Half a croissant, on a plate, with a sign in front of it saying '50c'
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Space Tube

Build a big vacuum tube* to space inflated with helium.
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The huge column would be neutrally buoyant so that it could rise to over 100km altitude. The central tube a couple of metres wide would be evacuated. The pipe is gently curved eastwards so that the exit angle is similar to that used by current rockets.

To allow the tube to extend above the altitude at which helium balloons cease to be effective, the tube would contain a number of smaller tubes that contain 'obstructions'. Air is blown through these pipes from the bottom and the resulting friction creates more lift. The air is then directed downwards at the lip of the main tube for an extra little push.

I lack the knowledge necessary to calculate the power available from the moving air or how feasible it is over such a long distance, but to improve the effective range, the air would start off hot and moving fast, then would be cooled as it gets higher in the tube. This system would rely on a large volume of air moving rather than high pressure. To avoid deceleration of the air due to gravity, the mass of the air cannot exceed that of the outside air.

This is similar to the Space Fountain concept, but much easier to engineer IMHO.

Inevitably, some air will get into the top of the tube. This is pumped out from the bottom continuously, but at the time of launch, a series of pre-launch disks are launched up the tube at relatively low velocity to force the air out the top of the tube. They then parachute back to earth to be used again. The last cleaning disk leaves the end of the tube seconds before the main launch vehicle.

This brings us to the launch mechanism. This is a simple magnetic rail, with the power coming from the rail rather than the launch vehicle. In fact the sled on the launch vehicle is a stage which detaches soon after leaving the top of the tube. It should not be a problem to transmit the power 100km or so, but this could be aided by wind turbines tethered near the tube at stages, solar power or the 'obstructions' mentioned earlier could be turbines.

The main problem is that the tube will need to be tethered to prevent it thrashing about in the wind. I think tethering it to the earth would be impractical due to the cable weight, but it could be tethered to itself to add stiffness. A few ground tether cables could be used by containing them within their own buoyancy tube and perhaps double up as the power cables. Anyway, these are mere details, way below my pay grade.

Back of envelope calculation: Assuming the structure weighs 100kg per metre, it would need to be about 35m in diameter at sea level. I have no idea how this would need to change at altitude.

<aside>Space elevators are not really like a conventional elevator in that it is not contained within a shaft. They should be called Space Cable Cars.</aside>

As a bonus, we can add arms to it and sell used cars to passing aliens.

*Can someone tell me the name of a tube with hollow walls, similar to a dewars flask but without a base? Tubule?

marklar, Jan 03 2011

Inflatable Space Elevator http://www.telegrap...-by-scientists.html
Vaguely similar idea [marklar, Aug 19 2015]

[link]






       Did you Google "space hose"?
MaxwellBuchanan, Jan 03 2011
  

       or HB Search. [edit:nvm}
FlyingToaster, Jan 03 2011
  

       //Did you Google "space hose"?// No, as that requires knowing that it is called a Space Hose. I have now though and got a relevant result, but I was far more interested by "How to Organize Closet Space with Vacuum Hose" which was disappointingly dull.   

       Anyway, the idea on Physics Forums is remarkably similar and it mentions the N-prize. There's even a mention of the flailing man, which makes me think I must have seen the idea before and regurgitated it from my subconscious.   

       Edit: Ah, just found the idea on the bakery.   

       Is this idea sufficiently different, given the different tubes (vacuum, lift hose, helium) and other details?
marklar, Jan 03 2011
  

       [+] A long skinny blimp/dirigible with an evacuated tube + linear accelerator in the middle is neither the fish of "Space Hose", nor the fowl of "strawberry launcher" though it uses (independently created) elements of both to produce something that's more viable than either.   

       I don't see any way something can be "tethered to itself" to limit contraction in any dimension unless you're planning on spinning up one of the components (in which case you're limiting expansion).   

       "double walled evacuated sleeve" ? "Dewar's sleeve" ?   

       Personally I wouldn't bother with the pre-launch disks: if the end of the tube is high enough, just evacuate it supersonically from the bottom... maybe have an airtight trapdoor at the top that's opened just prior to launch. (The downwards pressure on the closed hatch would be measured in pounds, not tons as at sea-level)   

       But... you're limited to about 50km altitude: even if your contraption only weighs 20kg per linear metre (since at this point you've ditched the blimp and the heavy vacuum container), every km adds 20 tonnes to the force necessary to keep the contraption from falling over. [edit: hmm... rocket motors aren't that heavy]
FlyingToaster, Jan 04 2011
  

       There are material limits as to how tall something can get.
RayfordSteele, Jan 04 2011
  

       [FlyingToaster] Yes, pumping 1000 tonnes of air per second is a tall order, not so much at sea level but once it is at 10 millibars that's a huge volume.   

       But still, the idea is to get past most of the atmospheric drag and be moving reasonable quickly without burning rocket fuel, so if this can be done by 50km altitude, it only needs to be this tall.   

       // tethered to itself // It is curved, so it would actually have to be attached to its elf along the inner side of the curve to prevent it straightening. I doubt this would help much with stability though, probably best to have the neutrally buoyant cables.   

       [RayfordSteele] Why, when it is neither piled or hung?
marklar, Jan 04 2011
  

       How is it then supported? I must've missed that part.
RayfordSteele, Jan 04 2011
  

       [RayfordSteele] The lower section is weightless due to it being filled with Helium. The upper part is lifted by drag from air pumped upwards and the thrust as the same air is directed downwards at the top.
marklar, Jan 04 2011
  

       Somehow I doubt that's enough structure to do the job.
RayfordSteele, Jan 04 2011
  

       Helium has weight, hence filling something with helium makes it weightless ? I think some laws of physics are being bent, or at least semantic abuse is occurring.
normzone, Aug 20 2015
  
      
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