Fourmilog: None Dare Call It Reason

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Wednesday, July 8, 2015

Reading List: StarTram

Powell, James, George Maise, and Charles Pellegrino. StarTram. Seattle: CreateSpace, 2013. ISBN 978-1-4935-7757-6.

Magnetic levitation allows suspending a vehicle above a guideway by the force of magnetic repulsion. A train using magnetic levitation avoids the vibration, noise, and rolling resistance of wheels on rails, and its speed is limited only by air resistance and the amount of acceleration passengers consider tolerable. The Shanghai Maglev Train, in service since 2004, is the fastest train in commercial passenger service today, and travels at 431 kilometres per hour in regular operation. Suppose you were able to somehow get rid of the air resistance and carry only cargo, which can tolerate high acceleration. It would appear that if the technical challenges could be met, the sky would be the limit. In this book the authors argue that the sky is just the start.

They propose a space launch system called StarTram, to be developed in two technological generations. The Generation 1 (Gen-1) system is for cargo only, and uses an evacuated launch tube 110 km long in an underground tunnel. This sounds ambitious, but the three tunnels under the English Channel total 150 km, and are much larger than that required for StarTram. The launcher will be located at a site which allows the tube to run up a mountain, emerging in the thinner air at an altitude between 3 and 7 kilometres. There will be an extreme sonic boom as the launch vehicle emerges from the launch tube at a velocity of around 8 kilometres per second and flies upward through the atmosphere, so the launcher will have to be located in a region where the trajectory downrange for a sufficient distance is unpopulated. Several candidate sites on different continents are proposed.

The Gen-1 cargo craft is levitated by means of high (liquid nitrogen) temperature superconducting magnets which are chilled immediately before launch. They need only remain superconducting for the launch itself, around 30 seconds, so a small on-board supply of liquid nitrogen will suffice for refrigeration. These superconducting magnets repel loops of aluminium in the evacuated guideway tube; no refrigeration of these loops is required. One of the greatest technical challenges of the system is delivering the electric power needed to accelerate the cargo craft. In the 30 seconds or so of acceleration at 30 gravities, the average power requirement is 47 gigawatts, with a peak of 94 gigawatts as orbital velocity is approached. A typical commercial grid power plant produces around 1 gigawatt of power, so it is utterly impractical to generate this power on site. But the total energy required for a launch is only about 20 minutes' output from a 1 gigawatt power station. The StarTram design, therefore, incorporates sixty superconducting energy storage loops, which accumulate the energy for a launch from the grid over time, then discharge to propel the vehicle as it is accelerated. The authors note that the energy storage loops are comparable in magnitude to the superconducting magnets of the Large Hadron Collider, and require neither the extreme precision nor the liquid helium refrigeration those magnets do.

You wouldn't want to ride a Gen-1 cargo launcher. It accelerates at around 30 gravities as it goes down the launch tube, then when it emerges into the atmosphere, decelerates at a rate between 6 and 12g until it flies into the thinner atmosphere. Upon reaching orbital altitude, a small rocket kick motor circularises the orbit. After delivering the payload into orbit (if launching to a higher orbit or one with a different inclination, the payload would contain its own rocket or electric propulsion to reach the desired orbit), the cargo vehicle would make a deorbit burn with the same small rocket it used to circularise its orbit, extend wings, and glide back for re-use.

You may be wondering how a tunnel, evacuated to a sufficiently low pressure to allow a craft to accelerate to orbital velocity without being incinerated, works exactly when one end has to be open to allow the vehicle to emerge into the atmosphere. That bothers me too, a lot. The authors propose that the exit end of the tube will have a door which pops open just before the vehicle is about to emerge. The air at the exit will be ionised by seeding with a conductive material, such as cæsium vapour, then pumped outward by a strong DC current, operating as the inverse of a magnetohydrodynamic generator. Steam generators at the exit of the launch tube force away the ambient air, reducing air pressure as is done for testing upper stage rocket motors. This is something I'd definitely want to see prototyped in both small and full scale before proceeding. Once the cargo craft has emerged, the lid slams shut.

Launching 10 cargo ships a day, the Gen-1 system could deliver 128,000 tons of payload into orbit a year, around 500 times that of all existing rocket launch systems combined. The construction cost of the Gen-1 system is estimated at around US$20 billion, and with all major components reusable, its operating cost is electricity, maintenance, staff, and the small amount of rocket fuel expended in circularising the orbit of craft and deorbiting them. The estimated all-up cost of launching a kilogram of payload is US$43, which is about one hundredth of current launch costs. The launch capacity is adequate to build a robust industrial presence in space, including solar power satellites which beam power to the Earth.

Twenty billion dollars isn't small change, but it's comparable to the development budget for NASA's grotesque Space Launch System, which will fly only every few years and cost on the order of US$2 billion per launch, with everything being thrown away on each mission.

As noted, the Gen-1 system is unsuited to launching people. You could launch people in it, but they wouldn't still be people when they arrived on orbit, due to the accelerations experienced. To launch people, a far more ambitious Gen-2 system is proposed. To reduce launch acceleration to acceptable levels, the launch tunnel would have to be around 1500 km long. To put this into perspective, that's about the distance from Los Angeles to Seattle. To avoid the bruising deceleration (and concomitant loss of velocity) when the vehicle emerges from the launch tube, the end of the launch tube will be magnetically levitated by superconducting magnets (restrained by tethers) so that the end is at an altitude of 20 km. Clearly there'll have to be a no-fly zone around the levitated launch tube, and you really don't want the levitation system to fail. The authors estimate the capital cost of the Gen-2 system at US$67 billion, which seems wildly optimistic to me. Imagine how many forms you'll have to fill out to dig a 1500 km tunnel anywhere in the world, not to speak of actually building one, and then you have to develop that massive magnetically levitated launch tube, which has never been demonstrated.

Essentially everything I have described so far appears in chapter 2 of this book, which makes up less than 10% of its 204 pages. You can read a complete description of the StarTram system for free in this technical paper from 2010. The rest of the book is, well, a mess. With its topic, magnetic levitation space launch, dispensed with by the second chapter, it then veers into describing all of the aspects of our bright future in space such a system will open, including solar power satellites, protecting the Earth from asteroid and comet impacts, space tourism, colonising Mars, exploring the atmosphere of Jupiter, searching for life on the moons of the outer planets, harvesting helium-3 from the atmospheres of the outer planets for fusion power, building a telescope at the gravitational lensing point of the Sun, and interstellar missions. Dark scenarios are presented in which the country which builds StarTram first uses it to establish a global hegemony enforced by all-seeing surveillance from space and “Rods from God”, orbited in their multitudes by StarTram, and a world where the emerging empire is denied access to space by a deliberate effort by one or more second movers to orbit debris to make any use of low orbits impossible, imprisoning humanity on this planet. (But for how long? Small particles in low orbit decay pretty quickly.) Even wilder speculations about intelligent life in the universe and an appropriate strategy for humans in the face of a potentially hostile universe close the book.

All of this is fine, but none of it is new. The only new concept here is StarTram itself, and if the book concentrated just on that, it would be a mere 16 pages. The rest is essentially filler, rehashing other aspects of the human future in space, which would be enabled by any means of providing cheap access to low Earth orbit. The essential question is whether the key enabling technologies of StarTram will work, and that is a matter of engineering which can be determined by component tests before committing to the full-scale project. Were I the NASA administrator and had the power to do so (which, in reality, the NASA administrator does not, being subordinate to the will of appropriators in Congress who mandate NASA priorities in the interest of civil service and contractor jobs in their districts and states), I would cancel the Space Launch System in an instant and use a small part of the savings to fund risk reduction and component tests of the difficult parts of a Gen-1 StarTram launcher.

Posted at 01:10