....
My guess is that the lunar variation of the 'Aldrin cycler' is the best bet for the foreseeable future: a spacecraft on a nonpropulsive swing-by trajectory around moon and earth, passing close to LEO on one leg of it's journey, then LLO and through a high-inclination half-orbit to 'hit' the moon again (half a lunar orbit later). It supposedly takes <10m/s dV for swing-by correction per complete sequence. Two cyclers would enable regular transfers, twice a month.
Since each cycler only has to be launched once, you could make it heavy: provide the crews with living space, radiation shielding and life support(including solar power). Use a minimum-volume chemical propulsion vehicle to transfer crew and cargo from LEO to the cycler and from the cycler to the lunar surface.
Wow, I never heard of that! Have to look it up!
{I tried--there are lots of online discussions of the Earth-Mars version but not of the Earth-Moon one; the only one I found had no details. Do you have links to a detailed treatment, one that specifies the orbits?}
Offhand, I'd think that the problem of rendezvous with it would be essentially the same as simply launching the spacecraft directly to the other world; the only advantage would be, traveling with the stuff on the "cycler." But I am prepared to learn otherwise!
And now I'm thinking of how that could have been part of an alternate Apollo program; essentially, the Apollo 8 mission (or a later one, Apollo 8 didn't include an LM) might have included, instead of an LM and about 16 tonnes of fuel needed for a lunar orbiting mission (if a 45 tonne stack had to be braked to lunar orbit) a 31 tonne mission module. Basically the proportions between the CSM and the mission module (the LM) would be reversed. That 31 tonne thing would include a stock of maneuvering fuel and a small engine, as well as a habitable section. The CSM is trimmed down to 14 tonnes, allowing the same maneuvering delta-V the standard Apollos had once they'd launched back to Earth from lunar orbit, and the astronauts set up the mission module for future uses as well as use it themselves while flying by the Moon. When they approach Earth they depart and brake for reentry.
I guess it is a slow mission because it uses essentially a Hohmann minimum energy orbit. I already figured the difference between that and the fast orbit they used in the OTL program was pretty negligible at launch--but looping around the Moon, it has distinctly lower delta-V to a 100 km orbit, by about 80 m/sec.
I figured that the 45 tonne Apollo stack needed to use 11.56 tonnes of fuel to brake to orbit in the OTL missions, a delta-V of 914.5 m/sec; needing only to brake 832 from the Hohmann orbit they save about 900 kg of that. So the LM can be beefed up a bit.
Or they need to lay in lots more supplies--the catch is, they have to wait several weeks for the module to come around again!
So it really requires a very different approach to the mission, and probably does not fit into early Apollo at all.
Technically there was no reason the Space Shuttle couldn't have reached lunar orbit. Practically there are tons. Specifically, 1,950 tons. That is the difference in "dry" weight between the Shuttle at roughly 2,000 tons and the roughly 50 tons for the combined CSM/LEM package.
The boost necessary to get that beast up to lunar escape velocity, much less Earth's, boggles the mind.
If you could refuel the external tank (I believe the Orbiter could reach orbit with the mass of the empty tank) in orbit, there would be plenty of delta-V to get to the Moon and back with the tank still holding lots of fuel.
Well, I think so--let's see, 851 tonnes, 721 of it fuel, ISP in the ballpark of 430--yep, it ought to have plenty to boost up to TLI, fly by the Moon on a free return orbit then brake back to orbital speeds as it returns to LEO altitudes. With something like 1000 meters/sec delta-V to spare; not enough to slow down to Lunar orbit and then boost up again to return to Earth and still have enough to quite brake down to low Earth orbital speeds though.
It might however brake itself enough on rocket thrust to be able to survive reentry.)
All you have to do is, boost 721 tonnes of hydrogen and oxygen into orbit and pump it into the tank!
Or have the tank pre-orbited and rendezvous with it and attach it.
That's the crazy part; it's about 7 times the payload a Saturn V could launch. Or 30 or 40 payloads of a typical modern "heavy" rocket.
It's the delta-V baby - the energy, expressed in kilometer per seconds.
Earth surface to Earth orbit (say, the ISS) is 7.2 km/s,
I believe you'll find the ISS is orbiting at more like 7660 m/sec, at an average altitude of 415 km;
plus the drag - crossing that thick atmosphere. Which brings the total to 9 km/s
Typical "mission delta-V" for an orbital launch is indeed in ballpark of 9-10 km sec, but it isn't the air drag that costs most of the difference, it's the gravity losses. If only we could glide along the ground at low friction, building up to orbital speed (on the surface of the Earth, higher than higher up) and then a bit more and gently coast up on an elliptical orbit! The air is the main reason we can't do that of course, but it isn't the air drag, so much as directly thrusting due up against gravity, and necessarily at the very time when the ratio of thrust to the mass to be lifted is at the worst, namely the start of the launch!
Another air effect that might cost as much as the drag is that air impedes the exhaust of the rockets, thus lowering both their efficiency (ISP) and thrust, just when we want both the most.
But that lumped together with drag is still only a few percent, maybe ten, of the total losses, it's mostly gravity loss.
Consider it as a vector problem; to achieve orbit we need about 8000 meters/sec (the orbital speeds are less but conservation of angular momentum means whatever horizontal speed we have lower down gets reduced) whereas a very good ascent profile might be as low as 9000 meters/sec; does that mean the losses are just 1000? No; gravity losses are due down (and so are the air drag losses, considering rockets boost pretty much straight up until they clear most of the atmosphere) so it's square root(9000^2-8000^2), a bit over 4000. Equivalent to the speed an object would pick up falling in Earth's gravity field (near the surface) for 400 seconds, which is in the ballpark of typical burning time--a bit low, but remember that as the craft nears orbital speed, centrifugal force offsets the gravity, reaching a perfect balance if it attains a perfectly circular orbit. So it isn't one G across one's path all the way, toward the end the weight counts less and less.
Once in Earth orbit, add 3 km/s to escape Earth gravity.
And the moon is most of the way out of Earth's potential field; down here on the surface we are at negative 62.568 megajoules below "zero" potential at infinity (considering just Earth, ignoring the Sun and everything else) whereas the Moon, orbiting with a semimajor axis of 384,399 km, over 58 times as far from Earth's center as we are, is at -1.07 megajoules, so it's over 98 percent of the way to infinity in potential terms. The difference between escaping Earth completely and barely reaching the moon is pretty small.
...The good news: once you have escaped the Moon, no more energy to reach Earth
[ - except, of course, if you want to return into Earth orbit, then a huge braking is necessary (say, to return to the ISS, but what's the point ?)]
The point would be if you wanted to reuse the rocket that lifted the spacecraft from the lunar surface or lunar orbit and set it back on trajectory to Earth, especially if that is the same rocket that one used to send that craft or another from low Earth orbit to the Moon.
I'm still trying to figure out if that's worthwhile or not; it would be so cool if aerobraking could do the trick. But I have to admit, most of the mass of a one-way rocket stage like the Saturn V third stage that did the job for Apollo is not the shell of the rocket nor its engine(s) but the fuel needed, and that fuel still has to be brought up from Earth anyway--in some kind of container. There might be no point in bothering to reuse the rocket, the penalties we pay trying to make it recoverable and reusable costing more than the price of just making a one-use version from scratch and discarding it.
For the nuke rocket, we almost certainly want to reuse it--but that means rocket braking, not aerobraking, and the cost of that pretty much offsets the advantage of its higher ISP, meanwhile we still have the problem of it being radioactive as hell--250 kilometers strikes me as one heck of a huge safety zone!
The huge amount of energy (11 km/s or so) is literally burned against the atmosphere - better to have a very thick heat shield.
The amount of energy is equal in both directions (Earth-to-Moon and Moon-to-Earth) the difference is that Earth has an atmosphere that provides free braking (free = no rocket motor, no propellant burned) when landing, something the Moon hasn't. Unfortunately, the Earth free braking is pretty brutal.
So If I have done the addition correctly (I hope so !) the two way trip is 17.4 km/s, which is truly enormous.
And why so much energy ? Because the Earth and Moon are so damn big (we are lucky not living on Jupiter, which escape velocity is accordingly apalling)
The bigger a planet or a moon, the stronger the gravity pull - making so much harder to land and liftoff from it.
By contrast some asteroids are so small, if an astronaut jumped he could escape the gravity pull...
I think the challenge of this thread is, to get a sense of whether or not pipelining a steady stream of missions to the Moon gets us any useful economies as opposed to separate moonshots a la Apollo.
I think one possibility is to use more hydrogen-oxygen engines and less hypergolic "storable" fuels. I've shared a personal horror story of hypergolics involving my late uncle elsewhere on the site; it didn't kill him right away but he struggled for decades with cancers resulting from his assignment cleaning up after a disaster involving a Titan missile--everyone else who went down into that silo with him died years before him, of the same diseases. Never mind that for now--"dragon's blood" is a pretty good fuel I have to admit, quite competitive with kerosene-oxygen, but hydrogen is really good stuff. Awkward in some ways, notably its bulk and its tendency to boil away.
Consider though something like a LEM that used hydrogen-oxygen instead of hypergolics. Allowing a decent safety margin and for gravity losses, it takes a delta-V of about 1800 meters/sec to go down to the Moon from a close orbit (I picked 100 km).
The LM descent stage used 8.2 tonnes of propellant, over half the mass of the whole LM, and an engine that had ISP of 311; the craft massed 15 tonnes approximately so the descent stage could propel it to a total delta-V of 2414. OK, NASA wanted a heck of a safety margin--understandably, since it might have to hover some time looking for a suitable landing site, also I might have underestimated the gravity losses.
Now if we had a hydrogen-oxy engine matching the ISP of the J-2 engines of the launcher, 436, the fuel to achieve the same delta-V (assuming the engine could match the thrust of the actual OTL LM descent engine) would be just 6.5 tonnes, a gain of 1700 kg of mass that could have some other purpose--supplies, equipment--or alternatively the LM could be made lighter, saving still more propellant.
But of course a hydrogen engine is impractical if it has to be loaded at Cape Canaveral, launched and then shipped across deep space for half a week or so plus however long the crew orbits the Moon before going down.
If however we have a sustained and expanding effort to explore the Moon using incrementally improved Apollo tech, we might have eventually sent out a space station of sorts, a rendezvous point and fuel dump. If instead of having to put 6.5 tonnes of liquid hydrogen and oxygen (mostly the latter by mass) in the tanks before launch, an earlier mission had carried out 6.5 tonnes of water, and then the station, using solar power, had split the water into the gases and then liquefied and stored them, the cold fluids could be loaded into the empty LM descent stage tanks at the last minute, and be kept chilled there by the station's facilities (venting boiling fuel into the station to be re-liquefied and then pumping in chilled fuel to replace it) until the landing is ready.
Thus, even if we are still launching standard Saturn Vs and sending the same payloads to the Moon the same way, with disposable stages and all, we can still improve the capabilities with this kind of infrastructure.
It would be impractical to chill the fuel for the ascent stage of a standard LM in the same way, unless the LM went to an established moonbase with extensive facilities including fuel maintenance equipment. A specialized moon shuttle that like an LM massed 15 tonnes fueled, that did not split into stages but kept all its fuel in one set of tanks fueling one engine, would if fueled with hydro-oxy require a bit under 10 tonnes of fuel leaving a bit over 5 tonnes for structure and payload--quite comparable to the total dry mass of the LM! If fuel re-chilling facilities existed on an established moonbase, then such a craft could shuttle between the base and the orbital fuel facility, as long as Earth kept shipping 10 tonne loads of water for conversion to fuel there. This saves 5 tonnes of payload to Lunar orbit.
OTL, NASA did consider the possibility of a hydrogen-oxygen fueled Service Module, which again would present opportunities either to save weight or to enhance mission payloads. Note that that did not assume the availability of any fuel re-cooling facilities; the design had to include inevitable boil-off losses. It helps that hydrogen has a lot of heat capacity and little mass, so launching with an excess of hydrogen could translate into a long shelf-life for the modules involved. I don't like the waste though.
And one thing I do like about reusable transfer vehicles, be they nuclear or chemical fueled (the latter relying on aerocapture at Earth to be reuseable) is that they hold out possibilities of using the more efficient engines to save mission mass at certain critical junctures. Even if we were to have a reusable chemical hydrogen-oxygen orbital transfer stage that simply boosts essentially a CSM-LM stack to TLI, and then leaves them to brake to Lunar orbit and then later have the CSM drive itself back to Earth from orbit alone, the big transfer stage can still enable the SM to be fueled with hydrogen, if it can dock with a fuel reconditioning station while the lunar mission proceeds. In fact it can also bring the fuel it will later need to return to Earth to such a station in the compact and easily stored form of water, or actually use fuel derived from a previous mission, passing its water on to the next mission, to be converted to fuel slowly at the leisure of the stations's presumably limited power supply.
By investing in facilities in Lunar orbit and on the surface then, the existing capabilities of the standard Saturn V, without exotic enhancements, could be leveraged into greater mission capabilities.
