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mass drivers for interplanetary commerce



 
 
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Old December 31st 16, 03:30 AM posted to sci.space.policy
William Mook[_2_]
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Default mass drivers for interplanetary commerce


http://www.nss.org/settlement/nasa/s...ctrofig16a.GIF

Constructed in 1976 and 1977, Mass Driver 1 was an early demonstration of the concept of the mass driver, launcher.

The mass driver launches payloads from a lunar base to L5. This is where a space colony is built to process lunar materials for use on Earth and in cislunar space - for example, power satellites in GEO.

The model consisted of a series of some 20 drive coils through which a small armature called the bucket traveled. This bucket is pushed by the pulsed magnetic fields of the drive coils. The bucket rides on four rails made from copper plumbing tubes, through which was fed direct current from several car batteries connected in series. The model was built around an epoxy glass cylinder, 10 cm in diameter by 20 cm long. It was wrapped with a coil of aluminium wire, energized by the batteries. The copper rails were mounted onto the interior of the drive coils, and the bucket was held onto the copper rails by spring-loaded beryllium copper strips with automotive carbon brushes mounted on the ends. The passage of the bucket depressed microswitches which triggered the discharge of capacitors through the drive coils. Optical switches could easily replace microswitches, and a DC power supply replace the car batteries.

The current in the bucket coil interacted, by the Lorentz force, with the pulsed magnetic fields from the drive coils to accelerate the bucket. When the bucket coil was cooled by liquid nitrogen to reduce its electrical resistance, it was able to achieve an acceleration of around 30 g (300 m/s²).

The mass driver was inspired and designed by Gerard K. O'Neill of Princeton University (who was on sabbatical at MIT during the 1976-77 academic year) and Henry Kolm of MIT. It was built under their direction by students at MIT, largely using material scavenged from the scrap heap at the Bitter Magnet Lab at MIT. A demonstration at the May 1977 Princeton University Space Manufacturing Facilities Conference was covered by Nova.

http://www.nss.org/settlement/manufa.../princeton.htm

Ultimately O'Neill and Kolm hoped to launch payloads to escape velocity from Earth, thus reducing the cost of space launch to only about ten dollars a pound, the cost of electric energy, by eliminating the need to launch rocket fuel. Further development awaits the day when the space launch market off-Earth is large enough to justify the construction cost.

The idea for the mass driver originated with Herman Oberth in the 1930s and was popularised in the 1966 Robert A. Heinlein book The Moon Is a Harsh Mistress. Heinlein referred to a similar device as an "induction catapult" and Oberth called it a "lunar catapult".

NASA has taken great interest in the concept over the years;

https://ntrs.nasa.gov/archive/nasa/c...0110007073.pdf

A 30 gee acceleration attains 3 km/sec requires 15.3 km long accelerator to achieve 3 km/sec. It takes 10.2 seconds to accelerate a round through the system. By rapidly recharging the capacitor bank powering each segment of the system multiple buckets may be accelerated simultaneously, achieving 30 rounds per second with 306 buckets in the driver at one time separated by 50 meters each.

A 3.1 meter diameter cylinder that's 12.2 meters long bucket has the same volume as an ISO high-cube shipping container. Each masses 1,000 kg and carries 25,000 kg, and contains 2,300 kg integral kick stage with 2,000 kg of propellant that imparts 330 m/sec delta vee to the system in transit.

30 buckets per second imparts 3,000 m/sec velocity to 849 tons per second. This requires a power rate of 3.82 trillion watts! Operating continuously over the course of a year, this system delivers 26.79 billion tons of materiel to everyone in the cislunar system.

A similar system operating on Mars imparting 6.3 km/sec to payloads requires a 67.5 km long track operating at 30 gees. It takes 21.4 seconds to bring a bucket up to this speed. Operating at 30 buckets per second 642 buckets are in the system, with an average separation of 105 meters. 849 tons per second over a 92 day period once every 780 days - produces an average rate of 3.16 billion tons per year of product for the same 3.82 trillion watts - produced only over the 92 day window.

Small bomblets made of subcritical flecks of fissile material, surrounding by Li-6 Deuteride, encased in fertile materials (thorium and U238) to convert the neutron flux of the initial explosion to charged particles, and detonated in series, through extreme compression using magnetic fields, to produce the required 3.82 TW in a compact lightweight system, to power the mass driver at the required rates.

https://ntrs.nasa.gov/archive/nasa/c...0010028795.pdf

When not driving the mass driver, the system can be used to process rock with an electric arc, and use time of flight mass spectrometry to separate out streams of pure isotopes which are processed into usable products. This is 15 GJ per ton, and at 849 tons per second - 12.8 terawatts is required.

So, to sustain a rate of 849 tons per second on the moon, requires 16.62 terawatts of power. To sustain 3.16 billion tons per year (100 tons per second) on Mars requires 1.96 terawatts average, peaking to 3.82 terawatts during synodic alighnment. A constant 3.82 terawatts on Mars sustains 150 tons per second for use on Mars, and 100 tons per second for use on Earth, with the Earth material ejected over a 92 day period every 780 days.

3.89 terajoules per kg of uranium and 23.01 terajoules per kg of Lithium-6 Deuteride, an a 3 gram capsule consisting of 23 micrograms of U235, 2,290 micrograms of Li-6 Deuteride, and 687 micrograms of U238, produces 55.45 gigajoules of explosive energy. Detonated at a rate of 100x per second - 300 grams per second - produces a raw energy of 5.5 terawatts, and a useful power of 3.9 terawatts. This process consumes 72.3 tons per year of U235, 2,168.1 tons per year of U238/Thorium, 7,226.9 tons per year of Li6D.



 




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