Flight Question Confirm understanding IMFD

[rich0]

It has been a year or two since I last flew orbiter and I wanted to try something a bit more complex, and noticed the apparently-new interplantary MFD. However, much of the documentation is fairly old - I could only find one tutorial that references the current version. As an added bonus this will be my first attempt at a slingshot.

I want to fly Earth-Venus-Jupiter, and using the trajectory optimizer I have dates for my launch window.

Right now I'm still stuck on the ground about 18 hours before my departure time trying to set up. I was able to set up a course to Venus with an arrival date corresponding to my intercept date. Beyond that I've been fumbling around with Orbit-eject, but that was based on what appears to be outdated documentation. It sounds like I should skip that program and go straight to surface launch, but in that program it just says no valid course when I select the course mode. Am I missing something obvious?

I'll go ahead and outline my meager understanding of what I need to do after launch so that you can correct any issues while you're reading:

1. After launch I'll be in an orbit aligned to my departure (hopefully). At that point I load up the orbit-eject program, wait until my optimized departure time, then at the next window do the prograde burn (or just hit autoburn I assume).

2. Once outside the Earth SOI I use the planet approach program to set the altitude and inclination in the report generated by trajectory optimizer.

3. Once inside the Venus SOI I use the course program to plot a course to arrive at Jupiter at the optimized target date, and then use the orbit eject program to calculate the burn and run it.

4. At that point I'm just flying to Jupiter - I can use Planet approach or the tool of my choice to go from there.

I'll just use off-axis mode for now, but to do better it looks like I could correct inclination at a node rather than at launch.

Is that about right? I see a slingshot program but looking at the only docs I could find it seems like that is mainly informational.

---------- Post added at 02:01 PM ---------- Previous post was at 12:32 PM ----------

Hate to self-reply, but one other question. In the output report of the trajectory optimizer, which inclination is the one to use when inserting into orbit around Venus? I have two legs, each with an inclination. I'm not sure if leg1 is the departure from earth, or the arrival at Venus. Granted, the inclinations are only a few degress apart so I could probably recover mid-course if wrong.

And of course it is entirely possible I've got something fundamental wrong to begin with.

Oh, the details in case anybody needs to know:

Depart Earth 24-Apr-2015 9:56, Venus 9-Oct-2015 11:04, arrive Jupiter 15-Dec-2017 14:05. c3 is 15.53, delV is 110 - I couldn't find any place with ship stats to get a sense of whether those figures are reasonable, but they're smaller than the figures for a direct transfer so I guess they must be. (Flying the XR-5 - if it turns out the fuel just isn't sufficient I can always try again in something smaller but the XR-2 won't carry that much O2.)

 

[dgatsoulis]

If you want to do this with IMFD, I think Mark&Tommy's [ame="http://orbithangar.com/searchid.php?ID=4142"]IMFD Full Manual/Playbacks [/ame] has an example of a flight with a slingshot.

It was an Earth-Jupiter-Saturn flight, or something similar IIRC, but it should help you see how those guys setup and flew a slingshot mission. It was written for an older version of IMFD but I think the method still applies for the latest version.

 

[rich0]

If you want to do this with IMFD, I think Mark&Tommy's IMFD Full Manual/Playbacks has an example of a flight with a slingshot.

It was an Earth-Jupiter-Saturn flight, or something similar IIRC, but it should help you see how those guys setup and flew a slingshot mission. It was written for an older version of IMFD but I think the method still applies for the latest version.

Thanks, I will take a look at that.

<snip question about version - after reading this new tutorial it looks like it is close enough to the current version. I'll give it another shot and see if I'm still getting errors.>

---------- Post added 01-29-13 at 03:58 AM ---------- Previous post was 01-28-13 at 03:25 PM ----------

So, I figured out what my surface launch problem was - I had set a target at some point, and that is apparently unnecessary.

I did watch the Earth-Jupiter-Saturn tutorial. It has you adjust the MFD as it goes along, and to be honest it could really give you a bit more time - you have to really jump on those buttons to keep up. It also starts out a few feet above ground, and when the ship drops the external cooling gets turned off, and then the ship is destroyed as they warp to the launch window. That required a bit of hacking the config file to disable oxygen depletion and internal heat.

While watching it I discovered why I might be having problems with midcourse corrections on the way to Venus - I haven't changed source to my own ship. Not sure if that is necessary. Or, maybe things were fine - I was getting panicky when I saw PeD's measured in AUs but after watching the tutorial I think those were relative to the Sun. I'm not sure I could find a way to tell what my intercept PeD was trending towards - I guess I can just trust the intercept program, or I guess I can run TransX to see what it is predicting.

 

[ronaldsi]

It was written for an older version of IMFD but I think the method still applies for the latest version




_____
It has been self-evident that Psych Box Set 1-6 has prone to pop culture without losing any of their edge. I am telling you the truth which is a classic theme song for it.

 

[rich0]

Well, my next attempt was so-so. I did mange to get to Venus, but on approach the best inclination I could do was 30deg, and I suspect that made the slingshot problematic. I was getting pretty high delta-Vs, but maybe that is to be expected (7k or so - whatever the units are - the trajectory optimizer predicted 110 km/s). I really struggled to figure out how to get target intercept to work right. The tutorial recording was of little help - the recording includes everything BUT the MFDs and it just gives you text instructions to manipulate eject/arrival times to minimize slingshot delta V (in about 75 seconds).

I'm not quite 100% clear on what the delta-Vs and times even represent in this mode. As I see it at the point I was at there are a few possible burns:

1. An immediate burn to set up the trajectory for the best possible slingshot (tweak inclination and periapsis time). This has a delta-v.
2. A prograde burn at periapsis - this also has a delta-v.
3. Some kind of burn beyond the SOI to adjust everything other than prograde velocity. This has a time and a delta-v.

The instructions indicate to work with the eject/intercept times in course mode and watch the delta-v in sling mode. I assume the delta-v in sling mode is the periapsis burn. The instructions don't mention anything about optimizing anything else.

I'm also suspicious that I might not even have the right window from the trajectory optimizer, though it seems like it got the Earth-Venus bit right. Maybe the inclination issues are a big part of the problem.

Ok, enough rambling - any ideas what is going wrong? Or does the burn just take that much fuel (I might need to tweak the spacecraft config in that case)?

 

[Tommy]

Well, there is an error in the Slingshot tutorial - caused by a miscommunication. I'll outline the correct method here.

1. While landed, set up Target Intercept with SRC = Earth and TGT = Venus. Then call up surface launch and set it to course. If it won't give you a solution it may be that your launch latitude is too high for the inclination of the parking orbit. In that case, just launch at 90 degree heading a day or so ahead of the ejection burn.

2. Once in Orbit, call up Orbit Eject, set to course. This will show you the nodes, and can be used to adjust your alignment if needed. The TN counter is only valid during the last orbit before ejection (will show you time since last node otherwise), so keep that in mind when setting normal/antinormal. Use Autoburn to perform ejection.

3. Once you leave Earth SOI, change the SRC in Target Intercept to self (hit src button then press "x" key, then <enter>) to perform any MCC's

4. Once near Venus (about 3x the SOI) use Planet Approach to match your pre-computed trajectory.

5. As soon as you enter Venus's SOI set Target Intercept to SRC = Venus, TGT = Jupiter. Use IMFD's Map program to find your current actual PeT at Venus, and set the TEj in Target Intercept to match this. Now open (in the other MFD than Target intercept, and linked) Slingshot Program (NOT Orbit Eject!), set to course. Adjust the TIn in Target Intercept while watching the Tot dV shown in Slingshot, and set to minimize. That last part is important - and is where the error is in the tutorial. Adjust Target Intercept but Watch Slingshot.

If you still get huge dV's it could be that IMFD is trying to follow a different trajectory than your planner - ensure you've chosen the correct plan in IMFD (off plane, two plane, etc).

 

[rich0]

Thanks for the tips. I just completed a sling-shot and confirmed that it basically put me right on an intercept for Jupiter. The delta-v was still pretty high to get lined up, but it didn't do any burn at periapsis (and the altitude was pretty high too). That makes me think my original plan wasn't all that great, or my PeT isn't quite right (though I doubt that should matter much) - or maybe I'm missing some other detail (Jupiter intercept maybe - the orbit isn't necessarily tangential).

However, the biggest issue I'm running into is that I'm simply not lined up right when I plan the slingshot. I set planet approach for the lowest inclination I could and an altitude of 450k and that took very little energy to execute (since I was far away). However, when I pulled up the predicted pass on transx it was at an inclination of something like 179 deg. As I approached the planet it was obvious transx was right - planet intercept was sending me way off course. The burn to set up the slingshot was 600 seconds and it was obvious from looking at the prograde vector on the hud that it had to move the vector all the way across the planet. If I could run the target intercept mode before I hit the SOI that would have probably been a few seconds of burn - it was 600 seconds of burn and that was when G=0.55 or so. Maybe I have some setting incorrect on planet approach, but it is putting me way off. Either way something is wrong with either the IMFD or my plan because the inclination was close to 15 degrees and my calculated course should have been closer to the ecliptic (which just makes sense).

So, if only I could line up the approach from outside the Venus SOI I'd probably complete the slingshot with an expenditure of only a few seconds of burn in total.

One other thing I did differently was set my Earth departure to two-plane instead of source plane. That had more delta-v, but it looks like it approaches Venus at a much lower relative inclination. Before I couldn't approach with less than 30 deg of inclination, and the IMFD calculated course was at 15 deg, so obviously that is going to waste fuel. I'd need to think about the modes a bit more, but I'm guessing that two plane follows the Earth plane until it intersects with the Venus plane and changes, while source just follows the Earth plane until you can switch to some plane that intersects Venus without the constraint that it be the plane of Venus itself (thus less delta V to change planes, but you approach at a higher inclination, which isn't a problem if all you want to do is enter an orbit and then wait for a window to do whatever your next plan is). So, when doing slingshots a two-plane approach is likely better up until your final destination (though if your final goal is a moon you might have to enter a parking orbit until things line up).

 

[Tommy]

I can't really help much with the Venus approach - I stopped using Planet Approach long ago. I just use Delta-V and Map (in plan mode) for MCC's and approach - it's very accurate and I can make the approach burn much farther out.

You should get the lowest inclination at Venus with a Target Plane plan, it's what I tend to use for sling approaches. The Venus - Jupiter leg will probably work best as Off Plane.

With Slingshot, it will always make the burn as soon as you push AB. Check the BurnView for Slingshot so you can see the vector breakdown. This way you can manually (using the stock Normal/AntiNormal APs) align to the sling plane early, and save the dVf component for the periapsis. Watch the dVp, it will likely either be rising or falling. If it's falling, it means you are approaching a node and should wait until it stops falling to make any plane adjustment. Toggle between actual and plan modes in Map to help figure the direction of the required change.

Sadly, IMFD is of limited help with slings, TransX can be better.

 

[rich0]

Thanks - those are good tips. With map mode you don't get any vectors - do you just tweak your orbit and watch the numbers change in realtime? That is actually what I used to do in transx for midcourse corrections - far out a little thruster burn is all it takes to make big changes.

I'm happy to do whatever works.

I like transx to a degree, but the big pain with that is optimizing anything is a big exercise in being a human 4-DOF root finder. With IMFD I can punch in an eject and intercept date and viola there is a course - I don't have to play with 4 DOF to get it to end up on the solution. It works well in realtime mode though to project paths. Map mode in IMFD works but it doesn't always give me the numbers I want.

Oh, does map work with any reference at any time without regard to your current position? That is, if I'm in Earth SOI and I just did an off-plane burn to Venus can I pull up map with a Venus reference and get the intercept periapsis/etc?

 

[Tommy]

When IMFD's Map is showing the course in green, it is showing your actual course. If you hit the [Plan} button, it will show your projected course (taking info from any of the course computers - including Delta-V) in blue. So you can plan a hypothetic MCC/Approach burn in Delta-V and watch Map to see the results of your burn (Map will "lag" a bit, it takes a couple seconds to update). Simply set the TEj in Delta-V high enough to give yourself some time (600 gives you ten minutes, and you can always increase it as you go) because you need TEj > 3 seconds when you hit [AB]. And watch out - Delta-V starts out with a default of 10 m/s dVf.

Map mode may not give you the numbers you want (although I'm not sure what you need that isn't provided by Map, somewhere, in one mode or another) but it gives you the numbers you NEED - the accurate ones. ALL other programs and MFD's calculate two-body solutions. Only Map calculates multi-body solutions and is MUCH more accurate than any other tool currently available.

And yes, you can set the REF, or the TGT or CNT anywhere you want, regardless of where you are. This is a big part of it's power, and you can get any number you need if you set it up correctly. This ability, coupled with Delta-V or Target Offsetting, will give you any result you desire.

On one flight, I made it from Earth to Titan (via Jupiter sling) in a stock DG with 35% fuel remaining at touchdown. The MCC's enroute to Jupiter were less than two seconds total (and both MCC/approach burns made before the halfway point). The sling used another 3 second burn. Once I left Jupiter, I used Map to target Titan directly (you can use Delta-V or Offsetting to do this) so no need for a SOI burn - just an aerobrake when I reached Titan. All in all, less than 30 seconds of burntime after the Earth Ejection burn. Yes, this takes a lot of fiddling around (much like TransX) but it is more accurate and you plan the burns hypothetically so you don't waste fuel figuring out which way you need to go.

IMFD can be used in a very simple manner when you don't need to be super-accurate, but it can also be used in a more complicated manner when special results are needed.

Map is complicated - it will take some practice and trial to get the hang of how to use it. What reference, center, mode, page, etc all affect the what is displayed and how it is displayed. Learning how to manipulate Map is the key to unlocking IMFD's true power and capabilities.

What you use for the "center" is more important than most people think - it affects the way IMFD displays the time dimension. You'll want to center "p-venus" (rather than just "venus") for instance.