what to expect from aerocapture?

[fommil]

I've been trying to find if anybody has made a chart or simulation for rules of thumb for how much deltav one can reasonably expect to shave off from a full insertion burn from an aerocapture around Laythe, Jool, Duna, Eve and Kerbin.

I can imagine there being too many degrees of freedom to make it realistic, especially since the vessel's drag and thermal characteristics are the limiting factor and hard to quantify (besides mass and Pe approach speed which are pretty easy to obtain).

Edited November 2, 2022 by fommil

[Lt_Duckweed]

With enough fine tuning, patience, and careful craft design, the answer is "all of it".

In general, you are limited by the heat tolerance of your craft, the body you are capturing at, and how much drag you can generate.

As long as everything is behind a good heatshield, you can usually get the capture pretty much for free.  With a spaceplane, you will have to propulsively capture at Eve, Jool, and Kerbin if coming from a distance much greater than a Duna or Eve return.  Once captured, you can of course do as many gentle aerobrakes as needed to do the rest of the aerobraking for free.

[Gargamel]

Moved to gameplay questions. 

[fommil]

2 hours ago, Lt_Duckweed said:

As long as everything is behind a good heatshield, you can usually get the capture pretty much for free. 

What I'm interested in is understanding the limits, so that I can design for it without leaning too much on the debugging cheats like set position/orbit (aka Human Space Program simulator). e.g. if I know my craft's drag at 10km at 1k / sec at kerbin (should be enough to extract the coefficients for the underlying cube based model), how does that translate into other planets. If I know that my craft starts to burn up at 15km at 1.5k / sec (i.e. the safe thermal zone of my cube based model) then I could use that to work out what my lowest altitude should be at Duna if I'm coming in at X k/sec, kinda thing. But I can appreciate that it would be too complex to simulate that. If the KSP model was more documented, I'd be tempted to code this up myself! (Disclaimer: I've worked on atmospheric reentry projects with ESA and I find this sort of thing to be more fun than I really should.)

I'm really only interested in the initial capture, not the subsequent aerobrakes because they are pretty much a given.

Edited November 2, 2022 by fommil

[Vanamonde]

There isn't a simple answer for this because it will depend on the drag the craft in question generates. 

[king of nowhere]

4 hours ago, fommil said:

What I'm interested in is understanding the limits, so that I can design for it without leaning too much on the debugging cheats like set position/orbit (aka Human Space Program simulator). e.g. if I know my craft's drag at 10km at 1k / sec at kerbin (should be enough to extract the coefficients for the underlying cube based model), how does that translate into other planets. If I know that my craft starts to burn up at 15km at 1.5k / sec (i.e. the safe thermal zone of my cube based model) then I could use that to work out what my lowest altitude should be at Duna if I'm coming in at X k/sec, kinda thing.

way too complicated to simulate. you are much better off going by trial and error here, reloading and trying with different periapsis.

I can, though, give a lot of pointers you may find useful.

Most important thing to consider is that when you arrive at a planet from interplanetary, you then fall towards the planet. so when you enter atmosphere you pick up speed roughly close to the escape velocity of the planet. which is about 1,4 km/s for duna, 2.7 km/s for laythe, 3.2 km/s for kerbin, 4.5 km/s for eve, give or take. you add that to your intercept speed.

so, for example, you simulate that you'd need a 600 m/s burn to capture around duna (and stay in a high elliptic orbit)? this means you'll enter duna's atmosphere at roughly 2 km/s. if your ship will survive entering kerbin's atmosphere at that speed, it will survive at duna too, and you'll be able to aerobrake all the way. on the other hand, doing the same on eve means facing a reentry at 5 km/s, which is only feasible with excellent thermal protection.

generally speaking, if your ship cannot survive the heat of your reentry, then going for a shallower trajectory is not worth the effort, you can at most save a few tens of m/s on capture. what you can do is rocket brake part of the way before entering atmosphere. for example, your ship can survive reentry at 2 km/s and you're coming to duna with 1 km/s intercept speed (which means hitting atmosphere at 2.4 km/s)? rocket brake for 400 m/s until you're at 2 km/s, then enter atmosphere. you'll get some inefficiencies because you're not burning at periapsis, but you can still get aerocaptured.

So the actual drag coefficient of your ship matters nothing. what matters is the maximum speed at which it can survive. which is something you can test on kerbin too. if you can survive reentry at a certain speed, then you can aerocapture all the way and it's just a matter of finding the right periapsis - which is something you can only do by trial and error, basically. calculating it would be too complex

normal altitudes for aerocapture are: 40-50 km for kerbin, 20-30 km for duna, 35-45 km for laythe. i never tried aerocapture at eve, but i guess 60-70 km would probably be it.

laythe is a bit different from the others because its atmosphere is weird, it gets very dense very fast, to the point that even though it is 20 km shorter than kerbin's, it is more dense at equal altitude for most of the way. while other atospheres start gradually, laythe atmosphere is like kerbin's atmosphere with the top 20 km removed, and it starts very abruptly. so aerobraking there requires some extra heat resistance.

[camacju]

If you're open to installing mods, there's a mod called Trajectories that will show you what your aerobrake path looks like based on your craft's aero characteristics.

[fommil]

7 hours ago, camacju said:

there's a mod called Trajectories that will show you what your aerobrake path looks

yeah it's pretty cool, especially to speed up the Human Space Program simulator (aka debug position/orbit) but it has a couple of flaws that make it unreliable for me. It doesn't support airbrakes (which was a critical part of my Eve missions), doesn't support upper stage projections (although I can always design accordingly so that I can adjust final approach with side mounted engines or RCS), and seems to get the inflatable heat shield projections completely wrong. But for spaceplanes, it might be just the ticket (airbrakes not feasible at this speed anyway). It also doesn't do the "your ship will start to break up here" projection, which is something I'm interested in.

[fommil]

7 hours ago, king of nowhere said:

I can, though, give a lot of pointers you may find useful.

they are, thank you!

7 hours ago, king of nowhere said:

when you enter atmosphere you pick up speed roughly close to the escape velocity of the planet.

Apologies, I think I was using the wrong terminology. When I was talking about approach speeds, I meant the orbital speed plus the dv needed for a circularisation burn. This is super easy to get from Alex Moon's calculator without even needing to experiment (and I think even some of the more crazy multi-gravity assist tools also give this info).

7 hours ago, king of nowhere said:

if your ship will survive entering kerbin's atmosphere at that speed, it will survive at duna too

but that's maybe a bit too much margin. Kerbin's atmo is much denser than Duna so the thermal increase happens over a much shorter period of time. What I'm also not sure about is what the maximum is at Duna before I run out of air. It's a pretty thin atmo so how much could I feasibly expect to shave off?

7 hours ago, king of nowhere said:

the actual drag coefficient of your ship matters nothing. what matters is the maximum speed at which it can survive. which is something you can test on kerbin too

Oh, wow, that is interesting. Is there a simple calculation to convert Kerbin max speed into Duna / Laythe / whatever max speeds? I would intuitively (but probably incorrectly) assume that I could go faster through lighter atmos like Duna, but would have to be more careful at Eve. Maybe it's a case of noting "I burn up at 1.5k/sec at 40km on Kerbin, which is equivalent to X on Laythe" by consulting the atmosphere pressure charts.

8 hours ago, king of nowhere said:

laythe is a bit different from the others because its atmosphere is weird, it gets very dense very fast

Thanks for the heads up, I was lazy and didn't read the charts! I will study it in detail now.

Thanks for your insights here, it's definitely helping me understand how the drag / atmosphere approximations are working! There is a lot of material out there but it is hard to know which version of the game it corresponds to so I find that even things from 2 or 3 years ago regarding SSTOs are out of data already.

BTW I found https://alterbaron.github.io/ksp_aerocalc/ in  https://wiki.kerbalspaceprogram.com/wiki/Calculation_tools which is a bit out of date, but I'll investigate to see how far off it is since any physics models got updated.

[king of nowhere]

47 minutes ago, fommil said:

but that's maybe a bit too much margin. Kerbin's atmo is much denser than Duna so the thermal increase happens over a much shorter period of time. What I'm also not sure about is what the maximum is at Duna before I run out of air. It's a pretty thin atmo so how much could I feasibly expect to shave off?

 Is there a simple calculation to convert Kerbin max speed into Duna / Laythe / whatever max speeds? I would intuitively (but probably incorrectly) assume that I could go faster through lighter atmos like Duna, but would have to be more careful at Eve.

 

the thing is, atmospheric density only depends on altitude. the atmosphere of duna, for example, is a lot thinner than that of kerbin; however, being at 50 km altitude on kerbin or at 30 km altitude on duna or at 70 km on eve makes little difference. it only means you have to go lower or higher. and this is why you can feasibly aerocapture no matter your speed, as long as your ship can take the heat. I did aerocapture from 10 km/s on kerbin with a 30 km periapsis, I could have done the same on duna with a 10 km periapsis.

there are some minor differences in atmospheric profile, but they are minor; except for laythe. btw, I did not realize laythe is different by reading the charts, but by trying to aerobrake there and basically hitting a wall.

Quote

Maybe it's a case of noting "I burn up at 1.5k/sec at 40km on Kerbin, which is equivalent to X on Laythe" by consulting the atmosphere pressure charts.

you could do that, but as i said, it's got little practical effect. there are only two basic scenarios:

- you burn before you slow down

- you slow down before you burn up

and in the first scenario, a passage in the atmosphere isn't going to help you much, so you may as well disregard it.

For example, if you come at kerbin too fast, you probably can set a periapsis around 55 km and survive. but such a periapsis will slow you by 10 m/s or less, so you can't aerocapture with it.

A good way to estimate aerobraking is to set a maneuver node at periapsis for 0 km/s. when your ship starts braking in the atmosphere, the game will change the maneuver node accordingly telling you that you have to accelerate to achieve the same trajectory. so the maneuver node will start at 0 and gradually go up, telling you how much you brake in the atmosphere.

generally speaking, for any atmosphere there is a sort of boundary line; above it, drag is small and you will make a fast passage and get out of the atmosphere again. below, drag is strong and you will lose hudreds of m/s. of course, the atmosphere changes gradually and so there isn't a well defined line, yet the difference between "you pass with low drag" and "you stop there" is only a few km. for example, on kerbin a 50 km periapsis can be used when returning from mun, and you will get out of the atmosphere having slowed down by a few tens of m/s. A 45 km periapsis will result in hundreds of m/s of braking, but if you're coming from mun it will take you a few passages before you are captured. A 40 km periapsis is generally enough for interplanetary capture. And even the time I came in at 10 km/s from a high energy trajectory, a 30 km/s was enough to stop my pod. Hard. we're talking some 40 g of deceleration. 

Similarly, i aerocaptured a lot of different ships on duna, but it was always with a periapsis between 22 and 27 km. below that, the atmosphere is so dense that it will stop you abruptly. above, it's not dense enough to make much of an effect.

this is why the speed at which you enter atmosphere - and whether your ship can survive it - is the only significant parameter.

 

 

 

[fommil]

45 minutes ago, king of nowhere said:

this is why you can feasibly aerocapture no matter your speed,

ok, I guess if 1km altitude on Duna isn't gonna capture you, then nothing will

47 minutes ago, king of nowhere said:

A good way to estimate aerobraking is to set a maneuver node at periapsis for 0 km/s. when your ship starts braking in the atmosphere, the game will change the maneuver node accordingly telling you that you have to accelerate to achieve the same trajectory. so the maneuver node will start at 0 and gradually go up, telling you how much you brake in the atmosphere.

oooh, this is a really nice hack, thank you. I've been noting my Pe velocity, then decreasing at Ap and noting what it is afterward, which is obviously inaccurate since I didn't measure at my true Pe (on either end of the maneuvre).

Basically my current mission is an SSTO to Laythe. I can carry 3k dv in that thing, without a tug. The Alex Moon orbital transfer cost is 2k from Kerbin (and I still haven't got the hang of gravity assists from the Mun/Minmus on a full tank), which I can do comfortably. The insertion burn to enter Jool (not Laythe!) on an elliptical orbit is quoted at just over 1k in the dv map. Converting to the relative speed to Laythe is tricky but in the worst case, I aerobrake around Jool until I get a pass across Laythe which should be another ~1k aerobrake. OK, so I really never need to aerobrake more than 1k at a time here, so I'm overthinking it. I can do that in pretty high orbital passes. And probably can just do it in one go with Laythe, given that I know I can aerocapture 1.5k dv on Kerbin already.

Refs:

http://alexmoon.github.io/ksp/#/Kerbin/100/Jool/210/false/optimal/false/33/1

https://wiki.kerbalspaceprogram.com/wiki/List_of_orbital_velocities_for_low_orbit

The confirmation I'm hearing from you is that I can aerobrake as much dv as I like if I go deep enough into the atmo, it's just that I need to be super careful about the exact Pe that I pick, so there's no alternative to trial and error once you get there. So no need to do endless Debug runs, like an Eve takeoff needs

Just gotta pick a landing site now. I'm assuming that Laythe has lots of natural runways. At least, Bradley Whistance always seems to be able to land anywhere on any body, so I'm probably overthinking that too.

[king of nowhere]

38 minutes ago, fommil said:

Basically my current mission is an SSTO to Laythe. I can carry 3k dv in that thing, without a tug. The Alex Moon orbital transfer cost is 2k from Kerbin (and I still haven't got the hang of gravity assists from the Mun/Minmus on a full tank), which I can do comfortably. The insertion burn to enter Jool (not Laythe!) on an elliptical orbit is quoted at just over 1k in the dv map. Converting to the relative speed to Laythe is tricky but in the worst case, I aerobrake around Jool until I get a pass across Laythe which should be another ~1k aerobrake. OK, so I really never need to aerobrake more than 1k at a time here, so I'm overthinking it. I can do that in pretty high orbital passes. And probably can just do it in one go with Laythe, given that I know I can aerocapture 1.5k dv on Kerbin already.

Refs:

http://alexmoon.github.io/ksp/#/Kerbin/100/Jool/210/false/optimal/false/33/1

https://wiki.kerbalspaceprogram.com/wiki/List_of_orbital_velocities_for_low_orbit

The confirmation I'm hearing from you is that I can aerobrake as much dv as I like if I go deep enough into the atmo, it's just that I need to be super careful about the exact Pe that I pick, so there's no alternative to trial and error once you get there. So no need to do endless Debug runs, like an Eve takeoff needs

Just gotta pick a landing site now. I'm assuming that Laythe has lots of natural runways. At least, Bradley Whistance always seems to be able to land anywhere on any body, so I'm probably overthinking that too.

the deltaV map is not accurate when it comes to jool. or rather, it technically is, but it assumes that you would capture in a low jool orbit, then circularize, then go from low jool orbit to a moon. which is the worst way you could approach that.

there are better ways to approach jool. the best one, in your case, would be to use a tylo gravity assist for capture, around jool, followed by arrival at laythe with other maneuvers; it could reduce intercept speed on laythe to almost 0 if done well, but it's very difficult to pull off; a mediocre execution, where you gravity capture around jool but end up in an imperfect trajectory for laythe, would cost several hundred m/s, which is still less than 1 km/s.. direct insertion around laythe is also another good way, it should cost no more than 400 m/s of intercept speed.

what I mean by "direct injection" is that you come to jool from interplanetary, you hit laythe, and you perform the capture burn there. Here's an example from another system, and it also did net a low deltaV for an interplanetary capture

the position of capture around the orbit is very important; as you see, I put a thorough care in intercepting the moon by being tangential to its trajectory. this way, the orbital velocity of the moon is subtracted from that of your vessel. You also get to use oberth effect from the moon in addition to oberth effect from the planet.

 

Either way, you can reduce your intercept speed towards laythe to less tha 500 m/s, so you'll be at 3 km/s when entering the atmosphere. then again, if your plane can survive a reentry at 4 km/s (which it apparently can, given your testing on kerbin), then there's no reason to bother finding the perfect intercept, because you'll areobrake all the excess speed away for free anyway.

 

As for periapsis, start with 35 km and see if it works. save the game as soon as you enter laythe's SoI; if the periapsis is too high or too low, you can reload to the beginning on the SoI and change the periapsis with a simple  radial burn; as long as the change is only a few km, the burn is cheap.

or, if you are sure your plane is extremely resistant, you can even go for a 30 km periapsis, which is guaranteed to capture you.

[fommil]

5 minutes ago, king of nowhere said:

direct insertion around laythe is also another good way,

this was plan A, but I've heard so many people talk about gravity assist from Tylo that I kinda want to do it. It feels like one of Those milestones in the game. Promoting Tylo GA to Plan A!

[18Watt]

5 hours ago, fommil said:

this was plan A, but I've heard so many people talk about gravity assist from Tylo that I kinda want to do it. It feels like one of Those milestones in the game. Promoting Tylo GA to Plan A!

There are several good methods of arriving at Jool, but I find Tylo assists to be the easiest to accomplish.  With a little fiddling I can make the resulting orbit around Jool be set up for an arrival to any of the other moons, from Laythe to Pol.  Being able to consistently and easily hit Tylo for a zero dv capture at Jool was a eureka! moment for me, for sure.

Vall and Laythe can also be used.  However, Laythe especially puts a bit more guesswork into the plan, at least if you are planning on using aero benefits- they are too random unless you are using a mod which helps wit that.

I suspect after you do a few captures using Tylo that you’ll prefer that arrival method at Jool in the future.   Good luck!

[magnemoe]

On 11/3/2022 at 2:00 PM, fommil said:

this was plan A, but I've heard so many people talk about gravity assist from Tylo that I kinda want to do it. It feels like one of Those milestones in the game. Promoting Tylo GA to Plan A!

The problem with aerobraking at Laythe is that you velocity get increased by the gravity of Jool and Laythe, Laythe also has an pretty small SOI. 
The benefit of Duna is that its low gravity don't speed you up much from entering its SOI to Pe. 

Here I'm coming in very fast using just a bit over 100 days from Kerbin, Pe ended up around 20 km, peaked at 7 g, who limited the braking more than overheating as torque because of uneven aerodynamic would start rotating the ship.  
Yes 3.8 Km/s is fast but I suspect its not that much faster than you will arrive with after an Hoffman transfer to Eve or Laythe. 

Make me thinking if doing the braking burn in low Jool orbit might make sense? its easy to set up and the high velocity should give you lot out of your fuel, note that you will then need an Tylo flyby to raise Pe and lower Ap. I wonder if this save more fuel than an tylo braking burn? 
 

[fommil]

just reporting back, my SSTO return to Laythe (with stops at all the other moons, except Tylo) was a success! Laythe, Val, Bop then Pol, then back home from there, my deltav return from Pol was probabyl incredibly inefficient because I tried to use Tylo for one last GA but totally messed it up, then realised it didn't matter because I had enough to burn from Pe anyway to get back to Kerbin.

That was the most fun I've had since an Eve return!

[Vanamonde]

This sounds like a successful Kerbal adventure.

[king of nowhere]

On 11/8/2022 at 10:50 AM, magnemoe said:

Make me thinking if doing the braking burn in low Jool orbit might make sense? its easy to set up and the high velocity should give you lot out of your fuel, note that you will then need an Tylo flyby to raise Pe and lower Ap. I wonder if this save more fuel than an tylo braking burn? 
 

braking in low jool orbit is normally a bad idea; not much because of the cost, as you remarked it is very cheap because Oberth (iirc 200 m/s to get captured arriving from kerbin), but because when you go to jool you always move to a moon afterwards, and to go to a moon a low jool insertion is bad. as you know, the more similar your orbit is to that of your target, the lower the intercept deltaV is going to be; and if you have a periapsis skimming jool's atmosphere, that's going to result into an expensive transfer to whatever moon you're aiming at.

this is the second reason you're better off using a gravity capture at tylo or laythe; not only you save the insertion deltaV entirely, you also end up in an orbit with a lower insertion deltaV for the moons.

I say it's "normally" a bad idea because there are corner cases where you want to do this: namely, when you come to jool with such a high intercept speed that using gravity capture is impossible, and what you gain by oberth effect far offsets the extra cost of injecting into a moon. not something that happens very often, but if for some reason you're using a high energy trajectory, then this is more efficient.

another case where this option is best is for gas giants other than jool; On jool you have two big moons perfect for gravity capture, but there are lots of modded planets out there lacking such luxury.

In the OPM system, it can be a good strategy to inject into urlum - though direct injection at wal is often preferrable. For Sarnus and Neidon, aerobraking at tekto and nissee respectively are generally the best options, but injection into low orbit of the gas giant is the next best thing if aerobraking is not possible.

In RSS, direct injection into low orbit is the best method of capture for uranus and neptune, because none of their moons are big enough for gravity capture or direct injection. with jupiter direct injection into a moon is often convenient, depending on the mission. as for Saturn, if you're performing a grand tour or similar mission and you're not coming from earth, you generally can aerobrake on titan; but if that's not possible, again, injecting into low orbit is often for the best.

so, injection into low orbit on a gas giant is actually the most efficient way to get captured in many cases; however, jool is just so conveniently set to give better options that you don't want to do it there. other modded planets, including the real planets we have in our solar systems, don't have such conveniences and justify that injection strategy

[magnemoe]

1 hour ago, king of nowhere said:

braking in low jool orbit is normally a bad idea; not much because of the cost, as you remarked it is very cheap because Oberth (iirc 200 m/s to get captured arriving from kerbin), but because when you go to jool you always move to a moon afterwards, and to go to a moon a low jool insertion is bad. as you know, the more similar your orbit is to that of your target, the lower the intercept deltaV is going to be; and if you have a periapsis skimming jool's atmosphere, that's going to result into an expensive transfer to whatever moon you're aiming at.

this is the second reason you're better off using a gravity capture at tylo or laythe; not only you save the insertion deltaV entirely, you also end up in an orbit with a lower insertion deltaV for the moons.

I say it's "normally" a bad idea because there are corner cases where you want to do this: namely, when you come to jool with such a high intercept speed that using gravity capture is impossible, and what you gain by oberth effect far offsets the extra cost of injecting into a moon. not something that happens very often, but if for some reason you're using a high energy trajectory, then this is more efficient.

another case where this option is best is for gas giants other than jool; On jool you have two big moons perfect for gravity capture, but there are lots of modded planets out there lacking such luxury.

In the OPM system, it can be a good strategy to inject into urlum - though direct injection at wal is often preferrable. For Sarnus and Neidon, aerobraking at tekto and nissee respectively are generally the best options, but injection into low orbit of the gas giant is the next best thing if aerobraking is not possible.

In RSS, direct injection into low orbit is the best method of capture for uranus and neptune, because none of their moons are big enough for gravity capture or direct injection. with jupiter direct injection into a moon is often convenient, depending on the mission. as for Saturn, if you're performing a grand tour or similar mission and you're not coming from earth, you generally can aerobrake on titan; but if that's not possible, again, injecting into low orbit is often for the best.

so, injection into low orbit on a gas giant is actually the most efficient way to get captured in many cases; however, jool is just so conveniently set to give better options that you don't want to do it there. other modded planets, including the real planets we have in our solar systems, don't have such conveniences and justify that injection strategy

Agree with you here, and as you state and was my main point here is that any sort of capture is better than none especially if you have other ships with ISRU. 
Once sent 4 bases to Jool and they come in fast and hard, two was not able to get an Pol or Bop intercept but the two others did and they sent tugs to refuel the others. 
The first to land on Pol was the last to enter Jool SOI, mostly because Pol was better placed. 

[Richmountain112]

Sometimes when you do an assist you might have to do a burn to get captured. There are some cases in which a Tylo Assist would require a collision course.

As for Sarnus I always do direct injection into an eccentric orbit then get to my destination by burning at apoapsis. Is that very efficient?

Also I wonder if it is possible to do a Slate assist, or is that moon too small?

Edited December 17, 2022 by Richmountain112

[Leganeski]

5 hours ago, Richmountain112 said:

As for Sarnus I always do direct injection into an eccentric orbit then get to my destination by burning at apoapsis. Is that very efficient?

For Kerbin to Sarnus specifically, the route you describe saves about 100 m/s over direct injection at the destination unless you're going to Slate, in which case directly injecting into Slate orbit is better. (In all cases, both routes are cheaper than arriving at the destination from low Sarnus orbit.)

In general, burning at apoapsis from an eccentric orbit is usually the most efficient route. However:

• It has a significant risk of accidentally encountering a large moon (Mun, Laythe, Tylo, Slate, etc.), which is why I don't usually use it.
• It's never much more efficient than burning at periapsis (for distant orbits) or direct injection at the destination (for close orbits or large moons). In the stock system, you can save at most maybe 250 m/s, and even that's only for really weird situations like going from Jool to Kerbin-synchronous orbit.
• When the destination is a large moon, it can be less efficient than direct injection because you're effectively making detours to irrelevant locations. For example, when going from Kerbin to Tylo, you spend more Δv capturing at Jool than you actually save from the better approach to Tylo.

5 hours ago, Richmountain112 said:

Also I wonder if it is possible to do a Slate assist, or is that moon too small?

Absolutely! Slate is huge; it's about as big as Laythe and will easily get you captured.

[Richmountain112]

So I just went to Sarnus again, and I did a direct injection into a mildly eccentric orbit before heading to Eeloo and going on an Ovok flyby. Then for my kerbin encounter, I just forced one.

For my first visit to Sarnus, I considered a Slate assist, but I didn't really see any major differences in velocity.

And about Urlum, I want to go near it and deploy an atmospheric probe before landing on Wal.

Plus I'm injecting directly from Kerbin to speed up travel time.

Sometimes there's another problem with aerobraking: Sometimes you can slow down too much and end up being "eaten" by the planet. I never really end up with this problem because I never aerobrake. Except at Kerbin, which obviously slowing down enough is likely the intention since you're returning from a long journey to the outer planets.

Edited December 18, 2022 by Richmountain112