On 2/27/17 10:37, Howie DeFelice wrote:
Thanks Phil, that explanation cleared up many questions. I had been thinking about ways to use the earths magnetic field to raise perigee by storing energy in capacitors and pulsing something along the lines of a magnetorquer with a high current pulse at the right time and vector. The problem, I think, is that I would need to generate the thrust at apogee where the earths magnetic field is weakest.
If you're going to use the earth's magnetic field to change orbit, you want a linear force, not a torque as you'd use for attitude control.
A straight conductor will experience a Lorentz force perpendicular to both the current flow and the ambient magnetic field. Problem is, how do you close the circuit to keep the current flowing? If you run a second conductor through the same magnetic field, it will experience an equal and opposite Lorentz force. If it's physically separated from the first conductor (i.e. forming a loop), the result will be a net torque, not a net linear force.
So you need one of two things. Option #1 is a gradient in the magnetic field. If your "forward" conductor is in a stronger part of the field, it will experience a greater Lorentz force for a given current than the "return" conductor in a weaker part of the field. You'll then get a net linear force, along with plenty of torque.
The other possibility is to use something not attached to your spacecraft as the return conductor so you won't feel the opposing Lorentz force. But what conductor is there in space??
LEO is actually within the ionosphere, and that's your return path. This is in fact how tethers have been used so far to either add or remove energy from a satellite orbit.
Some years ago N6NKF and I looked at how much linear force you could get from a practical magnetorquer coil. (I think it was when Oscar 13 was decaying, so that was 20 years ago!) I don't remember the exact result but I do remember it being negligible.
Here's a wild idea I just had. I wonder if it would be practical to build an ion thruster for use in the ionosphere that uses the ambient ions as your reaction mass? Think of it as vaguely analogous to the Bussard Ramjet in science fiction. You'd collect both protons and electrons and accelerate the protons rearward and the electrons forward to maintain charge balance. The protons, being much more massive, would result in a net forward thrust. You could also see this as analogous to an airplane, only the "propellers" are electric fields that work on an ambient atmosphere that is ionized.
Phil