Aerobraking in Real Life

[Nuke]

Duna is not Mars.

i am aware of this. but im familiar with the maneuver and use it whenever possible to save fuel. this is why im tempted to fire up orbiter and see what kind of g loads i end up with (though im too lazy to actually do this).

[KerikBalm]

@KerikBalm Are you saying that landing in stock duna is harder than mars??

Landing is not the same thing as reentry.

With updated numbers for the relative velocity (post #23), it seems one only needs to bleed off ~500 m/s in the first pass to aerocapture. You have roughly 10x the path length to bleed this off, so the acceleration required is lower.

On Duna, you can land just fine with chutes in many cases, and at places the atmospheric density is rather higher (over 15% that of Earth/Kerbin). That makes the actual landing a lot easier.

"Real world landing on Mars is more difficult by a crazy factor. It have taken tens of billions of dollars, half a century time and tens of thousands of scientists and engineers work years to land just a couple of primitive probes to Mars. "

Of course, almost everything in KSP is easier... like getting to orbit. I'm just examining the physics of the aerocapture maneuver. The larger scale height and bigger radius actually makes for better aerocapture conditions. I'm not discussing the difficulties landing, the inclined eccentric orbits, the testing that each part needs to have... and so on.

I'm just saying... it is most definitely possible to do an aerocapture at Mars.

The heating is significantly less than rentry from Earth orbit.

[AngelLestat]

But stock KSP "aerodynamics" if you compare it with FAR - RSS, you will see that in KSP stock you lost more deltav at launch in % to your final orbit speed than in FAR - RSS.

So the small scales on kerbin / duna with stock aerodynamics in fact reduce even more (comparing scales in %) the drag brake distance.

Also the only way to get close a bit more to reality is using FAR-RSS-DeathlyReentry-RealChute in Mars with 100 ton reentry and try to brake it with all kind of aerodynamics devices and just spending 500m/s in proppelent as you said.

First thing to notice the Deathly reentry config, to check if the default value is not an easy starting value.

Second, try to fulfill the safety NASA margins, is not the same claim that it can be done when we reach 10g, temperatures close to the limit and without accomplish our final orbit with the precision needed.

Use trajectories mod as a help for the last one. Far is now supported.

Edited January 15, 2015 by AngelLestat

[Nuke]

g loads dont only affect the occupants but also structural tolerances as well. a capsule might be able to handle a 5g re-entry just fine, and most stuff launched into space can usually survive up to 3g which is the standard for most payloads. a modular craft assembled in space however might have issues with such high g loads. the iss would probibly break if it had to handle even a fraction of a g (if you hypothetically wanted to perform an aerocapture with it, which you would never do irl). you would never want to exceed your craft's design tolerances. if you had a multi mission space craft that was manned and had a large nerva cluster and tankage or perhaps an orion drive, an aerocapture just might be out of the question at some places.

[PakledHostage]

Sorry, but can I drag us back to the original topic of discussion? The question was whether or not "any future manned spacecraft that goes to Mars [will] use aerobraking to save delta-V for circularising or is it far too risky/requires too much added weight for heatshields?". The issue of landing on Mars is irrelevant other than in response to another question about whether or not aerocapture had ever been used in the past. If you define aerocapture to include direct entry to a landing, then yes it has. Aerocapture has, to my knowledge, not yet been used to enter Mars orbit; only propulsive capture coupled with aerobraking has been used.

The paper I cited above seems to suggest that the relative risks of aerocapture are acceptable, should they ever be employed on a Mars sample return mission. A Mars sample return mission would be significantly larger and more massive than any other mission we've sent to Mars in the past, yet the paper's authors believe that aerocapture can be done with comparable reliability to propulsive capture and to a combination of propulsive capture and aerobraking. The paper also says the following about navigation targeting:

F. Navigation Targeting

This assessment addressed the risk of inaccurate targeting of a precise point in space prior to arrival at Mars leading to an unsuccessful capture. These points are different depending on the maneuver, however the accuracy of the prediction tools was assumed to be the same. The relative risk differences occur due to the various tolerances to this prediction for each of the capture methods. The accuracy data used for this analysis was based on the Mars Exploration Rovers for direct entry. The direct entry spacecraft of the MER missions hit their target for entry alignment to within +/- 200 meters without performing their final TCM. The assumptions for the tolerances of each capture method are as follows. For aerocapture, the tolerance is +/- 2 km, while the tolerance for aerobraking is +/-20 km. Finally, the tolerance for propulsive capture is +/- 200 km.

...

Given the high tolerance of all three capture methods relative to demonstrated accuracies, the probability of missing the predicted target with a given accuracy outside of its tolerance is insignificant for all three capture maneuvers.

It stands to reason that, since aerocapture into Mars orbit is a reasonable approach for a Mars sample return mission, it would also be a reasonable approach to entering orbit for a manned mission. As KerikBalm pointed out, we're only talking about scrubbing on the order of 500-1000 m/s from our arrival speed of 5400 m/s - 5900 m/s. That represents an energy dissipation (per kilogram) of roughly 10-20% of that required during reentry from LEO. Even considering the different heat transfer characteristics of Mars' CO2 atmosphere compared to Earth's mostly Nitrogen and Oxygen atmosphere, it isn't beyond our current technological limits to pull it off. Likewise, it stands to reason that it wouldn't be beyond our technological capabilities to use Aerocapture to enter Mars orbit during a manned mission.