Cometary rendezvous missions using ion propulsion is considered. The characteristics of the ion engine are discussed including the fuel efficiency and acceleration, and the design of the ion engine is described. The operation of the ion drive engine and an overview of its applications are presented

Atkins, K. L.

English

NASA Technical Reports Server

ION PROPULSION AND COMET HALLEY RENDEZVOUS
Kenneth L. Atkins
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 91103
197
https://ntrs.nasa.gov/search.jsp?R=19800003729 2020-03-21T20:47:26+00:00Z
I couldn't help thinking, as Professor Delsemme was talking, of an analogy
with Professor Delsemme as Captain Ahab and this great white ghost in the sky
as some "Moby Comet." And the mission I propose uses an instrumented probe as
your harpoon.
Ion drive, an advanced propulsion system, will provide the ship, taking
us out to the comet and allowing our "harpoon" to sample the comet. Some day
perhaps we'll bring the sample back and put it into a museum, or a zoo, which-
ever may be appropriate.
In speaking about a mission to a comet, specifically a Comet Rendezvous,
we are talking about a significant energy problem in terms of what it takes to
get there. The basic problem follows from an understanding of the cometary
orbits that were shown by Professor Whipple earlier in the day. They were
generally quite elliptic, or "egg-shaped", and most of them are highly inclined
to the ecliptic plane. Thus to intercept and match the cometary paths we must
change both the shape and the spatial orientation from a flat circular orbit to
an inclined elliptical one. This maneuver requires more energy than is generally
available from our conventional rockets. You see in Figure i. an estimate of
the launch mass capabilities of several Shuttle and Inertial Upper Stage
combinations. The launch energy parameter C3 is on the abscissa. This is a
measure of how much energy is put into the transplanetary trajectory.
If we overlay this capability with the requirements of a number of space
missions (Figure 2 ) -- and I apologize that the chart gets a bit busy -- we can
see that missions lying below the curves are within our general capability.
I have encircled general mission regions indicating the energy classes.
Basically, this shows that conventional chemical systems are unable to capture
comet rendezvous missions even if we projected four stage versions of the IUS.
Obviously we need some advanced propulsion capability to brighten the bleak
prospects for comet rendezvous.
198
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Fig. i. Shuttle/IUS capabilities.
199
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Fig. 2. General requirements for selected
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200
In the last several years NASA has funded the development of ion propulsion.
This system uses ion rocket engines that have a fuel efficiency, or a "miles-
per-gallon" improvement on the order of i0 times better than chemical rockets.
This says that you can deliver over four times the amount of total impulse than
you can with a chemical system while using less than half the fuel.
This allows us to talk about making impulse changes equivalent to that
required to achieve a rendezvous with Comet Halley.
Cometary rendezvous missions are not the only customers for ion propulsion.
This flight system has a number of applications in the planetary regime. Mis-
sions such as Mercury orbiters, Mars sample returns, and Saturn orbiters are
included along with bringing back the museum piece for Professor Delsemme.
It also has some applications in Earth orbit. I will not dwell on those today,
but I would like to have you recognize that this system is something that carries
a broader interest than applications in comet missions.
Let's talk about some of the characteristics of this system. I have
already alluded to the high fuel efficiency; a factor of i0 improvement. Ion
propulsion uses an inert fuel, liquid mercury. Liquid mercury looks nothing
like a comet so you do not mistake things you might see from your engine for
things you might see in a comet.
The acceleration is very low. You get only about 0.002 pound of thrust
per engine; but the system will be operated for significantly long periods of
time. Two or three years of continuous propulsion is something that appears
to be well within the capabilities of the technology. The integral will provide
very respectable vehicle velocities.
One of the reasons the engine can last so long is that it has no moving
parts. The only real wear-out mechanism that we worry about is the erosion
of the accelerator grid caused by the particles as they pass through and are
exhausted from the engine.
201
Ion engines provide a modular approach to spacecraft design. Several
engines can be clustered, each with its individual support equipment. Each
engine unit may be considered as a module. This allows a lot of flexibility
to vary the number of thrust units to match the mission requirements. Fewer
engines are required for some applications while more are necessary for others.
Further examining its operational characteristics, we recognize that an
electric system like this generates electric and magnetic fields. We have to
deal with electric and magnetic interaction with the rest of the spacecraft. The
charged exhaust particles lead us to concerns about deposition on surfaces and
attenuations if we communicate through the exhaust plume. I'll come
back to this later.
Now, I would like to describe the physical characteristics of an ion
propelled spacecraft. Figure 3 identifies the basic parts of the vehicle. It
is comprised of the thrust module, an interface unit and the large solar arrays
that collect sunlight and turn it into electricity for actually operating the
engines. Above the dotted line is the scientific spacecraft or the payload.
It consists of a mission module that carries all the command and control equip-
ment and a science package.
Figure 4 shows the design developed for a Halley rendezvous mission.
This artist's rendition displays the thrust subsystem with the ion engines to
the right. On the sides are the large, solar arrays with reflectors to collect
the sunlight and focus it on the solar cells at the bottom of the trough. The
technique of light concentration through use of the reflectors essentially fools
the solar cells into thinking they are closer to the sun than they really are,
and this mitigates the magnitude of the power loss experienced as we go away
from the sun. Thus, thrust performance stays at a relatively high level. Higher
thrust leads to larger payloads and shorter flight times.
202
___/_ MISSION MODULE*
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Fig. 3. Ion drive vehicle definitions.
Fig. 4. Artist's conception of ion drive vehicle required for a rendezvous
with Halley's comet.
The dimensions of the vehicle from "wing tip to wing tip" are something on
the order of 450 to 490 feet. The array wings are about 12 feet wide, so you
can see this is not a small machine. Each engine is 15 inches in diameter.
You may think that 450 feet is fairly long (it is about a football field and
a half from one wing tip to the other). However, on the scale of some systems
that have been considered for the Halley rendezvous, the ion drive vehicle is
relatively small. The solar sail, which was considered as an alternative
technique for accomplishing a Halley Rendezvous mission, was nearly nine miles
from wing tip to wing tip.
A better understanding of the ion propulsion technique is gained from
looking at the engine cutaway shown in Figure 5. The ion engine is deceptively
simple in its operation. It looks much like a coffee can about 15 inches in
diameter and about i0 inches deep. The fuel, in liquid form, is brought in
through a couple of heaters or vaporizers that transform the liquid mercury
fuel to a vapor and distribute it through this manifold. At the base of the
engine is an electron emitter or cathode. Electrons flow from the cathode
to the anode out around the circumference. The electrons pass through the
mercury vapor and cause ionizing collisions. Once charged, the mercury ions
are forced by magnetic and electric fields toward the two accelerating screens
over the exhaust end of the engine. A high electric field is placed between
these two separated screens so that as the ions drift into it they are accelerated
to a very high velocity and exhausted at speeds ranging from 50,000 to 75,000
miles per hour. The achievement of very high exhaust speeds at relatively small
expenditures of energy leads to the benefit of high fuel efficiency. The engines
thus offer a tremendous advantage in doing missions that we ordinarily refer to
as "high energy requiters". They allow achievement of these missions for
relatively small amounts of "fuel". Figure 6 shows a photograph of one
205
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Fig. 5. Model of ion engine.
F i g .  6 .  Photograph of an i o n  engine. 
of the engines. These engines have been tested both on Earth and in space and
are still undergoing tests at Lewis Research Center and its contractors.
The modular nature of the thrust subsystem is shown by the scale model in
Figure 7. This model has thrust units that combine two of these engines in each
module with the electric power processing equipment in two racks at the end
opposite the engines. The plates on the side are radiators which take away the
excess heat that can't be used in the power conversion process. The components,
put together in this fashion, become bimodular thrust units that can be
standardized and stacked tinker-toy fashion to form a thrust subsystem for the
ion drive rendezvous.
In the interface unit, just forward of the thrust modules, a propellant
tank and two roots for the connection of the solar arrays are housed. The inter-
face unit also provides the hard points for mounting the spacecraft.
Figure 8 depicts, in a series of scenes showing six different events, the
ion drive deployment from the shuttle. Basically, we start at the bottom left
with the ion drive stowed in the shuttle atop its twin-stage, solid, rocket
booster. This stack is then erected in the shuttle bay and separated. The
shuttle backs away to a safe distance and as the 3rd scene shows, the solid
rocket booster is ignited and drives the ion rocket to a positive escape
energy relative to Earth.
The fourth event shows burnout of the solid and separation of the ion
system. Event 5 shows the beginning of the deployment of the solar arrays
while the final scene at the lower right shows the partially deployed arrays
that signal the beginning of the ion thrust phase.
Let's now discuss science acquisition options. We understand your concerns
about operating with a system that has large electric and magnetic fields.
There are several modes for science acquisition in such an environment. First,
we have no difficulty in shutting the thrusters off, and in fact, the design
208
Fig. 7. Model showing possible modular construction
of an ion propulsion system.
209
bo
0
Fig. 8. Ion propulsion system deployment from the Shuttle/IUS.
of the Halley comet mission calls for exploration strategies where during most
of our time in the vicinity of the comet, the engines are shut down and in a
very quiet "coast" mode for taking the science data. Another option is to keep
one of the engine neutralizers operating. This achieves active control of the
spacecraft potential by providing a controlled source of electrons to balance
charge build-up.
In a third option, we could continue to operate the engines and take data
while both thrusters and neutralizers are operating. There are several ways to
handle the problems caused in that mode. We can use clever positioning of the
instruments, such as on booms, or we can shield the instruments.
We have also looked at the difficulties or concerns that might be seen
in handling a mercury propellant, both in loading it and launching. A number
of "worst-case" situations Such as reentry of the entire mercury tank after an
explosion during launch have been studied. These studies found that the effect
on the Earth's environment is equivalent to a temperature change resulting from
a trip of 300 miles; you deplete the ozone layer in a very, very limited vicinity
for a short period of time.
In talking aboutmissions to comets with ion drive there are several but
a limited number of options for targets. I will discuss two today - the Halley
Rendezvous and the Encke Rendezvous. Both have received considerable interest
from the Science Working Group.
Figure 9 is a picture of the trajectory to achieve rendezvous with
Halley's comet. The orbit of Earth is a dark circle. The launch would occur
somewhere around June, 1982. The spacecraft goes out, away from the Earth, and
begins to slow down much like a rock thrown up in the gravitational field. Then
it "hangs a left", or makes a big "U turn" and begins to thrust back toward the
sun. The orbit of Halley's comet is the dashed curve. The rendezvous occurs
211
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Fig. 9. Ion drive trajectory to attain a rendezvous
with Halley' s comet.
212
when the comet overtakes the spacecraft, just before Christmas, 1985. It will
make an interesting Christmas star as seen from the spacecraft.
The other alternative target is comet Encke. Figure I0 shows the
trajectory for Encke's 1987 apparition. The launch would occur in March, 1985.
The spacecraft would arrive 700 or 800 days later in May, 1987, some 50 to 60 days
before perihelion. During this particular apparitio n of Encke, the Earth is
across the solar system from the comet. This situation is not particularly
attractive for ground based optical observations of comet Encke, but we see no
real difficulties in communication with the spacecraft during the rendezvous.
One thing about the Encke mission that I think is significant is how
close it passes the sun during its perihelion, about 0.34 astronomical units.
That is going to be a very hot thermal environment, and it is going to take
some clever approaches on the part of engineers in order to solve the thermal
problem if this mission receives serious consideration.
In summary, I've introduced ion drive, discussed its characteristics and
operation, and briefly overviewed its potentially wide spectrum of applications.
Its modular nature and high fuel efficiency while operating from electricity
generated by collecting sunlight make it an ideal adjunct to the Shuttle
Transportation System. I specifically addressed its unique potential for
achieving comet rendezvous and used the examples of Halley's comet and Encke
as prime mission candidates. I hope from this brief introduction I've been
able to transmit something of the excitement I think is inherent in the combina-
tion of an exotic new propulsion technique, ion drive, with a mission to
investigate Halley's and other comets.
213
I
ORBITOFf ENCKEARRIVAL
EARTH _ 5/30/87(1"p-50}I
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Fig. I0. Encke rendezvous (1987) trajectory.
214


Ion propulsion and Comet Halley rendezvous

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