Flight Question An Accuracy Problem

[SystemsLock]

Hello! I'm relatively new here.

All has been great so far and I've been really enjoying myself. My first flight took me to orbit, which I did several times, strictly by the book. Now I'm looking to fly to, and dock with, the ISS using the DG.

I followed two tutorials. The one in the manual, which is more down to Earth and simple. And a tutorial I found [ame="http://www.orbithangar.com/searchid.php?ID=3074"]here[/ame], which is more in-depth and explanative.

I had to do it several times, but eventually I was able to execute all the steps properly. I waited, and waited until the rendezvous point, only to find myself off by more than 40km. The ISS was never more visible than a bright spec.

Either I'm supposed to manual maneuver closer to ISS (which I have no idea how to do) or I wasn't accurate enough. Which is hard to believe considering I was very careful. My RInc was 0.08. The guide says it should be within 0.01, which must be really tough because I aligned several times and couldn't get it closer. My DTmin was 5.53. The guide also says this should be within 0.01, which must be a joke because I set the sync orbit MFD to all 20 orbits, I came from below (speeding up), and I was very careful with gentle thrust.

Does anyone know what I might have done wrong? I can provide more flight detail if necessary.

 

[Wishbone]

5 seconds times 7 kilometers per second equals 35 kilometers. Use RCS and always remember space is not the place to be sloppy about a split second.

 

[SystemsLock]

So DTmin was the problem? I kept my thrust at the absolute minimum. The second I saw DTmin rise by a hair, I cut it off.

If it's typical to get it much below 5 seconds than there must be a more accurate method. Should I sync in less orbits? Or should I come in from above (slow down) rather than from below?

And what do you mean by use RCS? You mean like the prograde/retrograde autopilot? I definitely used them.

 

[Wishbone]

Going from above is a waste of propellant if you launch from below. Sync in one half orbit before the meeting, and remember to use Reaction Control System with Ctrl key (1/10 thrust) in LIN mode, operated by Numpad 6 and Numpad 9 keys if you are oriented prograde.

EDIT: Have you read "Go Play in Space"? There will be tons of abbreviations and acronyms from now on, and you gotta learn all of them. Actual astronauts/cosmonauts/taikonauts do, and they pass exams on that stuff

 

[SystemsLock]

Wait... you can use RCS thrusters for burns? I thought you use the main engines! I know you can use them in linear mode but isn't that for docking?

And what do you mean by sync half an orbit before? If I'm 1/4th of an orbit away from the ISS I'd need to sync many orbits (I used the max, 20) out in front or else I would have to be below Earths surface to orbit fast enough to catch up.

I'm pretty familiar with space/orbiter lingo. I've read most of the manual.

 

[Wishbone]

Please reread what I wrote. 1) It was not 1/2 orbit away from ISS, it was 1/2 orbit from the rendezvous. 2) "Go Play in Space" is NOT the manual.

You can use anything for "burns": main engines, RCS, spanners, even misbehavin' crew members (or those who have failed the exams, for that matter).

 

[SystemsLock]

Please reread what I wrote. 1) It was not 1/2 orbit away from ISS, it was 1/2 orbit from the rendezvous. 2) "Go Play in Space" is NOT the manual.

You can use anything for "burns": main engines, RCS, spanners, even misbehavin' crew members (or those who have failed the exams, for that matter).

Well sure you can use anything :lol: I was just wondering if it was typical. From now on I'll do one burn with my main thrusters and then a second with the RCS to perfect it.

Let me repeat myself about the sync orbit. Let's say I'm 1/4th of an orbit away from the ISS and we're traveling at the same speed. If I were to choose a rendezvous point 1/2 of an orbit away from current my position I would have to increase my speed to twice that of the ISS's in order to catch up in time. In order to increase my speed so drastically I would have to reduce the height of my orbit so low that I would crash into the surface of the Earth.

I think I get what you're trying to say, we're just not talking about the same thing.

I appreciate your help.

 

[Tommy]

When he say's "1/2" orbit from rendezvous, he means that you are on the last orbit before you actually rendezvous - not that you are on the opposit side of the Earth from your target.

When we say "use the RCS" we mean just that - use RCS in linear mode (foreward and "backward" thrust - using the 6 and 9 keys). RCS is less powerfull than Main engines, and allows much more precision - and using the <Ctrl> in conjuction with the 6 and 9 keys lowers the power of the RCS, allowing even more precision.

BTW, 40k is close enough. When you get to the point where you stop getting closer to the target, and start moving away, you need to eliminate the Relative Velocity (RVel). This can be done using the Docking HUD display - see the manual (or better yet, "Go Play in Space" - check the tutorials thread for a link) for insrtuctions on how to do this.

Once you have the RVel very low (less than 2 m/s), point your nose at the target and give a bit of thrust - and keep the RVel below 10 m/s. Then you should be able to see your "velocity vector ball" (the "x in a circle") and you can use linear RCS to keep it centered on the target until you get close enough to begin the actual docking process.

Tip : set Nav 1 frequency to the target's transponder, not one of the IDS frequencies. This will let you use Docking HUD at a greater distance from the target.

 

[Rtyh-12]

Well sure you can use anything I was just wondering if it was typical. From now on I'll do one burn with my main thrusters and then a second with the RCS to perfect it.

Personally, I use the main engines for larger burns, and after I get the parameter close enough to what I want it to be I use the RCS thrusters. Works like a charm

Also, do the parameters change while you orbit? I always lower them to almost zero, but by the time I reach the ISS they are already off. This is because of the realism settings. I play with all realism settings on, but I think that "non-spherical gravity sources" is the biggest problem. Have you got it on? The only way to correct it is to keep doing burns as you approach the ISS to correct your path. These are what we call Mid-course corrections or MCCs.

 

[PeriapsisPrograde]

Always try to be as accurate as possible. A rinc of .01 has a worst case scenario of 2km off:
Earth's radius: 6,371 km. Circumference formula: 2*pi*r. That gives an orbit length of ~40,000km. Tan(0.01)*40,000km = ~7km. Your climb from ascending node is only 1/4 orbit, though, so it's 1/4 the value. Still, 0.01 rinc is about a 2km miss.
For every 1 second of time, you are more than 7.5 km off!

 

[SystemsLock]

Yay!

Using the linear thrusters I was able to adjust the numbers considerably closer. The corrections brought me within 2km of the ISS and I was able to dock!

Thank you all for your help. Next stop, the moon!