Startram - maglev train to low earth orbit
Getting into space is one of the harder tasks to be taken on by humanity. The present cost of inserting a kilogram (2.2 lb) of cargo by rocket into Low Earth Orbit (LEO) is about US$10,000. A manned launch to LEO costs about $100,000 per kilogram of passenger. But who says we have to reach orbit by means of rocket propulsion alone? Instead, imagine sitting back in a comfortable magnetic levitation (maglev) train and taking a train ride into orbit.
All right, its not quite that simple or comfortable - but it should be possible using only existing technology.
Dr George Maise invented the Startram orbital launch system along with Dr James Powell, who is one of the inventors of superconducting maglev - for which he won the 2002 Franklin Medal in engineering. Startram is in essence a superconducting maglev launch system.
The system would see a spacecraft magnetically levitated to avoid friction, while the same magnetic system is used to accelerate the spacecraft to orbital velocities - just under 9 km/sec (5.6 miles/s). Maglev passenger trains have carried passengers at nearly 600 kilometers per hour (373 mph) - spacecraft have to be some 50 times faster, but the physics and much of the engineering is the same.
The scope of the project is challenging. A launch system design for routine passenger flight into LEO should have rather low acceleration - perhaps about 3 g's maximum, which then requires 5 minutes of acceleration to reach LEO transfer velocities. In that period, the spacecraft will have traveled 1,000 miles (1,609 km). The maglev track must be 1,000 miles in length - similar in size to maglev train tracks being considered for cross-country transportation.
Like a train, the Startram track can follow the surface of the Earth for most of this length. Side forces associated with the curvature of the surface can be accommodated by the design, but not the drag and sonic shock waves of a craft traveling at hypersonic velocity at sea level - the spacecraft and launching track would be torn to shreds.
To avoid this, the Startram track must be contained inside a vacuum tube with vents to allow air compressed in front of the spacecraft to escape the tube. A vacuum equivalent to atmospheric conditions at an altitude of 75 km (about 0.01 Torr) should suffice for the efficient operation of the Startram launch system. Rapid pumping to achieve this pressure will be provided by a magnetohydrodynamic vacuum pump.
If the entire Startram tube is at sea level, on exiting the tube the spacecraft will suddenly be subjected to several hundred g's due to atmospheric drag - rather like hitting a brick wall. To reduce this effect to a tolerable acceleration, the end of the Startram vacuum tube must be elevated to an altitude of about 20 km (12 miles). At this height, the initial deceleration from atmospheric drag will be less than 3 g's, and will rapidly decrease as the spacecraft reaches higher altitudes.
This new requirement begs the question - how do we hold up the exit end of the Startram vacuum tube? Well, the tube already contains superconducting cable and rings. Powell and Maise realized that the tube could be magnetically levitated to this altitude. If we arrange that there is a superconducting cable on the ground carrying 200 million amperes, and a superconducting cable in the launch tube carrying 20 million amperes, at an altitude of 20 km there will be a levitating force of about 4 tons per meter of cable length - more than enough to levitate the launch tube.
The vacuum tube would be held down against excess levitation force by high strength tethers. Dyneema (UHMWPE) is more than strong enough for this purpose. Redundant design would make a failure of the levitation system most unlikely.
The Startram launch system contains other technological wonders, such as a plasma window on the exit of the vacuum tube to prevent the inrush of the relatively dense air at that altitude from ruining the vacuum within the tube. However, all the required technology exists and is understood. The only engineering effort involved here is in increasing the scale.
Sandia National Laboratories has carried out a '"murder-squad" investigation of the Startram concept, whose purpose is to find any flaw in a proposed project. They gave Startram a clean bill of health. Estimates suggest that building a passenger-capable Startram would require 20 years and a construction budget (ignoring inflation and overoptimism) of about $60 billion.
Why take on such an enormous project? Simple - $50 per kilogram amortized launch costs. The total worldwide cost of developing and using rocket-based space travel is more than $500 billion. The Space Shuttle program cost about $170 billion. The International Space Station has cost about $150 billion to date. As yet, we are making very little commercial use of near-Earth space beyond deployment of communication and imaging satellites. Reducing the LEO insertion costs a hundredfold should finally start our commercial exploitation of the special resources of space. Not to mention making orbital hotels a travel goal for middle-class tourists!
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What might be more feasible would be a "hybrid" system that has a few miles of track sloping up a mountain to 5,000 MSL with a conventional rocket taking over at release. Such a system might accelerate a rocket to several hundred MPH instead of escape velocity. That would reduce the cost of the entire system to something reasonable. It would also fit on an existing military reservation.
I think the actual solution to exorbitant launch costs is what SpaceX is constructing: A 100% reusable space launch system.
The space elevator concept might be better, where if you built it high enough, eventually you can have the structure in centrifugal tension due to the earth's rotation, but the forces resulting from the winds would dictate an impossibly high tensile strength requirement.
Bob Tackett has the right idea - the mother ship with space-plane concept like Space Ship 1 white knight 2, but on a larger scale. This is the most cost-effective and most efficient approach.
However, if the payload was limited to cargo, much higher g's could be used. Then, the runway could be significantly shorter. The runway could be further shortened with coryatjohn's "hybrid" system. Now, we may have a system that is (1) feasible not only in terms of construction but also for maintenance and security and (2) commercially competitive. Perhaps, even new possibilities for ground-to-ground cargo transportation.