Advanced Question NC Burn

[StargazerBranden]

Okay, guys.
First off, thanks to everyone's input on our little Algonquin Round-Table discussion relating to Shuttle launches. Huge help.

Now on to the next astrophysical engineering discussion. This one relates to rendezvous.

The scenario is STS-71. Shuttle Atlantis docks with Soviet Mir space station for the first time. So, I'm in orbit on a catch up from behind trajectory. Apogee is 245.6 kilometers, perigee is 180 kilometers and each orbit I gain a little "ground" on the station. Now eventually I have to set up the NC burn. This is where I have trouble...

I use orbit target MFD and it has a DAP table for rendezvous with different targets. Indy91 explained the NC burn as such:

First I launched the Shuttle and aligned the planes (so RInc becomes 0) and also the line of apsides. This means, that the argument of periapsis (AgP) is the same as the AgP of the target. With all manual burns I made sure, that the AgP is always the same. Then I did a little phasing to catch up with the ISS. When I was not so far away anymore, I did a burn at the periapsis to raise the apoapsis to 400m below the targets apoapis.

Now a few calculations. Sync Orbit MFD only shows distance in time, so to be accurate I need to calculate the time distance for the NC burn, which is done ca. 245000ft (=74676m) behind the target. With the average velocity of the target e.g. 7700m/s (if the orbit is nearly circular you can use the current velocity) this is 74676/7700 = 9.7 seconds.

You should achieve this number with a burn at the apoapsis one orbit before the NC. You can use Sync Orbit MFD in Ship apoapsis mode or target apoapsis mode (its the same if your AgP is the same). The NC burn should aim for 48600 ft, so divide this number (in meters) by the targets velocity which results in e.g. 1.92 seconds. Your NC burn was perfect, if the line of apsides is still aligned, you are 400m below the targets apoapsis and you will arrive 1.92 seconds behind the target.
The NCC burn in my new scenario would have been only 0.02 feet/second, I was very surprised how accurate I was, because the script calculates more digits after the decimal point than i can count.
This is the method I always use for rendezvous. I still have to do more burns and adjustments (mostly for the AgP alignment) than the Shuttle would do, but the results are always good.

tl;dr

Use 9.7 and 1.92 seconds in Sync Orbit MFD.

[/QUOTE]


Now, I can align the Argument of Perigee well enough. Just a simple prograde burn when Mir's ApT reaches zero until my AgP equals Mir's AgP. As for the rest, however.... I could use some help from those familiar with shuttle operations. Am I using the 9.7 seconds value as the DTmin in Sync MFD or the 1.92 seconds? Or both at different points? When am I doing these burns? He says 1 orbit before NC burn but when is that NC burn? 1 orbit away from Rendezvous? My best educated guess is that I want to keep the DTmin value at 9.7 seconds until I'm 1 orbit away, then on my final orbit adjust DTmin down to 1.92 seconds, then start rendezvous preparations on orbit target MFD. But, as I said that is just an educated guess, more light on the educated part and a little heavier on the guess part.

Surely this Kansas bumpkin will get all this figured out at some point. (I bet I'm a major disappointment to Ron Evans, Joe Engle, and Steve Hawley! Lol!!)

 

[Thorsten]

I'm sorry, can't help you out here since this is a question about a particular tool I'm not familiar with.

In the real Shuttle, you'd use the ORBIT TGT display (SPEC 33) which has the ability to compute the burn parameters (including ignition time) by fitting the predicted trajectory to the intercept condition.

I'm not 100% sure about this, but I also think the real operation is to catch up with the rendezvous target on a lower circular orbit and then do a Hohmann-like transfer for the rendezvous (SPEC 33 usually computes a two-burn solution).

I don't think you need infinite precision though - you ought to be able to bridge a couple of 10 km by eye-balling it.

Edit: Sorry, SPEC 34 is orbital targeting, SPEC 33 is rendezvous navigation.

 

[StargazerBranden]

In regards to what indy91 told me there, and going off what information you gave, Thorsten, and going off of some NASA rendezvous ops documentation that I found, here's what I'm going to try tonight. I'm going to launch, circularize, and align planes and align argument of perigee. Then manipulate the spacecraft with sync MFD to catch up with Mir. Raise apogee to 400m below like he says, then at apogee from 76.2km away (250,000 ft. and that figure is from NASA documentation) bring the DTmin down to 1.22 seconds then at rendezvous start NCC burn with ORBTGT MFD
I'll post my results here, let you guys know if it works, the fuel requirement if it works, and all the technical data. Sorry for all the questions here lately but I'm just trying to get a grip on all of the technical details for the most realistic shuttle ops simulation as possible so I can maybe put together a detailed tutorial for future reference.

 

[ADSWNJ]

A couple of pointers for hand-flying the shuttle up to a docking (and of course - there's standard tools and autopilot modes to do it if you are not into dead-stick flying!):

With the Shuttle, as opposed to overpowered ships like a DG or a XR-2/XR-5, you have a relatively small amount of thrust, so you need to plan your RV approach carefully. I.e. don't have a closing velocity of 200 m/s and expect to stop on a dime!

Your approach looks exactly right: start low and behind the target. Lower orbit means you will catch up with the target, but you want to reel her in like a fish. What do I mean? Simply raise your orbits in steps each time you get an orbit or 2 away from you approach, raise your orbit to make it 2-3 orbits away and correspondingly less DV.

I agree with the point of aligning your periapsis with the target's AgP. This means that you are flying a similar oval loop to the target, and removes one element of dV when you approach the target.

Ideally, you want less than 10 m/s dV when you come up on station.

Have a look at RV Operations if you want. It was not written to be a faithful Shuttle approach path, but it approaches the target very carefully, staying outside of a spherical safety zone and tracking around the zone to the approach corridor direct tot he docking port. From there, it does a tighter and tighter tolerance approach, finishing up kissing the target at 4cm/sec or less. It's stable at 10x as well, if you want to see the speedy version of the same thing. (Oh and you can change the port whenever you want, and set the AP the challenge of backing off to a safe distance, rolling around the safety zone again, and then approaching the new port. E.g. you can do a grand tour of all ISS ports in as short a time as possible if you want!).

Enjoy!

 

[StargazerBranden]

Thanks for that information! Now I'm down to just playing around with altitudes to get my NCC burn with ORBTGT MFD correct and not have massive deltaV

 

[ADSWNJ]

Yup - that's the right approach. If you can chop your encounter dV by half on each burn, you will dial it in nice and gently. Get the apoapsis close to your target alt first, and use the raising of your periapsis to control the rate with which you catch up. E.g. final pre-encounter burn - bring the PeA to within 30km, and you will hopefully have a very gentle encounter dV.

 

[StargazerBranden]

Having a heck of a time getting ORBTGT to work. I get to about 76 km away, .5 km below at ApA, and about 15km PeA, and am getting huge deltaV values. 600 to 700 feet per second. Any ideas?

 

[Poscik]

Hi! 15km is way too low. Look at these charts. These are from STS-135 rendezvous checklists (http://www.nasa.gov/centers/johnson/pdf/567076main_RNDZ_135_F.pdf)


Summarizing - second last orbit has PeA 40kft(12km) below ApA. But last one orbit is only 9kft(less than 3km) below. If you do everything correctly, your final velocity difference will be less than 3 feet per second. You can even stop with vernier thrusters then.

Also notice, that your rendezvous point should occur just after sunrise, but this is detail I think.