Posted to sci.astro:
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From: Robert Clark )
Subject: Rockets not carrying fuel.
Newsgroups: sci.astro, sci.space.policy, sci.physics
Date: 2003-07-28 20:59:39 PST
From this web page, the weight of the shuttle external tank with the
liquid oxygen and hydrogen is 1.6 million pounds:
EXTERNAL TANK
http://science.ksc.nasa.gov/shuttle/...ewsref/et.html
But the amount of liquid oxygen that is burned is only 2,787 pounds
per second and the amount of hydrogen 465 pounds per second.
Nanotube productions methods are advancing quickly. Suppose it is
possible to make a fuel line of carbon nanotube material hundreds of
kilometers long. Could fuel be pumped up to a rocket accelerating to
orbital velocity?
What would be the fuel requirements for a rocket that did not carry
its own fuel? Say a rocket with the payload capacity of the shuttle
and with engines of the efficiency of the shuttle main engines?
Bob Clark
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From: Robert Clark )
Subject: Rockets not carrying fuel.
Newsgroups: sci.astro, sci.space.policy, sci.physics, sci.mech.fluids, sci.engr.mech
Date: 2003-07-30 22:03:44 PST
The total weight of the fuel and pipe would only have to be carried
near the end of the trip. For the lowest part of the trip where
typically according to the rocket equation most of the fuel gets
burned, little mass for the fuel and pipe would have to be carried.
What I wanted to see was how the rocket equation would be changed
when for the great majority of the trip there is little fuel "cost"
for the fuel weight itself.
As for the pipe, I'm estimating according to the strength vs.
lightness characteristics of carbon nanotubes that a thin walled pipe
composed of nanotube material even a hundred kilometers long whould
only weigh in the range of a few thousand kilos. The question then
would be the mass of the fuel that needed to be carried or supported
by the rocket.
The first thing to notice is that when you don't have that huge 1.6
million pound mass attached that needs to be accelerated you might not
need the high efficiency that a liquid hydrogen/liquid oxygen engine
offers. Then in that case you might be able to do with just gaseous
hydrogen and without an additional liquid oxygen oxidizer. This page
suggests hydrogen is used in liquid form to save weight and bulk:
Spaceflight :Principles of Rocketry
"Hydrogen and oxygen are gases at ordinary temperatures. But it is not
possible to store them as gases for use in a rocket. They would have
to be compressed to carry them in quantity, and these compressed gases
would have to be held in thick-walled tanks to withstand their
pressure. These tanks would add weight, which is a rocket designer's
enemy, for rocket builders always seek the lightest possible weight.
When these gases are liquefied at low temperatures, the rocket can
carry the largest possible quantities, and the tanks are light in
weight."
http://www.centennialofflight.gov/es...ockets/SP6.htm
Hydrogen gas is quite light at 1 atm pressure, about .08 kg/m^3. So
even if the pipe were 100km long and .1m wide giving it a volume on
the order of 1000 m^3, the mass of the hydrogen in the pipe would be
only 80 kg. But even liquid hydrogen is not very massive at about 71
kg/m^3 so it's mass within this 100km pipe would be only 71,000 kg,
still quite a difference from 1.6 million pounds.
But what if the rocket never even had to support the mass of the
fuel? This page gives an example of a type of pump known as a ram pump
that works from gravity alone and can raise liquids many times higher
than the distance of the fall of a gravity driven stream:
Contents for the pulser pump section of Gaiatech.
http://members.tripod.com/~nxtwave/g...lser/index.htm
This page gives a more general discussion of ram pumps:
Designing a Hydraulic Ram Pump.
http://www.lifewater.org/wfw/rws4/rws4d5.htm
This method may also be adaptable to work for pumping gases.
For a quite large fluid reservoir on the ground the force for raising
the fuel to the rocket would be provided by the pump on the ground not
the rocket. So the rocket would only be supporting the mass of the
fuel pipe itself. In this case you might even be able to use both
liquid hydrogen and liquid oxygen if the rocket did not have to
support the weight of these liquids.
Another possibility for pumping the fuel to the rocket might be to
use the principle of a hydraulic lift. As shown on this page a large
diameter piston moving a short distance can move a thin diameter
piston a long distance via an incompressible fluid:
How Hydraulic Machines Work
http://science.howstuffworks.com/hydraulic1.htm
(though in this case in order to drive the liquid 100km, the weight
you might need to apply to the large piston might be that of a
battleship.)
As for the speed of the fuel and whether it would have to be
accelerated to supersonic speeds. Note that what matters is its speed
with respect to the *pipe*. So you could have the pipe being
accelerated to high speed by the rocket while the fuel is moving at a
rather slow speed *with respect to the pipe*, this speed being
determined by the rocket's fuel requirements.
As for cryogenic requirements, if it turns out you wanted to use a
cryogenic fuel you might be able to insulate the fuel line with a
vacuum jacket and move the fuel fast enough to limit the loss due to
evaporation.
Bob Clark
Uncle Al wrote in message ...
Dan Tilque wrote:
Robert Clark wrote:
But the amount of liquid oxygen that is burned is only 2,787
pounds per second and the amount of hydrogen 465 pounds per
second.
Those numbers don't look right. Shouldn't the LOX be about 8
A boundary layer of raw fuel is pumped down the inner walls of each
combustion chamber to cool the walls. If you look at an apolitical
rocket system launching - the Saturn moon rockets - you note closeup
of the running engines at launch shows a black collar around each
exhaust that flashes white hot a bit later on down. The black is a
sheath of pyrolyzing kerosene that finally combusts.
Nanotube productions methods are advancing quickly. Suppose
it is possible to make a fuel line of carbon nanotube material
hundreds of kilometers long. Could fuel be pumped up to a
rocket accelerating to orbital velocity?
Got to give you credit for thinking outside the box. This
certainly is a unique idea.
But I can't imagine there exists a pump that could do this. Or
even come close. Especially since it has to operate at liquid
oxygen temps.
Turbopumping is no big deal, the Germans had it down pat for the V-2.
Pumping anything at sonic velocities through a long thin pipe is
really stooopid. You plug in the appropriate dimensionless number for
flow, you see where the turbulent flow regime begins, then you
carefully plan the project so you retire before the first shakedown
demo.
What insulates the cryogen from ambient temp? Nothing. Stooopid
idea.
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