by G. Harry Stine
Copyright 1981 by G. Harry Stine
Chapter 3: A True Space Transportation System
A big space transportation system will be necessary to lift the millions of tons of material into geosynch orbit required to build a powersat.
Because of all the publicity that was given to space launch costs by the news media during the Apollo manned lunar landing missions in the 1968-1972 time period, there is a stigma of "exorbitant cost" associated with every space mission.
Therefore, the difference between the Apollo flights and the SPS transportation system operation needs to be firmly established right at the start.
The Apollo Program was a politically-motivated project, funded by the government with cost a secondary concern to the primary goal of getting to the Moon before the Soviet Union. We know the Soviets had a manned lunar program; we also know now (and may have known at that time through intelligence sources) that the Soviets had the capability of sending a single cosmonaut on a circumlunar mission in December 1968, but that they apparently did not have the capability to land even a single cosmonaut on the Moon and return him to Earth alive. Even if we'd had this knowledge of the Soviet lunar capabilities in 1968-1970, we would have certainly gone ahead with Apollo anyway because the program was so large and so near completion that its very momentum would have carried it through. As a matter of fact, that is exactly what happened. The program's sheer size meant that it couldn't be stopped immediately, but it was brought to a halt in December 1972 with the return of Apollo-17.
National prestige was the primary driver behind Apollo. "Hang the cost! Hire another acre of engineers and print another billion dollars!" We had to beat the Soviets to the Moon. . . and we did. Once we'd done so, the whole space program was wound down because its function as an instrument of national prestige—not of scientific exploration and not as the exploitation of a new frontier—had been completed.
On the side, a lot of people made a lot of money as a result of Apollo, and they couldn't let it be stopped or fail. One of these was Lyndon Baines Johnson who "happened" to have some interest in some swampy land south of Houston and northeast of New Orleans . . . There were others, but that example serves the purpose here.
The space transportation system for the SPS program is not the same sort of animal. It is to the Apollo program as a transcontinental airline is to a single person flying a single-engined airplane across the country. Charles A. Lingbergh certainly pioneered transcontinental air transportation with his ' 'Spirit of St. Louis'' and other aircraft, but when United Airlines, American Airlines, and Transcontinental and Western Air (TWA) were in business ten years later, their operation and philosophy was greatly different . . . and so were their cost structures.
The SPS space transportation system depends upon other factors than have been paramount in the governmental space program to date: low cost, high reliability, ease of operation, inherent safety, and the capability for a high level of use. Without these factors, the SPS program cannot be carried out at all. Something must transport tons of material and hundreds of people into space at the lowest possible cost because investors can't afford a ' 'cost be damned'' program; with the highest reliability because project managers will count on each flight; with ease of operation because of the large number of flights required; with safety because human beings will beusing the system; and with versatility because of the wide variety of different types of cargoes that will have to be lifted to space in the SPS project.
If this sounds like a tall order in light of the history of space transportation thus far, it isn't.
As a matter of fact, such a space transportation system is on the drawing boards of at least three aerospace companies as of 1981.
An SPS space transportation system can be built starting today with technology that is either in existence or that can reasonably be expected to be in hand by 1990.
First of all, we must define what we mean by the term, "space transportation system." It is usually thought of as being rocket-propelled spacecraft. But a space transportation system is more than vehicles. This is true of any transportation system. This statement will be readily admitted by anyone who's worked in the transportation industry on Earth or who's engaged in the hobby of modeling transportation systems such as railroads. Some model railroad fans are interested in motive power—the locomotives. Others find their greatest interest in rolling stock. Still others enjoy track work while others delight in making realistic scenery. And yet another category of rail modeler enjoys the operation of the model system itself. Like a space transportation system, a rail transportation system is nothing if y6u've just got motive power and nowhere to go with it, no way to load or unload pay load, no terminals, no maintenance shops, and no control systems.
So our discussion of a space transportation system will have to include these additional support elements as well.
The DOE Reference SPS System or "Baseline System" that's being used as an example of how to get started in space power goes into considerable detail regarding space transportation. But, the space transportation system that we'll discuss here is a synthesis or amalgamation of the DOE study and the systems proposed by Rockwell International and Boeing Aerospace, combining systems to achieve the greatest ease of operation and the greatest possible versatility. The number of "throw-away" elements is minimized, thus reducing costs. Specialized vehicles have been replaced by spacecraft with a greater versatility in the sort of cargoes they can carry.
At this stage in the development of space transportation, it's simply not possible to place very large payloads at geosynch orbit 22,400 miles above the Earth's surface with a single launch vehicle having any reasonable size. It's like sending a cargo by ship from Liverpool, England to Pittsburgh, Pennsylvania. One sort of ship is required for the transatlantic portion of the trip—i.e.: a large ship can be used to cross the Atlantic Ocean, thus reducing costs. But to navigate up the Mississippi and Ohio rivers from New Orleans to Pittsburgh requires smaller ships with different characteristics. The analogy holds true for sending cargoes to geosynch orbit. It will have to be done in two steps.
The first step involves lifting payloads of cargo and people from the Earth's surface into low Earth orbit (LEO) about 150 miles up.
The second step lifts payloads from LEO to geosynchronous Earth orbit (GEO or GSO).
Two different types of space vehicles are required for the different steps.
The Earth-to-orbit step requires a lot of rocket thrust because the space vehicle or ship goes from the bottom of a very deep well—the Earth's gravity well—surrounded by the Earth's atmosphere up to the weightlessness and vacuum of LEO. To be reusable, the ship must be capable of repeatedly withstanding the very high temperatures associated with ramming back into the Earth's atmosphere, it must possess some way of being guided and controlled in the atmosphere, and it must be capable of safely landing on the Earth's surface in a condition that permits it to be used again with a minimum amount of time, money, and materials expended. To achieve the least costs, an Earth-to-orbit ship should be as large as it's practical to make it because, as its size goes up, the ratio between its structural weight and its loaded weight decreases. More of its gross weight can then be devoted to its payload: rocket propellants and cargo. For convenience, we'll call the Earth-to-orbit-and-return ship a "shuttle" even though it may not resemble the current NASA space shuttle. "Shuttle" will be a generic term for the sort of ship that operates between the Earth's surface and space.
For the second step in the trip, LEO to GEO, an entirely different ship is required, one that always operates in the vacuum of space and therefore does not need any aerodynamic shaping. This "deep space" ship also operates always in the weightlessness of orbit or under the accelerations imparted to it only by its own rocket engines. Therefore, the deep space ship can be designed and built quite differently to maximize the efficiency of its operation in the unique environment of orbital space. For example, its rocket engines do not need the tremendous thrust required for a shuttle, so they can be more modestly sized. The lower accelerations of its operations—a maximum of three gravities or three times the acceleration of Earth's gravity— permits engineering economies to be made in its structure. This deep space ship—oftimes called an "orbital transfer vehicle"—is the second category of spacecraft in our space transportation system.
However, the payload that both a shuttle and a deep space ship carries can be divided into two classes: (a) people, and (b) cargoes of materials and supplies. Each class of payload will require a different sort of care during transit.
People must have pressurized, temperature-controlled environments around them and cannot be subjected to any accelerations exceeding three gravities, this being considered the maximum permissible acceleration for people who have not undergone extensive training in high-gee maneuvers. (When I pull three-gees in my airplane during a maneuver, I know it, I don't like it, and I'm not trained to endure such acceleration for more than a few seconds because it becomes difficult to breathe and almost impossible to move.) People will require feeding and sanitary facilities on any trip lasting more than a few hours.
Cargoes of materials and supplies, on the other hand, may or may not require a compartment that is pressurized and temperature controlled; some cargoes may need only rudimentary temperature control with no pressurization requirements whatsoever. Some cargoes are acceleration insensitive, permitting boost accelerations that are higher and therefore more efficient in terms of how rocket propellant is used. In addition, a great deal more non-human cargo than human help is going to be required to build an SPS. Therefore, cargo freighters will probably be much larger and will transport heavier payloads.
A cursory look at a passenger liner versus a tramp steamer or a wide-bodied jet airliner versus a heavy-lift cargo plane reveals the simple truth that engineers design and build passenger vehicles differently from cargo vehicles. Although it's true that passengers often ride in cargo vehicles and that cargo is often transported in passenger vehicles, this swap of payloads usually occurs because of reasons of urgency, expediency, or economy.
We've thus divided our space transportation system into a basic requirement for four types of ships:
1. Earth-to-orbit-and-return passenger ships—Passenger shuttles.
2. Earth-to-orbit-and-return cargo shuttles—Freight shuttles.
3. Orbit-to-orbit passenger ships—Deep space passenger ships.
4. Orbit-to-orbit cargo ships—Deep space freighters.
Let's look at each of these in turn and discuss their basic characteristics because, regardless of who builds the ships, the technology used to build them, and when they're built within the next twenty years, each category will have certain basic characteristics.
The characteristics shared by all four classes will be (a) lowest possible operating cost, (b) highest possible reliability, and (c) maximum possible use rate. These three characteristics are shared by any commercial transportation system on Earth, and there's no reason why a space transportation system should be any different in these respects, or why a space transportation system cannot possess any of these three basic characteristics.
The size of each type of ship depends upon the nature of the SPS construction job and its engineering details. Up to a point, the bigger the ship in terms of numbers of passengers or tons of cargo, the more efficient it becomes. But in the case of the shuttles, a limit on size is reached because of the time and facilities required to handle them, load them, launch them, and recover them on Earth. This doesn't hold true for the deep space ships. Another limit on size is the amount of space transportation capability that's lost when a big ship must be repaired or maintained . . . or in case it's lost due to accident. (It would probably be most efficient to have one super-sized wide-bodied jet transport leaving each day from LAX to JFK, and vice-versa. But unless you've got a standby in case one of them needs repair, you 're in trouble. And if one of them happens to have an accident, you've lost not only a very large number of passengers but also a highly expensive airplane.)
The sizing of the space transportation vehicles in the DOE Baseline Study came about as a result of a series of reiterative trade-offs between all of the factors that were taken into consideration. This means that a "first cut" at sizing was taken, and the various effects upon system costs were determined as a result. Then changes were made in the sizing to determine effects on costs and other SPS elements.
The general ground rules followed were:
(a) The system elements were to be dedicated and optimized for the construction, operation, and maintenance of an SPS system. If other uses happened to "fall out" of these designs—as we will see that indeed they do—so much the better.
(b) The transportation system was designed for minimum total project cost—not just of the transportation system, but of the entire SPS system.
(c) The energy and therefore the rocket propellant costs would be minimized consistent with minimum overall system costs; energy costs money.
(d) The design of the transportation system would be made to minimize the environmental impact at the launch and landing sites and any environmental protective measures would be factored into the costs.
(e) The transportation system would require the minimum use of critical materials consistent with low cost, minimum energy, and environmental impacts.
The optimum spacecraft sizes that came out of the study were as follows:
The freight shuttle should be capable of lifting at least 424 metric tons (932,800 pounds) to LEO. This was not surprising. Space transportation planners have known for some years that the "million pounds to orbit" booster would be highly efficient for whatever large space projects were undertaken . N. B.: Both the Saturn 5 and the best-guess estimate of the Soviet Heavy Launch Vehicle (the Class G launch vehicle) are quarter-million pound to orbit launchers, but the Saturn 5 was designed with 1960 technology. The freight shuttle designed with 1980 technology is nowhere near as large, and the unique requirements of the space transportation system not only reduces the cost of such a launch vehicle but its size as well. For example, the Boeing version of the freight shuttle would be 76 meters (243 feet) tall and about 74 feet in diameter with a base diameter of 131 feet.
The passenger shuttle can be achieved by using the existing NASA space shuttle orbiter, putting a 75-passenger module in the cargo bay, and launching it as a two-staged vehicle with a reusable fly-back booster. The ultimate passenger shuttle is, as we'll see, a totally reusable single-stage spacecraft carrying up to 75 passengers.
The deep space passenger ships are also designed to carry 75 passengers in a variation of the 75-passenger shuttle module. Propulsion is provided by liquid hydrogen/liquid oxygen rocket engines.
But the deep space freighter is something totally new because its job is to propel SPS subassemblies put together in LEO out to GSO where the final assembly of the SPS takes place. Since such a craft is a freighter, and since it will be propelling a very large solar array, propulsion is by means of an electric rocket motor using solar electricity generated by the array itself. The thrust is not as great as for liquid propellant rocket engines, but the array doesn't have to be boosted up to GSO in a big hurry. As a matter of fact, because of its size and structure, it's more efficient to boost it very, very slowly with numerous electric rocket motors attached at various points of the structure itself. The deep space freighter becomes a very small electric rocket motor module capable of being attached to an array and capable of being remotely-controlled from LEO or GSO if necessary during its flight. There may also be a manned electric rocket module in case people are required to perform guidance, navigation, control, maintenance, or other tasks during trans-orbital flight.
And it was no surprise to the study participants who'd thought through the whole space transportation concept for years that the "easy" portion of the system was the deep space portion. Going back and forth between LEO and GEO is nowhere near as difficult in terms of technology, engineering, and energy as that first step: Earth to orbit.
There are several ways to crawl up through the Earth's thick atmosphere and out of her deep gravity well. Airplanes have been successfully (most of the time) doing it one way for more than 75 years. Their horizontal takeoff and horizontal landing (HTOHL) uses the aerodynamic lift generated by motion of the craft through the atmosphere. Until the advent of the NASA Space Shuttle, our space vehicles have been using vertical takeoff and vertical landing (VTOVL), this being the classic mode of rocket operation. The NASA Space Shuttle uses a new mode of operation: vertical takeoff and horizontal landing (VTOHL).
All three operational modes are useful for the Earth-to-orbit step of our space transportation system, and advanced designs using the three modes have left the drawing boards already.
The initial passenger shuttle will probably be based on an extension of current NASA space shuttle technology which, when operating at full-bore, will be capable of launching a space shuttle once a week utilizing VTOHL. But this operational mode requires separate facilities for takeoff and for landing. The passenger shuttle used in the DOE Baseline Study foresaw the extension of the current NASA shuttle with a reusable booster, but this still means that a large external tank is thrown away on each flight—and that's a cost item that can't be overlooked.
The ultimate for the shuttle task is single-stage-to-orbit (SSTO), a single vehicle that ascends to space and returns without dropping off anything and by using only rocket propellants to do the job. The SSTO approach can be done with either a winged vehicle such as the NASA space shuttle (albeit much, much larger) or with a ballistic vehicle.
A ballistic shuttle appears to be the way to go for cargo and freight using the VTOVL mode. Such a freight shuttle will be large—larger than the Saturn 5. Although an interim cargo shuttle can be cobbled-up using NASA space shuttle technology and hardware, the ultimate freighter shuttle for our space transportation system will be a ballistic VTOVL craft.
As a result of considerable study by Rockwell International and Boeing Aerospace in order to establish the DOE Baseline SPS System, three conclusions were drawn.
With both winged (VTOHL) and ballistic (VTOVL) modes, two-stage configurations required less development of technology and could be done with a great deal of existing know-how; such two-stage designs turned out to be less sensitive to differences in operations which in turn led to the possiblity of lower operational costs.
Winged vehicles (VTOHL) showed greater operational simplicity and reduced recovery and turn-around time when compared to ballistic vehicles (VTOVL), even when the ballistic shuttles were landed in special recovery ponds adjacent to the launch sites.
All types of shuttles, VTOHL and VTOVL, were more efficient if designed to use liquid oxygen and hydrocarbon fuel in a first stage and liquid oxygen and liquid hydrogen in the second stage. This is because of the greater bulk of liquid hydrogen which has low density and requires a very big tank to contain a given weight of this fuel.
SSTO came out second-best only on the basis of the available technology, in spite of the fact that SSTO is the way to go for Earth-to-orbit transportation, just as it was the only way to go for transatlantic airline transportation.
Before World War II and the advent of the four-engined Douglas DC-4 (C-54) transport plane, there was no airplane except special ships like Lindbergh's that could fly the Atlantic Ocean non-stop. Most transatlantic airplanes were flying boats, and all of them had to stop to refuel, usually at the Azores. There was a requirement for a very fast transatlantic mail plane, and Major R.H. Mayo of Imperial Airways of Great Britain came up with a solution. The biggest problem faced by a heavily-loaded airplane is taking off and climbing to its best cruising altitude. This is particularly true of a seaplane. Mayo suggested the use of a large Short Brothers Empire Class flying boat to lift a smaller seaplane pick-aback until cruising altitude was reached, whereupon the smaller airplane would release itself from it's first stage and proceed non-stop across the Atlantic Ocean with 600 pounds of freight and mail. On 21 July 1938, using the Empire Class flying boat Maia as a first stage, the little Mercury seaplane made it from Ireland to Montreal non-stop in 20 hours and 20 minutes. This ' 'composite'' was perhaps the first' 'two-stage airplane."
It didn't last because the development of the DC-4/C-54 airplane made the two-staged concept impractical, costly, and uneconomical.
History doesn't repeat itself, but historical patterns often do repeat themselves, and it is quite likely that we will see this pattern repeated in space transportation. SSTO designs—both winged and ballistic, VTOHL and VTOVL —are now under intensive study and could become reality within a decade. The economics of space transportation requires the development of the SSTO shuttle.
The ultimate in SSTO shuttles will be the horizontal takeoff, horizontal landing (HTOHL) ship. This, too, is foreseeable today although the technology to achieve such a ship isn't yet in our hands. But, by the year 2000, it would be. The HTOHL shuttle would take off from an ordinary airport runway and climb into space using wings, composite air-breathing engines such as turbofans converting to ramjets at altitude, plus rocket motors for power above the atmosphere. On its return, it would land on the same runway from which it took off. The HTOHL shuttle will be the ultimate because of its lower cost, its ease of operation, and a flight profile whose accelerations are comfortable to passengers. Because the HTOHL shuttle's payload isn't as great as that of a ballistic VTOVL or VTOHL shuttle, it will probably be used only for passengers.
How many of each type of spacecraft are going to be required to build two ten-gigawatt SPS units in geosynch orbit, every year?
Two passenger shuttles will be required, each making a flight every three weeks. This is one flight every twelve days, about the same rate as the programmed NASA space shuttle launches. But the passenger shuttles will be lifting or landing seventy-five people on every flight. The number of passenger shuttle flights is determined by the number of people required in LEO and GEO for SPS construction and assembly, assuming that crews will be rotated back to Earth every sixty days to prevent any possiblity of long-term physiological deterioration due to weightlessness.
Two of the deep space passenger ships will be required to shuttle people back and forth between LEO and GEO, carrying seventy-five people per flight.
The number of deep space freight ships required depends on whether it is decided to build an SPS using silicon solar cells or gallium arsenide solar cells. If silicon cells are chosen, twenty-three deep space freighters will be required because there will be thirty flights per year from LEO to GEO with SPS subassemblies.
This fleet of ships and the frequency of operation far exceeds the capabilities of any of today's 1980 space launch sites or facilities. There is literally no way that Cape Canaveral can be expanded to handle the job because of the simple fact that they've run out of room at the Cape. A new flight operations site, a true space port, will have to be built. It will require lots of land area located in a place remote from the high population density of cities. It will require several landing strips and several launch pads. It will require easy access from the nation's transportation network of railways and highways because of the thousands of tons of materials that must be transported to the spaceport. It will require electric power, which will mean that it must be sited near large existing power plants and high-capacity transmission lines. The huge volume of propellants needed will require that propellant manufacturing facilities be on the spaceport itself. Tons of liquid oxygen will be required along with tons of liquid hydrogen, both available from electrolytic decomposition of water and liquified by known industrial processes. The rocket fuel chosen by DOE for the launch phase of flight is methane because of the ease with which it can be obtained from either natural gas or from the gasification of coal; there are abundant reserves of both natural gas and coal in the United States, far more than are required for the entire long-term SPS project.
These basic requirements for a space port can be met either with a single spaceport or multiple sites. A single spaceport would be more efficient in terms of land, logistics, and environmental impact. And there is only one part of the United States where all conditions for a single spaceport could be met: the western region that includes the states of New Mexico, Arizona, southern Utah, and southern Nevada. There are several locations in this region that would be suitable for the nation's first real spaceport.
The size of the SPS project also means that there will be a large facility in Low-Earth Orbit (LEO): the LEO Base. This is where freight is off-loaded from the freight shuttles and people are transferred from passenger shuttles to deep space passenger ships if they 're going on out to GEO. LEO Base is a pass-through or staging base, but it will be large and will require the presence of at least a hundred people at all times for service, maintenance, etc.
At the far end of the line, the Geosynchronous Earth Orbital facility, GEO Base, will be the place where people assemble the SPS itself from subassemblies brought up by deep space freighters from LEO Base. As many as seven hundred people will be living at GEO Base at any given time, working three shifts around the clock, living in space, eating in space, and sleeping in space. They will require logistical services, personal services, health services, food services, waste disposal services, environmental services, entertainment and recreation services, religious services, and a host of everyday services that we take for granted here on Earth. In many ways, life in GEO Base will resemble living in the construction camps of the Trans Alaska Pipe Line or on an off-shore oil rig. There's nothing new about what needs to be done at GEO Base. What's new is the fact that, for the first time, we'll be doing it in space. But it's a matter of difference in kind, not degree. If the United States can keep three astronauts alive, healthy, happy and working for 84 days at a time in Skylab, and if the Soviets can keep two cosmonauts alive, healthy, happy, and working for six months at a time in Salyut-6, it's totally possible to keep seven hundred people alive, healthy, happy, and working at GEO Base, rotating the crews every sixty days to start with. As our experience grows, crews will be exchanged at longer intervals.
Thus, the transportation system for the Solar Power Satellite project is big. It will cost money. But its cost is something that must be considered in the total view of space power and all the consequences that flow from the possession of space power.