So I recently learned about the bi-elliptic maneuver, which is an alternative to the hohmann transfer that involves raising your apoapsis to a much higher orbit, raising your periapsis to the target periapsis at your apoapsis, and lowering your apoapsis to the target apoapsis. It's apparently more efficient than hohmann for target orbits much higher than yours. However, I tested the method by going from a 30km earth orbit to 10km moon orbit, but I didn't see said dv savings. How would I properly use it? Is the reason the maneuver was less efficient because of gravity losses? If so, can I use the bi-elliptic orbit to get more speed for lunar gravity assists?
How would bi-elliptic orbits be used properly in SFS?
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I'll check tonight, but from what I remember the bi-elliptic transfer is more efficient than the Hohmann transfer but not by much, the savings are very low in the case of the Moon. In this case it's generally not worth it as it implies an additional maneuver, a much longer transfer time, and the navigation doesn't allow you to plan your maneuver so that you get an optimal encounter with the Moon. Any correction will quickly negate the little savings you were expecting.
However it's in the case of the jovian system that it's very interesting. I made an example with a mission in which I try to reach Io orbit.
The first strategy is the Hohmann transfer for reference. Once I reach Jupiter, I burn at the periapsis to lower the apoapsis at Io level, then I insert the ship into ionian orbit:
With the bi-elliptic transfer, I first aim for a very elliptical orbit, then once at the apoapsis I raise the periapsis at Io level:
From there I can handle the insertion:
If you compare the fuel gauges, you'll notice that I saved a lot of fuel with the second method.
However it's in the case of the jovian system that it's very interesting. I made an example with a mission in which I try to reach Io orbit.
The first strategy is the Hohmann transfer for reference. Once I reach Jupiter, I burn at the periapsis to lower the apoapsis at Io level, then I insert the ship into ionian orbit:
With the bi-elliptic transfer, I first aim for a very elliptical orbit, then once at the apoapsis I raise the periapsis at Io level:
From there I can handle the insertion:
If you compare the fuel gauges, you'll notice that I saved a lot of fuel with the second method.
That's actually quite interesting, since Jupiter has a much higher orbital velocity when compared to earth. It makes sense raising your apoapsis would make it easier to slow down, since you have less speed total. This definitely seems useful for getting to a retrograde solar orbit or to jupiter (I've actually made a retrograde solar orbit using this method, though I didn't realize that I used the bi-elliptic at the time)
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There is a more fuel-efficient method of gaining retrograde orbit around the sun. You simply go to Jupiter and take retrograde gravity assist. Maybe with a little help near Jupiter's periapsis, depends on details and game difficulty level. And that already sends you directly to retrograde orbit around the sun, right from Jupiter, without spending much more fuel to gain high aphelion (apoapsis around the sun) first, then for slowing down to change orbit direction at that aphelion, and then even third time - for slowing down at perihelion (periapsis around the Sun) to reduce your extra unnecessary speed.
This method works with any other retrograde/prograde orbits around any other body, not only around the sun. Here is an example: link.
This method works with any other retrograde/prograde orbits around any other body, not only around the sun. Here is an example: link.