The Enzmann Starship: History and Engineering Appraisal

Abstract

During his student days Robert Duncan-Enzmann imagined a space vehicle design which he depicted in a watercolour painting and apparently dated 1949. In the 1960s he was heavily involved in space-mission design and introduced the concept of a fusion powered interstellar spacecraft design which utilised a 305 m diameter sphere of frozen Deuterium and a long cylindrical habitat/propulsion section joined onto it by a connecting structural column. The spacecraft was to be manned by a small community of people setting out to colonise nearby stars and the entire vessel would have a launch mass of between 3-12 million tons, most of which would be the propellant. Long time space advocate G. Harry Stine, presented the concept to a wider audience via ``Analog Science Fact & Science Fiction '' magazine in 1973. Stine envisioned the Starship to be part of a wider programme of interstellar exploration, beginning in the 1990s. Although the Enzmann Starship is relatively well known in science fiction circles, it is not well known within the interstellar research community and indeed just as little is known about its creator, Robert Enzmann. Very little has been written about the concept in the academic literature and no modern assessment of its engineering credibility exists. This paper sets out to reliably describe what is known about the Enzmann Starship design and also how the idea originated, based upon what is known to date. In this paper the engineering configuration is described, and a performance assessment is given in the context of modern scientific knowledge. Further information on the history and design of the Enzmann Starship is invited so that this concept can take its rightful place in the history of interstellar spacecraft design proposals.

Full text

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The Enzmann Starship: Hisotry & Engineering Appraisal
1. INTRODUCTION
A review of modern textbooks on interstellar travel will find
little mention of the Enzmann Starship design, a surprise to the
authors of this paper given its ingenuity. However, a review of
science fiction internet sources will reveal more about this
wonderful concept, which was first proposed by the American
Dr Robert Duncan-Enzmann. Little is known about Enzmann
except that he was an MIT Professor and an employee of
Raytheon Corporation. He is the sixth pioneering Robert in the
field of interstellar research, the other five being Robert Bussard,
Robert Forward, Robert Parkinson, Robert Freitas and Robert
Frisbee and he deserves as much credit for his idea. In realising
the dearth of information on this concept it was decided to
perform a literature research and to then write up a summary of
the results, describing the engineering design and the history
behind its development. This paper is the result of this effort
which took over a year. It is the intention of this paper to
provide greater awareness of the idea, within the technical
science community, and if other authors have information of
relevance this is invited.
If the 1960s were the golden age of human space explora-
tion, then including the 1970s and 1980s this was also the
golden age of interstellar Starship design. During this period
many colourful and original concepts were generated including
a number of fusion based propulsion designs such as Project
Daedalus [1] and the Bussard interstellar ramjet [2]. It is in this
context that the Enzmann Starship was also turned into a cred-
ible engineering concept, a period in history where scientists
began to apply rigorous engineering methods to speculative
proposals as a way of answering fundamental questions about
our universe and the possibility of intelligent life.
In writing this paper the authors have attempted to assemble
data on the Enzmann Starship wherever possible, including con-
tacting individuals (Robert Enzmann, Don Davis, Rick Sternbach)
involved with its early development. We first discuss what is
known about the history of the idea. We then give an overview of
the concept, its configuration layout and claimed performance. We
then discuss some follow up proposals to use the Enzmann Starship
for a wide scale program of interstellar colonisation. Finally, we
provide an engineering assessment of the proposal in the context
of four decades of subsequent research. It is the hope that this
paper will reveal more about this unique idea and perhaps advance
it to a more credible level.
2. HISTORY & ORIGIN OF
THE ENZMANN STARSHIP
In personal communications [3] to the authors of this paper
Robert Enzmann says that he developed an interest in space
flight during his very early school days, and he imagined the
distinctive configuration for the space vehicle on the 6th August
1945 during the WW2 bombing of Japan. He constructed paint-
ings of the idea back then and one of which is shown in Fig. 1
dated 1949. These authors are unsure if this is meant to be
“1969”.
THE ENZMANN STARSHIP: HISTORY & ENGINEERING APPRAISAL
JBIS, Vol. 65, pp.185-199, 2012
A. CROWL1, K.F. LONG2 and R. OBOUSY3
Icarus Interstellar, 20 Downlands Place, Boondall, QLD 4034, Australia.
Email: acrowl@icarusinterstellar.org1, kelvin.long@tesco.net2 and robousy@icarusinterstellar.org3
During his student days Robert Duncan-Enzmann imagined a space vehicle design which he depicted in a watercolour
painting and apparently dated 1949. In the 1960s he was heavily involved in space-mission design and introduced the concept
of a fusion powered interstellar spacecraft design which utilised a 305 m diameter sphere of frozen Deuterium and a
long cylindrical habitat/propulsion section joined onto it by a connecting structural column. The spacecraft was to be manned
by a small community of people setting out to colonise nearby stars and the entire vessel would have a launch mass of between
3-12 million tons, most of which would be the propellant. Long time space advocate G. Harry Stine, presented the concept to
a wider audience via “Analog Science Fact & Science Fiction” magazine in 1973. Stine envisioned the Starship to be part of
a wider programme of interstellar exploration, beginning in the 1990s. Although the Enzmann Starship is relatively well
known in science fiction circles, it is not well known within the interstellar research community and indeed just as little is
known about its creator, Robert Enzmann. Very little has been written about the concept in the academic literature and no
modern assessment of its engineering credibility exists. This paper sets out to reliably describe what is known about the
Enzmann Starship design and also how the idea originated, based upon what is known to date. In this paper the engineering
configuration is described, and a performance assessment is given in the context of modern scientific knowledge. Further
information on the history and design of the Enzmann Starship is invited so that this concept can take its rightful place in the
history of interstellar spacecraft design proposals.
Keywords: Enzmann starship, fusion propulsion, interstellar travel
This paper was presented at the British Interplanetary Society
symposium “World Ships - The Long Journey to the Stars”, 17
August 2011.

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A. Crowl, K.F. Long and R. Obousy
It is claimed by several sources, in particular internet
based, that a report was submitted to the New York Academy
of Sciences as early as 1964 detailing the idea although an
extensive literature research by these authors has revealed
no such reports in existence. Subsequent authors have claimed
that the Enzmann Starship was referenced in the book ‘Jour-
ney to Alpha Centauri’ by John McVey [4], however, search-
ing this book and the later 1969 republished version has
found no specific reference to Enzmann, although the book
does discuss century long missions to Alpha Centauri.
Enzmann also submitted several papers relating to ‘Mission
Planning’ to the New York Academy of sciences in 1966
although these were not pertaining to Starships [5]. It is
worth noting that in the 1960s America was in the full swing
of Project Apollo and indeed landed on the Moon in 1969.
Enzmann says that he took inspiration from these events and
was committed to humanity making its first steps towards
the stars. It is the view of these authors that it was during the
mid-1960s that the idea for the Enzmann Starship probably
began to crystallise in the mind of Robert Enzmann, al-
though he had the idea much earlier.
During 1972 the original concept for the Starship was modi-
fied in collaboration with the space artists Don Davis and Rick
Sternbach [6, 7]. In particular the original eight engine design
was changed to a 24 engine design and the modular sections
were made such that they could be split off from the main
vehicle. The new design also depicted smaller spheres and the
nested toroids at intervals along the hull, was to allow the ship
to split up into as many as three separate ships once a primary
destination had been reached. The torus became something of
an all-around connecting shape, able to latch onto most any
other module, along with the engines. This would require sig-
nificant undocking and redockings of the component parts. The
ability to rearrange modules would also afford the expedition
an added degree of safety in the event of a problem that
prevented a complete ship from completing the journey.
Sternbach, Dixon and Enzmann initiated, in those 1972 discus-
sions, the idea that parts from a disabled vehicle could be added
to a healthy one, and the trip could continue, albeit at some cost
in time and velocity. These creators also discussed the possibil-
ity of a dedicated tanker ship, but it never went anywhere; due
to the issue of having to push not only the fuel for itself, but for
the other ships. At the same time Sternbach, Dixon and Enzmann
worked to further develop the design and produce illustrations
of the concept, which included spraying metallic plasma onto a
balloon which mimicked the appearance of the Deuterium
sphere. Sternbach experimented with a number of different
shapes, volumes and masses to finally evolve the 24 engine
configuration. Sternbach reports in personal correspondence
[7] that the idea for the metal plasma fuel sphere fabrication
may have come from Richard Hoagland, who had been discuss-
ing the technique not only for big metal spheres, but also for
enormous telescope mirrors.
In recent correspondence, Sternbach also reports that: “As
far as this artist was concerned in 1972, the hardware added to
Enzmann’s initial design of unprotected frozen fuel, habitats,
and 6-8 engines was done mainly as an artistic exercise with
engineering practicality a secondary consideration. Isp, ex-
haust velocities, deuterium density at particular temperatures,
MHD taps, materials and processes, and the like were only
vaguely addressed when Don Davis and I were “improving”
the ship” [7]. He goes on to say: “A reconfigured ship that did
not use the large main deuterium tank would (from the top
down) consist of a spare operations deck, one of the cylindrical
habitats, one torus, one of the smaller gold-covered fuel/engi-
neering spheres, another torus, and eight engines. Such sepa-
Fig. 1 Enzmann Starship Painting (1949) by Robert Enzmann.

187
The Enzmann Starship: Hisotry & Engineering Appraisal
rate ships could conduct explorations of the destination star
system, support colony operations, hunt for refuelling sites,
and so on… Also, I seem to recall that there was some discus-
sion of each engine module being able to act as a separate
spacecraft, possibly even as a lander, though I have no clue
today as to how one of the primary heavy thrust units for the
entire ship could have been throttled down to perform less
brute manoeuvres down to a planet surface and back. I also
seem to recall that at one point the engine module was a
cylinder topped with a small fuel sphere of approximately the
same diameter. Presumably the small spheres, as well as the
three medium-sized engineering spheres, contained fuel process-
ing gear and storage tanks, kept full from the big main sphere”
[7].
One of the paintings from this period, made in early 1973,
by Don Davis is shown in Fig. 2, depicting a ‘Starbow, in the
background. Information from Davis reports that Richard
Hoagland is shown in the left foreground, Robert Enzmann is
exiting the central chamber and the rest show family members.
Davis depicted himself sitting on the right “with the longish
hair”.
Also in December 1972 was the 4th Conference on
Planetology and Space Mission Planning which took place on
board S.S Statendam going from New York to Cape Canaveral
to watch the launch of Apollo 17 [8]. The on board conference
included the likes of Isaac Asimov, Carl Sagan, Frederick Pohl,
Ben Bova, Rick Sternbach, Don Davis, Robert Heinlein, Marvin
Minsky, Theodore Sturgeon and Harry Stine to name a few. An
agenda for the conference also lists Robert Enzmann as one of
the speakers. The organizers of the conference were indeed
Robert Enzmann and Richard Hoagland. The conference had
several themes [9] which includes ‘Cornucopia of Space’, ‘Eco-
logical Niches’, ‘Propulsion Intelligent Machines & Socio-
Genetic Change’, ‘Energy & Propulsion’, ‘The Grand Design’.
Robert Enzmann gave several presentations including “Out of
the Cornucopia”, “Statement of Grand Design, & Galactic
Fertile Crescent”, “Force=dRo/dt (F≠ma) and e=hv(1-d/D) That
is an Intellectual Revolution”. Information via Rick Sternbach
to these authors also confirms that Robert Enzmann was in
attendance at the meeting and indications are that it is likely
Enzmann did discuss his Starship concepts during this confer-
ence. This appears to be the first time (and perhaps only) that
the Enzmann Starships have been presented at a conference
venue (except for the August 2011 British Interplanetary Soci-
ety World Ship symposium by Kelvin Long). The S.S Statendam
conference was also discussed by Isaac Asimov in his short
retelling ‘The Cruise & I’ published in the July 1973 issue of
The Magazine of Fantasy and Science Fiction [10]. It was also
recounted in a recent book on private access to space [11].
Several other illustrations have appeared over the years
describing the Enzmann Starship. One of the most iconic was
by Rick Sternbach and appeared on the front cover of Analog in
October 1973 and was painted after the S.S Statendam cruise
Fig. 2 Enzmann Starship painting (1973) by Don Davis.

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A. Crowl, K.F. Long and R. Obousy
[12]. This illustration showed two Enzmann Starships in orbit
around a giant gas giant with a star glowing in the background.
Sternbach also produced another image of the Enzmann Starship
showing it taken off from an asteroid factory. The original
artwork was reproduced in 2002 and is shown in Fig. 3.
In 1973, G.Harry Stine wrote the article that featured in the
October edition of “Analog Science Fiction & Science Fact”,
titled ‘A program for star flight’ [12]. It is this article which seems
to have given the Enzmann Starship its widest publicity. The writer
Stine seems to have altered the original design performance some-
what from that described by Enzmann. Stine argued that a mission
to the stars would be completed as part of a full programme plan
rather than as a one-off mission. The roadmap would involve three
phases which included identification of an astronomical target, the
launch of unmanned probes to the destination and finally the
launch of a full expedition fleet to the target system. A full pro-
gramme of exploration was proposed involving a fleet of 10
Starships to be launched, beginning in the year 1990 at a cost of
around $100 billion spread out over a couple of decades and based
on 1973 costs, one tenth of the then Gross National Product of the
United States. Each ship has a mass of approximately 12 million
tons in mass, although most of that was made up of the Deuterium
propellant. The Starships would be assembled in earth orbit and
capable of reaching speeds up to 30% of light speed. Any shocks
generated from the nuclear pulse engines would be mitigated by
absorbers on each of the eight propulsion units. Artificial gravity
would be provided for the crew by spinning the habitat cylinders
on their longitudinal axes. Thereafter, sometime in 1974 the space
artist David Hardy produced his first rendition of the Enzmann
Starship as a 35 mm slide for a popular publication [13], although
this has not been traced by these authors. However, a later version
of the painting was re-commissioned by one of these authors
(Long) in 2010 and is shown in Fig. 4 with only a minor modifica-
tion from the original.
In March 1977 M.A.G.. Michaud published a paper in which he
discussed the Stine Analog article on the Enzmann Starship and
referred to cruise speeds of 0.9c (unmanned) and 0.3c (manned)
[14]. Then in August 1977 Thomas R Schroeder wrote an article
for Astronomy magazine titled ‘Slow Boat to Centauri’ in which he
claimed cruise speeds of 0.1c were likely representative of the
design but cruise speeds up to 0.3c may be possible [15]. The
vehicle was said to have 12 million tons of fuel. Schroeder said
that the name that had been adopted for the Enzmann was the
“flying iceberg”. The ‘snowball’ was to give the added benefit of
radiation protection to the main body of the vessel. The outer
layers of the vehicle were comprised primarily of bulk material to
serve as radiation shielding for the inner decks. This bulk was to be
made up of the modules main nuclear reactor as well as various
store rooms, heat exchangers, airlocks, land craft storage, observa-
tion areas and communications equipment. Upon arriving at the
target star system the main vehicle was to remain in orbit while a
smaller, sturdy landing craft replenished the snowball for the
return trip home. The modules were designed to weather any
catastrophe so that if any one module was destroyed the two
remaining sections can be disassembled and transferred to another
ship – apparently a good blend between strength and flexibility for
maximum survival potential. Each module was separated by ‘mat-
ing collars’. The design of the engine was highlighted with an
unusual description worth quoting: “The nuclear pulse engines
used in the Enzmann starship differ from that suggested by
Dyson….the flying iceberg has eight units, each containing the
entire explosive energy of a nuclear blast inside a gargantuan
chamber, and venting that force afterward….With this more effi-
cient system, Enzmann believes his ship could reach a peak
Fig. 3 Enzmann Starships Painting (1973) by Rick Sternbach.
velocity of 30 percent of the speed of light”. It is worth noting at
this point that recent correspondence between these authors and
Freeman Dyson, show that Dyson had no involvement with the
Enzmann Starship concept [16].
As pointed out in the article, in order to reach these sorts of
cruise speeds, in all likelihood the vehicle would have to stage.
Ten percent of the speed of light was seen as more credible
reaching the nearest stars in half a century. One of the high-
lights of such a short trip is that much of the crew that started
out would still be alive when they arrive at the target star
system and with many years still ahead of them. Thomas
Schroeder created several depictions of the Enzmann Starship.
A 1978 publication by Ian Ridpath titled ‘Messages from the
stars: Communication and Contact with ET’ referred to the
Enzmann Starship as having been created in 1964 [17]. The
year 1980 saw the publication of the Roy A Gallant book
‘National Geographic Picture Atlas of our Universe’ which
contained a gorgeous Syd Mead painting of the Enzmann
Starship [18]. As reproduced in Fig. 5 the Syd Mead illustration
depicted a Daedalus Starship flanked by two Enzmann Starships
in the background.
In May 1983 Rick Sternbach put together some articles for
Science Digest, one of which by Robert Bussard included
considerable discussion on the Enzmann Starship which was
said to have been invented in 1969 [19]. The articles said that

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The Enzmann Starship: Hisotry & Engineering Appraisal
the vehicle was designed to carry enough fuel – nearly 3 million
tons of super cold deuterium with the vast amount of energy
generated by the fusion of deuterium, controlled using mag-
netic fields by a ‘magnetic bottle’. An enormous metal sphere
was said to house the fuel and also serve as a radiation shield
for three habitat cylinders attached behind it. Each habitat was
to be divided into 20 decks with over 100 rooms per level, and
some of the habitats would be rotated to simulate gravity. The
article further said that the Enzmann Starship would reach a
cruise speed of 0.09c flying to Alpha Centauri in 60 years.
The 1983 Science Digest painting (reproduced in Fig. 6)
showed a pair of ships encountering an icy moon around a gas
giant planet in a new star system. This painting was digitised in
2003. The spacecraft is shown orbiting in the vicinity of a gas
giant, with an icy moon in close proximity. It is conceivable
that the relevance of the moons were to act as a refuelling
station. The image also later appeared in a science book by
Arthur C Clarke [20].
Then in the early 1980s Robert Enzmann revisited his Starship
concept and indications were that he began to favour an alter-
native ‘Athodyd ramjet’ design over the original Enzmann
concept. This information was obtained through personal com-
munications with the family of Robert Enzmann [3].
In June 1984 Tony Martin and Alan Bond reviewed the
concept of World Ships. In one of those papers [21] they briefly
Fig. 4 ‘Enzmann at Jupiter’ painting (2010) by David A. Hardy. (David A. Hardy)
Fig. 5 Enzmann Starships painting (1980) by Syd Mead.

190
A. Crowl, K.F. Long and R. Obousy
discussed the Enzmann Starship concept and referred to the
‘Snowball’ design as having been conceived in the mid-sixties.
They referenced the eight engine design and said that the fuel
was delivered via a central load-bearing core running the length
of the cylinder. The vehicle was said to reach velocities of
around 0.01c. The outer layers of the habitats were said to be
composed primarily of bulk material to serve as radiation shield-
ing for the inner inhabited areas. The population was said to
start at 200 and then grow to 2,000 by the end of the journey.
The cylinder was to be spun on its longitudinal axis to establish
an artificial gravity. Apparently, Enzmann had suggested that 3-
10 vehicles would make a journey in a fleet; as such an arrange-
ment would have definite advantages in terms of mission secu-
rity and flexibility on long missions.
In February 1986 an image painted by Bob Eggleton of the
Enzmann Starship appeared on the front cover of Boskone
XXIII for the Boston regional science fiction convention [22].
Again it depicted two Enzmann ships, and the convention of a
‘fleet’ of Starships seems to have been followed by many an
artist; they always come in twos.
Fig. 6 Twenty-Four engine Enzmann Starship painting (1983/2003) by Rick Sternbach.
It is worth noting the various artists that have depicted the
Enzmann Starship over the years and these are summarised in
Table 1. The journey to discover details on the Enzmann design
has been an (enjoyable) detective story for these authors, al-
though often frustrating. But what kept them going was the
inspirational imagery provided by the space art community.
The research by the authors of this paper came to a head in
early 2010 when relatives of Robert Enzmann released an
‘Enzmann Starship’ web site [23] which included other Starship
ideas now favoured by Enzmann.
3. A MODERN ENGINEERING ASSESSMENT
3.1 Engineering Overview
Decades have passed since Robert Enzmann first proposed his
Starship concept and so it’s worth considering whether devel-
opments in science and technology allow us to assess the
concept’s merit as an engineering proposal. The proposed
Enzmann engineering concept had the unusual resemblance of
an augmented lollipop stick. This was a bare 305 m (1000 ft)
TABLE 1: List of Enzmann Starship Space Artists.
Artist Comments
Robert Enzmann 1946 Private Collection [3, 23]
Rick Sternbach 1973 Analog cover, 1983 Science Digest, repainted 2003 [7]
Don Davis 1972 Private Collection [6]
David Hardy 1974 A Popular Magazine (repainted 2010) [13]
Thomas Schroeder & Mark Paternostro 1977 Astronomy Magazine [15]
Syd Mead 1980 National Geographic Picture Atlas [18]
Bob Eggleton 1986 Boskone XXIII [22]

191
The Enzmann Starship: Hisotry & Engineering Appraisal
diameter sphere consisting of around 3 million tons of frozen
deuterium, a ‘snowball’, with a long 305 m spacecraft stuck on
the end of it containing the Starship modules. As has been
discussed the original concept had 8 nuclear fusion engines and
later versions had as many as 24 engines. The engineering
layout is illustrated in Fig. 7.
Because the Enzmann Starship is a colony ship, it would
also be prudent to provide for artificial gravity. Indeed in some
of the references found it was indicated that this was the
intention of Enzmann also. The equation for calculating the
gravity induced on a spinning cylinder is given by:
()
2
2
/30
9.81
Rrpm
gms
π
−
×
=(1)
We can use this equation to calculate the artificial gravity
induced by different spin rates and assuming a cylinder radius
of 45 m. The results for several spin rates are shown in Table 2.
We note that for 2 RPM we get 0.2 g which is greater than lunar
gravity. For 1 RPM we get 0.05 g which is approximately half
lunar gravity. A value of 4 RPM seems sensible to maintain a
near Earth gravity.
3.2 Structure & Materials
The thickness of the habitat sections is expected to be a lot less
than 1 m. This is from work performed by Bond and Martin in
1984 [24] where studies of wall thickness as a function of ship
radius were performed for both dry and wet World Ship con-
cepts.
Additions to the design by Enzmann (with assistance from
Rick Sternbach) saw that the total vehicle could, in fact, sepa-
rate into three separate craft with each consisting of a habita-
tion unit and 8 engines. The separation manoeuvre would be
performed once the vehicle had reached the destination star so
that each sub-ship could explore different parts of the solar
system. It is presumed (but not known) that each sub-vessel
would have a minimum quantity of fuel to enable individual
exploration of the target star system, prior to rendezvousing
with the lead vehicle and main propellant sphere for the next
interstellar leg.
Fig. 7 Enzmann Starship schematic engineering layout.
TABLE 2: Artificial Gravity as a Function of Spin Rate for the
Enzmann Starship.
RPM g’s
1 0.05
2 0.2
3 0.45
4 0.80
5 1.26
6 1.81
The spacecraft had a modular design so that if part was
damaged it could be ‘unplugged’ and replaced. The main fea-
ture of the spacecraft was a cylindrical habitation unit 91 m in
diameter and 91 m in length, with three of the modules coupled
end to end. The modules would be self sufficient, powered by
their own nuclear power plants. If any one was damaged it
could be ejected from the main vehicle and each vessel would
become a two-module spacecraft. The propulsion system is
contained within the engineering modules. A central load bear-
ing core, perhaps as thick as 15 m in diameter, would run from
the fore to the aft of the ship for structural strength.
The geometry of the entire structure has not been defined at
the sub-level for the Enzmann Starship. However, using this
gross geometry and mass figures as a starting point, this has
been attempted by these authors. The results are shown in Fig. 8
but the masses shown exclude the 3 million tons of Deuterium
sphere mass. No reference has been identified to indicate the
choice of materials used in the Enzmann Starship design. How-
ever, for this paper we have taken the liberty of selecting
materials which may seem suitable for the purpose. These are
shown in Table 3.
3.3 Propulsion
Enzmann seems to have proposed a Project Orion [25] type

192
A. Crowl, K.F. Long and R. Obousy
propulsion for his Starship concept, where nuclear bombs are
detonated external to the spacecraft. However, Enzmann also
seems to have proposed to detonate these devices internal to
the vehicle. He claimed that this was more efficient than Orion
and cruise velocities. The engine for the Enzmann Starship is
described as a “Orion Pulse Drive” but should be seen as more
of a place keeper than a specific engine choice.
Robert Bussard mentions the Enzmann Starship design briefly
in a Readers Digest article in 1983. He states that “The Enzmann
Deuterium rocket awaits magnetically confined fusion power
units that will provide constant thrust….Starflight will need a
hundred to a ‘million times more fusion power than any of the
present research is designed to produce…” [19]. It is not
known if Bussard was indicating a preference here for magneti-
cally driven fusion propulsion systems although given his later
interest in electrostatic confinement technology this seems un-
likely.
To make working, high-Isp deuterium fusion pulse units
needs something more akin to Daedalus [26, 27] for ignition
than Orion [25]. The Daedalus spacecraft was a two-stage
engine design. The first stage engine design had six D/He3
propellant tanks producing an exhaust velocity of 1.06×104
km/s equivalent to a specific impulse of 1.06 million s. The
2.84 gram inertial confinement fusion pellets were detonated
at a pulse frequency of 250 Hz with a mass flow rate of
0.711 kg/s producing a thrust of 7.54×106 N. Using a 2.7 GJ
electron driver energy would give rise to a 171.82 GJ energy
release from each pellet with an equivalent Q value of around
66.7. The first stage would use 46,000 tons of propellant, which
was 7,667 tons per tank.
The mass flow rate m
 is related to the total propellant mass
Mprop and burn time tb as follows:
p
rop
b
M
mt
=
(2)
We can then use this to calculate the start acceleration for
the vehicle, as follows:
p
rop
bHz
M
am
tf
=(3)
It has been previously discussed how the engine perform-
ance can be related to the pellet performance and design choices
[28]. The pellet mass mp is related to the mass of propellant
Mprop, burn time tb and pellet pulse frequency fHz by the follow-
ing equation:
p
rop
pell
bHz
M
mtf
=(4)
Using this equation we can work out the pellet masses
required assuming different pulse frequencies. This is shown
in Table 4 and for both a 8 or 24 engine configuration,
similar to that proposed by Robert Enzmann and later by
Rick Sternbach. Note that a pulse frequency of around 10 Hz
is consistent with the next generation of proposed commer-
cial fusion reactors [29]. Note also that a pulse frequency of
around 250 Hz is consistent with that proposed for the
Daedalus spacecraft [26, 27]. Also, for Daedalus the first
stage and second stage pellet masses were 2.84 g and
0.288 g respectively.
TABLE 3: Enzmann Starship (Suggested) Materials.
Fig. 8 Enzmann Starship configuration approximated by authors.
Component Material
Sphere Deuterium
Sphere Shell Titanium
Central Column Titanium
Pulse Chambers Molybdenum
Habitats Titanium/Aluminium
Collars Titanium
Shoulders Titanium
Nose Aluminium/Beryllium

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The Enzmann Starship: Hisotry & Engineering Appraisal
3.4 Deuterium Fuel Choice & Acquisition
Initially the fuel sphere was envisaged to be a naked ball of
frozen deuterium, but due to the non-zero vapour pressure and
low mechanical strength, Rick Sternbach seems to have sug-
gested the design placed a metallic shell around the propellant.
This would typically be a Titanium alloy. This would be around
1.3 mm thick and includes the necessary 50% safety factor for
maximum stress. A reflective plastic insulation blanket with a
mass of ~200 tons and 0.00001 m thick, in 50 layers with a bulk
density 1400 kg/m3 and an areal density of 0.7 kg/m2. So the
sphere would comprise of the internal Deuterium, surrounded
by the insulation layer and then surrounded by the metal shell.
The Deuterium sphere was assumed by Enzmann to be solid
Deuterium which as a density of around 180 kg/m3 at standard
temperature and pressure (0°C and 1.325 kPa). But instead it
was assumed better to change it to slush Deuterium which was
easier to handle. It had a mix of half liquid (170 kg/m3) and half
ice (205 kg/m3) leading to an assumed density of 190 kg/m3.
However, for a 3 million tons propellant this will then lead to a
revised geometry, with a radius of 155.63 m and a diameter of
311.26 m.
The choice of fuel is critical when designing a fusion pro-
pulsion system. Typically a plot of the reaction average cross
section as a function of plasma temperature will show that D/T
reactions will ignite at the lowest temperature, followed by D/D
and D/He3. A technical issue for any fusion based space propul-
sion is the choice of fuel to use. D/T has the lowest ignition
barrier due to the high ratio of neutrons (binds nuclei via strong
force) to protons (repels nuclei apart via Coulomb force).
Deuterium fusion was chosen as the propulsive reaction,
which produces roughly equal amounts of helium-3 and tritium
by the reactions:
22 3
11 2
(0.82 ) (2.45 )
H
H He MeV n MeV+→ + (5)
22 3
11 1
(1.01 ) (3.03 )
H
H H MeV p MeV+→ + (6)
As Deuterium was needed in vast quantities, Enzmann seems
to have proposed that it could be mined from the icy moons of
the gas giants Jupiter and Saturn, where it appears in abun-
dance. This assumes the existence of a solar system wide
economy where fuel mining vessels are travelling back and
forth from the gas giants to Earth anyway, for the purposes of
fuelling commercial nuclear power stations that work on the
principle of fusion burning.
Typical reactions considered for propulsion include DD, DT
and DHe3. DT has the lowest ignition temperature; however,
80% of the energy released goes into neutrons with only 20%
going into charged products. Since neutrons cannot be directed
via magnets, they transfer fusion energy directly to the engine
and vehicle structure, which must be handled by large cooling
systems. The neutrons can be thermalized and heat an inert
propellant which can then be expelled for thrust. However, due
to thermodynamic losses, and the high thermal load, it’s not as
efficient method of propulsion as with using ions.
The DHe3 has a very low level of neutron production, with a
large fraction of the reaction products being protons. However,
the greatest drawback of this reaction is the rarity of He3. Today
some believe there may be He3 on the Moon, and in the Daedalus
study, it was proposed that He3 could be mined from the gas
giants [30, 31]. The level of space based infrastructure that
would be necessary to mine He3 from these sources would
certainly increase the overall costs of the spacecraft.
DD reactions are an attractive option, since Deuterium (heavy
water) is abundantly available as a nuclear rocket fuel, on Earth
where it is used as a moderator in fission reactors as heavy
water for slowing down neutrons emitted from Uranium-235
decay, sufficient for fission reactions to occur. It is also found
in all comets, planets, asteroids and moons possessing water
ice. The reaction products of DD reactions, Tritium and Helium
3, undergo secondary reactions with the Deuterium, increasing
the total fusion energy, but also the neutron power.
The steps for extracting Deuterium from water can be sum-
marized in three steps:
1. Initially the water is spit into Hydrogen and Oxygen
electrolytically.
2. The by-products of the reactions are H2 and HD. This
gas is cooled, and eventually liquefies. Since H2 is
lighter it can easily be separated from the HD using
either a simple chilling process or even centrifuges.
3. The remaining HD is then heated, and then passed
through a catalyst which splits it into H2 and D2.
When one considers the DD reaction, and the secondary reac-
tions DHe3 and DT, from 6 Deuterium nuclei, 26.80 MeV (62%)
of the reaction products are mainly charged He3 and T and 16.55
MeV (38%) are in the neutrons. Some of the energy will also
reside in He4 daughter reactions. It is worth noting at this point the
potential for ultra-dense Deuterium (UDD) [32]. If this is success-
fully made in bulk and it can enable D/D→4He reactions, then this
may make the Enzmann Starship concept more feasible.
3.5 Mission Performance
The most widely popularised Enzmann design used Orion-style
TABLE 4: Enzmann Starship (Slow Boat) Pulsed Fusion Pellet Mass.
Pulse Frequency (Hz) Pellet mass (g) Pellet Mass (g) Pellet Mass (g)
1 Engine Design 8 Engine Design 24 Engine Design
1 5000 625 208
10 500 62 21
50 100 12 4
100 50 6 2
250 20 2 0.8

194
A. Crowl, K.F. Long and R. Obousy
nuclear fusion pulse engines, as described by Dyson [25] which
were estimated to achieve an exhaust velocity of 0.05c. This
can give us a yardstick to judge possible performance range of
the Enzmann Starships. First let’s examine Stine’s claimed
cruise velocity of 0.3 c. The following calculation will examine
how much mass the Enzmann Starship can deliver to a target
system given the following information:
1. The maximum speed of the ship will be 30% the speed
of light.
2. The ship will decelerate at the target.
3. The initial mass of the Starship’s propellant is 12 million
tons.
4. The exhaust velocity is 0.05c.
Using:
ln i
ex
f
m
vv m

∆= 

 (7)
Rearranging to obtain the final mass of the Starship:
/ex
i
fvv
m
me∆
=(8)
Inserting values we obtain:
6
0.6 / 0.05
12 10 73.73
fcc
tons
m tons
e
×
=≈ (9)
So, from the initial 12 million tons, only 73.73 tons remains
after the acceleration to 0.3c and a deceleration back down to
stellar orbital velocity at the target system. To achieve a more
reasonable mass-ratio, the exhaust velocity needs to be ~0.13c,
which is much too high for a pure deuterium rocket.
This analysis indicates the performance claimed in the Stine
[12] coverage of the concept is grossly over-optimistic. Later
sources [33] indicate a cruising speed of 0.09c, for a total pure-
rocket delta-v of 0.18c. Thus the mass-ratio can be computed to
be 36.6, which is more reasonable. Enzmann considered the
fuel would be mined from comets or moons. Assuming easily
extractable fuel in plentiful supply, this fuel to payload ratio
could be acceptable.
Dyson [25] estimated the exhaust velocity from using a
purely deuterium fuel. More feasible estimates of exhaust ve-
locity derived from “Project Daedalus” give exhaust velocities
of 10,000-12,000 km/s. This implies a mass-ratio of the order
of 100. From this mass-ratio estimate and a fuel mass of be-
tween 3,000,000-12,000,000 tons, the dry mass can be com-
puted as being in the range of 30,000-120,000 tons. The pay-
load of a small, growing population of 200 people would take a
considerable mass per person to sustain in a Closed Ecology
Life Support System (CELSS.) A mass-budget of 15-60 tons
per person is comparable to the 75 tons (sans inert mass-shield)
as assumed by the NASA Ames Space Colony study of 1975
[34]. Assuming a 120,000 tonne payload, the Starship’s volu-
metric density can be computed at ~60 kg/m3, which is compa-
rable with the 7.5 kg/m3 of the Standford Torus Space Colony.
With a full complement of 2,000 persons, and a space allotment
of 1,000 m3 per person, the Habitat Modules are relatively
spacious.
Table 5 shows the various Enzmann starship models de-
rived. Table 6 shows the final derived performance for the
Enzmann Starship, as determined by these authors. This is a
result of an extensive literature research as well as some sensi-
ble engineering decisions by the authors of this paper. The
essence of the Enzmann Starship as proposed by Robert
Enzmann has been preserved where possible, namely; 0.09c
cruise speed, 60 year total trip time, 3 million tons propellant,
population start and end size. All other parameters have fallen
out of these assumptions.
Calculations were also performed to estimate the pellet size
requirements for different pulse frequencies based on models
developed in an earlier publication [28] and assuming scaled
Daedalus pellets [27]. For the Enzmann Slow Boat, it was
estimated to be ~5,000 grams (1 Hz), ~500 grams (10 Hz),
~100 grams (50 Hz), ~50 grams (100 Hz) and ~20 grams
(250 Hz). For the Enzmann Slow Ship, it was estimated to be
~1,000 grams (1 Hz), ~100 grams (10 Hz), ~20 grams (50 Hz),
~10 grams (100 Hz) and ~5 grams (250 Hz). For the Enzmann
World Ship it was estimated to be ~1,100 grams (1 Hz),
~100 grams (10 Hz), ~20 grams (50 Hz), ~10 grams (100 Hz)
and ~5 grams (250 Hz). Although for an 8 or 24 engine design
these pellet masses could be reduced further still.
3.6 Comparison to Other Design Concepts
Since the creation of the Enzmann Starship concept there have
been many engineering studies using fusion based engines for
interstellar flight as shown in Table 7. This includes Projects
Daedalus [1], with its detailed propulsion system [26, 27] and
structure calculations [35, 36]. Other Inertial Confinement fu-
sion-based designs like VISTA [37] and Longshot [38] have
followed, VISTA being the most detailed, extended to interstel-
lar missions in a series of papers by Halyard [39, 40]. Halyard’s
analysis made explicit the need for high thrust for the greatest
range in a given mission time for interstellar vehicles, some-
thing which power-limited fusion engines struggle to do in sub-
century timescales. Thus Enzmann’s use of high-thrust, low
thermal load designs like the “Orion” external ignition system
is still a good option, though the likely exhaust velocity will be
less than the ideal we have indicated above.
We can also make some comparisons to the modern day inertial
fusion programme as proposed for the US National Ignition Facil-
ity (NIF) described in this reference [29]. These are briefly dis-
cussed in the context of the Enzmann design. Other facilities such
as the Laser MegaJoule (LMJ [41]) in France and the HiPER
project described in this reference [42], which are both under
consideration for construction in Europe. The key requirement to
attaining ignition is the fusion triple product which states that the
product of the temperature, particle density and plasma confine-
ment time must be ≥1021m-3sKeV as defined by the Lawson’s
criterion [43]. This is achieved by two main routes. Firstly, one can
utilise a low density (~1014cm-3) of particles and confining the
plasma magnetically for a long time (~seconds) – Magnetic Con-
finement Fusion (MCF) [44]. The other route is to utilise a high
density (~1023cm-3) of particles but for a short time (~ns) such as
using laser beams which deliver ~1MJ to the target – Inertial
Confinement Fusion (ICF) [29]. This was the route adopted for the
Daedalus study.
4. SCALED ENZMANN STARSHIPS
In this section we briefly look at scaling up the Enzmann
Starship design to larger versions. If The Enzmann Starship

195
The Enzmann Starship: Hisotry & Engineering Appraisal
TABLE 5: Evolution of the Enzmann Starship
Parameter Original Concept Imagined Concept Altered Concept Modern Concept
(Enzmann) (Stine) (Enzmann/Sternbach) (Crowl/Long/Obousy)
Length (m) 610 610 610 610
Sphere Diameter (m) 305 305 305 305
Total Habitat Length (m) 273 273 273 273
Individual Habitat Length (m) 91 91 91 91
Habitat Diameter (m) 91 91 91 91
Core Diameter (m) 15 15 15 15
No.Habitats 1 1 3 3
No.Engines 8 8 24 24
Propellant Deuterium Deuterium Deuterium Deuterium
Exhaust Velocity (km/s) Unspecified Unspecified Unspecified 12,000
Structure Mass (tons) Unspecified Unspecified Unspecified 30,000
Propellant Mass (tons) 3 million 12 million 3 million 3 million
Cruise Speed (km/s) 27,000 (0.09c) 90,000 (0.3c) 27,000 (0.09c) 27,000 (0.09c)
Time to Alpha Centauri (years) 60 years Not given 60 years 60 years
Starting Colony 200 200 200 200
Final Colony 2,000 2,000 2,000 2,000
TABLE 6: Enzmann Starship (Slow Boat) Configuration &
Performance
Parameter Value
Dry Spacecraft Mass (tons) 30,000
Propellant Mass (tons) 3×106
Start Population 200
End Population 2,000
Total Mass Ratio 101
Mass Ratio 10.05
Exhaust Velocity (km/s) 11,700
Total Delta.V (km/s) 54,000 (0.18c)
Cruise Velocity (km/s) 27,000 (0.09c)
Total Acceleration Time (years) 18.95
Mass Flow Rate (kg/s) 5.02
Start Acceleration (m/s2) 0.019 (0.002g)
Total Cruise Time (years) 41.05
Total Mission Time (years) 60
is classed as a ‘slow boat’ then we produce a ‘Slow Ship’ and
a ‘World Ship’ concept. These are examined in order to get
to a larger Enzmann concept which can be justified as a
much larger colony vessel comparable to World Ship param-
eters.
The technique for deriving the larger Starship concepts was
to start from the Enzmann Starship concept which was assumed
to have a dry habitat mass of 30,000 tons and then this was
scaled up by a factor of 10 and 100 for the Slow Ship and World
Ship respectively. A similar scaling was applied to the popula-
tion which for the Enzmann Starship went from 200 at mission
start to 2,000 at mission end. This meant that for all the con-
cepts the population distribution would be 150 tons/person at
journey start, but by the time of journey end would be 15 tons/
person. Several studies over the years have concluded that 15 –
65 tons would be appropriate for human habitats [34, 45]. So
the assumed mass distribution for the Enzmann concepts is at
least viable.
After some discussion and comparison to other concepts in
the literature the total mission times were fixed to 60 years, 150
years and 350 years for the Slow Boat, Slow Ship and World
Ship respectively. To calculate the performance of the scaled
concepts, the 0.09c cruise speed of the Enzmann Starship was
taken as a starting point. This was then used along with the
assumed dry and wet vehicle masses to estimate the mass ratio.
It was decided to fix the total propellant mass for all three
concepts. The exhaust velocity was then calculated via Equa-
tion (10). This then enabled the calculation of the acceleration
and thrust profiles as described above. For all the scaled
Enzmann Starship concepts, they are assumed to be ‘dry’ worlds,
that is, with no ocean component.
2
()
c
ex
v
v
L
nR
=(10)
,
ex
p
rop tot
v
dm
adt M
=(11)
ex
dm
Tvdt
=(12)
p
rop
pell
bHz
M
Mtf
=(13)
Tables 8 and 9 shows the possible pellet masses for the
scaled Enzmann concepts derived in a similar way to Table

196
A. Crowl, K.F. Long and R. Obousy
4. Once again the size of the pellets depends on the nature of
the engines and how the pellets are detonated. Table 10 and
11 shows the configuration and performance for the Slow
Ship and World Ship respectively. The Slow Ship has a total
mission duration of 150 years and it reaches a cruise veloc-
ity of 13,500 km/s or 0.045c. The World Ship has a total
mission duration of 350 years and it reaches a cruise veloc-
ity of 4,200 km/s or 0.014c. One of the decisions faced by
the authors of this paper was the choice of the exhaust
velocity for the three Enzmann concepts. For the Slow Boat
an exhaust velocity was assumed to be 11,700 km/s using
Equation (10) and this is a value not significantly different
from the ~10,000 km/s exhaust velocity assumed for the
Daedalus design which achieved a cruise velocity of 0.12c.
For the Slow Ship and World Ship a similar value was
derived to be 11,260 km/s and 12,119 km/s respectively,
driven by the choice of mass ratio. It is worth nothing at this
point that other authors [46] have found that the optimum
mass ratio for a fusion powered spacecraft is of order 4.92,
suggesting some optimisation would be required in any fu-
TABLE 7: Fusion Based Propulsion Concepts Proposed in the Literature.
Enzmann Daedalus Vista Longshot
Total Mass Tons 3.1×10654,060 6,000 350
Fuel Mass Tons 3×10650,000 4165 264
Payload Mass Tons 30,000 450 100 30
Cruise Speed km/s (%c) 27,000 (9) 36,600 (12.2) ~45 (0.015) 14,400 (4.8)
Fuel D D/He3DT/D D/He3
Engines 24 2 1 1
Dimensions m 610×305 190×100 170×100 65×15
Mission Interstellar Interstellar Interplanetary Interstellar
Nature manned unmanned unmanned Unmanned
Pulse Frequency Hz Unspecified 250 0-30 14-250
Pellet Mass (grams) unspecified 0.000288-0.00284 0.066 0.005-0.085
TABLE 8: Enzmann Starship (Slow Ship) Pulsed Fusion Pellet Mass
Pulse Frequency (Hz) Pellet Mass (g) Pellet Mass (g) Pellet Mass (g)
1 Engine Design 8 Engine Design 24 Engine Design
1 1000 125 42
10 100 12 4
50 20 2 0.8
100 10 1 0.4
250 5 0.6 0.2
TABLE 9: Enzmann Starship (World Ship) Pulsed Fusion Pellet Mass
Pulse Frequency (Hz) Pellet Mass (g) Pellet Mass (g) Pellet Mass (g)
1 Engine Design 8 Engine Design 24 Engine Design
1 1,100 137 46
10 100 12 4
50 20 2 0.8
100 10 1 0.4
250 5 0.6 0.2
ture work by others. Note however (Figs. 9 and 10) that even
the World Ship Enzmann concepts are dwarfed in size com-
parison to the World Ship vessels derived by Bond and
Martin [24].
5. A POSSIBLE COLONISATION TECHNIQUE
Whenever Robert Enzmann or others wrote about the Starship
design, the vehicle was often depicted not in isolation but at
least in twos and often referred to as part of a large fleet of
vessels. Upon arriving at the target star system such vessels can
choose to complete the necessary science investigations, re-
fuel, and then move onto another star system. However, eventu-
ally the colony will want to settle and this leaves the question of
what one does with the Starships. Rather than abandon them,
instead they could be used to construct giant orbiting colonies
by combining the vessels.
The vessels would be combined to form ‘Enzmann Rings’
by mating the central columns of each vehicle. Similarly, the

197
The Enzmann Starship: Hisotry & Engineering Appraisal
TABLE 10: Enzmann Starship (Slow Ship) Concept Derived
by Authors.
TABLE 11: Enzmann Starship (World Ship) Concept Derived
by Authors.
Parameter Value
Dry Spacecraft Mass (tons) 300,000
Propellant Mass (tons) 3×106
Start Population 2,000
End Population 20,000
Total Mass Ratio 11
Mass Ratio 3.32
Exhaust Velocity (km/s) 11,260
Total Delta.V (km/s) 27,000 (0.09c)
Cruise Velocity (km/s) 13,500 (0.045c)
Total Acceleration time (years) 98.67
Mass Flow Rate (kg/s) 0.96
Start Acceleration (m/s2) 0.003 (0.0004g)
Total Cruise Time (years) 51.33
Total Mission Time (years) 150
Parameter Value
Dry Spacecraft Mass (tons) 3000,000
Propellant Mass (tons) 3×106
Start Population 20,000
End Population 200,000
Total Mass Ratio 2
Mass Ratio 1.41
Exhaust Velocity (km/s) 12,119
Total Delta.V (km/s) 8,400 (0.028c)
Cruise Velocity (km/s) 4,200 (0.014c)
Total Acceleration time (years) 84.9
Mass Flow Rate (kg/s) 1.12
Start Acceleration (m/s2) 0.004 (0.0005g)
Total Cruise Time (years) 265.1
Total Mission Time (years) 350
Fig. 9 Comparison of derived Enzmann starship concepts.
rings could be combined further to make ‘Enzmann Cells’,
catering for a population of millions. Further, the cells could be
combined as more vehicles arrive until a giant ‘Enzmann Sphere’
is constructed, which is made up of hundreds of vessels and
holding a population of billions. The ‘Enzmann Spheres’ would
be the equivalent mass of a small moon and could remain in
permanent orbit about a gravitational body. Any work to re-
search these speculative ideas would need to address how each
section, ring or cell interacts with others, in relation to the
individual spins, torques and gravity fields. These concepts are
illustrated in Table 12 and Fig. 11.
6. CONCLUSIONS
In this paper we have reviewed and clarified the history and
design of the Enzmann Starship and have conducted a basic
engineering and physics assessment. The Enzmann Starship is a
concept since preceded by other engineering proposals such as
that examined for Project Daedalus, although unmanned. For
its time the Enzmann Starship was a visionary concept, requir-
ing engineering on a massive scale, presumably necessitated by
a large scale solar system wide economy. For any civilisation
that could build one of these Starships the galactic exploration
would be a real possibility. Having reviewed what is known
about the engineering concept from the literature, and under the
assumption it is constructed by an advanced interstellar society
with sufficient space infrastructure, the authors of this paper
conclude that the Enzmann Starship would work in theory.
Although a great deal of work is required to turn it into a
detailed design. The design was conservatively based on near-
term technologies, chiefly the thermonuclear version of the
“Orion” nuclear pulse drive, with sufficient mass and space to
sustain life when compared against later self-contained de-
signs, such as the Space Colony concepts of the 1970s. This
work is dedicated to Robert Duncan Enzmann, who now takes
his rightful place among the other ‘interstellar Bobs’ known by
the community, namely; Bob Forward, Bob Bussard, Bob
Parkinson, Bob Frisbee, Bob Freitas and Bob Enzmann.
ACKNOWLEDGEMENTS
The authors would like to thank Rick Sternbach and Don
Davis for answering questions on the history of the Enzmann
Starship proposal. David Hardy is also thanked for the in-
clusion of his painting ‘Enzmann at Jupiter’, commissioned
by one of the authors during 2010. Thanks go to Jay and
Michelle Snyder for giving assistance in communicating
with Robert Enzmann. Finally, the authors acknowledge the
important role that Robert Enzmann played in the history of
interstellar Starship design, by inventing a unique and vi-
sionary concept and for answering some of our questions
during this research project.

198
A. Crowl, K.F. Long and R. Obousy
Fig. 10 Comparison of World Ship (wet and dry) concepts to Enzmann starships.
TABLE 12: Colonisation Potential for Enzmann Starships
Size (km) Mass (tons) Population Equivalent Size
Enzmann Vessels Length 0.61 Tens of thousands 1,000s-10,000s Big Town
Diameter 0.091
Enzmann Rings Perimeter 10.4 Hundreds of thousands Hundreds of thousands Small City
Diameter 3.1
Enzmann Cells Perimeter 10.4 Millions Millions Large Asteroid
Diameter 9.4
Enzmann Spheres Perimeter 125 billions billions Small Moon
Diameter 40
Fig. 11 Colonisation Building with Enzmann Starships.

199
The Enzmann Starship: Hisotry & Engineering Appraisal
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* * *
(Received 25 April 2012; 24 August 2012)