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

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

(Greg) :

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

That is the same idea that I came up with and I see no reason why it should
not work, designing even an inefficient turbo-pump is still hard because you
still have a high speed spinning piece of metal, but my conculsion is you
don't need the tight clearances an efficient design needs, and wide range of
material can be used.

Earl Colby Pottinger


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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.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

(Henry Spencer) :

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.

The pump may have not be heavy, one design I thought of but never built so
you can take it with a block of salt is a H2O2/Coleman Fuel rocket. The H2O2
is fully decomposed then used to spin the turbine. The turbo-pump pumps both
the peroxide to the catalyst pack and the fuel to the combustion chamber.
Since I did not get access to the needed milling station I was unable to
build it to test the idea. The main advantages in the design are the large
clearances between the turbine blades and the housings. Comments - be
mercyless please, I may try to build it later and if you see a major flaw I
would like to know it now.

Earl Colby Pottinger

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"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

"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.

(Anvil) wrote in message
Just allow that there is some weight

penalty and select fairly efficient designs run at a non-optimum match.


I didn't think it was that easy to get a efficient design for a given
task. Unless the parameters of some previously designed pump/turbine match

very closely.

Greg

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

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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