Re: The Dual-stage 4-grid Ion Thruster
- From: meiza<meiza@xxxxxxxx>
- Date: Fri, 9 Dec 2005 21:00:53 +0200 (EET)
"frédéric haessig" <fhaessig@xxxxxxx> wrote:
> "meiza" <meiza@xxxxxxxx> a écrit dans le message de news:
> dnats5$pbb$1@xxxxxxxxxxxxxxxx
>> Rémy MERCIER <Rmy.MERCIER.1zqsxn@xxxxxxxxxxxxxxx> wrote:
>>
>>> with isp = 19200s
>> Huh, quite a great isp, but where do you need such a thing?
> Several possible reasons.
> 1) lower consumable mass. This means higher useful payload ( for a given
> launch vehicule )
As I explained, propellant mass ratios get ridiculously low but
the *power source mass* grows by the specific impulse.
(if you keep same thrust.)
Even with isp as low as 3100-3500 s (deep space 1), and a mass ratio
of 1.2, you get over 5 km/s of delta v. That is for example
a 1000 kg dry mass probe with 200 kg of propellant.
Deep Space 1 had total mass of 500 kg, of which used
fuel was 74 kg. (ratio 1.15) It gained 4.3 km/s of
speed when thrusting for over 400 days. The engine
used 2.1 kW of power.
Let's take that probe and do nothing but increase isp
5-fold to 17500.
Use basic Newtonian equations:
P = 0.5*(-dm/dt)*v_ex^2 , power
F = (-dm/dt)*v_ex , thrust
Since power stays the same, we can solve
mass flow:
-dm/dt=2*P/v_ex^2
and thus the thrust is:
F = 2*P/v_ex
So the thrust drops to one fifth.
When the original deep space 1 had thrusted for 400
days and gained 4.3 km/s of speed, our new probe
would have gained less than 1 km/s.
Only after 2000 days (5.5 years), we would be going
that speed. Sure, we would have used fuel only about 10 kg,
but whether it's 10 or 70 kg, that is only a vanishingly small
mass of the whole 500 kg probe.
Our new probe would eventually reach a 20 km/s delta v,
but it would take still 4 times more time, so it would
last in total close to 22 years to reach full speed.
So, one can say, let us increase power so that the high-isp
design can accelerate as fast as the original. But this
asks for 5X the solar cells. The original had two 5m long
"wings". Even if we saved 60 kg in
the form of fuel, it is not enough to make
four more of those for 8 kilowatts. It's hard to get
good figures for achievable solar cell power density, but
one quoted number is 10 kg/kW. This would make the
new cells weigh 80 kg. I don't know about the power
electronics.
> 2) Continuous thrust transfer. Hohman transfer ( aka minimum energy
> transfer ) take a lot of time when you go to target far away. Continuous
> thrusting reduces this by a big factor.
I don't know about this but are you sure you can get to a target
faster by thrusting low for one year compared to getting the same delta-v
in one hour? Or one month? Seems counter-intuitive.
> 3) In-space maneuvering. Either attitude correction or orbit changes. When
> you want to visit several celestial objects with the same mission ( like a
> planet and several of its moons ) or change orbit around a single object.
Deep Space 1 had hydrazine for attitude control (actually running out of that
forced mission end), as did Smart-1.
Ion engines are probably not very good for that (at least directly)... the
turning rate gets very slow, and if your solar arrays point in the wrong
direction and you'd need an attitude correction, you can't use the ion engine
because, uh, your solar arrays are pointing in the wrong direction. :)
If you want to visit several moons in one mission, you're talking Jupiter or
beyond, and then you don't do much with solar cells and have to use lower
power per kg nuclear energy. If you need more delta v, it makes less mass to
put in more fuel than to put more nuclear power and increase isp. Europe
so far hasn't invested in nuclear space power sources.
> The usual problem with ION thruster is not Too high an Isp ( as if there was
> such a thing ) but too low a thrust level level for some applications. It
> seems technology is slowly solving this.
The low thrust is precisely because of high power needed which is precisely
because of the high isp. This is Newtonian physics and it can't be solved any
other way than by higher mass efficiency power sources.
Higher isp in ion engines automatically means less thrust for same power.
There is no way around that.
Why bother optimising for fuel efficiency if your already oversize motor
is weighing more than the fuel tank. Only if you're going for a *really*
long trip so that the fuel tank actually starts to matter.
So that's why I was asking. High (>3000) isp is not useful unless you start
talking about much more than 5 km/s deltavees or alternatively really
low mass power sources.
I think Beppi-Colombo to Mercury will use Hall effect thrusters with
about 1500 isp, like Smart-1.
Does anyone have links or references to ion engine power system masses?
(Solar cells, electronics, thrusters.) It's really hard to find anything
in the net.
--
-meiza
.
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