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Aerobraking at Triton
nprev
post Jan 15 2007, 02:01 PM
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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.


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tty
post Jan 15 2007, 07:54 PM
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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
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nprev
post Jan 16 2007, 02:09 AM
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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.


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helvick
post Jan 16 2007, 07:37 AM
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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.
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edstrick
post Jan 16 2007, 12:20 PM
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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!
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tty
post Jan 16 2007, 07:26 PM
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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
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edstrick
post Jan 17 2007, 09:39 AM
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My *PREJUDICE* is that any atmosphere thick enough to have hazes is thick enough to aerobrake with.
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helvick
post Jan 17 2007, 06:00 PM
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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...
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nprev
post Jan 17 2007, 10:42 PM
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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).


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Bob Shaw
post Jan 18 2007, 12:06 AM
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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


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Greg Hullender
post Jan 18 2007, 12:38 AM
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Why use Triton, though? Neptune has all kinds of atmosphere.
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djellison
post Jan 18 2007, 12:44 AM
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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
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nprev
post Jan 18 2007, 02:43 AM
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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.


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Greg Hullender
post Jan 18 2007, 06:17 AM
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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.
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mchan
post Jan 18 2007, 08:41 AM
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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.
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