Flight Question Very basic question about launch direction

[shadowfax]

So I'm pretty good at launching, establishing an orbit, circularizing, matching planes, syncing, etc. What I'm not good at is figuring out the optimal direction to launch in, which means I end up with a Rinc of over 200 degrees and I either match planes (and then run out of fuel for the sync) or have enough fuel to de-orbit. Not both. You load a scenario that says "take off, heading x-degrees to establish an orbit with ISS." Great, but how do I figure out that "x" if the scenario doesn't feed it to me? I've read a lot of docs on it, but they all say some variation of "turn to the proper heading to establish orbit with the object," without saying how to compute that orbit, unless I've missed something.

Which I probably have. Any help?

 

[Wonderer]

Try this [ame="http://www.orbithangar.com/searchid.php?ID=2802"]Launch MFD - v. 1.4.5 for Orbiter 2010[/ame]

 

[garyw]

There is an equation to work out the launch direction for an inclination from the launch site.

Or you could just use [ame="http://www.orbithangar.com/searchid.php?ID=2802"]Launch MFD - v. 1.4.5 for Orbiter 2010[/ame]

Enjo also wrote this handy little calculator for it -> [ame="http://www.orbithangar.com/searchid.php?ID=2464"]Azimuth calculator v. 1.5 - GUI[/ame]

 

[SandroSalgueiro]

Here you can learn how to make that calculation: http://www.orbiterwiki.org/wiki/Launch_Azimuth

However, I strongly recommend you to use LaunchMFD instead of using a calculator. Notice that LaunchMFD gives you Equatorial Inclination, while the standard AlignPlaneMFD gives you Ecliptic Inclination. That once made my head hurt.

 

[Urwumpe]

The proper term for the "launch direction" is Launch azimuth, BTW. if you know this, you can find lots of more information about the topic.

The launch azimuth has a pretty complex relation to the inclination of your orbit. the simplest rule of thumb there: if you launch to the east, 90° azimuth, your orbit inclination(equatorial frame) will be the latitude of your launch site.

Think of the wave shaped ground track of your orbit over Earth - if you launch to east or west, your launch site will touch the ground track at its northern most/southern most extend. At the equator, your azimuth would be 90° +/- inclination (depending on which node you are looking at, since you cross the equator twice).

That said: Use the Launch MFD. It is doing the complex math for you, also already compensates for the rotation of Earth, which is much more complex math as in the coarse examples above.

 

[Tommy]

Part of launching in plane with the target is timing the launch. For trips to the ISS, you can use AlignPlanesMFD - and launch when Tn = 300. Assuming you are launching from KSC, If you are approaching the DN (ISS path crosses your base going North East), then your heading should be 42 degrees. If approaching the AN (ISS crossing to the South East) use heading 138 degrees.

For MIR (or the Moon) use a heading of 90 degrees - when the targets path is as close as it gets to KSC

 

[shadowfax]

Thanks guys - I worked on your advice (and the MFD) all weekend and things are going much better now.

 

[Moach]

you can do it without LaunchMFD too, if you got some courage and a good joystick

you need align-planes MFD, tho - launch roughly to the direction of the crossing orbital plane (you need to wait for the earth to "rotate your launch site under it") - then adjust your heading by looking at the rate and rInc displayes on the plane alignment thingy....

it takes some practice, but i generally get a more precise alignment doing this than with LaunchMFD when orbiting the G42-200 :hmm:


.and i'm also insane - i'm sure there are a million reasons why this would not be recommended by 4 out of 5 dentists :rofl:

 

[agentgonzo]

it takes some practice, but i generally get a more precise alignment doing this than with LaunchMFD when orbiting the G42-200 :hmm:

Have you used the off-plane correction in LaunchMFD? I've used it to get a 0.01° RInc at MECO. It's turned off by default.

 

[shadowfax]

you can do it without LaunchMFD too, if you got some courage and a good joystick

you need align-planes MFD, tho - launch roughly to the direction of the crossing orbital plane (you need to wait for the earth to "rotate your launch site under it") - then adjust your heading by looking at the rate and rInc displayes on the plane alignment thingy....

it takes some practice, but i generally get a more precise alignment doing this than with LaunchMFD when orbiting the G42-200 :hmm:


.and i'm also insane - i'm sure there are a million reasons why this would not be recommended by 4 out of 5 dentists :rofl:

Actually, I found myself doing this a few times last night. My Orbiter install has gotten crash happy, so I shut off all addon MFDs, which solved the problem. Now I'm turning them back on one at a time to see which one is the culprit. As of last night I hadn't turned Launch back on yet, so I was aligning planes at 50km or so. It actually did work really well.