So would it make the most sense to build a special docking station with something equivalent to two quick disconnect arms? It would sit between two starships, connected to both, and transfer fuel from one to the other exactly as the system does on the ground. Each starship would be responsible for the ullage burn (and keeping it balanced as the center of mass moved during fueling), but the pumps would be in the docking station. The docking station would need its own power, and big enough solar panels might be a problem, but perhaps big batteries would suffice with solar panels large enough to recharge them in 24 hours or so.That certainly minimizes the plumbing (no change to either starship), and if you can use the same QD structure that's used on the ground, then even that's not a new part.Does that all make sense? Or is that more or less what people have been talking about already and I'm just slow figuring it out? :-)
Quote from: Greg Hullender on 07/28/2022 03:28 pmSo would it make the most sense to build a special docking station with something equivalent to two quick disconnect arms? It would sit between two starships, connected to both, and transfer fuel from one to the other exactly as the system does on the ground. Each starship would be responsible for the ullage burn (and keeping it balanced as the center of mass moved during fueling), but the pumps would be in the docking station. The docking station would need its own power, and big enough solar panels might be a problem, but perhaps big batteries would suffice with solar panels large enough to recharge them in 24 hours or so.That certainly minimizes the plumbing (no change to either starship), and if you can use the same QD structure that's used on the ground, then even that's not a new part.Does that all make sense? Or is that more or less what people have been talking about already and I'm just slow figuring it out? :-)The problem with that is that the system lacks the medium term storage needed to enable flexibility in the schedule for the accumulation flights. This desire for flexibility with the scheduling of tanker flights seems to have driven SpaceX down the path they are currently pursuing, with dedicated orbital-only depots. From all appearances, the depot ships will be based off of the tankers, with as few changes as possible to suit their needs. Deleted entry hardware, additional thermal insulation, possible extra tank capacity to account for boiloff, and hardware for propellant transfer with docked ships (compatible QD plate and likely dedicated ullage thrusters) seem to be their medium-long term solution to the problem. In the long term, once they've mastered active cooling solutions to allow for minimal boiloff over longer time periods, the potential limitations from being locked into a specific inclination will be minimized by putting tankers in every useful orbit. In the shorter term (ie. HLS contracts), the relatively low-cost nature of the depot design should allow missions to be profitable even with single-use depots.
Given all this, it seems to me that there's a lot of functionality that's needed for refueling but probably isn't needed in any given Starship. Does it make sense for them to build a specialized "docking station" to handle all of that and avoid complicating the Starship design?The rough idea is a sort of cylinder that two Starships dock to which has quick disconnect arms that reach down to attach to each one. Lacking a clear idea of a) how they dock and b) where the QD ports are going to be, I can't be more specific. The dock would handle all the ullage burns and would need solar panels to provide power to pump the propellants around and keep them cool.That's a whole new structure that has to fit into a Starship and unfold itself in space, but maybe that's not so bad. It seems better than adding a whole lot of hardware to every Starship that would only be used during refueling. Does that all seem reasonable?
Yet another half-baked geometrical thought:If you put the depot's hardware (active side of the QD, cryocooler, pumps, etc.) in the ogive portion of the nose, it's straightforward to dock the depot and the other ship (either a lift tanker or a payload Starship to be refueled), dorsal-to-dorsal but nose-to-tail. However, as has been pointed out, then you have to deal with ullage management in two different directions.So what about this? Imagine sort of a backwards chomper, i.e. a fairing door whose bottom is at about the level where the ogive portion of the nose begins, whose top is a bit below the level of the LOX header tank, and whose sides are roughly at the midline of the fairing. Now put the whole fairing on an arm that can lift it clear of the depot and flip it over. On the inside of the removed door are solar cells and docking hardware. When a Starship wishes to use the depot, it slides into the (now concave) door segment and latches. The active QD then extends from the open ogive space and connects to the Starship.You now have both Starships pointing in the same direction, which removes the ullage issues. What you've sacrificed is some amount of torsional stability between the two Starships. Note that this is still a depot (or other kind of tanker) that can return to EDL, because everything will close up and stow, leaving the TPS alone.I'm not quite sure I have the geometry right in the attached very bad picture.
Quote from: TheRadicalModerate on 07/29/2022 09:01 pmYet another half-baked geometrical thought:If you put the depot's hardware (active side of the QD, cryocooler, pumps, etc.) in the ogive portion of the nose, it's straightforward to dock the depot and the other ship (either a lift tanker or a payload Starship to be refueled), dorsal-to-dorsal but nose-to-tail. However, as has been pointed out, then you have to deal with ullage management in two different directions.So what about this? Imagine sort of a backwards chomper, i.e. a fairing door whose bottom is at about the level where the ogive portion of the nose begins, whose top is a bit below the level of the LOX header tank, and whose sides are roughly at the midline of the fairing. Now put the whole fairing on an arm that can lift it clear of the depot and flip it over. On the inside of the removed door are solar cells and docking hardware. When a Starship wishes to use the depot, it slides into the (now concave) door segment and latches. The active QD then extends from the open ogive space and connects to the Starship.You now have both Starships pointing in the same direction, which removes the ullage issues. What you've sacrificed is some amount of torsional stability between the two Starships. Note that this is still a depot (or other kind of tanker) that can return to EDL, because everything will close up and stow, leaving the TPS alone.I'm not quite sure I have the geometry right in the attached very bad picture.This is what I've been proposing. The folding multi-use load-bearing fairing can probably be omitted for a simpler non load bearing panel, and the QD arm itself used for local grappling and mating. No need for two pieces of added hardware that can extend, actuate, and latch, when you can use one. Depending on the final 'chomper' geometry (would need to be able to fold far enough to clear, but this may be needed for payload deployment anyway) this could even mean the QD assembly is entirely self-contained within the payload bay, so the depot/tanker can have that unit removed (and the extra plumbing blanked off) and the vehicle be used for payload launches when no depots or tankers are needed in order to pay for itself, rather than sitting around doing nothing.
If you take that docking station and give it full sized tanks, it is a depot heavy. Tanks are the only thing missing from it.
- <thruster type(s)> (ion; cold gas; MethaLOX; MethOX; etc.)- <plane change capable?> (yes | no; time required for the plane change)
Quote from: groknull on 07/29/2022 08:32 pm- <thruster type(s)> (ion; cold gas; MethaLOX; MethOX; etc.)- <plane change capable?> (yes | no; time required for the plane change)Two questions: First, do ion engines provide anything like the thrust required for ullage? I know it doesn't take much, but even so.Second, I thought the cost of plane changes in LEO was prohibitive. Is that really on the table?
Quote from: OTV Booster on 07/29/2022 09:34 pmIf you take that docking station and give it full sized tanks, it is a depot heavy. Tanks are the only thing missing from it. Actually, I was thinking of the docking station as also being a way for crew to move from one Starship to another as well. My thought was that although you want to bring up the crew after you've already got your mission vehicle fully fueled, you really don't want to transfer all that fuel from one vehicle to another if you can help it. But if the docking station is a tube that two Starships dock to, why not make it a passageway for the crew and small amounts of last-minute cargo?Not sure if that's really necessary, but it makes the station rather different from yet another Starship variant.
This is what I've been proposing. The folding multi-use load-bearing fairing can probably be omitted for a simpler non load bearing panel, and the QD arm itself used for local grappling and mating. No need for two pieces of added hardware that can extend, actuate, and latch, when you can use one.
Depending on the final 'chomper' geometry (would need to be able to fold far enough to clear, but this may be needed for payload deployment anyway) this could even mean the QD assembly is entirely self-contained within the payload bay, so the depot/tanker can have that unit removed (and the extra plumbing blanked off) and the vehicle be used for payload launches when no depots or tankers are needed in order to pay for itself, rather than sitting around doing nothing.
I think you came up with some numbers on the amount of power needed to run a cooler. Don't remember what they were except they were big. I don't think what you're showing would be enough. If EDL is intended, Solar white paint is out. I'm not seeing an easy sun shade deploy. Same for PV. Maybe cheap flexible throwaway PV? Put it conformal over the heat tiles and let it burn away? Where does the radiator go?
Due to a lack of experimental and flight data of existing spacecraft, the Boil-off Monte Carlo program is not validated.
Largely agree here, although I still think that we're going to see an LSS with 1500t-1600t tanks, and you might as well use that layout for lift tankers and depots. But if everything remains at 1200t, then yes, you could conceivably have a single propulsion section layout that was good for everything.
Quote from: TheRadicalModerate on 07/30/2022 06:57 pmLargely agree here, although I still think that we're going to see an LSS with 1500t-1600t tanks, and you might as well use that layout for lift tankers and depots. But if everything remains at 1200t, then yes, you could conceivably have a single propulsion section layout that was good for everything.The Lunar Starship is the only vehicle that would need more than one 'standard' Starship's worth of propellant to be transferred, and those will be launching maybe once a year, if that. The rest of the time, a standard capacity Starship is sufficient as a tanker or depot for all regular launches (albeit most launches will likely require zero tankers, let alone a depot). Rather than using a custom stretch depot Starship that may-or-may-not be able to ever deorbit (and even if it can, that's a lot of extra R&D work to make sure it actually works) and require s abunch of unique handling equipment and procedures to accommodate its greater size; for that once-a-year occasion that actually requires that capacity you can launch two regular sized depots instead. In terms of cost and time, it's always preferable to make more copies of the same item than to make more one-off custom item variants whenever you can possibly help it.
