StarTram

The initial Generation 1 facility is proposed to launch cargo only from a mountain peak at an altitude of 3 to 7 kilometres (9,800 to 23,000 ft) using an evacuated tube remaining at local surface level.

More advanced technology is required for a Generation 2 system for passengers, with a longer track instead gradually curving up at its end to the thinner air at 22 kilometres (72,000 ft) altitude, supported by magnetic levitation, reducing g-forces when each capsule transitions from the vacuum tube to the atmosphere.

[1] Later, Powell co-founded StarTram, Inc. with Dr. George Maise, an aerospace engineer who previously was at Brookhaven National Laboratory from 1974 to 1997 with particular expertise including reentry heating and hypersonic vehicle design.

...The overall feasibility and cost of the StarTram approach was validated in 2005 by a thorough “murder board” study conducted at Sandia National Laboratory.The Gen-1 system proposes to accelerate uncrewed craft at 30 g through a 130-kilometer (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 pump.

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 kilometres per second (0.50 mi/s) from atmospheric drag.

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.

Except for probable usage of SMES 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.

[1][15] The Gen-2 variant of the StarTram is supposed to be for reusable crewed 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 kilometres (14 mi)) that peak aerodynamic deceleration becomes ≈ 1g.

[1] Though NASA test pilots have handled multiple times those g-forces,[17] the low acceleration is intended to allow eligibility to the broadest spectrum of the general public.

The cost for the non-elevated majority of the tube's length is estimated to be several tens of millions of dollars per kilometer, proportionately a semi-similar expense per unit length to the tunneling portion of the former Superconducting Super Collider project (originally planned to have 72 kilometres (45 mi) of 5-meter (16 ft) 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.

[1] An area of Antarctica 3 kilometres (9,800 ft) above sea level is one siting option, especially as the ice sheet is viewed as relatively easy to tunnel through.

[20] A 280-megaamp current in ground cables creates a magnetic field of 30 Gauss strength at 22 kilometres (72,000 ft) 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.

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.

[21][22] Alternatively, Gen-1.5 could be combined with another non-rocket spacelaunch system, like a Momentum Exchange Tether similar to the HASTOL concept which was intended to take a 4 kilometres per second (2.5 mi/s) vehicle to orbit.

[1][17][24] The StarTram ground facility concept is claimed to be reusable after each launch without extensive maintenance, as it would essentially be a large linear synchronous electric motor.

[1] Such would be novel in scale but not greatly different planned cost performance than obtained in other smaller pulsed power energy storage systems (such as quick-discharge modern supercapacitors dropping from $151/kJ to $2.85/kJ cost between 1998 and 2006 while being predicted to later reach a dollar per kJ,[26] lead acid batteries which can be $10 per kW-peak for a few seconds, or experimental railgun compulsator power supplies).

[1] For MagLifter, General Electric estimated in 1997-2000 that a set of hydroelectric flywheel pulse power generators could be manufactured for a cost equating to $5.40 per kJ and $27 per kW-peak.

[1] Yet, compared to some other potential implementations of a coilgun launcher with relatively higher requirements for pulse power switching devices (an example being an escape velocity design of 7.8 kilometres (4.8 mi) length after a 1977 NASA Ames study determined how to survive atmospheric passage from ground launch),[27] which are not always semiconductor-based,[28] the 130-km acceleration tube length of Gen-1 spreads out energy input requirements over a longer acceleration duration.

[29] The Gen-1.5 version of the StarTram for launch of passenger RLVs at 4 km/s velocity from the surface of a mountain would be significantly higher speed with a far more massive vehicle.

However, such would accelerate in a lengthy vacuum tunnel without air or gas drag, with levitation preventing hypervelocity physical rail contact, and with 3 orders of magnitude higher anticipated funding.

While the performance of niobium-titanium superconductor is technically sufficient (a critical current density of 5 x 105 A/cm2 under the relevant magnetic field conditions for the levitated platform, 40% of that in practice after a safety factor),[4] uncertainties on economics include a far more optimistic assumption for Gen-2 of $0.2 per kA-meter of superconductor compared to the $2 per kA-meter assumed for Gen-1 (where Gen-1 doesn't have any of its launch tube levitated but uses superconducting cable for a large SMES and within the maglev craft launched).

The preceding neglects how only the final portion of the track is levitated, but a more complex calculation only changes the result for force per unit length of it by 10-20% (fgl = 0.8 to 0.9 instead of 1).

[4] The active structure is calculated to bend by a fraction of a meter per kilometer under wind in the very thin air at its high altitude, a slight curvature theoretically handled by guidance loops, with net levitation force beyond structure weight exceeding wind force by a factor of 200+ to keep tethers taut, and with the help of computer-controlled control tethers.