ApolloHoax.net
Apollo Discussions => The Hoax Theory => Topic started by: benparry on June 28, 2018, 04:53:51 AM
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Good Morning All
can i just ask a question.
i read on one of the threads here and cannot bloody find it now that an object in space never stops accelerating.
is that correct.
i thought an object needed a force acting upon it to keep accelerating. i have read that gravity could be a force but can it keep accelerating it forever.
cheers
Ben
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Isn't that what a periodical orbit is?
An object continuously and perpetually accelerating towards its primary?
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I'm not sure. as an example if i were in space and fired a bullet out of a gun the power of the gun would allow it to accelerate for a bit, presumably until it reached its top speed but would it accelerate forever.
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Is someone confusing falling and acceleration? Object in orbit is perpetually falling towards primary but because of it's huge speed, surface of primary curves away from object before it hits the ground. Also in non-circular orbit the object is accelerating (trading potential energy to kinetic energy) from apogee to perigee and decelerating (trading kinetic energy to potential energy) from perigee to apogee.
Lurky
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Sorry Lurky that has gone straight over my head lol
so are you saying that objects cannot constantly accelerate without a force acting on it
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Sorry Lurky that has gone straight over my head lol
so are you saying that objects cannot constantly accelerate without a force acting on it
There is still gravity in space, and gravity would still apply a force.
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Sorry Lurky that has gone straight over my head lol
so are you saying that objects cannot constantly accelerate without a force acting on it
Yes, acceleration of mass always requires force.
as Trebor said, the force of gravity acts on our object. The object has both kinetic and potential energy. Kinetic energy is from the speed of the object and potential energy is from the height of the object from it's primary's center of gravity. An ellipse shaped orbit has a point of maximum distance called apogee and point of minimum distance called perigee.
At the perigee, the object has minimum amount of potential energy and maximum amount of kinetic energy. After passing the perigee, the height (potential energy) starts to increase and the speed (kinetic energy) starts to decrease until the apogee is reached. There the potential energy is at maximum and kinetic energy is at minum. After passing the apogee potential energy starts to decrease and kinetic energy to increase until perigee is reached again.
Total energy of the system stays the same but the ratio between kinetic and potential energy changes. Also notice that while the object is always under acceleration, only the "downhill side" of orbit increases the speed and the "uphill side" of orbit is deceleration or slowing down.
I hope this helps and sorry, English is not my primary language
Lurky
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ah ok so basically without gravity the object would simply move at the same speed not accelerate whereas if the object were in an orbit it would accelerate but not forever.
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Without the context it's impossible to determine what may have been meant by the prior statement you refer to. Usually when we say "in space" we mean in a purer Newtonian environment unaffected by nearby bodies or air resistance. In that case a bullet fired from a gun would accelerate to its muzzle velocity by the propulsion of the expanding gas and the continue in a straight line at a constant velocity. (There would be a force on the gun too, because of conservation of momentum. The gun would accelerate in the rearward direction until the bullet left the muzzle.) But in practical terms, anywhere in the solar system, motion is governed by orbital mechanics, of which gravity is a big part. In most cases a bullet fired from a gun anywhere in the solar system would enter some kind of solar orbit. And has has been explained, orbits involve constantly changing velocity.
When we fly actual spacecraft we differentiate between orbital flight and "accelerated" flight because different computer models are needed to maintain the spacecraft's state vector, its understanding of position and velocity. But velocity changes in both models. In orbital flight there is no reading on the ship's accelerometers because their reference masses are affected by the same orbital forces as the surrounding ship. Velocity changes are deduced based on where we compute the ship is along the orbits modeled in its flight plan, largely a function of time. In accelerated flight, the engines are used. The ship accelerates according to a force that is coupled to the reference masses in the accelerometers only in a way that allows acceleration to be measured, then integrated over time to obtain velocity.
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Without the context it's impossible to determine what may have been meant by the prior statement you refer to. Usually when we say "in space" we mean in a purer Newtonian environment unaffected by nearby bodies or air resistance. In that case a bullet fired from a gun would accelerate to its muzzle velocity by the propulsion of the expanding gas and the continue in a straight line at a constant velocity. (There would be a force on the gun too, because of conservation of momentum. The gun would accelerate in the rearward direction until the bullet left the muzzle.) But in practical terms, anywhere in the solar system, motion is governed by orbital mechanics, of which gravity is a big part. In most cases a bullet fired from a gun anywhere in the solar system would enter some kind of solar orbit. And has has been explained, orbits involve constantly changing velocity.
When we fly actual spacecraft we differentiate between orbital flight and "accelerated" flight because different computer models are needed to maintain the spacecraft's state vector, its understanding of position and velocity. But velocity changes in both models. In orbital flight there is no reading on the ship's accelerometers because their reference masses are affected by the same orbital forces as the surrounding ship. Velocity changes are deduced based on where we compute the ship is along the orbits modeled in its flight plan, largely a function of time. In accelerated flight, the engines are used. The ship accelerates according to a force that is coupled to the reference masses in the accelerometers only in a way that allows acceleration to be measured, then integrated over time to obtain velocity.
great stuff
thanks a lot Jay
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i thought an object needed a force acting upon it to keep accelerating. i have read that gravity could be a force but can it keep accelerating it forever.
Just worth a reminder here that, while 'acceleration' is commonly used to refer to increasing speed, acceleration is properly defined as any change in velocity, not speed. Velocity is a speed and a direction. Given this, slowing down is an acceleration, and so is changing direction. Essentially what that means is that any object not on a straight line path at a fixed speed is accelerating. An object in circular orbit is constantly accelerating even if its speed is constant because it is moving in a circle, i.e. constantly changing direction. In space, due to the gravitational infuences on every object, nothing really travels in a straight line at fixed speed.
This is one of those cases where lay use and actual definition vary, so care is needed when describing it.
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Ah well maybe thats what the original quote i remember actually meant Jay. thanks Jason.
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Could be. As Jason points out, the engineering use of "accelerated" flight is a misnomer. Acceleration, in the pure mathematics of physics, is nothing more exotic than the first-order time derivative of the velocity vector in some reference frame. It can be computed just as easily for pure orbital motion as it can be for powered flight. So from a pure physics standpoint some of those thought experiments still make sense. If the reference frame is, say, some part of the ISS, then some object moving within it and reckoned according to it could exhibit certain "pure" Newtonian motion -- if the measurement is casual and air is ignored -- even though the whole kit and kaboodle is whipping around the Earth in orbital motion. That's why any question regarding velocity and acceleration is met with the retort question, "...reckoned according to what?"
For powered flight very near some large astronomical body, the body is often considered the fixed point against which velocities (and their accompanying accelerations) are measured, because orbital motion around that body is what dominates your dynamics and engine maneuvers are most often designed to affect that orbit. You ignore the effects of other, more distant, orbiting bodies, or bodies around which your entire system is further orbiting. For orbits around the Earth, Earth can be considered fixed. Same for the Moon. For transfer orbits, you need a multi-body solution. In Apollo this was too much heavy lifting for the AGC, so it used an approximation of what's called the "restricted" three-body problem. In that model, the two principal bodies (Earth and Moon) together determine the orbital path of the third body (the spacecraft), and the spacecraft's mass is assumed to be negligible -- i.e., the paths of Earth and Moon are not measurably affected by the third body. In practice this was implemented first as an Earth-fixed model with adjustments for the increasing effects of the Moon's gravity, then as a Moon-fixed model with adjustments made for the decreasing effects of Earth's gravity. There comes a point at which the computer switches models and, due to the errors in approximation, seems to "jump" instantly from one reckoned position to another. Then of course you have observations from Earth of the spacecraft's position and velocity along the vector from the receiving antenna, so that provides a toehold for Mission Control mainframes to more finely fix the spacecraft's state vector, which can then be "poked" as needed into the AGC.
It's important in understanding Apollo engineering to realize that the computer had to use these various synthetic models deduced from orbital mechanics, and then separately a a closed-loop measured method when under power. The orbital model needed to update the state vector only once every several seconds in order to maintain an accurate knowledge of where it was in space. But under "accelerated" flight, you needed to poll the accelerometer data ten times a second or so in order to capture any vicissitudes of engine operation and update the state vector. They're qualitatively and quantitatively very different solutions -- both necessary in order to fly in space. But it masks the purity of what we sometimes consider in thought experiments carried out in space.
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And then there's Einstein's view of gravity, which is that it exists as an upward acceleration while you're standing on the earth, but not when you're free-falling in an orbit around it...
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And then there's Einstein's view of gravity, which is that it exists as an upward acceleration while you're standing on the earth, but not when you're free-falling in an orbit around it...
Upward? Shouldn't that be downward?
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Upward. As you stand on the earth, you feel a force pushing up on the bottom of your feet that's indistinguishable from standing inside a spacecraft in deep space far from any planet or star while a rocket engine under your feet accelerates you in the direction of your head at 9.8 m/s^2.
This is Einstein's "Equivalence Principle" at the heart of general relativity (GR). He assumed this was true and then worked out all the implications. One of those implications is that time passes more slowly in an accelerated frame relative to an inertial frame, and this has been demonstrated experimentally many times.
The atomic clocks on the global positioning satellites appear to run faster than atomic clocks here on the earth because we're being accelerated by gravity to a greater degree due to our lower altitude. That's "gravitational blueshift". At the same time, an observer on a GPS satellite comparing the local spacecraft clocks to time transmissions from clocks on the earth's surface would see the surface clocks appear to run faster than the local spacecraft clock; that's a gravitational red shift.
This is distinct from, but related to, the time dilation from special relativity (SR) that applies to observers in different inertial coordinate systems, i.e., in which all the observers feel "weightless". It also resolves the famous "twin paradox" from SR, in which an astronaut who flies rapidly away from the earth and then returns will be younger than his twin who stayed behind even though during the coasting phases of flight both perceived their twin's time as passing more slowly than their own. The reason is that the astronaut twin had to accelerate to achieve his high departure velocity, then he had to accelerate again to cancel that velocity and create a high velocity back to earth, and then the effects of GR kick in.
Note that in space flight, the effects of GR and SR are often in opposite directions, with one (usually GR) dominating; that's the case with GPS. On earth, GR is also easier to demonstrate with atomic clocks than SR though both have been done. It's easier to just go up a mountain and wait for a while than it is to maintain a high velocity relative to the earth.
Einstein did something similar when he derived special relativity (which he did before general relativity). He made a very simple assumption that the speed of light was exactly the same in every inertial reference frame regardless of the relative velocities of source and observer. And then he followed that to all its logical conclusions.
This is all pretty mind-bending, but both SR and GR are firmly supported by mountains of experimental evidence. That's the difference between real science and pseudoscience; real scientists will accept all sorts of weird and seemingly counter-intuitive things provided they're supported by the evidence.
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For a long time, I never understood how a gravity-assist planetary flyby could work to accelerate a spacecraft. To me it seemed that the gravitational attraction of the planet would remain the same before and after closest approach, so any velocity gained while approaching would be lost while receding. That was until I saw a graphical representation of the two Voyager encounters with Jupiter... it was an epiphany, or as Homer Simpson would say... Doh!.
I was thinking in terms of the planet (in this case Jupiter) being the reference frame, when in fact, the Solar System was the reference frame, and the planet itself is moving at orbital velocity. This meant that after closest approach, Jupiter was still moving along in its orbit, along with its gravity well, and not in the same place relative to Voyager as it was on approach, i.e. the approach and recession paths were not identical mirror images of each other....
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That's right. Actually, the term "slingshot" tells it all. A spacecraft flyby of Jupiter wouldn't do anything if Jupiter wasn't moving in its own solar orbit.
If you fly by the orbital trailing side of Jupiter (the usual case), then Jupiter "drags" it along as it orbits, transferring some of its (enormous) orbital kinetic energy to your spacecraft.
If you fly by Jupiter's leading side, the planet pulls back on you and you lose some of your own energy to the planet. You'd do that if you wanted to fall into the inner solar system (e.g., to closely approach the sun at high speed). Another reason would be to do a big plane change out of the ecliptic, as was done with Ulysses.
All these maneuvers would be very expensive to do on your own with chemical rockets. But if you do have a rocket, you can fire it at perijove and get an even bigger kick thanks to the Oberth effect. That's because the mechanical power a rocket delivers to its payload is directly proportional to its velocity, and you're going pretty fast on a close hyperbolic flyby of a planet as massive as Jupiter.
For the same reason, if you're in an elliptical orbit and you want to escape entirely, do your burn at perigee. I knew this when I saw the SpaceX Falcon 9 Heavy from San Diego as it approached second perigee but I didn't actually expect to see the escape burn because perigee occurred somewhat after LOS to our east. But it made sense given that they probably wanted to observe the burn from SpaceX in Hawthorne, near Los Angeles. Pretty neat sight.
If you really want to go out of the solar system in style you could do a flyby of Jupiter to remove most of your orbital energy, dropping you into a elliptical orbit with a very low perihelion. Then as you're racing around the sun (trying very hard not to burn up) you fire your own engine. I saw a paper proposal to do exactly this to catch up with I1, the first known interstellar asteroid that passed through our solar system last fall. (I forget its Hawaiian name.) The maneuver was actually more complicated than this, it required flybys of both Jupiter and Saturn to get the necessary plane change as well as the necessary velocity to escape the sun and catch up with that thing.
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My usual explanation, when people ask about gravity assists / slingshots, is to use the example of throwing a ball at e.g. a moving train. If the train is coming towards you at, say 50 mph, and you throw the ball towards it at 20 mph it bounces off at 120 mph! (Give or take, adjust for elasticity, velocity vectors etc.)
From the train's point of view, the ball approaches at 70 mph and leaves at 70 mph, but from the thrower's point of view, it gets a huge velocity change. (And the train loses a tiny amount of speed.)
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There is the small possibility that the context was more cosmological and the theory that the universe is undergoing accelerating expansion until everything is ripped apart.
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That's right. Actually, the term "slingshot" tells it all. A spacecraft flyby of Jupiter wouldn't do anything if Jupiter wasn't moving in its own solar orbit.
If you fly by the orbital trailing side of Jupiter (the usual case), then Jupiter "drags" it along as it orbits, transferring some of its (enormous) orbital kinetic energy to your spacecraft.
If you fly by Jupiter's leading side, the planet pulls back on you and you lose some of your own energy to the planet. You'd do that if you wanted to fall into the inner solar system (e.g., to closely approach the sun at high speed). Another reason would be to do a big plane change out of the ecliptic, as was done with Ulysses.
The way I explained gravitational slingshots to my kids was to point out the ramp they'd roll toy cars down. Roll the car down the ramp and it goes a certain speed when it leaves the ramp. But if the ramp is itself moving forwards the car leaves the ramp with extra speed. And if the ramp is moving backwards the car leaves the ramp with less speed. The moving ramp represents the planet moving around the Sun.
The other analogy which can be useful is the ice-skating relay races, where one skater grabs the hand of their partner and pulls them forward. Momentum is transferred from one skater to the other, one speeding up and the other slowing down; of course, when it's a planet "grabbing the hand" of a spacecraft, the planet loses only a tiny amount of speed and the spacecraft gains massively.
All these maneuvers would be very expensive to do on your own with chemical rockets. But if you do have a rocket, you can fire it at perijove and get an even bigger kick thanks to the Oberth effect. That's because the mechanical power a rocket delivers to its payload is directly proportional to its velocity, and you're going pretty fast on a close hyperbolic flyby of a planet as massive as Jupiter.
Is the Oberth Effect why Apollo and Space Shuttle launches would initially climb slightly above their intended orbits and then dive back down during powered flight?
If you really want to go out of the solar system in style you could do a flyby of Jupiter to remove most of your orbital energy, dropping you into a elliptical orbit with a very low perihelion. Then as you're racing around the sun (trying very hard not to burn up) you fire your own engine. I saw a paper proposal to do exactly this to catch up with I1, the first known interstellar asteroid that passed through our solar system last fall. (I forget its Hawaiian name.) The maneuver was actually more complicated than this, it required flybys of both Jupiter and Saturn to get the necessary plane change as well as the necessary velocity to escape the sun and catch up with that thing.
'Oumuamua?
Interesting article about it here: http://www.abc.net.au/news/science/2018-06-28/mysterious-interstellar-visitor-oumuamua-was-a-comet-after-all/9900114
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Is the Oberth Effect why Apollo and Space Shuttle launches would initially climb slightly above their intended orbits and then dive back down during powered flight?
No. They did that so that the upper stages have more time to get into orbit before falling back to earth since upper stage engines usually have low thrust. Without this you would have to pitch up more during the burn to compensate gravity and this would be overall less efficient.
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I had noticed and wondered about that myself; your explanation is almost certainly correct.