# Can electric propulsion be used for launch (theoretically) [closed]

Electric propulsion is driven by power. Are there any theoretical limitations to the amount of power that can be used for electric propulsion? If limits do exist, do they rule out electric propulsions as a launch mechanism? For simplicity, in talking about a vertical launch.

The perfect answer would also give details on power storage/production and if this rules out the method for launch.

Clarifying statement: My intension for the question was ion engines.

• Electrical propulsion only works in a vacuum. – Rikki-Tikki-Tavi Feb 12 '15 at 18:24
• Are you asking specifically about ion drives, or any means of propulsion that requires electricity? – Hobbes Feb 12 '15 at 20:31
• Arcjet should work, yeah? – Russell Borogove Feb 12 '15 at 22:36
• Please edit to clarify what you mean with "electric propulsion". There's so many definitions out there, most really vague, e.g. en.wikipedia.org/wiki/… Does there have to be direct electric interaction between spacecraft and its reaction mass? Does the power have to be stored or produced on spacecraft (ion, plasma drives,...) or can it be external (electrodynamic tethers, rail guns, magsails, beamed photon,...)? – TildalWave Feb 13 '15 at 14:26
• This question is ambiguous. Electric propulsion as in rail gun? Electric propulsion as in ion engine? In either case the answer's no. Rail guns from earth's surface have been shot down several times in space.stackexchange. Ion engines have a thrust to weight ratio much less than 1. If an engine weighing 10 pounds is only pushing .01 pounds, it's not going to get off the ground. – HopDavid Feb 13 '15 at 14:41

Edit: on second thought, you're probably referring to an ion drive instead of firing the spacecraft out of a rail gun.

You could use a rail gun on a spacecraft as a mass driver: gun fires a rock out the back end, and the reaction force drives the ship.

If you're talking about firing the spacecraft out of a rail gun: the main limits to this are common to all gun launch methods:

• either huge G-forces or very long barrels needed (500 m at 1000 G). G-forces sustainable for a human mean a gun that's hundreds of km long.
• atmospheric drag means you have to launch at speeds much higher than orbital speed.
• if the barrel is not a vacuum, you have to provide holes in the barrel so the air ahead of the projectile can escape. The supersonic shockwave means you can't build the gun near inhabited areas. Try drawing an uninhabited area 30 km wide and 300 km long anywhere in the First World.

Rail guns exist. The US Navy is experimenting with one that will fire a 3-kg projectile at 2.5 km/s. The most difficult part of this is getting the gun to not destroy itself. This gun works by placing a conductive dart on top of two parallel rails. The current goes up one rail, through the dart and down the other. The rails are heated by the huge current needed (on the order of 1 MA), to the point where they initially had to swap out the rails after a few shots. You're also setting up huge EM fields around the projectile.
There are other electromagnetic gun principles, e.g. you can place coils in series and pulse current through them to propel a metal projectile. I used to have fun firing metal pins from a coil scavenged from a set of toy train points.

• Atmospheric drag does more than just slow the payload. It would inflict severe punishment. For 8 km/s at the top of the earth's highest mountains I get 18,000 kilo pascals dynamic pressure. For comparison a class 5 hurricane inflicts about 3 kilo pascals and the max-Q of the shuttle was about 35 kilo pascals. – HopDavid Feb 13 '15 at 14:36
• OP edited the question after this answer was posted to specifically state ion engines, making this a non-answer. – user Jun 2 '15 at 12:30

Thrust is thrust and its effects are the same no matter where it comes from. In theory, yes electric propulsion such as hall effect thrusters, electrostatic ion thrusters and magnetoplasmadynamic (MPD) thrusters can be used to launch a satellite from the surface of the earth to orbit. However, due to the very high exhaust velocity of electric propulsion systems they have a very high specific impulse, which is a nice way of saying that in theory they can produce a large change in spacecraft velocity for a small amount of propellant.

While a higher specific impulse may sound advantageous, the gravity and atmospheric drag at Earth's surface can counteract the effects of the high specific impulse. By re-arranging the equations of motion and the equation for kinetic jet power we can see that as the specific impulse of an engine increases, the amount of power required to produce a pound of thrust also increases at a near logarithmic rate. Because of this increase in the ratio of power to thrust, electric propulsion systems would require an enormous power source to produce enough thrust to accelerate against gravity and minimize drag losses. Unfortunately, the specific mass of contemporary power systems (chemical, solar and nuclear) is too low to allow for this type of launch system.

So in summary, if we could build power sources that could produce an enormous amount of power for a very low structure mass (~3 kg/kWe or less), that were also compact then yes, it is possible to use electric propulsion systems for launch vehicle applications. However, this is beyond the current state of the art for power systems.