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nprev
Flying on Triton is no problem; you just need a chopper with blades a couple of kilometers long... rolleyes.gif

(Semi)seriously, would anything we think of as atmospheric flight work at all on Triton? I doubt that even a balloon "filled" with several cubic km of lab-quality vacuum would generate enough lift to get itself off the surface, much less a useful payload. Mars by comparison is a veritable pressure cooker.
tty
QUOTE (nprev @ Jan 15 2007, 03:01 PM) *
(Semi)seriously, would anything we think of as atmospheric flight work at all on Triton? I doubt that even a balloon "filled" with several cubic km of lab-quality vacuum would generate enough lift to get itself off the surface, much less a useful payload. Mars by comparison is a veritable pressure cooker.


The question is, would the atmosphere be dense enough for aerobraking? If so an aerobraking orbit with a peritritonium a couple of kilometers above the surface would presumably permit obtaining some fairly dramatic realtime video footage. In theory you should be able to come right down to daisycutter altitude, but the uncertainties of the local gravity field and relief would require some margin. It might also be a good idea to avoid geyser plumes. Hypersonic snowflakes can be nasty.

tty
nprev
Interesting thought, TTY. I guess wind isn't even a concern; not a lot of Newtons there compared to a "traditional" atmosphere.

(This should probably be moved to the Neptune/Triton thread), but here goes anyhow: Do you think a lifting/reentry body (by this I mean a solid structure) would provide any real benefit for this mission profile? I wonder if an initial entry using something like a big inverted-umbrella disc would provide enough deceleration in that thin air to save a good deal of fuel for getting into orbit around Triton. The effective surface area of a ballute for this application seems a bit large given that it's presumably not as resistant to heating.

EDIT: I probably didn't express my last point very clearly. What I meant was that a ballute large enough to do any good in Triton's atmosphere may suffer from asymmetric inflation problems, which in turn could cause localized heating problems (as well as aerodynamic instability)...just seems risky given our current lack of experience with them.
helvick
QUOTE (tty @ Jan 15 2007, 07:54 PM) *
The question is, would the atmosphere be dense enough for aerobraking?

Although there is plenty of drag available at approach velocities (~20km/sec) that might be useful initially the terminal velocity in that atmosphere would be very high - somewhere between 2 and 10km/sec depending on the coefficient of drag of your orbiter. I think that means that you definitely need to have a powered deceleration capability as Triton escape velocity is only 1;5km/sec.
edstrick
Without taking the <for me excessive> time to <figure out how to> do the numbers <and do them>...

I suspect that on Triton, you do have to deal with the atmosphere, but it's useless. You try to orbit through it and you'll get bits melted off your spacecraft, but it won't slow you down enough to be useful for entry and landing. Similar to the way Mars is for parachute landings... You slow down, but not enough to get by with just the chute. .... zip... CRUNCH!
tty
QUOTE (helvick @ Jan 16 2007, 08:37 AM) *
Although there is plenty of drag available at approach velocities (~20km/sec) that might be useful initially the terminal velocity in that atmosphere would be very high - somewhere between 2 and 10km/sec depending on the coefficient of drag of your orbiter. I think that means that you definitely need to have a powered deceleration capability as Triton escape velocity is only 1;5km/sec.


I was thinking more in terms of aerobraking in the MGS or MRO style, i e using it to circularize a highly elliptic initial orbit by repeated passes through the atmosphere. This would still mean an appreciable saving in weight as well as an opportunity to study the surface at very close range (though rather briefly).

tty
edstrick
My *PREJUDICE* is that any atmosphere thick enough to have hazes is thick enough to aerobrake with.
helvick
QUOTE (edstrick @ Jan 17 2007, 09:39 AM) *
My *PREJUDICE* is that any atmosphere thick enough to have hazes is thick enough to aerobrake with.

All this got me thinking about the fact that aerobraking happens kind of high up where the atmospheric density is quite low. Since Triton's gravity is fairly low Triton's atmosphere has a fairly large scale height of around 20km.

If the Tritonian surface atmospheric density is actually 100th of 1% of Earth's ( which is 1200g/m^3) then with that scale height of 20km you find that the density at 110km is very close to that for Mars at the same altitude with its 20g mean surface density and 11km scale height.

MRO's lowest altitude during aerobraking varied from 95-110km.

So maybe this isn't so mad after all.

Edited to add - this is now way OT for a Mars forum thread and I think we should either move this to the Uranus and Neptune forum or return to the topic of potential robotic helicopter probes on mars...
nprev
Hmm...Helvick saves the day again with his powerful secret weapon, deep knowledge of math & physics! smile.gif

That does sound perfectly feasible. I guess the major variables really are how much velocity needs to be shed before entering orbit, and whether savings can be realized via creative trajectories prior to attempting it. Perhaps we'd have to aerobrake once at Neptune during entry into the system & then do the same at Triton (f this is even feasible with the moon's retrograde orbit & 157 deg orbital inclination!) .

Still not a fan of ballutes yet, though. That technology needs to be VERY well tested before trying it on a mission that could only be re-flown a couple of times per century (transit time included).
Bob Shaw
QUOTE (nprev @ Jan 17 2007, 10:42 PM) *
That does sound perfectly feasible. I guess the major variables really are how much velocity needs to be shed before entering orbit, and whether savings can be realized via creative trajectories prior to attempting it. Perhaps we'd have to aerobrake once at Neptune during entry into the system & then do the same at Triton (f this is even feasible with the moon's retrograde orbit & 157 deg orbital inclination!) .



My back-of-the envelope calculations (of the helicopter-like armwaving variety) led me a-a-a-a-ges ago to speculate that there might be some very strange orbital mechanics going on there which could allow for some quite non-intuitive trajectories, eg from an interplanetary orbit to Triton making use of it's retrograde orbit. I also suggested that Triton, or other retrograde satellites, might be a safe repository for naturally-formed interplanetary material, such as interstellar grains and ejecta.

I have to say, though, the whole idea still makes my head ache!


Bob Shaw
Greg Hullender
Why use Triton, though? Neptune has all kinds of atmosphere.
djellison
That is what I was thinking - Neptune would really make a bigger, deeper, thicker, wider, and frankly - bluer way of doing the same procedure.

Doug
nprev
Limited as it is, Triton's atmosphere is still an aerobraking resource per Helvick's calculations. Whether it SHOULD be used as such is a whole other issue. It may conceivably give some flexibility in trajectory planning & fuel budgeting (i.e., a Neptune orbiter/Triton lander might be able to cast off the lander at some point during its own aerobraking sequence & let the lander do its own aerobraking @ Triton). That scenario is kind of complex in terms of mission-critical events, but may be worth thinking about in terms of fuel savings.

And actually, isn't it almost axiomatic that highly elliptical orbits are much cheaper to achieve in terms of delta-V? If that's a valid assumption, then this 'dual-aerobraking' scenario might allow us to fly a significantly larger lander payload.
Greg Hullender
Here's a question I've wanted to ask to anyone who might know the answer for sure: to do aerobraking at places like Neptune and Triton, is it necessary to send a few probes first to get exact measures of atmospheric density, or could we "wing it" based on what we know now? Our Mars experience suggests to me that this is a really delicate operation, and that implies that we need precise measurements to attempt it.
mchan
Some of the Neptune / Triton mission studies propose not just aerobraking but aerocapturing, i.e., use Neptune's atmosphere instead of a rocket engine to slow down from the hyperbolic interplanetary transfer trajectory to some kind of orbit. The illustrations of the aerobrake shell looks like a bloated surf board.
tasp
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}
djellison
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
helvick
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.
nprev
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.
JRehling
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.
tasp
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.
nprev
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).
JRehling
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.
tasp
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)
tasp
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 . . .
dvandorn
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
nprev
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.
tasp
{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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