Aerobraking mechanics

[Guest LoSboccacc]

Well, I\'ve some question about aerobraking: I did a retro burn from a 610km orbit and plunged down to atmosphere down to around 55k.

The apokee went down to 590km, but the perikee went up to 60k. I waited for another orbit and the apokee went down another bit, but the perikee went again further up.

I\'m a bit confused, is that supposed to happen to the perikee?

[Arthree]

Did you have winglets on it?

[Guest LoSboccacc]

just the command pod, I detached every other thing after the retro burn

[Cannon_Fodder]

Not sure just the capsule will cause enough drag for effective aerobraking.

But it sounds about right. The speed change from the minor speed change is putting you into a more ecentric orbit. As the perikee keeps dropping you with hit thicker and thicker atmosphere and get slower and slower and eventually not be able to get back out to orbit.

Once in atmosphere you can deploy your parachute. It works as a drag chute at high altitudes.

[Guest LoSboccacc]

the point is that the perikee rises, instead of getting lowered.

drag is enough to affect the orbit as the apokee lowers of quite a bit at each passage

[Cannon_Fodder]

That still sounds right, lower the speed at perikee and the orbit adjusts to match the new velocity.

The farther from Kearth you are the slower you need to be to stay in orbit.

You have too much velocity at perikee to 'fall' out of orbit and your orbit. Next time keep your retro burn rockets with you. The higher drag and mass will help you.

[rdfox]

Once in atmosphere you can deploy your parachute. It works as a drag chute at high altitudes.

I *sooo* wish the Mk16-3 had a top node on it.

Then I could use the standard Mk 16 as a drogue 'chute! ;D

[Entroper]

That still sounds right, lower the speed at perikee and the orbit adjusts to match the new velocity.

The farther from Kearth you are the slower you need to be to stay in orbit.

You have too much velocity at perikee to 'fall' out of orbit and your orbit. Next time keep your retro burn rockets with you. The higher drag and mass will help you.

Doesn't sound right to me, and doesn't match my experience. If you make a retro burn at perikee, it lowers your apokee without affecting perikee. If you do a retro burn near perikee (similar to the effect of aerobraking), it lowers apokee a lot and perikee a little.

When I've done aerobraking on an elliptical orbit, the apokee and perikee are both lower on every pass. This is what should happen as you reduce your orbital velocity. The only explanation for a raised perikee with a reduced velocity is that some non-tangential force was applied.

Perhaps try maintaining a heat-shield-forward orientation while in the atmosphere? I know that, once you get into reasonably thick air, the capsule will orient itself in this direction naturally due to drag, so obviously the drag model can apply torque. It makes sense, then, that the drag model can also apply force in a direction other than tangent to your direction of motion. In other words, the capsule might be producing lift if it's facing the wrong way.

[Guest LoSboccacc]

In other words, the capsule might be producing lift if it's facing the wrong way.

I'll try. Sadly it still takes too time to verify this kind of stuf... but with the next update....... ;D

[Hypocee]

This is dangerously outside my comfort zone, but from what I understand of mechanics I think you could see a circularizing effect (with rising perikerb) from drag even with no deflection lift, provided that there is a significant radial component to the entry velocity vector. It seems likely that this could be so in the case of a highly elongated 600-to-entry orbit.

I\'m thinking of it in terms of mechanical work. At first glance, it seems obvious that any work from a pure drag force must always occur exactly along the orbit, which should only reduce the perikerb. However, entering and exiting a spherical atmosphere may change that. I choose to ignore the variation in atmospheric density because a. it makes my brainmeats quiver and b. I think it may actually favor circularization but is at worst irrelevant in the case of pure drag.

Let\'s consider the work done on the capsule during the first half of the 'burn', to perikerb, and the second half back out to space. Consider the transit times in and out to be effectively equal for the sake of argument. This is the shakiest step in my chain of logic; obviously the outbound trip takes longer, but I\'m assuming that this increase is dominated by the decrease of the v² term in the drag equation.

Provided those assumptions, the work done on the capsule is greater during the inbound trip than the outbound because the magnitude of the velocity has been reduced before the outbound half. Thus, there is a net outward radial component to the work done, equivalent to an outward radial burn, raising the exit angle from what it would have been without atmosphere and thus increasing the next perikerb. (edit: ...and causing a retrograde shift in the argument of periapsis, which we\'re not currently well set up to measure but which could theoretically verify whether I\'m full of crap.)

This phenomenon would, of course, never raise perikerb beyond the atmosphere and would become less and less significant as the orbit became more circular with shallower entry angles.

[Guest LoSboccacc]

it makes sense. If that\'s true, it can be verifyed by starting the retrograde burn from the vertical of the space center and see if the apokee moves orbitwise at each passage.

[Guest LoSboccacc]

http://ssd.jpl.nasa.gov/jdg/papers/magellan.pdf

I\'ve found this, contains real data about the magellan mission to venus.

there is a graph of periapsis and apopasis during the various aerobraking orbits.

there is also a plotted graph of the argument of the periapsis.

the idea is exactly that being the drag force asymmetrical as speed slows down from the enter to the exit of the atmos, then the periapsis argument decreases and it\'s height rises.

if the orbit is elliptical enough then this effect is less evident: you pass faster in the atmos and the drag force is stronger and dominates the orbiting effects (see it in the first half of the magellan periapsis graph); in this situation the periaspsis lowers and its argument increases.

[Hypocee]

Hah, excellent. I knew aerobraking circularization had been done, but I didn\'t bother searching for citations since I couldn\'t trust the trickssy flight controllers not to use confounding factors like lift and supplemental circularizing burns. That paper has enough detail to eliminate those, though to be fair it doesn\'t explicitly spell out the reason for the rises in periapsis. Probably right on my little model, though.

Hanging out on this forum warps your brain by the way, with its perfectionist nerds practicing shaving meters for docking maneuvers around a tiny tiny planet. I LOL\'d when I met the pros\' '197 x 541 km near-circular orbit'

[Saaur]

Hanging out on this forum warps your brain by the way, with its perfectionist nerds practicing shaving meters for docking maneuvers around a tiny tiny planet. I LOL\'d when I met the pros\' '197 x 541 km near-circular orbit'

I\'m LOLing now. I\'m also almost certain I could do better.

Of course, the question then becomes: why? 'Close enough for government work' does have real meaning in the vastness of space.

[thorfinn]

http://ssd.jpl.nasa.gov/jdg/papers/magellan.pdf

I\'ve found this, contains real data about the magellan mission to venus.

there is a graph of periapsis and apopasis during the various aerobraking orbits.

there is also a plotted graph of the argument of the periapsis.

the idea is exactly that being the drag force asymmetrical as speed slows down from the enter to the exit of the atmos, then the periapsis argument decreases and it\'s height rises.

But the graph of apoapsis and periapsis at page 8 shows both apses decreasing throughout the aerobrake; the only anomalous interval is from about 21 to 27 July. From cursory reading of the document, I suspect influence of thruster firings and maneuvering on that. Less cursory reading finds the reason explained at page six: gravity field inhomogeneities acted to raise the perigee in that period of time.

Since gravity anomalies are obviously not modeled in KSP, the bug seems to be still there. (That, or the vanilla command pod does have some lift even though it shouldn\'t.)

[Entroper]

But the graph of apoapsis and periapsis at page 8 shows both apses decreasing throughout the aerobrake; the only anomalous interval is from about 21 to 27 July. From cursory reading of the document, I suspect influence of thruster firings and maneuvering on that. Less cursory reading finds the reason explained at page six: gravity field inhomogeneities acted to raise the perigee in that period of time.

Since gravity anomalies are obviously not modeled in KSP, the bug seems to be still there. (That, or the vanilla command pod does have some lift even though it shouldn\'t.)

http://www.quickmeme.com/meme/3543gq/

I\'ll be trying some aerobraking tonight with the command pod in various orientations.

[Hypocee]

Ah, true; I missed that, oops. I\'ve given up on expressing that I\'m not obsessed with my casual thought experiment being right; please attach some nonchalance to this -> I\'ll just say it one last time and then shut up pending testing: my thing could still be true. Kerbin\'s atmosphere and gravity field are radically curved compared to any real planet, and may display orbital phenomena that are negligible in the real universe. Part of the joke of that orbital eccentricity comparison is that it\'s actually comparing a (gah) 6247 x 6591-km orbit with (edit) a 1200 x 655-km one in equivalent units.

[The_Duck]

I believe that any drag force that acts directly opposite the velocity must lower both the perikee and the apokee. Neglect the atmosphere for a moment, and consider the ellipse the ship orbits in when in free fall. One focus of the ellipse is the center of the planet. Now, briefly apply some drag force. Afterwards the ship is travelling along a new ellipse, still with one focus at the center of the planet. The new ellipse intersects the old ellipse at the current position of the ship, since both orbits pass through this point in space. In fact, the two ellipses are tangent at this point, because in both orbits the direction of travel of the ship is the same at this point in space. This is true because the drag force slows the ship down without changing its direction of travel.

I think a few minutes drawing ellipses with a pencil and paper will convince one that if two ellipses share a focus, and are tangent at a certain point, then one ellipse lies entirely inside the other (except that they kiss at the tangent point). Therefore the new orbit has both a lower apokee and a lower perikee after the brief drag force is applied. And if a drag force is applied over an extended period, then the apokee will continuously decrease as long as the drag force exists.

--

You can also derive this mathematically (well, more mathematically). The approach I took is to express the (conserved) angular momentum and energy of the ship in terms of its altitude and velocity at perikee. A force applied against the velocity at some point in the orbit will produce a certain change in the angular momentum and energy of the ship. From the relationship between these quantities and the altitude and velocity of perikee, you can calculate the change in the altitude of perikee due to the drag force. The result shows that a drag force always lowers the perikee.

A possibly useful aspect of the result is that the rate of lowering of the perikee due to a given force is proportional to (V^2 - v^2), where v is the current velocity and V is the velocity at perikee.

--

I would speculate that the observed rise in perikee may result from slow buildup of numerical errors in the physics simulation. For example, when there is no atmospheric drag a coasting ship should orbit in an unchanging ellipse with fixed and ujnchanging apokee and perikee. However I think in practice in KSP people observe slight variations in apokee and perikee even when their orbits do not touch the atmosphere. In the several hundred kilometer fall from a high orbit to the atmosphere, you only need the numerical error to build up to ~1% to shift the perikee by several kilometers.

[Hypocee]

Again I don\'t necessarily disagree, but

Now, briefly apply some drag force. Afterwards the ship is travelling along a new ellipse, still with one focus at the center of the planet. The new ellipse intersects the old ellipse at the current position of the ship, since both orbits pass through this point in space. In fact, the two ellipses are tangent at this point, because in both orbits the direction of travel of the ship is the same at this point in space. This is true because the drag force slows the ship down without changing its direction of travel.

this is begging the question. The whole point is that a pass through the atmosphere cannot reasonably be treated as a 'brief' impulse, and that this may mean it cannot reasonably be treated as acting purely along the secant/mean/whatever orbital velocity vector.

[Iskierka]

Passing through an atmosphere in and of itself is not a brief impulse, but any given moment in the process of such is a brief impulse. His method shows that, for one brief retrograde impulse, both apses will either remain the same or reduce. Therefore, for a continuous set of brief impulses, such as passing through the atmosphere, both apses must still either remain the same or reduce. An increase in one is extremely anomalous.

[Guest LoSboccacc]

I guess that with the new orbital calculator we could have better measurement of parameters before and after we pass trough the atmosphere.

[Entroper]

Alright, I\'m testing this now. I\'m in an 875 km x 655 km orbit, coasting downward. I\'m going to point the capsule away from Kerbin while it coasts through the atmosphere, and see what happens.

EDIT:

Perikee #1 at 655 km, on the nose. Which suggests that the aerobraking didn\'t lower the perikee before I reached it. Could be promising.

Apokee #2 at 850 km, as expected.

Perikee #2 at 655 km, on the nose, again. I think we\'re onto something here, whatever non-tangential force is being applied by the drag is counteracting the reduction in perikee that should be taking place.

EDIT 2:

Yep. I orbited for 3+ hours with the perikee remaining at 655 km. As soon as I flipped the capsule around, my next perikee was 653.5 km, then 651.8 km. And it looks like my apokee for that last one is going to be 656.2 km, which is a massive decrease from the previous orbit. I\'m prepared to say that flipping the capsule around to point toward Kerbin actually pushed me into the atmosphere during aerobraking.

[rdfox]

...er, if perikee is at 655km, then you\'re not gonna be getting ANY aerobraking effect, because you won\'t be in the atmosphere. By definition, perikee is the point of closest approach to the surface...

[Entroper]

...er, if perikee is at 655km, then you\'re not gonna be getting ANY aerobraking effect, because you won\'t be in the atmosphere. By definition, perikee is the point of closest approach to the surface...

I initially gave the orbit dimensions as 875 km x 655 km, so subtract 600 km (Kerbin\'s radius) if you want altitudes. 55 km is well into the atmosphere.

EDIT: Even those dimensions are confusing, because they\'re radii as opposed to axis dimensions. Is it common practice to use altitudes, radii, or axes when giving orbit dimensions as x by y?

[Guest LoSboccacc]

I\'m used to altitude just because it\'s what the gauges reads and what the calculator uses. Dunno for the others, but it was clear after the first moments of duh.