[AR] Re: Flying to Orbit with Hydrogen?

From: Mike Smith <marine165@xxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Date: Fri, 27 Dec 2019 12:46:41 -0600

NOx formation in air requires the air to get *damned hot* - over 2200K at
the stoichiometric flame front.
[image: image.png]
Kerosene adiabatic flame temperature at STP initial conditions is only just
~2400K at STP initial conditions - at the dramatically reduced ambient
pressures in the high atmosphere the chemical kinetics are slowed way down
due to the low concentration of oxygen with which the (HC rich) hot exhaust
gas of the rocket must have to combust.  Up in the stratosphere where the
ozone layer resides the ambient pressure is 5 kPa or less.  The limiting
oxygen concentration for combustion of hydrocarbons (here I use CH4 as a
surrogate) is around 5% O2 at sea level - for equivalent oxygen
concentration in moles per cubic meter that corresponds to about 230 kPa
atmospheric pressure, or well below the stratosphere.  If I'm right
(probably a big 'if') the bright rocket plume you can see up to ~50 km
altitude is mostly hot glowing soot particles as opposed to actual
combustion of rich exhaust with atmospheric oxygen.  The rocket may create
NOx in the combusting exhaust stream at lower altitudes, but down there the
rocket's contribution to the overall NOx emissions from all other
terrestrial sources is negligible.

Mike

On Fri, Dec 27, 2019 at 12:10 AM FreeLists Mailing List Manager <
ecartis@xxxxxxxxxxxxx> wrote:

On Wed, Dec 25, 2019 at 10:10 PM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:

snip

Vertical-launch hydrocarbon rockets, at first glance, ought to be better
for ozone than a wing-supported LH2 airbreather.  They'll certainly spend
less time in the upper atmosphere, and should put out less water vapor
than a hydrogen burner and less nitrogen oxides than an airbreather.

It turns out that the chemistry is important.  Skylon engines run rich
and the hydrogen suppresses NOx formation.  The same trick is used to
take the NOx out of power plant exhaust.   So Skylon generates almost
no ozone-damaging NOx on the way up.  The NOx it makes is during
reentry on the way back.

(Although a careful look at the numbers would be needed to make sure.)

NOAA article is here:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016EF000399

The NOx problem from hydrocarbon rockets is due to air (nitrogen)
being entrained by the engine plume.  How bad the NOx problem is for
hydrocarbon rockets has not been analyzed.

Keith

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[AR] Re: Flying to Orbit with Hydrogen?

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