Kerbal Space Program - Using Gravity Assists To Save Fuel

[@scottmanley]

Pluto and Neptune are in a 3:2 resonance which makes them extremely stable with respect to each other.

[@meeast1169]

Surprised you did not pin yourself!

[@falconheavy809]

Cool!

[@twitchy.mp3]

@@meeast116910 years ago we didn’t have the whole pinning thing yet lol

[@MrMemetic321]

​@@meeast1169comments broke when they added Google+ And yes, that was a real thing

[@XavierBetoN]

I'm just here to revoke the old videos, keep up the good work boss

[@scottmanley]

And the faster you're going the less of an assist you get.

[@bredboi8416]

Ok thx you Scott manley

[@roelwieggers]

I have been using the Mun to get to Duna several times now. Also used it on my way to catch an asteroid that isnt in kerbins soi yet. I never realised that just using the oberth effect is more efficient... But at least someone thinks i'm clever. So i got that going for me, which is nice.

[@WofWca]

Wait where are you using the Oberth effect? On Kerbin's initial parking orbit?

[@RaimarLunardi]

So much work!!! I simply put MOAR BOOSTERS!

[@ZzmemeguyzZ]

Did you actually just do that using the stock Kerbal X? Wizard.

[@ShaboolieGaming]

Squad: Your a wizard scott. Scott: I'm a WHAT?! Squad: A wizard scott! Scott, Nah, just to mlg for you.

[@ZzmemeguyzZ]

You're* too*

[@ShaboolieGaming]

XD

[@ZzmemeguyzZ]

Shaboolie XDD

[@scottmanley]

I'm mainly using the maneuver node editor to show how tiny differences have big consequences

[@Tilion462]

Ah, great stuff as ever, Scott... Wonderful demo of those all-important energy-saving slingshots, almost tripling your ∆V at your target planet. Would have preferred to see it in a clean system I have to say - the numerous tracks got momentarily confusing once in a while... These manoeuvres are much better suited to robotic craft though: I was feeling sorry for the little green fellas by about day 200 (but I especially liked the sadistic, tantalising mid-trip return to Kerbin - nice touch!)

[@amazingteknique]

I seriously don't think I would have gotten into this game as much if it wasn't for your channel Scott. I know that I wouldnt have stuck with the learning curve had it not been for your vids. You sir are an amazing piece of the gaming community.

[@david_for_you_]

Hey Scott, love your videos! Since you mentioned this a couple of times already: There actually is a trick to let the game calculate multiple orbits ahead with the current maneuver node system. If you create additional nodes with 0m/s dv about an orbit later it will show you the resulting close approaches of the later orbit. The easiest way to create those nodes is directly after your "real" maneuver and than shift them with the maneuver node editor. Hope this helps in your future missions!

[@scottmanley]

The final burns were more because I was impatient, it would be more efficient to use another assist. Voyager grand tour requires more outer planets.

[@Logarithm906]

I sent a message to you before that one, the general gist of it was that most modern physicists would be able to programme a computer to do them sorts of calculations because a lot of physics these days is getting computers to solve your problem (instead of using pen and paper, which would take millennia to do). The message you replied to was clarifying that most other sciences also feature a lot of programming to solve problems (like research into protein folding).

[@gunerdown]

Scott your videos are always crazy(in a good way) and this is differently one of them. Now I go to go try this. Thanks for all the great videos!

[@wasteofspace1234]

For those who are interested there is now a mod called Slingshotter which helps a lot with these maneuvers.

[@MC3craze]

Yes, in most cases it does take more dV to get into a higher orbit and then a lower one. But when your ratio of your final semi-major axis to your starting semi-major axis is over about 12, a bi-elliptic transfer orbit is more efficent than a Hohmann transfer orbit. If that is difficult to understand, take the SMA of your target orbit, and divide it by the SMA of the orbit you start in. If the resulting number is greater than approximately 12 then a bi-elliptic transfer is more efficent.

[@TonboIV]

Thanks a lot Scott. This really brings home how missions like Cassini happen. KSP is such a great teaching tool for orbital mechanics. Something that takes pages of explanation and equations to sort-of-get, becomes obvious and intuitive when you actually DO it. :-)

[@MC3craze]

NASA does this quite often actually. A few examples are the two Voyager probes, the Juno probe, and the Cassini-Huygens probe. The Cassini mission was quite unique since it performed four gravity assists. Two at Venus, one at Earth, and finally one at Jupiter. These gravity assists gave the Cassini-Huygens probe enough energy to reach Saturn and complete its missions.

[@BrummieMorgan]

I'd like to think this video came about thanks to my suggestion of emulating the Voyager missions with their gravity assists. I doubt that's the case, but I'm still massively impressed and pleased to see this video. Well done, Mr. Manley.

[@Alex_Morrison]

I love the vids Scott they really interest me you deserve the 100k subs more than most people

[@Trehek]

I got here from the KSP Wiki's "Tutorial: Gravity Assist" page, and this video with "I used X gravity assists over Y hours to reach Jool" seems way too complicated. I'd really appreciate a more simple demonstration about the how-to and the effect of a single gravity assist pass.

[@_jelle]

This is rocket science man, did you expect something like 1+1=orbit-to-jool?

[@Trehek]

arizakaa In this video Scott messes around with mechjeb in order to get his trajectories to reach other planets. He performs gravity assists but he does not teach them. This video does not explain what kind of pass you need to get a succesful gravity assist from an orbital body. It does not explain the effects of passing before or behind a body etc. My comment didn't have anything to do with reaching Jool. Like I mentioned, this video was linked to in KSP Wiki's gravity assist page, so I expected a video which did a more detailed job at actually teaching the maneuver. Of course, all of this is irrelevant because according to a few comments Scott made, he did not intend this video to be a tutorial!

[@WarDaft]

Trehek He kind of is teaching them. The actual gravity assist itself is dead simple. If you fly close in front of the planet (ahead of it in it's orbit) you slow down. If you close fly behind it, you speed up. The closer you pass the more of a change you get. That's literally all of it, and the tutorial page you mention covers that. The hard part of it is actually chaining the gravity assists to get you where you want, which is exactly what this video is doing.

[@Watermark0n]

Gravity assists in KSP simply aren't easy to use, just how it is. Because you can't predict more than one orbit ahead, you're extremely limited in your options for how to approach a gravity assist. It basically boils down to trial and error, which is the summation of this video. It's honestly easier just to build a rocket with 1k more Dv, at the present time.

[@InsanestFoxOfAll]

The best way to get information on close encounters more than one orbit ahead is to put a maneuver node right after the close encounter.

[@scottmanley]

Depends on where you're going.

[@rvdm88]

Wow, i dont even "dare" to enter interplanetary space with such a fuel / ΔV budget, let alone try for Jool + land + return. You are the best at space!

[@thisisutubnottwitterwhyhandles]

i like whenever they get to the night side of the planet they start freaking out

[@scottmanley]

That is actually a good trick.

[@scottmanley]

It would depend on where you started, but i think it could help

[@davidchalfin3298]

I have lost count of the number of your videos I have watched in the past few days. You are great at KSP. I am a nuclear engineering major and I have an interest in physics outside of that, but the time you must spend to calculate these boggles the mind.

[@samsonguy10k]

Yes, the Kerbal X more serves as a foundation to build on to achieve anything beyond Kerbin orbit. I remember the Squad podcast (heralding 0.18, if memory serves) that a certain friendly, even manly, guy shared some time on, where one of the members of Squad added a lot of stuff to the Kerbal X, on camera, to get it to the Mun, and jokingly making a point that the ship was woefully made.

[@tavert]

He likely means a free return trajectory. If you fly by a planet/moon without making any burns, your speed when you exit the SoI will equal your speed when you entered the SoI. However the gravity assist will change the direction of your velocity vector. If you exit the SoI heading retrograde, then the gravity assist effectively slowed you down relative to the parent body.

[@andywonline]

Oh my... You have to follow this up!!!!!!! I vote for a jool landing & kerbin return... I struggle to get to duna with 1000 delta v.. But I'm learning every day.. Especially with your videos. Thanks for making these videos.

[@RedsBoneStuff]

Thank you for making this video. It has helped me quite a lot.

[@44kainne]

I'm pretty sure my brain just exploded. At present, working on a ssto heavy shuttle with a cargo capacity of 40t using b9 and far. Thus far limited success...thanks for the video, I'd be stoked to see some maneuvers in the Jool system.

[@Freak80MC]

Duna: "Ehh, it's there I guess" - Scott Manley

[@scottmanley]

Finally? I've done dozens of jool missions?

[@RollingThunder2020]

Remember that velocity is a vector - it has a magnitude _and_ a direction. You speed up in one direction then slow down in another, resulting in a large change in your final vector without having to burn any fuel to do it. The resultant new orbit can be effectively the same as one you would get with a much longer burn, which is like free acceleration.

[@TomottoSoup]

The Space Shuttle was designed to Military specs for cross range capabilities. Not to mention launching new military satellites (which you do mention) and so forth. There may not be much need, but the US military certainly seems to have its voice heard in design proceedings.

[@Dmitry633]

In orbiter space flight simulator there is a very powerful tool called TransX that allows you to plan multiply gravity assist with great precision.

[@MikeM8891]

A theoretical upper limit for a gravitational assist around the Mun is 464 m/s delta V.

[@Acheron538]

A good simile to answer your question would be to compare achieving escape velocity (fighting the pull of a planet's orbit) to climbing a steep hill, ethier in a vehicle or on foot. Not many vehicles (or people) are equipped to climb a hill straight from bottom to top. Due to the steep angle of travel, it requires far more exertion of energy. It is more efficient in some cases to climb a hill or mountain along a longer path, as it results in a less steep angle of ascent.

[@ragekwt1]

I think whilst heading out, you would ideally encounter every planet once (if they are line up perfectly), so you also get a gravity assist from every planet in the system.

[@woolfoma]

sorry to contradict you scott but I found a way to use a munar assist to save delta-v, heres how it works, basically make sure to align the planets for the interplanetary window (obviously), then get a munar assist lined up to kick you out to interplanetary space. then keeping your mun periapsis about 5 km, accelerate using the oberth effect until your trajectory has an intersect, this way you never have to burn other than in LKO but you still use the mun to accelerate your orbit.

[@zarblitz]

It doesn't take any more or less. It's all a matter of whether you're increasing or decreasing your velocity. A 500m/s increase in velocity takes just as much fuel as a 500m/s decrease in velocity.

[@TSLVMT]

This dude Does a grand tour with less delta V than landing on the moon. Let that sink in guys

[@samsonguy10k]

Interesting note about using the Mun to escape the Kerbin system. Robert Heinlein proposed in one of his books, The Rolling Stones, where one can launch to the Moon, orbit around to return to Earth, and then use the Earth to assist out of the Earth-Moon system. I aim to try that with Kerbin and the Mun, and maybe a follow-up with Kerbin and Minmus, but the theory does seem more sound that using more delta-v to just shoot past the Mun to interplanetary space.

[@TonboIV]

Depends. The fuel you'll spend getting stuff to Minmus is always more than what you'd save on your departure burn, but if you're sending that stuff with smaller rockets it might make things EASIER. If you mine Kethane on Minmus then it's a big savings. Whatever you do, if you're going further than Duna or Eve, head DOWN to LKO from Minmus and make your departure burn as low as possible above Kerbin's atmosphere to save fuel.

[@Epicshadow123456789]

Dude, no matter how little the effect, something like Cassini altered the orbit to a point that it would be different than what it would have been in a billion years.

[@ALLTHEAC3S]

0.21 is in testing right now, from what has been said by the devs it may not quite be a full career mode but a lot of the system behind it will be added. Kind of a back end heavy update it seems but it may allow modders to make much better career modes ad-hoc until squad adds it.

[@tristenperez2433]

I love you're vids. Your an inspiration to me. Keep up the good work!!!!

[@bananas401k]

scott, you can get a gravity assist from the moon out to minmus, then use minmus to get a really good gravity assist from kerbin

[@Toorkit]

Just wanted to play a fun space game, had to learn more physics than I did in a year of school. ಠ_ಠ I also love how you tell people to watch this video... at the end of the video. Good stuff! :)

[@Energya01]

Very nice demonstration, cool to see that gravity-assists are actually possible in KSP! I got lost in all the orbits a few times because of all the other probes flying around though.

[@MattDvc]

One of the best of your videos! More science videos like this plz :)

[@Epicshadow123456789]

I think he means getting close enough to the Mun that the mass of the body pulls you tight enough that it greatly lowers your velocity. I forgot what that technique was called.

[@MadeinHell2]

Well under the assumption that you build the ship in space and provide it with full tanks out there you are working under somewhat different conditions. Assuming your engines are efficient you'd probably have a lot of thrust to go around, unlike in situations where you have to make a rocket massive enough to get off Kerbin and THAN get into interplanetary space. If the ship was built around Minmus than you could probably use Kerbin for the escape "push"

[@FjrWizard]

I think he's pretty happy with that outcome, he did after all finance the whole thing.

[@scottmanley]

Landing on the mun and returning takes more fuel than the Kerbal X can provide.

[@GJames1212]

I usually use the Mun to assist me into interplanetary space, but only so I can ditch the last stage onto a collision course with it. Keeps the debris down and I get to see more explosions...

[@deepspacemachines]

Yup, that's exactly the same method as speeding up, with the only difference that you are transfering energy (momentum) from your ship to the planet. You slow down, the planet gets imperceptibly faster (because it is MUCH heavier) (If it wasn't around 3 am I'd be more precise, but in the meantime you could look it up somewhere) Sincerely Marrus with a Q

[@MC3craze]

You could use the Oberth effect while orbiting the Sun, since it has a deep gravity well. To actually get low enough to take maximum advantage, you would have to spend a great amount of fuel (however, a bi-ecliptic transfer can cut the fuel cost down.) If you're trying to escape the solar system, a more practical way would be to take advantage of Jool's strong gravity, and use the planet to perform a gravity assist.

[@Zorbeltuss]

For fuel efficency a munar assist might most of the time be a bad idea, but for an Ion probe you might want to do it to save time, also i would love a followup.

[@newunit18]

Scott, could we possibly get a video outlining various ways of optimizing delta V that you use? this was a great start but I want to know all the little tricks you use to make it to six planets on a tiny ass tank of fuel. Thanks! :)

[@bhaveshtailor3613]

Didn't know this game existed but wanna try now. Looked at this on ISRO website and wanted to know if I could replay the mission.... Third earth bound orbit raising maneuver for Chandryaan-2 spacecraft has been performed successfully today (July 29, 2019) at 1512 hrs (IST) as planned, using the onboard propulsion system for a firing duration of 989 seconds. The orbit achieved is 276 x 71792 km. All spacecraft parameters are normal. The fourth orbit raising maneuver is scheduled on August 2, 2019, between 1400 - 1500 hrs (IST).

[@AzureWind91]

Don't give up! It took me a month of playing this game to make it to the Mun and back successfully...which happened to be last night.

[@pierman84]

You're great Scott! Thank you for the advice!

[@MikeM8891]

The delta v is after burning from Kerban orbit towards the Mun. I calculated it based on the difference in velocity right before entering and after exiting the Mun's SOI.

[@TonboIV]

You spend fuel to go higher, or lower than your current orbit. Getting very low to sun directly from Kerbin takes a huge amount of fuel.

[@TyPeRHSDT]

If you mean, where does he get all that additional energy with the planet assists? then he uses planets with high mass and their gravity that adds that energy to your spacecraft. His spacecraft picks up more energy/speed because he flies through the perigee of the planets orbit. basically he keeps adding more and more speed with the planet encounters.

[@FatLobyte]

Are you a wizard? Because this looks like witchcraft

[@CheffBryan]

No, what's impressive is learning a very basic thing, and taking it to the point where we use it to throw ourselves to the surface of another world. We just lengthened the trip with some robots. Hell, we haven't even gone back to the Moon, even with the useful resources for cold Fusion up there.

[@Remus3777]

Once again scott, an amazing video

[@bar56]

this is just what i wanted. thank you Scott.

[@MadeinHell2]

After some point I'm pretty sure I'd be able to make breakfast, eat it, go to work, return and I'd still be docking.

[@tavert]

Did you see the Metaphor method of placing multiple maneuver nodes to forecast multiple orbits ahead? Was hoping you'd show that here.

[@acephantom903]

I do sometimes use the Mun to get into interplanetary space. I use a gravity assist to go to Minmus and then a 100m/s burn at 7km above Minmus and I can easily make it to Duna or Jool. I honestly haven't been able to make it to Eve or Moho.

[@MrArbreFleuri]

Hey Scott, big fan here, I'd love it if you showed us how to use gravity assists off of Jool's Moons!

[@Mallow-568]

The caps gave it away? :P The other 3 ships which didn't gravity assist past Kerbin's moons made it to Laythe with lots of fuel left, however the 2 that gravity assisted barely got any boost, and I was nearly close to running out of fuel on landing, heck one didn't even have any fuel on landing.

[@wraith444]

On the one hand I love the maneuver node editor, but on the other I don't want to be tempted to use the autopilot... Gah, Mechjeb, why must you tempt me so...

[@a7i20ci7y]

The gravity of Kerbin is greater than the moon. The oberth effect will also be greater. If you're in the neighborhood anyway, may as well swing around the big one.

[@sunjaeyoo7502]

the order i used: eve assist, 3 kerbin assists, and one jool assist to escape do a video on that!

[@mattkab2]

You might be able to slingshot around some of the moons, but (and I haven't done the math on this) I think the moons have too little mass to be really effective. It's probably better to rely on the Oberth Effect around Jool, since it's so massive

[@TheFaark]

You always have to burn until your orbit escapes kerbin's soi, if you want to travel further^^ The only problem with a straight up approach is that its periapsis is within kerbin, so you can't burn there. Not burning at periapsis is less efficient, esp if its 600-700 km away like at straight off kerbin. I just thought that this could be evened out by the d/v you save due to not raising your periapsis... at least in some cases/planets, maybe. But that obviously was wrong.

[@CheffBryan]

It is when compared to the vastness of space, or how long the universe has existed, and the ideals fought for are definitely petty in most cases.

[@Epicshadow123456789]

It's common practice in KSP, people actually use the moon as a source of a form of aero-breaking without an atmosphere.

[@CheffBryan]

We walked on the Moon 44 years ago, and the farthest we've come is games that simulate space travel. It took us just a bit more than that to go from the first powered flight to walking on the Moon. Kind of depressing we haven't progressed, as a species, at the same rate.

[@TrikeSquadron]

it would be nice to have a tool to calculate multiple encounter slingshots like that... but even with that, this kind of thing is realistically only useful for probes. I mean, if the kerbals actually have to breath and eat. and maybe would like to get back before the decade is out. Interesting seeing how you do it though.

[@daemonCaptrix]

It's terribly inefficient, but you can launch the interplanetary ship to Munar orbit, then launch a refueling craft. It doesn't save fuel to use the Mun this way, but it does increase leftover Δv.

[@MrJasper9393]

The thing is it's really not a lack of interest or money. The problem is efficient space travel. To send multiple astronauts to Mars and back takes an enormous amount of fuel as well as enough food for the entire journey. Without a more efficient method of travel it's very difficult to go much farther than we've already gone

[@SteveChisnall]

What's the greatest number of gravitational "keyholes" you've successfully lined up in a row?

[@HebaruSan]

+Steve Chisnall Allegedly the kids on the forums (mainly user PLAD) have been doing Kerbin-Eve-Kerbin-Kerbin-Jool for a while now. There are claims this can be done for 1011-1051 m/s.

[@trevoroutlaw6338]

Love your videos Scott.

[@jaiotu]

Cassini did this. It performed two flybys of Venus followed by a flyby of Earth and Jupiter before arriving at Saturn.

[@ZayneAdmiraal]

Most if not all NASA spacecraft have used gravity assists to some degree, especially the Voyager, Cassini and New Horizons probes.

[@AlternativeShae]

**looks up KSP slingshot in youtube** **this video goes to the very top video** **clicks it** **notices nothing is related to the slingshot part**

[@C0N72]

Not unless the mass of the ships getting assisted are massive. I'm talking a high percentage of the mass of the Earth. You have to remember that during a gravity assist, the gravity of the ship acting on the planet is so small it can be assumed to be zero so the effect on the planets orbit is so small it can be ignored. It's like saying the gravity of a helicopter in flight is towing the Earth. While technically true, the effect is so small it can be safely ignored.

[@A02034040]

Well done sir. More of the instructional (and entertaining) vids please.

[@sirloinofbeef9683]

sorcery! u got out and push didn't you

[@SirSnax-a-Lot]

Asking me to watch the video, after I already watched the video: Classic.

[@jeremiahdauphinais7186]

Scott Manley , I understand that you this video is 2 years old. I also understand that you took a lot of time to make us this wonderful video. After 200hrs in KSP, I'm finally at the point in my KSP experience to take advantage of this videos subject. I am still a little confused on how I can calculate the DV output from a gravity assist. Could you do a short video of maybe a mun gravity assist just to show the simple theory behind it and how much DV one could guesstimate from such a maneuver?

[@seanbush5313]

Jeremiah Dauphinais its hard to say, honestly you just have to try it your self with the infinite fuel cheat enabled

[@jeremiahdauphinais7186]

Thank you!

[@OrdinaryLatvian]

JUNO is going to use an Earth gravity assist to go to Jupiter, and Voyager used some gravity assists.

[@Hmshood1809]

Heya, just wondering g why you had to get so many assists for this. I got a probe to Jool with only 1 assist from Eve.

[@timfidotru]

In order to make a planetary gravity assist, you should first be able to escape a planet and to travel interplanetary, likewise, to make a stellar gravity assist, you ought to master interstellar travel first. (Which would probably have completely different principles, that make conventional maneuvering obsolete)

[@lanog40]

As coolguy2x10^7 said, there are errors. First off, there is a math error. It says the traveling will take ~68 years to complete. Secondly, Kerbol's SOI (Sphere of influence) is infinite, so your spacecraft also hits a bunch of physical errors.