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Old 09-07-2018, 12:30 AM   #1
Marijn
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Default Glideslope - BaseSync low energy problem

For landings on earth, I've been using BaseSync and AeroBrakeMFD for most of my flights. This always worked quite well, but I wanted some more control over the final approach with a pre-planned circuit. So, I had a go with the BaseSync and Glideslope combo. But I am not getting the results I was expecting. In fact, I end up almost 3000km short of the base. So I must be doing something very wrong.

Glideslope is configured for runway 15 on Cape Canaveral using the default 'XR Series' GS, which should be fine for an almost empty XR5 I'd say. The starting orbit is 190kmx190km with one orbit to go before passing over the base. I target 'GS2' in BaseSync and everything seems fine. I let de autopilot of BaseSync do the burn.

After leveling the horizon, I let the XR attitude autopilot follow the reference values as indicated on the DATA screen of the Glideslope MFD. AoA is around 41 degrees. At first things seem ok, but after a short while Glideslope indicated an energy problem. It starts as a small difference, but this escalates out of control quite rapidly.

The dEngy value becomes many times bigger than the reference value. During the whole re-entry, I am losing energy. This already starts at 100km of altitude. I was not expecting that. Why would a plane loose so much energy in such thin air? AeroBrakeMFD is confirming that following the AoA as dictated by the Glideslope reference value is a bad plan which will end up short. Another thing wich caught my attention is that Glideslope shows a reference value for VAcc=0. At 70km of altitude. How is it possible not to decellerate while flying at 70km?

Any ideas on what I could be doing wrong? Thanks.

GS v2.7
BS v3.3
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Old 09-15-2018, 03:12 AM   #2
ADSWNJ
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Very interesting problem! I built the Glideslope XR series glideslope on the XR-5, and I have flown a BaseSync to Glideslope with the XR attitude AP many times. I always target approx a 200 x 200 km initial orbit, a 120 km entry interface, and a 0.7 degree glideslope. From there, you can ride the reference slope quite nicely.

On the VAcc - note it's VAcc, not Acc. Meaning: vertical acceleration. You can bring the vessel to a neutral climb position to flatten out the slope pre-HAC, and then just burn off horizontal speed.

Have a look at the docs, to see if they help. If not, set up a Ctrl-S scenario, and save your glideslope and I'll take a look.

One day, we may have a full AP to help you with this. Probably on a Tuesday several years from now
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Old 09-15-2018, 05:27 AM   #3
NukeET
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Quote:
Originally Posted by ADSWNJ View Post
 ...and I have flown a BaseSync to Glideslope with the XR attitude AP many times...
Quote:
Originally Posted by ADSWNJ View Post
 If not, set up a Ctrl-S scenario, and save your glideslope and I'll take a look.
I hope this isn't off topic, but I'm looking to brush up on my re-entry skills by finally trying Glideslope. Would you consider posting a scenario on this thread and/or making a video/text tutorial? But, to be honest. I haven't yet RTFM.
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Old 09-15-2018, 07:39 AM   #4
Marijn
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In the meantime, I had quite a bit of practice and I did re-read the manual. I am getting better results, but I still feel that I am not getting what GS has to offer.

Quote:
Originally Posted by Marijn View Post
 I let the XR attitude autopilot follow the reference values as indicated on the DATA screen of the Glideslope MFD
I was trying to keep the AoA as indicated by the reference value. The manual speaks about two ways to control the descend by focusing on the vertical speed:
1) fixed AoA and S-turns (hard)
2) changing AoA (simple)

So I was doing it wrong. Method 2 basicly is the way I always used before, by adjusting the AoA. I still get better results when following Aerobrake for the main descent and ignore the warnings of Glideslope. When closing in, the values in GS seem to settle down in green. But I want to do the whole descent without the aid of Aerobrake.

When I aim for a 200x200km orbit when returning from the ISS, I usually end up with 190x190km. Any idea how much of a difference this would make when using a profile inteded for a 200x200km orbit? Perhaps more than I think. Setting up a scenario so you can have a look is a great offer. I'll do that, thanks.
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Old 09-15-2018, 11:40 PM   #5
ADSWNJ
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@NukeET - have a look at the docs with this MFD. I have a tutorial in there.

@Marijn - the S-turns approach is really not too hard, and is way more authentic to the Shuttle's reentry. You just need to have the confidence to hold the nose at 40-43 AoA, and roll up to 135 degrees of bank. The pattern down the glideslope should be a decreasing bank on each roll reversal, gradually reducing the azimuth error each time (e.g. from 15 degrees to 5 degrees over 5 or 6 turns).

The reference Glisdeslope I offer is only my preference. You are welcome to fly a different profile, then save your run and convert it to your reference profile. I'm also happy to add it to the distribution if it's a different way to achieve the same goal.

My long term dream is to add the HUD nav boxes like in RV Orientation, plus heads-up data, so you can run a full virtual cockpit and no MFDs. But that's a long term goal.
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