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ProfessorTomoe

Sci-Fi Physics Help Needed - Open Discussion

38 posts in this topic

56 minutes ago, Vorlonagent said:

Wouldn't the ship fly off the launchpad all on its own?  We think of the surface of the Earth as a static reference frame but it isn't.  At the equator, the earth's surface is moving at about 1000 MPH (1600 KPH) and gravity is what holds stuff in place at the surface. Eliminating gravity, inertia should take hold and toss the ship directly away from the Earth's center of mass, which ought to be the rough equivalent of straight up. 

Despite going from zero to a speed that breaks the sound barrier, the crew would be fine because it is not an acceleration but a function of inertia.  If the the ship ascends using only its inertia, the crew would experience a pull forward in the direction the ship was moving equal to the deceleration from air resistance.

Cut gravity on ship at either of the Earth's rotational poles, the ship would simply float there, but again if you can create an island of normal gravity in front of the ship or negative gravity behind the ship, there you go.

Negating gravity at various places on a planet can be especially nasty.  A bunker that could take a nuclear weapon hit is still transparent to gravity.  negate gravity in the interior of the bunker and everything not very securely nailed down splats against the ceiling.

The velocity the rocket would have wouldn't be straight away from the Earth's center of mass, but tangential to the surface of the earth at the moment the gravity is negated. Though it might appear that the vessel rises straight into the air by people on the ground. This is also ignoring the other gravity-dependent velocities, like the Earth's revolution around the Sun or the Solar System's revolution around the galactic core.

Also people and things in that hypothetical bunker wouldn't splat against the ceiling (though they would slam into the ceiling as if they had suddenly fallen up), as we are currently experiencing the acceleration of angular velocity from the Earth rotating and gravity holding us to the surface. Once the people and objects hit the ceiling, their velocity would be redirected by the structure, similar to how a seatbelt prevents people from flying forward if the car stops suddenly, though there could be problems if the ceiling isn't strong enough to support people and thing falling into it.

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1 hour ago, Vorlonagent said:

Wouldn't the ship fly off the launchpad all on its own?  We think of the surface of the Earth as a static reference frame but it isn't.  At the equator, the earth's surface is moving at about 1000 MPH (1600 KPH) and gravity is what holds stuff in place at the surface. Eliminating gravity, inertia should take hold and toss the ship directly away from the Earth's center of mass, which ought to be the rough equivalent of straight up. 

33 minutes ago, Don Edwards said:

That would depend on location and time of day, and in some parts of the world (polar regions) it would also depend on time of year.

On time of day especially, if it's at noon or midnight, the launchpad would either be facing towards or away from the sun, so the ship under no gravity, would want to "drift" east or west if it's momentum isn't kept with the Earth's movement. At dawn or dusk, the launchpad would face 90 degrees from the sun which would mean the lauchpad would either be facing into the path of the Earth, or away from the path. So the best time for a inertial assisted launch like that would be at the time when the pad is facing the opposite direction of the Earth's orbit.

 

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9 hours ago, Scotty said:

On time of day especially, if it's at noon or midnight, the launchpad would either be facing towards or away from the sun, so the ship under no gravity, would want to "drift" east or west if it's momentum isn't kept with the Earth's movement. At dawn or dusk, the launchpad would face 90 degrees from the sun which would mean the lauchpad would either be facing into the path of the Earth, or away from the path. So the best time for a inertial assisted launch like that would be at the time when the pad is facing the opposite direction of the Earth's orbit.

So, 6:00 p.m. launches, then. Right?

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If the Earth's rotation and it's orbit around the sun are both counter clockwise, I think so.

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You will want an equatorial launch site to maximize any inertial assisted launch.

Speaking of inertia, a dawn launch will also add the forward momentum of the planet as it orbits the sun.  Just like missiles fired from an aeroplane always launch forward.

 

 

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40 minutes ago, Pharaoh RutinTutin said:

Just like missiles fired from an aeroplane always launch forward.

Nope.  Off angle launches are possible on some aircraft and helicopters.  Granted some of those are because the thrust is aimed sidewards at launch, but it is still not a "forward" launch.

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3 hours ago, Pharaoh RutinTutin said:

Speaking of inertia, a dawn launch will also add the forward momentum of the planet as it orbits the sun. 

My thinking was that a dawn launch, even with the effect of gravity on the vehicle blocked, would still require a decent boost to get it moving faster than the Earth. A dusk launch would require little to no thrust from the vehicle, the boost could come from the launch platform by way of electromagnetic manipulation to basically reduce the vehicle's momentum in relation to the Earth, once at the right altitude, the vehicle would then use it's engines along with gravity to get into orbit.

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On 1/26/2017 at 0:06 AM, Scotty said:

If the Earth's rotation and it's orbit around the sun are both counter clockwise, I think so.

I checked. Both are counter-clockwise. Evening launches to take the best advantage of the gravity block seem to be the way to go.

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On ‎01‎/‎26‎/‎2017 at 1:06 AM, Scotty said:

If the Earth's rotation and it's orbit around the sun are both counter clockwise, I think so.

 

1 hour ago, ProfessorTomoe said:

I checked. Both are counter-clockwise. Evening launches to take the best advantage of the gravity block seem to be the way to go.

It depends on which pole you define as "up".

If you see Antarctica as the top of the world, the spinning is clockwise.

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For an inertialess drive, it depends on where you want to go.

If you are heading away from the sun, launch at sunrise to take advantage of how fast the earth is already moving.

If you're just going into orbit, launch more toward sunset and let the earth move out from under you, then juice to the desired speed and direction.. (There may be some particular time when progressing in a straight line as the earth continues on its curved orbit happens to be a really big help on getting the correct orbital speed, but the math to evaluate it is beyond me.)

Another advantage of inertialess/massless space travel is that, over reasonable distances, you can point your ship at the place you want to go and not worry about orbital mechanics.

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17 hours ago, Don Edwards said:

For an inertialess drive.

Ah, but we're not talking about eliminating inertia, only gravity.

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12 hours ago, CritterKeeper said:

Ah, but we're not talking about eliminating inertia, only gravity.

Yes. I doubt they'd be able to eliminate inertia. I presume the lack of gravity would mean they could use a smaller booster to put the same amount of payload into orbit. That's why this thread exists—to see how many holes can be poked in that presumption.

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The main direct effect you get from having a bunch of line blockers keeping the first 30000 feet gravity-free is that you no longer have a time limit on operations there - it becomes more like orbit. So, you can use a more efficient rocket. But since this is the first stage, it hardly makes a difference.

But the indirect effects might end up being pretty significant - you are going to have one HECK of a wind kicked up around the edge of the zone. Air will come in the bottom and find that it's suddenly facing much lighter gravity, so it expands upwards; at the top, it conversely suddenly finds that it is much too dense for the altitude it is, and it comes down. I expect this would become stronger over time, up to some limit; it is not clear to me how fast it would be, but it could be a very strong updraft. Maybe instead of firing a first stage rocket at all they let out parachutes and have that accelerate them up to the first 150 mph or something. Only fire up the rockets when you've reached peak upward velocity off of the wind.

… and again that's insignificant. The KE of 150 mph is just not that much compared to orbit.

BUT that's assuming the line blockers have a short range. Why would they? If they're blocking the lines like the name implies, they could cast a long-reaching gravity shadow up from where they are. And the wider the area, the further the shadow. Maybe it all comes back eventually, but you only need to make the region of effect teardrop-shaped (or more extremely, comet-shaped) to give ships a nice long upwards path. The tail would be pointed directly away from the attractive body, so the ship would have to stay above the column… in this case it might end up cheaper to go to Geosynchronous than Low-Earth orbit.

If you don't like that idea, let the line-benders operate much higher. Balloons can operate all the way out to the edge of space. Let them go up 50 km. That would be long enough that maybe a low thrust, high specific-impulse first stage would be a good idea.

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