AND WHEN TELEPORTATION ARRIVES?

[Wizlish]

gregc

Wizlish

Remember that there is a LIMITING speed associated with gravitational attraction, which is expressed in part in the formula for escape velocity; by direct implication the highest speed you'd achieve from rest toward Earth by gravitation alone would be about 7 miles per second, which is peanuts by interplanetary standards. 

 can you explain?

 

Look at how escape velocity is formally defined: it is the instantaneous speed at starting (and by extension a speed which, reached at any point during ascent) that will assure that a vehicle escapes a given gravity well 'completely' (meaning that it will never fall back directly to the body it ascended from).  Technically at the 'limit' this means the vehicle never *quite* stops when it gets out there, but moves only infinitesimally slowly.  Any speed above escape velocity, including any acceleration to or above the escape speed itself at any point after starting, assures that the vehicle will leave the gravity well entirely.

 

Now, if you turn the physics around, you can see that a body starting at rest any distance from a given gravity well, and accelerating toward it purely under the influence of that well's gravity, will not reach a speed greater than the escape velocity, any more than a Foucault pendulum pulled back to your nose and released will come back to actually hit your nose (although it may come scary close at first!).

 

Note that this is for straightforward radial acceleration.  In practice, of course, you pitch over and invoke orbital mechanics once you reach the appropriate speed.  That's another whole story, and a very interesting one, but notice that accelerating to escape velocity in the plane of any orbit results in ... escape, not a very-long-period orbit with the minor diameter at the original orbital diameter.

Wizlish

People see numbers like the number of weeks needed to reach relativistic velocity at 1g constant boost, or the velocities achievable with electric or laser propulsion even at milligal (sorry, I like the old expression!) peak acceleration.

I assume conventional gravity assist from the moon would be peanuts compared to the max velocity during the moon maneuver.   But "what if" that max velocity could be maintained because a "gravity shield" limits deceleration when exiting the moon's influence.    Would that be comparable to the velocity of a probe to the outer planets today? (don't compare it to "c").

 

The answer is yes, of course, and having the ability to shield gravitational attraction during recession from a gravity sling would be of great value.  (Remember that nominal escape velocity from Earth itself, with the gravity cancellation on, would be essentially zero, so a flyby would retain whatever state vector the vehicle had at the moment you activated the shielding (or whatever).  All that momentum would then be available for the next encounter, and so on. 

 

Although you also have to remember that if you are going anywhere but out of the solar system forever, you will need to decelerate at some point (principally by gravity assist with today's technology) in order to reach or orbit the target.  Cassini got up to nearly 28 mps at the peak of its second gravity assist, but lost most of that by the time it got to Saturn.  And here, again, is the problem inherent when you can only shield against gravitational attraction: you must use a harder counter-orbit to shed velocity (or more aerobraking when available) to make up for the energy you kept in recession from the prior well(s).

Would it lessen the time to destination compared to multiple gravity assists around the inner planets and sun?

Perhaps.  Perhaps not.  It would certainly accelerate part of the trip, but that might greatly extend what's needed to actually match velocity with the 'destination' rather than whiz past it like a kid who pedaled downhill to see how fast he could go and then grabbed onto a bus to go back uphill, but then went 40 mph past the McDonalds on ice with no brakes.  He'd go hungry for a while, and so might you.

Wizlish

Give me some time and perhaps I can find a way to tie railroading back into this somewhere. 

 this thread started out as a "what if" ...

 

But until we have railroads in interplanetary space, perhaps tracks on one of Aldiss's webs, the 'what if' has become tenuous.  At least in principle teleportation could supplant or augment some railroading-related things.  Now, my father can get away with some of this on his steam-engine group because after all the SSMEs used it for propulsion.  Escape velocity is not something at all needed in railroad applications!

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