Should have SpaceX do it
It will cost less and look cooler.
NASA has inked a $935m contract with Northrop Grumman to build the Habitation and Logistics Outpost (HALO) module for the Lunar Gateway. The contract will also require the aerospace outfit to integrate the module with the power and propulsion element being built by Maxar Technologies. HALO will be both a crew habitat and, …
What actually is "a near-rectilinear orbit "?
I mean I've heard of circular orbits, elliptical orbits, and precession of orbits due to Einstein's theory of relativity, but 'rectilinear' is something I've only ever associated with images, and whether lenses are 'rectilinearly corrected' (and if so, how well) or not.
Rectilinear basically means straight. A rectilinear lens is one for which the image of a straight line is still straight (most don't do this because it produces distortion away from the centre of the image, similar to the Mercator projection for maps). A halo orbit is an orbit around one of the Lagrange points, instead of around one of the actual objects in a three-body system (hence the backronym for this module). A near-rectilinear halo orbit is a specific type of halo orbit which looks like a bunch of somewhat curved lines connected together, rather than a more normal ellipse. It's a bit of an odd name because the lines aren't actually particularly straight at all, but I guess no-one has come up with a better name for it.
Thanks for that, I also came here to ask the same question. But my question goes further. Why? Why not a more or less circular orbit? Is this so the moon-bound ships can dock way out and "hitch a ride" down to low Moon orbit where the lander detaches for the final leg? Is it cheaper in fuel, even if further away much of the time in an emergency?
"Why not a more or less circular orbit? Is this so the moon-bound ships can dock way out and "hitch a ride" down to low Moon orbit where the lander detaches for the final leg? Is it cheaper in fuel, even if further away much of the time in an emergency?"
I would assume that having a non-circular orbit, means the flight controllers can use the Moons gravity to maintain it's orbit without requiring "booster burns", when the orbital height gets a bit low.
This can save the need for fuel to be carried to HALO, in order to increase it's orbital height every once in a while (the ISS has to do this, as it's skimming Earths atmosphere which slows it down a little and it needs a "burn" to raise it's height - but getting fuel to the ISS is easy. Less so with the Moon).
"when the orbital height gets a bit low."
What might cause that? Surely the lack of any noticeable atmosphere means it's theoretically possible to have a fast and low orbit such that you stay just high enough to avoid hitting any mountain tops? Might it be perturbations caused by TMA1? :-) Or does the the Earths gravity affect lunar orbits enough to matter?
There are a few advantages. Firstly, it's important to remember that the shape of an orbit depends on your frame of reference. A NRHO looks weird when you look at it in an inertial reference frame (ie. in this context, you'd take one in which the Sun is stationary and the Earth and Moon are moving), but it actually ends up effectively being a highly elliptical orbit around the Moon. Although it's not quite a real Moon orbit, since the Lagrange points effectively mark the point where you're outside any stable orbit of a body.
So you end up with an orbit that has a fairly close approach to the Moon for when you want it, but with much lower delta-v needed to get to the orbit, and with relatively small staion-keeping requirements. Since you're orbiting in the north-south plane rather than equitorial, you never have any loss of communication and only occasional, short eclipses for power to worry about. It's not the best choice in every respect, but overall it has a good combination of properties compared to the alternatives.
"Is this so the moon-bound ships can dock way out and "hitch a ride" down to low Moon orbit where the lander detaches for the final leg?"
Unfortunately that can't work. If you want to get close to he Moon, you need to change your momentum by a certain amount. Attaching yourself to something already in an orbit won't get you that change for free, it just means now you have a larger mass needing a smaller change in velocity, with overall exactly the same momentum change. This is why space elevators are a popular idea - you can't avoid the physics, but you can effectively bleed some of the momentum from the body you're attached to. As long as you're floating free, however, there's no way around needing some kind of rocket.
Why use NRHO(Near Rectilinear Halo Orbit) is because Orion only has ~1350 m/s of available Δv(change in velocity). Entering and exiting LLO(Low Lunar Orbit) on a direct trajectory(like how Apollo did it) requires at least 700 m/s each way. Entering and exiting NRHO requires ~450 m/s each way so NRHO is within Orion's operational range.
Orion does indeed "hitch a ride" down to LLO altitude but Δv of ~750 m/s needs to be applied to transfer from one orbit to the other. The plan is for the lander to provide the additional Δv for the orbit change. From LLO an additional ~2000 m/s is needed to reach the Lunar surface.
When we consider both Orion and lander propellant(fuel and oxidizer) going to the Moon via NRHO requires at least 33% more. At least is because lander propellant requirements depend on lander design. This is not ideal but since Orion isn't capable of operating in LLO and Congress mandates NASA must build SLS/Orion.
Since you mentioned time I'll briefly touch on the topic. Orion going to or from NRHO and Earth takes ~6 days versus the ~4 days needed if Orion could go to and from LLO. The lander going to or from NRHO and the Moon requires 12+ hours versus the 30 minutes or less for LLO. Minimum layover time on Gateway is ~7 day versus the 2 hours for LLO.
"Halo, halo, halo, what do we have here, then?"
NASA have now christened the Gateway the People Entrance and Recuperation Launch Yard, or "Pearly".
I dread to think what they will be doing off the belt of Orion, anyway. Barbecues that get out of hand, I suppose.
Shame there's no "I'll get my spacesuit" icon.
"1) for Beardy Branson
2) for Baldy Bezos
3) for Bitcoin Musk"
More like one for the Lunar lander, one for the incoming "astronaut taxi service" from Earth, and maybe one for a "lifeboat" or for a cargo mission.
I would also assume that at least one space craft would always be docked (whilst humans are inhabiting it, or working on the Moon) so as to allow a rapid escape, if something should go wrong.
I can hear it now...
I had that bloke Bezos in the back the other week, wanted to know if I thought it'd be worth setting up a Martian doorstep delivery service. So I says to him "Nah mate, won't get a look in there, bloody Martians have got it all sewn up. You should see what they want for a cabbie licence, criminal I calls it. And they gets preferential use of the saucer lanes anyway so there's yer profit margin straight out the airlock. And that bugger Musk got in first with delivering a vehicle for type approval so all the cabs have to come from Tesla, looks ok but you try finding a charging point that's working this side of Phobos..."