By Joseph Friedlander
By strength of materials arguments alone, staged (note that qualifier—a tower whose ‘payload’ is another smaller tower on top—in that sense skyscrapers with setbacks or even the tapered Eiffel tower is staged–) towers 30-to 100 kilometers high are quite doable.
New businesses and payback models would be enabled with each successive height regime (ie even 3 kilometers high would give superior relay capacity, a 30 kilometer tower could see either ocean from Mexico City, or all of the UK and Ireland from an English site near say Sellafield. A 100 kilometer tower would have the vast telecom coverage area of a satellite.
The higher towers would enable entirely new capabilities– for example, a backyard moon rocket is not practical from sea level (atmospheric drag is too great). By contrast a 4 stage moon rocket the height of a man is practical from 30 kilometers– because there you are essentially launching under Martian conditions (other than gravity). This would look like a Russian moon lander– necklaces of spherical tanks around a central engine– and four or five or even six stages. A man sized launcher of that sort would probably have a single kilo payload if doable at all– but obviously a larger one would have a larger payload. One three stories high would possibly be able to land a man on the Moon. (one way personnel replacement or colonist shipment to a lunar base)
Something of the same stacked necklace configuration (say 3 stages to 4 stages in this case) that merely reaches orbit around earth –minimum size from sea level is a large utility pole. From 30 km– a pencil (note that we speak of minimum here)–your payload will obviously vary with the size of the rocket, just illustrating that drag goes down with height. A three story high one of these could orbit– and return– a manned capsule.
From 100 km you literally could launch without a nose cone, with enormous weight and drag savings. That is a simple launch– you also could swing a rotary slingshot like launcher around with electric or other power. With existing materials (neglecting resonance effects, which could be isolated for) you could give up to a 4 km/sec boost which would cut the size of the needed rocket to a single stager which could be completely recycled (minimum savings in mass and cost AT LEAST 10 times over today’s costs per flight, with complete reuse for 10 flights another 10 times for a total of 1-2% or today’s spaceflight costs—that is the theory. If you are cynical about theory reaching practice note that Space X has recently orbited and returned the equivalent of the Gemini 1 mission (which did not deorbit its capsule) and did so for a few percent of the inflation adjusted cost of the Gemini program (at a guess, the equivalent in 1965 of $100 million, and the entire Gemini program was then in the low billions) So economical engineering is possible– and by launching air drag free spacecraft of very light construction (no perceptible max Q, no storms, etc, already dehumidified and deiced on launch) we may get it.
Long time readers of Heinlein’s science fiction will recall that in many of his stories written between 1940 and 1960 he postulated suborbital spacecraft as stepping stones to interplanetary (including lunar) space travel.
It may be that using rockets is simply too expensive for Earthly travel— because of the rocket equation, a single stage to orbit –or antipodes– rocket would be a flying gas can, almost all fuel. Engineering it for re-use would be very difficult. But there may be a cheat, just as a tower to get up and away from the atmospheric (and gravity!) drag losses that turn a theoretical 7.8 kilometers per second to orbit to an even more difficult realistic 9.2 kilometers a second to orbit—back to the theoretical 7.8 kilometers a second but this time achievable in real life! (and remember, no nose cone to jettison or lose the mission– its’ already off).
Lets cheat some more, then, and swing an electrically powered Zylon tether, with a tip velocity (the launch cradle hanging at that tip and then releasing on engine full power) of say 2.8 kilometers a second. Then 7.8 kilometers a second to orbit becomes 6 km/sec to orbit–(about the delta-V of a Trident missile) which means a single stage to orbit, single engine design is practical. This is huge, because if the engine is already ignited on release, there are no failure modes (simplifying here) but inflight explosion or failure to circularize (halfway around the world, you have to puff out another say 20-30 meters a second to raise your perigee above atmosphere or you reenter) It makes failures enormously less likely. And because the ratio of launch mass to fuel is no longer 30 to one but a more buildable 10 to 1, the chance of making something that could fly more than once just went up.
By staging the tether again (obviously with a bigger tower–) we might get to 4 km sec with a Zylon tether (many tricks used here, a stronger material– which may be forthcoming, would be enormously preferable)
Well that allows us to toss a capsule with just a heat shield, some very small steering jets, and a parachute to splash down in a circular landing water basin at a given target, with almost no rocket fuel aboard—which means that a hundred person capsule would be something like 100 tons (the tether might be as much as 4000 tons with Zylon, requiring a correspondingly massive tower, say half a million tons—but many large structures at least approach this mass already.)
(The lower 2.8 kilometer a second velocity would only enable a few hundred miles range, if just 2 km/sec then perhaps just 300 miles, like a V-2—but remember that we are launching at altitude so perhaps hundreds of miles of glide might be possible or not depending on the lift/drag ratio of the capsule design).
So we see that materials now available for both tower and tether could enable ballistic flights of 300 miles in 5 minutes and 1200-1500 miles radius in 15 minutes. This sounds very much like vintage Heinlein, say St. Louis or Budapest being the hub. Presumably any industries needing super-rapid transit to any site in the continental USA or Europe (rush industrial parts, organs for transplant, elite security forces, Fed Ex, etc) would ship to warehouses at the hub, then deploy to the target and recover the capsule by air or truck. (A 10-20 person capsule would be truck transportable depending on shape. As there would be no need for a pilot (automatic ballistics and landing program or stewardesses (15 minutes in flight)) it would be possible to cut personnel weight and expenses greatly, so smaller capsules would be practical.
Accelerating to say 2.8 km a second for a short flight, the motors would spin up and reel out the capsule, easily taking more time than the actual flight just to reach working radius, (in this case 100 km at 8 gs—not for weak or old people) then up the speed to launch velocity. There might be more than one tether on the axis, literally tens of smaller capsules could be reeled out and released at the same time as long as tower weight and power allowed it. So the effective launch rate might easily be one per minute. Ballistic considerations would complicate flight patterns (you can’t swing up two greatly different trajectories on the same tower axis)
From the perspective of the passenger, it will certainly take more time to elevator-climb the tower and then get swung up and released than even a 1500 mile hop will require for flight time. It might take say a 2 hour flight from London to Hungary, an hour climbing the tower—then a 15 minute joyride to say Greece. However, if London had its own tower, then things get interesting, and the calculation becomes, home to London Tower, 1 hour, up and packed and spun, 1 hour, Greece, 15 minutes. It may well be that super-rapid (at least 300 mph) magnetic capsule transport goes into demand because the tower swing capsules make us impatient with slow domestic travel!
At 10 cents a kilowatt hour, orbital speed would be around $1.50 a kilogram (I imagine the capsule loadings per passenger would be around a ton, so $1500 New York to Australia, , and half orbital speed (the 1500 mile hop) around a quarter of that. With cheaper nuclear electricity (thorium molten salt reactors) or space solar power (booted as part of this launch system’s space development, as we shall see) the cost could be far lower. If penny a kilowatt hour is practical, one can imagine very cheap travel indeed. The only drawback is Arthur C. Clarke’s famous, ‘Half the time the bathroom is out of reach, the other time out of order” comment. Add another 45 minutes to the travel time for sealed-in and spun up time (high G) and it is an hour and a half– for most people not undoable– but some people would have problems (I am thinking of pregnant women). Still nearly all conventional long distance air travel could be replaced, saving perhaps 7-8% of all oil consumption.
With next generation materials (double Zylon strength would do it) 7 -8 km sec with electric swing tethers have 6000 -12000 mile range (45 minutes to antipode) replacing airliners on superlong routes (777 class). This is not only good for oil consumption reduction (peak oil will happen someday, the question is when) but also for cheap space travel (12000 mile range is essentially orbital speed). If so we could finally get the emigration trail going.
A squadron of 30 such, flying once a day. (Distributed around the world because of need to match Station V’s orbit) It takes a year or so to lift the stuff for Station V, (~300,000 tons) and after that, in the next decade 10 times as much mass–3 million tons– of which 1 million tons is landed on the Moon. I think a generation ago, by comparison Antarctica logistics were on the order of 50,000 tons a year, (probably including fuel loadings) so this is double that but Clarke was thinking ahead. (The December 1970 Bulletin of the Atomic Scientists, on page 92 mentions a 35,000 ton tanker bringing in 7 million gallons of fuel and 2 10000 ton cargo ships bringing additional loads, for example) Clarke did not include major teleoperated robotic development in his plans, I do, so he undoubtedly had huge logistics for his human life support.
For that early 60’s massive spacelift perspective –the era of no aerobraking, largely propulsive (gas core reactors energizing liquid hydrogen) we can see that launching say 100 tons an hour per tower would lead to well over 300000 tons a year up. So hello, Space Station V!
That hypothetical space station, with its rotating artificial gravity, is remarkable for its scale (bigger than most aircraft carriers, and in three dimensions rather than its practicality. But a radiation proof station able to withstand any solar flare, and do so with a crew of thousands, would be quite practical with such mass loadings. (It might spin only fast enough for Lunar gravity, if any at all. But that would be enough to greatly simplify housekeeping)
It is noteworthy that if we had hibernation technology as postulated in the film, the Discovery class vessel used there would be best employed as a boost and quick-return tug, pushing a much larger capsule to a Hohmann trajectory (minimum energy) to the Outer System, say, Saturn, and then a perhaps 2 ton mass per person would suffice to supply them for the 6 years journey till another tug out there could intercept them. If so, loads of 500 people could be sent by the thousands per year (during a brief launch season when Earth and Saturn were on opposite sides of the Sun) leading in a few hundred years to a billion person population in Saturn System.
But the real incentive is space based solar power. If the cost to orbit were in the low dollar per kilogram range, the cost to geostationary (and the Moon) would be low enough to make space solar power practical. A sample of the economics might be those in Keith Henson’s plan He writes in his paper Beamed Energy and the Economics of Space Based Solar Power: For space based solar power to replace fossil fuel it must sell for 1-2 cents per kWh. To reach this sales price requires a launch cost to GEO of ~$100/kg. This system would do that and more. This would be self-booting after a while, each years power generation only lowering the costs of further tower launches.
Henson gives a figure in his paper Beamed Energy and the Economics of Space Based Solar Power (source of the above quote) a materials flow rate of ~1 million tonnes per year, enough to initially construct 200 GW per year of power satellites…so within a decade, complete electrical independence from fossil fuel dependency would be possible in this scheme. (With more towers, and more energy, in a self-booting process, within a generation even thermal use of coal would be gone because of simple economics– solar power would be cheaper)
Other visions of space development, including those of Professor David Criswell, require far less mass in space to boot complete independence from fossil fuel power sources. (Perhaps 100,000 tons would be sufficient in a longer range plan, but certainly the tonnages touched upon here would do the job)