Negative direction slingshot

[dlazerka]

I'm curious of a most effective way to launch ships that circle Kerbol in other direction all planets do.

Is it possible to use close flyby (aka slingshot) to actually slow down, than speed up?

Just imaging that in head tells me "no", as any gravitational body would try to "carry" me with them.

But I'm not sure...

[lajoswinkler]

Yes, it can be used. It depends on where do you put your periapsis.

http://wiki.kerbalspaceprogram.com/wiki/Tutorial:_Gravity_Assist

[FenrirWolf]

You're asking if it's possible to achieve a retrograde solar orbit? Yeah it is a thing that can happen, but I imagine the delta-v requirements are rather high. Chances are it would be more easily obtained via a bi-elliptic transfer instead of the usual hohmann transfer.

[lajoswinkler]

It would be fun to slam into Kerbin in a retrograde orbit. Someone needs to do that. It would probably invoke the Kraken.

[Dedjal]

I have only once done that. Tried to aerobrake in eves atmo.

Vessel ceased to exist. And since I had not saved for a long time, I never bothered to try again and see what went wrong.

[linkxsc]

Couple things. 1 like above a gravity assisted slowdown is doable. Its actually what the apollo program used between earth and moon, often known as a "free return trajectory" great for early game flybys of the mun and minmus.

As far as gerting a retrograde kerbol orbit... ive done it before, if you do slowdown passes of eve it can be done with less fuel, but i personally find it to be kinda useless past just seeing if it can be done.

[Armchair Rocket Scientist]

Couple things. 1 like above a gravity assisted slowdown is doable. Its actually what the apollo program used between earth and moon, often known as a "free return trajectory" great for early game flybys of the mun and minmus.

As far as gerting a retrograde kerbol orbit... ive done it before, if you do slowdown passes of eve it can be done with less fuel, but i personally find it to be kinda useless past just seeing if it can be done.

A gravity assist from Jupiter put the Ulysses probe into a polar orbit around the sun, so retrograde might be possible.

[K^2]

It would be fun to slam into Kerbin in a retrograde orbit. Someone needs to do that. It would probably invoke the Kraken.

Hah. Good idea, I'll have to try it. (It'd be easy to cheat this in, but where is the fun in that?)

[linkxsc]

A gravity assist from Jupiter put the Ulysses probe into a polar orbit around the sun, so retrograde might be possible.

You do have a point, a polar orbit gives you a bit more to work with than a regular... planar orbit I guess? Could maybe hit Eve with a polar retrograde assist, and do a burn while right on the opposite side of eve from kerbol.

[cantab]

Get apoapsis very high, burn retrograde at apoapsis. It's like sundiving except you burn a bit more.

What I don't know is what kind of gravity assists you can then get when you're going in the opposite direction to all the planets. Assuming here that you want to then establish a wider retrograde orbit than your comet-like initial one.

[Blind Dead McJones]

You'd need an absolutely absurd amount of delta-V to put something in retrograde orbit around Kerbol, probably about double what you need to fall into the sun in the first place. In fact, I can't imagine it even happening without cheating. With the physicsless parts you can get as much as 150 Gs of TWR, that's your best bet unless you want to watch it for hours.

[Yourself]

You'd need an absolutely absurd amount of delta-V to put something in retrograde orbit around Kerbol, probably about double what you need to fall into the sun in the first place. In fact, I can't imagine it even happening without cheating. With the physicsless parts you can get as much as 150 Gs of TWR, that's your best bet unless you want to watch it for hours.

I think you could definitely get away with less than double the delta-V of falling into the sun. Someone may have mentioned this already, but boosting to a higher orbit before then boosting to a retrograde orbit could actually be cheaper. Consider this little thought experiment:

The delta-V to put yourself on an escape trajectory is about 41% of what's required to fall into the sun. So let's say you put yourself on an almost escape trajectory. So close to an escape trajectory that by the time you reach apoapsis, the delta-V to go retrograde is negligible (a few m/s). Then all that's left to do is to fall back to periapsis and recircularize (which costs another ~41%). So in total, you could put yourself in a retrograde orbit for about 82% of the delta-V cost of actually completely nulling out your motion and falling into the sun.

Of course, this cost is limited by how long you're willing to wait.

If you work out the math for this transfer you can find that the total delta-V cost of this maneuver as a fraction of the "twice the fall into the sun" delta-V, you get:

ÃŽâ€V = 2 ÃŽâ€V_SUN * ( sqrt(2 * (R_p + R_a) / R_a) - 1 )

Where R_p is the periapsis radius (the original circular orbit radius) and R_a is the apoapsis radius (how high you place your apoapsis in the transfer). Note that this a decreasing function of R_a. If R_a = R_p, we recover:

ÃŽâ€V = 2 ÃŽâ€V_SUN

If we let R_a tend towards infinity then we get:

ÃŽâ€V = 2 ÃŽâ€V_SUN * ( sqrt(2) - 1 )

The time it takes to perform this entire maneuver and end up in the retrograde orbit is given by:

T = 2 À sqrt( (R_p + R_f)³ / μ )

Or, expressed in terms of the orbital period of your original orbit:

T = T₀ ( 1 + R_f / R_p )^3/2

[dharak1]

I'm not sure how much faster a retrograde Kerbol impact would be but I just crashed a probe into Kerbin at 11km/s and it hit the ocean at just 131 m/s. My god is aerobraking effective.

[Bacterius]

You'd need an absolutely absurd amount of delta-V to put something in retrograde orbit around Kerbol, probably about double what you need to fall into the sun in the first place. In fact, I can't imagine it even happening without cheating. With the physicsless parts you can get as much as 150 Gs of TWR, that's your best bet unless you want to watch it for hours.

Nah, it can be done way more efficiently. I did it with around 6.5 km/s delta-v by using a retrograde Jool assist (consisting of the transfer from Kerbin solar orbit, a main 4.3 km/s inversion burn, and minor correction burns) and I wasn't even trying. Someone mentioned an Eve assist, which I am not sure about because it is so deep inside Kerbol's gravity well (maybe a gravity assist ninja can try it out and see what kind of gains we're looking at) but I have no doubt it can be done very much more efficiently than just burning twice your orbital velocity backwards. A bi-elliptic transfer is also a possibility.

I'm not sure how much faster a retrograde Kerbol impact would be but I just crashed a probe into Kerbin at 11km/s and it hit the ocean at just 131 m/s. My god is aerobraking effective.

I got a record speed of 21.5 km/s on my retrograde Jool to Kerbin intercept. It just decelerated through the atmosphere like it was nothing, I tried it several times and got like 70 g's of deceleration at one point. I think once the drag system is improved the atmosphere should feel much less soupy and it won't be so easy to survive a head-on collision as opposed to a long aerobrake cutting through the upper atmosphere.

Edited August 12, 2014 by Bacterius