# Re: LUNA: Re: indigenous fuels

Doug Jones (random@qnet.com)
Thu, 22 Jul 1999 17:16:25 -0700

Eugene Leitl wrote:
>
> Doug Jones writes:
> > most critical design driver- solid state relays of the speed and
> > power needed aren't cheap. I can dig up detail information if
>
> How much power can we switch with a triggered arc discharge?

I've spent an hour or so trying to track down sources without much luck, but from what I recall from discussions on sci.space.tech the problem is turning *off* the current. V=Ldi/dt, and a large negative di/dt is a bitch to terminate- like opening a circuit breaker carrying a large DC current. Big sizzling arcs... A big inductor with a large current flowing through it (ie lots of stored energy) tries to be an ideal current source.

The ideal current profile in a stator coil is a square wave, going high when the bucket is about one coil diameter away, then dropping to zero as the bucket reaches the plane of the coil. This gives the bucket a good pull, and doesn't need maglev guideways. More realistically, you might get a roughly linear ramp up & ramp down with a Bi-FET driver switch and bridge diodes- the switch closes when the bucket is one diameter away, opens at 1/2 diameter, and the diodes divert the negative spike back into the supply capacitor. The cap driving each stage is oversized so that it approximates a constant voltage supply. When the coil current goes to zero, the diodes stop conducting, and the switch never sees more than 2x the drive voltage.

Coils at the high speed end would still need to have fewer turns and higher currents, to dump the stored charge faster- the total energy delivered to the bucket is the same for each coil (if acceleration is constant) but the time available for the current pulse goes as 1/v.

> > anyone is really truly interested.
>
> If this is not too much trouble on your part, please do.

Maybe after tomorrow morning... Rotary is supposed to fly the ATV in a hover test, then safe the system, put it away, close the doors, and send home everybody that's left. It's sad to see it go out with a whimper.

```--
Doug Jones, Freelance Rocket Plumber
Formerly with Rotary Rocket
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