TransX - The cruise function

The cruise function has a whole new set of variables and views. These are designed to help you to do course corrections and to aim accurately at the target.

Views

There are three views in this mode, and some of them are not always available.

Major view - This shows your current orbit, and also any hypothetical orbit, target body orbit or minor body orbit that may be set. It also shows an estimate for your closest approach to the target.

Encounter view - This only shows if you have a target set, and your current trajectory is close enough to that target to pass within its sphere of influence. It shows TransX's best estimate of how you will pass this body. It also shows any surface base you may have selected on the surface using the Map MFD. Like all these estimates, it will wander a bit at first, but becomes increasingly accurate as you approach the planet.

Course correction view. - This shows a target with some crosshairs. Aim your ship to put the crosshairs in the centre of the target, and burn. This view will help you burn off any larger course corrections with precision.

Cruise function variables

Major view variables

• View orbits - This one simply adjusts your view of things. If you are aiming to go to Mercury, you may prefer to see more of Mercury's orbit, and less of your current position out by Earth. If that's the case, pick 'Target', otherwise leave this one as it is.

• Base orbit - This one is important. The MFD has to have a baseline to compare any course corrections with. It does this by taking a snapshot of your orbit when the function starts, or right after you reset the MFD. You can also update this orbit by pressing the ++ key on this variable. I tend to use resets myself.

• Prograde vel. - This is the velocity you plan to add in your current direction of travel.

• Eject date - This is the time when you plan to do the actual manoevre. You can put this off into the future if you're in a tight orbit where it matters exactly when you manoevre. Course corrections in solar orbit, on the other hand, are not very time critical - if this date is on the right day, it'll be good enough! Resetting the MFD will reset this date to the current time. This is another good reason for resetting before a course correction.

• Outward vel. - This is the velocity you plan to add at 90' to your current direction of travel in a broadly outward direction.

• Ch. plane vel - This is the velocity you plan to add at 90' to the plane of your orbit. This (obviously) will change your orbital plane.

• Projection - Your viewing angle. There are only two options at the moment, the ecliptic plane, and your craft.

• Intercept with - This controls which way the targeting mechanism is set. If it's set to 'craft', it will work off your actual orbit. If it's set to 'hypothetical' it will work off the orbit you define using variables. Note the default is 'craft' in this function, but 'hypothetical' in the eject and orbit eject functions.

Encounter view variables

• Satellite. Pick a satellite of the body you plan to encounter, and it will display a grey line showing the point where you cross the orbital plane of the satellite on the encounter view.

You can use these facilities to completely change your orbit if you wish - even setting up things like lunar transfers from low earth orbit. But the most common use is to add the little course corrections needed to reach a planet.

Making a course correction

There are two schools of thought when it comes to course corrections. One is to set the whole thing up using a planned burn with calculated consequences, and then carry out the burn with precision. The other school of thought - which is particularly appealing for small corrections - is to just do it. If the adjustment is tiny, fuel efficiency doesn't matter much, and the immediacy of the feedback you get makes it worthwhile. I tend to do these tiny adjustments using linear thrusters in bursts of only a couple of seconds at the most.

Using the variables First of all, it's good to wait a while until you're well clear of the planet you started from. Its gravity causes changes to your orbit for a fair while. But eventually the figures will settle down. Once you decide to make a course correction using variables, there are a few standard steps to follow 1. Reset the MFD using the RST key (shift-X). This samples your current orbit,  sets the manoevre time to the present, and zeros everything else. 2. Switch the targeting system over to 'hypothetical' using the 'Intercept with' variable. 3. Start playing with the three velocity variables, looking to reduce the closest approach all the time. Try and get it down to only a few thousand kilometres the first time. This is my MFD after juggling the closest approch (in hypothetical mode) down to four thousand kilometres from Mars's core. And that figure doesn't even take Mars's gravity into account!

But of course, you can take Mars's gravity into account as well. To do this, change the view to Encounter. There you will see how your orbit around Mars looks. You'll see that your hypothetical orbit is shown there, in yellow. The MFD reckons that your current path dips to only 2,288 kilometres from the planet's centre. In other words, your current course correction will put you on a collision course! At this distance, that's nothing to worry about. As you get closer, the figures here wander less and less, but are still easy to adjust using tiny course corrections or thruster burns. This way you can line yourself up as accurately as you like, and use almost no fuel in the process.

After you're satisfied with all your variables, switch views again. This time, a target with some green crosshairs appears. Manoevre your vessel to put the crosshairs in the centre of the target. You'll also see the size of the burn you've got to do under Rel V. This was my first course correction burn (which is usually the largest), and it's about 30 metres per second. For this manoevre , the time doesn't matter much, which is why it's out by 5 minutes in this case! Burn until Rel V. is close to zero.

After the course correction is done, my actual path is now much closer to a Mars intersection. Do course corrections at regular intervals - especially as the target comes closer.

This screenshot was taken in the Encounter view just over 3 days from closest approach. The orbit skims close to the surface of Mars. This closeness makes retro burns more energy and fuel efficient. If you're feeling braver, you can put your minimum altitude a precise distance from the surface and aerobrake into orbit! For this type of fine adjustment, it's best to make regular tiny course corrections with manoevreing thrusters, and just watch the effect as you make the adjustment. I chose to make a simple retroburn, and ended up in low Martian orbit with 24% fuel remaining on board the Deltaglider. That's more than enough for a good controlled landing!

Next: Orbit Eject Function

Orbiter Mars - (C) Duncan Sharpe 2003