New Mars Forums

Tom, do you really think that some foreign power threatens to occupy and "rule over" the US?  Doesn't that seem absurd, even to you?

Who would it be, exactly?  Iraq?  Al Qaeda?  Afghanistan? 

For the sake of argument, say it's not absurd, say "they" can "rule over" anyone not wearing a tin foil hat.  The major cities of the US immediately succumb to "their" insidious influence.  Wouldn't succession be the worst possible strategy?  Then "they" would be right next door instead of ... wherever "they" are now.  Shouldn't you just nuke the cities and accept any refugees that got their tin foil hat on in time?

When a payload is accelerated by the ribbon, it exerts a force in the same direction as drag, deflecting the ribbon (I assume the payload is aerodynamic and provides it's own lift, but I ignore drag on the payload in the following).  Presumably that deflection needs to be limited to some extent, setting a limit on the payload.  The deflection is vaguely proportional to the ratio of the force exerted by the payload to the tension in the ribbon (high tension, less deflection).  It's hard to guess what the limit ratio might be - almost certainly < 1, very probably < 0.5, may be as low as 0.01 - I chose 0.1 for the following.

To accelerate to 8 km/s over 1000 km ribbon with

max stress (M Pa m^3 / kg) --- requiring tensioning mass (tons) --- max payload is (tons)

1.7 --- 25 --- 1.0
3.5 --- 50 --- 2.5
10 --- 150 --- 7.5
45 --- 700 --- 35
100 --- 1600 --- 78

3.5 M Pa m^3 / kg is the max stress that the high end Toray product can handle.  1.7 is that with a x2 safety factor.  100 is the figure assumed by the NIAC space elevator report.  45 is the figure quoted by liftport as the minimum economically feasible for a space elevator (and is roughly the figure implied by the LANL SuperThread press release).  10 is a predictable figure for near term CNT fiber composites.

If the limit ratio turns out to be 0.01, you'll have to divide the payload figures by 10, if 0.5 is doable, you can multiply them by 5.  If you double the length of the ribbon to 2000 km (and half the rotation rate) you can double the payloads, but you also have to double the tensioning mass figures.

I had a look to see if stresses were reasonable during steady state and deployment.  The stress/density ratio graph for the scenario from the previous post looks like this ...

http://www.geocities.com/noosfractal/st … seline.pdf

i.e., it peaks at 0.41 M Pa m^3 / kg.  This is well within the capability of modern carbon fiber composites such as those sold by Toray ...

http://torayusa.com/cfa/product.html

Their baseline T300 product has a tensile strength of 3.5 GPa at a density of 1760 kg/m^3 = 2.0 M Pa m^3 / kg.  Their high end T1000G product rates 3.5 M Pa m^3 / kg.

This density would give a ribbon thickness ~ 1 mm to maintain the 240g/m of the wikipedia article.

As a bit of an aside (since it doesn’t seem to be necessary and may be undesirable since it lowers ribbon tension), while playing around with passive control ideas, I found that if you can get some control over lift (and thus the climb angle), then a slightly different ribbon profile can cause the stress to peak at about half the baseline ...

http://www.geocities.com/noosfractal/stress_control.pdf

The 0.2 M Pa m^3 / kg figure would allow you to use high end Titanium alloys, but I'll stick with carbon fiber composites.

The T300 product wouldn’t make it through the 5 rpm deployment scenario, and the T1000G product goes right to the limit.  Halving the ribbon width for the first 50 kms deployed keeps the stresses under the T300 level.

Keeping the lift relatively constant is a challenge.  It doesn’t have to be rock steady, but it does have to be kept within bounds to keep drag under control.  There are also problematic failure modes like the ribbon twisting into a helix.  As well as a wing, the ribbon also has to be a launch rail, so payload transients have to be taken into account. 

The are some clever ideas out there for passive regulation (the space elevator needs something like this as well for the section of the ribbon in the atmosphere): “kite tails”, shaping the airfoil so that it generates counter-twist vortices and various anti-stall mechanisms.  A distributed version of Gurney microtabs might be an active control option.

===

Edit: oops, the stress curves were upside down.  They need to be zero at the free end unless there is a tensioning mass.  I've updated the graphs.

I did the simplest possible finite element analysis on the aerovator, and I think it is deployable as described in the wikipedia article – self lifting from the hub – if the hub can vary it’s rate of rotation.  I’ll describe what I did so that you can poke holes in it.

Each segment of the ribbon is at some angle with respect to the ground in the radial direction (the climb angle).  Lift is generated perpendicular to the lifting surface, so it will have a horizontal component (back towards the hub) and a vertical component that lifts the ribbon.  The horizontal component is balanced by the centrifugal force of the rotating ribbon mass, the vertical component is balanced by the weight of the ribbon.

The ribbon is light weight, and early on the air is dense, so the climb angle can be steep and still support the ribbon weight, even though the airspeed is also low.  The centrifugal force generated by a ribbon segment is proportional to it’s distance from the hub, while most of the opposing force generated by lift occurs early on generated by the steep climb angles.

When the above forces are balanced (I’ll get to drag next), the ribbon profile looks like this ...

http://www.geocities.com/noosfractal/aerovator.pdf

This is for a tip speed of 8 km/s ( = 13 minutes per rotation ).  I used a specific mass of  240g per linear meter (because the wikipedia article had the total mass at 240 tons) and a ribbon width of 100mm (less width = less drag and a 1000 km wing just doesn’t need that much lift).  For the coefficient of lift I used 0.5 (thin plate with an angle of attack of 5 degrees).  I used this simple model of the atmosphere.

The above profile isn’t practical because the forces are exactly balanced and unstable in the direction back towards the hub, but a stabilizing tensioning mass at the tip can be quite modest.  On the other hand, even larger tensioning masses don’t seriously deform the profile because induced small changes in climb angle generate large counterforces.

The profile isn’t optimized to reduce drag.  I used conservative coefficients of drag (0.04 for subsonic and 0.20 for supersonic, transitioning at 270 m/s) and the total drag generated was less than 50% of the radial tension – enough to cause the tip to trail, but not enough to prevent the hub from accelerating the tip.  I haven’t properly modeled drag – it should peak at Mach 1 and then drop again – and the coefficients are probably too high, so I didn’t look into reducing drag, but it is definitely possible to operate at higher altitudes than shown in the profile, and, as a reference, a tensioning mass of 60 tons at the tip produces a force of the same magnitude as the total calculated drag.

I used the same model to look at deployment scenarios.  Because most of the counterbalancing centrifugal force is generated after the 500 km mark, you need a tensioning mass for deployment.  At 1/13th rpm you need a peak tensioning mass of 450 tons (at altitude 16 kms, tip speed 45 m/s).  Not completely out of the question if the tensioning mass is a self lifting launch vehicle, but problematic.  However, if the hub can operate at up to 5 rpm, the required peak tensioning mass drops to 30 tons (at altitude 15 kms, tip speed 180 m/s) which is imaginable as a more or less unpowered wing designed to survive the entire deployment. 

Once you pass the peak tensioning requirement, the hub can gradually slow to the target velocity.  Note that after this point, there is no real rush to deploy since the situation is quite stable.  Also, the ribbon tip and tensioning mass are well clear of the troposphere before going supersonic.

I haven't looked much at payload launch dynamics, but the profile above provides a peak vertical acceleration of about 0.1g from about the 50 km mark, so the payload would have to provide for its own altitude gain.  After the 5 km mark, the radial acceleration rises linearly to 6g as advertised.

Sharp Views Show Ground Ice on Mars is Patchy and Variable
http://themis.asu.edu/news-groundice

Using observations by NASA's Mars Odyssey orbiter, scientists have discovered that water ice lies at variable depths over small-scale patches on Mars.
...
The new results were made using infrared images of sites on far-northern and far-southern Mars, where buried water ice within an arm's length of the surface was found five years ago by the Gamma Ray Spectrometer suite of instruments on Mars Odyssey. The smallest patches detectable by those instruments are several hundred times larger than details detectable by the new method of mapping depth-to-ice, which sees differences over scales of a few hundred yards or meters.
...
The resulting maps show that the nature of the surface soil makes a difference in how close to the surface the ice lies. Areas with many rocks at the surface, Bandfield explained, "pump a lot of heat into the ground and increase the depth where you'll find stable ice." In contrast, dusty areas tend to insulate the ice, allowing it to survive closer to the surface. "These two surface materials -- rock and dust -- vary widely across the ground, giving underground ice a patchy distribution," he said.

Computer models helped him interpret the temperature observations, he said. "They show areas where water ice would be only an inch or so under the soil, while in other areas ice could lie many feet below the surface."
...

Weldon: Democrats Set to Cripple Manned Space Program
http://www.spaceref.com/news/viewpr.nl.html?pid=22546

...
"It's increasingly clear that Democratic leaders have our manned space program in their crosshairs," said Weldon. Weldon noted that at the hearing to introduce his proposal Rep. Dennis Cardoza (D-CA), who sits on the powerful Rules Committee, said he opposed the amendment because he was 'not convinced' of the need for human space exploration.
...

http://www.physorg.com/news97337321.html

The University of Warwick team , Dr John Southworth, Dr Peter Wheatley and Giles Sams are the first people to calculate accurate measures of the surface gravity of all 14 known gas giant planets beyond our solar system that can be observed transiting (moving across the face of) their star.
...
All but one of these 14 known gas giant planets that can be seen transiting their star have a planetary radius bigger than Jupiter.
...
Despite all but one of the gas giants (HD 149026) being bigger than Jupiter all but one of them turned out to have surface gravities that are much lower than Jupiter’s. Only OGLE-TR-113 was found to have a surface gravity higher than Jupiter’s.

In fact they found that 4 of these planets actually have surface gravities close to or lower than that of Earth’s
...
University of Warwick researcher John Southworth said: "This research gives us a sense of the sheer variety of types of planet to be found beyond our Solar System. An understanding of the surface gravity of these worlds also gives us a clearer picture of the rate of in the evaporation of planetary atmospheres."

Full paper online at http://uk.arxiv.org/PS_cache/arxiv/pdf/ … 1570v1.pdf

[ it's cool when press releases include arxiv urls ]

US snubs Russian request for joint moon exploration: space chief
http://news.yahoo.com/s/afp/20070429/sc … usspaceiss

The head of Russia's space agency Sunday said the US has rebuffed an offer from Moscow to jointly explore the moon, while announcing a separate contract with NASA for nearly one billion dollars for the International Space Station.

Roskosmos chief Anatoly Perminov was quoted as saying by the Interfax news agency that Russia had proposed pooling resources to explore the moon.

"We were ready to cooperate but for unknown reasons, the United States have said they will undertake this programme themselves," he said.
...

They would rather redefine black as white than admit their pretense.

I didn't realize the effect the discovery would have on people.  I guess to me it was a sure thing that such planets existed, so the confirmation was less thrilling than for some.

BTW, under panspermia assumptions, Gliese 581 c, being so close, has had ample time to be seeded by single-celled life evolved on Earth & possibly vice versa (but probably not because the Earth had a 300 million year head start & single-celled life appeared 600 million years after the surface cooled).

You're way underestimating theologians.  They will do whatever it takes to enslave people's minds.

You could try presenting the project as an eco-investment ...

A Billion Years of Carbon-Neutral Power,
Beamed Down From Our Friend, The Moon
http://www.agci.org/energyPPT/criswell_agci.ppt

Weren't they tossing around multi-trillion dollar "fixes" recently?  You can save them a bunch and establish the private space industry at a stroke.

Researchers Shatter World Records with Length of Latest Carbon Nanotube Arrays
http://www.physorg.com/news96733560.html

18mm

Hey, that's not too bad for 2007.