NASA's New Upper Stage Rocket Engine Ready For Testing (J-2X)

wrote:
Fueled by liquid oxygen and liquid hydrogen, the J-2X engine will
generate 294,000 pounds of thrust in its primary operating mode to
propel a spacecraft into low-Earth orbit.

By changing the mixture ratio of liquid oxygen to liquid hydrogen,
the J-2X can operate in a secondary mode of 242,000 pounds of thrust
required to power a spacecraft from low-Earth orbit to the moon, an
asteroid or other celestial destination. The J-2X can start and
restart in space to support a variety of mission requirements.

What is the benefit from being able to change the mixture? Why
wouldn't one simply use the extra 52,000 pounds of thrust and merely
thrust for a shorter length of time when leaving orbit?

rick jones
--
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it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
- Rick Jones
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

Rick Jones wrote:

wrote:
Fueled by liquid oxygen and liquid hydrogen, the J-2X engine will
generate 294,000 pounds of thrust in its primary operating mode to
propel a spacecraft into low-Earth orbit.

By changing the mixture ratio of liquid oxygen to liquid hydrogen,
the J-2X can operate in a secondary mode of 242,000 pounds of thrust
required to power a spacecraft from low-Earth orbit to the moon, an
asteroid or other celestial destination. The J-2X can start and
restart in space to support a variety of mission requirements.

What is the benefit from being able to change the mixture? Why
wouldn't one simply use the extra 52,000 pounds of thrust and merely
thrust for a shorter length of time when leaving orbit?

Higher ISP. Hydrogen moves faster than water.

--
Mvh./Regards, Niels Jørgen Kruse, Vanløse, Denmark

Niels Jørgen Kruse wrote:
Rick Jones wrote:

wrote:
Fueled by liquid oxygen and liquid hydrogen, the J-2X engine will
generate 294,000 pounds of thrust in its primary operating mode to
propel a spacecraft into low-Earth orbit.

By changing the mixture ratio of liquid oxygen to liquid hydrogen,
the J-2X can operate in a secondary mode of 242,000 pounds of thrust
required to power a spacecraft from low-Earth orbit to the moon, an
asteroid or other celestial destination. The J-2X can start and
restart in space to support a variety of mission requirements.

What is the benefit from being able to change the mixture? Why
wouldn't one simply use the extra 52,000 pounds of thrust and merely
thrust for a shorter length of time when leaving orbit?

Higher ISP. Hydrogen moves faster than water.

Well then why didn't the press release *say* that and say it gets a
higher ISP as a result? How are we in the peanut gallery supposed to
ever learn?-)

thanks,

rick jones
--
Process shall set you free from the need for rational thought.
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

On 6/14/2011 11:01 AM, Rick Jones wrote:

Well then why didn't the press release *say* that and say it gets a
higher ISP as a result? How are we in the peanut gallery supposed to
ever learn?-)

I don't see how it's supposed to get a higher ISP using a combustion
chamber throat diameter and bell nozzle shape optimized for a higher thrust.

Pat

Re-SENT :

In sci.space.policy message , Mon, 13
Jun 2011 21:11:59, Rick Jones posted:

wrote:
Fueled by liquid oxygen and liquid hydrogen, the J-2X engine will
generate 294,000 pounds of thrust in its primary operating mode to
propel a spacecraft into low-Earth orbit.

By changing the mixture ratio of liquid oxygen to liquid hydrogen,
the J-2X can operate in a secondary mode of 242,000 pounds of thrust
required to power a spacecraft from low-Earth orbit to the moon, an
asteroid or other celestial destination. The J-2X can start and
restart in space to support a variety of mission requirements.

What is the benefit from being able to change the mixture? Why
wouldn't one simply use the extra 52,000 pounds of thrust and merely
thrust for a shorter length of time when leaving orbit?

Since hydrogen has lighter molecules than steam, they go faster for the
same energy. It could well make sense to reduce the oxygen flow and get
somewhat less thrust at more reduced propellant consumption.

For the best space performance, one wants the highest Isp. But for
launching against gravity, one also needs sufficient thrust/weight.
--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms and links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

Pat Flannery wrote:

On 6/14/2011 11:01 AM, Rick Jones wrote:

Well then why didn't the press release *say* that and say it gets a
higher ISP as a result? How are we in the peanut gallery supposed to
ever learn?-)

I don't see how it's supposed to get a higher ISP using a combustion
chamber throat diameter and bell nozzle shape optimized for a higher thrust.

According to Wikipedia, the J-2X runs at 5.5 mixture ratio in the upper
stage role, which is oxygen rich. The alternate mixture ratio then
doesn't have to be hydrogen rich and I should have written that water
goes faster than oxygen.

It is not clear though that the engine is optimized for thrust, running
oxygen rich may simply be to save weight of hydrogen tankage.

--
Mvh./Regards, Niels Jørgen Kruse, Vanløse, Denmark

Niels Jørgen Kruse wrote:

According to Wikipedia, the J-2X runs at 5.5 mixture ratio in
the upper stage role, which is oxygen rich.

No, 5.5 is fuel rich. Any mixture ratio below 8 (stoichiometric) is
fuel rich.

Jim Davis

=?ISO-8859-1?Q?Niels_J=F8rgen_Kruse?= wrote:
Pat Flannery wrote:

I don't see how it's supposed to get a higher ISP using a combustion
chamber throat diameter and bell nozzle shape optimized for a higher thrust.

According to Wikipedia, the J-2X runs at 5.5 mixture ratio in the upper
stage role, which is oxygen rich. The alternate mixture ratio then
doesn't have to be hydrogen rich and I should have written that water
goes faster than oxygen.

Exhaust velocity times mass of the exhaust gives the momuntum of the
fuel burned. The energy of the burn gives the energy, but the exhaust
can include more than just complete combustion products. It can also
include unburned but heated exhaust gas. This sounds to me like an
optimization problem among the linear momentum result, the quadratic
energy result and the temperature reduction by absorbing energy into the
unburned exhaust fluid.

It is not clear though that the engine is optimized for thrust, running
oxygen rich may simply be to save weight of hydrogen tankage.

Before orbit thrust matters greatly. After orbit ISP matters most. It
sounds like a compromise design to me.

On 6/16/2011 12:04 PM, Niels Jørgen Kruse wrote:
Pat wrote:

On 6/14/2011 11:01 AM, Rick Jones wrote:

Well then why didn't the press release *say* that and say it gets a
higher ISP as a result? How are we in the peanut gallery supposed to
ever learn?-)

I don't see how it's supposed to get a higher ISP using a combustion
chamber throat diameter and bell nozzle shape optimized for a higher thrust.

According to Wikipedia, the J-2X runs at 5.5 mixture ratio in the upper
stage role, which is oxygen rich. The alternate mixture ratio then
doesn't have to be hydrogen rich and I should have written that water
goes faster than oxygen.

It is not clear though that the engine is optimized for thrust, running
oxygen rich may simply be to save weight of hydrogen tankage.

It's odd they'd run oxygen rich, as most LH2/LOX engines run hydrogen
rich to avoid corrosive effects of the hot oxygen on the combustion chamber.
In the case of the RS-68 it runs hydrogen rich enough to turn the
exhaust plume red, unlike the SSME's bluish flame.


Pat

Jim Davis wrote:

Niels Jørgen Kruse wrote:

According to Wikipedia, the J-2X runs at 5.5 mixture ratio in
the upper stage role, which is oxygen rich.

No, 5.5 is fuel rich. Any mixture ratio below 8 (stoichiometric) is
fuel rich.

The idea that 5.5 is oxygen rich got into my head from this page
http://www.braeunig.us/space/propel.htm.

The note "LO2/LH2 and LF2/LH2 mixture ratios are higher than optimum to
improve density impulse" gave that impression at a hasty glance, but I
overlooked the note above that "optimum" is for sea level operation, and
it is still not clear if the priority is on ISP or density impulse. So
"optimum" was not a synonym for stoichiometric.

--
Mvh./Regards, Niels Jørgen Kruse, Vanløse, Denmark