<p>I recommend you to read the short paper(1) on gravitational machines by late physicist <a href="https://en.wikipedia.org/wiki/Freeman_Dyson" rel="nofollow noreferrer">Freeman Dyson</a>. As Dyson puts it:</p>
<blockquote>
<p>The difficulty in building machines to harness the energy of the
gravitational field is entirely one of scale. Gravitational forces
between objects of a size that we can manipulate are so absurdly weak
that they can be scarcely be measured, let alone exploited. To yield a
useful output of energy, any gravitational machine must be built on a
scale that is literally astronomical.</p>
</blockquote>
<p>Black holes aren't necessary, you could actually do the trick with binary stars, like <a href="https://en.wikipedia.org/wiki/White_dwarf" rel="nofollow noreferrer">white dwarfs</a>. A pair to accelerate your payload, and another to decelerate it at destination.</p>
<blockquote>
<p><a href="https://i.sstatic.net/CwVN7.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/CwVN7.png" alt="enter image description here" /></a></p>
<p>A double star has two components A and B, each with mass M, revolving
around each other in a circular orbit of radius R. The velocity of
each star is <span class="math-container">$$V = \sqrt \frac{GM}{4R}$$</span> where <span class="math-container">$$G = 6.7*10^{-8}
&gt; cm^3/sec^2g$$</span> is the gravitational constant. The exploiters of the
device are living on a planet or vehicle P which circles around the
double star at a distance much greater than R. They propel a small
mass C into a orbit which fall toward the double star, starting from P
with a small velocity. The orbit of C is computed in such a way that
it makes a close approach to B at a time when B is moving in a
direction opposite to the direction of arrival of C. The mass C then
swings around B and escapes with greatly increased velocity. The
effect is almostif the light mass C had made a elastic collision with
the moving heavy mass B. The mass C will arrive at a distant point Q
with velocity somewhat greater than 2V. At Q the mass C may be
intercepted and its kinetic energy converted into useful form.
Alternatively the device may be used as a propulsion system, in which
case C merely proceeds with velocity 2V to its destination. The
destination might be a similar device situated very far away, which
brings C to rest by the same mechanism working in reverse.</p>
</blockquote>
<p>Dyson estimates that a white dwarf version of this device could accelerate a payload to about 2000 km/sec.</p>
<p>The main limitations on this method, beyond the sheer scale involved, are tidal stresses on larger payloads, given the accelerations involved, and the suitable systems are quite short-lived, decaying due to gravitational radiation.</p>
<p>(1). <a href="http://www.ifa.hawaii.edu/users/barnes/ast242_s14/Dyson_Machines.pdf" rel="nofollow noreferrer">Dyson, F. (1963). Gravitational machines. Interstellar Communication, edited by AGW Cameron,(Benjamin Press, New York, 1963).</a></p>

CC BY-SA 4.0
2020-07-14T18:06:51.483Z
2020-07-14T18:06:51.483Z
ksousa
<p>Skyhooks or tethers are most useful for lifting mass from a surface into space, increasing velocity. Kerr blackholes have no appropriate surface for basing mass. Instead a gravity assist maneuver may take advantage of the high spin speed of a Kerr black hole, which induce frame-dragging where the space around the blackhole (within the ergosphere) is dragged at the speed of light in the same direction as the spin.</p>
<p><a href="https://nasa.fandom.com/wiki/Gravity_assist#Limits_to_slingshot_use" rel="nofollow noreferrer">https://nasa.fandom.com/wiki/Gravity_assist#Limits_to_slingshot_use</a></p>
<p>The Penrose process also offers a means of extracting energy from Kerr blackholes, if the rotational energy of the black hole is located not inside the event horizon but inside this ergosphere.</p>
<p><a href="https://en.wikipedia.org/wiki/Penrose_process" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Penrose_process</a></p>

CC BY-SA 4.0
2020-07-14T04:58:33.073Z
2020-07-14T04:58:33.073Z
gisol
<p>Could we use spinning black to act as a <a href="https://www.youtube.com/watch?v=dqwpQarrDwk" rel="nofollow noreferrer">skyhook</a> giving us enough energy boost to travel the galaxy?</p>

CC BY-SA 4.0
2020-07-14T07:25:34.013Z
there's [this](https://astronomy.stackexchange.com/q/36687/7982) but I don't think it's what you are looking for.
uhoh
CC BY-SA 4.0
2020-07-14T23:12:20.927Z
Nope.But thanks anyway.
Helena Wells
<black-hole>
Spinning black holes acting as a slingshot
Helena Wells
# Spinning black holes acting as a slingshot

(Asked by: username_0 on 2020-07-14)

Could we use spinning black to act as a [skyhook](https://www.youtube.com/watch?v=dqwpQarrDwk) giving us enough energy boost to travel the galaxy?

**Tags:** black-hole
> **Comments:**
> **username_1**: there's [this](https://astronomy.stackexchange.com/q/36687/7982) but I don't think it's what you are looking for.  
> **username_0**: Nope.But thanks anyway.  

---

## 2 answers

### Answer by username_2 on 2020-07-14. Score: 3

I recommend you to read the short paper(1) on gravitational machines by late physicist [Freeman Dyson](https://en.wikipedia.org/wiki/Freeman_Dyson). As Dyson puts it:

> The difficulty in building machines to harness the energy of the
> gravitational field is entirely one of scale. Gravitational forces
> between objects of a size that we can manipulate are so absurdly weak
> that they can be scarcely be measured, let alone exploited. To yield a
> useful output of energy, any gravitational machine must be built on a
> scale that is literally astronomical.

Black holes aren't necessary, you could actually do the trick with binary stars, like [white dwarfs](https://en.wikipedia.org/wiki/White_dwarf). A pair to accelerate your payload, and another to decelerate it at destination.

> [![enter image description here](https://i.sstatic.net/CwVN7.png)](https://i.sstatic.net/CwVN7.png)
>
> A double star has two components A and B, each with mass M, revolving
> around each other in a circular orbit of radius R. The velocity of
> each star is $$V = \sqrt \frac{GM}{4R}$$ where $$G = 6.7\*10^{-8}
> > cm^3/sec^2g$$ is the gravitational constant. The exploiters of the
> device are living on a planet or vehicle P which circles around the
> double star at a distance much greater than R. They propel a small
> mass C into a orbit which fall toward the double star, starting from P
> with a small velocity. The orbit of C is computed in such a way that
> it makes a close approach to B at a time when B is moving in a
> direction opposite to the direction of arrival of C. The mass C then
> swings around B and escapes with greatly increased velocity. The
> effect is almostif the light mass C had made a elastic collision with
> the moving heavy mass B. The mass C will arrive at a distant point Q
> with velocity somewhat greater than 2V. At Q the mass C may be
> intercepted and its kinetic energy converted into useful form.
> Alternatively the device may be used as a propulsion system, in which
> case C merely proceeds with velocity 2V to its destination. The
> destination might be a similar device situated very far away, which
> brings C to rest by the same mechanism working in reverse.

Dyson estimates that a white dwarf version of this device could accelerate a payload to about 2000 km/sec.

The main limitations on this method, beyond the sheer scale involved, are tidal stresses on larger payloads, given the accelerations involved, and the suitable systems are quite short-lived, decaying due to gravitational radiation.

(1). [Dyson, F. (1963). Gravitational machines. Interstellar Communication, edited by AGW Cameron,(Benjamin Press, New York, 1963).](http://www.ifa.hawaii.edu/users/barnes/ast242_s14/Dyson_Machines.pdf)


---

### Answer by username_3 on 2020-07-14. Score: 3

Skyhooks or tethers are most useful for lifting mass from a surface into space, increasing velocity. Kerr blackholes have no appropriate surface for basing mass. Instead a gravity assist maneuver may take advantage of the high spin speed of a Kerr black hole, which induce frame-dragging where the space around the blackhole (within the ergosphere) is dragged at the speed of light in the same direction as the spin.

<https://nasa.fandom.com/wiki/Gravity_assist#Limits_to_slingshot_use>

The Penrose process also offers a means of extracting energy from Kerr blackholes, if the rotational energy of the black hole is located not inside the event horizon but inside this ergosphere.

<https://en.wikipedia.org/wiki/Penrose_process>


---

