I don’t understand how a slingshot maneuver off of a central black hole would work. My understanding was that a slingshot never slows you down in the frame of the object you are slingshotting around, it only changes your direction. Since the central black hole is presumably stationary with respect to the rest of the galaxy, this wouldn’t help you in slowing down. Slingshotting around an intermediate mass black hole (if such things exist) out in the galactic disc seems like it would be more useful.
Or maybe there is something about general relativity that changes things?
I suspect “slingshot” here refers to Oberth effect. Passing close to event horizon of a galactic black hole would greatly increase the delta-V efficiency of the remaining reaction mass.
In principle this would apply to the main reaction mass too, but perhaps it can’t all be effectively used within the short timescale of a close pass with the black hole.
If I understand correctly, the Penrose process as such (i.e., actually extracting energy from the black hole’s rotation) only works if your exhaust is expelled fast enough, relative to you, that is is put on a negative energy orbit, which necessarily falls into the black hole. I’m not sure how you could perform a retrograde burn in which your exhaust somehow enters the black hole but you don’t, since in a retrograde burn your exhaust is getting extra orbital velocity.
I am still really curious whether it helps to execute the retrograde Oberth maneuver within the ergosphere of a Kerr black hole, and if so whether it is better or worse, or even possible, if you approach on an initially retrograde orbit. Of course the approach orbit is probably steeply inclined because you don’t want to spend any longer than necessary flying at 0.8c through the galactic disc.
slingshot never slows you down in the frame of the object you are slingshotting around
That’s true for one object. But if there are at least two, moving around fast enough, you could perform some gravitational dance with them to slow down.
In the typical case, there are (at least) two meaningful bodies other than the spacecraft doing the maneuver; in real-world use cases so far, typically the sun and a planet. An (unpowered) slingshot maneuver doesn’t change the speed of the spacecraft from the frame of the planet, which is the object that the spacecraft approaches more closely, but it does change the speed in the center-of-mass frame, and it works by transferring orbital energy between the planet-sun system and the spacecraft. But the key is that in order to change your speed as much as possible relative to the center of mass, the object which you approach closer (i.e., “slingshot around”) should be the object which is smaller, and thus has higher speed relative to the center of mass. Of course it still needs to be much larger than your spacecraft. In no case would that object be the central black hole of a galaxy, unless your goal is to reduce your speed relative to an even bigger nearby galaxy, or perhaps just to change direction.
Are you talking about some other type of situation? My orbital intuition is that if you are going to trade orbital energy with a system, you have to get close to it relative to the separation of the bodies in the system, so it will generally make sense to talk about slingshotting around one of the bodies in particular. This is especially true when you are approaching with much higher than escape velocity, so that an extended dance with more than one close approach is not possible unless the first approach already did almost all the work.
Ok, now I understand the type of maneuver you are talking about. That definitely does make sense. I wonder if our hypothetical probe has knowledge early enough about the orbital trajectories of the stars close to the black hole, such that it can adjust its approach to pull off something like that without too much fuel cost. Of course it’s a long trip and there is plenty of time to plan, but it seems that any forward-pointing telescope would tend to be at significant risk while traveling at 0.8c into a galaxy, let alone 0.99c before the primary burn. However, “not likely to survive if deployed for the whole trip” is not the same as “can be deployed for long enough to make the necessary observations.” One advantage to a “simple” powered flyby of the black hole is that at least you know well ahead of time where it’s going to be, and have a reasonably good estimate of its mass.
Alternatively, could it get that information prior to launch, and if so are the trajectories of those stars stable enough that they would be where they need to be after millions of years of travel? My guess is no.
Yeah, those star trajectories definitely wouldn’t be stable enough.
I guess even with that simpler maneuver (powered flyby near a black hole), you still need to monitor all the stuff orbiting there and plan ahead, otherwise there’s a fair chance you’ll crash into something.
I don’t understand how a slingshot maneuver off of a central black hole would work. My understanding was that a slingshot never slows you down in the frame of the object you are slingshotting around, it only changes your direction. Since the central black hole is presumably stationary with respect to the rest of the galaxy, this wouldn’t help you in slowing down. Slingshotting around an intermediate mass black hole (if such things exist) out in the galactic disc seems like it would be more useful.
Or maybe there is something about general relativity that changes things?
I suspect “slingshot” here refers to Oberth effect. Passing close to event horizon of a galactic black hole would greatly increase the delta-V efficiency of the remaining reaction mass.
In principle this would apply to the main reaction mass too, but perhaps it can’t all be effectively used within the short timescale of a close pass with the black hole.
I suspect another issue is that it’s too dangerous to fly at 0.99c as you are entering a galaxy. There’s too much gas and dust.
Yepp, that’s what I was thinking. And I just edited it to clarify, since “slingshot” seems like the wrong word.
Ahh, that makes more sense.
Yeah, perhaps one could use the Penrose process in reverse to slow down instead?
If I understand correctly, the Penrose process as such (i.e., actually extracting energy from the black hole’s rotation) only works if your exhaust is expelled fast enough, relative to you, that is is put on a negative energy orbit, which necessarily falls into the black hole. I’m not sure how you could perform a retrograde burn in which your exhaust somehow enters the black hole but you don’t, since in a retrograde burn your exhaust is getting extra orbital velocity.
I am still really curious whether it helps to execute the retrograde Oberth maneuver within the ergosphere of a Kerr black hole, and if so whether it is better or worse, or even possible, if you approach on an initially retrograde orbit. Of course the approach orbit is probably steeply inclined because you don’t want to spend any longer than necessary flying at 0.8c through the galactic disc.
That’s true for one object. But if there are at least two, moving around fast enough, you could perform some gravitational dance with them to slow down.
In the typical case, there are (at least) two meaningful bodies other than the spacecraft doing the maneuver; in real-world use cases so far, typically the sun and a planet. An (unpowered) slingshot maneuver doesn’t change the speed of the spacecraft from the frame of the planet, which is the object that the spacecraft approaches more closely, but it does change the speed in the center-of-mass frame, and it works by transferring orbital energy between the planet-sun system and the spacecraft. But the key is that in order to change your speed as much as possible relative to the center of mass, the object which you approach closer (i.e., “slingshot around”) should be the object which is smaller, and thus has higher speed relative to the center of mass. Of course it still needs to be much larger than your spacecraft. In no case would that object be the central black hole of a galaxy, unless your goal is to reduce your speed relative to an even bigger nearby galaxy, or perhaps just to change direction.
Are you talking about some other type of situation? My orbital intuition is that if you are going to trade orbital energy with a system, you have to get close to it relative to the separation of the bodies in the system, so it will generally make sense to talk about slingshotting around one of the bodies in particular. This is especially true when you are approaching with much higher than escape velocity, so that an extended dance with more than one close approach is not possible unless the first approach already did almost all the work.
You’re right, that you wouldn’t want to approach the black hole itself but rather one of the orbiting stars.
But even with high velocity, if there are a lot of orbiting stars, you may tune your trajectory to have multiple close encounters.
Ok, now I understand the type of maneuver you are talking about. That definitely does make sense. I wonder if our hypothetical probe has knowledge early enough about the orbital trajectories of the stars close to the black hole, such that it can adjust its approach to pull off something like that without too much fuel cost. Of course it’s a long trip and there is plenty of time to plan, but it seems that any forward-pointing telescope would tend to be at significant risk while traveling at 0.8c into a galaxy, let alone 0.99c before the primary burn. However, “not likely to survive if deployed for the whole trip” is not the same as “can be deployed for long enough to make the necessary observations.” One advantage to a “simple” powered flyby of the black hole is that at least you know well ahead of time where it’s going to be, and have a reasonably good estimate of its mass.
Alternatively, could it get that information prior to launch, and if so are the trajectories of those stars stable enough that they would be where they need to be after millions of years of travel? My guess is no.
Yeah, those star trajectories definitely wouldn’t be stable enough.
I guess even with that simpler maneuver (powered flyby near a black hole), you still need to monitor all the stuff orbiting there and plan ahead, otherwise there’s a fair chance you’ll crash into something.