Crew Dragon Mission Life Extension: Free-Flying, Uncrewed Duration

[TheRadicalModerate]

In innumerable Starship and HLS threads, we've discussed the possibility of a kludge, which would allow a Lunar Starship (aka LSS or HLS Starship) to act as an SLS/Orion replacement, when supplemented by an F9/D2.  The assertion for why this might be a good idea is that, by the time the LSS Option B work is complete, we have the makings of an Earth-NRHO-Earth system available off the shelf.  I don't think it's quite off the shelf, and I'd like to use this thread to discuss what's likely the long pole in the tent for implementing this:  D2 free-flying mission life.



There are plenty of variants on the conops for such an Artemis mission.  They're not the topic of this thread.  However, to get everybody on the same page for what the problem is, here's a reference conops:

1) LSS, which we'll call the "orbital transfer vehicle LSS" (OTV LSS) starts out in LEO, either freshly launched or returned from a previous Artemis mission.

2) OTV LSS uses a depot to take on enough propellant (it's <1000t) to perform an LEO-TLI-NRHO-RPOD-loiter-RPOD-TEI-LEOpropulsive mission.  (Note that it's the propulsive return to LEO, rather than using direct EDL or aerocapture, that makes the prop requirements considerably heftier--but not too hefty.)

3) Launch a crew of 4 on an F9/D2 to do rendezvous, prox ops, and docking (RPOD) with the OTV LSS.  Crew transfers to OTV LSS.

4) D2 undocks from the OTV, then goes into a low-consumption mode for a multi-week, uncrewed, free flight in LEO, waiting for the crew to return.  Note that this is different from what happens when D2 stays at ISS for a long duration, because ISS can provide power, ECLSS, and consumables while the D2 is docked.  In free flight, this support doesn't exist.  Free-flying crew Dragon mission life is currently about a week, but that's with the crew on board the whole time.  In this case, the crew would be on board for considerably less than a week (just long enough to do launch/RPOD with the OTV and then, after the OTV returned and did RPOD, long enough to take the crew to EDL).  But the free-flying time in LEO could be up to a couple of months while the D2 waited for the OTV's return.

5) OTV LSS now does LEO-NRHO (fast transit), then the crew transfers to the actual lunar lander HLS, does its potentially multi-week mission (per the Option B/SLD/SLT specs), then ascends back to NRHO.  Crew transfers back from the lander LSS to the OTV LSS.

6) OTV LSS does NRHO-TEI to return the crew.

7) D2 powers up back up and prepares itself to receive crew.

8 ) OTV does a propulsive insertion back into LEO, then does an RPOD with the D2.

9) Crew transfers to D2, which then returns crew through EDL.  OTV LSS waits in LEO for next mission.



The topics for this thread mostly revolve around steps #4 and #7 above:

a) Is such an uncrewed, low-consumption mode possible for a D2?
b) What modifications, if any, would need to be made to the current D2 to do this?
c) How much engineering and test work is needed to implement it?
d) What is the maximum duration of such a low consumption mode?
e) Are there any gotchas with RPOD with the LSS OTV, either post-launch or pre-EDL?
f) What contingency planning is needed for a failed post-return OTV-D2 RPOD?
g) If free-flying is impossible, is there a cheap widget that could support the D2 for the duration of the mission?  (ISS is obviously such a widget, but it's in a pretty high inclination for lunar departures and returns.



Topics that are not for discussion in this thread:

i) This is stupid, just get Starship crew-certified and you don't need to do this.  The premise is that Starship crew certification will be hard and take a long time.  That's a perfectly debatable premise; please don't debate it here.

ii) You can avoid the power-down if you use two D2s per mission, one for crew launch and one for crew EDL.  This is also a perfectly good discussion topic, but I'd like to focus on how to avoid doing this, since it'll be expensive to have two D2 launches per Artemis mission.

iii) Just use the D2 on a Falcon Heavy as the OTV instead of an LSS.  Again, a possibly viable idea, although it likely involves considerably more engineering.  Please, not here.

iv) This is all silly because NASA will never allow a competitor to SLS/Orion.  Possibly true, but it'll turn this thread into a food-fight.

v) Your reference D2+LSS conops is dumb.  It's much better to <insert your favorite variant here>.  Just stick to the essence of figuring out the D2 problems.

I've hopefully constrained this so that this is a relatively short thread.  The goal is to find out whether D2 mission life turns out to render this class of missions non-viable or really expensive to develop.  I don't know a lot about the relationship between mission life and consumables.  Hopefully people who do will chime in so we can figure out if this is viable.

Have at it.

[TheRadicalModerate]

IMO, most of the problems are going to be thermal:

1) What freezes without ISS ECLSS blowing around in the crew area?

2) What boils away without ISS consumables available while docked?

3) Are water or refrigerants actively consumed keeping things within operational limits during free flight?  Is water lost?  Is there an ammonia loop that needs a bigger reservoir?

4) Is there some limit to the radiators in the trunk that gets exceeded for long free flights?

5) Do the helium COPVs for the propulsion system exceed some critical temperature without some kind of expendable cooling?

6) What happens to ECLSS and cabin instrumentation if it doesn't stay within environmental limits that are satisfied by the ISS but not satisfied in free flight?

7) Are there mechanical components in the thermal management system (valves, pumps, etc.) whose cycle life is exceeded in a long mission?

If we can identify the gotchas, then we can figure out what needs beefing up.

[joek]

...
a) Is such an uncrewed, low-consumption mode possible for a D2?

Certainly.

b) What modifications, if any, would need to be made to the current D2 to do this?

A service module, likely stuck in, or replacing, the trunk.

c) How much engineering and test work is needed to implement it?

Unknown. However, DragonLab was advertised as a free flyer with a duration of "up to 2 years" and which would host pressurized and unpressurized cargo. Presumably SpaceX did the analysis for such a claim. Whether two years with full pressurized and associated ECLSS is anyone's guess. However have to expect that, given the 2-year claim, all systems would still be nominal-operational.

d) What is the maximum duration of such a low consumption mode?

Unknown. But if we take DragonLab's claims at face value, nominally 1-2 years.

There are undoubtledly additional clues in the various NASA requirements related to lifeboat and crew return vehicle requirements; unfortunately can't find them at the moment. Another clue would be ISS interface requirements--which might tell us what Dragon needs to stay alive as a free flyer, or what an attachment needs to provide. (Unfortunately the ISS interface requirements are not public AFAICT.)

edit: Are we talking about a free flyer in LEO or NHRO or ... ? Will make a difference in terms of thermal-nav-comm-etc. but doubt significant enough to materially change the equation.

[joek]

6) What happens to ECLSS and cabin instrumentation if it doesn't stay within environmental limits that are satisfied by the ISS but not satisfied in free flight?

As long as equipment stays within operational limits, should not matter. ISS limits are intended to satisfy humans; free flight or "stasis" does not need to satisfy the same. Just need to ensure that when crew boards (or is about to board), the spacecraft is capable of bringing the environment to within limits for crew. Orion works the same way.

[TheRadicalModerate]

b) What modifications, if any, would need to be made to the current D2 to do this?

A service module, likely stuck in, or replacing, the trunk.

If that's necessary, then the whole conops starts to look kinda unattractive, because that's a lot of work.

However, a D2 used for this conops doesn't have any payload in the trunk.  That means that, if all you need is a few tanks to super-size the consumables stores, you can build a specialized trunk payload that contains all that stuff.  You then still need some plumbing between the crew module and the trunk.  I don't know if that currently exists, although I suspect there's plumbing already there for radiators.

c) How much engineering and test work is needed to implement it?

Unknown. However, DragonLab was advertised as a free flyer with a duration of "up to 2 years" and which would host pressurized and unpressurized cargo. Presumably SpaceX did the analysis for such a claim. Whether two years with full pressurized and associated ECLSS is anyone's guess. However have to expect that, given the 2-year claim, all systems would still be nominal-operational.

Since DragonLab never had any customers, we don't really know if everything was already built in, or if it was an Elon fever-dream.  We also don't know if stuff to enable DragonLab was removed from D2.

I'm confident that D2 can be modified for long-duration uncrewed free flight, followed by a brief stint of crewed operation at the end of the mission.  The real question is whether the mods to do so are near-trivial, or a significant cost.  If they're trivial, then you have an SLS/Orion replacement that can be spun up very quickly, and the total mission cost is:

LSSmanufacturingCost/numberOfLSSmissions + LSSlaunchAndOpsCost + tankerLaunchCost*4or5 + D2launchCost

If they're non-trivial, then SpaceX has to bet considerably bigger on modifying the D2, which will annoy NASA, and the total mission cost is:

(LSSmanufacturingCost + D2reengineeringCost)/numberOfLSSmissions + LSSlaunchAndOpsCost + tankerLaunchCost*4or5 + D2launchCost

Or, alternatively, they could go the "use 2 D2's" route, which I'd rather not discuss here.

There are undoubtedly additional clues in the various NASA requirements related to lifeboat and crew return vehicle requirements; unfortunately can't find them at the moment.

Doesn't necessarily help, because any lifeboat never free-flies except during ascent and EDL, both of which take very little time.

Another clue would be ISS interface requirements--which might tell us what Dragon needs to stay alive as a free flyer, or what an attachment needs to provide. (Unfortunately the ISS interface requirements are not public AFAICT.)

I'm pretty sure that all that's implemented in NDS are power and data umbilicals.  IIRC, IDSS had room for some fluid transfer umbilicals, but I don't think anybody ever implemented them.

You can accomplish an awful lot with a blower duct and a human transferring the occasional bag of water back and forth.  Neither of those would be available during free flight.

6) What happens to ECLSS and cabin instrumentation if it doesn't stay within environmental limits that are satisfied by the ISS but not satisfied in free flight?

As long as equipment stays within operational limits, should not matter. ISS limits are intended to satisfy humans; free flight or "stasis" does not need to satisfy the same. Just need to ensure that when crew boards (or is about to board), the spacecraft is capable of bringing the environment to within limits for crew. Orion works the same way.

That all gets back to temperature.  If some consumable's rate of consumption is temperature-dependent (e.g. water), then a low temperature can cause condensation and a variety of yucky stuff.

[Jorge]

Another clue would be ISS interface requirements--which might tell us what Dragon needs to stay alive as a free flyer, or what an attachment needs to provide. (Unfortunately the ISS interface requirements are not public AFAICT.)

I'm pretty sure that all that's implemented in NDS are power and data umbilicals.  IIRC, IDSS had room for some fluid transfer umbilicals, but I don't think anybody ever implemented them.

The NDS IDD is public:
https://ntrs.nasa.gov/api/citations/20150014481/downloads/20150014481.pdf

joek may be referring to the ISS/COTS IRD (SSP-50808) which is controlled under EAR.

[DanClemmensen]

Another clue would be ISS interface requirements--which might tell us what Dragon needs to stay alive as a free flyer, or what an attachment needs to provide. (Unfortunately the ISS interface requirements are not public AFAICT.)

I'm pretty sure that all that's implemented in NDS are power and data umbilicals.  IIRC, IDSS had room for some fluid transfer umbilicals, but I don't think anybody ever implemented them.

The NDS IDD is public:
https://ntrs.nasa.gov/api/citations/20150014481/downloads/20150014481.pdf

joek may be referring to the ISS/COTS IRD (SSP-50808) which is controlled under EAR.

NDS is an implementation of IDSS. Crew Dragon uses an independent SpaceX active-only implementation of IDSS that is compatible with NDS. I don't think IDSS is controlled under ITAR.

The ISS has IDA ports, which implement passive-only NDS.

[Zed_Noir]

Not free flying, but a simple solution. Outfit the widget (depot tanker Starship) with a deck module that have a few radial docking ports above the propellant tanks. So the docked Crew Dragon capsule draws power and consumables from the depot waiting for the return of the OTV. Advantage of little modifications to the Dragon capsule.

[DanClemmensen]

Not free flying, but a simple solution. Outfit the widget (depot tanker Starship) with a deck module that have a few radial docking ports above the propellant tanks. So the docked Crew Dragon capsule draws power and consumables from the depot waiting for the return of the OTV. Advantage of little modifications to the Dragon capsule.

There are several ways to sustain the Crew Dragon in LEO.
   1) dock it to ISS
   2) Fly another unmodified Starship HLS that will sit in LEO and act as a minimal CLD to sustain the Crew Dragon. No new engineering needed.
   3) Add a second IDSS port to all the Starship HLSs. I think this qualifies as a minimal change, and it adds operational flexibility. One of these HLSs acts as a slightly less minimal CLD and sustains the Crew Dragon.
   4) Go ahead and build a real Starship CLD with multple IDSS ports.

The advantage of a CLD with two or more ports is that the Dragon and the Starship OTV can both dock to it for the crew transfers. The Dragon stays docked until the crew returns to LEO for the homeward transfer. No need for three separate Dragon dockings.

[TheRadicalModerate]

Not free flying, but a simple solution. Outfit the widget (depot tanker Starship) with a deck module that have a few radial docking ports above the propellant tanks. So the docked Crew Dragon capsule draws power and consumables from the depot waiting for the return of the OTV. Advantage of little modifications to the Dragon capsule.

There are several ways to sustain the Crew Dragon in LEO.
   1) dock it to ISS
   2) Fly another unmodified Starship HLS that will sit in LEO and act as a minimal CLD to sustain the Crew Dragon. No new engineering needed.
   3) Add a second IDSS port to all the Starship HLSs. I think this qualifies as a minimal change, and it adds operational flexibility. One of these HLSs acts as a slightly less minimal CLD and sustains the Crew Dragon.
   4) Go ahead and build a real Starship CLD with multple IDSS ports.

The advantage of a CLD with two or more ports is that the Dragon and the Starship OTV can both dock to it for the crew transfers. The Dragon stays docked until the crew returns to LEO for the homeward transfer. No need for three separate Dragon dockings.

1) ISS is too high an inclination.
2) Another LSS is a possibility, albeit an expensive one.
3) A second docking port violates the "almost off the shelf" rule.
4) As does a real-live CLD.

Here's one I haven't though through:  What if you take the D2 with you on the nose of the OTV-LSS?  You probably couldn't use more than one RVac and one low-throttle RSL if you want to stay within the torque and load envelope of IDSS, but that doesn't seem particularly difficult.

You'd likely have to undock the D2 for the OTV-to-lander RPOD, redock it after the lander left, and then repeat the process when the lander came back.  If you're really, really confident in your RPOD reliability, that might be acceptable.

[TheRadicalModerate]

Not free flying, but a simple solution. Outfit the widget (depot tanker Starship) with a deck module that have a few radial docking ports above the propellant tanks. So the docked Crew Dragon capsule draws power and consumables from the depot waiting for the return of the OTV. Advantage of little modifications to the Dragon capsule.

If you can build all the consumables and ECLSS support you need into the depot, you can build it into the D2 trunk in the first place.  The D2 mods are easier, faster to make, and cheaper.

[pochimax]

1) ISS is too high an inclination.

Maybe other CLD proposals will not have the ISS inclination (for example, Starlab? I don't know)

Just contract a private port to extend the Dragon life on orbit. Or at least, compare the cost of doing it to modifying Dragon.

[joek]

The NDS IDD is public:
https://ntrs.nasa.gov/api/citations/20150014481/downloads/20150014481.pdf

joek may be referring to the ISS/COTS IRD (SSP-50808) which is controlled under EAR.

Thanks (long time no see!). Yes, it's the IRD don't have access to.

Thought was that information about what the ISS provides to visiting vehicles (and vv.) might provide some insight into what is needed to keep the vehicle nominal as a free flyer beyond a few days.

[joek]

... I don't know if that currently exists, although I suspect there's plumbing already there for radiators.

Yes, for radiators and power; For Dragon 2, see Development of the Crew Dragon ECLSS, SpaceX, 2020. Would note that much of the ECLSS requirements are driven by crew occupancy--as you would expect (crew of 4 for 5 days as a free flyer).

[DanClemmensen]

3) A second docking port [on HLS-as-CLD] violates the "almost off the shelf" rule.

I am not a spacecraft engineer. However, the HLS is still in its design phase and I think it's likely that a second IDSS port is a simple design change.  This is especially true if the IDSS ports are on the airlock deck. It's also a useful addition to HLS in general in several scenarios. I think SpaceX may choose to design the very first HLS (uncrewed demo) as full-up Option B, skipping Option A altogether. A single HLS-as-CLD can be reused, first for Polaris 3, then as a CLD for any other demo missions, and then for the SLS/Priion replacement scenarios starting as early as Atemis III in the most aggressive case.  This allow for as many incremental test missions as desired, instead of the Artemis III "big bang" test.

This drifts very far from the topic of this thread, except in one sense: Why will a long-duration D2 ever be needed? IF (big IF) one or more crewed Starship types become available, what missions are left for D2 except to handle the Earth-LEO-Earth segment until Starship itself can handle it?

[joek]

...
This drifts very far from the topic of this thread, except in one sense: Why will a long-duration D2 ever be needed? IF (big IF) one or more crewed Starship types become available, what missions are left for D2 except to handle the Earth-LEO-Earth segment until Starship itself can handle it?

Think that violates OP rule #i (?) ...

i) This is stupid, just get Starship crew-certified and you don't need to do this.  The premise is that Starship crew certification will be hard and take a long time.  That's a perfectly debatable premise; please don't debate it here.

Thus the question: What would it take to provide a "long-duration D2" as an interim solution? Where "long-duration" means: (a) 2-3 months uncrewed in LEO; and (b) crewed for last few days of that time (LEO to Earth surface). Am I getting that right?

[joek]

...
That all gets back to temperature.  If some consumable's rate of consumption is temperature-dependent (e.g. water), then a low temperature can cause condensation and a variety of yucky stuff.

For a free flyer with no crew, what consumables? D2 has thermal (radiator) and power (solar cells) needed to maintain an uncrewed environment nominally indefinitely[1]. That "yucky stuff" is largely an artifact of crew presence; remove crew and you remove the problem during the free-flyer "stasis" period.

During "stasis" (uncrewed) ... Humidity control? Nothing going in-out; should not a problem[2]. N2-O2-CO balance-scrubbing? Nothing going in-out; should not a problem.

So maybe turn the question around: What is it that you see as a problem with extending uncrewed D2 LEO life to weeks-months as an uncrewed free flyer that may require substantive changes?

[1] Conjecture. Feel free to challenge if you have information to the contrary, but Orion and DragonLab suggest that, without crew, they can free fly for extended periods.
[2] Except maybe venting for some undefined purpose? Why would that be needed if existing thermal controls work?

[Zed_Noir]

...
This drifts very far from the topic of this thread, except in one sense: Why will a long-duration D2 ever be needed? IF (big IF) one or more crewed Starship types become available, what missions are left for D2 except to handle the Earth-LEO-Earth segment until Starship itself can handle it?

Think that violates OP rule #i (?) ...

i) This is stupid, just get Starship crew-certified and you don't need to do this.  The premise is that Starship crew certification will be hard and take a long time.  That's a perfectly debatable premise; please don't debate it here.

Thus the question: What would it take to provide a "long-duration D2" as an interim solution? Where "long-duration" means: (a) 2-3 months uncrewed in LEO; and (b) crewed for last few days of that time (LEO to Earth surface). Am I getting that right?

Also if SpaceX is willing to further developed the Crew Dragon.

Instead of using up resource on modifying the Crew Dragon. Think they will be more likely to mod some flavor of Starship for the purpose of the thread subject. Not necessary something crew rated.

[DanClemmensen]

In a desperate attempt to keep from dragging this thread off topic any further I started a new topic for
   Replacing SLS/Orion using Starship HLS and Crew Dragon
at
   https://forum.nasaspaceflight.com/index.php?topic=59662.0

[TheRadicalModerate]

...
This drifts very far from the topic of this thread, except in one sense: Why will a long-duration D2 ever be needed? IF (big IF) one or more crewed Starship types become available, what missions are left for D2 except to handle the Earth-LEO-Earth segment until Starship itself can handle it?

Think that violates OP rule #i (?) ...

i) This is stupid, just get Starship crew-certified and you don't need to do this.  The premise is that Starship crew certification will be hard and take a long time.  That's a perfectly debatable premise; please don't debate it here.

Thus the question: What would it take to provide a "long-duration D2" as an interim solution? Where "long-duration" means: (a) 2-3 months uncrewed in LEO; and (b) crewed for last few days of that time (LEO to Earth surface). Am I getting that right?

Yes, but with one additional constraint, which I should have made explicit in the initial premise:  The LSS / tanker / depot work needed to get to at least Option A, and probably Option B / SLT, can't be slowed down by whatever the D2/LSS kludge solution is.

Remember when Bridenstine yanked on Elon's chain in 2019 because he spent time on the Starship roll-out event while the CCtCap schedule appeared to be slipping?  Now imagine how NASA would react if the HLS schedule were on the critical path for a while, but SpaceX was making modifications to the LSS, or diverting resources to D2 mods, which caused a further slip in the schedule, solely for the purpose of being able to sabotage SLS/Orion in the not-too-distant future. 

I suppose if a SpaceX CLD solution were to magically appear before Option B was complete, that might be OK.  But I suspect NASA's reaction to that would be roughly the same:  "Why are you pouring so many resources into CLD, which we only care about a little, at the expense of Artemis, which we care about a lot?"

I think that rules out depot nannies, itty-bitty CLD precursors, second docking ports on LSS, and a whole host of other possible solutions.

However, using a second LSS as a nanny might be a solution, albeit a kinda expensive one.  Putting the D2 on the OTV-LSS's nose during the LEO-NRHO-LEO transit might be a solution.  And certainly some modest consumable extension that took the form of a trunk payload with a bit of plumbing could be solution, as would a pressurized extension package that didn't eat up too much pressurized cargo volume.

Best solution of all, of course, is that the uncrewed loiter just works as-is.  I don't rule that out, but it's always something, isn't it?

[TheRadicalModerate]

...
That all gets back to temperature.  If some consumable's rate of consumption is temperature-dependent (e.g. water), then a low temperature can cause condensation and a variety of yucky stuff.

...

[2] Except maybe venting for some undefined purpose? Why would that be needed if existing thermal controls work?

This is why I keep wringing my hands about thermal.  I could see the thermal system working great as a free-flyer for 7-10 days, using a little bit of venting when insolation thermal load got a little too extreme, but with enough consumable reserve that the system has plenty of margin.  When the D2 is docked at the ISS, the ISS could provide the extra margin, either by circulating heat into the ISS pressurized volume, where its thermal system would clean it up, or simply by somebody adding water to the D2 as needed.

But if the thermal system isn't perfectly closed during long-duration free-flight, that margin would (ahem) evaporate.

NB:  I don't know anything about the D2 thermal system, and I could just be a nervous nelly.  (It wouldn't be the first time.)  But that's why I started the thread:  What kinds of things could go wrong, and how hard would they be to patch?

[cohberg]

I think you may want to read the ECLSS paper linked to earlier in the thread.

using a little bit of venting when insolation thermal load got a little too extreme

Dragon doesn't vent atmosphere into space for thermal control when the trunk is attached.
Heat is removed via a closed loop radiator system. Only electricity is consumed to cool the craft.
Removing the human metabolic loads will decrease the heating of the craft, lowering the duty cycle for the TCS.

human transferring the occasional bag of water back and forth.  Neither of those would be available during free flight.

simply by somebody adding water to the D2 as needed.

Dragon doesn't have a water tank or consume water.

Humidity is produced by humans and is only removed, not created, by Dragon.

[edzieba]

Not free flying, but a simple solution. Outfit the widget (depot tanker Starship) with a deck module that have a few radial docking ports above the propellant tanks. So the docked Crew Dragon capsule draws power and consumables from the depot waiting for the return of the OTV. Advantage of little modifications to the Dragon capsule.

There are several ways to sustain the Crew Dragon in LEO.
   1) dock it to ISS
   2) Fly another unmodified Starship HLS that will sit in LEO and act as a minimal CLD to sustain the Crew Dragon. No new engineering needed.
   3) Add a second IDSS port to all the Starship HLSs. I think this qualifies as a minimal change, and it adds operational flexibility. One of these HLSs acts as a slightly less minimal CLD and sustains the Crew Dragon.
   4) Go ahead and build a real Starship CLD with multple IDSS ports.

The advantage of a CLD with two or more ports is that the Dragon and the Starship OTV can both dock to it for the crew transfers. The Dragon stays docked until the crew returns to LEO for the homeward transfer. No need for three separate Dragon dockings.

5) Land it, refurbish it, then launch it again.
This seems like a lot of extra R&D and HSF certification and complicated CONOPS purely to avoid reusing the proven reusable spacecraft.

[DanClemmensen]

Not free flying, but a simple solution. Outfit the widget (depot tanker Starship) with a deck module that have a few radial docking ports above the propellant tanks. So the docked Crew Dragon capsule draws power and consumables from the depot waiting for the return of the OTV. Advantage of little modifications to the Dragon capsule.

There are several ways to sustain the Crew Dragon in LEO.
   1) dock it to ISS
   2) Fly another unmodified Starship HLS that will sit in LEO and act as a minimal CLD to sustain the Crew Dragon. No new engineering needed.
   3) Add a second IDSS port to all the Starship HLSs. I think this qualifies as a minimal change, and it adds operational flexibility. One of these HLSs acts as a slightly less minimal CLD and sustains the Crew Dragon.
   4) Go ahead and build a real Starship CLD with multple IDSS ports.

The advantage of a CLD with two or more ports is that the Dragon and the Starship OTV can both dock to it for the crew transfers. The Dragon stays docked until the crew returns to LEO for the homeward transfer. No need for three separate Dragon dockings.

5) Land it, refurbish it, then launch it again.
This seems like a lot of extra R&D and HSF certification and complicated CONOPS purely to avoid reusing the proven reusable spacecraft.

Each Crew Dragon flight appears to cost about $200 million, so that's the cost of the second flight. You need to find an approach that costs less than $200 million extra per Moon mission if you want to avoid that second flight. In the specific context of this thread, your life-extending trunk would need to cost less than $200 million, because the trunk is expended. This includes the amortized cost of development and crew-qualification.  I have no idea what the cost would be. For discussions about other approaches, please go to the other thread.

[Barley]

Two things come to mind that might limit endurance.

Station Keeping

Either the quantity of propellent or the number of cycles for the thrusters.

Mold and Mildew

And other biologicals.  These are dealt with by air movement and filtering, cleaning, or by making the cabin inhospitable to life.  Perhaps it's as simple as opening a large can of silica gel before leaving for the moon.

[TheRadicalModerate]

I think you may want to read the ECLSS paper linked to earlier in the thread.

using a little bit of venting when insolation thermal load got a little too extreme

Dragon doesn't vent atmosphere into space for thermal control when the trunk is attached.
Heat is removed via a closed loop radiator system. Only electricity is consumed to cool the craft.
Removing the human metabolic loads will decrease the heating of the craft, lowering the duty cycle for the TCS.

human transferring the occasional bag of water back and forth.  Neither of those would be available during free flight.

simply by somebody adding water to the D2 as needed.

Dragon doesn't have a water tank or consume water.

Humidity is produced by humans and is only removed, not created, by Dragon.

I went through the paper at your suggestion.  Some thoughts:

1) I was surprised to see that D2 only supports 20 person-days of consumables.  That's 5 days of crewed use, not the 7-10 that I'd heard (or thought I'd heard) elsewhere.  That should still be fine for D2/LSS kludgery, though.

2) The paper never mentions what the thermal control system coolant is.  Does anybody know?  I assume it's a single-phase coolant?  If not, there could be some issues with free-flight thermal load that could trigger some kind of gas venting, which would need to be replaced from a consumable reservoir.

3) Care and feeding of the nitrox COPVs (2 sets of 3 COPVs each).  What you said about D2 not venting isn't completely correct.  The ECLSS will vent atmosphere if pressure limits are exceeded.  It's unclear to me whether that's an exceptional event in free flight or not.  Presumably, if venting does occur, then atmosphere is back-filled from the nitrox system as things cool down.  Note that adiabatically cooled nitrox is used to keep the cabin cool (well, not intolerably hot) during reentry, so maintaining an adequate nitrox reserve is essential.

I assume that venting never occurs while the D2 is docked to the ISS, because the ISS's ECLSS is more-or-less in charge then, and the ISS consumable budget would include any needed atmospheric backfill.

4) It must be the case that the nitrox COPVs themselves don't need to vent for pressure-relief while docked to the ISS, because the ISS can't replenish them, and everything works fine for long-term docking.  I think that means that, as long as free-flying temperature limits don't have weird spikes in them, the nitrox system itself wouldn't vent during extended free-flying.  But that kinda relies on the answer to the questions in point #2 above.

5) Are there non-ECLSS systems that might have consumables?  Helium COPVs would be an example of one such system.

6) A lot of the stuff above is going to depend on lighting conditions.  I would assume that a docked D2 gets at least partial shading from the ISS structure, which would make peak thermal loads (direct sunlight and near-equatorial Earth albedo on the day side) last for less time than it would experience in free flight.  I'd guess that the nitrox supply for 20 crew-days of free flight would be fine for accommodating >1-2σ lighting conditions, which might generate any or all of the following:

a) Cabin pressure relief venting from a pressure spike due to direct heating.
b) Nitrox adiabatic cooling release to cool the cabin down from direct heating.
c) Nitrox cooing, which increases the pressure, which causes a pressure relief venting.
d) Nitrox tank pressure high, which might release directly overboard.

None of these would be a big deal if they happened a handful of times per crew free-flight.  But they might add up to a serious nitrox depletion if that handful of times was multiplied by a factor of 10-20 over the course of uncrewed free flight.

I'm of course guessing on whether such thermal spikes actually exist, and what their duration would be.  But it does seem that they'll be worse in free flight than they would be if the ISS were partially shading the vehicle in day-side equatorial conditions.

7) Leaks.  The nitrox COPVs are sized to deal with a fairly big MMOD-induced leak, with the criterion being that it can backfill enough for an emergency de-orbit and EDL.  But itty-bitty MMOD damage, which would otherwise be considered nominal for a 4-day free flight, could add up to to a big draw-down over 30-60 days.

There's also of course some nominal amount of leakage through valves and fittings that pierce the pressure vessel.

Two things come to mind that might limit endurance.

Station Keeping

Either the quantity of propellent or the number of cycles for the thrusters.

This could also be another thing where popping out on day-side near the equator could be something that required mitigating the thermal spike with something like a barbecue roll.  (I'm on real thin ice here.)

Do we have any data on how often they swap out the Dracos on the D2 currently?  It's an excellent point that extended free flight is a worst-case stressor on those systems.

Mold and Mildew

And other biologicals.  These are dealt with by air movement and filtering, cleaning, or by making the cabin inhospitable to life.  Perhaps it's as simple as opening a large can of silica gel before leaving for the moon.

What's the alien script for "Do Not Eat" that needs to go on the can of silica gel?

It's possible that they could dry the cabin air substantially with a big nitrox pulse to drop the temperature down quite a bit, with a pressure vent at the same time.  Dry air doesn't necessarily eliminate the risk of some kind of overgrowth, but it certainly reduces it.

Such a purge would obviously eat into the nitrox reserves, but if there aren't any nitrox releases in nominal free flight (i.e., my nervous nelly list above isn't a problem), it shouldn't be too bad.  I guess the cold might put a pretty hefty thermal stress on the cabin systems, though.

[TheRadicalModerate]

Why is there a need for station keeping? The craft is free flying and in a stable parking orbit.

See my lighting conditions hand-wringing above.

quantity of propellent

F9 + Dragon likely will not be launching to 51.6°. The additional relative performance of F9 S1 / S2 can be used to more directly inject Dragon into the rendezvous orbit.
Both craft can actively phase their orbits.
OTV LSS can actively take part in prox op docking, reducing Dragon fuel consumption
Dragon already does port relocations during typical ISS missions, simulating the fuel consumption for the "extra" docking with the returning OTV LSS.

I'd think that you'd want to keep the waiting D2 within some range of orbital tolerance, and failure to do so could result in an abort of all or part of the lunar mission.  So there's at least some stationkeeping that needs to occur.

Dragon performs "propellant wasting" burns at the end of typical ISS missions, indicating a surplus of fuel.

Yes, but how much surplus?  Enough to extend the mission by a factor of 10-ish?

[TheRadicalModerate]

I think MMOD risk needs to go near the top of the list of issues.  It's cumulative, so a longer loiter jacks it up.

Not completely on-topic for here, but worth bearing in mind:  Something bad happening to the loitering D2, from any cause, would have to go into contingency planning.

My guess for how to handle this is some kind of "warm standby" for an F9 and D2 on the ground.  This could be as simple as coordinating CRS or private missions so you can get a mostly-ready D2 out of processing and onto a waiting F9 within a day or two, to be launched after the OTV-LSS winds up in whatever LEO it's gonna wind up in.

Also sorta O/T:  This is yet another reason why the "just use two different D2s for departure and arrival" strategy isn't great.  Now you'd need two different D2s,  at advanced stages in the processing pipeline, one for the return, and one in case something bad happened to the return D2.  (If the departure one has a problem, that's a straight-up abort back to EDL--assuming it's not a fatal problem.)

[TheRadicalModerate]

What you said about D2 not venting isn't completely correct.  The ECLSS will vent atmosphere if pressure limits are exceeded.  It's unclear to me whether that's an exceptional event in free flight or not. 

Yes, but I qualified my statement with two conditions

Dragon doesn't vent atmosphere into space for thermal control when the trunk is attached.

1. "for thermal control" - venting exclusively for the purpose of thermal control
2. "when the trunk is attached" - state dependent on if trunk is still attached

Let's take a look at what the SpaceX ECLSS paper has to say about overpressure venting and see if my statement is completely correct.

The only times when active cabin pressure control is expected to be needed is for the nitrox cooling purge during entry and possibly during suit leak checks

So two conditions for Dragon to vent:

1. nitrox cooling purge during entry - the trunk is no longer attached
2. possibly during suit leak checks - not for thermal control

Per your terminology, venting is exceptional. It will occur very infrequently (and at prescribed times) only when there are humans onboard.
There is no regular venting for temperature / humidity control. That is the purpose of (and handled by) the radiator.
The spacecraft does not vent atmosphere to control temperature when the trunk is attached.

Everything you have above is correct--as long as the TCS can handle all of the >2σ thermal loads.  At the very least, there have to be contingency plans (and software) to handle off-nominal heating events on orbit.  Losing one of the TCS loops would qualify.  If that's the case, might they also consider an abnormally cloudy day over the equator, just as the orbital node crosses between the Sun and the Earth, an off-nominal event?

(Yeah, I'm lawyering the bejeezus out of this.  But I want to be sure you can prove me wrong.)

Another random thought:  You have longer peak heating events in a 28º-35º inclination orbit (an arm-wave for something that's suitable for TLI and TEI return), than in the 51.6º ISS inclination.  How much extra heat does that dump into the system on a maximum-albedo day?

[TheRadicalModerate]

Apropos MMOD mitigation for a D2 free-flyer:  How much delta-v would you need to station-keep a D2 in VLEO (<375km circular altitude) for a couple of months?  Debris distribution by altitude spikes up very quickly at about 375km.  Below that, drag cleans stuff out very quickly.

I have a feeling that SpaceX is going to want most, if not all, depot operations as low as possible, because a bad accident with a depot is a catastrophically large debris generator.  Tankage for an OTV-LSS is more than generous, so refueling very low works fine, and the depot doesn't have to move.  Therefore, keeping the D2 more-or-less co-orbital with the big guys makes a certain amount of sense, and certainly dramatically reduces MMOD risk.

The problem, of course, is that it costs more propellant for station-keeping, which is the opposite of what we want if we're to keep the modifications to D2 at a minimum, and hopefully to none at all.

[Eer]

Off the wall variant that <might> violate the "free flying" OP requirement:

1) Launch a StarShip equipped with reclosable chomper or other wide open fairing around the cargo area.
2) In the back (base) of the cargo area, store consumables and a docking port for the D2.
3) When D2 loiter is needed, dock it to the cargo-hold-based docking bay for power and consumable refresh, and close the chomper for additional protection against foreign object collisions
4) when D2 loiter is no longer needed, undock, transfer crew, and return.  Optionally, transfer crew, undock, and return - the sequence depends on how much cargo space is crew-friendly, and whether there is a second docking port (externally accessible - not also in the cargo area).
5) return StarShip for refurbishment and before transfer to alternate orbit as needed.

I think of this as a "garage" or "covered StarShip Park" alternative. Leverage the size, resources, long loiter capabilities StarShip gives you.

[TheRadicalModerate]

Off the wall variant that <might> violate the "free flying" OP requirement:

1) Launch a StarShip equipped with reclosable chomper or other wide open fairing around the cargo area.
2) In the back (base) of the cargo area, store consumables and a docking port for the D2.
3) When D2 loiter is needed, dock it to the cargo-hold-based docking bay for power and consumable refresh, and close the chomper for additional protection against foreign object collisions
4) when D2 loiter is no longer needed, undock, transfer crew, and return.  Optionally, transfer crew, undock, and return - the sequence depends on how much cargo space is crew-friendly, and whether there is a second docking port (externally accessible - not also in the cargo area).
5) return StarShip for refurbishment and before transfer to alternate orbit as needed.

I think of this as a "garage" or "covered StarShip Park" alternative. Leverage the size, resources, long loiter capabilities StarShip gives you.

Not free-flying is OK, as long as it doesn't require a bunch of development work.

Assuming that chomper is still a thing, this isn't terrible.  I like using the Starship fairing as an MMOD first line of defense.  But there are some problems:

1) First, you have to design a D2-docking and -supporting payload package for Starship.  That's not huge amounts of work, but it's probably more than tweaking the D2 itself to free-fly for long enough.  Note that the assumption for D2 docked operation is that it has ECLSS from the host, as well as thermal support.  If you're going to close up the chomper, you also probably need to provide enough heat to the exterior of the D2 that you don't exceed minimum temperatures.  That's probably a fair amount of power.

2) Second, if you bring the Starship down after each mission, its launch cost gets factored into the overall mission cost.  If you've drunk the $5M/launch Kool-Aid, that's not a problem.  But if, as I believe, the actual cost will be closer to $50M/launch, it's not nothing.  It is better, however, than launching two separate D2s, which would cost an extra $200M-$300M.

3) You could leave the Starship in orbit permanently, but then it has stationkeeping prop to manage, and it'll be pelted with MMOD itself.

4) It's one additional mission-critical RPOD.  There's a contingency if it fails (launch a second, warm-standby D2), but NASA hates RPODs, for reasons I don't completely understand.  The SLS/Orion folks are gonna beat the D2/LSS kludge over the head with the two required LEO RPODs already.  Not giving them more ammunition would be desirable.

[DanClemmensen]

Off the wall variant that <might> violate the "free flying" OP requirement:

1) Launch a StarShip equipped with reclosable chomper or other wide open fairing around the cargo area.
2) In the back (base) of the cargo area, store consumables and a docking port for the D2.
3) When D2 loiter is needed, dock it to the cargo-hold-based docking bay for power and consumable refresh, and close the chomper for additional protection against foreign object collisions
4) when D2 loiter is no longer needed, undock, transfer crew, and return.  Optionally, transfer crew, undock, and return - the sequence depends on how much cargo space is crew-friendly, and whether there is a second docking port (externally accessible - not also in the cargo area).
5) return StarShip for refurbishment and before transfer to alternate orbit as needed.

I think of this as a "garage" or "covered StarShip Park" alternative. Leverage the size, resources, long loiter capabilities StarShip gives you.

I don't think this is minimal. You are solving problems that don't really need to be solved for an early Artemis mission. The lives of today's  ISS crews already depend on Crew Dragons that are more-or-less unprotected in LEO for six months. For the kludge SLS/Orion replacement missions, you "protect" D2 by having another D2 available on the ground. The astronauts can stay in the OTV until the replacement D2 can be launched.

Your "garage" appears to be an (at most) minimally-modified version of a "standard" cargo Starship. But the cargo Starship is a new development in the context of the OP. The OP kludge depends only on the Starship variants that are already needed for Artemis III, namely Depot, Tanker, and HLS. It does not assume that an EDL cargo starship exists.

Yes, we know that SpaceX wants an EDL Starlink dispenser Starship as soon as possible, but that's outside of Artemis and is also likely to be simpler than a chomper.

[TheRadicalModerate]

I think MMOD risk needs to go near the top of the list of issues.  It's cumulative, so a longer loiter jacks it up.

Update from the other thread:  In response to something that Twark_Main found, I did a back-of-napkin on a 4-month probability of collision for a D2.  At 25º inclination it looks like the probability of collision per m² per year would have to be so high that D2 is OK at pretty much any altitude.  See here.

Not sure how I feel about this result;  seems a bit optimistic.  Not sure what the model used for the definition of "collision", and even very small objects could cause an air leak that would render the D2 free-flyer unusable.  But the number has a lot of margin in it.

[OTV Booster]

There is one consumable in the ECLSS not yet discussed. The activated carbon filters.


I expect that there's a lot of plastic in the D2 interior. Plastic outgasses just the stuff that the carbon filter traps. Should be an easy fix. More filters.

[OTV Booster]

Does anybody have a clue how the trunk radiator and the D2 are connected? I seriously doubt that it's through the heat shield. That leaves a connection somewhere on the side of the craft.


The connection can't be too close to the bottom or there will be thermal problems during EDL. Even if the materials can take it there would be unnecessary thermal transmission to the interior that would call for increased nitrox flow for cooling. What I picture is QD connectors up high and the connection lines staying with the trunk when it disconnects.


From the conversation to date it doesn't look like there are any insurmountable problems in extending D2 loiter time (except maybe MMOD) but the trunk is there 'just in case'.


A hunch: The loiter limits are self imposed because they are what was planned for and qualified.

[OTV Booster]

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.


What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

[MDMoery]

Does anybody have a clue how the trunk radiator and the D2 are connected? I seriously doubt that it's through the heat shield. That leaves a connection somewhere on the side of the craft.

Here is a good shot of trunk jettison on the in-flight abort test (2:54 video, 2:24 MET).  You can see the umbilical "claw" at the bottom of the trunk as it falls away from the spacecraft.


You can see the umbilical attached to the spacecraft below and to the right of the hatch.  https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=41016.0;attach=1492554;image

Here you can see all the connection ports without the trunk attached.  https://www.aerospacemanufacturinganddesign.com/remote/aHR0cHM6Ly9jZG4uZ2llLm5ldC9maWxldXBsb2Fkcy9pbWFnZS9zcGFjZXgtdjItdW52ZWlsMDUyOTE0LXNwYWNleF82MjB4LmpwZw.lstuMblkq6I.jpg?w=948&h=533&mode=pad&anchor=middlecenter&scale=both&bgcolor=F0F1F2

[Asteroza]

Does anybody have a clue how the trunk radiator and the D2 are connected? I seriously doubt that it's through the heat shield. That leaves a connection somewhere on the side of the craft.


The connection can't be too close to the bottom or there will be thermal problems during EDL. Even if the materials can take it there would be unnecessary thermal transmission to the interior that would call for increased nitrox flow for cooling. What I picture is QD connectors up high and the connection lines staying with the trunk when it disconnects.


From the conversation to date it doesn't look like there are any insurmountable problems in extending D2 loiter time (except maybe MMOD) but the trunk is there 'just in case'.


A hunch: The loiter limits are self imposed because they are what was planned for and qualified.

There's a side clamp doohickey on the edge of the heatshield that provides connectivity.

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.


What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

If the power budget can cover it, maybe small reaction wheels in the trunk?

[Zed_Noir]

<snip>

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.


What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

If the power budget can cover it, maybe small reaction wheels in the trunk?

Installed a few Starlink Argon thrusters in the trunk as RCS for fine attitude control. As long as you don't need quick attitude adjustments.

Bonus of some orbit change capability.

[TheRadicalModerate]

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.

What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

Good point on the attitude control, especially since you're getting power from the trunk PV.

We could run out the per-orbit delta-v loss with a decent drag equation, assuming the nadir point remains constant.  But figuring out the torques on that nadir point probably requires CFD.

My main takeaway from this (incomplete) exercise is that the depot may be expensive to store in VLEO for months/years at a time.  But VLEO is probably the only place the depot is safe from MMOD--and other birds are safe from the depot, should it become MMOD.

If the power budget can cover it, maybe small reaction wheels in the trunk?

Might work.  D2's pretty big, though, and the bulk of the mass is above the trunk. 

You'd need the reaction wheels completely powered down and unloaded when the crew was on board.  Otherwise, crew-cert would be a nightmare.

If you're maintaining a constant nadir in orbit, do you come back to the same true anomaly with the reaction wheels approximately unloaded?

[TheRadicalModerate]

Installed a few Starlink Argon thrusters in the trunk as RCS for fine attitude control. As long as you don't need quick attitude adjustments.

Bonus of some orbit change capability.

Seems like a big change, especially with the plumbing.  And D2 is gonna be about 12t, which is at least a factor of 6 more than a Starlink v2 maxi.

[TheRadicalModerate]

There is one consumable in the ECLSS not yet discussed. The activated carbon filters.


I expect that there's a lot of plastic in the D2 interior. Plastic outgasses just the stuff that the carbon filter traps. Should be an easy fix. More filters.

Bear in mind that there are no crew to swap the extra filters, so you need a larger capacity manifold if LiOH really gets significantly depleted by non-CO2 organics.

Might be kinda shades of Apollo 13.  I wonder if SpaceX would insist on producing their own socks.

Worth thinking about.

[OTV Booster]

Does anybody have a clue how the trunk radiator and the D2 are connected? I seriously doubt that it's through the heat shield. That leaves a connection somewhere on the side of the craft.


The connection can't be too close to the bottom or there will be thermal problems during EDL. Even if the materials can take it there would be unnecessary thermal transmission to the interior that would call for increased nitrox flow for cooling. What I picture is QD connectors up high and the connection lines staying with the trunk when it disconnects.


From the conversation to date it doesn't look like there are any insurmountable problems in extending D2 loiter time (except maybe MMOD) but the trunk is there 'just in case'.


A hunch: The loiter limits are self imposed because they are what was planned for and qualified.

There's a side clamp doohickey on the edge of the heatshield that provides connectivity.

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.


What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

If the power budget can cover it, maybe small reaction wheels in the trunk?

If the power budget is too tight and trunk space allows, a deployable PV would be a solution. It adds system complexity but not directly to D2 and it admittedly increases the cost of the throw away part.

[OTV Booster]

There is one consumable in the ECLSS not yet discussed. The activated carbon filters.


I expect that there's a lot of plastic in the D2 interior. Plastic outgasses just the stuff that the carbon filter traps. Should be an easy fix. More filters.

Bear in mind that there are no crew to swap the extra filters, so you need a larger capacity manifold if LiOH really gets significantly depleted by non-CO2 organics.

Might be kinda shades of Apollo 13.  I wonder if SpaceX would insist on producing their own socks.

Worth thinking about.

Move one Nitrox COPV cluster from the service bay to the trunk along with as many more clusters as are necessary. Use the space opened up in the service bay for larger filter packs. This assumes that the one cluster still in the service bay stays untapped throughout the flight and one cluster is sufficient for EDL cooling. SpaceX, can we have your raw engineering data? Please? Pretty please?


There would be some rearranging in the service bay but in my mind that shouldn't be a complexity beyond what we're discussing.


What might be an issue is moving mass around might call for a recertification of LES. Counter to this is each current D2 launch carries different unpressurized cargo masses in the trunk, so maybe it's not a problem.


Socks? Naw. They might make their own duct tape though.

[OTV Booster]

Does anybody have a clue how the trunk radiator and the D2 are connected? I seriously doubt that it's through the heat shield. That leaves a connection somewhere on the side of the craft.

Here is a good shot of trunk jettison on the in-flight abort test (2:54 video, 2:24 MET).  You can see the umbilical "claw" at the bottom of the trunk as it falls away from the spacecraft.


You can see the umbilical attached to the spacecraft below and to the right of the hatch.  https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=41016.0;attach=1492554;image

Here you can see all the connection ports without the trunk attached.  https://www.aerospacemanufacturinganddesign.com/remote/aHR0cHM6Ly9jZG4uZ2llLm5ldC9maWxldXBsb2Fkcy9pbWFnZS9zcGFjZXgtdjItdW52ZWlsMDUyOTE0LXNwYWNleF82MjB4LmpwZw.lstuMblkq6I.jpg?w=948&h=533&mode=pad&anchor=middlecenter&scale=both&bgcolor=F0F1F2

Thanks. That's just what I was looking for.


So if they add services delivered from the trunk they'd add a superset of connectors on the claw. Any D2/trunk used for long duration would need minor connector modifications. Shouldn't be a big issue.

[Zed_Noir]

Installed a few Starlink Argon thrusters in the trunk as RCS for fine attitude control. As long as you don't need quick attitude adjustments.

Bonus of some orbit change capability.

Seems like a big change, especially with the plumbing.  And D2 is gonna be about 12t, which is at least a factor of 6 more than a Starlink v2 maxi.

There will power conduit cables, no plumbing. Local Argon reservoir tanks inside the trunk for each thruster.

The proposed add-on Argon thrusters takes longer burns to get results. But the Dragon capsule got plenty of time for extended thruster burns.

[TheRadicalModerate]

Move one Nitrox COPV cluster from the service bay to the trunk along with as many more clusters as are necessary. Use the space opened up in the service bay for larger filter packs.

I'm pretty sure the LiOH system is inside the pressure vessel.  But they could trade a bit of pressurized cargo space for another pack.

Even what you described above is enough to make the D2/LSS kludge less palatable.  Absolute minimum modifications are the way to go.

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

[DanClemmensen]

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

Apparently the pressurized volume of D2 is about 9.3 m³, while the pressurized volume of the HLS OTV is about 1000 m³.  Just open the hatch and set up a blower to dilute the volatiles by a factor of 100. And change the filters also, of course.

[TheRadicalModerate]

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

Apparently the pressurized volume of D2 is about 9.3 m³, while the pressurized volume of the HLS OTV is about 1000 m³.  Just open the hatch and set up a blower to dilute the volatiles by a factor of 100. And change the filters also, of course.

It can't be the full 1000m³, because you need some unpressurized volume for the airlocks and the rest of the "garage".  And there's nothing to say that SpaceX has to build out as much volume as possible in the rest of the payload bay.  Volume requires more ECLSS:  More fans, more dehumidification, more thermal control, etc.

I've been assuming that the pressurized crew module, with all of its consumables and the meatware, is about 15t.  That's probably at least twice the mass of the D2's pressurized system.  At a guess, that could allow many tens of times the volume of the D2.  But we obviously don't know how pressurized volume and system mass scale with each other.

I'd be a little surprised if the LSS pressurized crew module volume, excluding the airlocks, was more than 300-400m³.  Still, that's huge.  So yeah, any outgassing that couldn't get scrubbed from the D2 in free-flight probably wouldn't hurt the crew when diluted by the larger LSS volume.

There are probably contingencies where you'd want to minimize the time between LSS-D2 RPOD and when the D2 deorbited.  It's probably faster to let the D2 clean itself with new filters than to wait for the LSS to dilute things.

[Barley]

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

Apparently the pressurized volume of D2 is about 9.3 m³, while the pressurized volume of the HLS OTV is about 1000 m³.  Just open the hatch and set up a blower to dilute the volatiles by a factor of 100. And change the filters also, of course.

This assumes the air in the OTV is "good".  It's probably worse in at least some ways.  (Higher humidity and CO₂ for a start).  Mixing the air might dilute volatiles from the D2 but it could overwhelm parts of the D2 filters.  At the least you probably want to be able to change the filters after the transfer is complete and the hatch sealed.

For contingencies it would be interesting to know if the transfer could be done in spacesuits and the decent completed before the crew expire from heat exhaustion.  This could accommodate "bad" air or even a leak that leaves the D2 unpressurized.

[DanClemmensen]

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

Apparently the pressurized volume of D2 is about 9.3 m³, while the pressurized volume of the HLS OTV is about 1000 m³.  Just open the hatch and set up a blower to dilute the volatiles by a factor of 100. And change the filters also, of course.

This assumes the air in the OTV is "good".  It's probably worse in at least some ways.  (Higher humidity and CO₂ for a start).  Mixing the air might dilute volatiles from the D2 but it could overwhelm parts of the D2 filters.  At the least you probably want to be able to change the filters after the transfer is complete and the hatch sealed.

For contingencies it would be interesting to know if the transfer could be done in spacesuits and the decent completed before the crew expire from heat exhaustion.  This could accommodate "bad" air or even a leak that leaves the D2 unpressurized.

9.3 M³ of air at 1.2 kg/M³ has a mass of 11.16 kg. Just carry an extra reload of air. Vent the D2 to space and refill. Presumably D2 already has at least one refill, so an increase of the size of the refill tank should not be  big deal. For a mission that includes HLS OTV or any other large companion, the companion can carry this refill.

[TheRadicalModerate]

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

Apparently the pressurized volume of D2 is about 9.3 m³, while the pressurized volume of the HLS OTV is about 1000 m³.  Just open the hatch and set up a blower to dilute the volatiles by a factor of 100. And change the filters also, of course.

This assumes the air in the OTV is "good".  It's probably worse in at least some ways.  (Higher humidity and CO₂ for a start).  Mixing the air might dilute volatiles from the D2 but it could overwhelm parts of the D2 filters.  At the least you probably want to be able to change the filters after the transfer is complete and the hatch sealed.

There are people in the OTV, so I'd hope the air is good.  If there are differences in the humidity specs for HLS and D2, that sounds like a "Doctor, Doctor, it hurts when I do this" kind of problem:  don't do that.

For contingencies it would be interesting to know if the transfer could be done in spacesuits and the decent completed before the crew expire from heat exhaustion.  This could accommodate "bad" air or even a leak that leaves the D2 unpressurized.

As long as you can briefly unplug the suit umbilical, this is easy, as long as the OTV-LSS umbilicals are long enough to get into the D2 seats:

1) Plug into OTV-LSS suit air system with long umbilicals.
2) Depressurize the OTV-LSS.
3) Open hatch to depressurized D2.
4) Crew goes down tunnel, climbs into D2 seats.
5) Crew unplugs OTV-LSS umbilicals and plugs into D2 umbilicals.
6) Stupid problem:  You need a way to retract the OTV umbilicals back into the OTV so you can close the hatch.
7) Undock and get thee to EDL, quickly.

This is kinda why I was suggesting that the kludge conops might mandate a D2 on warm-standby.  It could be for the next crew mission to ISS or CLD.  It doesn't need to be mounted on an F9 yet, but it would have have the hypergolics provisioned, and any cargo would either have to be removable or in such a state that it's launchable in a day or two.

[OTV Booster]

<snip>

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.


What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

If the power budget can cover it, maybe small reaction wheels in the trunk?

Installed a few Starlink Argon thrusters in the trunk as RCS for fine attitude control. As long as you don't need quick attitude adjustments.

Bonus of some orbit change capability.

I noodled the problem of a dumbbell mass distribution on LEO a while back (30 years?), and mascons could set it tumbling. The problem is cumulative. Thrusters would have to counter the input of each mascon on every orbit. IIRC, the mass distribution is critical. If it's evenly distributed the problem is minimized - I think.


Not really sure how this impacts Dragon specifically. The starlinks are quite a bit lighter so I'm dubious. There's also the energy budget to look at. Reaction wheels or Control Moment Gyros might be more power efficient. It calls for a closer look than I'm capable of.

[DanClemmensen]

This assumes the air in the OTV is "good".  It's probably worse in at least some ways.  (Higher humidity and CO₂ for a start).  Mixing the air might dilute volatiles from the D2 but it could overwhelm parts of the D2 filters.  At the least you probably want to be able to change the filters after the transfer is complete and the hatch sealed.

CCP Dragons are currently sustained for six months by being connected to the ISS. The ISS has been in space and continuously occupied for about 23 years, with occupancy of seven or more for much of that time. I've never seen any comments on the subject, but I suspect it's fairly funky by now. An HLS acting as an OTV will be a new spacecraft with a pressurized volume about the same as the ISS. it will have been occupied by a crew of four for a month or less. I doubt its air will be worse than ISS air.

[OTV Booster]

A late thought. Station keeping propellant shouldn't be a problem. Even at VLEO the orbit decay shouldn't be all that much and there should be plenty of propellant for a small boost or two. That's strictly a gut estimate.

What would be a problem would be attitude control - especially at VLEO. I'd classify modifying the current tanks as non-trivial. Adding in a tapoff from extra tanks in the trunk? Again not trivial, but trivial enough?

Good point on the attitude control, especially since you're getting power from the trunk PV.

We could run out the per-orbit delta-v loss with a decent drag equation, assuming the nadir point remains constant.  But figuring out the torques on that nadir point probably requires CFD.

My main takeaway from this (incomplete) exercise is that the depot may be expensive to store in VLEO for months/years at a time.  But VLEO is probably the only place the depot is safe from MMOD--and other birds are safe from the depot, should it become MMOD.

If the power budget can cover it, maybe small reaction wheels in the trunk?

Might work.  D2's pretty big, though, and the bulk of the mass is above the trunk. 

You'd need the reaction wheels completely powered down and unloaded when the crew was on board.  Otherwise, crew-cert would be a nightmare.

If you're maintaining a constant nadir in orbit, do you come back to the same true anomaly with the reaction wheels approximately unloaded?

I don't think atmospherics is the problem for attitude control. It's mascons, especially down low.


Mascons cause pitch up/down, depending on vehicle mass distribution, with yaw added if the capsule is not passing over dead center. The problem is intermittent, so starting with stopped reaction wheels, spin one up for for pitch and another for yaw control as needed. When they eventually approach saturation, use the roll wheel to go nadir down temporarily, or spin 180 in either pitch or yaw to temporarily point the other way. In the new position pitch and yaw corrections desaturate the wheels.


One possible problem with this is that moving to the new temporary position will have to fight some gyroscopic procession that might in turn further saturate the saturated gyros. Got to thimnk on this.


The ISS uses CMG's and to desaturate they have to fire thrusters. That's what we're trying to avoid.

[OTV Booster]

Move one Nitrox COPV cluster from the service bay to the trunk along with as many more clusters as are necessary. Use the space opened up in the service bay for larger filter packs.

I'm pretty sure the LiOH system is inside the pressure vessel.  But they could trade a bit of pressurized cargo space for another pack.

Even what you described above is enough to make the D2/LSS kludge less palatable.  Absolute minimum modifications are the way to go.

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

All the filters and COPV's are in the service bay under the floor which, IIUC, is pressurized.


I think plastic out gassing is organics which would be filtered out by the charcoal filters. I don't think it would be all that much of a problem but it's the type of thing NASA pays attention to. The "new car smell" is an extreme example of what I'm talking about. Get a capsule with a few miles on the odometer and it's not a big problem unless it been parked in the sun all day.


I agree that it's better to not rearrange things if at all possible, but ya gotta do what ya gotta do. It's an option, if necessary.

[OTV Booster]

Hopefully, outgassing during uncrewed free flight just results in a little extra New Spacecraft Smell when the crew gets back.  Another possibility:  when they open the hatch, somebody zips in with an oxygen mask, swaps out the last dirty cannister, retreats back into the LSS, and closes the hatch for an hour or two.

Apparently the pressurized volume of D2 is about 9.3 m³, while the pressurized volume of the HLS OTV is about 1000 m³.  Just open the hatch and set up a blower to dilute the volatiles by a factor of 100. And change the filters also, of course.

This assumes the air in the OTV is "good".  It's probably worse in at least some ways.  (Higher humidity and CO₂ for a start).  Mixing the air might dilute volatiles from the D2 but it could overwhelm parts of the D2 filters.  At the least you probably want to be able to change the filters after the transfer is complete and the hatch sealed.

There are people in the OTV, so I'd hope the air is good.  If there are differences in the humidity specs for HLS and D2, that sounds like a "Doctor, Doctor, it hurts when I do this" kind of problem:  don't do that.

For contingencies it would be interesting to know if the transfer could be done in spacesuits and the decent completed before the crew expire from heat exhaustion.  This could accommodate "bad" air or even a leak that leaves the D2 unpressurized.

As long as you can briefly unplug the suit umbilical, this is easy, as long as the OTV-LSS umbilicals are long enough to get into the D2 seats:

1) Plug into OTV-LSS suit air system with long umbilicals.
2) Depressurize the OTV-LSS.
3) Open hatch to depressurized D2.
4) Crew goes down tunnel, climbs into D2 seats.
5) Crew unplugs OTV-LSS umbilicals and plugs into D2 umbilicals.
6) Stupid problem:  You need a way to retract the OTV umbilicals back into the OTV so you can close the hatch.
7) Undock and get thee to EDL, quickly.

This is kinda why I was suggesting that the kludge conops might mandate a D2 on warm-standby.  It could be for the next crew mission to ISS or CLD.  It doesn't need to be mounted on an F9 yet, but it would have have the hypergolics provisioned, and any cargo would either have to be removable or in such a state that it's launchable in a day or two.

Have a short umbilical extension of a few feet that reaches from the D2 seats to the hatch, plus a bit. Have a bracket just inside the hatch that holds the suit end of the extension with the other end already plugged into the seat. Have a corresponding bracket inside the OTV to hold the OTV side umbilical.


First through third crew entering the dragon takes an umbilical off the dragon side bracket that corresponds to their seat, disconnects the OTV umbilical and connects the Dragon umbilical, then passes the OTV umbilical to a crew member waiting to enter who stows the umbilical in the OTV side bracket. The last to enter goes in only far enough that another crew member can swap the connections, then they rise up back into the OTV far enough to allow stowing of the umbilical which has been placed in their hand by a cooperative crew mate. Last one in, close the doors.


Once in their seat each crew member disconnects the short extension and plugs into their normal receptacle, then they put the extension in the glove box. Don't all spacecraft have a glove box? Or maybe they can use a velcro cable wrap to snub up the extension and keep it on.

[TheRadicalModerate]

Or instead of all this extra unneeded "engineering", lets just use what SpaceX has already designed for and provided if there is an emergency.

1. Reach behind your seat for the SpaceX provided emergency breathing air bottle. Complimentary for any SpaceX flights from the Cape.
2. Attach the bottle via the standard quick attach suit umbilical port
3. Deal with contaminated atmosphere, perform any filter changes or relocate vehicle / seat with ease.

Depends on the contingency.  If you're trying to deal with the "bad air" problem, what you're suggesting probably works fine.  If you're trying to deal with a depressurized D2, the failure tree gets kinda bushy.  Three possibilities:

1) If the depressurization occurs early in the mission, or the OTV-LSS's ECLSS will handle a multi-day extension, have the crew wait for a replacement D2.

2) If a replacement D2 is unavailable, or the D2 depressurized close to or during RPOD, and the problem is fixable, the crew would need extensive time on the suit umbilicals.  Maybe the portable bottles let them get from the OTV-LSS to the D2 or vice-versa, but they'll need umbilicals long enough to do some difficult tasks.  The inside suits obviously aren't made for this kind of work, but one would think that it might be possible to do simple patching?

3) If the problem isn't fixable and the crew must use the bad D2, then the problem is fairly straightforward, albeit terrifying:  Unhook from the OTV-LSS system, use the portable bottles to transfer and seal the hatch, then plug into the D2's system, and hope that the air holds out long enough to get through EDL.

If the D2 is unpressurized, the OTV-LSS needs to be depressurized, because it's highly unlikely that there will be an airlock on the docking system.  How is that accomplished?  Does it just vent and replenish from storage, or does it have to pump down and save the cabin nitrox?  Is this new work for the OTV-LSS? 

There certainly isn't a happy ending if the HLS crew returns to find the Orion and/or Gateway depressurized.  That might militate toward NASA not caring very much about dealing with the problem, which in turn might create a problem porting the HLS-LSS's system over to the OTV-LSS.

[OTV Booster]

Or instead of all this extra unneeded "engineering", lets just use what SpaceX has already designed for and provided if there is an emergency.

1. Reach behind your seat for the SpaceX provided emergency breathing air bottle. Complimentary for any SpaceX flights from the Cape.
2. Attach the bottle via the standard quick attach suit umbilical port
3. Deal with contaminated atmosphere, perform any filter changes or relocate vehicle / seat with ease.

Depends on the contingency.  If you're trying to deal with the "bad air" problem, what you're suggesting probably works fine.  If you're trying to deal with a depressurized D2, the failure tree gets kinda bushy.  Three possibilities:

1) If the depressurization occurs early in the mission, or the OTV-LSS's ECLSS will handle a multi-day extension, have the crew wait for a replacement D2.

2) If a replacement D2 is unavailable, or the D2 depressurized close to or during RPOD, and the problem is fixable, the crew would need extensive time on the suit umbilicals.  Maybe the portable bottles let them get from the OTV-LSS to the D2 or vice-versa, but they'll need umbilicals long enough to do some difficult tasks.  The inside suits obviously aren't made for this kind of work, but one would think that it might be possible to do simple patching?

3) If the problem isn't fixable and the crew must use the bad D2, then the problem is fairly straightforward, albeit terrifying:  Unhook from the OTV-LSS system, use the portable bottles to transfer and seal the hatch, then plug into the D2's system, and hope that the air holds out long enough to get through EDL.

If the D2 is unpressurized, the OTV-LSS needs to be depressurized, because it's highly unlikely that there will be an airlock on the docking system.  How is that accomplished?  Does it just vent and replenish from storage, or does it have to pump down and save the cabin nitrox?  Is this new work for the OTV-LSS? 

There certainly isn't a happy ending if the HLS crew returns to find the Orion and/or Gateway depressurized.  That might militate toward NASA not caring very much about dealing with the problem, which in turn might create a problem porting the HLS-LSS's system over to the OTV-LSS.

AIUI, the suits are definitely not meant for EVA. They got no heat or cooling.


If a patch can be put on from the inside that's a plus. D2 pressure holds it in place. On the outside the pressure wants to blow it off.


Of course the hole will be inaccessible from the inside. Kinda like dropped toast always hits butter side down.

[TheRadicalModerate]

AIUI, the suits are definitely not meant for EVA. They got no heat or cooling.

Yeah, but the the umbilicals provide temperature-controlled air.  It isn't perfect, but it's better than nothing.

I'm more worried about dexterity in vacuum.

If a patch can be put on from the inside that's a plus. D2 pressure holds it in place. On the outside the pressure wants to blow it off.

Of course the hole will be inaccessible from the inside. Kinda like dropped toast always hits butter side down.

Yup.  It may be better just to assume that the only two responses are:

1) Wait for a new D2.

2) Climb in, use the umbilicals, and do an expedited de-orbit.  Of course, given that you have some piece of MMOD that went through the pressure vessel, you'd really like to do an extensive checkout before de-orbit, but you're on a clock.  There has to be enough nitrox to cool the cabin during reentry, MMOD hole or not.

[OTV Booster]

AIUI, the suits are definitely not meant for EVA. They got no heat or cooling.

Yeah, but the the umbilicals provide temperature-controlled air.  It isn't perfect, but it's better than nothing.

I'm more worried about dexterity in vacuum.

If a patch can be put on from the inside that's a plus. D2 pressure holds it in place. On the outside the pressure wants to blow it off.

Of course the hole will be inaccessible from the inside. Kinda like dropped toast always hits butter side down.

Yup.  It may be better just to assume that the only two responses are:

1) Wait for a new D2.

2) Climb in, use the umbilicals, and do an expedited de-orbit.  Of course, given that you have some piece of MMOD that went through the pressure vessel, you'd really like to do an extensive checkout before de-orbit, but you're on a clock.  There has to be enough nitrox to cool the cabin during reentry, MMOD hole or not.

Once they step out it's ~120C in the sun and ~100C ~-100C in the shade. I'd give them 10 minutes max on a Hail Mary EVA, probably less. Where is the hole? Where are the non-existent hand holds? Can they even assume the position and kiss it goodby in a suit?

[DanClemmensen]

AIUI, the suits are definitely not meant for EVA. They got no heat or cooling.

Yeah, but the the umbilicals provide temperature-controlled air.  It isn't perfect, but it's better than nothing.

I'm more worried about dexterity in vacuum.

If a patch can be put on from the inside that's a plus. D2 pressure holds it in place. On the outside the pressure wants to blow it off.

Of course the hole will be inaccessible from the inside. Kinda like dropped toast always hits butter side down.

Yup.  It may be better just to assume that the only two responses are:

1) Wait for a new D2.

2) Climb in, use the umbilicals, and do an expedited de-orbit.  Of course, given that you have some piece of MMOD that went through the pressure vessel, you'd really like to do an extensive checkout before de-orbit, but you're on a clock.  There has to be enough nitrox to cool the cabin during reentry, MMOD hole or not.

Once they step out it's ~120C in the sun and ~100C in the shade. I'd give them 10 minutes max on a Hail Mary EVA, probably less. Where is the hole? Where are the non-existent hand holds? Can they even assume the position and kiss it goodby in a suit?

There are two justification for having active-only IDSS ports on Crew Dragon today:
    1)The passive components add mass that can better be used for a bit of extra cargo.
    2)There is currently no realistic mission where Dragon-to-Dragon docking is useful because Dragons cannot survive long enough for a rescue to work.

But this is the "Mission Life Extension" thread. If Dragon can in fact loiter longer, then converting the active-only port into an active-passive port makes sense, and the Dragons can dock to each other.

[TheRadicalModerate]

There are two justification for having active-only IDSS ports on Crew Dragon today:
    1)The passive components add mass that can better be used for a bit of extra cargo.
    2)There is currently no realistic mission where Dragon-to-Dragon docking is useful because Dragons cannot survive long enough for a rescue to work.

But this is the "Mission Life Extension" thread. If Dragon can in fact loiter longer, then converting the active-only port into an active-passive port makes sense, and the Dragons can dock to each other.

Even in the case where a D2 goes bad loitering on orbit, the replacement would likely dock with the OTV-LSS, not the other D2.  There are cases where both OTV-LSS and HLS-LSS are passive-only (e.g., in the case where they're only going to be doing crew transfers at Gateway), but my guess is that OTV-LSS (and HLS-LSS, for that matter), will be active-passive.

[DanClemmensen]

There are two justification for having active-only IDSS ports on Crew Dragon today:
    1)The passive components add mass that can better be used for a bit of extra cargo.
    2)There is currently no realistic mission where Dragon-to-Dragon docking is useful because Dragons cannot survive long enough for a rescue to work.

But this is the "Mission Life Extension" thread. If Dragon can in fact loiter longer, then converting the active-only port into an active-passive port makes sense, and the Dragons can dock to each other.

Even in the case where a D2 goes bad loitering on orbit, the replacement would likely dock with the OTV-LSS, not the other D2.  There are cases where both OTV-LSS and HLS-LSS are passive-only (e.g., in the case where they're only going to be doing crew transfers at Gateway), but my guess is that OTV-LSS (and HLS-LSS, for that matter), will be active-passive.

In general there is no reason for long duration D2 when it has something to dock to. The thread is generic not just for support of the SLS/Orion replacement scheme. I was thinking about free-flying long-duration missions.