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Old 12-06-2011, 01:54 AM   #16
dgatsoulis's Avatar

Originally Posted by Tommy View Post
 BTW, how did you get the ascent autopilot to accept a heading with over 2 digits?
I'm sorry, I probably didn't understand the question very well, in my previous reply. The PRO904SPECxxx (hover take-off) and the PRO903... (normal take-off) autopilots, accept a 3 digit format for the heading. Writing PRO904SPEC090 is the same as PRO904SPEC90.
then i waited untill the speed was over 500 m/s
My comment was about the slight correction that was needed after the take off.

Originally Posted by Tommy View Post
 It would be interesting to know how long it took you to complete this, I took 1:45, with 7 minutes on the ground at the crash site. With a bit of practice I could easily shorten that up with more aggressive braking.
Very similar to yours. I think it was about 97 minutes, with 3 minutes on the ground at Antartica.

Originally Posted by Tommy View Post
 There will be a couple of "tricks" in my pdf, ways of using MFD's in ways they weren't really designed for, that will help you with that. The first flight is still pretty tricky.
This will be very interesting to see. I have tried to use IMFD for suborbital flights, but my results were not very good.
The way I go about it, is to try and make a relatively accurate estimate of how much time the flight will take me (in minutes) and then multiply that with 0.25 to see how many degrees to the east my target will "move", by the time I get there. (360 degrees/day ~ 0.25 degrees/minute).
Then I add the result to the target's longitude and find the bearing for the new coordinates.
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Old 12-06-2011, 02:19 AM   #17

When I tried to use the AP for the ascent it didn't seem to accept more than 2 digits - it's possible I have a different release of the DG-IV, or just don't know how to use it!

My "trick" with IMFD didn't work as well as I'd hoped - but another MFD from the same guy (hint, hint) proved very useful for directional corrections.

My launch heading adjustments were "seat of the pants", based on experience - and the heading was fudged "to the right" on both ascents! I also used a lower altitude "skimming" flight so minor corrections were possible.

Should be posting the files in this week sometime.
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Old 12-29-2011, 09:06 PM   #18

So, I haven't had a chance to put together that pdf, and now I can't find the paper with all the math on it, etc.

Still, I'll post the flight recordings, and explain the process I used to figure out the position of the crash site, etc.

ABOUT THE RECORDINGS: The DGIV doesn't record animations, so you will have to operate the landing gear. A note will tell you when. The turbo pump is also not recorded, so more fuel is used on the actual flights. Mission is broken into two flights, and I allowed 7 minutes on the ground at the crash site.

The DGIV's panel doesn't respond to the mouse during playback. You can use glass cockpit mode to set MFD's, etc. Shortly into the second flight (TommyReturn) the low fuel warning is triggered - and you won't be able to shut it off!


My math-fu is weak, so while I'm sure there is a more elegant and efficient way to do this, I just took a hammer to the problem. The second page that dgatsoulis gave (to movable-type.co.uk) was very helpful. A little ways down the page is a calculator that takes a starting location, direction, and distance, and finds the destination.

Starting location is easy. I fire up Orbiter, open the scenario editor, and place a DGIV on Pad one at KSC. Set the heading to 90, and take note of the latitude and longitude.

Direction isn't hard either. I simply add the relative direction provided to the current heading and get 200.495.

Distance is a bit trickier. I have a straight line distance (11799 km), but the calculator wants a geographical distance. I'm sure there's a simple way to convert a chord distance to an arc length - but can't remember how. I'll have to use an intermediary stage - convert to angular distance first.

Imagine, if you will, a circle. I'll place point A the the center. I'll place two more points on the circle (B and C) to represent my starting and ending destinations. It doesn't need to be perfectly to scale, so I'll place them on each side, about a third of the way up. I draw radial lines from A to B and from A to C, and draw a straight line from B to C. This makes an Isosceles triangle. Sided AB and AC are equal to the radius (6371 km) and BC is 11799 km). I need to determine the angle of A.

I suppose I could have googled up a site like this one: http://ostermiller.org/calc/triangle.html - but I didn't. I added a fourth point (D) in the middle of line BC, then add a line from A to D. This creates two right triangles. Looking at triangle ABD I see that I know the hypotenuse (side AB) and the side opposite (side BD, length is 1/2 the length of BC), and sine = opposite over the hypotenuse. My calculator converts that to an angle, which I double to get my angular distance. Then, Angular Distance is divided by 360, then multiplied by the circumference of the Earth to yield the arc length.

At long last, I can input the distance into that calculator we mentioned earlier, and it provides up with the coordinates of the crash site. It also provides our "Final Heading", which I also take note off. Later, I subtract 180 from that and use it for my initial heading on the return trip.

I'll also hit the "view map" link, so I can see my target and course on a map - how nice!


The headings that I have don't account for the Earth's rotation. I could make calculation, like dgatsoulis, or I could be lazy and just guess. The crash site is pretty close to the pole, so it won't move that far. I'll aim about 5 degrees to the right - maybe less, call it 205 degrees even. For the return I'll need a larger offset, about 15 degrees.

It's been quite I while since I drove one of these, so my ascents were far from perfect. I was also rather timid with the braking, and could easily shave at least 5 minutes or more off each leg.

As for navigation, the first leg is a bit tricky. There is no base to target, and the only stock MFD that can target a LANDED vessel is DockMFD! This at least provides us with a distance. AerobrakeMFD isn't much help without a target, but I know about where the site is on the map, so it can help a bit with energy management, but won't be very accurate. As for direction, the solution is BaseSyncMFD. It has the ability to target a co-ordinate, not just a base. One we are up to speed it will help us fine tune our heading. I'll use a skip re-entry method (rather than a single ballistic hop) to allow for heading corrections.

As you can see, I could have been much aggressive and delayed braking - saving time and fuel on the first leg, and more time on the second leg. The first landing (hover) took longer than it should have, and I screwed up a bit on the return trip - let myself get under the glideslope during the alignment turn and had to use a bit of power on final.

Still, it got the job done!
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Old 01-03-2012, 05:04 AM   #19
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Great job Tommy!

As the maths go, we did pretty much the same thing. Starting location and bearing were straightforward to find, but distance was trickier. I also used the pythagorean theorem, first finding half the angle and then doubling it to find the true distance. Any difference in our calculations is from roundoff errors and nothing else.

Once you have starting point-bearing-distance, it's not too difficult to calculate the target's coordinates. (link with the equations and shortcuts on post #1 of this thread).

For the launch azimuth, I tried to make a good estimate for my TOF and then see how much my target will move, by the time i get there. (Everything moves to the East at 0.25 degrees/min)
On the PDF attached here, I explain my reasoning and also give some examples of different ships and different departure/arrival sites. The first part may seem a little complicated at first, but with that equation, one can make a somewhat accurate estimate of how long the flight will take.

For the TOF, I got 50.67 minutes, multiplied it with 0.25 and added the result to my target's longitude. Then I calculated the bearing for the "new" coordinates. This would be my launch azimuth. The result was 204.25 degrees.

As for the flight, I took off with the PRO904SPEC204 autopilot and after reaching 500 m/s, I made a small correction to try and get that 0.25 degrees to the "right". - Important note: Must have the correct azimuth, before the ship reaches ~800-900 m/s speed.

I already knew the coordinates of my target and had a pretty good idea where it was on the map. I trusted my launch azimuth calculation so I let the AP burn the engines untill the groundtrack on MapMFD was above it. Then I pitched the ship to about 40 degrees AoA and used AerobrakeMFD to see if i needed more or less thrust to get to my target. The difficulty here is that there is no base to target.
I used the "map" page of AerobrakeMFD to see on what coordinates i was going to land and made the slight adjustments that were needed. DockMFD was on the other panel and after the reentry, I landed a few hundreds of meters away from the disabled DGIV.

The journey home was easier. I used the same method to calculate the launch azimuth, but this time, there was a base to target. Fuel was tight but enough to make it. To my surprise the same AP that I had used to get here, got me to a higher ApA than before, making my reentry more aggressive at first, but the DGIV could easily handle it.

I hope that the ones that tried this challenge enjoyed it, even if they were not successfull.

Playback attached. Remember to raise your gear right after the take offs and press "Backspace" to engage the turbo-pumps, ~40-50 secs later. Also lower the gears before the landings.

Have fun, happy (sub)orbiting
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File Type: zip RescueDisabledDGIV.zip (112.2 KB, 16 views)

Last edited by dgatsoulis; 01-03-2012 at 06:30 AM.
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Old 02-18-2012, 10:18 AM   #20
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I think this PDF can be really useful!

Page 3 General navigation
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