4 futuristic space technologies — and when they might happen

[Admin]

All four technologies are grounded in reality, but some are more feasible than others.

4 futuristic space technologies — and when they might happen : Read more

 

[Unclear Engineer]

Of these 4 technology concepts, the nuclear reactor on the Moon seems like a sure bet, but the others seem more fanciful than realistically evaluated.

Regarding the "space elevator" using a cable that is terminated on Earth and reaches past geosynchronous orbit, that just does not seem to have been thought through the dynamics properly. Even if we can eventually make a cable that can stand the load of tying the far end satellite to the Earth's surface, and assuming that a space "elevator" could "climb" that cable instead of being hauled up by another cable. there is still the issue of where the energy ultimately comes from to get the capsule to geosynchronous orbit.

Think about it this way: Assuming attachment of the bottom end at Earth's equator, it has a rotational speed to the east of 24,901 miles/24 hours = 1037.5 miles per hour. But, geosynchronous orbit is 6,876 mph. So, the capsule must be accelerated sideways by 5.838.5 mph.

How is that sideways acceleration going to be provided? If it is provided by a vertical cable pushing the capsule to the side as it climbs the cable, that will tend to move the cable to the west, which will lower the upper end of the fixed length cable as it takes on a more spiral path from Earth's surface to the upper end satellite. But, that will destabilize the geosynchronicity of the orbit, causing the upper end of the cable to eventually fall out of orbit.

So, maybe that could be fixed by putting rocket motors on the capsule that fire sideways to keep the side pressure on the cable to zero.

But, we also need to consider the effect on the upper satellite caused by hauling the capsule upwards. That will also haul the upper satellite downwards, again taking it out of geosynchronous orbit.

So, again, we could put rocket engines on the capsule to push it upwards, instead of "climbing" the cable by pulling itself up mechanically (and simultaneously pulling the satellite downward).

So, to have a vertical cable, we would need rocket motors that pushed the capsule sideways and upward in the right amounts to reach geosynchronous orbit. But, wait a minute, isn't that like what we already do without the cable? Are we really saving rocket fuel that way? Nope.

So, how would we design a cable stretching past geosynchronous orbit that could be used to haul mass to geosynchronous orbit without using any rocket fuel?

We would need to have that cable spiral in a path that is effectively tugging on the Earth's rotational energy directly along the axis of the cable at all points between the Earth's surface and the upper satellite. I am not sure that such a path even exists, but think that at least most of the energy could come from slowing the Earth's rotation a tiny amount but probably still requiring some rocket thrust on the upper satellite end of the cable to keep things stable, there.

And, besides the effects of hauling mass to geosynchronous orbit, there are also gravitational effects from the Sun and the Moon that need to be worked out. Just like the surface of Earth's oceans get pulled towards the Moon and the Sun, the satellite(s) in geosynchronous orbit and the cable will get pulled on a daily basis by the Sun and Moon with the Moon's pull changing into and out of phase with the Sun's pull on monthly period.

So, to show how this would need to be designed to really work, somebody needs to figure out the Lagrangian equation for the cable spiral, solve it, and determine how long that cable would really need to be. And, they need to determine what amount of rocket fuel would need to be hauled up that cable to keep it stably in geosynchronous orbit while people hauled masses up the cable and the Sun and the Moon affect it.

My guess is that it would need to be a much longer cable than the proponents of this concept envision for a vertical cable. And, I suspect that even thousand-mile-long nanotubes are not going to have the strength to weight ratio needed. That is the issue for a cable from Earth to orbit.

I have not thought about the idea of one from the Moon to lunar orbit. For one thing, it could be pointed directly at Earth, and use that for stability. The cable would be tidally locked, just like the Moon. And, it would have far less distance and far lower forces to contend with. Still, the dynamic effects of hauling loads up and down with just the cable tension needs more thought than I have time to give it. Pulling a mass to the satellite in lunar synchronous orbit is still going to be pulling that satellite closer to the Moon. Even the Lagrange point between the Earth and the Moon is not stable for a free satellite. So, pulling on a satellite in that location to haul loads off the lunar surface is not going to leave the upper end satellite in-place for continued service without some sort of compensating force.

Maybe somebody can show me how the energy to lift the loads off the lunar surface can be "harvested" from the energies of lunar orbit or lunar rotation or Earth's rotation, but I am not envisioning a process for that, at the moment.

The important points to remember are:
1. the energy gained by the hoisted capsule must be taken from somewhere; and
2. it cannot come from the satellite orbit on the upper end of the cable without reducing the energy of that satellite's orbit, so that it goes lower to the body that it is orbiting.