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inefficient turbo-pumps and Isp



 
 
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  #1  
Old August 8th 03, 07:08 AM
Greg
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Default inefficient turbo-pumps and Isp

I am trying to understand some of the loss mechanisms in a modern
rocket engine.

In a staged combustion cycle rocket engine, there are losses in the
turbine that drives the turbo pump/s. The friction losses in a turbine
show up in the gas as 'reheat'. Since this heat is available at the
trust chamber, it should not effect the Isp of the engine?

I know that this is the case for regeneratively cooled engines, the heat
taken out is still in the loop. So its no loss.

Am i correct? Can you get away with a very inefficient turbo-pump?

Thanks
Greg
  #2  
Old August 9th 03, 04:33 PM
Doug Goncz
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Default inefficient turbo-pumps and Isp

Can you get away with a very inefficient turbo-pump?

I think so, but it does need to be compact, so efficiency helps. These a
amazing machines, thousands of horsepower in a very small package.

The regeneratively cooled engine produces waste heat, effectively captured by
vaporization of fuel or oxidizer between turbopump inducer/impeller and
turbopump drive turbine. By running hot, Isp is increased.

Sorry I don't know the efficiency figures.



Yours,

Doug Goncz, Replikon Research, Seven Corners, VA
Unequal distribution of apoptotic factors regulates
embryonic neuronal stem cell proliferation

  #4  
Old August 19th 03, 09:33 PM
Henry Spencer
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Default inefficient turbo-pumps and Isp

In article ,
Greg wrote:
I know that this is the case for regeneratively cooled engines, the heat
taken out is still in the loop. So its no loss.
Am i correct? Can you get away with a very inefficient turbo-pump?


Yes and no. You're correct in thinking that the heat may not get lost,
although it may show up in unwanted places. More significantly, though,
an inefficient pump is likely to be a heavy pump, and pump mass can be
quite significant.
--
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  #6  
Old August 22nd 03, 02:03 AM
Zoltan Szakaly
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Default inefficient turbo-pumps and Isp

"Christopher M. Jones" wrote in message ...
"Henry Spencer" wrote:
Greg wrote:
I know that this is the case for regeneratively cooled engines, the heat
taken out is still in the loop. So its no loss.
Am i correct? Can you get away with a very inefficient turbo-pump?


Yes and no. You're correct in thinking that the heat may not get lost,
although it may show up in unwanted places. More significantly, though,
an inefficient pump is likely to be a heavy pump, and pump mass can be
quite significant.


In other words, heavy pumps play on the other side of the
rocket equation, not in Isp but in achievable mass ratios
and in the dry mass fraction of stages.

A couple of quick calculations using data from
astronautix.com, I get an average of about 20% of the
dry mass and a negligable fraction of the gross mass
(~0.2%) as engine mass for the first stages of the
Atlas V, Arianne 5, and Delta IV med. I.e. the
difference in mass ratios of an engineless stage and
an engined stage is about 25% (0.998/0.8), which
means that engine mass decreases the first stage
delta V by *very roughly* 0.22 * Isp * g, or around
1/2 to 1 km/s for most liquid chemical propellants
(again, very roughly).


The optimum thrust to weight at takeoff for a rocket is around 1.5,
and if the engines have a thrust to weight ratio of 100 then the
engine mass is 1.5 percent of the total mass at takeoff. This is more
than 20% of the dry mass.

Zoltan
  #7  
Old August 23rd 03, 03:45 AM
Christopher M. Jones
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Default inefficient turbo-pumps and Isp

"Zoltan Szakaly" wrote:
The optimum thrust to weight at takeoff for a rocket is around 1.5,
and if the engines have a thrust to weight ratio of 100 then the
engine mass is 1.5 percent of the total mass at takeoff. This is more
than 20% of the dry mass.


Hellooo, McFly, I was using *actual* numbers from *actual*
rockets. If my numbers conflict with your estimates then
guess which one is most likely to be in error? The thrust
to weight ratio of the RD-180 (Atlas V first stage engine)
is about 78, and the weight is about 1% of the GLOW, yet
that's still only 24% of the first stage's mass.

Perhaps you are misestimating the dry mass fraction of
the stage.

  #9  
Old August 23rd 03, 05:36 PM
Ian Woollard
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Default inefficient turbo-pumps and Isp

Zoltan Szakaly wrote:

The optimum thrust to weight at takeoff for a rocket is around 1.5,


Actually no, it depends on the rocket, mainly on the staging scheme;
some rockets (especially two or more stage rockets) can optimise at
around 3g takeoff.

Zoltan

  #10  
Old August 25th 03, 01:45 PM
Earl Colby Pottinger
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Default inefficient turbo-pumps and Isp

Ian Woollard :

Zoltan Szakaly wrote:

The optimum thrust to weight at takeoff for a rocket is around 1.5,


Actually no, it depends on the rocket, mainly on the staging scheme;
some rockets (especially two or more stage rockets) can optimise at
around 3g takeoff.

Zoltan


When I did some simple modelling of my designs I found 4.5g to be the best,
since I had a very simple drag model that I don't trust I would also agree
the 3g does a very good job for some designs.

Earl Colby Pottinger

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