Starship On-orbit refueling - Options and Discussion

[Twark_Main]

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

where are you getting an Isp of 250s off a cold gas thruster?

100-125 range would be doing quite well for a cold gas thruster out of 5 bar.

I'm assuming combusting gas.  Cold gas simply doesn't close for settling, unless transfer times are unreasonably short.  It especially doesn't close if you're doing pressure-fed transfers.

It would be interesting to pass all our proposals by the data we have on what SpaceX is declaring and building. As far as I can tell, none of these proposals fit that set of data:

1. Hot gas thrusters
2. Thermospheric surfing
3. axial coupling
4. Some strange method of taking advantage of surface tension (as demonstrated on the Space Station)

Which leaves us "how are they not violating the laws of physics?".  Which they aren't,  That's #5, they profess to always follow the laws of physics.

?Shrug?

SpaceX is often wrong before they get it right. 

Iterative design is the closest thing (that really exists) to design infallibility, but we should remember that it's not actually the same thing.

[butterwaffle]

Just some off the wall ideas:

+ Two tankers and one ship, all end-to-end and spinning about an axis perpendicular to the common long axis would drive liquids to the ship. This doesn't work for a single tanker+ship because the center of gravity would drive liquids away from their junction. Any additional mass at the tip of the tanker would help shift the cg.
+ O2 is paramagnetic. Is it enough to induce significant centrifugal pressure? How about liquid methane?
+ Equip the tanker with pistons in the tanks. There are a lot of piston configurations to consider.

[InterestedEngineer]

Average on-pad Starship prop load rate comes out to something like 450kg/s, and it takes a bit more than 45min.  Let's say that on-orbit prop transfer is only a tenth as fast, so we're at 9.3hr for a 1500t transfer.  Estimate the coupled system mass at...  2000t?¹  So, for 1E-4m/s², we need 200N of thrust, which, at Isp=250s, would be a mass flow of 0.08kg/s.  That's 2.7t of methalox.  That's a rounding error on any kind of sensible margins, at least to start.

where are you getting an Isp of 250s off a cold gas thruster?

100-125 range would be doing quite well for a cold gas thruster out of 5 bar.

I'm assuming combusting gas.  Cold gas simply doesn't close for settling, unless transfer times are unreasonably short.  It especially doesn't close if you're doing pressure-fed transfers.

It would be interesting to pass all our proposals by the data we have on what SpaceX is declaring and building. As far as I can tell, none of these proposals fit that set of data:

1. Hot gas thrusters
2. Thermospheric surfing
3. axial coupling
4. Some strange method of taking advantage of surface tension (as demonstrated on the Space Station)

Which leaves us "how are they not violating the laws of physics?".  Which they aren't,  That's #5, they profess to always follow the laws of physics.

?Shrug?

SpaceX is often wrong before they get it right. 

Iterative design is the closest thing (that really exists) to design infallibility, but we should remember that it's not actually the same thing.

or maybe they toss tons of prop out the door with cold gas thrusters, the best part being no part and calling it good enough.

At an average mass of 1000t, and a higher acceleration of 1mm/s² to alleviate our fears of too much surface tension/not enough settling, we end up with 1000N being the required force for say 33 minutes of fuel transfer (which is 100kg/sec at velocity of 10m/sec or 1000N as well.  Those should at least balance against the thrusters, right?)

With a cold gas thruster exhaust velocity of 1000m/sec, that leaves us with a mass flow rate of 1kg/sec.  so 2t of ullage gas thrown away per refuel.  That's not completely horrible.  It's a 1% loss rate.

"Premature optimization is the root of many evils" is likely a SpaceX mantra.

[TheRadicalModerate]

It would be interesting to pass all our proposals by the data we have on what SpaceX is declaring and building. As far as I can tell, none of these proposals fit that set of data:

1. Hot gas thrusters

SpaceX was planning on a combusting methox thruster for HLS, as of mid-2020.  It was going to be used for lunar landings and takeoffs.  After the IFT-1 crater, I can't see much chance for a Raptor-propelled landing, so that probably still exists.

That may be too hefty a thruster for settling, but once you've got the plumbing for one kind of thruster, how hard is it to make a little one that runs at the same feed pressure?

or maybe they toss tons of prop out the door with cold gas thrusters, the best part being no part and calling it good enough.

At an average mass of 1000t, and a higher acceleration of 1mm/s² to alleviate our fears of too much surface tension/not enough settling, we end up with 1000N being the required force for say 33 minutes of fuel transfer (which is 100kg/sec at velocity of 10m/sec or 1000N as well.  Those should at least balance against the thrusters, right?)

Your numbers seem optimistic.

Your worst-case transfer is from the depot into the HLS-LSS.  That's going to be at least 1800t system mass (1500t prop, 300t inert).  For 1E-3m/s² acceleration (I agree that's better to plan to), that'll be 1800N.  As an approximation, it's OK to assume that the receiving Starship starts empty and squeezes all of its ullage out as thrust, as it slowly fills.¹  But all of the ullage in the source depot will be needed for pressure-feeding the prop--and then some.

Prop to transfer: 1500t
Coupled System mass:  1800t
Settling acceleration:  1E-3m/s²

Settling thrust: 1800N
Cold gas Isp: 100s (~1000m/s)
Thruster mass flow:  1.8kg/s

Prop transfer rate:  Let's go hog-wild and say 100kg/s (22% of pad transfer rate)
Prop transfer time for 1500t prop:  15,000s (4.2h)
Cold gas needed:  27t

LCH4 tank volume (assumes subcooled prop):  730m³
Gas temperature:  112K
Moles in empty tank @3bar (a guess), 112K, 730m³: 2.35E5
Molecular weight: 16g/mol
Cold gas mass in empty tank:  3.8t

LOX tank volume: 950m³
Gas temperature:  90K
Moles in empty tank @3bar, 90K, 950m³: 3.81E5
Molecular weight: 32g/mol  (note that cold GO2 will have a lower Isp than cold GCH4)
Cold gas in empty tank:  12.2t 

Total cold gas mass @average 3bar:  16.0t.  And we need 27t in this scenario.

You can, of course, heat prop to make more cold gas.  But the Raptors aren't running, so you'll have to do it with either a resistive heater or an APU.  How much plumbing is that?  Where are the heaters?  How much power do they consume?

"Premature optimization is the root of many evils" is likely a SpaceX mantra.

That's true, but it cuts both ways.  How much optimization do you want to do on a pressure-fed, cold gas system before it becomes more complex than the combusting gas system, which we already know we need for lunar landing, and which would come in awfully handy for midcourse translunar maneuvers?

__________
¹If you had good numbers for both GCH4 and GO2 cold gas thrusters, it'd be interesting to know what liquid propellant mass flows into the tank balanced the settling thrust outflows.  I haven't done that computation, but you need to keep the destination ullage space in a fairly narrow range:  Too low, and the thrusters won't choke, too high, and your propellant transfer time goes up as the pressure differential between source and destination narrows.

[Greg Hullender]

Welcome to the discussion!

Just some off the wall ideas:

+ Two tankers and one ship, all end-to-end and spinning about an axis perpendicular to the common long axis would drive liquids to the ship. This doesn't work for a single tanker+ship because the center of gravity would drive liquids away from their junction. Any additional mass at the tip of the tanker would help shift the cg.
+ O2 is paramagnetic. Is it enough to induce significant centrifugal pressure? How about liquid methane?
+ Equip the tanker with pistons in the tanks. There are a lot of piston configurations to consider.

As you might guess, in a 5-year thread, a lot of stuff has already been discussed. The most important thing to remember is that we're talking about something SpaceX can do in the next few years, so ideas involving huge amounts of technical risk and (likely) years of development don't get much attention. In particular, things that add a lot of mass to the tankers are really unpopular, since they impose a cost on every single tanker launch. Adding mass to the depot is much less controversial.

It turns out that cryogenic plumbing is a really big deal. As a result, proposals that require a lot of new plumbing are high-risk. At this point, any kind of docking other than back-to-back (dorsal) using the same refueling ports that are currently used on the ground is probably not practical. This fact sank a lot of my own early proposals in this thread.

Despite its popularity in science fiction, there has been essentially zero work on rotating systems in space. As a result, nothing depending on a rotating system is likely to happen in the next ten years. Almost everyone (including me) has some sort of rotational proposal when they first post here. NASA let us all down by never making any effort to study this.

I like the piston idea, although I called it a "syringe." You can even buy cryogenic syringes. But the extra mass and complexity rule them out, particularly when a zero-parts system to just let pressure do the transfer seems to be adequate. But I think you're the only one besides me to suggest this, so now I don't feel so lonely! :-)

Using magnets to manage liquid O2 has been discussed before, and there have been studies on it. Current thinking is that it'd weigh too much to be useful in the near future. But a bigger problem is that methane is not paramagnetic, so this won't work for it at all.

[InterestedEngineer]

Your worst-case transfer is from the depot into the HLS-LSS.  That's going to be at least 1800t system mass (1500t prop, 300t inert).  For 1E-3m/s² acceleration (I agree that's better to plan to), that'll be 1800N.  As an approximation, it's OK to assume that the receiving Starship starts empty and squeezes all of its ullage out as thrust, as it slowly fills.¹  But all of the ullage in the source depot will be needed for pressure-feeding the prop--and then some.

where did the "one huge transfer at once" requirement come from?

[Twark_Main]

or maybe they toss tons of prop out the door with cold gas thrusters, the best part being no part and calling it good enough.

Good enough for now maybe, but SpaceX isn't the sort of company to leave iterative improvements on the table for long.   

At an average mass of 1000t, and a higher acceleration of 1mm/s² to alleviate our fears of too much surface tension/not enough settling, we end up with 1000N being the required force for say 33 minutes of fuel transfer (which is 100kg/sec at velocity of 10m/sec or 1000N as well.  Those should at least balance against the thrusters, right?)

Probably not. That's on;y a factor of 2 reduction in free-stream arcing height, whereas earlier I guessed a reduction of 10x or more is probably necessary to keep sloshing under control.

This is the part where on-orbit testing becomes essential. We just don't have enough data on those important parameters.

[TheRadicalModerate]

Your worst-case transfer is from the depot into the HLS-LSS.  That's going to be at least 1800t system mass (1500t prop, 300t inert).  For 1E-3m/s² acceleration (I agree that's better to plan to), that'll be 1800N.  As an approximation, it's OK to assume that the receiving Starship starts empty and squeezes all of its ullage out as thrust, as it slowly fills.¹  But all of the ullage in the source depot will be needed for pressure-feeding the prop--and then some.

where did the "one huge transfer at once" requirement come from?

What else would you do?  Do multiple RPODs between your multi-hundred-million-dollar HLS-LSS and the depot?

[InterestedEngineer]

Your worst-case transfer is from the depot into the HLS-LSS.  That's going to be at least 1800t system mass (1500t prop, 300t inert).  For 1E-3m/s² acceleration (I agree that's better to plan to), that'll be 1800N.  As an approximation, it's OK to assume that the receiving Starship starts empty and squeezes all of its ullage out as thrust, as it slowly fills.¹  But all of the ullage in the source depot will be needed for pressure-feeding the prop--and then some.

where did the "one huge transfer at once" requirement come from?

What else would you do?  Do multiple RPODs between your multi-hundred-million-dollar HLS-LSS and the depot?

if this LEO (I'm confused where this requirement is taken place) you fill the HLS from earth surface launches directly.

[Twark_Main]

Your worst-case transfer is from the depot into the HLS-LSS.  That's going to be at least 1800t system mass (1500t prop, 300t inert).  For 1E-3m/s² acceleration (I agree that's better to plan to), that'll be 1800N.  As an approximation, it's OK to assume that the receiving Starship starts empty and squeezes all of its ullage out as thrust, as it slowly fills.¹  But all of the ullage in the source depot will be needed for pressure-feeding the prop--and then some.

where did the "one huge transfer at once" requirement come from?

What else would you do?  Do multiple RPODs between your multi-hundred-million-dollar HLS-LSS and the depot?

if this LEO (I'm confused where this requirement is taken place) you fill the HLS from earth surface launches directly.

TRM's point is, do you want to plan for 10 risky rendezvous, proximity operations, docking. and transfer sequences (once for each launch from the surface), or do you want to reduce your risk and only have a single docking with a pre-filled depot?

[InterestedEngineer]

Your worst-case transfer is from the depot into the HLS-LSS.  That's going to be at least 1800t system mass (1500t prop, 300t inert).  For 1E-3m/s² acceleration (I agree that's better to plan to), that'll be 1800N.  As an approximation, it's OK to assume that the receiving Starship starts empty and squeezes all of its ullage out as thrust, as it slowly fills.¹  But all of the ullage in the source depot will be needed for pressure-feeding the prop--and then some.

where did the "one huge transfer at once" requirement come from?

What else would you do?  Do multiple RPODs between your multi-hundred-million-dollar HLS-LSS and the depot?

if this LEO (I'm confused where this requirement is taken place) you fill the HLS from earth surface launches directly.

TRM's point is, do you want to plan for 10 risky rendezvous, proximity operations, docking. and transfer sequences (once for each launch from the surface), or do you want to reduce your risk and only have a single docking with a pre-filled depot?

best part is no part.  If the heavy-heavy dock adds parts, take the risk of docking.  When is the last major docking incident in outer space that resulted in major damage to spacecraft?

[Twark_Main]

TRM's point is, do you want to plan for 10 risky rendezvous, proximity operations, docking. and transfer sequences (once for each launch from the surface), or do you want to reduce your risk and only have a single docking with a pre-filled depot?

best part is no part.  If the heavy-heavy dock adds parts, take the risk of docking.

Spoken like a true zealot.   What ever happened to your sensibility for engineering trade-offs?

"A man with one Elon quote always knows what time it is. A man with two Elon quotes is never sure..." 

Everything possible must be done to ensure astronaut safety.

https://twitter.com/elonmusk/status/1829297836988706976

[InterestedEngineer]

TRM's point is, do you want to plan for 10 risky rendezvous, proximity operations, docking. and transfer sequences (once for each launch from the surface), or do you want to reduce your risk and only have a single docking with a pre-filled depot?

best part is no part.  If the heavy-heavy dock adds parts, take the risk of docking.

Spoken like a true zealot.   What ever happened to your sensibility for engineering trade-offs?

"A man with one Elon quote always knows what time it is. A man with two Elon quotes is never sure..." 

Everything possible must be done to ensure astronaut safety.

https://twitter.com/elonmusk/status/1829297836988706976

I think my memory is fading, or the requirements aren't clear.  There are assumptions being made that are not in our heads.

Which is why "your requirements are wrong" is another Elon quote.  3 is the number of completion, and the next stop is 7...

Why are we refueling something with a crew in it?

[OTV Booster]

Raising orbit between tankers is reasonable. It doesn't look like a good move while a tanker is hooked up.

Using drag to lower the orbit means you still need to repay all that delta-v using thrusters anyway. The tanker has to reach the higher initial starting orbit, so there's no real savings here. You're just constraining your altitude regime and re-orienting into a high-drag attitude (wrecking any pre-settling) for no reason.


What happens if you add small methox thrusters that re-use the large vacuum bells as their expansion nozzle? In theory you should be able to reach extremely high Isps with a very minimal system. Naturally this helps a lot for both propellant transfer and long-term orbit maintenance at lower (safer) altitude.

https://ntrs.nasa.gov/citations/20160001041

https://ntrs.nasa.gov/citations/20110014049

I think we've got our wires crossed.

The point of transferring from tanker to depot at VLEO is that it saves tanker propellant. Each tanker that goes a meter higher than it has to has to drag its dry mass along. (Dry mass)x(number of tankers) If VLEO gives free drag induced ullage settling, all the better.

If circumstances demand the depot boost higher because it's getting too low or the weather changed, it's gotta do what it's gotta do. It's only raising one dry mass, albeit a higher mass than a tanker. The propellant it raises would have been otherwise raised by the tankers along with their dry mass. Follow on tankers will have to spend more to reach it. Ce La Vie.

Should the outbound ship match depot VLEO orbit or should they both go to a higher more stable altitude for transfer? Open question. Too many variables to judge this far out for a system that has not yet reached orbit or attempted wholesale propellant transfer.

If depot has to raise up to meet an outbound ship it's still only dragging one dry mass along. Dropping back down to VLEO for another campaign, it will be dragging one dry mass and little propellant. TANSTAAFL, but sometimes you can find a cheap one.

Getting back to my original question, how fast can the weather (VLEO atmospheric density) change? That coupled with the time it takes for a tanker transfer dictates how low VLEO can be. I suspect a significant raising of orbit in the middle of a transfer would be a fustercluck.

[InterestedEngineer]

The point of transferring from tanker to depot at VLEO is that it saves tanker propellant. Each tanker that goes a meter higher than it has to has to drag its dry mass along. (Dry mass)x(number of tankers) If VLEO gives free drag induced ullage settling, all the better.

It's not just the dry mass.  The propellant mass too.  With an impulsive burn most of the propellant mass is expended out the back at the lower orbit, so the wet mass never makes it to the higher orbit.

[OTV Booster]

The point of transferring from tanker to depot at VLEO is that it saves tanker propellant. Each tanker that goes a meter higher than it has to has to drag its dry mass along. (Dry mass)x(number of tankers) If VLEO gives free drag induced ullage settling, all the better.

It's not just the dry mass.  The propellant mass too.  With an impulsive burn most of the propellant mass is expended out the back at the lower orbit, so the wet mass never makes it to the higher orbit.

I figure one way or another that propellant will get raised from VLEO. It can get raised in individual tankers, the depot, or the outbound ship. If in the depot or the outbound ship, only one dry mass gets raised. Or am I missing the point?

[OTV Booster]

TRM's point is, do you want to plan for 10 risky rendezvous, proximity operations, docking. and transfer sequences (once for each launch from the surface), or do you want to reduce your risk and only have a single docking with a pre-filled depot?

best part is no part.  If the heavy-heavy dock adds parts, take the risk of docking.

Spoken like a true zealot.   What ever happened to your sensibility for engineering trade-offs?

"A man with one Elon quote always knows what time it is. A man with two Elon quotes is never sure..." 

Everything possible must be done to ensure astronaut safety.

https://twitter.com/elonmusk/status/1829297836988706976

I think my memory is fading, or the requirements aren't clear.  There are assumptions being made that are not in our heads.

Which is why "your requirements are wrong" is another Elon quote.  3 is the number of completion, and the next stop is 7...

Why are we refueling something with a crew in it?

For HLS, LEO and NRHO ops needn't have crew on board. For HEO there would probably be crew. For LLO there would be crew. Which will be used? There's too much turmoil right now. The bookies aren't taking any new bets.

A big advantage of using a depot in LEO is timing. Fill the depot and do it early in case there are hiccups. Do a late topoff if necessary. When alignment is coming up launch the uncrewed HLS and do the propellant transfer with plenty of time to spare. Some of the alternative conops that have bubbled up would transfer crew to HLS after the topoff and before the first exit burn.

The "classic" Artemus conop would have crew transfer to Starship at Gateway where one option is to have a tanker/depot available to do another topoff before the crew gets on board.

IMO, even the classic approach had some wiggle room where later topoffs might happen. With the recent turmoil it's impossible to say exactly how they'll do it.

[Twark_Main]

... "your requirements are wrong" is another Elon quote.

I can't help noticing how this quote cuts equally both ways. 

So in (what is apparently  ) the Battle Of Comparing Elon Quotes, we're one quote opposed, one quote in favor, and one abstention. The quote in favor is about saving lives, whereas the quote opposed is about saving cost. I know which one I'd put more stock in.

Why are we refueling something with a crew in it?

Did you have something particular in mind?

[InterestedEngineer]

... "your requirements are wrong" is another Elon quote.

I can't help noticing how this quote cuts equally both ways. 

So in (what is apparently  ) the Battle Of Comparing Elon Quotes, we're one quote opposed, one quote in favor, and one abstention. The quote in favor is about saving lives, whereas the quote opposed is about saving cost. I know which one I'd put more stock in.

Why are we refueling something with a crew in it?

Did you have something particular in mind?

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

Now there's no need for a depot.  Just refuel the HLS like you would a depot, put the crew on it, launch to the LHRO

[Twark_Main]

... "your requirements are wrong" is another Elon quote.

I can't help noticing how this quote cuts equally both ways. 

So in (what is apparently  ) the Battle Of Comparing Elon Quotes, we're one quote opposed, one quote in favor, and one abstention. The quote in favor is about saving lives, whereas the quote opposed is about saving cost. I know which one I'd put more stock in.

Why are we refueling something with a crew in it?

Did you have something particular in mind?

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

Now there's no need for a depot.  Just refuel the HLS like you would a depot, put the crew on it, launch to the LHRO

How quickly we forget what's immediately upthread! 

This wasn't just in the context of crew safety, but also (TRM's original, stated) consideration of

where did the "one huge transfer at once" requirement come from?

What else would you do?  Do multiple RPODs between your multi-hundred-million-dollar HLS-LSS and the [tankers]?