OHM Precession MFD

[hribek]

Thanks for the observations. The CG shift was primarily a hack (not touching the visuals at all), and I found it to be of little help if atmo flight was involved.

That depends on what vessels you use it with! Just make the mesh adjust as it should and it will work fine. Vessels like stock DG just don't have an aerodynamic model that puts CG shift to good use. Try it with an XR vessel, it will work there. There may be others.

I imagine that if a vessel's lift/drag model consists only of a function that returns lift, drag and torque based on AoA, dynamic pressure and so on, it will not be enough for the CG shift to make any difference. Does that make sense to you?

 

[Wishbone]

I'll try to make the CGS function more user-friendly as time allows, thanks for the input. And yes, aerodynamic models of most (all?) ships don't know if CoG was shifted. Orbiter sure does, at least for RCS and aerosurface torques.

 

[hribek]

Doug said that the XRs actually shift the center of lift for the wings :lol:

 

[Wishbone]

A long-awaited (by me, at least) update: you can specify aimpoint offset in the Intercept page, so you should (ideally) arrive at a point say 300 meters below the target. Rendezvous speed matching predictions are now operationalized into steering directions. Works like a treat with BTC 2.0 in the other MFD (later BTCs are way too buggy for my liking).

 

[jgrillo2002]

Any idea how the intercept mode works. I read the manual but I just dont get it

 

[Wishbone]

At the moment I use the following sequence: when in phasing orbit, fire up Precession MFD, push TGT, enter target's name. Set NOR to say 20 to see if your closest approach comes within the next 20 orbits. If inclination is more or less aligned, the closest approach will be on the order of altitude difference between you and the target. Then press WAK, enter number of orbits to intercept minus 3 or 2 (let's say "17 revs"), set high time acceleration and wait till you are woken up by Precession MFD. Then, for the actual burn, the easiest way is to use IMFD. Set Target Intercept, enter target name, set TIn with SET button to approximately the time to intercept shown in Precession MFD, vary TEj and TIn to get the lowest total dV, and do the ejection burn within IMFD.

Afterwards, power off IMFD and bring up Burn Time Calculator. Set Target, set offset to your desired standoff distance. In Precession MFD after IMFD burn the closest approach will be on the order of a few kilometers, set NOR to 0.5 (assuming you're half an orbit from the rendezvous), and set OFF to whatever aimpoint you need - if you're arriving ahead of the target (V-bar), set 0 300 0.

Now use RCS in linear mode to null closest approach distance that shows in the top line. Once you've done that (and be prepared for further minor corrections along the way), note the dV value from Precession MFD and enter it into Burn Time Calculator, then press ARM. Once that is done, you can use PITCH UP/DOWN, YAW LEFT/RIGHT indications from the bottom line to orient for rendezvous burn.

Oh, and by the way. The minimum realistic miss distance that shows up in the screen is on the order of 0.3-0.8 meters, finetuning by RCS with Ctrl key pressed down.

It also makes sense to reduce NOR value as intercept time nears, since sometimes the optimizer locks on the wrong local minimum, from the initial guess at the middle of the interval.

 

[Cras]

Wishbone,

I am very interested in using your method trying to get the Shuttle (Davids Shuttle Fleet 4.7), to the ISS. Never crossed my mind to use IMFD to do rendezvous.

Could you elaborate, or more like take me through it in more baby steps. I have no experience thus far using either Precession nor Burn Time Calc, but I think I got the basic feel for it follow your notes.

One thing I have to ask, is what do you mean by using RCS to null the closet approach distance. I understand wanting to clear the residuals fromthe IMFD burn, but how do you know which directions need nulling out.

Ideally I would like to get the Shuttle around 650 feet under the station on the R-bar. My first attempt got me on the r-bar, or pretty darn close to it, and i must admit I was a bit surprised at how close I got to it to be honest, not knowing really what I am doing with these two new MFDs, and I was about 25000 feet under the station.

 

[Wishbone]

Currently I use RCS quite inefficiently, literally groping in the dark by small bits in all six directions and looking at the increase or decrease in closest approach distance. Three rows marked +X +Y +Z etc. represent the expected increase/decrease of miss distance when you're aligned prograde (+Z) and add 1 meter of dV along the corresponding axis.
This is like numeric optimization, one does "steepest descent" to the point of zero miss distance. I'll try to make it simpler in the next build since the machine can calculate the direction and magnitude of the optimum corrective burn.

 

[Cras]

Ok then, as my efforts with the RCS was very much as you described them, I just sort of fired away in all directions trying to get the number to go down.

This is a fantastic and powerful tool I might add. Wish I had found this earlier when I messed about getting the DGIV in a GEO-stationary orbit. I can see having a lot of fun using this and burn time calc, adds a more finer touch to orbital ops than the procedures I have been using

Thanks for the reply

 

[Wishbone]

An update of work-in-progress. This morning managed to find the direction that corresponds to the instantaneous correction vector and display it in the screen. Things to do:
a) find the "best" time to do the MCC in terms of total dV;
b) ditto for correction dV only (handy when you're doing a kinetic kill);
c) let the user choose time for the MCC, which incidentally fixes the problem of jittering optimal direction;
d) select engines for automatic burn (main/rcs fore with attitude maneuver/multi-axis rcs without attitude maneuver), based perhaps on fuel expenditure or user-specified thresholds.

 

[Wishbone]

Meanwhile, a grave geometric error has been detected, effectively rotating the Earth 90 degrees (conversion to right-handed KOST found necessary, by swapping y and z components of position and velocity vectors). Am sorry, will be fixed in the next build.

EDIT: thus, the claims of accuracy were exaggerated.

 

[jgrillo2002]

dumb question but I saw this sentence on your description of the addon

Have you ever wanted to set up a perfect Sun-synchronous orbit, or a Molniya-type HEO that would not suffer from perigee rotation?

How would set up something like that or at this case a geostationary orbit and those other two orbits using the precession MFD? Read the manual but didnt describe how.

 

[Wishbone]

Molniya: the inclination is 63.4 with some decimal degrees, so first you have to launch into this inclination. Now watch ArgP predicted figure on Page 1. By RCS or OMS burns you can gradually move it to zero.

Sun-synchronous (I assume you're flying with inclination >90 degrees): make precession of the orbit (top row left number on Page 1) match the number on the right, by a judicious combo of normal, anti-normal, prograde and retrograde burns. To get your ground track re-visit requirement fulfilled, push NDY, enter, e.g., 1 (1 day between revisits), and then NOR (set it to 14 or 13 or whatever number of orbits there should be between ground track repeats). There will pop a number saying prograde or retrograde or synchronous. If it says prograde just burn prograde until it is zero, same for retrograde. If you're flying a Molniya type, set NDY to 1, NOR to 2 and you're ready to finetune just the same. If it is a geosynchronous orbit, simply match orbital period with the planet's Sidereal rotation period.

This is predicated on having Non-spherical gravity on...

 

[jgrillo2002]

I have that off so will it do problems?

 

[Wishbone]

For true sun-synchronous orbits - yes, there'd be a problem, if you want to keep the track in sunlight for a while. For anything else - not much, since without oblate Earth you'll have no problems with rotating perigee.

 

[jgrillo2002]

well im only interested on Geosync orbits. the MFD works but my orbit isnt circular but its synced. kinda odd

 

[Wishbone]

It is synchronous but not stationary. The MFD itself grew out of this page which grew out of my small orbit design tool...

 

[Wishbone]

Here goes the "update" with the geometry problem corrected and instantaneous intercept burn "optimization"...

EDIT: the DLL goes into Modules\Plugin directory.

 

[Cras]

The update is giving me a pop up message.

Unable to Locate Component

This application has failed to start because libnlopt-0.dll was not found. Re-installing the application may fix this problem.


It occured on loading Orbiter. I downloaded the zip, replaced the old dll with the new one. When I uncheck Prescession MFD in the modules tab, no problem. The second I check the box, the pop up shows up. I am going to try and use it anyhow and see if it still works.

EDIT

No it doesn't. It doesn't show up in the sim. I also re-downloaded it to see if that was the issue, but no-go.

 

[Wishbone]

Attaching libnlopt-0.dll... It's :facepalm: time... The OHM version will be updated as time allows.

EDIT: put this DLL where lpsolve55.dll is (in Orbiter root folder).