Related to the blimp thread...

Comments

• Phi <font color=#FF0000>C</font><font color=#FF9900>o</font><font color=#FFFF00>l</font><font color=#00F

  January 2005

  I remember reading about vaccum blimp planes... I think the trick is to make a container light enough yet strong enough to keep everything else out.
  
  -Φ
• Random Chaos Actually Carefully-selected Order in disguise

  January 2005

  yes, vaccume is "lighter" then helium.
  
  Think of it this way: Two rocks are heavier then one rock.
  
  Helium has two protons and is heavier then hydrogen that has one proton.
  
  In compaison vaccume has NO mass becuase there are NO "rocks" in it, so it is lighter then everything.
• Biggles <font color=#AAFFAA>The Man Without a Face</font>

  January 2005

  You can also explain it by simply saying that a vacuum is less dense. :)
• sleepy_shadow Ranger

  January 2005

  Hydrogen is lighter. Vacuum would be even lighter. However... there is one big problem.
  
  Lift generated by an airship is constrained by the mass of displaced air... from which the airship's own mass (including that of filler gas) is subtracted. This means that after the weight of filler gas has become negligible compared to surrounding air... further tweaking offers very little reward.
  
  Normal air weighs about 1 gram per liter. Reasonably hot air weights about 0.5 grams per liter -- leaving notable space until the point where all available lift is fully exploited.
  
  Hydrogen and helium however... weigh approximately nothing compared to normal air... for which reason the optimality model of blimp lift... is for most purposes, satisfied by them. On surface altitude, they lift about 1 gram per liter. One cannot get notably more, and the cost of getting it... would be very high.
• JackN <font color=#99FF99>Lightwave Alien</font>

  January 2005

  What? You mean I can't compress a vacuum? :p :D ;)
  
  j/k
• Biggles <font color=#AAFFAA>The Man Without a Face</font>

  January 2005

  Sure you can! Just decrease the volume of its container! ;)
• shadow boxer The Finger Painter & Master Ranter

  January 2005

  I wonder if you could get any appreciable extra lift from superheated helium.
  
  Ive also thought that a blimp should be shaped to also provide good ole fashioned aerodynamic lift. With a nice big thick chord you could stuff alot of gass into the wings and provide a hybrid lift vehicle. With all that volume up there you might as well shape said volume to be an asset rather than an additional burden on the craft.
• Capt.Montoya Ranger

  January 2005

  The problem with vacuum might be that you need a rigid container to hold it...
  That is likely to mean higher weight than what a blimp needs.
  With H2 and He you can use a flexible container.
  
  But in fact I'm not even sure if a vacuum would provide buoyancy...
• Freejack Jake the Not-so-Wise

  January 2005

  Captain hit the exact point, H and He are lite enough that they can be held at a higher pressure than 1 bar and still create bouyancy, which in turn means that in the design of the craft, you not only have your source of lift, but you also have a source of structural support.
  
  Where as a container holding a high enough level of vacuum would be have to be built with such a structure that it would overcome any bouyancy that a lack of air might provide.
  
  Jake
• Sanfam I like clocks.

  January 2005

  Theoreticaly...
  
  Could a perfect sphere, made in a vacuum as to contain a vacuum upon completion, be made of a thin and light enough material, say...aluminum, to maintain its form under, say, one atmosphere?
• sleepy_shadow Ranger

  January 2005

  Depends on how thin... and how high vacuum. Sorry... I'm really bad at *that* kind of calculations.
  
  It's a real pain in the butt to convert inward pressure pointing into a sphere... into forces applied along the surface of the sphere. With my current knowledge of mechanics, I cannot do this within a reasonable amount of time.
  
  But in very approximate terms, I might be able to solve this optimality model.
  
  [b]Crude problem:[/b]
  
  1) You have one liter of volume, and one gram of aluminum to build.
  2) For sake of simplicity, let's assume it's a cube (in reality, you would want a sphere)
  3) Given that one liter of displaced air lifts one gram of weight... how thin would the cube's walls have to be?
  
  [b]Crude solution:[/b]
  
  1) Since here, volume=thicknes*surface... to calculate thickness, I need to divide the volume of the aluminum wall by its surface area.
  2) Since the cube would have 6 sides, each 10x10 cm, the surface of the 1 liter cube should be 6x10x10=600 cm2.
  3) Volume can be found by dividing mass by density. Since the density of aluminum is 2.7g/cm3, wall volume should be 1/2.7=0.37 cm3 of aluminum.
  4) Therefore, thickness would be 0.37cm3 divided by 600cm2, meaning approximately 0.0006cm.
  5) Presuming that the vacuum would need to achieve helium-grade lift, it should be somewhere around 10x below atmospheric pressure.
  6) My intuition tells: aluminum of this thickness does not have particularly good chances of standing up to such pressure.
  
  To make a vacuum blimp, I suspect one would need a super-material. Unless you have time to wait until human nanotech gets better... at very least, you would have to ask the Centauri. :)