Starship On-orbit refueling - Options and Discussion

[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!

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.

It's not the your tension-cable-snapping-hazard peanut butter is more "exotic" than my thin-mission-critical-pressurized-tin-can chocolate. It's that we don't have any experience (flight heritage) mixing the two.

You can use your same logic to casually dismiss this entire thread, saying "I like how we're talking about pumping liquid as if it's more exotic than rockets." With an example that makes it so obvious, can you now see the problem with this line of reasoning?   

[dondar]

 cables under load produce torque. "We" don't see it because we live in significant gravity well.

[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.

It's not the your tension-cable-snapping-hazard peanut butter is more "exotic" than my thin-mission-critical-pressurized-tin-can chocolate. It's that we don't have any experience (flight heritage) mixing the two.

You can use your same logic to casually dismiss this entire thread, saying "I like how we're talking about pumping liquid as if it's more exotic than rockets." With an example that makes it so obvious, can you now see the problem with this line of reasoning?   

False. It was done on Gemini 11.

The issue isn’t to say there aren’t engineering issues. Every bridge needs proper engineering. The issue is acting like it’s super exotic and shouldn’t be touched.

[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!

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.

It's not the your tension-cable-snapping-hazard peanut butter is more "exotic" than my thin-mission-critical-pressurized-tin-can chocolate. It's that we don't have any experience (flight heritage) mixing the two.

You can use your same logic to casually dismiss this entire thread, saying "I like how we're talking about pumping liquid as if it's more exotic than rockets." With an example that makes it so obvious, can you now see the problem with this line of reasoning?   

False. It was done on Gemini 11.

I almost mentioned Gemini 11 too (I edited and took it out), figuring it would be obviously eliminated from a discussion of high tension lines.

Even at 1 milli-G you're looking at 15-20 tonnes of tension in the line. Don't stand behind that when it goes!!

The issue isn’t to say there aren’t engineering issues. Every bridge needs proper engineering. The issue is acting like it’s super exotic and shouldn’t be touched.

Then that "proper engineering" needs to be built into the estimated cost of this proposal. People are acting like tethers are free (or just doing elementary tension calculations), which was the original issue.

I expect the total system mass ends up being 10-100x the mass you calculate just looking at tension and material strength by itself, and maybe more.

[Greg Hullender]

I posted a query to Reddit's /r/AskEngineers, since I'd seen discussions on "snapback" there in the past. Here's what I said:

Summary: I'm part of a lengthy discussion about challenges of in-orbit refueling, where I have a proposal for something that involves a pair of fuel depots, massing about 3,000 metric tons each, connected by a cable (presumably 19 mm wire rope) about 6 km long with a maximum of 36 kN of tension on it. Others have raised the objection that if a micrometeoroid strikes the cable it will snap and the snapback will destroy one or both depots. What is a sensible engineering solution to mitigate this problem?

Details: One challenge of in-orbit refueling is to settle the cryogenic propellants so the liquid part goes to the bottom of the tanks and the gas (aka "ullage") goes to the top. This only requires an acceleration of about 1 mm/s^2, but it needs to be sustained for the duration of fueling. We know that SpaceX is planning to use "ullage burns" to accomplish this, but that requires venting cold gas or firing a little rocket for extended period of time.

My proposal was to connect two depots with a cable and let tidal forces do the ullage settling for free. That is, a line from the center of the Earth always passes through both depots and along the cable, so the imbalance between gravity and centrifugal force creates a small tidal acceleration away from the center in both depots. Note that SpaceX already needs to fill two depots, so the extra depot isn't an extra cost.

I've computed that at an orbital height of 287 km (where SpaceX plans to put their depots), if an empty depot has 150 metric tons of mass and a full one has 3000, then the cable needs to be 6 km long to guarantee at least 1 mm/s^2 in the full depot. Given that length, maximum tension is when both depots are full and comes to 36 kN. A single wire rope of 19 mm thickness should handle this, at a cost of about 35 tons, but, obviously, you'd want more than one cable, give a single hit could sever it. I envisioned three cables in a well-spaced equilateral triangle, since even a very lucky hit wouldn't hit more than two of them at once. Or run more cables to mitigate against another hit while you're in the process of replacing the one(s) that got hit. And probably have a regular schedule to replace cables every few years.

The objection has been raised that the snapback from a severed cable could puncture one or both of the two depots. Searching online, I see lots of concern about snapback, but most of the mitigation seems to revolve around keeping the cable from snapping in the first place. I don't think that's viable in this case.

So what is the best way to mitigate this risk? Is there anything comparable in terrestrial engineering?

N.B. I double-checked some of the numbers, so, to the extent that this disagrees with numbers I had before, this is correct.

So far, I think the most interesting suggestion was to use a type of cable where snapback essentially doesn't happen. Assuming such a thing is possible.

[mikelepage]

The 250m cable setup with two depots is importing many of the considerations from the spin gravity discussion, just to make use of the gravity gradient. If we're going to go there, I think it's worth comparing with the alternative where we eliminate the cable and spin the assembly:

Two 50m depots *docked* nose to nose, can spin once every 6 minutes 15s (0.16 rpm) to achieve >1mm/s acceleration in the relevant tanks (r=35m). The starship being filled/emptied, could hang off the side of one of the depots, weighing 11310N when full of propellant (~1.2 tons). If this third ship was in the spin plane, this should also negate any intermediate axis issues.

Approximating the whole setup to a 100m long cylinder, 2x 400N Draco thrusters at each end could achieve this spin rate, with a 18 minute burn, using 584kg of prop. (obviously it will be the Starship equivalent thrusters, but I don't think we have Isp for them yet)

Attached: a modified version of my spreadsheet from the rotational gravity thread.

[rsdavis9]

The 250m cable setup with two depots is importing many of the considerations from the spin gravity discussion, just to make use of the gravity gradient. If we're going to go there, I think it's worth comparing with the alternative where we eliminate the cable and spin the assembly:

Two 50m depots *docked* nose to nose, can spin once every 6 minutes 15s (0.16 rpm) to achieve >1mm/s acceleration in the relevant tanks (r=35m). The starship being filled/emptied, could hang off the side of one of the depots, weighing 11310N when full of propellant (~1.2 tons). If this third ship was in the spin plane, this should also negate any intermediate axis issues.

Approximating the whole setup to a 100m long cylinder, 2x 400N Draco thrusters at each end could achieve this spin rate, with a 18 minute burn, using 584kg of prop. (obviously it will be the Starship equivalent thrusters, but I don't think we have Isp for them yet)

Attached: a modified version of my spreadsheet from the rotational gravity thread.

Yes this is what I have been trying to promote in numerous (>1) posts and haven't gotten any traction(recognition).

I think with slow rotation that it should be able to dock the receiving vehicle while maintaining the spin of the depot pair.

[Greg Hullender]

The 250m cable setup with two depots is importing many of the considerations from the spin gravity discussion, just to make use of the gravity gradient. If we're going to go there, I think it's worth comparing with the alternative where we eliminate the cable and spin the assembly:

What's nice about the gravity-gradient approach is that it's very stable, and, since it turns so slowly (once per orbit, so once in 90 minutes), it's quite easy to dock with. Gravity stabilization has already been used in other missions, so the technology isn't completely new.

The trouble with a rotating assembly that's not gravitationally stabilized is that it's not inherently stable. Among other things, the center of mass will move dramatically during fueling and unloading. No one has ever made anything like it work. (Not that I know of, anyway.) It's something someone might do someday, but not very soon. Not unless all else fails.

I think with slow rotation that it should be able to dock the receiving vehicle while maintaining the spin of the depot pair.

In the gravity-gradient case, I've computed the Lagrangian and simulated it numerically. Docking has zero immediate effect on the spin rate, but it does make the orbit slightly elliptical and changes the period, which results in a small amount of swinging. But even for the case of a 3000 ton depot disconnecting from an empty depot, the amplitude is microradians. Something easily cleaned up during the regular altitude boosts.

[mikelepage]

What's nice about the gravity-gradient approach is that it's very stable, and, since it turns so slowly (once per orbit, so once in 90 minutes), it's quite easy to dock with. Gravity stabilization has already been used in other missions, so the technology isn't completely new.

The trouble with a rotating assembly that's not gravitationally stabilized is that it's not inherently stable. Among other things, the center of mass will move dramatically during fueling and unloading. No one has ever made anything like it work.

The gravity gradient might be stabilizing, but it's incredibly weak relative to the inertias involved.

To be fair, this is profoundly unintuitive territory, so feel free to tell me why the following analogy doesn't apply, but picture this: 2x 3000 ton locomotives, 250m apart on a perfectly straight, level track with no wind. Rolling resistance is low enough that with one or a few 8mm steel cables, one locomotive can accelerate the other at 1 mill-G. But both locomotives have large reserves of near boiling liquid, and the smallest amount of force either locomotive can apply is the same order of magnitude as the tension on the cable (Draco engines at 400N ~= the tension on the cable).

It's precisely because of the fact these are not rigid bodies that cable solutions get shot down on the spin gravity threads whenever they come up. The slightest movement within either body can cause liquid sloshing/jerks to the tension on the cable. The stabilising force of the gravity gradient is nowhere near strong enough to not want to have a solid coupling between two vessels that big.

Yes this is what I have been trying to promote in numerous (>1) posts and haven't gotten any traction(recognition).

I think with slow rotation that it should be able to dock the receiving vehicle while maintaining the spin of the depot pair.

Glad you agree, but I'd strongly recommend against trying to dock to a spinning depot, when approach & docking procedures typically take (at least?) an hour, and the propellant required to do a spin-up/spin-down cycle is a rounding error on the amount of propellant being transferred. It's an extra complication that isn't necessary.

[rsdavis9]

Glad you agree, but I'd strongly recommend against trying to dock to a spinning depot, when approach & docking procedures typically take (at least?) an hour, and the propellant required to do a spin-up/spin-down cycle is a rounding error on the amount of propellant being transferred. It's an extra complication that isn't necessary.

haven't done the calc to compare. Glad to hear that it is "a rounding error"

[DanClemmensen]

Yes this is what I have been trying to promote in numerous (>1) posts and haven't gotten any traction(recognition).

I think with slow rotation that it should be able to dock the receiving vehicle while maintaining the spin of the depot pair.

Glad you agree, but I'd strongly recommend against trying to dock to a spinning depot, when approach & docking procedures typically take (at least?) an hour, and the propellant required to do a spin-up/spin-down cycle is a rounding error on the amount of propellant being transferred. It's an extra complication that isn't necessary.

How do you dock to a spinning target, even at very low rotation rates? For each of the two spacecraft, if the port is not on the axis of rotation, then at least one of the two spacecraft will need to translate continuously, not just rotate. The only way I know of to dock without  continuous translation is to rotate both spacecraft about an axis that is perpendicular to the docking plane and centered on the dock, but this requires that each spacecraft can adjust its CoM to be on that axis. How do you do this?

[mikelepage]

Glad you agree, but I'd strongly recommend against trying to dock to a spinning depot, when approach & docking procedures typically take (at least?) an hour, and the propellant required to do a spin-up/spin-down cycle is a rounding error on the amount of propellant being transferred. It's an extra complication that isn't necessary.

haven't done the calc to compare. Glad to hear that it is "a rounding error"

I did the calc in this post above when I attached the spreadsheet. 584kg on 150 tons transferred is 0.4%. A rounding error. And that’s for the extreme case with two 3000 ton depots.

How do you dock to a spinning target, even at very low rotation rates?

Although rsdavis9 was suggesting docking to a slowly rotating target, I was recommending against it for the reasons you mention, and having calculated that the thrust required to spin up or down isn’t worth worrying about. Just dock as normal and start and stop the spin each time.

[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.

[BN]

Glad you agree, but I'd strongly recommend against trying to dock to a spinning depot, when approach & docking procedures typically take (at least?) an hour, and the propellant required to do a spin-up/spin-down cycle is a rounding error on the amount of propellant being transferred. It's an extra complication that isn't necessary.

haven't done the calc to compare. Glad to hear that it is "a rounding error"

I did the calc in this post above when I attached the spreadsheet. 584kg on 150 tons transferred is 0.4%. A rounding error. And that’s for the extreme case with two 3000 ton depots.

How do you dock to a spinning target, even at very low rotation rates?

Although rsdavis9 was suggesting docking to a slowly rotating target, I was recommending against it for the reasons you mention, and having calculated that the thrust required to spin up or down isn’t worth worrying about. Just dock as normal and start and stop the spin each time.

currently cost per kg to orbit via falcon 9 is estimated at ~$4,000.

584kg x $4,000 = $2,336,000

oof. not sure what the cost will be for starship long term, but that's still going to be a lot of money.

[Robotbeat]

F9’s internal cost is $1000/kg. Starship is hoped to eventually be $10/kg. So less than $6k.

[Greg Hullender]

To be fair, this is profoundly unintuitive territory, so feel free to tell me why the following analogy doesn't apply . . .

I don't think the locomotive analogy is helpful, since that problem is dominated by friction. Are you trying to argue that there will be failure modes in which the cables go slack? Given the masses involved, that will take a good bit of force. That force must come from somewhere. Short of an out-of-control docking accident, I don't see where that kind of force is coming from. Again, there's no friction here, so your intuition really doesn't apply. Do you have papers that analyze this? I've found lots of papers about the instability of tethers, but they always make an exception for the gravity-gradient case.

It's precisely because of the fact these are not rigid bodies that cable solutions get shot down on the spin gravity threads whenever they come up. The slightest movement within either body can cause liquid sloshing/jerks to the tension on the cable. The stabilizing force of the gravity gradient is nowhere near strong enough to not want to have a solid coupling between two vessels that big.

I'll admit that, up until now, I've been modeling both depots as point masses. I'm going to have to give some thought to the question of how sloshing might affect things. Just a quick survey of papers makes me think sloshing will actually tend to damp out whatever harmonic motion there is. A lot, of course, depends on what force causes the sloshing. And again, good papers would help.

[Greg Hullender]

How do you dock to a spinning target, even at very low rotation rates? For each of the two spacecraft, if the port is not on the axis of rotation, then at least one of the two spacecraft will need to translate continuously, not just rotate. The only way I know of to dock without  continuous translation is to rotate both spacecraft about an axis that is perpendicular to the docking plane and centered on the dock, but this requires that each spacecraft can adjust its CoM to be on that axis. How do you do this?

In the gravity-gradient case, the upper depot is always at the perigee of a slightly elliptical orbit and the lower depot is always at the apogee of one. Docking with either one should be no more difficult that docking with a satellite in such an orbit.

[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.

[mikelepage]

To be fair, this is profoundly unintuitive territory, so feel free to tell me why the following analogy doesn't apply . . .

I don't think the locomotive analogy is helpful, since that problem is dominated by friction. Are you trying to argue that there will be failure modes in which the cables go slack? Given the masses involved, that will take a good bit of force. That force must come from somewhere. Short of an out-of-control docking accident, I don't see where that kind of force is coming from. Again, there's no friction here, so your intuition really doesn't apply. Do you have papers that analyze this? I've found lots of papers about the instability of tethers, but they always make an exception for the gravity-gradient case.

No papers I can refer to unfortunately, but more or less, yes.

I don't think the cables don't have to go slack for sloshing to be a major problem though. My understanding is that OMS systems don't have any kind of throttling capability, and those thrusters are just on or off, and their force will be a significant fraction of the total weight of the vessel at 1 milli-G.

Say you have a thruster valve that sticks in the open position for a couple of extra seconds and induces any kind of pendulum or twisting motion relative to the cable (i.e. orthogonal to the stabilizing force of the gradient). >95% of the mass of the vessel is now sloshing around chaotic pendulum style. Difficult enough to get that under control without also being worried about snapping the cable.

[Greg Hullender]

Say you have a thruster valve that sticks in the open position for a couple of extra seconds and induces any kind of pendulum or twisting motion relative to the cable (i.e. orthogonal to the stabilizing force of the gradient). >95% of the mass of the vessel is now sloshing around chaotic pendulum style. Difficult enough to get that under control without also being worried about snapping the cable.

Before we try modeling sloshing, let's just look at the effect on the pendulum motion. That means the cable is massless, and the depots and tankers are point masses. Worst case is with empty depots, of course.

So, assume two empty depots weighing 150 tons each and a cable 6 km long. A tanker, weighing 150 tons carrying 300 tons of fuel docks with the lower depot, but a single raptor engine stays on for two seconds too long. (It can't really do that in a horizontal direction, but ignore that for now.) That's 2.75 MN of thrust vs. 600 tons of mass (lower depot plus tanker--the upper depot doesn't count for this purpose) for 1.82 m/s^2 of transverse acceleration. In two seconds, the depot/tanker pair reaches a velocity of 3.67 m/s. The distance to the COM after docking is 1200 m, so the angular velocity is 3.06E-03 radians/sec. Using that as the initial conditions for the differential equation that describes the motion of the depots, 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, but it can be damped out over time with whatever system does the station keeping. (E.g. ion thrusters.)

For the case of vertical acceleration instead of transverse, it will travel 3.66 meters. This is comparable to the elastic stretch of the cable, so (depending on exactly what it's made of) it won't go slack. Again, this is a worst case. If either depot holds more fuel, the stretch will be greater. As a result, It will slowly bounce up and down for a while, but the steel cable is a damped oscillator, so it will stop over time. Again, I don't see this as a big deal by itself.

Anything else will fall between these two cases.

As for sloshing, one expects the tanks to contain some anti-sloshing baffles. That should both minimize sloshing and mean that it's damped, so it will eventually stop. Sure, the motion may be chaotic, but it will also be small and not last very long. Since we're not spending fuel to generate gravity, we can afford to wait 15 minutes or an hour, if that's what it takes.

In reality, I think they'll use cold gas or a dedicated OMS with far less thrust than a Raptor, so these really are worst-case scenarios.