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ProfessorTomoe

Sci-Fi Physics Help Needed - Open Discussion

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I'm in need of some help in the realm of physics. We've got some smart folks here, so I hope someone can lend a brain cell or two.

First, some background. In the novel I'm working on, technology makes it possible to manipulate "lines" of the four fundamental forces (gravity, electromagnetism, weak nuclear, strong nuclear). The one most frequently manipulated is gravity. (Remember, this is science fiction, but I'd like it to be rooted in some amount of facts.)

One of the jobs I've dreamt up involves making it easier and cheaper to launch rockets into space. I picture a group of people surrounding a launch pad (from a safe distance) and "blocking" the lines of gravity that keep the rocket grounded. The blocking would allow the rocket to rise into the air without firing off a huge booster, but there's only so high it can go before the blockers lose the ability to track it and to block the right lines. At some point, rocket engine power is going to be needed, to let it rise farther and/or to give the rocket the proper angular momentum so that it stays in orbit.

If you've read through all of that, thank you and congratulations. Now for my question: what would happen if the rocket fired a main engine while the gravity lines are blocked? Let's say it's pointed straight up. Would it be able to continue into space (and then into orbit) without the need for today's massive boosters?

Alternatively, if the rocket is fired while aimed horizontally (moving it out of the blocked domain), would it be able to gain orbit, again with a smaller booster, or would it plummet the moment it crossed out of the blocked domain and back into the influence of gravity?

Again, please remember that this is science fiction, somewhat rooted in real physics. What say you, those of you who know the subject? My knowledge in the field is lacking, and my imagination can't decide if this is really something that I need to write about.

Thanks!

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Would, or could, the manipulated force lines form a column of very low atmospheric pressure (vacuum?) in the blocked gravity zone?

That could greatly reduce the amount of fuel needed.

I wanted to link a video of an Apollo Lunar Excursion Module launching off the moon to demonstrate the very quick ascent.  But almost everything lunar on YouTube seems to be people trying to "prove" that it was fake.

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2 minutes ago, Pharaoh RutinTutin said:

Would, or could, the manipulated force lines form a column of very low atmospheric pressure (vacuum?) in the blocked gravity zone?

Most likely not, unless the atmosphere was affected by the lack of gravity in the zone. I would imagine that natural wind pressure inrush would fill any vacuum created. Then again, I could be wrong.

What if it didn't reduce atmospheric pressure? The manipulators could probably lift and guide the rocket into at least the upper stratosphere, if not higher.

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My take on this is that you are basically creating an ability to launch a real rocket from an imaginary platform X feet above the ground - X being the altitude where the people on the ground can't *reliably* block the gravity lines - and with some small amount of upward momentum.

This will take less energy than launching from the ground - however, X would have to be quite large to make a BIG difference. Probably at least 30,000 feet, and the higher the better. (This also has the advantage of significantly reducing air resistance, because you're starting out with fewer miles of air above you and thinner air around you.)

So you might want to look at ways of enlarging X. Like, oh, make sure the rocket is well-lit even when its engine is not going full-blast, and launch on clear nights. Put some people in helicopters or specially-designed lighter-than-air craft (give them observation rooms on the *tops* of the craft, so they can look *up*) at a fairly high altitude in a ring around the launch site.

Starting the engine on/near the ground will kick up a lot of dust and smoke at ground level, which might be a problem for the blockers.

The other factor is that a huge amount of the energy spent on getting into orbit is not about getting *up* there, it's about getting up to orbital speed. Lots of things in low orbit have orbital periods of 3 hours or less, and they travel many more miles in one orbit than a spot on the equator travels in one day, so they have to move horizontally a LOT faster. Blocking gravity won't directly help on that, although it could indirectly. (If you can temporarily turn off gravity, the spacecraft - in space - will travel in a straight line at constant speed, and you can arrange for the direction to be away from earth. Then when you let gravity return, the spacecraft will fall toward earth and accelerate, at no energy cost. You'll still have some trickery to get it going in exactly the right direction at exactly the right speed at exactly the right altitude, but you have that *anyway*.)

(By the way, the above reveals one of the problems with antigravity. If you have antigravity, you have a perpetual-motion machine and free energy. There are a variety of reasons that physicists give a firm NOPE to that.)

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Part of the problem with getting into space isn't it's so far away, but it's so fast.  If you don't have the orbital velocity you are going to fall back to earth fairly soon.  Low earth orbital speed is about 7.8 kilometers per second.  Delta-V is a harsh mistress.

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4 hours ago, Don Edwards said:

(By the way, the above reveals one of the problems with antigravity. If you have antigravity, you have a perpetual-motion machine and free energy. There are a variety of reasons that physicists give a firm NOPE to that.)

That's if you have antigravity in which it costs less energy to raise an object than can be extracted from its descent. If however your antigravity method still costs at least 10 joules to lift one kilogram by one meter (at Earth's surface), i.e. the same amount of energy as the gain in potential energy from lifting it, then any such antigravity "perpetual motion machine" will run at a deficit and thus not be "perpetual".

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4 hours ago, Don Edwards said:

(By the way, the above reveals one of the problems with antigravity. If you have antigravity, you have a perpetual-motion machine and free energy. There are a variety of reasons that physicists give a firm NOPE to that.)

… as an aside, "free energy" is an actual jargon term for something completely different.

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6 hours ago, Don Edwards said:

The other factor is that a huge amount of the energy spent on getting into orbit is not about getting *up* there, it's about getting up to orbital speed. Lots of things in low orbit have orbital periods of 3 hours or less, and they travel many more miles in one orbit than a spot on the equator travels in one day, so they have to move horizontally a LOT faster.

... and ...

3 hours ago, mlooney said:

Part of the problem with getting into space isn't it's so far away, but it's so fast.  If you don't have the orbital velocity you are going to fall back to earth fairly soon.  Low earth orbital speed is about 7.8 kilometers per second.  Delta-V is a harsh mistress.

Both of these are concerns, yes. However, my goal in the book is to launch rockets into space at a fraction of what it "used to cost" (we're talking near the end of this century here). There will be some booster activity, but it won't fire until something like 30,000 feet is reached.

Now, if they can get the rocket that high and fire a booster (I guess with the rocket oriented at an angle so the booster gives Delta-V as well as a further altitude boost), do you think it could make it to some kind of orbit more cheaply than simply firing the whole thing from the ground?

1 hour ago, ijuin said:

That's if you have antigravity in which it costs less energy to raise an object than can be extracted from its descent. If however your antigravity method still costs at least 10 joules to lift one kilogram by one meter (at Earth's surface), i.e. the same amount of energy as the gain in potential energy from lifting it, then any such antigravity "perpetual motion machine" will run at a deficit and thus not be "perpetual".

Energy usage for blocking lines (of any of the 4 fundamentals) is going to be a handwaved item in the book. It's a cheap out, but necessary so as to not bog down the story. Like I said, this is "somewhat" rooted in real physics. ;)

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8 hours ago, ijuin said:

That's if you have antigravity in which it costs less energy to raise an object than can be extracted from its descent. If however your antigravity method still costs at least 10 joules to lift one kilogram by one meter (at Earth's surface), i.e. the same amount of energy as the gain in potential energy from lifting it, then any such antigravity "perpetual motion machine" will run at a deficit and thus not be "perpetual".

The proposed method of turning off gravity isn't significantly affected by the mass of the thing being lifted. An examination of the formula* of gravity - (M1+M2)G/D2 - in relation to the mass of the earth and the mass of anything humans would want to lift reveals that the energy cost at ground level is 99.999999% determined by the mass of the earth.

So if antigravity is feasible for the proposed use, it's a source of free energy.

And the proposed launch system is based on the presumption that it's feasible.

* Ignoring relativity, since we'll be dealing with speeds low enough that we would need extremely sensitive instruments to detect its effect at all.

6 hours ago, ProfessorTomoe said:

However, my goal in the book is to launch rockets into space at a fraction of what it "used to cost" (we're talking near the end of this century here).

Oh, that's easy.

A few major alternatives:

1) Leave the energy source on the ground. Rockets carry both their reaction mass and their energy source - usually the same substances. But there is no substance that is a really good high-density source of energy *and* is, or produces in combustion, really good reaction mass; everything we have is a compromise. But put your spacecraft on top of a really big ice cube, and blast the bottom of the ice cube with very-high-energy pulsed lasers, and you can get quite a bit of acceleration. The lasers will convert ice to plasma on one pulse, and cause the plasma to hugely expand on the next. And there may be better choices than ice cubes.

2) Leave the energy source *and* the guidance system on the ground. have your spacecraft climb a tether that reaches from the ground to beyond geosynchronous orbit. By sticking to the tether there is no need for guidance, and energy can be delivered through components of the tether. The energy cost of getting up there can be regained, at least in part, by extracting energy from things coming down via the tether. Since the tether has to extend beyond geosynchronous orbit (for balance, so it doesn't fall down), it can be used to assist space launches further - anywhere past geosynchronous, the tether is traveling *faster* than orbital speed.

3) Have *part of* the energy source and guidance system already in orbit. Since we don't (yet) know how to build the tethers needed in #2, let's use much shorter tethers centered in LEO and spinning, with the bottom of the spin in the upper atmosphere - within reach of aircraft. The end of a tether near the bottom of its spin would be traveling relatively slowly (compared to the satellite) relative to the ground, sufficient that an aircraft could dock with it. This system will lose energy, so the tethers will occasionally need to be spun-up and therefore refueled...

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17 hours ago, ProfessorTomoe said:

First, some background. In the novel I'm working on, technology makes it possible to manipulate "lines" of the four fundamental forces (gravity, electromagnetism, weak nuclear, strong nuclear). The one most frequently manipulated is gravity. (Remember, this is science fiction, but I'd like it to be rooted in some amount of facts.)

8 hours ago, ProfessorTomoe said:

Energy usage for blocking lines (of any of the 4 fundamentals) is going to be a handwaved item in the book. It's a cheap out, but necessary so as to not bog down the story. Like I said, this is "somewhat" rooted in real physics. ;)

Question. Does the launch have to use only the manipulation of gravity? You have electromagnetism in there as well so why couldn't that be used in conjuction with gravity.

I would suspect that a launch tower built like a giant railgun, could also have gravity dampening capabilities built in that would be able to create an electromagnetic field strong enough with a low amount of energy to get a craft up to escape velocity.

 

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In trying to look at cost and such, we're overlooking one crucial detail: acceleration. When you launch a rocket from the ground and get it to, say, 30,000 feet, it spends all that time burning the engines and accelerating. So when it's at 30,000 feet it's already going at a significant velocity and has significant horizontal thrust already, giving a good start to a stable orbit. If you started at 30,000 feet, though, it's velocity at 30,000 feet is: 0. Meaning it has to make up for lost time by burning the engines, getting the rocket up to the velocity and acceleration it should already be at that altitude, and then trying to get to orbital velocity. So you're going to need the same amount of fuel just to get it to the right velocity, in addition to the line blockers.

But since the line blockers are out of range at this point, they can't keep gravity from affecting the rocket. This adds the wrinkle of a deadline to the launch: the rocket has to reach sufficient velocity to stay in orbit before the gravity of Earth causes the rocket to crash. And since the rocket lacks that initial thrust that in ground-based launch would have given it, and since it lacks a stable platform to start from zero at, this basically means that as soon as the engines ignite, the rocket is going to start falling, giving a bit more work for the engines to do before the rocket gets to orbital velocity and altitudes.

Now, whether or not the fuel consumed in overcoming the downloads acceleration is balanced by the fuel saved by starting in the thinner atmosphere is a topic for people for people who are way better at math than I am.

Having a line blocker on the ship might lessen the worries about falling, but the fuel is still needed to reach orbital velocity, so depending on how big the blockers are, that might not be feasible (too large,  and we're gonna need a bigger fuel tank).

And we'll I can't calculate the thrust versus atmospheric friction issue, I can calculate this much: we have to spend the same amount of money on the fuel, in addition to the cost of the line blockers.

It would not be cheaper.

 

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1 hour ago, Scotty said:

Question. Does the launch have to use only the manipulation of gravity? You have electromagnetism in there as well so why couldn't that be used in conjuction with gravity.

I would suspect that a launch tower built like a giant railgun, could also have gravity dampening capabilities built in that would be able to create an electromagnetic field strong enough with a low amount of energy to get a craft up to escape velocity.

The only problem with this is that the craft (and people inside, if any) would have to be able to handle the tremendous acceleration that a railgun would produce. I don't want to put crumpled people into space. ;)

1 hour ago, Zorua said:

In trying to look at cost and such, we're overlooking one crucial detail: acceleration. When you launch a rocket from the ground and get it to, say, 30,000 feet, it spends all that time burning the engines and accelerating. So when it's at 30,000 feet it's already going at a significant velocity and has significant horizontal thrust already, giving a good start to a stable orbit. If you started at 30,000 feet, though, it's velocity at 30,000 feet is: 0. Meaning it has to make up for lost time by burning the engines, getting the rocket up to the velocity and acceleration it should already be at that altitude, and then trying to get to orbital velocity. So you're going to need the same amount of fuel just to get it to the right velocity, in addition to the line blockers.

A valid point. So, firing at 30,000 feet would be a waste of everything, really.

How about this: ignite the rocket at ground level and hold it, like with the Saturn V's hold-down clamps. Release it once it's built up X amount of thrust, and simultaneously block the gravity lines, allowing it to ascend without gravity holding it down. Let it go up Y number of feet, gradually letting gravity lines reattach during ascent, and have it pitch over to gain angular momentum once all the lines are unblocked. Would that work?

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2 minutes ago, ProfessorTomoe said:

The only problem with this is that the craft (and people inside, if any) would have to be able to handle the tremendous acceleration that a railgun would produce. I don't want to put crumpled people into space. ;)

Maybe there could be something within the crew cabin that would block the lines of gravity inside and act like an inertial dampener?

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5 minutes ago, Scotty said:

Maybe there could be something within the crew cabin that would block the lines of gravity inside and act like an inertial dampener?

Hmm. That's something for me to think about. I don't know if I could describe this in the book without a massive infodump, though. Still thinking about it, though.

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1 hour ago, ProfessorTomoe said:

Hmm. That's something for me to think about. I don't know if I could describe this in the book without a massive infodump, though. Still thinking about it, though.

"Inertial blockers active?" "Check."

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If the technology is capable of manipulating gravity, one has to ask if it can bunch gravitic lines or invert them.  Bunched gravity allows the ship to "fall up" towards a local gravity well.  Inverted (repelling) gravity could be set behind the ship to make it "fall" away from the negative gravity source.  Depending on the intensity with which one could concenrate to dissipate gravity one could build an Albiquerre Warp Drive, peek past the Swatrzchild radius of a black hole, do your own gravitational lensing telescopes or create quantum black holes.

This is before we talk about manipulating electromagnetism.  The potential here is huge.  All chemistry is based on electromagnetism.  Zero out electromagnetic interactions and normal matter turns into plasma.  Admittedly unnaturally cold plasma, but still plasma.  That junk on the inside of your oven that nothing can get rid of?  This would do it.  Such as device could change the amount of energy required for a chemical reaction either for the greater or lesser, alter the emission spectra for elements, create absolute-zero temperatures. 

Manipulating the strong and weak nuclear forces would allow you to tear elements apart, down to their component quarks, or put them back together to create custom atomic nuclei not found in the natural world.  We know what Uranium 233 does and what 235 does.  What about 234?

Depending on exactly what you can do, things go really nuts compared to our world.

 

 

 

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5 hours ago, Zorua said:

So you're going to need the same amount of fuel just to get it to the right velocity, in addition to the line blockers.

<snip>And we'll I can't calculate the thrust versus atmospheric friction issue, I can calculate this much: we have to spend the same amount of money on the fuel, in addition to the cost of the line blockers.

It would not be cheaper.

Um, that makes no sense to me.  Whatever velocity you are trying to reach, it costs more to reach it if you are fighting both inertia and gravity than if you are fighting inertia alone.  You can figure out what direction you need to be pointed in, and arrange the gravity blockers and your thrust to best achieve it.

Treat it as if you're already in space, and you know that at a certain point, a rocket is going to turn on pointing a certain direction, with a certain amount of thrust, and figure out what you need to do to accomodate and/or counteract that.  But you're not comparing that situation to not having to fight that rocket at all -- you're comparing to having it running, working against you, the whole trip!

Picture a rocket launch from the surface of a planet, but with a separate rocket engine glued to the top of the rocket, pointing straight up.  There's some mechanism which will always keep it pointing straight up.  Will it take just as much fuel to reach orbit with that engine firing as it would without?  Launching with gravity in effect is the same as having that engine constantly firing downward.  Launching without gravity is turning that engine off.

Yes, you still have to figure out how to get to orbital velocity, but that's just a matter of figuring out what direction and rate to fire engines to achieve it.  Orbit is basically just constantly falling at an angle that matches the curve of the orbit.  As was pointed out, it should be possible to overshoot the target height while gravity is blocked, then make use of gravity to gain acceleration to help achieve orbital velocity while burning even less fuel.

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2 hours ago, CritterKeeper said:

Um, that makes no sense to me.  Whatever velocity you are trying to reach, it costs more to reach it if you are fighting both inertia and gravity than if you are fighting inertia alone.  You can figure out what direction you need to be pointed in, and arrange the gravity blockers and your thrust to best achieve it.

Treat it as if you're already in space, and you know that at a certain point, a rocket is going to turn on pointing a certain direction, with a certain amount of thrust, and figure out what you need to do to accomodate and/or counteract that.  But you're not comparing that situation to not having to fight that rocket at all -- you're comparing to having it running, working against you, the whole trip!

Picture a rocket launch from the surface of a planet, but with a separate rocket engine glued to the top of the rocket, pointing straight up.  There's some mechanism which will always keep it pointing straight up.  Will it take just as much fuel to reach orbit with that engine firing as it would without?  Launching with gravity in effect is the same as having that engine constantly firing downward.  Launching without gravity is turning that engine off.

Yes, you still have to figure out how to get to orbital velocity, but that's just a matter of figuring out what direction and rate to fire engines to achieve it.  Orbit is basically just constantly falling at an angle that matches the curve of the orbit.  As was pointed out, it should be possible to overshoot the target height while gravity is blocked, then make use of gravity to gain acceleration to help achieve orbital velocity while burning even less fuel.

I know how orbit works. Thing I was saying is if the line blockers are ground-based (and how big they are may enforce that), and they have a maximum effective range, gravity will kick in once the rocket leaves that range. Like in your example, getting to 30,000 feet and then firing that downwards rocket. 

Which might not be a factor for different reasons entirely...

 

6 hours ago, ProfessorTomoe said:

A valid point. So, firing at 30,000 feet would be a waste of everything, really.

How about this: ignite the rocket at ground level and hold it, like with the Saturn V's hold-down clamps. Release it once it's built up X amount of thrust, and simultaneously block the gravity lines, allowing it to ascend without gravity holding it down. Let it go up Y number of feet, gradually letting gravity lines reattach during ascent, and have it pitch over to gain angular momentum once all the lines are unblocked. Would that work?

Holding it in place until it built up enough thrust sounds like flooring it in a car with the back wheels suspended in the air. The Mythbusters tested that one (the "hit the ground running" idiom), and it didn't work.

I think only a few seconds of restraint is enough to get the engines flooring it before releasing the clamps. Not too significant a difference. 

The thing is, if you have the tech to turn gravity off, why bother with conventional fuel? You are not limited by weight here. Therefore the size of the craft and how much fuel it can carry is irrelevant. No matter how large a line blocker is, and how big the ship would have to be to carry one, it can weigh pretty much nothing. 

And the line blockers affect all four fundamental forces, giving you inexhaustible resources (strong nuclear force) to build with (think the replicators from Star Trek, and inexhaustible energy (weak nuclear force, electromagnetic) to power it all. 

At that point, I think cost is irrelevant once you have the first few blockers built. Just 3D print the whole rest of the thing, turn gravity off, 3D print some plasma in controlled directions, and you'll be in space in no time, and then you can worry about orbit at your leisure (it's not like you're gonna fall back down if you take too long, right?). The only cost is paying people at that point. 

Now, if this novel is set early enough to where they haven't figured that out yet, all this about cost and fuel and momentum is still valid. 

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You can make a ship weigh almost nothing, but you can't make its mass any less than it is.  Moving it around is still going to take energy of some sort.o

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11 hours ago, Vorlonagent said:

Depending on exactly what you can do, things go really nuts compared to our world.

This is pretty much the idea behind my book. :)

7 hours ago, Zorua said:

Holding it in place until it built up enough thrust sounds like flooring it in a car with the back wheels suspended in the air. The Mythbusters tested that one (the "hit the ground running" idiom), and it didn't work.

I think only a few seconds of restraint is enough to get the engines flooring it before releasing the clamps. Not too significant a difference.

That's about as long as the Saturn V was held—just enough to get the engines up to sufficient power to lift that behemoth off of the launch pad. Ignition sequence start took place at around T-7 or T-8 seconds before liftoff.

7 hours ago, CritterKeeper said:

You can make a ship weigh almost nothing, but you can't make its mass any less than it is.  Moving it around is still going to take energy of some sort.

That's the reason for the booster. I think. :)

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I once read a story where they achieved immunity to both gravity and certain aspects of relativity (so they could do at least light-speed, possibly FTL, I don't recall that detail with confidence) by temporarily shifting an object's mass - but not the object - into an alternate universe.

Unlike turning off the earth's gravity in a small area, the energy cost of doing this would likely be proportionate to the mass of the object.

However, the notion that this wouldn't have some nasty short-term side effects, possibly even on the nuclear level, is... problematic. The story did not address any such side effects.

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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.

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49 minutes ago, Vorlonagent said:

Wouldn't the ship fly off the launchpad all on its own?

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.

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