http://en.wikipedia.org/wiki/Tether_propulsion#Tether_cable_catapult_system

It's an idea by the late Dr. Forward- a mass driver where the mass
driver coils pull themselves down the length of a conductive cable
that powers it, and then before they reach the end the coils slow and
stop themselves and the payload carries on.

Payload could be quite heavy, and g's are low, so payload could be human.

Top speed > 30 km/s (enough to get to Mars or Jupiter- if you can work
out how to stop!)

--
-Ian Woollard

"I think we all would have been a lot happier if they hadn't landed a
man on the moon. Then we'd go: 'They can't make a prescription bottle
top that's easy to open? I'm not surprised they couldn't land a man on
the moon. Things make perfect sense to me now.' "

> From: Ian Woollard
>
http://en.wikipedia.org/wiki/Tether_propulsion#Tether_cable_catapult_system

How does a calculated top speed of thirty times the speed of sound translate
to 30kmps? How much engineering will be required to hold the cable rigid?
Would it work inside out, if the electromagnets were inside a pipe? At 1G,
the cable would have to be 45,000km long to achieve a final velocity of
30kmps. At 2G acceleration upwards that would fit on a space elevator. But
I'm not sure how rigid the elevator woudl be under such loads, now how
magnetic it is possible to make the tether. With current or near future
technology, any 1G engine is going to be very heavy.

John

On 01/08/07, ANTIcarrot wrote:
> How does a calculated top speed of thirty times the speed of sound translate
> to 30kmps?

30 times the speed of sound in *metal*- metals have a *much* higher
speed of sound.

> How much engineering will be required to hold the cable rigid?

You need a *big* counterweight at one end, and some way of tugging the
other. Not too hard in zero-g, or using tidal forces I think. Also you
would want some dampers along the length of the cable.

> Would it work inside out, if the electromagnets were inside a pipe?

Probably.

> At 1G,
> the cable would have to be 45,000km long to achieve a final velocity of
> 30kmps. At 2G acceleration upwards that would fit on a space elevator.

No, it wouldn't, the cable would be way to heavy; its a highly
conductive non self-supporting cable intended for low-g only.

I reckon you could pull 3g though with humans. It would take 20
minutes, but a fit person could handle that pretty well. 15000km
cable. The cable length goes as the square of the final speed. To get
to/from Mars orbit you probably only need a few tens of kilometers.

> With current or near future
> technology, any 1G engine is going to be very heavy.

No, not at all. It's just a glorified electric motor. Think Tesla
roadster without the heavy batteries.

> John

--
-Ian Woollard

"I think we all would have been a lot happier if they hadn't landed a
man on the moon. Then we'd go: 'They can't make a prescription bottle
top that's easy to open? I'm not surprised they couldn't land a man on
the moon. Things make perfect sense to me now.' "

From: Ian Woollard

> It's an idea by the late Dr. Forward- a mass driver where the mass
> driver coils pull themselves down the length of a conductive cable
> that powers it,

It's the mass-driver-like coils pulling themselves along the cable part
that I don't get. What would there be, magnetically speaking, for the
coils to grab onto on a continuous, unvarying conductive cable?

Regards,

Mike Combs

On Aug 01, 2007, at 11:11 UTC, ANTIcarrot wrote:

> How does a calculated top speed of thirty times the speed of sound
> translate to 30kmps?

It's 30 times the speed of sound *in the cable*, which I guess must be
1 km/s.

> At 1G, the cable would have to be 45,000km long to achieve a final
> velocity of 30kmps.

Yeah, that was my concern with the proposal of launching humans on it,
too.

Cheers,
- Joe

Joe Strout -- joe@...
Strout Custom Solutions, LLC

On 01/08/07, Combs, Mike wrote:
> It's the mass-driver-like coils pulling themselves along the cable part
> that I don't get. What would there be, magnetically speaking, for the
> coils to grab onto on a continuous, unvarying conductive cable?

Eddy currents. You drag a magnetic field backwards down the cable. The
cable generates eddy currents that oppose the motion of the field.
It's like the magnetic brake some adventure rides use, except the
magnetic field moves.

I'm not sure *exactly* what the cable would optimally look like, it
might need to be quite thick or it might consist of a ladder shape to
give the eddy currents some area and to generate enough thrust to give
good acceleration and minimise the power and cable length.

> Regards,
>
> Mike Combs

--
-Ian Woollard

"I think we all would have been a lot happier if they hadn't landed a
man on the moon. Then we'd go: 'They can't make a prescription bottle
top that's easy to open? I'm not surprised they couldn't land a man on
the moon. Things make perfect sense to me now.' "

On 01/08/07, joe@... wrote:
> On Aug 01, 2007, at 11:11 UTC, ANTIcarrot wrote:
>
> > How does a calculated top speed of thirty times the speed of sound
> > translate to 30kmps?
>
> It's 30 times the speed of sound *in the cable*, which I guess must be
> 1 km/s.

If you use steel, speed of sound in steel is ~5 km/s.

> Cheers,
> - Joe

--
-Ian Woollard

"I think we all would have been a lot happier if they hadn't landed a
man on the moon. Then we'd go: 'They can't make a prescription bottle
top that's easy to open? I'm not surprised they couldn't land a man on
the moon. Things make perfect sense to me now.' "