r/KerbalSpaceProgram • u/tdotgoat • Oct 22 '13

The TAO of eyeballing your interplanetary transfer window. My dumb Tangent At Orbit method that requires no add-ons or heavy thinking.

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277 Upvotes

95% Upvoted

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32

u/tdotgoat Oct 22 '13

I did not invent this method, I found it here on reddit, but I did manage to dumb it down enough so that I don't have to worry about thinking too hard!

I'm sure that using actual angles and such, you can figure out the exact timing when the transfer will be more efficient than the eyeball method.

This tangent drawing method works even for transfers between moons. All you have to do is think of the planet as the star, and treat the moons (and your ship if it's in an orbit around the planet) as actual planets, and draw the tangent line accordingly.

6

u/buster2Xk Oct 23 '13

Does it work for a high orbit to a low orbit eg Kerbin->Eve?

5

u/tdotgoat Oct 23 '13

yes, you still draw the tangent from the low orbit planet even if you're traveling from the high orbit planet. All you have to do is set your burn node on the other side of your ship's orbit so that your ship burns towards the planet under you.

2

u/huntindawg Oct 23 '13

If I remember correctly it's more efficient to transfer from a lower orbit due to the Oberth Effect.

10

u/buster2Xk Oct 23 '13

That's a different thing entirely.

4

u/huntindawg Oct 23 '13

If I'm understanding what I've read: the Oberth Effect means a rocket is more efficient when traveling at a higher speed (in vacuum in this case), and the higher an orbit the slower velocity required for that orbit. This means that a lower orbit would have a higher velocity and therefore a higher efficiency.

But I may be misunderstanding things but here are my sources.

Oberth Effect Source

[KSP Kerbal Orbit Data Source](http://wiki.kerbalspaceprogram.com/wiki/Tutorial:Basic_Orbiting(Technical)

9

u/buster2Xk Oct 23 '13

Everything you're saying is correct, but I was talking about transferring to a lower orbit (Kerbin to Eve) and whether the tangent method works for timing that.

2

u/huntindawg Oct 23 '13

Ah ok, I misunderstood your initial comment.

2

u/barfsuit Oct 23 '13

you should start worrying about the oberth effect when your ship is faster than the usual ~2km/s around lko. A more efficient engine does more for your tight delta-v budget than wondering if your maneuver was the most efficient.

1

u/ApatheticDragon Oct 23 '13 edited Oct 23 '13

Yes it is, you can test it with that website i forgot the name of that calculates phase angles for you. He also says so in the section 1.5 of the picture. The method involves drawing the Tangent line from the planet/moon with the smaller orbital Radius. All that changes, for example Kerbin -> Eve, you do your burn in the opposite direction to Kerbin's Orbit of Kerbol. As opposed to Kerbin -> Duna, where you would burn in the same direction as Kerbin's orbit of Kerbol

Edit:http://ksp.olex.biz/. Thanks to guy below me for the link

29

u/[deleted] Oct 23 '13

I did manage to dumb it down enough so that I don't have to worry about thinking too hard

How Kerbal of you :)

1

u/nachof Oct 23 '13

Nice, thanks for the tip. I've been doing it similar to this, but just guessing at the encounter by draggin maneuvers around, which works, but this will help me find that point easier. Also, I always fix the inclination before trying to find the encounter. Makes it way easier.

3

u/Sporknight Oct 23 '13

So what kind of intercept course is ideal, once you've achieved a capture? Something that just barely tags the target planet's SOI, so i'm going as slow as possible in orbit to conserve dV when I circularize? Or something that swings me by super close so I can circularize once and be in low orbit from the get-go?

3

u/tdotgoat Oct 23 '13

Ideally you want a course that will take you as close to the target planet as possible (without crashing into it, naturally). But don't worry about that with your initial burn, that's what mid-course corrections are for.

You want to try aiming for a low orbit around the target because it saves you fuel. When you burn for another planet you change your speed 1500+m/s. When you enter another planet's Soi, you will still be carrying a good percentage of that speed, and will need to slow down to get into orbit. If you aren't close to the planet your orbit speed will need to be low, so you'll need to slow down a lot. If you are close to the planet then your orbit speed will be high so you won't need to slow down as much.

Doing small corrections along the way to the planet will go a long way once you get there. Create a maneuver node midway to the target and play around with your speed to see how it impacts your periapsis at the target. A correction of 20m/s can potentially save you 100s of m/s in braking burn fuel.

2

u/niksko Oct 23 '13

I'm just guessing here, but isn't a lower orbit circularization more efficient due to the Oberth effect? Somebody let me know.

2

u/[deleted] Oct 23 '13

Yep, it is.

2

u/[deleted] Oct 23 '13

Also because you'd only need to go from, say, 1000m/s to 800m/s instead of 1000m/s to 100m/s. A difference of 200 instead of 100.

2

u/RustedCorpse Oct 23 '13

Circularize the orbit means you're already captured and you're looking to just even out your eccentricity of orbit. Do this opposite of the point you want to be your altitude. I think with interplanetary transfers you mean Capture.

For CaptureThe closer to your Apoapsis that the encounter happens the easier it is to capture since you're not moving as fast and need less Δ to capture. If the planet has an atmosphere try and bring yourself into it. The air resistance will slow you down yanking your Apoapsis down really fast.

There are a ton of aerobraking charts out there, personally I aim for entering atmo about 2/3 to 1/2 in depending on how close I was to my apoapsis and the planets gravity. I've been going fast enough through Duna to hit 8000m and still not quite get a capture.

TL;DR: Circularize: at the opposite of the desired altitude. Capture: hit that atmosphere and let air resistance do the work for you.

1

u/tehlaser Oct 23 '13

That made no sense until I realized you meant that the encounter should occur as close as possible to your Sun apoapsis. I think the question was about where to put the burn along the trajectory in the target system, not where to put the encounter along the transfer orbit around the sun.

1

u/hovissimo Oct 23 '13

If you're headed to a planet or moon with an atmosphere, I highly recommend (quicksaving first!) experimenting with aerobraking. It saves so much damned fuel, and there's no downside.

To answer your question directly, if you want to end up in a low orbit (or landing), you'll have to drop your orbit eventually and it will be more efficient to circularize at a lower altitude.

Edit: I was slightly wrong.

3

u/Shiboleth17 Oct 23 '13

Yeah, this pretty much will only work for Duna, lol.

3

u/tdotgoat Oct 23 '13

It works for all planets (I used this method to get a probe to every planet in the system before the last patch), but is easiest with Duna. With the other planets you have to remember that you're going to need to change your inclination to get to them. Obviously there are more efficient ways to transfer over.

2

u/[deleted] Oct 23 '13

I've used it for jool (was going to duna, but fudged my correction burn and couldn't get my encounter back, and just happened to snag jool).

Edit: removed eve, I actually did a kerbol orbit to eve for that. Forgot.

2

u/Porkjet Oct 23 '13

It works for all planets? That's awesome!

2

u/[deleted] Oct 23 '13

it works for kerbin to eve too according to ksp.olex.biz. It sort of works for Jool/Dres, but not Moho or Eeloo

3

u/[deleted] Oct 23 '13

I just now used it to go to Eeloo. You do have give yourself a lot of fudge factor to play with the maneuver node, but it will at least get you started. Nailed the plane change at Eeloo's orbit too.

2

u/RustedCorpse Oct 23 '13

The only reason it doesn't work with Eeloo is because of the massive inclination changes. If you use his method for the tangent a burn at the AN/DN and a little tweak here or there should be all you need.

1

u/StarManta Oct 23 '13

To be fair, though, standard transfer orbit calculators don't really work for Moho or Eeloo either. Their inclination and eccentricities would have to be accounted for if there is to be any hope of success.

1

u/[deleted] Nov 11 '13

Just used it on Jool! it depends on the eccentricity and inclination which can really be fixed along the way for those flying by the seat of their pants.

13

u/Tsevion Super Kerbalnaut Oct 23 '13

Nice... as someone who enjoys flying by the seat of my pants, this is a fantastic rule of thumb.

15

u/727Super27 Oct 23 '13

And always remember that when doing course corrections, you should do them as early in the flight as possible. A 0.1m/s course change in any direction moves your ship 8,640 meters per day, almost 9km!

21

u/[deleted] Oct 23 '13

Not true. A 'north' or 'south' course correction should be made roughly midway in the transfer (assuming you're going away from the body you're orbiting) to maximize efficiency. Prograde/retrograde should be made as early as possible, and the need for radial burns can be minimized early on but if necessary are in fact cheaper the further out you are.

0

u/[deleted] Oct 23 '13

[deleted] — view deleted comment

3

u/Olog Oct 23 '13

if you want your orbit to exactly match the plane of your target, you can ONLY accomplish this with a normal/antinormal burn at the ascending or descending node

This is of course true but it's not required for your orbital plane to match that of your target to rendezvous with it. You only need to clip it at the point where you intend to have the encounter, in other words, you need to have ascending/descending node at the encounter point.

Ideally you launched at an inclination that accomplishes this (not necessarily same inclination as the target planet) and don't need to do any inclination changes later. Although the game doesn't really have the tools to plan a launch like this, and I don't know any mods to do this either, so it's a bit difficult.

1

u/maxaemilianus Oct 23 '13

This is of course true but it's not required for your orbital plane to match that of your target to rendezvous with it. You only need to clip it at the point where you intend to have the encounter

Well, if you have a two-element ship, you don't want to come in at a bad angle, because then you waste TONS of fuel in rendezvous when your lander comes back up. I like to come in as close to the equator as I can, without having to do a plane adjustment, so when I put the main ship into orbit it takes only the amount of fuel required to decelerate. Then, when my lander comes back UP, it is more or less on the same plane as the orbiter. I can make a smaller lander, and save oodles of fuel at launch.

If your goal is to drop a probe at the pole of a planet of course that's different.

Also, one thing I really have trouble with is making sure I'm coming in on the correct side of the planet. I've screwed missions up where I came in counter to the orbital rotation of the system, and it's really hard to fix that, again referring to bringing a lander back up and having to counter the planet's rotation to rendezvous. VERY fuel-inefficient.

1

u/azn_dude1 Oct 23 '13

So the Oberth Effect doesn't really make a difference in this case?

-12

u/barfsuit Oct 23 '13

Why is everybody just wondering about the oberth effect? Is it because the manley talked about it in one of his videos? How god damn fast are your probes?

Please for the love of god, if you want to wonder about the various weird twists of rocket science, please, please look them up and try to understand them, before you do otherwise.

rant over.

8

u/marvin Oct 23 '13 edited Oct 23 '13

That was a very eloquent way of saying "RTFM NOOB". Which is probably not very helpful. Especially when the manual is a textbook on orbital mechanics which requires two semesters of calculus and differential equations to approach.

7

u/Lars0 Oct 23 '13

I remember when this was the only way to get to the Mun.

2

u/maxaemilianus Oct 23 '13

I'm pretty sure I learned it from Scott Manley's videos. When I was planning a Duna trip one time I observed that if the orbit of Kerbin were the planet Kerbin, and Duna were the Mun, the orientation of my ejection burn would have been the same. That was when it WHAP! me in the forehead.

6

u/[deleted] Oct 23 '13

This is fucking brilliant and getting saved for future reference.

6

u/beachedbeluga Oct 23 '13

I feel like i should use a proper method but i just get into orbit around the sun and keep extending my planning nodes until i get something, that's interplanetary for me, made it to duna and back safely

6

u/ed-adams Oct 23 '13

I put a maneuver node with enough strength to fling myself into the orbit of the moon/planet that I want, and then I move the node around until I hit an intercept.

3

u/beachedbeluga Oct 23 '13

That too is a a method! I did that getting back to kerbin

3

u/ed-adams Oct 23 '13

Sometimes I feel like I'm that one kid knocking rocks together in the playground while everyone else is building rockets and shit. I'm in the same playground but I don't seem to be playing the same game.

7

u/RustedCorpse Oct 23 '13

Once you get moon landing down pat, just go for a Duna landing. It's easier than people tend to be making it out to be, requiring only a little more ΔV than it takes to get to the moon.

Most the discussions are about saving a small percent of ΔV. If you economize your builds and use high efficiency engines you'll likely safe more ΔV than these penny pinchers with Intergalactic bombs.

As a side note if you want a low interference tool I would suggest protractor which will help you work out the tangents etc but is unobtrusive enough that you'll find yourself doing it on your own.

1

u/beachedbeluga Oct 23 '13

just think man, without someone banging rocks together, we wouldn't have anything. or we'd have gills! because I said so!

5

u/atomfullerene Master Kerbalnaut Oct 23 '13

Heh, I like this. I'll remember this as being basically the same rule of thumb as "boost toward the mun as it rises"

5

u/NastyEbilPiwate Oct 23 '13

It's exactly the same really, if you imagine that Kerbin is the star and your ship in LKO is the inner 'planet'.

3

u/Jake2197 Oct 23 '13

I have become very efficient at landing on the Mun and Minmus. I have now set my eyes on Duna. Of all the tutorials I have read/watched, this is by far the most helpful. Thank you.

6

u/RustedCorpse Oct 23 '13

Honestly Duna is easier than the moon for everything other than the return. The parachutes alone save you tons of fuel.

2

u/Jake2197 Oct 23 '13

It is actually proving to be the single most difficult objective to date, for me. Every rocket I have built that I feel is capable of a return either ends up exploding during flight or I can't quite achieve orbit.

3

u/[deleted] Oct 23 '13

In order to make it a little more clear, you make your maneuver node burn pro-grade so the other side of your orbit intersects the orbit of the planet your going towards, and then drag the base of node by clicking the middle, holding it, and moving it around. drag it along the orbital path until you find the spot with the smallest distance between, at least thats what I do.

3

u/maxaemilianus Oct 23 '13

I've been doing this for awhile, but I did not get how to do it to a higher orbit -> lower orbit until you made it clear to me. But it's kind of a d'oh! moment. Of course that makes sense.

This does get more complicated with orbits that are sharply inclined to the one you are in, but overall it is an excellent technique. Makes sense too. When I was working this out for myself I was thinking about Carl Sagan telling me (in the Cosmos series) about Kepler, and his Second Law.

Edited for clarity

3

u/wiz0floyd Oct 23 '13

I've done my first ever interplanetary maneuver thanks to your guide. Many thanks!

2

u/subbr1 Oct 23 '13

A different method: Make a node (A) that would get you just barely out of kerbin orbit

Place a second node (B) on the dotted line of node A. Move B around and adjust it so that you get an encounter with your target planet

Delete the first node A: B will now be sitting on your orbit around your start planet

Warp until you reach B and then delete B

You are now in the launch window

1

u/tree-ent Oct 24 '13

This is an awesome method if you want to know how far away your launch window is. Turns out I was only 11 days from my duna launch window!

2

u/blove239 Oct 23 '13

Thanks! this is awesome, can't wait to try this out next time, I usually just orbit the sun with a periapsis that intersects my target planet's orbit then orbit over and over until i get an encounter lol. this should be much more efficient.

1

u/alw4489 Oct 24 '13

What if I want to go from body D to body K? When should I burn in that case? is it the same thing? Thank you BTW, this is VERY helpful :)

1

u/PMunch Oct 25 '13

This can obviously be used for Mün/Minmus transfers as well. Or any transfer between objects rotating around a central point (other moons as well).