Trajectory Office backroom

[garyw]

Welcome to the Trajectory Office backroom.

This thread is here to provide mission support for missions beyond Earth, mostly the staff here will be concentrating on Mars missions but they'll be able to lend a hand with a few other missions as well.

First question we need to have answered by our trajectory backroom guys - Can you please give us some launch windows to Mars + dV requirements?

Thanks guys!

 

[Arrowstar]

Coming right up!

EDIT: Okay, here's what a few runs of the TOT gives me. I've done some analysis on the next three launch windows. They are as tabulated below.

Launch Window | Depart C3 [km^s/s^2] | Arrival Speed [km/s]
12/10/2013 |9.1051|3.28
3/7/2016|8.1229| 5.0556
5/13/2018| 7.7498|2.9746

Now, the actual departure dV will be determined by the orbit the spacecraft is in before the TMI burn. A very brief analysis seems to show that polar and/or retrograde (inclination around 110 degrees) orbits provide the easiest way to depart Earth. What I'll do, once we've settled on the launch date, is determine what a decent orbit to inject into is for each window.

At arrival, I'm thinking aerocapture maneuvers for everything but the comm sats (which may not be built for that sort of thing). If you can tell me how much dV those sats are capable of, I can find a trajectory that minimizes the arrival velocity to below that amount.

That all said, my recommendation for what should be launched to Mars and when is as follows:

OFMM-7/8 - 2013 window (2x launch vehicles)
OFMM-14/15/16/17 - 2016 window (4x launch vehicles)
OFMM-20/21/23 - 2018 launch window (3x launch vehicles)

Porkchop Plots
Below I've posted porkchop plots that show the required C3 energy (the hyperbolic excess speed squared) for the three Earth-Mars launch windows. These are merely a graphical representation of what was found in the table above and are shown mostly for the visually-inclined.

2013 Window

​

2016 Window

2018 Window

 

[Arrowstar]

Can I get some feedback on this from anyone else interested in doing trajectory work (and maybe the pilots, too)? Heck, if someone wants to run a "simulation" with Orbiter to verify, that would be quite appreciated.

 

[Wood]

A couple of things:

At arrival, I'm thinking aerocapture maneuvers for everything but the comm sats (which may not be built for that sort of thing).

I don't think that's going to work, the James Cook doesn't have a heat shield. I'm not sure what the Mars robotic landers are, but aerocapture makes sense for them.

A very brief analysis seems to show that polar and/or retrograde (inclination around 110 degrees) orbits provide the easiest way to depart Earth.

I don't think that's a good idea at all. The payload penalty would more than cancel out any possible benefit, and KSC cannot launch into either polar or retrograde orbits. If there are any russian launches then I think about the same inclination as the ISS would be ideal, else somewhere between 28.5 and around 40 degrees inclination would be good.

Other than that, it looks good. I'll have a play with TransX in the next few days based on those dates.

 

[Cras]

I would have guessed that the ideal orbit would be as equatorial as possible. The proposal that a retrograde orbit would be most ideal certainly caught me by surprise.

Why would the retrograde orbit/polar orbit benefit in the transfer?

 

[Arrowstar]

I don't think that's a good idea at all. The payload penalty would more than cancel out any possible benefit, and KSC cannot launch into either polar or retrograde orbits. If there are any russian launches then I think about the same inclination as the ISS would be ideal, else somewhere between 28.5 and around 40 degrees inclination would be good.

Fair enough. I didn't really like it either, but it was the way the numbers were coming out. In any event, I figured we'd launch eastward regardless as I'm sure our launch vehicles are capable of the C3 necessary to get us to Mars so long as they launch properly...

Thanks for the feedback! :tiphat:

---------- Post added at 12:49 AM ---------- Previous post was at 12:48 AM ----------

Cras: I couldn't tell you why specifically, it's just the way the numbers were working out. I may investigate that tomorrow if I get the chance.

 

[garyw]

I don't think that's going to work, the James Cook doesn't have a heat shield. I'm not sure what the Mars robotic landers are, but aerocapture makes sense for them.

One could be fitted but no one seemed to raise this when the missions were up for discussion.

As for landers - why aerocapture? Why not direct entry?

 

[insanity]

Direct entry for the landers makes the most sense. I'll plug the numbers into TransX sometime (hopefully when I'm at work today) and see what I get in terms of a reasonable parking orbit from KSC (and WIN)

 

[garyw]

I figure that with direct entry it should be slightly easier to target the primary and secondary landing sites.

 

[Pipcard]

One could be fitted but no one seemed to raise this when the missions were up for discussion.

As for landers - why aerocapture? Why not direct entry?

I'm wondering how people can even do a direct entry (from a interplanetary trajectory) towards a specific landing site.

 

[PhantomCruiser]

Those TransX and IMFD ninjas can do it. I usually have to get into a parking orbit first.

 

[garyw]

I'm wondering how people can even do a direct entry (from a interplanetary trajectory) towards a specific landing site.

Carefully?

Seriously though I don't see why you'd want to waste prop in slowing down into Mars orbit. Your goal for landers is to.... well, land so just hit the atmosphere and land.

Landing at a specific spot shouldn't be too hard, you'll need to be aware of timing more than anything because you'll see to make sure that the correct face of mars is presenting itself and/or that your angle is such that you don't overshoot your target.

 

[Pipcard]

Landing at a specific spot shouldn't be too hard, you'll need to be aware of timing more than anything because you'll see to make sure that the correct face of mars is presenting itself and/or that your angle is such that you don't overshoot your target.

I'm wondering how people time that months in advance (in the simulation of course).

 

[garyw]

Why would you? Any mission to another planet is all about breaking it into pieces.

First piece get into LEO.
Second pierce is the Mars injection burn.

After that you can fine tune the approach with Mid course correction burns.

 

[Pipcard]

Why would you? Any mission to another planet is all about breaking it into pieces.

First piece get into LEO.
Second pierce is the Mars injection burn.

After that you can fine tune the approach with Mid course correction burns.

Still, a mid-course correction is a few or several weeks (or a few days) in advance.

 

[Cras]

IMFD can get pretty accurate when targeting a base. Heck, you can even tell it in how many orbits you want to pass over the base

 

[Pipcard]

Well, I can only use TransX. Never really figured out how to use IMFD (especially for BaseApproach).

It doesn't really matter, since I'm not even doing that mission.

 

[Arrowstar]

The reason I never liked direct entry is because it doesn't consider local weather at the time. Once you're on that flight path, you're stuck. There is no stopping or turning around. Now, I realize this is Orbiter and we don't have local weather, but I still like to pretend it's a factor. If you enter an orbit first, you can hold out for dust storms or the like. A number of Russian probes (at least 1 anyway) have been lost because they descended into a Martian storm.

Anyway, it's your call, Gary. It doesn't matter to the interplanetary trajectory which way we go about setting down.

 

[Cras]

That is a good point. In Orbiter (at Earth anyway), with the D3D9 client, there is no runway lights, so there is a practical reason for not performing the inverted direct return and landing. I always have to get into LEO to wait until my landing site is on the day side, otherwise I cant see the runway.

With Mars, not so much an issue since we are not dealing with runways, but I would like to consider the dust storms being an issue.

 

[Pipcard]

That is a good point. In Orbiter (at Earth anyway), with the D3D9 client, there is no runway lights, so there is a practical reason for not performing the inverted direct return and landing. I always have to get into LEO to wait until my landing site is on the day side, otherwise I cant see the runway.

With Mars, not so much an issue since we are not dealing with runways, but I would like to consider the dust storms being an issue.

You don't have to use the D3D9 client.