Of course it occurs to me after writing all that that with two Orbiter launches we could design a compound unit made of two Orbiter loads, a dedicated and safe corridor docking extension and then piggy-back a separate dedicated hypergol fuel tank unit, my intuition says bracket it onto the corridor module in the -X direction, out of the "wind" from the station's nose-forward orbital attitude. We would still have some thrusters (well, one, the -X thruster nozzle facing into the "wind") on the docking corridor, but it would be inert until the tank unit with the other three thrusters attached arrives. This would allow simpler and lighter propellant tanks and a possible helium tank separating them to be simple cylinders with hemispheric caps, holding considerably more volume and mass of propellant.
Flipping it around, if we don't mind the corridor being a flimsy inflatable thing, we might be able to include it in a single launched module, now it is the corridor that goes in the lee. Later launches, or a previous one, can supply Whipple shield panels to erect around the corridor, which helps with thermal management of the corridor too.
Some interesting thoughts,
@Shevek23 . An interesting point, of course, is how much balanced thrust matters for space stations (or doesn't!). Much like ISS (or KSP) large Control Moment Gyroscopes can be used for most reorientation, and reboost will likely be confined as much as possible to visiting orbiter's thrusters to keep time off
Enterprise's systems. However, the question of where and how Enterprise's pressurized volume, power generation, radiators, and other primary systems might be expanded is an interesting one we'll be touching on in the next few posts!
As the station grows, however it grows, its drag will increase. We need some thrusters working, or anyway capable of working, to maintain orbit. I presume design has to allow for the possibility that Orbiter launches might be suspended with crew stranded on the station, or even a whole Orbiter load of them suffering what Discovery suffered, damaged TPS. The station can only be safe harbor for Orbiters with the delta-V to get to SSE's inclination and altitude of course, but odds are fair it can be done. My speculation as to the standard maximum crew capacity for sustained operations in normal conditions is pretty conservative, based on power generation and the reflection that Orbiter launches will not be terribly frequent, probably 8 times a year as OTL, and only some of those launches are for Enterprise operations, others are free flyers for other missions. Say my guesstimate of 14 is low, and crew is more like 21, plus three visiting flight crew for an Orbiter when one is docked, and we add another 8 to these 24 for 32. Even with just the LOX tank in use for hab/lab they won't be crowded. The issue would be vital supplies. CO2 scrubbers would go first except I suppose SSE involves developing reusable scrubbers. Check. Then breathing oxygen, but it would not be hard to have lots of that in stock. Check. Power is solar so a certain power budget is always available without limit, fine. Water can be recovered from human exhalations as our bodies metabolize food and oxygen into excess water, we don't even have to look into recycling urine, though that is doable too, so food becomes the hard limit. If there are plans to ship up food in such emergencies, stranded crew can wait out long holds on American crew launch capacity, also it might be possible to rely on European, Japanese, Chinese or Soviet/Russian capsules being sent up uncrewed and docked by remote control to give at least some of the 32 the chance to return to Earth and lower burdens on life support for the rest. (The Russians probably can't send an empty Soyuz to SSE, though they might be able to from Russian launch sites, but setting up to launch Soyuz from Kourou is possible, or Cape Canaveral for that matter. Your posts have mentioned the European capsule program, so perhaps I should just strike Soyuz and the Russians from the list and focus on those European things out of Kourou.
In such a circumstance, with a couple dozen or more crew stuck aboard SSE for perhaps a year or more, the station had better be able to maintain altitude!
And reaction wheel/gyro systems can jam or otherwise break down. I like the use of such systems, in fact given the basic problem of maintaining attitude on a metastable axis, I suppose fluctuations that need to be corrected will be chaotic and as likely to go one way as the other so the major control load will be desaturating itself for the most part, cutting the net accumulation down to a filtered drift. Yaw and roll perturbations seem as likely to be either way so the gyro system won't require a great deal of desaturation. When it works, great. Someday it might not work, then we need strong reaction jet control until it can be fixed.
Another thing worth remembering, if we are building along the Z axis, dorsally, ventrally or both, is that the more mass we shift out of the XY plane, the higher the negative feedback tidal torque is, which offsets the positive feedback metastable tendency of the station as currently laid out to tumble out of orientation, thus saving the number of attitude maintenance thrusts we need. Drag rises, but we have this offsetting consideration. Building mass into the LOX tank will skew the metastable X axis to an angle to the station nominal X axis, thus to stay in metastable balance the attitude will be skewed and drag area increased. The easiest way to bring it back would be if our dorsal tower were forward, off the intertank ring, but alas Leonardo's port is way aft. While if we build ventrally from the ventral port, the axis is pretty far forward, the intertank ring being about 1/4 of the way from the nose (or 1/3 bearing in mind the nose is pointed). The result of doing both would be to have the mass X axis diagonal to the hull shape axis.
One thing we could do--dock a module to Leonardo parallel to it, lying just "above" it in the frame of the Orbiter, securing the end near Enterprise built in habitation, and have a second port on the dorsal side above the estimated X location of the center of mass projected for the station. Similarly though with more difficultly we could shift the ventral axis of growth aft by putting a suitable module at the ventral ET port. Securing its aft end would have to involve some sort of new truss, running I might think as a half circle arch to the hoped for side trusses. Once there, if secured strongly enough, we have two towers each growing where we expect the CM to settle, somewhat aft of Enterprise's nose I guess.
But we'd have to add a lot of modules to totally nullify the pitching moment. The resulting towers would have fairly high drag. That's OK if we can keep raising the control.
...However, the question of where and how Enterprise's pressurized volume, power generation, radiators, and other primary systems might be expanded is an interesting one we'll be touching on in the next few posts!
Looking forward to that!
I might as well toss out how I've been thinking about it.
Volume--where and how to grow. Obviously I favor using the ET volume to the greatest degree as soon as possible. Versus the OTL Orbiter alone system, the tank brings liabilities of higher drag and mass, we should start benefiting to leverage those costs ASAP. Since I've harped on the wisdom of using the tank volume and avoiding module expansion, let me acknowledge the drawbacks!
a)With space experiments, isolation is often desirable. One experiment will be doing things that might interfere with another. With lots of separate modules, managing this is easier. Against that, I think I already mentioned the idea of using the tail end of the ET as a large ultrahard vacuum volume. I didn't realize when I was suggesting that that the hydrogen tank bottom already has an inspection hatch built in, so the idea of using the tail end of the tank as an EVA hangar seems pretty good. If the inspection hatch is narrow for an EVA suited astronaut (and still worse if someone else in an EVA suit is trying to bring an injured spacewalker in to safety) then perhaps we should put an inflatable EVA cabin back there, that can gradually depressurize by replacing full pressure air with low pressure pure oxygen, allowing EVA crews to decompress for EVA in a shirtsleeves setting, with an inflatable airlock added on to that. Though actually EVA is supposed to be out of Leonardo's tail end, adding a new airlock in the same general zone is probably not a great idea.
Anyway, if we do all the lab work that happens in normal atmosphere within either tank, they are all more liable to interfere.
b) proofing against major catastrophe. We know from Mir experience that breakdowns and mishaps in space stations are no remote possibility and no joke. With Mir's dissected structure it was at least possible to abandon specific modules and seal them off, or vent them. But what if at some intermediate stage of operations, essentially everything is happening in the LOX tank? (It won't be, we still also have Leonardo and Enterprise's built in hab section from the get go even if we have nothing else, and surely something is going onto Leonardo's expansion port real soon. OK anyway say half or more of everything, habitation and lab work, is in the LOX tank). And something bad happens--a fire for instance.
Clearly leaving the LOX tank one big room is not going to work. We need to partition it. How? Obviously any partitions should be fireproof, I am not sure how one accomplishes that without introducing other hazards. Asbestos panels would be nicely fireproof but we know better than to subject the free fall crew to breathing asbestos dust! Kevlar is essentially the rich man's asbestos, it has similar fire resistance and heat tolerance, and I suppose it poses similar hazards if little flakes or dust of it come off. Fiberglass won't burn (well I guess anything will if hot enough and exposed to enough oxygen concentration, but it is pretty well fireproof anyway) but little flakes of it, little broken off strands, would hurt whoever breathes it much as would asbestos. I imagine NASA had some pretty good solutions already.
The partitions should be sound-attenuating and thermally insulating too I suppose. Thermal management of the tanks I suspect is best done primarily by blowing in cool cleaned air, and removing stale warmed air, with forced draft circulation of some kind. Not only do we want to change the air and cool it down probably (we might need to warm it up instead, especially in early days) but in free fall exhaled carbon dioxide would accumulate around an immobile person's head; people would die of hypoxia and CO2 poisoning in their sleep. One needs to blow the air around so there are no still pockets. A single blower pumping suitably cooled air in would probably not cut it but several valves in the right place might serve. But if we cut the tank up into pockets, each one needs its own circulation, both input fresh air and removal by suction.
I am thinking each crew member should have a tiny rack, 5 cubic meters, they sleep on one side under a net or straps and have the rest of the space for limited motion and privacy, and should be able to adjust temperature and humidity as they see fit within a certain range. The rest of habitation is common spaces, but under this catastrophe proofing consideration, they all need fast evacuation routes out of a given major pressure space. Compartmentalization can slow the spread of hazards and buy time but it cannot guarantee long term safety. If the crew all sleep in the ET LOX tank, their only way out of that, in current configuration, is via a single narrow hatch.
I am very very leery of making new cuts in the ET structure for a variety of reasons, structural and because sawing through the materials is likely to create dust hazards hard to contain in free fall. But I do think some kind of second way out of the ET nose section is needed, and the obvious place is the nose tip. Ideally a docking port for an escape vehicle should be provided there, and better yet have this atop some kind of corridor that leads to some other habitable space. Earlier I suggested building a tunnel to the Enterprise built in side hatch, not realizing that the Intertank Ring is so close to the nose of the orbiter and in fact ahead of it, and that therefore the built in transfer tunnel to the Enterprise forward habitation set is already in place. (I thought intertank ring access was via a port in Leonardo). A long tunnel back from the nose tip of the ET to the side hatch, which can also have a second airlock and perhaps decompression porch branching off this access, seems in order--and this would give crew sealing off Leonardo due to a fire or the like there a direct route to a nose mounted escape vehicle that need not divert through the intertank and LOX tank volume.
If we had the side trusses and extended them forward along the LOX ET nose in an arc, attached to any installed hardpoints there, we'd have a nose structure to attach the cut out nose hatch and escape vehicle dock to, and an anchor for the bridge to the Orbiter hatch.
My notion is that the LOX tank should be dedicated to crew habitation, that we pack all the crew racks into rings opening onto corridors giving ready access to the intertank ring, and have common crew volume for recreation, hanging out, eating and infirmary equipment forward of the crew spaces, everything laid out to give easy and fast access to the nose hatch, and that we initially treat the hydrogen tank as space for all kinds of lab work. It already has a tail escape hatch, which should have at least an emergency ingress/egress air lock, a second crew escape vehicle parked there, air inflated corridors perhaps back to whatever is docked to Leonardo. If we do a lot of dorsal building, we should take care to always have two paths in or out of any inhabited module and a third escape vehicle as an alternative to an Orbiter that might not be in dock, and similarly any ventral build should have the same considerations.
The hydrogen tank as noted is where tank installed lab racks should be from the get go. If we build lab installations from the tail forward, we counter the shift of CM due to filling up the nose tank--both by slowing the buildup of rack installation there, and shifting the lab work masses as far aft as possible. Persons who might be trapped by containment measures in the rear can hope to escape out the tail hatch, if not to an escape vehicle than to external installed light corridors back forward to the dorsal or ventral ports. Initially crew will enter the cavernous space and find nothing but fireproof tensioned gripping lines to cross to the rear lab zone, partitioned off with a movable heavy containment curtain. As station work expands that curtain comes forward, or a new one is installed to subdivide the lab zone; each cylinder section can be radially subdivided by radial curtains to segment the volume into separate compartments. Here we won't generally need soundproofing or temperature control, wherever a given experiment requires this, we use thicker curtains. Before we can fill the rear tank, the empty forward space can be used for exercise or other ad hoc purposes, and an escape route to the intertank ring should be adequate.
Anyone in the hydrogen tank is presumably alert and able to respond rapidly to any alarms and mission planning will have drilled them as to their escape paths. The LOX tank is more critical as this is where tired people and sleeping people will generally be and in general everyone is relaxed and off their A game there.
I feel justified in bringing up escape craft both because others have, and because the OTL 1991 ODSSF paper provided for one (taken over from other Freedom plans) and indeed we have had hints about the European crew vehicle, which might well be repurposed into modules launched uncrewed and perhaps not fully supplied that could be designed to sit for years on standby drawing off modest amounts of station power. Of course I just suggested four where the '91 plan thought one would be plenty.
But there is more to discuss spinning off your post that is not related, and this subject deserves a post of its own. Suffice it to say that guesstimating as many as 32 people might be stuck on SSE without being able to return by Orbiter, 4 escape vehicles each capable of holding 8 people (exactly the spec in the OTL Orbiter-alone station plan) is hardly excessive! Capacity should be greater than crew as some people might be stuck unable to get to a vehicle with room for them. Then again, it should not be too hard for escape vehicles to be able to maneuver and redock elsewhere, so an empty one cut off by a failure on its access path should be able to move over to pick up excess refugees when a full one vacates its port. More than 4 might be excessive.
c)--positive cases for added Orbiter bay modules, and possible large modules shipped up by Shuttle C--some experiments will want amenities that won't be easily provided in the common tank atmosphere. Suppose we want to use direct sunlight for some purpose; we want the module where that is to be mounted to get it. Since the station is rotating over its orbit, and the axis it rotates about runs in the Y direction, port and starboard, and ideally would be perpendicular to the plane the Sun seems to revolve in, but in real life this is tricky. I believe if the station were in an equatorial orbit, zero inclination, the Y axis would remain aligned with Earth's rotational axis, but obviously with Earth being tilted 23 degrees relative to the normal of the ecliptic plane, we get the same seasons Earth gets, and the cylinder surface of a module parallel to the Y axis would see the sun apparently oscillating from solstice to solstice, only at the equinoxes would we get the ideal illumination revolving around the cylinder. I am not sure I correctly visualize how the 39 degree inclination relative to Earth's axis complicates this, but I believe it would end up involving a precession of the orbital axis relative to Earth's over a period of many years, and relative to the ecliptic, the inclination would oscillate between +62 and -16 over those years. (The orbit would always remain 39 degrees relative to Earth's axis).
I have several ideas what we might want direct solar illumination to do in various experiments, but plainly step one is to get the module outside the ET.
Other things we might want to mess around with include experiments involving the ultra-hard vacuum in the station wake--this is a bonus of this station design that large volumes are compactly consolidated to make a particularly large "wake." (The OMS +Z axis thrusters will tend to contaminate the wake zone right behind the ET, maybe we can either shield the experiment zone or shut them down in favor of other thrusters elsewhere). For this we don't actually want a separate module, we want a porch of some kind attached right behind the ET. Similarly for experiments measuring the effect of full on atmospheric impact, we want them mounted on another porch in front of the ET.
As I've noted if we make long and massive enough booms along the Z axis, we nullify the tidal potential that the gyros and thrusters are always fighting to keep the station oriented. Make them longer, with more mass more distant from the CM in those directions, and we get natural tidal stabilization, at the price of higher drag of course.
I've proposed a way to get a centrifuge bio-lab inside the ET, in rather severe circumstances to be sure especially for human investigators within the thing. We might instead send up two modules, or one payload of two half-modules, with a rigid truss to separate them a bit, and set these spinning for a dumbbell centrifuge lab, outside on another boom presumably composed of Legoed-together modules. The obvious axis for these would be parallel to the station spin axis, that is the Y axis, which puts them in the same plane (or more or less Z offset) as the solar panels. Or we could mount it on a Z axis boom, but that will complicate Orbiter and other approaching or receding spacecraft.
By and large though I believe most purposes, even a Lunar gravity bio-centrifuge, can be installed within the ET. (My particular notion of such a centrifuge did involve the section of the ET it spins in to be in vacuum to minimize drag, but I suppose we could spin it within atmosphere and just deal with the drag issues).
Power Generation---I figured the obvious place to install a third upgrade panel set which might be all we ever need to add, would be forward of the existing set, on my proposed SRB mounted trusses, far enough forward so they full clearance versus the installed set. They'd enjoy some Z offset, being mounted astride the ET centerline rather than Enterprise's. Now the trouble with installing two arrays in a line is that as the station pitches over its orbit on the day side, one array will shadow the other. Deploying them farther out on the Y axis would be nice, but I thought that would be mechanically demanding, requiring a heavy cantilever truss for each wing. But then again, noting that it is solar panels that suffer the most from trace reaction engine impingement, we might want to make such a wide Y axis paralleling wing set after all--and perhaps we can get dual use for such long offsetting trusses by installing modules or other operational stuff on them.
Z axis towers with their drawbacks but also benefits could be another place, but then we'd hardly want to have a thruster on their mounting tower or mast, unless we can stretch the tower farther to reduce trace gas impingement by pushing the thruster cluster that much farther out. We would not want to make Orbiters or other visiting craft dock so very far out!
Another idea, probably too far out there for SSE, maybe good for a future station, would be to set up a solar array as a separately orbiting object, say a kilometer or so off, and then use a rotating microwave beam antenna to beam power to the station, and thus require a simpler, lighter yet higher power density receiver which of course must track the solar array free flyer--but in the same orbit, the solar unit would tend to stay in the same relative angle to the station structure; the rectenna picking up the power would only need minor aim tweaking. Over time the separate objects would drift and thus both need to actively maintain their orbits to stay in synch. (Mutual gravitational attraction is part of why they would drift but I suspect very minor compared to other perturbations).
Another approach would be to sacrifice the efficiency of sun-tracking panels and mount them fixed, in planes parallel to the station spin axis, and perhaps tilting them over more limited ranges to track seasonal offsets of the apparent circle of the apparently revolving sun. Say we had six panels, say they are squares. If we could have a truss running across the nose of the ET and another across the tail diameter, we might (foregoing other ideas like mounting an escape capsule there, just mount the solar panel flat, in the YZ plane, and fixed, and the second one similarly on the tail. A third might be mounted over the forward part of Enterprise at 60 degrees from the nose/tail ones, that is 30 degrees to the Z axis, a fourth behind and flipped the other way, and another such pair on the ventral side. Now if the station comes from behind Earth nose first, the nose panel is in full direct sunlight, whereas the two that are forward on dorsal and ventral side are at 60 degrees and thus generate about half full power each, while the other three are in shadow and generate nothing. As we come 30 degrees past "dawn," the dorsal forward panel has dropped to zero power, and the nose panel is at 86 percent, but the ventral forward panel is also at 86 percent, at 60 degrees past it is now the ventral forward panel head on, the ventral rear one has ramped up to 1/2 power and the nose unit has dropped to 1/2. At 90 the two ventral panels deliver 86 percent each, and as we keep going the tail panel powers on and then the dorsal rear. The station goes into shadow with the three rear panels generating power. Thus power fluctuates between 1.72 and 2 full on panel power generation, and if we delete the dorsal panels we have 1.5 at dawn, rising to 1.72 and then to 2 and not dropping below 1.72 until we are 30 degrees away from going into shadow. So for 1/3 an orbit it averages 1.82 panels full on worth, and for 1/6 an orbit (2 1/12 periods that is) 1.61. That works out to 1.75 panels worth for half the orbit, and zero the other half. But of course for our standard array installed, even if it can track the sun for full power all the time in sunlight, half the time it is getting none either, so if each square panel element has the same area as the sun-tracker array does, we generate an additional 1.75 times as much power. Of course we had to install 4 panels, not 1.75, to get that. We can scale the panels down in area to each be 4/7 the area of the legacy sun tracker set, then the mass of solar panel array is 16/7 or just over 2 times the moving array mass, but the panels are rigidly and simply mounted in an arc over the ventral face of the ET, doubling the net power generation. Well, until an Orbiter comes to dock at the ventral port, which probably involves some thrust impingement on the fixed panels and then shadowing the array. There is probably a better way to do this, but I think the point is clear that if we are willing to spend somewhat more than twice as much on panel area and thus mass, but save on complicated, fallible active panel pointing machinery, the trade-off might be worth it. Also I suspect part of what limits solar panel element life is intensity of power generation--that is if we had a panel that was always 60 degrees off direct pointing at the Sun, it would generate only half the power it could, but might last considerably longer, if not quite twice as long. (Solar panels are also subject to being hit by micrometeoroids and thus their useful area gets eroded. This form of deterioration will proceed however lightly the panel is generating).
Heat radiators are somewhat more flexible. At first glance it might seem vital to avoid any sunlight shining on them at all, but this depends on the temperature of the radiator. If the radiator is a lot hotter than the black body temperature of full on sunlight, the solar input is just a minor offset impeding a given rate of heat rejection. The additional solar input raises the temperature necessary to radiate the desired output plus that input, or we can face being able to reject less heat while the sun is impinging on it. At a certain critical radiator temperature this "less" equals zero. But if it can run hotter than that even full solar flux does not limit to zero.
As noted, I suspect the ET being maintained at a standard 295 K will itself serve as a major radiator area.
