Slingshots FTW!

[Tokay Gris]

I began experimenting with slingshots. Mostly Mun, because it is easy to reach and easy to hit.

The first two probes launched (with IMMENSE delta-V) surprised me... I didn't really have to aim. Roughly one slingshot with Mun between Kerbol and Kerbin... Voila! Almost Eve Orbit and the first probe ended up there. (Pretty sharp angle, though. Aerobraking worked at 55 km... WOAH!))

Second slingshot while Kerbin was between Mun and Kerbol.... Hm... almost Duna orbit. Lets... see... Eeloo? Hey, why not....

And it is suprisingly simple to do. Just hit the right spot so that your vessel is either going along your orbit or against. Usually, that means aiming for Mun, when it is in front in Kerbins orbit or behind.

No idea if I actually safe delta-V that way, but it makes handling of my large ships a lot easier.... also, it is fun to do a low slingshot (3-4 km) with a 600 ton ship....

And no, this is not a mission report. This is me boasting!

*grin*

[Motokid600]

I've yet to go interplanetary. I have to learn this as much as possible.

[Tokay Gris]

You could try it with a simple probe. Put a nuclear engine underneath an X-200-32 tank, aim for Mun and see what happens. The difference between the orbital velocity before you enter Muns SOI and after you leave it is your gain... After that it is just to hit the right spot so that this velocity is in the right direction.

[Vanamonde]

It's simple if you stumble into it by accident. When trying it by design, waiting for one world to line up the right way with another world and then plotting a burn to come out of the slingshot on the right trajectory ends up being extremely difficult and time-consuming.

[maackey]

Hitting another planet out of a slingshot burn would take a lot of effort, but flying off in the general direction is not the hardest thing in the world. Maneuver nodes can tell you what your exit trajectory will be, and you can make small correction burns while in solar orbit.

[rryy]

Well, not really, because the rotation of the Mun is quite fast compared to the rotation of the planets around the sun. And even still, there is a window of opportunity for an escape window. You may need a lot of quicksaves to get the escape burn right, though.

[Crush]

A slingshot around Mun usually doesn't save you that much delta-v, because due to the Oberth Effect accelerating in Kerbin orbit is more efficient than accelerating in open space, which you usually still have to do after a Mun slingshot. When you are lucky you can save about 100m/s with a Mun slingshot.

What's much more efficient is a slingshot around Duna when you are heading for Jool or around Eve when you want to get to Moho. This can save you 1000 m/s and more.

Another great application of gravity slingshots is entering the Jool system with a gravity slingshot around Tylo. Tylo's gravity is strong enough to get you from transit speed into an orbit around Jool. Well, you could also aerobrake in Jool's atmosphere, but a slingshot is much more elegant and safer, because your final orbit is a lot more predicatable than with an aerobrake.

I haven't done a return mission from Jool yet, but a close slingshot around Tylo should be able to bring you pretty close to Kerbin.

Edited June 9, 2013 by Crush

[Vanamonde]

You can also do some really useful deceleration slingshots at Ike and the moons of Jool. That's really a matter of the luck of your arrival time, but Ike is large and there are several Joolian moons, which means opportunities are not rare.

[Crush]

You can also do some really useful deceleration slingshots at Ike and the moons of Jool. That's really a matter of the luck of your arrival time, but Ike is large and there are several Joolian moons, which means opportunities are not rare.

You can help your luck by planning the course correction for the fly-by as early as possible, preferably even before entering the SoI of the target planet.

I was, however, never able to get an Ike fly-by which got me into an orbit around Duna without also performing an aerobrake or expending additional fuel. Ike is only large compared to Duna. Compared to the other moons it is rather one of the smaller ones. It also has a very low orbit around Duna, which means less kinetic potential to tap into. And Duna isn't a heavyweight either, which means that you need to lose a lot of speed to get below escape velocity. Another (albeit minor) problem is Ike's geography. While Mun allows you to almost scratch the surface during an equatorial fly-by, Ike with its irregular shape doesn't allow you to be so brave.

Edited June 9, 2013 by Crush

[jheriko]

When at the lowest point during the slingshot you can make some very large changes to trajectory cheaply so that when you exit you have a close encounter with a target. The effect is so strong that you will often have to wait a bit later before making fine adjustments since even the fine control on rcs will send your trajectory towards/away from encounter far too much.

[bsalis]

Another great application of gravity slingshots is entering the Jool system with a gravity slingshot around Tylo.

I used a Tylo sling to help me get to Eelloo. It worked great for radial change needed.

[rkman]

Roughly one slingshot with Mun between Kerbol and Kerbin... Voila! Almost Eve Orbit

Second slingshot while Kerbin was between Mun and Kerbol.... Hm... almost Duna orbit.

Doesn't mean much without specifying how much delta-V you saved; what orbit would you have ended up in if the Mun would not have been there? For all we know the Mun can have only changed the direction of the trajectory without adding much speed.

Gravity can be a cheap way to change direction, which can be very useful.

But a change in direction is not what makes it a slingshot.

Edited June 10, 2013 by rkman

[PakledHostage]

I hosted a “Minimum Delta-V to Duna†challenge thread this past winter that, unfortunately, was lost along with so many good threads.

Delta-V maps, such as the one that is the subject of the current Delta-V map thread, predict a minimum delta-V for a transfer from LKO to Duna of about 1060 m/s. One entrant in the challenge managed to reach Duna's surface from LKO using about 1100 m/s for a direct transfer.

I managed to save about 80 m/s off the minimum predicted by the delta-V charts using a gravitational assist off the Mun. I managed to get from a 100 km orbit about Kerbin to Duna's surface using just 980 m/s. I could have maybe saved a dozen m/s or so more delta-V had I been a bit more precise, but not much more than that.

I still have the screen shots:

Starting Fuel

Starting Orbit

Post transfer burn fuel

Planned Munar slingshot trajectory

Actual Munar slingshot trajectory

Munar periapsis

Transfer trajectory – Post gravitational assist

Transfer trajectory – Duna arrival

Arrival fuel

Landed on Duna

Math

[Ford_Prefect]

Is there any way to calculate this stuff? It would be really useful if you could predict it...

[PakledHostage]

Is there any way to calculate this stuff? It would be really useful if you could predict it...

You can. I just takes a fair bit of math. I used Lambert's method when planning my Duna slingshot mission. I also picked my transfer window such that I'd be arriving at Duna when Duna was near its ascending node. (If I remember correctly, the second Duna transfer window after starting a new campaign is almost perfect.) This made it a 2D problem because no significant plane change manoeuvres were required.

There was a bit of discussion recently in the How To sub forum about Lambert's method. Meyst and Tavert gave a good description of the method to use in the How does one execute and calculate Delta V for non optimal planetary transfers thread.

Edited June 10, 2013 by PakledHostage