IMFD Does IMFD account for "full stack" mass?

[MetalMania]

I've been messing around with IMFD on and off for a while, tried a bunch of tutorials, read the "full manual", watched playbacks, etc. When I follow a tutorial, generally I have no problem. I think I understand the concepts and the basics of what does what, but when I try to use it on one of my own flights things don't go so well. At best I get where I'm trying to go but it's ugly, at worst I completely miss my target.

Most recently I've been having a go at "Jupiter Rockets (DIRECT)" by dougkeenan. I'm doing a "dual launch" scenario where I dock an Earth Departure Stage booster with a docked Orion crew vehicle and Lunar Module in LEO, and use the EDS to do a TLI burn. My RInc to the Moon in LEO is 0.5 degrees. Despite trying to follow the principles I've learned (or at least what I think I've learned) from tutorials etc, I either end up way short in my burn, miss the intercept with the Moon, or can't get my Lunar PeA as shown in the IMFD Map display raised from deep beneath the surface. Yes, I'm trying to do MCC burns. The last couple of times I've tried it I set up a burn in Target Intercept that at least should make it out to the Moon's orbit, yet the auto-burn cuts off well before my new orbit reaches that size. Which leads me to the title of this post.

Does IMFD account for the total mass of all components of a docked "stack"? I'm wondering if the burn is coming up short because it only considers the mass of teh EDS stage?

Just to try something different I tried with a DGIV, figuring I'll take the whole "stack mass" issue out and use a single vessel. Well, this time I tried to plot a course for a plain Jane prograde intercept, and ended up with AB burning well past the planned trajectory, and the map view now shows a free return figure 8. But if I tried for a free return, unless I was following an Apollo tutorial, I'd never get it to work.


I'm starting to feel like IMFD for me stands for "I aM F#$!^& Dumb".

 

[Tommy]

IMFD will consider the mass of vessels that are Docked, but not ususally the mass of vessels that are "attached".

I'm not familiar enough with the Jupiter Direct add-on, but it's possible that it does something funny with the way it handles it's engines that would cause IMFD to miscalculate the burn time.

More likely, there is a problem with orienting the full stack, resulting in an incorrect thrust vector. You may need to lower the AutoBurn MaxRate on IMFD's main config page.

As for the problem with the DGIV, I just don't have enough information to help. If you could try again, and start recording after you set up the trajectory (but before you actually hit AB), and attaching the recording, it would help. Even just some screenshots of the set up, and the results.

 

[MetalMania]

Thanks for the reply. I'll try the AutoBurnMaxRate adjustment - I do notice it seems to get a bit "wobbly" when orienting the stack before the burn with the Jupiter addon.

I'll give the DGIV another try and try to capture some info - I don't even remember what I did the last time I tried it to be honest. But if I may, here's a general rundown of what I do to set up a flight with the DGIV (this is from memory so I might have some glaring omissions):

1. Launching from KSC, wait until RInc to moon in Align Plane MFD is at its lowest and use the DGIV ascent auto pilot set to heading of 90, making small adjustments during ascent. I usually end up in a LEO of around 230 - 240 km with RInc < 0.5 degrees.

2. Open IMFD in left display, select Course > Target Intercept, set target to Moon.

3. Set burn guidance mode to Off-Axis, I think the transfer type is left at Off-Plane. I thnk I then start adjusting TEj to lower the DV Tot a bit, the "default" TIn is 416 K or so (almost 5 days I think). I know if I move that earlier I'm asking to get there faster thus needing more Dv. I know the DGIV has plenty but I'm trying to understand the concepts and how to set up properly if I had to find efficiency.

4. Open IMFD in right display, set op-shared 0, open Map program and set to "course" so I can see my flight plan, set target to Moon and Sel until Moon Pe is the focus.

5. Back to left IMFD in Course, I enable Vel Frame offset. I am aware that offsetting is more of an "advanced" IMFD technique - but I'm under the impression it's a good idea for an Earth>Moon flight because there isn't as much time and distance involved to make large Pe adjustments as one might have on a longer interplanetary voyage. Obviously the Apollo tutorials all use this as it appears to be essential for setting up a free return - I'm not trying to do that here. I'm trying to set up a Moon prograde approach, coming in just behind it, and braking into a prograde parking orbit. I'm not even currently trying to change my inclination along the way - I'll take whatever my launch trajectory and MCCs get me (which looks to be almost equatorial). So back to the offsetting - I tweak the Rad and Lon values so the intercept vector in the Course display looks like it's bringing me in on a trajectory like I want instead of aiming me for dead center, and PeA in the Map program is around 200 km or so. I'll note here that usually the trajectory in Map looks nothing like the one in Course. Course is an ellipticall orbit that goes out to the Moon's orbital distance and loops back. In Map it goes out to the Moon's distance, does a sharp hook to the right and does what looks like an S-turn over the moon.

6. I engage AB and wait. While Ttb counts down the PeA in Map changes but usually not too bad. Then the burn happens, sometimes near the end the ship's orientation takes a sharp turn towards Earth (!) but usually it's only the last few seconds of the burn. Within half an hour after the burn is done, my PeA in Map has dropped to well below the surface of the Moon.

7. I wait until just outside Earth's SOI to make a MCC, but it seems like no matter what I do I can't raise the PeA from deep within the surface - either that or its several thousand km away and I can't bring it closer. This is based on what I'm seeing in the Map program making adjustments to the Course program parameters. Sometimes I get lucky and just burn retrograde when I think I'm close enough and it gets me captured but I know I shouldn't feel like I'm doing it by the seat of my pants. The last time I tried making a correction - it sort of worked, but I ended up turning my trajectory into a retrograde approach instead of prograde leading me to believe the initial TLI burn didn't do a very good job of putting me where I thought it would. Basically it looked like I was actually arriving ahead of the Moon in its orbit instead of just behind as I thought I set my offset vector to.

I'll try to get some actual captures/scenarios next time I try it but from memory that's roughly how I set it up. Should I just forget offsetting and try using Planet Approach to fix the PeA? It just seems like if I do a TLI with the initial trajectory aimed dead center for the Moon it's too much to overcome in such a relatively short flight. I'm probably just doing something fundamentally wrong.

 

[Tommy]

I wouldn't worry about offsetting. It would increase the efficiency a bit, but there's planty of time/distance to use Planet Approach.

If you do use offsetting, you will need to readjust the offset values before making a MCC. Also, switch to "Realtime" burn guidance for MCC's.

As for Map, I recommend setting TGT = "moon", CNT = "p-moon", and enabling Plan, SOI, and Int. This will provide a bit better display.

Do you have "unlimited fuel" enabled? This could explain the funny orientation during the burn - since IMFD makes it's calculations assuming a reduction in mass during the burn.

 

[orbekler]

IMFD will consider the mass of vessels that are Docked, but not ususally the mass of vessels that are "attached"....

Time for a question within a question...
For "attached" you mean also XR-5 main fuel payload?

This, just because doing a Moon-Mars trip with XR-5 + several main fuel payloads, I noticed that while Target Intercept planned correctly and crossed Mars, after Orbit Eject, with (IMFD) Map, real trajectory wasn't even touching Mars orbit. I had to perform a considerably high MCC (about half of Orbit Eject) to get the right path.

I'd be happy to know that it was just my bad planning...:lol:

 

[jarmonik]

It is very unfortunate that Orbiter's Vessel interface doesn't have GetSuperstructureMass() function, only GetSuperstructureCG() is implemented. IMFD and LTMFD won't take attachments in account in the total mass. Looks like I need to rebuild the MFDs and do it in the hard way. I would rather do something else with that time.

 

[Wishbone]

The code is available inside the Precession MFD sources; the functions are in the "autopilot.h" file. They collect all attachments and docked vessels in a stack. I don't know if there's double counting going on, though - some folks increment their vessels' mass with the one from attachments, some don't.

 

[MetalMania]

Do you have "unlimited fuel" enabled? This could explain the funny orientation during the burn - since IMFD makes it's calculations assuming a reduction in mass during the burn.

Hi, I haven't had the opportunity to try another flight yet but no, I don't have unlimited fuel enabled. Especially when I'm trying the Jupiter Direct addon, I end up having just barely enough fuel for the burn.