General Question Correct Launch Time

[Thundersnook]

Greetings Masters of Orbiter

Just let me first thank you for this forum!:thumbup: I've been playing orbiter since 2004 I think, but never checked about some communities, but this one is great!

So here is my question:

Is there any way to calculate my launch time with the given parameters Inclination, Latitude of my launchplace and maybe some other parameters I could get from the MFDs (Like speed on earth or orbital speed)

To calculate the correct launch-azimuth I used this formula:
a=arctan(tan(arcsin(cos(Inc)/Cos(Lat))))-SOE/(OS*cos(arcsin(cos(Inc)/cos(Lat))))))
where
a=Azimuth
Inc=Desired orbit-inclinatin
Lat=Latitude of launchplace
SOE=Speed on earth(through earth rotation)
OS=Desired final orbital speed (of course its connected with the final orbital altitude)

So this is what I use to get the Inclination right, but we also want to get the LAN of our ship congruent with our targets LAN (Longitude Of Ascent Node)

So for this if tried a lot of things with the Align-Plane MFD And the best result I couldget was that my orbit differs 0.04° from my targets orbit.

For this I took off when the crosspoint of my orbital plane with the targets was exact that time I need to obtain half of my Orbital speed. (So lets say the DG needs 500 seconds to climb on orbit and gained half of its orbital speed after 350 seconds, I started when the crosspoint my orbital plane was 350 seconds from my targets plane)

BUT: Is this the correct way to go? Of course it might be a silly question, because it seemed to work (I was very satisfied about my RInc of 0.04° )
But I just want to ask you, how you determine you correct launch-time for an intercept with ISS or any other station.

For me there is some beautie in an perfect accent so I wasn't very hapy with an RInc of 0.20° or so, so let me know if this is the way to go.

Thank you for any of you answers, and now my official "Hello" to all of you!

Greetings!

(And sorry for my english ... it might be not the best^^)

 

[garyw]

Is there any way to calculate my launch time with the given parameters Inclination, Latitude of my launchplace and maybe some other parameters I could get from the MFDs (Like speed on earth or orbital speed)

[ame="http://www.orbithangar.com/searchid.php?ID=2464"]Azimuth calculator v. 1.5 - GUI[/ame]

and

[ame="http://www.orbithangar.com/searchid.php?ID=2802"]Launch MFD - v 1.3.3 BETA for Orbiter 2010[/ame]

(And sorry for my english ... it might be not the best^^)

It's not a problem, it's been perfectly understandable!

Oh and welcome

 

[insanity]

I tend to be a bit lazy and use launch mfd, which does both these calculations.

Using align planes, you want to wait until the ISS is at a node, then launch to the correct azimuth (northern heading for DN, southern heading (azimuth-180degrees for AN).

 

[Tommy]

I launch when TN (shown in AlignPlanesMFD) is 300. While 350 seconds is correct, AlignPlanes doesn't seeem to accurately predict the time to node when you are landed, and I've found 300 seconds works well. But a RInc of 0.04 degrees is a bout as good as I can get, so your method obviously is working just fine.

The above mentioned LaunchMFD might be a bit more accurate, but if you have non-spherical gravity sources enabled you'll get a touch of RInc just from the difference in precesion so I wouldn't put too much effort into getting the launch exact, a few hundreths of a degree are easily dealt with.

 

[Thundersnook]

Thank you for your thoughts! I never realized LaunchMFD :blink: What a mistake! This one is great and exactly the thing I've been looking for!

Thank you very much!

 

[Enjo]

I'm glad it could help. Welcome to the forums, BTW.

The above mentioned LaunchMFD might be a bit more accurate, but if you have non-spherical gravity sources enabled you'll get a touch of RInc just from the difference in precesion so I wouldn't put too much effort into getting the launch exact, a few hundreths of a degree are easily dealt with.

Actually I was working on a fix for that, and there's some draft code inside LaunchMFD.cpp, which involves integration, but I forgot to include it after Agentgonzo fixed the Time part. Anyway he also brought offplane correction which minimises these LAN errors on the fly.

BTW, Agentgonzo should be back from his safari already. I hope that everything is OK with him.

 

[Tommy]

Actually I was working on a fix for that, and there's some draft code inside LaunchMFD.cpp, which involves integration, but I forgot to include it after Agentgonzo fixed the Time part. Anyway he also brought offplane correction which minimises these LAN errors on the fly.

That may help if I can specify parking orbit PeA and ApA, and it can figure out (or allow me to specify) the number of orbits to intercept. I've noticed that if my parking orbit is low (PeA around 200k, which is often the most efficient) then even if my launch is perfectly aligned, a very slight RInc will develop during the sync process if it takes more than a couple orbits. It's never much, nothing a couple seconds of RCS can't fix, so I never considered it a problem that really needed fixing!

I consider LaunchMFD the best thing that ever happened for tail sitters - being able to rotate to the correct heading before pitching over makes launching the shuttle so much easier and more efficient it's hard to remember how I ever got by without it!

 

[Enjo]

That may help if I can specify parking orbit PeA and ApA, and it can figure out (or allow me to specify) the number of orbits to intercept.

Do you know that you can do all of these in ecliptic frame, except the last one? From the UI point of view, it shouldn't be much different than the current ecliptic frame orbit params input - just with an additional parameter which would be the number of orbits. From the internal point of view, it may be as simple as adding something proportional to (2*PI)*number_of_orbits, although my question is - what would exactly the number of orbits mean? Not the number of the target orbit's passes over your launch site?

I consider LaunchMFD the best thing that ever happened for tail sitters - being able to rotate to the correct heading before pitching over makes launching the shuttle so much easier and more efficient it's hard to remember how I ever got by without it!

Then it's a good thing that the MFD is FOSS, because otherwise, after I get really mad on the cold world so that I take the MFD from OH, you could recreate it (this happened to some non-FOSS addons in the past...)

[EDIT]
386'th post!

 

[Tommy]

The idea behind being able to specify number of orbits is to allow LaunchMFD to account for difference in precession caused by non-spherical gravity sources (and would only apply if this option is enabled - don't know if the API provides an add-on with a way to determine this). It could just as easily be entered in time units (seconds or hours, etc).

For those who wonder what I'm talking about, a vessel in a lower orbit will have it's LAN move westward a tiny bit faster than one in a higher orbit. The higher the Inc, the more pronounced this will be (in an equatorial orbit this doesn't apply. The greater the difference in altitude the more pronounced this will be. For most LEO ops, this is so slight that it isn't even noticable unless you will be in the parking orbit for a day or more.

I've never considered this to be a weakness in LaunchMFD. From my own point of view, trying to account for this would be fixing something that isn't broken. Most of the time, you won't know how long the sync will take until you are already in orbit. Since any value I could input would be a guess, I would be just as likely to make things worse than better anyway! I probably wouldn't use that "extra" feature even if you did include it - the issue is so minor I'm kind of sorry I even mentioned it.

I certainly never mean to imply that there was anything wrong or incomplete about LaunchMFD. It's performance already exceeds any reasonable expectation, and is one of the best designed and most usefull MFD's available. Thank you for the work you have put into this, and also for releasing it as FOSS. It is a gift to us all, and I hope you never thought I didn't appreciate it.

 

[Enjo]

That's absolutely fine. I have some free time now to spend on my private projects, where Launch MFD is my Master Thesis, so it deserves some love.

Now I understand what you mean and I agree that such a feature would be a bit beyond, if not against, the current philosophy of Launch MFD - providing as little planning as it's required. You may feel sorry, but actually you reminded me to get back to the feature that I had to comment out a year ago. What it does is simpler than you were talking about - it only fixes the satellite's time remaining to orbit pass, which, as you know, is smaller by 50-100s than currently displayed Time to Intersection, if the nonspherical is enabled (Yes, API lets you learn this via NonsphericalGravityEnabled()). What I was eventually thinking about was letting Launch MFD know that you don't want to launch on the next possible target's orbit pass but in some further passes, but other folks would have to tell me if they'd find it useful. I have some doubts.

It was my ambition to include the LAN drift but I had to comment it out because the time readout failed at other point, which was later fixed by Agentgonzo. This reason, and a new, general integration function that I've recently developed, give me an opportunity to do it the right way this time.

 

[Tommy]

Sounds grat, and I'm not sorry at all. It looks like you are working on issues that are far more important than the difference in precession rates (which are negligable, really).

To be honest, I never really considered that was much room for improvement, but I suppose there always is. I look forward to what ever improvements you can find to make.