I'm on a Free Return Trajectory (And so can you!)

[Rascal Nag]

After trying burning later, it did in fact work as intended, bu the Periapsis still was not above Kerbin. If I wait a bit longer it would work, but because there\'s really no way of creating an easily repeatable result this way I don\'t think it\'s desirable. After trying lowering the Apoapsis a bit from 14,500 km, I believe it was to 14,225 km, the periapsis coming back was well above the atmosphere, at about 300,000 meters. I think that somewhere between 14,500 and 14,300 km as an apoapsis may get a result with a periapsis above the atmosphere, probably closer to 14,300. I\'ll keep testing this.

[closette]

Here are my results to date, all starting with just over 150km prograde circular orbits and boosting just after Munrise: distances given are those on the display, which are measured from Kerbin sea level (add 600 km to get the distance from Kerbin\'s center).

apoapsis at 12, 500 km above Kerbin sea level - failure to free return (Mun impact)

apopapsis at 13 000 km above Kerbin sea level - failure to free return (Mun impact)

apoapsis at 14 250 km above Kerbin sea level - success! Final periapsis within Kerbin\'s radius.

apoapsis at 15 000 km above Kerbin sea level - success! Final periapsis within Kerbin\'s radius.

(Edited to fix my terrible units error, and to emphasize that the values given are those shown on the display, i.e. measured from Kerbin sea level, not its center of gravity).

The last result may not be consistent with Rascal\'s, depending on the exact moment and duration of the TMI boost.

And I mis-spoke earlier. Spacecraft mass may have a small indirect effect, since a more massive craft may take longer to accelerate up to the required speed for a given apoapsis (depending on the engines you are using). I suspect that timing the exact moment to start boosting may be more critical though. I hope that success or failure for free-return isn\'t so finely tuned that it\'s tough to reproduce, but please check and post your own experiences.

I didn\'t realize that missing the Mun would be so interesting!

[Rascal Nag]

Here are my results to date, all starting with just over 150km prograde circular orbits and boosting just after Munrise:

apoapsis at 12.5 million km - failure to free return (Mun impact)

apopapsis at 13.0 million km - failure to free return (Mun impact)

apoapsis at 14.25 million km - success! Final periapsis within Kerbin\'s radius.

apoapsis at 15.0 million km - success! Final periapsis within Kerbin\'s radius.

The last result may not be consistent with Rascal\'s, depending on the exact moment and duration of the TMI boost.

And I mis-spoke earlier. Spacecraft mass may have a small indirect effect, since a more massive craft may take longer to accelerate up to the required speed for a given apoapsis (depending on the engines you are using). I suspect that timing the exact moment to start boosting may be more critical though. I hope that success or failure for free-return isn\'t so finely tuned that it\'s tough to reproduce, but please check and post your own experiences.

I didn\'t realize that missing the Mun would be so interesting!

15 million meters also was inside Kerbin\'s radius for me. I just tried to have a higher orbit around kerbin before TMI (now 200 km), but at 14,500 km out it did not raise the periapsis much at all, still well within Kerbin. What\'s left to do would just be tweak when TMI occurs, probably just a bit later (I\'m actually thinking maybe when the Mun, if the spacecraft is looked at from directly behind the engines, is blocked out), or tweak apoapsis height, but that\'s extremely sensitive.

[closette]

Thanks for making me realize my units mistake in my earlier posts - 15 million meters, not kilometers! And that\'s measured above Kerbin sea level, not its center!

Raising the starting orbit should change the 'sweet spot' moment to boost a little bit, but not as much as the bigger changes to apoapsis. The semi-major axis of the elongated Kerbin orbit is (assuming that periapsis does not change after boosting)

0.5 x (starting orbit height + 600km + apoapsis height + 600km), so for a 150km start to a 15000 km apopasis it works out to 8175 km, and for 200km of course it is 8200 km.

Using Kepler\'s 3rd law for Kerbin, I find those values result in transit times from circular orbit to post-boost apoapsis of 10.86 hours from 150km and 10.96 hours from 200km - six minutes difference, but it depends exactly when you start and stop boosting. Anyone want to check my math?

[PD]

I wish my brain was still capable of Physics.. Why WHY wasn\'t this game designed all those years ago?! I\'m too rusty for anything but semi-scientifically throwing rockets towards the Mun and seeing if they come back!

I\'m going to collate the results posted here into the wiki article now, and I hope that soon enough we should have a fairly comprehensive list! Thanks for your help with the research guys, we\'re really using our Space Program for some science here, which is pretty damn cool.

PD

EDIT: Also, thank you closette for the praise, very much appreciated!

[Rascal Nag]

I do not think, even if we find a solution to get us to re-enter at a shallower angle/get a periapsis above the surface, that we will be able to reliably recreate it. A few m/s at the Mun\'s distance can be the difference between home sweet home and lost in space.

The idea that it was impossible with current instrumentation was incorrect, but the current instrumentation does make it virtually impossible to consistently maintain similar trajectories over and over. If we do eventually have the ability to see how other bodies will affect our trajectory in real time, then it will certainly become possible for one to reliably create a free return trajectory with similar qualities to others over and over. But for now, a steep, steep re-entry is all we can get consistently. We\'ll just have to hope our guys can survive double digit G\'s for now.

[closette]

I think I see what you mean, Rascal. One way to test this is to boost out to 12-14 thousand km apogee away from the Mun on purpose, coast up to apokerb, then provide a small, random impulse (in a random direction), and see what that does to the perikerb. (Yes it can be calculated but it\'s more fun to do!).

In my simplistic understanding, swinging into and out of the Mun\'s sphere of influence is equivalent to providing such an impulse (well not a random one, but one that\'s hard to predict/reproduce). Does that make sense?

[Rascal Nag]

I think I see what you mean, Rascal. One way to test this is to boost out to 12-14 thousand km apogee away from the Mun on purpose, coast up to apokerb, then provide a small, random impulse (in a random direction), and see what that does to the perikerb. (Yes it can be calculated but it\'s more fun to do!).

In my simplistic understanding, swinging into and out of the Mun\'s sphere of influence is equivalent to providing such an impulse (well not a random one, but one that\'s hard to predict/reproduce). Does that make sense?

Yes, and if that data could be used to figure out the apoapsis needed to obtain the same results with the Mun, then even better, but the last thing that would have significant effect would be the time of the TMI and the height above Kerbin when it is performed. This is an easier thing to do though I think, since working with one orbit is a lot easier than two. I think that the standardized height for these tests should be at about 160 km, to give us a comfortable height over the 150km mark so we\'re not waiting forever to start the burn.

I don\'t think it needs to be random though - the Mun gives us an impulse retrograde to its own orbit and towards Kerbin, so it would need to be narrowed down to that. On top of that, we could simulate being captured by the Mun by doing this impulse when we are not quite at the apoapsis, since it never makes it that far before it enters the SOI. The direction of the impulse tests should start at just slightly towards Kerbin but mostly retrograde, to pretty much half and half of the two. When the best match is found though it would be difficult to figure out what kind of interaction with the Mun would cause the effects seen in the tests.

Actually, maybe not. The speed at the entrance of the Mun\'s SOI should be equal to that when we leave the Mun, so if we can figure out our velocity relative to the Mun just as we get transferred to its field of influence, we can (I think) begin to figure out what apoapsis will net us the desired impulse. But for this, knowing the Mun\'s SOI would help a lot. How far out does it extend? This would be a big help. Also, we would need to look at our direction of travel as we escape the Mun. We might have the right speed but not the right angle, and everything would be wasted.

[closette]

From the Kerbal Space program Wiki http://kerbalspaceprogram.com/~kerbalsp/wiki/index.php?title=Mun, it states the Mun\'s sphere of influence (SOI) is ~2430 km, which I am guessing is from the Mun\'s center, so 2230km above its surface (which is the altitude displayed).

Also when you say:

The speed at the entrance of the Mun\'s SOI should be equal to that when we leave the Mun

...just to clarify for others, this statement is true in the Mun\'s reference frame, but not Kerbin\'s, which I think is what you meant.

For those who want to play around with - and better understand - the so-called 'slingshot' effect in our solar system (more properly a gravity assist), as viewed in two reference frames, here\'s a Flash applet that I\'ve found instructive: http://galileoandeinstein.physics.virginia.edu/more_stuff/flashlets/Slingshot.htm.

[PD]

That\'s a fun little app, but to be honest, what more entertaining way is there to learn about Gravitational Slingshots than throwing Kerbals towards the Mun at high speed?

8) PD

[4 IN 1]

Just pointing out, the OP have been ninja\'d

And the best part is, reentry is also counted as a factor

After a few try on it I find that the lag angle was not the biggest problem as I once through in my own quest, its the timing of the burn that matter the most ???

So, like the man in my sig said: This is not rocket science... this is precision.

[samstarman5]

Thanks to this thread I was able to accomplish a FRJ with zero hiccups. I think the only issue is the trip into the atmosphere. Those G\'s are ridiculous!

I wonder if RCS is enough to cut down your descent speed so you don\'t turn your pod into a sardine can? (Baked sardine can when heat is incorporated)

[Ydoow]

Yeah the reentry looks pretty damn steep lol

Thanks for pointing out how to do a FRT though, I\'ll be trying this out soon.

I\'m going to try keeping RCS units with me to hopefully be able to add some orbital energy to round out the ellipse and give it a shallower reentry.

Or perhaps I\'ll try to make it do a few Aerobraking orbits to slow it down to a respectable speed

[PD]

I am yet to try this, but one on the return leg, RCS should be enough to alter your trajectory to reduce the angle. It would be worth experimenting with

PD

[gtmattz]

After finding this topic, all I want to do is experiement with FRT\'s, thanks alot ???

But seriously, I have found that from a nice circular orbit at 120km at a heading of 90 degrees, if I burn at \'munrise\' till my Ap is 14.5M km, that a free return is almost guaranteed, and for some reason I derive so much satisfaction from doing this over and over, it is almost pathetic... :\'( To the point where I have not even bothered with continuing to try to land on the mun, and have instead been working on fine tuning my rocket so that I have just enough fuel to get into orbit and do the munshot burn, and no more...

[MaxMurder]

Ive tried this a couple times... It is a very thrilling challange because once you make your TMI burn and decouple you are taken out of control. You prettymuch just have to ride it out and hope that you are the right course. The first time i tried for a FRT it went perfectly! The next couple times... not so much. One mission slammed into the munar surface and the next ended with three kerbals lost in space.

I like doing my TMI deploying the chute then letting the game run on 1x and coming back a few hours later to find the kerbals safely landed on Kerbin... or with the mission failure screen up.

[Pingonaut]

I found it really difficult to orbit the Mun until I found this! I get a perfect mission every time!

Now what would be cool is slingshoting from Kerbin to Mun and repeating that.

[WX_Echo]

You genius... Do you realise how many 'experts' were saying an FRT was impossible with just 1 8-ball and the other information we currently have availible?!?!

I\'m so pleased to see our community constantly pushing the bounderies of what is 'possible'. I remember when people said we would never get to Mun on the instrumentation!

I think most folks were more worried about the delta-v required to accomplish Kerbin-Mun transfers. Hohmann transfer techniques are actually very easy to calculate and perform (which my calculator will perform for you), and require no special navigation aids.

The nav-ball we have is quite useful and will get us to far-flung places in the kerbal-verse. What we could use, however, is more orbital parameters in the map that allow us to estimate a spacecraft\'s position in greater detail (e.g. true anomaly, relative angular separation, etc.).

All this aside, nice work with the free return!

[PD]

Oh don\'t get me wrong, I\'d \'like\' more instruments. I\'m just constantly amazed at how much we actually can do.. against the odds.

IMHO that really is what 'being kerbal' is all about. As opposed to just blowing up rockets on the launch pad.. although I hear the smell of burning kerbal-flesh in the mornings is a common occurrence at the Launch site.

C\'est La Vie.

PD

[Kosmo-not]

Sign me up! I\'ve already got an idea on how to achieve a reliable free return trajectory. It would require some minor correction burns right after the TMI burn.

[Kosmo-not]

Ok, it wasn\'t all that hard to do. I screwed around for a bit to find a set of parameters that would work and not provide me with a steep reentry. After that, it was time to do it for reals. I started my TMI from around 160km. After a couple correction burns immediately after, and setting my apokee to 13.5Mm, it was time to say goodbye to powered flight. I got the correct angle by using map view, and a paper sheet with the angle I needed on it (21.8°).

I even took a video of the mission:

Homeward Bound (KSP Free Return Trajectory)

[LambdaCactus]

That was a beautiful video, the music was a nice touch.

What does the 21.8° refer to?

[dashcunning]

I presume it\'s an indicator for when to start the TMI burn. But like you, I can\'t figure out what it\'s referring to. I want to say the angle between line segments Mun-Kerbin and Spacecraft-Kerbin, but that\'s not right. Maybe that would be 111.8° (90+21.8)

[WX_Echo]

That\'s a wonderful video! Great music selection as well.

[Kosmo-not]

*edit* My bad, everyone. Angle I used is 68.2°.

*edit* (I figure I can word this better)

The angle to which I\'m referring is the angle from apokee to the Mun with Kerbin at its apex. I have this drawn on a piece of paper which I hold up to the screen when in map view looking from above. I make a correction burn to adjust this angle until it\'s right (68.2° for 13.50Mm apokee, TMI burn at around 160km).

For example, if the angle from apokee to the Mun is too small, turn 90° from velocity vector towards Kerbin and burn a little. Then recheck the angle. Once the angle is good, adjust apokee to desired altitude.

I found a set of parameters by trial and error flying a 'cheat ship' with unlimited fuel. When it came time to do it for real, I recorded it for the video I posted. I made 2 burns to get the angle correct, and then one more to get my apokee correct.