RE: starship-design: Plasma Engine (VASIMR)

["L. Parker" <lparker@cacaphony.net>]

Kevin,

> Okay, I vote we accept VASIMR as our interplanetary vehicle engine of
> choice.  The design seems advanced enough to suggest that we will have
> off-the-shelf versions by 2050.  And it is powerful enough to get us
> around a star system.  If the above delta V numbers are accurate, then
> there is enough thrust to send 20000 Kg (including engine weight and
> fuel, of course) at 10 m/s^2 as long as the Reaction Mass holds out.

The stated delta v's assume particular payload and mass ratios which include
the fuel. Simply adding engines does not give an equivalent increase in
delta v. Typical payload ratio are 0.2 which means each engine (with no
increase in payload) would only add 40 km/sec.

>
> I wonder if a cluster of such engines would work as an interstellar
> drive?  As I've noted before, constant 1G acceleration/deacceleration
> applied over the course of the entire trip would build up a huge time
> dilation effect as even a relatively short range trip to tau ceti (12
> LY) would get up to .9925 C

No, see above.

>
> As i am writing this , another message from Lee says.
>
> > Correction, AIMSTAR is claiming delta v in excess of 900 km/sec, making
it
> > almost four times better than the nearest plasma engine.
> >
>
> Allows 90000 Kg masss to be driven at 10 m/s^2
>
> > For interstellar use, we would need to boost this to at least 90,000
> > km/sec....
>
>
> So would a cluster of 100 AIMSTAR drives give us the thrust we would
> need?  Or have I misunderstood the math?

No, they won't. Too bad.

Power isn't really the problem. It is a combination of power (as thrust) and
ISP. Current engines with sufficiently high ISPs don't have sufficient
thrust and vice versa. You sort of put your finger on it with the constant 1
g acceleration thing. It isn't really necessary to accelerate at 1 g for the
whole trip - just until we have achieved a sufficiently high cruising
velocity to get there in a reasonable time period. To me that means 0.9 c or
better, other people on the group will settle for as little as 0.3 c.

Lee