Wednesday, November 26, 2008

Space Needle

to get into space: use a balloon - heat or helium - to rise the craft as far as possible. then you start moving as fast as you can using a solar power or maybe a rocket. the balloon should be shaped like a very, very large needle so that it can be aerodynamic. if you go fast enough maybe you can rise above the atmosphere and start skipping across the surface. keep going faster and faster until you hit escape velocity.


Sylvain Poirier said...

First, I think we can quickly forget about hot air: it requires a much larger volume than helium to make the same force, and a lot of energy to keep it hot all along the way up, since the pressure decreases and the balloon must therefore increase in volume to keep the density lower than outside air, and dilation makes it colder.

Now with helium, well maybe...
One thing you should not forget when thinking on this subject: normally the escape velocity you need to reach is not a vertical one but a horizonal one to get into orbit, as soon as you escape the atmosphere and its resistance. The main reason for this, is that while "in the absolute", the escape velocity does not depend on the direction of speed, the effective value of the object's velocity you have to enter in this equation is the one defined in the geocentric frame of reference, where the speed of an object is the sum of its speed with respect to the nearby ground, with the speed of the nearby ground wrt the one of the center of the earth. In other words, for saving energy you have to be near the equator and speed to the east, to have the speed of the rotation of the earth work for you.

But anyway, remember that the wheight to send up is usually big, so that the required volume of helium will be big too. You need about 1 meter cube of helium per kilogram, just for a slow lift that won't give any speed.

I think you can forget about aerodynamism too: a high speed of a big volume would produce a small resistance to the movvement of a big mass going at that speed, but here precisely the mass will be very low for that volume, so that the same force stops it very quickly.
One good point still, is that, if you manage to go high enough and keep the helium in the same volume resisting the difference of pressure, you can then try to use the pressure of the helium you're starting to throw away, as a contribution to rocket your craft - or directly include it in the fuel, I'm not sure what formula would be best.

inhahe said...

The idea is that a sufficient horizontal velocity to escape might be achieved with little force, by a) being extremely aerodynamic, and b) after sufficient height and speed is reached, accelerating while skipping atop the atmosphere so as to minimize friction. but that's the main point that i'm unsure of, because i don't know if a sufficient speed can be reached to do that while still within atmospheric friction, and i don't know if the top of the atmosphere is clearly enough defined to skip across it, and i don't know if it can be raised high enough to do that with little force, given the extremely low density of the air at that height, relative to the density of the needle's material. so it's a lot of if's, but i thought i'd post it just in case it's a good idea.

the resistance given to a heavy and a light craft is actually the same, in proportion to energy required to keep it going, it's just smaller for a heavier craft in the sense that with more momentum the effects of resistance take more time to speed up or slow the craft, and thus are less obvious where the speeds of the compared crafts are not constant. but to your argument's credit, more available propulsion force generally means more weight, so a heavier craft would have more force to counteract an equal amount of resistance. this fact, however, is offset by the fact that we hopefully would not have to move very fast until we are in little or no air density to cause friction.

about the second point regarding the feasibility of hot air, i don't think that it matters that the dilation causes it to lose heat, because the whole point of heating it is to make it dilated in comparison to the outside air. so to say that that the dilation would cause it to lose heat would hamper our ability to dilate the air relative to the outside, reduces to saying that the dilation of the air hampers our ability to dilate the air. while my argument is presented semantically, i think it reflects that in a mathematical analysis the variables would cancel out so that no extra energy is needed to compensate for mutual dilation of the inner and outer air, with the possible exception that some of the heat-energy would be lost due to escaping air, on account of the fact that the balloon would not actually be constructed to expand but rather to allow air to escape when necessary, as it does in a hot air balloon.