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Post by capricorn1 on Mar 4, 2011 16:23:47 GMT -4
One thing about the TLI that has always intrigued me is the weighlessness aspect.
I get this completely when they are orbiting, but what gravitational effects are there when the craft performs the TLI and for the next few days?
I would love this to be explained to us lay-people.
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Post by LunarOrbit on Mar 4, 2011 16:33:41 GMT -4
I can't give you exact numbers, but I think the only time the astronauts would experience anything like artificial gravity would be when the engine was actually firing and accelerating the spacecraft. As soon as it shut down they would be weightless again. Is that what you mean?
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Post by Jason Thompson on Mar 4, 2011 18:07:00 GMT -4
It's no different from being in orbit. The force due to gravity acts on the whole spacecraft and everything in it to the same degree, hence the crew are still effectively travelling at the same speed and accelerating at the same rate as the spacecraft throughout. The only time they feel anything approaching a gravitational attraction to something inside the spacecraft is when they fire the engine and apply an acceleration over and above the force of gravity they are all feeling.
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Post by Kiwi on Mar 4, 2011 21:37:08 GMT -4
One thing about the TLI that has always intrigued me is the weighlessness aspect. Do you understand the situation? During Translunar Injection they would not be weightless due to the acceleration, but they would be weightless before and after the TLI burn. For instance, the Apollo 11 TLI burn lasted from GET 2:44:16 to 2:50:03, just 5 minutes and 47 seconds. Mike Collins says in his book "Carrying The Fire — An Astronaut's Journeys" (Michael Collins, Cooper Square Press, New York, 1974), pages 372 and 373, Except for any tiny forces during docking with the LM, mid-course corrections, and stopping and starting the barbecue roll, they would not have felt any significant g-force again until the six-minute lunar orbit insertion burn which started at GET 75:49:50, and Collins says on page 389, "The acceleration is only a fraction of one G". For the remaining 73 hours of the translunar coast the spacecraft decelerated due to earth's pull then accelerated due to the moon's pull, but in each case the forces were very gradual and they were moving with it so would have remained weightless and, without looking out the windows at the earth or moon, would have had no sense of motion inside the spacecraft. |
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Post by capricorn1 on Mar 5, 2011 9:03:52 GMT -4
One thing about the TLI that has always intrigued me is the weighlessness aspect. Do you understand the situation? During Translunar Injection they would not be weightless due to the acceleration, but they would be weightless before and after the TLI burn. For instance, the Apollo 11 TLI burn lasted from GET 2:44:16 to 2:50:03, just 5 minutes and 47 seconds. Mike Collins says in his book "Carrying The Fire — An Astronaut's Journeys" (Michael Collins, Cooper Square Press, New York, 1974), pages 372 and 373, Except for any tiny forces during docking with the LM, mid-course corrections, and stopping and starting the barbecue roll, they would not have felt any significant g-force again until the six-minute lunar orbit insertion burn which started at GET 75:49:50, and Collins says on page 389, "The acceleration is only a fraction of one G". For the remaining 73 hours of the translunar coast the spacecraft decelerated due to earth's pull then accelerated due to the moon's pull, but in each case the forces were very gradual and they were moving with it so would have remained weightless and, without looking out the windows at the earth or moon, would have had no sense of motion inside the spacecraft. Yes, that explains it. ;D I read somewhere that the TLI is in essence a very wide orbit of the Earth, that changes to a very wide orbit of the Moon.....hence the figure of eight. Is that right? Am I also right in saying that the huge speed generated by the TLI is clawed back by Earth gravity and slows it significantly by the time the Moon gravity is stronger? |
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Post by chew on Mar 5, 2011 9:28:45 GMT -4
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Post by Kiwi on Mar 5, 2011 23:00:30 GMT -4
I read somewhere that the TLI is in essence a very wide orbit of the Earth, that changes to a very wide orbit of the Moon.....hence the figure of eight. Is that right? In essence that is correct, but as I suspected from your first post where you mentioned weightlessness during TLI, you are using the wrong term. Look up the definition of TLI. It is only the burn of a few minutes that gets them from low earth orbit to translunar coast, which is part of what you are talking about. The other part is TEI (the trans-earth injection burn) and trans-earth coast. To prevent misunderstandings it's important to use the correct terminology. It is rocket science! At least you understand what the author of the first "hoax" book I read didn't understand, and nor did I at the time, although I knew that his claims about photos were nonsense. It was William L. Brian's book "Moongate." He claimed that because the astronauts crossed the gravitational neutral point much further from the moon than the text books stated, the moon's gravity had to be much more than one-sixth earth's. But he was taking the straight line between the earth and the moon and the neutral point distance on the line, whereas any spacecraft going to and from the moon would hardly ever be near that straight line. Its figure eight orbit would take it mostly out to one side, and the further out it is, the greater the distance the neutral point would be from both earth and moon. Also, it take would two straight lines at different angles to join earth, moon, and spacecraft at the neutral point. Simple, when you think of it, but Brian didn't. |
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Post by mccomb on Mar 6, 2011 3:23:35 GMT -4
That's an awesome web page. And unlike some others I've seen, its trajectories obey the conservation of energy principle  |
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Post by capricorn1 on Mar 6, 2011 12:07:27 GMT -4
In essence that is correct, but as I suspected from your first post where you mentioned weightlessness during TLI, you are using the wrong term. Look up the definition of TLI. It is only the burn of a few minutes that gets them from low earth orbit to translunar coast, which is part of what you are talking about. Yes of course the translunar coast is what I meant after the main burn. So the fact that it is an ever decreasing wide orbit.....that is what makes them weightless for the duration, and only during rocket burns do they actually have weight from the acceleration against gravity. Is that essentially it? |
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Post by chew on Mar 6, 2011 14:01:36 GMT -4
and only during rocket burns do they actually have weight from the acceleration against gravity. Is that essentially it? Remove the "against gravity" part and it will be exactly right. P.S. I remember hearing that Jules Verne got it wrong in his book about the trip to the Moon. The voyagers didn't experience weightlessness until they were at the equigravisphere. |
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Post by Data Cable on Mar 6, 2011 14:02:04 GMT -4
So the fact that it is an ever decreasing wide orbit.....that is what makes them weightless for the duration A transfer orbit is essentially an extremely eccentric (elliptical) orbit around the source body which deliberately intersects an orbit around the destination body, but it's still an orbit. The astronauts and the spacecraft they're inside are in perpetual freefall at the same rate, there is no differential in force between them and their spacecraft, therefore they're "weightless." Gravity is irrelevant in this context. They have "weight" from the acceleration of the spacecraft, period. Gravitational force is indistinguishable from constant acceleration. Standing on the earth, you feel the resistance of the ground pressing against your feet (and verse visa) from gravitational attraction of the two masses toward each other. If you were in interstellar space standing on a rocket engine which was accelerating at 9.8m/s 2, you would feel exactly the same resistance, even though there is negligible gravitational attraction between your mass and that of the engine. So while the SPS is performing the TLI/TEI burn, the craft is accelerating against the astronauts, and they feel "weight" proportional to the acceleration rate. But once the burn stops, the spacecraft and its contents cease accelerating and "weightlessness" resumes. |
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Post by Nowhere Man on Mar 6, 2011 15:12:22 GMT -4
P.S. I remember hearing that Jules Verne got it wrong in his book about the trip to the Moon. The voyagers didn't experience weightlessness until they were at the equigravisphere. You remember correctly. Balmer and Wylie did the same in When Worlds Collide. Probably along with many other contemporary and forgotten writers. Fred |
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Bob B.
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Post by Bob B. on Mar 8, 2011 0:58:17 GMT -4
At least you understand what the author of the first "hoax" book I read didn't understand, and nor did I at the time, although I knew that his claims about photos were nonsense. It was William L. Brian's book "Moongate." He claimed that because the astronauts crossed the gravitational neutral point much further from the moon than the text books stated, the moon's gravity had to be much more than one-sixth earth's. But he was taking the straight line between the earth and the moon and the neutral point distance on the line, whereas any spacecraft going to and from the moon would hardly ever be near that straight line. Its figure eight orbit would take it mostly out to one side, and the further out it is, the greater the distance the neutral point would be from both earth and moon. Also, it take would two straight lines at different angles to join earth, moon, and spacecraft at the neutral point. NASA refers to the "equigravisphere" (they don't use the term "neutral point"). The equigravisphere is the boundary where the spacecraft trajectory is considered to transition from earth-centered to moon-centered; however, it has an arbitrary definition. NASA defines the equigravisphere as being 40,000 statue miles from the center of the Moon. It is commonly believed by people outside NASA that the equigravisphere are all points in space where Earth and lunar gravity are equal, but this is not how NASA uses the term. If someone performs calculations assuming that the equigravisphere reported by NASA is the gravitational neutral point, then they are going to get garbage results. The actual neutral point is much closer to the Moon than the distance of the equigravisphere as defined by NASA. |
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Post by ka9q on Mar 8, 2011 9:07:46 GMT -4
Perhaps the better term is "free fall". When you are in a spaceship orbiting a planet, you and the ship are actually continuously falling around it without hitting it. All parts of the spaceship (including you) fall together, so you and the ship experience no forces relative to each other. So you simply float around inside the ship in what you perceive as "zero gravity". This is true no matter what kind of orbit you're in -- a low orbit just above the earth or in a long elliptical orbit out to the moon.
Now let's fire the ship's rocket while you're still floating around inside. Because you're (momentarily) unattached to the ship, the rocket will begin to accelerate it past you until its inside wall hits you. For as long as the rocket fires, that wall will continue to press on you and carry you along with the rest of the ship. You will experience what seems just like gravity until the rocket shuts down. Then you'll be "weightless" again.
Your experience while the rocket fires is actually indistinguishable from sitting inside your ship as it sits motionless on the earth waiting for launch. In that case the forces acting to push you into your couch are being transmitted from the ground up through the body of the rocket.
Einstein used this very same thought experiment to work out the details of general relativity. He hypothesized that there was no way to tell the difference between acceleration due to gravity and acceleration due to actually being pushed by a rocket in deep space. As far as we can tell, he was right; no experiment has ever been able to distinguish the two except for an effect very close to a massive body called a "gravity gradient". It's a very small effect that comes from the fact that not all parts of the ship are equally close to the planet.
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Bob B.
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Post by Bob B. on Mar 8, 2011 10:12:02 GMT -4
Einstein used this very same thought experiment to work out the details of general relativity. He hypothesized that there was no way to tell the difference between acceleration due to gravity and acceleration due to actually being pushed by a rocket in deep space. Although in Einstein's thought experiment there was no difference, in practice there is a difference. On the surface of the Earth the acceleration of gravity is constant, while the acceleration of a spaceship being propelled by rocket power is variable. When an astronaut is sitting on the launch pad, he/she will be pushed down into the couch with an acceleration of 1 g. All of us feel a constant 1 g as we go about our daily lives and this never changes (although there are tiny almost imperceptible differences depending where on Earth you are located - such as at sea level versus on a mountaintop). When a spaceship is accelerating under rocket power, the acceleration is constantly changing. For instance, if the rocket engine is providing constant thrust, the acceleration will gradually increase as propellant is burned. Burning propellant decreases the mass of the spaceship, thus acceleration increases in accordance with Issac Newton's equation F=ma. In other instances, an engine may be throttled to produce a variable thrust. Or, in the case of a multi-engine rocket, some engines may be shut down prior to others, thus causing an abrupt change in thrust. Any change in thrust or mass will change the acceleration, and these changes will be felt by the astronauts. |
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