Flight Question Figure eight orbit and rendezvous

[Jarod]

Hi,
I'm taking time to decide what exactly I want to achieve in my personal space program and I was thinking about :
- building an orbital space station (with centrifuge modules) and then
- send it in a perpetual figure eight orbit around the Earth-Moon system
(so that people can transit from Earth to Moon and back with the comfort of artificial gravity)
but how am I going to dock with it ? What MFD will be the most helpful to catch the station as it approachs Earth or the Moon ?
Maybe I'm wrong about how it would need to be done ?

Thanks for your help.

 

[Grover]

i think transx does this kind of intercept, but since youre trajectyory is affected by more than two bodies (earth, sun AND moon) it will get very complicated as you get close to the moon

alternatley, IMFD calculates based on more than this I THINK, but im not sure if it does that kind of intercept, (though it should)

 

[RisingFury]

When you say perpetual figure eight, do you mean a free return trajectory around the Moon?

You'll only be able to fly around once. For the second fly around, you'll need to correct your course significantly.

 

[Jarod]

yes, I know that I'll need to do some "station-keeping" if it can be called like that at this scale, so that the station will stay on track.
But I don't know how much energy will be necessary to do it, and to dock with it I guess that will be a nightmare even worse than trying to catch ISS at the first orbit.
The goal was really to have a vehicle to do Earth-Moon trip without having to spend too much energy.

 

[RisingFury]

The goal was really to have a vehicle to do Earth-Moon trip without having to spend too much energy.

Every time the station approaches Earth, it'll need to spend a lot of fuel to rotate it's orbit to catch up with the Moon again.

If you want to dock with the station, you'll need to catch it. That means the ship will need to be put into a trans-lunar orbit, just like the station is. So... what's the point of having the station, then?

 

[flytandem]

There is a way to make repeating returns of the moon an Earth. It is retrograde around the moon then retrograde around the earth. The period of the return trip is about 8 days as I recall. Someone with a good knowledge of TransX should be able to find it. It's been a long time since I flew this trip. Also the height at the Earth was fairly high, pehaps about geostationary but remember it is opposite geos' and in plane but opposite the moon's orbit.

 

[Grover]

so you would sling back in such a way as to return to the moon

but with the gravitational perbutations, your MCCs would have to be numerous and possibly large

and as RisingFury said, a station here is rather obselete, since to dock to it, you're on trans-lunar trajectory anyway, youre better off with a lunar space station to refuel prior to descent burns if necessary

 

[Jarod]

But the energy needed to put a Dragon on a trans-lunar trajectory would be way lower (less mass) than the energy needed for a complete station.
This is why I thought of keeping the station on its figure eight orbit while only the Dragon would need to use a lot of energy.
I think it would be more fuel efficient than a complete station which will need to decelerate/accelerate at Earth and Moon to close its orbit.
I mean for commercial transport, I don't see people staying several days in a Dragon or similar vessel, it's efficient to bring people in orbit but it's not adequate to bring families to the Moon.

 

[Izack]

Every time the station approaches Earth, it'll need to spend a lot of fuel to rotate it's orbit to catch up with the Moon again.

If you want to dock with the station, you'll need to catch it. That means the ship will need to be put into a trans-lunar orbit, just like the station is. So... what's the point of having the station, then?

There would be at least some savings, as the rendezvous craft could be much lighter and easier to accelerate than a large cycling station. For a trip as short as a lunar one though, I'd be amazed if it were any advantage at all. If you want lots of upmass to the Moon best to have large one-way stages, IMO.

 

[Tommy]

If you want lots of upmass to the Moon best to have large one-way stages, IMO.

Actually, there are better ways, but we would need an expensive infrastructure to use them. For instance, a HASTOL, coupled with a few more rotors and lunavators, would be capable of sending cargo to the Moon with only about 4.1km/s of acceleration required from the launch vessel (uses JP-7, slush hydrogen, LOX), and the rest comes from the rotors (which can use solar power, electrodynamic pumping, and ion engines to maneuver). That gets cargo to the Moon for a fraction of the cost of getting it into LEO - and cargo from the Moon back is free, actually required to keep the system in balance!
http://www.niac.usra.edu/files/library/meetings/annual/jun01/391Grant.pdf



However, to get back on topic, it sounds like Jarod is talking about a lunar cycler. It makes sense, even for the relatively short lunar trip.
http://cbboff.org/UCBoulderCourse/documents/LunarCyclerPaper.pdf

So far, human spaceflight has not been exactly luxurious. Even the Shuttle wasn't up to economy class airline standards. Everything else is worse. So far, we have been flying "Baggage Class" because of the need to keep the mass down. Someday, I'd like to think we'll do better - and a cycler makes sense. People aren't going to want to be stuck in a cramped seat for days, eating MRE's. so you'll need a kitchen, some space, comfort, entertainment - pehaps even privacy. But you don't want to have to accelerate all that mass four times every round trip. You'ld still be stuck in a tin can for a just a few hours at each end, but that wouldn't be worse than airline travel.

Wonder if you could combine a cycler with HASTOL?

 

[n72.75]

It's hardly energy efficient but it's an interesting orbit none the less.

 

[Tommy]

Cycler's are VERY effiecient over the long run, you only need to accelerate the really heavy stuff like the life support machinery and kitchen, etc, once. The "taxi" vessels that carry the passengers and food, etc, are lightwieght because they are only for short (a few hours) flights.

The main advantage, though, is that a cycler can be larger and more comfortable for extended trips. Not everyone would want that for a short lunar trip, but they could come in handy for interplanetary someday.

 

[agentgonzo]

There is a way to make repeating returns of the moon an Earth. It is retrograde around the moon then retrograde around the earth. The period of the return trip is about 8 days as I recall. Someone with a good knowledge of TransX should be able to find it. It's been a long time since I flew this trip. Also the height at the Earth was fairly high, pehaps about geostationary but remember it is opposite geos' and in plane but opposite the moon's orbit.

I remember flying a similar trajectory ages ago and the ranges that Flytandem is suggesting is about what I expect. Essentially you can think of it as multiple free-return trajectories tied together and can be done with very little delta-V at each closest approach to the Earth or Moon. The general way that I did it was as follows.
Set up a multi-stage TransX plan as follows:

1. Leave Earth manoeuvre
2. Moon Encounter (free-return)
3. Earth Encounter
4. Moon encounter (about 1/3 of a revolution around Earth from the first encounter).

Then do the normal back-and-forth between stages in TransX to get the Earth Encounter to be at the correct altitude to accomplish the stage-4 encounter. As Flytandem says, this altitude is a lot higher than LEO. Do CC burns at each encounter (maximum efficiency) or shortly afterwards and as you do each encounter then add either another moon or earth encounter to the end of your plan. You can keep doing these slings in perpetuity as each CC takes of the order of 10s of m/s Delta-V.


As for the 'why' that people are saying - yes it won't save you much energy as you still need to accelerate your dragon craft from LEO speeds to the speed of your station (what's been termed the 'lunar shuttle' in previous discussions on this) and then decelerate when you get to the moon so you're essentially not saving anything. However, it does mean that you can launch your station once with lots of supplies and use this as a mid-point refuelling (both in terms of propellant and consumables such as water/food). It will increase the comfort of the astronauts (they don't need to be cooped up in a capsule for the 2-4 days it takes to get to/from the moon) and also allows for transfer of equipment from one mission to the next (by stowing in on the station) rather than having it burn up on reentry or recovered and then re-launched next mission (cameras, multi-mission equipment etc). You can also dump a lot of your extra (or contingency) fuel at the end of your mission onto the station as you only need to perform a slight CC to get from the station to reentry at the end of your mission, but normally have quite a bit of contingency fuel that just gets vented/wasted before reentry.


Plus it's a fun thing to do!

 

[flytandem]

Had a few minutes to read up on the backflip. I opened orbiter and started with the DG Mk 4 in orbit stock scenario. TransX maneuver to go to moon, sling with "backflip" over to moon again 1/2 orbit later then sling back to Earth. Here's a save of the setup. Haven't flown it yet. Cheers. (sorry for the extra ships, just too lazy to trim the stock scenario down)

BEGIN_DESC
Orbiter saved state at T = 518609 (backflip example)
END_DESC

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Mir:Mir
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  IDS 0:540 10 1:542 10 2:544 10
  XPDR 482
END
Luna-OB1:Wheel
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  RVEL 1379.622 -533.239 0.550
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END
GL-01:DeltaGlider
  STATUS Orbiting Earth
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SH-03:ShuttleA
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PB-01:ShuttlePB
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END
GL-02:DeltaGlider
  STATUS Landed Mars
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  AAP 0:0 0:0 0:0
END
SH-01:ShuttleA
  STATUS Landed Moon
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END
END_SHIPS

BEGIN_VistaBoost
END

---------- Post added at 07:19 PM ---------- Previous post was at 08:23 AM ----------

Wow. Got back from dinner and decided to take the back flip Earth Moon Moon Earth flight. When I opened the maneuver target I saw a huge amount of deltaV. Ridiculous. Figured someone probably saw the error or at least poor flight planning and was ahead of me in fixing it. But went ahead and redid the maneuver. Not sure why it was over 4K. That's what happens when you are looking more at the arrival at the moon than the departure thrust and doing it in just 3 or 4 minutes.
Anyway I found a more typical 3120 m/s eject deltaV. What I found strange was after setting up a nice sling from moon to moon to earth and made a save the restarting of the save would not show the moon to moon sling as planned. The angles were correct but what TransX thought was going to be the closest approach was nowhere in the same ballpark. But I did find a fix.

In the scenario below, you will probably have to toggle FWD to stage 3. Then VW to Sling Direct. Then VAR to Inherit Velocity. Then + from Yes to No then again back to Yes. It should find the closest approach as being 1/2 orbit after the sling instead of 1 full orbit after it.
Here's the scenario...

BEGIN_DESC
back flip take 2
END_DESC

BEGIN_ENVIRONMENT
  System Sol
  Date MJD 51989.3704834732
END_ENVIRONMENT

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  Ship GL-01
END_FOCUS

BEGIN_CAMERA
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  FOV 50.00
END_CAMERA

BEGIN_HUD
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  REF AUTO
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END_MFD

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  AFCMODE 7
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END
Mir:Mir
  STATUS Orbiting Earth
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  AFCMODE 7
  IDS 0:540 100 1:542 100 2:544 100
  XPDR 482
END
Luna-OB1:Wheel
  STATUS Orbiting Moon
  RPOS -628614.80 2153426.78 -2025.01
  RVEL -1418.821 -414.600 0.068
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  AFCMODE 7
  IDS 0:560 100 1:564 100
  XPDR 494
END
GL-01:DeltaGlider
  STATUS Orbiting Earth
  RPOS -5771803.98 -2966554.13 2287241.70
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END
SH-03:ShuttleA
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  HEADING 70.00
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  NAVFREQ 0 0
  XPDR 0
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END
PB-01:ShuttlePB
  STATUS Landed Earth
  BASE Habana:1
  POS -82.4000000 22.9994604
  HEADING 22.00
  AFCMODE 7
  PRPLEVEL 0:1.000000
  NAVFREQ 0 0
END
GL-02:DeltaGlider
  STATUS Landed Mars
  BASE Olympus:3
  POS -135.4300000 12.7366196
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END
SH-01:ShuttleA
  STATUS Landed Moon
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  AFCMODE 7
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  NAVFREQ 0 0
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  AIRLOCK 0 0.0000
  GEAR 0 0.0000
  PAYLOAD MASS 0.0 0
END
END_SHIPS

BEGIN_VistaBoost
END

 

[flytandem]

I had some fun flying a repeated backflip trajectory and did about 6 months worth of cycles. I have found it not exactly the 2 month cycle as suggested in the paper. Instead it seems to run a 42 day (1.5 month) cycle.
Here's a rough timeline of the route.

Day 0- eject Earth, toward the moon
Day 3- swing by the moon onto moon to moon backflip orbit.
Day 17- swing by moon and head toward Earth. It is now on an orbit around the Earth with a low Pe, Ap around 480,000 Km and a 14 day period. It is also in plane with the moon's orbit around the Earth.
Day 20- swing by Earth with low Pe.
Day 34- swing by Earth with low Pe.
Day 45- swing by the moon onto moon to moon backflip orbit. This is a repeat of the day 3 encounter meaning we are on a 42 day repeating cycle.

The backflip while going moon to moon was difficult to plan and needed on average 50 to 100 m/s total corrections. But the eccentric double orbit of the Earth heading for the moon was able to be corrected with minimal deltaV especially if doing the corrections near the Earth Pe.