A 1,000 meter Solar Power Bubble would generate 10 MW of power, roughly 12 hours a day. My earlier estimates are 11.1 million kg of bubble, for about $33M. Per StarTram, I need my mass-driver up at 22 km altitude, where air density, mass and potential buoyancy has halved four times, to 1/16th, so the air in the 1 km SPB masses about 40 million kg. We need to heat the air up a lot more, but it's already cold at that altitude, and since there's less of it to heat, we can afford to divert power into heating the air, beyond the passive amount we've been counting on. We need to heat the air up, on average, by 90 degrees C, from around 270 K (probably less; -3 C is ~27 degrees F). Even if we are dealing with 96 degrees, divided by sixteen, and multiplied by ten, we only have to come up with the equivalent of 60 degrees worth of heat, whereas the 100 meter SPB needed 73 degrees. Thermal mass, and the heaviness of same, is probably more important.
I'm going to wave my hands some more about the mass of the mass driver. I really can't see it adding more than a few thousand tonnes per km, but I could be wrong... the thing is going to stick out like a sore thumb in thermal, is all! These things only really make sense as remote power generation, communications and space launch platforms for microsats, which I expect to be the norm. $100M is too much, if someone can provide orbital launch services from a spaceplane first-stage, or a high-altitude aerostat. A demonstration mass driver, 300 km long, launching at 60 m/s^2, would cost over ten billion dollars, and still need a kicker stage to get it from 6 km/s to orbit...
36 GWhr of power per day. A 3 tonne orbiter to 6,000 m/s is 108 billion joules, 30 MWhr of electricity. We can do 1200 of these a day, 50 an hour, one every 72 seconds, or 30 per hour during the daytime hours, every 36 seconds. Six gees is high, especially with working parts, rocket engines that must fire to get the orbiter safely into orbit. But we should be able to put a tonne of payload on orbit at a time, and reuse the orbiters, or cannibalize them for spare parts.
The best thing would be to not have to get the orbiter all the way to orbital velocity, using hypersonic skyhooks. The center of mass is in a higher, slower orbit, with the lower end at 100 to 200 km altitude. The high end is moving too fast for it's altitude, and objects released from there go into escape trajectories. The lower end of the tether experiences about half a gee of acceleration, a little over 4 meters, and the outer tether is under roughly the same, outward, drag. An electromagnetic motor, powered remotely by SPB or by solar power in orbit, pushes against the Earth's magnetic field, making up the losses from cargo going up and out.
If we already have the skyhook, we can lower the acceleration by half to 30 m/s^2 on passenger flights, increase the launch time but reduce the velocity, and then use on-board thrusters to accelerate to match the 6 kps of the lower end of the skyhook. Or we could double the length of the mass driver, and double the through-put, too. Each launch takes twice as long, 200 seconds, under half the acceleration for twice the distance. But we could double the cargo capacity of an orbiter which is going up to the skyhook, and have a few passengers in each 3 tonne orbiter, which returns and deploys a para-wing to land, perhaps even under power.
I don't really see the demand for this thing, beyond tourists to the lower end of the skyhook, who then go up to an orbital hotel at midpoint/midway station, for a spectacular view of the Earth. Say about twelve flights per day, eighty per week, four thousand a year, mostly for a few thousand tourists, a few hundred scientists, engineers and space workers going up and coming back down a few times a year... $10 billion plus for that works out to $2.5 million per flight, for the first year, plus operating costs, then just operating costs. If you need it to pay for itself and replace itself inside of 5 years, that's $4 billion a year and $1 million per flight, $1,000 per kg or $1/2 million per seat, assuming each orbiter is a two-seater, or a three-seater with one pilot (and the 'pilot' is most likely a company man, dead-heading while the computer flies).
These are expensive propositions, but if this thing takes off, the cost goes down, the more the cheaper it gets. At five percent of capacity and twenty thousand launches per year, that's $200/kg and $100,000 per seat. I think we could get there in five years, at which point the thing can afford to operate at cost, and prices fall, use soars... well, probably. Is thirty to sixty thousand tourists in the first five years reasonable? Not really. Eventually, but not soon. Even with 500 tourists, and as many added every year for ten years, that's less than 28,000 in that time, more likely and more reasonable.