Gravity assists: do you use them?

[Hyomoto]

I like using them, especially if I'm coming back. Take Minmus for example, a burn that would take 57 seconds to reenter Kerbin's atmosphere and still have a 1,500km AP can use a 27 second burn to meet up with the Mun, and use that to slow you down enough to reentry with an AP of 400km. Like anything, sure you can build a rocket to do X by just making it larger, but a lot of times it's just fun to not. I almost ALWAYS use a free return trajectory for my trips to the Mun so I leave no debris in space, just as I almost always use the Mun to exit Kerbin's SOI. Using planets to push yourself out further and further is also just fun, it's like playing galactic pinball (or something). It's a game all in and of itself.

I even had one trip where I was using Duna to pull myself into Eve's orbit (for science!), and I caught not only Duna's SOI, but also Ike's. That was a sweet mission.

[PLAD]

I use flybys all the time, maybe a little too much. As people have pointed out, it's quick to add more dV to a craft, but it takes a lot of precious real-world time to execute an overly complicated mission.

But oh the things that flybys can do...

I've made it from Low Kerbin Orbit (LKO, for me 75kmx75 or less) to Jool orbit for 1011m/s.

I've made it from LKO to Eeloo's surface and back for 2897 m/s.

I've made it from LKO to Mun's surface on 1644m/s and back to Kerbin for 813m/s. (That took 26 game days and about 20 real-world hours!)

Metaphor made it from Kerbin's surface to Moho's surface for 7217m/s.

And for the ultimate in using difficult flybys to save a little dV check out where Vector made it from LKO to Eve's surface for 873m/s! I haven't had the guts to try and use his trick to get to Jool for around 900m/s but I bet it could be done.

I use Arrowstar's TOT and my own Flyby Finder and Lambert spreadsheet to plan the paths. Note that I know of no tool that can plan flybys of moons, just the planets.

Some of the links go into great detail on how to use flybys. Good luck!

Edited November 14, 2014 by PLAD
typo

[Red Iron Crown]

^^ That is pretty darned impressive, both in planning and patience.

Edited November 14, 2014 by Red Iron Crown
How do I grammar?

[problemecium]

In my dreams, I use gravity assists out the wazoo.

In real life, well... I don't have all that much more patience than other KSPers, and I end up projecting that onto my Kerbals. I don't want them sitting around bored for months while I timewarp, so, uh... long story short I finished my 0.25 career in one Kerbal week. No really, I checked the UT: Day 7. So I have yet to do anything that would involve a gravity assist xD

[turkwinif]

The only time I've used gravity assists is when I'm sending a Voyager-like probe out of the system using Jool

[Slam_Jones]

I never really plan on using them, but if I encounter one while making a burn, I'll usually take advantage of it to save some dV.

[Stratzenblitz75]

I use gravity assists every time I can. They can greatly reduce the amount of Dv required to perform any given transfer, and therefore, you can use a smaller ship. Yes, I agree that gravity assists can be tedious to set up, but this is offset by how satisfying it is, at least for me, to execute them. Plus, I find it entertaining to flyby a planet/moon 1 km over the surface at 2 km/s; makes for some thrilling scenes.

This is why I like the Jool system, there's just so many opportunities for gravity assists. Tylo, Laythe and Vall are so massive and so close together (relative to the other planets), it makes gravity assists easy and powerful. Entering the Jool system at an inclination and want an equatorial Jool orbit? Gravity assist + aerobrake over the pole of Laythe. Leaving the Jool system? Gravity assist at Tylo or Laythe. Want to go from Bop or Pol to an equatorial orbit around Laythe? Gravity assist at Tylo to fix inclination and lower periapsis, then aerobrake at Laythe. If you plan it right, you can get close flybys (close enough to get "in space near" science reports) of Tylo, Laythe, and Vall while spending less than 50 m/s Dv. Its fun!

The Mun is also a useful place to gravity assist. It can save you a decent amount of Dv when going to and from Minmus. The Mun is also useful when transferring to Eve or Duna. Set it up correctly, and you can shave about 100 m/s off of your Eve or Duna departure burn, which can mean a lot if you're carrying heavy equipment (such as an Eve lander). I've personally never tried a free return gravity assist off the Mun, but its on my ever growing to-do list.

Planet gravity assists are much harder to set up as they require a lot more precision than moon gravity assists. I've only done one planet-planet gravity assist before, and that was Duna-Duna gravity assist because my lone NERVA wasn't powerful enough to get a Jool transfer from Duna orbit (even after multiple periapsis kicks). I basically just increased by Solar apoapsis so that my Solar orbit had double the period of Duna's orbit. Therefore, when I reached Solar periapsis, I would encounter Duna and perform a gravity assist. Even then, it wasn't enough to get a Jool intercept so I had to perform a small burn. The whole ordeal saved me about 150 m/s of Dv, which isn't a lot but I had no other choice since my Dv budget was so strict.

That being said, I'm looking forward to doing more planet gravity assists in the future. PLAD is definitely inspiring me...

[OgreMagi]

haven't used them much, used jool to fling out a few probes but mostly its rockets or aerobreaking

[Mitchz95]

I use them to get captured in, and ejected from, the Joolean system. Aerobraking is a big no-no since I use DRE, and my ships tend to be relatively light delta-v wise.

[Aragosnat]

I know I should use them. Just most of the time I forget about them to don't want to try and find out when to do a burn as I'm keeping my mod count down to very very little.

[Jarin]

I hardly ever used gravity assists until I did my first Joolian expedition. I had so much fun pinballing around the Joolian moon system that I got addicted to gravity assists.

[LABHOUSE]

Use them were th my orion for rss mars missions (still needs refuelling).

[numerobis]

PLAD, that's impressive.

One thing I worked out is that the conic patch approximation improves gravity assists rather a lot compared to IRL: V_inf on one side will be equal to V_inf on the other. If you were to enter Mun SOI going directly away from Kerbin, and exit on the other side after 30 minutes still going directly away, you'd have saved about 40 m/s from Kerbin not pulling on you during the transit. If you bend 90 degrees at Mun, still, you're saving 20 m/s from the conic patch approximation.

[NFUN]

This thread partly inspired me to do one earlier today. I had little cash on my career save, so I built a cheap vehicle that flew by Mun, Minmus and then Mun again. So many crew reports, so much science. I really should try to learn how to use them better.

[Kamuchi]

I always try to do an aerobreak on Kerbin, even just for fun

I don`t go out that much yet, so much to screw around with on Mun and Minmus and base building

[Kamuchi]

Assists are also great to help fix your orbit inclination:

Fixing a 78 degree inclination with a 367ton vessel over Kerbin would cost you nearly 2900dV and then you still need to add the dV for the geo burn and circularization.

Wait till the Mun is in position and tinker the best node you can get out of it, you can save a couple thousand dV with heavy ships/asteroids and getting into the desired orbit is peanuts as you`re far out

[PLAD]

One thing I worked out is that the conic patch approximation improves gravity assists rather a lot compared to IRL: V_inf on one side will be equal to V_inf on the other. If you were to enter Mun SOI going directly away from Kerbin, and exit on the other side after 30 minutes still going directly away, you'd have saved about 40 m/s from Kerbin not pulling on you during the transit. If you bend 90 degrees at Mun, still, you're saving 20 m/s from the conic patch approximation.

That is an interesting insight. Almost every time I do a Munar flyby on my way to Eve I have to do a 10-20m/s correction afterwards, no matter how precisely I try to do it. It reduces the 100m/s or so savings it looks like I'm going to get to only 80-90m/s when corrections are included. If KSP's path-predictor doesn't take this into account it would match the magnitude of the error quite nicely. Some experiments and number crunching are in order here...

And thanks. At least 90% of what I know about KSP I got from reading other people's posts.

[Ikaneko]

I used one in career mode once.

I had little fuel to return from Minmus, so I looped the pe down to 11mill and got an assist down to kerbin from the Mun.

I got lots of science from that mission: entire kerbin system flyby FTW!

[Red Iron Crown]

One thing I worked out is that the conic patch approximation improves gravity assists rather a lot compared to IRL: V_inf on one side will be equal to V_inf on the other. If you were to enter Mun SOI going directly away from Kerbin, and exit on the other side after 30 minutes still going directly away, you'd have saved about 40 m/s from Kerbin not pulling on you during the transit. If you bend 90 degrees at Mun, still, you're saving 20 m/s from the conic patch approximation.

Can you expand upon this? It seems to me that the velocity of the Mun would have changed direction in the time it takes to transit Mun's SoI, approximating Kerbin's gravitational influence on the vessel when it reemerges from Mun's SoI and has the velocity added back on.

[lajoswinkler]

Yes, very often, but you need to be very precise in order not to change the inclinations a lot.

[Caelib]

I would like to use them, but I feel like the maneuver node system needs to be expanded to some sort of pre-launch "mission planning" phase where you can setup a chain of maneuver nodes by phase-angle instead of orbit/time.

KSP is played in so many variety of ways, but I would love something like this:

1) I want to send a probe to Tylo

2) Open "Mission Plan" UI (use your imagination on how this might appear)

3) Plot THEORETICAL maneuver nodes: Kerbin > Moon (gravity assist) > Duna (gravity assist) > Jool (gravity assist) > Tylo.

4) When pressing "launch" button from VAB, SPH, Runway or Launchpad, user would be given an option to select a "Mission Plan". The game would then time-warp to 10 minutes before the launch window and user would see the first maneuver node counting down while on the launchpad/runway.

In summary, this would be a procedurally-generated set of maneuver nodes. It would also be amazing if it would list the Delta-V requirements for the mission and if stock KSP would show stage-based Delta-V and TWR in the VAB/SPH.

Some may argue this is very MechJeb-like ... I don't disagree with that assessment, but it removes a lot of the guesswork from mission planning.

Edited November 15, 2014 by Caelib

[Superfluous J]

Can you expand upon this? It seems to me that the velocity of the Mun would have changed direction in the time it takes to transit Mun's SoI, approximating Kerbin's gravitational influence on the vessel when it reemerges from Mun's SoI and has the velocity added back on.

In a simple 2-body solution (which we have in all situations in KSP) the speed you are going at a certain altitude, you will be going every time you are at that altitude.

So, you enter Mun's SOI at the near side of Kerbin going (say) 1000m/s relative to Mun, you will exit the other side going 1000m/s relative to Mun. Relative to Kerbin, you are now (about) the full diameter of Mun's SOI further away and going the same speed you were when you went in. If Mun hadn't been there, Kerbin's gravity would have slowed you down. Instead, Mun sped you up (on the way down) and then slowed you down (on the way up) to end up at exactly that same 1000m/s.

[Red Iron Crown]

So, you enter Mun's SOI at the near side of Kerbin going (say) 1000m/s relative to Mun, you will exit the other side going 1000m/s relative to Mun. Relative to Kerbin, you are now (about) the full diameter of Mun's SOI further away and going the same speed you were when you went in. If Mun hadn't been there, Kerbin's gravity would have slowed you down. Instead, Mun sped you up (on the way down) and then slowed you down (on the way up) to end up at exactly that same 1000m/s.

I get that part, but you're not accounting for the change in the SoI's velocity vector direction. The velocity subtracted when you enter the SoI is not in the same direction as that added when you exit, because the SoI has moved relative to Kerbin.

[Superfluous J]

I get that part, but you're not accounting for the change in the SoI's velocity vector direction. The velocity subtracted when you enter the SoI is not in the same direction as that added when you exit, because the SoI has moved relative to Kerbin.

You can make it close, though, if you go fast enough (Going to Moho, for example, takes a pretty hefty burn and you can fly by Mun pretty straight). And they don't perfectly offset or anything, it's not a 1:1 thing. In fact you can tweak your Mun encounter to maximize the benefit or (even better) come out going forward along Mun's orbit and get the added benefit of Mun's orbital velocity to your own.

[Red Iron Crown]

You can make it close, though, if you go fast enough (Going to Moho, for example, takes a pretty hefty burn and you can fly by Mun pretty straight). And they don't perfectly offset or anything, it's not a 1:1 thing. In fact you can tweak your Mun encounter to maximize the benefit or (even better) come out going forward along Mun's orbit and get the added benefit of Mun's orbital velocity to your own.

If you're going fast enough, then Kerbin's gravity wasn't going to have much effect on your speed during the time you are in Mun's SoI (reflected by the small change in direction of Mun's velocity). I understand about using the Mun's orbital velocity to your advantage, I just don't see how patched conics saves you from Kerbin's gravity to any significant degree.