Question Rendezvous Help

[Grover]

Not a factor. You can always stay low for longer. It doesn't make that much difference in time, and NO difference in fuel consumption.

staying low for longer is what i do as well, but its no good staying in orbit, living in a cramped spacecraft for 6-8 hours longer than necessary, you can always just launch tomorrow if the target is on the far side of earth

Huge waste of fuel. If you get in front of your target, the only way to get back is to go to a higher sma (then your target's) and back down again for rendezvous.

when i said this i just meant to stay in a lower, stable orbit to make sure you are close to the targer (in terms of angles alone). although approaching from behind and slowing down is more fuel efficient, i now realise

Completely unnecessary. You only need to be that accurate during the last ½-1½ orbit. (except for plane alignment. You might save some fuel doing plane corrections early, but not always)

not always completely unnecessary, perbutations can damage your orbit greatly, depending also on where you're orbiting, ive had times where the DTmin can go up by 5-6 per half orbit

If you're using a Deltaglider and don't care too much about fuel economy, you can come in screaming with your tail on fire and disregard all of the above.

or that :rofl:

 

[C3PO]

not always completely unnecessary, perbutations can damage your orbit greatly, depending also on where you're orbiting, ive had times where the DTmin can go up by 5-6 per half orbit

I used to chase those perturbations until I realized that I was just wasting time and fuel. The major part of those perturbations even out over one complete orbit, leaving a small "permanent" change. The change is proportional to the difference in the orbits. (Your and your target's)

That's why I don't clean up the alignment until I reach the final "chase orbit". I usually do a small plane change every time I raise the orbit by burning ~2-5° from prograde, depending on my RInc. I like to watch how RInc changes over one orbit, to judge how it's moving. If you align too early, you're just as likely to make it worse, and you'll have to align all over again.

"In-plane" variations can be pretty much ignored until the last few orbits, because you'll need to adjust the last orbit anyway to get a good approach. Chasing that DT is quite futile, and you're much better off using that fuel to set up a good final orbit.

But the main object is to enjoy yourself. :thumbup:
Any docking you can float away from is a good docking, right? :rofl:

 

[Poscik]

Few days ago I managed to do such approach with all perturbations enabled:

First I checked station's altitude at certain point of orbit. Then I set my ApA to that altitude. In next few orbits, I watched if my ApA matches station's altitude at this point. If not, I raised my ApA to correct height difference. And here some tips. DTmin of ~10 will place you at NC point, after one orbit, DTmin of ~2 will place you at Ti, then DTmin 0 directly under the station. You can adjust your orbit in Sh Apoapsis mode to reach NC, but starting from NC, I used Manual Axis, because I didn't want to meet the station at the other point of orbit. In this rendezvous type, you're minimising RVel as possible. On RBAR I had ~1m/s. Also I had to do small plane change somewhere after NC burn. Usually I'm launching with offset to let gravity do her job, but this time offset was too small.

 

[C3PO]

Few days ago I managed to do such approach with all perturbations enabled:

First I checked station's altitude at certain point of orbit. Then I set my ApA to that altitude. In next few orbits, I watched if my ApA matches station's altitude at this point. If not, I raised my ApA to correct height difference. And here some tips. DTmin of ~10 will place you at NC point, after one orbit, DTmin of ~2 will place you at Ti, then DTmin 0 directly under the station. You can adjust your orbit in Sh Apoapsis mode to reach NC, but starting from NC, I used Manual Axis, because I didn't want to meet the station at the other point of orbit. In this rendezvous type, you're minimising RVel as possible. On RBAR I had ~1m/s. Also I had to do small plane change somewhere after NC burn. Usually I'm launching with offset to let gravity do her job, but this time offset was too small.

Very nice! That's a bit weird looking approach because it starts on the -Vbar. You can't catch up to anything from there. I usually approach on a straight(ish) line below the -Vbar, and use RendezvousMFD to judge the correct time to raise my ApD so that I "top out" on the -Rbar below the station.

BTW on that diagram you can see 2 "large" maneuvers and 5!!! correction burns.

 

[Chub777]

Just watched Tex's DG IV to ISS tutorial. I attempted the same thing. DTmin was 0 and RInc was 0. When I ended up at my rendezvous point, I was ~100 km behind the station. Have I done something wrong? Also the negative velocity indicator did not "float" towards the station.

 

[Mister Mxyzptlk]

Chub, yes you did something wrong but I don't know what. How and when did you execute your burn to match velocity with ISS? Did you try using the velocity match function of the orbital mode in IMFD?

 

[C3PO]

Just watched Tex's DG IV to ISS tutorial. I attempted the same thing. DTmin was 0 and RInc was 0. When I ended up at my rendezvous point, I was ~100 km behind the station. Have I done something wrong? Also the negative velocity indicator did not "float" towards the station.

Is non-spherical gravity on? Are you sure that 100 km was the closest approach? Is your joystick calibrated?
You should post a scenario where the transfer burn is completed.

 

[Chub777]

Is non-spherical gravity on?

No. I turned it on and tried it again. With non-spherical gravity on, DTmin drifts to around 5.00.

Are you sure that 100 km was the closest approach?

106.1 km.

Is your joystick calibrated?

I don't use a joystick, only numpad.

You should post a scenario where the transfer burn is completed.

This isn't when I completed my transfer burn, but it's half an orbit until the DG-IV and ISS intercept:

BEGIN_DESC
Orbiter saved state at T = 22989
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 60246.9257903663
END_ENVIRONMENT

BEGIN_FOCUS
  Ship GL-01
END_FOCUS

BEGIN_CAMERA
  TARGET GL-01
  MODE Cockpit
  FOV 49.53
END_CAMERA

BEGIN_HUD
  TYPE Docking
  REF ISS 0
END_HUD

BEGIN_MFD Left
  TYPE OSync
  TARGET ISS
  MODE Sh apoapsis
  LISTLEN 5
END_MFD

BEGIN_MFD Right
  TYPE Docking
  TARGET ISS 0
END_MFD

BEGIN_PANEL
END_PANEL

BEGIN_SHIPS
ISS:ProjectAlpha_ISS
  STATUS Orbiting Earth
  RPOS 2622786.67 5798602.44 -2145292.71
  RVEL 6950.526 -3302.166 -414.968
  AROT 110.00 -10.00 80.00
  AFCMODE 7
  PRPLEVEL 0:1.000000
  IDS 0:588 100 1:586 100 2:584 100 3:582 100 4:580 100
  NAVFREQ 0 0
  XPDR 466
END
Mir:Mir
  STATUS Orbiting Earth
  RPOS -6601329.69 -53886.60 -871183.20
  RVEL 1014.542 -468.018 -7654.161
  AROT 0.00 -45.00 90.00
  AFCMODE 7
  IDS 0:540 100 1:542 100 2:544 100
  XPDR 482
END
Luna-OB1:Wheel
  STATUS Orbiting Moon
  RPOS 1398262.15 1750891.55 1558.41
  RVEL -1155.828 922.877 1.416
  AROT -0.00 -0.00 119.05
  VROT 0.00 0.00 10.00
  AFCMODE 7
  IDS 0:560 100 1:564 100
  XPDR 494
END
GL-01:DeltaGliderIV
  STATUS Orbiting Earth
  RPOS 2151936.14 5859569.35 -2071586.31
  RVEL 7229.924 -2854.128 -596.041
  AROT -98.73 -68.85 23.71
  VROT 0.00 0.07 0.01
  RCSMODE 2
  PRPLEVEL 0:0.397721 1:0.939699 2:0.983333
  IDS 0:45 100
  NAVFREQ 94 524
  XPDR 0
  ;-------------------------- Skin parameters must contain directory name of skin (without space) 
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  CabinCO2Level 600.0003
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  CabinO2Setting 21.4000
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  CabinPressSetting 14.7000
  AntennaTarget no_target
  O2ConsumptionSetting 1
  FuelConsumptionSetting 1
  MainenginePower 1
  ;------------------Crew parameters  UMMUCREW Function-Name-Age-CardiacPulse-WeightKg (fonction of Pilot must be: Capt) 
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  UMMUCREW Capt-Christopher_Coles-47-60-70
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  UCGO @@0,1,0,0,@@1,1,0,0,@@2,1,0,0,@@3,1,0,0,
END
END_SHIPS

BEGIN_Attachment Manager
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BEGIN_ReFuelMFD
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END

 

[C3PO]

Non-spherical gravity should be off for now. Sorry for the confusion.

Your ApA is almost exactly 100 km below the station's PeA, so you will not intercept it. You need a higher ApA to make the orbits intersect. Put the SyncMFD in intersection mode and try again.

Your PeA is ~200 km, and that makes your relative speed high at interception. You should try chasing in a higher orbit. It takes more time to catch up, but you can accelerate that.

 

[Poscik]

Very nice! That's a bit weird looking approach because it starts on the -Vbar.

No It don't. Diagram starts from here, but as I said - NC is point which is ~10DTmin behind the station. This have to be maneuvred some time earlier.

BTW on that diagram you can see 2 "large" maneuvers and 5!!! correction burns.

MC corrections are not performed if not needed. NCC is always performed as far as I know.

 

[Mister Mxyzptlk]

Chub, There is a "mod" button on sync orbit mfd. You need to click it til it is on "intersect one" or "intersect two" and then do a prograde burn until the orbits intersect. Then once they intersect you adjust your orbit when you are at the intersect point or when you are 180 degrees from the intersect point to get the Sh-Tor and the Tg-Tor countdown timers to read the same. Since there are 2 intersect points you can flip between them and use the one that has the closest set of readings. BUT, and this is a big but, you have to start the process when you still have 4 or more orbits til rendezvous/intersect. In the scenario you posted you are in your last orbit before rendezvous and it is too late get a good sync. Do you understand all of the salient points in my post?

 

[Urwumpe]

DTMin of 5.0 is actually 35 km distance (and thus perfectly close for doing a small correction to intercept the target) not 100 km. Are you sure you are not reading the instruments wrong, for example using the wrong intersection mode?

 

[Chub777]

Chub, There is a "mod" button on sync orbit mfd. You need to click it til it is on "intersect one" or "intersect two" and then do a prograde burn until the orbits intersect. Then once they intersect you adjust your orbit when you are at the intersect point or when you are 180 degrees from the intersect point to get the Sh-Tor and the Tg-Tor countdown timers to read the same. Since there are 2 intersect points you can flip between them and use the one that has the closest set of readings. BUT, and this is a big but, you have to start the process when you still have 4 or more orbits til rendezvous/intersect. In the scenario you posted you are in your last orbit before rendezvous and it is too late get a good sync. Do you understand all of the salient points in my post?

I see. I was using the Sh-apoapsis mode. Also by "you have to start the process", do you mean the reducing of the DTmin?

 

[PeriapsisPrograde]

A quick reference when trying to dock
0.01 Rinc ~ 1 km distance at rendezdous
0.01 Dtmin ~ 80 m distance at rendezvous
1 DTmin ~ 8 km distance at rendezous

You need to be pretty accurate to fly a good approach.

 

[Wishbone]

Or use TransX/KOST 0.75 to predict and minimize miss distance.

 

[Urwumpe]

I see. I was using the Sh-apoapsis mode. Also by "you have to start the process", do you mean the reducing of the DTmin?

That means "The time difference is measured at the apoapsis of your ship". This is NOT the same as the intersection with the target orbit.

before you have intersection, you should use Sh-Apoapsis because it is the closest available. You should better use "Intersection #1" or "Intersection #2" as reference, once your orbit crosses the target orbit. If you are at DTMin < 1.0 at one intersection, your distance will be less than 10 km.

You can correct the intersection easily...just switch to the next intersection, and use RCS for getting it to zero. if your future arrival is behind the target, you need to fire retrograde, so you do a bigger "semiloop" below the target. If you are arriving in front of the target, burn prograde to make the "semiloop" smaller.

 

[Mister Mxyzptlk]

The process I mean is the process of doing prograde or retrograde burns to get the yellow Sh-Tor and Tg-Tor readings to be the same. Those readings represent the time in seconds till "Sh" your vessel, and "Tg" the target vessel will be at the intersect point. When both readings are exactly the same both vessels will be at the intersect point at the same time.

For now forget DTmin., Just make sure that Rel. inc. is 0. Right now I am going to explain a little more regarding Sh-ToR and Tg-ToR so you will understand how to do your burns to get them to be the same and stay the same.

If Sh-ToR is a larger value than Tg-ToR, your vessel is going to be late to the intersect point by however much the difference in value is, and conversely If Sh-ToR is a smaller value your vessel will be early to the intersect point. If Sh-ToR is a smaller value than Tg-ToR then you are early and must do a prograde burn till the values are equal at the intersect point.
Conversely if Sh-Tor is a larger value then you are late and must do a retrograde burn till the values are equal at the intersect point. To really fine tune the burn and get the numbers to coincide you will need to use the translation thrusters on fine control (hold "control" down on the keyboard) when the numbers are getting close.

Once you understand how to fine tune your your intersect time at the intersect point, which will most likely be your apoapsis or close to it, experiment with prograde and retrograde burns at your periapsis. Just make sure you don't use the translation thrusters that would cause you to move normal or antinormal as this will change your rel. inc.

Any questions?

 

[blixel]

RInc is usually 0.0° but DTmin drifts to 0.1.

One problem I had when I was learning to rendezvous was that I would get the numbers all set, and then I'd time warp ahead to get to the rendezvous orbit ... and that would often be at least 3 or 4 revolutions, sometimes many more.

What I found was that during those revolutions, my DTmin and/or RInc would sometimes drift a bit. (I'm not entirely sure why since I don't have nonspherical gravity sources enabled.) But anyway, what I started doing was taking more time with the rendezvous effort.

Instead of blasting ahead several revolutions at a time, I would accelerate time 1 revolution at a time ... stopping at each Ascending/Descending node to RE-CHECK my RInc, and stopping at each rendezvous point to RE-CHECK my DTmin. I would also RE-CHECK the altitude of the ISS at my rendezvous point once per revolution and make sure that my ship's altitude was still aligned.

Since I started doing that, rendezvousing has become a total piece of cake. Now when I show up at the ISS, I am sliding in as smooth as silk.

 

[Tex]

It sounds like you're not setting up your rendezvous point correctly. As I explained in my tutorial, you set your rendezvous point up by burning just after crossing orbital sunset so that the rendezvous point is actually on the other side of the planet in daylight:

 

[Chub777]

Ok. Let me just try this.