Editing Mass driver (section)

==Fixed mass drivers==
{{see also|Space gun}}
Mass drivers need no physical contact between moving parts because they guide their projectiles by dynamic magnetic levitation, allowing extreme reusability in the case of solid-state power switching, and a functional life of&nbsp;– theoretically&nbsp;– up to millions of launches.  While marginal costs tend to be accordingly low, initial development and construction costs are highly dependent on performance, especially the intended mass, acceleration, and velocity of projectiles.  For instance, while [[Gerard K. O'Neill|Gerard O'Neill]] built his first mass driver in 1976–1977 with a $2000 budget, a short [[Mass Driver 1|test model]] firing a projectile at 40&nbsp;m/s and 33 [[g-force|g]],<ref>Compare:
{{cite journal
 |last1        = Henson
 |first1       = Keith
 |author-link1 = Keith Henson
 |last2        = Henson
 |first2       = Carolyn
 |author-link2 = Carolyn Meinel
 |title        = 1977 Space Manufacturing Facilities Conference
 |url          = http://www.nss.org/settlement/L5news/L5news/L5news7706.pdf
 |journal      = L5 News
 |publisher    = L-5 Society
 |date         = June 1977
 |volume       = 2
 |issue        = 6
 |page         = 4
 |access-date  = 2017-11-27
 |quote        = The stars of this conference [...] were Professor Henry Kolm of Massachusetts Institute of Technology and the group of student volunteers who built the first mass driver [...] In its best test, the mass driver prototype produced an acceleration of thirty-three gravities. This is more than Dr. O'Neill [...] had considered necessary for a lunar surface mass driver. [...] The mass driver was demonstrated several times during breaks between conference sessions, each time with a round of applause for the team who built it in less than four months on a budget of $2,000.
 |archive-url  = https://web.archive.org/web/20170505100448/http://www.nss.org/settlement/L5news/L5news/L5news7706.pdf
 |archive-date = 2017-05-05
 |url-status   = dead
}}</ref>
his next model had an order-of-magnitude greater acceleration<ref>Compare:
{{cite journal
 |last1        = Snow
 |first1       = William R.
 |last2        = Dunbar
 |first2       = R. Scott
 |author-link2 = R. Scott Dunbar
 |last3        = Kubby
 |first3       = Joel A.
 |last4        = O'Nell
 |first4       = Gerard K.
 |author-link4 = Gerard K. O'Neill
 |title        = Mass Driver Two: A Status Report
 |url          = https://classes.soe.ucsc.edu/ee070/Fall07/UNSECURE/Pictures%20of%20the%20Mass%20Driver%20discussed%20in%20class/Mass%20Driver%20Two%20-%20A%20Status%20Report.pdf
 |journal      = IEEE Transactions on Magnetics
 |date         = January 1982
 |volume       = Mag-18
 |issue        = 1
 |page         = 127
 |access-date  = 2017-11-26
 |quote        = Mass Driver Two combines for the first time all the essential features of an operational mass driver, with the exception of bucket recirculation and payload handling. Its nominal design acceleration is 5000 m/s2, for a final velocity of 112 m/s.
 |doi          = 10.1109/tmag.1982.1061777
 |bibcode      = 1982ITM....18..127S
 |archive-url  = https://web.archive.org/web/20120722020519/http://classes.soe.ucsc.edu/ee070/Fall07/UNSECURE/Pictures%20of%20the%20Mass%20Driver%20discussed%20in%20class/Mass%20Driver%20Two%20-%20A%20Status%20Report.pdf
 |archive-date = 2012-07-22
 |url-status   = dead
}}</ref>
after a comparable increase in funding, and, a few years later, researchers at the University of Texas estimated that a mass driver firing a 10 kilogram projectile at 6000&nbsp;m/s would cost $47 million.<ref name = massdriver2>[http://www.soe.ucsc.edu/classes/ee070/Fall07/UNSECURE/Pictures%20of%20the%20Mass%20Driver%20discussed%20in%20class/Mass%20Driver%20Two%20-%20A%20Status%20Report.pdf IEEE Transactions on Magnetics, Vol Mag-18, No. 1]{{Dead link|date=March 2020 |bot=InternetArchiveBot |fix-attempted=yes }}, January 1982. Retrieved May 10, 2011.</ref>{{qn|date=November 2017}}<ref>[http://www.utexas.edu/research/cem/IEEE/PR%2083%20Gully%20Publications.pdf Electromagnetic Launchers for Space Applications]. Retrieved May 10, 2011.</ref>{{failed verification|date=November 2017}}

For a given amount of energy involved, heavier objects go proportionally slower. Lightweight objects may be projected at 20&nbsp;km/s or more. The limits are generally the expense of energy storage able to be discharged quickly enough and the cost of power switching, which may be by semiconductors or by gas-phase switches (which still often have a niche in extreme pulse power applications).<ref>{{cite web |url= http://www05.abb.com/global/scot/scot256.nsf/veritydisplay/8a528b3efd5df655c12578470029b1f6/$file/eml08aw_high%20current%20high%20voltage%20switches%20for%20electromagnetic%20launch%20%28eml2008%29.pdf |title= High Current, High Voltage Solid State Discharge Switches for Electromagnetic Launch Applications}}</ref><ref>{{cite web|url= http://www.electricstuff.co.uk/pulse.html|title= Pulse Power Switching Devices&nbsp;&ndash; An Overview}}</ref><ref>{{cite web|url= http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA428435|title= Scanning the Technology: Modern Pulsed Power|access-date= April 27, 2011|archive-date= December 1, 2012|archive-url= https://web.archive.org/web/20121201183324/http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA428435|url-status= dead}}</ref>  However, energy can be stored inductively in superconducting coils. A 1&nbsp;km long mass driver made of superconducting coils can accelerate a 20&nbsp;kg vehicle to 10.5&nbsp;km/s at a conversion efficiency of 80%, and average acceleration of 5,600 g.<ref name=L5news>{{Cite web |url=http://www.nss.org/settlement/L5news/1980-massdriver.htm |title=L5 news, Sept 1980: Mass Driver Update |access-date=2009-07-28 |archive-url=https://web.archive.org/web/20171201042921/http://www.nss.org/settlement/L5news/1980-massdriver.htm |archive-date=2017-12-01 |url-status=dead }}</ref>

Earth-based mass drivers for propelling vehicles to orbit, such as the [[StarTram]] concept, would require considerable capital investment.<ref name = startram>{{cite web|url= http://www.startram.com/resources|title= StarTram2010: Maglev Launch: Ultra Low Cost Ultra High Volume Access to Space for Cargo and Humans|access-date= 2011-04-28|archive-url= https://web.archive.org/web/20170727013646/http://www.startram.com/resources|archive-date= 2017-07-27|url-status= dead}}</ref> The Earth's relatively strong gravity and relatively thick atmosphere make the implementation of a practical solution difficult. Also, most if not all plausible launch sites would propel spacecraft through heavily-traversed air routes. Due to the massive [[turbulence]] such launches would cause, significant [[air traffic control]] measures would be needed to ensure the safety of other aircraft operating in the area.

With the proliferation of reusable rockets to launch from Earth (especially first stages) whatever potential might have once existed for any economic advantage in using mass drivers as an alternative to chemical rockets to launch from Earth is becoming increasingly doubtful. For these reasons many proposals feature installing mass drivers on the [[Moon]] where the lower [[gravity]] and [[Atmosphere of the Moon|lack of atmosphere]] greatly reduce the required velocity to reach lunar orbit.{{Citation needed|date=September 2024}} Lunar launches from a fixed position are much less likely to generate issues with respect to matters such as traffic control due to its low population and difficulties in lunar air travel.

Most serious mass-driver designs use superconducting coils to achieve reasonable energetic efficiency (often 50% to 90+%, depending on design).<ref>{{cite journal|bibcode= 1980ITM....16..719K|title= Electromagnetic Launchers|doi= 10.1109/TMAG.1980.1060806|journal=IEEE Transactions on Magnetics|volume=16|issue=5|date=September 1980|last1= Kolm|first1= H.|last2= Mongeau|first2= P.|last3= Williams|first3= F.|pages= 719–721}}</ref> Equipment may include a superconducting bucket or aluminum coil as the payload. The coils of a mass driver can induce [[eddy current]]s in a payload's aluminum coil, and then act on the resulting [[magnetic field]]. There are two sections of a mass driver. The maximum [[acceleration]] part spaces the coils at constant distances, and synchronizes the coil currents to the bucket. In this section, the acceleration increases as the velocity increases, up to the maximum that the bucket can take. After that, the constant acceleration region begins. This region spaces the coils at increasing distances to give a fixed amount of velocity increase per unit of time.

Based on this mode, a major proposal for the use of mass drivers involved transporting lunar-surface material to space habitats for processing using [[solar energy]].<ref>[http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html NASA, 1975: Space Settlements: A Design Study] {{Webarchive|url=https://web.archive.org/web/20170625000423/https://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html |date=2017-06-25 }}. Retrieved 2011-05-09.</ref> The Space Studies Institute showed that this application was reasonably practical.

In some designs, the payload would be held in a bucket and then released, so that the bucket can be decelerated and reused. A disposable bucket, on the other hand, would avail acceleration along the whole track. Alternatively, if a track were constructed along the entire circumference of the Moon (or any other celestial body without a significant atmosphere) then a reusable bucket's acceleration would not be limited by the length of the track&nbsp;&ndash; however, such a system would need to be engineered to withstand substantial [[centrifugal force]]s if it were intended to accelerate passengers and/or cargo to very high velocities. 

===On Earth===
In contrast to cargo-only chemical [[space gun|space-gun]] concepts, a mass driver could be any length, affordable, and with relatively smooth acceleration throughout, optionally even lengthy enough to reach target velocity without excessive [[g-force|g forces]] for passengers. It can be constructed as a very long and mainly horizontally aligned [[launch track]] for spacelaunch, targeted upwards at the end, partly by bending of the track upwards and partly by [[Earth's curvature]] in the other direction.

Natural elevations, such as mountains, may facilitate the construction of the distant, upwardly targeted part. The higher up the track terminates, the less resistance from the atmosphere the launched object will encounter.<ref>{{cite web |url=http://spacemonitor.blogspot.com/2007/03/magnetic-launch-system.html |work=The Space Monitor |title=Magnetic Launch System}}</ref>

The 40 [[megajoule]]s per kilogram or less [[kinetic energy]] of projectiles launched at up to 9000&nbsp;m/s velocity (if including extra for drag losses) towards [[low Earth orbit]] is a few [[kilowatt-hours]] per kilogram if efficiencies are relatively high, which accordingly has been hypothesized to be under $1 of electrical energy cost per kilogram shipped to [[Low Earth Orbit|LEO]], though total costs would be far more than electricity alone.<ref name = startram/>  By being mainly located slightly above, on or beneath the ground, a mass driver may be easier to maintain compared with many other structures of [[non-rocket spacelaunch]].  Whether or not underground, it needs to be housed in a pipe that is [[vacuum pump]]ed in order to prevent internal air [[Drag (physics)|drag]], such as with a mechanical shutter kept closed most of the time but a [[plasma window]] used during the moments of firing to prevent loss of vacuum.<ref>{{cite web |url=http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA426465&Location=U2&doc=GetTRDoc.pdf |title=Advanced Propulsion Study |access-date=2011-05-03 |archive-date=2012-12-01 |archive-url=https://web.archive.org/web/20121201194510/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA426465&Location=U2&doc=GetTRDoc.pdf |url-status=dead }}</ref>

A mass driver on Earth would usually be a compromise system.  A mass driver would accelerate a payload up to some high speed which would not be enough for orbit. It would then release the payload, which would complete the launch with rockets.  This would drastically reduce the amount of velocity needed to be provided by rockets to reach orbit.  Well under a tenth of orbital velocity from a small rocket thruster is enough to raise [[perigee]] if a design prioritizes minimizing such, but hybrid proposals optionally reduce requirements for the mass driver itself by having a greater portion of [[delta-v]] by a rocket burn (or orbital [[momentum exchange tether]]).<ref name = startram/>  On Earth, a mass-driver design could possibly use well-tested [[Maglev train|maglev]] components.

To launch a space vehicle with humans on board, a mass driver's track would need to be almost 1000 kilometres long if providing almost all the velocity to [[Low Earth Orbit]], though a lesser length could still provide major launch assist. Required length, if accelerating mainly at near a constant maximum acceptable [[g-force]] for passengers, is proportional to velocity squared.<ref name = physics>{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#mot1 |title=Constant Acceleration}}</ref> For instance, half of the velocity goal could correspond to a tunnel a quarter as long needing to be constructed, for the same acceleration.<ref name = physics/> For rugged objects, much higher accelerations may suffice, allowing a far shorter track, potentially circular or [[Helix|helical]] (spiral).<ref>{{cite web |url=http://techfreep.com/magnets-not-rockets-could-fling-satellites-into-space.htm |title=Magnets, Not Rockets, Could Fling Satellites Into Space |access-date=2008-05-04 |archive-date=2017-12-01 |archive-url=https://web.archive.org/web/20171201041611/http://techfreep.com/magnets-not-rockets-could-fling-satellites-into-space.htm |url-status=dead }}</ref> Another concept involves a large ring design whereby a space vehicle would circle the ring numerous times, gradually gaining speed, before being released into a launch corridor leading skyward.

Mass drivers have been proposed for the disposal of nuclear waste in space: a projectile launched at much above Earth's [[escape velocity]] would escape the Solar System, with atmospheric passage at such speed calculated as survivable through an elongated projectile and a very substantial heatshield.<ref name=L5news/><ref>
{{cite book
|editor-last=Horton |editor-first=T. E.
|title=Thermophysics of Atmospheric Entry 
|publisher=[[American Institute of Aeronautics and Astronautics]]
|date=1982
|isbn=978-0-915928-66-8
|first1=Chul |last1=Park
|first2=Stuart W. |last2=Boden
|chapter=Ablation and deceleration of mass-driver launched projectiles for space disposal of nuclear wastes
|pages=201–225
|doi=10.2514/5.9781600865565.0201.0225
}}
</ref>{{Verify source|date=October 2019}}