# Linear motion reaction wheels?

So, as I've recently learned, reaction wheels rotate a craft with Newton's Third Law, remain spinning while the craft should rotate, then stop, resulting in no rotational speed, but after having rotated the craft, yes?

So what's to stop you from tying a brick to your space craft, and throwing it, as to send you forward and the brick back relatively, until the brick pulls on the rope, and nulls to change. Would this work? If not, why not? Could this be used for maneuvering position wise rather than velocity wise? Could you create a massive object with an incredibly long/strong rope to say, go to the moon(but have no change in velocity other than gravity and other expected things).

I figure that in order to pull the brick back, and not end up in the same position you started at, you'd have to anchor to something to increase your mass, say the moon.

• The problem is reliably anchoring yourself to something to increase your math. (Also, check out RCW saturation and so forth to see practical problems with this.) Commented Aug 24, 2015 at 22:11

## 1 Answer

In principle, yes, you can use this technique to change your position in such a way that you return to your initial velocity at the end of a maneuver. In practice, it doesn't scale well.

For an Earth-to-moon trip, you need a 770,000km rope if the brick is the same mass as your spaceship, and if you want to get there in a matter of days, you need to throw that brick away at 6km/s (or 3km/s for the brick and -3km/s for the spaceship, if you prefer to look at it that way).

For an Earth-to-moon trip, going back to your starting velocity at the moon end of the trip is definitely not what you want. Earth orbital speed in LEO is about 7.8 kilometers per second; orbital speed at the moon's distance is about 1 kilometer per second. So for the brick trick, you have to harpoon the moon at a relative speed of no less than 6.8 km/s. Good luck with that!

Without the rope in the way, using the usual rocket techniques, your moon-relative speed on the approach is (I think) more like 700 m/s. The harpoon technique is still not recommended, but using a rocket to get into orbit is very practical.