Question about launch windows for manned missions

[Kelderek]

How would NASA and others go about determining a launch window for a manned mission to Mars or somewhere else far away from the Earth and Moon?  For an unmanned mission it would be much simpler, they just choose a window that gives a fuel efficient (delta-V) journey.  But for a manned mission you would have to add time efficiency as another very important consideration due to limited life support resources.

When playing KSP I couldn't help but notice that if I send a mission to Duna at the optimal launch window and stick around at Duna for a couple of game days, if I were to attempt to return home to Kerbin after that it would be an awful time and highly inefficient for delta-V.  The planets are not in the right phase angle for going from Duna back to Kerbin.  I usually have to calculate a new launch window from Duna back to Kerbin and use time acceleration a fair bit before I start my journey back home.

So for NASA, how would they go about solving this problem?  Does planning a trip that lasts a reasonable amount of time require them to use higher than normal delta-V to balance it out?  Do the launch windows coming and going work out in such a way that force the mission to last a certain duration on the planet like Mars?  In other words, would a manned mission have to plan on being on the surface many more months just so that the return voyage happens in a reasonable window (for time and delta-V efficiency)?

I believe that the launch windows are based on the synodic period between Earth and Mars which is over 25 months.  This would be the case for going in either direction, so going from Earth to Mars has windows every 25 months and going from Mars to Earth has different windows (a different phase angle) that are also 25 months apart.  The important question would be what is the time offset between the two?

I suppose people who play with some type of life support mod in KSP may have already addressed this, but as I haven't used any of those myself, I am curious how this problem is solved.

EDIT: here's an example of what I mean for KSP with Kerbin and Duna: Leave Kerbin Y1 D236, Arrive Duna Y2 D69 (258 days flight).  The next available launch window from Duna is Y3 D242, arriving back at Kerbin Y4 D84.  You have to sit on Duna roughly 600 days before you can have a good launch window to leave.  It just so happens that then previous launch window to leave Duna is only about 59 days before you arrive, so you just miss it and have to wait a long time for the next one.  Since time is such an important commodity for manned spaceflight, I'm guessing they have to choose less than perfect launch windows and I'm curious how those are determined.

Edited June 21, 2016 by Kelderek

[Shpaget]

They (probably) use porkchop plots.

 

Then they decide the optimal time to launch, taking into account that while longer transfers requiring lower dv, require more supplies, larger habitation modules, increase radiation exposure and the shielding necessary. Those things, while not increasing dv, do increase spacecraft mass which means bigger rocket.

Edited June 21, 2016 by Shpaget

[Kelderek]

Are there porkchop plots that account for both the arrival and departure legs of the mission?  The one you linked looks like it only shows a one-way trip.  I assume that when send people to Mars, we'll also try to bring them back home   That's what I'm really getting at, how do they plan for the entire mission including: time it takes to get there, time to screw around on the surface and time it takes for a return trip back home.

[Nibb31]

There haven't been any manned missions to Mars, so there is no predetermined textbook way to do it. When they do get around to sending humans to Mars, they will probably proceed by doing multiple iterations of porkchop graphs and selecting the best window each way. The time between the two windows then determines the duration of the stay on Mars.

It's expected that a typical Mars round trip would be approximately 18 months long.

Edited June 21, 2016 by Nibb31

[Bill Phil]

They might just screw the Delta V, and go for time. That's the usual trade-off, I hear.

[benzman]

If I remember rightly, and someone will correct me if I am wrong, the problem is that if you use a minimum energy Hohmann transfer orbit to get to Mars you arrive about two months AFTER you should have left to come home.  This obliges you to wait on Mars for the next return window in roughly fourteen months time, which is why it is not practical to think of a manned Mars mission lasting less than about two and a half years.   As an aside, this is also why it is not feasible to do a manned fly-by of Mars without a huge ship with masses of DV.

 

[PB666]

1 hour ago, Bill Phil said:

They might just screw the Delta V, and go for time. That's the usual trade-off, I hear.

Well if you are solely interested in time, its a suicide mission, uless there is a return vehicle already on Mars waiting to take you into orbit. 

[Kelderek]

19 minutes ago, benzman said:

If I remember rightly, and someone will correct me if I am wrong, the problem is that if you use a minimum energy Hohmann transfer orbit to get to Mars you arrive about two months AFTER you should have left to come home.  This obliges you to wait on Mars for the next return window in roughly fourteen months time, which is why it is not practical to think of a manned Mars mission lasting less than about two and a half years.   As an aside, this is also why it is not feasible to do a manned fly-by of Mars without a huge ship with masses of DV.

 

In that scenario, I wonder if it makes sense to leave a month or two before that optimum window, so that you would get to mars and be able to leave within a month or so after the optimal return time.  I realize that by leaving earth early it may take longer to get there, but a month or two of time efficiency for each leg of the trip would be a lot less than being forced to spend a year or more on the surface.  I know that every extra day before or after a launch window makes the time/energy efficiency worse, but I'm willing to bet there is some wiggle room there so that the timing could work out for a full mission.

[DBowman]

@benzman there are two main classes of Mars mission conjunction and opposition. A conjunction mission uses low energy Hohmann transfers with a 550 (ish) day stay over. An opposition class takes a high energy leave early, stay very briefly, leave late strategy.

Low energy flybys are very possible, it takes much less energy not to capture and then escape Mars. With timing etc you can arrange for dual Venus & Mars flybys with low energy departure and only a small correction burn at Mars. I explored some options for a one man mission to keep it cheap.

People have also proposed various hybrid profiles. For example a flyby where a small lander detaches part way through the approach, accelerates to arrive at Mars early enough to do a quick landing, and finally rendezvous with the flying-by mother ship. Or missions that use Mars encounter to go into an off plane orbit that stays close to Mars for half a Mars year (for low latency tele-robotic surface operations ) before encountering again and using gravity assist to help get an Earth encounter.

[cantab]

On 21/06/2016 at 7:20 AM, Kelderek said:

Does planning a trip that lasts a reasonable amount of time require them to use higher than normal delta-V to balance it out?

Correct. The "opposition class" mission profile uses a slightly higher energy departure and a significantly higher energy return, with the trip back to Earth cutting inside the orbit of Venus - and in some plans using a Venus gravity assist.

I've not seen any that make the higher-energy trip the outward journey. I assume this is because on the way back the capsule can just slam into Earth's atmosphere and land and the rest of the ship is ditched, whereas on the way out you need to capture the whole thing into Mars orbit. If we could confidently aerocapture in real life though, then making the higher-energy trip outwards would let the expensive Earth ejection burn be done with efficient hydrolox engines.

http://www.collectspace.com/ubb/Forum39/HTML/000121.html

[benzman]

10 hours ago, DBowman said:

@benzman there are two main classes of Mars mission conjunction and opposition. A conjunction mission uses low energy Hohmann transfers with a 550 (ish) day stay over. An opposition class takes a high energy leave early, stay very briefly, leave late strategy.

Low energy flybys are very possible, it takes much less energy not to capture and then escape Mars. With timing etc you can arrange for dual Venus & Mars flybys with low energy departure and only a small correction burn at Mars. I explored some options for a one man mission to keep it cheap.

People have also proposed various hybrid profiles. For example a flyby where a small lander detaches part way through the approach, accelerates to arrive at Mars early enough to do a quick landing, and finally rendezvous with the flying-by mother ship. Or missions that use Mars encounter to go into an off plane orbit that stays close to Mars for half a Mars year (for low latency tele-robotic surface operations ) before encountering again and using gravity assist to help get an Earth encounter.

That all looks quite interesting.  I was not aware of your other posting on the subject.  I shall read it at my leisure. 

 

[DBowman]

10 hours ago, cantab said:

I've not seen any that make the higher-energy trip the outward journey. I assume this is because on the way back the capsule can just slam into Earth's atmosphere and land and the rest of the ship is ditched, whereas on the way out you need to capture the whole thing into Mars orbit.

good point, for KSP you can aerocapture at Duna easily enough