Jump to content

Mun flyby in interplanetary missions


MedwedianPresident

Recommended Posts

If you can refuel in Mun orbit, it makes sense to start your interplanetary missions there (or even better, from Minmus). Refuel, burn as if you're returning to Kerbin (80km periapsis), and finally, burn at Kerbin periapsis to get an intercept with wherever you're going.

Link to comment
Share on other sites

Scott Manley did a video on this, the Mun isn't particularly useful for most interplanetary destinations with a traditional gravity slingshot because you need more than you gain from the Mun to get there, and burning after you leave the Mun looses too much efficiency due to the Oberth effect.  If you just want to hit the solar SoI though, it's great.

If I hit the Mun or Minmus on an interplanetary mission, I do it as sh1pman describes.

Hmmm.... there's something I should try, a gravity slingshot off the Mun that rather than throwing me up, throws me down.  If you can hit LKO with more velocity than you had when you finished your transfer burn, that could be a win.  Of course, getting the timing for an interplanetary transfer right under those conditions would be a nightmare :-)

Link to comment
Share on other sites

2 hours ago, The Aziz said:

Yes. I always postpone my burn by an orbit or two to avoid meeting with Mun. 

^ This.

34 minutes ago, Eric S said:

If you can hit LKO with more velocity than you had when you finished your transfer burn, that could be a win.  Of course, getting the timing for an interplanetary transfer right under those conditions would be a nightmare :-)

Use this: 

Of course hitting the Mun isn't really necessary, only getting your apoapsis higher in order to increase your orbital energy.

Edited by regex
Link to comment
Share on other sites

30 minutes ago, Eric S said:

Scott Manley did a video on this, the Mun isn't particularly useful for most interplanetary destinations with a traditional gravity slingshot because you need more than you gain from the Mun to get there, and burning after you leave the Mun looses too much efficiency due to the Oberth effect. 

While I watched the video, I can't remember the reasoning.  I'm guessing that you won't get much deflection angle (and thus little slingshot effect) once you have enough delta-v (less slingshot effect) to get to Duna/Eve (and it only gets worse by going further).  This gets compounded as without the deflection angle you are much more limited to when you can leave, which means you lose more from changing your launch window than you gain from the slingshot.

While I find it a lot easier to use the Mun to slow down from Minmus, I suspect that the math doesn't work out for Aerocapture from outside of Kerlbalsoi.  But it still might be worth trying (assuming you have the delta-v to correct from the Mun).

Link to comment
Share on other sites

On 10/12/2017 at 10:55 AM, wumpus said:

While I watched the video, I can't remember the reasoning.

It comes down to two things.  First, the most delta-v you can get from any flyby is twice the relative velocity.  And it retains the sign, so if you're moving faster than the Mun is orbiting, you won't get anywhere near that amount of delta v because the optimal change in delta-v would slow you down.  Because of orbital math, what a flyby is really doing is changing the direction of your relative velocity with respect to the body you're flying past. The maximum change this can yield is if the new direction is directly opposite your current direction.

The second thing that limits this is the gravity of the body in question.  The more gravity it has, the more capable it is of offering significant deflection, and the faster you're moving relative to the body, the more gravity it takes to deflect the craft's path.  The Mun is a bit on the light side in this respect, making it difficult to reach the limits set by the first limiting concern.

 

Edited by Eric S
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...