Moon is a Black Hole

[SchweinAero]

41 minutes ago, GoSlash27 said:

I don't think there'd be much advantage there, since you leave with the same relative velocity you came with. 

Gravity assists harness the body's orbital velocity, which wouldn't change.

Best,

-Slashy

Couldn't you still get far closer to the centre than with our current Moon, allowing for a wider selection of changes in direction for a given relative speed?

[GoSlash27]

31 minutes ago, SchweinAero said:

Couldn't you still get far closer to the centre than with our current Moon, allowing for a wider selection of changes in direction for a given relative speed?

Yes, that's the advantage. It's not all that much of an advantage though, since we can get nearly any direction we want now. My point is that the most DV you can gain from an encounter is 2Vorb, regardless of whether it's a moon or a black hole.

Best,
-Slashy

Edited July 3, 2018 by GoSlash27

[p1t1o]

If you split it into two tiny, equal-weight black holes and have them orbit each other in a binary pair, they could be made to orbit each other very rapidly, increasing 2Vorb significantly.

[Superfluous J]

31 minutes ago, GoSlash27 said:

Yes, that's the advantage. It's not all that much of an advantage though, since we can get nearly any direction we want now. My point is that the most DV you can gain from an encounter is 2Vorb, regardless of whether it's a moon or a black hole.

Best,
-Slashy

I haven't done the math but i wonder how much Oberth benefit you'd get from burning there as opposed to doing all the burning in LEO.

Edited July 3, 2018 by 5thHorseman
Removed Swypeos

[razark]

Y'all are missing the most important change here.

If the moon were to be replaced by a black hole, we'd have to put up with an endless¹ stream of moon hoaxers asking "How could Apollo land on the moon if there is no moon?!?"

 

¹Ok, even more endless than it is now.

[kerbiloid]

If bring an asteroid and put it in a near-blackmoon orbit, we could get a visible eccentric minimoon, doing circles in sky.
Ptolemy would be glad.

Also if it were an iceteroid, it would be a Moonmus.

[Bill Phil]

It seems like most orbits around the Moon would be more stable, since there would be no mascons...

[p1t1o]

You could build a space station in orbit around it, and orbit it at a distance much less than normal-lunar radius giving the station a very low orbital period making it seem like the Earth and sun are constantly zooming around the sky. I'd call it the "Cosmic Emporium of Motion Sickness" or "CEMS".

***

Dammit I should have called it the "Cosmic Emporium of Rotational Nausea"....CERN

Edited July 3, 2018 by p1t1o

[kerbiloid]

The Moon will not be made of cheese.

[NSEP]

It will be darker at night, and i have nothing to look at with my telescope, so i will probably sell it and might use that money to buy a drawing tablet, and one thing i will tell you guys is that i NEED a drawing tablet! My stuff will look so much better if i were able to draw using a tablet. And i HATE drawing on paper, because its so limited and you need alot of supplies, and they got lost constantly. Its also really hard to color stuff the way you want, but when you draw digitally, you can make very clea.... oh wait, am i getting a little of track here? This is a thread about... black holes? Oh.

Maybe scientists could probe/crash into the thing, for science, you know. Probably going to be hard considering the 'hole' would be the size of a grain of sand or something.

[RealKerbal3x]

15 minutes ago, NSEP said:

Maybe scientists could probe/crash into the thing, for science, you know. Probably going to be hard considering the 'hole' would be the size of a grain of sand or something.

Although it'd be very tiny, it would still have the same gravity as the Moon so if you just slow your orbital velocity around it enough you should hit it.

I think.

Edited July 3, 2018 by RealKerbal3x

[NSEP]

51 minutes ago, RealKerbal3x said:

Although it'd be very tiny, it would still have the same gravity as the Moon so if you just slow your orbital velocity around it enough you should hit it.

I think.

You need to make sure your orbital velocity is perfectly still, a tiny nudge off-course and you would need to go back for another round.

[magnemoe]

6 hours ago, razark said:

Y'all are missing the most important change here.

If the moon were to be replaced by a black hole, we'd have to put up with an endless¹ stream of moon hoaxers asking "How could Apollo land on the moon if there is no moon?!?"

¹Ok, even more endless than it is now.

No the real issue is how an small stable body with not even serious geology suddenly turn into an black hole at an moment with no energy release. 

Most probably explanation would be that its an true god and its probably an trickster god like Loki or Hermes. It could also be godlike aliens making fun of us. 

Expect more weird stuff, next week you wake up with an tail and your car only run backwards but has stopped use fuel.
In short the trickster make  fun of us to laugh at the response, not wanting to destroy us but not very concerned about the damage. 
A bit like kids tying an nut on the end of an cat tail, it got balance problems, the problem was removing it as the cat was not amused.   

Edited July 3, 2018 by magnemoe

[Lisias]

6 hours ago, razark said:

If the moon were to be replaced by a black hole, we'd have to put up with an endless¹ stream of moon hoaxers asking "How could Apollo land on the moon if there is no moon?!?"

On a side note, there's a beneficial side effect of such hysteria: political support for the Man going back to Moon.

We need a reprise for this show.

[Confused Scientist]

https://what-if.xkcd.com/129/

[Gargamel]

18 hours ago, kerbiloid said:

Unsure what about mentally challenged persons. Is their fullmoonophilia caused by light or by tide.
Probably by light, so they would be more calm. (Or redistribute their feats more uniformly).

HAHAHA... reading down your list of scientific stuff... and then this... wait what?  LOL. 

I will say, that I spent many years as a medic, in a truck and in an (very) Urban Trauma ER, the full moon effect is real.  Even accounting for observer's bias/paradox, We tended to see a lot of weird stuff on the full moons. 

[linuxgurugamer]

22 hours ago, cubinator said:

I really hope that isn't a screencap from an actual movie with that exact scenario as a plot point...but I know deep down that it is...  

Ever hear of "The Core"?

[GoSlash27]

10 hours ago, 5thHorseman said:

I haven't done the math but i wonder how much Oberth benefit you'd get from burning there as opposed to doing all the burning in LEO.

HMV,

 It's always cheaper to burn direct from LEO unless you can refuel in lunar orbit using reaction mass collected from the moon itself. Since the moon is a black hole, that's not an option. AFA the Oberth advantage, it's offset by the gravity well. Every destination has a gate orbit; a radius where the Oberth and gravity well balance at a DV minimum. The closer destinations have a gate orbit above the current moon's surface. The more distant ones would have a gate orbit below the moon's surface, and thus would save some DV. But again, you can't acquire fuel from a black hole.

 TL/DR: No advantage there.

Best,
-Slashy
 

[K^2]

30 minutes ago, GoSlash27 said:

It's always cheaper to burn direct from LEO unless you can refuel in lunar orbit using reaction mass collected from the moon itself.

That's not actually true. Obereth effect absolutely continues to work with a black hole. You can think of it as converting gravitational energy of your fuel into kinetic energy of your rocket. And with a black hole, that can be a lot of energy. If your goal is ejecting yourself from Solar System with as much velocity as possible, having Moon-mass black hole instead of the Moon would make it a lot easier. You still have to get yourself to trans-Lunar, but from there, the ride out of Sol is almost free. There is definitely a limit to how much you can exploit it, because you'd have a limited window for a burn, but it'd still be quite useful.

[Superfluous J]

42 minutes ago, GoSlash27 said:

HMV,

 It's always cheaper to burn direct from LEO unless you can refuel in lunar orbit using reaction mass collected from the moon itself. Since the moon is a black hole, that's not an option. AFA the Oberth advantage, it's offset by the gravity well. Every destination has a gate orbit; a radius where the Oberth and gravity well balance at a DV minimum. The closer destinations have a gate orbit above the current moon's surface. The more distant ones would have a gate orbit below the moon's surface, and thus would save some DV. But again, you can't acquire fuel from a black hole.

You seem to be expecting me to get into orbit around the Black hole moon* before escaping. I'm thinking launching yourself from Earth to Moon such that you use the least fuel possible to get a crazy-close encounter with the black hole, basically as close as you can get without being ripped to shreds, and then at that Pe burn to escape. Or doing a combination of Oberth-affected LEO burn and Oberth-affected LMBH burn to get a faster Earth-escape speed than if you had just burned in LEO.

Seems you'd be going so fast on your lunar flyby that Oberth would give you quite a boost, but I don't know how much that is

If it's literally mathematically impossible in all situations to get better results then fine. I just don't KNOW that the math says that it is. Or isn't.

*Black hole moon, won't you come, and wash away the rain?
Black hole moon, won't you come. Won't you come?

Edited July 4, 2018 by 5thHorseman

[GoSlash27]

23 minutes ago, 5thHorseman said:

I'm thinking launching yourself from Earth to Moon such that you use the least fuel possible to get a crazy-close encounter with the black hole, basically as close as you can get without being ripped to shreds, and then at that Pe burn to escape.

HMV, see below...

28 minutes ago, K^2 said:

If your goal is ejecting yourself from Solar System with as much velocity as possible, having Moon-mass black hole instead of the Moon would make it a lot easier.

K^2,
Actually, it doesn't work that way. Any velocity you pick up from the Mun's gravity well is lost escaping it. If you come in (say) at 1 km/sec with respect to the moon, you will leave with that same 1 km/sec with respect to the moon.... even if it *temporarily* accelerates you to relativistic speeds. Think of it as coasting down a hill and up the other side, except with no drag. It doesn't give you anything for free. You leave with the same speed you had when you came in, no matter how fast you're going at the bottom. Whatever you gain on the way in, you lose on the way out.

 The reason gravity assists work is because the body is in motion, not the gradient of it's gravity well. Turning the moon into a black hole doesn't affect it's orbital velocity, so it has no effect on that.

Best,
-Slashy

Edited July 4, 2018 by GoSlash27

[Superfluous J]

5 minutes ago, GoSlash27 said:

The reason gravity assists work is because the body is in motion, not the gradient of it's gravity well. Turning the moon into a black hole doesn't affect it's orbital velocity, so it has no effect on that.

Yes, but I'm not talking about a gravity assist. I'm talking about an Oberth maneuver at Moon's Pe to take advantage of your massive velocity. I'm basically offering an alternative to the gravity assist idea that you correctly point out won't help. At least, it won't help any more than it does with the normal Moon in place.

I can set it up. And if I type "Kerbin" and "Mun" somewhere below I apologize. I literally had to edit every post in this thread while typing them

Also, let's assume KSP's SOI system for simplicity. I don't think it will affect the actual answer though the details will be slightly different.

Proposal: You want your speed as you leave Earth's SOI to be V m/s. This will eject you to - say - Jupiter.

Known: To get that velocity in LEO, you need to do a burn of X m/s. This is a pretty simple calculation that you can do in KSP with a maneuver node and not do the calculation.

Known: To do that in this "black hole moon" system, you need to burn Y m/s in LEO, and Z m/s as you pass by Moon. There are various strategies of how to do your burns, and it should be obvious that these burns are possible to do. I don't know the best possible way to burn but suspect that it's likely "burn the least you can at Earth to reach the Moon" and I also suspect that there is some way to mathematically figure it out (though I'll admit that's more a faith in math than anything else).

Unknown: Is there any possible way, given that Moon is a point mass and you can get as close as you want, that Y+Z could ever be less than X?

 

[GoSlash27]

HMV,
"Yabut"

 That's what I'm trying to get across... Oberth isn't what most people think it is. It's not like "free parking", where you automatically get bonus DV for going faster. It's simply math behaving correctly, but in a counter-intuitive way. For every V_inf, there is an ideal R for the burn. Above it, and you waste kinetic energy. Below it, you waste potential energy. Going too low is *much* worse than going too high.
 The idea of "the lower and faster, the better" is a gross over- simplification, and leads to a false conclusion.

 Best,
-Slashy

[Superfluous J]

6 minutes ago, GoSlash27 said:

For every V_inf, there is an ideal R for the burn. Above it, and you waste kinetic energy. Below it, you waste potential energy. Going too low is *much* worse than going too high

Hmmm. Okay then it gets stickier but I still have a wonder.

To get a specific V_inf for Earth, there is some specific V_inf at Moon. Does there exist an R at Moon (under its surface but not under the black hole's radius) where you can burn Z at it after burning Y in LEO, such that you can get the same Earth V_inf as a burn in LEO of X, such that Y+Z < X?

Again, I don't know.

Edited July 4, 2018 by 5thHorseman

[K^2]

55 minutes ago, GoSlash27 said:

Actually, it doesn't work that way. Any velocity you pick up from the Mun's gravity well is lost escaping it. If you come in (say) at 1 km/sec with respect to the moon, you will leave with that same 1 km/sec with respect to the moon....  [...] The reason gravity assists work is because [...]

We're not talking about a free gravity assist here. We're talking about approaching "Moon" at 1km/s on trans-Lunar, picking up 100km/s in free-fall, kicking in engines to pick up 3km/s of extra delta-V at periapsis, loosing that 100km/s worth of KE climbing back out, and being left over with enough KE to be departing "Moon" at 25km/s, which is just about right for escaping Sol all together.

This is the whole point of Obereth effect. Your fuel use determines your velocity changes, while orbits are all about energy and angular momentum. A small change in velocity when you are already going fast equates to a huge change in kinetic energy. If you have a sufficiently deep gravitational hole to play with, all transfers that intercept it are effectively free.