Possible gravity reduction due to near-orbital speeds?

[HarvesteR]

Ok, so, I'm going by the assumption that the OP question was this: If you take an aircraft that can go fast enough in the atmosphere, does it experience less G force as it approaches orbital velocity?

Yes. I've had this happen here myself.

This has nothing to do with centrifugal force and whether or not it exists (it does in a rotating reference frame), it's simply the fact that the mechanics of orbiting don't switch off just because you're in the atmosphere.

I built a space plane one day that had enough engine power to fly at an appreciable fraction of orbital velocity. IIRC, it could do over 1000m/s at around 20km altitude, which is nearly 50% of orbital speed at Kerbin. Flying level above the surface (0 vertical speed), I noted the G meter indicated around 0.5 G.

I was perplexed by that for a while, until I realized that we really were flying at almost half the speed it takes to orbit the planet, and it makes sense then that if you're moving that fast and maintaining altitude, the G force will tend to zero as you approach vCirc.

Note though, that this is G force I'm talking about, not the gravitational pull of the planet. The force of gravity acting on the ship is still the same (or roughly the same as at the surface, because 20km is peanuts to space).

Zero-G is achieved once your acceleration matches that of gravity. We experience 1G at the surface because we are opposing gravity already just by the fact that we're not falling through the ground. Orbiting however, isn't opposing gravity, it's falling along with it.

As you fly forward very fast then (assuming level flight in relation to the surface), you experience less G force on you because you're approaching a state where your acceleration matches that of gravity. You don't experience zero G immediately however, because you're still opposing it by maintaining level flight. The closer you are to orbital speed then, the closer to zero G you get, and the less wing load is required to stay level.

If you go faster than vCirc then, you actually have to pitch down to maintain level flight, because that trajectory would send you away from the surface. You then experience G force again as you have to oppose the acceleration of gravity.

I hope this explanation made sense... I tried to explain it by as many angles as possible, but there's only so much you can do without having to start drawing up charts (I'm not going to start drawing up charts).

Cheers

[DChurchill]

You're always falling, even when orbiting.

The reason you feel weightless in orbit is that you're in perpetual freefall because of your orbital path. The vomit comet flies a parabolic path that in effect gives a few moments of "weightlessness" because you are the one falling in the parabola, and the airplane flies around you. Since it can't really stop flying, it has to fly the parabola that you're "falling" in.

Make sense?

But you're not going to feel weightless in an aircraft approaching orbital speeds unless that aircraft is flying a parabolic arc. It's not the speed alone that makes you "weightless" it's also the trajectory.

Edited July 12, 2013 by DChurchill

[HarvesteR]

You're always falling, even when orbiting.

The reason you feel weightless in orbit is that you're in perpetual freefall because of your orbital path. The vomit comet flies a parabolic path that in effect gives a few moments of "weightlessness" because you are the one falling in the parabola, and the airplane flies around you. Since it can't really stop flying, it has to fly the parabola that you're "falling" in.

Make sense?

But you're not going to feel weightless in an aircraft approaching orbital speeds unless that aircraft is flying a parabolic arc. It's not the speed alone that makes you "weightless" it's also the trajectory.

Very true. In fact, the Vomit Comet needs to be piloted just right to maintain that parabolic trajectory, which isn't that easy on a craft specifically built to not free-fall.

But you don't have to be perfectly parabolic to start to feel weightless. Close enough will do, and this is what happens if you fly close to orbital speeds while maintaining altitude. You're not perfectly in free-fall, so you experience something in between zero and 1G.

Cheers

[TonboIV]

Thank you Harv! Excellent explanation, and I quite like your point about centrifugal force being legitimate in a rotating reference frame. One thing that probably should be stated, is that we aren't actually talking about flying level (in a straight line) here. If you did that, you'd eventually leave the atmosphere. To maintain altitude, you must fly in a circle! A very big circle that goes all the way around Kerbin (or earth). Moving in a circle always requires acceleration, and if you're at orbital speed gravity will provide exactly the right acceleration and you'll fall in a perfect circle. If you use modded engines (or infinite fuel) you should be able to fly faster then orbital speed, and experience a net upward force within the atmosphere.

[(An Original Name)]

Well, that was exactly the explanation that I was looking for Harv Thanks for everyone's help in clearing this up, and sorry if I wasn't very clear about my exact question

[TonboIV]

Infinite fuel to on!

Here's my best attempt hand flying. It's actually very hard to hold altitude exactly and I could only get my g-force down near zero for seconds at a time.

I'm in a near circular orbit here at 21km. G-force 0.065 and note my nose angle is zero since I don't need any lift!

Added MechJeb, and using transletron plus smart-SAS I was able to maintain 3000m/s orbital speed at 28km. Note on my HUD, vertical speed is near zero, flight engineer shows g-force 0.584 and you can see I'm inverted and nose significantly below the horizon. All that g-force is up and I'm flying down to fight it!