Why are the LFO burn rates so high in comparison with jets?

[Branjoman]

With liquidfuel and intakeair the liquid fuel goes down at barely a crawl, while with liquid fuel and oxidizer they burn at a ferocious rate. Shouldn't they burn at same rate, considerng that the oxidizer is basically onboard intake air...right? Or am I just being silly?

Edited October 16, 2014 by Branjoman

[Red Iron Crown]

It's because jet engines are a bit broken in KSP. They have a really high Isp, which is realistic as Isp is only supposed to account for mass the vessel loses and jets harvest much of their reaction mass from the atmosphere. But, in KSP and not in real life, that high Isp is also applied to the intake air that is also used as reaction mass. This results in the jet engine consuming fuel at a much lower than accurate rate (about 1/15th of what fuel consumption should be IIRC).

So the short answer is the rocket mode is working properly but the jets are consuming fuel at a much lower rate than they should.

[sal_vager]

I think you're overdoing the explanation there Red

Jet engines are more fuel efficient than rocket engines by a large margin, both in real life and in KSP so fuel use with jets will be a lot lower.

[Srpadget]

The way jets and rockets create thrust is to fling reaction mass out the back end at high speed.

The primary difference is that the reaction mass that gets flung out the back end of a rocket is ENTIRELY the fuel/oxidizer mix. So the contents of the tank are both energy source and reaction mass.

Whereas in a jet engine, the reaction mass is basically composed entirely of intake air. The fuel is ONLY there as a way of making the intake air really hot so that it shoots out the back end at high pressure/speed.

Using what we in engineering refer to as "rough order of magnitude estimation", about half (ish) of what shoots out the back end of a rocket is fuel--rocket exhaust is a noxious brew of chemical combustion products. About 1/100 (ish) of what shoots out the back end of a jet engine is fuel -- jet engine exhaust is essentially just really hot air with a teensy bit of combustion products as more-or-less minor impurities.

[Pecan]

I think you're overdoing the explanation there Red

Jet engines are more fuel efficient than rocket engines by a large margin, both in real life and in KSP so fuel use with jets will be a lot lower.

And to explain that in KSP specifically - look at the "Isp" figures for the jet and rocket engines in the VAB/SPH. The jets' figures are much higher, meaning they use much less fuel for a given amount of thrust.

[Red Iron Crown]

I think you're overdoing the explanation there Red

Jet engines are more fuel efficient than rocket engines by a large margin, both in real life and in KSP so fuel use with jets will be a lot lower.

Maybe I overdid it a bit, especially for a new user (Welcome aboard, Branjoman!).

Jet engines are more fuel efficient than rockets, but not by the margins that currently exist in KSP. The OP was intuitively thinking that intake air would be a 1:1 replacement for oxidizer so that LF consumption would be similar in both modes, and you are right to say that in air-breathing mode LF is consumed at a lower rate than that. The reason it stands out so much is that the fuel consumption rate is so much lower than it should properly be.