Double-Layer Ion Thruster

Australian engineers have just announced what they claim is a much
more efficient ion thruster:

http://www.e4engineering.com/item.as...ews&ch=e4_home

It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for launching
to orbit. By a factor of how much are ion thrusters too weak for
achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

Am 11 Jul 2003 13:00:41 -0700 schrieb "sanman":

It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for launching
to orbit. By a factor of how much are ion thrusters too weak for
achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

They are strong enough to be used to accelerate masses - even to solar
system escape velocity (and by principle even up to relativistic
speeds) - but they are not usable for lift purpose, because their
mass-to-thrust ratio is too bad. Their strength is their durability.

I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

I would say No. But never say never :-)


cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker)
--
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sanman wrote:
Australian engineers have just announced what they claim is a much
more efficient ion thruster:

http://www.e4engineering.com/item.as...ews&ch=e4_home

It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for launching
to orbit. By a factor of how much are ion thrusters too weak for
achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

Efficiancy isn't the problem.
Physics is.
Current ion thrusters are well over 10% efficiant, (I think it's
around 70%) in converting electrical energy to kinetic energy of
the exhaust.

The problem is that for an ISP of 5000 seconds, you've got to
provide at least 2.5 billion joules per kilo of propellant.

A large power station provides about 2.5 gigawatts, and will provide
enough power to run about a kilo a second through your thruster,
for a thrust of about 5000Kg.
Most power stations weigh considerably more than this.

Compare this with typical chemical systems which only need around
25 million or so, to get an ISP of 500.

Now, add the huge amount of electrical circuitry, generators, ...
You might be able to do SSTO from eros, but nothing much bigger.


I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

No.
Electricity generation plant is HEAVY.


--
http://inquisitor.i.am/ | | Ian Stirling.
---------------------------+-------------------------+--------------------------
What a wonderfull world it is that has girls in it! -- Robert A Heinlein.

(sanman) wrote in message
. com...
It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for
launching to orbit. By a factor of how much are ion thrusters too
weak for achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

Saying that ion thrusters are too weak for achieving escape velocity
is not really accurate, since once in space they are really good
at getting high velocities. The problem is just getting off Earth.

The Russian SPT-290 has 1.5 Newtons of thrust and mass of 23 Kg.
With F=MA and A=9.8 m/s^2 it would take 23*9.8 = 225.4 Newtons
to lift off. So we need about 150 times more thrust, not
counting power supply. It takes 30 kw to run this thruster.
It would be really amazing if you could get a 30 kw power source
down to 23 Kg, so that the total mass was 46 Kg. But even in
this case we would need 300 times more thrust. But suppose
that our thruster magically could do 300 times more thrust and
still weigh only 23 Kg, we would still need 300 times more power.
That would mean getting 9 megawatts in 23 Kg. This is just
not going to happen anytime soon. :-) So high ISP thrusters
(like ion drives and Hall Thrusters) are not going to help
with SSTO. It just takes too much power.

High ISP thrusters could help with getting to orbit by reboosting
a tether which lifts a single stage to tether rocket (SSTT).
The mass of the solar power and thrusters actually helps the
tether store momentum/energy, so here the high mass/thrust
ratio does not bother you. You want a bunch of mass anyway.
The SSTT would only need to get to like 5 km/sec. It is far
easier to make a reusable rocket that only goes to 5 km/sec because
of the needed delta-V and the lower reentry heat problem.
This seems a very workable idea. We at spacetethers.com are
working on it.

-- Vince

(sanman) wrote in message . com...
Australian engineers have just announced what they claim is a much
more efficient ion thruster:

http://www.e4engineering.com/item.as...ews&ch=e4_home

It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for launching
to orbit. By a factor of how much are ion thrusters too weak for
achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

I seem to recall that excluding fuel, these drives produce 100-1000
times less thrust than the force exerted by gravity on the drive
itself at the Earth's surface. Hall effect thrusters have more thrust,
but I don't think it's more than a factor of five or ten improvement.
Many current drives are intended to operate continuous for months or
years at a time, or for station keeping on satellites that stay in
orbit for years (ie, need an occasional boost to stay in position).

Another significant problem is that ion drives and hall effect
thrusters can only work in near vacuums. I just don't see this as a
technology that can work on Earth.


Karl Hallowell

"sanman" wrote:
Australian engineers have just announced what they claim is a much
more efficient ion thruster:

http://www.e4engineering.com/item.as...ews&ch=e4_home

It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for launching
to orbit. By a factor of how much are ion thrusters too weak for
achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

I think you're somewhat confused. Escape velocity is not
what's important, and the high Isp of ion engines makes it
very easy for them to achieve delta Vs as high or much
higher than escape velocity. What's important for being
able to launch is thrust, and especially thrust to weight
ratio. If you don't have sufficient thrust to counteract
the force of gravity on the launch vehicle plus a little
bit more to get it off the ground, then your rocket won't be
able to get off the pad. Then you need to factor in the
other aspects such as Isp and the delta V needed to get
into orbit and launch profiles and stuff like that, but
that gets a lot more complicated. Suffice it to say that
electric rockets don't have near high enough thrust to
weight ratios to be suitable as first-stage propulsion on
a launch vehicle, or probably on any stage except higher
stages for use after the payload is in orbit. Chemical
rocket engines have thrust to weight ratios of several
hundred thousand times greater than ion engines.

(Vincent Cate) writes:

(sanman) wrote in message
. com...
It's been pointed out that ion thrusters are more efficient on Isp,
but they produce less thrust -- too little to be of use for
launching to orbit. By a factor of how much are ion thrusters too
weak for achieving escape velocity?
By how many times would the above system have to be improved to be
used for SSTO?

I presume that nuclear power can be used to augment the
thrust/efficiency of electric propulsion. Would it be enough?

Saying that ion thrusters are too weak for achieving escape velocity
is not really accurate, since once in space they are really good
at getting high velocities. The problem is just getting off Earth.

That's not entirely true. Even if one is starting in orbit, acceleration
(and therefore thrust) is still important in that if the vehicle produces
significantly less than the local gravitational acceleration, one cannot
get up to escape velocity in less than a single orbital period, which means
that a low-thrust vehicle must slowly "spiral" up out of the gravity well.
(An early nuclear-powered ion-drive proposal would have spent more time
spiraling out of the Earth's gravity-well and spiralling down into Mars's
gravity-well than it spent in transit between Earth and Mars...)
This "spiralling" out of the Earth's gravity-well is a Very Bad Thing,
not only because it incurs huge gravity-losses and wastes time in this
process, but even worse, it spends a long time "soaking" in van Allen belt
radiation, which is not good for either its passengers or its electronics...
(One of the proposed advantages of VASIMR is that it can run in a high-
thrust mode for quick escape-burns from gravity wells, then shift to
a more economical low-thrust, high-ISP mode for the "cruise phase"...)


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

(Karl Hallowell) wrote in message . com...
I seem to recall that excluding fuel, these drives produce 100-1000
times less thrust than the force exerted by gravity on the drive
itself at the Earth's surface. Hall effect thrusters have more thrust,
but I don't think it's more than a factor of five or ten improvement.
Many current drives are intended to operate continuous for months or
years at a time, or for station keeping on satellites that stay in
orbit for years (ie, need an occasional boost to stay in position).

Another significant problem is that ion drives and hall effect
thrusters can only work in near vacuums. I just don't see this as a
technology that can work on Earth.


Karl Hallowell


Alright, what about ion thrust for a craft shuttling between the lunar
surface and lunar orbit? That's effectively vacuum conditions, and the
gravity is less.
I mentioned the fullerene fuel, with its higher molecular weight,
being able to proportionally increase the thrust. I'd imagine that
buckyonions, being concentrically layered buckyballs, would have even
higher molecular weights. At least a fuel-economizing thruster would
help reduce the damage to the fragile lunar vacuum whose preservation
may be desirable for industrial manufacturing purposes.

(sanman) writes:

(Karl Hallowell) wrote in message . com...
I seem to recall that excluding fuel, these drives produce 100-1000
times less thrust than the force exerted by gravity on the drive
itself at the Earth's surface. Hall effect thrusters have more thrust,
but I don't think it's more than a factor of five or ten improvement.
Many current drives are intended to operate continuous for months or
years at a time, or for station keeping on satellites that stay in
orbit for years (ie, need an occasional boost to stay in position).

Another significant problem is that ion drives and hall effect
thrusters can only work in near vacuums. I just don't see this as a
technology that can work on Earth.

Alright, what about ion thrust for a craft shuttling between the lunar
surface and lunar orbit? That's effectively vacuum conditions, and the
gravity is less.

Still too wimpy. Typical ion drive thrust-too-mass ratios limit ion-drive
spacecraft to accelerations of less than a milligee; hence, at most you can
lift off smallish asteroids in an ion-drive spacecraft. To lift off the Moon
in an electrically-propelled spacecraft, you need somthing with a much higher
thrust-to-mass ratio --- say a plasma drive, or at least a resistojet...


I mentioned the fullerene fuel, with its higher molecular weight,
being able to proportionally increase the thrust. I'd imagine that
buckyonions, being concentrically layered buckyballs, would have even
higher molecular weights.

....However, unless they all have =EXACTLY= the same atomic weight, and can
be given =EXACTLY= the same charge (or at least, that they all have exactly
the same charg-to-mass ratio) they will be very poor propellants for an ion
drive, since ion drives are very picky that way...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

sanman wrote:

(Gordon D. Pusch) wrote in message ...
Still too wimpy. Typical ion drive thrust-too-mass ratios limit ion-drive
spacecraft to accelerations of less than a milligee; hence, at most you can
lift off smallish asteroids in an ion-drive spacecraft. To lift off the Moon
in an electrically-propelled spacecraft, you need somthing with a much higher
thrust-to-mass ratio --- say a plasma drive, or at least a resistojet...


I mentioned the fullerene fuel, with its higher molecular weight,
being able to proportionally increase the thrust. I'd imagine that
buckyonions, being concentrically layered buckyballs, would have even
higher molecular weights.

...However, unless they all have =EXACTLY= the same atomic weight, and can
be given =EXACTLY= the same charge (or at least, that they all have exactly
the same charg-to-mass ratio) they will be very poor propellants for an ion
drive, since ion drives are very picky that way...


-- Gordon D. Pusch

Hmm, I remember reading about how VASIMR engines are supposed to use
microwaves to heat the plasma, and how they can produce more thrust. I
never realized that they can also vary the specific impulse and thrust
(I should have looked more closely at the acronym!)

Regarding the buckyballs/buckyonions, though -- wouldn't these also be
a more useful propellant for a VASIMR rocket, since they offer a
better, more uniform cross-sectional area for the microwaves to hit
and ionize? Any opinions on that?

Also consider that nanotubes have recently been shown to be very
efficient for generating microwaves and terahertz radiation. Would
these make a non-nuclear VASIMR rocket feasible? Or do you absolutely
need nuclear-scale energy to power it?

Use of carbon nanotubes/buckyballs as supplemental mass in the thrust of
a VASIMR engine is in the realm of possibility, but then that begs the
question, why the hell didn't you just go with simple graphite dust? One
of the big issues with VASIMR is that it needs significant cooling of
its components to operate reasonably. That is why liquid hydrogen is
used as both coolant and fuel. The only other reasonable cryogen is
neon, but that has somewhat unfavorable ionization properties despite
the improvements in mass and handling it would bring.

As an aside, there has been some interesting discussion on the use of
beamed power to provide the energy for VASIMR, either through microwaves
or laser from a series of ground or orbital power stations. Regardless,
the electrical consumption of VASIMR is horrendous, but that is
typically the price you pay for high efficiency propulsion.

JunkBoy