Re: NASA's Tethered Generator

To: tesla-at-poodle.pupman-dot-com
Subject: Re: NASA's Tethered Generator
From: Tesla List <tesla-at-stic-dot-net>
Date: Sat, 10 May 1997 23:52:17 -0500
Approved: tesla-at-stic-dot-net
Subject:      Re: NASA's Tethered Generator
      Date:   Sat, 10 May 1997 19:12:56 GMT
      From:   jim.fosse-at-bjt-dot-net (Jim Fosse)
        To:   Tesla List <tesla-at-pupman-dot-com>
References: 
           1


On Fri, 09 May 1997 01:15:33 -0500, Tesla List <tesla-at-pupman-dot-com>, you
wrote:

>Subject:  Re: NASA's Tethered Generator
>  Date:  Fri, 9 May 1997 00:00:59 +0000
>  From:  "John H. Couture" <couturejh-at-worldnet.att-dot-net>
>    To:  Tesla List <tesla-at-pupman-dot-com>
>
>
[snip]
>
>It appears that NASA was apparently misguided by people who had never
>heard of Nikola Tesla. I understand that the necessary high voltage is
>obtained by electronics using an electron gun. This device has limited
> voltage and power capability. A Tesla coil can do much better. The 
>problem is that the Tesla coil must be redesigned to be feasible for 
>this application, a challange for future Tesla coilers.
>
>  Some day NASA might fund this research. Nikola Tesla would then be
>vindicated regarding his world electrical system concepts.
>
>  For those interested in this subject try 
>
>            http://liftoff.msfc.nasa.gov/tss1r/
>

John,
        Thanks for the URL. 

All,

Part way through reading this reference, I was struck by the horrible
thought - they are building a hugh DC excited TC with a linear
secondary instead of a coiled secondary! No wonder the last tether
smoked itself, the current in the tether went to I * Q ! 

Last year, Richard Hull posted about one of his experiments using a HV
DC feed straight into the base of a TC secondary. On the toroid, he
placed a needle point to bleed off the DC. This "gas discharge"
exhibited negative resistance (normal for any gas discharge,
classically shown using a NE2 lamp) which caused the TC secondary to
oscillate at it's resonant frequency given only a DC feed! (Richard,
please feel free to correct any errors I stated about your experiment)

The tether experiment will use an electron gun to bleed off excess
charge from the shuttle into the surrounding plasma. If any negative
resistance IV characteristics are display by this discharge, the
entire tether would ring at it's 1/4 wave frequency. 20.7km =>
3.62kHz.

On page 13 (ish) of the tether article I noted the characteristics of
the tether. The DC resistance, 0.12 ohms per meter struck me as high,
but I skipped over it for a while.

I then closed netscape an continued reading mail from the tesla list.
The next post JUMPED out at me, given the absurdly high R of the
tether wire.

------include from Malcolm's post on Q----------

FYI,
     Here is the mathematical derivation for the critical value for
Q in a "resonant" system - you will see why I have highlighted
resonant in a moment. We start by saying we want to find the value of 
Q at which the frequency of a system has dropped to 0 Hz. I start 
with an electrical system since I am familiar with the necessary 
equations. However, this is true for all systems which can store 
energy.

[snip]
[Now introduce Q into the equation using the identity:
Q = SQRT(L/C)/R (this is easily derived using Q = wL/R = 1/wCR)
Squaring both sides and multiplying each side by R^2 gives:
Q^2 x R^2 = L/C]

Substituting for L/C in our original equation gives: Q^2 x R^2 = R^2/4

Dividing each side by R^2 gives Q^2 = 1/4    which => Q = 1/2 = 0.5
(negative square root discarded).

This is the value of Q, at and below which the circuit can no longer 
ring. In other words, a circuit with this value of Q loses energy at 
the same rate as it can take it up.

Think that's all OK but comments welcomed as always,
Malcolm
------------- end include -----------------------

I went back and calculated the inductance of this 20.7km wire.

        L = 2 * 10^-4 * [ ln(2 * l/r) - 3/4] in microhenries.

        l = 20.7km = 20,700,000 mm
        r = 1 mm (i guessed on the conductor's radius)

        L=> 69.5 mH.


        R = 20,700 m * 0.12 ohms/meter = 2.48 kohms.

therefor:
        Q = wL/R = 2 * pi * 3.63kHz * 69.5mH / 2.48 kohms = .639

Bingo! given the correct diameter of the wire, I'd bet that the Q of
the system HAS BEEN set to 0.5!

Nasa has decided not to let this tether experiment turn into a giant
shock excited oscillator.

I have not seen the specs for the first tether, but I'll speculate
that the resistance of it was low.


        jim