Orbiting a black hole.

[hawkinator]

So, imagine your orbiting a black hole at a distance high enough that your not going all that particularly fast. Then, you burn retrograde till your periapsis is close to the event horizon. Now, a few questions:

1. Does this mean that gravity will accelerate us to a large fraction of C?

2. What happens if we burn prograde at the apoapsis periapsis (lol auto correct)? Does the Oberth effect become ridiculously powerful here?

Edited May 12, 2015 by hawkinator
Auto correct fail.

[luizopiloto]

we will probably die...

[Jovus]

1. Yes.

2. Probably, but the Oberth effect is classical, and the relativistic effects are very relevant here. I'm not sure what the math would look like off the top of my head, but I suspect there's at least something you could point out as analogous.

[hawkinator]

Yeah, the weird way the classical physics interfaces with relativity in this scenario is the primary reason it makes my brain hurt...

[K^2]

So, imagine your orbiting a black hole at a distance high enough that your not going all that particularly fast. Then, you burn retrograde till your periapsis is close to the event horizon. Now, a few questions:

1. Does this mean that gravity will accelerate us to a large fraction of C?

2. What happens if we burn prograde at the apoapsis periapsis (lol auto correct)? Does the Oberth effect become ridiculously powerful here?

As I've been corrected in another thread, your lowest periapsis can actually be no lower than 3/2 of the Schwarzschild Radius. In other words, well above event horizon. But yeah, if you drop that low, you'll be moving almost at c.

The Obereth effect gains aren't going to be nearly as insane as they would be classically, because rocket formula is quite different at relativistic speeds.

[Bill Phil]

K^2, could you explain how the rocket formula is different at relativistic speeds?

[K^2]

Comes down to relativistic addition formula. For a classical rocket, if it's traveling at velocity v, and exhaust velocity relative to rocket is v_p, then exhaust is traveling at v - v_p. That is no longer the case for a relativistic rocket. So trying to exploit conservation of momentum becomes a bit complicated.

Another complication is that there are two different velocities to be considered. From perspective of inertial observers, map velocity is the relevant quantity, and it's limited to c. So as you get close to c, your rocket's efficiency drops dramatically. The faster you go, the more fuel you need to burn for the same dV. On the other hand, if you care about how fast you get somewhere by ship's clock, then the relevant quantity is the proper velocity. And that's not limited to c. The reason for that is that while you can't go faster, by burning more fuel you make the distance you need to travel shorter, thanks to Lorentz contraction. And if all you care about how fast you get there, it's just as good. And from perspective of fuel efficiency, it's better. Once you're going fast enough, the hyperbolic nature of Lorentz contraction starts to compensate for the logarithm in the rocket formula. And that has some absolutely insane consequences, such as it becomes actually feasible to build a matter-antimatter rocket large enough to cross the galaxy in a matter of decades. By ship's time, of course. On Earth, tens of thousands of years would pass anyways.

The actual derivations involve serious math. You can take a look at Wikipedia article on proper acceleration to get some insights.

[Bobnova]

The tidal forces will rip you apart, but the radiation will have already killed you.

[K^2]

Tidal forces of large black holes are quite mild. Hawking Radiation, likewise, completely harmless. So if you can find a supermassive black hole with no significant accretion disk, you can safely orbit it at distances being discussed here.

[Onchyophora]

In kerbal space you find super massive black hole with no accretion disk, in real space super massive black hole finds you!

and accretes you.:cool:

[AngelLestat]

In kerbal space you find super massive black hole with no accretion disk, in real space super massive black hole finds you!

and accretes you.:cool:

Is that a joke or you want to point a serious case in reality?

From what I know.. an active accretion disk will be the exception, not the rule of today super massive black holes, when we see quasars, there are all supermassive black holes with an active accretion disk and a jet particle stream pointing to us. But that is because we are seeing the first billions of years of galaxies, when they had a lot of gas to consume.

Also I dont see how can you fall in a super massive black hole, the galaxy is huge in comparison, and you only fall if your periapsys is super close.

[magnemoe]

One fun idea would be to make an super massive but small object in KSP. Think miniature black hole or an tiny neutron star. Think Eve mass, this would enable us to do some crazy maneuvers.

[hawkinator]

One fun idea would be to make an super massive but small object in KSP. Think miniature black hole or an tiny neutron star. Think Eve mass, this would enable us to do some crazy maneuvers.

Crazy cool transfers? Yes. very much yes.

Landing on it? Ultimate hard mode.

Realism? Ehhhh , not so much. Putting an object like that into the game exploits the things the game doesn't simulate (like n-body gravity and relativity) On a related note, I've wondered for a long time now if it would be possible to make a relativity mod for KSP.

[K^2]

On a related note, I've wondered for a long time now if it would be possible to make a relativity mod for KSP.

It'd be very hard to pull off without the source. You can try and fake it by adjusting time and positions of things, plus maybe hijacking the shader, but it'd be iffy. To do it right, you need to replace a lot of core functions.

[hawkinator]

It'd be very hard to pull off without the source. You can try and fake it by adjusting time and positions of things, plus maybe hijacking the shader, but it'd be iffy. To do it right, you need to replace a lot of core functions.

Yeah, Faking it with time/position would be the (only) way to go about it. The reason I would think it would be possible would be the fact that there is a lot of control over time in KSP. On the other hand, I understand neither coding nor relativity well enough to do it. On the other other hand, I also seem to be derailing my own thread lol.

[NFUN]

One fun idea would be to make an super massive but small object in KSP. Think miniature black hole or an tiny neutron star. Think Eve mass, this would enable us to do some crazy maneuvers.

Gregroxmun tried that a few months ago. The game really didn't like it, physics broke down a bit, and as far as I know, he stopped the project.

[Xannari Ferrows]

I can give you a Newtonian analysis, but it wont help much. Our space works differently form what we can put on paper.

[K^2]

Gregroxmun tried that a few months ago. The game really didn't like it, physics broke down a bit, and as far as I know, he stopped the project.

Yeah, that's about what I'd expect form it.

The C# code for the game is obfuscated, but not too badly. In principle, it should be possible to use a mod to set up a bunch of hooks to override core functions. If somebody really wanted to do this right, I could provide pointers.

[magnemoe]

Gregroxmun tried that a few months ago. The game really didn't like it, physics broke down a bit, and as far as I know, he stopped the project.

Can imagine it would be serious kraken bait

You don't get tidal forces on an ship, however numbers cange very fast at Pe and this should cause problems

[quietghost]

The short answer with approaching a black hole is that the concepts of apoapsis and periapsis and orbit do not exist. Because of relativity (and as someone pointed out earlier), one cannot come closer to the black hole than the photon sphere, or for those of you who have seen Interstellar, the "critical orbit". While certain mechanics still apply (like conservation of momentum), they are extremely skewed due to the gamma factor caused by speeds near those of the speed of light.

An aside note and interesting fact: The photon sphere is where the black hole starts from the "black" void perspective. It is NOT the event horizon. What the photon sphere means is that any photons on an INWARD trajectory to the black hole will never escape, but those on an OUTWARD trajectory can (and will). So while radiation can indeed escape from inside the photon sphere, there is not very much of it. The general concepts applying to the event horizon still apply. Also, inside the event horizon there is no such thing as conservation of momentum or energy, so anything goes!