Starship On-orbit refueling - Options and Discussion

[steveleach]

refueling vehicles with crew in it short of emergencies is an anti-requirement.

Woah, hang on a sec.

Surely that's an inherent part of the long-term conops? Launch a ship full of colonists from Earth, refuel in orbit, then send them on their way to Mars.

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

[Greg Hullender]

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

I think that's the direction the discussion is headed, although there hasn't been much talk about Mars per se. It tends towards the idea that vehicles capable of EDL would stay close to the planet they serve while vehicles that don't or can't EDL travel between planets. (Counting the moon as a planet for this purpose.)

It has the advantage that you can fully fuel and provision the transport vehicle before loading crew into it. I suppose it also has the advantage that you can specialize the various vehicles for their purposes. It has a drawback that you'll have to find some way to inspect and service the transports in space.

[DanClemmensen]

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

I think that's the direction the discussion is headed, although there hasn't been much talk about Mars per se. It tends towards the idea that vehicles capable of EDL would stay close to the planet they serve while vehicles that don't or can't EDL travel between planets. (Counting the moon as a planet for this purpose.)

It has the advantage that you can fully fuel and provision the transport vehicle before loading crew into it. I suppose it also has the advantage that you can specialize the various vehicles for their purposes. It has a drawback that you'll have to find some way to inspect and service the transports in space.

The refueling campaign for a Mars fleet will take awhile, possibly months. I think it makes more sense to fuel them all up before ferrying the crewed up, topping off the ships just prior to crew arrival and ship departure for Mars. The ferries can be optimized to carry lots of crew in economy-class (or even steerage class) conditions for the few hours needed to get to the Mars ships, which must be comfortable for the long-duration flight to Mars. I think one ferry load can accommodate five or more Mars ship crews.

[steveleach]

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

I think that's the direction the discussion is headed, although there hasn't been much talk about Mars per se. It tends towards the idea that vehicles capable of EDL would stay close to the planet they serve while vehicles that don't or can't EDL travel between planets. (Counting the moon as a planet for this purpose.)

It has the advantage that you can fully fuel and provision the transport vehicle before loading crew into it. I suppose it also has the advantage that you can specialize the various vehicles for their purposes. It has a drawback that you'll have to find some way to inspect and service the transports in space.

SpaceX could always have gone with a transport system with multiple specialised vehicles, but instead went for surface-to-surface with refuelling. Starship is designed from the ground up around that concept.

I get the benefit of minimising RPODs for ships with people in, but the depot deals with that well enough.

Plus, we have precedent with SpaceX convincing NASA that "load and go" was safe enough for crew on F9.

[Norm38]

The refueling campaign for a Mars fleet will take awhile, possibly months. I think it makes more sense to fuel them all up before ferrying the crewed up, topping off the ships just prior to crew arrival and ship departure for Mars. The ferries can be optimized to carry lots of crew in economy-class (or even steerage class) conditions for the few hours needed to get to the Mars ships, which must be comfortable for the long-duration flight to Mars. I think one ferry load can accommodate five or more Mars ship crews.

I have no prediction for how the sausage will finally get made, but where this ends is a true orbital starbase.  Tanker docking section, long term fuel storage, docks for Mars departure ships, hab space and docking ports for the "economy class" surface-LEO ships.
Some folks are going to have a lot of fun designing that. 

[Twark_Main]

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

I think that's the direction the discussion is headed, although there hasn't been much talk about Mars per se. It tends towards the idea that vehicles capable of EDL would stay close to the planet they serve while vehicles that don't or can't EDL travel between planets.

Traveling between planets also greatly benefits from the ability to perform EDL. By using aerocapture (and possibly multi-orbit aerobraking) you greatly reduce the propellant requirements to travel between planets. This includes the Moon even, which still benefits from EDL capabilities on the return trip to Earth.

[Greg Hullender]

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

I think that's the direction the discussion is headed, although there hasn't been much talk about Mars per se. It tends towards the idea that vehicles capable of EDL would stay close to the planet they serve while vehicles that don't or can't EDL travel between planets.

Traveling between planets also greatly benefits from the ability to perform EDL. By using aerocapture (and possibly multi-orbit aerobraking) you greatly reduce the propellant requirements to travel between planets. This includes the Moon even, which still benefits from EDL capabilities on the return trip to Earth.

I didn't mean to rule out aerocapture. But EDL includes a lot more than that, I think.

[Twark_Main]

Is there a suggestion that SpaceX plan to transfer the people from the launch ship to a pre-fuelled Mars ship?

I think that's the direction the discussion is headed, although there hasn't been much talk about Mars per se. It tends towards the idea that vehicles capable of EDL would stay close to the planet they serve while vehicles that don't or can't EDL travel between planets.

Traveling between planets also greatly benefits from the ability to perform EDL. By using aerocapture (and possibly multi-orbit aerobraking) you greatly reduce the propellant requirements to travel between planets. This includes the Moon even, which still benefits from EDL capabilities on the return trip to Earth.

I didn't mean to rule out aerocapture. But EDL includes a lot more than that, I think.

"Sufficiently advanced aerocapture is indistinguishable from EDL." 

For cargo ships, you just use regular EDL. For crewed vehicles you want to reduce transit time, which means high aerocapture velocities.

This is one of those "distinction without a difference" situations.

[TheRadicalModerate]

Something I haven't seen covered here (or elsewhere) is a comment at 7:55 in this Spaceflight Now video:



My transcription of the money quote:

Where we have been doing some in-house testing of behalf of SpaceX is on their MMOD thermal tiles for in-space. So not the heat shield for reentry, not the hot side, but being able to have the MMOD tile and the reflective for keeping the cryos cold.  We brought a couple of those tiles in and did some testing.  SpaceX was very happy with the capability, both at Glenn Research Center and Marshall's test capability.  And so we've expanded that relationship and being able to do some testing for them.

Bolding mine, to emphasize the most "WTF!?" parts of this.

So... this puts a pretty different spin on things.  Ain't no stinkin' Solar White paint involved if the whole depot and possibly HLS are covered with tiles.

I've been worried about MMOD for a while.  Indeed, I think that's the most compelling reason to keep the depot in VLEO--so the consequences of both impact and production of MMOD are minimized.  If they're building a completely different tile technology for this, that's pretty interesting.

Did anybody know about this?

[InterestedEngineer]

Something I haven't seen covered here (or elsewhere) is a comment at 7:55 in this Spaceflight Now video:

<youtube link edited out>

My transcription of the money quote:

Where we have been doing some in-house testing of behalf of SpaceX is on their MMOD thermal tiles for in-space. So not the heat shield for reentry, not the hot side, but being able to have the MMOD tile and the reflective for keeping the cryos cold.  We brought a couple of those tiles in and did some testing.  SpaceX was very happy with the capability, both at Glenn Research Center and Marshall's test capability.  And so we've expanded that relationship and being able to do some testing for them.

Bolding mine, to emphasize the most "WTF!?" parts of this.

So... this puts a pretty different spin on things.  Ain't no stinkin' Solar White paint involved if the whole depot and possibly HLS are covered with tiles.

I've been worried about MMOD for a while.  Indeed, I think that's the most compelling reason to keep the depot in VLEO--so the consequences of both impact and production of MMOD are minimized.  If they're building a completely different tile technology for this, that's pretty interesting.

Did anybody know about this?

MMOD is a much bigger risk if you want to keep vehicles in orbit for months while you fuel them.  especially in LEO.

Probably a bigger risk than launch, refuel ASAP, and leave.  Which favors VLEO as well.  You get 50 orbits to refuel, then scoot.

I dub this "launch and scoot".  Minimal time spent in or traversing high MMOD orbits.

[TheRadicalModerate]

MMOD is a much bigger risk if you want to keep vehicles in orbit for months while you fuel them.  especially in LEO.

Probably a bigger risk than launch, refuel ASAP, and leave.  Which favors VLEO as well.  You get 50 orbits to refuel, then scoot.

I dub this "launch and scoot".  Minimal time spent in or traversing high MMOD orbits.

You still need to leave the depot in VLEO in between fueling campaigns.  It could sit empty for months or even years.  I guess you can deorbit them and build a new one, but that's a non-trivial additional expense to the fueling campaign.

If the tiles can act as something like Whipple Shields, that could extend depot lifetime a lot.

[InterestedEngineer]

MMOD is a much bigger risk if you want to keep vehicles in orbit for months while you fuel them.  especially in LEO.

Probably a bigger risk than launch, refuel ASAP, and leave.  Which favors VLEO as well.  You get 50 orbits to refuel, then scoot.

I dub this "launch and scoot".  Minimal time spent in or traversing high MMOD orbits.

You still need to leave the depot in VLEO in between fueling campaigns.  It could sit empty for months or even years.  I guess you can deorbit them and build a new one, but that's a non-trivial additional expense to the fueling campaign.

If the tiles can act as something like Whipple Shields, that could extend depot lifetime a lot.

Maybe a Depot is a bad requirement.  Let's remove it and see what happens.

Then again, moving 100t of steel tank to a higher orbit is 1-2t of fuel (basically a burp), so if you are leaving it empty for months at a time it's not a problem, esp, if you have the equivalent of Whipple shields.

It's the sensitive re-entry shield that needs to avoid MMOD.  So those need to be in orbit as little time as possible, as low as possible.  That would be fuel tankers, mars bound cargo, and all human rated transport.

[Twark_Main]

MMOD is a much bigger risk if you want to keep vehicles in orbit for months while you fuel them.  especially in LEO.

Probably a bigger risk than launch, refuel ASAP, and leave.  Which favors VLEO as well.  You get 50 orbits to refuel, then scoot.

I dub this "launch and scoot".  Minimal time spent in or traversing high MMOD orbits.

Again, at 200 km VLEO you can remain in orbit indefinitely with a couple of Starlink thrusters. This keeps you well within the low MMOD VLEO regime, without sacrificing the ability to "flatten the curve" and spread out your Mars launch capacity across many months.

Maybe a Depot is a bad requirement.  Let's remove it and see what happens.

"Maybe [my favorite opinion this week is right]" isn't a real argument, I'll have you know. 

If you want to claim that it is actually a bad requirement, you have all your work ahead of you.

Then again, moving 100t of steel tank to a higher orbit is 1-2t of fuel (basically a burp), so if you are leaving it empty for months at a time it's not a problem, esp, if you have the equivalent of Whipple shields.

"Burping up" to a higher orbit nullifies your MMOD advantages, to the same degree as it extends your orbital lifespan. If you extend your lifespan by 10x, you also increase your MMOD risk per orbit by 10x.

It's the sensitive re-entry shield that needs to avoid MMOD.  So those need to be in orbit as little time as possible, as low as possible.  That would be fuel tankers, mars bound cargo, and all human rated transport.

So basically, everything. 

[InterestedEngineer]

So basically, everything. 

Everything but a depot.

[InterestedEngineer]

Maybe a Depot is a bad requirement.  Let's remove it and see what happens.

"Maybe [my favorite opinion this week is right]" isn't a real argument, I'll have you know. 

If you want to claim that it is actually a bad requirement, you have all your work ahead of you.

No, the burden of proof is on the one who wants to keep the part (in this case, a custom depot Starship).

Elon isn't being flippant when he says the requirements are probably wrong.  30 years of software engineering tells me the same thing.

[Greg Hullender]

It occurs to me that one big advantage of VLEO should be that the impacts that do occur will mostly be from behind or on the sides. My reasoning is that anything in a circular orbit only has a lifetime of weeks, so the only MMODs one is likely to find below 200 km are ones with much higher apogees, which means they'll be moving faster than the depot. If they were orbiting in the same plane, they'd only hit it from behind.

Of course, if an object's orbit were inclined with respect to the depot, its forward velocity would be the cosine of that angle, so the depot could run into those, but most of those should impact the sides, not the front.

If one is worried about a tanker's heat shield suffering MMOD damage during fueling, it would make sense to plan to do the refueling such that the heat shield faced the direction of motion of the depot. A relatively small amount of shielding could expand the depot's MMOD shadow, if desired.

Apologies if this was already obvious. (Double apologies if it's wrong!) :-)

[InterestedEngineer]

It occurs to me that one big advantage of VLEO should be that the impacts that do occur will mostly be from behind or on the sides. My reasoning is that anything in a circular orbit only has a lifetime of weeks, so the only MMODs one is likely to find below 200 km are ones with much higher apogees, which means they'll be moving faster than the depot. If they were orbiting in the same plane, they'd only hit it from behind.

Of course, if an object's orbit were inclined with respect to the depot, its forward velocity would be the cosine of that angle, so the depot could run into those, but most of those should impact the sides, not the front.

If one is worried about a tanker's heat shield suffering MMOD damage during fueling, it would make sense to plan to do the refueling such that the heat shield faced the direction of motion of the depot. A relatively small amount of shielding could expand the depot's MMOD shadow, if desired.

Apologies if this was already obvious. (Double apologies if it's wrong!) :-)

The primary benefit for VLEO in regards to MMODs is the relative low density.  They just don't last very long, so there's no accumulation.

[Greg Hullender]

The primary benefit for VLEO in regards to MMODs is the relative low density.  They just don't last very long, so there's no accumulation.

In circular orbits, yes. I mentioned that. Density is pretty low up to almost 400 km. But debris after a collision will include particles in elliptical orbits that spend only a little time at 200 km. Yes, those will circularize fairly quickly and then decay, but they're still an issue.

[Twark_Main]

It's the sensitive re-entry shield that needs to avoid MMOD.  So those need to be in orbit as little time as possible, as low as possible.  That would be fuel tankers, mars bound cargo, and all human rated transport.

So basically, everything. 

Everything but a depot.

Hard to have a depot if "fuel tankers" never go into the same orbit. 

No reason is given (or exists) to exclude the depots from this list.

[Twark_Main]

Maybe a Depot is a bad requirement.  Let's remove it and see what happens.

"Maybe [my favorite opinion this week is right]" isn't a real argument, I'll have you know. 

If you want to claim that it is actually a bad requirement, you have all your work ahead of you.

No, the burden of proof is on the one who wants to keep the part (in this case, a custom depot Starship).

Elon isn't being flippant when he says the requirements are probably wrong.  30 years of software engineering tells me the same thing.

As for the whole "burden of proof" silliness, attempting to seize the high ground Null Hypothesis isn't a real argument, it's an epistemological sleight-of-hand trick. 

Nobody is suggesting Elon is being flippant about requirements. I'm just observing that your idea of the requirements can be wrong too. Hence why the quote "cuts both ways." 



With the miscellaneous rhetoric issues out of the way, as far as actual engineering we come back to:

Everything possible must be done to ensure astronaut safety.

Elon isn't being flippant when he talks about astronaut safety.