Aerobraking at Jool, still possible in 1.04?

[johnnyhandsome]

Ever since the 1.0 update, my speed as I am for a Jool aerobrake at around 192-198 miles in altitude is around 8,000 meters per second. Previously, pre 1.0, I think it was around 3-4000 K, even at the braking altitude of 130.

Is it still possible to aerobrake at Jool and if so, what am I missing?

[Thegamer211]

I guess now you will have to slingshot by one or more moons to slow down instead.

[Wjolcz]

I don't know about you, but from my experience in 1.0 the Jool atmosphere is instant death for anything that touches it.

[Geschosskopf]

Is it still possible to aerobrake at Jool and if so, what am I missing?

There was a long thread on this subject a couple months back. The consensus ended up being that while you CAN build some monster heatshield and survive very shallow areo passes at Jool these days, that's a lot of time and effort. OTOH, you can do a retrograde flyby of Tylo which will capture you into the Jool SOI no problem (other than arranging said flyby).

The underlying problem is that except at Kerbin, atmospheres are actually more like oceans because their densities don't fade away to produce a fairly wide upper layer with very low density, in which you can aerobrake. Instead, the density is high all the way to the top. Duna's density is so low even at its highest level that this isn't a problem, but Eve, Jool, and Laythe can be killers. As mentioned above, Jool and Laythe can be dealt with via gravity braking but at Eve, there's nothing to brake on. Thus, for Eve, you either need to bring about 1800 extra dV to capture on thrust or build some ginormous heat shield.

[Jett_Quasar]

I was able to do a small aerobraking manouver above 195KM, but I still had to do a burn to get capture. Engine burns at that low altitude is actually quite efficient due to the Oberth effect as well.

JR

[tater]

Yeah, the problem is that in order to make Kerbin reentry have a non-zero chance of reentry problems, denser atmospheres become lethal. It's an artifact of the mini-sized Kerbol system.

[panzer1b]

I have been aerobraking bloody CAPITAL SHIPS around laythe for ever (my last fleet arrived around laythe 2 days ago). The trick is to hit laythe at the point where it is moving away from you (so to the right of jool and your trajectory). This makes you hit it below 4km/s, which is surviveable if you have enough drag and dont go too low. Also, just performed aerobrake at laythe with a SSTL (single stage to laythe) starfighter that didnt have any issues at all with teh maneuver. The trick is to point radial or antiradial and let your wings do the work, wings that arent in line with your velocity vector (and even fuselages) have enormnous drag, so much that you can actually end up getting a reentry without burning up if you dont level out at the right time.

Essentially the only 100% suicidal maneuver seems to be coming in to a direct laythe aerobrake if its heading towards you (speeds in excess of 6km/s are pretty much instant suicide). Jool seems surviveable too, although i havent tested this in 1.04 as its less dV to go to laythe directly, or if you prefer Tylo grav assist to slow u down, but jool has teh advantage you dotn need to go deep into it as its very wide and you get a much longer time to brake, allwoiung you to just graze the upepr atmo which according to my knowledge isnt instant vaporization. The one thing i really love about Jool is the fact that once there, you have theoretically infinite potential to alter your orbit with just small maneuvers due to teh sheer amount of gravity assists you can get. All id want in stock is faster timewarp, so getting to Jool doesnt require 10 minutes of wait at max time warp.

Also, this comes from someone that has never used airbrakes once since ive not found any use for them, extra mass and drag that doesnt benefit your dV at all. Stick to radial or antiradial (assuming it has a hull or wings and isnt a sphere, spheres are suicide to try to brake with as you cat really increase drag), and just come in not too low. Always going too low is suicide as you wont slow enough before u burn.

As for Duna and Eve, its also possible and ive aerobrakes on these 2 a few times as well. Duna seems very forgiving, even super large warships have been braked for me without issues of them frying, and even takes some more trouble, but again, isnt excessively hard to brake on. Eve only becomes a major pain in the arse when you are trying to get to it from outer planets like jool or eeloo (not that ive ever done this), but from kerbin, the velocity isnt enough to vaporize anything at least from my experience.

[OhioBob]

Aerobraking at Jool, still possible in 1.04?

It is still possible with the right spacecraft design, but it’s really tricky. Below is a re-post from another thread in which I describe my experiments on this problem:

I did some experiments to see if it were possible to do a Jool aerocapture under v1.04. I found that it is possible under just the right conditions, but barely. The entry corridor is so narrow that if you miss the correct periapsis altitude by just a couple hundred meters you will either blow up or fail to capture. The design of the spacecraft is also very important. The spacecraft requires a large ablator and a low ballistic coefficient, i.e. low mass per cross-sectional area. A low ballistic coefficient means that the spacecraft will decelerate more quickly for every unit of dynamic pressure produced. This allows aerocapture to occur higher in the atmosphere and with less aerodynamic heating.

The vehicle I used in my tests had a mass at atmospheric entry of only 2915 kg and was equipped with a 2.5 m heat shield. This means its mass was less than 600 kg per square meter of ablator surface. I don't think that anything much higher than this would have a very good chance of survival. My velocity at atmospheric entry (200 km altitude) was 9783 m/s, which is about typical for a Kerbin-to-Jool transfer. For this vehicle and conditions, I found that the ideal periapsis was about 196,200 m ±200 m. On one attempt I came in with a periapsus a little below 196,000 m and just barely made it, having burned off all but 3 kg of my ablator (a few meters lower and I probably would have been destroyed). On another attempt I came in with a periapsis just a little below 196,500 m and failed to aerocapture (I actually got my eccentricity below 1 but was still on an escape trajcetory due to the patched conics).

Although, in this example, a periapsis altitude >196.5 km would not result in an aerocapture, the vehicle would be significantly slowed down to the point that just a small burn would be necessary to finish off the capture. This might be a good strategy to use - a combination of aerobraking and propulsive braking. Obviously the higher the periapsis altitude, the less the heating and the greater your chance of survival.

I've proven that small probes can survive a Jool aerocapture with proper design and targeting. Unfortunately, I think that large massive vehicles are a greater challenge. The need for a low ballistic coefficient means that a large craft would have to be equipped with an unwieldy number of heat shields. For Jool aerocapture you want to use a "pancake" design.

The underlying problem is that except at Kerbin, atmospheres are actually more like oceans because their densities don't fade away to produce a fairly wide upper layer with very low density, in which you can aerobrake. Instead, the density is high all the way to the top.

While that is effectively true, atmospheres in KSP do tapper off to zero pressure and zero density at the upper boundary level. However, the atmospheres of Jool and Eve do so in a rather artificial way in which they drop off quite rapidly as the boundary nears. This means that the atmospheres thicken very quickly, and in an unrealistic manner, once dipping below the boundary.

[johnnyhandsome]

Cool, interesting info. Anyone have any idea why the approach speed seems so much higher now? 8K m/s just isn't healthy!

[problemecium]

^ I second this query. Kerbin orbits at around 9 km/s, and Jool presumably orbits at a few km/s itself (though I don't remember how many), so if you come in parallel to Jool's path, then by subtracting a positive number from another positive number you should get a smaller number than 9 km/s. Uhh... right?

*is bad at math*

[Green Baron]

just tried with a small probe and gave up. Coming in at 8km/h the craft exploded instantly at 198,500m altitude. I had no shields but doubt that would have changed something. Standard settings, 1.0.4. PE was 185km.

k

[OhioBob]

Cool, interesting info. Anyone have any idea why the approach speed seems so much higher now? 8K m/s just isn't healthy!

I don't think the approach speed is any higher than it use to be. Mathematically it just isn't possible to approach Jool any slower.

^ I second this query. Kerbin orbits at around 9 km/s, and Jool presumably orbits at a few km/s itself (though I don't remember how many), so if you come in parallel to Jool's path, then by subtracting a positive number from another positive number you should get a smaller number than 9 km/s. Uhh... right?

*is bad at math*

You can consider approaching a planet as just the reverse of escaping it. To escape a planet you must increase your velocity to at least escape velocity. Therefore, when a spacecraft approaches a planet from deep beyond its sphere of influence, the spacecraft must be traveling at no less than the planet's escape velocity. As the spacecraft is pulled in by the planet's gravity, its velocity increases. At all points along its trajectory, the spacecraft's velocity will be greater than the escape velocity at that altitude. At the top of Jool's atmosphere, escape velocity is 9,547 m/s. Therefore, any spacecraft entering Jool's atmosphere from a hyperbolic escape trajectory must hit the atmosphere going at least 9,547 m/s.

[Rune]

I will pint out one gramatic correction that, I think, underlines the whole issue. Aerobraking is not the same as aerocapturing.

Aerobraking is a very gentle procedure that has actually been done a bunch of times in RL, where an unshielded spacecraft dips repeatedly into the higher parts of an atmosphere to lower its apoapsis, being "captured" all the time (in an elliptical orbit), and over a period of months. This is how we actually dispose of cubesats and low-orbit crap, too.

Aerocapture, on the other hand, means lowering your velocity form a hyperbolic escape trajectory, into an elliptical one, in a single pass. A very different cookie. The (I think) only closest example in real life would be the Galileo probe entry into Jupiter. That little bugger hit the atmosphere of the king of planets going some 47km/s, and burned off 80kgs out of an initial mass of 152 in the heatshield breaking in its atmosphere (the heatshield being half of the mass of the probe, BTW), experiencing a peak of 230Gs in deceleration. Another close example were the Apollo reentries, coming from a shade under interplanetary speed, lots of flames and acceleration too.

So, in KSP terms, I plan to keep on dipping into the atmosphere to save dV when possible, which means I can do it almost unshielded from Mun if I pick high heat-tolerance parts, but I will have to use really big heatshields if I'm truly interplanetary, or capture in another way (hello there, Tylo) and lower my apoapsis the slow way, over multiple passes. I will refrain from aerocapture except in a very few selected cases, probably on light probes, and my game will improve in realism for that.

Rune. TL,DR: this is realistic, and I'm happy about it.

Edited August 23, 2015 by Rune

[Green Baron]

Rune, the galileo probe was not captured into orbit but sank down through the atmosphere until it was destroyed. Just thought that might be important to note . . .

:-)

k

edit: You didn't say that, i just felt eager to note :-)

Edited August 23, 2015 by kemde

[Rune]

Rune, the galileo probe was not captured into orbit but sank down through the atmosphere until it was destroyed. Just thought that might be important to note . . .

:-)

k

edit: You didn't say that, i just felt eager to note :-)

Duly noted and edited

[purpleivan]

Aerobraking/capturing is certainly possible now as I did it a few days ago with what was intended as an entry in the "New Home" challenge (sent my 105 Kerbal lander with only 1 kerbal in it, so I need to fly it again).

Pics... so it happened

BTW... I had the engines running as I was overshooting the landing site, not actually required to be captured by Laythe... just avoid a soggy rocket.

[Fearless Son]

The consensus ended up being that while you CAN build some monster heatshield and survive very shallow areo passes at Jool these days, that's a lot of time and effort.

But it would be quite Kerbal:

[KerikBalm]

^ I second this query. Kerbin orbits at around 9 km/s, and Jool presumably orbits at a few km/s itself (though I don't remember how many), so if you come in parallel to Jool's path, then by subtracting a positive number from another positive number you should get a smaller number than 9 km/s. Uhh... right?

*is bad at math*

As you cross the SOI, your relative velocity is much much lower. Then Jool's gravity accelerates you. Orbital velocity for jool is already about 8km/sec - obviously if you *slow down* to caputre into orbit, then at perapsis, you'll be doing over 8km/sec. If you put your apoapsis out to the edge of Jool's SOI, you'll be doing over 9km/sec at perapsis. As you *slow down* to capture, momentarily you'll be in the same orbit... with apoapsis right there at the SOI boundary.

Anything crossing into Jool's SOI going along a ballistic path, with a perapsis at or below Jool's atmosphere, will be going over 9km/sec when it reaches perapsis... even if the relative velocity before crossing the SOI boundary was 1m/s (or 0.00001 m/s).

Personally, I never did an aerocapture at Jool, I always used Laythe.

I tried doing an aerocapture at laythe with a probe behind a heat shield, it worked fine, didn't use much ablator either.

But I also had a survey scanner probe with no heat shield, and I find the tylo or laythe gravity assists for capture to be pretty easy to get as well.

Yeah, the problem is that in order to make Kerbin reentry have a non-zero chance of reentry problems, denser atmospheres become lethal. It's an artifact of the mini-sized Kerbol system.

Nah, the RL Jupiter is also pretty deadly... see the gallileo probe example. Eve would be much more forgiving if it had kerbin's gravity. Its not its thick atmosphere as much as it is its large gravity well that will ensure any object encountering it is travelling much faster than something encountering kerbin.

The underlying problem is that except at Kerbin, atmospheres are actually more like oceans because their densities don't fade away ...

Don't forget Duna. As others said, it does fade away at the top of Eve and Jool too, but that transition is quite fast and not at all like the rest. Laythe is somewhere in between I guess.

The (I think) only closest example in real life would be the Galileo probe entry into Jupiter.

I think every mars surface mission since the viking program has also done direct capture and descent.

The viking program had the orbitersestablish orbit before deploying the landers, and they deployed the landers based on what they saw, but since then, its been direct capture and landings.

They could probably get the landing elipses smaller if they went to orbit first, but I guess the accuracy is good enough already