Generally speaking there are two approaches to manned interstellar flight. One uses high technology to build a small(realativly) starship that can achieve a high fraction of the speed of light, making the journey in a few decades or less. This has the disadvantage of relatively small payloads, making colonisation difficult without a fleet of ships. The second alternative is to use a ship that travels at only a few percent of the speed of light or less, making the voyage in possibly hundreds of years. These are the slowships. Most commonly they are seen as worldships, each one capable of supporting thousands of people for hundreds of years. This approach has the advantage that the ship is elf is a colony, making the initial settlement no different than the voyage that came before. However, the ships are truly huge, and may be beyond the ability even of a united star system to construct. One of these ships, and perhaps the most interesting for a author of hard SF, is the Enzmann Starship.
The Enzmann starship is simple in concept. A cluster of thermonuclear pulse engines, a habitat section holding ~200 people, a massive ball of frozen deuterium, and a cruising velocity of up to 0.3 c. Conceived in 1964 by Dr. Robert Enzmann there have been several variants in which the size, number of drive units, and method of storing the fuel were explored. Detailed information on the Enzmann is hard to find, however, so I am unable to venture into a discussion of the differences between the variants.
The most iconic feature of the starship is the several millions tonnes of deuterium propellant stored as a gigantic sphere on the nose of the craft. Originally this was to have been unconfined, the deuterium 'ice' supporting its own weight. However, this was shown to be unfeasible and later designs used a seamless metal shell. This was created by inflating a thin plastic balloon, on which metal plasma is deposited to form a flawless sphere. I have no idea if this is really feasible, but it could be perfect for manufacturing habitats, starship hulls, planetary domes on airless worlds, and giant reflectors for photon starships. It might also prove to be a viable technique for working with refractory metals, extremely difficult in the normal way of things.
Annoyingly, the exact engines use are never discussed in any of the literature I managed to find. That they are nuclear pulse units is always stated, but this is never clarified. The deuterium fuel suggests that a arrangement similar to the Daedalus starship is intended, but with multiple smaller units, although a reason for this is not given, perhaps for redundancy. Note that deuterium requires an external ignition system, unlike an Orion drive, which cannot be used in a cluster of engines - I think, again this was something I was unable to find a definitive answer on. They could also be internal pulses, like the drive used in the Helios designs exempt that this has a Isp far to low for a serious interstellar starship other than, perhaps, a multigenerational starship. The more modern development of the magnetic inertial confinement drive might fit the bill perfectly, although requiring a change of propellant. Thinking about the number of engines used it occurred to me that as the starship is likely to be turned into a habitat when it arrives at the destination system, the drives could be stripped off and fitted to smaller vessels composed of other pieces of its structure, allowing the more rapid expansion of the colony.
In some sources a cruising velocity of 0.3 c is stated. This may well be attainable with a Daedalus style drive with high Isp, but seems unlikely for a thermonuclear pulse like Orion. An interstellar Orion starship is often given a cruising velocity of 3% c, although it could be made slightly higher if the fuel fraction is increased considerably. Magnetic/inertial confinement fusion, steady state or otherwise, could well provide the necessary performance. The daedalus starship does not have a capacity large enough for a multi-generation voyage, which would be required with a 0.03 c cruise, but could carry sufficient range of ages to make a 0.3 c voyage possible.
The final plausibility of the Enzmann starship is debatable, and depends on many assumptions about technology and construction that are answerable only through practical groundwork. Such a large vessel would also require a very robust economy to produce. However it basic configuration - multiple engines, large spin habitats, and a single huge fuel tank - have several advantages. In a follow up post I will look at the what these advantages are, and the way they could be applied to a SF 'Verse to create a setting for interstellar colonisation.