Starship On-orbit refueling - Options and Discussion

[DanClemmensen]

I do not really understand how Ship and Depot can mate without moving plumbing parts. I suspect there are ways to minimize them, but not eliminate them.

We're in microgravity. The spacecraft is the moving part.

We already have hardware to perform docking and then active 6-axis relative control. It's called NDS. The linear actuators make up a Stewart platform, so you can just "muscle" the two ships into the correct relative orientation and then actively pull them together in a controlled way.

The problem is that something must stick out. In the case of NDS or any other IDSS, the actual IDSS port on a visiting vehicle is at the tip of a cone, so it sticks out sufficiently, and the IDA on the ISS also sticks out. I did not see a way to do the same for the Depot.

On a Starship, the nose is a possible pointy part that sticks out enough, but that is not the part most folks are considering. If using a stewuart platform for the initial ("soft") dock for refill, then where on the Ship and Depot is it located so that it protrudes enough?

[Narnianknight]

I do not really understand how Ship and Depot can mate without moving plumbing parts. I suspect there are ways to minimize them, but not eliminate them.

We're in microgravity. The spacecraft is the moving part.

We already have hardware to perform docking and then active 6-axis relative control. It's called NDS. The linear actuators make up a Stewart platform, so you can just "muscle" the two ships into the correct relative orientation and then actively pull them together in a controlled way.

The problem is that something must stick out. In the case of NDS or any other IDSS, the actual IDSS port on a visiting vehicle is at the tip of a cone, so it sticks out sufficiently, and the IDA on the ISS also sticks out. I did not see a way to do the same for the Depot.

On a Starship, the nose is a possible pointy part that sticks out enough, but that is not the part most folks are considering. If using a stewuart platform for the initial ("soft") dock for refill, then where on the Ship and Depot is it located so that it protrudes enough?

The depot interface only needs to stick out a few centimeters to provide enough room for the raceways and other hardware along the ships' dorsals. The soft docking can be done with four 3-dof (fore/aft, left/right, extend/retract) actuators on the depot (two fore, two aft), each which interface with a corresponding jack on the tanker, or some similar (probably better) system. It would take a bit more thinking to come up with an optimal soft-dock system.

[TheRadicalModerate]

In particular, If a depot has a 9-meter docking interface as its forward end that looks like the top of a Booster, it can dock to a Ship's engine end. If that Depot has an articulated SQD stored below its docking ring, it can swing it out to connect to the Ship's QD. This Depot gets fueled for launch just like any other Ship, and it can be refilled from another Depot if needed. It refills from tankers using its swing-out SQD, and it refills other ships using its swing-out SQD. It launches using an expendable nosecone.

Is there any evidence that they're doing this?

This is basically what the tail-to-tail but dorsal-to-ventral docking scheme was.  We haven't seen artwork for anything like that in a long time.  Furthermore, that requires that both the depot and the target have substantially different fill/drain hardware.

Swing-out QDs have a massive amount of surface area that needs to be pre-chilled.


I still like my "depot kit" idea:  You do whatever you're gonna do for insulation, but the depot has a few grappling posts on its dorsal side, and you deploy a payload that unfolds to provide:

1) The QD gender change.
2) A couple of berthing arms to grab the targets (using similar grappling posts) and soft-berth them into position.
3) Lots of solar panels (if needed)
4) Cryocoolers (if needed)
5) Heat rejection panels (if needed)
6) Sunshades (if needed)

The only plumbing runs are through the gender change and the cryocooler (if needed), so your chill-down surface is very small.

I'll bet you could design a depot kit that was deployable through a Pez door on a separate Starship.  That allows you to iterate its design on the same depot.  Just throw away the old version and install a new one.  Then you can use the Starship that brought the new one up for depot ops testing.

[DanClemmensen]

In particular, If a depot has a 9-meter docking interface as its forward end that looks like the top of a Booster, it can dock to a Ship's engine end. If that Depot has an articulated SQD stored below its docking ring, it can swing it out to connect to the Ship's QD. This Depot gets fueled for launch just like any other Ship, and it can be refilled from another Depot if needed. It refills from tankers using its swing-out SQD, and it refills other ships using its swing-out SQD. It launches using an expendable nosecone.

Is there any evidence that they're doing this?

None whatsoever. It's "nose" to tail, not tail-to-tail. This hare-brained scheme requires that the top end of the Depot looks like the top of a booster. I have never seen any description or render for anything like this. The concept requires nearly zero modification of Ship or Tanker.

I do not think we have seen any credible render or description of the Depot from SpaceX, so while there is absolutely no evidence for my hare-brained scheme, there is little evidence for any other scheme.

[Greg Hullender]

Here is an off-the-wall concept (I cannot use the term "design" here) that minimizes the flex in the plumbing. Put the SQD on a long arm that is mounted on the dorsal side of the Depot. It runs nearly the length of the Depot and is hinged at the bottom. It's range of motion is a few degrees, like the TVCs. The top of the arm is above the  9-meter docking ring and carries the QD plate. The arm is about 45 meters long. The entire arm is launched under a disposable fairing and the top is under the disposable nosecone, which is extended dorsally to cover it.

Won't that prevent normal docking then? Or is this long arm on the opposite side of the depot?

[DanClemmensen]

Here is an off-the-wall concept (I cannot use the term "design" here) that minimizes the flex in the plumbing. Put the SQD on a long arm that is mounted on the dorsal side of the Depot. It runs nearly the length of the Depot and is hinged at the bottom. It's range of motion is a few degrees, like the TVCs. The top of the arm is above the  9-meter docking ring and carries the QD plate. The arm is about 45 meters long. The entire arm is launched under a disposable fairing and the top is under the disposable nosecone, which is extended dorsally to cover it.

Won't that prevent normal docking then? Or is this long arm on the opposite side of the depot?

What is "normal" docking? The long SQD arm starts far enough above the Depot's "normal" QD plate that it does not interfere with it. But you are correct: since the Deport has no TPS, It may be simpler to put it on the ventral side. That also lets it be longer, which further reduces the max angle for the swing out, and it may (or may not) simplify the plumbing.

Again: this Hare-brained scheme is nowhere near being a valid concept, just food for thought.

[TheRadicalModerate]

I do not think we have seen any credible render or description of the Depot from SpaceX, so while there is absolutely no evidence for my hare-brained scheme, there is little evidence for any other scheme.

The attached is from November, complete with a Block 2 target Starship.

[DanClemmensen]

I do not think we have seen any credible render or description of the Depot from SpaceX, so while there is absolutely no evidence for my hare-brained scheme, there is little evidence for any other scheme.

The attached is from November, complete with a Block 2 target Starship.

Thank you. I have seen this. I cannot determine whether or not it comes from SpaceX.

Do you think it comes from SpaceX? If so, why? Have you seen a description from SpaceX? If so where? Do you think this image is credible? I cannot determine from the image how this arrangement can work.

[TheRadicalModerate]

Thank you. I have seen this. I cannot determine whether or not it comes from SpaceX.

Do you think it comes from SpaceX? If so, why? Have you seen a description from SpaceX? If so where? Do you think this image is credible? I cannot determine from the image how this arrangement can work.

I think it comes from NASA, but it's clearly SpaceX artwork that's been passed through.

Update:  Here's proof that it comes from SpaceX.  Look at the credit in the caption.

I spent a lot of time blowing up those attachment points, and I can't figure them out either.  However, I played with a capture system where the target Starship kinda nestled into some flexible booms, which could then snap into some latches on the target.  (The amount of time I spent on this made your blunt-nosed depot look fully-baked.)

I'm still kinda betting that they'll settle on a berthing solution, but arms and grapples are certainly more complicated than sorta-sticky booms.  SpaceX's MO is always to try the simplest, dumbest thing possible, and then work their way up until something works well enough.  Booms are simpler and dumber.

FWIW, what you're describing fits very well with the "pusher/tanker", aka "StarPusher" idea that Twark and I fiddled with a zillion years ago.  If you arrange the depot and target in-line, nose-to-tail, the depot starts looking a lot more like a first stage, which allows all fueling RPODs to occur in VLEO, where you don't have weird orbital mechanics or radiation issues for crews.  The target does the RPOD, the depot transfers as much prop as is needed, and then uses the rest of the prop to begin the TLI (or TMI) maneuver.  When the depot is almost dry, it stages off and returns to VLEO, while the Starship immediately continues with the burn.

Something to keep in mind, but I doubt it's the first--or even fifth--iteration.

[Narnianknight]

The attached is from November, complete with a Block 2 target Starship.

The soft docking can be done with four 3-dof (fore/aft, left/right, extend/retract) actuators on the depot (two fore, two aft), each which interface with a corresponding jack on the tanker, or some similar (probably better) system. It would take a bit more thinking to come up with an optimal soft-dock system.

I had forgotten the details of that render. It turns out what I was thinking of (approximately) seems to be exactly what they are planning. Maybe that render was why I had that in my head.

Thank you. I have seen this. I cannot determine whether or not it comes from SpaceX.

Do you think it comes from SpaceX? If so, why? Have you seen a description from SpaceX? If so where? Do you think this image is credible? I cannot determine from the image how this arrangement can work.

It first appeared on the Flight 6 livestream.

I spent a lot of time blowing up those attachment points, and I can't figure them out either.  However, I played with a capture system where the target Starship kinda nestled into some flexible booms, which could then snap into some latches on the target.  (The amount of time I spent on this made your blunt-nosed depot look fully-baked.)

It appears to me that the booms are attached to the target, opposite of what I suggested. I was at first confused about this for mass reasons, but on second thought, it makes sense. If the system breaks on a depot, docking could become impossible, and all the prop in the depot is wasted. However, if the system breaks on a tanker, the tanker can just land for repairs, and the prop accumulated in the depot is preserved. If the depot side of the mechanism has no moving parts, it cannot break. The disadvantage is that HLS will need the booms.

I suspect the booms fold against the side of the tanker during launch for aerodynamics.

FWIW, what you're describing fits very well with the "pusher/tanker", aka "StarPusher" idea that Twark and I fiddled with a zillion years ago.  If you arrange the depot and target in-line, nose-to-tail, the depot starts looking a lot more like a first stage, which allows all fueling RPODs to occur in VLEO, where you don't have weird orbital mechanics or radiation issues for crews.  The target does the RPOD, the depot transfers as much prop as is needed, and then uses the rest of the prop to begin the TLI (or TMI) maneuver.  When the depot is almost dry, it stages off and returns to VLEO, while the Starship immediately continues with the burn.

It would be more efficient to transfer all the prop to the target than to do any burns with twice the dry mass. This could only be useful if the LSS tanks are full, such as if the depot had larger tanks and held more prop than the LSS, so could not transfer all of it to the LSS. Are you suggesting this, or a LSS which has existing prop fill from a previous transfer?

[OTV Booster]

In particular, If a depot has a 9-meter docking interface as its forward end that looks like the top of a Booster, it can dock to a Ship's engine end. If that Depot has an articulated SQD stored below its docking ring, it can swing it out to connect to the Ship's QD. This Depot gets fueled for launch just like any other Ship, and it can be refilled from another Depot if needed. It refills from tankers using its swing-out SQD, and it refills other ships using its swing-out SQD. It launches using an expendable nosecone.

But this introduces a lot of new cryogenic plumbing, does it not? Anything involving refueling from the nose instead of the base is going to be a big deal. And cryogenic plumbing with moving parts sounds like something that needs human techs available on site to do repairs.

You may be correct about the complexity. I think the bulk of the repairs we see at Starbase are caused by the launch and not by the moving parts per se. I do not really understand how Ship and Depot can mate without moving plumbing parts. I suspect there are ways to minimize them, but not eliminate them. Note that Booster and Ship already have moving cryogenic plumbing for the TVC engines.

Here is an off-the-wall concept (I cannot use the term "design" here) that minimizes the flex in the plumbing. Put the SQD on a long arm that is mounted on the dorsal side of the Depot. It runs nearly the length of the Depot and is hinged at the bottom. It's range of motion is a few degrees, like the TVCs. The top of the arm is above the  9-meter docking ring and carries the QD plate. The arm is about 45 meters long. The entire arm is launched under a disposable fairing and the top is under the disposable nosecone, which is extended dorsally to cover it.

Methinks this is more complex than needed. Mount the plate inside a doghouse on an extendable track much like that used on drawers but with multiple segment to extend its range. Mount it near the point of use.

In your concept it would mount near the booster simulating 'nose' with plumbing trailing behind. (Plumbing details are currently limited to arm waving). Alternatively, it could be a gender bender mounted in a doghouse above the QD plate and all plumbing would be internal.

In both cases the plate/bender would need to move axially to the ship to engage the target plate.

IMO the problems inherent in dragging along a bunch of flex plumbing make that idea a no-go.

The downsides of a gender bender shouldn't be minimized. It would have the weight of two QD plates plus the mass of all the crossover plumbing albeit without the complexity of deployable cryo plumbing. Thickness is also an important consideration. Thickness would be, at minimum, twice the OD of the largest fluid connection and possibly 3-4 times that diameter. This would impact any standoff arrangement between ships.

There are only so many combinations of plates & benders. They all have downsides and all the downsides can be overcome with varying design and operational efforts. Operational effort has to be part of the equation.

IMO, KISS says to duplicate the ground GSE on the depot with a gender bender temporarily installed on the ground side. The down side to this is that no transfers can happen without a depot as an intermediary. The upside is it's operationally simple in most use cases.

There are use cases where this is inadequate. Not many, but they're important ones. IIU the arguments correctly it mostly involves way above LEO transfers to lunar and translunar missions.

Sending a depot for these missions is doable but because of the depots presumed high dry weight, not ideal. Even a standard tanker might not be ideal because of the mass of the heat shield and fins. Ignoring all other concerns the ideal looks like a light weight stripped down tanker with a QD plate mimicking the launch GSE.

The problem here is it would, in many Lunar conops, need a large propellant load available from either a depot or multiple tanker loads. The QD plate would support transfers from tankers but not depots. Filling directly from tankers implies an extended stay at LEO or VLEO where thermal issues demand a depot with its high mass and thermal management. Doesn't sound viable.

Ideally this stripped down tanker with depot QD, which I dub 'Transfer Tanker', would visit LEO or VLEO only long enough to load up then raise itself to a more favorable thermal environment. Operationally, the easiest way to do this would be for it to loiter near a depot awaiting the last tanker of the mission. When this tanker arrives, the depot instead of accepting the load of propellant, transfers its contents to the tanker which in turn moves on to fill the transfer tanker.

The downside is an extra transfer. The alternative is deployable gender benders of some ilk. This introduces novel hardware while a transfer tanker uses already deployed hardware in novel combinations. It has few or no fabrication challenges and has no more need to return to earth than a depot.

Another use case might be to move propellant from a VLEO depot where transfer is efficient to ships at LEO. The depot need not move its mass other than to periodically boost orbit.

[DanClemmensen]

In particular, If a depot has a 9-meter docking interface as its forward end that looks like the top of a Booster, it can dock to a Ship's engine end. If that Depot has an articulated SQD stored below its docking ring, it can swing it out to connect to the Ship's QD. This Depot gets fueled for launch just like any other Ship, and it can be refilled from another Depot if needed. It refills from tankers using its swing-out SQD, and it refills other ships using its swing-out SQD. It launches using an expendable nosecone.

But this introduces a lot of new cryogenic plumbing, does it not? Anything involving refueling from the nose instead of the base is going to be a big deal. And cryogenic plumbing with moving parts sounds like something that needs human techs available on site to do repairs.

You may be correct about the complexity. I think the bulk of the repairs we see at Starbase are caused by the launch and not by the moving parts per se. I do not really understand how Ship and Depot can mate without moving plumbing parts. I suspect there are ways to minimize them, but not eliminate them. Note that Booster and Ship already have moving cryogenic plumbing for the TVC engines.

Here is an off-the-wall concept (I cannot use the term "design" here) that minimizes the flex in the plumbing. Put the SQD on a long arm that is mounted on the dorsal side of the Depot. It runs nearly the length of the Depot and is hinged at the bottom. It's range of motion is a few degrees, like the TVCs. The top of the arm is above the  9-meter docking ring and carries the QD plate. The arm is about 45 meters long. The entire arm is launched under a disposable fairing and the top is under the disposable nosecone, which is extended dorsally to cover it.

Methinks this is more complex than needed. Mount the plate inside a doghouse on an extendable track much like that used on drawers but with multiple segment to extend its range. Mount it near the point of use.

In your concept it would mount near the booster simulating 'nose' with plumbing trailing behind. (Plumbing details are currently limited to arm waving). Alternatively, it could be a gender bender mounted in a doghouse above the QD plate and all plumbing would be internal.

The Hare-brained concept (HBC) is intended to reduce the amount of flexible plumbing to a minimum and reduce the range of motion to a minimum. There is the alleged reasoning:
goal: minimal changes to Ship, Tanker, and GSE. basically all required new hardware is part of Depot.
    *Depot has a booster-style top to mate to the Ship or tanker bottom in a nose-to-tail arrangement
    *this requires a GSE-style QD on Depot to mate to the Ship/tanker QD plate
    *this requires plumbing from near the aft end of Depot to this QD arm plate.
    *add the plumbing in a long arm from aft end of Depot.
    *the only flex joint in the entire arm is at the aft end connection. The rest of the plumbing is rigid.
    *the flex joint only flexes in one direction. The range of flex is small, probably about four degrees.

I am a retired telecommunications system architect. I'm not  any other kind of expert, and especially not a cryogenic plumbing expert. However, it's possible that the aftermost 4 meters of the cryo lines in the arm could be semi-rigid instead of completely rigid and could accommodate a flex of one degree per meter of pipe.

   

[OTV Booster]

In particular, If a depot has a 9-meter docking interface as its forward end that looks like the top of a Booster, it can dock to a Ship's engine end. If that Depot has an articulated SQD stored below its docking ring, it can swing it out to connect to the Ship's QD. This Depot gets fueled for launch just like any other Ship, and it can be refilled from another Depot if needed. It refills from tankers using its swing-out SQD, and it refills other ships using its swing-out SQD. It launches using an expendable nosecone.

But this introduces a lot of new cryogenic plumbing, does it not? Anything involving refueling from the nose instead of the base is going to be a big deal. And cryogenic plumbing with moving parts sounds like something that needs human techs available on site to do repairs.

You may be correct about the complexity. I think the bulk of the repairs we see at Starbase are caused by the launch and not by the moving parts per se. I do not really understand how Ship and Depot can mate without moving plumbing parts. I suspect there are ways to minimize them, but not eliminate them. Note that Booster and Ship already have moving cryogenic plumbing for the TVC engines.

Here is an off-the-wall concept (I cannot use the term "design" here) that minimizes the flex in the plumbing. Put the SQD on a long arm that is mounted on the dorsal side of the Depot. It runs nearly the length of the Depot and is hinged at the bottom. It's range of motion is a few degrees, like the TVCs. The top of the arm is above the  9-meter docking ring and carries the QD plate. The arm is about 45 meters long. The entire arm is launched under a disposable fairing and the top is under the disposable nosecone, which is extended dorsally to cover it.

Methinks this is more complex than needed. Mount the plate inside a doghouse on an extendable track much like that used on drawers but with multiple segment to extend its range. Mount it near the point of use.

In your concept it would mount near the booster simulating 'nose' with plumbing trailing behind. (Plumbing details are currently limited to arm waving). Alternatively, it could be a gender bender mounted in a doghouse above the QD plate and all plumbing would be internal.

The Hare-brained concept (HBC) is intended to reduce the amount of flexible plumbing to a minimum and reduce the range of motion to a minimum. There is the alleged reasoning:
goal: minimal changes to Ship, Tanker, and GSE. basically all required new hardware is part of Depot.
    *Depot has a booster-style top to mate to the Ship or tanker bottom in a nose-to-tail arrangement
    *this requires a GSE-style QD on Depot to mate to the Ship/tanker QD plate
    *this requires plumbing from near the aft end of Depot to this QD arm plate.
    *add the plumbing in a long arm from aft end of Depot.
    *the only flex joint in the entire arm is at the aft end connection. The rest of the plumbing is rigid.
    *the flex joint only flexes in one direction. The range of flex is small, probably about four degrees.

I am a retired telecommunications system architect. I'm not  any other kind of expert, and especially not a cryogenic plumbing expert. However, it's possible that the aftermost 4 meters of the cryo lines in the arm could be semi-rigid instead of completely rigid and could accommodate a flex of one degree per meter of pipe.

I love HBC.


IMO the arm can be done away with and all the flex stay at the top near the action. The engines flex 15deg at cryo temps and all the flexing in this system could be done before thermal conditioning. Arm vs drawer sliders? Who knows. Sounds like food for in flight investigation.


Looking at down sides: what is the mass being added to an already heavy depot? Would the LEO or VLEO depot boost up maybe as high as NRHO to do a transfer? I can't run the numbers but maybe it would only boost up to HEEO and await a top off on its way to NRHO. RadMod probably ran the numbers but he's run so many, with obscure multi letter acronyms it's impossible to follow.


The translatable QD plate (however done) is a new mechanical system with everything that implies. It might work right out of the box or it might be an ongoing PITA.


A transfer tanker would have no novel systems and a low dry mass to suck propellant when moving to a high energy location. TANSTAAFL, but sometimes it can be a cheap meal.


Which would work out best? I've got a beverage of choice saying it's a transfer tanker. If neither, we each owe the other a beverage.

[Narnianknight]

In particular, If a depot has a 9-meter docking interface as its forward end that looks like the top of a Booster, it can dock to a Ship's engine end. If that Depot has an articulated SQD stored below its docking ring, it can swing it out to connect to the Ship's QD. This Depot gets fueled for launch just like any other Ship, and it can be refilled from another Depot if needed. It refills from tankers using its swing-out SQD, and it refills other ships using its swing-out SQD. It launches using an expendable nosecone.

But this introduces a lot of new cryogenic plumbing, does it not? Anything involving refueling from the nose instead of the base is going to be a big deal. And cryogenic plumbing with moving parts sounds like something that needs human techs available on site to do repairs.

You may be correct about the complexity. I think the bulk of the repairs we see at Starbase are caused by the launch and not by the moving parts per se. I do not really understand how Ship and Depot can mate without moving plumbing parts. I suspect there are ways to minimize them, but not eliminate them. Note that Booster and Ship already have moving cryogenic plumbing for the TVC engines.

Here is an off-the-wall concept (I cannot use the term "design" here) that minimizes the flex in the plumbing. Put the SQD on a long arm that is mounted on the dorsal side of the Depot. It runs nearly the length of the Depot and is hinged at the bottom. It's range of motion is a few degrees, like the TVCs. The top of the arm is above the  9-meter docking ring and carries the QD plate. The arm is about 45 meters long. The entire arm is launched under a disposable fairing and the top is under the disposable nosecone, which is extended dorsally to cover it.

Methinks this is more complex than needed. Mount the plate inside a doghouse on an extendable track much like that used on drawers but with multiple segment to extend its range. Mount it near the point of use.

In your concept it would mount near the booster simulating 'nose' with plumbing trailing behind. (Plumbing details are currently limited to arm waving). Alternatively, it could be a gender bender mounted in a doghouse above the QD plate and all plumbing would be internal.

The Hare-brained concept (HBC) is intended to reduce the amount of flexible plumbing to a minimum and reduce the range of motion to a minimum. There is the alleged reasoning:
goal: minimal changes to Ship, Tanker, and GSE. basically all required new hardware is part of Depot.
    *Depot has a booster-style top to mate to the Ship or tanker bottom in a nose-to-tail arrangement
    *this requires a GSE-style QD on Depot to mate to the Ship/tanker QD plate
    *this requires plumbing from near the aft end of Depot to this QD arm plate.
    *add the plumbing in a long arm from aft end of Depot.
    *the only flex joint in the entire arm is at the aft end connection. The rest of the plumbing is rigid.
    *the flex joint only flexes in one direction. The range of flex is small, probably about four degrees.

I am a retired telecommunications system architect. I'm not  any other kind of expert, and especially not a cryogenic plumbing expert. However, it's possible that the aftermost 4 meters of the cryo lines in the arm could be semi-rigid instead of completely rigid and could accommodate a flex of one degree per meter of pipe.

Remind me again what the advantages are of nose-to-tail docking. I can't see what it accomplishes other than a bunch of mass, an expendable nosecone, an expendable fairing, and a few dozen points of failure. The smallest possible amount of flexible plumbing is zero, and the scheme described in SpaceX's render has just that.

[DanClemmensen]

I love HBC.


IMO the arm can be done away with and all the flex stay at the top near the action. The engines flex 15deg at cryo temps and all the flexing in this system could be done before thermal conditioning. Arm vs drawer sliders? Who knows. Sounds like food for in flight investigation.

Looking at down sides: what is the mass being added to an already heavy depot? Would the LEO or VLEO depot boost up maybe as high as NRHO to do a transfer? I can't run the numbers but maybe it would only boost up to HEEO and await a top off on its way to NRHO. RadMod probably ran the numbers but he's run so many, with obscure multi letter acronyms it's impossible to follow.

The translatable QD plate (however done) is a new mechanical system with everything that implies. It might work right out of the box or it might be an ongoing PITA.

A 45-meter arm is fairly spectacular and blatantly obvious, but I don't think it's very heavy. If you accept nose-to-tail at all, you will need those two 45-meter cryo-pipes anyway, so the only add-on is the arm's stiffening structure. The cryo-pipes are rigid for most of their length, so they contribute to the stiffness and may even be the primary structure. Since the arm is strapped to the body for launch and also after it is connected to the Ship during propellant transfer operations, it only needs to be stiff enough to handle the swing-out before docking and the swing-in after docking, which occurs only under very low acceleration. My guess: it's lighter than a more elaborate system. Depot is not heavy to begin with: no TPS, no aero-control surfaces, and the nosecone and arm cover are discarded after launch.

[DanClemmensen]

Remind me again what the advantages are of nose-to-tail docking. I can't see what it accomplishes other than a bunch of mass, an expendable nosecone, an expendable fairing, and a few dozen points of failure. The smallest possible amount of flexible plumbing is zero, and the scheme described in SpaceX's render has just that.

There is a lot of mass and complexity in the docking mechanisms shown (vaguely) in the render. I don't think the render "describes" anything. That's OK, SpaceX owes us nothing, and we can invent a description that is consistent with the render.  I think the render implies a Depot with two QDs, so that mass is the same either way. To me, the render implies at least some modifications Ships and Tankers.

The advantages of the HBC are:
  *zero changes to Tanker and Ships.
  *simple reboost of Ship/Depot as a unit
  *simplified plumbing within the Depot.

[Greg Hullender]

Does anyone have a good idea how difficult it would be to just build depots with both male and female refueling ports? One dorsal and one ventral, say. The idea is to duplicate some of the piping for the current fueling ports, but still down near the tail and thus as short as possible.

You could then adopt an architecture that all tankers are male and all mission ships (e.g. HLS) are female. A sequence of tanker launches fills a pair of depots. Then a final tanker plus a mission ship fuel up from those two depots and launch together into HEEO for the final fueling.

There must be some reason why no one is discussing this idea. Is there public info anywhere that shows the current plumbing?

[TheRadicalModerate]

FWIW, what you're describing fits very well with the "pusher/tanker", aka "StarPusher" idea that Twark and I fiddled with a zillion years ago.  If you arrange the depot and target in-line, nose-to-tail, the depot starts looking a lot more like a first stage, which allows all fueling RPODs to occur in VLEO, where you don't have weird orbital mechanics or radiation issues for crews.  The target does the RPOD, the depot transfers as much prop as is needed, and then uses the rest of the prop to begin the TLI (or TMI) maneuver.  When the depot is almost dry, it stages off and returns to VLEO, while the Starship immediately continues with the burn.

It would be more efficient to transfer all the prop to the target than to do any burns with twice the dry mass. This could only be useful if the LSS tanks are full, such as if the depot had larger tanks and held more prop than the LSS, so could not transfer all of it to the LSS. Are you suggesting this, or a LSS which has existing prop fill from a previous transfer?

You're right that it's not quite as prop-efficient.  For Block 2-sized depots and LSSes (with the depot consuming the payload cylinder with extra prop), it costs about 80t of prop more for an Option A-like mission.

The real advantages are operational:

a) If you have a crewed mission from LEO, you never need to refuel anywhere but VLEO.  Abort options are plentiful if refueling doesn't work.

b) Crews experience minimal radiation, because they don't have to take at least one additional orbit through the Van Allen Belts.

c) Circular VLEO refueling orbits only, where both lunar and martian orbital mechanics are relatively tolerant of delays.

d) If the LSS launch is delayed past the point where boiloff requires the depot to need another tanker, the tanker replenishment is easier in VLEO.  Very small prop margins needed in the depot, because you know exactly how much prop is available in VLEO before you commit.  (In an HEEO FTO, you don't know how long the depot will have to hang out before the LSS arrives.)

e) No managing depots for multiple missions in weird HEEOs.  Every mission begins and ends in VLEO.

[Narnianknight]

Remind me again what the advantages are of nose-to-tail docking. I can't see what it accomplishes other than a bunch of mass, an expendable nosecone, an expendable fairing, and a few dozen points of failure. The smallest possible amount of flexible plumbing is zero, and the scheme described in SpaceX's render has just that.

There is a lot of mass and complexity in the docking mechanisms shown (vaguely) in the render. I don't think the render "describes" anything. That's OK, SpaceX owes us nothing, and we can invent a description that is consistent with the render.  I think the render implies a Depot with two QDs, so that mass is the same either way. To me, the render implies at least some modifications Ships and Tankers.

The advantages of the HBC are:
  *zero changes to Tanker and Ships.
  *simple reboost of Ship/Depot as a unit
  *simplified plumbing within the Depot.

I'm not sure what you mean by schanges to tankers. Tankers don't exist yet. If you mean changes between Starlink ships and tankers, there already have to be major changes such as tank and payload bay size. I'm also not sure where you're seeing a lot of mass and complexity in the render. Other than the SQD sticking out ~0.3 m as expected, the only mass and complexity I see are the four small arms connecting the ships, certainly orders of magnitude less than that of the HBC.

Now that I look closer at the render, it does seem that the tanker is situated ~5 m higher than the depot, which would lend credence to the idea that there are two QDs, a female at the normal position and a male for docking a few meters up. However, the SQD is already pretty close to the aft dome, so there's not much room to work with there.

Does anyone have a good idea how difficult it would be to just build depots with both male and female refueling ports? One dorsal and one ventral, say. The idea is to duplicate some of the piping for the current fueling ports, but still down near the tail and thus as short as possible.

You could then adopt an architecture that all tankers are male and all mission ships (e.g. HLS) are female. A sequence of tanker launches fills a pair of depots. Then a final tanker plus a mission ship fuel up from those two depots and launch together into HEEO for the final fueling.

There must be some reason why no one is discussing this idea. Is there public info anywhere that shows the current plumbing?

This gives me the idea that a male QD and docking hardware could be on the ventral side, removing the issue of squishing it between the female QD for launch and the aft dome (which doesn't explain the render, but whatever). Tankers would still be female though, and a second depot would go to FTO. There's no need to drag EDL hardware past VLEO.

[TheRadicalModerate]

Remind me again what the advantages are of nose-to-tail docking. I can't see what it accomplishes other than a bunch of mass, an expendable nosecone, an expendable fairing, and a few dozen points of failure. The smallest possible amount of flexible plumbing is zero, and the scheme described in SpaceX's render has just that.

There is a lot of mass and complexity in the docking mechanisms shown (vaguely) in the render. I don't think the render "describes" anything. That's OK, SpaceX owes us nothing, and we can invent a description that is consistent with the render.  I think the render implies a Depot with two QDs, so that mass is the same either way. To me, the render implies at least some modifications Ships and Tankers.

It absolutely requires some mods to ships and tankers.  It remains to be seen whether they're more significant that changing the plumbing around.

I don't understand why you think this requires two QDs.  It either requires that the depot have a male QD (which implies some GSE weirdness, or a jettisonable female-female QD adapter), or that some kind of male-male QD adapter is added to the standard female QD.  But in both cases, the prop transfer mechanism is the nexus of plumbing that already exists behind the QD plates.

  *zero changes to Tanker and Ships.

Not true.  You have to go from a stage-joining mechanism that's mediated by the chopsticks and humans eyeballing things to one that has to reliably dock an engine skirt filled with fragile engine bells to a staging mechanism that doesn't exist, protected during launch by a jettisonable fairing that doesn't exist.

Both nose-to-tail and tail-to-tail are hard!

  *simple reboost of Ship/Depot as a unit

I like this one, but it's really not needed at this point in the program.

  *simplified plumbing within the Depot.

I assuming you're talking about an single deployable arm that's 52.1m long (assuming v2 Starships), with male QDs on either end?  Otherwise, the plumbing is anything but simplified.

Even then, you're talking about an arm that contains multiple pressure vessels that's much, much longer than any possible payload bay.  How does it get deployed?  Is it assembled on-orbit?  Can it be popped out of a Pez door?  Does it have to be brought up in pieces on an F9?  How does it get maneuvered into place?¹

I did do a back-of-napkin on how much prop you need to vaporize during chill-down, and it's about 100kg, which isn't terrible.  (This assumes that the arm has two 30cm stainless steel pipes with a 2.5mm wall thickness, is covered in MLI, and reaches an equilibrium temperature of about 120K while it's idle.)  I thought that number would be much larger, so the whole system is a lot less sensitive to plumbing length than I thought.

Still... 104.2m of piping, capable of handling 2-6bar, deployed and mated to one spacecraft, with the thermal expansion allowances to mate it to a second vehicle, over and over?

_________
¹Mind you, the "depot kit" has some of these problems, but it doesn't require a pair of 52.1m pressurized pipes.