If Gravity Could Be Temporarily Cancelled How Much Cheaper Would Space Travel Be?

[magnemoe]

On 5/5/2023 at 5:01 AM, sevenperforce said:

Well, begrudgingly to @Spacescifi's credit, this is one of the only workable magic solutions he's come up with so far.

And it matches most of the depictions of levitation and spaceflight in science fiction.

This, the magic gravity plates who are so common in  science fiction. And yes using it to hover or get into space would be an valid use case for them. It would be much more efficient than an helicopter unless the process was pretty inefficient. It would be very inefficient for creating gravity inside an ship as it would require the energy to hover the inside all the time. Worse you have the gravity 90 degree of the direction of trust. 
Why? because planes and ships are oriented this way  

[kerbiloid]

Antigravity minefields of antigravity plates.

[sevenperforce]

On 5/28/2023 at 11:10 PM, magnemoe said:

This, the magic gravity plates who are so common in  science fiction. And yes using it to hover or get into space would be an valid use case for them. It would be much more efficient than an helicopter unless the process was pretty inefficient. It would be very inefficient for creating gravity inside an ship as it would require the energy to hover the inside all the time. Worse you have the gravity 90 degree of the direction of trust. 
Why? because planes and ships are oriented this way  

I believe that in the Firefly universe the ships like Serenity have two systems that work like this: a g-screen and a g-field. The g-screen insulates the bulk of the ship from the effects of gravity, so that the engines don't have to fight against gravity during the climb to space. The g-field, on the other hand, produces a directional gravitational field for discrete objects within the ship to allow the use of decks, chairs, bunks, and so forth. It's reasonably clear that although these use similar technology, they are two separate systems. Both the g-field and the g-screen, while requiring a lot of energy to set up, require very little energy to keep going and decay slowly after a power loss, slow enough that issues like oxygen and temperature are more pressing.

When the ship's main pulse drive is activated, the surge of power allows for the g-field and the g-screen to be aligned together, which effectively cancels the inertia of the entire vehicle and allows for very high acceleration. Feels very much like the sort of of stuff that @Spacescifi using goes on about. He should really just borrow the Firefly mechanic and be done with it.

One unanswered question (I think) is how far the effects of any g-screen system actually extend. Ordinary objects are in free-fall, but free-fall itself is actually an extremely eccentric orbit around Earth's center of mass. If you cancel that, are you floating relative to Earth, or are you also floating relative to the sun? Relative to the galaxy? 

[darthgently]

57 minutes ago, sevenperforce said:

should really just borrow the Firefly mechanic and be done with it.

One unanswered question (I think) is how far the effects of any g-screen system actually extend. Ordinary objects are in free-fall, but free-fall itself is actually an extremely eccentric orbit around Earth's center of mass. If you cancel that, are you floating relative to Earth, or are you also floating relative to the sun? Relative to the galaxy

It seems unintuitive that a g-screen could tell the difference between galactic, solar, and planet/moon gravity and would simply screen the sum of all of them.  Hard to be logical about something not very logical though

+1 on the Firefly mechanic being the most "sensible"

Edited May 30, 2023 by darthgently

[kerbiloid]

Maybe the Firefly stays still.
Just applies a transformatrix to the surrounding Univers, making it move around the ship.

When you play KSP, you don't move, the Kerbal universe does.

[sevenperforce]

3 minutes ago, darthgently said:

It seems unintuitive that a g-screen could tell the difference between galactic, solar, and planet/moon gravity and would simply screen the sum of all of them.  Hard to be logical about something not very logical though

Yeah, there's a problem here.

I'm currently sitting on a chair being pulled toward the center of the Earth at 9.81m/s², which is the equivalent of being accelerated at 9.81m/s² for an object in free-fall. More importantly, this is the equivalent of being at the apogee of a highly eccentric 6378 km by 6.7 km orbit around the center of the Earth, with an apogee velocity of around 361 m/s (if you're clever, you can use that to determine my latitude). I rapidly come to notice this orbital path (and the change in orbital velocity relative to my fixed surroundings) if I jump off a ledge. A device or machine that could "screen" gravity and cancel my weight, leaving my inertial mass intact, would be the equivalent of stopping my orbit from being an orbit: transforming my apogee velocity of 361 m/s into a pure tangential velocity.

Thus if gravity was "screened" and turned off for me, I would continue moving at 361 m/s in a straight line, while the surface of the Earth continued to move under me at 361 m/s along the 39th parallel, in a circle. After four minutes, both of us would have traversed 86.6 km, but Earth's surface would have done so along one decree of circular arc, causing it to drop away underneath me by 972 meters. After twenty minutes, both of us would have traversed 433 km, but Earth's surface would now be 24 km beneath me. After an hour, Earth's surface would be a whopping 225 km beneath me.

A g-screen, then, would result in a perceived vertical levitation effect which increases quite rapidly with altitude for the first three hours, then increases more slowly, reaching a maximum of 361 m/s (or more, if you're at a lower latitude) after 6 hours.

However, this becomes a problem when you start looking at other orbits. Earth is orbiting the sun. If I "screen" gravity for my spaceship, my 30 km/s of orbital velocity I share with Earth is transformed to tangential velocity. After ten minutes, my straight trajectory will have drifted 1068 meters away from Earth's near-circular trajectory. After an hour, it will be 38 km. So my rate of drift relative to the surface of the Earth will be different if I am on the daylight side or the night side. (Fortunately, the rate of the sun's rotation around the center of the galaxy is slow enough (in rad/sec) that drift between my vehicle and the sun is not a significant issue.)

I suppose the easiest way to fix this is to posit that the g-screen, while very efficient, is not QUITE 100% efficient, and so portions of the ship still experience SOME gravitational attraction, which anchors the ship to the planetary surface at least a little. That would fix the drift problem, but I'm not sure what it does to orbits themselves, since the inertial mass of the vehicle is still intact.

3 minutes ago, darthgently said:

+1 on the Firefly mechanic being the most "sensible"

Agreed. It was a good balance. Not Star Wars handwavium, where everything floats for no visible reason, but also not so detailed that it introduces obvious contradictions or otherwise adversely impacts the plot.

Another reasonably good system (which I suppose approximates the Star Wars thing to some degree) is the "gravity rail" approach. There's some sort of superconducting loop which "locks" a vehicle to whatever spot in the local gravitational gradient where it finds itself, without using up any energy. That allows an object to experience gravity (and have the objects in or on it also experience gravity) while floating, but also while following the local curvature of space so that it doesn't drift. However, such a system doesn't allow you to follow the terrain; if you run off a cliff you'll stay at the same elevation, and if you try to drive up a slight hill you'll plow straight into it.

Perhaps the superconducting hoverloop thingy stores energy relative to your own gravitational potential, so when you drop off a cliff, it absorbs more energy in cushioning your fall, and when you need to ascend a hill, you lose potential energy in your hoverloop while gaining gravitational potential energy. But this suggests some maximum altitude to which a speederbike can ascend...

[kerbiloid]

29 minutes ago, sevenperforce said:

I'm currently sitting on a chair being pulled toward the center of the Earth at 9.81m/s²

It's the Earth is pulled to you at 6.674*10^-11 * 80 / (6.371*10⁶)²  ~= 1.3*10^-22 m/s².

So, "your" fall from 1 m altitude will last for sqrt(1 * 2 / 1.3*10^-22) ~= 1.24*10¹¹ s ~= 4 000 years.

Ph: Phyzix.

[darthgently]

My assumption was that as the g-screen ramped up, orbital velocity would become less necessary, so as the g-screen ramped up to max, orbital velocity would be ramped down to zero.  So no tangential velocity to deal with.  And, as implief by @sevenperforce, any mutually exclusive mix of g-screen and orbital_velocity could be dialed in one would assume

Edited May 30, 2023 by darthgently

[sevenperforce]

1 hour ago, darthgently said:

My assumption was that as the g-screen ramped up, orbital velocity would become less necessary, so as the g-screen ramped up to max, orbital velocity would be ramped down to zero.  So no tangential velocity to deal with.  And, as implief by @sevenperforce, any mutually exclusive mix of g-screen and orbital_velocity could be dialed in one would assume

The trouble is with how inertial and gravitational mass are handled, I think.

Everything in Earth's SOI is attracted toward the center of Earth, regardless of its mass. Even photons, with no rest mass, are deflected by Earth's gravitational well, albeit only slightly due to their great speed.

And so reducing the effects of gravity on an object -- "screening" some portion of its gravitational mass -- wouldn't change the free-fall behavior of the object. If you "screen" 99% of the mass of a 1-tonne object, you will be able to lift that object 10 meters off the ground with relative ease, exerting just slightly more than 98 Newtons of force and expending only 981 Joules. However, if you then release that object, it will accelerate downward at 9.81 m/s² just as if there was no gravity screen at all, and so all 1000 tonnes of its inertial mass will impact the ground at 44.3 meters per second, releasing 981000 joules of energy.  This creates an infinite energy glitch and I think it also potentially violates the Copernican principle.

The natural "fix" is to reduce the vehicle's felt perception of gravity. But this really mucks things up, because now you start to drift away from Earth relative to the sun, etc., because if the felt perception of the sun's gravity dropped by 99%, your 30 km/s orbital velocity around the sun would immediately become an escape trajectory.

[magnemoe]

3 hours ago, sevenperforce said:

I believe that in the Firefly universe the ships like Serenity have two systems that work like this: a g-screen and a g-field. The g-screen insulates the bulk of the ship from the effects of gravity, so that the engines don't have to fight against gravity during the climb to space. The g-field, on the other hand, produces a directional gravitational field for discrete objects within the ship to allow the use of decks, chairs, bunks, and so forth. It's reasonably clear that although these use similar technology, they are two separate systems. Both the g-field and the g-screen, while requiring a lot of energy to set up, require very little energy to keep going and decay slowly after a power loss, slow enough that issues like oxygen and temperature are more pressing.

When the ship's main pulse drive is activated, the surge of power allows for the g-field and the g-screen to be aligned together, which effectively cancels the inertia of the entire vehicle and allows for very high acceleration. Feels very much like the sort of of stuff that @Spacescifi using goes on about. He should really just borrow the Firefly mechanic and be done with it.

One unanswered question (I think) is how far the effects of any g-screen system actually extend. Ordinary objects are in free-fall, but free-fall itself is actually an extremely eccentric orbit around Earth's center of mass. If you cancel that, are you floating relative to Earth, or are you also floating relative to the sun? Relative to the galaxy? 

Sneaky, I assume the high energy cost on startup is that you can not use this to create energy or make an stealthy reactionless drive. 
Still the spaceships are planes or boats is so integrated now it will not go away before we have large spaceships flying around all the time. 
Then it dies like other bad memes like cars detonates because they bumped into each other. 

[Gargamel]

I’m getting the TLDR from this thread to be “Don’t explain the ubiquitous”. 

[sevenperforce]

2 hours ago, Gargamel said:

I’m getting the TLDR from this thread to be “Don’t explain the ubiquitous”. 

Not just this thread.