None
of these variables really make much difference to your interplanetary
trajectory, but they can be useful for adjusting the first part of your
trajectory - the part that's around the Earth.This function is used when you wish to leave the current minor body. The best time to call it up is before you take off. If you are already in orbit, there are times when it is better to use the Orbit Eject mode instead.
In this tutorial we will be using the default 'Cape Canaverel' scenario that comes with the December 2002 build.
The Eject function has quite a few variables, and it's worth quickly running through them. These instructions all assume you're planning to leave Earth - if you're using TransX on some other planet, just substitute it in Earth's place.
Prograde vel. - This is one of three variables that set the direction in which you leave Earth. This sets your planned velocity change along the direction of Earth's movement. This is the most efficient direction for increasing or decreasing the size of your planned solar orbit, so it should usually be the variable you adjust first.
Eject date - This is the date when you're planning to leave Earth behind. This is the launch window date. Adjusting the launch window is normally the second thing you do. As you adjust it, you'll see any
Outward vel. - This is the second variable that specifies how you leave Earth. This one specifies the velocity at which you move outward (away from the Sun). In fact it isn't quite outward - it's at 90' to the direction of Prograde velocity. This variable can be used to give you some flexibility in your eject date. If you use positive outward velocity, you can use a later eject date, and if you use negative outward velocity, you can use an earlier one. It's normally more fuel efficient to set the date first, and leave this fairly close to zero. But if you're in a hurry, this is the way to go.
Ch. plane vel. - This stands for 'change plane velocity'. This is velocity at 90' to the plane of Earth's rotation. You can add or subtract this to change the plane of your planned orbit. I'll show you how and why to use this as the tutorial progresses.
That's all the variables you need to specify an interplanetary transfer. But there are some other, less important variables as well, which alter your view of proceedings.
Projection. This adjusts the position from which the orbit graph is displayed.
Intercept with. This controls the targeting system. Currently the targeting system is set to work on the hypothetical orbit (the one we're going to plan). The other option is craft - this is useful once you've achieved escape velocity. This uses the actual orbit of your craft as the input to the targeting system.
View orbits. Default is all, and you won't normally want to change that. This changes the orbits that are used to scale the diagram. For flights like Neptune to Earth (for example) it can be useful to suppress the display of Neptune to concentrate on the Target orbit.
None
of these variables really make much difference to your interplanetary
trajectory, but they can be useful for adjusting the first part of your
trajectory - the part that's around the Earth.
Pe distance. This sets the Periapsis distance for your hypothetical orbit. A hypothetical orbit is generated when you set some velocity using the major view variables. You should aim to set Periapsis distance close to the surface, but safely above any atmosphere.
Ej orientation. This sets the orientation of your hypothetical orbit. Basically, every orbit you see on this view has one thing in common. It's hyperbolic, and the outgoing arm of that hyperbola goes in the direction you specified in the major view. But you can still rotate this bit of your orbit, using your exit direction as an axis. That's what this variable does. There are times when adjusting it can make your life easier.
Projection. This adjusts how the orbit graph is displayed. There are four options - Ecliptic, Craft and Target show the orbit from the point of view of the ecliptic plane, the craft's orbit (your current orbit) and the Target orbit (the orbit you're planning). These three projections are almost identical to the projections found in Martin's Orbit MFD. The fourth projection is equatorial. This shows your current planet with the North pole at the top. This projection rotates around with the planet.
View orbits - All, or hypothetical. If your craft is in a big elliptical orbit, there may be times when you don't want to see it all.
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The first job is to tell the MFD where you are, and where you want to go.
After you've made these adjustments, TransX displays the orbits of Earth and Mars (Mars is the outer one of the two). The green line indicates the current position of each planet.
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 The
next stage is to set some prograde velocity. When you set even a little
prograde velocity, the targeting system will switch on, and you will start
to see some facts and figures about your planned orbit's closest approach
to the target. At the moment those facts are that we don't get much closer
than 100 million kilometers. We need quite a bit more prograde velocity.
Closest approach is in metres. As is usual in computing circles, K stands for 'kilo-' thousand, M for 'Mega-'a million, G for 'Giga-' a billion, and T for 'Tera-' a trillion. 107G is 107 billion metres, or 107 million kilometres. |
 Now
we've added quite a bit more prograde velocity, and you can see the effect
it's had. Our plan now misses Mars by only about 7 million kilometres.
Much better, but still not quite there. The main problem is the
inclination of our orbit. We haven't changed our orbital inclination at
all, and so our hypothetical orbit will be coplanar with Earth's. Mars
isn't coplanar with Earth, so we now need to adjust this, and get
everything else tuned up.
As you can see, my hypothetical orbit doesn't go miles beyond the orbit of Mars. This helps to keep the velocity at which we will finally encounter Mars down to something reasonable. At the moment, TransX predicts our plan will have us reach Mars at 4.482k per second. And that we will get there when MJD is 52163.80 |
 This
is how my major view looked after I'd finished adjusting all the
variables. The final figures I went for are:
As you can see, closest approach is now only 450 thousand kilometres. That's definitely good enough. As you'll see, I can't fly accurately enough to make it worthwhile setting the trajectory any more precisely at this stage. |
Next: Setting up the minor view
Orbiter Mars - (C) Duncan Sharpe 2003
Most
of the planning work actually takes place before you take off. This is because
the plan you create affects both your takeoff direction and time.