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drhex
2012-Sep-21, 10:28 AM
In Ian Stewart's "17 Equations that changed the world" (p72), I read this:
"In 2005, Michael Dellnitz, Oliver Junge, Marcus Post, and Bianca Thiere used tubes to plan an energy-efficient mission from the Earth to Venus. The main tube here links the Sun/Earth L1 point to the Sun/Venus L2 point. As a comparison, this route uses only one third of the fuel required by the European Space Agency's Venus Express mission, because it can use low-thrust engines; the price paid is a lengthening of the transit time from 150 day to about 650 days."

where "tubes" refers to some particular path and speed through space that economizes on fuel use by utilizing the planet's momentum and gravity in an optimal way.

The claim to reduce fuel consumption by a factor three surprised me though - I always thought that getting off the ground was the most expensive part and that moving around in space after that was cheap?

Hornblower
2012-Sep-21, 10:56 AM
Reducing the amount of fuel needed in space by 2/3 will greatly reduce the total initial mass of the orbiting spacecraft, which in turn will greatly reduce the total mass of the booster that is needed to get it off the ground and into orbit.

Shaula
2012-Sep-21, 11:40 AM
Getting somewhere requires two burns - one to speed up, one to slow down. And any manoeuvres in flight require burns too. The 'tubes' minimise that by using the objects in the solar system to perform a lot of the delta V required.

cjameshuff
2012-Sep-21, 07:49 PM
The claim to reduce fuel consumption by a factor three surprised me though - I always thought that getting off the ground was the most expensive part and that moving around in space after that was cheap?

Getting to orbit from Earth's surface is the most expensive part in terms of propellant usage, but maneuvering in space isn't free, and the propellant can easily make up a big chunk of the spacecraft's mass. Without orbital propellant factories, it takes just as much propellant to lift propellant as it does any other part of the spacecraft's mass, so reducing the propellant needed for the spacecraft itself still makes a big difference.

Ara Pacis
2012-Sep-21, 08:01 PM
It sounds like they're referring to the Interplanetary Transport Network (http://en.wikipedia.org/wiki/Interplanetary_transport_network) (note, it may also go to non-planets).

It can take a long time to use that network, so it's only really good for probes or freight, but isn't really useful for passenger craft.