This page is for starships that travel at conventional speeds. The fact that interstellar distances are so astronomically huge means the main problem is the voyages will take many thousands of years. And that's for the nearby stars, others will take millions of years.
The main problems are that human astronauts will die of old age long before the voyage ends, and most spacecraft are not built to last that long.
The starships in this page come under the headings of "Go Slow", "NAFAL" (not as fast as light) and "Apocee" (far from c). For arbitrary reasons I am defining an Apocee starship as one which cruises at a speed below 14% of the speed of light (0.14c). This is because that is the speed where the relativistic gamma factor reaches 101% (γ = 1.01). I warned you it was arbitrary.
Distance between stars is huge, traveling said distance slower-than-light will take a huge amount of time, human beings have a very limited lifespan. And it is much easier to travel at 10% the speed of light than it is to travel at 99.99999% the speed of light
"Go Slow" means to focus on the limited human lifespan problem, and be content to travel slowly at 10% c or so.
There are several ways of dealing with the lifespan issue. See sub-sections below
The paper World ships: Feasibility and Rationale suggested this terminology. I am unsure though how common this useage is .
|< 1,000||< 100,000||> 100,000|
|< 10%||Slow Boat||Colony Ship||World Ship|
|< 1%||—||Colony Ship||World Ship|
|Design||Population Size||Dry Mass|
|Enzman world ship ||20,000 - 200,000||300,000||3×106||0.9%|
|Torus world ship ||100,000||107||5×107||1%|
|Dry world ship - Mark 2A ||250,000||2.0×1011||8.2×1011||0.5%|
|Dry world ship - Mark 2B ||250,000||5.7×1011||2.3×1012||0.5%|
|Wet world ship ||250,000||2.2×1012||9.0×1012||0.5%|
 A Bond and AR Martin. World Ships-An Assessment of the Engineering Feasibility. Journal of the British Interplanetary Society, 37:254, 1984.
 A. Crowl, K. F. Long, and R. Obousy. The Enzmann Starship: History & Engineering Appraisal. Journal of the British Interplanetary Society, 65(6):185, 2012.
 Andreas M Hein, Mikhail Pak, Daniel Pütz, Christian Bühler, and Philipp Reiss. World Ships Architectures & Feasibility Revisited. Journal of the British Interplanetary Society, 65(4):119–133, 2012.
Since every atom of mass is a penalty, the logical starship would just carry a master computer and no human crew. This avoids the payload mass of the crew, the habitat module, the life support system, food, water, and everything. The starship might be under a meter long, which would make this concept the lowest mass of all the slowboat starships.
However, nobody wants wants to read about the adventures of a computer (yes, I know there have been a couple of SF stories on this theme, but it requires extraordinary skill on the part of the author, and the stories are not wildly popular. With the exception of the Bolo stories by Keith Laumer et al.).
Enter the "digital crew" concept. You postulate technology capable of "uploading" human brain patterns into a computer. In essence, the ship's computer is running incredibly advanced simulations of the crew, creating a virtual reality much like that found in the movie The Matrix. This also allows the author to pontificate upon the nature of reality, ask if we are actually unaware virtual people in a virtual reality, and stuff like that. Authors who have used this concept include Sean Williams, Shane Dix, and Greg Egan.
The point is the author is allowed to write stories about human beings, but the digital humans and their digital environment take up zero mass.
One could add equipmment capable of manufacturing artificial bodies for the crew from local materials upon arrival at the destination. However, the advantage of a digital crew ship over a seed ship is the lower ship mass due to the absence of frozen embryos, artificial wombs, and robot mommies. Adding artifical body manufacturing facilites would reduce or remove the advantage. The only remaining advantage is that the new bodies inhabited by adults instead of babies.
You could regain the advantage if the manufacturing equipment is really tiny. Say a couple of grams worth of nanotechnology self-replicating machines, intended to work on handy asteroids or other free materials lying around the destination solar system. The nanotechnology bootstraps itself by replicating using in-situ resources as feedstocks until it has mass of a few tons, then shifts gears to start manufacturing artificial bodies.
The next higher mass class of slowboat is the Seed ship aka Embryo Space Colonization via an embryo-carrying interstellar spaceship (EIS). It will tend to have more mass than a Digital Crew ship and less than a Sleeper Ship.
The starship is tiny, containing a payload of millions of frozen fertilized eggs, artificial wombs, robot factory, and a master computer. No mass is needed for life-support, habitat modules, or any human crew.
After traveling for thousands of years, the ship lands in a good spot for a colony. The robot factory starts cranking out robots. Robots build the settlement buildings and start growing food (if the planet is really nasty they might have to spend a few centuries terraforming the planet first). Then the master computer thaws out enough eggs for the available artificial wombs, brings the babies to term, then tries to convince the babies that the robots are mommy and daddy.
I don't know about you but I suspect that the first generation is going to grow up a little bit emotionally stunted.
The most straightforward method is to cryogenically preserve human embryos. The more difficult but more flexible method is to carry frozen sperm and egg cells, and do in vitro fertilization at the destination. The most unobtainium method is to carry genetic information in computer files, then synthesize the required genetic sequences at the destination.
As with all interstellar colonization proposals, there are quite a few technological challenges to solve:
The ship robots will have to be advanced enough to raise and nurture the children, as well as building the settlement and growing crops. They could be teleoperated drones controlled by the ship's computer.
The side problem is they will have to be manufactured at the destination using in situ resources. The whole idea behind the Seed Ship is to minimize the payload mass, carrying an army of robots negates this.
Examples of Seed Ships in science fiction include The Songs of Distant Earth by Sir. Arthur C. Clarke, 2001 Nights chapter Night 4 by Yukinobu Hoshino, Long Shot by Vernor Vinge, Voyage from Yesteryear by James Hogan and the movie Interstellar.
Sleeper ship tend to have more mass than a Seed Ship and less than a Generation Ship.
The crew is frozen into suspended animation, so they do not age nor require food and oxygen during the thousand year journey. Or spacious living accomodations. The Sleeper Ship does require the mass of the crew, enough mass for a spartan habitat module, and only enough consumables for the time the crew will be awake.
Poul Anderson warned that frozen crew have a limited shelf life. Naturally-occurring radioactive atoms in the human body will cause damage. Normally the body will repair such damage, but one in suspended animation cannot. After a few hundred years, enough damage will accumulate so that a corpse instead of a living person is thawed out at journey's end. This may force one to thaw each crew member every fifty years or so to allow them to heal the damage, then freezing them again.
The highest mass type of slowboat tends to be the Generation ship. This is because it has to carry the mass of an entire community as crew, a habitat module at the minimum the size of a small town, and enough life support for the people for however many hundreds of years the journey takes. As the ship crawls to its destination, generations of people are born, have children, and die of old age.
A flying asteroid can be defined as a spaceship, using the principle that anything can be a spaceship if you throw it hard enough.
If the generation ship is escaping from some Terra-destroying catastrophe; carrying Terra's scientific and cultural heritage, a representative sample of animal species, colony equipment and supplies, and a fertile representative sample of humanity, the craft is termed an Interstellar Ark.
Problems include the later generations refusing to cooperate with their forefather's vision, civil wars that wreck the ship, failure of the closed ecological life support system, ship arriving at the destination but the current generation does not want to colonize an icky dirty planet because they like living in their ship and the later generations forgetting where they came from, forgetting where they are going, and indeed forgetting the fact that they are in a starship. The classic "forgetting you are on a ship" stories are Robert Heinlein's Orphans of the Sky (1941) and Brian Aldiss' Non-Stop (1958).
In Larry Niven and Jerry Pournelle's FOOTFALL, the aliens deal with the "forgetful generation" problem by including a group of original crew frozen in suspended animation. Members of the original crew are periodically woken so they can ensure that the generational crew keeps the faith. The concept is sort of a combination of sleeper starship and generation starship.
The concept was sort of touched on in Don Wilcox's The Voyage that Lasted 600 Years (1940), though in that story only the captain was frozen. Since he was only a single person he had a limited influence on the generational tribes.
There are also disturbing ethical questions about the morality of condemning several generations of people to living inside a space-going rock. This can lead to political problems starting a Generation ship development project in the first place.
Noted SF author Kim Stanley Robinson apparently got very annoyed at people who dismissed the generation ship ethical questions while simultaneously being infected with a blind faith techno-utopianism over the feasibility of building a generation ship. Currently our technology has utterly failed to build a closed ecological life support system that can operate longer than a year or so, much less a couple of centuries. The same goes for spacecraft components. The second law of thermodynamics is a harsh mistress. The techno-utopians have to understand that there is no plan B.
As a sort of wakeup call, Mr. Robinson wrote the deliberately pessimistic novel Aurora about a disastrous generation ship heading for Tau Ceti and all the things that could and did go wrong. And the novel featured some techno-utopian characters in a most unfavorable light. The more optimistic members of the science fiction community have been raging at Mr. Robinson ever since.
A variation of the "Increase Lifespan" technique was in Charles Sheffield's Between The Strokes Of Night. A technique was discovered that would allow human metabolism to enter the "S-state." In this state, humans age at a rate 1/2000th normal, and perceive things at the same rate. There was also a protocol that would return an S-state person back to normal metabolism.
So with ships traveling at a slow 10% light speed, the trip to Proxima Centauri seems to take only a few weeks to an S-state person. Of course to a human in normal state, the trip will take about forty years.
As far as the S-state person is concerned, the ships are travelling faster than light. As long as they always stay in S-state.
To an S-state person, a normal state person moves so fast that they are invisible. To a normal state person, an S-state person appears to be immobile, though they are actually moving very very slowely. Of course to an S-stater all those normal state persons grow old and die 2000 times faster.
Finally there is the "Methuselah" concept. Advances in medical technology might increase human lifespan to thousands of years. So prolonged interstellar trips are more a problem of boredom instead of life-span.
The advantage is the starship does not have to carry the mass of the engine and the propellant, you leave it at home. This makes the task of designing the starship merely incredibly difficult, instead of utterly impossible.
The home system can also add to the laser batteries gradually after the starship's journey starts, as needed (the inverse-square law will weaken the beam as the range increases). You cannot do this with a self-contained starship, all of its engines have to be built before the journey starts.
And if a home system's laser battery or two break down, no problem! The resources of the home system are available to fix it. If a self-contained starship engine breaks down on the other hand, they are in trouble. They do not have the resources of the home system to help, all alone in interstellar space. They have to fix it themselves with whatever spare parts they managed to bring along. Or die all alone in the night.
The disadvantage is the starship is at the mercy of whoever is in charge of the laser station back in the Solar System. If there is a revolution back home and the Luddites seize power, the starship and crew are up doo-doo pulsar with no gravity generator. Dr. Forward came up with two clever ways of using the home system's lasers to decelerate the starship into the target solar system. In Larry Niven and Jerry Pournelle's classic The Mote In God's Eye the Motie aliens laser sail starship rather pointedly do NOT use Dr. Forwards deceleration methods, because they absolutely do not trust the laser station controllers back at their homeworld. In Dr. Forward's The Flight of the Dragonfly aka "Rocheworld" political foot-dragging and short-sighted policies almost lead to disaster for the laser station and starship. In Buzz Aldrin and John Barnes's Encounter With Tiber politics does kill the laser station and starship.
Starwisp is an ultra-low mass interstellar probe, a tiny sail driven by a beam of microwaves. The concept was invented by Dr. Robert L. Forward, and expanded upon by Dr. Geoffrey A. Landis.
Dr. Forward assumed that the microwave beam would be efficiently reflected by starwisp, so he calculated it would be a superconducting metal mesh with a sail mass of 16 grams and a payload mass of 4 grams; total mass of probe is 20 grams. Dr. Landis found this turned out not to be the case, it would absorb quite a bit of microwaves and heat up (i.e., the design is thermally limited). In Dr. Landis' design the starwisp is woven out of carbon wires with a sail mass of 1,000 grams, a payload mass of 80 grams, and a diameter of 100 meters.
Acceleration is 24 m/s2, microwave lens 560 km in diameter transmitting 56 GW of power, accelerating the probe to 10% of the speed of light.
Yes, it probably could be weaponized. See Accelerando by Charles Stross.
So your gigantic laser battery at home pushes the laser sail starship to its destination, accelerating it to about half the speed of light. Presumably you want to stop at your destination instead of streaking through it at 0.5c. But how?
If you were going about an order of magnitude slower, you might be able to use the sunlight from the destination star to put on the brakes. However that ain't gonna be enough at 0.5c. You'll just pancake into the star at a substantial fraction of the speed of light and be vaporized.
Dr. Philip Norem had a clever idea. Interstellar space has large magnetic fields. So one can use large electrical charges on the starship to make huge light-year wide sweeping turns by the Lorentz force.
Say you were going to Alpha Centauri. You aim the starship not at the destination, but instead off to one side. How far off depends upon the starship's turning radius. The laser battery back at the solar system pushed the starship up to relativistic velocities over the next 27 years or so. Then the lasers turn off.
The starship deploys one hundred metal cables, each about 50,000 kilometers long. It then charges them up to 800,000 volts and 3.7×104 coulombs. This is timed to interact with the interstellar magnetic field (as mapped) so that the starship makes a huge gradual turn, until it is approaching Alpha Centauri from the back door. That is, so that a line drawn from the starship to Alpha Centauri will pass directly through the solar system and the laser battery.
Meanwhile, the solar system laser battery starts up its barrage long enough in advance so that the leading edge of the laser wavefront will reach the starship just as it is aligned properly. It then continues the barrage for the years required to bring the starship to a halt exactly at Alpha Centauri.
In Mallove and Matloff's The Starflight Handbook, they note that if the interstellar magnetic fields have not been well mapped, this scheme could potentially doom the starship to a lonely death. If the starship misses the beam, it just goes sailing off into the Big Dark. The Starflight Handbook has the equations for a starship using the Lorentz force, if you are interested.
Bottom line: a starship using this propulsion could manage velocities of about 0.5 c, and expends energy at about the same rate as a laser lightsail. It is suitable for carrying massive payloads on multi-century long missions. It could manage to propel a generation ship or bulk materials for an interstellar colony.
Of course a generation ship would have to hope that the people back at Sol will be willing to constantly supply them with 10 terawatts of laser beam for 370 years.
Spacecraft with interstellar medium momentum exchange reactions (SWIMMERs) is another propulsion system attempting to avoid the need to carry reaction mass. Much like the laser light sail. Because when you are trying to delta-V your spacecraft up to fractional c velocity the Tyranny of the Rocket Equation will kneecap you like a loan shark's henchman, and for similar reasons. The "interest rate" on the propellant is just too freaking high.
SWIMMERs are actually more energy efficient than light sails. It is very similar to Robert Zubrin's dipole drive, and the two might actually be different designs for the same concept. Zubrin's design has a constant electric field while the SWIMMERs use a pulsed electric field.
SWIMMERs take advantage of the fact that while the interstellar medium (ISM) could pass for a good vacuum in a laboratory, that is NOT the same as having no atoms in it at all. It still has about a million atoms per cubic centimeter.
Electric sails are propelled by solar wind, Magsails are propelled by the solar magnetic field. Both are very low mass structures composed of charged or current carrying wires. They both can use the heliosphere or ISM as drag, to slow the sail down.
SWIMMERs attempt to use low mass structures composed of charged wires to somehow interact with the ISM and create thrust instead of drag. The exact opposite of electric sails.
The idea behind SWIMMERs is to set up "inhomogeneities" in the surrounding ISM that the SWIMMERs electric field can push upon to create thrust.
The pusher plate is composed of tether segments, much like a standard electric sail. However, unlike the e-sail, the tethers are two identical layers of wire separated by strong insulator. The insulator keeps the wire layers physically apart and electrically isolated. The tethers are constructed with fine superconducting wire. The pusher plate is spun to create tension and keep the tether grid extended without requiring a heavy support structure.
In the Primer phase of the operational cycle, the front layer of the tethers are charged up to positive potential φ, where φ is a bit above the stopping potential of the ions in the local interstellar medium. The back layer of tethers are charged to negative potential -φ.
Due to something called the "edge effects of the finite plates and the self-shielding behavior of plasmas", ISM ions streaming toward the front positively charged layer slow down. This creates an over-dense clump in front of the pusher plate. Simultaneously an underdensity forms between the clump and the front of the tethers.
Next comes the Pull phase of the operational cycle. The charges on the front and back layers are reversed and significantly increased. The ion clump in front of the pusher plate is attracted to the (now) negative front layer, pulling the spacecraft forwards (creating thrust).
However, if the ion clump actually strikes the pusher plate the plate charge will create drag and negate the thrust. This has been taken into account. Remember the underdensity that was formed right over the pusher plate? There ain't hardly no ions in that zone, so they will cause only minimal drag. This gives time for the approaching ion clump to create plate thrust. When the ion clump is close to actually striking the pusher plate, the plate charge is turned off. This allows the ion clump to pass through the plate, again with only minimal drag.
Finally comes the Push phase of the operational cycle. The plate charge is turned back on with the same polarity. Since the ion clumps are now behind the plate, they are repelled and create more thrust by the law of action and reaction.
Due to symmetry, while the positively charged ions are moving front to back, the negatively charged electrons are moving back to front. Ordinarily this would cause reverse thrust that would cancel out the forward thrust, with a net thrust of zero. Luckily, ions weigh 1,836 times more than electrons, so they create 1,836 times more thrust. The 1/1,836 reverse thrust of the electrons is negligible.
It may be possible to use multiple layers of pusher plates to accelerate ions to a higher thrust. The report says that may be worth looking into.
Laser sail can only be pushed by the laser beam from Sol. This means if the laser sail has to brake to a halt at the destination, or return back home, it will have to do all sorts of weird things.
Since SWIMMER can direct its thrust independently of the energizing laser beam, it has no problem at all either braking to a halt at destination nor returning home. Or even braking to a halt back home at journey's end. Normal mode is used for traveling to destination, Destination Braking mode is used to halt, Tractor Beam mode is used to return to Terra, and Home Braking mode is used to stop at Terra.
|tcruise||time to traverse 1 parsec (3.26 ly) in the ISM, ignoring time spent in Sol's heliosphere and time spent decelerating near α Cen A.|
|tcruise/tsail||compares cruise time with time required by ideal massless light sail with equal payload pushed by equal amount of power|
|Pdelivered||power delivered to SWIMMER by laser beam based back at Sol|
|Mpay||mass of payload|
|Mpower||mass of power conversion system (converts laser light into electricity)|
|Summed tether length||total of all the individual pusher plate tethers added together|
|Mpusher||mass of pusher plate tethers (decreases during journey as excess plate mass is jettisoned)|
|Space probe||Ark ship|
|Vmax||0.020 c||0.014 c|
|tcruise||260 years||370 years|
|Pdelivered||10 MW||10,000 GW|
|Mpay||1000 kg||8×109 kg|
|Mpower||2500 kg||1×109 kg|
|Initial pusher plate parameters|
|Summed tether length||4.1×109 m||2.0×1016 m|
|Mpusher||7400 kg||3.7×1010 kg|
|Final pusher plate parameters|
|Summed tether length||2.9×108 m||1.0×1015 m|
|Mpusher||520 kg||1.9×109 kg|
For the sample missions, the paper assumes that a standard interstellar sail laser array will work equally well with SWIMMER. Since there have been quite a few papers about such laser arrays the paper doesn't bother doing any analysis.
The paper didn't go in deep analysis of the power conversion system that transforms the laser light into electricity. It assumes the system will have a specific power of 4 kW/kg. If the system has a worse specific power, the trip times will be increased.
For the Sol to α Cen A trip, a simplified model of the ISM was used. It was assumed to be uniform with a density of 0.07/cm3 and a temperature of 7000 K. The heliospheres of both Sol and α Cen A were assumed to have a density of 7.3/cm3 and a temperature of 140,000 K. Furthermore the solar wind in both heliospheres are assumed to be uniformly streaming outward at 5×105 m/s to a distance of 100 AU, at which point the medium abruptly transitions to a stationary ISM.
This is to deliver a 1,000 kg payload to α Cen A, decelerating it into gravitational capture to become a permanent space telescope observing the α Cen system. It will only require 10 megawatts of laser beam. Constantly supplied for 260 years of course. If back at Terra there is a war obliterating the laser array, a drastic cutback of the space exploration budget, or other interruption of the beam; the space probe will just go sailing into the big dark. Which will be a waste of space probe dollars but that's life in the big city.
SWIMMER would work well for a generation ship due to its extremely favorable performance at lower powers and velocities. For purposes of example the paper sets the payload mass at 8×109 kg, the mass of Freeman Dyson's Super Orion ship. The paper assumes that for such an ambitious mission the power conversion system would have an improved specific power of 10 kW/kg.
It will require a 10,000 gigawatt laser beam. Constantly operated. For 370 years. If anything interrupts the beam the space ark will also go sailing into the big dark. Only this time there will be generations of people living inside. For a while at least...
A related issue is mechanical reliability. Currently the best space probe NASA can build cannot be guaranteed to properly function past about forty years. The starship will need an extensive self-repair capability or have some way of having humans periodically available to fix things.
A common science fiction gag is the "jumping the gun" plot. A slower than light ship departs on a 500 year journey to Alpha Centauri. About 100 years after launch, some joker on Terra invents a faster-than-light starship. Fleets of FTL ships fly to Alpha Centauri and colonize the place. The slower than light ship arrives to find not the virgin planets they were expecting, but instead 400 year old colonies. Har, har, silly slowboat.
The earliest example of this trope that I could find was A. E. van Vogt's "Far Centaurus" (1944)
In 2006, scientist Andrew Kennedy actually studied the problem. He published his analysis in a paper called Interstellar Travel: The Wait Calculation and the Incentive Trap of Progress. In it, he introduced his solution: the Wait Calculation.
The Wait Calculation allows future space explorers to avoid the "jumping the gun" problem (and also avoid being paralyzed with indecision by terror of jumping the gun). The equation shows that, assuming technology develops in such a way that there is exponential growth in the velocity of travel, there is an optimal departure time for arriving earliest. Kennedy states that the equation applies even if somebody invents faster-than-light travel.
You see, there comes a time when although technological advances continues to produce higher speeds, the waiting time for that advance is too long to make up the velocity difference. If you wait too long for a higher speed, a slower ship launched sooner will have enough of a head start to beat you.
Design Population Size Dry Mass
Dry world ship - Mark 2A  250,000 2.0×1011 8.2×1011 0.5% Dry world ship - Mark 2B  250,000 5.7×1011 2.3×1012 0.5% Wet world ship  250,000 2.2×1012 9.0×1012 0.5%
The Bond and Martin  world ships are the largest proposed world ships in terms size and mass. Figs. 3 and 4 show a artist conceptions of the Bond and Martin world ships . In particular Fig. 4 provides a size comparison of the ”Dry World” (habitable area is mostly land) and ”Wet World” (habitable area is mostly water) world ships.
It can be seen that the habitat (large cylindrical section) and the propulsion module (second cylinder in the back with nozzle) are dominating the designs. In addition, a flat, circular dust shield is attached to the front of the world ship. A Bond and AR Martin. World Ships-An Assessment of the Engineering Feasibility. Journal of the British Interplanetary Society, 37:254, 1984.
This is from Colonized Interstellar Vessel: Conceptual Master Planning by Steve Summerford (2012)
The focus of the report is on the habitable space of the starship. It is assumed that any suitable interstellar propulsion system can be attached, so that is outside the scope of the report. Some sort of fusion power generation is assumed, since the inverse-square law will solar photovoltaic panels worthless long before the edge of the solar system is reached.
Mr. Summerford notes that all those images of generational starships built inside hollow asteroids are very pretty, but it will be centuries before our technology is up to actually building one. Let alone paying for it. Yes, it would be real nice to have a starship environment that looks like Terra. But in the spirit of learning how to walk before you run, perhaps it would be best to design a more modest starship. True, the enviroment will probably look more urban and less like rural. However it will be pleasant to have a design we can actually build. And pay for.
The drawback is that while the other methods only have to worry about breakdowns of the ship's technology, generational ships have to also worry about social breakdowns of generations of inhabitants. So attention must be paid to preservation of the physical, spatial, and psychological needs of the colony. It wouldn't do for the starship to fail because the fifth generation snapped psychologically due to living for 125 years inside miserable little steel boxes. These design problems are just as important as matters of radiation shielding, fuel suppply, and food production.
Therefore the report does devote some space to issues such as architecture, healthy community living, privacy needs, and other "soft" problems. Many of the principles come from basic urban planning.
To expand a Standford Torus, the logical method would be to stack several toruses. But this would be an engineering nightmare, and a wasteful increase of mass. Instead, a banded cylinder design is used. Flat-bottomed faceted cylinders called "bays" are used. This allows sufficient colonial living space while maintaining a compact vehicle design.
Each bay contains dwellings, agicultural areas, civic structures, open spaces, an places of work and research.
To avoid spin nausea, the spin rate should be no more than about 2 rpm. The design goes with 1.5 rpm, spin radius of 400, meters, and an artificial gravity of 1.0 g. A rate of 2.0 rpm was looked at to reduce the spin radius, but the increase in the coriolis effect was considered unacceptable.
For urban planning, they use a metric called "floor-to-area ratio" or FAR. In a site, you take all the buildings therein, and add up the surface area of all the floors in all the buildings. This is the "floor". Divide by the surface area of the site and you have the FAR. Example: if a site with a surface area of 10,000 m2 had a single bulding, and it was a four story building with each floor containing 2,500 m2, then the FAR would be 1.0. The FAR of the generation ship was set to from 1.25 to 1.75 in an attempt to enforce some park like open areas with no buildings.
The Enzmann starship is a concept for a manned interstellar spacecraft proposed in 1964 (date is disputed) by Dr. Robert Enzmann. Over the years the basic design has evolved, and there were several types in the initial design. It was quite popular in the science fiction community. An analysis of the Enzmann starship can be found here.
In 1972 space artists Don Davis and Rick Sternbach worked with Dr. Enzmann to develop the idea. This refined the "lollypop" look of the ship. For some odd reason most paintings of the Enzmann starship show two of them in formation. The original design had a naked sphere of frozen deuterium as fuel. Calculations with Sternbach and Davis revealed that the deuterium could not be kept frozen and was too structurally weak to be accelerated. So the redesign encased the deuterium in a huge tank.
The Enzmann exploded into the science fiction community with the October 1973 issue of Analog magazine. G. Harry Stine wrote an extensive article about the concept, accompanied by a stunning piece of cover art by Rick Sternbach. Stine said the ships were 12 million tonnes, could reach 0.30 c (highly unlikely), had 8 engines, and used spinning habitats for artificial gravity.
- Command center 30 meters in diameter
- Central core load bearing struture 15 meters diameter
- Frozen deuterium 300 meters diameter
- Living modules 90 meters diameter × 90 meters long
- Engineering compartments 70 meters diameter
In Science Digest, Rick Sternbach's 1972 piece depicts a pair of Enzmanns departing from an asteroid factory. The number of engines was increased from 8 in the original design to 24. Modular sections were created that can separate from the starship. Height of starship was 690 meters. 3 million tonnes of deuterium, with metal shell (doubling as a radiation shield). Magnetic confined fusion propulsion. 20 decks per habitat, 100 rooms per deck. Cruising speed 0.09c.
Thomas Schroeder wrote an article entitled "Slow Boat to Centauri" in Astronomy Magazine. Claimed a cruising speed of 0.1c, and an advanced design might reach 0.3c. 12 million tonnes deuterium. The outer layers of the habitats were composed of bulk material as radiation shielding for the inner layers. Bulk means nuclear reactor, store rooms, heat exchangers, airlocks, landing craft, observation areas, communication equipment. Eight Project Orion nuclear pulse units.
- Frozen deuterium 305 meters diameter
- Height 609.6 meters
- Living modules 91.5 meters diameter × 91.5 meters long
- From bottom of fuel sphere to top of Orion engines 305 meters
In the 1980's Dr. Enzmann started designing variants.
In 2011, K. F. Long, A. Crowl, and R. Obousy did a study on the Enzmann starship, and tried to rationalize it with recent developments in astronautics. First they took the historical concepts:
|Sphere Diameter||305 m||same||same|
|Total Habitat Length||273 m||same||same|
|Individual Habitat length||91 m||same||same|
|Habitat Diameter||91 m||same||same|
|Core Diameter||15 m||same||same|
|Propellant Mass||3×106 metric tons||12×106 metric tons||3×106 metric tons|
|Cruise Speed||27,000 km/s|
Long et al created three variants. The primary difference is the size of the population carried. The rest of the design was re-sized to handled the population. As near as I can calculate, the cruise and mission times for all three are for a mission to Alpha Centauri.
|Specific Power||11.5 MW/kg|
|Thrust Power||344 terawatts|
|Length||620 m||979 m||1752 m|
|Dry Mass||30,000 MT||300,000 MT||3,000,000 MT|
|Propellant Mass||3 × 106 MT||3 × 106 MT||3 × 106 MT|
|Exhaust Velocity||11,700 km/s||11,260 km/s||12,119 km/s|
|Cruise Velocity||27,000 km/s|
|18.95 yrs||98.67 yrs||84.9 yrs|
|41.05 yrs||51.33 yrs||265.1 yrs|
|60 yrs||150 yrs||350 yrs|
|Mass Flow Rate||5.02 kg/s||0.96 kg/s||1.12 kg/s|
|Thrust||58,730 kN||10,810 kN||13,573 kN|
|Mission Duration||48 years|
|Cruise Phase||44 years|
|Boost Phase||2.05y (stage 1)|
+ 1.78y (stage 2)
|Wet Mass||5.4×107 kg|
|Propellant Mass||5.0×107 kg|
|Dry Mass||4×106 kg|
|Payload Mass||450,000 kg|
|Min Kinetic Energy to cruise speed||3.5×1022 J|
|Min Power to cruise speed||291TW|
|Pellet pulse freq||250Hz|
|Stage Dry Mass||1.69×106 kg|
|Num Propellant Tanks||x6|
|Propellant Tank Mass||7.666×106 kg|
|Total Propellant Tank||4.5996×107 kg|
|Stage Wet Mass||4.7686×107 kg|
|Accel (1st stage ignition)||0.14 m/s2|
|ICF capsule Q||63|
|Exhaust Velocity||1.06×107 m/s|
|Specific Impulse||1.08×106 sec|
|Stage Dry Mass||9.8×105 kg|
|Num Propellant Tanks||x4|
|Propellant Tank Mass||1.0×106 kg|
|Total Propellant Tank||4.0×106 kg|
|Stage Wet Mass||4.98×106 kg|
|Accel (2nd stage ignition)||0.13 m/s2|
|Accel (2nd stage burnout)||0.676 m/s2|
|ICF capsule Q||35|
|Exhaust Velocity||9.21×106 m/s|
|Specific Impulse||9.39×105 sec|
This was a study conducted between 1973 and 1978 by the British Interplanetary Society to design a plausible uncrewed interstellar spacecraft. Yes, it is naught but a mere space probe, but it is a starship.
Given the mission parameters and state-of-the-art of various propulsion designs, the study chose an inertial-confinement fusion engine. In an effort to cut down on radiation shield mass, they selected the close-to-aneutronic deuterium—helium-3 fuel. A neutron radiation shield protecting a starship 190 freaking meters long will savagely cut into the payload mass. Since there isn't enough helium-3 in the entire world, the study proposed a titanic mining operation to gather the stuff from Jupiter's atmosphere. The study details about 3He skimming is why the Traveller role-playing-game allows "Wilderness Refueling". But I digress.
Even using a two stage fusion propulsion system, Daedalus could only get up to 12% the speed of light, and could not slow down. The good news is it only takes 48 years to reach Barnard's Star (so some of the scientists who sent it might actually live long enough to see the data). The bad news is since it cannot slow down, it will streak through the Barnard's system at 12% c and have only a couple of days to frantically take pictures and do all the science it possibly can. Basically if Daedalus was designed so it could come to a halt at the Barnard system, the trip time would double to about 96 years. This would guarantee that all the scientists who sent the blasted ship would be long dead of old age, which makes it hard to work up any enthusiasm to work on the project. Or to convince the public to fund it.
|Inertial Confinement Fusion|
|1||Pellet Injection Gun|
|2||Superconducting field coils (4)|
|3||Electron beam generators|
|4||Plasma exhaust jet|
|6||Energy extraction coils|
|7||Frozen nuclear pellet|
Project Icarus is a design study to create an unmanned probe starship to investigate a nearby star. It was inspired by the old Project Daedalus study. In 2013 they had a "contest" for research groups to submit proposed designs given some pre-defined limitations.
|Wet Mass||17,800 metric tons|
|Dry Mass||2,365 metric tons|
|Payload||150 metric tons|
|Propulsion||Z-Pinch DD Fusion|
|Exhaust Velocity||1.289×107 m/s|
|Accel time||4 years|
|Coast time||93 years|
|Decel time||1 years|
|Wet Mass||45,000 metric tons|
|Dry Mass||3,000 metric tons|
|Payload||150 metric tons|
|Propulsion||Z-Pinch DD Fusion|
|Specific Impulse||one million seconds|
|Accel time||25 years|
|Coast time||70 years|
|Decel time||5 years|
Most of the other Icarus fusion designs use inertial confinement fusion. That's because IC fusion is easier to get halfway worthwhile power levels. Magnetic confinement fusion would be nicer but once you get enough nuclear fusion going to to be worthwhile, the magnetic bubble pops like a cheap balloon.
The drawback to IC fusion is that the confinement time is pathetic. The longer you confine the fusion reaction, the more of the fusion fuel actually burns and generates energy. But in IC fusion the first bit of fusion acts to blast the pellet apart, scattering the un-burnt fuel to the four winds.
Back in the olden days of fusion research, the darling was Z-Pinch fusion. You send a bolt of electricity (about 5 mega-amps) down the center of a long tube full of ionized plasma, creating magnetic field which compresses the plasma enough to ignite nuclear fusion. One of the big advantages with Z-Pinch was that the confinement time (and net energy output from the burn) can be increased by simply making the reaction chamber longer.
Unfortunatley, the disadvantage is that Z-Pinch fusion suffers from several hydrodynamic instabilities which disrupt the plasma. So researchers stopped working on it in.
But in 1998 Dr. Uri Shumlak discovered you could eliminate the instabilities if you made the plasma move at high velocities. Based on this work, Z-Pinch was selected for the Icarus design.
The Firefly's long thin tail is the Z-Pinch tube, frantically fusing and radiating x-rays like a supernova. So the starship was given its name for similar reasons as the one on the TV show: it is a flying thing whose tail lights up.
The spacecraft profile is long and skinny, for two reasons:
- Its cruise velocity is a substantial fraction of the speed of light (4.5% c for the 2013 version). This make interstellar dust grains impact with about 9.1×10-4 joules worth of damage, the equivalent of 46,000 cosmic ray photons. You want to reduce the ship's cross section as much as possible to minimize the number of grain impact events.
- The longer the ship is, the farther the payload can be placed from the deadly radioactive Z-Pinch drive, taking advantage of distance shielding.
Many other starship designs use 3He-D fusion, because all the reaction products are charged particles that can be easily shieldied. The drawback is that 3He is rare, you'd have to harvest the atmosphere of Jupiter for twenty years in order to get enough.
Instead, Firefly uses D-D fusion, since deuterium can be easily found in common seawater. Of course then you have to deal with all the nasty neutrons and x-rays produced by that reaction. Firefly's approach is to forgo the use of massive radiation shields, and instead try to let as much of the radiation escape into space. The Z-Pinch core is almost totally open to space with only a triad of support rails connecting the aft electrode and magnetic nozzle to the rest of the vessel.
Even with that, the waste heat is going to be titanic. That's where the heat radiators come in. Notice how they are the bulk of the ship. Makes the thing look like a garantuan lawn-dart. The radiators use beryllium phase-change technology, and are positioned as close as possible to the heat loads on the tail.
A long conical shield forwards of the reactor core deflects x-rays away from the payload using shallow-angle effects. The electrodes, rails, and other structure near the core are constructed of zirconium carbide (which is capable of surviving the intensely radioactive environment.
The 2014 design had a total length of just under one kilometer, half of which is the fuel tanks. The forward part of the ship uses the old fuel tank in lieu of spine trick in an effort to save on ship mass.
A fission reactor provides secondary power.
|Pellet Mass||2 grams @|
|Ignition Rate||150 Hz|
|Specific Impulse||540,240 s|
|Exhaust Velocity||5,299,750 m/s|
|Dry Mass||7,444,000 kg|
|Propellant Mass||247,100,000 kg|
|Wet Mass||263,991,000 kg|
|Mission Duration||118.5 years|
|TOTAL TANK||927,000 kg|
|TOTAL PAYLOAD||150,000 kg|
|TOTAL SUBSYSTEM||4,670,000 kg|
|Magnetic Sail||1,780,000 kg|
|Dust Shield||15,000 kg|
|Tritium Prod (decel)||95,000 kg|
|D/T tanks, pellet manufacture||20,000 kg|
|TOTAL FUSION ENGINE||11,144,000 kg|
|Laser system||1,000,000 kg|
|Neutron Shield||82,000 kg|
|TOTAL DRY MASS||7,444,000 kg|
|TOTAL PROPELLANT||247,100,000 kg|
|TOTAL WET MASS||263,991,000 kg|
This was the winner in Project Icarus' 2013 contest to design an interstellar starship using current technology. The entry was created by the Munich Ghost Team headed up by Andreas Hein. The basic rules were to design a spacecraft which was mainly fusion powered and on a mission to Alpha Centauri carrying a 100 to 150 tonnes payload and reaching the destination in no more than 100 years.
The design uses Deuterium-Deuterium fuel, even though it has only half the exhaust velocity of Deuterium-Tritium and Deuterium-Helium3, and about 38% of the energy expresses itself as nasty neutron radiation. They rejected D-T because blasted Tritium has a freaking half-life of only 12 years so most of it would decay away during the 15.6 year acceleration phase and the 54 year coast phase. You'd have to carry a huge excess penalty mass of extra Tritium to allow for decay. They also rejected D-He3 because there probably isn't enough He3 on all of Luna, and harvesting it from a gas giant's atmosphere would require a huge space infrastructure.
Forced to use D-D, the designers looked for ways to turn that pesky neutron flux from a liability into an asset.
Standard inertial-confinement fusion engines use a circular firing squad of lasers to implode the fuel pellet. The compression ignites the fusion fuel. The designers note this is a bit inefficient. By analogy, it is possible to detonate a stick of TNT by squeezing it but you have to squeeze real hard. It takes a lot less energy to use a match to light the fuse on the TNT.
So the designers used a so-called "fast ignition scheme". The circular firing squad just has enough laser power to confine the fusion fuel, but not the extra energy needed to compress it to ignition. A secondary high-powered laser acts as the fuse, piercing the pellet and igniting it. You get the same energy from the pellet, but you need a whole lot less input laser energy.
Alas, a "whole lot less" is still freaking huge. Lasers are power hogs.
The standard method is to harvest some of the fusion energy and convert it into electricity. This is stored in huge banks of heavy capacitors, to be used for the next laser pulse. The initial capacitor charge comes from a nuclear reactor or something which trickle charges the capacitor banks. The problem is the mass of all those capacitors is a punishing amount of penalty-weight.
That's when the designers turned the D-D waste neutron flux into an asset. Have you ever heard of Nuclear-pumped lasers?
All lasers consist of a lasing medium which emits laser light when it is pumped. Conventional lasers pump the medium with electricity or light. Nuclear lasers on the other hand pump with the awesome might of nuclear fission. Uranium-235 is exposed to neutrons, undergoes fission, the energy pumps the lasing medium, and a rather powerful laser beam emerges.
The main draw-back of nuclear-pumped lasers is the lack of convenient sources of high neutron flux. A nuclear reactor can provide a bit of neutrons, and in theory a fission warhead detonation can provide lottsa neutrons (see Bomb-Pumped Lasers). The light-bulb went off over most of your heads while you were reading the previous paragraph. Yes, the neutron flux from the detonating D-D fuel pellets would work splendidly.
The designers used a solid-core nuclear laser instead of liquid-core, since solid-core is more suitable for generating extremely short high-powered pulses. A ring-shaped chamber circles the thrust chamber, centered on the fusion pellet detonation point. The chamber is filled with a uranium dioxide aerosol and a fluorescent gas acting as the lasing medium. Some of the neutrons from the fusion detonation enter the chamber, causing fission reaction with the uranium 235 atoms, the fissile products then excite the fluorescent gas thus pumping it. The light flash from the fluorescent gas is transmitted through a light pipe into the laser amplifier. This creates the laser beam.
This system is about 8% efficient, which is pathetic for a general device but actually fantastic for a laser. And it is using all those otherwise worthless neutrons. The drawback is the uranium dioxide aerosol and fluorescent gas are expended with each laser bolt, that is, they are consumables. Which adds to the mass load.
However, even with the consumables the total mass of the ignition system is less than 1,000,000 kilograms, which is far less than all those banks of capacitors.
The report states an exhaust velocity of 0.018 c, which is considerably smaller than the theoretical maximum of D-D fusion (0.043 c). Which is probably very realistic.
As with all high-energy propulsion there is huge amounts of waste heat to get rid of, and this system does not lend itself to open-cycle cooling (not enough mass in the exhaust to carry off enough heat). Meaning you actually need plenty of heat radiators. The design use liquid-droplet radiators with a total area of 7.6 square kilometers. It has a very high heat rejection rate of 500 kW/kg by using liquid aluminum as the heat-conducting liquid.
The spine of the ship is a cylindrical truss structure composed of carbon nanotubes. This material has an exceptionally high tensile strength at a very low density, prime spacecraft building material. And you are going to need it. An engine thrust of 1.6 megaNewtons is 160 metric tons of compressive force which the spine has to endure for a bit more than 15 years. Thin spines tend to buckle so the design has a spine with a fat diameter of 100 meters. I did some analysis of the above image, if the spine is 100m in diameter, the pictured ship is about 1.1 kilometers long.
Starting wet mass is 153,940,000 kg, of which 150,000,000 kg is propellant. Outrageous mass ratio of freaking 45.9!
It accelerates for 25.63 years, reaching a velocity of 0.05c. It then coasts for 56.43 years.
Upon approaching Alpha Centauri, it deploys a magnetic sail. This drags on the interstellar medium, decelerating the spacecraft for 36 years. Once the velocity is down to 0.01c the fusion engine is used to finish the job of bringing the ship to a halt. The ship is now totally out of fuel. It then deploys lots of scientific probes and drones to gather as much scientific information as it possibly can, and transmits it back to Terra.
|Distance||0.37 ly||3 ly||0.02 ly||0.0024 ly|
|End Velocity||5% c||-||3,470 km/s|
|Duration||25.63 yrs||56.43 yrs||36 yrs||0.229 yrs|
|Num Propellant Tanks|
|Num Propellant Tanks|
|Bombardment Shield||53,650 kg|
|Bombardment Shield Thickness||9.01 mm|
|Engine Mass||318,000 kg|
|Fusion Pellet||0.000288 kg|
9.97 mm dia
|Pulse Rate||150 Hz|
|Exhaust Velocity||9,210,000 m/s|
|Mass Flow||0.0432 kg/sec|
|Jet Power||1.832 TW|
|Boost Phase||15.184 yrs|
0.274 ly from Sol
|Dist Boost Burn||0.274 ly|
|Cruise Phase||81.475 yrs|
3.936 ly from Sol
|Deburn Phase||2.861 yrs|
4.4 ly from Sol
|Dist Deburn||0.189 ly|
|Total Duration||99.519 yrs|
|Cruise Velocity||14,481,476 m/s|
|Excess Velocity at Dest||1,280 m/s|
The original Project Daedalus starship was a two-stage rocket with each stage having an inertial confinement fusion rocket. Starship Resolution is a conservative extrapolation. Instead of two engines each in a stage, it has a single engine fed by lots of drop-off tanks.
If for some obsessive-compulsive reason you need to know how many fuel pellets each stage has, simply divide the propellant mass in a stage by 0.000288.
|Specific Impulse||550,000 s|
|Exhaust Velocity||5,400,000 m/s|
|Propellant Mass||75,000,000 kg|
|Cruising Velocity||15,000,000 m/s|
This was the fourth entry in Project Icarus' 2013 contest to design an interstellar starship using current technology (it didn't win, the Ghost ship did). The basic rules were to design a spacecraft which was mainly fusion powered and on a mission to Alpha Centauri carrying a 100 to 150 tonnes payload and reaching the destination in no more than 100 years.
For reasons similar to those raised by the Ghost ship, this design also uses the relatively feeble Deuterium-Deuterium fusion reaction. Both designs use laser ignited inertial confinement fusion engines.
The main drawback to IC fusion engines is since beams of light do not push very hard, you need metric-assloads of laser energy to crush the fuel pellet to fusion ignition. Which requires lots of heavy lasers, savagely cutting into your payload mass budget. Since the laser pulse has to be microscopically short, the lasers have to be powered by huge banks of weighty capacitors, further slashing your payload budget.
The Ghost ship gets around this by replacing the capacitor banks with nuclear-pumped lasers, using the waste neutrons from the prior detonation.
The Ultra-Dense Deuterium starship gets around this with something even more tricky. It uses a weird fuel called, you guessed it, ultra-dense deuterium.
Ultra-dense deuterium (UDD) is an exotic form of metallic hydrogen called Rydberg matter. As you can probably figure out from the name the stuff is dense. Real dense. As in 1028 to 1029 grams per cubic centimeter dense. About a million times denser than frozen deuterium.
For our purposes the interesting point is it is about 150 times as dense as your average pellet of fusion fuel when laser-compressed to peak compression. Yes, this means do you not need metric-assloads of laser energy to crush the fuel pellet, a pellet just sitting on the table is already at 150 times the needed compression. It is pre-compressed. All you need is a miniscule 3 kilojoules worth of laser energy to ignite the stuff. That is pocket-change compared to what 200-odd compression lasers require. In fact it is so little that a single laser can handle the job. This results in a vast savings on laser mass and capacitor mass.
The laser pulse has to be quick, so the power rating is a scary 1 petawatt. But by the same token since the pulse is quick, it only require the aforesaid 3 kilojoules of energy.
Since you do not have to compress the fuel you can avoid all sorts of inconvienient hydrodynamic instabilities and plasma-laser interation problems.
You also have virtually unlimited "fusion gain". Meaning that with a conventional IC fusion engine there is a maximum fuel pellet size due to the hydrodynamic instabilities and the geometric increase in compression laser power. With UDD you can make the fuel pellet as large as you want (well, as large as the engine can handle without blowing up at any rate). With other laser intertial confinement fusion, if you make the pellets larger, you have to make the laser array larger as well. Not so with the UDD drive. The fusion gain depends solely on the size of the pellet, you do not have to make the lasers bigger.
An important safety tip: since UDD has such absurdly low ignition energy, there is a statistical change a large number of UDD atoms would undergo fusion spontaneously. This dangerous instability means the spacecraft will carry ordinary deuterium fuel and only convert it into UDD immediatly before use.
The cherry on top of the sundae is UDD fusion does not produce deadly neutron radiation. Instead it produces charged muons, which are not only easier to deal with, but also can be directly converted into electricity. Left alone, the muons quickly decay into ordinary electrons and similar particles.
And since deuterium is plentiful in ordinary seawater, you do not have to go strip mining Lunar regolith or set up atmospheric scoop operations around Jupiter were you to use a fusion reaction requiring Helium-3.
Sounds too good to be true, I hear you say. Well, there are a couple of drawbacks.
The minor drawback is that D-D fusion has a specific impulse (and exhaust velocity) which is about half of what you can get out of D-T fusion or D-He3 fusion. This drastically increases the mass ratio required for a given mission delta-V. Having said that it is still much better than what you'll get out of chemical or fission engines.
But the major drawback is UDD might not even have that magic ultra-density.
You see, the vast majority of the UDD-related papers has been published by a single scientific group at University of Gothenburg, Sweden, led by Dr. L. Holmlid. Currently there are no third-party confirmations about UDD observations and generally very few discussions about it in the scientific community. Until the density figure is confirmed, it might be all a pipe-dream.
The spacecraft has two stages, kinda-sorta.
It accelerates for ten years using Stage One, reaching a velocity of 0.04c. Stage One is then jettisioned.
It accelerates for an additional two years using Stage Two, reaching a velocity of 0.04c. Stage Two stops burning, it still has fuel left. It jettisons about 68% of its heat radiator mass which is no longer needed.
The spacecraft proceeds to coast for the next seventy-five years.
At the end of the coast phase, the spacecraft is 0.378 light-years (0.368 + 0.010) away from the destination (Alpha Centauri). About 24,000 astronomical units. It then deploys a Magsail drag (with a mass of 238,000 kg). Over the next twelve years it decelerates the spacecraft to a velocity of 0.012c.
The spacecraft is now 0.01 light-years from destination. It jettisions the Magsail, bringing the spacecraft mass down to 612,000 kg. Stage Two's engine starts burning (in the diagram this is marked as "3rd Stage"). It burns for the next two years, bringing the spacecraft to halt at the destination. The spacecraft mass is now 320,000 kg, of which 150,000 kg is scientific payload.
This is from the paper Interstellar Flight JBIS Vol. 11 (1952) by the legendary Les Shepherd The Journal of the British Interplanetary Society (JBIS) proudly proclaims this to be the first technical paper on interstellar flight.
Lamentably I am still trying to obtain a copy. In the meantime I will make do with Mr. Shepherd's popularization of his paper which appeared in Science-Fiction Plus April 1953.
Mr. Shepherd points out that when it comes to interstellar colonization, the problem is not transporting a man across stellar distances, it is more a problem of transporting an adequate community. If the transport is not moving at relativistic velocities you are probably talking about a generation ship (I suspect the sleeper ship concept had not been conceived of as early as 1952). Shepherd opines that interstellar explorers or colonists, faced with the knowledge that they will not only never see Terra again but also never see their destination, should adopt a similar philosophy to that of a soldier setting out on a suicide raid. There will be no personal gain, but instead the dying knowledge that some will survive to benefit from their action. This is calling for the sacrifice of entire generations in the depths of space, which admittedly will require a revolution in society. But Shepherd says this may be necessary if we are ever to become a galactic people.
Shepherd does not mention the problem of generation born en route being angry at their forebearers presumptuously committing them to this role. He does point out that the society will have to be a bit regimented. There will be specific population goals (overpopulation or underpopulation is a problem) so procreation is strictly regulated. Civilization has to be preserved, knowledge and culture will have to be carefully handed from generation to generation. New developments in science and art will be needed since Shepherd is of the opinion that "stagnation is the first step to degradation."
Shepherd figures that generation ships should not be used until the state-of-the-art allows transit times less than one thousand years. However he apparently didn't think of the "jumping the gun" problem.
For the journey to Alpha Centauri Shepherd figures that a fission-powered generation ship with an amount of fission fuel equal to 2.4 times the ship's dry mass, plus enough hydrogen propellant to rase the total mass ratio to 5.0 would have an exhaust velocity of 6,000 km/s and a deltaV of 10,000 km/s (about 0.03c). It accelerates to 5,000 km/s, cruises, then brakes down to zero at Alpha C. The transit time should be about 250 years.
If the transit time was lengthened to 350 years, the acceleration and deceleration phases could be increased to 50 years each, with 250 years of coasting in the middle. This would reduce the required acceleration to 0.00327 m/s2 (about 1/3000 g). Which would reduce the required engine power output per short ton of wet mass (specific exhaust power) to "only" 10 megawatts. E.g., if the wet mass was 10,000 short tons the engine power would be 100,000 megawatts (100 gigawatts). Shepherd admits that designing engines which can crank out 100 gigawatts for fifty years will be a bit of a challenge.
Shepherd says the transit time can be cut to 140 years if "lithium-hydrogen" fusion is used, but I think Mr. Shepherd was unaware that there are much better fusion reactions that can be used. I'd be more sure if I could read his actual paper.
Shepherd tries to put a spin on matters, pointing out that while a thousand years sounds like a long time to us, it is actually a small interval in terms of geological time. Which will fool nobody with an I.Q. higher than room temperature. He also mentioned that it would be a real good idea if astronomers made quite sure that the target star indeed had a habitable planet. Otherwise it would be a most tragically ironic ending to a very long mission.
Shepherd also acknowledges that the biological problem of maintaining a life support system for thirty generations is a major engineering challenge, but that isn't his department. Conservation of resources is important since losses can really add up over a thousand years. E.g., a million ton vessel losing 100 milligrams of air per second doesn't sound too serious, but over a 1,000 years that adds up to about three thousand tons of atmosphere loss.
The ship should also transport an entire ecosystem to be transplanted to the new world. This would turn the ship into a veritable Noah's Ark, and might force the ship to be a hollowed-out asteroid in order to carry everything. An asteroid has such a large radius that it could be spun up for artificial gravity at a low enough rate to prevent spin nausea.
Design Population Size Dry Mass
Torus world ship  100,000 107 5×107 1%
More recently Hein et al.  have presented a world ship design with stacked Stanford Tori for population sizes on the order of 104 to 105, shown in Fig. 2. Similar to , this world ship design is based on the Daedalus fusion propulsion system and a habitat design borrowed from an O’Neill colony, in this case the Stanford Torus. Fig. 2 also shows the dust shield put on top of the Stanford Torus facing flight direction.
The authors of  have subsequently further developed the design, in order to reduce the overall mass of the spacecraft, which is dominated by the shielding mass for the habitat (>90%).
One possibility would be to use the deceleration propellant as the shielding material. The propellant mass mainly consists of Deuterium, which has similar shielding characteristics as hydrogen . The propellant is used up during the last years of the trip for decelerating the spacecraft and would serve as a shielding up to this point. The two disadvantages of this approach are that the complexity of the spacecraft increases. The fuel needs to be transported from the shield to the fusion engine. The fuel pellets either need to be manufactured on-board or the shielding is already in the form of fuel pellets. In both cases additional equipment has to be installed. Andreas M Hein, Mikhail Pak, Daniel Pütz, Christian Bühler, and Philipp Reiss. World Ships Architectures & Feasibility Revisited. Journal of the British Interplanetary Society, 65(4):119–133, 2012.
 GL Matloff. Utilization of O’Neill’s Model I Lagrange Point colony as an interstellar arc. Journal of the British Interplanetary Society, 1976.
 M T Simnad. Nuclear Reactors: Shielding Materials. pages 6377–6384. Elsevier, Oxford, 2001.