3 ms·

There's no tether material nearly strong enough to withstand the forces from wind and the tether's own weight and drag at 30km altitude - plus 15km additional l

There's no tether material nearly strong enough to withstand the forces from wind and the tether's own weight and drag at 30km altitude - plus 15km additional length for lateral drift.

Nitrolo 4y ago
There's a concept in engineering called "Reißlänge" (≈ breaking length), which is how long a cable of a certain material can be before it would rip due to its own weight if suspended by one end [0]. The values for some materials are actually much higher than I remembered, with Kevlar reaching ~250 km. From a purely hold-its-own-weight perspective it seems like it might actually be possible! Obviously there's a ton of other problems that wouldn't make it feasible, the first I can think of is the weight of a 50 km tether wouldn't allow the balloon to even take off. I'm sure you can think of another million reasons it wouldn't work. [0]: https://de.wikipedia.org/wiki/Rei%C3%9Fl%C3%A4nge https://de.wikipedia.org/wiki/Rei%C3%9Fl%C3%A4nge
- lovemenot 4y ago
  That's interesting and surprisingly long for Kevlar. The highest altitude recorded by a single kite is less than 5km. I understand the biggest constraint for higher flights is tether strength-to-weight ratio. Nylon is typically used - fishing line. Tether diameter is another factor as the load from wind, on the tether alone, is considerable over such lengths.
1letterunixname 4y ago
It's a materials manufacturing to applications and lifecycle problem, not a technical one. Graphene does 50 GPa in bulk. A single atom sheet of it does 10 MPa on its own. Take 75 metric tonnes of cable / space elevator ribbon. That expressed as tensile pressure at an attachment point(s) is conservatively 7.5 GPa over an area about 10 cm^2. With sensible engineering, the entire craft (a very large one) would possess a web of attachment points across its perimeter to distribute forces more-or-less evenly.
anon223345 4y ago
Dynema might work, the more tension it receives the stronger it gets
trilbyglens 4y ago
Not to mention the incredible hazzard it would present to aircraft

There's no tether material nearly strong enough to withstand the forces from wind and the tether's own weight and drag at 30km altitude - plus 15km additional length for lateral drift.

Nitrolo 4y ago

There's a concept in engineering called "Reißlänge" (≈ breaking length), which is how long a cable of a certain material can be before it would rip due to its own weight if suspended by one end [0]. The values for some materials are actually much higher than I remembered, with Kevlar reaching ~250 km. From a purely hold-its-own-weight perspective it seems like it might actually be possible! Obviously there's a ton of other problems that wouldn't make it feasible, the first I can think of is the weight of a 50 km tether wouldn't allow the balloon to even take off. I'm sure you can think of another million reasons it wouldn't work. [0]: https://de.wikipedia.org/wiki/Rei%C3%9Fl%C3%A4nge https://de.wikipedia.org/wiki/Rei%C3%9Fl%C3%A4nge

lovemenot 4y ago
That's interesting and surprisingly long for Kevlar. The highest altitude recorded by a single kite is less than 5km. I understand the biggest constraint for higher flights is tether strength-to-weight ratio. Nylon is typically used - fishing line. Tether diameter is another factor as the load from wind, on the tether alone, is considerable over such lengths.

1letterunixname 4y ago

It's a materials manufacturing to applications and lifecycle problem, not a technical one. Graphene does 50 GPa in bulk. A single atom sheet of it does 10 MPa on its own. Take 75 metric tonnes of cable / space elevator ribbon. That expressed as tensile pressure at an attachment point(s) is conservatively 7.5 GPa over an area about 10 cm^2. With sensible engineering, the entire craft (a very large one) would possess a web of attachment points across its perimeter to distribute forces more-or-less evenly.

anon223345 4y ago

Dynema might work, the more tension it receives the stronger it gets

trilbyglens 4y ago

Not to mention the incredible hazzard it would present to aircraft