01-17-2004, 05:31 PM
Hi,
er... for something to be launched from the moon, it has to get up there first. How does it get up there? it has to be launched from earth. Surely launching stuff from the moon would cost more, since it has to escape the gravity of 2 pretty-hefty rocks.
I think the idea is to set up the production plants on the Moon. Then the stuff launched from the Moon is simply material that was already there. Of course, setting up an industrial complex on the Moon that is sufficiently advanced to make spacecraft might take more than a few weeks :)
On the rail guns, there are a few not so minor problems. First is that the strength of materials available means that rail guns capable of sufficient energy to be useful will, at the very least, need to be extensively rebuilt after each shot. Second is that we can't get enough electrical power to the guns. We can get the energy, but not in a short enough time. Inductive reactance slows the rate at which a voltage can change. Every circuit has some inductance. Even in theory, it is not possible to reduce the inductance sufficiently to toss viable payloads at orbit achieving speeds. Third is the fact that we live at the bottom of an atmosphere. Since the velocity required at the mouth of a rail gun for the payload to achieve orbit is in the hyper-velocity range, when the payload leaves the gun it will be subjected to a huge deceleration, as well as heating great enough to need a heat shield (which adds to the throw weight but not to the payload). While this third problem applies only to a launch from Earth, the other two apply anywhere and are enough to make the rail gun concept is pretty much useless as a launch system.
Now, if you are thinking of a linear induction system, similar to what Heinlein was discussing in The Moon Is a Harsh Mistress, then that's another kettle of fish. But that's not a rail gun. Different technology, different problems.
--Pete
er... for something to be launched from the moon, it has to get up there first. How does it get up there? it has to be launched from earth. Surely launching stuff from the moon would cost more, since it has to escape the gravity of 2 pretty-hefty rocks.
I think the idea is to set up the production plants on the Moon. Then the stuff launched from the Moon is simply material that was already there. Of course, setting up an industrial complex on the Moon that is sufficiently advanced to make spacecraft might take more than a few weeks :)
On the rail guns, there are a few not so minor problems. First is that the strength of materials available means that rail guns capable of sufficient energy to be useful will, at the very least, need to be extensively rebuilt after each shot. Second is that we can't get enough electrical power to the guns. We can get the energy, but not in a short enough time. Inductive reactance slows the rate at which a voltage can change. Every circuit has some inductance. Even in theory, it is not possible to reduce the inductance sufficiently to toss viable payloads at orbit achieving speeds. Third is the fact that we live at the bottom of an atmosphere. Since the velocity required at the mouth of a rail gun for the payload to achieve orbit is in the hyper-velocity range, when the payload leaves the gun it will be subjected to a huge deceleration, as well as heating great enough to need a heat shield (which adds to the throw weight but not to the payload). While this third problem applies only to a launch from Earth, the other two apply anywhere and are enough to make the rail gun concept is pretty much useless as a launch system.
Now, if you are thinking of a linear induction system, similar to what Heinlein was discussing in The Moon Is a Harsh Mistress, then that's another kettle of fish. But that's not a rail gun. Different technology, different problems.
--Pete
How big was the aquarium in Noah's ark?