LOX/LH2 ratios

[n0mad23]

I've been browsing Encyclopedia Astronautica, specifically looking at LOX/LH2 engines and am curious about the mix ratios.

Looking at the Ottobrunn lox/lh2 rocket engine, the Vulcain2 for example, the Oxidizer to Fuel Ratio: 6.7.

The Pratt and Whitney Cobra has a Oxidizer to Fuel Ratio: 6.

The Ottobrunn HM-7-A has an Oxidizer to Fuel Ratio: 5.15

Is the difference in these ratios based on the engines themselves? In other words, does the HM-7-A use less LOX than the Vulcain2 because the latter's engine is more efficient at burning the mixture?

And does anyone have any links for calculating both LH2 and LOX mass per m^3 handy?


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Found the answer to the second part of my question here http://www.orbiter-forum.com/showthread.php?t=23809

 

[N_Molson]

To begin with there are several sub-formulas between the generic "LOX LH2 terms". From Encyclopedia Astronotica :

Lox/LH2

Oxidizer: LOX. Oxidizer: LOX. Fuel: LH2. Fuel: LH2. Propellant Formulation: LOX/LH2. Propellant Formulation: LOX/Slush LH2. Optimum Oxidizer to Fuel Ratio: 6. Optimum Oxidizer to Fuel Ratio: 4.

"slush LH2" is a more "compact" version of LH2 : [ame="http://en.wikipedia.org/wiki/Slush_hydrogen"]http://en.wikipedia.org/wiki/Slush_hydrogen[/ame]

And if you look on (french) Wikipedia, the results are differents (and make a little more sense, for once). You have this line inside a table a the end :

Version Vulcain 1 (Vulcain 1B) Vulcain 2
Hauteur 3 m 3,45 m
Diamètre 1,76 m 2,10 m
Masse 1 686 kg 2 100 kg
Propergols Oxygène liquide (LOX) et Hydrogène liquide (LH2) dans un rapport 5,9:1 Oxygène liquide (LOX) et Hydrogène liquide (LH2) dans un rapport 6,1:1

Which means :

Propellants : LOX/LH2, ratio 5,9:1 for Vulcain 1 ; LOX/LH2 ratio 6,1 for Vulcain 2.

The SSME uses the ideal 6:1 mixture.

The RS-68 (Delta IV) uses also 6:1

La chambre de combustion brule de l'oxygène et de l'hydrogène liquide sous une pression de 102 bars lorsqu'il fonctionne à 102% de sa puissance nominale avec un ratio de mélange de 1 pour 6.

So, to sum up, the ideal LOX/LH2 ratio seems to 6:1.

 

[Wishbone]

Stoichiometric ratio (6:1 for LOX/LH2) is not always optimal. Sometimes you go for an increase in the lowest molecular mass component if it would give you higher Isp in the specific operational conditions of the engine (altitude etc.). Attaching the pic from the 1987 Russian book on chemmotology facepalm: no kidding)...

 

[n0mad23]

More pieces to put into the puzzle. Thank you gentlemen.

It's quite surprising just how much volume LH2 actually requires, especially in comparison to Lox. Conceptually, I "grok" it, but it's not quite the same as actually constructing the meshes for the tanks and actually seeing how much physical space LH2 demands.

I've been contemplating a Lunar base in the Malapert Mountain area with an eye on converting H20 into Lox/LH2 and a creating some sort of fuel base there. I'm now thinking everything has got to be a lot bigger than I'd originally envisioned.

 

[Urwumpe]

The main effect of the different mixture ratios comes actually from the average molecular mass of the exhaust: More H2 means "lighter" exhaust, and thus at the same energy, the molecules have a higher average speed.

Similar with other propellant combinations.

 

[USISIT]

Urwumpe HAS IT RIGHT... REF: http://www.braeunig.us/space/comb.htm clearly lists optimum ratios between 5 and 6 under optimal pressure/volume differences. 4.3 seems to be ideal in theory but based on experience the optimum lowers thrust.

As to n0mad23: I too am envisioning the H2O-LH2/LOX problem and as mentioned in these posts the math is often scewed with variables. Hence the following question (I'll search for an answer after posting, but i think this thread dealing with power and thrust bespeaks my issue):

OK, I GIVE UP. I HAVE BEEN SEARCHING FOR DAYS FOR THE ANSWER TO MY QUERY AND ALTHO I THINK I DID THE MATH BACK IN THE 1970'S WHEN STUDYING PHYSICS AND CHEMISTRY, I'M AT A LOSS NOW...

I USED TO SAY THAT LIQUID WATER WAS THE BEST WAY TO STORE EXCESS HYDROGEN AND OXYGEN, I.E. A GALLON OF WATER IS EASIER TO STORE AND TRANSPORT THAN AN EQUIVALENT AMOUNT OF LH2 AND LOX. MY QUERY NOW IS BOTH PHYSICALLY AND ENERGETICALLY WHAT IS THE VOLUME AND ENERGY EQUIVALENT BETWEEN WATER AND THE GASES IN LIQUID STATE - IDEALLY. THE MATH CONTAINS SO MUCH LOSSES AND TRIG AND CALCULUS AND VARIABLES THAT I BROKE MY CALCULATOR. EIGHT HOURS OF SEARCHING ONLINE HAS NOT YIELDED AN ANSWER YET. FIRST MY QUESTION, THEN MY TAKE ON THE MATH SO SOMEONE WITH A BETTER CALC. MAY FIGURE IT OUT:

DOES A GALLON OF WATER AT ROOM TEMP. YIELD A GALLON OF LH2+LOX IDEALLY AND CONVERSELY DOES A GALLON OF LH2 AT SAY -450 DEG. F. COMBINED WITH A GALLON OF LOX AT SAY -300 DEG. F. EQUAL HOW MUCH WATER AND ANY LEFT OVER H2 OR O. IDEALLY. MATH?: I GUESS YOU MEASURE THE MASS OF H2 AND O IN 1 GAL OF WATER AND COMPARE TO THE MASS OF H2 IN A GALLON OF LH2 AND THE MASS OF O IN A GALLON OF LOX. I'M HAVING TROUBLE FINDING THAT. CONVERSELY ALTHO NASA MIT AND STANFORD HAVE PAGES AD-NAUSEAM ONLINE ABOUT THE COMBINATION AGAIN THE MATH INCLUDES SO MANY FRICTIONAL COEFFICIENTS LOSSES DUE TO ALTITUDE TEMP AND VELOCITY AND CONVERSION OF ENERGY THAT I BROKE MY CALC. TEARDROP... IDEALLY THEORETICALLY HOW MUCH WATER CAN YOU EXPECT TO GET FROM A GALLON OF LH2 + A GALLON OF LOX?

PHILOSOPHICALLY DOES A GALLON OF WATER EQUAL A GALLON OF LIQUID GAS OR DOES A GALLON OF LH2 + A GALLON OF LOX EQUAL THREE GALLONS OF WATER WITH A HALF GALLON OF LOX LEFT OVER? AND THO I'VE TRIED MANY DIFFERENT VOLUMES FROM LITERS TO CUBIC CENTIMETERS NASA STILL MEASURES THE LIQUID PAYLOAD DELIVERED TO SPACECRAFT IN GALLONS PER MINUTE AND AT DEGREES FAHRENHEIT.

Sophmoric, maybe. Could someone point to an answer while i fix my calc. :facepalm:

 

[Col_Klonk]

Like aircraft/car engines.. maybe they also adjust for a 'richer' mixture ratio to keep the engine cooler, allowing for a bit of waste.