LEO-Saturn

[penlu]

LEO to Saturn with only 5 km/s Delta-V. I'm wondering whether or not this is possible.

The Hohmann transfer from Earth to Mars requires 3 km/s Delta-V, not counting escape. It takes 258 days, a bit long. So it 1.) becomes desirable to speed things up a bit and 2.) makes it so that in the process we spend even less fuel.

Right, so we go and sling by the moon. Since the total velocity change can be as much as two times the speed of the orbiting planet (or the moon) we can gain up to 2 km/s of speed from the moon's stately waltz across the sky. I'd put the total velocity gain at 1.5 km/s, so a Saturn V could easily lob a spacecraft on this trajectory.

From here, we continue to Mars. Sling around, and gain about 24 km/s from Mars's speed in its orbit at least (assuming that we get the full speed of Mars in). That is enough to put us on a Hohmann to Saturn, or Jupiter if you want to go a bit faster.

Then aerobrake an be done with it. Is it possible?

 

[Woo482]

that sounds fun ! but wont you get a bit hot if you aerobrake ?

 

[Andy44]

How much delta-V did it take for Cassini to do it's transfer? It started off going coreward, towards the inner planets, slung past Venus, did two Earth slings, which pushed it to Jupiter, and from there onward to Saturn. There was also a deep space propulsive burn in there somewhere to set up one of the encounters.

I have never heard anyone using the Moon to sling; I think flytandem tried it in Orbiter on his way to Mars and reported it didn't save him much. I may have to give this a try. Now that I'm thinking about it, you may be able to set up a series of repeating slings past the Moon to get extra velocity before leaving Earth's SOI.

 

[RisingFury]

Yea, I've been thinking if this would work...

If it does work, how come it's not being used by real launches? It's got to have some sort of a drawback... or maybe it just doesn't gain you enough to be worth the extra course corrections...

 

[agentgonzo]

... or maybe it just doesn't gain you enough to be worth the extra course corrections...

I think this is the reason. Slingshots around the Moon and Mars don't get you a massive amount of delta-V.

 

[Urwumpe]

From here, we continue to Mars. Sling around, and gain about 24 km/s from Mars's speed in its orbit at least

No. You won't get so much. Make it about 240 m/s, that is more realistic.

Mars has a very low gravity and you need to do a very close swing-by to get at least a bit of velocity from it (The more you change the direction of the velocity vector in the local frame of reference, the more speed you gain by a swing-by).

A very effective swing-by around Jupiter (162.4° change in direction), gives you "only" about 9.2 km/s speed extra.

EDIT: I just used the scenario for a small GMAT test:

As you can see, a mars gravity is not strong enough for causing a really pretty swing-around even with minimal excess velocity - the more excess velocity you have, the closer you need to get to mars for the same angle. And closer means lithobraking in that case.

Angle was 144.39° , that gives a maximum velocity change of about 1 km/s

 

[cjp]

Some time ago I tried to launch something to the outer planets using an Ariane 3. There was hardly enough delta-v to reach Mars, but I thought I could gain some speed by doing multiple inner-planet slingshots. Somehow I managed to do 6 or 7 slingshots (always planning something like 2 slingshots ahead), but I finally ran out of slingshot opportunities, and at an earlier stage I was already forced to do a slingshot that would decrease my energy, but that was the only way to continue to another planet. I didn't reach Jupiter or Saturn.

My experience is that Mars and the moon aren't really useful, unless they just happen to be at the right place and you only need a little bit of extra speed. I'd say you can try to do it Cassini-style, using Venus and Earth.

Slingshotting repeatedly with the same body is also interesting. For this you'd need to have an orbital period ratio that is a rational number, e.g. 1:1, 1:2, 2:1, ... It should be possible to make a program to show all harmonic orbits you can reach from an approximate slingshot situation.

 

[agentgonzo]

...but I finally ran out of slingshot opportunities...

Slingshotting repeatedly with the same body is also interesting. For this you'd need to have an orbital period ratio that is a rational number, e.g. 1:1, 1:2, 2:1, ...

What you can do is perform a MCC when you are at ApA to target the planet again for a further sling. On eccentric orbits this normally doesn't require a lot of Delta-V and can increase your slinging opportunities quite a lot as you won't need to wait 15 orbits if you get into a 3:5 resonance.

 

[Belisarius]

How much delta-V did it take for Cassini to do it's transfer? It started off going coreward, towards the inner planets, slung past Venus, did two Earth slings, which pushed it to Jupiter, and from there onward to Saturn. There was also a deep space propulsive burn in there somewhere to set up one of the encounters.

Here's the flightpath of Cassini, I've been trying to replicate it but I haven't got my slingshot technique quite right yet.

I think, like the others, that slingshotting off the Moon is hardly worth the trouble. It would be more efficient to swing back towards Earth and slingshot there, but that would have to be after a Venus swingby... or maybe not?

Maybe a free-return type trajectory could be planned to set up an Earth slingshot shortly after Earth launch?