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About Space Travelling

  1. Jan 25, 2012 #1
    The question may sound silly,but I want to know why we still need to do all sorts of calculations to go outer space...I mean why cant we just...like science fiction....launch our spacecraft and go outer space from anywhere???Why do we need specific launch site.I mean aside from huge amount of fuel emission,is there any other reason?

    I was just curious to know it...Thanks for replying
  2. jcsd
  3. Jan 25, 2012 #2


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    If you mean "why are rockets launched from specific locations" the answer is- mostly for safety from crashing rockets. The Russian base is in a fairly sparcely populated area in Siberia. The United States base, Cape Canaveral is in a more densely populated area of Florida, but it is on the coast and rockets are launched over the Atlantic Ocean.

    There are some other considerations The closer a base is to the equator, the more "assist" one can get from the rotation of the earth. The higher a base is, in altitude, the less atmospher one must go through. However, neither the Russian nor the United Space bases are particularly close to the equator nor very high.
  4. Jan 25, 2012 #3
    Rockets typically don't carry enough fuel for willy nilly coruse corrections...and if they do carry enough fuel it is VERY expensive to lift it inton space.
    It's curdely analogous to why ships plot a course before leaving port....or why you pick a
    route before you embark on a car trip....
  5. Jan 25, 2012 #4
    Thanks for replying...that means one reason is safety and another is earth's forces on spacecraft...okay so can we sum up that aside from safety the main problem is fuel consumption and engine power???
  6. Jan 25, 2012 #5


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    Yes, and fuel = money
  7. Jan 25, 2012 #6


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    The reason they don't just blast off in to space, then decide "let's go over there" is because of fuel. In a car on earth, it is fairly easy to stop the car and go in the reverse direction. But for a spacecraft, doing something like this would take a ridiculously large amount of fuel.

    Wikipedia's page is pretty good: http://en.wikipedia.org/wiki/Rocket especially useful is the stuff about the delta-v capacity.
    [tex] \Delta v = \nu_e \ ln( \frac{m_0}{m_1} ) [/tex]
    Where delta-v is the change in velocity which can be achieved, given that the spaceship initially has [itex]m_0[/itex] total mass, and later has [itex]m_1[/itex] total mass (due to propellant being propelled into space). And [itex]\nu_e[/itex] is the effective exhaust velocity (of the order of a few thousand m/s).

    So what the equation tells us is that a significant fractional loss of the spaceship's total mass is required to change the spaceship's velocity by a few thousand m/s. Now this might sound like a great speed, but considering that the distance from the earth to the sun is of the order 10^12 m, this means that a few thousand m/s is the kind of speed which we expect our spaceship to go at.

    So this means that turning our spaceship round mid-flight would require a large fraction of its total mass to be used as propellant. So we could only imagine being able to do this once (even once might be too difficult).

    If the total payload of the spaceship was zero (i.e. any of it could effectively be used as propellant), then there would be no problem. But obviously, this is not true, since we at least want the spaceship to take photos of planets, e.t.c. and some of the spaceship's weight will be its hull.

    So using rockets, we simply aren't able to travel the solar system on the fly. They must plan the journey beforehand, so the change in velocity is as small as possible during the 'space journey'. Also, they can use the gravitational pull of planets in calculations, so that some of the change in velocity is given 'for free' by the gravitational pull of the planets.
  8. Jan 25, 2012 #7
    Going up is quite energy intensive, the more the ship weighs the more thrust is needed to create the same lift (i.e. you have to burn more fuel per second to generate the same motion upwards). Thus, you can't just tack on a whole bunch of fuel tanks and put the pedal to the metal, you'll burn most of it up getting off the ground. Thus it's really about finding a balance between fuel you have to carry and still making it off the ground.
  9. Jan 25, 2012 #8
    Actually Baikonur is in Kazakhstan. The location was choosen to be as far south as possible while keeping the round track over SU territory (didn't want to drop spent stages on China). The other launch facility, Plesetsk, is in northern european part, not that far away from Finland.

    Thats true, but the main reason is lattitude of the launch site gives minimum orbital plane inclination which can be achieved directly without plane change. Changing direction once in orbit is horrendously expensive. For high-inclination orbits, such as polar and sun-synchronous orbits used by earth observation sats the location does not really matter but for geostationary comsats you really want to be as close to the equator as possible. That's why French built Guiana Space Centre. Sea Launch is also worth mentioning, they launch comsats into geostationary orbit from converted oil rig stationed at 0 deg lat in the pacific.

    At a few $100M a pop better be safe than sorry :)
  10. Jan 26, 2012 #9
    In a way, you can think of a spaceship like a gun. You pretty much only control where the barrel is pointing. After you fire it, you basically leave it up to the laws of nature to take it to the target. You might have a tiny bit of fuel for course correction, but that's the jist of it.

    At close range you can fire from the hip, but if you want to shoot something further away, you need to take careful aim. Indeed, snipers need to do some calculations in their heads and possibly on paper if they hope to hit anything. If you want to hit a moving target, you need to do even more calculations so that the bullet and target end up in the same place. That's what...hundreds/ thousands of yards.

    A spaceship is kind of like that, except the targets are usually millions/ billions of MILES away. AND they're moving.
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