Lox/LH2 rocket engine exhaust pollution

[Tommygun]

Would anyone happen to know what the typical average percentages are of the different compounds in the exhaust of a Lox/LH2 rocket engine?

I know it's mostly water vapor with a bunch of H2, some CO2 and maybe a few other things, but I can't find any references.

Thanks.

[NovaSilisko]

It's literally just water vapor, from the combination of hydrogen and oxygen. Where would the carbon for carbon dioxide even come from?

I mean, the only pollutant I can think of would be chunks coming off of an ablative nozzle such as the one possessed by the RS-68, which would I believe be just chunks of graphite (I am unsure of the actual material) and not really any environmental concern.

You don't really get fuel and oxidizer with "impurities" beyond the parts per million level at worst. Substantial impurities in fuel or oxidizer quickly make your rocket stop being a rocket.

Edited April 11, 2015 by NovaSilisko

[Elthy]

Where does the CO2 come from? A normal hydroden engine shoukd only emit H2O and H2 (depending on the fuel mixture), other stuff can inly come from ablative cooling...

[Tommygun]

I'm assuming the carbon is coming from the engine bell and impurities, but was just wondering if there was anything else created even if it's just trace amounts.

[Streetwind]

Most US engines run fuel rich, especially the hydrogen burners... elemental hydrogen in the exhaust increases the Isp due to its low atomic mass. In such a fuel mixture you don't have any free oxygen left to react with any carbon, even if there was some on hand.

You can however get some extremely freaky chemistry going in a multi-thousand degrees, high pressure reaction chamber. Compounds that are neither stable nor sensible can and absolutely will form for a short period of time and get exhausted. They'll then quickly decay once outside.

[Tommygun]

OK thanks, that's what I was wondering about.

I didn't realize that they decayed afterward.

[lajoswinkler]

It's not pure water vapor as the mixture is so hot it reacts with the engine and the nozzle itself, but those compounds are so negligible in amount that for all intents and purposes you can say it's water vapor with small amount of hydrogen.

There are indeed some freaky species inside the flame, because it's a flame and flames are like that, but as soon as it condenses it's water.

Also that's if you ignore the total life cycle of the fuel production. Most hydrogen is made from fossil fuels, so it's called black hydrogen.

[StrandedonEarth]

The real polluters are SRB's, with all the aluminum and chlorine spewing out. I don't think the STS SRB's got high enough to affect the ozone layer, but I did read the Shuttle punched a huge hole through it with every launch.

Of course, then there's the Proton rocket with its highly toxic propellant stew of N2O4/UDMH

[lajoswinkler]

The real polluters are SRB's, with all the aluminum and chlorine spewing out. I don't think the STS SRB's got high enough to affect the ozone layer, but I did read the Shuttle punched a huge hole through it with every launch.

Of course, then there's the Proton rocket with its highly toxic propellant stew of N2O4/UDMH

There is no chlorine being released by SRBs. Perchlorates are. Entirely different thing. Aluminium compounds would even be ok on their own, but perchlorates are a serious threat to the local environment which includes people. They are soluble which means they are mobile.

"Punching holes through ozone" is a stupid media buzzphrase. Effect is minimal, very local and ephemeral.

Compared to UDMH, I'd say perchlorates are way more detrimental in the long run as they're endocrine disruptors.

[architeuthis]

For LOX, LH2 there are a few other equilibrium species: hydroxide, and decomposed monatomic hydrogen and oxygen. Apart from LOX/LH2 combustion chemistry can quickly become very complicated, but engineers still have to do the analysis to determine rocket isp (from the adiabatic flame temperature and mixture ratios). If you're curious about the basic process for determining these I made some google slides a while ago. In practice people used more sophisticated numerical tools, such as NASA's CEA.

[Iskierka]

LOx/LH₂ engines only produce H₂O and some intermediate byproducts in any meaningful quantities - reactions with the combustion chamber and nozzle are minimal. What does happen, though, is that the reaction between these two very quickly gets so hot that it can't complete, which is the actual reason for using a higher LH2 quantity - H₂ and H⁺ ions in the exhaust do make it lighter and travel faster, but they don't travel as fast as a H₂O molecule that's had its energy released. The trouble is that H₂O has to share its energy with everything else that hasn't reacted, slowing it down, so the LH₂ ratio is increased to make sure that everything else is faster.

The ideal exhaust from a LOx/LH₂ rocket would be pure H₂O, but because it burns far too hot (reaction tapers off around 3000K, theoretical temperature would be around 6200K), we instead use a mixture that produces H₂O, lots of H₂ and H⁺ ions, and some trace amounts of O₂, O^2- ions, and HO^- ions. The excess hydrogen mostly tidies up unreacted oxygen parts, but small amounts get left in.

[Tommygun]

How much H2 is left unreacted?

Would it be like 97% water and 3% H2 or is it really quite small.

I'm guessing the ions left over would be in the parts per million/billion?

[Guest]

Only thing I can imagine, is atmospheric nitrogen and oxygen coming into contact with hot exhaust plume and react, producing some variety of oxides. I don't know, if it really happens.

[NovaSilisko]

On the subject of SRBs, the shuttle SRBs used asbestos for the lining. I'm not sure if any of that got into the exhaust plume, but yeah...

Thankfully, for the SRBs on SLS, they've redesigned it not to use asbestos.

[Sandworm]

LOx/LH₂ engines only produce H₂O and some intermediate byproducts in any meaningful quantities - reactions with the combustion chamber and nozzle are minimal. What does happen, though, is that the reaction between these two very quickly gets so hot that it can't complete, which is the actual reason for using a higher LH2 quantity - H₂ and H⁺ ions in the exhaust do make it lighter and travel faster, but they don't travel as fast as a H₂O molecule that's had its energy released. The trouble is that H₂O has to share its energy with everything else that hasn't reacted, slowing it down, so the LH₂ ratio is increased to make sure that everything else is faster.

The third reason for running the mixture slightly rich (low in 02) is that hot 02 will burn most anything made of metal. An even slightly lean mixture would eat through any metal engine parts it touched.

[Bill Phil]

H2O2 is a product of the inefficiencies in a few engines?

[morat242]

How much H2 is left unreacted?

Would it be like 97% water and 3% H2 or is it really quite small.

I'm guessing the ions left over would be in the parts per million/billion?

The rule of thumb is about 50% unreacted for LH2, 5-20% for hydrocarbon.

Efficiency goes down when the mass of the exhaust products goes up, so adding enough oxygen to burn all the hydrogen would cramp down on performance as well as melt/burn the engine.

H2O2 is a product of the inefficiencies in a few engines?

I would be really surprised if any rocket ever output hydrogen peroxide as an exhaust product. The reason it never became more than a niche propellant is its tendency to exothermically decompose, which is catalyzed by way, way too many common substances. At exhaust temperatures, there is no way it could stay together.

[Tommygun]

The rule of thumb is about 50% unreacted for LH2, 5-20% for hydrocarbon.

Efficiency goes down when the mass of the exhaust products goes up, so adding enough oxygen to burn all the hydrogen would cramp down on performance as well as melt/burn the engine.

OK, so I'm guessing the unreacted hydrogen comes out so hot that it then burns with the atmospheric oxygen and then turns to water as well?

Edited April 13, 2015 by Tommygun

[Bill Phil]

I would be really surprised if any rocket ever output hydrogen peroxide as an exhaust product. The reason it never became more than a niche propellant is its tendency to exothermically decompose, which is catalyzed by way, way too many common substances. At exhaust temperatures, there is no way it could stay together.

It's a similar compound. Probably shows up in the lower temp engines.

[kerbiloid]

"Ecological" Hydrogen, btw, does not appear from nothing.

Usually Hydrogen is produced from natural gas (with all its ecological surprises)using methane pyrolysis/reforming (producing much CO and CO2).

So, LH2/LO2 fuel itself makes only a water vapour, but its production is maybe even more harmful than unpoetic kerosene.

[lajoswinkler]

"Ecological" Hydrogen, btw, does not appear from nothing.

Usually Hydrogen is produced from natural gas (with all its ecological surprises)using methane pyrolysis/reforming (producing much CO and CO2).

So, LH2/LO2 fuel itself makes only a water vapour, but its production is maybe even more harmful than unpoetic kerosene.

I see both of our countries have the same moronic officials which misuse the term "ecology". It has become a term similar to what "organic" is in USA now. It makes the actual ecologists very mad.

[Kaezan]

That moment when you search for Lh2 Lo2 reaction and this thread comes up before wikipedia

[notfruit]

Isn't water vapor better at trapping heat in the atmosphere than CO2?

[YNM]

Isn't water vapor better at trapping heat in the atmosphere than CO2?

Yeah, but they condense into liquid droplets unlike carbon dioxide.

[MinimumSky5]

Also, water won't contribute to acidification of the oceans, so if we ignore the production of the hydrogen, it is a clean fuel source.