Pressure fed versus pump fed rockets

The advantage of a pressure fed rocket, as I understand it, is much
cheaper (and somewhat smaller and lighter) engines, but at the cost of
very heavy tanks that hold propellant at 250-300 psi, compared with the
20-30 psi tanks used in pump fed rockets.

Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage of
very heavy tanks? Or am I missing something?

(Apologies if I have posted this twice -- email is acting up)

-- Larry

"Larry Gales" wrote in message
news:Pine.WNT.4.56.0311141016110.2448@homecomps...

The advantage of a pressure fed rocket, as I understand it, is much
cheaper (and somewhat smaller and lighter) engines, but at the cost of
very heavy tanks that hold propellant at 250-300 psi, compared with the
20-30 psi tanks used in pump fed rockets.

Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage
of
very heavy tanks? Or am I missing something?

(Apologies if I have posted this twice -- email is acting up)

-- Larry

My opinion is that the main thing holding back the cheap pumps is
the concept that they are complicated and expensive in the first place.
A reasonable effort can show that the pumps you descibe are doable
by changing a few of the assumptions in the design. One of the first
assumptions to be changed is that the thrust chamber is served by
a pump system that must be designed for it. By doing low level
systems engineering of the entire thrust package in parallel, several
options can be made available.

One of the simple ones is leaving the thrust chamber ablative or
radiative in cooling, with the pump rotors stacked directly above
the injection manifold, feeding it out of coaxial bowl volutes. Drive
the tip turbine with gasses taped off the combustion chamber.
A little thought will produce better geometries.

Larry Gales wrote in message news:Pine.WNT.4.56.0311141016110.2448@homecomps. ..
The advantage of a pressure fed rocket, as I understand it, is much
cheaper (and somewhat smaller and lighter) engines, but at the cost of
very heavy tanks that hold propellant at 250-300 psi, compared with the
20-30 psi tanks used in pump fed rockets.

Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage of
very heavy tanks? Or am I missing something?

You are missing something. Maximizing rocket engine power is all about
maximizing mass flow rate of propellants.

Basic fluid dynamics laws:

.
m = Pv (mass flow rate = density * volumetric flowrate.),


In other words, if you want to move the same amount of mass per unit
time at a lower pressure (hence lower outlet density), you need a
larger volumetric flow rate--i.e., the pump has to be physically
larger, and thus weighs more. I know it's somewhat counterintuitive,
but lower pressure = bigger pump. This has a double effect, because
the bigger the volume is, the greater the surface area of associated
piping, thus the thicker the piping has to be for the same pressure,
thus mass of the engine goes up again. Counterintuitive and ugly, but
true. For equal mass flow rates, the higher pressure system is smaller
and lighter (although it requires exponentially more power to drive
it.)

Pressure fed systems take the performance hit to avoid having a pump
at all, as that gets rid of both weight and complexity on the engine
side, although adding mass to the tank. Two things that make
turbopumps so ugly is that 1. they have to be powered by
something--i.e. you need a turbine of some sort to power the pump, and
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

Larry Gales wrote in message
Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage of
very heavy tanks? Or am I missing something?

The main reason that very high chamber pressure is used is because it
produces an overall lighter engine for a given thrust. Well for sea
level engines anyway. Higher chamber pressure means higher expansion
ratio and hence better performance, it also means a smaller engine
although stronger (hence heaver per unit area, its still a net win).
But high chamber pressure means heaver turbo-pumps and as the SSME
have shown they can be problematic to develop and operationally
expensive (although this could have a lot to do with hydrogens low
density ie the same performance turbo-pump for RP1 should be
lighter/cheaper).

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

I think that if commercial considerations are more of a design driver
you would end up with pump fed engines, but with lower chamber
pressures. To 'recover' some performance, some kind of altitude
compensation is probably going to have lower operation costs than very
high chamber pressure engines. This is not really going to work for a
SSTO where sea level T/W of the engines is much more critical and
turbo-pumps really are worth there weight in gold.

Personally i think if you stay away from liquid hydrogen then high
performance turbo pumps with reasonable operational and development
cost should be possible. But so far history has proved me wrong.

greg

Greg wrote:

Larry Gales wrote in message

Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage of
very heavy tanks? Or am I missing something?


The main reason that very high chamber pressure is used is because it
produces an overall lighter engine for a given thrust. Well for sea
level engines anyway. Higher chamber pressure means higher expansion

How much better would it be if they, say, launched at 12,000 feet?

On Fri, 14 Nov 2003, Greg wrote:

Date: 14 Nov 2003 21:29:27 -0800
From: Greg
Newsgroups: sci.space.tech, sci.space.policy
Subject: Pressure fed versus pump fed rockets

Larry Gales wrote in message
Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage of
very heavy tanks? Or am I missing something?

The main reason that very high chamber pressure is used is because it
produces an overall lighter engine for a given thrust. Well for sea
level engines anyway. Higher chamber pressure means higher expansion
ratio and hence better performance, it also means a smaller engine
although stronger (hence heaver per unit area, its still a net win).
But high chamber pressure means heaver turbo-pumps and as the SSME
have shown they can be problematic to develop and operationally
expensive (although this could have a lot to do with hydrogens low
density ie the same performance turbo-pump for RP1 should be
lighter/cheaper).

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

I think that if commercial considerations are more of a design driver
you would end up with pump fed engines, but with lower chamber
pressures. To 'recover' some performance, some kind of altitude
compensation is probably going to have lower operation costs than very
high chamber pressure engines. This is not really going to work for a
SSTO where sea level T/W of the engines is much more critical and
turbo-pumps really are worth there weight in gold.

Personally i think if you stay away from liquid hydrogen then high
performance turbo pumps with reasonable operational and development
cost should be possible. But so far history has proved me wrong.

greg

------------------------
Thanks for your very informative reply -- I learned a lot.

-- Larry

Tom Merkle wrote:
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.

It's my understanding that quite a lot of rocket impellers cavitate over
the whole flight. It's only got to last a few minutes...

Tom Merkle

"Tom Merkle" wrote in message
om...
Larry Gales wrote in message
news:Pine.WNT.4.56.0311141016110.2448@homecomps. ..
The advantage of a pressure fed rocket, as I understand it, is much
cheaper (and somewhat smaller and lighter) engines, but at the cost of
very heavy tanks that hold propellant at 250-300 psi, compared with the
20-30 psi tanks used in pump fed rockets.

Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would
get
the low cost advantages of pressure fed rockets without the disadvantage
of
very heavy tanks? Or am I missing something?

You are missing something. Maximizing rocket engine power is all about
maximizing mass flow rate of propellants.

Basic fluid dynamics laws:

.
m = Pv (mass flow rate = density * volumetric flowrate.),

I believe you are missing much of his point about creating cheap components
as opposed to high performance ones. You are also confusing gas compression
and flow with pressurising incompressable liquids. 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.

In other words, if you want to move the same amount of mass per unit
time at a lower pressure (hence lower outlet density), you need a
larger volumetric flow rate--i.e., the pump has to be physically
larger, and thus weighs more. I know it's somewhat counterintuitive,
but lower pressure = bigger pump. This has a double effect, because
the bigger the volume is, the greater the surface area of associated
piping, thus the thicker the piping has to be for the same pressure,
thus mass of the engine goes up again. Counterintuitive and ugly, but
true. For equal mass flow rates, the higher pressure system is smaller
and lighter (although it requires exponentially more power to drive
it.)

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.
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
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.

Pressure fed systems take the performance hit to avoid having a pump
at all, as that gets rid of both weight and complexity on the engine
side, although adding mass to the tank. Two things that make
turbopumps so ugly is that 1. they have to be powered by
something--i.e. you need a turbine of some sort to power the pump, and
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.

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.

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
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.

Tom Merkle

John Hare

Joseph Oberlander wrote in message ink.net...
Greg wrote:

Larry Gales wrote in message

Would it not be possible to build very cheap low pressure pumps with 250
psi instead of the 1400-3600 psi used in pump fed rockets, so we would get
the low cost advantages of pressure fed rockets without the disadvantage of
very heavy tanks? Or am I missing something?


The main reason that very high chamber pressure is used is because it
produces an overall lighter engine for a given thrust. Well for sea
level engines anyway. Higher chamber pressure means higher expansion

How much better would it be if they, say, launched at 12,000 feet?

Yep. Its reasonable gain, which is why there are so many proposals
using some form of high altitude launch. But 12,000 feat seems a
little low if your going to all that effort.

Larry Gales wrote in message
You are missing something. Maximizing rocket engine power is all about
maximizing mass flow rate of propellants.
[snip]

In other words, if you want to move the same amount of mass per unit
time at a lower pressure (hence lower outlet density), you need a
larger volumetric flow rate--

This is not quite right. Most fuels and oxidisers can be considered
incompressible. So the density is a constant. The only exception IIRC
is liquid hydrogen. Going down in pressure won't always mean a smaller
pump, but always a lighter pump. Pumping power is proportional to
presser and volume (for incompressible liquids) so lower pressure
means smaller turbine, low tip velocity etc. and thus everything can
be made lighter.

greg