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NFunky

Low lunar/munar flyby

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Hey guys
 
Got a follow on question to something I asked before.  I'm playing with RO/RSS, but I believe this topic should apply to stock as well.
Basically, I'd like to know how to get a perilune below 50 km, that will allow me to correct for an Earth return with 300 m/s dV or less.  I have a bare bones system with Mk1 pod, lunar heatshield, and service bay, on top of a rocket that can comfortably send it to any type of lunar transfer orbit.  I've done a free return successfully now, but I didn't manage to get close enough to complete the contract or get low-space lunar science.  I belive I can increase service module dV by 100 m/s or so, but not much more without designing a whole new rocket.
 
(NOTE: I guess if using stock system, think about the service module as holding 100 m/s or so, and the perimun being less than ~30 km)
 
Thanks!

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Posted (edited)

I still don't quite get it. You set up any old free return orbit that goes equatorially around the mun/moon retrograde. You do a midcourse correction to lower your Pe as low as you like. You fly by the mun/moon taking all the data you like. Once you leave the SOI, you do a small retrograde burn to lower your Kerbin/Earth Pe to some nice reentry altitude and profit. Or you can do it as a prograde burn while you are still zooming past the mun/moon. What part of that is hard? Pictures?

 

Edited by bewing
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I guess I'm being ham handed, cause the smallest correction burns I can get add up to more than 300 m/s.  Is it just not possible to do a low flyby->return unless I have the dV equivalent to get into lunar orbit?

Approximately where, around the moon, should I try to put my periapsis?  I still don't have an intuitive grasp on how the radial vs. retrograde acceleration (from the moon's gravity) works.

I've been assuming that the closer my perilune is to the line of the moon's orbit (in front), the stronger the retrograde acceleration, but it doesn't appear to really work like that.  It looks like a combination of a retrograde and anti-radial assist is what I nneed, but I'm not sure how to go about getting it.

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Posted (edited)

Well, for a free-return orbit, the more hyperbolic the better at the destination. That is, you want to pick up the least gravity slingshot possible. Now, trying to get low altitude science is directly contrary to that, of course. Because the lower you get the more gravity you feel, and the longer you feel it.

So basically, the orbit you start with takes you from low orbit around your homeworld out to your Ap. If you meet nothing while you are there, you come back to a nice low Pe around your homeworld. But (unfortunately) you encounter a celestial object on your voyage that gives you a slingshot -- prograde and somewhat radial in (from your homeworld's perspective). To get back home once you leave this SOI, you need to get back to that nice low Pe around your homeworld that you had when you started. So you have to cancel out whatever gravity slingshot you picked up.

Which means that (from your homeworld's perspective) you want your burn to be retrograde, and maybe somewhat radial out. And for oberth reasons, you want to do the majority of your burn near your Pe if you want to maximize efficiency. This has the added benefit of getting you out of the Mun/Moon's SOI quickly, which reduces the slingshot. So, the spot where you are moving retrograde WRT the homeworld is exactly over the backside of the mun/moon, and probably the best place to start burning (prograde, WRT the local SOI) is just before that point. The exact location of the Pe doesn't really matter much though, as you'll be going quite fast though that whole encounter.

Edited by bewing

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Posted (edited)

This is totally possible. I'll assume you're in a circular orbit around the Earth, at 303km (the lowest orbit cheat menu will put you :P) and matching inclination with the Moon.

Here is an example of what it looks like:

uZhJT6F.png

This kind of gravity assist is super "threading the needle" type business (especially when it comes to the resulting perigee), so a couple of few m/s correction burns will be needed to get it completely perfect, but it's a basic free return with a low perilune and a perigee near Earth's atmosphere. A few m/s in correction burns will make it perfect.

The most important thing is the amount of overshoot you have relative to the Moon's orbit, if I rotate the maneuver node about the orbit to get the Moon out of the way, this is about how much overshoot there should be:

sUM8wLX.png

The free return works because essentially the Moon performs an orbit-lowering gravity assist (by passing behind the moon), so first you need to overshoot to give the Moon's gravity something to lower. If you have too much overshoot (i.e. hyperbolic) you might be able to still achieve your goals, but you'll end up in a retrograde orbit around Earth after the gravity assist rather than a prograde orbit, getting the prograde orbit is unbelievably sensitive, in  contrast, it's relatively easy to get a retrograde orbit (this will reduce trip time to moon though). If you have insufficient overshoot you won't be able to get both a low perilune and perigee.

Since it's super-sensitive to the exact timing and the exact deltaV of the ejection burn, some kind of maneuver node editor (which I didn't actually use here) helps though you're still going to need at least some tiny correction burns. And of course, because the Moon's orbit is elliptical those exact numbers aren't going to work for you unless the moon is in exactly the same place! Nevertheless starting with about that amount of overshoot should save some time in finding the exact burn required…

Edited by blakemw

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Thank you guys!  I'll give it another go today.

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Well, I found a really interesting and unique flight path that does get me both a low perilune and basically a return trajectory (tiny correction needed).  Only problem is, it take almost a month to complete the entire journey.  A whole month in a tiny capsule is a bit too much to ask of my pilots.

Anyway, I've got Precise Node installed, so I'll keep messing with maneuver nodes and see what I can come up with.  Here's a link to a picture of the trajectory I stumbled upon.  I am definitely doing something wrong, so please feel free to let me know what!

https://www.dropbox.com/s/e4mrnbgt749j8nr/FreeReturnREF.GIF?dl=0

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You want to meet the Moon on the outbound leg of your first orbit.

Look, here's a pic of the Free Return trajectory, so that we can make sure we are all talking about the same thing.

xcZx1Rh.jpg

 

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Really appreciate the help.  After many attempts, I've gotten to the point now where I can plot an ordinary free-return (with a moderately high perilune), but I'm still having trouble finding a trajectory with a perilune under 100 km that sends me on a proper return.

What side of the moon should I be trying to put the perilune?  If the Earth-Moon line is 12 o'clock, I've been trying to place perilune around 5 o'clock, but I don't think that's right.  Is there a particular technique I should use for node tweaking to help me find the correct path?

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Posted (edited)

6 o'clock should always work.

Please show us a picture of your maneuver node at the Moon, and your final orbit.

 

Edited by bewing

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I think I'm still not getting it.  The only way I was able to put the perilune at 6 o'clock, I had to reduce my overshoot, and ended up with this:
 
 
 
(So sorry about the links rather than just embedding the pics, but I for some reason I can't seem to embed the links.)

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1 hour ago, NFunky said:

I think I'm still not getting it.

That looks perfectly good for your first burn. :) Now you need at least one more. That Pe looks perfectly workable.

So a little bit past that Pe, in the Moon's reference frame (close to 6 o'clock), place another maneuver node with a small prograde burn. Adjust it so that your resulting trajectory hits Earth and you're done!

 

 

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Sorry for the delayed response.  I gave it a shot, but did not have the delta-V to lower my perigee enough for a return.  I only have about 295 m/s to begin with, but I usually have to use about 30-40 m/s for correction burns & attitude control.  In total, I used around 250 m/s and was only able to lower my perigee to ~160,000 km.
 
I think I have to get as low a perigee as possible from the initial lunar encounter, so my perilune burn is more on the order of tens of m/s.  Unfortunately, I still don't think I fully understand how gravity assists work, and I'm still not able to replicate (or come close to) blakemw's first pic.
 
Help!

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Posted (edited)

Once you have the perilune you want make a small adjustment to the timing of the burn (like 0.1s) - this will change perilune (because the ship ejects at a slightly different angle and so will pass Mun closer or further), then adjust the prograde magnitude of the burn (in increments of like 0.1m/s) so that perilune returns to what you want it to be, through this process perigee will have changed, either getting higher or lower, make note of whether it go better or worse, and either do the same thing again, or the opposite thing.

If you get both the timing and the prograde magnitude exactly right you won't need ANY burn at perilune, in practice the level of precision is too great to do in a single ejection burn from Earth, but a single ejection burn plus a correction burn to fix the error (might involve a radial component) should be enough to put you on a free return all the way back to Earth's atmosphere. It is much cheaper to make corrections when just leaving Earth than later on.

 

Edited by blakemw

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Did it!  Thank you so much for the help.  I feel like, through the node tweaking process, I've learned a ton about the way a celestial body's gravity influences a trajectory.

One final note, I didn't realize quite how important inclination is to this whole process.  My inclination differed from the Moon's by ~0.5 degrees, but prevented me from getting my perigee below ~14,000 km.  With ~90 m/s anti-normal added to the injection burn, I came down to less than 500 km.  So yeah, I'm learning a lot.

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