Aerobraking /Aerocapture in 1.05. Things You've Noticed....

[Seyvern]

[quote name='dantedarkstar']Geschosskopf, I still hold that you are wrong. Let me explain it again.

Assuming no irreversible processes happen (like atmospheric braking) then spaceflight is *fully* reversible. There is no time arrow defined -- if time is reversed, we couldn't tell (as we can for example for thermodynamic processes like gas escaping from container into space since reverse process is HIGHLY unlikely), since the dynamics would be exactly described by the same physics.
This means that considering going from different APs to same PE is *exactly* equivalent of going from PE to different APs. Of course this ignores atmosphere, but since you are going to the same predicted PE, it ignores atmosphere anyway. So let's assume that Kerbin has no atmosphere for a moment, and just track how much of the trajectory is "below 70km" or any other set altitude. Then my previous claims hold true. The higher velocity at given fixed PE, the higher the AP, but at the same time less curve and therefore trajectory always above the lower-velocity trajectory.
We have 2 trajectories (both are after any burning you did at AP, so both are ballistic trajectories from that point): A) from AP 12Mm to PE 37km, B) from AP 1.5Mm to Pe 37km
This is exactly equivalent to trajectories: C) from PE 37km to AP 12Mm, D) from PE 37km to AP 1.5Mm. It should be obvious to anyone playing KSP that trajectories A and C have the same velocity at periapsis, and that velocity (Va=Vc) is higher than the periapsis velocity of trajectories B and D (Vb=Vd<Va=Vc) (can't cheat conservation of energy without burning engines, which I assume you don't do except at AP to adjust PE). If your velocity is higher, then the curvature of the trajectory (at a given altitude) MUST be smaller. This is because the gravitational force is proportional only to masses and the distance from the body that you are interacting with. And the mass of the object itself cancels out when calculating acceleration, so it only depends on mass of the planet and distance form the planet center -> altitude. Given higher velocity, the curvature must be smaller = larger curvature radius, as described by r = V^2/a (this is simple relation for circular motion, but if we consider local curvature then it works as well), since a = acceleration from gravity which is the same for both A/C and B/D trajectories.
While it is obviously possible to draw an ellipse of higher eccentrity (and higher AP) with same PE, that is also narrower, such ellipse is not a physical trajectory and will not have Kerbin as one of focal points (which is always true for trajectories in point gravitational field, as calculated in KSP).

In the below picture, the low AP orbit (actually lowest possible, since it's circular) is blue. The high AP orbit is NOT the red one that indeed would spend more time in atmosphere, but the green one, which is always above blue one, except at PE. Black points are supposed focal points of the ellipse. Note that the red one does NOT have focal point at the center of Kerbin (cyan circle), since PE is always *closest* point to the focal point which must be (according to physics) the body with gravity field (so by definition PE is closest to focal point). An even higher AP ellipse would be again wider around kerbin and always above green trajectory.
[url]http://i67.tinypic.com/24o9b0o.png[/url][/QUOTE]


Dante is correct. Another way to think of this is that the higher the AP the shorter the time in the atmosphere - as he said - as seen in the limiting cases: In the limiting case of a circular orbit, the lowest possible AP for a given PE, the time in the atmosphere is infinite (assuming no drag). The other extreme, a near inifinate velocity hyperbolic pass, has the shortest possible time in the atmosphere, as the orbit becomes a line tangent to the circle which describes the circular orbit with the same altitude as the PE of the hyperbolic pass.

[spikeyhat09]

yeah... kind of hoping they add some sort of procedural heat shields that you can easily add to the underside of spacecraft (especially spaceplanes), for a weight penalty of course. I would imagine it could be made costly as well in career mode, if you swing that way. It would even eliminate the need for having multiple types of heat shields if there were just a "one size fits all" option. Might be kind of hard to integrate into a part, but maybe it could be a tab up at the top with action groups and kerbals. Call it "Surfacing", and they could even add other types of surfaces for enhanced effects. Anyway, that's my little ramble. Landing anything of interest on eve now requires an annoyingly elaborate array of circular heat shields, which is disappointing. Edited November 19, 2015 by spikeyhat09

[Geschosskopf]

[quote name='dantedarkstar']Geschosskopf, I still hold that you are wrong. Let me explain it again.[/QUOTE]

OK, you've convinced me I'm wrong. There's a reason I'm not a rocket scientist.

[cephalo]

Sorry to necro this thread after a couple of weeks, but I have an observation to add. I tried aerocapture on Laythe at 8000ms, 45k altitude, and all of my ship parts exploded at once. I have heat shields covering what I thought were the sensitive bits, but everything is the sensitive bits.

[DancesWithSquirrels]

On 11/15/2015, 10:36:43, GoSlash27 said:

The important thing I'm seeing:
A simple Mk.1 capsule with 'chute is too clean when equipped with a heat shield. Once it gets subsonic, you have to wait forever for safe deployment of the 'chute. You run the risk of smacking into the ground before the 'chute deploys.
I logged a bug report about it. In the meantime, I recommend attaching the heat shield with a decoupler and jettisoning it once the heating subsides.

I've had good results in reducing my ablator resource from 200 to 60-80 or so. Basically the level of ablator used in a test reentry plus a bit for safety. Otherwise you're carrying extra weight that you don't need for a Minmus return, at a rate of 0.1 per 100 ablator. So removing 120 ablator saves 0.12 tons of weight.

That makes it a bit easier to slow down for safe parachute deployment before encountering the cumulogranite clouds. 

[Aethon]

Wow.  I'm glad I saw this thread.  I had a ship returning from Mun that I forgot to set an alarm for.  Whoever bumped this saved Hanleys' life and 550 science for my career.

I've been using a 30km periapse for Mun return and haven't had any problem.  I think at that alt. you spend less time in the hot zone.

 

This is my standard lander which is a bit overkill, but the leftover fuel allows me to (usually) land the whole thing.  From Mun, I hit the atmosphere at about 3200 m/sec.  She gets hot, but nothing 'splodes.  

It starts getting squirrelly at around 2000 m/sec and wants to flip around, so I go full throttle and the LV-909 gimbal helps keep it pointed retro.  It is very difficult, but I can usually do it.  If I start to lose retro while it's still flaming, I'll stage off the bottom and let the heat shield deal with it.

 

 

 

 

I lost it this time but the flames were almost gone.  Preparing to manually deploy drouge.  Nothing worse than getting all the way through the hot stuff and staging away the bottom part of the ship when I only want my 'chutes.

 

One more to manually deploy.  

 

It's annoying.  Every time I take a screenshot, the Aerodynamic Forces Overlay comes up. 

 

..And there was much rejoicing.

 

 

Edited December 7, 2015 by Aethon

[bewing]

After a few experiments having low-tech reentry vehicles explode into bits at 35Km or hit the ground at 300+ m/s, I decided that passive reentries just don't work very well anymore (with heat shields or whatever). So I either put a descent engine on everything, or make sure to have a bit of fuel left in my last liquid fuel stage. This gives the advantage that I don't need to discard the last engine before landing, and recover some money on landing.

Another thing I did notice was that there is a big difference between a vehicle coming in stabilized tail first, or one that is unstabilized and intentionally rocking 90 degrees to either side or tumbling slowly. A craft coming in tail first will accelerate as it falls because of the current very low drag coefficient on engines. One that is tumbling or rocking will slow reasonably quickly.

 

[I_Killed_Jeb]

From 85x85, you can pretty much bellyflop onto KSC without too much trouble. De-orbit at around 45 degrees west of KSC, aim about two thirds of the way in between KSC and the island runway, hold attitude about halfway between radial and prograde

Edited December 8, 2015 by I_Killed_Jeb

[Temstar]

Is it possible to aerobrake into LKO from Mun/Minmus without using a heat shield?

Of course I know the answer is yes if you do a billion pass through the atmosphere to achieve the aerobrake, but i mean practically, say with 2 passes. Supposedly interplanetary aerocapture to Kerbin is possible, which means aerobrake from Minmus is possible too. Question is if it can be done with a reusable craft without resorting to the complexities of replaceable heat shields using docking ports.

[I_Killed_Jeb]

47 minutes ago, Temstar said:

Is it possible to aerobrake into LKO from Mun/Minmus without using a heat shield?

Of course I know the answer is yes if you do a billion pass through the atmosphere to achieve the aerobrake, but i mean practically, say with 2 passes. Supposedly interplanetary aerocapture to Kerbin is possible, which means aerobrake from Minmus is possible too. Question is if it can be done with a reusable craft without resorting to the complexities of replaceable heat shields using docking ports.

I'd imagine it's a very narrow margin you are working with.

[CalculusWarrior]

6 hours ago, Aethon said:

's annoying.  Every time I take a screenshot, the Aerodynamic Forces Overlay comes up. 

This is due to Steam's 'Screenshot' key being F12, the same as the Aerodynamic Forces Overlay. You can either use KSP's built-in screenshot button, F1 (the pictures are found in the game's Screenshot folder in its main directory), or install the ScreenMessageHider mod, which gets rid of all those annoying messages which pop up when the UI is hidden. Unfortunately, it doesn't hide the Aerodynamic Force Arrows, so you may want to use the F1 key anyway (or rebind Steam's screenshot key).

[NathanKell]

2 hours ago, Temstar said:

Is it possible to aerobrake into LKO from Mun/Minmus without using a heat shield?

 

Yes. A Mk1 capsule and parachute can survive a single pass direct descent (although the window is only ~5km wide), so an aerobrake is also survivable.

[Hannu2]

On 16.11.2015 klo, Kerbart said:

*Right now the goal of reentry is to get below 250m/s before you run out of time to open the chutes. I hope to see the day where the goal of reentry is that, and getting below 1km/s before your heatshield runs out.

That day is today if you adjust amount of ablation mass low enough. About 150-200 units is enough for 3 kerbal pod reentry at about 3200-3300 m/s (PE 20 km). I use 250, there are good safety marginal. Full heatshields are intended to descent massive stacks of fuel tanks on Eve.

[Benji]

On 16.11.2015, 09:56:43, Warzouz said:

At Duna, I found a bug : Chutes safely opens at 220m/s instead of nearly 600 or 700m/s in 1.0.4. As the atmo is very thin, this speed can only be achieved by retro burning now.
...

As Duna is inspired by Mars and you look at the problems Nasa faces with aerobraking in Mars`s atmosphere, I don't think it's a bug.

[jros83]

[About the F12 thing and screenshots: you must be using Steam version, where F12 is defaulted to taking a screenshot, a Steam default setting. You can either change Steam's setting, or change KSP's bind for aerodynamic overlay.]

[OhioBob]

On ‎12‎/‎7‎/‎2015‎ ‎3‎:‎23‎:‎12‎, DancesWithSquirrels said:

I've had good results in reducing my ablator resource from 200 to 60-80 or so. Basically the level of ablator used in a test reentry plus a bit for safety. Otherwise you're carrying extra weight that you don't need for a Minmus return, at a rate of 0.1 per 100 ablator. So removing 120 ablator saves 0.12 tons of weight.

For returns from Mun/Minmus, I've been using an ablator mass ≥5% the mass of the vehicle.  For example, a Mk1 command pod with parachute and 1.25m heat shield has a mass of 1240 kg.  However, I don't want to include the ablator, so I run the ablator slider down to zero, which gives a mass of 1040 kg.  5% of that number is 52 kg, which is the amount of ablator I want the dial back in.  Since the ablator increases in 20 kg increments, I set it to 60 kg.

So far it has worked pretty well, though I haven't accumulated enough experience to give the method my final endorsement.  Surely for high-speed interplanetary intercepts, the ablator percent will have to be higher.

[ment18]

Ablator is odd becasue i've never built a vessel that used more than 50% of max units of ablator, even in extreme scenarios.  Albator might be unnecessary for most reentries.  I built a 13 ton lander that could reenter from lko with 4 1.25m heat-shields empty of ablator) and a nerv on the bottom.  The heat-shields got a little hot but nothing exploded.  My eve lander also doesn't use most of it's ablator, though I don't usually care.

Edited December 11, 2015 by ment18

[panzer1b]

Well a few more tests i did with various craft aerocapture at laythe, and ive concluded that speeds below 3500m/s are rather safe (and even allow direct aerocapture to surface in a single pass from kerbin directly.  Just make sure laythe is moving in your direction when you aerocapture or 6000+m/s relative velocity is an instant detonation.

Overall the following areas ive managed to aerocapture in 1 shot without frying a basic spaceplane with somewhat lowish wing area:

 

Duna (from kerbin):

A complete joke, nothing i send here even gets yellow let alone explodes.  I cant think of a single craft that would be unable to aerocapture on duna unless its some super sleek and extremely heavy vessel with no wings or drag surfaces.

 

Laythe (from kerbin):

Can be very easy to do if you come in at a perfect trajectory, otherwise it can be very hard if not impossible to accomplish.  Make sure laythe is heading away from you when you enter its SOI, doesnt need to be exactly away from you but the closer its velocity vector is to your craft's the less dangerous it gets.

 

Jool (from kerbin):

Just stay at high altitide and dont go too deep, its dangerous but possible to aerocapture into the jool system with 1 good pass from kerbin trajectory.

 

Kerbin (from jool/laythe)

Can get very dangerous but sofar ive managed to aerocature here from jool directly, just dont expect it in 1 pass, usually you end up with a AP above mun unless your craft is extremely high drag and lightweight.  For a full fueled craft forget it.  From duna or eve it is a joke even for crappy made craft though as the relative velocity you need to kill is very low.  The velocity from eve or duna is roughly that of minmus, a bit more but if you dont absoluteley have to reenter in 1 pass then anything can pretty much aerobrake from duna/eve to kerbin, dres is moderate, jool is hard but again quite possible.

[The Optimist]

You can do some amazing things with flat-bottomed fairings (flat bottom, aerodynamic top) and sepratrons, which can subtract 500 m/s easily.