IMFD Silly question about Mars launch window

[bloodtoes]

Hello,

After using transx for most of my interplanetary flights, I'm slowly learning to use IMFD for such trips and doing OK with it. However, I'm curious about a discrepancy between orbiter/IMFD and reality. Maybe this is more of an Orbiter issue, but according to :

http://en.wikipedia.org/wiki/Exploration_of_Mars#Launch_windows

The next launch window for an Earth-Mars journey is during December 2009. My assumption is this means this is the next point when a trip to mars would require the minimum energy to get to Mars, as stated on the website. However, in orbiter and calculating the trip with IMFD, the minimum energy launch that I could find was more in the mid-october area. That's pretty close, but off by a couple months.

What is the likely cause of this? Is the orbit of Mars or Earth in Orbiter off? Is it because of the dynamics of the ship I am using? Or is there something else at work here? Are there IMFD settings that might be responsible?

Thank you kindly.

 

[Oceanic]

i am using this Hohmann Microsoft Excel spreadsheet to check for launch windows.
the lowest delta-v launch window to mars would be october 15th 2009.

i think wikipedia is wrong on this one

 

[tblaxland]

i am using this Hohmann Microsoft Excel spreadsheet to check for launch windows.
the lowest delta-v launch window to mars would be october 15th 2009.

i think wikipedia is wrong on this one

I concur. Also the October 2007 window is two months after Phoenix actually launched.

 

[insanity]

Hello,

After using transx for most of my interplanetary flights, I'm slowly learning to use IMFD for such trips and doing OK with it. However, I'm curious about a discrepancy between orbiter/IMFD and reality. Maybe this is more of an Orbiter issue, but according to :

http://en.wikipedia.org/wiki/Exploration_of_Mars#Launch_windows

The next launch window for an Earth-Mars journey is during December 2009. My assumption is this means this is the next point when a trip to mars would require the minimum energy to get to Mars, as stated on the website. However, in orbiter and calculating the trip with IMFD, the minimum energy launch that I could find was more in the mid-october area. That's pretty close, but off by a couple months.

What is the likely cause of this? Is the orbit of Mars or Earth in Orbiter off? Is it because of the dynamics of the ship I am using? Or is there something else at work here? Are there IMFD settings that might be responsible?

Thank you kindly.

I did my mars flights in the last two weeks. I had the same wiki/flight issue and just assumed wiki was wrong

 

[bloodtoes]

I did my mars flights in the last two weeks. I had the same wiki/flight issue and just assumed wiki was wrong

Interesting. Well, in this case I suppose the guide/wiki was wrong, and reality/orbiter got it right. Thanks! :thumbup: I'll have to set my scenarios back a couple months to do some real lowest-energy transfers to Mars.

 

[2552]

i think wikipedia is wrong on this one

Not anymore

 

[bloodtoes]

Not anymore

Hah! Maybe a more thorough update will be needed to synch the other launch windows, but at least the October 2009 one is right. =)

edit: Spoke too soon, someone went and reverted the edit. I guess a [citation] was [needed].

Double edit: http://nssdc.gsfc.nasa.gov/planetary/mars/marslaun.html

This might be a good start.. heh.

 

[kwan3217]

You have to be careful with launch windows, and the simple Hohman 180deg transfer is rarely best in real life. Things like the orbit eccentricity of Mars and the orbit inclination of Venus screw up our nice circular coplanar assumptions.

If you look at the porkchop plot to one of those planets during any particular launch window, you will see a minimum launch energy significantly (~1month) before the Hohman launch date and another one significantly after. The only thing the Hohman window is good for is finding the center of the general range to build your porkchop plot.

Check out Wikipedia on Porkchop Plot, they have a nice picture of one. http://en.wikipedia.org/wiki/Porkchop_plot

Also, the best trajectory for a DeltaGlider will be different from the best trajectory for a real spacecraft. On a DeltaGlider, you use the same engine and the same fuel supply both to leave Earth and arrive at Mars. It doesn't matter how you divide the energy to a DG. A real orbiter is launched with a huge launch vehicle, and enters orbit with a puny wimpy engine, so they try to minimize arrival speed and give the spacecraft engine less work. A real lander arrives at Mars with a heat shield instead of an engine, and it doesn't really matter how fast it arrives, so they try to minimize launch speed. This is why even though a Hohman transfer takes about 8.5 months, landers almost always arrive in 7 months and orbiters in 10-11 months.

 

[Shadow Addict]

i am using this Hohmann Microsoft Excel spreadsheet to check for launch windows.
the lowest delta-v launch window to mars would be october 15th 2009.

i think wikipedia is wrong on this one

I've also been using this spreadsheet. I just take the date it gives me, put it through an MJD calculator, put that into IMFD and use that as a base to find a the most efficient transfer. The spreadsheet never seems to give a precise date, but it helps narrow it down.

 

[tblaxland]

If you look at the porkchop plot to one of those planets during any particular launch window, you will see a minimum launch energy significantly (~1month) before the Hohman launch date and another one significantly after. The only thing the Hohman window is good for is finding the center of the general range to build your porkchop plot.

All true. Taking your comments on board, I have made some amendments to the article in an attempt to improve its accuracy. Minimum energy launch dates were computed using Piper's porkchop generator.

 

[bloodtoes]

Fantastic, I love seeing revision in action. =)
BTW, Wiki's page on MJD just links 3 or 4 possibilities of what MJD might stand for, it might be best to link to the http://en.wikipedia.org/wiki/Julian_day#Alternatives page instead.
Good stuff.

 

[tblaxland]

Thanks for the heads-up. Fixed.

 

[chad_s]

Here is a Java app that can be used to calculate porkchop plots:
http://astrojava.com/btp/