StarTram

startram

StarTram is a proposal for a maglev space launch system. The initial Generation 1 facility would be cargo only, launching from a mountain peak at 3 to 7 km (1.9 to 4.3 mi) altitude with an evacuated tube staying at local surface level; it has been claimed that about 150,000 tons could be lifted to orbit annually. More advanced technology would be required for the Generation 2 system for passengers, with a longer track instead gradually curving up at its end to the thinner air at 22 km (14 mi) altitude, supported by magnetic levitation, reducing g-forces when each capsule transitions from the vacuum tube to the atmosphere.

American physicist James R. Powell invented the superconducting maglev concept in the 1960s with a colleague, Gordon Danby, also at Brookhaven National Laboratory, which was subsequently developed into modern maglev trains. Later, Powell co-founded StarTram, Inc. with Dr. George Maise, an aerospace engineer was at Brookhaven from 1974 to 1997, with particular expertise in reentry heating and hypersonic vehicle design.

A StarTram design was first published in a 2001 paper and patent, making reference to a 1994 paper on MagLifter. Developed by John C. Mankins, who was manager of Advanced Concept Studies at NASA, the MagLifter concept involved maglev launch assist for a few hundred m/s with a short track, 90% projected efficiency. Noting StarTram is essentially MagLifter taken to a greater extreme, both MagLifter and StarTram were discussed the following year in a concept study performed by ZHA for NASA’s Kennedy Space Center, also considered together by Maglev 2000 with Powell and Danby.

The Gen-1 StarTram system proposes to accelerate unmanned craft at 30 g through a 130-km (81 mi) long tunnel, with a plasma window preventing vacuum loss when the exit’s mechanical shutter is briefly open, evacuated of air with an MHD (magnetohydrodynamic) pump. In the reference design, the exit is on the surface of a mountain peak of 6,000 meters (20,000 ft) altitude, where 8.78 km per second (5.46 mi/s) launch velocity at a 10 degree angle takes cargo capsules to low earth orbit when combined with a small rocket burn providing 0.63 kilometres per second (0.39 mi/s) for orbit circularization. With a bonus from Earth’s rotation if firing east, the extra speed, well beyond nominal orbital velocity, compensates for losses during ascent including 0.8 km per second (0.50 mi/s) from atmospheric drag.

A 40-ton cargo craft, 2 meters (6 ft 7 in) in diameter and 13 meters (43 ft) long, would experience briefly the effects of atmospheric passage. With an effective drag coefficient of 0.09, peak deceleration for the mountain-launched elongated projectile is momentarily 20 g but halves within the first 4 seconds and continues to decrease as it quickly passes above the bulk of the remaining atmosphere. The peak heating rate  is very high, yet comparable magnitude to some prior experience, such as the Galileo atmospheric entry probe to Jupiter. Transpiration water cooling is planned, briefly consuming up to ≈ 100 liters/m2 of water per second. Several percent of the projectile’s mass in water is calculated to suffice.

The tunnel tube itself for Gen-1 has no superconductors, no cryogenic cooling requirements, and none of it is at higher elevation than the local ground surface. Except for probable usage of SMES (superconducting magnetic energy storage) as the electrical power storage method, superconducting magnets are only on the moving spacecraft, inducing current into relatively inexpensive aluminum loops on the acceleration tunnel walls, levitating the craft with 10 centimeters clearance, while meanwhile a second set of aluminum loops on the walls carries an AC current accelerating the craft: a linear synchronous motor.

Powell predicts a total expense, primarily hardware costs, of $43 per kilogram of payload if with 35 ton payloads being launched 10+ times a day. Present rocket launch prices are $10,000 to $25,000 per kilogram to orbit. The estimated cost of electrical energy to reach the velocity of low earth orbit is under $1 per kilogram of payload.

The Gen-2 variant of the StarTram is supposed to be for reusable manned capsules, intended to be low g-force, 2 to 3 g acceleration in the launch tube and an elevated exit at such high altitude (22 km) that peak aerodynamic deceleration becomes ≈ 1g. Though NASA test pilots have handled multiple times those g-forces, the low acceleration is intended to allow eligibility to the broadest spectrum of the general public. However, with such relatively slow acceleration, the Gen-2 system has to be 1,000 to 1,500 km (620 to 930 mi) in length. The cost for the non-elevated majority of the tube’s length is estimated to be several tens of millions of dollars per km, proportionately a semi-similar expense per unit length to the tunneling portion of the former Superconducting Super Collider project (originally planned to have 72 km of 5-meter diameter vacuum tunnel excavated for $2 billion) or to some existing maglev train lines where Powell’s Maglev 2000 system is claiming major cost-reducing further innovations. An area of Antarctica 3 km (1.9 mi) above sea level is one siting option, especially as the ice sheet is viewed as relatively easy to tunnel through.

For the elevated end portion, the design considers magnetic levitation to be relatively less expensive than alternatives for elevating a launch tube of a mass driver (tethered balloons, compressive or inflated aerospace-material megastructures). A 280 megaamp current in ground cables creates a magnetic field of 30 Gauss strength at 22 kilometres (14 mi) above sea level (somewhat less above local terrain depending on site choice), while cables on the elevated final portion of the tube carry 14 megaamps in the opposite direction, generating a repulsive force of 4 tons per meter; it is claimed that this would keep the 2 ton/meter structure strongly pressing up on its angled tethers, a tensile structure on grand scale.

An alternative, Gen-1.5, would launch passenger spacecraft at 4 km per second (2.5 mi/s) from a mountaintop at around 6000 meters above sea level from a ≈ 270 km (170 mi) tunnel accelerating at ≈ 3 g. Though construction costs would be lower than the Gen-2 version, Gen-1.5 would differ from other StarTram variants by requiring 4+ km/s to be provided by other means, like rocket propulsion. However, the non-linear nature of the rocket equation still makes the payload fraction for such a vehicle significantly greater than that of a conventional rocket unassisted by electromagnetic launch, and a vehicle with high available weight margins and safety factors should be far easier to mass-produce cheaply or make reusable with rapid turnaround than current 8 km per second (5.0 mi/s) rockets. Dr. Powell remarks that present launch vehicles ‘have many complex systems that operate near their failure point, with very limited redundancy,’ with extreme hardware performance relative to weight being a top driver of expense. (Fuel itself is on the order of 1% of the current costs to orbit).

Alternatively, Gen-1.5 could be combined with another non-rocket spacelaunch system, like a ‘Momentum Exchange Tether’ similar to the HASTOL (Hypersonic Airplane Space Tether Orbital Launch) concept which was intended to take a 4 km per second (2.5 mi/s) vehicle to orbit. Because tethers are subject to highly exponential scaling, such a tether would be much easier to build using current technologies than one providing full orbital velocity by itself. The launch tunnel length in this proposal could be reduced by accepting correspondingly larger forces on the passengers. A ≈ 50 to 80 km (31 to 50 mi) tunnel would generate forces of ≈ 10-15 g, which physically fit test pilots have endured successfully in centrifuge tests, but a slower acceleration with a longer tunnel would ease passenger requirements and reduce peak power draw, which in turn would decrease power conditioning expenses.

The current land speed record of 2.9 km per second (6,487 km/h or 4,031 mph) was obtained by a sled on 5 km (3.1 mi) of rail track mostly in a helium-filled tunnel, in 2003, in a US$20 million project at Holloman Air Force Base (which has also been running a maglev high speed track development program for general DoD hypersonic test applications.

One Comment to “StarTram”

  1. Reblogged this on krypticweb dot net.

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