Think of the cheapest fuel to use to propel flight machines.

[senoga andrew]

fossil fuel

[Libin Daniel]

The air should be the cheapest.

[Felipe Garcia]

LNG

[Sean Zimmerman]

Unleaded? But, not any of the premium typ, just regular.

[Siddharth Kedare]

Hydrogen perhaps? at 45,000 ft, I suppose there is enough oxygen to burn it... problem is storing it... metal hydrides anyone?

[danita]

dirt =]

[Rajmohan D. Waghela]

only if it has some Thorium in it.

[Felipe Garcia]

I'm sure that at some point we'll use methane obtained from organic breakdown of matter. In the end, it's renewable, and I'm sure that we can produce enough organic matter to satisfy our needs. Reduce, Reuse, Recycle.

[senoga andrew]

thank you all for attempting to answer.we ask questions to know and discover beyond us.
cacth you later

[danita]

im tellin you felipe, dirt. its the future.

[Libin Daniel]

Why not just use air in such an engine that gives exhaust gases as purer air- The platinum rule.
Easy on paper, though ;)