Gravitational Currents Could Slash Fuel Needed For Space Flight 177
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by
CmdrTaco
from the this-doesn't-smell-right dept.
from the this-doesn't-smell-right dept.
Hugh Pickens writes "BBC reports that scientists are mapping the gravitational corridors created from the complex interplay of attractive forces between planets and moons that can be used to cut the cost of journeys in space. 'Basically the idea is there are low energy pathways winding between planets and moons that would slash the amount of fuel needed to explore the solar system,' says Professor Shane Ross from Virginia Tech. 'These are free-fall pathways in space around and between gravitational bodies. Instead of falling down, like you do on Earth, you fall along these tubes.' The pathways connect Lagrange points where gravitational forces balance out. Depicted by computer graphics, the pathways look like strands of spaghetti that wrap around planetary bodies and snake between them. 'If you're in a parking orbit round the Earth, and one of them intersects your trajectory, you just need enough fuel to change your velocity and now you're on a new trajectory that is free,' says Ross. 'You could travel between the moons of Jupiter essentially for free. All you need is a little bit of fuel to do course corrections.' The Genesis spacecraft used gravitational pathways that allowed the amount of fuel carried by the probe to be cut 10-fold, but the trade off is time. While it would take a few months to get around the Jovian moon system using gravitational currents (PDF), attempting to get a free ride from Earth to Mars on the currents might take thousands of years."
old idea (Score:5, Informative)
Getting out of Orbit (Score:5, Informative)
The vast majority of fuel usage is simply getting out of orbit. I imagine this would be musch more useful for vehicles that are simply motoring around the solar system, but not dropping to the planet, or even going into LEO.
Re:n-body problem (Score:5, Informative)
Depends on your time horizon. Millions of years, no. Human time horizons, however, we can handle.
A good, modern, numerical integrator at quadruple precision can handle the Sun, planets, and hundreds of asteroids with very small numerical errors (microns over decades). Bigger errors are introduced by observational uncertainty, primarily in the masses of the asteroids. But, even with that, errors are 100's of meters over decades.
Re:old idea (Score:5, Informative)
Or if you'd RTFA you'd see the part where they talk about the Apollo missions and how it is not the same concept.
Re:So if there was a 10-fold decrease (Score:4, Informative)
Re:Fly part way. (Score:3, Informative)
"Up" is not the problem in getting to space; it is the velocity that is the problem. A 747 can reach a top speed of 567 miles per hour, while orbital velocity is 17,500 miles per hour. So, even if you could make a 747 carry a fully-loaded shuttle (it can't), you'd still need to accelerate an additional 17,000 miles per hour (which would still require the solid rocket boosters and the external tank, which are the majority of the weight, which a 747 certainly couldn't carry).
The shuttle passes a speed of 567 miles per hour in the first 20-30 seconds of flight IIRC. They are already throtting back the engines by that point to reduce aerodynamic stress on the vehicle.
Re:You can't dumb down rocket science (Score:3, Informative)
From TFA
Wouldn't that be the opposite of what you just said?
Re:You can't dumb down rocket science (Score:1, Informative)
If you actually read about this stuff, it's more or less the case. Nothing BS about it. The basic idea is that if you can get to one of the (unstable) Lagrange points (#1-3), only a very small impulse is needed to go anywhere in the solar system. It's a chaotic system where small changes to initial conditions will over time lead to an infinite range of outcomes. This has been known for decades, but is only now becoming somewhat practical as we have the computing power to calculate the trajectories.
It's only "somewhat" practical because the transfers can take impractical amounts of time, but the system has been used. In one case, a Japanese probe (Hiten) that was not designed with nearly enough fuel to get from Earth orbit to Lunar orbit was nevertheless salvaged for use as a Lunar probe (after their dedicated Lunar orbiter failed) by the way of an "Interplanetary Superhighway" or low energy transfer. It took six months to achieve Lunar orbit but practically no fuel.
not gravity assist! (Score:3, Informative)
A gravity assist trajectory is using the gravitational field of a large planet to divert a spacecraft to it's final destination. Since you are falling down a gravity well with this trajectory, you generate acceleration. The reason this works is that you are essentially "stealing" some of the momentum from the planet (think billiard balls colliding and exchanging momentum, but this is just without the collision).
This technique is almost the dual of the gravity assist in that it has the spacecraft follow the 3 dimensional paths of zero-net gravitational acceleration. Think of this like walking between two mountains mostly along the isolines (instead of taking a path where you are walking down into a valley and have to walk back up). The path might be long and windy to walk across the iso lines, but you reduce the total energy you have to expend (except to get from your starting point to the iso-line and from the iso-line to your destination). The reason these paths are called currents is that it really isn't a 2-d isopath with minimum energy you are following, but really a 6-d iso path (position and velocity thus a "current"). This is where the analogy breaks down with the 2d isopath.
BTW, this is really, really old news... http://www.jpl.nasa.gov/releases/2002/release_2002_147.html [nasa.gov]
And also a DUPE http://science.slashdot.org/article.pl?sid=03/03/07/215211&mode=thread&tid=160 [slashdot.org]
Re:not gravity assist! (Score:4, Informative)
Just thought I'd add another clarifying point.
It's often hard to visualize this, but even though a gravity current path (the minimum energy path) in a 6-d manifold (position+velocity) has time-varying velocity 3d velocitu (because the path isn't straight in 3d space it implies some acceleration from a 3d perspective), the velocity change is still essentially zero energy because sometimes the energy for the required velocity changes can come from gravity interaction itself (imagine a "valley" of some sort in a 6-d manifold), although some may require being very near the optimal path (imagine a "ridge" of some sort in a 6-d manifold) and thus require small corrections to prevent "butterfly-effects" from pushing the spacecraft further away from the optimal path (which these small course corrections are still better than fighting gravity all the way to the destination).
Re:You can't dumb down rocket science (Score:3, Informative)
A few months to get around the Jovian Moons sounds a lot like "a few months to get TO the Jovian moons." The general public doesn't know how long it takes to get there. They think we can get there in a couple hours using impulse engines. We can't. Hell, we're even going to miss the 2010 deadline. Dave Bowman will NOT be pleased.
not 4-d, 6-d (Score:3, Informative)
The "tubes" are really iso-paths in 6-dimensional (3d position + 3d velocity).
The "tubes" happen to connect the LaGrange points in 4d, though.
You do NOT have to navigate spacetime in your own power if you stay in these "tubes", although since they are 6-d isopaths, their "minimum energy" aspect to the path is really at their intrinsic velocity (which is why they are slow).
Let's try to get this one right...