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... Continued from Part 4.
The Sands of Duna
Part 5: On your mark, get set, ...
Now that we're in a low, roughly circular, Kerbin orbit, we need to find the position within that orbit that we will choose for are upcoming periapsis. We need to find the place in that orbit that we will choose for all our prograde thrust maneuvers.
Recall the discussion a few posts ago around Figure 13. It's going to be right around there.
So let's set up a maneuver node there, and drag out the prograde symbol until it extends all the way out to Duna's orbit. We won't be following through on this maneuver, but we use it to choose the future, central location of our maneuvers. Figure 30 shows the map view.
[Figure 30: Maneuver node to help pinpoint location of new periapsis around Kerbin's orbit.]
Now zoom in and make sure that the projected path is roughly symmetric around Kerbin's solar orbit. The goal is to adjust the maneuver node around the low Kerbin orbit such that the projected orbit always stays outside, on other side, of Kerbin's orbit around the sun.
[Figure 31: Make sure projected orbit (purple, dotted line) is symmetric around Kerbin's solar orbit. The projected orbit should never cross Kerbin's orbit, on either side.]
Keeping the projected orbit as such means that maneuver's location around Kerbin's low orbit is a fine location to ensure a good Hohmann transfer orbit, as that location becomes the new periapsis.
It's okay now to reduce the prograde adjustment of the maneuver node, now that we know the maneuver node's location. We won't be able to perform the entire Duna injection/Kerbin escape burn in a single burn anyway.
[Figure 32: Location of manuever node with respect to Kerbin's day/night sides (1/2)]
[Figure 33: Location of manuever node with respect to Kerbin's day/night sides (2/2)]
Figures 32 and 33 show the maneuver node's location with respect to Kerbin's day and night transition. For a Hohmann transfer between planets, the maneuver node's location will always be roughly around there if the target planet has a higher, solar orbit than the starting planet. You might need to adjust it slightly using methods outline above, depending on the starting and target planets, initial orbital elevation, etc., but it will always like somewhere around that general location.
Adjust the prograde symbol of the maneuver node such that the burn time acceptably short. Starting at a low Kerbin orbit, I'd suggest that the burns be kept down to less than a minute. Thirty second burns will be even more efficient.
If you haven't figured it out yet, we are going to gradually increase the apoapsis over the course of several orbits. Each orbit we will burn at periapsis only.
This is the same, general strategy used by the Mars Orbiter Mission (MOM) of the Indian Space Research Organization (ISRO).
[Orbit trajectory diagram (not to scale) of MOM.]
For more information on the MOM, see http://www.isro.org/pslv-c25/mission.aspx
The idea here is that we don't have enough thrust (particularly when we use our main engine cluster) to perform the entire Duna injection/Kerbin escape burn in a single burn. But we can break it up over several obits, keeping are periapsis low, and thus fully exploiting the Oberth effect for each burn.
[Figure 34: ... Go! And we're off.]
To be continued...
The Sands of Duna
Part 5: On your mark, get set, ...
Now that we're in a low, roughly circular, Kerbin orbit, we need to find the position within that orbit that we will choose for are upcoming periapsis. We need to find the place in that orbit that we will choose for all our prograde thrust maneuvers.
Recall the discussion a few posts ago around Figure 13. It's going to be right around there.
So let's set up a maneuver node there, and drag out the prograde symbol until it extends all the way out to Duna's orbit. We won't be following through on this maneuver, but we use it to choose the future, central location of our maneuvers. Figure 30 shows the map view.
[Figure 30: Maneuver node to help pinpoint location of new periapsis around Kerbin's orbit.]
Now zoom in and make sure that the projected path is roughly symmetric around Kerbin's solar orbit. The goal is to adjust the maneuver node around the low Kerbin orbit such that the projected orbit always stays outside, on other side, of Kerbin's orbit around the sun.
[Figure 31: Make sure projected orbit (purple, dotted line) is symmetric around Kerbin's solar orbit. The projected orbit should never cross Kerbin's orbit, on either side.]
Keeping the projected orbit as such means that maneuver's location around Kerbin's low orbit is a fine location to ensure a good Hohmann transfer orbit, as that location becomes the new periapsis.
It's okay now to reduce the prograde adjustment of the maneuver node, now that we know the maneuver node's location. We won't be able to perform the entire Duna injection/Kerbin escape burn in a single burn anyway.
[Figure 32: Location of manuever node with respect to Kerbin's day/night sides (1/2)]
[Figure 33: Location of manuever node with respect to Kerbin's day/night sides (2/2)]
Figures 32 and 33 show the maneuver node's location with respect to Kerbin's day and night transition. For a Hohmann transfer between planets, the maneuver node's location will always be roughly around there if the target planet has a higher, solar orbit than the starting planet. You might need to adjust it slightly using methods outline above, depending on the starting and target planets, initial orbital elevation, etc., but it will always like somewhere around that general location.
Adjust the prograde symbol of the maneuver node such that the burn time acceptably short. Starting at a low Kerbin orbit, I'd suggest that the burns be kept down to less than a minute. Thirty second burns will be even more efficient.
If you haven't figured it out yet, we are going to gradually increase the apoapsis over the course of several orbits. Each orbit we will burn at periapsis only.
This is the same, general strategy used by the Mars Orbiter Mission (MOM) of the Indian Space Research Organization (ISRO).
[Orbit trajectory diagram (not to scale) of MOM.]
For more information on the MOM, see http://www.isro.org/pslv-c25/mission.aspx
The idea here is that we don't have enough thrust (particularly when we use our main engine cluster) to perform the entire Duna injection/Kerbin escape burn in a single burn. But we can break it up over several obits, keeping are periapsis low, and thus fully exploiting the Oberth effect for each burn.
[Figure 34: ... Go! And we're off.]
To be continued...