Transfer Orbits

[SpaceEnthusiast23]

I don't understand the difference between a hohmann transfer orbit and a bi-elliptic transfer orbit. Can anyone explain please?

[munlander1]

This could be wrong, but my understanding is that a bi-elliptical orbit uses 2 burns. A hohmann transfer only has one if you don't circularize. 

Edited May 20, 2017 by munlander1

[Joseph Kerman]

4 minutes ago, munlander1 said:

This could be wrong, but my understanding is that a bi-elliptical orbit uses 2 burns. A hohmann transfer only has one if you don't circularize. 

A bi-elliptic transfer maneuver has 3 burns. One for the initial orbital burn to a high apoapsis, one for the transfer, and one retroburn to circularize.

A Hohmann transfer is very simple. One burn to raise the orbit to the target one, and another to circularize.

 

I don't get how you got that wrong considering you've done that as a lander.

[SpaceEnthusiast23]

11 minutes ago, munlander1 said:

This could be wrong, but my understanding is that a bi-elliptical orbit uses 2 burns. A hohmann transfer only has one if you don't circularize. 

I also think that, but on Wikipedia, it says that a bi-elliptic orbit can be more efficient sometimes, bit I don't understand why 3 burns would be more efficient than 1.

4 minutes ago, Joseph Kerman said:

I don't get how you got that wrong considering you've done that as a lander

Did you mean me? I've only gotten to LKO so far. I'm a rookie.

Edited May 20, 2017 by SpaceEnthusiast23

[munlander1]

9 minutes ago, Joseph Kerman said:

A bi-elliptic transfer maneuver has 3 burns. One for the initial orbital burn to a high apoapsis, one for the transfer, and one retroburn to circularize.

A Hohmann transfer is very simple. One burn to raise the orbit to the target one, and another to circularize.

 

I don't get how you got that wrong considering you've done that as a lander.

Well, I did put down with out the burns to circularize your orbit. 

[Joseph Kerman]

12 minutes ago, SpaceEnthusiast23 said:

Did you mean me? I've only gotten to LKO so far. I'm a rookie.

*facepalm*

[SpaceEnthusiast23]

34 minutes ago, Joseph Kerman said:

*facepalm*

Oh ok you mean him.

[0111narwhalz]

A bielliptic can be more efficient than a Hohmann because it makes better use of the Oberth effect. It raises its apoapsis when it's going very fast, then raises its periapsis from a long way away, and finally drops its apoapsis while going fast. It also opens the door for plane changes at very low speeds (at the apoapsis of the transfer).

It should be noted that bielliptic transfers are more efficient only when the difference between the origin and destination orbits is very large, though the plane change might make it viable despite lesser differences. Note also that bielliptic transfers let you leave whenever you like, since changes in the period of the transfer orbit require only trivial amounts of delta-v.

[Pand5461]

@SpaceEnthusiast23, there is no such thing as bi-elliptic transfer orbit, at least not in Kerlerian sense.

Hohman transfer is a procedure to get from one circular orbit to another one. To do that, you burn at some point of the initial orbit to get to an elliptical orbit that has the opposite apsis at the target orbit height. This elliptical orbit is the Hohmann transfer orbit. But transfer is not completed yet. To complete the transfer, you need to circularize at the target altitude bu the second burn.

Bielliptic transfer is also a procedure to get from one circular orbit to another one. But first burn sends the ship to a very eccentric elliptical orbit (let's say, with Ap at the border of SOI). From there, you change the periapsis height to the target altitude and then circularize at periapsis. Therefore, bielliptic transfer involves two transfer orbit, as one may guess from its name. The idea behind it is that with a really high apoapsis, big changes of periapsis may be achieved by very small burns.

This all really comes into play when you need to change the inclination. Transfer from an equatorial to a polar orbit requires V_orbital×√2 deltaV, while at the apoapsis of a higly elliptical trajectory inclination change reduces to almost nothing. Therefore, it clearly becomes more efficient to get to high elliptical orbit, change inclination at apoapsis and circularize back at periapsis than to change inclination at constant altitude (please note that bielliptical inclination change is better for big changes, small ones <20° are more efficient at constant altitude).

For better explanation, read the Wiki page, it has examples with concrete figures.

Edited May 20, 2017 by Pand5461

[SpaceEnthusiast23]

10 minutes ago, Pand5461 said:

@SpaceEnthusiast23, there is no such thing as bi-elliptic transfer orbit, at least not in Kerlerian sense.

Hohman transfer is a procedure to get from one circular orbit to another one. To do that, you burn at some point of the initial orbit to get to an elliptical orbit that has the opposite apsis at the target orbit height. This elliptical orbit is the Hohmann transfer orbit. But transfer is not completed yet. To complete the transfer, you need to circularize at the target altitude bu the second burn.

Bielliptic transfer is also a procedure to get from one circular orbit to another one. But first burn sends the ship to a very eccentric elliptical orbit (let's say, with Ap at the border of SOI). From there, you change the periapsis height to the target altitude and then circularize at periapsis. Therefore, bielliptic transfer involves two transfer orbit, as one may guess from its name. The idea behind it is that with a really high apoapsis, big changes of periapsis may be achieved by very small burns.

This all really comes into play when you need to change the inclination. Transfer from an equatorial to a polar orbit requires V_orbital×√2 deltaV, while at the apoapsis of a higly elliptical trajectory inclination change reduces to almost nothing. Therefore, it clearly becomes more efficient to get to high elliptical orbit, change inclination at apoapsis and circularize back at periapsis than to change inclination at constant altitude (please note that bielliptical inclination change is better for big changes, small ones <20° are more efficient at constant altitude).

For better explanation, read the Wiki page, it has examples with concrete figures.

 

14 minutes ago, 0111narwhalz said:

A bielliptic can be more efficient than a Hohmann because it makes better use of the Oberth effect. It raises its apoapsis when it's going very fast, then raises its periapsis from a long way away, and finally drops its apoapsis while going fast. It also opens the door for plane changes at very low speeds (at the apoapsis of the transfer).

It should be noted that bielliptic transfers are more efficient only when the difference between the origin and destination orbits is very large, though the plane change might make it viable despite lesser differences. Note also that bielliptic transfers let you leave whenever you like, since changes in the period of the transfer orbit require only trivial amounts of delta-v.

Thanks guys.