Grand Ship Builder

What is rocket exhaust formed of?

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Is the exhaust a mixture of the propellants? If so, then is the exhaust of a liquid hydrogen/liquid oxygen rocket technically water in plasma form?

What about RP1 rockets?

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Not water in plasma form, just very hot water vapor, plus some leftover O2 for extra thrust. RP1 would add a lot of CO2 to the mix.

Also, thread moved, since this is about science :)

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LH2/LOx: suerpcritical steam. (previous comment about plasma was found to be wrong after a little research. *facepalm*)

LOx/ Methane, LOx/ RP1: Steam and carbon dioxide

N2O4/Aerozine 50: Nitrogen gas, steam, and carbon dioxide.

Solid propellant: Depends on the precise mix of the fuel grain, but it's full of a lot of particulate as well as gases.

Edited by MaverickSawyer

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Figured Liquid Hydrogen/Liquid Oxygen mixture resulted in simply very hot steam, just decided to call it plasma instead because it's easier.

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Well, I was corrected that it's borderline plasma under certain conditions, so you still get points.

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2 minutes ago, Grand Ship Builder said:

Figured Liquid Hydrogen/Liquid Oxygen mixture resulted in simply very hot steam, just decided to call it plasma instead because it's easier.

Well, unless the exhaust is hot enough to strip the electrons from the atoms and molecules comprising the exhaust plume, it's not a plasma, but simply a gas heated to incandescence.

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8 minutes ago, monstah said:

Not water in plasma form, just very hot water vapor, plus some leftover O2 for extra thrust. RP1 would add a lot of CO2 to the mix.

Also, thread moved, since this is about science :)

I thought it was better off with a hydrogen rich mixture, as it gives better specific impulse?  As well as not being horrendously oxidising?

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13 minutes ago, 1101 said:

I thought it was better off with a hydrogen rich mixture, as it gives better specific impulse?  As well as not being horrendously oxidising?

Oxygen is heavier than both hydrogen and water - if you want thrust, you want more of it flying out the nozzle. At the sacrifice of some specific impulse, of course, but that's likely less of a certain for launch engines.

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11 minutes ago, GluttonyReaper said:

Oxygen is heavier than both hydrogen and water - if you want thrust, you want more of it flying out the nozzle. At the sacrifice of some specific impulse, of course, but that's likely less of a certain for launch engines.

But water has oxygen in it!
I guess you are talking about the oxygen we breathe, which is 2 oxygen atoms. I guess that works.

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6 minutes ago, Grand Ship Builder said:

But water has oxygen in it!
I guess you are talking about the oxygen we breathe, which is 2 oxygen atoms. I guess that works.

Yup, that's the important bit - oxygen atoms have an atomic weight of 16, but oxygen comes in pairs in this case.

As such, oxygen molecules have an atomic weight of 32, while water molecules (with only one oxygen atom) have an atomic weight of 18. :) 

Edited by GluttonyReaper

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4 minutes ago, GluttonyReaper said:

Yup, that's the important bit - oxygen atoms have an atomic weight of 16, but oxygen comes in pairs in this case.

As such, oxygen molecules have an atomic weight of 32, while water molecules (with only one oxygen atom) have an atomic weight of 18. :) 

That's why teaching yourself science using the Internet paying attention in science class really pays off!

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True, but you never get a truly perfect mixture ratio... you'd destroy the engine. You either run ox-rich, or fuel-rich. It lowers the combustion temperatures to a level that the regenerative cooling can keep up with.

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3 hours ago, Grand Ship Builder said:

Is the exhaust a mixture of the propellants? If so, then is the exhaust of a liquid hydrogen/liquid oxygen rocket technically water in plasma form?

What about RP1 rockets?

Well, it's the reaction products for sure. Whatever that bizzare reaction is. Might include bits of molten metal if it was failing. :P

But to a more technical point, most rocket engines doesn't react their reactants in the perfect stoichiometric ratio, as the maximum temperatures sustained would be likely to obliterate the engine. So instead there's an excess of propellant or oxidizer.

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3 hours ago, 1101 said:

I thought it was better off with a hydrogen rich mixture, as it gives better specific impulse?  As well as not being horrendously oxidising?

I'm out of likes but this is most hydrolox engines. I won't say all hydrolox engines but the ones I know the stats for all run considerably hydrogen rich. RS-25, RS-68: O:F 6:1, RL10: 5.5:1, these are both by mass but not as high as I thought they were. Brain fart.

As for the trade offs of running hydrogen rich, running closer to stoichiometric increases the temperature of the exhaust gasses which increases specific impulse but decreasing the the molar mass of the exhaust gas also increases the specific impulse. In other words, there is a balance between fuel rich and maximum energy input that gets you the peak efficiency. So, for the RS-25, RL10, and the RS-68, they all run way more fuel than oxidizers. 

It's also worth noting that the RS-25 and the RL10 are regeneratively cooled while the RS-68 is ablatively cooled so the ablative material is in the exhaust as well which slightly decreases the specific impulse. 

Edited by Racescort666

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4 minutes ago, Racescort666 said:

It's also worth noting that the RS-25 and the RL10 are regeneratively cooled while the RS-68 is ablatively cooled so the ablative material is in the exhaust as well which slightly decreases the specific impulse. 

Interesting... I didn't know the RS-68 was ablative. That could explain the orange color of the exhaust plume at liftoff.

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2 minutes ago, MaverickSawyer said:

Interesting... I didn't know the RS-68 was ablative. That could explain the orange color of the exhaust plume at liftoff.

The nozzle is, the combustion chamber is channel wall construction. They did this to save cost over tube wall construction like RS-25 and RL10.

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7 minutes ago, Racescort666 said:

The nozzle is, the combustion chamber is channel wall construction. They did this to save cost over tube wall construction like RS-25 and RL10.

Ahhhh. Okay, so similar to the LR-87/91-11, but with channel wall instead of brazed tube. Sensible, as Rocketdyne was uniquely qualified to know just how labor intensive brazed tube was. :sticktongue:

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1 minute ago, MaverickSawyer said:

Ahhhh. Okay, so similar to the LR-87/91-11, but with channel wall instead of brazed tube. Sensible, as Rocketdyne was uniquely qualified to know just how labor intensive brazed tube was. :sticktongue:

!!!

Still out of likes. Is this a 24 hour curse or what?

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Just now, Racescort666 said:

!!!

Still out of likes. Is this a 24 hour curse or what?

Something like that. :wink:

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Hydrocarbons (incl. Kerosene and Methane) + LO → water steam, CO, CO2, C (particles of white-hot soot making the flame bright), traces of the oxidized kerosene additives, traces of sulfur oxides.

Hypergolics → water steam, nitrogen, (toxic) nitrogen oxides, traces of additives, CO2 (because (...)methyl-...), insignificant amount of CO, sometimes iodine oxide.

LH + LO → bath air (because all poisons listed above have been already exhausted into atmosphere, and in greater amounts, while producing LH from the natural gas).

Solid fuel - water steam, CO2, CO, soot, nitrogen, (toxic) nitrogen oxides, alumina, chlorine compounds, mix of numerous poisonous things due to additives and pollutions.
(Because powder is used only in small rockets, while others mostly use a mix of aluminium, rubber and ammonium nitrate)

Pentaboran + fluorine → Audience Choice Award.

Edited by kerbiloid

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6 hours ago, kerbiloid said:

LH + LO → bath air (because all poisons listed above have been already exhausted into atmosphere, and in greater amounts, while producing LH from the natural gas).

 

Not sure about the exact composition of the plume for Hydrolox, but 'Ignition!' lists basically any combination of Hydrogen and Oxygen you like for the combustion chamber.  Presumably once it's cooled down a bit in the plume, you'll just get the normal stuff, but:

Quote

If you put two moles of hydrogen and one of oxygen into the chamber you do not come out with two of water. You will have H2O there, of course. But you will also, because of the high temperature, have a lot of dissociation, and the other species present will be H, H2, O, O2, and OH. Six species in all, and you can't know, a priori, in what proportions they will appear.

 

 

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15 hours ago, 1101 said:

I thought it was better off with a hydrogen rich mixture, as it gives better specific impulse?  As well as not being horrendously oxidising?

Yes.  The rocket equation is all about exhaust *velocity*.  Hydrogen (and anything with low mass) gives the highest exhaust velocity for any temperature so works well.  It also doesn't oxidize everything nearby.  This is important for nuclear thermal rockets, which want to use hydrogen for exactly this reason, but unfortunately hydrogen tends to escape quickly under all current technology* and eventually under all foreseeable technology.

So add nuclear (as planned) and beamed power (like escape dynamics couldn't sell) as pure hydrogen exhaust.  Don't expect to see any soon.

* hopefully the James Webb telescope will have the problem of long-term storage/recycling of helium solved before launch (it depends on that).  That should be very close to solving long-term hydrogen storage.  Still no answer to the fact that eventually (even liquid) hydrogen leaks through *everything*.

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On 8/11/2017 at 6:39 AM, wumpus said:

Yes.  The rocket equation is all about exhaust *velocity*.  Hydrogen (and anything with low mass) gives the highest exhaust velocity for any temperature so works well.  It also doesn't oxidize everything nearby.  This is important for nuclear thermal rockets, which want to use hydrogen for exactly this reason, but unfortunately hydrogen tends to escape quickly under all current technology* and eventually under all foreseeable technology.

So add nuclear (as planned) and beamed power (like escape dynamics couldn't sell) as pure hydrogen exhaust.  Don't expect to see any soon.

* hopefully the James Webb telescope will have the problem of long-term storage/recycling of helium solved before launch (it depends on that).  That should be very close to solving long-term hydrogen storage.  Still no answer to the fact that eventually (even liquid) hydrogen leaks through *everything*.

F=m-dot * velocity, so the mass flow of the exhaust is also quite important.

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11 hours ago, mikegarrison said:

F=m-dot * velocity, so the mass flow of the exhaust is also quite important.

This comes up in ion drives when you are directly accelerating the mass instead of exciting velocity by heating it up.  You want the heaviest atoms possible, thus xenon is preferred (even though argon is more common than CO2, you typically want noble gasses to avoid having them react once stripped down to ions).  For thermal rockets, hydrogen still works best.  If you somehow decide to build a railgun (throwing large masses is a silly idea, only used to explain how rockets work), you would probably use U238 (depleted uranium) with an iron sabot (if you needed the thing magnetic).

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3 hours ago, wumpus said:

 If you somehow decide to build a railgun (throwing large masses is a silly idea, only used to explain how rockets work),

Still, a consideration in Hard Scifi for deflection from shooting, and also as an emergency backup if for some reason, you have no propellant but can still shoot.

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