Index Home About Blog
From: (Henry Spencer)
Subject: Re: Tsiokovskii - Rocket Equation
Date: Wed, 8 Nov 1995 00:43:50 GMT
Organization: U of Toronto Zoology
Lines: 31

In article <47h6fj$> (Lamont Granquist) writes:
>What is the story on Ion drives for interstellar voyages?  They've got
>higher exhaust velocities, but you need the added mass of your
>energy source, which complicates the calculations slightly.  How do
>figure out the ratio between the mass expelled and the mass required
>for your energy?

A given power-source technology typically has a fairly well-defined W/kg
figure of merit.  Given that, you can compute the optimum exhaust velocity
for a given mission.  Since higher exhaust velocities use less fuel but 
need more power, for a given delta-V there is an optimum exhaust velocity 
which minimizes total initial mass, and it generally *isn't* "as high as 

Ion rockets are pretty much non-starters for interstellar work, by the
way.  The power sources are too heavy.

In a very rough sort of way, you can characterize Earth-to-orbit launch
as dominated by thrust, solar-system travel as dominated by exhaust
velocity, and starflight as dominated by energy.  (This *is* rather
an oversimplification.)  Interstellar flight in reasonable amounts of
time requires such horrendous amounts of energy that the biggest
problem is energy storage and handling.  Electric rockets simply do
not work for interstellar flight because the equipment needed to
generate and handle huge amounts of electrical energy is so heavy.
For this application, you have to go back to thermal rockets -- which
means high-performance nuclear ones at a minimum -- to avoid having
to do energy conversions.
The problem is, every time something goes wrong,   |       Henry Spencer
the paperwork is found in order... -Walker on NASA |

From: Henry Spencer <>
Subject: Re: Opinions Please: Ion drives and Solar Sails
Date: Sun, 24 Dec 1995 03:28:33 GMT
Organization: SP Systems, Toronto

In article <4b8ubi$> (Edward K. Gilding) writes:
>existance?????? ...

Depends on what you mean by "real working ion drive".  If you mean a
*complete* *system*, space-rated and ready to fly, power source plus power
conditioning plus ion thruster(s)... no, not yet, although the first ones
will fly on Hughes comsats (for low-fuel-consumption stationkeeping)
within a year or two.  If you mean just the ion thrusters, yes, working
ion thrusters have been in research labs (and, once or twice, in space)
since the 1960s.

>...I know that electrons can be used to 
>push something forward on earth, although it is weak.  Does the concept 
>work better or worse in the vaccuum of space?

Electrons are pretty useless for propulsion, they are too light.  Heavy
ions are what you want.  The concept is really workable only in vacuum.
Thrusts are low and power consumption high, but fuel consumption is very
low, so for certain purposes they look good.

>...What would its effecientcy be...

Depends on details of the design.  It can be fairly high.

>Now on to the solar sails.  How far from the sun could one get in a 
>reasonably sized solar sail before you get too far from the sun...

Too far from the Sun *for what*?  It's quite possible to use a solar
sail to achieve solar escape velocity.  On the other hand, it is also
true that solar-sail maneuvering -- never exactly rapid -- is so slow
as to be essentially useless once you're out beyond, oh, say, the
asteroid belt.

>...Is it really just tin foil?

It's the thinnest reflecting surface you can make and handle, basically.
Aluminum can be very thin and still reflective; the limits are currently
set by practical considerations.  Today's sail designs typically use a
thin sheet of Kapton plastic with a very thin aluminum layer on the
surface.  Given in-space assembly and some other complications, it may
eventually be possible to dispense with the Kapton and just use aluminum
30nm or so thick.  Ordinary aluminum foil is far too thick (and hence
heavy) to be useful.
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |

From: (Henry Spencer)
Subject: Re: Ion Engine
Date: Thu, 29 Oct 1998 15:36:34 GMT

In article <>,
Jason Dugas  <> wrote:
>I've been trying to get more information off of the web (see sites
>listed below) about the operation of these rocket engines.  Could anyone
>explain why it is the "positive" particles that are being expelled?

In fact, an ion engine has to eject positive and negative particles in
equal numbers, because it's very undesirable to build up an electric
charge on the spacecraft.  The "neutralizer" in an ion engine emits
electrons to neutralize the ion beam.  The reason why you accelerate the
ions and just toss the electrons out in the vicinity of the beam is that
the ions carry most of the momentum, and momentum is what counts for
rocket propulsion.  Accelerating electrons is really easy, but yields very
little momentum per unit of energy invested.

>Is there a particular reason for this, or would it be equally feasible to
>eject negative particles out of the accelerating electrode?

Accelerating negative ions is just as easy as accelerating positive ones.
But acceleration is the easy part.  The area where lots of effort and pain
has gone into ion-engine design -- and where today's methods are still not
all that great -- is rapidly, thoroughly, and efficiently converting the
propellant into ions.  And it is *much* harder to generate negative ions
than positive ones.  Generating a negative ion requires adding an electron
to an atom gently enough that it will stick; generating a positive ion
just requires knocking an electron loose.
Mass-market software technology has |  Henry Spencer
been deterioriating, not improving. |      (aka

From: (Henry Spencer)
Subject: Re: Deep Space 1. Why use Xenon?
Date: Tue, 16 Feb 1999 15:33:55 GMT

In article <7aa6ug$rq$>,
Stuart Forbes <> wrote:
>I'm trying to find out why NASA's Deep Space One probe uses xenon as it's
>fuel in the ion drive. I understand that xenon is the heaviest
>non-radioactive noble gas, but that it is also very expensive to produce.
>Would another gas be suitable? Argon? Nitrogen? Is it just that xenon merely
>the best gas to use and so they're using it in the prototype? If so, why
>would it be the best gas?

Most fundamentally, for good performance in practical ion-rocket systems,
you want a very high mass/charge ratio... which means heavy atoms.  (Some
work has been done on heavy molecules, but they tend to break up when
ionized, which causes various problems.)

An important secondary issue is that the propellant should be easy to
ionize, which generally means the heavier elements (whose outer electrons
are weakly bound to the atom).  Acceleration is the easy part; efficient
ionization is the *big* problem of ion-rocket design.

It's also helpful if the propellant can be stored at high density without
excessive pressures or cryogenic temperatures.

Finally, for practical reasons it's good to have propellants which are
safe to handle and which are unlikely to either condense on or react with
spacecraft outer surfaces (since an ion rocket operates in a haze of
neutral propellant vapor).

The paper-design preference in the old days was cesium, but it loses big
on the "practical reasons" issues, and it's expensive too.  Mercury was
the experimental favorite for a long time, although it too has "practical
reasons" problems, and its electrical conductivity is a nuisance.  Of
late, the combination of public relations and worries about spacecraft
surface contamination (possibly exaggerated -- mercury ion thrusters have
been tested in space without such problems) has swung the preference
toward the noble gases.  And xenon is best of the noble gases; the lighter
ones can be used, and probably would be used if much larger quantities of
propellant were needed, but there are penalties in both thruster
performance and tank mass.

The cost of xenon is a relatively small component of the cost of an ion-
propulsion space mission, at current space-hardware costs.  The reason for
switching to a lighter noble gas if large quantities were needed would be
shortage of supply -- there just isn't much xenon on Earth -- rather than
high cost, because a big mission would have high hardware costs too.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: Plans for ion propulsion?
Date: Wed, 17 Feb 1999 04:15:21 GMT

In article <7a77gd$b7g$>,
Bernhard Schmidt  <> wrote:
>> Current ion-rocket systems, and reasonably-likely future ones, do not have
>> enough thrust to be used in Earth-to-orbit launchers.
>Don't forget that ion - engines don't work inside atmosphere.

Nor do a lot of the rocket engines used in upper stages.

Also, while I expect it's true of today's ion engines, I don't think it
represents an inherent law of nature, to the point that it just couldn't
be overcome.  Electron-beam welding "doesn't work" in atmosphere either...
but in fact it is possible to do it on the shop floor (rather than in a
vacuum chamber), by excluding air as much as possible from the work area,
and pumping like mad to keep at least a partial vacuum within the electron
gun itself.  It might well be possible to do something similar for ion
engines, if there was a *reason* to.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

From: (Henry Spencer)
Subject: Re: Deep Space 1. Why use Xenon?
Date: Wed, 17 Feb 1999 16:24:55 GMT

In article <7ac8bi$51u$>,
blair patrick bromley <> wrote:
>...In the 1960's, consideration was given to
>colloids for ion propulsion, which are simply a conglomerate of hydrocarbon
>molecules that can carry a charge, and which have a much higher mass to
>charge ratio than xenon, but it appears that there were problems with
>producing quantities of colloid propellant that were consistent in there
>structure and nature.

Although I haven't really gone digging, the literature of the time seems
to contain a number of reports of successful operation of colloidal-ion
thrusters.  I suspect that this is another of those ideas which just went
out of fashion for no good reason before being tried properly, rather than
hitting any significant technical snags.  A lot of good ideas died in the
budget squeezes of the late 1960s and 1970s, not because there was
anything wrong with them, but because money was tight and applications
were scarce and largely-arbitrary decisions had to be made about which
things to continue funding.
The good old days                   |  Henry Spencer
weren't.                            |      (aka

Index Home About Blog