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Aerobraking at Triton
tasp
post Jan 18 2007, 03:22 PM
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With the Voyager 2 data in hand, and the capability of the craft to do a stellar occultation of Triton prior to aerobraking, I think a first time demonstration of this technique would be very likely to succeed.

This would also apply to Pluto.

{seems like there is a thread here somewhere were it is stated possible decel rate at Triton could be ~40 Gs, the atmosphere may be thin, but it is deep}
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djellison
post Jan 18 2007, 03:26 PM
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To conduct aerobraking at Mars - they used much of the instrumentation from other spacecraft ( or the actual spacecraft doing the braking ) to calculate the safe altitude to use. I would be more confident in a system that relied on in-situ density measurements from which to determine the best atmospheric path.

Doug
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helvick
post Jan 18 2007, 06:45 PM
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This is particularly true for Triton where there is some evidence that there is long term variability in its temperature and atmospheric parameters. You would end up with a very dead spacecraft if you tried aerocapture or aerobraking at Triton and weren't able to dynamically sense the in situ parameters of relevance (temperature\density profile by altitude) and respond to them on the fly so to speak.
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nprev
post Jan 19 2007, 01:36 AM
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In engineering terms, how adaptable can an aerobraking system be made to address potentially variable conditions, assuming that the payload is designed to survive worst-case deceleration & heat-load scenarios?

In Triton's case, I would assume that the density variation is pretty long term due to Neptune's orbital period & axial tilt (please correct me if I'm wrong; Helvick, I unfortunately cannot view the article you posted). The scary possiblity is that some atmospheric constituents may abruptly sublimate or freeze out during temp-triggered phase-change events, or enhanced geyser activity during the long trip out for a mission.

Hate to eat crow here given my previous skepticism of ballutes, but that technology MAY offer enough flexibility to overcome these issues; seems as if the desired decel surface area would be easier to control, provided that uniform inflation could be achieved regardless of size.

Alternatively, maybe the lander needs "solar panels" (really speed brakes w/variable pitch) after all! What's nice about that is that they could be designed for aerodynamics rather than for maximizing surface area for power generation.


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JRehling
post Jan 24 2007, 05:09 PM
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It seems to me that one way to get control would be to design a system that could provide more deceleration than you need, with the ability to cut the ballute loose when the desired delta-v has been achieved. If need be, the delta-v could be measured by doppler shift of a signal with known frequency (eg, from Earth). Cutting the cord to the ballute would instantly end the "manuever", and it would probably fall to the surface. The nice thing is, you don't have to do any prior sensing or modeling of Triton's atmosphere. The deceleration itself is what you measure, eliminating the "middleman" analysis.

Begs for a small Ranger-style package to make the trip to Triton's surface (maybe surviving with the ballute as a parachute?) while the Neptune Orbiter zipped away, perhaps acting as a relay.
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tasp
post Jan 24 2007, 06:03 PM
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You're idea has merits, but the resulting path around Neptune is going to be pretty indeterminate. If the Triton aero-braking takes 60 degrees of Tritonian longitude or 150 . . .

You would want to be on a path that returns to Triton for further orbit shaping. Perhaps a ballute with flaps or tethers that adjust for length.
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nprev
post Jan 25 2007, 01:40 AM
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I like your idea, JR. What would be REALLY cool is if two or three of these miniprobes could fly on one mission. Not only would this increase the probability of at least one success, but if we truly score and all three survived they'd undoubtedly be dispersed across a very broad landing ellipse (or even retargeted a bit during/after the orbit shaping Tasp suggests).


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JRehling
post Jan 25 2007, 04:22 PM
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QUOTE (tasp @ Jan 24 2007, 10:03 AM) *
You're idea has merits, but the resulting path around Neptune is going to be pretty indeterminate. If the Triton aero-braking takes 60 degrees of Tritonian longitude or 150 . . .

You would want to be on a path that returns to Triton for further orbit shaping. Perhaps a ballute with flaps or tethers that adjust for length.


Hopefully, the degree of uncertainty would be less than 60 vs 150! If it were 60 vs 62, then a manuever via chemical propulsion could clean up the residuals.
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tasp
post Jan 26 2007, 03:47 AM
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Presumably, the uncertainties in braking would increase with the approach velocity. If our craft is trying to out warp New Horizons, we might have some considerable speed to burn off. I am not smart enough to work out these orbits in my head, but maybe we could utilize some hypersonic bankings alternating to the the left and right to modulate the braking manuvre and stay on track. The amplitude of the sinusoidol deflections could be fine tuned on the spot to keep our craft on track and to modulate the amount of braking while emerging at the correct Tritonian longitude. We could also 'corkscrew' through the atmosphere, even if we are down range too far at the conclusion of braking, a final 'pitch up' manuvre would approximate us back to the right path. Trick would be for the on-board guidance to do the right thing on the first try.

(this could be a pretty wild ride)
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tasp
post Jan 26 2007, 03:55 AM
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Seems like repeated banking during decel could increase our path length through the atmosphere by quite a bit, once we decel enough for a straight line path to take us where we want to go, stop the left and right bankings. Might be tremendous flexibility in combining banking and variable geometry ballute configurations during this phase of the flight.

Variable geometry ballute airfoil might be simulated by a flat disk that could be angled left/right/up/down by variable angle to the flight path just by adjusting the lengths of the tethers. That configuration might just amaze everybody in what kind of path it could fly in the Tritonian exosphere . . .
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dvandorn
post Jan 26 2007, 04:11 AM
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If you're talking about bleeding off NH or Voyager-like speeds, then you're talking about something going between 10 and 20 km/sec. Even with carefully shaped trajectories, you're talking about only a minute or two of passage through Triton's extremely, extremely thin atmosphere.

Just how many fancy maneuvers you think you'll have time to do in such a time frame?

-the other Doug


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nprev
post Jan 26 2007, 04:22 AM
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I don't think direct entry is an option, given that the atmospheric density @ Triton seems to require at least a rough-order-of-magnitude current measurement before even the most adaptable EDL systems should be allowed to proceed. The Cassini/Huygens method seems much more appropriate, given the risks.

To clarify, unnecessary risk avoidance is a core heuristic for UMSF, and that's particularly important for an almost literally once-in-a-lifetime mission like this. Relatively speaking, we can go to Mars almost any time we want to; getting to (and succeeding at) Neptune takes considerably more time & effort.


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tasp
post Aug 20 2009, 09:57 PM
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{sorry, I have no idea how to link this in a post, so I'll just bump it for the benefit of the current discussion}

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