Question Simple question,which MFD is more accurate to intercept vessels?

[downloaderfan]

Hello guys,
I wanted to know which MFD provides the most accurate solution to intercept an orbiting vessel.Since IMFD's map cannot be used to see our closest approach to orbiting vessels.

So far i have used transX,encounter MFD and IMFD's target intercept program.

TransX and encounter MFD seem to provide the same accuracy,IMFD's target intercept program seems to be more accurate than transX.

The ones which i have not used is rendezvous MFD and Precession MFD.

I would really appreciate if anyone can share his/her experiences with the above MFDs.

Thanks.

 

[Belisarius]

I'm too stupid to understand TransX so I always use IMFD. The rendezvous is very good with this, good enough for me to switch to manual on approach and dock without any other maneuver.

So IMFD works fine for me. Can't really compare.

 

[ADSWNJ]

I'd suggest just using Align Planes, Sync MFD and Docking MFD to start with, until you are really comfortable with the orbital mechanics and you can match a slightly elliptical target orbit (e.g. ISS's) precisely to come up on station with less then 20 m/S DV.

Nothing against the one-burn and hey presto you are there, but I think the fundamentals are important too, especially in your first 20-30 docking attempts.

 

[Andy44]

Rendezvous MFD works great, but you have to have your orbit plane at a low angle to the target's first, and be within a few degrees of true anomaly before the Hill's equations can do their magic. [ame="http://www.orbithangar.com/searchid.php?ID=1199"]Rendezvous MFD 050621[/ame]

 

[downloaderfan]

I'd suggest just using Align Planes, Sync MFD and Docking MFD to start with, until you are really comfortable with the orbital mechanics and you can match a slightly elliptical target orbit (e.g. ISS's) precisely to come up on station with less then 20 m/S DV.

Nothing against the one-burn and hey presto you are there, but I think the fundamentals are important too, especially in your first 20-30 docking attempts.

Thank you for your suggestion ADSWNJ,i have executed the docking procedure many times,i think i have a good understanding of the fundamentals.(Its only an assumption)I understand the importance of fundamentals while using sync MFD.

I've used sync MFD in combination with transX(its more easier that way,as sync MFD doesn't consider the altitude),whenever i needed to save fuel.But most amount of fuel can only be saved with better accuracy,so i decided to move one step ahead.

Thanks.

---------- Post added at 04:56 PM ---------- Previous post was at 04:26 PM ----------

Rendezvous MFD works great, but you have to have your orbit plane at a low angle to the target's first, and be within a few degrees of true anomaly before the Hill's equations can do their magic.

So rendezvous MFD is the most accurate MFD to intercept vessels in orbiter?How good would it be before a few degrees to the true anomaly?

 

[Cras]

rendezvous will get you there, but like it has been said, you really need to use it right otherwise you are just gonna end up performing a really horrid rendezvous.

IMFD can do a pretty good job, but on its own, it is pretty hard to control how you will approach the target.

Precession MFD used in conjunction with IMFD works pretty well. You know you are on the last orbit of phasing, use IMFD to intercept, and then fine tune it using Precession MFD, because you can target the offset and trim for that easier than with IMFD.

Or, use Sync MFD. Just need to do a little math to determine the DTime for each pass but you can pretty much do a perfect rendezvous with that.

 

[sorindafabico]

After orbital alignment, I have a procedure with three stages:

sync+transx - sync to track encounter time, transx to track encounter distance.

rendezvous+imfd - rendezvous to fine-tune the approach, imfd to kill relative velocity.

pursuit (for auto docking) or pursiut+docking (for manual docking) - pursuit mfd has a nice docking autopilot and can help manual docking too - you can check axis alignment with it.

 

[Cras]

http://livestre.am/4u6cj

This is a recording of a rendezvous and docking that I streamed. It is not polished video, but you can get an idea of how it can be done. This is a full STS ORBT rendezvous, I aimed for the same waypoints as the real Shuttle did, as well as real corrections.

 

[downloaderfan]

Thanks a lot guys for sharing your views and experiences.I tested all the MFDs which can used to intercept vessels in two different scenarios-One in low earth orbit,other in high earth orbit.All the realistic parameters were on.

I wanted to make least corrections as possible to intercept the vessel.

Plane alignment has to be very low,less than 0.1 degrees.

Rendezvous MFD has a good autopilot,but it is very difficult to perform an intercept using least fuel with rendezvous MFD,we have to find the time to intercept manually for which we consume least fuel.And finding it is proved much difficult to me as it was not as convenient as IMFD to do the same.

The best way to intercept a target as per me:

Use sync MFD to find the orbit in which u have the least DTmin.
Once in that orbit,use IMFD's target intercept program,adjust TIn so dv hits a least value and hit autoburn.
Then u do tiny tranlation corrections regularly to correct ur trajectory by watching the closest approach in either precession MFD,encounter MFD or transX,all 3 seem to provide same accuracy.
Then u can null your relative velocity with the help of attitude MFD.

If in some case,ur encounter velocity is too high,u can use burn time MFD to find the correct time to burn so as to stop just before the target.It works very well even if ur approaching your target at 4 km/s.

 

[statickid]

Personally I start by having the best launch as possible. Its easy to fly into the correct plane with the wing-to-orbit type vessels, but for vertical launch I think Launch MFD is the way to go to minimize error. After a good launch, sync orbit is always fine for me.

Any rendezvous procedure really just "corrects" the "error" left after launching. The "error" may be by design (dangerous to launch directly to station), or just forced by time constraints (can't wait for PERFECT launch window), but the very best way to conserve fuel is by simply launching correctly, IMO

 

[downloaderfan]

but the very best way to conserve fuel is by simply launching correctly, IMO

Correct,i've always used winged vessels and before i started using launch MFD,my inclination on an average with respect to ISS was over 1 degree after MECO,it just wouldn't go down below a certain value no matter how much i bank.

But after I started using launch MFD and began to launch at the correct time,my inclination with respect to the ISS after MECO was always below 0.5 degrees.

 

[Belisarius]

The best way to intercept a target as per me:

Use sync MFD to find the orbit in which u have the least DTmin.
Once in that orbit,use IMFD's target intercept program,adjust TIn so dv hits a least value and hit autoburn.
Then u do tiny tranlation corrections regularly to correct ur trajectory by watching the closest approach in either precession MFD,encounter MFD or transX,all 3 seem to provide same accuracy.
Then u can null your relative velocity with the help of attitude MFD.

If in some case,ur encounter velocity is too high,u can use burn time MFD to find the correct time to burn so as to stop just before the target.It works very well even if ur approaching your target at 4 km/s.

That's the way I do it, except I tend to do course corrections and retro burns by eye. I enjoy "seat of your pants" style. All the previous about LaunchMFD giving you a best launch window with minimum Rinc, that's good advice too.

 

[Cras]

The key really, is to make it so you intercept your target at the right time.

And that time is at Apogee. I assume you are already in the right plane with your target, Launch MFD will make this easy to do every time. But then make sure your apogee is at the same point as your targets. Then you phase from there.

You will find that when it is time to match up your velocity with your target, you will not need things like IMFD to do so. Your burn would be so small IMFD may not even work. You just use a few puffs of translational RCS to fine tune your position under your target and the rest is a cake walk.

If you do this right, you can literally rendezvous using translational RCS only. You wont even need to touch your OMS. You may use it once or twice just to get the burn done more quickly, but that is it.

 

[downloaderfan]

And that time is at Apogee. I assume you are already in the right plane with your target, Launch MFD will make this easy to do every time. But then make sure your apogee is at the same point as your targets. Then you phase from there.

Yes,interception at apogee reduces encounter velocity if u intercept the ISS from an orbit lower than its own orbit,like after the ascent.

But from a higher orbit,interception at perigee seems to reduce encounter velocity.

 

[Cras]

Yes,interception at apogee reduces encounter velocity if u intercept the ISS from an orbit lower than its own orbit,like after the ascent.

But from a higher orbit,interception at perigee seems to reduce encounter velocity.

I suppose that makes sense, I try to avoid rendezvous from above if I can help it. I much prefer coming in from underneath, for a number of reasons. 1, I am not wasting fuel getting higher than I need to be. 2, I can see the target as I approach, with it being set on the black of space. Harder to see that initial dot when you are looking at the Earth. 3, that is how the Shuttle did things, so that is how I do it.

 

[Andy44]

For bonus points, set up your approach such that when you reach you come to a stop a few meters away from it before drifting away again. From this point you can use the RCS to gently approach the docking port. Real approaches are done this way to reduce the danger of collisions.

Also you want to avoid hitting the target with your RCS jet wash to prevent damage; this is also a function of how you set up your approach. I have not practiced enough to do these things perfectly, but it's fun to try.

---------- Post added at 12:43 AM ---------- Previous post was at 12:40 AM ----------

Another thing: Gemini crews practiced making visual approaches by getting into an orbit a few km below the target, and then when the target is in front of and above the vehicle at an elevation of about 40 degrees, they would point the nose at it and thrust forward. This is done best with the sun behind you so the target is nice and bright. I don't think it's the most efficient way to do it, but it sure is fun to do in Orbiter.

 

[Enjo]

but the very best way to conserve fuel is by simply launching correctly, IMO

But if you have SCRAMs, you'll be better off by using them to align planes and gain some "free" DV

Correct,i've always used winged vessels and before i started using launch MFD,my inclination on an average with respect to ISS was over 1 degree after MECO,it just wouldn't go down below a certain value no matter how much i bank.

For winged ascents, you may still use Launch MFD, with its pitch guidance disabled, by pressing PTC button or [Shift + i]. It will retain the facility of Rinc minimization from both sources - inclination itself, and the LAN difference, that you were unable to minimize by banking.
But that's not to say that even for winged ascents, timing the launch doesn't help. Otherwise, the Off-Plane correction (LAN error minimization) isn't able to converge.

 

[downloaderfan]

For winged ascents, you may still use Launch MFD, with its pitch guidance disabled, by pressing PTC button or [Shift + i]. It will retain the facility of Rinc minimization from both sources - inclination itself, and the LAN difference, that you were unable to minimize by banking.
But that's not to say that even for winged ascents, timing the launch doesn't help. Otherwise, the Off-Plane correction (LAN error minimization) isn't able to converge.

Yes i only follow the heading in launch MFD,i have my own pattern of pitch angles during the ascent.I guess the little bit of error in LAN is because of different time to reach half orbital velocity,time to reach half orbital velocity changes each time i load up launch MFD even if it was the same scenario.
I have done ascents with 0.00 degrees Rinc wrt to the target after MECO with the help of launch MFD.

 

[Enjo]

The time changes because it's taken from the previous simulation session. But still, although there is a perfect time to launch indicated by 0° off-plane correction, and this assuming that the ascent times are the same, the off-plane correction will adapt to any situation once it's operational, costing some fuel, depending on how big the correction had to be.

And maybe I didn't make myself clear: after pressing the PTC switch you may still enable the autopilot that will leave the pitch alone, and will control the bank for you, so that you may focus on the pitch control only, and some sexy indicators in your XR bird.

 

[downloaderfan]

And maybe I didn't make myself clear: after pressing the PTC switch you may still enable the autopilot that will leave the pitch alone, and will control the bank for you, so that you may focus on the pitch control only, and some sexy indicators in your XR bird.

Yes,i did read the entire manual that comes with Launch MFD after u suggested me to use Launch MFD for the ascents back in the math and physics section of this forum.It almost cancelled out the fuel that i used to waste before on plane changes after ascents.Before i never understood its importance and used it only to get the initial heading,but then after realizing all of its features,i never do ascents without launch MFD.And it totally eliminates our dependence on align planes MFD or map MFD.

Launch MFD=Absolutely awesome and a must use addon.

Really appreciate that you put your precious time in building it.