# Frame drag to accelerate spaceships?

1. Mar 12, 2013

### makc

As far as I understand, a spaceship going around rotating black hole should gain additional acceleration due to frame drag. Right? So, would there be max speed gain, like in gravity assist, or could you accelerate infinitely? Thanks.

2. Mar 12, 2013

### pervect

Staff Emeritus
Robert Forward had some proposal to accelerate objects via frame dragging, but I don't recall the details. A large torus of some sort, I believe.

Last edited: Mar 12, 2013
3. Mar 12, 2013

### Bill_K

Geodesics in the Kerr metric have the usual constants of the motion: energy and angular momentum. A particle falling in from infinity would re-emerge with the same energy it went in with.

4. Mar 12, 2013

### makc

is it not the same as with gravity assist, these constraints should apply to spaceship and black hole together?

I guess I'm asking if falling body can borrow some of black hole energy.

5. Mar 12, 2013

### Bill_K

makc, Sorry, I forgot about the ergosphere!

6. Mar 12, 2013

### pervect

Staff Emeritus
More on Forward's scheme -- which I never quite followed. (The original was in "Indistinguishable from Magic", a collection of non-fiction essays.).

Last edited: Mar 12, 2013
7. Mar 12, 2013

### Bill_K

Ok, I guess I really do need some education here. I've seen Forward's device before - in fact I remember a rather long Am J Phys article he wrote - but I never realized before how much I just don't get the point of it. It all comes back to my earlier comment:
This continues to be true in Forward's torus, does it not? It's stationary and axially symmetric. Hence by general principles, a particle traveling along a geodesic will have constant energy and angular momentum. So you can't simply drop a particle through the hole and expect it to be accelerated! :grumpy:

The ergosphere in the Kerr metric only works a benefit if you do something nongeodesic while inside it: either (a) fire your rockets or (b) split in two, as in the Penrose Process. I'd have to say the torus is similar - on a freely falling object the frame dragging has only a temporary effect, and when you emerge from the other side and move away you'll find to your disappointment that nothing has changed.