Advanced Question XR1 Ascending really close to orbiting object.

[Muybonito523]

Hey,

I have been messing around with orbiter the past little while. Something that I thought would be cool, is if I could ascend into orbit right next to the object I want to be in orbit with. In other words, instead of getting into orbit, aligning planes, syncing, then waiting; I was wondering if I could do it in such a way that I ascend and obtain orbit very near the object such as ISS or MIR or another orbiting object.:hmm:

Anyone know how?

 

[Dambuster]

to the forums!

I'm not sure if this is what you're looking for, but try searching for the threads of 'XR2 to ISS in 10 minutes', or similar.

 

[Spike Spiegel]

Correct me if I'm wrong, but I believe what you're trying to do is known as "direct ascent". There's a bit of discussion here:

http://orbiter-forum.com/showthread.php?p=153699

as well as a project with this flight plan in mind:

http://orbiter-forum.com/showthread.php?t=897

 

[Muybonito523]

Hey, Thanks for the terminology Spike, "Direct ascent" was exactly what I was looking for.

The threads gave me some input as well abot launch MFD and such. Any opinions about what works the best is welcome.

Also thanks for the tutorial name DamBuster, I found a tutorial here:

http://www.orbithangar.com/searchid.php?ID=3691

for anyone else that might want it. It helped me get the basic idea. However, it still didn't give me precise information on when to takeoff! I was hoping that maybe there was a way to do it so that you almost come right up underneath the ISS or other station, but that may be asking too much.

Thanks for the help!

 

[Tommy]

However, it still didn't give me precise information on when to takeoff!

It's a little hard to give real precise instructions on that because there is such a major variable - how long does it take to make the ascent? It's hard to get a perfectly repeatable ascent unless you use an autopilot, and even a small difference ( a few seconds) difference makes a big difference at rendezvous. Remember that the target is moving about 7.5 kilometers per second ( ~18,000 mph) so even a couple seconds variation in ascent time means a difference of hundreds of kilometers in the end.

That said, it's not impossible by any means. As it is with most flights, timing is essential to efficiency. You'll want to be a bit picky about launch windows - both the station's position and it's orbital plane need to be suitable. Theoretically, a direct ascent is possible once per target's orbit (Just before the target passes your longitude - how much more depends on target's period and how long it takes you to get into orbit, etc). Realistically, it's best to choose an opportunity when the planes are aligned as closely as possible as this lowers the RVel at intercept - it's no good to fly by because you don't have the fuel to null the RVel.

What you want to qualifies (IMHO) as "advanced", and you should get your ascents down to where you can reliably fly the same profile before attempting this (unless you are using an ascent AP such as the DGIV's.)

If it sounds as though I'm trying to discourage you, I'm not! It just takes practice. Expect to fail at first, but alway try to understand WHY you failed. The flight recorder is your friend - you can review a flight and find mistakes you didn't catch in "realtime".

ExternalMFD is also your friend - the more info you have the better. I'll typically have IMFD (using Surface Launch slaved to Target Intercept) open in one MFD, IMFD's Map open in the other, and SyncMFD and SurfaceMFD (changing to Orbit) open in externals.

Good luck, and let us know if you encounter any specific problems. Try to give us as much info as you can (screenshots can be helpfull), and there are many people here who will try our best to share what we've learned.

 

[TMac3000]

For direct ascent to ISS or Mir, this is how I do it in a DGIV:

Wait until the target matches your longitude, and is about 3500 - 4000 km behind you in its orbit. Then take off and launch into orbit using the 230 km autopilot PRO903SPECnn. Note: For ISS, the heading you use will be either 042 or 132 depending on whether ISS is in the northern or southern half of its orbit. If you do the ascent right, you should reach orbit within 1000 km of the station.

After reaching orbit, select Synch Orbit MFD, and turn normal or anti-normal as needed to reduce RInc. You need not wait for a node. Do a burn until until your RVel cancels out (don't worry about getting RInc to zero; as long as your Y-axis burn reduces RInc, it will gradually straighten itself out).

Now tune your COM/NAV to the station and select the docking MFD. Use translational thrusters to put the negative-V cross on the target box, and continually make small adjustments to keep it there. Pick an approach speed you are comfortable with, and use forward/back translation (you might have to use main/retro engines at first) to maintain it.

Keep making braking burns and translational adjustments until you reach the station. You know what to do from there:thumbup:

 

[Muybonito523]

Hey thanks for all th input guys it has really helped.

Currently, I can do pretty well with the Xr1 and Xr2 when doing direct ascent.

Now I am trying to do it with the shuttle endeavor from Shuttle Fleet. I've mastered the takeoff almost, getting the incline to be minimum, getting the ISS station to be within 200-300k. My problem is that I can't seem to get it were my shuttle is in orbit near enough to the ISS to make burn corrections and approach it, that is, without losing all fuel in the process. It seems to always lose track and ISS seems to blast ahead of me every time.

Mainly the frustrating thing is I have the Sync orbit MFD open, RInc around 0 and .01, and there is an intersection point where ME:550 ISS:600, something like that. No matter what type of manipulation I can think off, the numbers are always 50 off, prehaps I have forgotten how to manage sync orbit, but it just kills me that no matter what I do, those numbers stay out of sync. Any ideas at what I am doing wrong, it's probably misuse of the sync orbit mfd lol.

Thanks for all the help!

---------- Post added 05-13-10 at 04:58 AM ---------- Previous post was 05-12-10 at 10:04 PM ----------

After many failed attempts, I finally got my shuttle to the ISS station in just under one orbit. I did it twice in a row, both successful so I guess it works.

I wish I could share the recording of the second one, but I don't know how to make movies. If anyone knows ow and how to make comments within them, please let me know.

Basically, the key factor is that I want the ISS to pass me up some during the middle of my ascent. If in front of ISS at the end of ascent, it's pretty much a fail because the shuttle doesn't have enough fuel to do the required burns.

I took off with the ISS going at the near 040 direction, just when it was over south america. My ascent was pretty standard, at least it matched what the automated ascent in Shuttle Fleet would do, except that I was doing it manually.

#1 When my ApA reaches target altitude, I kill engines, but do not yet jettison external tank, which at this point has almost 10% fuel left. Don't worry I get rid of it later.

#2 Using the external tank, I then do as TMac3000 suggested with the -V and +V on the ducking HUD. I made the cross match up and sometimes lead the ISS station and kept accelerating towards it at 400 m/s, this required some manual burns in various directions because it will not stay there by itself.

#3 At around 100k from the target, I would use the remaining external fuel to slow my acceleration toward the target in the docking MFD to 100 m/s or lower because the normal boosters of the shuttle have trouble slowing down when going 400 m/s towards something. I overshot multiple times trying to speed up the process and get their faster, it's not worth it. At this point, my orbit is still an elliptical, headed for a crash course into the atmosphere if no corrections made. So being me and just wanting to be sorta realistic, this is where I dump the external fuel tank so that it goes back and burns in the atmosphere. Then I would match the cross -V in the Docking HUD again because it would get out of sync while I do this.

#4 Now this is the most boring part, basically it takes almost a full quarter orbit to reach the target by using translation, and burns in various directions depending on how good your inclination was after ascent. I usually kept the acceleration 10 m/s above the K distance maximum, in other words, if it was 50k away, I would be accelerating at it at 60 m/s. Magically, by keeping the cross near the ISS and doing these random burns, my orbit would slowly match the ISS orbit, and I would come right up on it.

#5 that pretty much is it, then you just do you're basic approach and what not and dock with the ISS.

Thank you for all your input. It helped alot! It was fun ( minus the many frustrating parts ) figuring it out and was a neat challenge.