Pressure fed versus pump fed rockets

(Tom Merkle) wrote in message


2. the pump impeller has to be VERY carefully machined,and staged to
prevent cavitation, as this would under most circumstances shake the
engine apart and destroy the pump.

Tom Merkle

correction to my post(thanks Ian Wollard):
I oversimplified quite a bit.

should read "2. the pump impeller has to be VERY carefully machined to
minimize cavitation", not prevent it. Some cavitation takes place in
every pump, and is only disasterous when it occurs a lot. The damaging
effects are not just from vibration but also from affecting the
output. The impeller is also just one piece of an expensive pump. The
turbine driving it, bearings, and associated piping are just as
demanding in terms of tolerance as the impeller.

Tom Merkle

Greg One problem is just how much pressure gets wasted in cooling passages
Greg etc. IIRC the SSME turbo-pumps run at a pressure of about 5000psi but
Greg the thrust chamber runs at only 3500psi or thereabouts.

Even worse than that:

From page 166, Design of Liquid-Propellant Rocket Engines:
HPFTP discharge: 6024.8 psia
HPOTP discharge: 6952.2 psia

Page 92: (following the H2)
Main combustion chamber
Coolant inlet pressu 5890 psia
Coolant exit pressu 4800 psia

Back to page 166:
HPFTP turbine inlet: 4933 psia (?!?)
HPFTP turbine exit: 3376 psia
Throat stagnation pressu 3010 psia

I don't understand the rise in pressure from chamber coolant exit
to HPFTP turbine inlet. Presumably this happens through the fuel
preburner... I would have expected a pressure drop there (mostly
from the injectors).

On page 113 the book suggests that a 20% pressure drop across the
injectors is a good value because it gives good isolation between
chamber pressure variation (NASA says 5% variation is "stable
combustion") and propellant flow rate. It looks like the SSME
gets by with a 12% drop instead, but maybe that's because they're
injecting hot turbine exhaust rather than cold dense propellants.

"johnhare" wrote in message om...
"Tom Merkle" wrote in message

I believe you are missing much of his point about creating cheap components
as opposed to high performance ones.

nope, got the point but questioned it's applicability to engine
design.

You are also confusing gas compression
and flow with pressurising incompressable liquids.

Guilty as charged.

A one inch pipe will
flow 10 times as much gas at ten times the pressure at constant temperature.
The same size pipe will flow almost identicle masses of incompressable
liquid at 10 times the pressure. A 1% difference is not a consideration.

[snipped my now pointless blather]

The pump impeller and volute are about the only fuel handling
components that increase in size, though not necessarily in mass as
they can be thinner materials for lower pressures to be handled.

oops. good point. Again, I was apparently lost in the weeds between
higher and lower pressure engines, as well as gas vs. incompressible
liquid flow.

The thrust chamber increases in size for lower pressures. However,
he was discussing 250 psi pumped vs 250 psi pressure fed, which
makes that point a non issue. Liquid flow pipe diameters are a function

true. I have no response for that.

of of velocity and density, so for the same velocity, the same mass
will flow. With similar masses flowing at lower pressures, thinner
pipes can be used.
A low pressure pump fed system does not have to be much more complex
than a full up pressure fed system. A slightly higher thrust chamber
pressure
can save mass on the thrust chamber, canceling the mass penalty of the
pump. Just as long as you stay off that slippery slope of max possible
performance at all costs. Tank masses are only part of the problem,
pressurant
gas and systems are a mass and cost driver. I am willing to argue that they
are more
expensive than some pumps. XCORs' EZ-Rocket had helium as one of the main
cost drivers per flight.

On point 1. So what if you need a turbine. Turbines are not as super tech
as some people tend to believe. The turbine off of a semi truck engine
turbocharger makes a dandy test bench model for a small rocket engine
turbopump. Mine came from a blown engine for $20.00.

Were you able to hook an engine up so that its turbopump was not
independantly powered? That's amazing if so.
Using a turbocharger to work as a turbine for a bench model is a far
cry from turbines being 'easy,' though. A lot of research, money, and
years has gone into getting combustion engine turbo chargers right.
I'm sure if as much commercial money was poured into turbopumps,
they'd be every bit as cheap and reliable. If turbine technology was
really that generally easy, though, we'd all be driving jet cars,
wouldn't we? (cheaper fuel, higher fuel efficiency!)


On point 2. Carefully machined impellers are available at your local
industrial pump supplier. Scapped ones can be machined locally in an
hour on old lathes at a normal rate of ~$55.00 an hour. Designing around
cavitation is more of a consideration for very high performance pumps
than for the ones under consideration here, though some attention must

Won't that make interesting (bad) things happen in the thrust chamber?

be paid to it. Also, it needs to be pointed out that the impeller tip speeds
required here are 300-350 feet per second. These are the tip speeds of
hand held demolition saws with $5.00 abrasive blades that are frequently
out of balance and always loaded off center when working.

Although those bearings never have to work in a LOX environment,
right?

John Hare, thank you for the informative and unsarcastic correction.
From now on I'll leave explaining the engine science to people who
have obviously built one. (not me)

(chagrin)
Tom Merkle

(Iain McClatchie) wrote:
[...]
From page 166, Design of Liquid-Propellant Rocket Engines:
HPFTP discharge: 6024.8 psia
Page 92: (following the H2)
Back to page 166:
On page 113 the book suggests that a 20% pressure drop across the


Iain, I haven't seen this one listed on the group (though I missed
Scott's jpegs this time around). Could you add author info, and maybe
date?

Tnx

/dps

"Tom Merkle" wrote in message
om...
"johnhare" wrote in message
om...

On point 1. So what if you need a turbine. Turbines are not as super
tech
as some people tend to believe. The turbine off of a semi truck engine
turbocharger makes a dandy test bench model for a small rocket engine
turbopump. Mine came from a blown engine for $20.00.

Were you able to hook an engine up so that its turbopump was not
independantly powered? That's amazing if so.

I was trying to work out a propellant supply system. At the time
I tried that, I was not interested in building the actual engine. If
a supply system can be built on the cheap, I have friends with
engines, stands, and experience. One spontaneous disassembly
due to ignorance is enough for me.

Using a turbocharger to work as a turbine for a bench model is a far
cry from turbines being 'easy,' though. A lot of research, money, and
years has gone into getting combustion engine turbo chargers right.
I'm sure if as much commercial money was poured into turbopumps,
they'd be every bit as cheap and reliable. If turbine technology was
really that generally easy, though, we'd all be driving jet cars,
wouldn't we? (cheaper fuel, higher fuel efficiency!)

I deal with people that do jet dragsters, and others that deal with
turbine drives through gear boxes. Turbines are not a good match
for automotive velocities for the most part. In many ways, turbopumps
for the low end pressures under discussion are easier than turbochargers.

On point 2. Carefully machined impellers are available at your local
industrial pump supplier. Scapped ones can be machined locally in an
hour on old lathes at a normal rate of ~$55.00 an hour. Designing around
cavitation is more of a consideration for very high performance pumps
than for the ones under consideration here, though some attention must

Won't that make interesting (bad) things happen in the thrust chamber?

Yes it would if it gets out of hand. The lower speeds here are just easier.

be paid to it. Also, it needs to be pointed out that the impeller tip
speeds
required here are 300-350 feet per second. These are the tip speeds of
hand held demolition saws with $5.00 abrasive blades that are frequently
out of balance and always loaded off center when working.

Although those bearings never have to work in a LOX environment,
right?

Right. Though I believe that may not be much of a problem.

John Hare, thank you for the informative and unsarcastic correction.
From now on I'll leave explaining the engine science to people who
have obviously built one. (not me)

I have not yet built succesful ones. The #1 problem I share with other$,
i$ lack of fund$. I have been agressively addressing that issue to the
exclusion of hardware for a couple of years. Homework and theory
has not stopped. In many of the texts, they assume everyone is after
max performance and you can find the easy, discard solutions with
minimal effort. The easy ones are good enough for now. When I $olve
problem #1, it's hardware again.

(chagrin)

Unearned, I overstated my case.

Tom Merkle

(dave schneider) wrote in message . com...
(Iain McClatchie) wrote:
[...]
From page 166, Design of Liquid-Propellant Rocket Engines:
HPFTP discharge: 6024.8 psia
Page 92: (following the H2)
Back to page 166:
On page 113 the book suggests that a 20% pressure drop across the


Iain, I haven't seen this one listed on the group (though I missed
Scott's jpegs this time around). Could you add author info, and maybe
date?

Huzel and Huang, "Modern Engineering for Design of Liquid-Propellant
Rocket Engines", Fourth printing, 1992, AIAA

I may have misled you by dropping the first three words. Sorry about
that. Where (and what) are Scott's jpegs?

BTW, I read
http://yarchive.net/space/rocket/roc...ign_books.html
and then hit amazon.com. My wife was NOT happy with the bill!

(Iain McClatchie) wrote:

Huzel and Huang, "Modern Engineering for Design of Liquid-Propellant
Rocket Engines", Fourth printing, 1992, AIAA


Thanks!
[...] Where (and what) are Scott's jpegs?

His bookshelf pictures. Scott Lowther, that is.


BTW, I read
http://yarchive.net/space/rocket/roc...ign_books.html
and then hit amazon.com. My wife was NOT happy with the bill!


But you'll actually read the books, and that's even scarier!

/dps