Advanced Question [IMFD] Moon/Luna-OB1 to ISS/Mir

[Goth]

What's the best way to go from the Moon (or Luna-OB1) to the ISS (or Mir) using IMFD?
Is it possible to intercept the target in a single course?
I did:
* Planet Approach -> ref Earth, tgt Mir, src Moon and, to match Mir parameters, EqI 26.92 and PeA 305k;
* mid-course corrections with Planet Approach using x as source when out of Moon's SOI;
* Orbit Insert program; after the insert my orbital plane was not aligned with Mir's one because I would have had to wait like six days for the Planet Approach program to be able to put it aligned (or I could have used the PeT optimization but it's the same, you'll just spend those days traveling instead of waiting on the Moon's surface/Luna-OB1);
* rendezvous with the Mir space station, very fuel expensive.

Is there a cheaper and faster way to reach a space station on a planet different from the planet where you start? Like planning a course that from the Moon brings you directly to the Mir space station? Or is it just impossible?
If it is, there is at least some way to reduce all those days of waiting for the correct alignment?

Anyway, the stock (default) vessels and space stations in Orbiter are able to give you very fun flights with no addons at all. Great sim.

 

[Tommy]

I'm not at home, so I can't do any tests (maybe after work), but I can think of a couple options.

You could use Target Intercept (Ref Earth, Tgt Mir, Src Moon), and use Surface Launch and Orbit Eject as usual, followed by MCC's with Target Intercept (Src set to self). You would want to use Map's Plan feature to ensure that you are entering Earth's orbit in a prograde direction, and your course is tangent to the target's radius. Unfortunately, Map's Intercept mode doesn't work with vessels as targets, IIRC. Not sure this would offer any improvement on your method using Planet Approach.

Either way, there are two problems to overcome. One is that your velocity at intercept will be about 10.3k m/s - so the RVel will be about 3k. You would want to calculate the RVel and use BurntimeCalcMFD so that you can start the insertion burn at the correct time (one half the burntime before the intersection - and you'll need to "fudge" the timing so that your course has you initially arriving at the intersection 1/2 the burntime ahead of the target. You'll also have to anticipate the amount your PeA will change during that insertion burn - with a burn that long it will be noticable.

The big difficulty is plane alignment - and that will require leaving the Moon at the correct time. Even if you have the same Inclination, getting the LAN to match is all about the timing. With the ISS it's worse - since the ISS's inclination is higher than the Moons. So, chances are, you'll have some plane change to make during the insertion/rendezvous burn - making it even longer and harder to be accurate.

If you are using a winged vessel like a DG or XR, then I would suggest setting a course that leaves your Pe at a node with the Target - and setting the PeA to about 68k alt. Then you would make a combination of aerobraking and aerodynamic plane change so that your planes are matched - and your ApA is at the Intercept point. You would need to figure out when the target will reach the intercept - and how long it will take to complete the aerobrake/PlC and reach the intercept. SyncMFD (using manual axis) could help with the timing for the target - but you'll have to guess the timing for the PeT of your vessel.

Any way you look at it this will be very difficult, and probably quite expensive. Focus on the plane alignment since that will keep the fuel cost low - at 10+ km/s plane changes are VERY costly.

Another idea for plane alignment would be to set a course that has the node just inside the Earth's SOI and make the PlC there, where your velocity is low. Keep in mind that if you have Non-Spherical Gravity enabled your vessel will precess less than the target, so you won't want to match the plane exactly during that burn - you'll need to anticipate the difference in precession so that the planes are matched at intercept.

Just about any method you use will require some "external" calculations.

Another problem if you are leaving from Luna OB 1 station - the station is usually in a polar orbit so IMFD will want you to make a plane change before or during the ejection burn. In that case, you might be better off using Delta Velocity (and Map) to perform the ejection from the plane you are in, rather than changing to the ejection plane IMFD prefers.

 

[Goth]

Even if you have the same Inclination, getting the LAN to match is all about the timing. With the ISS it's worse - since the ISS's inclination is higher than the Moons. So, chances are, you'll have some plane change to make during the insertion/rendezvous burn - making it even longer and harder to be accurate.

Yep, I suspected that there were no solutions apart the plane change manouvre or to wait. So to save fuel the best thing is to wait some days for the correct alignement.

Target Intercept revealed to be even worse, especially because (as an old IMFD manual I have says) it points at the center of the planet no matter what the target is. So it's totally not useful for the final stage; and for the mid-course stage it's better to replace it with Planet Approach or something else.

Your other various solutions seemed cool though. In general it seems it's a very complex flight anyway.

EDIT: I finally made it waiting the necessary days to allow the Planet Approach to match visually the planned orbit with the Mir orbit. Setting the inclination to 26.92° was not enough as I said because of the... LAN? It would actually be cool if you guys tell me why that happens (the theory part) because all the thing I know is that for some reason I have to wait for the Moon to put me to a decent orbit to make those 26.92° to be actually that.
However, after 11 days (that's a lot) the Planet Approach program allowed me to escape from the Moon. I did some corrections with the same program and data (remembering to set the target to my vessel when out of Moon's SOI) and finally I did an orbit insert, five days later (the traveling time was shorter lol). My final relative inclination with the Mir space station is 0.18°, which is not so bad. The rendezvous only took half an orbit and was perfect.

I've then tried to do again the trip launching immediately (without the 11 days waiting) and do some plane change manouvres but the plane change still doesn't allow you to match the target orbit: you can reach RInc equal to zero but the orbit looks "inverted".
So again I ask why and possible solutions.

By the way, I did not found problems starting from the Luna-OB1 how you said.
From 90.17° ECL I went to 104.34° ECL after the burn. dV was 2.597k, which to me seems the normal dV to escape from the Moon.

 

[Tommy]

Target Intercept targets the center of the target - so it should work since Mir is the Target - but as I said I don't know that it offers a better solution than Planet Approach. Also, I haven't had a chance to try it - so I could be wrong.

An orbital plane is defined by both the Inclination and the LAN (Longitude of Ascending Node - where the object crosses the reference plane (either the ecliptic or the equator, depending) in a "northerly" direction). It may help to try and visualize it as The Inclination indicates how much the orbit is "tilted" from the equator, while the LAN indicates the direction of that "tilt" So, if my vessel and my target both had an inclination of 30 degrees, and our LANs were 180 degrees apart, the RInc would be about 60 degrees - I could be crossing over Texas and my target would be crossing over South America. Changing the LAN is best done by timing the arrival, and it's not always possible to place the nodes in a good place for a plane change manuever to change the LAN by any significant amount. Remember - we are starting from the Moon's orbital plane - so it's best to wait until the Moon's LAN is suitable.

I'm not sure what you mean by the "the orbit looks 'inverted'". If you have a RInc of zero then your Inc and LAN both match the target's. I suspect that you had matched the Inc, but had "opposing" LANs (LAN 180 degrees apart, or there abouts.)

I wish I could explain this better, and in more detail, but sadly I don't have internet at home. That means I can't fire up Orbiter and check my facts before I post - and I don't want to give you bad information. Just keep in mind that I'm working from memory here - and memory isn't exactly my strong point!

So, if memory serves ... Since we are talking about a Moon - Earth (or Moon - LEO) trip, we should be using the Equatorial frame rather than the Ecliptic Frame. This means your LAN will change as the Earth rotates - about 360 degrees per day. If you watch your LAN in OrbitMFD while in LEO you will see the LAN change - but if you watch the LAN while in a lunar orbit (and have OrbitMFD referenced to the Moon, of course) your LAN won't change nearly as fast since the Moon doesn't rotate as fast.

However, there is another reason why your LAN changes - precession. This is caused by the non-spherical gravity created by the Earth (this can be disabled in the launchpad, but it's less realistic). How fast your orbital plane will precess depends on your altitude - the closer you are to the Earth the faster it will precess. So to get a "perfect" RInc without a plane change you need to account for the difference in precession rates. Otherwise, even if your LAN matches the target's when you leave the Moon it will be a bit different by the time you get to Earth. There's an add-on at OH called PrecessionMFD that will prove helpful in figuring that out - but I haven't used it myself so can't tell you how to use it. Just so you know, IMFD assumes that non-spherical gravity is enabled, so it tries to account for it in it's predictions.

BTW, getting the RInc as low as 0.18 degrees is pretty good!

 

[Goth]

Target Intercept targets the center of the target - so it should work since Mir is the Target -

That's what the logic says, but in the practice you're indeed wrong. Dunno why of course, but this is what it did in my tests.

I suspect that you had matched the Inc, but had "opposing" LANs (LAN 180 degrees apart, or there abouts.)

Yes, that may be the case. Basically, if you rotated manually the orbit graph in the IMFD you were able to obtain an X with the two orbits: one was specular/inversed respect the other one. Starting after those 11 days made everything perfectly aligned instead.

Just so you know, IMFD assumes that non-spherical gravity is enabled, so it tries to account for it in it's predictions.

Nice!

(all the rest)

Thanks man for your explanations, far from your Orbiter installation or not that's been very useful.

 

[Tommy]

I just realized that I missed something the first time around:

By the way, I did not found problems starting from the Luna-OB1 how you said.
From 90.17° ECL I went to 104.34° ECL after the burn. dV was 2.597k, which to me seems the normal dV to escape from the Moon.

That's a 14.17 degree plane change that was "included" in your transfer burn. The dV you show is about right (perhaps a bit low) IF you had started on the surface. From Low Lunar Orbit, dV should be a bit under 1.0k. So you had about 1.6k of plane change in that burn - over half the burn.

You can get a good idea of how efficient an ejection/transfer burn is by checking the values in the Burn Vector display. Anything other that dVf is inefficient. You can get a rough idea just by seeing how IMFD orients the vessel during the burn. For maximum efficiency the vessel should be aligned very closely to the prograde velocity vector. So, if the "ball and cross" isn't straight in front of you, you are being inefficient. Remember, velocities tend to be rather high when ejecting, so it's relatively cheap to change the AMOUNT of your velocity, and relatively expensive to change the DIRECTION of your velocity.

Performing the ejection using Delta-V (and using only dVf) would have saved you over 1.5k during the ejection. The MCC's would have been a bit higher this way, but only by 100 - 200 m/s.

 

[Goth]

Ahah you're right, that dV is ok only if you launch from the surface. :lol:
It's very interesting for me to know that I can still save some fuel because this trip is only a tiny part of long journey that I'm doing with the sock DG, so with more fuel I could still go to a lot of other places.

So thanks man but I fear I don't know how to do what you suggested.
I'm not even sure about the theory.
I mean, if you are in such a bad orbit (almost polar), you have to do a plane change (or have a planar change in the ejection vector), otherwise how can I reach my target?
Doing something like a "source plane ejection" won't be so useful because I'll have to change the plane anyway.
So what you actually mean with that? It's not really necessary that you take me through all the IMFD Delta Velocity program thing, but I ask at least for the theory.
Thanks again for the interest in this topic.

 

[Tommy]

You can use Delta-V to perform the ejection/TEI burn using only dVf. Your (Earth) inclination will depend on the timing of the burn - but try to get the node between your transfer orbit and the ISS's plane as close to the edge of Earth's SOI as possible. Then, you will make a MCC which also aligns the planes at that time - when your velocity is low and the plane change is relatively inexpensive. Doing this will reduce the cost of the plane change from 1.5k down to perhaps 100 -200 m/s.

It could be a bit more efficient to have the node just outside the Moon's SOI - velocity will be at it's lowest at that point - but I'm pretty sure that waiting until just inside Earth's SOI will provide more accuracy, and you will end up making less MCC's (should only need one using this method) and end up being a bit more efficient overall.

Keep in mind that you can get from the Moon to Earth from ANY lunar orbit - the polar orbit ejection will only cause your inclination to be higher. This is countered by the rather low cost of plane changes at the very low velocities you have at the edge of the Earth's SOI, and the fact that a smaller deviation to your course will manifest in a bigger difference at that distance. A small, one or two degree, deviation in your course can result in a large change (20 to 30 degree) to your inclination.