Science Forums 2

[CraigD]

Originally Posted by goku

what about ram jet engines?
resistance is energy

Originally Posted by Moontanman

The main problem with Ram Jets is they have to be moving to work, you cannot take off with a ram jet only. Ram Jets are also difficult to throttle and can flame out easily, they also use more fuel than a good turbofan jet.

Another problem with ramjets (or scramjets) is not only must they be moving to work, they must be moving fast. Although it’s possible to run one at airspeeds as low as 100 MPH or so, they are very inefficient below about mach 0.5, and only approach or even exceed turbojet efficiency around mach 3.

Resistance does requires energy, and resistance varies about with the square of airspeed, so usually, higher speed aircraft takes more energy to carry the same load the same distance. So, as a general rule, improving energy the energy efficiency of an aircraft (or any vehicle where most of its drag is parasitic drag) requires lowering its speed. So, rather than motors that work better at higher speeds, motors that work better at lower speeds, such as turboprops, are needed.