Starship On-orbit refueling - Options and Discussion

[Robotbeat]

I also think that the cost of just settling the propellant with thrusters is not a big deal, either. Especially as you can thrust in a direction to increase the energy of your orbit.

By my calcs, linear acceleration is actually significantly worse than the spin-G option.

Check my math:
F= ma (where a is 0.0098m/s² for 1 milli-G)
Let's say its 3180 ton to accelerate (3000 tons of depot, plus 180 ton HLS).
F = 31,164 N
at an Isp of 300 that's 10.6kg/s mass flow rate
Depending how long it takes to fill HLS (30-90 minutes) it will cost at least 19 tons of prop (for 30 min transfer), and you're obviously not doing the entire transfer at periapsis, so it's not a very efficient method of raising the orbit. Whether that's a deal-breaker or not I'm not sure.

you can do it with about 1/10,000th of a gee according to ULA’s work, and there’s no reason it has to take longer to transfer from one tank to another than to empty an entire super heavy booster into its engines.

And actually you CAN choose to do the transfer at perigee. That you arbitrarily just decided you can’t is not convincing. Why intentionally do it in an inefficient way??

[Robotbeat]

Doing rapid prop transfer while under ullage thrust near perigee is essentially like a weird version of cross feed. Cross feed has been studied extensively, including by SpaceX.

[Paul451]

the maximum amplitude of the swing is 3.2° to either side of the vertical, with a period of 1 min 55 seconds. This will not go away by itself

Why won't it go away? Won't tidal effects remove the sway? That's what causes the vertical stability in the first place.

Aside: Another type of unwanted motion would be twist. Essentially each end acts independently like a spring pendulum, and since the cables are the spring, there won't be a lot of dampening, and any dampening requires the motion to change from rotation to wave/whip and linear/bounce, creating weird vibration modes (autoparametric resonance?) AIUI, that's why trusses are preferred, or any combined compression/tension structure.

[mikelepage]

I also think that the cost of just settling the propellant with thrusters is not a big deal, either. Especially as you can thrust in a direction to increase the energy of your orbit.

By my calcs, linear acceleration is actually significantly worse than the spin-G option.

Check my math:
F= ma (where a is 0.0098m/s² for 1 milli-G)
Let's say its 3180 ton to accelerate (3000 tons of depot, plus 180 ton HLS).
F = 31,164 N
at an Isp of 300 that's 10.6kg/s mass flow rate
Depending how long it takes to fill HLS (30-90 minutes) it will cost at least 19 tons of prop (for 30 min transfer), and you're obviously not doing the entire transfer at periapsis, so it's not a very efficient method of raising the orbit. Whether that's a deal-breaker or not I'm not sure.

you can do it with about 1/10,000th of a gee according to ULA’s work, and there’s no reason it has to take longer to transfer from one tank to another than to empty an entire super heavy booster into its engines.

Er, what? Pretty sure the raptor turbopumps are powered by combustion of prop, at >1 gravity of acceleration. In the absence of that it's going to go slower. And if it takes ~30 min for the GSE to do it, I think you're being pretty optimistic if you think it will be faster in space.

If ULA has shown that 10^-4 gees is sufficient for settling, then great. But in that case the spin-G solution also gets cheaper on prop. Once you have the lever arm the length of two starships joined nose to nose, the prop costs of spin-G transfer are 1-2 orders of magnitude less for a given acceleration level.

And actually you CAN choose to do the transfer at perigee. That you arbitrarily just decided you can’t is not convincing. Why intentionally do it in an inefficient way??

I could have been clearer, I was just saying that a 30-90 minute prop transfer will take up a substantial proportion of an orbit in LEO, if not the entire orbit. You could do the transfer in several steps to maximise Oberth benefit by only transferring when you're close to perigee. It just introduces a different set of inefficiencies to be starting and stopping the process each time the depot comes around.

[Greg Hullender]

the maximum amplitude of the swing is 3.2° to either side of the vertical, with a period of 1 min 55 seconds. This will not go away by itself

Why won't it go away? Won't tidal effects remove the sway? That's what causes the vertical stability in the first place.

This is where we'd need a good way to model energy lost due to sloshing and to changing tension on the cable. I think that means finding the dissipation function and then computing the Lagrangian with that. Without a dissipation function, the sway will never go away.

Aside: Another type of unwanted motion would be twist. Essentially each end acts independently like a spring pendulum, and since the cables are the spring, there won't be a lot of dampening, and any dampening requires the motion to change from rotation to wave/whip and linear/bounce, creating weird vibration modes (autoparametric resonance?) AIUI, that's why trusses are preferred, or any combined compression/tension structure.

Yes, I haven't modeled twist yet. (I was wondering when someone would bring that up.) Since my model is strictly one-dimensional, it can rotate freely about the long axis. I was thinking of adding some solar panels to both depots to fix that problem and to see how much that stabilizes things.

Having three cables, spaced as widely apart as possible, with plates every 100 m will make it more rigid (and more like a truss), but, again, we need to know the dissipation function to really model it.

Again, though, the thing needs station-keeping thrusters anyway, and since we're talking fairly low-frequency movements, they ought to be able to take care of such things.

A larger question--one that bothered the guys on Reddit's /r/askengineers was the very low orbit, which is going to require quite a bit of thrusting to keep the depots up. You could put the ensemble at, say, 500 km without adding hugely to the refueling costs, but the MMOD risk will be a lot higher. (Such a huge trade-space!)

[TheRadicalModerate]

Can somebody explain to me how the target Starships are going to do RPOD with a pair of Starships dangling on the end of a rotating tether?


A couple of times up-thread, I've mentioned the possibility of minimizing ullage thrust by using propellant management devices, but I haven't gotten anybody to take the bait yet.

The idea is you settle the prop intermittently, relying on the PMDs to keep a big enough glob of prop near the inlet that transfer can occur unaccelerated.  You'd probably need multiple cycles of ullage burn / PMD capture / transfer to get a full load transferred, but it would still be a huge savings in prop, without the complexity of anything rotating.

[mikelepage]

Can somebody explain to me how the target Starships are going to do RPOD with a pair of Starships dangling on the end of a rotating tether?

To be clear, Greg's gravity tether has the same rotation rate as the ISS (once per orbit), and mine expends propellant to stop spinning for the purpose of RPOD. Not sure if you were conflating the two proposals, but I don't really see the problem with either as far as nominal RPOD goes. I just think the tether causes problems with potential failure modes.

A couple of times up-thread, I've mentioned the possibility of minimizing ullage thrust by using propellant management devices, but I haven't gotten anybody to take the bait yet.

The idea is you settle the prop intermittently, relying on the PMDs to keep a big enough glob of prop near the inlet that transfer can occur unaccelerated.  You'd probably need multiple cycles of ullage burn / PMD capture / transfer to get a full load transferred, but it would still be a huge savings in prop, without the complexity of anything rotating.

Great if it works, but it sounds pretty hand-wavy to me. My understanding is that PMDs use surface tension to hold prop together, which necessarily means there will be lots more surface area, which adds mass and/or slows down prop flow rate - in a vehicle which still has to get to orbit in the first place.

All this to avoid the "complexity" of rotation. When rotation is inconvenient, we can avoid it by stopping the rotation.

In the setup I'm proposing, you have two depots docked nose to nose, oriented end-on to the sun for the vast majority of the time. Tanker arrives, docks, whole assembly spins up to 0.16rpm* using a few hundred kg prop, transfers propellent for 30 minutes, then the whole assembly slows down and stops using a few hundred kg prop. Tanker leaves. Whole process is done in a couple of hours or so. The rest of the time, the pair of depots stays end on to the sun.

If you really cared about keeping everything balanced, you could fill the depots up with tankers, two by two. As we were seeing in the "replace SLS with starship/dragon" thread, having two depots full of prop helps for most scenarios anyway.

*side note: Gemini 11's tether experiment spin rate was up to 0.15rpm. Greg, check out the wikipedia article as to how the gravity gradient wasn't able to keep a 30m tether taught.

[Oersted]

Count me among those who think you shouldn't deal with all the potentially chaotic motions a tether could induce. I have much more confidence in a truss structure if prop transfers are done centrifugally.

And here's a thought. Maybe Starship can actually launch with such a nose-mounted truss? With the gridfins SpaceX has already shown that aeroloads are not a significant factor on launch. Why not simply launch a depot version of Starship with a long truss already in place on the nose of the ship? With long I mean just about the length of the ship.

Such a truss would come with a grapple/docking fixture already attached. Minimal assembly required.

Musk once said that tanker versions would look really weird, or words to that effect. Maybe this is what he was talking about. Mind you, not very probable, but just throwing the idea out there.

[Twark_Main]

Can somebody explain to me how the target Starships are going to do RPOD with a pair of Starships dangling on the end of a rotating tether?


A couple of times up-thread, I've mentioned the possibility of minimizing ullage thrust by using propellant management devices, but I haven't gotten anybody to take the bait yet.

The idea is you settle the prop intermittently, relying on the PMDs to keep a big enough glob of prop near the inlet that transfer can occur unaccelerated.  You'd probably need multiple cycles of ullage burn / PMD capture / transfer to get a full load transferred, but it would still be a huge savings in prop, without the complexity of anything rotating.

There's interesting software and control theory here.

I don't picture this problem like "settling." It's more like herding a very-slightly-sticky blob floating in zero-g, using a "magician's cup" that just happens to fully enclose the fluid. If it helps you envision the manipulation task in your hands, picture the tank being transparent, non-cryogenic, and shoebox sized. How would you do it?

Just blindly pushing the tank in one direction seems inefficient. The tank will build up some speed, then smack into the blob jostling it everywhere, needing lots of time under ullage to settle back down.. You really want to move the tank "around" the blob, and then (instead of smacking into it at speed) gently slowing and holding the tank in a location where the PMD is wicking the blob.

I expect retrothrust wouldn't cancel 100% of your initial kick, due to some viscous momentum transfer into the blob. And of course there can be multiple blobs, bubbles, etc.

So the optimal settling algorithm might paradoxically include prograde thrust (to move toward the blob) followed by retrograde thrust (controlled velocity ramp based on blob geometry, to gently 'dock' the PMD to the blob), followed by more prograde thrust (to let bubbles separate out). 

Estimating and optimally controlling this, based on IMUs and in-tank cameras, sounds like a fun problem.

[TheRadicalModerate]

Can somebody explain to me how the target Starships are going to do RPOD with a pair of Starships dangling on the end of a rotating tether?

To be clear, Greg's gravity tether has the same rotation rate as the ISS (once per orbit), and mine expends propellant to stop spinning for the purpose of RPOD. Not sure if you were conflating the two proposals, but I don't really see the problem with either as far as nominal RPOD goes. I just think the tether causes problems with potential failure modes.

I was indeed conflating them, and that's wrong.  You're right that the RPOD in your nose-to-nose situation is no different than any other target-to-depot RPOD.

However, you're still going to have some interesting dynamics when you pump prop, because the CoG is going to change.  When the system is rotating, a CoG change applies a torque, which will cause the angular momentum vector to precess, which will make the system tumble out-of-plane.  That may or may not be a deal-breaker, but it certainly needs to be analyzed.  It's not a corner case; it'll happen with every single prop transfer, in either direction.

As for the tethered version:  the CoG of the tethered system is halfway between the two depots.  That's the spot that dictates the orbital speed of the system.  So, if the system has been tidally stabilized (i.e., the rotation rate of the system is equal to the orbital period), then the lower depot is moving faster than a free-flying body at that altitude, and the upper depot is moving slower.  That means that the free-flying body (aka the target Starship) has to accelerate all the way to docking, which sounds like a terrible way to run prox ops.

Furthermore, if you change the CoG of a tethered system by adding the mass of the target Starship, all kinds of bad stuff is going to happen.

As for the ISS:  its nadir-to-zenith length is only about 20m, and the docking ports are near the CoG.  Not the same problem.

A couple of times up-thread, I've mentioned the possibility of minimizing ullage thrust by using propellant management devices, but I haven't gotten anybody to take the bait yet.

The idea is you settle the prop intermittently, relying on the PMDs to keep a big enough glob of prop near the inlet that transfer can occur unaccelerated.  You'd probably need multiple cycles of ullage burn / PMD capture / transfer to get a full load transferred, but it would still be a huge savings in prop, without the complexity of anything rotating.

Great if it works, but it sounds pretty hand-wavy to me. My understanding is that PMDs use surface tension to hold prop together, which necessarily means there will be lots more surface area, which adds mass and/or slows down prop flow rate - in a vehicle which still has to get to orbit in the first place.

PMDs don't affect flow rate.  All they do is ensure that something aggregates the prop near the inlets.

Regular payload-carrying Ships wouldn't need them, because the prop always goes into them.  Tankers, however, would need them.  That'll reduce prop to orbit by a bit.  So you'd have to do the full-up calculation on how much prop you lose from ullage acceleration vs. the mass penalty of the PMDs.

I don't think there's a prayer of a rotational solution.  I'd be happy to be wrong, but I don't think I am.

[Twark_Main]

However, you're still going to have some interesting dynamics when you pump prop, because the CoG is going to change.  When the system is rotating, a CoG change applies a torque, which will cause the angular momentum vector to precess, which will make the system tumble out-of-plane.  That may or may not be a deal-breaker, but it certainly needs to be analyzed.  It's not a corner case; it'll happen with every single prop transfer, in either direction

For a transfer end-to-end the torque will be entirely in-plane, so all that happens is the axis shifts to the new "see-saw" balance point. It's not like we're transferring fuel between two adjacent ships, with one above and one below the spin axis normal plane.

[ThatOldJanxSpirit]

This LinkedIn post has started to do the rounds on social media.

https://x.com/spacesudoer/status/1915767110309171681?s=46

I can’t verify the reliability of the source, but it could be considered evidence that SpaceX has started development of turbo pumps for propellant transfer.

[Narnianknight]

This LinkedIn post has started to do the rounds on social media.

https://x.com/spacesudoer/status/1915767110309171681?s=46

I can’t verify the reliability of the source, but it could be considered evidence that SpaceX has started development of turbo pumps for propellant transfer.

SpaceX is not using pumps for propellant transfer. This is probably for waist landing engines or something else.

[ThatOldJanxSpirit]

This LinkedIn post has started to do the rounds on social media.

https://x.com/spacesudoer/status/1915767110309171681?s=46

I can’t verify the reliability of the source, but it could be considered evidence that SpaceX has started development of turbo pumps for propellant transfer.

SpaceX is not using pumps for propellant transfer. This is probably for waist landing engines or something else.

My initial thought too, but the phrases ‘all starship missions beyond low earth orbit’ and ‘most exiting project at SpaceX’ seem a bit of a stretch for the lunar landing engines. I really can’t think what the something else would be.

[rsdavis9]

Hot thrusters would satisfy both of those needs. Fuel efficient attitude control and also when ganged together(Lots of them) lunar landing waist thrusters.

[Twark_Main]

This LinkedIn post has started to do the rounds on social media.

https://x.com/spacesudoer/status/1915767110309171681?s=46

I can’t verify the reliability of the source, but it could be considered evidence that SpaceX has started development of turbo pumps for propellant transfer.

SpaceX is not using pumps for propellant transfer. This is probably for waist landing engines or something else.

Also even if they do, they wouldn't use a turbopump, just an electric motor driving a pump.

[TheRadicalModerate]

...it could be considered evidence that SpaceX has started development of turbo pumps for propellant transfer.

A turbopump (i.e., a centrifugal pump driven by a gas turbine) seems like swatting a fly with a nuclear weapon.  However:

This is probably for waist landing engines or something else.

I've been thinking that waist engines would probably be pressure-fed, but a single electric pump feeding a bunch of combustion chambers makes a lot of sense, and it's much more mass-efficient.

Let's find out:  Back-of napkin on pump power, assuming that you want a common design for both lunar and martian waist thrusters:

Isp=300s
Chamber pressure at injector plates = 200bar, so... 220bar at the pump outlet?
Thrust needed for Mars:  (150t dry + 100t payload) * 3.72m/s² = 930kN.
Thrust needed for Moon: (120t dry + 160t prop + 100t payload) * 1.62m/s² = 620kN.
Mars mass flow (mdot) = 316kg/s
Mars methane mdot = 69kg/s
Mars LOX mdot = 247kg/s

If I did the algebra right:

pumpPower = Δpressure*mDot/(density*efficiency)

If we ignore the inlet pressure to the pump and use 80% for the efficiency:

methanePumpPower = (22,000,000Pa * 69kg/s) / (422.8kg/m³ * 80%) = 4488kW
loxPumpPower = (22,000,000Pa * 247kg/s) / (1141kg/m³ * 80%) = 5953kW
Total power = 10,441kW

Figure that the waist thrusters need to be able to fire for 15s = 0.0042h, and you need a battery capable of storing 43.5kWh of energy.  Lithium-ion batteries have a specific energy of about 270Wh/kg, so we're looking at 161kg of batteries to run the system.

In practice, the Starship power system is going to have batteries that are shared across a bunch of different loads.  I suspect that the waist thrusters aren't the biggest energy hog in the system (that would be the electric motors driving the elonerons during entry and descent), so the extra battery mass might be less than 161kg.  (Or not:  the waist thrusters have to fire immediately after the elonerons are finished doing their thing.)

You can use the same architecture for prop transfer, for hot gas ullage thrusters, and for attitude thrusters.  You may need different pumps for the different applications, because of the wildly different pressure differentials, mass flow rates, and location on the vehicle:

Prop transfer:
2150t (block 3 depot guess) in, say, an hour = 597kg/s (130kg/s LCH4, 467kg/s LOX)
Pressure differential is really close to zero, as long as you tie the ullage spaces of the source and target together.  Say 10kPa?

methaneXferPumpPower = (10,000Pa * 130kg/s) / (422.8kg/m³ * 80%) = 3840W
loxXferPumpPower = (10,000Pa * 467kg/s) / (1141kg/m³ * 80%) = 5120W
Battery mass equivalent = 33kg


Ullage thrusters:
Fully loaded depot mass = 120t dry + 2530t prop (boiling) = 2650t
Target Starship mass = 150t dry + 200t payload = 350t
System mass = 3000t
Ullage acceleration = 0.001m/s²
Thrust = 3kN
Mass flow @ Isp=300s = 1.02kg/s (0.22kg/s LCH4, 0.80kg/s LOX)
Pressure differential = 220bar

methanePumpPower = (22,000,000Pa * 0.22kg/s) / (422.8kg/m³ * 80%) = 14kW
loxPumpPower = (22,000,000Pa * 0.80kg/s) / (1141kg/m³ * 80%) = 19kW
Total power = 33kW
Battery mass equivalent = 122kg


Attitude control thrusters are harder to figure out.  You need to know your max required angular rates, the x-y-z moments of inertia, and the duty cycle.  My guess is that the ullage thrusters and the attitude control thrusters can be the same, and you probably need a duty cycle of 2500 thruster-seconds between battery charges.

The other problem with attitude control is that you need the system armed for long periods of time, which requires keeping it pressurized.  This is not a good application for an electric pump that's pumping liquid.

That might be a good reason to use COPV-base pressurant.  The best way to do that is to flow LCH4 or LOX into a COPV, seal it, and heat it to criticality, something like 500bar.  That only requires the tiniest of pumps. 

But you might want to use an electric pump to backfill liquid prop into the system, under pressure, as the thrusters fire.  That gets... complicated, but it keeps the COPVs very small.

[InterestedEngineer]

...it could be considered evidence that SpaceX has started development of turbo pumps for propellant transfer.

A turbopump (i.e., a centrifugal pump driven by a gas turbine) seems like swatting a fly with a nuclear weapon.  However:

This is probably for waist landing engines or something else.

Figure that the waist thrusters need to be able to fire for 15s = 0.0042h, and you need a battery capable of storing 43.5kWh of energy.  Lithium-ion batteries have a specific energy of about 270Wh/kg, so we're looking at 161kg of batteries to run the system.

First, the 270Wh/kg is for the 100% to 0% range, nobody runs lithium batteries that way.  They degrade quickly when you charge them past 80% and you want the low side to be 20% so there's some spare.

Practically speaking the energy density of Lithium-ion is half of their rated spec (60% if you want to be picky).

Second, 10MW is quite a lot of output, for a 800V battery that's 12,500 amps.   Batteries can't sustain that kind of output.  A Tesla under max load is 360kW (450A, 480HP).  So you've far exceeded the maximum output of modern batteries.

In practice, the Starship power system is going to have batteries that are shared across a bunch of different loads.  I suspect that the waist thrusters aren't the biggest energy hog in the system (that would be the electric motors driving the elonerons during entry and descent), so the extra battery mass might be less than 161kg.  (Or not:  the waist thrusters have to fire immediately after the elonerons are finished doing their thing.)

I agree you'd share, which mitigates the battery density problem, but the biggest hog on max output isn't going to be elonerons, it's going to be 10MW worth of pumps.

I haven't addressed the 80% efficiency yet, it's probably more like 60%.

[KilroySmith]

First, the 270Wh/kg is for the 100% to 0% range, nobody runs lithium batteries that way.  They degrade quickly when you charge them past 80% and you want the low side to be 20% so there's some spare.

Practically speaking the energy density of Lithium-ion is half of their rated spec (60% if you want to be picky).

While what you say is true for an EV battery where you'd like to minimize the capacity loss over 10 years and 200,000 miles of driving, it doesn't really apply to spacecraft.  Maintaining the battery at 50% between uses, charging to 100% when needed, and discharging down to 5% (leaving a small safety margin) would work fine for long-lived orbital craft.

The bigger issue is that 270 Wh/kg is for bleeding-edge consumer products, not space rated, derated batteries.  Taking that into account, I might buy into your 60% derating.  But this IS SpaceX, and they might very well use lightly modified consumer batteries for non-human-spaceflight purposes simply because it's easier and cheaper.

  Using 95% of the battery (100->5%, leaving a small safety margin) would work fine for space use, especially if you left the batteries at 50% for any long-duration quiescent periods, and only fully charged when necessary. 

[InterestedEngineer]

Using this equation

pumpPower = Δpressure*mDot/(density*efficiency)

If we ignore the inlet pressure to the pump and use 80% for the efficiency:

methanePumpPower = (22,000,000Pa * 69kg/s) / (422.8kg/m³ * 80%) = 4488kW
loxPumpPower = (22,000,000Pa * 247kg/s) / (1141kg/m³ * 80%) = 5953kW
Total power = 10,441kW

and knowing that a single battery system can output about 400kW max, we can back calculate the flow rate for 22MPa

mDot = pumpPower*density*efficiency/Δpressure

mDot = 400kW * 1141kg/m³  * 60% / 22,000,000Pa = 12.4 kg/sec (LOX)

Which would be 3.4 kg/sec of Methane (at slightly less power)

F = mdot * v = 42.2 kg/sec * 2800m/sec = 118kN.

So each full size Tesla Battery on board can pump methalox up enough to get 118kN of thrust.  Assuming we are running 4+1 redundancy we have 236kN of thrust.  At 1.5kW/kg that's 1.3t of batteries.

for a 250t Starship, that's 1 m/sec of acceleration.  Not enough for the Moon, let alone Mars.

Plenty for maneuvering thrusters.