Flight Question "Proper" ISS docking?

[fraxudemspas]

I've just installed Orbiter today, I'm going through tutorials and the "checklists" scenarios, and I've actually made it to the ISS!

However, I can't help but feel it just wasn't how it's "supposed" to be done. I'm using the default delta glider, which seems pretty overpowered (made it to orbit with half the fuel left), and I still spent most of my fuel chasing the station. I've flown by it at over 100 m/s a couple times before finally figuring out how to slow down and close orbits. How is this done with real spacecraft? I figured out aligning orbital planes and orbiting below the ISS to "catch up", but what then? After I've closed orbits as much as I could with the sync display, I just switched to docking hud, pointed at the ISS and fired the engines until I was too close, overshot, fired again, flew through the ISS at 50 m/s, turned around and kept at it until I eventually managed to come to a stop meters away.

Also, how do you actually dock? I've got the nosecone open, but aligning with the docking bays is near impossible since the station itself won't stop spinning. is this even realistic?

 

[garyw]

Why not change views to use kill rot on the ISS? I do this to simulate 'free drift' to make docking easier.

 

[fraxudemspas]

That did make it much easier, thanks.

Also, how do you figure out precisely how you should be launching to reach the desired orbital plane? I figure the final inclination depends on the latitude of the cosmodrome and the launch azimuth, but I'm unable to derive the precise relationship between the three.

 

[sorindafabico]

If the ISS is spinning too fast, go to the scenario editor and kill its rotation once (if you want to be a little realistic).

You said you were approaching at 50 m/s - it's too fast. You can do some simple math to get the right time to sync your speed with ISS speed - or instal some preety good MFDs which do the math to you.

Use RCS thrusters to align with the docking port and linear thrusters to the final approach - like the tutorial says.

The Deltaglider is a good learning vessel, but, as you said, unrealistic. Once you managed to do a good rendezvous/docking with the DG, try with Shuttle fleet and Soyuz.

---------- Post added at 11:24 PM ---------- Previous post was at 11:18 PM ----------

Also, how do you figure out precisely how you should be launching to reach the desired orbital plane? I figure the final inclination depends on the latitude of the cosmodrome and the launch azimuth, but I'm unable to derive the precise relationship between the three.

Open Map MFD and look for the groundtrack of the station. Wait until its orbit passes above the launch site and takeoff to the correct azimuth. Open the Align Plane MFD, select the station and try to minimize RInc (it's a bit hard to do manually).

LaunchMFD has an autopilot to this.

If you're using a vessel with native launch autopilot, try this tutorial: http://www.orbiter-forum.com/showthread.php?t=27675

 

[statickid]

on the docking hud use the vector cross to fly to the station. it will tell you your relative velocity to the station. also, you want the cross to be DIRECTLY on top of the station, this means that you are traveling towards it for rendezvous. use linear burns to adjust your trajectory until the cross is over the station. you can travel to it as fast as you want, but don't forget to decelerate well before you get there. even a relative speed of 50 m/sec is quite fast for final approach. also, when you get close, aim for some of the furthest docking windows and not the station, that way, if you can't slow down fast enough, you won't hit it. however, if you can't slow down fast enough you are going to fast hahah.

 

[ADSWNJ]

Hi Frax! Welcome aboard and congratulations getting to the ISS so soon into your Orbiter career. I'll try to give you some pointers to get you thinking in the right direction, but to be honest, if you are comfortable "wrestling down" the ISS by brute force, then you already had a good feel for what you are doing, and with practice and lots of reading of this forum, you will be an expert real soon.

I'm using the default delta glider, which seems pretty overpowered (made it to orbit with half the fuel left), and I still spent most of my fuel chasing the station.?

The default Delta Glider (DG for short) is definitely super-powerful. It's ideal for getting around with the minimum of hassle, where you don't necessarily want to bother with the subtleties of managing fuel to with 5% accuracy. In the Orbit Hangar, you'll find lots of different ships that offer much bigger challenges to get to orbit and docking. Check out the awesome XR-series ships (XR2, XR5) to see how to manipulate SCRAM jets and control max temperatures. Look at the shuttles for very fuel-limited ascent profiles. And check out the Russian ships (e.g. Soyuz) for big challenges with pulse thrusters and very small Delta-V capabilities.

I've flown by it at over 100 m/s a couple times before finally figuring out how to slow down and close orbits. How is this done with real spacecraft? I figured out aligning orbital planes and orbiting below the ISS to "catch up", but what then? After I've closed orbits as much as I could with the sync display, I just switched to docking hud, pointed at the ISS and fired the engines until I was too close, overshot, fired again, flew through the ISS at 50 m/s, turned around and kept at it until I eventually managed to come to a stop meters away.

The real experts in the forum could write whole books to answer thos one paragraph of questions!! I'm by no means a super-expert, but I have done loads of dockings, so I can give you some general pointers and then answer follow-up questions. The process of arriving in close proximity (e.g. 1km) of another ship, in the same orbit, with the same eccentricity, apoapsis, periapsis, argument of periapsis and so on is as sophisticated as you have patience (and Delta V) to achieve. Firstly, you need to launch into a planar alignment that is very similar to the target. Definitely less than 5 degrees, and for fuel-limited ships, try to get it below 1 degree as you achieve orbit. Several tools help with this in the ascent phase - including Orbit MFD and Launch MFD. Try to initially get to a 200km x 200km perfect 0.0000 eccentricity orbit, with the Inc within 5 degrees of the target to start with. Next - use Align Plane MFD to get the relative Inc (RInc) down to zero. You will be turning to Normal or Anti-Normal Attitude with the autopilot as you come to the "nodes" (i.e. the planar intersect axis between your plane and the target plane. Remember "Anti-Normal (AN) on the Ascending Node (AN)" and the opposite (Normal) on the Descending Node. Burn the main engines when the Align MFD tells you to. The perfect alignment gets you to a RInc of zero, and a nice perpendicular positioning of you versus the AN and DN node points.

OK - so you are going round the Earth now in the same plane, but with a different low-point (Periapsis), high point (Apoapsis) and Eccentricity (e.g. ISS is not a perfect circular orbit). As you are lower than the ISS, you will be traveling faster (i.e. faster time T to complete an orbit). So you can catch up with the ISS, regardless if it is even just 10 degrees behind you. Use Sync MFD to achieve a perfect proximity approach to the ISS. There are lots of ways to do this, but I want you give you a picture of how to do this generically, and you can then explore more advanced concepts later. You want to think about "reeling in the target" like you are doing some deep-sea big fishing expedition. You want to have the Sync MFD length (LEN) on a big number like 18 orbits. You'll see the closest orbit will be the last one (unles sby chance you are close to docking). As you are going faster round your orbit, you will gradually catch up with the ISS. This will show up on the Sync MFD with the closest approach orbit (highlighted yellow) moving through the list (18, then 17, then 16, etc). As it comes down to say 10 orbits, then you want to do a PROGRADE burn at the PERIAPSIS of the ISS (look at ORBIT MFD), to raise your Periapsis up to say 50km below the ISS. Your orbit is now egg-shaped, with the furthest away point hopefully aligned close to the same point for the ISS (check AgP is very similar). As your orbit is now bigger, you will take more time to go round the orbit, so your closure to the nearest point to the ISS will now be more gentle. So the number of orbits to closest approach goes out from say 10 orbits to maybe 16 orbits. This is what I think about with the "reeling in of the fishing line" ... as you get your orbital time closer and closer to the ISS, it takes more orbits to close in on the target, but your relative velocity at the closest approach comes down to a much smaller and more realistic number. (E.g. if you get TOO comfortable to hot-dogging up to the ISS at high closure rate, then blasting your mains to come to a screaming halt, then you will be hopelessly lost in a little Soyuz with pulse thrusters.). Back to the reeling in ... continue doing this game of chasing around the orbits to get to within 4-10 orbits of closest, then gently nudge Apo and Peri alternatively up to get closer to the ISS. Assuming you are doing a SH-Peri (shifting your Periapsis), then get your PeA (Peri Altitude) to be identical to the ISS, and then work the ApA up to within 20km of the ISS ApA. Once you get within 4-6 orbits, you want to get the Sync MFD time down to zero on your closest orbit, by doing small translation thrusts to tune your orbit to come to proximity at the perfect time. On the final orbit, you will null out the final delta to the ISS (sub 10 m/s), and you will be on station.

As I said - this is the basic method. There are TONS of enhancements to this once you are comfortable with this.

Also, how do you actually dock? I've got the nosecone open, but aligning with the docking bays is near impossible since the station itself won't stop spinning. is this even realistic?

LOL! Well - this is a tough challenge. Even more so if you allow the target to free-drift. Think what you are trying to do first ... you are trying to align two multi-billion dollar ships in 6 degrees of freedom, to a precision of 2-3 cm. Why six degrees? You have to get your docking port to the right place in relative 3-D space to the target, with translation thrusters (up-down, left-right, fore-aft). You then need to get the right orientantation (pitch, roll, yaw). The problem is - adjusting your orientation also messes up your translation location, so it's a real dance between rotation and translation thrusters to get this to work. FINALLY - if you don't jump in the ISS first and engage an autopilot (e.g. prograde autopilot), then you will have a degree of drift on the target ship that will continually mess up your approach. (Learn from my fail ... I once spent HOURS trying to dock to a piece of the OFSS ship without realizing that you needed to stabilize the target!!).

Have a close look at the Rendezvous MFD and any docking tutorials to help with this. When you complete your first docking, it's just an awesome feeling of achievement!

Hope this helps get you going. Happy orbitnauting ...

-Andrew :thumbup:

 

[statickid]

p.s. the "retro burn" for your relative velocity will be based on the docking vector and not the orbital HUD's retro reticule.

 

[fraxudemspas]

Yeah, I think I've got the docking display figured out, that does make reaching the actual docking bay much easier. This time I made it to within 50 km using orbital maneuvers before switching to visual approach. I killed the station's rotation when I was a couple km out, then went in on RCS, using the HUD's visual guide to close in then aligning my orientation with the docking port using the docking computer display.

Open Map MFD and look for the groundtrack of the station. Wait until its orbit passes above the launch site and takeoff to the correct azimuth. Open the Align Plane MFD, select the station and try to minimize RInc (it's a bit hard to do manually).

Yes, but what's the actual formula for the launch azimuth? Also, how do you determine precisely when you should be launching to match orbital node longitude?

 

[Hlynkacg]

First off, welcome to the forum!.

In the real world this gets very complicated (atmospheric drag, coriolis effect, etc...) but if you really want to crunch the numbers here are some basic formulas you can do on a pocket calculator to get into the ball park.

The problem breaks down into two parts Launch time, and launch azimuth. (direction).

Launch Azimuth = arc sin( cos(tgtInc)/cos(latitude) )

where "tgtinc" is the orbital inclination of your target and "latitude" is the latitude of your launch site.

To calculate your launch time you need to know when the target will be overhead and velocity of it's orbit.

Velocity = sqrt( (gConst * body Mass) / (orbit radius) )

where gConst is the gravitational constant (6.67e-11) body mass is the mass of the planet (5.97219e24 kg for Earth) and orbit radius is the distance of your periapsis from the center of the planet (altitude - planet's radius).

The the rest is dependant on the vessel being flown.

Given the velocity and your vessel's rate of acceleration calculate how long it will take you to reach the target velocity/orbit. This is how much you need to "lead your target" by.

Hope this helps. :tiphat:

 

[blixel]

Yes, but what's the actual formula for the launch azimuth? Also, how do you determine precisely when you should be launching to match orbital node longitude?

I use [ame="http://www.orbithangar.com/searchid.php?ID=2802"]Launch MFD[/ame] for this.

See screenshot:

When Time to Intersection reaches 300 seconds*, I launch and head toward the Req. Heading. The one that is highlighted in White is the heading you want to use. (If the ISS is passing overhead coming out of the south and heading north, you want the northern heading. If it's coming out of the north and heading south, you want the southern heading.) Note that you just use the heading to give you an idea of what direction to aim for after take off. But while riding to orbit, you will want to make adjustments to your heading as needed to keep your Relative Inclination as low as possible.

* Why 300 seconds? Because that is approximately how long it takes to reach 1/2 orbital velocity. Read the LaunchMFD-doc.pdf that comes with Launch MFD for more information about the Time to Intersection.

 

[sorindafabico]

Wow, ADSWNJ post is excellent!

Yes, but what's the actual formula for the launch azimuth? Also, how do you determine precisely when you should be launching to match orbital node longitude?

Hlynkacg explained well, I'll just add the azimuth from Cape Canaveral (the only one I remember now): 42 degrees if the target is crossing the cape heading north.

If you are launching manually, blixel explained very well about the timing. You can do that only with MapMFD too, launching when the station is passing above the launch site (this is not a rule, only a reference point), but take a look on Align Plane MFD to do course corrections when on launch phase and get a low RInc.

LaunchMFD and RedezvousMFD are superb, I would add InterplanetaryMFD, which has an autopilot for matching your speed with target speed (useful when you're close) and A LOT of useful tools. You can find the download links in the "Addons" section of the forum.

But if you want more challenge, go with the standard MFDs. It's more rewarding when you do the hard way.