Aerobraking in Real Life

[Argylas]

Just a quick thought - will any future manned spacecraft that goes to Mars use aerobraking to save delta-V for circularising or is it far too risky/requires too much added weight for heatshields? I used this all the time in KSP, but when I started using the Deadly Reentry mod I found that suddenly I was much more inclined to chalk up the increased delta-V cost to just circularize around the planet I just wondered if it is the same in RL and whether NASA has even considered using aerobraking as a possible scenario for orbiting Mars.

[Nibb31]

Space probes already use aerobraking for orbital insertion. Magellan used it first at Venus in 1993 and pretty much every mission to Mars orbit has used it since MGS in 1997.

There is no need for a heat shield as simply skimming the upper atmosphere with solar panels deployed causes enough drag to slow down the spacecraft without generating too much heat. It doesn't work like in KSP where the atmosphere has a hard limit. They usually do it over several weeks or months with many periapsis passes.

Edited January 13, 2015 by Nibb31

[Cirocco]

NASA and other agencies already perform aerobraking manouvres, but usually not for aerocapture. Don't shoot me on this if my info isn't fully correct (I'm an engineer, but no spaceflight engineer) but I believe they perform insertion burns and then try to attain the desired apoapsis with several aerobraking passes. If I recall correctly either ESA or NASA performed a test with an inflatable "doughnut" aerobrake to increase surface area so they can perform the manouvre in the future on heavier payloads. Can't seem to find what it was called or which agency performed it right now.

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Space probes already use aerobraking for orbital insertion. Magellan used it first at Venus in 1993 and pretty much every mission to Mars has used it since MGS in 1997.

they use it to change the apoaps, but I'm fairly certain orbital insertion is still done with a burn. The speeds from interplanetary travel make it too dangerous to attempt an insertion purely with aerobraking. You'd have to go too deep into atmosphere where the aerodynamic forces and heat from air compression would be far too dangerous.

again though, not an aeronautics or spaceflight engineer, could be wrong.

[KerikBalm]

As I understand it, aerobraking is used when:

* They are landing a probe - I'm not sure if this is just deorbiting after a capture burn, or straight capture and deorbit - IIRC, the relative velocity was similar to the apollo capsule on reenntry - and the scale height of Mars' atmosphere is larger than Earth, so there's more time to slow down before thicker atmosphere.

* They need to lower their apoapsis (this obviously requires you have an Apoapsis - ie have captured)

I see no reason that you couldn't do a little aerobraking and a capture burn. If you pick an altitude that doesn't require adding a heat shield, maybe it will only slow you down a few hundred m/s when you need a few thousand.... but that would still save a few hundred m/s

[K^2]

The larger the ship, the less heat shielding, proportionally, you'll need for aerocapture. I'm not going to promise anything, since first few manned missions to Mars are likely to be extra-cautious, and who knows what sort of developments we'll have by the time we turn flights to Mars into a routine. But if we wanted to establish regular traffic between Earth and Mars right now, with current tech, I would absolutely expect aerobraking to be a big part of capture. (Also, a cycler. But that's a separate topic.)

[Hannu]

NASA and other agencies already perform aerobraking manouvres, but usually not for aerocapture

This is true. They can not aerocapture, because probes does not have any heat shield. Aerocapturing would need a shield because high g and high temperature. But they make Mars orbit insertion burn to very eccentric orbit and lower apoapsis during tens of very gentle brakings in very thin air. It takes months of time and saves hundreds of meters per second.

It is impossible to say anything about possible manned Mars missions. Current plans are very rough and surely real mission will be different. I think that there will not be manned Mars mission before we get some new propulsion technology. For example some kind of fission reactor powered high power electromagnetic propulsion. But it takes decades of time and only future shows what are dv capabilities when someone truly decide to pay the costs of mission.

[KerikBalm]

This is true. They can not aerocapture, because probes does not have any heat shield. Aerocapturing would need a shield because high g and high temperature.

The landers, for obvious reasons, do carry heat shields.

http://en.wikipedia.org/wiki/Mars_Polar_Lander#Launch_and_trajectory

It turns out the relative velocity is even lower than simple Earth reentry - 6.9 km/sec

That spaceccraft linked above did not survive due to a landing mistake, but all the rover and landers we've landed had heat shields.

Curiosity was a direct aerocapture and landing, without establishing orbit first. I don't know how they did the orbiters for the viking landers, but those landers had heat shields

[Cirocco]

Curiosity was a direct aerocapture and landing, without establishing orbit first. I don't know how they did the orbiters for the viking landers, but those landers had heat shields

oooooh, I didn't know that. Thanks for the factoid

[crubs]

http://en.wikipedia.org/wiki/Aerocapture#In_practice

According to Wiki, aerocaptures have never been performed. Do any of you have sources to show otherwise?

[PakledHostage]

It turns out the relative velocity is even lower than simple Earth reentry - 6.9 km/sec

It may be that high for some spacecraft, but the atmosphere relative entry speeds anticipated for Mars Science Lab were only about 5.4 - 5.9 km/s, depending on the timing of the transfer from Earth. (ref: Mars Science Laboratory Mission Design Overview, Table 1 and Table 2). That's only about 3/4 that of reentry from LEO, despite the fact that MSL entered directly from an Earth-Mars transfer trajectory (i.e. it did not enter Mars orbit first).

According to Wiki, aerocaptures have never been performed. Do any of you have sources to show otherwise?

It depends on whether or not you restrict the definition of aerocapture to mean capture into orbit, or if you allow it to include direct atmospheric entry to a landing. The Mars Pathfinder mission used direct entry to a landing back in 1997, as did the Mars Exploration Rover missions and the Mars Science Lab mission.

As Nibb pointed out, other Mars orbiter missions have used propulsive capture followed by several months of aerobraking to reach their final working orbits.

Edited January 13, 2015 by PakledHostage

[AngelLestat]

You are all missing a big point.

Mars atmosphere is only a 0,6% our atmosphere.

It may be enoght to brake things no more than 1 ton.

But beyond that is not enoght.

This is due the relation between volume, surface and mass.

Volume grows exponentially with the mass, meanwhile surface remains almost the same.

Not even with the new inflatable heat shields, they should be many times bigger than our current prototypes.

That is not the only issue, parachutes does not work with higher mass than 2 tons either.

A manned mission needs modules from 50 to 150 tons in the surface.

The only way is doing a supersonic retro propulsion, and if you do that, then you dont have time to combine with another aerobraking technique.

Edited January 13, 2015 by AngelLestat

[B787_300]

Actually Angel, Parachutes would work, You would need a series of them though, a supersonic one and then a subsonic one, but the simpler thing to do it to land propulsively

[PakledHostage]

Actually Angel, Parachutes would work, You would need a series of them though, a supersonic one and then a subsonic one, but the simpler thing to do it to land propulsively

Also, Argylas' question was "will any future manned spacecraft that goes to Mars use aerobraking to save delta-V for circularising". That doesn't preclude some combination of propulsive capture and aerobraking to save delta-V.

[GreeningGalaxy]

Robert Zubrin's Mars Direct plan involves aerobraking over Mars in multiple passes, using a deployable skirt type of thing on the bottom of the capsule to increase drag. It's not a replacement for rockets, especially not for landing, but it does save a lot of delta-V. Mars's atmosphere is thin, but not so thin as to be useless for aerobraking.

If you think about it, most of the rovers sent to Mars have used parachutes during some part of their descent, which slowed them down a lot - if you brought your orbital periapsis within a kilometer or so of the surface, it's hard to imagine that you'd stay in orbit, so aerocapture over Mars is probably possible too, if at a considerably lower altitude than Earth.

[Hannu]

The landers, for obvious reasons, do carry heat shields.

Of course, but I thought that we spoke about aerocapture to orbit. As far as I know any spacecraft have never used aerocapture from transfer orbit to orbit around planet. Probably it is cheaper and safer to include fuel than carry heavy heat shield. If you want to aerocapture to orbit you need large dv during one pass. Apoapsis adjustment can be done one or few meters per second per atmosphere pass and that does not need heatshield.

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You are all missing a big point.

Mars atmosphere is only a 0,6% our atmosphere.

It may be enoght to brake things no more than 1 ton.

But beyond that is not enoght.

If you mean aerobraking from interplanetary trajectory to Mars orbit, density is not problem. 6 km/s in the gas of 6 mbar pressure is practically the same thing as lithobraking. Larger problem is that Mars is so small sphere that braking length is short. You have to use very high deceleration which is a stress to ship and possible crew.

Thin atmosphere is severe problem when something heavy try to descent to surface. Atmosphere is too thin to decelerate ship to subsonic speed but too dense to disturb engines and burn structures.

The only way is doing a supersonic retro propulsion, and if you do that, then you dont have time to combine with another aerobraking technique.

What do you mean? It is impossible to avoid all aerobraking if you land on Mars. The most practical is to aerobrake most of several km/s and decerate last hundreds of meters per sec with engines. You save order of magnitude or more landing mass compared to Mars sized planet without atmosphere.

[AngelLestat]

As PakledHostage said, parachutes is kinda derail of topic.

But let me clarify that.

Curiosity parachute was huge, now nasa is testing with a parachute twice that size (not to lands humans) to land the next heavy missions.

That parachute does not enter in any wind tunnel. So testing is more expensive.

This parachute will be enoght to 5 tons or a little more maybe. But the weight, size and complexity of a parachute to 50 tons is not possible with current technology. In fact it will take so much time to open that will crash before it ends.

But I mistake in one thing with supersonic retro propulsion.

You can do both, aerobraking and retro propulsion at the same time.

The last plan that is mentioned in the documentary "man on mars - mission to the red planet 2014" , it said that you can use your rocket height entering in side way (not retrograde) to drag and brake meanwhile you use supersonic retro propulsion to keep your atmosphere altitude. Of course that burn normal to the planet is not efficient as in retrograde, but this combination is better than full retro propulsion.

The problem with aerocapture, is that is not so easy to calculate like in earth or venus, in these planets you can do it and you can have a final apoapsys of 500 km or 550 km.

In mars is very easy to end crashing or with escape trajectory after cross the atmosphere.

Because the layer that can decelerate you enoght is so thin that few meters down or up can be very significant to your final orbit.

Hannu: we are talking of brake things without breaking them and with precision.

Edited January 13, 2015 by AngelLestat

[Nuke]

my experience with aerocapture at duna from an interplanetary trajectory usually involves getting dangerously close to the surface and pulling a lot of gs. not to say aerocapture at mars wouldn't be possible, but it might push the danger margins far beyond what is considered acceptable for a manned mission.

[Hannu]

my experience with aerocapture at duna from an interplanetary trajectory usually involves getting dangerously close to the surface and pulling a lot of gs. not to say aerocapture at mars wouldn't be possible, but it might push the danger margins far beyond what is considered acceptable for a manned mission.

It is problem in Mars too. Both bodies have relatively small radius and have thin atmospheres. Surface is not worst safety factor. You must have much more accurate altitude than just avoid hitting the surface, because too high velocity in too dense gas is as bad.

I think that there will not be manned Mars mission before we learn to accept risks which are unavoidable in pioneering work. During past centuries kings sent tens of ships to explore world and only small fraction of them came ever back. Investors, crew members and public knew and accepted that it is impossible to achieve anything without taking huge risks. Now we have lost all that pioneer spirit. Space exploration is expected to be as safe and predictable than boring average office work. Any accident causes panic in public, investors and politicians and they pull back whole projects. Actually no one have even the courage to try any new achievements. They just produce empty words and nice animations.

[KerikBalm]

http://en.wikipedia.org/wiki/Aerocapture#In_practice

According to Wiki, aerocaptures have never been performed. Do any of you have sources to show otherwise?

Well, its often a bit hard to show because its rarely explicitely stated that there is no orbital insertion burn. They never specify that atmosphric entry is also aerocapture (as the other post here noted, they may not count this as its not going into an orbit).

But if you look at the mission payloads.... the cruise stage doesn't have anything for a retroburn.

All the mission animations also show direct entry.

20-25 seconds, cruise stage separation while near earth.... then its just the aeroshel and a little guidance package with solar panels

Same thing seen at 50 seconds, followed by a message "8 1/2 months later" showing the separation of the cruise stage, and then atmospheric entry

Of course, but I thought that we spoke about aerocapture to orbit.

You said "They can not aerocapture, because probes does not have any heat shield. Aerocapturing would need a shield because high g and high temperature."

We've sent probes that did have heat shields, it can be done.

my experience with aerocapture at duna from an interplanetary trajectory usually involves getting dangerously close to the surface and pulling a lot of gs. not to say aerocapture at mars wouldn't be possible, but it might push the danger margins far beyond what is considered acceptable for a manned mission.

and

It is problem in Mars too. Both bodies have relatively small radius and have thin atmospheres. Surface is not worst safety factor. You must have much more accurate altitude than just avoid hitting the surface, because too high velocity in too dense gas is as bad.

Duna is not Mars. Mars is 10x the radius, which basically translates into 10x the distance you can spend aerobraking. While the relative velocity is higher, this doesn't really change the time ones needs to spend in the atmosphere for aerobraking, thanks to quirks of physics - basically, you just need to travel through a mass of atmosphere equivalent to the mass of your craft

http://en.wikipedia.org/wiki/Impact_depth#Applications

In KSP, Duna's Scale height is less than that of Kerbin's. In contrast, Mars scale height is almost twice that of Earth (10.8km vs 6km)

In KSP, if you have 2 aerobraking trajectories that differ by 2km, the amount of air you travel through is reduced by about half. On Mars, tht results in onl a change of 20%.

A 12km braking height in KSP is 4 scale heights... 1/54th the density of "sea level"

A 12 km braking height on Mars is barely over 1 scale height... roughly 1/3rd the density of sea level... but you get to travel through it roughly 10x longer due to the difference in radius.

even if duna's atmosphere is ~20x denser at sea level, 1/54 * 20 = 0.37, it does not compare to Mar's aerobraking power of 1/3 * 10 = 3.333... roughly 10x the atmosphere to break with at a perapsis of 12 km

In KSP, it uses atmospheric pressure as a proxy for atmospheric density - the two are not the same, a colder gas, of higher molecular weight (CO2 has a higher molecular weight than N2, which is 78% of Earth's atmosphere) is singificantly denser - of course this difference may not sound huge... a place on mars may have 0.6% the pressure of earth, but 1% the density... but that is proportionally a very significant change.

Which... menas you really have about 16x the breaking power with Mars at a 12km PE.... which means you can move your PE up another 2.8 scale heights... and get a pe 30 km higher, or roughly 42km...

A 42km PE on Mars should give you the same braking power as a 12 km PE on Duna.

Note that while planet scale is different by a factor of 10 between KSP and real life, for Kerbin, the atmospheric thickness is very similar (5km scale higher for Kerbin, 6km scale height for Earth)

The escape velocity for mars is 5 km/second. The incoming transfers have a velocity of 6.9 km/s

They only need to bleed off 1.9 km/second in the first pass, or 27% of your incoming velocity

Edited January 14, 2015 by KerikBalm

[Nuke]

aerocapture is really just aggressive aerobraking. in ksp i usually go for a highly eccentric near-escape orbit, the bare minimum of what would constitute an aerocapture maneuver. i follow up on this by using slower and more controlled aerobraking passes to reel in the apoapsis to where i want it. on some planets the g load is very small, oddly jool usually offers a more gentle breaking than does laythe, where the gs are usually in the red. once in a blue moon (i see what i did there) i can hit both jool and laythe with only minor corrections, pick up a partial deceleration job at jool, and finish at laythe without redlining the g meter. but this is a rare occurrence in a real solar system. one of these days i might load up orbiter and see what some more realistic aerocapture maneuvers look like.

hitting terrain is something you can figure out ahead of time and plan a mission around (for example timing the approach so that periapsis doesn't intersect olympus mons), but g loads can break things and kill astronauts. so you might do partial powered deceleration where the orbit remains solar, but results in less orbital energy needing to be dissipated during the aerocapture phase. this might bring the margins to manned safe levels.

[AngelLestat]

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

Try RSS with FAR and deathly reentry, not sure if the atmosphere parameters match or not, but that at least would be a bit closer to reality.

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I am agree with Nuke.

I wonder if we can sent a wing shape capsule (dreamchasser style) and try to use a different kind of surface control (I know they dont work at those speeds) or rockets to control the angle and remain in the atmosphere until we brake enoght.

If we just protect the botton part of the craft, then if we bounce too much from the atmosphere and we need to go back, we rotate 180 degress and we point our nose to ground, so the surface towards the wind is the same.. Not matter if we try to gain height or if we try to lower it.

[Hannu]

You said "They can not aerocapture, because probes does not have any heat shield. Aerocapturing would need a shield because high g and high temperature."

We've sent probes that did have heat shields, it can be done.

Only lander probes have done that. They must have heat shield in any case. Therefore it is practically use it also for braking and saving fuel, because you have not an option to avoid risks of the atmosphere. But any orbiter probe have never made an aerocapture. Orbiter can choose if it take risks and costs of aerocapture or bring enough fuel for controlled braking burn. Of course it is technically possible to aerocapture, but so far no-one have decided that it is the way of smaller costs and risks.

Duna is not Mars.

Of course not, but it has some analog properties because it is small and have thin atmosphere in KSP's scale. 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.

[PakledHostage]

The incoming transfers have a velocity of 6.9 km/s. They only need to bleed off 1.9 km/second in the first pass, or 27% of your incoming velocity

You keep citing that value (6.9 km/s) but where do you get it from? The atmosphere relative entry speeds anticipated for Mars Science Lab were only about 5.4 - 5.9 km/s, depending on the timing of the transfer from Earth. (ref: Mars Science Laboratory Mission Design Overview, Table 1 and Table 2). As you point out, MSL did not execute propulsive braking before it entered Mars atmosphere directly.

Edit: I recalled reading a paper that compared the relative risks of aerocapture vs. propulsive capture for Mars orbiters. I found it after a brief search on Google. It is titled "Assessing the Relative Risk of Aerocapture Using Probabilistic Risk Assessment". It was published by The American Institute of Aeronautics and Astronautics. There's no date on the .PDF copy of the article but it seems to have been published in about 2005. The article uses as a case study, a Mars Sample Return mission that was under investigation at the Jet Propulsion Laboratory (JPL) at the time. I mention it because it might be interesting reading for some here.

Edited January 14, 2015 by PakledHostage

[KerikBalm]

http://en.wikipedia.org/wiki/Mars_Polar_Lander#Launch_and_trajectory

" The complete burn sequence lasted for 47.7 minutes after a Thiokol Star 48B solid-fuel third stage booster placed the spacecraft into an 11-month, Mars transfer trajectory at a final velocity of 6.884 kilometers per second with respect to Mars."

Maybe I misread that, maybe that was the relative velocity at the end of the injection burn

[Hannu]

Polar lander had different transfer orbit than MSL. Orbits of Mars and Earth are eccentric (especially Mars has significantly eccentric orbit) and inclined to each other. There are quite large variations between launch windows. Probably they also selected orbit with easy re-entry to very heavy MSL.