Starship On-orbit refueling - Options and Discussion

[daavery]

I suspect that any refueling plan will ( for now)  require the 2 vehicles be in the same orbit, not just meeting at one point common between a circular orbit and an elliptical orbit. I will be very surprised if the fuel transfer takes less than 1 orbit. right now it takes over an hour to fuel a starship using 100s of HP driving the fueling pumps.

[InterestedEngineer]

I suspect that any refueling plan will ( for now)  require the 2 vehicles be in the same orbit, not just meeting at one point common between a circular orbit and an elliptical orbit.

to anyone who's played KSP for more than an hour would say 'now and forever'.

Or simply calculate the periapsis speeds of a circular and then an elliptic orbit.

[Robotbeat]

Clearly it needs to be the same orbit.

But KSP (which is fun, don’t get me wrong!) is a really bad guide for what is practical or not. It makes building new, many-stage launch vehicles super easy. It makes rendezvous and docking really hard (even though we’ve done this literally hundreds of times with ISS, etc). And it makes vertical landing recovery of multiple stages for reuse nearly impossible without modding, even though the vast majority of US rocket launches use this approach without problem. It also barely makes reuse worth it even when you do manage to make it happen. (And your point is valid, just wanted to mention that.)

There are also other methods that could allow the vehicles to be in separate orbits, like tethers. So “for now” is defensible.

[TheRadicalModerate]

I suspect that any refueling plan will ( for now)  require the 2 vehicles be in the same orbit, not just meeting at one point common between a circular orbit and an elliptical orbit. I will be very surprised if the fuel transfer takes less than 1 orbit. right now it takes over an hour to fuel a starship using 100s of HP driving the fueling pumps.

Always the same orbit.

As for refueling time:  If you're in a circular VLEO, it doesn't really matter.  If you're in, say, LEO+2500m/s, that's about a 12hr orbit.  I doubt that two Starships that have boosted independently to HEEO can do rendezvous and docking in less than an orbit--even a 12hr orbit. 

If we allocate 2hr for undock and checkout before the remainder of the TLI burn, that would give us 10hr to pump up to 1200t.  That would require 33kg/s.  That's not an unreasonable transfer rate--especially in microgravity, with a few bar of ullage pressure and a set of pretty modest pumps.

On the other hand, if you want to do it in one orbit, and RPOD and undock/checkout can be reduced to, say, 8hr, then you'd need 83kg/s.  That's probably getting up there.

[OTV Booster]

That depends largely on the operating realities of the company. If tankers are sitting idle, and actual launch costs are as low as some speculate, then keeping them busy with propellant launches may make business sense. Somewhat as Falcon launches Starlinks instead of sitting idle. Or as a cargo truck, plane, or ship is an expense sitting idle at the terminal. 

It's always cheaper to let something sit idle than it is to send it out for no reason.  Yes, it will always make sense to launch full tankers, even if the mission only requires a fractional tanker.  But you're not just going to launch prop to LEO depots for no reason.

You should also consider that if things are really that cheap, two things will happen:

1) Demand will spike, which will first soak up spare capacity and then increase prices.

2) The market will be hot enough that Starship will get some competition.  That may reduce prices somewhat, but it will also make SpaceX want to be as ops-efficient as possible.

In both cases, nobody's gonna be launching prop just for grins and giggles.

If more propellant than is needed is accumulating in LEO, and tankers are idle,  then sending some of it to cislunar can make sense.

No.  It never makes sense, absent a conops with sufficient cadence to generate the demand.  It's always better to leave excess prop in LEO until you need it.  Even if there's boil-off, the boiled-off prop costs a fraction of what it costs to replace the equivalent amount of prop in cislunar.¹

There are many missions enabled by large quantities of propellant in HEO. Any high energy mission would benefit by leaving HEO with full tanks.

Which missions do you have in mind?  And in what HEEO, with what RAAN and argument of perigee?  Are they crewed missions?  What are their abort contingencies?

One of the nice things about using cislunar depots for high-energy missions is that, unlike an HEEO, which is in a near-constant sidereal reference frame, the cis-lunar ones are in an Earth-Moon rotating reference frame, which means that you have a not-terrible window to any departure asymptote once a month, instead of once a year.

I do think that if refueling in HEEO is really the best way to do lunar missions, then an HEEO depot might make sense.  But even then, you don't move the prop to it until you need it, for exactly the same reason you don't move prop to cislunar depots until you need it:  boil-off costs more at high energy than at low energy.

____________
¹Weasel words:  If all you have is passive boil-off management, then cislunar is colder than LEO.  Whether it's enough colder that boil-off is cheaper would require some figuring.  But if there's enough traffic going to cislunar to make this at all useful, I'm pretty sure that SpaceX will have a zero boil-off depot technology, and LEO will then be just as good as anywhere else to store excess prop.

Wellll, zero boiloff in LEO calls for more massive cooling hardware and PV panels than HEEO or cislunar. Same operating costs but not the same overall costs.


I agree that squeezing a tight launch schedule to do propellant launches for speculative use makes no sense. OTOH, if it's known that propellant will be needed within a known timeframe and there is a real honest to gosh lull in the launch schedule, it can make sense. If spaceflight is ever going to become routine, infrastructure has to be available on an ad hoc basis. Build it, and they will come.

[launchwatcher]

It makes rendezvous and docking really hard (even though we’ve done this literally hundreds of times with ISS, etc).

MechJeb does rendezvous & docking quite nicely.

[TheRadicalModerate]

Wellll, zero boiloff in LEO calls for more massive cooling hardware and PV panels than HEEO or cislunar. Same operating costs but not the same overall costs.

Amortized over hundreds of launches?  I'd be surprised if beefier power and heat rejection moved the fully burdened cost by more than 1%.  The same can't be said for forward-deploying prop in cislunar without it being earmarked to a mission.

[sdsds]

[...] forward-deploying prop in cislunar without it being earmarked to a mission.

Again we agree. I've not noticed anyone suggesting that though, so you might be fighting a straw man.

I think we likely agree the value of propellant is only realized when it is used for propulsion of a vehicle conducting a mission. And obviously for that it needs to be in the tanks of the mission vehicle.

For the class of missions (and it's quite large) where all the propellant needed for the mission can be loaded into the vehicle's tanks in LEO, that's by far the most efficient way to proceed. I just don't see SpaceX forever limiting itself solely to missions in that class.

[edzieba]

It's always cheaper to let something sit idle than it is to send it out for no reason.  Yes, it will always make sense to launch full tankers, even if the mission only requires a fractional tanker.  But you're not just going to launch prop to LEO depots for no reason.

Actually it is not cheaper to let equipment and personnel sit idle if there is any revenue, or potential revenue work for them to do. Idle personnel cost as much as busy ones, and building inventory in good locations is potential revenue work. The alternatives are to eat the expense for losses, or lay them off. Neither is good for the long term if something useful can be done.

Only if someone is paying you to sit loaded propellant depots in orbit.
Otherwise, all you have done is moved from groundside unsold raw materials (unlaunched Starship missions) that could be used for multiple services (prop launches, satellite launches, lunar missions, etc) serving a large market into an unsold in-orbit product (orbiting propellant) with a small or possibly non-existent market.

For using "spare" Starships to speculatively fill prop depots specifically, you're not using idle assets to generate revenue but instead spending money to build up idle inventory with no buyer.

[InterestedEngineer]

It's always cheaper to let something sit idle than it is to send it out for no reason.  Yes, it will always make sense to launch full tankers, even if the mission only requires a fractional tanker.  But you're not just going to launch prop to LEO depots for no reason.

Actually it is not cheaper to let equipment and personnel sit idle if there is any revenue, or potential revenue work for them to do. Idle personnel cost as much as busy ones, and building inventory in good locations is potential revenue work. The alternatives are to eat the expense for losses, or lay them off. Neither is good for the long term if something useful can be done.

Only if someone is paying you to sit loaded propellant depots in orbit.
Otherwise, all you have done is moved from groundside unsold raw materials (unlaunched Starship missions) that could be used for multiple services (prop launches, satellite launches, lunar missions, etc) serving a large market into an unsold in-orbit product (orbiting propellant) with a small or possibly non-existent market.

For using "spare" Starships to speculatively fill prop depots specifically, you're not using idle assets to generate revenue but instead spending money to build up idle inventory with no buyer.

Great arguments if it's a business.

There's no profit to be had on the Moon, it's all government.

IF government had any sense, they'd replace the billions they are spending on SLS and custom lightweight space probes/satellites with Starship and heavy off-the-shelf components for probes/satellites, and use the saved billions for logistics instead.

Logistics was a large part of the western forts system cost in the 19th century America, for example, and it worked out well.

But modern government has no sense, so I'm just pipe dreaming.

[OTV Booster]

Wellll, zero boiloff in LEO calls for more massive cooling hardware and PV panels than HEEO or cislunar. Same operating costs but not the same overall costs.

Amortized over hundreds of launches?  I'd be surprised if beefier power and heat rejection moved the fully burdened cost by more than 1%.  The same can't be said for forward-deploying prop in cislunar without it being earmarked to a mission.

This is the amorphous time frame problem that we've both complained about. Both of us owe Mia Culpa's.


Mea Culpa.


I expect early depots to have one or two campaign lifetimes. The mission pace of Artemus will be low. If a HEO or cislunar depot is called for, with boiloff management easier in the high energy orbits, why not stockpile well in advance and keep operations smooth (see earlier comments on opportunistic tanker launches)?


If a depot is going to see hundreds of transfers this will most certainly be well down the road. Under these circumstances I seriously doubt keeping a depot filled will be a mission specific event, and inventory turnover will be high enough that the trade equations change in ways impossible to predict this early.


When SX has a system that works end to end, demonstrating launch, propellant transfer and EDL, we'll have a clearer idea of what the future holds. Wanna see a launch! Wanna se a launch!

[TheRadicalModerate]

This is the amorphous time frame problem that we've both complained about. Both of us owe Mia Culpa's.

Mea Culpa.

I expect early depots to have one or two campaign lifetimes. The mission pace of Artemus will be low. If a HEO or cislunar depot is called for, with boiloff management easier in the high energy orbits, why not stockpile well in advance and keep operations smooth (see earlier comments on opportunistic tanker launches)?

If a depot is going to see hundreds of transfers this will most certainly be well down the road. Under these circumstances I seriously doubt keeping a depot filled will be a mission specific event, and inventory turnover will be high enough that the trade equations change in ways impossible to predict this early.

When SX has a system that works end to end, demonstrating launch, propellant transfer and EDL, we'll have a clearer idea of what the future holds. Wanna see a launch! Wanna se a launch!

There's going to be an Era of Low BEO Cadence, and an Era of High BEO Cadence.  At low cadence, you should assume that anything left in orbit will boil dry, and timing lift tanker launches for specific missions is important.  At high cadence, prop becomes a lot more fungible, and you manage prop to LEO as a logistical problem that's separate from (albeit dependent on) the variety of LEO and BEO missions that need to be satisfied.

I'm concentrating on the Era of Low Cadence for the time being.  I don't think anybody knows how the market shakes out at high cadence.

[OTV Booster]

This is the amorphous time frame problem that we've both complained about. Both of us owe Mia Culpa's.

Mea Culpa.

I expect early depots to have one or two campaign lifetimes. The mission pace of Artemus will be low. If a HEO or cislunar depot is called for, with boiloff management easier in the high energy orbits, why not stockpile well in advance and keep operations smooth (see earlier comments on opportunistic tanker launches)?

If a depot is going to see hundreds of transfers this will most certainly be well down the road. Under these circumstances I seriously doubt keeping a depot filled will be a mission specific event, and inventory turnover will be high enough that the trade equations change in ways impossible to predict this early.

When SX has a system that works end to end, demonstrating launch, propellant transfer and EDL, we'll have a clearer idea of what the future holds. Wanna see a launch! Wanna se a launch!

There's going to be an Era of Low BEO Cadence, and an Era of High BEO Cadence.  At low cadence, you should assume that anything left in orbit will boil dry, and timing lift tanker launches for specific missions is important.  At high cadence, prop becomes a lot more fungible, and you manage prop to LEO as a logistical problem that's separate from (albeit dependent on) the variety of LEO and BEO missions that need to be satisfied.

I'm concentrating on the Era of Low Cadence for the time being.  I don't think anybody knows how the market shakes out at high cadence.

Yup. Back in the day, folks probably had to order gasoline from Hiram's Hardware Store a month in advance. When volume got high enough Hiram put a pump out front.


It's not a perfect analogy (none are) but following it a bit further, unless a farmers needs met some threshold, it made more sense to go out of route for gas than to install a pump out on the farm. The tradeoffs work different for depot locations but it's the same problem with the same decision points.

[InterestedEngineer]

This is the amorphous time frame problem that we've both complained about. Both of us owe Mia Culpa's.

Mea Culpa.

I expect early depots to have one or two campaign lifetimes. The mission pace of Artemus will be low. If a HEO or cislunar depot is called for, with boiloff management easier in the high energy orbits, why not stockpile well in advance and keep operations smooth (see earlier comments on opportunistic tanker launches)?

If a depot is going to see hundreds of transfers this will most certainly be well down the road. Under these circumstances I seriously doubt keeping a depot filled will be a mission specific event, and inventory turnover will be high enough that the trade equations change in ways impossible to predict this early.

When SX has a system that works end to end, demonstrating launch, propellant transfer and EDL, we'll have a clearer idea of what the future holds. Wanna see a launch! Wanna se a launch!

There's going to be an Era of Low BEO Cadence, and an Era of High BEO Cadence.  At low cadence, you should assume that anything left in orbit will boil dry, and timing lift tanker launches for specific missions is important.  At high cadence, prop becomes a lot more fungible, and you manage prop to LEO as a logistical problem that's separate from (albeit dependent on) the variety of LEO and BEO missions that need to be satisfied.

I'm concentrating on the Era of Low Cadence for the time being.  I don't think anybody knows how the market shakes out at high cadence.

Yup. Back in the day, folks probably had to order gasoline from Hiram's Hardware Store a month in advance. When volume got high enough Hiram put a pump out front.


It's not a perfect analogy (none are) but following it a bit further, unless a farmers needs met some threshold, it made more sense to go out of route for gas than to install a pump out on the farm. The tradeoffs work different for depot locations but it's the same problem with the same decision points.

Another similar situation was coal fired ships in the 19th century.  The British Empire had a network of refueling stations all around the world on the empire where "the sun never set".

How they built out that network of coal stations would be an interesting piece of history to research.

We still have a few remnants of that culture:

https://www.azlyrics.com/lyrics/nathanevans/wellermanseashanty.html

[OTV Booster]

This is the amorphous time frame problem that we've both complained about. Both of us owe Mia Culpa's.

Mea Culpa.

I expect early depots to have one or two campaign lifetimes. The mission pace of Artemus will be low. If a HEO or cislunar depot is called for, with boiloff management easier in the high energy orbits, why not stockpile well in advance and keep operations smooth (see earlier comments on opportunistic tanker launches)?

If a depot is going to see hundreds of transfers this will most certainly be well down the road. Under these circumstances I seriously doubt keeping a depot filled will be a mission specific event, and inventory turnover will be high enough that the trade equations change in ways impossible to predict this early.

When SX has a system that works end to end, demonstrating launch, propellant transfer and EDL, we'll have a clearer idea of what the future holds. Wanna see a launch! Wanna se a launch!

There's going to be an Era of Low BEO Cadence, and an Era of High BEO Cadence.  At low cadence, you should assume that anything left in orbit will boil dry, and timing lift tanker launches for specific missions is important.  At high cadence, prop becomes a lot more fungible, and you manage prop to LEO as a logistical problem that's separate from (albeit dependent on) the variety of LEO and BEO missions that need to be satisfied.

I'm concentrating on the Era of Low Cadence for the time being.  I don't think anybody knows how the market shakes out at high cadence.

Yup. Back in the day, folks probably had to order gasoline from Hiram's Hardware Store a month in advance. When volume got high enough Hiram put a pump out front.


It's not a perfect analogy (none are) but following it a bit further, unless a farmers needs met some threshold, it made more sense to go out of route for gas than to install a pump out on the farm. The tradeoffs work different for depot locations but it's the same problem with the same decision points.

Another similar situation was coal fired ships in the 19th century.  The British Empire had a network of refueling stations all around the world on the empire where "the sun never set".

How they built out that network of coal stations would be an interesting piece of history to research.

We still have a few remnants of that culture:

https://www.azlyrics.com/lyrics/nathanevans/wellermanseashanty.html

And there's a bit of a feedback loop too. Where you go impacts where refueling is needed. Where refueling is available impacts where you can go. It's not tightly bound but it is there.

[Asteroza]

This is the amorphous time frame problem that we've both complained about. Both of us owe Mia Culpa's.

Mea Culpa.

I expect early depots to have one or two campaign lifetimes. The mission pace of Artemus will be low. If a HEO or cislunar depot is called for, with boiloff management easier in the high energy orbits, why not stockpile well in advance and keep operations smooth (see earlier comments on opportunistic tanker launches)?

If a depot is going to see hundreds of transfers this will most certainly be well down the road. Under these circumstances I seriously doubt keeping a depot filled will be a mission specific event, and inventory turnover will be high enough that the trade equations change in ways impossible to predict this early.

When SX has a system that works end to end, demonstrating launch, propellant transfer and EDL, we'll have a clearer idea of what the future holds. Wanna see a launch! Wanna se a launch!

There's going to be an Era of Low BEO Cadence, and an Era of High BEO Cadence.  At low cadence, you should assume that anything left in orbit will boil dry, and timing lift tanker launches for specific missions is important.  At high cadence, prop becomes a lot more fungible, and you manage prop to LEO as a logistical problem that's separate from (albeit dependent on) the variety of LEO and BEO missions that need to be satisfied.

I'm concentrating on the Era of Low Cadence for the time being.  I don't think anybody knows how the market shakes out at high cadence.

Yup. Back in the day, folks probably had to order gasoline from Hiram's Hardware Store a month in advance. When volume got high enough Hiram put a pump out front.


It's not a perfect analogy (none are) but following it a bit further, unless a farmers needs met some threshold, it made more sense to go out of route for gas than to install a pump out on the farm. The tradeoffs work different for depot locations but it's the same problem with the same decision points.

Another similar situation was coal fired ships in the 19th century.  The British Empire had a network of refueling stations all around the world on the empire where "the sun never set".

How they built out that network of coal stations would be an interesting piece of history to research.

We still have a few remnants of that culture:

https://www.azlyrics.com/lyrics/nathanevans/wellermanseashanty.html

And there's a bit of a feedback loop too. Where you go impacts where refueling is needed. Where refueling is available impacts where you can go. It's not tightly bound but it is there.

If there were a few more nodes in the problem, simulating a solution using a slime mold network to look for solutions may be interesting.

[LMT]

If there were a few more nodes in the problem, simulating a solution using a slime mold network to look for solutions may be interesting.

Or you might setup the nodes, depots, and campaigns in SpaceNet, and share your results and example apps in forum.  That would be useful.

[OTV Booster]

From Ars Technica https://arstechnica.com/science/2022/11/spacex-is-now-building-a-raptor-engine-a-day-nasa-says/

SpaceX plans to keep its low-Earth orbit propellant depots topped off with fuel for missions other than Artemis, Kirasich added. "So it's not like every time we go to the Moon we're going to start with an empty depot," he said.

The key points in this paragraph are 'depots' plural, and depots will be kept topped off for multiple uses.

It's not clear if the plural means finite lifetime with replacement or multiple depots on orbit at the same time. I'd go with multiple on orbit with the understanding that not right from the get go.

[Robotbeat]

Pretty interesting. So they’d be able to use really lightweight missions that would normally launch on Falcon 9 (or even Falcon 1), offer it for basically the marginal cost of launch, then the “profit” would be propellant they can load into a depot for other missions.

[wes_wilson]

Pretty interesting. So they’d be able to use really lightweight missions that would normally launch on Falcon 9 (or even Falcon 1), offer it for basically the marginal cost of launch, then the “profit” would be propellant they can load into a depot for other missions.

That's slick.