Starship On-orbit refueling - Options and Discussion

[Twark_Main]

Elon's Orbital Garage! "Best prices for Meth and Lox above the Karmen Line!"

Next stop for methalox in 1.3 light minutes!

This is astrodynamics. 0.16 A.U. or bust.   

[TheRadicalModerate]

Yes, if you're sure an engine failure isn't going to blow up the depot you just spent ten launches to fill up. it may take quite a bit of experience with reuse before anyone is confident of that though.

Of course you're not sure.  You're not even sure with brand new engines.  The question is what the probability of catastrophic failure is.  And that's not just limited to engine restarts; you have to factor in the RPOD failure tree, and the QD connection tree, and the prop transfer tree.  All of those things have to happen with each lift tanker, while the engine restart only has to happen mission-critically once.

Just to get a handle on this:  Let's assume that the catastrophic RPOD/refueling risk is 0.2% per tanker, the catastrophic maintained restart risk is 0.05%, and the unmaintained risk is 5x worse: 0.25%.  For 10 tankers, that gives your maintained pLOM=1 - (1-.002)¹º(1-.0005) = 2.03%, and the unmaintained pLOM = 1 - (1-.002)¹º(1-.0025) = 2.22%.

Seems OK to me.
 

And depots are cheap.  If they have even a 10-mission lifetime, that's going to be about $1.5M/mission in costs. 

How do you figure this? Estimates I find just searching the web suggest it would cost ~$30 million to build one plus (if it's full) the cost of all the launches to fill it up, say, another $100 million. So if you lose a full one, you're out $130 million plus whatever the cost of the delay getting another one up and fueled. That's assuming it doesn't require lots of custom stuff like extensive solar panels for active cooling.

Fair enough, and that should probably burden a full depot restart more heavily than near-empty RPODs. I'll accept $30M as a manufacturing cost, so each lunar mission (the whole campaign) puts $3M of depot at risk for 10 reused missions.  But remember that you have the burn no matter what.  All we're arguing about is how likely one less-recently maintained engine is going to be to generate a catastrophic failure over one that's more recently maintained.  At that particular failure node, it's probably a non-trivial difference.  But you're more interested in getting through the whole tree with no problems.

[Greg Hullender]

All we're arguing about is how likely one less-recently maintained engine is going to be to generate a catastrophic failure over one that's more recently maintained.  At that particular failure node, it's probably a non-trivial difference.  But you're more interested in getting through the whole tree with no problems.

Again, there are just too many variables we can only guess at.

I will confess that part of my obsession with stationary depots is that I see them as the core of a useful space station, particularly if you can eliminate ullage burns. I'm visualizing a space drydock that can maintain vehicles that never EDL with two depots attached by cables (one up, one down) that are long enough for tidal forces to do the ullage settling. (250 m gets you 1 mm/sec). But that doesn't work if the depots need to fly off.

[TheRadicalModerate]

I will confess that part of my obsession with stationary depots is that I see them as the core of a useful space station, particularly if you can eliminate ullage burns. I'm visualizing a space drydock that can maintain vehicles that never EDL with two depots attached by cables (one up, one down) that are long enough for tidal forces to do the ullage settling. (250 m gets you 1 mm/sec). But that doesn't work if the depots need to fly off.

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

[Paul451]

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

You've not visited a small town.

[TheRadicalModerate]

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

You've not visited a small town.

Quite the contrary.  And, while there used to be lots of gas-pump/garage combos, they've all disaggregated.  Convenience stores do the gas pumps, and the garages are standalone.  I don't know why.  Maybe it's a lot more expensive to insure all your mechanical equipment and tools against catastrophic fire than it is to insure your stock of Cheetos?

Slightly more on-topic, there's something to be said for being able to visit your repair station (assuming one exists, which I think is a ways off) and your fueling station with little maneuvering.  But you can give them all the same orbital parameters with the exception of true anomaly, and you can phase between them for a handful of m/s.

[Greg Hullender]

I will confess that part of my obsession with stationary depots is that I see them as the core of a useful space station, particularly if you can eliminate ullage burns. I'm visualizing a space drydock that can maintain vehicles that never EDL with two depots attached by cables (one up, one down) that are long enough for tidal forces to do the ullage settling. (250 m gets you 1 mm/sec). But that doesn't work if the depots need to fly off.

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

I was going to put them at least 250 m apart. :-) Even a couple of km of cable probably presents no problem, and it makes the problem a bit easier in cases where one depot is empty but the other is not.

[StraumliBlight]

The Technical Annex in SAT-LOA-20241218-00288 discusses propellant transfer.

Low Earth Orbit.

[...]

Missions beyond LEO will also require a tanker version of Starship for propellant aggregation.  During these missions, SpaceX will launch one or more propellant tanker versions of Starship.  Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches. LEO operations will occur in a circular orbit at 281 km altitude (+/- 100 km) and an inclination ranging from equatorial (0 degrees) to polar.

Medium-Earth Orbit/High-Earth Orbit/Final Tanking Orbit. 

Missions beyond LEO will also require space station operations in medium-Earth orbit (“MEO”) to high-Earth orbit (“HEO”). For example, crewed lunar missions will include a secondary propellant transfer in MEO/HEO, the Final Tanking Orbit (“FTO”).  Operations in MEO/HEO will occur in an elliptical orbit of 281 km x 34,534 km and an altitude tolerance of +116,000/-24,000 km apogee and +/- 100 km perigee, with inclination between 28 and 33 degrees (+/- 2 degrees).

[mikelepage]

The Technical Annex in SAT-LOA-20241218-00288 discusses propellant transfer.

Low Earth Orbit.

[...]

Missions beyond LEO will also require a tanker version of Starship for propellant aggregation.  During these missions, SpaceX will launch one or more propellant tanker versions of Starship.  Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches. LEO operations will occur in a circular orbit at 281 km altitude (+/- 100 km) and an inclination ranging from equatorial (0 degrees) to polar.

Medium-Earth Orbit/High-Earth Orbit/Final Tanking Orbit. 

Missions beyond LEO will also require space station operations in medium-Earth orbit (“MEO”) to high-Earth orbit (“HEO”). For example, crewed lunar missions will include a secondary propellant transfer in MEO/HEO, the Final Tanking Orbit (“FTO”).  Operations in MEO/HEO will occur in an elliptical orbit of 281 km x 34,534 km and an altitude tolerance of +116,000/-24,000 km apogee and +/- 100 km perigee, with inclination between 28 and 33 degrees (+/- 2 degrees).

This does seem to suggest that SpaceX still doesn't see depots being a separate build, or going beyond LEO.
I think this pushes us towards there being some kind of "depot kit" as TRM mentioned upthread?

[DanClemmensen]

The Technical Annex in SAT-LOA-20241218-00288 discusses propellant transfer.

Low Earth Orbit.

[...]

Missions beyond LEO will also require a tanker version of Starship for propellant aggregation.  During these missions, SpaceX will launch one or more propellant tanker versions of Starship.  Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches. LEO operations will occur in a circular orbit at 281 km altitude (+/- 100 km) and an inclination ranging from equatorial (0 degrees) to polar.

Medium-Earth Orbit/High-Earth Orbit/Final Tanking Orbit. 

Missions beyond LEO will also require space station operations in medium-Earth orbit (“MEO”) to high-Earth orbit (“HEO”). For example, crewed lunar missions will include a secondary propellant transfer in MEO/HEO, the Final Tanking Orbit (“FTO”).  Operations in MEO/HEO will occur in an elliptical orbit of 281 km x 34,534 km and an altitude tolerance of +116,000/-24,000 km apogee and +/- 100 km perigee, with inclination between 28 and 33 degrees (+/- 2 degrees).

This does seem to suggest that SpaceX still doesn't see depots being a separate build, or going beyond LEO.
I think this pushes us towards there being some kind of "depot kit" as TRM mentioned upthread?

Those appear to be related to FCC filings for RF transmissions to support these missions. The RF stuff needs to be precise, but the exact nature of the Ship does not. We cannot infer that there is only one "tanker variant". "Variant" may mean one variant of the generic Ship, or it may mean multiple variants of "Tanker".

[Twark_Main]

I will confess that part of my obsession with stationary depots is that I see them as the core of a useful space station, particularly if you can eliminate ullage burns. I'm visualizing a space drydock that can maintain vehicles that never EDL with two depots attached by cables (one up, one down) that are long enough for tidal forces to do the ullage settling. (250 m gets you 1 mm/sec). But that doesn't work if the depots need to fly off.

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

You've not visited a small town.

I think the more relevant analogy is, "nobody builds their house next to the Starbase tank farm."

Yes obviously this is due to legal and ownership reasons. But the (ultimate) reason for those (proximal) reasons boils down to safety.

[Twark_Main]

I will confess that part of my obsession with stationary depots is that I see them as the core of a useful space station, particularly if you can eliminate ullage burns. I'm visualizing a space drydock that can maintain vehicles that never EDL with two depots attached by cables (one up, one down) that are long enough for tidal forces to do the ullage settling. (250 m gets you 1 mm/sec). But that doesn't work if the depots need to fly off.

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

I was going to put them at least 250 m apart. :-) Even a couple of km of cable probably presents no problem, and it makes the problem a bit easier in cases where one depot is empty but the other is not.

A couple kilometers of cable isn't going to last long in LEO. MMOD isn't kind to long tethers.

[Robotbeat]

If that was true, ISS wouldn’t exist. The debris issue is proportional to cross sectional area, not length.

[eriblo]

If that was true, ISS wouldn’t exist. The debris issue is proportional to cross sectional area, not length.

The ISS has shielding so sure, tether should be fine as long as you include appropriate shielding.

[Greg Hullender]

I will confess that part of my obsession with stationary depots is that I see them as the core of a useful space station, particularly if you can eliminate ullage burns. I'm visualizing a space drydock that can maintain vehicles that never EDL with two depots attached by cables (one up, one down) that are long enough for tidal forces to do the ullage settling. (250 m gets you 1 mm/sec). But that doesn't work if the depots need to fly off.

Even on Earth, nobody puts the mechanics' garage next to the gas pumps any more.

I was going to put them at least 250 m apart. :-) Even a couple of km of cable probably presents no problem, and it makes the problem a bit easier in cases where one depot is empty but the other is not.

A couple kilometers of cable isn't going to last long in LEO. MMOD isn't kind to long tethers.

I suppose the assumption is that the cable is destroyed if it's hit by anything, so the longer the cable, the shorter the lifespan.

For the sort of forces we're talking about here, a single 8 mm steel cable would support 7500 metric tons of depot at either end--more than a full depot AND a full tanker docked together while massing around a ton itself.

I'm not the right kind of engineer, but if this is a big concern, it'd seem you could run three cables for, say, 100 m, then attach all three to a metal plate, and run three cables from the other side. To fail, the ensemble needs three failures in the same 100 m length.

Now run three of these triple cables. (You're only adding about 10 tons of mass to the depot structure.) If there's a break in one cable, you can have someone disconnect that segment of the triple cable and attach a replacement. You only have to replace a 100-m segment, and any single strand of either of the other two triple cables would easily support all the weight. So repairs wouldn't be an urgent matter; they could just be routine.

[Robotbeat]

It’s not that big of a problem. The depots themselves are FAR bigger targets than some little tethers. 8mm wide by 250m long is just 2 m^2 in cross sectional area.

I’d love it if people concern trolling about issues ever bothered to do the most basic arithmetic about the issue first.

[Twark_Main]

In the three-cable arrangement, I wouldn't want to be nearby when one of those cables snaps. I definitely don't want to be in a fragile tin can that's holding my air in. The first rule of cable safety is that you don't stand directly in-line with the cable. 

The Shuttle tether experiment failed due to a broken cable, and this wasn't even caused by a MMOD strike.

[Greg Hullender]

In the three-cable arrangement, I wouldn't want to be nearby when one of those cables snaps. I definitely don't want to be in a fragile tin can that's holding my air in. The first rule of cable safety is that you don't stand directly in-line with the cable. 

The Shuttle tether experiment failed due to a broken cable, and this wasn't even caused by a MMOD strike.

I think it'll need a mechanical engineer to offer a useful opinion here. It doesn't seem like a big deal to me, given the small forces involved, but an expert would know--and know how to mitigate risks like that.

[Twark_Main]

In the three-cable arrangement, I wouldn't want to be nearby when one of those cables snaps. I definitely don't want to be in a fragile tin can that's holding my air in. The first rule of cable safety is that you don't stand directly in-line with the cable. 

The Shuttle tether experiment failed due to a broken cable, and this wasn't even caused by a MMOD strike.

I think it'll need a mechanical engineer to offer a useful opinion here. It doesn't seem like a big deal to me, given the small forces involved, but an expert would know--and know how to mitigate risks like that.

Cable snap-back isn't exactly a controversial hazard when it comes to lines under tension.



The mitigation is to not let the cable snap. To reduce damage you can reduce the strain energy, but ultimately you're limited by the cable material. A thin pressurized steel tank covered by a lightweight whipple shield isn't going to stand up well.

I hope you can let us know what your mechanical engineer friend says. I'll be very interested to hear it!

[Robotbeat]

In the three-cable arrangement, I wouldn't want to be nearby when one of those cables snaps. I definitely don't want to be in a fragile tin can that's holding my air in. The first rule of cable safety is that you don't stand directly in-line with the cable. 

The Shuttle tether experiment failed due to a broken cable, and this wasn't even caused by a MMOD strike.

I think it'll need a mechanical engineer to offer a useful opinion here. It doesn't seem like a big deal to me, given the small forces involved, but an expert would know--and know how to mitigate risks like that.

Cable snap-back isn't exactly a controversial hazard when it comes to lines under tension.



The mitigation is to not let the cable snap. To reduce damage you can reduce the strain energy, but ultimately you're limited by the cable material. A thin pressurized steel tank covered by a lightweight whipple shield isn't going to stand up well.

I hope you can let us know what your mechanical engineer friend says. I'll be very interested to hear it!

skill issue.

I like how we’re talking about cables as if they’re more exotic than rockets. Oh, a tether broke one time? Weird. Rockets never fail. Well forget that idea, it’s obviously impossible.