Why don't hydrogen and oxygen detonate when burned in the Space Shuttle engines?

[boab]

When hydrogen and oxygen burn they form steam, or H2O. I was thinking of quick and easy way to make steam would be by burning H2 gas and "air" in a simple rocket engine type burner at the pressure I needed the steam. Disregarding that the resulting flame is about at the top of the list for heat liberated from combustion, this wouldn't be a disadvantage. So let's ignore the burner/boiler meltdown aspect.
Chemistry is not my strong point, so as I started looking into the idea I was immediately confronted with the fact that even at very low pressures, (say 3 atmospheres) H2 and O2 gases can be extremely explosive. Not just burning, but can suddenly detonate at "extremely high velocities". Often at 6000 fps + range. This aspect wouldn't be good on the burner/boiler.
However it suddenly occurred to me that the Space Shuttle main engines run on liquid H2 and O2, a much more concentrated form, and do not detonate even when injected into the engines at several hundred PSI, and are burned.
My question is simple, why? Obviously I'm overlooking something, probably basic, and would appreciate anyone's help in understanding what that is.

boab

 

[Astronuc]

FYI - http://www.netl.doe.gov/technologies/coalpower/turbines/refshelf/GE%20Hydrogen-Fueled%20Turbines.pdf

As to the question on LH2/LOX:

f. Explosion Analysis: The method followed for the explosion analysis in Appendix B primarily came from references used in a seminar on the calculation and evaluation of fire and explosion hazards sponsored by the American Institute of Chemical Engineers. For confined gas explosions, the deflagration pressure wave is generally assumed to be 10 times the initial pressure. Calculations for a stoichiometric hydrogen-oxygen mixture show the deflagration pressure is approximately 143psia. When a pressure wave strikes a surface, a reflected pressure wave is developed. This reflected pressure is greater than the incident pressure and results from a momentum change, due to a change in direction when the moving air strikes a dense surface. The reflected pressure of a deflagration wave striking a surface normal to the incident pressure wave is approximately twice the deflagration pressure or, 285psia.[11,13]

[11] Tunkel, Steven J., "Methods for the Calculation of Fire and Explosion Hazards", AIChE Today Series. Course notes and excerpts, including:

[11a] Handbook of Compressed Gases, Third Edition, Compressed Gas Assn., Inc., Van Nostrand Reinhold, NY, 1990.

[11b] Kuchta, Joseph M., Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-Related Industries - A Manual, U.S. Department of the Interior, Bureau of Mines, Bulletin 680, 1985.

[13] Grelecki, Dr. Chester, "Fundamentals of Fire and Explosion Analysis", AIChE Today Series. Course notes and excerpts, including:

[13a] Glasstone, Samuel (Editor), The Effects of Nuclear Weapons, Chapter III: "Air Blast Phenomena", United States Atomic Energy Commission, April, 1962.

[13b] Cook, Melvin A., The Science of High Explosives, Appendix II, American Chemical Society Monograph Series. Reinhold Publishing Corp., New York, 1958.

Ref: http://www.dnfsb.gov/pub_docs/rfets/sir_19941201_rf.txt

With respect to the Shuttle Main Engines (SSME), please refer to:
http://www.pw.utc.com/vgn-ext-templating/v/index.jsp?vgnextoid=ef4f34890cb06110VgnVCM1000004601000aRCRD (Click on tab Characteristics)
Chamber Pressure: 2,994 psia

The 143 deflagration pressure is approximately 10 atm, but only 5% of the SSME chamber pressure.

 

[boab]

Astronuc:
I thank you for the very informative reply, to say the least. As the old saying goes, "Ask an you shall receive!" All the information and references you provided will keep me busy for some time. Very good material, right along with what I needed. Again my thanks for the trouble.
boab

 

[Astronuc]

BTW, note the fact that SSME runs with a rich (in hydrogen) mixture, i.e. the hydrogen content is greater than stoichioetric mix for pure combustion. The excess hydrogen absorbs the energy and increases the specific impulse, which is a measure of propulsive efficiency.

 

[Q_Goest]

Hi boab. Thought I'd chime in just to help the understanding along...

Detonation has a specific meaning. This Wikipedia definition is pretty solid IMO:

Detonation is a process of supersonic combustion in which a shock wave is propagated forward due to energy release in a reaction zone behind it. It is the more powerful of the two general classes of combustion, the other one being deflagration. In a detonation, the shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration, which propagates at a subsonic speed (i.e., slower than the sound speed of the explosive material itself), and without a shock or any significant pressure change.

Ref: http://en.wikipedia.org/wiki/Detonation

There is a difference between what's called ‘steady burning’ (such as is found inside the Space Shuttle’s main engine) and ‘unsteady burning’. Steady burning is characterized by a (relatively) slow flame front propogation. When gasoline is burned with air inside an internal combustion (IC) engine for example, even though that burning is extremely rapid, the actual flame front moves relatively slowly in comparison to the sonic velocity, so the air/fuel mixture inside an IC engine does not "detonate", it actually burns at a relatively slow (compared with sonic) velocity. If you put a high speed camera inside the cylinder of an IC engine for example, you'd see this boundary between the burnt gasses and unburnt fuel/air mixture, and that boundary would propogate outward from the source of ignition (spark plug).

Detonation or deflagration on the other hand, is a type of unsteady burning wherein the flame front travels at supersonic velocity through a mixture of flammable gasses. Detonation or unsteady burning can be eliminated by not providing a flammable mixture to propogate through, which is essentially what is done inside the Shuttle engine. In that case, hydrogen and oxygen are separate prior to their mixing in the flame front. By eliminating the flammable mixture, the flame front can not propogate and there can be no detonation. Ahead of the flame front, there is no ignitable mixture. Behind the flame front is only spent product (water and excess hydrogen as Astronuc points out).

Try this paper. Look especially at the bottom of the third page where it talks about steady and unsteady hydrogen combustion:
http://eprint.iitd.ac.in/dspace/bitstream/2074/212/3/dashyd96.pdf