One of the places our more astronautically expert fans felt the series already took leave of any pretense of technological rationality was in the reveal of Space Shuttle Orbiters used to shuttle between LEO and low lunar orbit. While this is a bit hair-raising and does not seem like the most probable evolution from the POD to 1983, I think perhaps it is less insane than it seems at first glance.
To be sure, in just about every phase of flight, including many featured on screen, it would be impossible for an Orbiter derived vehicle to be there without an external fuel tank--this is the same problem I had with Pathfinder being allegedly a "Nerva" or for that matter any sort of plausible nuclear powered ship. (Among others--I didn't even touch much on the terrible problem of intense gamma and neutron radiation coming from a nuclear core powerful enough to achieve the sort of flight shown, nor on the massive weight such a reactor, never mind the shielding mass necessary to avoid killing the crew in seconds. While all these problems are severe, it was the lack of visible container for the reaction mass that made it flat out impossible).
But what if we retcon in a propellant tank?
We have after all Sea Dragon, which apparently in show canon is in a version putting up "550" tonnes of payload into LEO. Presumably it is not crew-rated, so we cannot have a straightforward launch out of LEO to land cargo on the Moon directly, not in missions involving crew going to the Moon anyway. The show does imply that this is exactly the role of Sea Dragon normally, to send great tonnages of cargo to Jamestown base.
Looking at that, I estimate that using the most economical Hohmann orbit, the delta V involved is 3140 m/sec (or less, this is to Lunar apogee) with an encounter speed between craft and Luna that amounts to 840 or so m/sec, which combined with Lunar surface potential gives a landing speed (for direct descent or ascent) of 2525 m/sec. Combining these into two burns of one rocket system, where we have hydrogen-oxygen burning engines with Isp of 450 sec, the landed mass on the Moon would be 152 tonnes. If we increase the Isp a bit, we can add maybe 10 tonnes to that.
But let's come at it another way. Suppose we have modified a Shuttle Orbiter similar to OTL's in these ways:
1) we revise the SSMEs so that they can be restarted in space. This might be a matter of putting external support equipment similar to that used on the ground into a separate tank attached to the Orbiter.
2) we provide for much better radiation protection for the crew. This is not necessary for LEO orbiters because they are under the protection of Earth's magnetic field, but going beyond the Van Allen belts and as far out as Luna, there is no such protection from solar wind particles, and these I think are the most deadly single source of ionizing radiation hazards. There are also cosmic rays, but on one hand there is little that can be done to greatly reduce them, and on the other while their particles are individually far more energetic than all but the most extreme solar particles, their numbers are lower and the nature of high energy interactions actually limits the damage they do when passing through. Whereas a modest thickness of mass can stop the less energetic but numerous solar particles completely. So it is shielding against these and not cosmic rays or other particles moving at relativistic speeds I am talking about.
3) it is probably a good idea to reengineer the thermal protection system, but not strictly necessary I think. The Orbiter is only designed to survive reentry from low Earth orbit, while a craft falling down from the Moon would have double the kinetic energy--but I think it would be possible for a Shuttle to use its aerodynamic features to safely brake off that excess half of its energy in a skip-entry, where it dips into a moderately low altitude perigee to brake from escape speeds down to orbital, and then rises again to LEO to cool off before descending again, this time to enter pretty much the way any normal Orbiter reenters. Or it could instead maneuver to dock with something in LEO.
I would think the structural modifications might be accomplished in a mass-neutral way, but the radiation shielding upgrades would have to cost in mass terms. With the Orbiters massing about 80 tonnes empty, with a 20 tonne cargo capacity to orbit and up to 10 tonnes of OMS propellant to be loaded in (which also serves as the supply of reaction control rocket fuel as well) if we add 10 tonnes fixed for the radiation shielding, we are left with just 10 tonnes to LEO instead of 20. This is the big distinction between Lunar Orbiters and regular LEO ones.
But anyway, with the reduced LEO upmass, this should still work just as well as standard Orbiters to reach LEO.
At this point, we also launch a Sea Dragon payload of 550 tonnes to LEO, which the LO docks with.
This payload is a mix of oxygen-hydrogen fuel tank and Lunar payload.
Now if we want the Shuttle to be able to return to Earth, the normal limit for it to reenter and land from LEO is 100 tonnes entering the atmosphere. Therefore when the Lunar Orbiter has delivered its cargo to Low Lunar Orbit, this is all the mass we need to send back to Earth.
I estimate that if it has at this point a 25 tonne small propellant tank, containing 24 tonnes of oxygen and hydrogen, and massing 1 tonne empty (which is proportional to the ratio of tank mass to propellant found in the Shuttle ET OTL) that should be plenty to send it back to Earth, to do a skip-brake off the upper atmosphere and then either make LEO orbit or reenter back to Earth's surface. Of course the TEI propellant tank is ejected after the TEI burn is complete; course corrections depend on the LO's Orbital Maneuvering System using hypergolic fuels hitherto untapped, except insofar as the Reaction Control System, which taps into the same propellant mix, have depleted it. Any excess mass as the LO approaches its skip-brake with the upper atmosphere is burned off on this approach, leaving just enough for course corrections between the brake maneuver and final reentry to Earth's surface. (Orbiters have to land, to be checked out and refurbished on the ground, though it might sometimes be possible for them to divert to a LEO space station first, perhaps with their OMS/RCS supply topped off to reentry requirements).
It follows that whatever is left over after the 550 tonne tank/payload module launched on Sea Drago plus the 120 tonne on orbit has first sent itself on a translunar trajectory (a Hohmann orbit) and then braked to LLO is 1) 120 tonnes of Shuttle, including a 10 tonne payload; 2) the separate reserve 25 tonne TEI propellant tank; 3) empty tankage for whatever amount of propellant is needed for these two burns to put the remnant into LLO and 4) cargo for Jamestown base.
As it happens the combination of burns from LEO to LLO is such that with engines getting 450 sec Isp, the mass ratio is 2.5, so 40 percent of whatever boosts out of LEO arrives at LLO. Out of 670 tonnes, this is 268 tonnes, thus we burned 402 tonnes--a full Shuttle ET holds 750, so the Sea Dragon tank would mass 16 tonnes dry (versus 30 for the launch ET). The 25 tonne TEI tank set is separate. All this accounts for 141 tonnes, the rest is payload launched on Sea Dragon and 10 tonnes from the Orbiter, totaling 137 tonnes.
Now we need a vehicle to rise up from the Lunar surface, and rendezvous with the LunarOrbiter. By 1983, Jamestown should be producing and storing hydrogen and oxygen fuel, so we have a surface-LLO and back again shuttle vehicle--much bigger than the LSAM shown on the show--that we load full of hydrogen/oxygen propellant. Much of this is used up sending the rest up into LLO. I guessed some 25 tonnes of dry mass would be enough to cover any tankage, engines, structural reinforcement and an emergency escape vehicle the crews ride in. To land the cargo, and haul up 10 tonnes (including the mass of Jamestown personnel rotating home) we'd need to load in a considerable amount of oxygen/hydrogen propellant, close to 200 tonnes of it, from Lunar sources. I envision this thing being a bit like the Eagles in Space: 1999; the tank is the long core element, with landing gear and engines and a suspension girder system on one side, with the crew vehicle on one tip (presumably the LOX tank tip, it being less cold). The LO would dock with the crew vehicle on the nose, transfer crew, then maneuver to transfer the cargo rack riding on the opposite side of the TLI/LLO ET. The Super-Eagle would then bring the payloads down. The design can compensate for uneven mass distribution by throttling engines appropriately.
Note that if the Super-Eagle is sized to carry more propellant, we can fuel the 1-tonne TEI tank set with Lunar sourced propellant, raising the downmass cargo to Luna surface by 24 tonnes to 161 tonnes; this of course requires considerably more than 24 tonnes of propellant to be added, to boost the larger mass to LLO and then land the larger cargo as well as any added structural mass--I do think 25 tonnes dry still allows for plenty of that though.
Thus, by combining a Sea Dragon and Shuttle launch, we can in fact deliver essentially the same downmass to Luna while enabling a Shuttle to be the vehicle crew ascend and descend, to Luna orbit and back to Earth--provided we have first invested in the mining and processing and storage of hydrogen and oxygen on the Moon, and the Super-Eagle to shuttle between Lunar surface and LLO.
Of course this system has some problems--for one thing, what to do with the expended TLI/LLO transfer 400 tonne empty propellant tank, now stranded in LLO? We could just leave it there, but the orbit would start getting cluttered with these 16 tonne units of space junk. For modest propellant cost, we could deorbit them to crash on the lunar surface, and maybe someday harvest the debris for useful metal. If we just leave them in the easiest Lunar orbit that passed over Jamestown (practically at the South Pole) they would drift out of it pretty fast, due to the orbital perturbations from Lunar mascons, which would probably have the damn things crashing to the Lunar surface at random within months or anyway years.
But what if we included their mass in the down-mass payload to the Lunar surface? The SuperEagle I envisioned would be somewhat dwarfed by its vast bulk--but the 16 tonne mass is not a tremendous burden to add, it just increases the downmass load by 10 percent, so make the SE's a bit bigger and load them with more propellant at launch from Jamestown, and they can haul them down pretty handily. Then upon landing, we would have the crewed lifeboat vehicle met with a ladder (possibly even pressurized so crew don't have to suit up to transfer into Jamestown), drop the cargo rack to be wheeled away to a loading dock, drain out any residual oxygen and hydrogen from the SE, and using some cranes unhitch the spent TLI/LLO tank and stash it away nearby, perhaps under a light tarp to cut down on deterioration from raw sunlight and the occasional micrometeor. There would be no point in doing this if the tanks didn't have potential uses of course--but they clearly do! For one thing we are in the business of manufacturing liquid hydrogen and oxygen, and behold these tanks are just for the purpose of storing the stuff, so there we are--just run some insulated lines out to the tank parking lot and fill them up with surplus propellant, running ullage lines back for any hydrogen boiloff to re-compress it back to liquid.
For another, it would be possible to use them as structural units for habitation or lab/workshop space. To cut down long term radiation exposure to Earth-surface levels, we have to do something about cosmic rays, which are attenuated by the 10 tonnes of Earth atmosphere above each square meter of sea level. Well, I believe raw regolith is about 2.5-3 times the density of water, so 3-4 meter thick layers should pretty well duplicate the radiation shielding effect, and into the bargain would handily stop even the most intense Solar output particles too. All the inhabited, pressurized workspaces and residence in Jamestown should be under these regolith layers. So, if we have a tank holding 400 tonnes of oxygen and hydrogen liquid fuel, in a 6:1 ratio, it should bulk about 1000 cubic meters. We can just pile up regolith right on top of it--one might fear what 10 tonnes per square meter would do to the structure, but it only weighs less than 2 tonnes would on Earth, and the tanks are designed to contain several atmospheres of pressure--so if we fill a tank with pure gaseous oxygen at about 1/5 atmosphere, the internal pressure would balance the weight of regolith pretty exactly, or we can add to that 4 parts more nitrogen (I gather ammonia is one of the volatiles we'd find with Lunar ice, so there is one source of that) for a full sea level style atmosphere, and the tank can easily handle the 0.8 atmosphere overpressure. Once we have done this, and attached some kind of airlock or linking tunnel to other pressure zones, we have a cavernous space many meters tall (over 7 if we shrink the tank in proportion keeping similarity with launch ET, but I think it might be smarter to preserve the 9 meter diameter and just shorten it) and tens of meters long.
One way or another it seems well worth bringing the things down to Jamestown or possibly to other sites. So that is a fair solution to the space junk problem! (The little TEI tank set would simply burn up in Earth's atmosphere).
There would be other ways to go of course. One might leave a few of them in LLO to develop an orbital tanking station--hauling Lunar propellant product up to fill them, and with incoming vessels from Earth or LEO topping off with them.
Note that if we want to maintain anything in orbit above Jamestown, as opposed to brief rendezvouses which are all we see on the show, it will not do to just pull into a polar orbit. As noted, mahscons will derange these orbits and probably crash anything put there. And if we somehow had an orbit where things did not crash from, as a general thing, orbits maintain their axis due to conservation of angular momentum--so an orbit that was perfectly good for a ship coming from LEO to enter from a Hohmann orbit will hold still versus the fixed stars, while the Moon orbits the Earth and thus another vessel coming in will find this parking orbit at a severe angle. Jamestown based ships could reach it anyway, but it would be a bad orbit to boost back to Earth from, until half a month had passed.
But wait! in real life, orbits don't preserve their axes, because there are generally perturbing torques operating on them, which cause them to precess like a gyroscope tipped over on Earth. In fact, it is possible for the mascons themselves to cause the orbit to precess as fast as the Moon spins and orbits Earth, and then the orbit would maintain a track over the same terrain on the Lunar surface indefinitely. In fact the orbit would not be an ideal circle, it would be battered, as the orbiting object slalomed side to side and up and down due to the layout of various mascons it passes more or less over--but sometimes it happens that the net effect is to return it to the same altitude and heading it started at. But this means it will pass over the same terrain again and again, ideally on the exact same path each time.
These types of Lunar orbits are called "frozen orbits," and if we could find one that passes over Jamestown and clings more or less to the limb of the Moon as seen from Earth, that orbit would be ideal for receiving ships coming in from a Hohmann transfer, and sending vehicles returning to Earth on such a transfer orbit. If we put a space station there, it would wobble around the ideal circle, speeding up, slowing down, veering east or west, but it would always pass over Jamestown at the same height and heading and would in general hang around the limb of the Moon as seen from Earth, thus in l line of sight. We could set up a fuel depot there and also a bunch of relay satellites, and the entire strip of limb visible to one or another at any given time would be in close radio contact. In real life I imagine frozen orbits are just metastable, the object never quite returns to the same parameters over the same point on the surface, and someday it will start diverging more and more and eventually crash. But a fuel depot at least will have plenty of propellant to correct its course actively, and we can design the comsats to have reserves to maneuver too, which we can periodically renew--or just capture and replace the satellites with upgraded ones, and refurbish or junk the old ones.
There is no need to have recourse to a frozen orbit if we are not maintaining a station in LLO, but I do think that is the logical direction of the Jamestown project after all.
For general Lunar exploration, the idea of setting up a base at Lagrange 2 (there are various reasons L1 is a lot less popular) is widely touted, but I think this is a bit dubious. The Lagrange points are a long way from Luna, about as far as geosynch satellites are from Earth, about 1/10 the way from Earth to the Moon, or near 40,000 km. On low energy more economical transfer orbits, it takes a long time to get from the surface to such a point, many days in fact, and these are days the transfer vehicle is exposed to Solar particle radiation. And it takes serious delta-V to get there--very nearly Lunar escape velocity or so I estimate, so a round trip out to L2 and back to Shackleton (or any other point on the Moon, but Shackleton is where the fuel supply is!) would involve delta-V of 4900 m/sec or more--more than it takes to go from LEO to LLO in fact. For a Lunar shuttle such as the show's LSAM to go out and back, exchanging identical mass cargoes, would involve a mass ratio of 3 or so--we'd have to load in twice the mass of propellant if we are using oxygen-hydrogen chemical engines, versus craft dry mass and payload each way.
The ideal Frozen orbit I desire might not exist, but I think there is a fair chance one does; we already know of several, and perhaps some of the known orbits are close enough already. If not, I think it is doable to find one.
It would not give easy anytime access to any point on Luna whatsoever, but it would give such access to a band of Luna spanning all latitudes, along the limb. Surely that is a good piece of the Moon, and transfers from surface to it are much cheaper and much much faster than to the Lagrange points, whereas the particular range I hope to find one in is also ideal for transfer to and from Luna/Earth Hohmann transfer orbits. It is more economical to go to L2 than to this "FLO," but that advantage is eaten up and worse transferring to the surface from there. (And it is very very difficult to go from a Lagrange point to FLO or vice versa, the angles are all wrong!
In the context of FAM, where the USA and USSR are focused on developing Shackleton mainly for the water to use as rocket fuel, I think LEO-FLO is the obvious line of development. Initially masses will be landed on the Moon from Earth, and then direct landing from the Hohmann transfer orbit is an excellent option, going one-way. Given though that the programs involve developing an expanding inhabited Moonbase for each power, both will want to use in situ Lunar resources to at the very least save mass to be landed for returning crew back to Earth by using Lunar propellant, and in this context, shipping propellant up to FLO to maximize delivery from LEO to FLO, and enabling all transfers up or down between surface and FLO with lunar stocks, is the intelligent thing to do.
I think I have shown how it is possible for the Yankees to indeed use a variant of the Shuttle Orbiter for human passage between LEO and FLO and then back to Earth from FLO. It is not settled at all that this is a rational course versus better alternatives the same technology might enable! The advantage of using the Orbiter is that, with additional investment of structure mass in radiation protection for the crew (not that the show seems to be paying attention to that in the least, the design of Jamestown base is all wrong, with all those modules parked on the surface and no regolith piled on them, or very little, for instance) and assuming as I do the aerobraking skip maneuver is reliable with a vehicle with the hypersonic aerodynamics of the Orbiter, the return to Earth from FLO (or just any old LLO passing over Shackleton, if the rendezvous is quick enough) is cheap even for the high mass involved, whereas if we have hundreds of tonnes of propellant boosted up from Earth on Sea Dragons, the Orbiter mass is modest compared to the payload delivered to FLO (about the same, favoring the latter). The complex and critical element that provides safe habitation for humans and carries the elaborate, expensive but high performance reusable Shuttle Main Engines returns to Earth with each sortie, to be inspected and refurbished for another mission.
It is not crazy--if only the aired canon had bothered to show us a big ET on the belly of the Shuttle arriving at Luna or en route there! We might be able to get away with pretending we were only seeing the Lunar Orbiters coasting back to Earth, at which point they would have ditched the miniature (but still pretty bulky!) auxiliary TEI tank.
And if the LSAM had in fact been accompanied by a huge Super-Eagle, which I am tempted to call a Roc. (Lord of the Rings aside, Eagles are not mythically viewed as transport vehicles).
This same form of transfer could work with smaller ETs and payloads than the 550 tonne canon Sea Dragon payload of course. If we limit the Shuttle upmass to Luna to the 20 tonnes capacity standard (which recall I intruded on with radiation shielding for the crew) and thus downmass from LLO as before to 100 tonnes, but keep a single medium sized ET, to place that minimum 125 tonnes into FLO (pointlessly, with zero cargo up to Luna) requires 187 tonnes beyond the 23 or so, or 210 all up. Such a tank would mass maybe 2 tonnes dry, so that is another 3 tonnes or so. Now to boost the measly 20 tonne upmass to FLO as well requires another 30 tonnes, so we are looking at 240 tonnes. A Sea Dragon could launch two such tanks, but not three. This is silly of course--if anything we'd want to go the other way!
That is, suppose the program developed the potential inherent in the Space Transport System OTL to orbit the full 750 tonne propellant capacity ET needed to reach orbit all the way to LEO. This was avoided OTL to prevent accumulation of space junk in LEO, but with a moon-focused high budget program, we could reasonably focus on developing an orbital propellant depot. 3 or more ET's could be an excellent starting point for such a LEO station! Then develop a fuel tanker version of STS--we can do this by developing a form of "Shuttle-C" as most recently explored in the collaborative To Boldly Go TL that starts with transforming the test article Enterprise Shuttle plus its ET retained into orbit as a nearly "instant" Space Station Freedom. With development of an uncrewed engine module to recover the SSMEs, an ET lifted by such a thing can retain perhaps a hundred or more tonnes of propellant. Thus a dozen or so launches can provide propellant for a couple ET refills on orbit. If we then launch a Lunar Orbiter and refill its full tank, and separately on other Shuttle-C launches accumulate a FLO payload for it, we can reserve 33 or so tonnes of propellant to return the LO and a full 30 tonne ET back to Earth, disposing of the tank in the atmosphere. This leaves some 720 tonnes of propellant to be expended placing 2/3 that amount, or 480, into FLO--of this 163 is the reserve to return to Earth, leaving a whopping 317, double the above Sea Dragon launch based payload, down to Luna. And if we supply the 33 tonnes of propellent for the LO and tank to boost back to Earth, that becomes 350 tonnes to be landed on the Moon. Shuttle-C might manage to deliver 80 tonnes or so a launch, so we'd be looking at 5 of those (with surplus stored toward a future sortie at the LEO space station), one Lunar Orbiter launch, and maybe 15 or so Shuttle- C tankers. Aside from the Lunar Orbiter launch, we have to place 1100 tonnes in orbit altogether--so actually we'd do well to develop Sea Dragon anyway and use two launches. Now we have only one type of ET, the same standard we use for ordinary Shuttle orbital launches, so sticking to that the Roc concept of surface to FLO lunar shuttle would be built around a landed 30 tonne dry standard ET. With 720 tonnes of propellant to use up from Lunar sources (30 of 750 loaded go to the Lunar Orbiter's ET that we use twice then dispose of, recall) to put through 4000 m/sec delta V (up to FLO then back) we could boost 500 tonnes up and 500 down! Now that would mean on takeoff it massed 1220 tonnes, which even at Lunar gravity would mean we'd need over 200 tonnes (Earth weight force) of thrust, or a full SSME, just to hover it, and thus a full Orbiter complement of three to lift it briskly. And in fact with a full complement of such engines plus a 30 tonne tank structure, that's only 40 tonnes dry. To manage a solid grip on 350 tonnes downmass, we might well indeed need say 50 more tonnes structure all up, so call the dry mass 100 tonnes to be generous, and bearing in mind we don't have 400 to haul down, just 350, it would seem we have overkill. But wait! If we have a propellant depot in FLO, which we can keep augmenting by the way by hauling spare ETs that otherwise would just be burnt up or otherwise disposed, we can usefully haul up spare propellant and deposit it. If we have only 400 tonnes all up to land on the Moon from FLO, we need 224 to land it, call it 240 for safety margin. Over this we need to deliver, again call it 40 for safety margin, to the returning LO's ET, so we need to deliver, along with 10 tonnes of mass returning to Earth, 280 of propellant, 10 of cargo, 100 of structure, or 490 to FLO. Starting with 110+the full 750 tonnes of propellant, we actually will deliver 551, so this arrangement leaves a surplus of 60 tonnes to park in the FLO propellant depot. And at other times, when there is no payload coming in from Earth, we would take off massing 880 but require only 493 to reach FLO, then to land the 100 tonne dry mass again, just 56 tonnes more--leaving 200 tonnes to bank in the depot. Out of every 750 tonnes produced then we can use 200 in deeper space. I believe we'd find if we tried this same trick to stockpile fuel at L2, we'd use up a lot more sending it there.
Meanwhile the sorts of vehicles that benefit the most from being launched interplanetary from Moon-Earth L2 are those that do not use high thrust, low Isp engines that would be using a mix of oxygen and hydrogen. It is quite possible a nuclear thermal rocket that uses only hydrogen would be most suitably launched from there, but this is a later evolution of the program. As of the mid-80s, the US program remains chemical (until in canon Pathfinder launches, but as I have posted elsewhere that is a pretty ASB and imponderable super-technology, quite unlikely to be workable in anything remotely as shown. And with all its other gee-whiz superpowers we can infer from its canon trajectory, it is also high thrust!)
As a general thing, high thrust vehicles benefit from the Oberth Effect, whereby it is generally more effective in terms of delta-V achieved to fire rockets low in a gravity well than higher. As long as NASA is using hydrogen-oxygen chemical engines, and even when using some kinds of more advanced ones, it makes more sense to assemble interplanetary vehicles for launch from low Earth orbit than from a distant lunar base.
It might indeed make sense to ship propellant down to LEO from Luna, rather than launch it up to LEO from Earth. This might be the case if production of it on Luna is inexpensive enough and the sources are massive enough that prodigal overall consumption is not going to deplete the supply any time soon.
I believe that it will be when we look into that, that we will find that the case for Lunar Orbiter becomes weakest and we would see how a very different program than shown in the show canon would be far more rational.
