Shuttle engines chemistry

Can anyone help with a Chemistry question? I understand that the Shuttle
engines burn hydrogen/oxygen in a ratio of 3/1. What is the equation for
this reaction? I teach high school chemistry and quote the equation 2H2 +
O2 -- 2H2O. e.g ratio of 2/1. I assume the 3/1 ratio means that the fuel
mix burns rather than explodes like the 2/1 ratio

Rod Stevenson
England

Rod Stevenson wrote:
Can anyone help with a Chemistry question? I understand that the Shuttle
engines burn hydrogen/oxygen in a ratio of 3/1. What is the equation for
this reaction? I teach high school chemistry and quote the equation 2H2 +
O2 -- 2H2O. e.g ratio of 2/1. I assume the 3/1 ratio means that the fuel
mix burns rather than explodes like the 2/1 ratio

Nope.
2:1 will burn, not explode in a rocket engine.
3:1 (I diddn't think it was that high) is somewhat better for use in
rockets, as it provides a bit higher thrust per unit of weight of fuel.
(Called ISP.)

"Rod Stevenson" wrote in
:

Can anyone help with a Chemistry question? I understand that the Shuttle
engines burn hydrogen/oxygen in a ratio of 3/1. What is the equation for
this reaction? I teach high school chemistry and quote the equation 2H2 +
O2 -- 2H2O. e.g ratio of 2/1. I assume the 3/1 ratio means that the fuel
mix burns rather than explodes like the 2/1 ratio

Rod Stevenson
England



The SSME runs at a oxidiser/fuel ratio of 6:1 (by mass)
By molar ratio as your figures are, this is a 3/1 Hydrogen to Oxygen

Why do this?
The burn is a bit cooler and totally non-oxidising, making all the
engineering aspects easier.
The average molecular mass of the exhaust is a bit lower, resulting in a
faster exhaust velocity despite the lower temperature, thus resulting in a
higher ISP. i.e. more impulse from the same mass of reactants.

the equation:
2/1 ratio:
2H2 + O2 -- 2H2O mean molecular mass = 18

3/1 ratio:
3H2 + O2 -- 2H2O + H2 mean molecular mass = 12.7

(note that these are simplified. You will allways find traces of other
forms such as H, OH, O3 etc in the exhaust. but in trace amounts only)

The reaction at 3/1 ratio delivers the same energy from 38/36 of the mass
as the 2/1 ratio, thus about 5% less energy-of-combustion per mass. But the
much lighter exhaust molecular mass means about 15% higher ISP.

As for burning/exploding nature of the mix..
A perfectly mixed gaseous form will indeed detonate.
But in a typical rocket engine the mixture is not perfectly mixed, indeed a
great part of the fuel is in the form of microdroplets of liquid rather
than gas form. Rocket combustion is a highly turbulent burning of partially
mixed materials.

Marvin wrote in :

3/1 ratio:
3H2 + O2 -- 2H2O + H2 mean molecular mass = 12.7

(note that these are simplified. You will allways find traces of other
forms such as H, OH, O3 etc in the exhaust. but in trace amounts only)

Minor additional note: within the atmosphere, you will also find nitrogen
compounds in the exhaust.


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3/1 ratio:
3H2 + O2 -- 2H2O + H2 mean molecular mass = 12.7

What is the role of the lonely H2 ? Is it just dead weight that provides
thrust just because it is accelerated by the burning of the active
"ingredients" ?

In article , John Doe wrote:
3/1 ratio:
3H2 + O2 -- 2H2O + H2 mean molecular mass = 12.7

What is the role of the lonely H2 ? Is it just dead weight that provides
thrust just because it is accelerated by the burning of the active
"ingredients" ?

Yep. There are two different processes going on here. If all you cared
about was the amount of energy you got out of burning your fuel, you
could go stoichiometric and have a 2/1 ratio. But we don't directly
care about how hot the combusion chamber gets.

A rocket nozzle is a fantastic machine for converting the heat energy
in burning fuel to kinetic energy of molecules all moving in the same
direction. You want the exhaust to be moving as fast as possible when
it comes out of the nozzle, so you want the combustion chamber
pressure and temperature as high as you can manage. But once you do
that, you can still win by making the exhaust lighter- if you can
lower the mass, the same amount of 'push' will make it go faster.
Hence we leave that extra hydrogen in, since H2 weighs less than H2O.

cheers,

Steven

--


"M-Theory is the unifying pachyderm of the five string theories."
- Brian Greene, _The Elegant Universe_

SIOL wrote:

But still, that hydrogen does not add up any energy to the equation, it just
moves the average velocity of the molecules in the exhaust stream to a
higher value.
And since one has to carry excess H2 with the rocket, which means extra
weight and air drag, I wonder where does the net benefit come from ?

The important point is that the chamber temperature of a stoichiometric
mixture will be very high, so high that the combustion is incomplete
(due to dissociation). So you are actually not getting as much chemical
energy out as you think.

Adding hydrogen reduces the chamber temperature, which reduces dissociation.
and increases the efficiency at which the available chemical energy is turned
into heat. It turns out (doing the thermochemical calculations) that
the optimal mixture ratio is somewhat fuel rich. The exact value depends
on the details of the engine (pressure, size, expansion ratio). The high
pressure of the SSME inhibits dissociation, which reduces the optimal H/O
ratio.

Lower temperature fuel/propellant combinations (for example, with hydrogen
peroxide as the oxidizer) are optimal near the stoichiometric ratio.

Paul

Steven Kasow wrote:

SNIP
A rocket nozzle is a fantastic machine for converting the heat energy
in burning fuel to kinetic energy of molecules all moving in the same
direction. You want the exhaust to be moving as fast as possible when
it comes out of the nozzle, so you want the combustion chamber
pressure and temperature as high as you can manage. But once you do
that, you can still win by making the exhaust lighter- if you can
lower the mass, the same amount of 'push' will make it go faster.
Hence we leave that extra hydrogen in, since H2 weighs less than H2O.


But still, that hydrogen does not add up any energy to the equation, it just
moves the average velocity of the molecules in the exhaust stream to a
higher value.
And since one has to carry excess H2 with the rocket, which means extra
weight and air drag, I wonder where does the net benefit come from ?

"Paul F. Dietz" wrote:

SIOL wrote:

But still, that hydrogen does not add up any energy to the equation, it just
moves the average velocity of the molecules in the exhaust stream to a
higher value.
And since one has to carry excess H2 with the rocket, which means extra
weight and air drag, I wonder where does the net benefit come from ?

The important point is that the chamber temperature of a stoichiometric
mixture will be very high, so high that the combustion is incomplete
(due to dissociation). So you are actually not getting as much chemical
energy out as you think.

Adding hydrogen reduces the chamber temperature, which reduces dissociation.
and increases the efficiency at which the available chemical energy is turned
into heat. It turns out (doing the thermochemical calculations) that
the optimal mixture ratio is somewhat fuel rich. The exact value depends
on the details of the engine (pressure, size, expansion ratio). The high
pressure of the SSME inhibits dissociation, which reduces the optimal H/O
ratio.

Lower temperature fuel/propellant combinations (for example, with hydrogen
peroxide as the oxidizer) are optimal near the stoichiometric ratio.

Paul

Is the combustion chamber pressure of a SSME still the highest of any
current liquid fuel rocket engine?


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"SIOL"
And since one has to carry excess H2 with the rocket, which means extra
weight and air drag, I wonder where does the net benefit come from ?

The term that describes this is called specific impulse. The hydrogen atom
is the lowest mass atom on the periodic table. All of your rocket fuels are
going to produce the odd atom or molecule here or there. The specific
impulse tells you the amount of go per unit of rocket juice.