February 08, 2015

Elon Musk tweeted on the space elevator, electric rockets and railguns

Earth based Space elevators would be 62000 miles long. This about 2.5 times the 24,000 mile circumference of the earth.

At 200 kilometers per hour, a 30-passenger climber would be able to reach the GEO level after a 7.5 day trip. The climbing module would need to climb through Van Allen Radiation Belt for possibly days.

On February 13, 2006 the LiftPort Group announced that, earlier the same month, they had tested a mile of "space-elevator tether" made of carbon-fiber composite strings and fiberglass tape measuring 5 cm (2.0 in) wide and 1 mm (approx. 13 sheets of paper) thick, lifted with balloons. The carbon fiber was not strong enough for a real full blown earth based space elevator.

In 2007, Elevator:2010 held the 2007 Space Elevator games, which featured US$500,000 awards for each of the two competitions, (US$1,000,000 total) as well as an additional US$4,000,000 to be awarded over the next five years for space elevator related technologies. No teams won the competition, but a team from MIT entered the first 2-gram (0.07 oz), 100-percent carbon nanotube entry into the competition.

Liftport Group is working on a Lunar Space elevator to be deployed before 2020. This is technical feasible with materials available today. They have detailed plans on their approach. The issue is they can gather sufficient funding and operationally execute to deliver the plan.

Nextbigfuture covered the Space Elevator contests and has reported on various Space elevator and tether proposals. It would take a book to cover them all by collecting and updating hundreds of posts.

The Lunar Space Elevator Infrastructure is going to serve many purposes for the human race. The first and most obvious is the opening up of the Moon for tourism and colonization purposes. LSEI can be built with current materials, but it would not have a lot of throughput. The goal of the study would be to determine a method of increasing transport throughput to enable humans to go back to the Moon. LiftPort’s LSEI architecture provides consistent, safe, and high-volume lunar transportation. Each Ribbon attached to the lunar surface allows for an additional 260 kg of cargo. Built up over time, 5, 10 or 15 such Ribbons allows for human-rated heavy-cargo capacity. By building a complete, reusable, and expandable infrastructure, LSEI can send three astronauts to the lunar surface every four weeks. In addition, the LSEI expands the capabilities of the Deep Space Habitat envisioned by the Global Exploration Roadmap and endorsed by NASA. Accessing the Moon also means accessing the minerals of the Moon. The Moon’s suspected to possess a motherlode of helium3, which many (ourselves included) believe it could be used to energize nuclear fusion reactions and provide vast amounts of energy in a process which avoids the radioactive waste of nuclear fission (the process used in nuclear power on earth currently.)

Elon is correct in terms of any simple plan. There are technical feasible plans which would involve decades of development.

StarTram Generation 2, a megastructure more ambitious than Gen-1, reaching above 96% of the atmosphere's mass

This is why any remotely feasible proposal for magnetic launch like the StarTram proposal will have the launch vehicle exit into very thin atmosphere.

An alternative, Gen-1.5, would launch passenger spacecraft at 4 kilometres per second (2.5 mi/s) from a mountaintop at around 6000 meters above sea level from a ≈ 270 kilometres (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 kilometres 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 concept which was intended to take a 4 kilometres 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 kilometres (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

SOURCES - Liftport Group, Elon Musk Twitter, Wikipedia

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