Author Topic: Rocket Engine Q&A  (Read 382931 times)

Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #520 on: 10/11/2014 01:36 am »
If the other two engine families where not also fist stage engines, I would say it had something to do with a sealevel optimized engine, but that is not the case here.

Look beyond the engines to the LVs which use them. Ariane and H-II have solid boosters which do most of the lifting at low altitudes. Thus you don't have to worry about core engine sea level Isp so much. Situation is different with Delta IV, good sea level performance is required from RS-68.

sea level Isps:

Vulcain 1: 326s
Vulcain 2: 318s
LE-7: 349s
LE-7A: 338s
RS-68: 365s

So your opinoin is that the other 4 engines are optimized for vacuum performance?

It makes sense.

« Last Edit: 10/11/2014 02:16 am by nimbostratus »
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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #521 on: 10/11/2014 02:27 am »
Just a guess? Thrust. The low area ratio implies a big throat. Have you compared the propellant mass flow of the engines?


For a given propellant combination, the theoretical area ratio is generally related to combustion pressure.
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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #522 on: 10/11/2014 02:48 am »
Remember the RS-68 was designed for cost. Hence the use of an ablative nozzle instead of a regeneratively cooled nozzle. I wonder if it had something to do with manufacturing costs.

If the other two engine families where not also fist stage engines, I would say it had something to do with a sealevel optimized engine, but that is not the case here.

Could it be the weight of a larger nozzle would have negated the ISP gain?

Weight or t/w has no influence on thrust, inproper area ratio does.

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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #523 on: 10/11/2014 09:12 am »
As we all know, most liquid hydrogen rocket engines have extraordinary Isp compared to other chemical rocket engines. I think the reason is generally the low average molecular mass of the resultants of combustion. The resulant water vapor has small molecular mass compared to CO2 or CO, also fuel rich combustion allows plenty of  hydrogen molecules to be left over, which results in  hot mixture of low average molecular mass, usually 12-13, compared to over 18 for cases of hydrocabon fuels.

Now the rocket engine function

c=ve+(Pe-Pa)Ae/q

The low average molecular mass of the resultant mixture contributes to both "ve" and "(Pe-Pa)/q" as follows:

For the "ve" part,



Low average molecular mass means high ve, which is the case for flights in designed ambient pressure.
But even for a vacuum optimized liquid hydrogen engine burning in the vacuum, this part of Isp can hardly make it over 420s, which you can deduce yourself.

For the "(Pe-Pa)Ae/q" part, low average molecular mass means larger exit area even for the  same thrust and optimzed for the same ambient pressure, which can be seen as in the case of rd-0120 and rd-170.




Larger exit area means larger value for the  "(Pe-Pa)Ae/q" part of Isp.
I also conclude that Isp part over 400s are mainly the result of larger exit area induced by the low average molecular mass of resultant of combustion.

What do you think of my understanding? Welcome to reply.


 
« Last Edit: 10/11/2014 01:12 pm by nimbostratus »
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Offline smoliarm

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Re: Rocket Engine Q&A
« Reply #524 on: 10/11/2014 10:14 am »
>>mocular mass
??
It should molecular mass
Or, better yet - molar mass.

The answer to your question is in chemical thermodynamics. Which is bad, because in general opinion this is very complex matter. But the good news is that here you need only the simplest part of all the thermodynamics - its First Law and the term Enthalpy.

The answer - in a short -
LH2 has very high combustion Enthalpy (which is here energy per mole) AND the lowest possible molar mass.
This gives to the LH2-LOX pair the highest specific combustion energy -- per gram of propellant - among all the things which can burn (in the Universe:)

Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #525 on: 10/11/2014 10:33 am »
>>mocular mass
??
It should molecular mass
Or, better yet - molar mass.

The answer to your question is in chemical thermodynamics. Which is bad, because in general opinion this is very complex matter. But the good news is that here you need only the simplest part of all the thermodynamics - its First Law and the term Enthalpy.

The answer - in a short -
LH2 has very high combustion Enthalpy (which is here energy per mole) AND the lowest possible molar mass.
This gives to the LH2-LOX pair the highest specific combustion energy -- per gram of propellant - among all the things which can burn (in the Universe:)

I Did mean molecular. Thanks for bringing it out.

As for the thermodynamics, one doesn't have to understand thermodynamics thoroughly to design rocket engines, what he need to do is learn to read relative charts.

« Last Edit: 10/11/2014 11:21 am by nimbostratus »
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Offline R7

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Re: Rocket Engine Q&A
« Reply #526 on: 10/11/2014 01:49 pm »
Larger exit area means larger value for the  "(Pe-Pa)Ae/q" part of Isp.

Too simplified statement. For instance engine optimized for vacuum should have as low Pe as possible. Play with engine simulation tools, the greater the area ratio and the lower the Pe the closer optimum expansion and vacuum Isps are, meaning pressure thrust component shrinks.

Optimum would be not to have the pressure thrust component at all but in practice you always have some. At sea level it is usually negative, meaning optimum expansion doesn't happen at sea level but at higher altitude (~4-5km for many kerolox, even higher for hydrolox core engines you listed (and SSME)).
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Offline Jim

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Re: Rocket Engine Q&A
« Reply #527 on: 10/11/2014 01:57 pm »

As for the thermodynamics, one doesn't have to understand thermodynamics thoroughly to design rocket engines, what he need to do is learn to read relative charts.


Quite wrong.

Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #528 on: 10/11/2014 02:11 pm »
Larger exit area means larger value for the  "(Pe-Pa)Ae/q" part of Isp.

Too simplified statement. For instance engine optimized for vacuum should have as low Pe as possible. Play with engine simulation tools, the greater the area ratio and the lower the Pe the closer optimum expansion and vacuum Isps are, meaning pressure thrust component shrinks.

Optimum would be not to have the pressure thrust component at all but in practice you always have some. At sea level it is usually negative, meaning optimum expansion doesn't happen at sea level but at higher altitude (~4-5km for many kerolox, even higher for hydrolox core engines you listed (and SSME)).

I can't say you are wrong, you just missed the point.

I was not talking about optimizing a liquid hydrogen engine, but comparing liquid hydrogen engines with other chemical rocket engines.
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Offline R7

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Re: Rocket Engine Q&A
« Reply #529 on: 10/11/2014 02:45 pm »
I can't say you are wrong, you just missed the point.

I was not talking about optimizing a liquid hydrogen engine, but comparing liquid hydrogen engines with other chemical rocket engines.

The point was exit areas / pressures as such aren't reason why hydrolox excels. Smoliarm already gave you the short answer.


Furthermore statements like this...

For the "(Pe-Pa)Ae/q" part, low average molecular mass means larger exit area even for the  same thrust and optimzed for the same ambient pressure, which can be seen as in the case of rd-0120 and rd-170.
 

... are baffling at best because RD-0120 and RD-170 are neither same thrust nor optimized for same ambient pressure.
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Offline R7

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Re: Rocket Engine Q&A
« Reply #530 on: 10/11/2014 03:01 pm »
At lease one thing is for sure, the optimized ambient pressure for rd-0120 and rd170 is approximately sea level pressure.

Incorrect. ~0.5atm for RD-170 and ~0.16atm for RD-0120.
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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #531 on: 10/11/2014 03:20 pm »
At lease one thing is for sure, the optimized ambient pressure for rd-0120 and rd170 is approximately sea level pressure.

Incorrect. ~0.5atm for RD-170 and ~0.16atm for RD-0120.

Can't believe that.
Perhaps you are right.
And as for the thrust, I meant thrust of a chamber of RD-170 and that of rd-0120.


« Last Edit: 10/11/2014 03:22 pm by nimbostratus »
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Offline R7

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Re: Rocket Engine Q&A
« Reply #532 on: 10/11/2014 03:49 pm »
Incorrect. ~0.5atm for RD-170 and ~0.16atm for RD-0120.

Can't believe that.

It's not a matter of belief, that's how it is.
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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #533 on: 10/11/2014 04:24 pm »
Incorrect. ~0.5atm for RD-170 and ~0.16atm for RD-0120.

Can't believe that.

It's not a matter of belief, that's how it is.

OK,and thanks for sharing these sceenshots.

It seems that you have very good simulation software.

Can you also provide the exit area of a chamber of rd-170 and that of rd-0120 optimized for a common  ambient pressure, say 0.5atm?


« Last Edit: 10/11/2014 04:55 pm by nimbostratus »
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Offline Jim

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Re: Rocket Engine Q&A
« Reply #534 on: 10/11/2014 05:44 pm »

Can't believe that.


An understanding of thermodynamics would open your eyes.

Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #535 on: 10/12/2014 01:47 am »

Can't believe that.


An understanding of thermodynamics would open your eyes.

Of course.
And I do know a little.
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Offline R7

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Re: Rocket Engine Q&A
« Reply #536 on: 10/12/2014 10:14 am »
OK,and thanks for sharing these sceenshots.

It seems that you have very good simulation software.

Can you also provide the exit area of a chamber of rd-170 and that of rd-0120 optimized for a common  ambient pressure, say 0.5atm?

You are welcome. It is a good software indeed, developed by a real Russian rocket scientist Alexander Ponomarenko. Go ahead and try it yourself. RD-170 configuration file is included in the software examples, RD-0120 you can quickly build from public information but I'll attach configuration file for that to speed things up. Remove the ".txt" from the filename, forum software requires it.
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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #537 on: 10/12/2014 12:39 pm »
OK,and thanks for sharing these sceenshots.

It seems that you have very good simulation software.

Can you also provide the exit area of a chamber of rd-170 and that of rd-0120 optimized for a common  ambient pressure, say 0.5atm?

You are welcome. It is a good software indeed, developed by a real Russian rocket scientist Alexander Ponomarenko. Go ahead and try it yourself. RD-170 configuration file is included in the software examples, RD-0120 you can quickly build from public information but I'll attach configuration file for that to speed things up. Remove the ".txt" from the filename, forum software requires it.

It is a file of a not usual format, it will take some time to read it.

And I have a new question, is a combustion chamber temperature of over 3800K normal?I haven't seen so high a value before.
« Last Edit: 10/12/2014 01:05 pm by nimbostratus »
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Offline R7

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Re: Rocket Engine Q&A
« Reply #538 on: 10/12/2014 04:44 pm »
@nimbostratus

You are not supposed to read the attached file but load it into the software I provided link for :)

Chamber temperature is connected to chamber pressure. RD-170 Pc is very high, therefore the Tc is high too. IIRC only RD-180 has higher Pc among kerolox engines, the RPA tool calculates Tc nearly 3900K for it.
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Offline nimbostratus

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Re: Rocket Engine Q&A
« Reply #539 on: 10/13/2014 01:50 am »
@nimbostratus

You are not supposed to read the attached file but load it into the software I provided link for :)

Chamber temperature is connected to chamber pressure. RD-170 Pc is very high, therefore the Tc is high too. IIRC only RD-180 has higher Pc among kerolox engines, the RPA tool calculates Tc nearly 3900K for it.

Oh, I didn't realize that.

I visited that site several times, but never found that software.

As for chamber temperature, as far as I know, the chamber termparature can hardly exceed 3700K. I guess the software has problem with the temperature part.

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