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Author Topic: Direct ascent  (Read 2753 times)

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Offline DocHoliday

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09 March 2004, 11:18:54
Hey all,

How would I execute a direct ascent and rendez-vous with a space station from Earth?

I do remember they did it on Apollo missions to launch the LM and directly dock it to the CSM on
the first orbit, but how would you do it on Earth with ISS for example?

In theory you have two problems:
1. the relative plane alignment has to be as accurate as possible, that is no problem, you just sit
tight and wait.
2. the ISS has to be at the right location at the same time that you have the minimum RInc, which
is a bit more rare, but I guess that too would happen if you waited long enough. :)

Assuming you have that. What then??? Where exactly would the ISS have to be and how would
you launch correctly. With the Sync MFD?

Anyone have any experience with this?

Cheers,


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Offline harmsway

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Reply #1 - 09 March 2004, 13:43:32
This all depends on what ship you are flying. A crude method to fiqure this out would be to launch
your chosen ship to the same orbit that the ISS is in and watch your time. So now you know the
time needed to reach station with the same orbital elements.

For the next launch do what you said, wait. Only this time you will launch the proper time before
station passes over your launch site. Now do the math, fiqure out what distance station would
have to be from your base in that time frame. That's when to launch. This takes care of the sync
problem and you can always align plans on the way up.

Of course we are back to your orginal question. Exactly what is that distance?

Gene


Offline harmsway

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Reply #2 - 09 March 2004, 16:29:37
O.K. I just took my own advice and tried this crude method. Using the scenario "DG to ISS" in the
checklist folder I launch from Cape Canaveral in the Delta Glider and discovered this. By the time
the DG made it to an orbit similar to the ISS the ISS had moved some 40 degrees in it's orbit.

Thus I reloaded the same scenario and waited for the ISS. The Cape is at 80W so the ISS needed
to be at 120W before I would launch. I then launch with a heading of ~ 135 and align planes on
the way up. Whew...This is kind of crude so I didn't get quite the direct flight you are asking for but
I did end up within 350K from the station. From this distance I was able to close in and dock to the
ISS before I made even my first orbit around Earth. Cool.

I found myself shaking hands with the ISS crew when I was over Alaska. Not to bad. I'm sure if
you refine these numbers a little better you could get there within half an orbit. So much depends
on the skill of the pilot getting into the proper orbit the quickest way possible.

Thanks for the challenge
Gene


Offline harmsway

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Reply #3 - 09 March 2004, 19:49:42
The first time I came up a little bit short. So on my next try I launch when ISS was 122W.

This time I came to a parking orbit just 183k from ISS. From here it didn't take long to close in and
dock. From launch to ISS dock it took a mere 45 minutes and I was just passing the southern tip of
Africa.

Gene


Offline harmsway

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Reply #4 - 09 March 2004, 20:06:58
A launch with ISS at 120W put me within 350k, then
A launch with ISS at 122W place me with 183k, well that's 167k closer

Therefore a launch with ISS at 124W should place the DG within 20K of ISS

Give it a try, let me know

Gene


Offline DocHoliday

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Reply #5 - 10 March 2004, 08:42:39
:) you obviously got a kick out of this!

Thanks for the experience sharing. I haven't tried it in a planned way before, I only managed to
get it right occasionally. I really got the idea from Dan's IO scenario with the DG, when you need to
pick up Joe Drunk from orbit.

What I would like is to come up with some sort of theoretical model, equipped with sufficient
formulae to be able to calculate the thing for any planet and any orbit. So I guess we're talking
angular velocity of a target vessel, which depends on it's orbit, which in term depends on the its
altitude and the mass of the planet.

Now the real challenge as you also said is to get the orbital planes aligned (which can be easy
enough, with the right launch time and azimuth) and to put the ship into a proper orbit. Which is
my next question. How do you go about targeting your periapsis and apoapsis? I usually just
takeoff and take whatever orbit I get into, or use the dg's launch program. I don't have an
adjustable pitch program to be able to get into any kind of circular orbit (fuel permitting of course).
I tried to, but I end up with a need for an iterative formula for pitch adjustment, because the pitch
angle determines the vector of the thrust and this has to be constantly adjusted depending on the
current state of the ship (alt, pitch, vertical and longitudinal component of speed).

I'm low on time, but I will give all this a try, as soon as I can.

Thanks again,
Janez


~~~

"Mood is a matter of choice. I choose to have fun!" -Vidmarism No 15

Offline harmsway

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Reply #6 - 10 March 2004, 16:00:18
After further trials I have not been able to improve upon my above results. Another words I am not
able to get much closer then a couple hundred kilometers yet. But I am able to reproduce this and
get a pretty much direct flight to Station. This means I can be docked within 45 minutes to an hour
after launch. Remember this is only being done with the DG.

Now to answer your question. I always pictch up 60-70 degrees until I'm out of the atmosphere.
Then I begin to slowly bring the pitch down to build up velocity. In the case of ISS which is well
above 300k you have a lot of climing to do. So don't flatten out too quickly. I then will watch the
apoapsis in the orbit MFD. The apoapsis of the ISS is 6.740 so before mine reaches 6.700 I begin
to flatten out and push to get into orbit. At this point you will be up close to 300k in the surface
HUD.

This takes some experence but you want to end up in orbit with your apoapsis close to 6.740. I
normally change to the Docking HUD even before I get there. If by chance the station is within a
few hundred kilometers you can immediately begin closing in on station.

To target any station, just bring the station elements up in the orbit MFD. This will tell you what
the stations apoapsis is. Now allow your ship to climb setting that apoapsis as your own.

One last thing. I assume you know this but for those that don't. To align to target you simple have
the align MFD up during the whole ascent. A little common sence will clue you in on the intitial
heading. Don't let yourself get suck into chasing the Rinc though. No matter what you do you will
always have some 2 or 3 Rinc remaining. Normally you would clean this up latter, but with a direct
ascent leave it be.

Gene

« Last Edit: 10 March 2004, 16:00:18 by harmsway »