| Topic: |
Science > Physics |
| User: |
"Brian White" |
| Date: |
23 Oct 2003 12:59:55 AM |
| Object: |
Rockets not carrying fuel. |
I missed the reference to pulser pumps in the old posting below.
Anyway, I am just pointing out an error in the posting. Mr Clark says
that pulser pumps are a type of ram pump. This is not correct. They
work by a different principle.
They are not as efficient as ram pumps but the absolute distance that
they can pump is probably quite a bit higher.
So for those of you who followed the thread, it may or may not be
relevent.
Thank you (and thanks Mr Clark for mentioning them)
Brian White
From: Robert Clark (rgregoryclark@yahoo.com)
Subject: Re: 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/essay/SPACEFLIGHT/rockets/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/gaiatech/pulser/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.
.
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