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 Post subject: G-Force in SPAAACE!!
 Post Posted: Sat Oct 15, 2011 3:30 pm 
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Mysterious Masked Dude
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I've been wondering something. Does one pull noteable Gs in space when undergoing rapid acceleration? Specifically, if we put a pair of jets in space and had them dog fight, would the pilots be effected by G forces similar to combating earth-side or what?

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Sat Oct 15, 2011 4:04 pm 
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I would imagine not, as the "G" in "G-force" stands for "gravitational" and that g-force refers to acceleration relative to free-fall - and AFAIK it's impossible to free-fall in space/zero-gravity.

Of course I'm not a physics major. But that's what I think.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Sat Oct 15, 2011 4:20 pm 
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YES!!!

G's are a measure of acceleration and really have nothing to do with the actual effect of gravity. (1 G is the equivalent of the acceleration caused the earth's gravity, roughly 9.8 m/s^2) Whenever one speeds up, slows down or turns one will experience G's depending on how sharp/rapid the maneuver is.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Sat Oct 15, 2011 4:33 pm 
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Im going to second Sume. Thats why there are also concepts of creating artificial gravity for spaceships using rotation.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Sat Oct 15, 2011 8:03 pm 
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As reference points, the Space Shuttle pulled 3 Gs on average during ascent and the Saturn V 3rd stage would pull at least 1 G acceleration during Trans-Lunar Injection.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Sun Oct 16, 2011 7:47 am 
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Location: In the Zaku's cockpit. Yes, the one that just exploded.
Thanks for the quick replies. I had been wondering about that for a time. Another question related to the first. Would the G-forces produced by the two space jets dog fighting be any more pronounced then they are planet-side?

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Sun Oct 16, 2011 10:57 am 
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G-forces don't work that way in space.

The G-forces sustained by pilots in aggressively maneuvering aircraft within an atmosphere are the result of the pilot rapidly shifting the direction of movement; the G-forces can be thought of as resistance to these moves, since the aircraft, per Newton's First Law, wants to keep going in its original direction.

In space, the only way to change your movement vector is to burn your thrusters, which will produce G-forces on the pilot, but while F-16 with a F110-GE-100 engine on afterburner can produce ~28,600 pounds of thrust, and has a loaded weight of ~26,500 pounds producing a thrust-to-weight ratio of ~1.079 Gs from thrust alone, it can maneuver tightly enough to pull upwards of 9 Gs. Burning your thrusters to change your vector (mmm, vector addition...in 3D!) in space won't add any more G-force to the pilot than if the pilot was simply trying to go faster in a straight line.

The G-force an aircraft can pull can be thought of as a factor of its maneuverability, while the only G-forces which will act upon a spacecraft will be the result of its raw thrust.


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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Mon Oct 17, 2011 9:32 am 
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In space, there is no air. Wings don't give you lift, nor do engines give you any thrust. Dead in the water, so to speak.

Pardon the obligatory inane reply :lol:


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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Tue Oct 25, 2011 7:57 pm 
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Maneuver thrusters are for changing attitude the way control surfaces do on an airplane. Just like an airplane, they make it so the engines are facing the same way you want to go (or at least the direction you want to change your vector by the most).

I wish I could find it, but I can't seem to; there's a video somewhere of the inside of a Space Shuttle as it maneuvers. Stuff goes flying everywhere, people are banging their heads, and it's general chaos. To be sure, it's nothing like the Gs experienced by fighter pilots, but that's not really what the Space Shuttle is for.

You know how, when you're on one of those merry-go-rounds at a playground and it gets harder and harder to hold on as it goes faster and faster? That's the force of acceleration, increasingly perpendicular to the direction of gravity as the big kid spins it faster and faster. When you lose your grip and fly off into the sandbox, you go in a straight line. That line is your vector at the moment you lost your grip. It's in excess of 1G because it's the acceleration imparted by Earth's gravity (9.8 m/sˆ2 ; i.e. 1G), plus the acceleration that was pulling you in a circle. If you took away Earth's gravity, you'd still shoot off to the side; you just wouldn't hit the ground, and you'd fly away until something else changed your vector.

In space, there are two usual ways (and lots of other neat, exotic, experimental ways) to move around: explosives (rockets) and cold gas jets. Cold gas jets are like a fire extinguisher, or the canisters of CO2 used by paintballers, or scuba tanks. They're a certain amount of pressure that you can release in a direction. The faster you throw that gas in a direction, the more it pushes back at you. Explosives, like the liquid rockets on the Shuttle, combine chemicals that generate heat and rapidly change density at the same time. When they change density, the rocket squirts the exploding gasses in one direction and the whole mass moves in the other direction.

So, to answer your question!
If you took an "aircraft", replaced its air-breathing jets with liquid rockets that produced the same thrust as the jets, then replaced its control surfaces with maneuver thrusters that produced the same effect as the control surfaces of the plane, you'd get the same kinds of G forces affecting the pilot. You just wouldn't have the effect of gravity to contend with, so unlike a real airplane, it wouldn't maneuver differently if its orientation were different.

On the other hand, if you're actually worried about G effects in spacecraft, you're probably trying to think rationally about space combat, and space combat wouldn't be anything like that at all. There's no horizon to hide behind, no atmosphere to hide your heat signature, so you can see each other from so far away that by the time my bullets arrived you'd have simply stepped to the side. Fortunately, there's a whole website for thinking about that stuff! There's even a page devoted to fighting in space. It has nothing to do with giant robots, natch. As much as I love hard SF, I'm willing to suspend my disbelief if there's a humanoid machine involved.

(If I recall, Planetes addresses a lot of this stuff. It's a damn good manga.)

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Wed Oct 26, 2011 12:55 am 
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Joshua wrote:
You know how, when you're on one of those merry-go-rounds at a playground and it gets harder and harder to hold on as it goes faster and faster? That's the force of acceleration, increasingly perpendicular to the direction of gravity as the big kid spins it faster and faster. When you lose your grip and fly off into the sandbox, you go in a straight line. That line is your vector at the moment you lost your grip. It's in excess of 1G because it's the acceleration imparted by Earth's gravity (9.8 m/sˆ2 ; i.e. 1G), plus the acceleration that was pulling you in a circle. If you took away Earth's gravity, you'd still shoot off to the side; you just wouldn't hit the ground, and you'd fly away until something else changed your vector.

I understand what you're trying to get across here, but a lot of this also looks very weird to me.

What is throwing the kids off the merry-go-round is the "fictitious" inertial force where they will want to travel in a straight line, as you said. In a merry-go-round, this force is always perpendicular to gravity and doesn't become "increasingly perpendicular".

When the kid loses his/her grip, why does that happen at 1G? I think it can happen at any acceleration. The force of gravity doesn't have anything to do with the merry-go-round example if the kids are holding on with their hands. Are you talking about if the kids are just standing on the merry-go-round with the force of friction holding them in place?

At the instant that a kid loses his grip, his acceleration has to be exactly 1G, purely due to gravity because at that point there are no other interactions occuring (no other forces acting on the kid) to impart additional acceleration. The inertial force the kid experiences is due to the contact interaction with the merry-go-round. As soon as a grip on the merry-go-round is lost, that acceleration is also lost.

f=ma

(force of gravity only) = mass x (acceleration due to gravity only) after losing grip.

(gravity + inertial force of merry-go-round) = mass x (acceleration greater than 1 g) while still on merry-go-round.

I think this example can get slippery for most folks though because it mixes acceleration that is due to change in direction (the merry-go-round) and acceleration that is a change in speed (gravity). You can spin the merry-go-round at a constant rate and still experience that inertial force.

What's interesting though is if the merry-go-round spins fast enough that the kids' feet dangle in the air, why aren't they pulled down by gravity?

Crash, when dealing with acceleration, and experiencing G's specifically due to maneuvers, you can almost ignore sources of gravity like a planet or whatever (because they are constant) and concentrate only on the motions of the craft. Changes in speed (braking or hitting your boosters) and changing direction (making sharp turns) are the only real things your pilots should be concerned about.

*edit:

Actually, let me correct/elaborate on the previous paragraph.

We're all saying pilots can experience g-forces due to sharp maneuvers, regardless of there being a source of gravity in the vicinity.

Thinking back on it, I think what Joshua was trying to say was that in addition to the force felt from the maneuvers, if there is a body of gravity present, it will also act on you and their forces will have a combined effect (depending on the directions each force is pointing.)

If gravity is present, what you have to most worry about is if you ever pull a maneuver that causes both the force of gravity and the inertial force to point in the same direction. This happens when you accelerate away from the planet. Both forces pull you down towards the planet. This is a dangerous maneuver that causes a pilot's blood to pool near their feet (away from their brain) and causes blackouts.

Of course, this means that your motion can also cancel how you feel the effect of gravity. If you accelerate towards the planet at 1 g, the force of gravity and the inertial force point in opposite directions and are of equal magnitude, so they cancel out. This is the feeling of weightlessness during free-fall.

So, I take the earlier statement back: be mindful of bodies of gravity, but they do not need to be there for pilots to experience g-forces from maneuvers.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Wed Oct 26, 2011 9:48 am 
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Seraphic wrote:
Thinking back on it, I think what Joshua was trying to say was that in addition to the force felt from the maneuvers, if there is a body of gravity present, it will also act on you and their forces will have a combined effect (depending on the directions each force is pointing.)


Yeah, that's the aside that I made at the end. Under the force of gravity, if you were to do a perfect loop, going the same speed all around (impossible, I know, but it's a thought experiment) then at the bottom of the loop you'll feel precisely 1G greater acceleration than at the top because of the gravity. In deep space, that doesn't have any effect, and in orbit (where there's gravity in effect or you wouldn't be orbiting, but it's precisely canceled by forward momentum) you also wouldn't feel it (though rotating bodies do weird things in orbit. Think of a spirograph.)

Quote:
If gravity is present, what you have to most worry about is if you ever pull a maneuver that causes both the force of gravity and the inertial force to point in the same direction. This happens when you accelerate away from the planet. Both forces pull you down towards the planet. This is a dangerous maneuver that causes a pilot's blood to pool near their feet (away from their brain) and causes blackouts.


Conversely, if a plane does an inverse loop, it causes a redout where the blood pressure increases in your head until you pass out. Because it's contrary to the way we've evolved to deal with pressure, our bodies really don't deal with it well.

This is why astronauts (and fighter pilots) wear G-suits that squeeze blood into the relevant parts of their bodies under high-G maneuvers and avoid inverted high-G maneuvers altogether.

Quote:
Of course, this means that your motion can also cancel how you feel the effect of gravity. If you accelerate towards the planet at 1 g, the force of gravity and the inertial force point in opposite directions and are of equal magnitude, so they cancel out. This is the feeling of weightlessness during free-fall.


Totes.

Quote:
So, I take the earlier statement back: be mindful of bodies of gravity, but they do not need to be there for pilots to experience g-forces from maneuvers.


Agreed.

... and just to clarify this:

Quote:
What's interesting though is if the merry-go-round spins fast enough that the kids' feet dangle in the air, why aren't they pulled down by gravity?


They are pulled down by gravity. You add all vectors together. You can't ever spin fast enough to go truly straight out; one of the forces is always pulling down. You'd have to spin infinitely fast to cancel the gravity effect completely, which I don't recommend. So if you're spinning fast enough that the merry-go-round produces 1G of acceleration, your body will hang at exactly 45° because the momentum of the spin equals the pull of gravity, making a combined force. The neat thing is that you can figure this out with trigonometry: c^2 = a^2 + b^2, so you're experiencing 1.4G.

(I'm not particularly talented at math, btw; I only know the parts that are really simple, like that. Also, I know that vector math is extremely simple, no matter how many dimensions. You simply tack each vector front to back like a train on a crazy track, then calculate the distance from the beginning of the first to the end of the last, and that's the vector. Google just showed me this handy tutorial/calculator!)

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 9:14 am 
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Joshua wrote:
The neat thing is that you can figure this out with trigonometry....


oh no, my brain. [insert sound of a watermelon exploding here]

but seriously, thanks for all the info everyone. really cleared up most of the little nagging questions I had about the issue.

one last bit from me though. based on what you guys have explained, would I be correct in believing that during any of the battles that occured during reentry proceedures that we've seen in gundam that the pilots -assuming they are pulling the same maneuvers) where dealing with a higher g load than they would be use to in the "normal" battles in space? would third difference in the g load be enough to notably affect the pilots?

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 12:43 pm 
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I'm not positive, but I don't believe that that's the case. "Atmospheric re-entry" is basically shorthand for "slowing down enough so that you notice gravity again". During re-entry, you would feel some gs from the reduction in speed, and there would be a gradual transition from zero-g to normal gravity, but I don't think there would be any major g-forces like there would be from violent maneuvers.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 1:16 pm 
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It's the deceleration from impact with the atmosphere. "Re-entry" refers to entry into the atmosphere, not just de-orbiting.

So, for instance, when the Apollo astronauts landed on the moon, they didn't re-enter to the moon; they de-orbited. When they returned to Earth, they de-orbited, then hit the atmosphere, resulting in very high accelerations like we're talking about.

Oo! Check it out! Maximum Gs experienced by Apollo astronauts!

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 2:21 pm 
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Shuttle re-entry is about 3 Gs. That means that, if you weigh 175 lbs, for the time you're re-entering, you weigh 525 lbs. It's also directional, in the direction you're moving. So in the ballute re-entry scene, everyone's on their back, which is distributing the force across their body, except for the one guy on the hang glider shuttle wings. That means all of his weight is on his chest. If the average weight for a human head is 15 lbs., that means he's somehow holding up a head that weighs 45 lbs. I don't know about you, but I've got a niece who weighs that much. If I lay down face down on a bed with my head hanging off the edge, there's no way I could hold it up if she sat on the back of my head.

Here's a pilot holding up to 9 Gs. It's straight down, as though she's pulling up really hard (though this looks like it's in a centrifuge). Notice that she can't even keep her eyelids up. This is a known thing for fighter pilots: you might have the training for your body to take the Gs, but you can't see because your eyes are closed.

You'll also notice this "hup! hup!" thing they do. It's to increase blood pressure so blood keeps going to the pilot's brain.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 3:02 pm 
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As Joshua pointed out reentry G forces can get pretty nasty. Apollo astronauts returning from the moon were going at twenty-five thousand mph. That's about eight thousand mph faster than orbital velocity. They had to fly a special "hopping" reentry sequence which caused them to pull 6 to 7 Gs at some points. I don't know about you but I'd feel less comfortable trusting my safety to the slide rules and computers that had less calculating power than pocket calculator that they used back then! :)

Some Soviet cosmonauts briefly pulled 16 Gs during an emergency reentry when their Soyuz booster failed to put them in orbit.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 4:08 pm 
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Location: In the Zaku's cockpit. Yes, the one that just exploded.
...damn. that's freaking intense compared to what I would have thought.

You guys actually missed my question though. I was asking about dog fighting before actual reentry but still getting close to the point of no return in the reentry sequence, like we've seen in a few of the various Gundam series.

...that's worded weird, sorry.

Taking the example you provided from Zeta, I'm wanting to know about the g-forces the pilots where experiencing in the battle before they started blowing their ballutes versus what they'd encounter dog fighting further away from earth.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Thu Nov 03, 2011 6:05 pm 
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That depends entirely on the re-entry burn they do. Moving around in space is all vector addition: you can do it in small, sharp bursts (high g for short periods of time) or in slow, gradual pushes (low g for long periods of time). In most situations, both will get you there pretty much identically. (Human spaceflight tends to opt for short periods of high acceleration, because it's easier to design engines that way, and it's cheaper in terms of propellant, but there's no reason you have to do it like that.)

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Fri Nov 04, 2011 2:38 am 
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The forces experienced in a dogfight, as noted, are 3-6 G. The forces during entry are also about 3-6 G. The big difference is that entry is sustained but predictable, where in a dogfight it comes in shorter bursts but is in all sorts of crazy directions, given that MSes seem to be able to move in any direction.

It occurs to me that you could actually calculate this based on knowing how big a Suit is and how quickly it diminishes into the distance when dodging/getting punched. My guess is that the number is impossibly high, and that the cockpit would contain one sphere of Haro and another of zero-G human soup.

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 Post subject: Re: G-Force in SPAAACE!!
 Post Posted: Fri Nov 04, 2011 9:07 pm 
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I now know everything I ever wanted to know about G-forces in space combat now.


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