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Rocket efficiency -- some moments to clarify.

Posted by: SarK0Y - Fri Jan 23, 2015 8:45 pm
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Rocket efficiency -- some moments to clarify. 
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Post Rocket efficiency -- some moments to clarify.   Posted on: Fri Jan 23, 2015 8:45 pm
Hi there.

i got a bit wonder of this:
Quote:
Rocket engine efficiencies are much better, up to 70%, because they operate at very high temperatures and pressures and can have very high expansion ratios.

https://en.wikipedia.org/wiki/Internal_ ... efficiency

Frankly, it's fully untrue: efficiency of rocket is [kinematic energy of payload] / [energy of burnt fuel] & the're far below than 70%. :D

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Fri Jan 23, 2015 10:52 pm
Read the text referenced.

http://www.ewp.rpi.edu/hartford/~ernest ... ements.pdf

The chemical combustion efficiency can be very high, as high as 99%.

The conversion to propulsive force is typically 50% for a chemical rocket, though it can be higher.

50% is cited as typical of chemical engines producing an ISP of 300.

The Space Shuttle Main Engine has a claimed 99.6% chemical combustion efficiency.

Its turbo pumps run at up to 80% efficiency.

http://large.stanford.edu/courses/2011/ ... TATION.pdf

Remember these systems are running at extremely high temperatures and pressures, have excellent heat recovery from the exhaust, and are on the verge of melting down during their entire operating life which is only measured in minutes.

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Fri Jan 23, 2015 11:13 pm
let's take payload w/ mass 20 000kg & velocity 8 000 m/s, then its kinetic energy is mv2/2 = ~ 6,4e+11 J. Now, let's burn 30 metric tonnes of kerosene (its energy density gets about 40MJ per kg), so 30 000 kg X 40 000 000J/kg == 1,2e+12 J. has anyone seen such a lightweight rocket??? :roll:

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sat Jan 24, 2015 6:30 am
Are you counting the engine, or the entire vehicle?

The Space shuttle has an efficiency of only 17% when you count the entire vehicle.

But you don't include an arbitrary vehicle and destination when measuring efficiency.

An Otto cycle engine would take how long to get to 8000 m/s when pushing a 20 tonne payload? What would the mass of that system be?

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sat Jan 24, 2015 7:45 pm
Quote:
Are you counting the engine, or the entire vehicle?

Actually, we always have the most concerns to get maximal overall efficiency. i have discussed about entire rocket, not just parts of this. In practice, to measure efficiency just with temperature in combustion chamber & at output ain't adequate way, because we deal w/, f.i., endothermic reactions + the needs to cool engines + we must damp parasitic vibrations…

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sat Jan 24, 2015 11:00 pm
Well, then your are not talking about the engine efficiency. If measurements are arbitrary then comparison is impossible.

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sun Jan 25, 2015 3:17 am
idiom wrote:
Well, then your are not talking about the engine efficiency. If measurements are arbitrary then comparison is impossible.

not sure what you mean. the launch weight of rocket derives straightly from engine efficiency. And simple calculations show that kerosene engine needs greater than 800s of ISP to gain efficiency ~50%.

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sun Jan 25, 2015 3:28 am
The launch weight of a rocket depends on far you want it to go.

If an engine has to make it to orbit, then all other engines are 0% efficient.

I don't think you mean to argue that jet engines and v8's are zero percent efficient?

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sun Jan 25, 2015 3:44 am
idiom wrote:
The launch weight of a rocket depends on far you want it to go.

If an engine has to make it to orbit, then all other engines are 0% efficient.

I don't think you mean to argue that jet engines and v8's are zero percent efficient?

you can measure kinetic energy & energy of burnt fuel for whatever engine. no problems at all.

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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Sun Jan 25, 2015 3:33 pm
SarK0Y, I believe you are referring to propulsive efficiency. This is the amount of kinetic energy added to the rocket as a function of the kinetic energy of the exhaust. This starts out at zero percent when the rocket is standing still, and increases to 100 percent when the rocket's velocity is equal to the exhaust velocity.


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Post Re: Rocket efficiency -- some moments to clarify.   Posted on: Mon Jan 26, 2015 7:38 pm
DaveHein wrote:
SarK0Y, I believe you are referring to propulsive efficiency. This is the amount of kinetic energy added to the rocket as a function of the kinetic energy of the exhaust. This starts out at zero percent when the rocket is standing still, and increases to 100 percent when the rocket's velocity is equal to the exhaust velocity.

Dave, actually, many sources get too inaccurate about rocket efficiency. In fact, we have two figures on topic:

1. [power of engine] / [power of burnt fuel]. Yes, it can be 70% & even higher.
2. [kinetic energy of payload] / [energy of burnt fuel]. for chemical rocket engines, it barely takes 17%. electrical ones are capable to leap up to 60% & even higher.
====================================
Pe == Fe * Ve (Pe -- power of engine, Fe -- thrust, Ve -- exhaust velocity)

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