When is weightlessness felt?

[Codey737]

Hello!

I've been pondering the question of when do astronauts on launch feel weightless? Is it after engine shutdown or at AP? I'm never been to space nor am I rocket scientist,can anyone answer this?

-Codey

[The Silent Majority]

While I am neither as well, what you called weightlessness is free fall, therefore it is at engine shutdown, but it starts (?) when the engine shuts down.

Hopefully someone more qualified than me will come along and correct me if I am wrong.

I would also think that at stage separation there would be a brief moment of free fall (depends on the vehicle of course).

[Nuke]

you feel acceleration so long as the engine is on. this is not gravity, though the astronauts cant really tell the difference (well they can look at the instruments and tell that the engines are on, and they might feel vibrations from the engines, but you know what i meant). id say when the engines turn off and you are far enough out of the atmosphere to not feel any major deceleration from drag (and this can just be a suborbital hop too). put simply, when the ship is in freefall.

[Skyler4856]

Astronauts in LEO don't really feel weightlessness, they encounter micro-gravity, their free fall is interrupted by several factors, the most significant of which is drag.

[R0cketC0der]

Feeling weightless actually only means that you are accelerating at the same speed as everything around you. If you would be in a plane that goes into a straight vertical dive and somehow gets zero drag, you would feel weightless. The same applies for anything that isn't accelerating or decelerating.

[lajoswinkler]

Astronauts in LEO don't really feel weightlessness, they encounter micro-gravity, their free fall is interrupted by several factors, the most significant of which is drag.

Yes, they feel weightlessness. Microgravity is related to the pseudoforces arising from separation of two objects on various orbits. For example, if you put two steel cubes together, one closer to the Earth than the other, after one orbit they will be separated. If you were to attach a very sensitive dynamometer on them, there would be a really tiny force registered. That force causes the objects inside a space station to cling to the walls of it after a while. That is microgravity, and the astronauts can't feel it.

[Tarrow]

If you would be in a plane that goes into a straight vertical dive and somehow gets zero drag, you would feel weightless. The same applies for anything that isn't accelerating or decelerating.

It doesn't need to be a vertical dive to be honest. Or even involve a drop in altitude for that matter. The "reduced gravity" flight profile of the vomit comets and other related aircraft provides an effective zero g before it completes it's ascent (starting with when it begins to nose back down to level flight from it's climb, then continuing as it noses down for a 45 degree dive).

The equivalent near-zero point in a spacecraft launch would be on main engine cutoff I guess - the point where the only external forces acting on the vessel are gravity and air drag, with the crew experiencing the exact same acceleration due to gravity as the vessel.

Edited March 30, 2014 by Tarrow

[KerikBalm]

I'd also say it happens at engine cutoff, and ends upon reentry.

I suppose for suborbital flights, it could possibly be more gradual, if the engine cuts off while there is still noticable atmospheric drag(of course this would result in going from positive Gs to slight negative Gs, and theb back to zero as the craft ascends through the atmophere and out into space) - but that wouldn't happen on an orbital flight, so engine cutoff would occur last.

[Sillychris]

You feel weightless when your acceleration vector is equal to the gravitational field's vector, ie freefall. That feeling in your stomach when you first drop on a rollercoaster is the same phenomenon.

As pointed out previously, thrust and friction are two things that can prevent your acceleration and gravity vectors from matching. However, if you are careful, you can utilize these things to match and achieve temporary "weightlessness". This is the principle of parabolic microgravity flights.

KerikBalm phrased it nicely: "happens at engine cutoff, and ends upon reentry"

[Rocketscience101]

I see you have a ton of people explaining this to you, but you asked a question which goes right into my field. The answer is when they begin falling, or shortly after the engine shuts down. While in space, you are never truly weightless, you are in what is called a free fall. A constant falling state. Because you travel so high, so far so fast, that you fell over the curve of the earth. Which is why astronauts appear they are floating, when in fact they are just falling, but not hitting the ground. I think that's the first rule of space travel. Yep, right here, begin falling but do not hit the ground until planned.

[Nuke]

technically speaking the vectors will never match perfectly. there are other factors that prevent perfect zero gravity. any difference between the two vectors is acceleration that you can feel, but its usually really small. thats why we call it microgravity. [/useless factoid]

Edited March 31, 2014 by Nuke

[lajoswinkler]

I see you have a ton of people explaining this to you, but you asked a question which goes right into my field. The answer is when they begin falling, or shortly after the engine shuts down. While in space, you are never truly weightless, you are in what is called a free fall. A constant falling state. Because you travel so high, so far so fast, that you fell over the curve of the earth. Which is why astronauts appear they are floating, when in fact they are just falling, but not hitting the ground. I think that's the first rule of space travel. Yep, right here, begin falling but do not hit the ground until planned.

Yes, they are weightless by the very definition of weightlessness. They are falling and, relative to the compartment they're in, are weightless. "Relative to" is a very important phrase.

[Moon Goddess]

when in fact they are just falling, but not hitting the ground. I think that's the first rule of space travel.

I thought that was the secret to flying.

"There is an art to flying, or rather a knack. Its knack lies in learning to throw yourself at the ground and miss. ... Clearly, it is this second part, the missing, that presents the difficulties."

[KerbMav]

I thought that was the secret to flying.

"There is an art to flying, or rather a knack. Its knack lies in learning to throw yourself at the ground and miss. ... Clearly, it is this second part, the missing, that presents the difficulties."

"Don't Panic", it applies here as well.