About the floating natutre of floating in space

[Thesla]

Im curious on the nature of microgravity. I understand that in space, orbit to be , more precise, everything is constantly falling, thus the felling of no acceleration. But, how and when exactly during the flight does the craft feel no gravity? I remember a very outrageous scene in a superman movie in wich a plane flies so high up that in a given moment it passes the magical line of gravity and everyone iside it starts floating. I know that the scene is really stupid, but it got me thinking.

So, do you feel the zero g only after the craft circularizes? Or as a gradual loss of weight, or what? Im curious.

[Leszek]

Gravity is an acceleration force that pulls you towards the center of the mass. It is important to start with this fact before all else. The acceleration due to gravity on earth is 9.81 ms^2. This is also about the same on Kerban.

As you move towards orbit, the engines are pushing you ever faster, as a result the people in the craft will feel acceleration from the engine and it is identical to them as gravity would be in every way except magnitude. When the craft reaches orbit, the engines stop, the crew feels instant weightlessness.

If you ignore what the crew feels and look at the physics of the ship itself, as it approaches orbital speeds the effect of gravity seems to dissipate. That is the ship doesn't have to try as hard to keep altitude, but that is only because the ground is curving away from the craft at a rate approaching the acceleration of gravity. Or put another way, in near orbit gravity is accelerating you downwards at say 8 ms^2 but the ground is curving away from you at 5 ms^2. The ship only has to make up the remaining 3ms^2. The net effect is that the ship experiences less and less gravity until orbit.

[Mr Shifty]

Here's how Chris Hadfield described a Shuttle launch:

Launch is immensely powerful, and you can truly feel yourself in the centre of it, like riding an enormous wave, or being pushed and lifted by a huge hand, or shaken in the jaws of a gigantic dog. The vehicle shakes and vibrates, and you are pinned hard down into your seat by the acceleration. As one set of engines finishes and the next starts, you are thrown forward and then shoved back. The weight of over 4 Gs for many minutes is oppressive, like an enormous fat person lying on you, until suddenly, after 9 minutes, the engine shut off and you are instantly weightless. Magic. Like a gorilla was squishing you and then threw you off a cliff. Quite a ride

Edited October 14, 2014 by Mr Shifty

[-Velocity-]

It's when you're in free fall- when there are no external forces acting on you. We're not in free fall with respect to the Earth when we're standing on Earth because the ground repels us and pushes us up. Thus, we "feel" Earth's gravity. We're in free fall around the Sun, so we don't feel the Sun tugging on us, other than some very minute tidal forces. Same goes with the Moon, but the tidal forces are just a bit stronger.

I don't know what movie scene you're referring to, but it's probably not outrageous at all. If a plane goes into a free fall, no one inside will feel the force of gravity and everything will start floating around. http://en.wikipedia.org/wiki/Reduced_gravity_aircraft

However, with a plane, since there is significant drag (an external force that will try to keep you from being in free fall) I believe that significant amounts of engine thrust is needed to counteract the building drag and maintain the 9.81 m/s^2 acceleration towards Earth's center for any significant period of time.

Edited October 14, 2014 by |Velocity|

[Nibb31]

You can experience microgravity without going to space by booking a seat on a parabolic flight (there are several aircraft in the world equipped for this, often nicknamed "vomit comet"). You sit in the aircraft while if flies up and down on parabolic trajectory. At the top of each curve, you get a few seconds of microgravity.

In fact, you can get the same phenomenon riding an elevator going down, or when you drive over a speed bump.

It has nothing to do with altitude, of course. If you fly straight up, as soon as the engine stops, your rocket falls straight down. You will experience microgravity inside the rocket, just like in the elevator, until you hit the ground (ending your day with great unpleasantness). Astronauts start to feel the effects of microgravity in the same way, only as their rocket accelerates toward the horizon, instead of falling straight down, it simply goes fast enough to fall *beyond* the horizon, meaning that the fall never stops. When you are freefalling while moving fast enough to always miss the ground, that's called orbit.

Edited October 14, 2014 by Nibb31

[Leszek]

A lot of waitless scenes in the movie Apollo 13 where shot on NASA's Vomit Commet.

Here is a youtube video that shows the Vomit Commet in action.

http://www.youtube.com/watch?v=VX1RoQe6ZPo

[Mr Shifty]

To add to what's been said above, it shouldn't be surprising that weightlessness would be sudden -- at the moment the engines turn off. As in KSP, the TWR of a rocket will increase as fuel burns off and the weight of the rocket decreases, so maximum G loading occurs right before MECO. Then the engines turn off and, provided there's no atmospheric drag, the rocket (and astronauts) will be in instantaneous free-fall. Note that it doesn't matter whether the rocket is thrusting 'vertically' or 'horizontally'; it will still feel like weight.

Edited October 14, 2014 by Mr Shifty

[Thesla]

Thats really cool, but how about a rocket with REALY big TWR? I mean, sometimes in ksp i build rockes with such violent TWR that the optimal apoapsis is reached far before the craft leaves the upper atmosphere, and the apoapsis starts dropping ever so slowly, while the craft is still in sub orbital flight. Would then the pilots feel some sort of acceleration?

Ps. I know that humans tend to die upon high acceleration, hehe

[Leszek]

If the vehicle and the people are experiencing different acceleration then the the people feel gravity.

In the case of coasting to apoapsis, while in the atmosphere the ship is slowing down slightly. The people aren't. But the difference is so small that it would feel about the same as weightless anyway.

[Thesla]

Cool, thanks!

[MockKnizzle]

In a nutshell, "weightlessness" or microgravity occurs whenever an object is in free fall.

There are three basic criteria for an object to be freely falling:

1) The object is not in contact with the ground.

2) The object is not in aerodynamic flight or subject to large aerodynamic forces.

3) The object's engines are off.

With this in mind, you can experience a second or so of microgravity from the comfort of your own home... all you have to do is jump off the roof!

[78stonewobble]

With this in mind, you can experience a second or so of microgravity from the comfort of your own home... all you have to do is jump off the roof!

I live in a ten-story building so, pass.

[Camacha]

Here is a youtube video that shows the Vomit Commet in action. http://www.youtube.com/watch?v=VX1RoQe6ZPo

Interesting video, even if the sound is terrible

[Thesla]

Now thats a very kerbal thing to do!

[lajoswinkler]

In order for the object to experience weightlessness, its acceleration (scalar and vector) must be zero. Aside from climbing through troposphere, when air drag is a significant counter force, engines being on mean you're under acceleration and therefore your mass resists it. You are constantly pushing against your seat, and the seat pushes into you.

When the engines are turned off, you feel like you've been fired out of your seat. Your whole body feels as it's falling down and it would be very unsettling to anyone unprepared. Ever been in an airplane that flew through an air current that made it drop down? A fast elevator starting to go down? A drop tower? That's the feeling, but a lot worse, and it lasts indefinitively.

Acceleration can be concerned with a vector, such as when you're spinning around a center point and held by a centripetal force. You will experience centrifugal force. However, when orbiting Earth (even in LEO) is concerned, the distance between your body and the barycenter of you-Earth system is so huge that you behave almost as one material point and no apparent forces are observed. They still exist, though, and are called microgravity. Strap two steel balls together using one metre tethers to a sensitive dynamometer and let them be; soon you'll read the force on the device. The ball closer to the Earth is in a lower orbit and thus orbits faster. That's why any loose objects in ISS will eventually cling to the walls.

In case of much more concentrated masses, the acceleration of the vector part of velocity is so dramatic it pulls apart molecules. Small and medium sized black holes are like that.

Edited October 15, 2014 by lajoswinkler

[Motokid600]

Your whole body feels as it's falling down and it would be very unsettling to anyone unprepared. Ever been in an airplane that flew through an air current that made it drop down? A fast elevator starting to go down? A drop tower? That's the feeling, but a lot worse, and it lasts indefinitively.

Which I must admit is why I'd never be able to go to space. I'm sure I could get used to the perpetual feeling of having "your stomach dropped" but that coupled with the tight spaces and constant humming of machinery ( I imagine what it feels like onboard the ISS is alot like being inside an airplane, but with microgravity ) forget it. I'm not very claustrophobic, but it gets bad when I can't stretch my legs out after so long. ( I would probably turn down an invitation for a ride in the Soyuz )

[Thesla]

If anyday you get such an invitation and decline it, plz give it to me!

[Brotoro]

If the vehicle and the people are experiencing different acceleration then the the people feel gravity.

In the case of coasting to apoapsis, while in the atmosphere the ship is slowing down slightly. The people aren't. But the difference is so small that it would feel about the same as weightless anyway.

Do the pilots of a vomit comet not use a little engine thrust during the weightless arc to compensate for the air drag so that the plane follows the desired parabola more perfectly?

[Beowolf]

In the case of coasting to apoapsis, while in the atmosphere the ship is slowing down slightly. The people aren't. But the difference is so small that it would feel about the same as weightless anyway.

He described a high TWR launch where he reached his desired apo while still well inside the atmosphere. Air resistance goes up with the square of the speed, so in this special case I'm thinking deceleration from drag would be quite noticeable to any passengers who survived the launch. In a more realistic manned launch, I'm sure you'd be right.

Do the pilots of a vomit comet not use a little engine thrust during the weightless arc to compensate for the air drag so that the plane follows the desired parabola more perfectly?

Yes they do. "At that point, the thrust was reduced so that it compensated the air drag." Source: http://www.scilogs.com/go_for_launch/riding-the-vomit-comet/