[ExI] specific thrust

[David]

On Fri, 10 Dec 2010 23:39:19 -0800
Adrian Tymes <atymes at gmail.com> wrote:

> On Fri, Dec 10, 2010 at 11:10 PM, spike <spike66 at att.net> wrote:
> > Ja, ion engines are only good for enormous specific thrust.
> 
> Why is this?
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E = 0.5* M * V * V

Enormous specific thrust means very high exhaust velocity.  That means
large amounts of energy per unit mass of exhaust.  To go from an
exhaust velocity of 3km/s (good chemical rocket) to 30km/s (low end ion
engine) increases your energy requirements by a factor of 100, while
only increasing your thrust by a factor of 10.

You can't do it with chemical based power.  The energy simply isn't
there. You need either nuclear, solar, or ground-based beamed power.

To get above 1G on an ion rocket would require a staggering amount of
power.

BOEC :
Assume:
	100 ton liftoff mass
	2G acceleration
	30 km/s exhaust
Leads to: 
	65 kg/s exhaust mass
	300 Gigawatts energy requirement

It gets worse as your ion drive gets better.

-David.