's gargantuan Oberth effect.

[Guest]

Thanks a lot this will be useful

[Guest]

so another question. How much Velocity does an ablative heat shield survive