Meadville Space Center Forum Index Meadville Space Center
Know the future through the past
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Virtual AGC
Goto page Previous  1, 2, 3 ... 11, 12, 13 ... 16, 17, 18  Next
 
This forum is locked: you cannot post, reply to, or edit topics.   This topic is locked: you cannot edit posts or make replies.    Meadville Space Center Forum Index -> Programming
View previous topic :: View next topic  
Author Message
LazyD



Joined: 10 May 2005
Posts: 406

PostPosted: Tue Mar 07, 2006 7:56 pm    Post subject: Reply with quote

It's reasonably close for the missions where the parking orbit is close to coplanar with the moon's orbit. Apollo 12 was 60 degrees out of plane - the timing is quite unforgiving, and it is difficult to get into an orbit that passes over the landing site.

For coplanar parking orbits, you only need to get to the moon's orbit radius close to the time the moon gets there.

For non-coplanar transfers, the above has to happen at the same time and location as the moon intersects your transfer orbit plane. This is quite sensitive to the parking orbit inclination and LAN.

What I'm trying to do is write a transfer tool that get you to the moon from any earth orbit. If there are differences between the simulation and reality my tool will do what works in the simulation. We'll see how it turns out.

D.
Back to top
View user's profile Send private message Send e-mail
Christophe



Joined: 09 Feb 2005
Posts: 280
Location: France

PostPosted: Tue Mar 07, 2006 8:19 pm    Post subject: Reply with quote

Thanks Lazy D for that. Keep up that work!
Back to top
View user's profile Send private message Send e-mail
irnenginer



Joined: 29 Jan 2006
Posts: 64

PostPosted: Wed Mar 08, 2006 6:31 am    Post subject: Reply with quote

Looking at the documentation P31 was considered the standard pre-burn program for LOI,TEI, some others. See GSOP Section 5.3-22.

The question on coordinate systems has good timing. I have been working on a way to transform orbiter state vectors to AGC state vectors. One of the little secrets of the AGC code is that it uses almost standard SI (they use csec alot for time). I have had no luck transforming one to the other though. I have also had to deal with the problem of some error between the two. If youd like to take a look here is a snapshot of ones I have.
Code:
MJD:40211.5640197667
RPOS    6194272.22 -1170682.14 -1789569.45
RVEL 282.121 -6033.366 4942.58

From AGC
RN 4982360 4264460 245132
VN 4409.556103 4937.121355 -4126.031065
Mission Time: 196200

The units are the same.
Delta of magnitude
Position 9725.406086m
Velocity -4.274992664 m/s
Back to top
View user's profile Send private message
Tschachim



Joined: 26 Nov 2004
Posts: 1272

PostPosted: Wed Mar 08, 2006 3:20 pm    Post subject: Reply with quote

Sorry for the lack of updates, but meanwhile I have sinusitis and need some rest. Sad

Fabrizio Bernardini wrote something about state vectors, no clue if this helps you?
http://www.aec2000.it/~fb/SV_Quest_1.html

Cheers
Tschachim
Back to top
View user's profile Send private message Send e-mail Visit poster's website
Christophe



Joined: 09 Feb 2005
Posts: 280
Location: France

PostPosted: Fri Mar 10, 2006 9:40 pm    Post subject: Reply with quote

irnenginer wrote:
Looking at the documentation P31 was considered the standard pre-burn program for LOI,TEI, some others. See GSOP Section 5.3-22

Sorry, still not sure...
look at this: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750065848_1975065848.pdf
Page 1.2, fig1.
Back to top
View user's profile Send private message Send e-mail
irnenginer



Joined: 29 Jan 2006
Posts: 64

PostPosted: Sat Mar 11, 2006 2:16 am    Post subject: Reply with quote

Christophe wrote:

Sorry, still not sure...


Ya got me there. I bow to your documentation.
Thats a good one too. Smile
Back to top
View user's profile Send private message
Christophe



Joined: 09 Feb 2005
Posts: 280
Location: France

PostPosted: Sat Mar 11, 2006 9:32 am    Post subject: Reply with quote

Hi irnenginer.
It seems that there are some difference sometime between GSOP (MIT doc) and NASA doc.
Where is the truth? Wink
I guess that MIT is the builder and NASA is the user, so perhaps NASA doesn't use the AGC exactly as MIT planned to...
Anyway it's more confusing for us Shocked
Back to top
View user's profile Send private message Send e-mail
Christophe



Joined: 09 Feb 2005
Posts: 280
Location: France

PostPosted: Sat Mar 11, 2006 9:44 am    Post subject: Reply with quote

I have a question for AGC specialists here:
When starting a virtual AGC scenario, computer is off until you put the breakers in.
Then, if you check the CMC clock (V16N65) you see a difference between CMC time and orbiter time roughly equal to the time you take before closing the breakers.
Is it important, or may be we don't care with that?
Refer to the doc, the CMC clock can only be updated via V73 with P27, that means that you cannot reset the clock, manually.
Moreover it's in octal, something very unfamiliar for me. Shocked
Back to top
View user's profile Send private message Send e-mail
movieman



Joined: 25 Nov 2004
Posts: 1018

PostPosted: Sat Mar 11, 2006 2:49 pm    Post subject: Reply with quote

I don't think it matters: I believe the AGC saves away the clock value at liftoff and uses that to calculate mission time.
Back to top
View user's profile Send private message
irnenginer



Joined: 29 Jan 2006
Posts: 64

PostPosted: Sat Mar 11, 2006 4:00 pm    Post subject: Reply with quote

The clock resets automatically at liftoff.

The time is stored in the memory in centiseconds in to octal words.

Time was very important to navigation so everything during a mission is referenced to the time of liftoff, ie the mission clock.

The only reason you would want to change the clock is if you had a power failure.
Back to top
View user's profile Send private message
Christophe



Joined: 09 Feb 2005
Posts: 280
Location: France

PostPosted: Sat Mar 11, 2006 4:14 pm    Post subject: Reply with quote

Ok guys thanks for your explanations.
I've just checked that: Any clock that is displayed before liftoff doesn't matter. The clock is reset at liftoff. A few seconds after liftoff occurs I V16N65E and see the time is counting exactly like the event timer.
I could answer myself if I was more thoughtful before posting. Embarassed
Back to top
View user's profile Send private message Send e-mail
irnenginer



Joined: 29 Jan 2006
Posts: 64

PostPosted: Wed Mar 15, 2006 4:02 am    Post subject: Updating State Vectots Via Orbiter Reply with quote

As per my earlier post I have been working on transferring Orbiter State Vectors to AGC State Vectors for updating the AGC. I have had some success. Here is how I have been doing it:

While in orbit and in P00. Do a quicksave and open the scenario file in a text editor. Take the orbital state vectors and EMEM locations 1170-1205. Decode the EMEM as follows
Code:
 1170-75 three double precision numbers multiply by 2^29(in Earth orbit)  to get position vector in meters (x y z)
1176-1203 three double precision numbers multiply by 2^7 and by 100(in Earth orbit) to get velocity vector in m/s ( x y z)
1204-1205 one double precision number multiply by 2^28 and divide by 100 to get mission time in seconds.


Take to MJD in Orbiter and subtract the launch time MJD to get the mission time at the time you saved. At this point you will notice that the mission time from the state vectors and the orbiter mission time are not the same. That is because the AGC does not update the state vectors continually. But are typically close enough to compare to close in on the right rotational values. Use rotation matrices to convert the Orbiter state vector to the AGC. See
http://www.euclideanspace.com/maths/algebra/matrix/orthogonal/rotation/index.htm
on how to do this. This is where things get fuzzy. I have gotten pretty close in position by using Y rotation 0degrees Z rotation 180degrees and X rotation about 178degrees. This makes some sense because orbiter uses a left handed coordinate system and the AGC uses a right handed on. But the velocity is not right, and use different values to get close. That confuses me. Can anyone clue me in?
Anyway I convert those numbers back into octal and use V71E to update the state vectors using 1 for 1501 See http://www.aec2000.it/~fb/SV_Quest_1.html for how to do that. Then do a program 21 and V82 to see how close you got. Most of the time I end up with pretty crazy orbits reported from the AGC.

Of course this is very time consuming. I have a EXCEL spreadsheet that does the whole thing, I will work on cleaning it up and make it available to download for those brave souls that might want to play with this.

I have also been working to get the error needles to work for launch monitoring. I have the numbers and the locations for specifying the pitch and roll payloads but nothing happens.Still working on it.
Back to top
View user's profile Send private message
Tschachim



Joined: 26 Nov 2004
Posts: 1272

PostPosted: Wed Mar 15, 2006 3:58 pm    Post subject: Reply with quote

I'm still trying to get P37/40 with Lambert steering working with the hope to be able to fly a simple mission, but had no further success. The "velocity to be gained" is still going mad during the burn and the engine doesn't cut off.

So I would like to ask if you figured out something new about that or if you think if it's possible that these problems are related to the state vector problems you mentioned?

Cheers
Tschachim
Back to top
View user's profile Send private message Send e-mail Visit poster's website
dseagrav



Joined: 03 Dec 2005
Posts: 216

PostPosted: Wed Mar 15, 2006 4:27 pm    Post subject: Reply with quote

Hrm; I wonder if it's a bug in the IMU... Maybe the cheat they use to avoid pulse-counting isn't working somehow?
Back to top
View user's profile Send private message
Tschachim



Joined: 26 Nov 2004
Posts: 1272

PostPosted: Wed Mar 15, 2006 5:04 pm    Post subject: Reply with quote

dseagrav wrote:
Maybe the cheat they use to avoid pulse-counting isn't working somehow?

Sorry, I don't understand what you mean? Embarassed
Back to top
View user's profile Send private message Send e-mail Visit poster's website
Display posts from previous:   
This forum is locked: you cannot post, reply to, or edit topics.   This topic is locked: you cannot edit posts or make replies.    Meadville Space Center Forum Index -> Programming All times are GMT
Goto page Previous  1, 2, 3 ... 11, 12, 13 ... 16, 17, 18  Next
Page 12 of 18

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group