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Pluto/Triton Lander Deceleration, using thin atmospheres for EDL
gallen_53
post Jul 4 2015, 10:31 PM
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Pluto has a very thin atmosphere. I believe it was originally measured using stellar occultation from ground based telescopes. Many years ago, I was asked to do a quick pre-Phase-A study for a Pluto atmospheric probe to be carried by the New Horizons spacecraft. At the 11th hour, someone raised the reasonable point that if we were going to travel all the way to Pluto then maybe we should drop a probe into Pluto's atmosphere. Unfortunately, the rules defining the study were impossible to satisfy. The entry speed in the inertial frame at 700 km altitude was 15 km./sec. The maximum allowed entry mass was 15 kilograms. To get the vehicle to slow down, I had to assume a very large base radius to yield a tiny ballistic coefficient. The base radius that I was forced to assume was 2 meters. That assumption yielded a ballistic coefficient of 0.62 kg/m^2 at peak dynamic pressure occurring at 36.5 km altitude. The assumed free stream density at that altitude was about 5.5e-6 kg/m^3. For purposes of comparison, the Stardust probe had a ballistic coefficient of 60 kg/m^2 at peak dynamic pressure (two orders-of-magnitude greater). The density of air at the Earth's surface is 1.225 kg/m^3. Under the study requirements, the proposed Pluto probe was effectively made out of "cotton candy" but still had to shield against a significant peak heat flux of 36 watts/cm^2. That sort of heat flux meant the "cotton candy" had to be some sort of carbon fluff (how do I deploy it with a total mass constraint of 15 kg?). However the peak g-load was 33g. Under that sort of g-load, the carbon fluff would have crushed with the aerodynamics becoming unstable. The design refused to close so we knew it was game-over and walked away from the problem. That was an unfortunate conclusion. It would have been very cool to have obtained images from the surface of Pluto like what the Huygens probe acquired for Titan along with an atmospheric model based upon in-situ data.
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qraal
post Jul 4 2015, 10:43 PM
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Wasn't there a balloon decelerator studied for a Pluto Lander? I guess that doesn't fit your mass-budget, but it does make the prospect of a lander-mission carried by a fly-by bus at least conceivable. That'd be a perfect fit for an E-Sail propelled bus.
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gallen_53
post Jul 5 2015, 12:20 AM
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The design that I earlier described assumed a spherical section geometry. Given the vehicle's low density, that was effectively a balloon. I wanted to use an umbrella design because that could have withstood the peak deceleration and have a straight forward deployment scheme. However the 15 kg constraint almost immediately removed an umbrella from consideration. People are always suggesting some sort of inflatable concept for entry vehicles but inflatables usually have issues with dynamic instability, i.e. the surface vibrates until it tears itself apart. Dynamic pressure scales as velocity squared. Imagine the force your hand feels when you stick it out your car's window while driving down the freeway. Now scale that dynamic pressure up by three or four orders of magnitude.
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Nafnlaus
post Jul 5 2015, 12:45 AM
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Did you consider any sort of trap to have a cloud of ions or ionized dust as your drag surface? I've long pondered that for solar sail purposes - trapping such as:

* Superconducting dust in a magnetic field
* Ions in a magnetic field
* Ionized dust in a magnetic field
* Ions in a RF trap
* Ionized dust in a RF trap

... and so forth. Tiny particles means rapid radiative cooling, and suspending them magnetically or electrically means easy deployment and no issues of structural strength, while having a surface area to weight ratio that no macroscopic sheet could hope to meet. Then again, any such method would certainly introduce new challenges... It's kind of tempting to try to model such a system to see what sort of drag to weight ratio one could realistically get. Maybe I should pull out Geant4 and play around with trap geometries...

How much do we actually know about Pluto's atmosphere? I've seen some diagrams before (which I can't seem to locate any more) showing that despite its extremely low pressure, its change in pressure with altitude is very small (aka it extends very high off the surface).
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nprev
post Jul 5 2015, 01:14 AM
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New topic established for this discussion. Please remember rule 1.9 re 'SF engineering'; gotta keep things in the realm of the feasible. Have fun! smile.gif


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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qraal
post Jul 5 2015, 01:25 AM
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Here's the paper I was thinking of (has been a while since I read it too)...

A LIGHT-WEIGHT INFLATABLE HYPERSONIC DRAG DEVICE FOR PLANETARY ENTRY
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qraal
post Jul 5 2015, 01:31 AM
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There's been some work on using plasma drag devices for decelerating/aerobraking. Have you looked at them?

I've always had a lot of respect for your re-entry opinions, ever since you said (years ago) that "Beagle 2" would dig a hole, due to the modeling being done using the wrong atmospheric composition - N2 instead of CO2 dominated atmosphere.

I'd really like to see a low-mass decelerator option for Pluto and Triton lander missions. Both destinations need up-close study, but orbiters would take too long to arrive. A flyby bus for a lander payload would open up the option space IMO.
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qraal
post Jul 5 2015, 01:42 AM
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Now this is funny. Gary and I have had this conversation before, as quoted on NASA Spaceflight's Forum...

http://forum.nasaspaceflight.com/index.php...33293#msg633293

...originally on Centauri Dreams about 5 years ago.

Now it's somewhat more relevant. I guess we'll have better models of the Plutonian atmosphere shortly.
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gallen_53
post Jul 5 2015, 04:00 AM
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Ah yes, the Internet never forgets.... Along this topic, last year I repeated that same line of speculation about Nereid, Triton, Pluto and Charon coming from a common body while I was on a business trip at the Applied Physics Lab (APL). I was there to present some results concerning a Uranus probe study at the Workshop on the Study of the Ice Giant Planets. Many of the guys at the workshop were high level planetary scientists. One of the planetary scientists made the obvious response that there was no reason to assume that Triton and Pluto came from the same body since objects were raining down from the Kuiper belt all the time. Since many different objects are coming in all the time, it makes more sense that Triton and Pluto should have originated from separate objects rather than assume the unlikely event that they came from a single object after going through complicated orbital mechanics.

The Ice Giant Planets workshop was held in Building 200 at APL where apparently they do unclassified work like the New Horizons spacecraft. The entry room for Building 200 has full scale models of their interplanetary spacecraft hanging from the ceiling including New Horizons and Messenger. I was struck by how small a spacecraft New Horizons was. If you go to the museum at JPL, they have a full scale model of the Cassini spacecraft on display. Cassini is huge and many times larger than New Horizons. While I was looking at the New Horizons spacecraft, it occurred to me that there was no way I could have stuck an entry probe on it. New Horizons really is just a minimalist heliocentric flyby spacecraft.
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gallen_53
post Jul 5 2015, 04:35 AM
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QUOTE (qraal @ Jul 5 2015, 01:31 AM) *
I've always had a lot of respect for your re-entry opinions, ever since you said (years ago) that "Beagle 2" would dig a hole, due to the modeling being done using the wrong atmospheric composition - N2 instead of CO2 dominated atmosphere.


I'm wrong in my opinions more often than I am correct. I was fairly certain that MSL would auger-in because Sky Crane was too complicated. That opinion was widely but quietly held in the community. In hind sight, I should have had more confidence with Sky Crane because one of its designers was Keith Comeaux. Keith is one of the smartest aerospace engineers that I have ever met. The root of my error with Sky Crane was that I visualized it as being fixed in an inertial frame. In an inertial frame, Sky Crane would have behaved like a compound pendulum which is dynamically chaotic and impossible to control. However Sky Crane was in a decelerating frame which made it dynamically stable. I still think MSL was a mistake because the money spent on it would have been better spent mass-producing MER clones but that's another story.

I have no idea how one would use plasma drag devices for an entry vehicle. Entry vehicle designers are extremely conservative. Your typical spacecraft heat shield is very prosaic in its design and made out of ordinary stuff like cork dust mixed with resin. Convincing people to do something as radical as instrumenting a heat shield was politically difficult to do. Fortunately NASA did do that with MSL and found out that the design methodology was too conservative. We significantly over predicted the turbulent heat flux and carried too much thermal protection mass to Mars. They've also made a point of properly instrumenting the heat shield for the Orion capsule. When the Orion Capsule is actually carrying people, it should be a very efficient design.
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mcaplinger
post Jul 5 2015, 04:56 AM
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QUOTE (qraal @ Jul 4 2015, 06:31 PM) *
...ever since you said (years ago) that "Beagle 2" would dig a hole, due to the modeling being done using the wrong atmospheric composition - N2 instead of CO2 dominated atmosphere.

Of course, all the evidence suggests that Beagle 2 landed successfully and simply failed to unfold, so the EDL system isn't implicated.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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gallen_53
post Jul 5 2015, 05:16 AM
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QUOTE (qraal @ Jul 5 2015, 01:25 AM) *
Here's the paper I was thinking of (has been a while since I read it too)...

A LIGHT-WEIGHT INFLATABLE HYPERSONIC DRAG DEVICE FOR PLANETARY ENTRY


I just looked at the PDF that you linked. The author was Angus D. McRonald. He was the guy from JPL who tasked us to do our study of the Pluto lander. I was previously unaware of this paper and know nothing about its history. I assume that the work behind the paper was done independently from our study since we were not acknowledged in the paper. It's not uncommon to commission two independent studies for the same problem to enhance confidence in the conclusion. Years ago, I did the thermal protection system (TPS) sizing for the Mars Exploration Rover aeroshell while another team did the same study within Lockheed Martin. The Lockheed guys reproduced my sizing to two significant digits using a different software (they used RECAP) while I used my own software, Traj. The fact that two separate groups independently produced the same result to two significant digits gave everyone a "warm and fuzzy feeling" during the comprehensive design review of MER's aeroshell.
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TheAnt
post Jul 5 2015, 05:21 AM
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QUOTE (gallen_53 @ Jul 5 2015, 12:31 AM) *
...... That sort of heat flux meant the "cotton candy" had to be some sort of carbon fluff (how do I deploy it with a total mass constraint of 15 kg?). However the peak g-load was 33g. Under that sort of g-load, the carbon fluff would have crushed with the aerodynamics becoming unstable. The design refused to close so we knew it was game-over and walked away from the problem.


Aha, I suspected that at someone at least had taken a look at the problem of getting a lander to Pluto together with NH. Turned out to be 2 studies done even. =)
Even without knowing any details I suspected it would be a near impossible feat, cotton candy subjected to 33g sounds pretty much like that.
Thank you for the info that that suggest it might be possible to send a low mass lander some time in the future.
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gallen_53
post Jul 5 2015, 08:26 AM
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Pluto is interesting mainly because it is a Kuiper belt object. Neptune's moon Triton is also a Kuiper belt object. I suspect that Triton and Pluto are very similar. Triton also has a very tenuous atmosphere. I don't know if Triton's atmosphere is dense enough to significantly slow down an entry vehicle. The idea has been kicked around to aerocapture a spacecraft into the Neptune system using Neptune's atmosphere. The problem with aerocapture is one must do a periapsis raising manuever at apoapsis otherwise the vehicle reenters the atmosphere on the first return periapsis. An idea that I like is to use Triton's atmosphere for the periapsis raising maneuver. One could theoretically get a spacecraft into a Triton orbit with no fuel consumption by doing clever aerocapture maneuvers off of Neptune's and Triton's atmospheres. The problem with the concept is the spacecraft needs to spend a long time in Neptune's atmosphere to aerocapture to Triton (about 10 minutes). Supposedly the heat load is so great from doing that maneuver that conventional thermal protection materials like carbon-phenolic will not work, i.e. the heat soak eventually convects through the carbon-phenolic and fries the payload. Also, the planetary scientists have told me that Triton may not be all that interesting as a Kuiper belt object because it got cooked during the capture process that put it in orbit around Neptune. The hard truth is that any world past Saturn is too far away and requires too much travel time. That situation might change if they could get solar electric propulsion sorted out (the Dawn spacecraft is very exciting because it has fully demonstrated the capability of solar electric propulsion). We found in our Uranus probe study that solar electric propulsion was an enabling technology that not only got us to Uranus quicker but also freed us from the tyranny of planetary assist orbital mechanics. My suspicion is that if we want to study Kuiper belt objects then we should probably focus on Saturn's moon Phoebe. Saturn is a fantastic system for "one stop shopping". Not only is Saturn interesting in its own right but there is also Titan, Saturn's rings, Phoebe and Enceladus. This fixation over Jupiter's moon Europa is something that I have never understood. If one wants to do a "water world" then Enceladus is the obvious choice. Enceladus has water geysers and does not have the problem of Jupiter's extreme radiation environment. If I was king, I'd send a flagship class spacecraft to Saturn. While entering the Saturn system, I'd drop a lander on Titan that could float on one of Titan's ethane lakes, split off a separate lander to Phoebe, do the Saturn orbit insertion burn and then send the main spacecraft to Enceladus to look for sea monsters.
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antipode
post Jul 6 2015, 07:01 AM
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QUOTE
I suspect that Triton and Pluto are very similar.


Except that Triton has had 4[?] billion years of being poked and prodded by Neptune, so even if they started off similar, they probably aren't now. Hopefully we will know in a few weeks!

P
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