Gravity Assists | KSP (Not) Beginner's Guide

[@MikeAben]

Up Next: Aerobraking and Aerocapture - kzbin.info/www/bejne/Z6LEl56wmN59qqM

[@SuperiorThirteen]

Incredible, I was JUST thinking how gravity assists just don't click with me, hoping for this to pop up in my sub box. Thanks Mike!!

[@davidfordyoyoguy]

You really are the best KSP teacher, IMO! I had just recently figured all of this out, and you kind of confirmed that I was thinking about it right.

[@atlas4733]

If your gravity assist is stronger if closer to the body, and the ground is limiting how close you can get, imagine a gravity assist skimming past the event horizon of a black hole...

[@therickroller2358]

Just don't touch an inch past the event horizon or there won't be any need for the gravity assist bc your not leaving

[@samael4550]

I mean, if future humans ever manage to make it all the way to a blackhole, it’s safe to assume they’ll have figured out a way to travel near or at the speed of light. Just imagine how fast the resulting black hole assist would be… and imagine the G force at periapsis 😂 I feel like you could escape a galaxy like that, though that’s just a guess I have no clue how far you’d actually be able to get

[@Storywalker4]

Well, tidal forces and radiation go brrr. With something that dense, being that close to it will rip things apart.

[@neighbourhoodguy8644]

Bro you are the man, all these years i have just been doing gravity assists by predicting my resulting trajectory. I have never found a better tutorial that shows that the conic patch limit settings exist. Thank you so much.

[@alartor]

Finally a video with clear indications on how to plan a gravity assist! (beyond the "trial-and-error / see what would happen if..." approach). That "retrograde from inside / prograde from outside to slow down - and vice versa to speed up" is really the key to it (on a model with orbits and rotations as in KSP). Though some other videos around go deeper on the actual physics and math of it, none had nailed it as clearly in a practical aspect as this one. Once again, thanks, Mike! Awesome job! Side note / question: for mid-course corrections like the ones in this video, isn't it usually better to use radial and normal instead of pro/retrograde? (note this is just the top off my head, without giving it much thought to this specific case). Thanks!

[@MikeAben]

Thanks. As to the question. I usually play with both and see which is more efficient.

[@JohnWilliamNowak]

Let's also remember that stock KSP uses patched conics to approximate gravity. So it's very possible that something which would work in real life wouldn't in KSP.

[@Torbjorn.Lindgren]

@@JohnWilliamNowak At least as importantly, KSP doesn't even model gravity "correctly" in the first place. To avoid having to do N-body problem (which has no "solution") it always only applies gravity from the most important body at any one time, that's what SOI is - the point where which body KSP uses to model gravity changes. In real life there's no "SOI" as such and any trajectory needs to account for multiple gravity sources, the more precision you need the more bodies you need to include and because there's no analytic solution for N-body problems the result can only be approximated via an iterative approch - the smaller the time-step used the more processing power is required but the closer the number will be to reality. This gets really complicated (IE slow) when there's lots of nearby bodies like near Jupiter. There's a number of other relevant simplifications in KSP, for example all maneuvre node calculations assume the thrust is instantaneous, this is why the delta-V figures in the earlier parts were so far off what the burn actually needed (since they weren't instanteanous the code vastly over-estimated the Oberth effect on the burn). However, it's my understanding that the basic principles involved in these gravity assists do work in real life, it's just that none of the numbers are "real" even if you use the "Real Solar System" KSP mod. Which is fine, it's a game, not an universe simulator. There IS the Principia mod for KSP which according to the documentation modifies KSP to do proper N-body simulation and adds many related things including "flight planning" (which includes that burns take time), but as expected the documentation hints that things gets a LOT more complicated. I've not tested it so I can't say how this actually plays out. So I expect that RSS plus Principia should be way closer to reality but I wouldn't be surprised if there's still enough differences that NASA and other space agencies still have to use their programs to do fly-by planning rather than via KSP+RSS+Principia... But who knows.

[@thegamesforreal1673]

​@@Torbjorn.Lindgren N-body problems indeed cannot be solved analytically, but they can be solved numerically up to desired accuracy if you're willing to put up with long computations. To handle a problem like Jool's system numerically with proper gravity calculations, you can easily argue that it's no big deal to disregard the low-mass moons like val, bop and pol unless you are specifically getting close to them, leaving you with a (mostly) 5-body problem: Jool, Laythe, Tylo, your own spaceship and the sun. Such a problem can be solved numerically well enough.

[@_RandomPea]

Almost as though you predicted my next question! Thanks Mike this will keep me busy

[@_RandomPea]

You nailed that burn ! Nice job 👍

[@FranklySean]

Finally, I found a video that explains gravity assists. Scott Manley was less than useless. Great job, sir.

[@OwOraTheWitch]

Thank you so much for making this video specifically. Up until this point I had no idea functionally how gravity assists worked, and I watched basically every video Scott Manly put out on the topic. But after watching this it finally clicked, so I just wanted to say thank you so much and keep up the good work. As someone who plays KSP I'm sure you realize how great of a feat it is to make a video that's more educational than Scott's.

[@MikeAben]

Glad it helped.

[@bikingchad]

You really have a flair for these tutorial videos! Keep up the great work until KSP2 drops and your channel should take off. 🤞

[@ryuk5673]

Thank you for your extremely helpful videos. I thought this technique was too advanced and complicated for me and u do a great job simplifying things

[@mgalyean]

Thanks for the vids, great job. I do have a different understanding of gravity assists though. If you overtake the moon in the same direction as it is orbiting you will get a boost, if you approach going the other direction, head-on, it will slow you down. The rotation of the body only matters if you are aerobraking as in KSP the atmosphere rotates with the surface. So you get a stronger aerobrake effect at the same altitude of you hit it on a retrograde trajectory

[@federationairspacefas3808]

15:18 I'm so glad I'm a nerd because I know what you were talking about it looks complicated but it looks pretty easy

[@anthonygamer]

Thank you for these guides! This game is tough to learn.

[@jimcrall]

Mike Aben has the best KSP tutorials on youtube by far. Thanks for another great one!

[@MikeAben]

You're welcome.

[@stardaggerrihannsu2363]

Professional development, thanks again to the Great Master!

[@dansiegel995]

Mike et al, RCS can indeed be thrust limited, so its always the thruster of choice for final precision. Also, consider disabling unneeded engines, so as long as you can maintain symmetric thrust through your CoM. This is even easier to accomplish with RCS, as large ships will have many RCS thrusters all over the place. You only need 2 of those RCS thrusters on if burning in a single direction. Well, actually 2 directions, forward and backwards (which is useful to correct overcooking the burn).

[@MikeAben]

Good idea.

[@tTRITUS]

6:52 "slowing down side" and "speeding up side" are certainly interesting layman terms. would probably make more sense if you just said that the orbit line goes opposite of the planets spin.

[@MikeAben]

Which is exactly what I begin talking about after this moment.

[@Ion115]

Now I can finally read those sci-fi screens with orbits in star wars movies

[@Bretaxy]

Been looking forward to this one.

[@emiliogarza6446]

really great tutorial as always

[@MikeAben]

Thanks

[@_Zeicom_]

Best youtuber ever

[@thechorus8981]

Thanks you so much, for years I tried to find a way to show on screen the red trajectory with mods and it's just an option 😭😭 why did they no enable this by default ?

[@thestrangecrisismalachi4121]

I was thinking about slingshoting from Eve to Jool.

[@Jicklenop]

I'm working on my first ship to send off to Jool and this will be incredibly helpful. One question I have is when you built yours, did you still include all the delta-v for a full jool capture? Or how much did you budget for the overall capture? (Based of the delta v map you shared earlier in the series that is)

[@MikeAben]

It's got 6128 m/s of delta-v. By the delta-v map, without any aerobraking or gravity assists, the total to get to low orbit about Lathe and back is 7390 m/s. Even with maximizing gravity assists and aerobrakes, the total from the map is 5380 m/s. I think I just took that number and added a buffer.

[@Jicklenop]

Thanks for the reply! Helps a lot, as well as all your series, keep it up

[@ferguson4096]

Does my moon gravity assist need to be after my Jool Periapsis? All of my moon intercepts are before the Periapsis and any burn after a moon gravity assist will need to be hundreds of dV to plot another moon intercept. None of the available trajectories seem to be helping me out at all. Thanks.

[@MikeAben]

The assist can be on either side of Jool's periapsis

[@ferguson4096]

@@MikeAben Thanks!

[@matthewp4046]

Not sure I see how the spin of the body has any effect on things orbiting it… I had assumed it was all about angle relative to path

[@MikeAben]

It doesn't, but the orbital direction does and the spin happens to be in the same direction.

[@MystDaLow]

@@MikeAben Why mention the spin at all then?

[@MikeAben]

@@MystDaLow It's an easy reference for which side of the body you are passing on.

[@MystDaLow]

@@MikeAben oh, i see :-) thanks!

[@kosairox]

Can I ask what's the reason to approach Jool from the "outside" (effectively slingshoting us to a higher orbit)? Wouldn't it be better to approach Jool from the "inside" to slow us down?

[@MikeAben]

If by outside and inside you mean the prograde and retrograde sides of Jool, it's definitely better to come around prograde (which I did) as that is the direction all the moon's are orbiting. If you're concerned that I swing around Jool before encountering Lathe, that doesn't matter. You don't get a "slingshot" effect from Jool unless you leave Jool's SOI. My encounter with Lathe would be at the exact same speed whether I encounter it before or after my closest approach to Jool.

[@scottwilliams846]

I want to make a scenario for my younger brother to play and I need to know how to set up a series of gravity assists from Jool's moons to create an orbit that doesn't take you out of Jool's SOI, and instead just passes you around between the moons in an infinite loop. How might I do that? Also the scenario if aliens contact you from Jool and you must prepare for war with no respawning Kerbals. There will be 4 Joolean motherships with 9 starfighters each (I don't know how to make them shoot fireworks when you're not directly controlling them) and you must either eradicate them or get them to run out of weapons and they surrender. I have no idea how to do that though, since I can't find a mission creator. I also wanted to start the player at a low tech level, but don't know how to do that, either.

[@geoffreyporter6373]

There is a function in the Mission builder that lets you pick a random part to explode, or damage. So you could set it up to cause a fuel tank to lose fuel, or change the point of control to a different direction, so you have to figure out what's the new reference.

[@scottwilliams846]

@@geoffreyporter6373 how do I access the mission builder? Can I even without a DLC?

[@geoffreyporter6373]

@@scottwilliams846 Oh, sorry, the mission only comes with the Making History DLC. If you're not on console, you can look into BDArmory.

[@MadalinCH.]

for some reason every time i do gravity assists i still need about 3-4k d-v to circularize at laythe. Not helpfull...

[@tabbycat6802]

0:38 A ton of fuel is not that much in rocketry... more a droplet.

[@sebdapleb1523]

this mike guy is such a sigma male, on his #tutorial grindset

[@n111254789]

What the hell does that even mean?

[@windowsxseven]

@@n111254789 lmao clueless

[@n111254789]

@@windowsxseven Yes, it's why I asked. Tf is a sigma male on a grindset