Long cables to power plasma rockets to orbit.

[exoscientist]

RichAlex, that superconducting cable you mentioned probably is that weight because of the complicated design of a superconducting cable, as shown in the reference you linked.<br /> Overhead high tension power lines are typically just uninsulated aluminum a few millimeters to centimeters thick and may run for hundreds of kilometers:<br /><br />Electric power transmission.<br />http://en.wikipedia.org/wiki/Electric_power_transmission#AC_power_transmission<br /><br /><br /> Bob Clark <div class="Discussion_UserSignature"> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>RichAlex, that superconducting cable you mentioned probably is that weight because of the complicated design of a superconducting cable, as shown in the reference you linked. <br />Overhead high tension power lines are typically just uninsulated aluminum a few millimeters to centimeters thick and may run for hundreds of kilometers<p><hr /></p></p></blockquote>What is the amperage through those cables? I am certain that it is very, very low (that is the whole point in jacking up the voltage so high). The total power run through the cables is not high enough for our rocket. And, though those cables run for hundreds of kilometers, they are supported every hundred meters or so. <br /><br />Superconducting cable generally is 20% lighter than conventional copper cable rated for the same load, according to several sources. I think the number might be greater, because I saw load densities up to 150x conventional cable. Do not expect a cable that is capable of conducting 40 MW of electricity to be light weight! <br /><br />The bottom line, I think, is the electricity is a really crummy way to transfer large amounts of power. If you want a lot of power, you need chemicals (or nukes), not electricity. The MHD rocket is only going to be useful in space, at least in orbit, not in take-off from Earth. That said, I'm wondering if electrons freed from chemical reactions could be used in place of an external electrical supply? It might be that a practical improvement on launch vehicles could be made by using fancy and clever chemical reactions to augment standard rocket chamber design.

 

[exoscientist]

RichAlex, those high tension wires have been carrying hundreds of megawatts long distances for many decades now. Since the voltages are in the hundreds of kilovolt range that puts the amperage in the 1,000 A range. These are just aluminum wires perhaps an inch thick and less.<br /> At 2700 kg/m^3 density for aluminum, and 3 cm thickness that gives a mass of about .03m x .03m x 1m x 2700 kg/m^3 = 2.43 kg for 1 meter.<br /> You could reduce the thickness to 1 cm and reduce the weight to .243 kg/m reducing the amperage to 100 A, and increase the voltage to 1 megavolt.<br /> There is a long distance power line operating at a megavolt:<br /><br />Powerline Ekibastuz-Kokshetau<br />http://en.wikipedia.org/wiki/Powerline_Ekibastuz-Kokshetau<br /><br /> The reason there is a push to superconducting cable to carry electricity over long distances is to eliminate the power losses that normal conductors have over many kilometers, not because it is lighter weight.<br /> A superconducting cable being lighter by 20% over copper means it is lighter by a factor of .8. However, aluminum already weights less than 1/3 the weight of copper.<br /><br /><br /> Bob Clark<br /> <div class="Discussion_UserSignature"> </div>

 

[ihwip]

There is an idea that nobody suggested yet.<br /><br />Why not create two threads stretching out into orbit like a space elevator but much much thinner and only designed to handle current.<br /><br />Then your ship is launched imbetween these threads powered by energy pumped to it through the threads into connectors and into the plasma rocket.<br /><br />Wouldn't this hybrid space elevator approach be a little more feasible?

 

[MeteorWayne]

Designed only to handle current (and voltage and resistance, see Ohms Law) is exactly what we've been talking about. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[ihwip]

Well yes but the current idea is cables that are connected to the shuttle itself and flexible/mobile.<br /><br />My idea is this:<br /><br />Create two space elevators maybe 100 meters apart. These go off into space 100km and are wide enough to handle the current without evaporating.<br /><br />Then the shuttle is launched imbetween them with wires leading out to each post. I'm thinking of something that looks like O---<O />---O and the current fed to the lines is conducted to the plasma rocket. As the shuttle moves up, its lines would move up each thread to prevent the weight of all that cable.<br /><br />I am trying to describe a system that works more along the lines of those electric race tracks I always got for Christmas as a kid. The cars themselves had no power but the leads on the track conducted to their onboard engines.<br /><br />Its crazy but it just might work!

 

[vogon13]

And it might not.<br /><br />It looks like it might cost a few trillion dollars to find out.<br /><br />Are you going to write out a check for this ??<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[ihwip]

That is what the government is for. Another option would be to construct it sideways so that the rocket takes off horizontally. It would be easier to build and could have power fed from multiple nuke plants on the way around.<br /><br />I am thinking that this project might simply be more difficult and more costly to perform than creating a space elevator.

 

[billslugg]

The diameter of the cable must be on the order of 1 meter. It must be about 5000 miles long to get to orbit. It will weigh about 8 million tons or about 50,000 times more than the payload. It will NOT WORK! GET IT! <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[scottb50]

What might make more sense is a variation of a Space Elevator. A very light continuous fiber to carry the electrical energy while the vehicle engines carry the weight. Zip up the cable and shoot off the end.. <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

A very light cable can not carry the current required. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[scottb50]

Carbon nano-tubes of course. <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

I repeat, light cables can not carry the required current, no matter what they are made of. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[Pooua]

exoscientist, using the numbers you give, I get the following results: <br /><br />1 MV * 100 A = 100 MW <br />100 MW * 22.5 lbs/MW = 22500 lbs = 10227 kg thrust<br />1 ton capsule = 909 kg<br />0.243 kg/m = 243 kg/km <br /><br />10227 kg thrust - 909 kg capsule = 9318 kg<br /><br />Ignoring the weight of the fuel (maybe included in that 1 ton capsule weight) and using all the reserve lifting capacity of the engines running on 100 MW means<br /><br />9318 kg thrust / 243 kg / km weight of cable = 38 km<br /><br />If I did my calculations correctly, and using your numbers, I show that a 1-ton capsule could at most lift 38 km of cable. If the cable were suspended from a balloon 38 km in the air, the cable still would only reach an altitude of 76 km (48 miles). At the peak of its flight, the rocket thrust would equal the weight of the cable, meaning that acceleration would have dropped to 0 by that point, and the rocket would be coasting. <br /><br />We would need to double the power to the engines to double the altitude at least to the edge of space. Of course, that still means the rocket isn't accelerating when it reaches the end of its flight, and it is pulling the weight of a cable that is several times heavier than the capsule. We would probably want at least twice again as much power (now 400 MW) to get at least a bit of acceleration up to the apex of the trajectory. Maybe a bit more for a safety factor. So, at least 500 MW of power down a 1 cm-cross section of cable that weighs about 41 Mg (about 45 tons). Can a 1 cm cable even lift that much weight, particularly while carrying 500 MW of electricity down its length? Somehow, I doubt it.

 

[igorma]

Pay a visit to electrical substation, look at size and weight of equipment there, and stop dreaming of high voltage transmission lines to spaceships. Besides, if you can make cable short enough with supporting posts made of unobtainium, why do you need cable at all? You end up with a rather strange implementation of space elevator.

 

[exoscientist]

Pooua, your calculations are correct except for one thing. The great length of the cable is not supported by the rocket. It is supported by heliums balloons or its own independent thrusters. <br /> It is only a short conducting tether, perhaps 100 meters long, that has to be supported by the rocket.<br /><br /> Bob Clark <div class="Discussion_UserSignature"> </div>

 

[exoscientist]

High tension power lines are at most an inch wide, made of uninsulated aluminum and carry hundreds of megawatts over distances of hundreds of kilometers.<br /><br /><br /> Bob Clark <div class="Discussion_UserSignature"> </div>

 

[billslugg]

That is correct. However, they are at voltages of close to a million volts. Very large separations or very heavy insulation must be used. Also, megawatts is not going to do the job. For a shuttle type launch you need close to a tera watt. <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>

 

[exoscientist]

It's easy to calculate the pure energy budget to get to orbital velocity from the kinetic energy equation: (1/2)*m*v^2. The energy cost to get to high altitude it turns out is only a small percentage of this, which you can calculate from the equation m*g*h.<br /> So for a 100,000 kg body like the shuttle orbiter itself the kinetic energy required is (1/2)*100,000*8,000^2 = 3.2 trillion joules. But this is spread out over the time the vehicle is being powered. If it had a 1,000 second flight time, the power required would be 3.2 gigawatts each second.<br /> Also for most launches such as for satellites you wouldn't need a shuttle sized craft. If you only wanted to launch 1,000 kg, it would take 32 megawatts for 1,000 seconds.<br /><br /><br /> Bob Clark <div class="Discussion_UserSignature"> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>The great length of the cable is not supported by the rocket. It is supported by heliums balloons or its own independent thrusters.<br />It is only a short conducting tether, perhaps 100 meters long, that has to be supported by the rocket.<p><hr /></p></p></blockquote><br /><br />Well, it can't be helium balloons. There is no way to fly helium balloons more than 50 miles altitude. The record for an unmanned balloon is 31 miles altitude. So, something else would have to support the cable the remaining 50+ miles. <br /><br />Rockets would not be too practical, either. I mean, what are you trying to accomplish, anyway? You wouldn't be saving any fuel, or gaining any velocity or lift advantage by having to fly several rockets (in formation, no less) to lift all the cabling. <br /><br />It is fairly obvious from the performance of this type of rocket engine that it is best suited for deep space travel, not for lifting off from a planetary surface. It has a very nice, high speed exhaust, and a bit more thrust than most other electric thrusters. It just cannot compete with old-fashioned chemical rockets in terms of force.

 

[exoscientist]

<blockquote><font class="small">In reply to:</font><hr /><p>Well, it can't be helium balloons. There is no way to fly helium balloons more than 50 miles altitude. The record for an unmanned balloon is 31 miles altitude. So, something else would have to support the cable the remaining 50+ miles.<p><hr /></p></p></blockquote><br /><br /> A key fact about getting to orbit is that it's the orbital velocity that is the hard part. The energy for getting up to 100 km or 200 km or even 400 km altitude is a small part of the kinetic energy needed to get to 8000 m/s. Plug in some numbers in those equations for the kinetic energy and the potential energy for altitude that I gave to see this yourself. The reason I suggested for going to high altitude for this proposal was to reduce the atmospheric pressure/density at the high Mach numbers required for orbit. At 30 km the air pressure is only about 1/100 that at ground level. At 50 km it's only about 1/1000 that at ground level.<br /> Then to get to the higher altitude just point the rocket upwards and give it a small extra velocity push.<br /> In regards to keeping the several kilometers long cable portion static but at high altitude, remember getting to altitude is easy. You could do this for the static cable as slow as you want, such as by helium balloons. Even if you used thrusters since you could make the speed be as small as you wanted this also would be easy to do.<br /><br /><br /> Bob Clark <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

That's an interesting point.<br /><br />Altitude is not what gets you in orbit, horizontal velocity is.<br /><br />So a space elevator gets very squirrely as a mass gains altitude but not horizontal velocity.<br /><br />I haven't seen that issue addressed in any Space Elevator threads AFAIR.<br /><br />Hmmmm. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[richalex]

<blockquote><font class="small">In reply to:</font><hr /><p>So a space elevator gets very squirrely as a mass gains altitude but not horizontal velocity.<br /><br />I haven't seen that issue addressed in any Space Elevator threads AFAIR.<p><hr /></p></p></blockquote>I saw a post a few days ago (maybe an old post) in which someone claimed that the work-around would be to take the would-be satellite up to GEO, then drop it. While it falls, rockets accelerate it into orbit. <br /><br />As for the proposal to launch at high altitude, I think it would be more cost effective just to hoist a conventional rocket on a huge balloon platform than to attempt to string line and power it electrically.

 

[richalex]

exoscientist, what is it that you are trying to accomplish by making this rocket electric? If nothing more than simply being able to say that it can be done, then maybe we could fly a rocket up some distance short of orbit. But, this is far from a practical idea. The cable can't even get it to orbital altitude, and you noted that it would take more energy for the rocket to reach orbital velocity.

 

[kelvinzero]

<font color="yellow">So for a 100,000 kg body like the shuttle orbiter itself the kinetic energy required is (1/2)*100,000*8,000^2 = 3.2 trillion joules. </font><br /><br />This is the energy required if you have something solid like the earth to push against, eg a mass driver.<br /><br />When the mass of the propellent is not hugely more than the craft you have to count the energy that goes into the propellent as well. In the case of ion or plasma propulsion far more energy goes into the propellent than the craft I would guess.<br /><br />Since these wires are going to be so heavy you might as well push against them. In that case what you have is a mass driver with one end hauled up to the upper atmosphere by balloons. Actually it sounds very like a rail gun. Simplistically all you need to do is lay a bar between these two rails and off you go.<br /><br />Someone on this site (probably Docm) mentioned a rocket going very fast at ground level because it was travelling through helium? If so, that gives an interesting possibility: The same helium that hauls up your mass driver could provide a hollow tube that the craft could accelerate through with much more ease than through atmosphere of the same pressure?<br /><br />ah, it was in fact RichAlex