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Exhaust temperatures

Posted by: pfrit - Wed Feb 25, 2009 6:01 am
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Exhaust temperatures 
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Post Exhaust temperatures   Posted on: Wed Feb 25, 2009 6:01 am
OK, lets me start with the standard disclaimers. I am a complete idiot. I know absolutely nothing. I talk to squirrels and do what they tell me.

I have been long confused by rocket engines. I was taught that a rocket engine is the most simple example of a heat engine, and one of the most efficient. The efficiency of the rocket is ultimately the difference between the ambient temperature and the exhaust temperature. Also a major loss of fuel (read loss of efficiency) is using the fuel for cooling the throat to prevent burn-through. It has left me confused by rocket design. Why make the engine out of such heat intolerant materials? I understand that it has to handle a great deal of vibration, so you don't want a overly brittle material. That still leaves an awful lot of superior materials. Lets leave SHARP metals out of the discussion. How about sapphire? It is easy to make, a real pain in the arse to work, but can be exceptionaly strong (if worked correctly) and there is quite probably no fuel combination that could burn through it short of a plasma torch from a fusion engine. You can buy sapphire boules in excess of a foot in diameter and 2+ feet long now. The armadillo crew is using/used a carbon fiber throat. Nearly all carbon allotropes react uncontrolably with oxygen when the temp passes 600c. But not glassy carbon. Couldn't you coat your engine's interior with glassy carbon? It doesn't melt or burn past 3000c. OK, I know nothing about using glassy carbon or binding it to carbon fiber. But how about sintering sapphire dust to sinterered nanotubes? That would be hard, strong and resist temps well past 2000c. Granted, it is possibly literally impossible to work with once made, so it would have to be formed in the final shape to be used.
The other issue is that the reason that ceramics don't work in rockets was always explained to me as a problem with vibration. Does this mean that a rocket engine can't be tuned to prevent hard vibration? In any other engine, vibration means the engine is not balanced correctly. There does not seem to me to be any intrinsic reason to have large vibration in a rocket. It may, however, be impossible to acheive in reality (engineering raises its ugly head once again). But is it unreasonable to think a rocket engine could be tuned enough to allow fairly brittle materials to be used? There are a large number of fairly strong glasses available.
My question comes down to, we have been building real rockets for close to 100 years. Why don't we have exhaust temps past 2000c by now? Material science has moved tremendously in that time. Even if you didn't raise the exhaust temps, you could at least hope to develop a 5000 hr engine.

In conclusion, I am obviously denser than osmium and have the IQ of a slighly damp dish rag. Please answer anyway.


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Post    Posted on: Wed Feb 25, 2009 7:05 am
im not really sure if increasing the temperature really matters that much in a lot of engines. for example, in plasma thrusters the temperature is theoretically millions of degrees (its a plasma), but in those cases the whole concept of temperature doesn't even make sense, so it doesn't matter. the rocket's thrust is determined by the exhaust velocity and burn rate, so while these are indirectly related to the temperature there's not necessarily a direct benefit to having a hotter engine. i don't really know that much about propulsion yet but my guess is engines aren't hotter for 3 reasons : a) cooling, b) there's not necessarily a direct benefit to a hotter engine, and c) the reactions themselves aren't that high temperature.

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Post    Posted on: Wed Feb 25, 2009 7:35 am
My last thermodynamics was 20 years ago for me, but I was taught that any heat engine's efficiency was directly linked to the diffence in output and ambient temps. A simple way to look it in rockets would be if you had a rocket that had an exhaust temp of 450c. On earth, it could lift and fly, but on venus it would just cool the rocket. That is one reason why there was never any talk of a land and return mission to venus. Now I realize that thermodynamics NEVER gives you an accurate prediction of actual conditions, but it is usually wrong in the other direction.


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Post    Posted on: Wed Feb 25, 2009 4:05 pm
You initial assertion is incorrect: Rockets are not heat engines.

A simple glance at wikipedia on "heat engine" and "rocket engine" explains the difference. Even if the overall factual quality is suspect, the basics are there.

Heat is a side affect of the combustion process in rockets, or better, propellant based engines. One can design a propellant engine that can lift a person (or more!) with ambient or less than ambient temperatures. Grab a firehose, point it straight down, turn it on. The principal of rocket engines is similar.

Best yet: http://www.youtube.com/watch?v=0snTqLQLpBA


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Post    Posted on: Wed Feb 25, 2009 5:11 pm
I stand corrected. A rocket engine does not need to be a traditional heat engine. However, to be useful it does. Imagine if Pixel vented its propelent out of the engine without igniting it. It would develop some thrust, but not enough to lift off (or maybe it would have enough thrust to jump. Interesting, but expensive experiment). A normal rocket engine is an internal combustion engine. OK, an attitude thruster is not an IC. But remember it is not called an "Altitude" thruster. :) I leave it to people with more letters after their names to debate whether a water rocket is a true heat engine. All of that is besides the point. It is axiomatic that the hotter a rocket's jet, the more thrust it develops. By running a rocket's propelant leaner, you get more thrust at the expense of engine life.


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Post    Posted on: Wed Feb 25, 2009 6:12 pm
Basic physics is E=MV^2 energy = mass * velocity squared. Rocket motors use burning fuels to increase the velocity of the mass, since, when it gets hot, a material expands according to the ideal gas law, PV=NRT: http://en.wikipedia.org/wiki/Pv=nrt SO increasing the temperature (T) increases the pressure (P) since volume(V) is held basically constant by the combustion chamber. The gas then migrates thru the nozzle to an area of greater volume (free air or space) and less pressure. The pressure difference between the two determines the velocity (since the mass is conserved) which is fortunate since velocity, not mass is the driving force of the energy equation.
The nozzle is shaped to provide a more effecient coupling between the exhaust gas and the surrounding medium, and therefore rocket nozzles differ when designed for air versus the vacuum of space.
http://en.wikipedia.org/wiki/Rocket_nozzle
So, in short, we get things hot because it's easiest :)


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Post    Posted on: Wed Feb 25, 2009 7:17 pm
I believe I understand where I failed to communicate. I am talking thermodynamics, not newtonian physics. The two are completely compatible (in this discussion, at least), but they deal with very different perspectives. Newtonian physics deal with what forces do and interact (in this case, 3rd law). Thermodynamcis deals with why forces are. Newton dealt with forces pushing masses. Maxwell dealt with why does the force exist in the first place. Newtonian physics give accurate numbers but in narrow forms. Chemistry's gas laws (boyle, charles, ...) come, mostly, from newtonian physics. I believe that ideal gas calculations come from this as well. Real gas calculations blend the two, but are still mostly newtonian. If you just look at the newtonian perspective, you want a great mass of fast moving gas exiting the engine. To get better performance, you increase the pressure of the chamber (heat it) or increase the mass of the mass of the exhaust (denser fuels). From a thermodynamics view it is simpler. Add more heat. You can increase the temperature of the exhaust or increase the amount of matter in the exhaust to hold the heat. As an aside, I think the formula you wanted was E=MV2/2. Half of the energy goes in each direction. Otherwise energy is not conserved.


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Post Exhaust Temperatures   Posted on: Fri Feb 27, 2009 11:00 pm
I had to pull up my thermo notes from two decades ago to refresh my memory as well.

There were a number of errors made in the previous posts. The ambient temperature means nothing to a rocket engine, although a rocket engine is indeed a heat engine, even water leaving a nozzle has a lower static temperature than the water in the high pressure supply. The temperatures that matter are those inside the combustion chamber and in the nozzle at the exit. The temperature at the nozzle exit is the low temperature end and is where the heat is being rejected. As this heat is associated with the mass that is also leaving the system, it matters not at all what the temperature of the medium that it is located in. Ambient pressure does matter though, as on the surface of Venus. This factor limits the expansion allowable in the nozzle as noted below. Also, using the fuel to cool the hot bits of the engine is not a loss mechanism. It is a energy reclamation system described as regenerative cooling and “sweepsâ€

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Post Re: Exhaust temperatures   Posted on: Thu Jan 28, 2010 1:11 pm
Wow my head is spinning. There's quite a few contradictions in this thread that I still don't understand the answer. One thing I know for sure is that rockets are not heat engines and everything falls from there.

There's lots of Racing Tips online.


Last edited by KevinSeed on Wed Feb 27, 2013 8:02 pm, edited 6 times in total.



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Post Re: Exhaust temperatures   Posted on: Fri Jan 29, 2010 5:33 pm
KevinSeed wrote:
One thing I know for sure is that rockets are not heat engines


The Carnot equation works for rocket engines if you think of their job as being converting chemical bond energy into kinetic energy of the exhaust.


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Post Re: Exhaust temperatures   Posted on: Sun Jan 31, 2010 7:02 pm
Jabowery wrote:
The Carnot equation works for rocket engines if you think of their job as being converting chemical bond energy into kinetic energy of the exhaust.
and then mentally subtracting the amount of work needed to compress the exhaust back to the chamber pressure, isothermally at the exhaust temperature and....which isn't actually happening and makes no logical sense. Carnot cannot give you a number which even bounds the efficiency of conversion of chemical to kinetic energy, becuase a rocket engine is not a heat engine working even approximately in a cycle.


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Post Re: Exhaust temperatures   Posted on: Mon Feb 01, 2010 2:48 pm
I disagree,, A rocket engine may not necessarily be a closed cycle system (depending on engine type), but it still will have a defined efficiency curves and limitations.
How would you characterize a Ramjet engine? or a turbojet or tubofan engine?

http://en.wikipedia.org/wiki/Jet_engine ... efficiency
http://en.wikipedia.org/wiki/Cycle_effi ... Efficiency

besides, wouldn't a rocket engine more closely fit the Brayton Cycle like other jet engines?
http://en.wikipedia.org/wiki/Brayton_cycle
(I know I know, wikipedia blech, lol, but its all i have at my fingertips at the moment)

Now granted, i am not a propulsion engineer, so forgive me if I misunderstand some things, however, saying
nihiladrem wrote:
becuase a rocket engine is not a heat engine working even approximately in a cycle.

I believe couldn't be farther from the truth.


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Post Re: Exhaust temperatures   Posted on: Mon Feb 01, 2010 8:38 pm
Quote:
I disagree, A rocket engine may not necessarily be a closed cycle system (depending on engine type), but it still will have a defined efficiency curves and limitations.
I've hardly said there are no efficiency limitations, all I have said is that Carnot's law/equation (whatever) does not bound the efficiency of conversion of chemical to kinetic energy in a rocket. It simply does not apply.

Quote:
How would you characterize a Ramjet engine? or a turbojet or tubofan engine?

The powerplant part is fairly similar to the Brayton cycle. I would be forced to use equations I suppose.


I noticed something in the wiki article about efficiency being limited by the coldest and hottest part of the cycle. Maybe we ought to do the same with rocket and determine the efficiency by the ratio between the fuel temperature and the combustion temperature.

Quote:
besides, wouldn't a rocket engine more closely fit the Brayton Cycle like other jet engines?
Brayton is or can be near a (closed) cycle, in the sense that it's often almost cold air in and hot air out, and the discharge and inlet pressure are closely matched. You can mentally put in a cooling system with some imaginary cooling temperature, (worst case all at the inlet temp), substitute cooled exhaust for fresh air, and call it a cycle. Then it's a reasonably close approximation of a (closed) thermodynamic cycle and can be bound the efficiency using Carnot without further consideration.

The situation is not similar in rockets however, when the discharge pressure is generally far lower than the inlet pressure. In essence, the difference here is that the exhaust is allowed to reach enormous volumes, but the cost of incorporating that part into a cycle (reducing the volume again) which might bound efficiency to Carnot, is not paid.

A simpler example. If you imagine some mixture of gasses being burned in a cylinder as the piston expands outwards, you can hold the temperature relatively constant by adjusting the rate of burning until all the gas is burned. There is no significant temperature ratio involved, ought not Carnot efficiency be zero? You've definitely done work nonetheless. What limits are there on conversion of chemical to mechanical energy when there isn't an upper limit on volume or a lower limit on pressure in sight?

Carnot's law is not an answer to everything in thermodynamics, and was never intended as such. It couldn't have simplified much wrt rockets in any case.

Quote:
I believe couldn't be farther from the truth.
By a cycle I (and I think quite obviously given the previous sentence) meant something where the state returns to what it was.


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Post Re: Exhaust temperatures   Posted on: Tue Feb 02, 2010 10:03 pm
nihiladrem wrote:
Jabowery wrote:
The Carnot equation works for rocket engines if you think of their job as being converting chemical bond energy into kinetic energy of the exhaust.
Carnot cannot give you a number which even bounds the efficiency of conversion of chemical to kinetic energy, becuase a rocket engine is not a heat engine working even approximately in a cycle.


Try this thought experiment:

Measure the temperature in the combustion chamber of a rocket. Now go down stream and travel at exactly the same speed as the exhaust and stick a thermometer into the exhaust at zero relative velocity to it. That's another temperature. Plug those into the Carnot equation. What efficiency do you get?


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Post Re: Exhaust temperatures   Posted on: Wed Feb 03, 2010 8:44 am
Hello from Spain! (this is my first post)


If you divide the proccess in two steps you'll see how Carnot is (omni :) )present.

First the combustion transforms quimical energy into heat, so the gas is at a new temperature Tmax.

The next process is the gas expansion that makes some works giving the propulsion of the rocket. This transforms the internal enegy into work.
For what I remember of my college years, ideally, this should be an adiabatic process whithout variation of entropy.

If you see the formulas of this proccess you'll see that work is proportional to the temperature difference.

W = constant* (Tmax - Tmin).

So the maximum work will be with Tmin = 0kelvin.

Divide by maximum work and there is carnot

performance = 1 - (Tmin/Tmax)

So, for a fixed Tmin, the greater the Tmax the better performance.

(Sorry by my bad and long forgotten english)


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