Challenge Mars - Retrieve Payload from Phobos and Deimos (hard)

[orbekler]

Yesterday I completed the challenge scenario from XR2 Ravenstar 1.4 installation.

Personally I didn't find very difficult to get Mars reentry, provided there was no AeroBrake MFD restriction, instead, the tanks search was to me very unnerving.

If you completed this challenge too, it would be nice to share methods and experiences, for I'm sure I didn't follow the most efficient way.

I landed on Olympus Pad 1 with 7.5% main fuel and 51.9% RCS fuel.

(Multiple) Playback attached, but just as proof, cause I wouldn't take it as model, for the many many mistakes and distractions included. :lol:

This is basically the playback in few steps:
1) take-off from Mars, 90° heading;
2) reach apoapsis of about 20M Alt, making sure orbit of Deimos is crossed;
3) reach the first node (DN here), then plane align;
4) at apoapsis, get low DTmin with sync orbit, Interesction method (not effective, but easy - not apsis method);
5) again, in the last orbit before rendez-vous (it's just a stone, after all!!!), refine DTmin;
6) at meeting point, IMFD->Orbital->oVM, target Deimos, Voilà - XR2 & Deimos side by side;
7) point to Deimos, horizon level, search for coordinates, pick up tank.

Then it's just a repeating story, lowering orbit to cross Phobos's one, same kind of sync, same search, same picking up.
Then IMFD->Base Approach, AeroBrake MFD and landing.

http://www.mediafire.com/?wo69m4lcchamr5v

 

[dgatsoulis]

Thanks for reminding me of this one. I hadn't tried it.
It's a really tough mission, if you try it with expert settings:
-MainFuelISP=0
-LOXLoadout=0
-APUFuelBurnRate=5

The dV is almost enough to complete the mission but not exactly.
I had to take some weight off the XR2, but i didn't want to leave the CHM or any of the crew on the base. First: they are mandatory and second: it's all that extra weight that makes this mission so difficult.

So, i cut a few kg off the APU fuel and the LOX, leaving the ship with only
30% APU and 1 day's worth of oxygen for the crew of 14. This gave me a dV of 7303 m/s to start with.

Then i waited until 51990.745 so that i could launch directly for Deimos, using IMFD. The launch went like this:
Full hover until i reached 60 m/s vertical, then cut the hover engines and press AB on IMFD. The rendezvous was setup for an intercept time of about 6.15 hours. The launch used about 4860 m/s of dV.

After a negligable 0.5 m/s correction i arrived 50km away from Deimos and i was closing in fast. With IMFD's Match Velocity program, i used about 800 m/s to match my speed.
And then another ~100 m/s -this was way to much, i should have done it with half the dV- to locate the tank and land on top of it. As soon as i loaded it in the bay, i lost another 15m/s because of the extra weight.

I took off from the rock with my linear RCS thrusters (another ~10 m/s) and setup an ~360 m/s manouvre to rendezvous with Phobos. The corrections were a bit more costly here, but only about 15 m/s.
This time i got a little closer (40km) and again, i used the Match Velocity program (a ~450m/s burn).

It took another 72 m/s to land on top of the second tank, this time loading it to the bay, cleaned a good 20 m/s from my estimated dV. Then another 10 m/s to take off from the rock.

Now the real difficulty was to try and land back at the Olympus base. (At a supposed 10km long runway, on the side of the base)

Let's look at the results:
Launch : 4860.0
1st MCC : 0.5
Matching Speed with Deimos : 800.0
Picking up the 1st tank : 100.0
Weight of 1st tank : 15.0
Launch from Deimos : 10.0
Intercept Phobos : 360.0
MCCs : 15.0
Matching Speed with Phobos : 450.0
Picking up the 2nd tank : 72.0
Weight of 2nd tank : 20.0
Launch from Phobos : 10.0
Straight back to Olympus : 700.0
---------------------------------------------------------
Total: :7412.5
Initial estimate :7303.0


Bringing the node closer to the Equator and performing a "tiltted" aerobrake, almost did the trick. I managed to land 72 km away from the base. Gear and hover, were shot to hell, but the payload was ok and none of the crew was hurt.

Lot's of improvments to try out, in this flight.
Every m/s of dV you can save in one part of the journey, will be invaluable in the next part.

Have fun, happy orbiting!

 

[orbekler]

Well, I think this is a "professional" way to perform, I'm just doing things at "eyeball"!

I'm afraid that if I'll use those cfg settings, I won't make it. I'll try in anyway. :thumbup:

I dropped the IMFD solution to reach Deimos, cause it would have required an approach plan "by the numbers", that I'm not yet able to do (relative speed was too high, so it was the insertion energy required).

So I decided to use the classic method (Orbit + Sync), as it was the ISS.
The only exception was IMFD Orbital tools, in which Orbital Velocity Match seems to be one of the most powerful tool that I ever used to sync orbits.

I'm preparing a clip for the challenge "Rescue Disabled XR1-01 (hard)" where with this tool I approached the disabled XR1 at apoapsis, making that "hard" scenario actually at most "average"
Of course, with default settings

...Full hover until i reached 60 m/s vertical, then cut the hover engines and press AB on IMFD....

VERY interesting, I never thought to use IMFD that way...

 

[jthill]

It turns out the cargos have transponders, so I used dock and surface to orient the approaches, landed 26m away from the container on deimos, it was fun doing that in the dark. Yay local lights!

So then I got distracted landing on Phobos, bounced at about 3-4m/s. That didn't work real well, but fortunately we were level, didn't breach, and Kara went out and said it was just bent up, not broken. This is Mars not Earth, so that meant we were still good for reentry. We just went ahead and loaded the cargo while floating a couple meters in the air -- didn't need the engines for standoff, not even RCS. Just floating, 6m above the Phobos surface. Kara said it was almost worth wrecking the gear bay doors for that.

Reentry was real nice. Half the planets in the system were all gathered to greet us in the sky as we deorbited. I had to keep the DNP low so aerobrake alone wasn't enough, and I wasn't about to do multiple orbits so I just flipped and tail-stood it. Another lousy landing, Kara's not speaking to me, but we're all alive and the ship'll fix.

 

[orbekler]

It turns out the cargos have transponders, so I used dock and surface to orient the approaches, landed 26m away from the container on deimos, it was fun doing that in the dark....

One of the most difficult thing to do for me was to keep an altitude that wouldn't "sink" in the satellites surface, as collisions don't respect their irregular shape, and a simple RCS fart would disrupt the almost nonexistent orbit.

I didn't think to use dock, but it was almost the same using the payload range detection.

I'm trying to do the same challenge with expert settings, but I got 21% main fuel left just to reach Deimos, it will be VERY hard to complete all.

@dgatsoulis: could you please provide fuel percentage instead of dv?
It's much easier to see if I'm within checkpoint limits.

BTW: your "Pull & Shoot" IMFD take-off works like a breeze!

 

[jthill]

I circularize in low orbit then raise ApA at the next alignment node, that pushes the high node out as far as possible for a big win. I get to Deimos with about 40% fuel remaining. For climbout I don't even use hover, just RCS the nose up to about 10deg and light the engines while it continues rotating, when it passes vertical we're pointed in about the right direction for this orbit so it's easy. I've always done steep climbouts, 60-70-80 degrees, I confess I haven't looked for anything better.

 

[dgatsoulis]

@dgatsoulis: could you please provide fuel percentage instead of dv?
It's much easier to see if I'm within checkpoint limits.

I am so used to counting my fuel in terms of dV, that i don't think i even looked at the fuel indicator. I always use BTC 2.0 to check how much juice i still have.
I believe i had around 25% after the launch/Deimos transfer and about 10% after the Deimos-Phobos transfer, but i'm not sure if that's correct.
Anyway, i recorded a succesfull flight, i'm attaching the playback on this post, if you want to see it.
I used the CONFIG_OVERRIDE option in the scenario, so you should be able to see the flight with the settings i had.

BTW: your "Pull & Shoot" IMFD take-off works like a breeze!

Not exactly ideal for Mars. It works much better on smaller airless bodies.
All you have to do is check to see if the burn vector is above the horizont an hit the AB button.

 

[jthill]

Oh, Lord, I had all the config entries in Config\XR2RavenstarPrefs.cfg, but they weren't having any effect. I should have listened to the "this is too good to be true" feeling I was getting even after checking that I had the entries. Using the overrides in the scenario file turns this into a real challenge. dgatsoulis, you have "CONFIG_OVERRIDE_LOXConsumptionMultiplier 0" in there, is there something about your crew you're not telling us?

 

[dbeachy1]

For technical reasons the number of settings you can override in the scenario file itself is quite limited; the preferred way to override default XR config settings is to use a corresponding foo.xrcfg file for the XR vessel in your scenario. Check out the section titled Configuring Your XR Vessel on page 17 of the XR Flight Operations Manual for details.

EDIT:
That being said, there's nothing wrong with using overrides in the scenario file if they are sufficient for your needs. :tiphat:

 

[jthill]

That's what I did. I just checked with a virgin install, XR2Ravenstar-1.4.zip, UCGO20_100114.exe and nothing else. Changing Config\XR2RavenstarPrefs.cfg to MainFuelISP=0, LoxLoadout=0, APUFuelBurnRate=5 in has no effect (20M+ ApA at 50%+ fuel), but then adding CONFIG_OVERRIDE_MainFuelISP 0, CONFIG_OVERRIDE_SCRAMFuelISP 0, CONFIG_OVERRIDE_LOXLoadout 0, CONFIG_OVERRIDE_APUFuelBurnRate 5 to the scenario file has the expected efect (30% fuel at 150km circular).

 

[dgatsoulis]

The file that controls the settings for this particular ship (XR2-PhobosDeimosPayloadMission) is the XR2-PhobosDeimosPayloadMission.xrcfg file in the Config directory. The CONFIG_OVERRIDE lines in the scenario that i posted, override these settings but also create a small conflict, producing that white debug line on the lower left corner of the screen.

dgatsoulis, you have "CONFIG_OVERRIDE_LOXConsumptionMultiplier 0" in there, is there something about your crew you're not telling us?

:lol: No, the crew was ok, it's the captain who messed up.
The LOXConsumptionMultiplier was set to 4 (realistic) and the LOXLoadout was set to 0 (7 days). The oxygen that i started with was 23hours 59min and landed with 3 hours remaining.

-edit-
If you want, you can change the values in the .xrcfg file and delete the CONFIG_OVERRIDE lines from the scenario. This way you can fly the mission with "proper" settings.

 

[jthill]

I see it now: an xrcfg comes pre-installed for that specific ship. Many apologies for wasting so much time with my blindness.

 

[orbekler]

I see it now: an xrcfg comes pre-installed for that specific ship. Many apologies for wasting so much time with my blindness.

Actually if you leave its xrcfg installation settings, it's not a terribly difficult challenge.
Dgatsoulis made it at the very edge. Clean, fast, PERFECT.
Chapeau. :tiphat:

@Dgatsoulis:
One thing: I saw that you didn't land on pad, but you had a pretty good reserve of RCS fuel that cross-fed, (maybe,) could have help.
Is it part of the challenge not cross-feeding?

 

[dgatsoulis]

No, cross-feed is fine. Besides i had already cross-fed fuel from RCS to Main, right before the base-alighnment/deorbit burn.

My Olympus base has a 10 km runway 90/270 direction, right next to the pads. I was aiming for that and eventhough i didn't exactly nail it, i was quite happy with the result. (Considering the dV).