4 ms·

Not having to carry oxidizer with you is part of it, yeah. But in a vacuum, it's not just reaction energy that's a limiting factor -- it's reaction mass. Once y

Not having to carry oxidizer with you is part of it, yeah. But in a vacuum, it's not just reaction energy that's a limiting factor -- it's reaction mass. Once you get into space, the only way you can gain momentum is by ejecting material that you brought with you. But in an atmosphere, you have a free supply of mass to push against. I don't know how to estimate the behavior of a jet engine, but plugging in the numbers for a Cessna 172 shows that the mass of air flowing through the propeller per second is several hundred times higher than the amount of air actually used by the engine.

ethbro 12y ago
I wasn't thinking about it that way. Good perspective! For props, I imagine this would be a fundamental difference, as they're not ejecting mass, no? They're expecting energy to pull themselves through something more fluid-like.
- teraflop 12y ago
  Not sure if I'm understanding you right, but this isn't really a fundamental difference between piston "prop" engines and turboprop "jet" engines. The mechanical design of the combustion chamber is very different, but in both cases, the combustion produces mechanical energy, which spins a fan, which propels air backwards.
  - ethbro 12y ago
    (I-am-not-an-aerospace-engineer) I was thinking the fundamental difference would come from viewing rocket vs prop from the mass perspective you outlined in your previous comment. In a rocket, and also in the various derivatives of a jet engine, forward thrust is generated by the expansion and acceleration of mass away from you. Either fuel + oxidizer in the former, or fuel + compressed air taken from the atmosphere in the latter. And the corresponding effect of that fuel+ acceleration on your mass. In a prop, forward thrust is generated by linking a powerplant to a blade that interacts with air. The exhaust mass from the powerplant burning fuel in this isn't directly connected to the forward thrust. In that, if you had a non-propeller method of propulsion, you could hook the powerplant to that and still have effectively the same system.

ethbro 12y ago

I wasn't thinking about it that way. Good perspective! For props, I imagine this would be a fundamental difference, as they're not ejecting mass, no? They're expecting energy to pull themselves through something more fluid-like.

teraflop 12y ago
Not sure if I'm understanding you right, but this isn't really a fundamental difference between piston "prop" engines and turboprop "jet" engines. The mechanical design of the combustion chamber is very different, but in both cases, the combustion produces mechanical energy, which spins a fan, which propels air backwards.
- ethbro 12y ago
  (I-am-not-an-aerospace-engineer) I was thinking the fundamental difference would come from viewing rocket vs prop from the mass perspective you outlined in your previous comment. In a rocket, and also in the various derivatives of a jet engine, forward thrust is generated by the expansion and acceleration of mass away from you. Either fuel + oxidizer in the former, or fuel + compressed air taken from the atmosphere in the latter. And the corresponding effect of that fuel+ acceleration on your mass. In a prop, forward thrust is generated by linking a powerplant to a blade that interacts with air. The exhaust mass from the powerplant burning fuel in this isn't directly connected to the forward thrust. In that, if you had a non-propeller method of propulsion, you could hook the powerplant to that and still have effectively the same system.