The Wrong Way Round

[RizzoTheRat]

Clearly I should read the small print on contracts, I've been caught out before on launch satellite missions wanting to orbit Kerbin the wrong way, but I now have one wanting to orbit the sun the wrong way.

I'm guessing I need to set up very elliptical orbit to get a nice slow Apoapsis where I can change direction for a minimal thrust but that leads me to a few questions.

- Is that the most efficient way of doing it?

- How close to the sun can I get doing an Oberth maneuver without melting my probe?

- How complicated is the maths to work out the tradeoff between the amount of dV I expend pushing the Apoapsis out, and the amount I'd need to change direction, other than just playing the nodes and seeing what happens? (I have an engineering degree but I haven't really got the hang or orbital mechanics yet )

- Anyone got a rough idea how much dV this is likely to need? I don't have Ion drives yet so this may have to be a slightly larger mission than I originally expected.

[mhoram]

Keep in mind that you could perform a gravitational slingshot. Jool would be the best candidate since it has the highest mass.

This is a post about the maths of 90° planechanges. But the calculations should also be appliable to your setting.

[RizzoTheRat]

Ooo, some good stuff there, might need to spend an evening with a pencil and probably Excel to work through it.

Interesting thought on using Jool. I've not played with gravitational slingshots yet but presumably if I went for an elliptical orbit with a nice slow Apoapsis in Jool's SoI I'd get the most benefit.

I've spent most of the KSP careers messing about between Mun and Minimus, I'm just starting to realise how much more complicated interplanetary space is

[selfish_meme]

also make sure to initial orbit retrograde, then you don't need a planechange

[cantab]

The most efficient way will be a Jool gravity assist. Key is to enter a transfer orbit with an apoapsis somewhat beyond Jool's, so that your trajectory crosses Jool's instead of just touching it.

The next most efficient will be to push your Sun apoapsis way far out, like beyond Jool's orbit far out, and reverse direction there and get your periapsis touching the target orbit. The further the apoapsis is the less the delta-V needed but the longer the flight takes, make sure the contract doesn't expire! The delta-V requirement is the delta-V to escape the Sun from Kerbin, plus 41% of the orbital speed at your target orbit's distance from the Sun, plus a margin for the orbital reversal and safety. Estimate that target orbital speed based on the speeds of the planets' orbits.

[GoSlash27]

also make sure to initial orbit retrograde, then you don't need a planechange

selfish_meme,

He's talking about a solar retrograde orbit, not Kerbin. Don't worry, I've made the same mistake once or twice...

Solar retrograde is gonna be expensive

Best,

-Slashy

[Kryxal]

Actually, to enhance the Jool slingshot, consider using a Laythe slingshot ... I remember trying to do a Laythe flyby once for science and wound up solar-retrograde by accident!

[selfish_meme]

selfish_meme,

He's talking about a solar retrograde orbit, not Kerbin. Don't worry, I've made the same mistake once or twice...

Solar retrograde is gonna be expensive

Best,

-Slashy

I know, I just thought it would cost less dV starting from a Kerbin retrograde orbit?

[Wolf Baginski]

I know, I just thought it would cost less dV starting from a Kerbin retrograde orbit?

A Kerbin retrograde costs delta-V in the launch, and most of the delta-V is just getting out to Jool.

An example of the general class of orbit is a free-return orbit to Mun. This is similar to what the Apollo capsules did. There's a Scott Manley video on YouTube. Standard transfer orbits have you trailing the target. The free-return passes in front of the target and returns to Kerbin as retrograde. The track has a figure-8 pattern.

The difference in orbit needed to get to Jool would be tiny, and I can't rule out having to do a burn at Jool periapsis to adjust the SoI exit so you miss Kerbol on the correct side. Depending on the final orbit needed, there would be another burn at Kerbol periapsis, pretty huge to reduce the apoapsis again.

Luckily I have a Kerbal Space Program simulator on my computer. It's called Kerbal Space Program.

[jwbrase]

Ooo, some good stuff there, might need to spend an evening with a pencil and probably Excel to work through it.

Interesting thought on using Jool.

IRL, the cheapest way to drop something into the Sun is to send it by Jupiter first.

I've not played with gravitational slingshots yet but presumably if I went for an elliptical orbit with a nice slow Apoapsis in Jool's SoI I'd get the most benefit.

Not really. You want to send yourself in a direction opposite to the way Jool is orbiting. If your Apoapsis is in Jool's SOI, your Jool-relative velocity vector approaching Jool will be opposite to the way Jool is orbiting. Your Jool-relative speed leaving Jool's SOI will be the same as your speed entering Jool's SOI, only the direction will change. As a consequence, a slingshot cannot provide you with a Kerbol-relative boost in the same direction as your Jool-relative velocity entering Jool's SOI (conservation of momentum* is a royal pain in the neck ). Thus, you want to arrange your Jool encounter so that you're approaching Jool from a different direction than you want to be boosted in, and preferably for a fairly high encounter velocity. One thing that you may want to look at, time and transfer windows permitting, is one or more Eve slingshots to boost you onto an orbit that sets up your Jool encounter. Another thing to consider is that you may be able to get around the "no boost in your approach direction" bit with a sling off of one of Jool's moons.

I know, I just thought it would cost less dV starting from a Kerbin retrograde orbit?

Nope. Same dV to leave Kerbin, more dV to launch into that orbit.

[cantab]

One more thing to consider: depending on the timeframe of this contract, the kind of other missions you're doing, and your tolerance for long burns, it may be an option to wait until you unlock ions then brute force it. Getting 20-25 km/s out of ion engines is fairly straightforward.

[Laie]

I know, I just thought it would cost less dV starting from a Kerbin retrograde orbit?

It's like going to Eve, only more so: you launch backwards out of the Kerbin system. If you begin with a retrograde orbit you may do that burn on the dark side of Kerbin, but I don't think that would provide any benefit.

[Red Iron Crown]

- How complicated is the maths to work out the tradeoff between the amount of dV I expend pushing the Apoapsis out, and the amount I'd need to change direction, other than just playing the nodes and seeing what happens? (I have an engineering degree but I haven't really got the hang or orbital mechanics yet )

Here's a quick formula to calculate how high you should set apoapsis for an orbit or radius r and plane change amount θ:

Ap = r * sin(θ/2) / (1 - 2 * sin(θ/2))

Less than 39 degrees of change and it's not worth raising at all, over 60 degrees and it's worth raising as high as your patience allows.

More math here.

[Sharpy]

It's like going to Eve, only more so: you launch backwards out of the Kerbin system. If you begin with a retrograde orbit you may do that burn on the dark side of Kerbin, but I don't think that would provide any benefit.

Trying to turn around near Kerbin will cost something of order of 20,000m/s; closer to 25,000 if you include launch and Kerbin escape. If you raise your solar apoapsis, you can do this in half the delta-V, and even less if you use Jool assist.

[mrclucks]

Depending on how long left I on the contract, I would burn from kerbin trying to get an orbit intersecting eve with solar apoapsis as high as possible, then proceed to get a slingshot from eve and exploiting the oberth effect to make the solar orbit even higher. Repeat with other planets and when I'm satisfied with the height of my apoapsis, hopefully higher than eloos orbit, I'd burn retrograde at apoapsis.

As for your dv problem, I'd use asparagus staging and get rid of tanks as you use them but only use one high ISP engine. I've built a rocket with +15k total dv in a career mode where I had just unlocked the poodle. It flew like an epileptic fish on crack and had fairly low twr, but it also landed on the mun and minmus.