If a 4% concentration of hydrogen in ambient air goes BOOM...

....why do the SSME's have an ignition system? Why don't they just mix a
small amount of H2 and O2 in an ignition system?

"Alan Erskine" wrote in
:

...why do the SSME's have an ignition system? Why don't they just mix
a small amount of H2 and O2 in an ignition system?

The SSME has an internal ignition system, in both gas generators;
they're literally spark plugs. Necessary since a precisely controlled
startup sequence is required and just mixing the two propellants won't
result in ignition, unlike hypergolic propellants.

If you're referring to the external spark generators, that's only to
ignite venting hydrogen before it builds up to explosive levels, which
might happen if the engines are shut down on the pad.

The visible venting on the engines prior to ignition is oxygen; vented
hydrogen is piped away and burned off some distance from the pad
on a flare stack.

--Damon

Damon Hill gebruikte zijn klavier om te schrijven :
"Alan Erskine" wrote in
:

...why do the SSME's have an ignition system? Why don't they just mix
a small amount of H2 and O2 in an ignition system?

The SSME has an internal ignition system, in both gas generators;
they're literally spark plugs. Necessary since a precisely controlled
startup sequence is required and just mixing the two propellants won't
result in ignition, unlike hypergolic propellants.

If you're referring to the external spark generators, that's only to
ignite venting hydrogen before it builds up to explosive levels, which
might happen if the engines are shut down on the pad.

The visible venting on the engines prior to ignition is oxygen; vented
hydrogen is piped away and burned off some distance from the pad
on a flare stack.

--Damon

Damon: Do you have any source on that?

IFAIK The shuttle does not have an onboard ignition system build in the
SSME's.
Otherwise the crew should be able to ignite an shutdown SSME during
lift-off. If a SSME is shut down for whatever reason, it can not be
ingited wothout an external ignition source!

André

"Damon Hill" wrote in message
...
"Alan Erskine" wrote in
:

...why do the SSME's have an ignition system? Why don't they just mix
a small amount of H2 and O2 in an ignition system?

The SSME has an internal ignition system, in both gas generators;
they're literally spark plugs. Necessary since a precisely controlled
startup sequence is required and just mixing the two propellants won't
result in ignition, unlike hypergolic propellants.

But isn't that ignition/combustion?

"Alan Erskine" wrote in news:dUqBj.24424
:

"Damon Hill" wrote in message
...
"Alan Erskine" wrote in
:

...why do the SSME's have an ignition system? Why don't they just mix
a small amount of H2 and O2 in an ignition system?

The SSME has an internal ignition system, in both gas generators;
they're literally spark plugs. Necessary since a precisely controlled
startup sequence is required and just mixing the two propellants won't
result in ignition, unlike hypergolic propellants.

But isn't that ignition/combustion?

Yes. What's the problem?

--Damon

=?ISO-8859-15?Q?Andr=E9,_PE1PQX?= wrote in
:


Damon: Do you have any source on that?

IFAIK The shuttle does not have an onboard ignition system build in the
SSME's.
Otherwise the crew should be able to ignite an shutdown SSME during
lift-off. If a SSME is shut down for whatever reason, it can not be
ingited wothout an external ignition source!

Absolutely not!

If a SSME shuts down it's off, period. There is no restart capability
once it's in the air because the engine requires equipment that is on
the ground. There is no ground restart for a whole bunch of safety
reasons.

The engine ignition is internal: (from Wikipedia)

quote:

The oxidizer and fuel preburners are welded to the hot-gas manifold. The
fuel and oxidizer enter the preburners and are mixed so that efficient
combustion can occur. The augmented spark igniter is a small combination
chamber located in the center of the injector of each preburner. The two
dual-redundant spark igniters, which are activated by the engine
controller, are used during the engine start sequence to initiate
combustion in each preburner. They are turned off after approximately
three seconds because the combustion process is then self-sustaining. The
preburners produce the fuel-rich hot gas that passes through the turbines
to generate the power to operate the high-pressure turbopumps. The
oxidizer preburner's outflow drives a turbine that is connected to the
HPOTP and the oxidizer preburner pump. The fuel preburner's outflow
drives a turbine that is connected to the HPFTP.

end quote

I don't know of a single rocket engine of any type that is ignited
externally, aside from fireworks.

--Damon

"Damon Hill" wrote in message
...
"Alan Erskine" wrote in news:dUqBj.24424
:

But isn't that ignition/combustion?

Yes. What's the problem?

That's what I mean; why have spark ignition when they could combine O2 and
H2? Surely there's a way.

André, PE1PQX wrote:

Damon Hill gebruikte zijn klavier om te schrijven :
"Alan Erskine" wrote in
:

...why do the SSME's have an ignition system? Why don't they just mix
a small amount of H2 and O2 in an ignition system?

The SSME has an internal ignition system, in both gas generators;
they're literally spark plugs. Necessary since a precisely controlled
startup sequence is required and just mixing the two propellants won't
result in ignition, unlike hypergolic propellants.

If you're referring to the external spark generators, that's only to
ignite venting hydrogen before it builds up to explosive levels, which
might happen if the engines are shut down on the pad.

The visible venting on the engines prior to ignition is oxygen; vented
hydrogen is piped away and burned off some distance from the pad
on a flare stack.

--Damon

Damon: Do you have any source on that?

Too lazy to google on "SSME igniter"?

D.
--
Touch-twice life. Eat. Drink. Laugh.

http://derekl1963.livejournal.com/

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL

"Alan Erskine" wrote in
:

"Damon Hill" wrote in message
...
"Alan Erskine" wrote in news:dUqBj.24424
:

But isn't that ignition/combustion?

Yes. What's the problem?

That's what I mean; why have spark ignition when they could combine O2
and H2? Surely there's a way.

Because oxygen and hydrogen don't self-ignite. Externally igniting
propellants will and have damaged the SSME on the pad during ignition
aborts; as I quoted from the Wikipedia article on the SSME, the ignition
sequence begins at the gas generator/preburners.

--Damon

Maybe it would go bang rather than whoosh?

Brian in a slightly silly mood.



--
Brian Gaff -
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Alan Erskine" wrote in message
...
...why do the SSME's have an ignition system? Why don't they just mix a
small amount of H2 and O2 in an ignition system?