Wednesday, September 23, 2015

Tall Ships and Trading Posts: Tall Ships verses Spacecoaches

Welcome back, RocketFans, to another installment of Tall Ships and Trading Posts.  This time we're dealing with the specific design issues of Spacecoaches and Tall Ships, and what they are meant to do.

First of all, you may have noticed I wrote Spacecoaches and Tall Ships.  I'm beginning to see these as two different types of solar electric spacecraft.  Just as you have Piper Cubs or Lears on one end of the aviation spectrum, with A380s and 787s on the other, there will be "small" Spacecoaches for Ma and Pa immigrants and larger Tall Ships for commercial purposes.

"Small" - bit of a misnomer, as the engines and solar sail systems must be of a certain size to work at all.  I've been working on the larger Tall Ship version for the time being, and the numbers are humbling.  It will take 1200 MET units, arranged in a 900 square meter array to push the habs, propellant and eight hundred thousand square meters of thin solar film, carbon composite masts and yardarms, and automated rigging for same.  These  Solar Sailers have masts that are kilometers long, with hab sections that are almost too small to see when the Ships are shown in scale.

"But Ray", I hear no one ask, "if the number of passengers is smaller, and the payload is smaller, why can't the sails and engines be smaller?"  Ah, my hypothetical straw man, you must understand that in this case, it is not about payload, or even about mass.  It is about travel time.  If you make a smaller MET array, the thrust is lower, therefore the acceleration is lower, therefore the travel time is longer.

But still, lets look at some numbers.  Consider the following a real-time look at the creative process, meaning I'm making it up as I go along.

A full-sized Tall Ship has a MET array that pumps out 48000N of thrust and takes 240 Mw of electricity to run.  In order to power that system in the orbit of Mars, and assuming a SFnal PV efficiency of 50%, you'll need about 800,000 square meters of sail. That will be arranged on two masts, each a kilometer long, and about four hundred meters wide.  With all the stuff I mentioned above in terms of mast and rigging, that's about 900 tons for the power train.  You'll need 3000  tons of propellant, but that's okay, because it's water and can be drunk, bathed in, flushed, then used as propellant in a model of combined utility.  Habitats run you 270 tons, payload about seventy-two for a two-year mission.  This Tall Ship is more like Rick Robinson's gossamer winged spacecraft, with about 36 passengers and crew, two big habs, and a 400-ton lander/ transfer craft.

A more conventional Spacecoach design will lose the lander, have a hab suitable for about six or so, cargo for a family of pioneers, and that's about it.  If we try to quarter the power train, we get 400 METs, 60 Mw of power, 12000, and a sail array 200,000 square meters.  That would be a pair of masts about five hundred meters by two hundred along the yardarms.  But, the mass will still be 450 tons, because while you quarter the area, you can only halve the mass. Still, try a thousand tons of water propellant, and habs at only 135 tons  you'll keep the same payload reqruirements, because these folks are moving to stay.

So, Tall Ship: Total "wet" Mass of 4672 Tons, and a Mass Ratio of 2.8 (wow!).  With a Delta V of 10.1 kps.  The Space coach will have a wet mass of 1657 tons, a mass ratio of  2.5 and a Delta V of 8.9 kps.  So our Spacecoach has shorter legs than a Tall Ship, but not overly so - especially with such a significant reduction in cost (assuming you measure cost by x-thousand dollars a ton).

So now we can figure travel time. All the current figures assume enough supplies for a two year mission, so if the Spacecoach can't make it to it's destination in that time, you have a problem.  Tall Ships are golden - they accelerate at a blistering 0.01 m/s.  They also have a long burn duration, on the order of four days (750000 seconds) and can zoom out to Mars in 16 months.  That may sound slow, but you can make the trip in gravity, with showers everyday, so it ain't all bad.

Spacecoaches, however, have the following numbers:  Acceleration is only 0.007m/s, so they are slower and their burn durations is only a quarter of a million seconds (69 hours) before coast-and-flip.  The average velocity will also be only be half that of a Tall Ship. With those numbers, it will take our Spacecoach thirty-three months to travel to Mars.  Having enough supplies is not a problem, fortunately, because we kept the 72 ton payload of the Tall Ship.  It will mean that you can carry less pioneer stuff, however...

But how much do you need?  Look at your ship:  You have enough solar sails to run your hab's power systems and your propulsion.  Another thing, your propulsion system doubles as a mining system, with the MET thrusters making handy drills on the surface of, say, Deimos or Mars herself.  The habs you already live in?  They have structure, bracing, and plumbing for use in gravity already.  So it sounds to me, for the cost of an aerobraking transport to the surface that the folks at Cape Dread would be happy to charge you for, you could turn that Spacecoach from a slow boat from Terra to a instant homestead/mining outpost.  Convenient.

These numbers are very much back-of-the-envelope, so take them with a grain of salt.  What the number do show is that you can have small slow Spacecoaches for individual families that cost less and provide an instant homestead on the other side.  You can also have larger commercial Tall Ships that economically yet quickly ferry dozens of people from here to the Red Planet.  And what's more, these numbers are scaleable - you can add more cargo to a Tall Ship if you don't mind going slower, and you can get a Spacecoach to go faster with the addition of more METs and studding sails on the existing mast.

What I'd like to have is Spacecoaches and Tall Ships move at closer to the same travel times. I have definate ideas brewing in my fevered brain, and they involve not only these majestic sail ships of space, but their successors, Microwave beam-riders. Those  however, are a topic for another post. 

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