Challenge XR-2 Moon and Return, a little different

[2Gm over cxc]

I read a few earth-moon-return challenges and I think this one is a little different.

Goal: 1) Take off from KSC in the XR-2. 2)Dock and refuel with Mir. 3)Land on the moon. 4)Return to earth and land anywhere.

The Challenge:
1) Fuel ISP, SCRAM ISP, APU all set to expert (0). Oxygen set to 14 days with default crew.

2) Use whatever MFDs are needed to reach the Moon.

2) Once landed on the moon, the only MFDs allowed for the return trip are Orbit MFD, Map MFD, and Surface MFD. The purpose of this is to simulate a failure of the hypothetical guidance computer. Thus, only real time MFDs are allowed for return. This means that one must plan the return conceptually, before precious fuel is actually burned. The balance of remaining fuel and LOX is crucial to planning.

3) Use crew habitat module and two Fuel tanks as payload.

4) No refueling on the Moon. Only allowed 1 refuel at Mir after takeoff.

5) As you cannot refuel on the Moon, land anywhere you want. Keep in mind your landing position so that a costly plane shift is not needed.

6) As this is an emergency situation, land anywhere you want to on Earth.....just not in the ocean.


Tips:

1) I might not understand this correctly, but using transX, it is not that crucial to align planes with the Moon prior to burning. I think this is because transX factors in three spacial dimensions (prograde, plane, outward). If someone knows the reasons I'd like to know.

2) The most important factor in return is PeA. After Getting my PeA to 65km, I had 0.8% main fuel, and 14% RCS. I entered earth's atmosphere with an orbital V of around 10.2 km/s. Only way to survive was to aerobrake down to about 8.2 km/s, lower my ApA to 6M km, and do it again. I found I could not not enter and land in one go, and I wanted to save what little fuel I had left for attitude control. My second reentry, I had to use center of gravity and elevator trim to maintain attitude, as RCS was down to about 3%.

I did this because I wanted to see what the lowest settings on the XR-2 ISP were actually needed to get to the Moon and back. I used the Mir refuel and two AUX fuel tanks as I wanted to make is more realistic (assuming there is no base on the moon). After I tried it on ISP 1, I returned with enough fuel to circularize my orbit, and land back at KCS. So I tried ISP of 0 and that is what happened! It was a lot of fun trying to survive the reentry with so much V, and was very satisfying when I landed on west coast of Africa with 0.1% Main, 0.1% RCS, and very little LOX remaining.

I'm curious if I can do better and want to see what other more experienced orbiters can do.

, long may he reign.

 

[Tommy]

Is it allowed to top off O2 at MIR?

 

[2Gm over cxc]

Yes, but I found it didn't make a real difference as it takes 1-4 hours to dock with Mir, depending on launch time.

P.S. The "hypothetical guidance computer failure" sounds cool..... The truth is that half the time I reset transX by clicking "FWD" on the first stage when "no target" is set, I get a CTD. I did not want to risk that, so yes.... guidance failure it is.

Oh another tip: To save fuel, GS does not to be zero on the moon, as we are not landing at a base. I found landing was possible at 250 m/s with a VS of 1m/s. I find its easier to land with a specific VS if you watch the VS in the HUD, not surface MFD, as the refresh rate is much higher, and you can keep an eye on your attitude. Also, I like to set hover engines so VS acceleration is positive and small, (ie, about 0.1m/s/s) and use translation thrusters to keep VS negative. This allows for a natural positive correction when you don't give the spacecraft any input. Sorry if this is common knowledge!

 

[Tommy]

To be honest, the main reason I was asking is because I've made the trip from KSC to ISS/MIR so often it's boring. Since I can leave Mir with full tanks, I'll just start there - it won't actually make any difference as far as the challenge goes.

O2 is not a concern for me, as I can easily do a direct re-entry from the Moon and land anywhere I choose. I'll have more than enough O2, even if I start from KSC, since I won't have the long "extra" orbit around earth when I get back.

Might even have time to try later tonight when I get home.

 

[2Gm over cxc]

Good point..... It has gotten to the point where I take off with Align Orbit MFD, and the Hud set to dock, and eyeball it.... it is a little boring. So yes, by all means start from Mir, just remember those two AUX tanks. I don't know if it is possible to do it without them.

I have only been playing around with Orbiter for the past couple months, so this may not even be a challenge. I figured using expert ISP, and no Moon refueling would be tough. Mods, feel free to delete this thread if its not.

P.S. Tommy, I can't tell you how many times I have searched the forums for an answer, and saw your advice. Thanks for your posts.

 

[Tommy]

It's still a challenge. Not a huge challenge for some of us, perhaps, but still a challenge for many. Getting back efficiently without TransX or IMFD requires some ability, and should be a learning experience even for veterans.

If I didn't think it was a worthwhile challenge, I wouldn't consider doing it!

With every challenge, when someone scores better than me it becomes a learning opportunity.

 

[2Gm over cxc]

With every challenge, when someone scores better than me it becomes a learning opportunity.

Humility laced with a subtle sense of superiority. I can appreciate this. :tiphat:

Let me know what your results are, if you decide to try it. I would like to refine my fuel efficiency and conceptual understanding.

 

[Tommy]

Didn't get a chance last night, but sometime in the next week - I hope. I'll record the flight, perhaps even add a couple notes if time permits.

It's not so much that I feel superior - it's just that I've been Orbiting for about 6 years, longer than many - so I should hope that I've learned a few tricks along the way. I'm certainly not "The Best", but I seem to be "above average"! One of the things I like about Orbiter is that the more I learn, the more there is to learn.

BTW, I can add a few pointers to the two "tips" in your first post.

1: It's possible to get to the Moon from ANY Earth orbital plane - if you time it right. The "trick" to an off-plane intercept is that you have to intercept the target at a node. It's very hard to do this with TransferMFD because it's basically two dimensional. It doesn't show the nodes - and I seem to recall that it errors it the RInc is too large.

I've gotten from the ISS to the Moon using only a prograde burn (and an MCC, of course), no plane or outward velocity. It's all in the timing.

2; The main obstacle here, I think, is the limited APU fuel. I suspect that is going to limit my options on the direct re-entry and will limit my choices on where I land.

During a normal re-entry, the vessel is sort of "balanced". If you have a bit too much lift you will gain altitude - where the thinner atmo will reduce your lift. A bit too little lift, and you'll descend into thicker air, and get more lift. Once you get the right AoA, you can power down the APU and the vessel will slowly descend as you loose lift (due to reduced velocity) without needing many adjustments.

In a direct re-entry, you have to start out inverted and use lift to keep you from rising back out of the atmosphere. When you are inverted, your vessel is "unbalanced". Too much lift and you will loose altitude, where the thicker air will give you even more lift, etc. The Inverted re-entry requires frequent small adjustments to the AoA, so you have to keep the APU running the whole time until you get down to sub-orbital. That means I'll need to get down to sub-orbital as fast as I can, which means a High AoA. That will reduce my range and cross-range compared to a low AoA re-entry. I'd like to land back at KSC- but that may not be possible using those settings and stock MFD's.

BTW, the current version of the XR-2 supports inverted re-entries in the attitude hold AP - which is quite efficient and uses very little RCS (it uses mostly COG and trim).