Quote from: Twark_Main on 08/29/2022 08:19 amIf there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.I wouldn't say no advantage. At a minimum, you could use the paint (not the tiles) on the parts of the ship that don't have to be jet black and that would reject 90% of the heat from sun exposure at very little cost. But it definitely doesn't buy you ZBO. Not with a standard Starship, anyway.At this point, I'd like to talk a little bit about exactly how the fuel is going to be transported. (Apologies if this has already been discussed. Extra apologies if it's already been discarded with good reason!) :-)I know Elon doesn't like depos, but I'm starting to think it makes more sense to have a special "fuel depot" (for want of a better word) that a standard-issue Starship would carry as cargo rather than have a special tanker version of Starship. That would mean that two Starships would never dock to each other directly--not for purposes of fueling, anyway.My reasoning is as follows:First, that means you don't have an extra version of Starship with extra-large tanks. Second, the fuel depot would have the hardware required to mate to the QD ports on a Starship and do the refueling, so you wouldn't need a version of Starship with that equipment on it somewhere. (Or, worse, add that hardware to every Starship.)The basic fuel depot would just be fuel tanks, the docking attachment, and pumps. It wouldn't ever leave the cargo bay of the Starship that brought it up to orbit. It's the sort of thing you'd use to refuel the Lunar Starship, assuming that only needs one extra load of fuel. You'd launch the two Starships at about the same time so boiloff wouldn't be a big concern.A more complex fuel depot would be larger and have a cryocooler, radiator, extra insulation, and QD ports identical to those on a Starship. It would be put in orbit by itself and get filled up on multiple visits by Starships carrying basic depots full of fuel and LOX. (Note that you wouldn't need to develop this until after the lunar missions.)That sort of assembly probably would benefit from extensive use of Solar White tiles. Yes, it would still need a cryocooler, but the tiles ought to reduce the mass of the cooler, the radiator, and the solar panels needed to power it.You would cope with the orbital precession problem by timing the initial orbit and the refueling orbits so the depot is in the right place at the right time to refuel the Starship dedicated to the mission. After the mission Starship departed, you could send up another Starship to retrieve the empty depot. If you wanted to use the same depot for multiple missions, you could use something like the RAAN-Agnostic 3-Burn Departure Methadology for Deep Space Missions from LEO Depots that has been talked about earlier.Again, apologies if this has already been debated/discarded.
If there's no advantage to using a selective solar coating (which seems to be true), then we're back to MLI.
I am opposed to something that stays inside a cargo bay. It could work if the ship carrying it is nosed up between the tanker and the receiving ship and just acts as a fuel pump but that has safety issues for a crewed receiver.
An important point you addressed is GSE plate gender mating. The four solutions so far discussed were: 1) the tanker deploying a gender bender on top of its GSE plate; 2) a tanker having a second, gender bent GSE plate; 3) the tanker being already gender bent and a temporary adapter mounted on the ground GSE for launch; 4) A free flyer with all transfer kit mounted and two gender bent GSE fittings.
The idea of snaking a couple of hoses is not going to work. At minimum there also needs to be ullage transfer to minimize pump size and power, with the benefit of minimizing wastage. Past that, looking long term, it's not unreasonable to expect a transfer of any other fluids needed on later builds, and juicing up batteries has a certain appeal if a transfer campaign get drawn out. That and to my knowledge, fluid transfer has never been done between two bodies not firmly connected. Might as well take advantage of the GSE hardware that's already there and not get too fancy.
Quote from: Twark_Main on 08/29/2022 05:56 pmDelete the tanks, use Starship's. Now you just two pumps and two hoses. That's a win.But is that a win? Now you've got another kind of Starship--one that can't be used for any other purpose. Is that desirable?
Delete the tanks, use Starship's. Now you just two pumps and two hoses. That's a win.
Quote from: OTV Booster on 08/29/2022 09:39 pmI am opposed to something that stays inside a cargo bay. It could work if the ship carrying it is nosed up between the tanker and the receiving ship and just acts as a fuel pump but that has safety issues for a crewed receiver.What are the safety issues? Is there a real concern that a vehicle being refilled might explode?
Quote from: Paul451 on 08/29/2022 04:57 amQuote from: Greg Hullender on 08/28/2022 06:06 pmWhat I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.Or use boil-off. [...] Maybe I'm missing something, but how is this different from just doing nothing?
Quote from: Greg Hullender on 08/28/2022 06:06 pmWhat I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.Or use boil-off. [...]
What I would say is that, for a Starship in LEO, there is no completely passive ZBO solution. Not at the moment, anyway. There must be some cryocooler, with all the extra hardware that implies. The best that Solar White can do for you is reduce the mass of the cryocooler.
Thanks! That was hugely helpful. For the first time, I think I actually understand a few of the terms I saw in discussions further upthread. Quote from: OTV Booster on 08/29/2022 09:39 pmI am opposed to something that stays inside a cargo bay. It could work if the ship carrying it is nosed up between the tanker and the receiving ship and just acts as a fuel pump but that has safety issues for a crewed receiver. What are the safety issues? Is there a real concern that a vehicle being refilled might explode? Quote from: OTV Booster on 08/29/2022 09:39 pmAn important point you addressed is GSE plate gender mating. The four solutions so far discussed were: 1) the tanker deploying a gender bender on top of its GSE plate; 2) a tanker having a second, gender bent GSE plate; 3) the tanker being already gender bent and a temporary adapter mounted on the ground GSE for launch; 4) A free flyer with all transfer kit mounted and two gender bent GSE fittings.Hmm. I actually visualized a "free flyer" depot with two sets of fittings: one male and one female. My thought was that you'd fill it up through the female fittings and it would fill a Starship via the male fittings. A basic depot would get filled on the ground before loading into the cargo bay. An advanced depot would get repeated loads of fuel while in orbit and then (when full) would unload into a Starship. There would never be a time when two Starships were connected directly to each other. (Although the case where the depot is secured inside the cargo bay is, admittedly, not very different from that.) You could even chain multiple depots together, if there were any advantage to be had from that. (E.g. greater capacity but sharing the same power source/coolant or something.)Quote from: OTV Booster on 08/29/2022 09:39 pmThe idea of snaking a couple of hoses is not going to work. At minimum there also needs to be ullage transfer to minimize pump size and power, with the benefit of minimizing wastage. Past that, looking long term, it's not unreasonable to expect a transfer of any other fluids needed on later builds, and juicing up batteries has a certain appeal if a transfer campaign get drawn out. That and to my knowledge, fluid transfer has never been done between two bodies not firmly connected. Might as well take advantage of the GSE hardware that's already there and not get too fancy.I guess I'm just concerned that the GSE hardware either won't be long enough to reach or else it'll protrude too much at launch time. Perhaps that's a silly thing to worry about, but I've had trouble visualizing how it would actually work. Likewise, the free flyer with two male GSE adaptors that bridges two Starships troubles me because I don't see how it ever gets into place without someone in a space suit moving it around. Of course, given the name, I guess it could have its own propulsion and guidance system, but, man, that seems really complicated. At that point, it really is a vehicle. (It's a beautifully symmetrical solution, though.)Thanks again for your clear and helpful responses.
Quote from: Robotbeat on 08/29/2022 06:24 pmI already ran the numbers. That’s in my post. It’s just proportional to absolute temperature to the fourth power. 5.67e-8W/(m^2*Kelvin^4) is the Proportionality constant, the Stefan-Boltzmann constant.I'm doing something different, actually. See upthread.
I already ran the numbers. That’s in my post. It’s just proportional to absolute temperature to the fourth power. 5.67e-8W/(m^2*Kelvin^4) is the Proportionality constant, the Stefan-Boltzmann constant.
As you can see, the curves never cross. That means there's no upper wavelength cutoff beyond which a totally emissive blackbody would "beat" a totally reflective surface.
You're right about the safety issue with kit in nose design. I was having cranial flatulence. Still, it adds another GSE hookup and has to pump up hill. The plumbing has to reach down to the main tanks. I just don't see any advantage over the dedicated depot or the free flyer except for deploying, and enough downside to give it the hairy eyeball.
Interesting factoid: one ton of propellant takes up 0.016m of tank height. A nominal 150t of propellant would need a 2.4m tank stretch. Just a matter of moving domes, wiring, plumbing and vents. This would make the tankers their own variant and save the weight and operational inconvenience of cargo bay tanks. Some see that as a problem. I don't. Opinion: Hardware rich won't end when operations start. Tanker use will be slow at first but my money says use will pick up as the utility proves itself. A 40t probe to the Oort cloud? No problemo.
The free flyer would need two of one type of GSE plate. It would exactly mimic the plate on the ground GSE, which would be exactly what the ship GSE plate expects to mate up with. The gender of a quick disconnect doesn't dictate flow direction.
Quote from: Twark_Main on 08/30/2022 06:55 amAs you can see, the curves never cross. That means there's no upper wavelength cutoff beyond which a totally emissive blackbody would "beat" a totally reflective surface.Does such a surface exist, though?
(snip)Btw SPHEREx is precisely the special case I described earlier: the cone always points up, so it never has a view of the Earth. It also needs to be located in a dusk/dawn polar orbit, such that it never has a view of the Sun. There are downsides to using a disk/dawn orbit for a depot: it's less efficient than launching eastward, and it has less flexibility for targeting a specific right ascension for departure.
Quote from: OTV Booster on 08/30/2022 06:53 amYou're right about the safety issue with kit in nose design. I was having cranial flatulence. Still, it adds another GSE hookup and has to pump up hill. The plumbing has to reach down to the main tanks. I just don't see any advantage over the dedicated depot or the free flyer except for deploying, and enough downside to give it the hairy eyeball.I still don't think I'm clearly conveying what I'm visualizing. Let me give it another try. First, remember that I'm assuming no modifications to the cargo Starship whatsoever. It has a female GSE hookup down at the bottom of the tanks and that's all. There is no extra plumbing running up to the nose of it.The basic depot has a male GSE hookup. It fits in the cargo hold of a standard starship and is firmly braced in there. It can move out a little bit (relative to the cargo hold), probably on rails or something, so it's always firmly attached. It does not have any external plumbing.When cargo starship A refills starship B, A opens the cargo doors, lines up the depot GSE with B's GSE and connects them. Once the cargo doors are open, the depot rolls out, and the two GSE's can mate without any other contact between the two vehicles. Both vehicles do a gentle ullage burn while the propellants are transferred. Because the docking is asymmetrical, the ullage burns will have to be calculated to generate net zero torque. Note, though, that even a symmetrical docking will have this problem because the masses of the two starships won't be the same and the center of mass will change dynamically during the refilling operation.After the depot is depleted, they disconnect and the depot retracts. A goes home, and B goes on its mission. (Assuming it only needed one load.) As a result, there's no plumbing that reaches down to the main tanks; they are refilled from the bottom, just as they are on the launch pad. Is that clearer?Quote from: OTV Booster on 08/30/2022 06:53 amInteresting factoid: one ton of propellant takes up 0.016m of tank height. A nominal 150t of propellant would need a 2.4m tank stretch. Just a matter of moving domes, wiring, plumbing and vents. This would make the tankers their own variant and save the weight and operational inconvenience of cargo bay tanks. Some see that as a problem. I don't. Opinion: Hardware rich won't end when operations start. Tanker use will be slow at first but my money says use will pick up as the utility proves itself. A 40t probe to the Oort cloud? No problemo.I just can't help thinking that if you're trying to save money by mass-producing these things, it will help a lot to have fewer versions. Right now I'm seeing a cargo version, a crew version, a tanker version, a lunar version, etc. and that just seems like a problem to me. Obviously it's easier if the lunar version is just the crew version minus the last steps (that attach tiles), and maybe the cargo version and the crew version share all the work up to the point where the upper part of the vehicle is configured, but the tanker version seems like a very different work flow. Also, there probably won't be very many lunar and crew versions built, so maybe it's okay that they're expensive to build; almost everything will be cargo starships, after all. But you'd likely need a lot of tankers.Hence my thinking that it's a win if the basic depot is as simple as possible and just fits into the cargo hold. Again, I'm no expert here--just trying to think the whole thing through.Quote from: OTV Booster on 08/30/2022 06:53 amThe free flyer would need two of one type of GSE plate. It would exactly mimic the plate on the ground GSE, which would be exactly what the ship GSE plate expects to mate up with. The gender of a quick disconnect doesn't dictate flow direction. Yeah, I realize that. I had been thinking you'd want a female GSE on any depot just so you could fill it on the ground using the same hardware that fills a starship, but perhaps that's unnecessary. Also, it would let you chain multiple depots together--if that had any value. It also occurred to me that it might be easier to make pumps that only work in one direction, but that's sheer speculation on my part.In that case, though, why does a depot need two GSE's? Why not just one male one that it uses to fill up from one starship (or on the ground) and then later pump the propellant into another one? It only seems you'd need two attachments if you wanted to be connected to two starships at the same time, but with a depot, I don't think you'd ever want to do that.If a single refill (of 150 tons) is sufficient for the lunar missions, then a basic depot is all that would be required for a while. But I'm still fascinated by the idea of a "jumbo depot" that could hold much more propellant indefinitely.Using your 0.016 m/mT figure, it would seem that an empty jumbo depot that filled the 18m cargo bay could hold 1125 tons of propellant. It would take 7.5 flights to fill such a thing, and three such depots could completely refill a starship (if there were a reason to do that--maybe if you wanted to take months hauling propellant up so you could quickly fill up a fleet of starships all heading to Mars in the same launch window). If we were going to construct such a "depot complex," I could imagine dividing the functionality between "storage modules" (just tanks and plumbing) and a single "command module." The command module would have solar panels, cryocooler, radiator, communication links, ullage engine etc. (but minimal fuel tanks) that would attach to one or more storage modules.But that's starting to be pretty pie-in-the-sky. :-)
Personally, I like box-in-a-box. There's a reason shipping containers revolutionised transport. But it's against SpaceX's design philosophy, and dedicated tankers aren't going to be sitting around wasted, so there's no reason for it.
IIUC, the cargo/tanker has GSE up in the nose along with propellant tanks. It noses up to the QD plate on the receiving ship, its QD plate extends, and it pumps over the propellant from the cargo bay tanks. Is this right?
The actual numbers will change as the ships capabilities evolve. Twark or Rad Mod or maybe Robo ran some numbers awhile back claiming that a tanker with a small stretch could load a total of 1600 or 1650t of propellant and deliver quite a bit more than 150t. A dedicated depot would most probably be a tanker variant with extra kit so this plays into supporting early Artemus missions where the extra propellant can be put to good use. Don't ask. It was a very detailed discussion.
There is one point in your cargo/tanker idea where safety would be a concern. This may be where my brain fart happened. If a crewed ship needs refueling it should only happen once. Multiple small transfers each carry the same safety risks as one big gulp. This can be got around by waiting for the transfers to be finished before the crew launches and transfers over to the refueled ship. Another complexity, but maybe the best way.
You're correct in expecting the depot to have what you call female and what I call gender bent GSE. My terminology comes from electronics adapters (called gender benders) used to reverse a cables gender.
A depot would have only the one QD plate. It's the free flyer that would have two of the same gender as the ground side GSE. One side would mount to a tanker with standard QD plate that becomes a temporary accumulator. The other would mate to the tankers (also standard QD plate) arriving to fill the accumulator, and ultimately to the receiving ship (also with standard QD plate). Keep in mind, the free flyer and depot are not complimentary. The are competing concepts with the same goal.What you call a command module is pretty much what the free flyer would be. It packs in whatever functionality a depot needs but allows plain Jane tankers to become storage tanks (accumulators). If you build the command module into a tanker you have - a depot. The free flyer concept that's been thrown around would be a new design with all that implies.
IMO it's way too early for hard decisions in depot vs free flyer, let alone the variants. Just gonna set back and enjoy the show.
The basic depot [...] fits in the cargo hold of a standard starship and is firmly braced in there. [...]When cargo starship A refills starship B
I just can't help thinking that if you're trying to save money by mass-producing these things, it will help a lot to have fewer versions. Right now I'm seeing a cargo version, a crew version, a tanker version, a lunar version, etc. and that just seems like a problem to me.
But you'd likely need a lot of tankers.
Quote from: Paul451 on 08/29/2022 11:07 pmPersonally, I like box-in-a-box. There's a reason shipping containers revolutionised transport. But it's against SpaceX's design philosophy, and dedicated tankers aren't going to be sitting around wasted, so there's no reason for it.Another reason to go with "box-in-a-box" is that it would let them refill vehicles that used different kinds of fuel. If they draw straight from the tanks of a special tanker starship, then the only thing they can ever refuel is something that uses CH4 and O2. But if the propellants are carried in separate tanks in the cargo hold, then they could fill up (in theory) a vehicle that used H2 and O2, or a nuclear-thermal vehicle that just used H2 or an ion drive that wanted xenon or krypton or whatever. Obviously the depots would be different, but the cargo starships would be the same.Of course, that assumes there will ever be very many vehicles like that needing to be refueled.
Of course, that assumes there will ever be very many vehicles like that needing to be refueled.
Quote from: Greg Hullender on 08/30/2022 07:40 pmOf course, that assumes there will ever be very many vehicles like that needing to be refueled.LOL. Get in your way back machine to around 1915 and try to explain a megaplex truck stop with 800 parking spots, a movie theater, an 8 bay repair facility, a hotel, a full service restaurant, half a dozen fast food joints and three C stores. You'll be locked up as a lunatic.
I think there is a world market for maybe five computers.-- Thomas Watson, IBM Founder, 1943
I don't think the analysis in the thread has touched on all the available coatings.