TransX Unable to intercept ISS; dV not stable

[Kaito]

I apologize if this has already been covered; I have not been on the forums for a while, and I could not find a topic on this.

I am using the latest version of TransX ([ame="http://www.orbithangar.com/searchid.php?ID=3039"]TransX[/ame]) with Orbiter 2010p1. I have Gravity Gradient Torque, Radiation Pressure, and Non-spherical gravity sources turned on.

What I am trying to do is use TransX to intercept the ISS. I am following the tutorial on youtube: Planes are aligned, below the ISS, so I bump up the Prograde to overlap the ISS orbit, and then I start changing the launch date. Sometimes, it's about 83k seconds, but fine. Hit VW to get the alignment right, fine. dV, 80, fine. Fast forward time...by the time my launch window has arrived, dV is now at 1.3k. When I used TransX with 2006p1, this problem did not occur.

If someone can help me with this, I would be very grateful.

Thank you,
~Kaito

 

[edsupagood]

I believe Non-spherical gravity and Gravity Gradient Torque are behind this, try to turn them off and see if it helps.

 

[Tex]

It's probably only Non-spherical gravity sources. Gravity gradient torque doesn't mess with your orbital parameters much, just the vehicle orientation.

 

[Kaito]

Yup, turning off Non-spherical gravity sources and Gravity Gradient Torque solved the problem. (sigh) I was hoping I would be a better orbinaut by having those on, only to find out I have to turn them back off...

 

[Wishbone]

Am afraid you are using a very very wrong approach. First off, you should launch your craft IN ISS ORBITAL PLANE. That's what Launch and Map MFDs are for. Next, you enter a phasing orbit with RInc on the order of 0.1-0.7 degrees and start catching up with the station (it is wasteful to enter a higher orbit and then decelerate). Here the best way to time a transfer burn is with IMFD by changing ejection and arrival times to minimize total dV. TransX is only useful after the transfer burn has been done and you want to do corrections to arrive to the velocity matching burn as close to the ISS as possible.

 

[Kaito]

Am afraid you are using a very very wrong approach. First off, you should launch your craft IN ISS ORBITAL PLANE. That's what Launch and Map MFDs are for.

I might not have made it clear, I already did that. Sorry for the confusion

Here the best way to time a transfer burn is with IMFD by changing ejection and arrival times to minimize total dV.

Im using TransX for this part, not IMFD


Now, I guess a for a follow up question: How would I use transX to intercept the ISS with Non-Spherical Gravity on?

 

[flytandem]

Yup, turning off Non-spherical gravity sources and Gravity Gradient Torque solved the problem. (sigh) I was hoping I would be a better orbinaut by having those on, only to find out I have to turn them back off...

I just did a run at the ISS with non-spherical on. A bit like golfing with a lot of winds changing the ball trajectory. But it does work, just a bit tougher than with non-spheric off.

I set up a preliminary maneuver and had an approximate time and amount. As I did the 3 or 4 orbits toward maneuver time the ClApp did wander around a lot going from a few meters to 20 Km or more. No worries,... just got to 10 minutes before the maneuver and toggled it off and on and reset it. It still wandered but not too much in 10 minutes. I did a slight adjustment of amount about 60 seconds before the maneuver. After the maneuver it had my ClApp at about 1.5 Km which was to happen in about half an orbit. Then nudging the LIN on the fly took just a little bit to get it to a few dozen meters as I was then only 5 minutes from ClApp.

 

[Kaito]

I just did a run at the ISS with non-spherical on. A bit like golfing with a lot of winds changing the ball trajectory. But it does work, just a bit tougher than with non-spheric off.

I set up a preliminary maneuver and had an approximate time and amount. As I did the 3 or 4 orbits toward maneuver time the ClApp did wander around a lot going from a few meters to 20 Km or more. No worries,... just got to 10 minutes before the maneuver and toggled it off and on and reset it. It still wandered but not too much in 10 minutes. I did a slight adjustment of amount about 60 seconds before the maneuver. After the maneuver it had my ClApp at about 1.5 Km which was to happen in about half an orbit. Then nudging the LIN on the fly took just a little bit to get it to a few dozen meters as I was then only 5 minutes from ClApp.

Thank you flytandem, that helped a lot. I was worried that I was missing something
I can now fly in peace

 

[Bonanza123d]

I barely use any tools for my intercept. I am able to get it on the dot every time with the orbit mfd and a calculation in my head. Barely any fuel consumption too!

 

[flytandem]

I barely use any tools for my intercept. I am able to get it on the dot every time with the orbit mfd and a calculation in my head. Barely any fuel consumption too!

Love it!
Reminds me of my first day with Orbiter back in 2004. Had no patience for all the tools but did look at Map and Orbit and just went round and around until I docked. For me the problem was not even knowing about the HUD and the ISS transponder so I had to get close enough to see the ISS out the window. Each pilot has his her own cup of tea. Mine was to use a more planned approach and TransX seemed to fit my tastes.

 

[Bonanza123d]

Love it!
Reminds me of my first day with Orbiter back in 2004. Had no patience for all the tools but did look at Map and Orbit and just went round and around until I docked. For me the problem was not even knowing about the HUD and the ISS transponder so I had to get close enough to see the ISS out the window. Each pilot has his her own cup of tea. Mine was to use a more planned approach and TransX seemed to fit my tastes.

When you first start orbiter, people know Sync MFD for use. Since i know the velocities of me and the ISS.All i do is a simple burn and do some adjustment burns to get to the ISS. Final result: 30K

 

[jthill]

people know Sync MFD

That's still what I use. Get myself in a 200km circular orbit, shoot for target apoapsis intercept and tweak to arrive at tg periapsis when my next tg apoapsis arrival is early by half the difference in orbital periods. The burn to raise ApA lengthens the orbit by exactly that much. I play with all the oddities, nonspherical, pressure, whatnot. The feeling of being in the grip of forces far beyond any actual control is astonishing. Sailing without a keel and _still_ nailing the approach tg apoapsis angle can wander a lot, but it only translates to a few hundred meters of distance. Once you burn to match orbit periods at rendezvous the easy part's over.