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Old 8th February 2013, 20:12   #76
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Thanks AK once again for taking us this far. I have been able to go through you post only today. I thought I picked up those two errors on escape velocity and earth's inclination but then saw that Mr Kala has already been there. Great work, but then when are you taking us "boldly to the place where no man has ever gone before", interglactic travel, I love this word, gives me a feeling of ultimate travel adventure.

Lastly, a query about the eccentricity of these satellites who don't believe in going around a parent in proper circles, where do you measure the height of the orbit, at apogee or perigee.
Cheers

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Old 8th February 2013, 23:30   #77
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Very interesting article, many thanks.

I've always struggled with the concept of escape velocity. I can calculate it, it's a simple formula, but I've never been able to really understand it.

Say for argument sake, I was able to built a rocket that takes of vertically from earth and keeps going up at a steady speed of say 1500 km/h. And it just maintains that vertical climb of 1500 km/h, why wouldn't it just 'escape from earth?

I'm probably overlooking something pretty elementary, but in my mind if I move away from something at a steady pace, I just, well move away from it!

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Old 8th February 2013, 23:59   #78
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Hi Jeroen, regarding escape velocity, when you convert 1500km/h into km/s it comes close to 0.416 km/s. This is significantly lesser than the 11.2 km/s required to "escape" the earth's gravitational pull. The reason being that according to the law of conservation of energy, the higher levels of potential energy cannot be attained at speeds lesser than the critical escape velocity which in turn determines how high the rocket will travel. In other words, the faster an object travels, the more kinetic energy it has, the higher the rocket goes, the more potential energy it has. The escape velocity also depends upon how and where the rocket is launched. That is the reason a rocket is made to come into orbit by altering its flight course once it reaches the mesosphere(if i remember properly).
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Old 9th February 2013, 01:47   #79
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Say for argument sake, I was able to built a rocket that takes of vertically from earth and keeps going up at a steady speed of say 1500 km/h. And it just maintains that vertical climb of 1500 km/h, why wouldn't it just 'escape from earth?
Jeroen
You are quite correct in your understanding, for as long as your rocket has propulsion it will continue to move away from earth. All it needs thrust greater than the force of gravity of earth ie 9.8N . You need escape velocity to move out of earth's sphere of influence when you launch an object from the surface of earth and it has no propulsion thereafter.
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Old 9th February 2013, 07:25   #80
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You are quite correct in your understanding, for as long as your rocket has propulsion it will continue to move away from earth. All it needs thrust greater than the force of gravity of earth ie 9.8N . You need escape velocity to move out of earth's sphere of influence when you launch an object from the surface of earth and it has no propulsion thereafter.
Cheers
Thanks, but if I keep moving at 1500 km/h why wouldn't I ultimately move out to the earth's sphere of influence?

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Old 9th February 2013, 10:07   #81
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Thanks, but if I keep moving at 1500 km/h why wouldn't I ultimately move out to the earth's sphere of influence?

Jeroen
You will certainly carry on and on as long as the propulsion system continues to spew out thrust from the rear. When you run out of fuel or have a midway breakdown you will have to decide what to do then, depending on your location.
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Old 9th February 2013, 19:18   #82
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Thanks, but if I keep moving at 1500 km/h why wouldn't I ultimately move out to the earth's sphere of influence?
Jeroen
Actually, this escape velocity applies to all bodies aspiring to "escape" the clutches of our dear lady earth. If you keep the 1500 kmph speed up for a long time, you will certainly be able to leave the earth. However, stop the thrust and you will tumble back into her arms . Bodies having the escape velocity will be neither able to go into orbit nor will tumble back to earth when the thrust is cut out.
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Old 9th February 2013, 23:02   #83
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Actually, this escape velocity applies to all bodies aspiring to "escape" the clutches of our dear lady earth. If you keep the 1500 kmph speed up for a long time, you will certainly be able to leave the earth. However, stop the thrust and you will tumble back into her arms . Bodies having the escape velocity will be neither able to go into orbit nor will tumble back to earth when the thrust is cut out.
How does that work than? I go for 1000 hours at 1500 km/h. At that moment I cut my engines. I'm 1.500.000 km from earth. Will I still fall back to earth. What if I leave the engine running for 10.000 hours? Will I still fall back to earth. This escape velocity doesn't make any sense, because if I just keep going at a steady pace, ultimately I'll find myself in a different galaxy. So what's with the escape velocity??

The further I move away from earth (or any mass) the less gravitational pull I'll be experiencing. So it get easier and easier, or not?

Again, I'm probably overlooking something pretty fundamental and or simple, but I just don't get it.

Thanks, Jeroen
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Old 9th February 2013, 23:33   #84
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The answer would be yes to both your assumptions. The tumbling back to earth happens only when you maintain proximity with the object (in this case, earth). Escape velocity is just a measure of the velocity required by an object to completely escape the earth while assuming 0 retardation due to friction. This is not related to anything else. If you think escape velocity is the velocity required to move an object from point a to b somewhere away from earth, that is not what it represents. It is just a theoretical measure of the velocity required by an object(not propelled-lets assume an object being slingshotted or being a projectile of a huge cannon). When an object is propelled, the concept of escape velocity is only applied to it when its thrust stops and the residual velocity it possesses with regards to its position relative to the planet determines whether it will continue on the same path or be affected by the planet's gravity.
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Old 10th February 2013, 07:30   #85
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The answer would be yes to both your assumptions. The tumbling back to earth happens only when you maintain proximity with the object (in this case, earth). Escape velocity is just a measure of the velocity required by an object to completely escape the earth while assuming 0 retardation due to friction. This is not related to anything else. If you think escape velocity is the velocity required to move an object from point a to b somewhere away from earth, that is not what it represents. It is just a theoretical measure of the velocity required by an object(not propelled-lets assume an object being slingshotted or being a projectile of a huge cannon). When an object is propelled, the concept of escape velocity is only applied to it when its thrust stops and the residual velocity it possesses with regards to its position relative to the planet determines whether it will continue on the same path or be affected by the planet's gravity.

thanks, I think I'm beginning to understand.
Would it be fair to say, that giving how rockets work and contrary to "my rocket" they don't have unlimited fuel" it's advantageous to reach escape velocity asap? The quicker you get there, the less fuel you need, the less weight etc. Of course, accelerating quickly to the escape velocity will take fuel itself, so there must be some way to calculate a certain optimum on rocket mass, fuel and acceleration rate.

Thanks Jeroen

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Old 10th February 2013, 11:35   #86
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Now, in the ideal case, it would be advisable to reach the escape velocity as soon as possible. However, rockets face some amount of friction on its surfaces. Therefore making the process of acceleration a little delayed(not by much, but yes it is present). There are various parameters on which a rocket is made. Firstly, the type of payload it will carry, this might be a manned or a satellite payload or could be a miscellaneous one (nuclear waste might be sent out of the earth. This is just an example).

The second thing would be the type of rocket motor design. This is based on either solid propellant( Ammonium perchlorate, potassium nitrate are some examples) or a liquid propellant( Oxygen+Fuel(can be kerosene, Hydrogen contained under high pressure liquid form) or some hybrid types. So obviously, we have a lot of variables to play around. Rocket mass will be determined by all the factors listed above. This is another reason why booster rockets are added for providing additional lift.
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Old 10th February 2013, 20:11   #87
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Lastly, a query about the eccentricity of these satellites who don't believe in going around a parent in proper circles, where do you measure the height of the orbit, at apogee or perigee.
Cheers
Frankly speaking, both are measured. Perigee cannot be below earth's radius As such no orbit is a proper circle!

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Very interesting article, many thanks.
I'm probably overlooking something pretty elementary, but in my mind if I move away from something at a steady pace, I just, well move away from it!
Jeroen
Very simple elementary it is. Remember nothing goes up 90° vertical in earth! You always go in a parabolic path, why? Answer to your question lies in the answer to this question!

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Now, in the ideal case....
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Old 10th February 2013, 21:18   #88
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Here we go! The much anticipated "Inter-planetary/Inter-stellar/Inter-galactic" navigation techniques.

Statuary warning: I will show some basics, and I don't guarantee that you will arrive at your destination planet/galaxy. If you get lost mid-way or never arrive at your destination or unable to return to Earth - please don't blame me! To be travelled at your own risk and finances please!

What do we need first?

1. Trajectory - flight path to our destination - first we should determine where we want to go and try to sketch a path to our destination. Its the same way we navigate in cars during our long-distance trips. (now after this post, I hope no one can complain that TBHP gives travelogues for only inter-earth travel! I am expecting some inter-planetary travelogues in the coming months once this post is complete )

2. Orbit determination - our current location with respect to our origin and to our target destination. Meanwhile, enroute certain reference points/Way points are also used as reference to cross check the orbit and to make sure that our preplanned route matches with where we are proceeding currently.

3. Maneouvre's and course changes - do we travel just straight towards our target or should we make some directional changes enroute. If so, where, how much, when?

So armed with above information we now set out to actually develop the plan and the trajectory towards our destination. During the above process it is not necessary that the information available to be very precise, rather, it is enough if we get the basic information correctly and a rough estimation of various possibilities. This knowledge will help us in exploring various other paths if the one we intially chose ends up being very-fuel inefficient/dangerous.

Now what are the basic questions we might encounter while trying to find out answers for the above steps:

1. Trajectory: How far is our destination? What lies inbetween? Has there been any mission which previously attempted to go there? If yes, what was the issues faced by them? How did they solve it? If no, why not? What all could be the possible pit falls?

2. Orbit determination: What will be the environment which we may need to encounter en-route? For ex. Jupiter the largest planet, has the largest attraction force, so what will be its effect on our trajectory? how can it be avoided or be even used to our advantage? How complicated will our trajectory(orbit) is going to look like? How do we keep in touch with the Earth while on the way? Power? How to feed the on-board equipments with power? Remember the farther we go away from the Sun the less effective the solar panels become. Alternate?

3. How many course changes/deviations are to be expected? Where? how energy consuming are they? how complex are they? How do we know exactly when those maneouvres need to be executed and how do we receive the confirmation of the same?

These are some basic questions that will arise while trying to plan our way to other celestial bodies. Since it will be a long post, I will put up back-to-back posts to avoid sleepiness! Hope mods don't mind.
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Old 10th February 2013, 21:51   #89
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Here we go! Since it will be a long post, I will put up back-to-back posts to avoid sleepiness! Hope mods don't mind.
Press on regardless! Great we are ready for the blast off tonite itself. I am interjecting only to ensure that all mandatory checks are in order for you to launch the rest.
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Old 10th February 2013, 22:14   #90
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Basically the two modes of inter-planetary mission are:

1. Earth-return: The capsule or astronauts returns back to earth with samples collected or after performing experiments.

2. Deep space exploration mission: Simply the data is relayed back to earth in form of Images or scientific data.

Previously we have learned about Hohmann transfer, now adding one more method to called "Gravity-assist". What is this? Lets see what does Wiki says:

Quote:
Gravitational slingshot



Over-simplified example of gravitational slingshot: the spacecraft's velocity changes by up to twice the planet's velocity


The gravitational slingshot technique uses the gravity of planets and moons to change the speed and direction of a spacecraft without using fuel. In typical example, a spacecraft is sent to a distant planet on a path that is much faster than what the Hohmann transfer would call for. This would typically mean that it would arrive at the planet's orbit and continue past it. However, if there is a planet between the departure point and the target, it can be used to bend the path toward the target, and in many cases the overall travel time is greatly reduced. A prime example of this are the two crafts of the Voyager program, which used slingshot effects to change trajectories several times in the outer Solar System. It is difficult to use this method for journeys in the inner part of the Solar System, although it is possible to use other nearby planets such as Venus or even the Moon as slingshots in journeys to the outer planets.
This maneuver can only change an object's velocity relative to a third, uninvolved object, – possibly the “centre of mass” or the Sun. There is no change in the velocities of the two objects involved in the maneuver relative to each other. The Sun cannot be used in a gravitational slingshot because it is stationary compared to rest of the Solar System, which orbits the Sun. It may be used to send a spaceship or probe into the galaxy because the Sun rotates around the center of the Milky Way.
Idea how gravity assist works:






Some fantastic read: http://saturn.jpl.nasa.gov/video/pro...tationsSlides/


Keep this gravity assist in mind, because it will be used very extensively in the coming days.

Last edited by AlphaKilo : 10th February 2013 at 22:35.
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