Gravity satellite feels the force

But Gravity is just a theory!

 

Ummmm NO...

Gravity IS a LAW.

The theory was proved to be correct for mass...

Now what we're trying to do is tie gravity in with the other physical laws for a UNIFIED THEORY of everything...

Gravity is the real problem with this so far, but I think once we get a better handle on the "missing particles" and why acceleration is occurring instead of the big crunch, we'll be closer to being able to tying everything together into one equation.

 

I have a fraternity brother who worked on a similar Ion engine design, only much, much bigger - it was going to be outfitted onto a NASA probe to Jupiter's moons, but the mission got scrubbed before they could build it. stupid funding.

 

Speaking of satellites, I'd be interested in hearing your thoughts on a geostationary satellite with a tether, also known as a 'space elevator'. Are we close to having a strong enough 'rope'?

 

The carbon nanotube stuff hasn't been cultured to any usable lengths yet as far as I had heard... but I remember reading somewhere they have increased the size by a factor of 50.

The problem being currently discussed in some detail is the weight of the line, and the atmospheric/tidal currents moving the line.

It's a great concept... but I don't really think it is doable in todays technological condition... there needs to be another breakthrough in manufacturing for the cable.

 

I agree with Amm0bob here... it's a great idea, but in the end probably not all that doable with today's technology. Think about what they had to go through to keep this satellite up, and it's just skimming through the atmosphere. Now imagine miles and miles of rope being blown around attached to it. Not to mention that the satellite would need to provide enough upward force (a constantly firing engine) to counteract the weight of the rope AND of whatever was riding up it.

 

Well, i certainly learned a thing or two there so if you pull in a weight to counteract the mass moving up the line, you have to use thrusters to send the weight back out when the mass moves down, right?

 

The science fiction of Arthur C Clarke (Fountains of Paradise) specifically dealt with space "elevators." Like a lot of SF from Clarke, it will probably be a reality - eventually

Jerry Pournelle and Larry Niven have had similar ideas. A short story by Hans Moravec (Cable Cars in the Sky) actually had the math worked out, for stable points, needed material strength, etc.

Just as supersonic flight was considered "impossible" until proper materials and engineering could be applied, I have no doubt a synchronous skyhook will eventually be developed

 

No, you don't. Hans Moravec figured out most of the math, actually a Soviet era guy by the name of Y.N. Artsutanov had it figured out, Moravec had the good common sense to actually *read* the original Russian and properly translate it

A lot of good engineering was wrapped up in early science fiction. The classic "20,000 leagues under the Sea" was laughed at when first translated into English. Many errors were made translating French, and also between the metric system and the Imperial system of measurement.

Correctly translated, "20,000 Leagues" is very much good engineering with the math correct

Anyway, a space elevator

The ballast would have to be about 150,000 km from the center of the earth. That is a LONG cable!. Assuming such a cable could be made that is many, many times stronger than carbon fiber, orbital centrifugal force would keep constant tension on the cable through the ballast

It's assumed some sort of solar power system is in use to power the "cab" upwards until it reaches the synchronous point. Beyond the synchronous point, some sort of braking system (Eg, magnetic with feedback to the cable, like recovering energy during Prius braking) is also assumed to be employed.

By playing around with ballasts at the opposite end, it's assumed the entire cab trip could be powered by earth's angular momentum, with only small applications of power to start and then eventually stop the cab.

As the ballast is beyond the synchronous point, just releasing it the ballast will run back out along the cable. Again, with some sort of regenerative braking system (Oddly enough, only briefly covered in the original works), most of this energy is captured for later use

The actual math is a bit scary to those who aren't very good in calculus. To summarize: if the cable had to be made from present high strength steel, it would have to be 10 to the 50th bigger in the middle than at the taper, and it would weigh 10 to the 52nd what it could support

At that point, the original author made a comment about claims from other scientists who claimed the "impossibility" of such a system, using just steel cable. The author pointed out that rocket flight into orbit would likewise be impossible if we depended on coal fired, steam powered rockets, as it would weigh 100 billion times as much as its payload in order to achieve escape velocity

If the cable system were made from kevlar, it would be 10 to the 10th bigger in the middle, and would weigh 10 to the 13th what it could safely carry. Much better than steel, but still clearly impractical

There is much technical discussion of single crystal graphite whiskers, with a taper of only 10:1, and a mass ratio of only 400:1. That already is well within the realm of being doable

One neat thing about the synchronous cab system - if it ever becomes a reality - is that it solves many problems with launching vehicles and payloads out of earth's gravity well. At the very end of the ballast, the orbital velocity is about 10 km/sec, so a payload released from there would easily achieve a Hohmann minimum energy trajectory to reach most points within our own system

 

This is fascinating. Is it too late to be an engineer?

I remember being introduced to the wonderful worlds of Arthur C Clarke as a little kid when my parents took me to see 2001. I had to explain the ending to my Mom, who just didn't get where the floating baby came from. That one afternoon matinee sparked a lifelong interest in science fiction, and I devoured everything I could find. The librarian thought I was such a good kid for bringing home all those books for my Dad, and wondered how he had time to read them all.

The space elevator idea came many books later, but I loved it instantly. The math was beyond me, I'm sure, and it took awhile to realise how something could stay so far up there without moving. And all these decades later, it's still not built! Bucky Fuller was another childhood hero, so it seems highly appropriate to me for his new form of carbon to be the tether. He invented a damn fine map, too, which is inexplicably little-known, but should be the only one used.

Well, enough babbling. Here's another question from a still-boyish imagination that should probably know better, but doesn't. How about using a bicycle to get up and down the cable? They're light weight, very efficient, and the 'passenger' comes with its own energy storage. Sure, the oxygen tanks will be heavy, and the gearing range will be kinda funky to accommodate the varying payload, but....OK, be nice. Once you've stopped laughing at the image of astronauts in tights, you can tell me why it won't work.

 

Well, yeah, it would take a month or two. But other than that, the mechanics might be feasible? Far out. A wing suit could make the trip home a little faster.

 

I think the only issue with bike idea is the distance involved. Plus, at the bottom, where you are still under the sway of earth's gravity, you would be peddling up a verticle hill.

 

The first leg, in the thickness of the atmosphere and the greatest gravity, would be the most difficult. But for most of the way, there'd be zero air friction, which takes about 90% of a cyclist's energy at higher speeds on Earth. The velocities attained could be considerable, even pedalling, and strong braking could well test the limits of the cable. An unintentional 'launch' due to a snapped cable could ruin your whole day.

What power systems and energy sources are being considered now?

 

No, you need a wench

Oh, wait a minute ... wrong thread. Never mind. Oh great, I spilled my stirrers all over the floor ... grumble grumble