Laythe to Kerbin transfer?

[waterlubber]

Is there a way for me to calculate a transfer from Laythe to Kerbin, directly, without going into Jool orbit? I don't want to waste the DeltaV as I only have 3000 left in my SSTA. (Refueled on Laythe, if you're surprised )

I'm in a low 70km Laythe orbit. Most calculators don't want to do the math--how can I, manually?

Help appreciated.

Solved! Solution: Manual. Computers suck at everything. Just drag the node around till you get an encounter...

Edited April 25, 2015 by waterlubber

[Capt. Hunt]

Actually, I suspect a grav-assisted Joolian slingshot would probably take less delta-v then the trajectory you're describing

[waterlubber]

Yes, but I'm not sure how to align the planets because none of my transfer calculators want to do the math

[Commissioner Tadpole]

Do it the same way as you do in a Kerbin-to-Moho/Eve transfer. Wait for Laythe to be positioned where you'd normally be if you were orbiting Jool and burning back to Kerbin.

Unrelated, but what exactly is a "SSTA"?

[waterlubber]

SingleStage to Anywhere

It's a plane that can transfer to any body on a single stage---mine kinda fits the description if it refuels in orbit and on the planet

Thanks guys.

[Empiro]

You can't gravity assist off of Jool since you're already in orbit around Jool.

However, the quick answer is "not very much" past escape velocity if you're already in an orbit around Laythe (around 2000 m/s).

The quickest way to calculate this is to use the equation for how much velocity you keep after escaping: V(infinity)^2 = V^2 - V(escape)^2.

By using the transfer calculator, if you're in an orbit at Laythe altitude around Jool, it takes 1581 m/s, so that is how much speed we need after escaping Laythe. The escape velocity for Laythe is 2800 m/s, so you just solve and get 3215 m/s. Since you're already in orbit (2000 m/s) around Laythe, it means that you'll need just under 1300 m/s to make it. As an interesting note: this is less than you need if you're in Jool orbit, because of the Oberth effect.

You can save a bit more if you instead use Tylo for a gravity assist, but not that much (since you'll need about 800 m/s to escape Laythe anyway). It sounds like your 3000 m/s should be plenty, in any case.

Edited April 25, 2015 by Empiro

[waterlubber]

You can't gravity assist off of Jool since you're already in orbit around Jool.

However, the quick answer is "not very much" past escape velocity if you're already in an orbit around Laythe (around 2000 m/s).

The quickest way to calculate this is to use the equation for how much velocity you keep after escaping: V(infinity)^2 = V^2 - V(escape)^2.

By using the transfer calculator, if you're in an orbit at Laythe altitude around Jool, it takes 1581 m/s, so that is how much speed we need after escaping Laythe. The escape velocity for Laythe is 2800 m/s, so you just solve and get 3215 m/s. Since you're already in orbit (2000 m/s) around Laythe, it means that you'll need just under 1300 m/s to make it. As an interesting note: this is less than you need if you're in Jool orbit, because of the Oberth effect.

You can save a bit more if you instead use Tylo for a gravity assist, but not that much (since you'll need about 800 m/s to escape Laythe anyway). It sounds like your 3000 m/s should be plenty, in any case.

How does this math check out? After you get escape velocity (2800) from my orbital velocity (1855). Do you mean 2800-1855? I'm not sure what you mean.

In other words, I understand I need to subtract the 1855 m/s DeltaV from my laythe burn. How do I add the escape DeltaV, assuming the planet "steals" some due to gravity losses?

[luizopiloto]

wait for laythe to be aligned between Kerbol and Jool, then, burn prograde from the Kerbol iluminated side of Laythe...you will eject directly into Kerbol orbit from the right side to return to Kerbin

[davidpsummers]

Is there a way for me to calculate a transfer from Laythe to Kerbin, directly, without going into Jool orbit? I don't want to waste the DeltaV as I only have 3000 left in my SSTA. (Refueled on Laythe, if you're surprised )

I'm in a low 70km Laythe orbit. Most calculators don't want to do the math--how can I, manually?

Help appreciated.

If you want to know when, you can use any of the calculators for Jool => Kerbin. The window from Laythe will be basically the same time.

You can then put in your burn mode and see how much dV you need to bring the periapsis of the resulting solar obit in to the distance of Kerbin's orbit (the optimal transfer will have a solar orbit where the periapsis just touches Kerbin's orbit and if you do the burn at the optimum time, then you want an optimum orbit).

Then you fiddle with it. Maybe add some normal or anti-normal component so the game shows you the closest approach (if it doesn't already) or just burn to an orbit that graze's Kerbin and do a correction once you leave Jool's SOI?

[waterlubber]

Alright, I'm going to execute the burn as if Laythe was my spacecraft, so everything is in line. How much energy does the escape from laythe from a 70km to Jool orbit take from my ejection DeltaV? For example, if at peri I was 3km/s, and at the SOI change I was 2km/s. How much is this number?

[davidpsummers]

BTW, you can to a gravity assist off another moon. I don't know how much this will give (not a lot?) It will be hard to calculate a burn. Maybe sling off into a retrograde direction (with respect to Jool's orbit, you will always want to apply your dV retrograde wrt Jool. A transfer to a more inner planet is applying the right amount, at the right time, of dV to retrograde) and see what you get. Once you leave Jool's orbit, you can adjust the solar orbit? (you would gain the gravity assist, but loose some Oberth effect, the solar burn will be a low velocity).

[waterlubber]

It's okay. I fiddled a ton by adjusting my node in increments of Laythe's orbital period, so I was ejecting from jool at approximately the same location each iteration. Eventually I found one that got me kind of close to Kerbin---going to refine this later!

Edit: DAMMIT! KSP decided to bork my orbit and now I have to re-create the node. ARGH!

Edit Edit: Finally got the transfer set up...only 5 years to go! *sigh*

Edited April 25, 2015 by waterlubber

[Empiro]

How does this math check out? After you get escape velocity (2800) from my orbital velocity (1855). Do you mean 2800-1855? I'm not sure what you mean.

In other words, I understand I need to subtract the 1855 m/s DeltaV from my laythe burn. How do I add the escape DeltaV, assuming the planet "steals" some due to gravity losses?

After you escape Laythe, you need have about 1581 m/s left over (in the correct direction of course!) to escape Jool and transfer to Kerbin -- that's your target V(infinity). You solve for V in the equation above (and use 2800 m/s for V(escape)). You'll get the answer 3215 m/s (note the squares). Since you're already going 2000 m/s around Laythe, you need to spend 1215 m/s to get to 3215 m/s.

Because Laythe has a tiny SOI compared to the size, you actually might be able to get away with less, as your test seems to indicate.

Edited April 25, 2015 by Empiro