Launching satellites by a rail gun running up the tether to a balloon
This sounds to me like the dumbest idea ever, but it just might work:
- Make a 1km in diameter balloon, fill it with H2,
and have it hover at 30km. Air at sealevel is
1.22km/m3, and air at 30km is 1/100 as dense as at
sealevel, so this provides lift of around 5 million kilograms.
500*500*500*pi*4/3*0.01*1.22 = 6387905, but the H2 and
balloon weigh something. If there is a surrounding shell of
CH4, then another shell of N2 beyond that,
then adjacent gasses are mutually not flammable.
- Tether the balloon to the ground by an 80km tether of Kevlar
or Zylon that start close to level but end at 30 degrees. If it
were straight it would be 60km, but it will bow some. It should
bow mostly by the ground and be straight as it goes up under the
- Build a railgun on the tether.
- Surround the railgun by an airless tunnel, about 2 meters in diameter,
near vacuum, that can open at the balloon end. Enforcing vacuum requires
very heavy tunnel walls at sea level, but the walls can be much lighter
30km up. Near as I can tell the walls are always going to be much heavier
than the lift provided by the vacuum even 30km up, so to make it
lighter make the tunnel narrower not wider. If the tunnel averages
100kg per meter, this weighs 8 million kilograms, of which the
balloon supports 4 million kilograms.
- Better yet, support each little bit of the tunnel with a series
of smaller balloons along the entire length. This makes each
little piece of the tunnel self-supporting.
- Even better yet, stratospheric winds above about 30 degrees latitude
blow from west to east, pulling on the balloons and keeping the tube
in tension. Shape balloons as steerable streamlined kites. They can
steer to quickly compensate for wind direction, and they can provide
aerodynamic lift in addition or instead of being lighter than air.
- Launch satellites via the railgun with an acceleration of 50
gravities. sqrt(80000m * 2 / (9.8*50)) = 18 seconds, 9.8*50*18
seconds = 8854m/s, so at release it is going a bit faster than
the 7800m/s typical of low earth orbit.
- At that speed, air burns on contact and heat is a problem,
even 30km up. So the cars need to be long and heavy and with a
pointy heat shield in front.
- Once that works smoothly, build another one 10 meters in
diameter 2000km long that can launch people. If it can launch 100
people per car and 1 car every 10 seconds, it would take 23 years
to launch the entire human population of the earth into space.
The main advantages of this approach are it gives you a long inclined
railgun without digging any holes or owning any mountains, and it
avoids the atmosphere until it reachs a point many times higher than
any mountain. Unlike a space elevator, the tether can be built with
You still need to alter the orbit significantly once you get out of
the atmosphere, because the original orbit is elliptical and passes
through the earth. That takes rockets or some other means of propulsion,
and it has to get the job done in about an hour, because the orbital
period isn't very long.
Bob Predicts the Future
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