# Has anyone suggested using an electrically powered sling or centrifuge mass thrower as a form of electrically powered spacecraft propulsion?

Has anyone suggested using an electrically powered sling as a form of electric propulsion for a space craft? It might be a centrifuge type arrangement with a way to release part of or all of the contents into space at high speed, or an actual sling made of carbon fiber, say, being very light and long, or something else. Conservation of momentum would mean the spacecraft went the opposite way.

The direction of rotation of the machine could alternate between one way and the opposite way to prevent a build up of angular momentum. Or two slings or centrifuges might spin in opposite directions, releasing at the same moment.

Or it could be a thread, cord, or rope (or pairs of them) that are spun and let out until it/they are full length and then released.

I imagine that the speed of the projectiles would be somewhere between ten kilometers per second and one thousand kilometers per second. But I have no idea how to calculate that. The limiting factor would be the strength of the centrifuge or string, at a guess. Since there are centrifuges that produce one million g's (you read that right), that strength is likely to be high.

The ultracentrifuge is a centrifuge optimized for spinning a rotor at very high speeds, capable of generating acceleration as high as 1 000 000 g (approx. 9 800 km/s²).1 There are two kinds of ultracentrifuges, the preparative and the analytical ultracentrifuge. Both classes of instruments find important uses in molecular biology, biochemistry, and polymer science. (Ultracentrifuge - Wikipedia)

The fastest centrifuges use a vacuum to reduce friction. Space is a vacuum.

The idea came to me just now as I read of pulsed electric propulsion in this article: https://en.wikipedia.org/wiki/Electrically_powered_spacecraft_propulsion

• Is your question specifically about centrifuge guns as opposed to linear mass drivers, as described at en.wikipedia.org/wiki/Mass_driver#Spacecraft-based_mass_drivers? Commented Apr 25, 2021 at 18:05
• Making a spacecraft that can withstand the extreme G-force and a tether that can withstand the extreme tension is not necessarily easy. Commented Apr 25, 2021 at 21:23
• @ChristopherJamesHuff Absolutely. Also, I just realized I'd better add a weapon tag to this. Commented Apr 26, 2021 at 14:02
• Can't forget the Kzinti Lesson, after all. A while back there was a guy pushing his "DREAD" centrifuge gun, with one of the claimed advantages being lack of recoil, IIRC with someone suggesting that this would make it a useful spacecraft weapon. At the time, I pointed out how physically impossible that claim was and suggested that it could actually be used for spacecraft propulsion. I'm not aware of any serious work on such things though. Commented Apr 26, 2021 at 15:26
• @ChristopherJamesHuff You were right. There must be recoil because of conservation of momentum unless mass with an equal amount of momentum goes in the opposite direction, i.e. backwards out of the gun, which arguable is a sort of recoil or the equivalent. Commented Apr 26, 2021 at 18:09

Both things you mention have been worked on, conceptually or actually. The paper I'm familiar with on sling launchers is one by Geoffrey Landis, but he states he is expanding on earlier work. His paper is on ResearchGate - Journal of the British Interplanetary Society, Vol. 58, No. 9/10, pp. 294-297 (2005). Analysis of a Lunar Sling Launcher. That version is the long cable version you mention.

There's now an enterprise looking to develop a centrifuge launch system for launch to low Earth orbit - Spinlaunch. Their website is currently a landing page, an email address, and a page that lists around 30 open positions for which they are hiring. They have raised \$80 million. From their Wikipedia page -

The technology uses a vacuum-sealed centrifuge to spin a rocket and then hurl it to space at up to 8,000 kilometres per hour (5,000 mph). The rocket then ignites its engines at an altitude roughly 200,000 foot (61 km) to reach orbital speed of 17,500 miles per hour (28,200 km/h).

• ...where did you hear that Zubrin was part of SpinLaunch? He's written about other similar concepts for using slings on the moon, but those are unrelated to SpinLaunch (and far more feasible and potentially useful). Commented Apr 25, 2021 at 20:10
• – uhoh
Commented Apr 25, 2021 at 22:16
• @ChristopherJamesHuff from Jon Goff of Altius Space. Commented Apr 25, 2021 at 22:54
• @ChristopherJamesHuff I'll take that out as it's part of a recalled conversation. Commented Apr 26, 2021 at 18:25

This would have a very low specific impulse, closer to firing projectiles from a machine gun than to ejecting gas at several km/s. It would need an awful lot of bullets to build up speed. Another way to say this is that the mass of the fuel would be an impractically high fraction of the craft's total mass.