Proof of concept Duna Cycler trajectory

[PakledHostage]

In preparation for when I send Kerbals to Duna, I decided to plan and execute an unmanned free-return Duna flyby trajectory. The principle of this type of trajectory is similar to a Mars cycler trajectory, even though it isn't a true cycler trajectory.

I intend to use this type of trajectory for my manned mission to Duna because it will give my Kerbals the best chance of getting home if something goes wrong en route to Duna, just as Apollo 13 took advantage of a similar contingency to get back to Earth. The only catch is that it will take them almost 180 days to get home if they do have to use this option. Another catch is that it is less fuel efficient than a simple Hohman transfer. I calculated that I needed 260 m/s more delta-V to get to Duna using this method than I would have if I’d used a simple Hohman transfer. And it will take even more fuel to align them for Duna capture when they arrive. That being said, the outbound trip to Duna took just shy of 33 days. This shorter trip has obvious benefits.

And while some may question the practicality of this type of trajectory due to the high fuel burn, it was an interesting challenge to plan and execute. It requires a very high degree of precision that would not be possible without accurate navigation. Even so, I did not use any mods to help me navigate the mission. Only the command pod’s part.cfg file was modified to set the crew capacity to zero. Instead, I planned the mission prior to launch and relied on rudimentary celestial navigation and ded reckoning for navigation.

Here are the screen shots for anyone who’s interested:

The inter-planetary escape burn was timed to begin 28 seconds after Kerbol set. This would give me the correct ejection angle.

The patched conic renderer in the map view was flicking back and forth between a Duna intercept and Kerbin intercept because I had intentionally set the spacecraft up to just graze Duna’s SOI, thereby minimising the effect of Duna’s gravity on my trajectory.

[Nibb31]

I think what you did was a Duna free return.

A cycler would orbit Kerbol with an orbit that would do both Kerbin and Duna flybys on a regular basis. The main advantage of the cycler concept is to provide comfort for repeated long journeys, something like an interplanetary cruise ship. It allows you to launch the crew and cargo on small shuttle crafts at the departure and arrival points. The hab modules, cabins, entertainment, kitchen, exercice equipment, zero-g swimming pool (!), and so on, go on the cycler ship and are only launched once and reused for many round trips.

The shuttle crafts still have to carry the same delta-v as the interplanetary cycler (because they have to rendez-vous on the cycler's interplanetary orbit), but they can be lighter and therefore use less fuel on each round trip. If your cycler is a one-man cabin designed for a single round-trip, then there is no real gain.

But it is interesting as a challenge

Edited October 18, 2012 by Nibb31

[vexx32]

A "free return" is where you go from one body to another without burning for corrections, which is not what he did. Also, it's usually from a parent body to a child one (for example, it's pretty easy to get a Munar FRT that goes around the moon and lands you back in Kerbin's atmosphere).

[bsalis]

Wow, great work there. If you do get a Duna Cycler going I want to know about it. I suspect the corrections needed to keep it going will be high though. It's an interesting exercise, but I don't see much of a benefit to a cycler in the game. Since as far as I know, the cycler concept is mostly about having re-usable habitation modules.

[nhnifong]

So it's just a matter of leaving kerbin at 1665 m/s? What was your solar semimajor axis? I'd like to try this at some point

Good job! Kerbals are not known for their precision. This is a revolutionary way of thinking for them

[PakledHostage]

@nhnifong: No, the ejection speed and angle are important, but you also need to time your escape such that you arrive at Duna's orbit at the same time Duna does. And because you're crossing Duna's orbit at a significant angle, your relative speed is very high. As a result, the "window" during which you're within range of Duna's SOI is quite short. During a Hohman transfer intercept, you have a fairly long period where you are basically flying parallel to the target's orbit, so you don't need to be as precise.

As for the semi-major axis of the transfer orbit, it needs to be the (cube root of 32 divided by 2) times Kerbin's semi-major axis. This yields an orbital period of 2 times Kerbin's orbital period, so you'll intercept Kerbin again after it makes two complete trips around Kerbol.

@Nibb31: Thanks. I am aware that this wasn't a true cycler trajectory and that they aren't very useful for a single trip to Duna. I just wan't sure what to call it because, as Vexx32 points out, it is unclear whether it is really called a free-return trajectory either. Instead, I called it a "proof of concept Duna cycler trajectory", because it is that. It wouldn't take much fiddling with these initial conditions to turn it into a true cycler trajectory. But maybe someone out there knows the proper term for a trajectory like this?

I will use this trajectory when I send my Kerbals to Duna. For the time being, this was just an unmanned "dry-run" so I could prove my calculations and navigation before risking my Kerbal hero's lives on the real mission!

[Frederf]

My preferred free return trajectory makes use of gravitational braking. The benefit is that if the craft or a portion thereof wants to capture it may do so making efficient use of the Oberth effect. Skimming the SOI of the target body avoids the complication of gravitational assist (or de-assist) but misses on the benefits available.

This is the classic Apollo style profile that swings around the target body (Moon) and has its trajectory bent back toward the origin (Earth) right down the atmospheric corridor if one can be that precise.

[PakledHostage]

My preferred free return trajectory makes use of gravitational braking. The benefit is that if the craft or a portion thereof wants to capture it may do so making efficient use of the Oberth effect. Skimming the SOI of the target body avoids the complication of gravitational assist (or de-assist) but misses on the benefits available.

This is the classic Apollo style profile that swings around the target body (Moon) and has its trajectory bent back toward the origin (Earth) right down the atmospheric corridor if one can be that precise.

An "Apollo style trajectory" is fine when you're making a free return to the parent body in the system (like in the case of an Earth-Moon-Earth or Kerbin-Mun-Kerbin trajectory) but it isn't possible for a Kerbin-Duna-Kerbin trajectory.

I chose to skim Duna's SOI in this case because I didn't need a gravitational assist to return to Kerbin. If you look at the video below, you'll see that my spacecraft was already on an intercept trajectory with both Kerbin and Duna even without the gravitational assist. Also, this mission was flown in version 0.17 of the game without the help of MechJeb or other such mods. And we didn't have maneuver nodes then, so the degree of precision possible wasn't as high as it is today.

Feel free to take up the challenge, however, and use the added precision of the maneuver nodes to set up a gravitational assist into a true cycler trajectory.

Edited June 1, 2013 by PakledHostage

[Frederf]

Yeah I realized post-posting that earth-moon and earth-mars dynamics are different. I didn't know if that made free return with significant gravity braking more or less possible. My gut feeling is that gravity assist is usable in the Duna scenario for some benefit. It certainly would improve the usefulness of a cycler if you didn't have to get off the taxi on the edge of the body's SOI.