Slingshot to slow down.

[rocketman768]

Has anyone here used a slingshot around the Moon to slow down when returning from Mars or any of the outer planets? How did you do it? Thanks!

 

[lennartsmit]

I thought slingshots were there to speed you up?

 

[rocketman768]

I thought slingshots were there to speed you up?

In his paper “Тем кто будет читать, чтобы строить” [To whomsoever will read [this paper] in order to build [an interplanetary rocket]],[1] which he dated “1918-1919,”[2] Yuri Kondratyuk suggested that a spacecraft traveling between two planets could be accelerated at the beginning of its trajectory and decelerated at the end of its trajectory by using the gravity of the two planets' moons.

-- Wikipedia, 13 Nov 2008

 

[agentgonzo]

You can use gravity assists (slingshots) to either gain or loose velocity. To use a slingshot to lose speed when coming in from Mars, you need to approach the Moon from 'in front' of it in its orbital path, so the Moon's gravity slows you down.

 

[rocketman768]

You can use gravity assists (slingshots) to either gain or loose velocity. To use a slingshot to lose speed when coming in from Mars, you need to approach the Moon from 'in front' of it in its orbital path, so the Moon's gravity slows you down.

Yeah, so how do you time your departure from Mars correctly so that the Moon will be in the right location for you to do the retro slingshot?

 

[agentgonzo]

Yeah, so how do you time your departure from Mars correctly so that the Moon will be in the right location for you to do the retro slingshot?

Planning with TransX. Even en-route, minor course corrections can adjust your time of arrival by days at a time to ensure that the moon is in the correct position when you encounter it.

 

[tblaxland]

You can use gravity assists (slingshots) to either gain or loose velocity.

MESSENGER is a good example of using slingshots to slow down.

There have been discussions before about using the moon to slingshot to Mars. IIRC the general consensus was that the gravity of the Moon was so low as to make it not worthwhile. You may even expend more delta-V manoeuvring to set up the slingshot than you gained from it. I strongly doubt that the case for an inbound slingshot would be any different, but don't let me be the wet blanket, by all means try it and report back. :speakcool:

EDIT: Here is the thread I was thinking of. It seems even Mars does not have sufficient gravity for a decent slingshot:
http://www.orbiter-forum.com/showthread.php?t=4510&highlight=moon+mars+slingshot

 

[C3PO]

There have been discussions before about using the moon to slingshot to Mars. IIRC the general consensus was that the gravity of the Moon was so low as to make it not worthwhile.

IMHO the smart thing would be to start from the moon, and use an Earth sling shot to get to Mars.

 

[tblaxland]

IMHO the smart thing would be to start from the moon, and use an Earth sling shot to get to Mars.

Sure, is OK if you happen to live on the Moon

 

[Urwumpe]

IMHO the smart thing would be to start from the moon, and use an Earth sling shot to get to Mars.

Should not bring you any gain from directly escaping from first moon and then Earth with a single maneuver.

Using resonance with the moon to get away from Earth would be more effective.

 

[agentgonzo]

IMHO the smart thing would be to start from the moon, and use an Earth sling shot to get to Mars.

You won't be able to get any additional velocity from slinging around the Earth starting at the moon as the Earth is the centre of the system you're orbiting in. In order to gain/lose velocity, you need to sling around an co-orbiting body (if coming from Venus, you are essentially orbiting the Sun and so can sling around the Earth in that case as the Earth is orbiting the Sun, but when coming back from the Moon to the Earth, the Earth is the centre of the system and so essentially at rest, so you can't sling around it).

What you can do though is to drop from the Moon to the Earth and then perform a large ejection burn when at a low perigee about the Earth and thus at high velocity so saving fuel (burning at higher velocities is more fuel efficient). FlyTandem wrote a nice tutorial about correctly targeting the orbit to send you from Moon-Earth-Mars with a big burn at low PeA around the Earth - http://flytandem.com/orbiter/tutorials/Surrogate/index.htm

 

[C3PO]

What you can do though is to drop from the Moon to the Earth and then perform a large ejection burn when at a low perigee about the Earth and thus at high velocity so saving fuel (burning at higher velocities is more fuel efficient). FlyTandem wrote a nice tutorial about correctly targeting the orbit to send you from Moon-Earth-Mars with a big burn at low PeA around the Earth - http://flytandem.com/orbiter/tutorials/Surrogate/index.htm

That was indeed the way I meant. You just did a way better job explaining it.:lol:
Tnx!

 

[HiPotOk1978]

After reading this thread, I did some experiments, took me a few tries to get it just right but what I did was enter Earth space at an angle that took me right in front of the moon... The moons gravity tugged on me and I lost a tiny bit of speed as I passed though the SOI. Not sure if that is the proper way but it appears that I spent more fuel trying to get near the moon that if I would have burned retrograde that would have been more fuel efficient, but chances are I just didn't do it right and forced it to happen instead of doing my timing better

 

[MajorTom]

By all means, DO be careful. When you gain momentum slingshotting 'round a planet, the planet loses some momentum.

Slingshot with care, lest all our planets spiral into the sun

 

[HiPotOk1978]

hmm, maybe we can bring Mars closer then so over time It will get warmer

 

[flytandem]

Attn all those with some Buck Rogers in them....

A tricky maneuver is to to an aerosling which would be a slingshot but with assistance from lift in the atmosphere to enhance gravity. For example, coming back from the outer planets you might find that Earth isn't lined up really well but Mars is. And a sling around Mars to hit Earth would need to be far below the surface, ie; Pe/Planet Radius is between zero and 1 in value.

Setting up the sling, you need to make "inherit velocity" be "No" and look at your encounter velocity and estmate about how much speed you will lose in the atmosphere and enter the reduced speed as your exit, or sling velocity. Then adjust the sling angles with that value. For example, the encounter velocity might be 6,000 m/s and you might then estimate a loss of 1,000 m/s in the atmosphere so you enter 5,000 as your sling velocity.

Then when you arrive, you need to slice into the atmosphere at a shallow angle and roll inverted as you level off at a nice comfortable maneuvering altitude. 30 Km is typical at Mars. 90 Km at Venus as I recall. As you decelerate in the atmosphere you will be looking at the speed difference between what you estimated you will leave the atmosphere at (shown as Delta V) and also watch the Pe of the sling rise up to match your height, shown as "Pe Ratio". When it reaches your height (Pe Ratio = 0) simply climb out of the atmosphere.

I have a replay of an aerosling at:
http://www.flytandem.com/orbiter/playbacks/aeros1.zip
Unzip the files into your Orbiter folder and it will place the files correctly into the particular data and scenario folders. The readme file explains it.

But... I made this with TransX 3.10 . So if you want to play the playback you need to place a copy of 3.10 in your modules/plugin folder and activate it before startup. 3.10 is available here...
http://www.flytandem.com/orbiter/challenges/transx.dll

Hey agentgonzo.... I tried to manually modify the playback scenario coding for TransX to make it accepted by 3.13 but no success. It just opens with no plan or sling which makes the playback useless. Any ideas? It's not really that important, so no worries if it's not possible.

 

[mjessick]

Originally Posted by agentgonzo
What you can do though is to drop from the Moon to the Earth and then perform a large ejection burn when at a low perigee about the Earth and thus at high velocity so saving fuel (burning at higher velocities is more fuel efficient). FlyTandem wrote a nice tutorial about correctly targeting the orbit to send you from Moon-Earth-Mars with a big burn at low PeA around the Earth -


This is essentially the trip described by Heinlein in "The Rolling Stones," 1952. He adds an interesting plot complication to the planning for the burn around Earth. The book has some other trajectory stuff involving a space search and rescue for a disabled craft later, after having gone on to visit the asteroid belt.