All textual content is by Stephen Ashworth, Oxford, UK,
unless attributed to a different signed author.
Images are credited when the source is known.
Growth Options (1)
Today we begin an occasional series considering the basics of human civilisation’s options for growth in space.
Is an expansion of industrial civilisation into space feasible, or is it just a childish dream? The essays in this series examine particular aspects of the astronautical vision and use simple calculations to distinguish feasible options from fantasy.
Let the reader beware: speculative engineering calculations and estimates of feasibility have an irreducibly subjective element. Engineering is not an armchair pursuit; the physical world is complex enough that one cannot design a new project – nor can one prove that it will never work – by calculations and theoretical arguments alone. The only way to be sure is to roll one’s sleeves up and get one’s hands dirty: to try out an idea in practice.
Any discussions concerning an engineering idea which has not been tried before are therefore inevitably slanted to suit the writer’s preconceptions. Either one wants something to succeed, and so when problems arise one looks for solutions to those problems, or one wants to damn the whole project, and so one goes looking for problems, excuses and denigratory put-downs and avoids any mention of solutions.
If the idea under consideration violates the law of conservation of energy or some other well-established fact, then the problems may indeed be insuperable. But all too often the distinction between raising legitimate objections and indulging in mendacious stonewalling is a blurred and murky one.
Author’s declaration of interest: where space is concerned, while open to reasonable debate, in the end I am looking for solutions.
The Basics of Getting into Space
Before anyone can do anything in space, they first have to get there. If transport to and from space is too expensive, it sets up a barrier to any kind of activity there, and makes the natural resources of space inaccessible.
Therefore we must first consider ways and means of overcoming that barrier by getting from Earth’s surface into orbit and back regularly, safely and affordably. Since space transport is currently dominated by government vehicles and missions, we must also ask whether they are capable of growth, or whether more commercial activity is required.
It must be clear that an astronautical civilisation cannot arise unless and until routine operations in space are possible for all kinds of payloads, including people.
The cost of transporting payloads into space on current ballistic missile based launch vehicles is on the order of $10,000 per kg, or $10 million/tonne. The best-known launcher, the US Space Shuttle, cost about $1.5 billion per flight (in 2010 dollars). With a payload of about 25 tonnes, this comes to $60 million/tonne.
This is because, although intended to be partly reusable, the Shuttle was not actually seriously operated as a reusable vehicle; in its later years each orbiter spent about a year on the ground being refurbished inbetween flights. Only once did an orbiter achieve as many as four flights in a single year (Discovery, in 1985). Real reusable vehicles such as cars, trains, ships and aircraft, make daily trips. So for the present, at the extremely low present-day usage rates, throw-away rockets based on or derived from military ballistic missiles such as Atlas, Delta, Ariane, Soyuz and Proton have proven cheaper.
At a cost of $10 million/tonne, commercially self-sustaining activities are confined mainly to robotic communications satellites. Earth observation and satnav are not economically self-sustaining, since they require heavy government subsidy, particularly noticeable in the American GPS and European Galileo satnav systems.
If the actual human presence in space is restricted to government officials (and occasional special guests of the government) flying on government-built, owned and operated vehicles, then the large-scale permanent human occupation of space and other worlds would depend upon a large-scale government programme with those goals. Such a programme has not yet been officially adopted by any government, and under current global economic and political conditions it seems most unlikely that it ever would be adopted.
If progress in manned spaceflight is gauged by the annual man-days logged in space, overwhelmingly by government astronauts on government missions, then there has in fact been continuous, though erratic, growth from 1.13 man-days in 1961 up to a record level of 2190.11 man-days in 2010 (corresponding to an average annual growth rate of 16%). (Note for pedants: as will be obvious to most native English speakers, “man” and its derivatives are here used in the sense of man (male or female, child or adult) as opposed to machine, not man (adult male) as opposed to woman: Vostok 6 was a manned spacecraft.) But given falling space agency budgets and the lack of any coherent strategy for building on current successes with the Shuttle, the International Space Station and Soyuz, it is not at all clear that this rate of progress can be maintained in the future.
Plans for expansion of the ISS, for example, are conspicuous by their absence. The space agencies currently seem more interested in promoting schemes to repeat the Apollo project (to the Moon, Mars or the asteroids: known as the global exploration strategy) than in consolidating and extending our existing foothold in space. The irresponsible view has arisen that the ISS is merely another government mission, with an end date at which the capability that has been built up over a quarter of a century at great expense of public funds will be deliberately destroyed, not an infrastructure programme which can and should be permanently maintained and used to extend our capabilities further.
Unless new actors appear on the scene, the annual graph of man-days logged in space therefore seems likely to plateau out over the next 5 to 10 years while the ISS is maintained at a constant manning level, and then to fall after the ISS is abandoned, whether deliberately, or as a result of management incompetence – particularly in failing to provide it with a sufficiently frequent and responsive transport service to maintain it through major malfunctions or collision damage.
Construction of a space economy supporting a permanent space population therefore cannot take place without the introduction of new launch vehicles which can lower the cost of flights to orbit and raise their frequency, regularity and reliability sufficiently to allow new economic activities to begin, activities which create infrastructure which is not only sustainable in the long term but also capable of growth.
But what sort of a vehicle would that be?
The key point here is that commercial human passenger spaceflight is already here. Millions of people fly in space every year. They deserve astronaut wings.
Sounds like a joke? Well, what is the definition of space? The usual official definition is that space begins at an altitude of 100 km above sea level, though the US Air Force has adopted the altitude of 80 km. The reasoning is that above a certain altitude, aircraft can no longer use their wings and must rely on thrusters to control their orientation and rocket engines to support their weight as well as drive them forward. Although there is no exact altitude at which this happens, and the air pressure fluctuates with day to day weather conditions, it is in the region of 80 to 100 km.
Another, equally possible, equally reasonable, definition might be that space begins at the altitude above which unprotected human life is impossible. Commercial passenger jets fly close to the tropopause at around 11 km altitude. I’ve flown there; you probably have too. Yet at this height above the ground, atmospheric pressure is only a quarter of sea level pressure. People begin to suffer from altitude sickness, due to a lack of oxygen, at only 2.4 km, and passenger aircraft fly 4.6 times higher than that, 1.25 times higher even than the summit of Mount Everest (8.8 km). Meanwhile, the average air temperature in the tropopause is –56.5 °C.
Our current civilisation therefore already enjoys routine, comfortable, safe and economic passenger access to space in the sense that air travellers fly well outside the region of the atmosphere hospitable to human life. If they were exposed to conditions outside their cabin, they would die from cold and oxygen starvation, almost as quickly as if they were outside the atmosphere altogether, in real outer space.
Since we already have a well developed aircraft industry which carries travellers to the fringe of space, it must be clear that the logical way to get into space proper, if it is technically possible, would be to design an aircraft able to fly faster and higher until it ended up in orbit.
This is exactly what a number of engineers have proposed, from the 1940s onwards (with the German pioneer Eugen Sänger). Despite the world’s space agencies acting more to protect vested interests in legacy throw-away rockets than to serve the public by developing progressive technologies, the Skylon programme in the UK is making considerable progress towards realisation of aeroplane-style access to orbit. Skylon’s overall layout resembles an aeroplane; it takes off and lands horizontally on a runway, but is capable of reaching orbit and returning to Earth.
The flight profile of a launch to space is quite different from that of an airliner. After the airliner takes off it accelerates to Mach 0.8 = 0.236 km/s, after which it must maintain its cruising speed for several hours with its engine thrust. Reaching space by contrast requires much greater acceleration, to a speed of about 9.0 km/s (including losses), 38 times faster, but once in orbit a vehicle maintains its orbital speed without requiring any more engine thrust.
The extra acceleration required to get into orbit requires greater energy expense per flight than is saved by not needing propulsion once one gets there, so the spaceplane is intrinsically more expensive than the aeroplane.
The Skylon approach has to be the right one for large-scale economic access to space. The key technology is the combined jet-rocket engine, which can breathe air on the way up, and switch to onboard oxygen once the atmosphere is too thin. Liquid oxygen is much heavier than liquid hydrogen fuel, so any saving in the mass of oxygen carried onboard results in a major saving in the weight of the entire vehicle.
Meanwhile the horizontal take-off and landing flight profile has the advantage that a new vehicle can make numerous test flights before committing to go into orbit, starting with short hops and graduating to greater speed and altitude as experience is gained. Testing of a new vehicle is a crucial factor: a new commercial airliner might make several hundred to a thousand test flights during its test programme prior to going into service. The Space Shuttle made a total of 135 flights in its entire career (two of which ended in loss of the vehicle and crew): not enough to qualify it as ready for regular use by airliner standards, but a good start. Then of course the programme was cancelled. But at over $1 billion a go, the Shuttle was hardly likely to be properly flight tested.
For comparison, the test programme for Concorde involved 5335 hours of flight, 2000 of which were at supersonic speeds, equivalent in duration to about 25 Shuttle flights of 8 days each. But since each test flight would not have lasted longer than around 10 hours, and the earlier ones may only have lasted an hour or two, the actual number of individual test flights made must have been in the region of 1000. Even at its maximum annual launch rate of 9 in the year 1985, the Shuttle would have to have been operated for over a century just to qualify it for service, according to airliner test standards. This emphasises the impracticality of vertical launch, semi-throw-away systems for any serious access to space.
Getting back down again from orbit, the only realistic method involves shedding one’s orbital speed through air resistance. This of course causes the enormous heating of re-entry, and any chink in the protection against such heat is liable to be disastrous, as it was in the case of Space Shuttle Columbia in 2003. However, it must be realised that the severity of the heating depends on the ballistic coefficient, i.e. the mass per unit surface area of a re-entering vehicle, because the kinetic energy which must be dissipated is proportional to the mass, but if one doubles the surface area exposed to frictional heating while keeping the mass the same, then the amount of frictional energy per unit surface area is halved.
Any fully reusable vehicle returning from orbit consists mainly of empty hydrogen tanks, and thus has a low ballistic coefficient. The Shuttle, of course, threw away its main tank at the end of every ascent, and thus threw away this potential advantage. A vehicle like Skylon therefore suffers less re-entry heating per unit surface area, and is consequently cheaper and safer: it is easier to insulate a larger area against weaker heating than a smaller area against stronger heating.
In order to find the comparative cost of getting into orbit, I shall compare the Skylon C-1 design with the Airbus A-319 (chosen because it has a similar payload and because I have flown on this aircraft myself). The key data we shall need are as follows:
|Airbus A-319||Skylon C-1|
|Maximum take-off weight:||75.5 tonnes||275 tonnes|
|Fuel:||24,210 litres (standard fuel load) of Jet A-1 fuel = 19.5 tonnes||66 tonnes liquid hydrogen|
|Fuel energy content:||34.7 MJ/litre = 43.15 MJ/kg (density = 0.804 kg/litre)||141.86 MJ/kg|
|Fuel cost:||~$1000/tonne (2011)||$0.32/lb manufactured on site (2002); $1.00 to $1.40/lb manufactured elsewhere and transported to site (2002); 1 tonne = 2204.6 lb whence maximum value in 2002 about $3000/tonne|
|Oxidiser:||Atmospheric oxygen||Atmospheric oxygen supplemented by 150 tonnes liquid oxygen|
|Oxidiser cost:||Free||$0.21/kg (2001), but sensitive to energy price changes; say $300/tonne now|
|Payload:||15.2 tonnes passengers and cargo||12 tonnes passengers and/or cargo|
|Passenger capacity||156||40 to low orbit with no cargo; 20 to space station orbit with 1.5 tonnes cargo|
|Cost per passenger:||~$250||~$500,000 estimated by manufacturer (Reaction Engines Ltd)|
First, let us compare the energy costs. A flight of Airbus which uses all the fuel in its standard tanks consumes 0.84 million megajoules. A flight of Skylon uses 9.36 million megajoules. A Skylon flight is therefore 11 times more energy intensive than an Airbus flight, moreover, in its passenger-carrying configuration it carries 7.8 times fewer passengers (taking the more realistic lower value of 20 passengers for Skylon), making it 87 times more energy intensive per person.
Next, the monetary fuel costs. Airbus costs $19,500 for a fuelling stop. Taking the high end of the range for hydrogen costs, but assuming efficiencies can be found which compensate for rising costs since 2002 (in particular, given large-scale use, hydrogen fuel is likely to be manufactured at the launch site), the cost to fill its tanks is $198,000 (hydrogen) plus $45,000 (oxygen) coming to $243,000 in total. Skylon is 12.5 times more expensive to fuel, or 97 times more expensive per passenger.
Making the reasonable assumption that the overall cost of operating a fully reusable vehicle scales roughly in parallel with the fuel costs, we would therefore expect a passenger seat on Skylon to cost about 100 times greater than one on an Airbus A-319.
The cost per seat estimated by Reaction Engines is actually 2000 times greater than an Airbus ticket, though still at a level affordable by a significant fraction of well-to-do people today. The difference of a further factor of 20 is reasonable: the airline industry is a mature one, having been transporting large numbers of passengers with jet aircraft for half a century. Skylon will be the first of its type, and has to develop its own market.
As that market grows, however, we should expect to see prices per passenger seat fall from around $500,000 towards $25,000. They might not end up quite this low because the spaceplane still requires special equipment which the aircraft does not, notably attitude control thrusters and better insulation against the more extreme heat and cold of space. We might take $100,000 as a reasonable compromise: the price of a passenger ticket to space after the industry has had a decade or two to get going.
Note that this is a higher estimate than that obtained by Gerard O’Neill. In The High Frontier, he proposes that ticket prices for a one-way trip between Earth’s surface and a colony at L5, the distance of the Moon, will fall within a range of $6,000 to $30,000 (p.100-103). However, he is assuming a system which has matured to the stage that his ships can refuel at the L5 station, which is looking further ahead than I am doing at the moment.
What about cabin pressurisation and temperature control? As noted above, from the point of view of the living, breathing, warm-bodied human traveller, the Airbus already flies in space. Its cabin is pressurised to the equivalent of 2.4 km altitude, and kept comfortably warm. With an interior pressure of 0.75 sea-level atmospheric pressure and an exterior pressure of about 0.25 atmospheres, the pressure differential is half an atmosphere, easily enough to sustain life if the proportion of oxygen is increased.
Concorde flew higher and enjoyed a higher cabin pressure: the pressure was 0.81 inside and 0.065 outside, thus a differential of 0.745 atmospheres, equal to the value in the Airbus cabin. If a Concorde cabin reduced its internal presssure to match that of the Airbus 319, it would be suitable for occupation even when surrounded by the effectively total vacuum of space. The Airbus A-380, too, has a higher cabin pressurisation of about 0.835 atmospheres, which again would comfortably sustain its passengers in space.
Radiation is a different kind of problem. No space capsule or passenger-carrying vehicle has had or could have radiation protection, due to the enormous weight that would need to be added. Radiation, however, is tolerable for short periods, up to and including the six months to a year which astronauts spend in space at present. Up to an altitude of about 500 km, including low Earth orbit where space stations operate, Earth’s magnetic field provides some protection against the worst of the danger.
A regular space passenger industry would tolerate low radiation doses in transit, just as passengers in high-flying aircraft have already done for many years. The radiation protection on the space stations they visit would be built up over several years, taking the form of screens of plastic or plastic containers filled with water. Once in orbit, such screens could be reused indefinitely.
One of the major objections raised to any proposal for large-scale space passenger transport schemes is that there is nowhere to go: there is no “there” there.
The reality is actually the reverse: the destination is more advanced than the available transport. The attraction of low Earth orbit is indisputable: astronauts who have flown in space are unanimous that the journey is an impressive and worthwhile one from the point of view of personal experience alone. They comment on the beauty of Earth seen from space, a heightened appreciation of the cosmic perspective of their planet in the astronomical universe, and a sense of universal human brotherhood. The experience of weightlessness, while it often takes a little getting used to, answers the dream of flying like a bird.
Meanwhile, as the series of space stations from Salyut 1 to the ISS has demonstrated, the technologies for living in space are more advanced than those for travelling to and from orbit. American entrepreneur Robert Bigelow has already placed two privately developed habitat modules in orbit. When the transport is ready, there will be space hotels to fly to, and people willing to spend time on them.
One of the most significant results of the ISS programme so far is that it has proved that rich people will spend money in order to spend time in space. One of them liked it so much he even flew twice. While socialists may object to multi-millionaires paying enormous sums (between $20 million and $40 million a ticket) on activities which provide jobs and develop technologies towards the point where larger numbers of people can afford to fly themselves, it must be stated that leisure travel on Earth too started out as a luxury which only the extremely wealthy could afford, but which thanks to the development of rail, sea and air travel is now accessible to people at all economic levels. Tourism has become a trillion-dollar global industry (personal travel and tourism valued at $2.8 trillion in 2002), and there is every prospect of it extending to low Earth orbit in the near future.
A one-day symposium, held at the British Interplanetary Society on 10 November 2005 and organised by British aerospace engineer David Ashford, called space tourism “the key to low-cost access to space”, and concluded with the following points:
Clearly, the public side of that equation is doing its best to ignore commercial passenger spaceflight. In America at least, its preferred focus is on saving jobs in legacy industries and planning costly exploration ventures from which public participation is excluded. The Shuttle programme was intended to reduce costs to space and open space up to new applications, but no attempt has been made to evolve the (admittedly extremely flawed) design of the Shuttle in this direction. Proposed Shuttle successor programmes such as the X-33 have been impractical and unsustained. The goal of the Shuttle programme was finally abandoned by president George W. Bush in January 2004.
It is therefore mainly up to the private sector, with a low level of public support in the form, for example, of NASA contracts for ISS supply and lukewarm European support of Skylon research, to consolidate the human foothold in low Earth orbit.
While some have expressed skepticism that non-governmental space programmes can operate safely (while presumably not hesitating to book seats on commercially owned airlines flying commercially developed aircraft), it must be understood that only commercial spaceflight can achieve levels of economy and safety compatible with a large-scale extension of human civilisation into space.
The key point here is that economy and reliability require large-scale use. You learn to fly by flying, not once or twice a year, but every day. Technologies and operating procedures have to mature over a long process of trial and error. A space programme flying once every six weeks (the Shuttle in a good year) – let alone one flying only once or twice a year (the likely flight rate of the currently planned Space Launch System, if it ever gets built) – cannot hope to match the economy or reliability of one flying every day or hour.
Frequency of flights, large-scale market, full reusability of all hardware, verification of hardware through a comprehensive flight test programme, regularity and reliability of the resulting service – these factors all fit together. They are illustrated today by the airline industry. If spaceflight can emulate this model, then we have the basis for a civilisation expanding into space. If it cannot, for whatever reason, do so, then we are confined to Earth forever, no matter what scientific discoveries may yet be made by occasional scientific probes to the planets or the stars.