rocket engine efficiency

[Luiz Kuntz]

why are rockets burning most of their fuel outside the engine


======================================= MODERATOR'S COMMENT:
Not sure the evidence for this, but it's technical, so will permit so the question can get answered.

[jkn]

On 12 joulu, 17:01, Luiz Kuntz <lckunt...@gmail.com> wrote:
> why are rockets burning most of their fuel outside the engine

No they don't.

All hydrogen will burn if there is eight kg oxygen for each kg of
hydrogen. Shuttle used mixture ratio 6:1, so 75% of hydrogen is
burned inside of the engine.

For thermo dynamical reasons about 4:1 (?) would give highest exhaust
velocity (ISP).

Perhaps you are thinking white area in picture:
http://en.wikipedia.org/wiki/File:Shuttle_Main_Engine_Test_Firing.jpg

I think it is caused by pressure change, not fuel burning. Notice
that it does not produce much light.

- jkn

[VinceMulhollon]

On Dec 12, 9:01 am, Luiz Kuntz <lckunt...@gmail.com> wrote:
> why are rockets burning most of their fuel outside the engine

Theres about fifty layers to this onion:

1) Chemically perfect ratio is too hot. So don't do that.

2) Fuel rich is MUCH easier on the metal combustion chamber than
oxidizer rich. That only explains why its 51:49ths rich not "most"

3) Max thrust ratio technically has nothing to do with max Isp ratio.
So right at takeoff when you need the most you can get as early as
possible, you'll tend to crank the O2 open until it practically flames
out (well ... exaggeration). Lighter engine for a given takeoff mass
means better mass ratio at burnout. But cranked open O2 at a poor
Isp, means for better fuel economy, as soon as you can you, you go
back to normal, blowing lots of H2 out the exhaust. So it varies over
time, but yes, most of the time, you're running extremely H2 rich.

4) People will incorrectly claim the higher hydrogen fraction means a
better exhaust velocity, but simplistically that ruins temp quicker
than it gains low molecular weight improvements in exhaust velocity,
so its a net loss. They're at a deep layer of the onion but there's
much deeper to go...

5) The actual reason some run richer is H in the exhaust improves
nozzle efficiency (insert thermodynamic mumbo jumbo about H2 vs CO2
thermodynamics in the exhaust) so much its worth it to the overall
system, even if on paper it ruins the exhaust velocity. So you start
with a little less, but keep much more of the total, for a net win.
Or rephrased, running rich doesn't make a theoretically perfect nozzle
work any better, but a real world one with real world exhaust in it
does because of thermodynamic mumbo jumbo.

6) The ideal ratio also varies with atmospheric pressure, which is
about 1 bar on the pad and about 0 in orbit, which is another
complication. Not much variation, but some.

"Most of" implies over 50% which I guess can be true for a H2/O2
engine, sometimes. That species runs extremely rich sometimes.
"most" engines do burn "most" of their fuel inside the chamber "most"
of the time.