Starship On-orbit refueling - Options and Discussion

[JIS]

Some detail thoughts on propellant transfer involving micro acceleration. The various spin schemes (IMO) may be great in the future but need too much R&D for a time sensitive program.

It takes higher G to settle props than it takes to keep them settled. Settling has to overcome viscosity along the tank walls and other impedimenta. Viscosity aids in keeping settled that propellant which is already settled. Settling props will have at least some small amount of slosh that must be damped while settled propellants, by definition, have all slosh damped out.

It has been experimentally determined that 0.0001g will settle props. To the best of my knowledge (which isn't all that great) it is unknown what it takes to keep it settled. It would be great if it was another order of magnitude but it will be lower than 0.0001g.

A side issue that may impact settling g is that foamy propellants that would be unacceptable for transfer or an engine burn would have little impact on ROPD and might even be beneficial. Not big gobs of bubbles. More like beer foam. I think a layer of foam would help damp small sloshes.

Some dynamic viscosities and surface tensions, for comparison (and mention of foam):

  Substance    Dyn Visc (Pa-s)    Surf Tens (mN/m)
Boiling LCH4:  1.1E-04                   14.0
Boiling LOX:    1.9E-04                   13.2
NTO:              4.7E-04                 ~21
MMH:             8.8E-04                 ~54.3
Water:            1.0E-03                   72.0
Beer:              2.5E-03                   ??   (my surface is less tense after consumption)

I don't think you're going to have much problem with foam or even bubbles.

I'm still kinda hoping that propellant management devices can eliminate continuous settling acceleration completely.  Surface tensions of LOX and LCH4 are significantly lower than MMH and NTO, which is what most PMDs have been designed for.

There are some solutions how to overcome problem with two phase transfer. Here are some after giving the topic few minutes of thoughts:

1. vapour/liquid separator which work in zero gravity - mainly using a centrifugal force or some movable parts.
2. Allow some limited mount of two phase transfer - gas blow-by through transfer line would cause more rapid pressure equalisation between the tanks.

Eventually, the propellants transfer will result into equalisation of pressure between the tanks. So the receiving tanks needs to be vented and supply tank pressurized. To make the process more efficient a vapour recovery compressor can be used to compress excess vapour and store it for later use - for example pressurisation of the tanks or use in thrusters.

[JIS]

A couple of screenshots from the preso attached below.  Three things:

1) Still looks like the docking / berthing / stabilization mechanism is the four struts, slotted into four receptacles.  Compare that to the shots narianknight posted here, here, and here, which I presume are an early attempt at implementing the receptacles.

2) In the first screenshot below, it looks kinda like the two Ships (both depot and target) are vertically aligned.

3) In the second screenshot, it looks like they're significantly offset from one another.  This could be a perspective problem, or it could be evidence that the depot's presumably male QD is separate from the female QD used with the GSE.  It also could be an indicator that the depot is longer than the target, but then you'd expect the rear flaps to be more-or-less aligned.

Have we seen any more recent renders of docking? I think these are current. They show a non-androgynous system, so one of the two participants must always be the Depot. A non-Depot can only dock to a Depot. However, in these renders the Depot is depicted with EDL hardware (TPS and control surfaces) so the renders are inconsistent, since a Depot should never EDL.

It's been said that SX intends to transfer directly from the tankers. With a maybe kinda chance at a mars shot late next year and Artemus needs, this makes sense from a minimum viable first hack PoV. I've not seen anything from SX explicitly claiming this but that means little. To avoid ambiguity, that assertion is about me, not SpaceX.


In some ways getting loiter figured out seems harder than working out the transfer itself. Each new tanker build gives an opportunity to explore this. At some point the tanker will evolve into a depot in everything but name - with tiles and fins. That's probably when tankers and depots will become different ships.

HLS bid specifically mentioned propellant depots. This obviously tries to minimize boil-off during several weeks to accumulate ~1300t of propellants for HLS Artemis III mission - assuming tanker flight rate of about 100 starship per year during the time of the mission. The demo mission will require probably only about half of the propellants, but the tanker flight rate and capacity will probably by lower at the time. So still several weeks would be required.

The initial propellant test will be done even earlier at flight rate 10 per year, using two tankers launched several weeks apart. The first tanker will be subject to a significant boil-off so it is going to be a nice long term loiter experiment.

[DanClemmensen]

   imagine a half full vehicle being thrusted in one direction, setting the fluid to one side.  In this condition, let's say the Force is 10, the Mass is 1000 (10 struct+990 prop), and then the accel is 0.01.  Now you went to far and need to thrust a little in the opposite direction.  Now the fluid is just floating in the middle.  Your Force is still 10, but your mass is 10, so your accel is 100 times higher at 1.  This is obviously extreme and might be completely physically wrong.  If it's not wrong, then your GNC is really going to struggle with the non-linear behavior.

It's never been tested before, and can't be tested on earth.  No space docking has ever had so much fluid mass, unattached to the walls.

After reading this, my perception of the difficulty of docking any Starship has gone from "it's trivial" to "it's really, really hard".  When some of your mass is fluid that can slosh, the acceleration due to your thrusters will have variable and complex delays, and these delays are far more than minor nuisances. The smaller the needed velocity corrections, the more the delay and I suspect the uncertainty in the delay also  increases as the velocity change decreases. This makes the effect on the final stages of docking disproportionately severe.

This affects all Starship docking in zero g. The more propellant, the bigger the problem, but even actively docking Starship to Gateway is likely to be an issue. Docking a Tanker to a Depot will be "interesting".

Don't think it is a big deal as long you know the total spacecraft mass. If you want to change the velocity by some dV, you just calculate impulse your thrusters need to give your spacecraft and you perform the manoeuvre. At the beginning, when the propellants are sloshing the dV of the spacecraft can be all over the place, but after some time the impulse has as time to propagate through the propellants and the final dV should match your prediction.

So the problem is the transient time. With more propellants (nearly full tanks) the effect of sloshing is bigger but transient time is shorter. With less propellants the effect is smaller but longer.

Doesn't look to me like something unsolvable. Chaotic systems could actually have some "simple" solutions.       

Sure. But take a look at the videos of a Dragon docking. You must hit a thirteen-dimensional hyperspherical target, three spatial dimensions, three rotational dimensions, the six associated velocity dimensions,  and time. Simplified, for IDSS this means you need to get to zero relative velocity (less than about 3 mm/s), aligned to about 40 mm in three spatial dimensions and maybe 10 degrees in three orientations. Those last little bursts are the difficult ones. For SS, I think those four long legs are needed to create a much larger hyperspherical capture space than IDSS does. For example, the legs might be able to extend and compress over a length of (say) 2 meters, with ball joints with a thirty-degree range of motion, and with variable force in tension and compression  up to (say) one tonne.

[JIS]

   imagine a half full vehicle being thrusted in one direction, setting the fluid to one side.  In this condition, let's say the Force is 10, the Mass is 1000 (10 struct+990 prop), and then the accel is 0.01.  Now you went to far and need to thrust a little in the opposite direction.  Now the fluid is just floating in the middle.  Your Force is still 10, but your mass is 10, so your accel is 100 times higher at 1.  This is obviously extreme and might be completely physically wrong.  If it's not wrong, then your GNC is really going to struggle with the non-linear behavior.

It's never been tested before, and can't be tested on earth.  No space docking has ever had so much fluid mass, unattached to the walls.

After reading this, my perception of the difficulty of docking any Starship has gone from "it's trivial" to "it's really, really hard".  When some of your mass is fluid that can slosh, the acceleration due to your thrusters will have variable and complex delays, and these delays are far more than minor nuisances. The smaller the needed velocity corrections, the more the delay and I suspect the uncertainty in the delay also  increases as the velocity change decreases. This makes the effect on the final stages of docking disproportionately severe.

This affects all Starship docking in zero g. The more propellant, the bigger the problem, but even actively docking Starship to Gateway is likely to be an issue. Docking a Tanker to a Depot will be "interesting".

Don't think it is a big deal as long you know the total spacecraft mass. If you want to change the velocity by some dV, you just calculate impulse your thrusters need to give your spacecraft and you perform the manoeuvre. At the beginning, when the propellants are sloshing the dV of the spacecraft can be all over the place, but after some time the impulse has as time to propagate through the propellants and the final dV should match your prediction.

So the problem is the transient time. With more propellants (nearly full tanks) the effect of sloshing is bigger but transient time is shorter. With less propellants the effect is smaller but longer.

Doesn't look to me like something unsolvable. Chaotic systems could actually have some "simple" solutions.       

Sure. But take a look at the videos of a Dragon docking. You must hit a thirteen-dimensional hyperspherical target, three spatial dimensions, three rotational dimensions, the six associated velocity dimensions,  and time. Simplified, for IDSS this means you need to get to zero relative velocity (less than about 3 mm/s), aligned to about 40 mm in three spatial dimensions and maybe 10 degrees in three orientations. Those last little bursts are the difficult ones. For SS, I think those four long legs are needed to create a much larger hyperspherical capture space than IDSS does. For example, the legs might be able to extend and compress over a length of (say) 2 meters, with ball joints with a thirty-degree range of motion, and with variable force in tension and compression  up to (say) one tonne.

Yes, it will be fun to watch it sped up. Even some bumping could be involved, crushing some tiles in process. I wonder whether those "tentacles" will be moving around autonomously trying to hit the right "place".

Edit:
Hold on, there are no heat tiles at the leeward side. So it should be a dent only. Stainless steel is pretty thought and flexible so don't think it will be a big deal. More likely the tentacle or refueling coupling can get damaged.   

[DanClemmensen]

The initial propellant test will be done even earlier at flight rate 10 per year, using two tankers launched several weeks apart. The first tanker will be subject to a significant boil-off so it is going to be a nice long term loiter experiment.

Boiloff is not a problem during the transfer demo. The pad will almost certainly be able to launch the ships on two consecutive days: that's what it's built to do. Commodities are the limiting factor initially.  Even if the second Ship (prototype Tanker) is long delayed, the first ship (prototype Depot) only needs to retain enough propellant to de-orbit if the Tanker fails to orbit and transfer. Depot can boil off the rest of its propellant to keep the tanks cool, probably for weeks. Tanker arrives and transfers propellant to Depot for the demo. While they are up there, they can separate and then do a Second RPOD and transfer the propellant back the other way to demonstrate that the system works in the other direction.

[OTV Booster]

Some detail thoughts on propellant transfer involving micro acceleration. The various spin schemes (IMO) may be great in the future but need too much R&D for a time sensitive program.

It takes higher G to settle props than it takes to keep them settled. Settling has to overcome viscosity along the tank walls and other impedimenta. Viscosity aids in keeping settled that propellant which is already settled. Settling props will have at least some small amount of slosh that must be damped while settled propellants, by definition, have all slosh damped out.

It has been experimentally determined that 0.0001g will settle props. To the best of my knowledge (which isn't all that great) it is unknown what it takes to keep it settled. It would be great if it was another order of magnitude but it will be lower than 0.0001g.

A side issue that may impact settling g is that foamy propellants that would be unacceptable for transfer or an engine burn would have little impact on ROPD and might even be beneficial. Not big gobs of bubbles. More like beer foam. I think a layer of foam would help damp small sloshes.

Some dynamic viscosities and surface tensions, for comparison (and mention of foam):

  Substance    Dyn Visc (Pa-s)    Surf Tens (mN/m)
Boiling LCH4:  1.1E-04                   14.0
Boiling LOX:    1.9E-04                   13.2
NTO:              4.7E-04                 ~21
MMH:             8.8E-04                 ~54.3
Water:            1.0E-03                   72.0
Beer:              2.5E-03                   ??   (my surface is less tense after consumption)

I don't think you're going to have much problem with foam or even bubbles.

I'm still kinda hoping that propellant management devices can eliminate continuous settling acceleration completely.  Surface tensions of LOX and LCH4 are significantly lower than MMH and NTO, which is what most PMDs have been designed for.

Talk about rabbit holes. Started learning about PMDs and found this:
https://ntrs.nasa.gov/api/citations/20110000503/downloads/20110000503.pdf
I made it through the executive summary and looked at the centerfold graphics. It talks about LOX & CH4 specifically but failed to state what the specific design goal was. It talks mostly about thermal and mentions a little about vapor control and nothing about slosh. Conclusion: PMD is a general term with more than one sub type. Gotta go hurt my brain some more.


Another conclusion is you're drinking some fine beer. Coors would be closer to water. I wonder how lager and IPA compare.

[DanClemmensen]

For example, the legs might be able to extend and compress over a length of (say) 2 meters, with ball joints with a thirty-degree range of motion, and with variable force in tension and compression  up to (say) one tonne.

OK, maybe it's not as bad as I thought, but it's still pretty serious.
Momentum (a vector) is mass times velocity: p = mv.
Consider the momentum of the lighter ship relative to the heavier ship. If the lighter ship is 400 tonne and the closing velocity is 1 cm/s, then the momentum is 4 tonne*m/s. To stop it using one leg it needs to push back at 400 tonne for one second or 4 tonne for 100 seconds, etc. At 1 tonne for 400 seconds, it will compress by 2 meters.
EDIT: bad math. try again:
Say we wish to stop a ship traveling at v =1 cm/s in a length s = one meter. average velocity will therefore be 0.5 cm/s and the ship travels the distance in 200 seconds.
    s=0.5 a t²
    1 m= a * (200 s)²
     (1 m)/(200 s)² = a
    a = 1/40000  m/s²

now f=ma

f = 400*1000 kg * 1/40000 m/s²
  = 10 kg*m/s²
  = 10 N.

Not very much. A  small RCS thruster (e.g., on a Starliner) is about 400 N, so let's up our game and stop in 10 cm instead of 1 meter. we now need 100 N.

Of course, we have four legs, but we are correcting in six dimensions, so some may be in tension, which throws more compression at the other legs. Compression will also be variable and change gradually, so use one tonne 1000 N as the average. We are also still sloshing and perhaps getting assistance from the thrusters on both ships, but I suspect we are basically OK here. After the ships are brought to rest relative to each other, the legs can slowly retract to compete the hard docking.

[OTV Booster]

There are two lines of thought on ship turnaround time, specifically the tanker. One line is it'll happen soon. The other is that it won't.

Reentry is a tough nut. My thought is that it will be many flights before turnaround is down to even a week. If true, this bodes ill for a refueling campaign using only a few tankers. If it calls for 10 loads it might need 10 tankers. On the bright side, the tanker will be the simplest and least expensive build.

If a fleet of tankers is needed one thought is to store them on orbit. The difficulties here is keeping them alive long enough to reenter and be used for the next campaign. Bringing them back creates a storage problem but ISTM less of a problem than orbital storage.

An alternative would be to splash them. Stripping tiles and fins increases load and drops the number needed. This also reduces the load on the pad although this may also be achieved by storing on orbit only until the campaign is over. It's the classic engineering "it depends".

If the tanker build is close to its final form or expected changes can be reasonably be applied to them, by all means bring 'em back to an expanded rocket garden. If they can easily be designed for 2-3 weeks loiter, and a trade study show this to be worthwhile, do it and keep the pads clear for the campaign.

It might even be a split decision. Most probably several tankers of increasing sophistication will be built and tested before a full tilt refueling campaign is needed. Even an obsolescent tanker can be useful if it saves building another one. Strip the tiles and fins and splash it. Return the ones that are worthwhile.

Of course if the first V3 that returns for a grab can be recycled in a couple of days, the whole picture changes. That's the extreme positive outcome. The reality will be somewhere between that and no ship being recycled by the time of the first refueling campaign.

Stay tuned. Interesting times ahead.

[Twark_Main]

After the ships are brought to rest relative to each other, the legs can slowly retract to compete the hard docking.

Is there a hard docking phase, or is the plan to just transfer fuel through the legs themselves?

It might be beneficial to actually avoid hard-docking, because then the legs can act as energy-damping shock absorbers through the entire duration of the filling operation. Any bumps or transients would be easier to soak up.

[OTV Booster]

For example, the legs might be able to extend and compress over a length of (say) 2 meters, with ball joints with a thirty-degree range of motion, and with variable force in tension and compression  up to (say) one tonne.

OK, maybe it's not as bad as I thought, but it's still pretty serious.
Momentum (a vector) is mass times velocity: p = mv.
Consider the momentum of the lighter ship relative to the heavier ship. If the lighter ship is 400 tonne and the closing velocity is 1 cm/s, then the momentum is 4 tonne*m/s. To stop it using one leg it needs to push back at 400 tonne for one second or 4 tonne for 100 seconds, etc. At 1 tonne for 400 seconds, it will compress by 2 meters.

Of course, we have four legs, but we are correcting in six dimensions, so some may be in tension, which throws more compression at the other legs. Compression will also be variable and change gradually, so use one tonne as the average. We are also still sloshing and perhaps getting assistance from the thrusters on both ships, but I suspect we are basically OK here. After the ships are brought to rest relative to each other, the legs can slowly retract to compete the hard docking.

The M60 tank main gun recoil mechanism offered increasing resistance as the gun moved rearward. Totally passive and it kept the gun from bottoming out. I tried googling it with no luck. Our Uncle Sugar trained me on these. It's a real thing.


If the arms/legs use this technique the transfer of momentum between ships stays relatively smooth. Even without this, once the ships are mated, as long as the momentum transfer stays within the strength limits of the arms and their attaching points, propellant movement is not catastrophic. Just settle it again then start props transfer.


Treat settling for hookup and settling for transfer as two distinct problems.

[DanClemmensen]

After the ships are brought to rest relative to each other, the legs can slowly retract to compete the hard docking.

Is there a hard docking phase, or is the plan to just transfer fuel through the legs themselves?

It might be beneficial to actually avoid hard-docking, because then the legs can act as energy-damping shock absorbers through the entire duration of the filling operation. Any bumps or transients would be easier to soak up.

We have pictures of the socket on the passive side. It's just a big hemispherical hollow with no internal connectors. It looks very robust. I think a separate hard dock would make more sense. Movement after a hard dock is probably no big deal. it's like the BQD on the pad after you get the Booster latched down.

I do wonder how Starship HLS will dock to Gateway, however.

[Twark_Main]

Variable-strength springs would mean the attachment point needs to be stronger than it needs to for the same momentum absorption capacity.

The better approach is the one taken by NASA and IDSS, which uses electric motors with torque limiters (similar to an electric drill/driver) that will simply push back with the maximum allowable force for the entire stroke length. This is the efficient way to get the most "bang for your buck" out of the extra structural reinforcement required on each docking hard point.

Of course now it's active and not passive, and you still want a passive backup (I believe IDSS has one too). It's always a tradeoff!

[DanClemmensen]

Variable-strength springs would mean the attachment point needs to be stronger than it needs to for the same momentum absorption capacity.

The better approach is the one taken by NASA and IDSS, which uses electric motors with torque limiters (similar to an electric drill/driver) that will simply push back with the maximum allowable force for the entire stroke length. This is the efficient way to get the most "bang for your buck" out of the extra structural reinforcement required on each docking hard point.

Of course now it's active and not passive, and you still want a passive backup (I believe IDSS has one too). It's always a tradeoff!

I was not attempting to describe the mechanism, as I do not have the expertise. My internal mental model is a shock absorber in an adaptive suspension system.

[Twark_Main]

Variable-strength springs would mean the attachment point needs to be stronger than it needs to for the same momentum absorption capacity.

The better approach is the one taken by NASA and IDSS, which uses electric motors with torque limiters (similar to an electric drill/driver) that will simply push back with the maximum allowable force for the entire stroke length. This is the efficient way to get the most "bang for your buck" out of the extra structural reinforcement required on each docking hard point.

Of course now it's active and not passive, and you still want a passive backup (I believe IDSS has one too). It's always a tradeoff!

I was not attempting to describe the mechanism, as I do not have the expertise. My internal mental model is a shock absorber in an adaptive suspension system.

No worries, and no correction intended.

I just find it impressive how the NASA solution effectively "min-maxes" the fundamental underlying engineering constraints. Really smart design from really smart folks!

[Vultur]

The initial propellant test will be done even earlier at flight rate 10 per year, using two tankers launched several weeks apart. The first tanker will be subject to a significant boil-off so it is going to be a nice long term loiter experiment.

Boiloff is not a problem during the transfer demo. The pad will almost certainly be able to launch the ships on two consecutive days: that's what it's built to do. Commodities are the limiting factor initially.  Even if the second Ship (prototype Tanker) is long delayed, the first ship (prototype Depot) only needs to retain enough propellant to de-orbit if the Tanker fails to orbit and transfer.

Given that even Ship 39 (which isn't supposed to go to orbit at all) seems to have the "receiving" hardware, I would kind of expect the initial ship to not even be really a prototype Depot.

[DanClemmensen]

The initial propellant test will be done even earlier at flight rate 10 per year, using two tankers launched several weeks apart. The first tanker will be subject to a significant boil-off so it is going to be a nice long term loiter experiment.

Boiloff is not a problem during the transfer demo. The pad will almost certainly be able to launch the ships on two consecutive days: that's what it's built to do. Commodities are the limiting factor initially.  Even if the second Ship (prototype Tanker) is long delayed, the first ship (prototype Depot) only needs to retain enough propellant to de-orbit if the Tanker fails to orbit and transfer.

Given that even Ship 39 (which isn't supposed to go to orbit at all) seems to have the "receiving" hardware, I would kind of expect the initial ship to not even be really a prototype Depot.

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

[TheRadicalModerate]

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

Are you sure about that?  The socket thingees might weigh more than the leg thingees.  You can put a lot of stuff on the dorsal side of a Starship and still get it to do launch and EDL.

[DanClemmensen]

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

Are you sure about that?  The socket thingees might weigh more than the leg thingees.  You can put a lot of stuff on the dorsal side of a Starship and still get it to do launch and EDL.

The mass on both side is primarily the stuff needed to brace against the applied forces, and those forces are by definition the same on both sides. Thus to a first approximation just the base structure on the legs side will be the same mass as the socket side. The legs and associated actuators are in addition to this passive mass.

[jarmumd]

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

Are you sure about that?  The socket thingees might weigh more than the leg thingees.  You can put a lot of stuff on the dorsal side of a Starship and still get it to do launch and EDL.

The mass on both side is primarily the stuff needed to brace against the applied forces, and those forces are by definition the same on both sides. Thus to a first approximation just the base structure on the legs side will be the same mass as the socket side. The legs and associated actuators are in addition to this passive mass.

First off, I have no idea what their refueling system is like.  But, it's typical that the Chaser or Active vehicle has the soft capture mechanism, which in this case would be their probes (leg thingys).  That would mean that the Target or Passive vehicle has the cones.  Now, the reason for this, is so that the vehicle which is approaching (Chaser), has all the capabilities for aborts.  In other words, when Dragon 2 comes to the ISS, the ISS doesn't abort and move out of the way in case of an issue, Dragon does.  So the Chaser wants to have the ability to abort from GNC, and if you get partial capture or a capture with failures, be able to release any mechanisms.

It doesn't have to be this way... For HLS we did "reverse role" where the docking system was on the Target, but it implies that you have to have really redundant comms so that the Chaser can tell the mechanism on the Target to abort.  Can be done, just not simple.

Furthermore, all things being equal, you would want the passive side to stay in orbit (cones).  While this is more mass (assuming the complexity of deployment is on the probes), it also means that if the probe mechanism fails, it might be a loss of mission to refuel, but not a loss of tanker (or mission to the moon).

[OTV Booster]

Call it whatever you like. It will have the four leg thingees. In my universe, only Depot has the leg thingees. All other Ships (tankers, HLS, etc.) have the socket thingees. Thus, that first Ship is a Depot. It does not need to be finalized design, so in my terminology, it's a prototype Depot.

Are you sure about that?  The socket thingees might weigh more than the leg thingees.  You can put a lot of stuff on the dorsal side of a Starship and still get it to do launch and EDL.

I've got a hunch the socket thingies and the strut base attachment are the same.


The Canada arms are the same at both ends. How they grab onto the ISS attachment points and how they grab an end effector is the same. And their motive power comes from their wrist and shoulder, not the attachment points.


It's sockets vs posts with a knobber on the end but the parallel is there. For the early trials any ship could take on any role. Is this the SpaceX way or what.