Jump to content

gravity assist and ship mass


Recommended Posts

hi,
I'm try to plant a flag on moho, I've unsuccesfully tried several ship configurations.
everytime I try to reach moho I do almost the same eve gravity assist manouver.
at this point I've noticed that the result orbits are much different each others.

when I do gravity assit manouvers, the mass of the ship matters so much?

Link to comment
Share on other sites

The mass of your ship does not affect how much the planet pulls on you as you go by. If it did, when Jeb was EVA'd outside his craft in LKO he'd go into a different orbit.

The mass of your ship though DOES affect how much your engines can push it, so if you are doing a gravity assist with a burn, then the result will be different with the same burn for 2 ships.

Without knowing more about your situation, I can't really say any more. However, maneuver nodes help you abstract away the engine burn time, so you just burn as much as the node tells you to.

Link to comment
Share on other sites

ok thx.
I don't burn while in eve SOI, so maybe it's cause of different node position, when i start engines...i don't know...
in one attempt, this morning, I did a manouver that, when out of eve SOI, the ship went aligned with moho and a periapsis about 7 or 8 Mkm, but now I can no longer reproduce it.
I think it depends on which direction I enter the eve SOI.  I continue to try. thx

Link to comment
Share on other sites

The altitude and latitude of the gravity assist, as well as passing (exiting) 'in front' or 'behind' the planet all affect the final outcome. A bit over simplified but

  • infront/behind ---> if you speed up or slow down after exiting the SOI
  • altitude/Pe ---> how much you speed up/slow down + new heading in plane of orbit
  • lattitude ---> inclination change of plane of orbit

So reproducing a gravity assist need quite a bit of tweaking - you'd probably be better off tweaking for the outcome to set the gravity assist rather than trying to reproduce the exact same assist conditions to get the same outcome (which as you've experienced can vary widely from minor differences in the assist conditions)

 

 

Link to comment
Share on other sites

2 hours ago, antipro said:

I think it depends on which direction I enter the eve SOI.  I continue to try. thx

It completely depends on which direction you enter and exit the SOI. That is the whole point of gravity assists: you have the same Eve-relative velocity going in and out, just in different directions.

When you enter Eve SOI, your Sun-relative velocity vector is Eve's velocity vector, plus your Eve-relative velocity vector. When you exit, your Sun-relative velocity vector is Eve's velocity vector, plus a new Eve-relative velocity vector of the same magnitude but different direction.

So, if you're using Eve to brake into Moho, what you want to do:

Enter Eve's SOI from behind Eve, go around the front side, and go back out from behind Eve.

Let's say you have an Eve-relative velocity of 2 km/sec, and the gravity assist does a perfect U-turn. Eve orbits at roughly 11 km/sec.

So, before the intercept, you have 13 km/sec of velocity relative to the Sun. After Eve turns you around, you now have 9 km/sec of velocity relative to the Sun.

In practice, you're not going to get perfect U-turns, but the theory remains the same: speedy thing goes in, speedy thing comes out, but in a different direction.

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...