why are turbofan engines more efficient than rocket engines?

Question

why do they use rockets for the shuttle and turbofans for airplanes

Answer 1

To burn one unit of mass of hydrogen requires 8 times the mass in oxygen. A rocket has to bring all that extra oxygen along with the fuel, and that means huge heavy tanks. An aircraft takes the oxygen in the air outside. (Airplane typically burn hydrocarbon, which are a compound of hydrogen and carbon,with jet fuel, the ratio of oxygen to fuel weight is still 3.5 to 1).
Your question is about turbofan, actually. Turbofan have a small part of the air taken from the outside being directed to the combustion chanber, most of the air is simply compressed and pushed out, trading the small mass of gas at high speed of a turbojet with a larger mass at lower speed; basically the low pressure fan is working somewhat like a ducted propeller. By recapturing the high pressure gas energy into a turbine that drives the fan, the efficiency is increased, the tradeoff is the more complex inetrnal arrangement and the more demanding conditions in the combustion chamber requiring a more carful design and exotic material.

So, to get back to you initial question: a rocket engine is less efficient because it needs to bring along the oxydizer. A tubrofan engine would not work in the vacuum of space. At lift off, the shuttle would benefit from turbofan, the problem is that it could use them only for a few seconds (as they are also limited in their maximum speed) and would have to have an air intake (needing some design compromise) and the shuttle would have to bring the weight and complexity of the extra engine system all the way to orbit as dead weight.

Answer 2

Turbo fan engines are 'air breathers' the require an outside source of oxygen for combustion. The shuttle flies above the level where sufficient oxygen is available. Rocket engines use liquid oxygen carried by the rocket itself. The extra weight would be to much for airline travel.

Answer 3

You have a vehicle into which you are trying to impart momentum. Momentum is the product of mass and velocity; for a particular momentum, you can push gently on a lot of mass and move it slowly, or push hard on a little mass and move it rapidly. But energy is the product of mass and velocity squared, so if you want to get momentum with as little expenditure of energy as you can manage, you push as gently as possible on lots of mass. Which is what a turbofan does: it moves a lot of air -- far more than enters the combustion chamber -- but the exhaust velocity is relatively small so the energy consumption is low. In a rocket, all of the ejected mass comes from the fuel, so since there is no outside mass, you have to push on the exhaust gas as hard as you can -- and that means LOTS of energy.

Answer 4

i would think rockets are a bit extreme for airplanes.