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Orbital Hovercraft

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Complementing an equator-girdling spaceport runway on the Moon¹, each shuttlecraft has been built with a flat titanium underbelly, and fitted with an air tank that vents through it, providing a cushion of gas for speeding over smooth surfaces at barely suborbital velocity².

For a hovering medium, Oxygen or Calcium vapour is used (Ca's boiling point is less than Ti's melting point) ³.
______

¹ A runway on the Moon sounds like a ridiculous idea: simply a method of almost certainly impacting a random mountain at 1.7 kilometres per second. And it is.. unless

• the runway's built along the equator,
• it's quite long (**much** preferably completely around), and
• the loop it forms around the Moon, or segment thereof, is completely circular, varied slightly only where necessary to mitigate the influence of subsurface gravitational anomalies (mascons), ensuring a constant vertical displacement to any object above in a circular orbit... ie: no bumps.

Under those specific circumstances, landing on a lunar runway, or indeed any airless body within a Goldilocks zone of mass, is completely transformed into a procedure that could very realistically be completely eyeballed from orbit to spaceport bar, and back again.

Approaching spacecraft need only assume an equatorial orbit, then gradually reduce forward speed. Departing spacecraft find themselves automatically in a circular lunar equatorial orbit. And, if anything mechanical goes wrong during the procedure, just sit back, coast through space and wait for Triple-A to show.

For braking-regeneration and launch-assist (1.45 MJ/kg each way), integrate at least one linear accelerator along the runway's length. A 50 km long 3g mass-driver will get you to or from orbital velocity in less than a minute.

² - Ground travel at orbital velocity will void most tire manufacturers' warranties.

³ - Oxygen, Titanium and Calcium are among the most common elements found in the Lunar regolith.

FlyingToaster, Mar 11 2012

Moon VLO Device
[xaviergisz, Mar 12 2012]

Earth to Moon lifter, aka moon string The_20Hindenburg_2c_20but_20in_20space
[not_morrison_rm, Mar 12 2012]

Charles Stross - Generation Gap http://www.antipope...fiction/gengap.html
prior art concerning tangential space runway, keyword: "Cannonball Express" [FlyingToaster, Jul 28 2012, last modified Feb 03 2015]

[link]






       This feels like I'm attending Part 2 of the wrong seminar...
Alterother, Mar 11 2012
  

       erm yeah, sry... most of Part I in the footnotes now.
FlyingToaster, Mar 11 2012
  

       // A runway on the Moon //   

       ??   

       I knew I should have studied the supplementary material.
Alterother, Mar 11 2012
  

       Instead of landing like a helicopter, land like an airplane. You'd be moving a bit faster than a Cessna when you first touched down or just before you left the ground, of course.
FlyingToaster, Mar 12 2012
  

       I started to say that equatorial wasn't needed, as the moon only rotates once a month, but a bit of thought shows it is a good idea. [+]
baconbrain, Mar 12 2012
  

       Of course, why didn't I think of this...a perfect match for the moon string idea
not_morrison_rm, Mar 12 2012
  

       //I started to say that equatorial wasn't needed//
I almost did the same thing: pretty well gotta be equator in both location and orientation for any semblance of superiority or even equity compared to radial landing, and should definitely be completely circumferential for that full "24/7" convenience.
  

       //moon string// in the sense of the elephants' march you mean. That's... hmm... would you need stretchy string?
FlyingToaster, Mar 12 2012
  

       I missed the Moon VLO 'bake. Makes more sense now. Actually, it doesn't, but at least it has context.
Alterother, Mar 12 2012
  

       The runway goes around the Moon perfectly circular so "VLO", on the order of an inch off the surface, is possible.   

       So we slow the shuttle down until it's an inch off the runway and at that point we kick some gas into the plenum and it becomes a 7,000km/h hovercraft which can continue to be decelerated in some manner without ever actually touching the ground. Like an air-hockey puck. Eventually it gets slow enough that we can put the wheels onto the runway, and the gas gets turned off and then it's basically a RV moseying along.   

       It might seem that you'd need more "air" than would be available on the shuttlecraft, but when the gas hits onto the pavement at 2.4km/s it's going to heat up (and expand) quite a bit. And at 3g deceleration (using a forward facing retrorocket for instance) it takes less than a minute'n'half to stop.   

       For a runway without a linear accelerator(s), you'd need a forward-facing rocket to slow down. With LA's, you just VLO around 'til you're near one then use that to stop.
FlyingToaster, Mar 12 2012
  

       A supersonic hovercraft - if it worked at all - would work very differently from a subsonic one. Pretty much by definition, the lifting force can only be transferred between the craft and the ground at the speed of sound in the gas. It's not necessarily a deal breaker, but worth a mention. The mechanics might be more akin to aquaplaning than to a hovercraft.
spidermother, Mar 12 2012
  

       There's ports in the plenum which push the gas out of the vehicle. The gas (or mist) is moving along with the vehicle until it hits the ground at which time it gets really excited and expands more.   

       Speed of sound @ 2,000C (for air anyways) is almost 1km/sec, so with a plenum depth of 1" and a clearance of 1", the balance would be moved back 5" or so at near-orbital speed. Maybe point the ports forward a bit instead of straight down.   

       Still wondering if a plenum or simply a flat surface would be better.
FlyingToaster, Mar 12 2012
  

       //Speed of sound @ 2,000C (for air anyways) is almost 1km/sec//   

       Wouldn't that figure depend upon the pressure?
AusCan531, Mar 12 2012
  

       No. Pressure does not affect the speed of sound in a gas. The effects of pressure on modulus and density exactly cancel each other.
spidermother, Mar 12 2012
  

       The eponymous "they" say that it doesn't, but at very high temperatures it probably does [edit: okay it doesn't].   

       Precise math to figure out how much of a gas would be required is a bit out of my league and of course is dependent not only on vehicle size&weight, but air-cushion<>ground friction.   

       Hmm... decided to go with a flat plate instead of a plenum at least for illustration purposes. The only real reservations I have about a flat-plate is it puts the body of the vehicle closer to the ground, ie: if you come across a stone that's bigger than ground clearance allows for...   

       Robot broomsmen, muttering mechanically about inconsiderate pilots, keep the runway free of large stones.
FlyingToaster, Mar 12 2012
  

       Thanks [spidey] and [FT]. I will have to go away and think about that. (Somewhere where there is no pressure.)
AusCan531, Mar 12 2012
  
      
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