Re: The Dual-stage 4-grid Ion Thruster

Rémy MERCIER <Rmy.MERCIER.1zqsxn@xxxxxxxxxxxxxxx> wrote:

> This is a new esa' ion thruster, THE 'DS4G' THRUSTER,
> with isp = 19200s
> nice!
> http://www.esa.int/gsp/ACT/propulsion/safe_test_diaries_wk1.htm

Huh, quite a great isp, but where do you need such a thing?

If a vehicle had just one third of it's mass fuel,
this engine could accelerate it to 80 km/s! (In space of
course, and it would take a lot of time.)
A cheat **** tells the trip to Mars from LEO needs less
than 5 km/s.

But the power consumption must be big compared to thrust and
total impulse too. The higher the exhaust velocity, the more
energy you actually give to the expelled fuel instead of the
vehicle. So the crafts with these motors would need to have
either big solar cells (makes them heavy, reduces mass
fraction and acceleration), or then really low thrust
requirements (i.e. these would be used on stationkeeping).

Has someone calculated optimal isp:s for certain trips
and ion engines / hall thrusters?

Thrust is proportional to exhaust velocity but power
is proportional to exhaust velocity squared.
There's no point in having a huge isp craft
if it reaches it's good velocity only after 10
years of thrusting, when the other one with
worse isp would already be at the destination.


I guess it boils down to the rocket equation.
Deltav is proportional to both exhaust velocity
and the logarithm of mass ratio.

With low mass ratios (only a bit above 1), the
derivative of the logarithm is close to 1, and
it's easier to grow that than isp.
Acceleration drops then with increasing fuel mass.

If instead isp (v_ex) was grown, and the thrust
kept at constant, the power would grow to the
power of two - the solar arrays balloon faster
than the fuel tank. So acceleration would
drop faster.

Only at high mass ratios does the logarithmic
behaviour begin to show and you hit diminishing
returns. Only then it's wiser to increase isp.

But with this high isp (20 000) and for example a
mass ratio of 2.7 (1 part rocket, 1.7 part fuel),
you'd get the 200 km/s delta v. Where do you need
that?
And how long would even a no-payload thingy
take to accelerate to that speed?

I guess for a trip to Jupiter or further you'd need
nuclear energy, which isn't very power-dense, so you're
better off just having a worse isp and more fuel. (I'd
guess this gives smaller total mass at same triptime.)

Not even trips to asteroids or comets and back, all close
to Sun...

So the main problem with ion engines to me seems to be
*too high* isp!

*) handy "cheat ***":
http://www.pma.caltech.edu/~chirata/deltav.html
.

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