Solar-electric deep space drive engines, according to Isaac Kuo at sfconsim-l, may soon achieve a power output density of about 400 watts per kilogram, when operating near Earth distance from the Sun. If you do not see what this sort of technical information could possibly have to do with so lovely an image as gossamer wings, you probably reached this blog by accident, have no poetry in you, or both.
What makes it potentially relevant as well as beautiful is that 400 watts/kg is in hailing distance of the 1 kw/kg that Isaac and I independently chose as a benchmark for nuclear-electric drive, and generally as needed for relatively fast interplanetary travel. A spacecraft using solar electric drive can thus reach the same interplanetary speeds as its cousin, though it will take somewhat longer to reach cruising speed, and somewhat longer to slow down. It is a fair prospect that with a few decades' further progress, by the time we're actually building interplanetary ships the performance of the two drives will be comparable.
This is a big deal, because solar-electric space drive is technically and operationally elegant, while nuclear-anything drive, and especially nuclear-electric drive, is not. A solar electric drive has almost no moving parts. A nuclear-electric drive has lots of complex internal plumbing to draw energy from the reactor and incidentally keep it from melting. This plumbing operates under very nasty conditions, radioactivity being nothing to sheer high temperatures.
Plumbing is a big part of what makes spaceships so expensive, because it is complicated, full of parts that can jam, and as there is never a plumber around when you need one, it has to work perfectly for months at a time. (Even if you have a plumber in the crew, taking a nuclear reactor apart en route is a pain.) Robinson's Second Law: For each gram of physics handwavium in futuristic space tech, expect about a ton of plumbing handwavium.
Nuclear drives are also full of nasty fissionable stuff, tricky and dangerous to work with, requiring heavy shielding to get anywhere near (and radiation goes a long ways in space), requiring extreme security measures in handling and storage, and socially uncomfortable no matter how careful your procedures are.
In short, anything that gets rid of nuclear reactors in space is a huge plus on every level of operation, from spacecraft construction and maintenance to obtaining funding. Solar electric drive with comparable performance banishes nuclear reactors from the inner Solar System. You don't need them for travel, and you certainly don't need them for anything else, because one thing the inner Solar System has an ample and endless supply of is sunshine. Those skies are never cloudy all day.
Solar electric power does gasp for air, or for sunshine, as you move outward from the Sun. At Mars, thrust is about half as much as near Earth. In the asteroid belt it is about a fifth to a tenth, at Jupiter one twenty-fifth, at Saturn one percent. To give this some context, a one-milligee drive, baseline performance near Earth, nudges a ship along at about 1 km/s per day, reaching orbital transfer speeds in a week or two. At Jupiter, the drive delivers some 40 microgees, and a ship puts on about 1 km/s per month, thus the better part of a year for orbital transfer burns.
The time lost due to sluggish acceleration is only half as much, some six months, and a Jupiter mission would likely be upwards of a year each way even for a nuke-electric ship. So until we have regular bus service to Jupiter, the time cost is not dreadful. The inner Solar System, through the asteroid belt, can be efficiently traveled by solar-electric drive, which ought to hold us through this century and into the next.
Of course nuclear-electric ships can be built, but Isaac also pointed out a subtle effect that could sideline them. Over the decades to come we will build solar-electric probes, and later ships, steadily developing the technology, while nuke-electric remains a paper tech, falling further and further behind. A serious advance into the outer system will require a faster drive in any case - by that time perhaps a fusion drive, which can still be two orders of magnitude below the magical performance level of a 'torch.'
Let's mentally sketch-design a solar electric ship. Departure mass with full propellant load is 400 tons, broken down as follows:
Payload, 100 tons
Structures and fitting, 50 tons
Drive engine, 100 tons
Propellant, 150 tons
The drive engine we make an advanced one, meeting the baseline standard of 1 kw/kg. Thus rated drive power is 100 megawatts. If the exhaust velocity is 50 km/s (specific impulse ~5000 seconds), 80 grams of propellant is shot out the back each second. Thrust is 4000 Newtons, about 1000 lbs, giving our ship the intended 1 milligee acceleration at full load. Mass ratio is 1.6, so total ship delta v available on departure is 23.5 km/s, enough for a pretty fast orbit to Mars.
We could 'overload' this ship with a much bigger payload, another 400 tons (thus 500 tons total payload). Max acceleration falls to half a milligee, and mission delta v to 10 km/s - still ample for the Hohmann trip to Mars, for slow freight service. Since we want to go there ourselves, we will stick with the faster version and configure it as a passenger ship. Each passenger/crewmember requires cabin space, fittings, life support equipment, provisions and supplies for the trip, plus the mass of the passenger and baggage - in all, say, about 3 tons per person, so our ship carries some 30-35 passengers and crew.
The cabin structure of this ship might be about the size of a 747 fuselage, divided into berthing compartments or roomettes, diner/lounge area, galley, storage spaces, and life support plant. If the propellant is hydrogen, the tankage will be about the same size; if other stuff is used, the tankage will be smaller. All in all, the hull portion of our ship is comparable in size and mass to a jumbo jet. As space liners go this is a modest-sized one, as its modest passenger/crew capacity shows.
Now, finally, the gossamer wings part. We accounted for the mass of the drive engine, including solar collectors, but have not yet looked at the physical size of the solar panals. They are big. Big. If we assume that about 35 percent of the sunlight that hits them is converted into thrust power, they capture some 500 watts per square meter at 1 AU - meaning that for a 100 megawatt drive you need 200,000 square meters of solar panels, a fifth of a square kilometer.
This trim little interplanetary liner is physically enormous, or at least its solar wings are. The 'wingspan' might well be one kilometer, 'wing chord' then being 200 meters. In sheer size our ship is much bigger than any vehicle ever built (though freight trains can be up to about 2 km long).
Angular, squared-off, an instrument of technology - but how can this ship be anything but a thing of beauty, an immense gleaming-black butterfly? If that is too fluttery, say a dragonfly, or to be prosaic an equally immense gleaming-black kite. Indeed the prototype configuration is much like a box kite, likely for later versions as well.
Something is magical about such ships and travel aboard them. The drive thrust and power performance is the same as for a nuke-thermal ship, but now the milligee acceleration feels appropriately gentle, not merely weak, as our ship glides from world to world on its great sun-wings. (This is not, however, solar sailing, but a sun-powered 'steamship.')
The modest capacity of this immense little ship adds to the charm. With only about 35 passengers and crew this is no tawdry impersonal cruise ship. It all has somewhat the flavor of airship travel as we imagine it - perhaps encouraged by the zeppelin-like proportions of the vehicle, the gondola dwarfed by the feather-light structure that carries it. In early decades the ship will be much more utilitarian, a transport rather than a liner - don't ring for the steward; it's your turn in the galley. But if we go to the planets we will eventually go in liners.
The scenery out the viewports* won't change much after the first week or so spiraling out from Earth. (In fact you probably ride a connecting bus up through the Van Allen belts.) By then it is time for reading, cards, conversation, and flirting, till Mars looms close and the ship begins its long graceful swoop down to parking orbit.
* I disagree with Winch. All but the most utilitarian spaceships will have a few viewports, because while there is often nothing to see, when there is it is breathtaking. And fundamentally, why else are we going into space?