Flight Question Arriving from Jupiter - sling around Earth to Moon

[kevlvm]

Trying something different. Im coming in from Jupiter(or any outer planet) and wondering if you can set up to come around the earth to help slow down and sling out to the moon to capture for landing at BB? I was able to follow a tutorial and am able to get close to earth, but am unsure how to set this up while flying. Any help would be appreciated.

 

[Urwumpe]

Trying something different. Im coming in from Jupiter(or any outer planet) and wondering if you can set up to come around the earth to help slow down and sling out to the moon to capture for landing at BB? I was able to follow a tutorial and am able to earth, but am unsure how to set this up while flying. Any help would be appreciated.

Yes, you can... BUT! you should remember that the moon also has to be at the right location. It requires careful timing... I better make you a drawing of the navigation and how you alternate for getting a bigger window.

You will likely be pretty fast when reaching the moon, because Earth isn't the best planet for a gravity assist.

Which planet you are coming from and where the moon is determines how much speed you can recover from a retrograde gravity assist by Earth. It won't be much. Maybe you have to include a tiny aerobraking sequence for better landing.

But: Thank you for raising Earths orbit by a few nanometers if you do that. :lol:

 

[kevlvm]

Thanks, I thought it would be pretty cool to try it, but realized the location of the moon would be critical and didnt know if any of the planning tools(IMFD,Transx) had that capability. Give a sweet view out the window of the ARROW

 

[Urwumpe]

Thanks, I thought it would be pretty cool to try it, but realized the location of the moon would be critical and didnt know if any of the planning tools(IMFD,Transx) had that capability. Give a sweet view out the window of the ARROW

Both have the capability, I am not sure though, how you plot it properly in TransX, I have always been a IMFD Fan. I am still doing some final tests here at work before returning to my lair, so I can make you a cheat sheet.

 

[dgatsoulis]

I don't see how a retrograde Earth approach will help you slow down relative to the Earth-Moon system.
Yes, you can either speed up or slow down relative to the Sun, but relative to Earth you will only speed up. The more you drop inside Earth's gravity well, the more you will speed up.

The Periapsis velocity will be sqrt(Venc² + Vesc²), where Venc is the heliocentric Earth encounter velocity and Vesc is the escape velocity of the geocentric periapsis altitude.

The only way that I know of, would be to drop from Jupiter, sling Earth once and then arrive back at Earth with a lower encounter velocity.

Am I missing something here?

 

[asbjos]

Am I missing something here?

I think so. As I understood it, he was going to sling by the Moon, which would remove some energy relative to the Earth.

 

[n122vu]

Maybe you have to include a tiny aerobraking sequence for better landing.

I agree, and would almost go as far as to say it's a requirement. As dgatsoulis pointed out, whether you come in pro- or retrograde, you're going to pick up some energy when you round Earth for a sling. If you don't counter that with some atmospheric drag, you're going to make one big smoking hole in the moon unless you do a hefty burn.

 

[dgatsoulis]

I think so. As I understood it, he was going to sling by the Moon, which would remove some energy relative to the Earth.

The OP said:

wondering if you can set up to come around the earth to help slow down and sling out to the moon to capture for landing at BB?

That sounds like an Earth sling to me, not a Moon sling.

Then Urwumpe said:

Which planet you are coming from and where the moon is determines how much speed you can recover from a retrograde gravity assist by Earth. It won't be much.

This is what I am asking. A retrograde Earth approach (whatever the braking method is, either a burn or an aerocapture), would have you arriving at the moon moving at the opposite direction. The Moon going prograde and you approaching retrograde. Thus making the Moon encounter velocity much higher than a prograde Earth approach.

For arriving at the Moon, there is no benefit from a retrograde Earth approach and capture.

The only way it could work, would be to sling Earth with a retrograde approach, go around the Sun once and then arrive back at Earth with a lower encounter velocity and with a prograde approach.

I agree, and would almost go as far as to say it's a requirement. As dgatsoulis pointed out, whether you come in pro- or retrograde, you're going to pick up some energy when you round Earth for a sling. If you don't counter that with some atmospheric drag, you're going to make one big smoking hole in the moon unless you do a hefty burn.

It depends on the spacecraft and whether an aerocapture is an option. The OP mentioned the Arrow, so I guess a burn at periapsis would be a more realistic approach.

I wouldn't say the burn is too hefty. Dropping from Jupiter to Earth you have a heliocentric encounter velocity ~9.1 km/s. If we assume a Periapsis altitude of 100 km, the Earth periapsis velocity is ~14.35 km/s.
The Periapsis velocity of an orbit that has a periapsis altitude of 100 km and apoapsis at the Moon is ~10.95 km/s. So you need to lose 14.35-10.95 = ~3.4 km/s . Either with a burn or dropping inside the atmosphere is a matter of choice/spacecraft.