Editing Mass driver (section)

===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}}