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Using Sling Shot Power to Hurl Into Orbit
the_2nd_coming writes "space.com has an article
about a new application of a very old technology.
NASA is putting money into Momentum-eXchange/Electrodynamic Reboost tether technology -- MXER for short -- an innovative concept that if implemented would station miles and miles of cart-wheeling cable in orbit around the Earth. Then, rotating like a giant sling, the cable would swoop down and pick up spacecraft in low orbits, then hurl them to higher orbits or even lob them onward to other planets."
It seems that the greatest two motivators of technology are SciFi authors, and sales people.
This approach was used by Robert Heinlein in several books; it is a pleasure to see his vision honored.
As for sales people, I can't count the number of times that I have had to create what they have sold.
I noticed on one of the diagrams that the orbit of the slingshot itself degrades after each launch pick-up. Maybe the decrease in orbit isn't very significant, but would this system require self-adjustment? How would the system stay in service over the long term?
True believers seek redemption from the sin of death.
In some ways the neatest thing about it is that it does away with the need for reaction mass, which is is nothing else an environmental improvement.
Lacking <sarcasm> tags,
This thing is going to transfer momentum to the space craft it is boosting, right? Where is it getting it from? Something has to get that cable spinning, and I don't see how to get the tether started/respun after it has be put in orbit/used to boost without using just as much energy as you have saved by using it.
I am sure I am missing something, but I don't know what, could someone fill me in?
P.S. I did RTFA, doesn't explain it...
"I'll have a Guinness, no wait, make that a Coors Light" -Grad student I work with, who shall remain anonymous...
What are you talking about, we're WAY more willing to sacrafice life then them.... Oh wait, you ment our lives. Never mind.
"Have you ever thought about just turning off the TV, sitting down with your kids, and hitting them?"
Do you think this could boost payloads delivered from small non-NASA suborbitals like Rutan's:
SpaceShipOne?
It would be great if you could just fly up to the edge of space, chuck your payload up, have a tether catch it and then land. Very cheap compared to rockets.
Also I wonder if the tether guys are working with: Carbon Fiber 60% stonger than steel
If these tethers work as claimed, they could significantly reduce the cost of getting satellites into geosynch orbits and high earth orbits, not to mention interplanetary travel as you mention.
"I wonder how many satellites will happen to stray close enough to those cables to get the crap beat out of them."
:-)
The chances of that happening are vanishingly small. I'd be more worried about orbital debris damaging the cable - but it seems they've already thought of that (multistranded). If the cable did get damaged, the worst that would happen is the satellite (if already "picked up") would get shot off in an unstable orbit.
A plus with this would be you could use the power generated by a conductive wire thru the cable to power ion thrusters or similar to keep the cable rotating (air friction at the pickup would require some boost occasionally). Less need for solar cells
Like others have mentioned, gravity slingshots are a whole nother beast.
I remember reading about this idea many many years ago - and I consider it something that will happen well before a space elevator will.
SB
It's old. The more humans I meet, the more I like my cats. At least they are honest.
I am a rocket scientist. In fact I've worked both on space tethers and giant space guns
professionally.
Electromagnetic tethers work on the same principle as an electric motor - put a current
through a wire in a magnetic field and you get a force. In earth orbit, you can make electrical
contact with the ionosphere so that you have a
one-way current in your wire, and thus a net force. The wire will accelerate one way, and the
ionospheric plasma accelerates the other way, but there is plasma all around the earth, so you
don't run out.
The force you get is IL x B, where I is current
L is the length of the wire, and B is the magnetic field. Since the strength of the
Earth's magnetic field is a given, you can only
play with the current in the wire and the length of the wire to get more force.
The only consumable you have is a bit of gas
that is ionized and squirted out to make your electrical contact with the ionosphere. It turns out you only need about 2% as much gas as a normal rocket would use for the same push, and only 1/8 as much as an ion thruster, so it is very mass-efficient. It can be powered by solar panels.
The downside is it only works well up to about 600 miles. Above that the ionosphere gets too thin to be of much use. That's where the momentum exchange tether comes in.
Vertical cables, or tethers, can be built in a wide range of lengths and spin rates. Any long vertical object in orbit tends to want to remain vertical because the Earth's gravity changes with the inverse square of the distance from the center of the planet.
So the bottom of the object, being closer to the Earth's center is tugged by gravity more than the middle, and the top is tugged less. This is the same effect that causes tides.
Left to itself, then, a vertical cable will stay vertical. The entire thing takes the same amount of time to orbit the earth. So the bottom end, which is moving in a smaller orbit, is moving slower, and the top end is moving faster.
A free object in a lower orbit actually moves
faster, thus if you let go at the bottom of
the cable, you will find yourself at a suborbital speed and re-enter. Similarly, if you let go at the top end, you were moving faster than the local orbital speed, and are thus flung into a higher orbit.
So if you are heading to, say, the Moon, you could ride up in a suborbital rocket that gets you to a landing platform at the bottom of the tether, ride an elevator to the top, then let go and get flung outwards.
While you were riding up the elevator, the rest of the tether is moving down due to Newton's law. Thus the electrodynamic motor, which is typically 10 km long and attached to the much longer momentum tether, is used to make up the altitude lost.
If the momentum exchange tether is short, i.e.
hundreds of km long, the difference in gravity
between the top and bottom isn't too great and
you can build it out of ordinary strong materials. When it gets sizeable in relation
to the Earth's radius, then you need materials
somewhat stronger than what we have available
in quantity.
Because the Earth's orbit has both natural and
manmade objects flying around, you need to be
able to tolerate damage to the tether. At a
minimum you need something like 6 cables, spaced
far enough apart that no single object can
take out more than 2 at a time (you can always
get 2 if you are aimed just right), and you need a way to replace damaged sections and transfer the tension around the damaged area in the mean time. The Tethers Unlimited design uses a fine mesh of many strands.
In the limit of a very long tether, you can get the bottom end to be stationary relative to the ground, and you get the space elevator. But it turns out that one that large, even using insanely strong nanotubes, weighs so much it would never make sense economically. A practical one would be in the 100s to a few 1000 km long.
Daniel