Other Planet Orbit - rendezvous (and optional return)

[khyron42]

While there are no other planets yet, we do have some proposed ones from NovaSlisko, as well as real planetary orbits from Earth\'s solar system. I\'m proposing a challenge with two different Levels of accomplishment, and a choice of targets. If you can do them with just stock, Slisko, or NovaPunch parts, submit them here! Please include which part set you were using.

The Other Planet Orbit challenge level 1 is to get into the orbit that another planet would be in if it existed in the game. You will need to launch from Kerbin, leave its sphere of influence, and put yourself on a trajectory relative to Kerbol that will raise or lower your orbit appropriately; Then you\'ll need to speed up / slow down to put yourself in a circular orbit at the target altitude.

To be a successful 'Level 1' competitor, all you need to do is supply screenshots of the map view showing your new orbit Ap and Pe. It doesn\'t need to be completely circular - get the orbit to within 1% of the target planet\'s orbit. Feel free to include other screen shots showing off your spacecraft too.

Example Level 1 OPO challenge, Mercury orbit, stock parts:

I was initially hopeful that this ship could manage a level 2...

Level 2 is much, much harder. Return to Kerbin after taking that screenshot and land safely. The best tip I can give for this is timing it right: take a note of how long your flight out to your new orbit was or use an orbit calculator to find the transfer time. Then, eyeball your position relative to Kerbin on its orbit. Kerbin\'s orbit is 106 days, so 1/4 of the way around that orbit is 26.5 days - you can eyeball smaller subdivisions of the orbit to find when you\'ll need to start your return burn, to line up with where Kerbin will be in the right number of days.

The Other Planet Orbit challenge applies to any of these orbits around Kerbol:

From Nova\'s creative efforts:

The Fire Star: 2,255,740 km

Tiberius: 7,519,134 km

Eve: 9,774,874 km

From reality (semi-major axis of orbit from Wikipedia):

Mercury: 57,909,100 km

Venus: 108,208,930 km

Earth: 149,598,261 km

Mars:227,939,100 km

Plus any others you want from reality.

[Cepheus]

I\'m using my standard Deep Space Launch System (a mismash of IFE parts.)

• 
  [li]Launch from KSC into a 300 km parking orbit. Mine was a 304 km by 42 km orbit.[/li]
  [li]At Apoapsis, accelerate to 2,781.9 m/s. You\'ll be in an escape trajectory.[/li]
  [li]Good job! You are now orbiting Kerbol.
  [li]Have some fun.
  [li] Ready to go home? Yes? SHOW ME THAT INTERSECT ORBIT!

[/li]

[/li]

[/li]
[li]Alright, now get rid of that velocity, and wait for Kerbin to get to you.

[/li]
[li]Since, relative to Kerbol, you aren\'t moving, Kerbin is going at 9284.5 m/s. When you enter Kerbin\'s SOI, that\'s how fast you will be going.
[li]You\'re gonna want to lower your orbit in steps. You don\'t want to come careening into the atmosphere at 9 km/s.[/li]
[li]Deorbit burn, then you\'re home.[/li]

[/li]

[semininja]

You know, if you have the patience for it, it takes much less fuel to just put yourself into a circular orbit slightly (1-3 terameter, or 1bil km) inside or outside the orbit of Kerbin, then wait for it (or you) to catch up so you can just drop in.

[khyron42]

I forgot to include the important 'Slisko/stock/NovaPunch only.' My example was done with pure stock parts.

[samstarman5]

Added challenge: To those who do this (it\'s gonna be a while before I am up to snuff), make sure you jot down the orbit distances you make. If any upcoming planets match those within hmmm, maybe 50,000 km, share a screenshot of said planet with a big sign(shopped, of course) saying, \'<insert name here> was here.\' Bonus points if your handle is Kilroy.

[lebull]

Since, relative to Kerbol, you aren\'t moving, Kerbin is going at 9284.5 m/s. When you enter Kerbin\'s SOI, that\'s how fast you will be going.

Not to mention you are accelerating towards Kerbin when you enter it\'s SOI (in game, anyway).

[khyron42]

Yeah - the need to burn fuel to slow yourself into Mastado\'s suggested approach and then slow down again when you entered Kerbin\'s influence (or at least adjust course to hit atmosphere) is why I suggested a well-timed return orbit instead. Semininja\'s approach is better if you miss, unless you really do have ridiculous amounts of fuel to spend.

[semininja]

There\'s actually a very useful utility that can figure out all the numbers for you, to get you from the outer circular orbit to a Kerbin intercept transfer orbit. It\'s produced by WX_Echo, and can be found here: http://kerbalspaceprogram.com/forum/index.php?topic=4707.0

[khyron42]

Anyone else have a successful level 1 attempt? I tried for the Fire Star and ran out of fuel while braking into a circular orbit for a level 1 attempt.

[WX_Echo]

There\'s actually a very useful utility that can figure out all the numbers for you, to get you from the outer circular orbit to a Kerbin intercept transfer orbit. It\'s produced by WX_Echo, and can be found here: http://kerbalspaceprogram.com/forum/index.php?topic=4707.0

Thanks to semininja for the endorsement!

Level 2 is much, much harder. Return to Kerbin after taking that screenshot and land safely. The best tip I can give for this is timing it right: take a note of how long your flight out to your new orbit was or use an orbit calculator to find the transfer time. Then, eyeball your position relative to Kerbin on its orbit. Kerbin\'s orbit is 106 days, so 1/4 of the way around that orbit is 26.5 days - you can eyeball smaller subdivisions of the orbit to find when you\'ll need to start your return burn, to line up with where Kerbin will be in the right number of days.

In addition to standard Hohmann and bi-elliptic transfer analysis, my calculator should prove quite helpful in satisfying the intercept conditions necessary to complete a level 2 mission. Specifically, the user simply has to enter the details of their mission (i.e. initial circular orbit altitude, target orbit altitude/radius ratio, or - in the case of bi-elliptic transfers - midcourse impulse altitude/radius ratio), and it will provide all the Delta-Vs needed to execute the transfer. Additionally, it will solve the intercept condition for you and provide the intercept angle (relative lead or lag angle required at the burn time to achieve intercept) and synodic period (time between intercept opportunities).

The calculator will also allow you to select high-to-low transfers; this will be useful when attempting to bring your brave kerbonauts back home from a target altitude that is greater than Kerbin\'s (i.e. r2 > r1).

I\'m still on Christmas holiday, but I will make a point of incorporating this challenge into the how-to YouTube videos I\'m planning for v1.2 of KSP Orbit Mechanic when I return. These videos are intended to show folks how best to use the calculator, and I\'m sure it would be even more meaningful if they actually had relevance to a community challenge.

Finally, I\'m hoping that interplanetary mission analysis will be the next significant addition to the calculator. Patched conics will be especially useful for challenges like this one; that is, interplanetary transfers executed within the SOI of a planet (like Kerbin) will yield Delta-Vs different than those predicted by the standard massless transfer equations (e.g. the standard Hohmann/bi-elliptic results). An example of this can be found here.

Best wishes to all participants. I wish I could get started sooner!

[PakledHostage]

Finally, I\'m hoping that interplanetary mission analysis will be the next significant addition to the calculator. Patched conics will be especially useful for challenges like this one; that is, interplanetary transfers executed within the SOI of a planet (like Kerbin) will yield Delta-Vs different than those predicted by the standard massless transfer equations (e.g. the standard Hohmann/bi-elliptic results). An example of this can be found here.

Best wishes to all participants. I wish I could get started sooner!

I haven\'t had a chance to look at your 'Orbit Mechanic' calculator so I don\'t know what you\'ve done so far, but I think the challenge with developing something that handles interplanetary transfers within a planet\'s SOI will be developing the user interface. As you obviously already know, effective transfers initiated from within a planet\'s SOI require accurate timing so that your spacecraft not only ends up going the right speed, but also the right direction.

Myself, I use a sort of 'celestial navigation' technique where I position my spacecraft into a circular orbit at an altitude that results in Kerbol or the Mun rising/setting at some useful angle relative to Kerbin / the Mun\'s orbital trajectory. I then time my pre-calculated burns off that reference. In interplanetary space, I use the time that Kerbin / my spacecraft passes periapsis / apoapsis to determine how much of an orbital adjustment I need to make to rendezvous correctly. For calculating the actual burns, I use a MathCAD spreadsheet and a home-brew numerical model, but neither can be said to be user friendly because I end up having to manually enter a lot of parameters...

I admire people who can create intuitive user interfaces. Hats off to you for offering to take it on! Feel free to let me know if you want another set of hands to help out with the development. I might be able to port some of the work that I have already done to your project.

[thorfinn]

Alright, now get rid of that velocity, and wait for Kerbin to get to you.

But.... this is completely unphysical. How can this be? You should fall into the star, not just hover there waiting for Kerbin to get near. The ship, like Kerbin, is subject only to the gravity of Kerbol...

[PakledHostage]

Further to my last post, here\'s some screen shots of a mission I flew recently. I didn\'t meet the objectives of this challenge but I thought that people might be interested anyway.

On the pad. I\'m using my Kerballo 8 stack with the minor addition of an extra decoupler below the RCS tank at the top.

Climb to orbit

Decoupling following post interplanetary orbit insertion burn

Orbital trim maneuver of +4.8 m/s upon crossing Kerbin\'s SOI into inter-planetary space reslted in this orbit about Kerbol

Periapsis (here I did another orbital trim maneouver adding +0.1 m/s to raise my apoapsis by about 1000 km.

Approaching Kerbin rendevous after 212 days in space. I sized my orbit so that my spacecraft would complete 3 orbits in 5 hours less time than it took Kerbin to complete 2 orbits of Kerbol. This put me just slightly out in front of Kerbin as I neared Kerbol apoapsis, with Kerbin closing at aprox 1500 m/s.

Re-entering Kerbin\'s SOI, I had to use about 1/3 of my RCS tank to lower my Kerbin Periapsis to 10 km for aerobraking. My initial Kerbin Periapsis upon entering Kerbin\'s SOI was about 11000 km. I probably could have been more accurate if I had a better estimate of Kerbol\'s mass. As it is, my orbit about Kerbol was about 60 minutes quicker than expected, so I ended up about 5.5 hours ahead of Kerbin at apoapsis, rather than the planned 2.5 hours.

Jettisoning my trusty RCS tank and nozzles prior to re-entry

Final descent

[samstarman5]

212 days? That\'s a lot of socks to knit!

[PakledHostage]

But.... this is completely unphysical. How can this be? You should fall into the star, not just hover there waiting for Kerbin to get near. The ship, like Kerbin, is subject only to the gravity of Kerbol...

If this works, it is probably because Kerbol\'s gravity is miniscule at Kerbin\'s orbital altitude. It is only on the order of 0.00634 m/s (0.065% of g) at Kerbin\'s 13.5 million km altitude above Kerbol.

If he\'s got the Delta V available to kill on the order of 9-10 thousand m/sec, then he could do that and thereby put himself into a highly elliptical orbit (essentially setting himself up to fall straight in). He would then only have to perform the occasional tiny burn to hold altitude while he waits for Kerbin to come around.

[thorfinn]

If this works, it is probably because Kerbol\'s gravity is miniscule at Kerbin\'s orbital altitude. It is only on the order of 0.00634 m/s (0.065% of g) at Kerbin\'s 13.5 million km altitude above Kerbol.

Minuscule as it can be, it\'s the only thing that\'s keeping Kerbing going in a circle. Do you remember that mechanics thought experiment with an evacuated tunnel going through the center of the Earth? (They usually teach it in 12th grade, if I remember correctly...) The period of the oscillation is exactly the same of that of a circular orbit. In that picture Kerbin was more than 90 degrees away... more than 10 km/s of delta V and LOTS of burns would be needed to hover 'mostly there' for all that time.

[PakledHostage]

Minuscule as it can be, it\'s the only thing that\'s keeping Kerbing going in a circle. Do you remember that mechanics thought experiment with an evacuated tunnel going through the center of the Earth? (They usually teach it in 12th grade, if I remember correctly...)

I think we\'re talking past each other a bit here... we both seem to agree that it isn\'t the most efficient way to return to Kerbin.

His screen shots here suggest that he did it successfully, however. The delta V that I mentioned is needed just to set him up to fall straigt in. I have no idea (and make no claims about) how much he would have used while waiting for Kerbin after that. As you correctly point out, we don\'t know how long he had to hover there waiting for Kerbin.

[khyron42]

In spite of the control issues at high speeds far away from kerbal, I decided to try an Eve level 2. (Since everyone else is talking theory, thought I\'d just try it and see if it was so impossible that it was a bad challenge.)

Eve is the easiest of the other planets to do this for, so here goes. Only control mechanisms are gimballing engines and an ASAS, so all course corrections use low thrust from the engines to allow me to spin the ship around.

On the pad, ready to go:

Done setting up the transfer orbit to Eve (anything from 9,677,125 km to 9,872,622 km acceptable):

Got it to an acceptable Eve orbit, still about 20 tanks of fuel left:

Ok, let\'s eyeball this return orbit... took me 35 days or so to transfer inwards... carry the three, add a fudge factor... I think I should burn when my ship is at 6\'o\'clock on its orbit and Kerbin is at about 4:00 on the orbit.

Wait... maybe it should have been 4:30 or 5...

I can salvage this, just ride it around one more orbit and see how it lines up, then go for it!

Frak. Too far ahead now. Fine, go circular slightly outside of Kerbin orbit, still lots of fuel left, just ride it out...

Much, much additional corrections later including fixing the orbital plane to correct a slight tilt...

Closing in! Still have about 5 fuel tanks left, just one more correction, about a 30 second burn straight in towards Kerbol to move the periapsis, and...

YES! now to bring it in.

Too fast first pass through the atmosphere, had to ride it a second loop, and the ground is all screwed up, and jumping around by up to a km every 5 seconds or so, just a real wild ride and scary that the random ground jumps would keep me from ever aerobraking into re-entry.

It doesn\'t show up, but the ground was flickering like mad in that shot. But here I am on the, um, ground. I think. I passed through the tilted apparent surface of the water at about 100 feet altitude.

At the end, I jettisoned an extra 4 1/2 fuel tanks right before the first re-entry, so complete success with stock parts, just so sloppy I feel like never piloting again. Extra screenshots: http://imgur.com/a/PYHww

[Cepheus]

To be honest, we\'re gonna need time compression far above 10,000x. Even at that compression, it takes forever when you interact with Kerbol.

I would also really like to know how you manage to get BACK to Kerbin, without using my 'mock lagrange point.'

[PakledHostage]

I would also really like to know how you manage to get BACK to Kerbin, without using my 'mock lagrange point.'

It requires fairly precise timing and orbital trim manoeuvres but it is entirely possible to work out with only some simple math.

Try setting yourself up with an apsis (either peri or apo) at Kerbin\'s orbital altitude, then watch and record the time that Kerbin passes that apsis. Try to be as accurate as possible. I slow the warp rate progressively down to 5x while watching for the Kerbin Icon to be centred in the PE or AP icon. Compare the time at which Kerbin passes that apsis to the time when you pass it and then adjust your orbital period so that you catch it on the next orbit.

For example, if Kerbin is ahead of you by 2.5 days when you reach the apsis, then you\'ll need to adjust your orbit so that it is 2.5 days shorter than Kerbin\'s orbit. If it is too far away to catch up/let it catch you in a single orbit, then divide the difference by two, three or some other integer and then complete that many orbits before it catches you/you catch it. You could use a tool like Wx_Echo\'s 'Orbit Mechanic' mentioned earlier in this thread to determine the required speeds. You have to be fairly accurate though. Kerbin moves the radius of its SOI every 2.5 hours, so ideally you\'ll want to time your arrival at the apsis within that tolerance.

You might also be able to catch Kerbin by passing within about 84100 km of its centre of mass on some other trajectory, but that would be harder to set up because you\'d have to be even more accurate about the timing.

[khyron42]

To be honest, we\'re gonna need time compression far above 10,000x. Even at that compression, it takes forever when you interact with Kerbol.

I would also really like to know how you manage to get BACK to Kerbin, without using my 'mock lagrange point.'

I agree completely on the higher time compression.

For me, it\'s all a matter of orbital mechanics. I\'ve started typing a description of transfer orbits and how to time them to intersect the actual object twice now, but I can\'t seem to describe it clearly without turning into a 20-page essay. The simple version: wait in a circular orbit until your intercept object is positioned roughly the right number of days behind where your Hohmann intercept point will be, then make the orbital changes to start the Hohmann. Then if you missed get into a circular orbit that\'s only slightly different than the target (less than 80,000 km ideally for Kerbin) - further out from Kerbol to let it catch up, closer in to catch up to it. It might take 100 days or more of game time depending on how good you are at setting that up, but you will eventually be so close to Kerbin that you can eyeball a burn that will get you within Kerbin\'s SoI, at which point you just re-enter like any other flight.

[PakledHostage]

The simple version: wait in a circular orbit until your intercept object is positioned roughly the right number of days behind where your Hohmann intercept point will be, then make the orbital changes to start the Hohmann.

Good idea. I wonder if we couldn\'t also use the target reticle on the nav ball to estimate our angular position relative to Kerbin? It would require a bit of trigonometry but it might be more accurate than eyeballing it?

Then if you missed get into a circular orbit that\'s only slightly different than the target (less than 80,000 km ideally for Kerbin) - further out from Kerbol to let it catch up, closer in to catch up to it.

The only problem that I can see with this approach is the low rate of closure. At Kerbin\'s orbital altitude plus or minus 84100 km, you\'re only closing at ±29 m/s. That amounts to only 1 day per Kerbin year... If you\'re 2.5 days behind (about 8.5 degrees of angular displacement), it will take you on the order of 260 days to rendevous. Compare that with about 104 days until rendevous if you were to set yourself up in a 13,108,000 km x 13,534,442 km orbit instead.

[semininja]

I think the readers of this thread would be interested in the upcoming updates to the aforementioned orbit calculator...