Starship On-orbit refueling - Options and Discussion

[BT52]

Hmm dint dude from Masten space joined SpaceX?

If yes then then they got small nozzle ISP solutions at their hands already. 

[TheRadicalModerate]

TL;DR - hot gas thrusters don't have any advantage over ullage thrusters as long as acceleration is low.

I still don't think this works.

I guessed on Isp for cold/warm gas.  It's obviously going to be dependent on main tank pressure if you're just using ullage gas.  I'd assume we're using methane, since it's lighter than molecular oxygen.

I'm way too stupid right now to figure out what the pressure in the depot and tanker methane tanks will be.  Let's assume that the tanker hasn't undergone condensation collapse yet, and the depot has.  I'm gonna guess that, together, they're at something like 3bar.  What does that yield for a reasonable nozzle exit velocity?  I'll bet it isn't Isp=70s.  Maybe... 20s?  (Please, somebody who has these pressure and Isp spreadsheets, chime in.  Let's assume that T=260K for the depot and T=300K for the tanker.)

Worst case, we have a tanker that's (1200t - 150t)/1200t = 87.5% empty, and a depot that's 150t/1614t = 9.3% empty.  Assuming O:F=3.6, that should have about 722m³ of methane ullage.  Using 3bar common tank pressure and T=280K common temperature (I'm guessing again, but things ought to mix pretty well when a warm tanker with lots of empty space and higher pressure gets connected to a cold depot with not very much space), then, by Boyle's Law:

300,000Pa * 722m³ = 8.314n280K
n = 93,044mol
startUllageMass = 1490kg

But wait!  There's more!  We can't use all of that mass, because there's some minimum pressure where you won't have choked flow anymore, and Isp will drop into the low single digits.  Again, too stupid to calculate, so let's guess it's at 1.5bar.  That will leave:

150,000Pa * 722m³ = 8.314n260K (I'm guess on the adiabatic temperature)
n = 50,100mol
endUllageMass = 802kg

usableUllageMass = 688kg

Using Isp=20s, xferTime=2000s, and depotProp=1600t, that limits your max acceleration to a=0.0058mm/s².

That's... pretty small...

NB:  There's a fair amount of hand-waving going on here.  If anybody can provide better numbers for Isp and before-and-after temperatures, please do!
_______________

On top of all that, there are other reasons why you need lots of high-pressure gas, with the biggest one being spin-up gas.  So, once you've gone to the expense of developing a renewable pressurization system for some bag of COPVs, why wouldn't you at least use warm gas being fed from supercritical reservoirs to drive non-combusting thrusters?

And if it turns out that the LSS really does need waist landing thrusters, which will almost certainly have to be combusting gas, why wouldn't you use those for ullage burns?  They'd have to throttle down pretty low, but that doesn't seem like a terrible problem for pressure-fed methox.

Mind you, I still think you'll need non-combusting warm gas thrusters for prox ops and docking, but if they feed off of the COPV high-pressure manifolds, they're easy, too.

[TheRadicalModerate]

Hmm dint dude from Masten space joined SpaceX?

If yes then then they got small nozzle ISP solutions at their hands already.

Unless he and SpaceX are willing to get sued by Astrobotic for theft of intellectual property, they don't.

[BT52]

Hmm dint dude from Masten space joined SpaceX?

If yes then then they got small nozzle ISP solutions at their hands already.

Unless he and SpaceX are willing to get sued by Astrobotic for theft of intellectual property, they don't.

Well they cant get sueed by knowledge of building small nozzle engine. Ahh i see. I was under impression they bought also some designs with him. But i guess not.

[TheRadicalModerate]

Hmm dint dude from Masten space joined SpaceX?

If yes then then they got small nozzle ISP solutions at their hands already.

Unless he and SpaceX are willing to get sued by Astrobotic for theft of intellectual property, they don't.

Well they cant get sueed by knowledge of building small nozzle engine. Ahh i see. I was under impression they bought also some designs with him. But i guess not.

I don't think SpaceX has any lack of knowledge about how to build a small engine.  So unless this guy came with plans and test results (which absolutely would be theft of intellectual property), he doesn't help very much.

[Paul451]

I mentioned way back in the thread that SpaceX tested a small methane/GOx engine with the initial intent of using them in place of conventional nitrogen cold-gas RCS. Musk later said they're sticking with cold-gas for RCS. But there's no reason to assume they'd do an long ullage burn in a stupid way for no good reason.

[InterestedEngineer]

It turns out one might be able to do zero-g siphon tricks:

https://twitter.com/astro_pettit/status/719490255682842626?lang=en

[TheRadicalModerate]

But there's no reason to assume they'd do an long ullage burn in a stupid way for no good reason.

Agreed.  My point was that it doesn't look like the mass of CH4 in ullage space will be adequate for the ullage impulse.  (I didn't do the computation including the O2 ullage, but things were bad enough that roughly doubling the mass wouldn't be adequate either.)

[oldAtlas_Eguy]

But there's no reason to assume they'd do an long ullage burn in a stupid way for no good reason.

Agreed.  My point was that it doesn't look like the mass of CH4 in ullage space will be adequate for the ullage impulse.  (I didn't do the computation including the O2 ullage, but things were bad enough that roughly doubling the mass wouldn't be adequate either.)

If you add the O2 ullage the total available mass is a factor of 4X for mass over that of the CH4 alone.

[TheRadicalModerate]

But there's no reason to assume they'd do an long ullage burn in a stupid way for no good reason.

Agreed.  My point was that it doesn't look like the mass of CH4 in ullage space will be adequate for the ullage impulse.  (I didn't do the computation including the O2 ullage, but things were bad enough that roughly doubling the mass wouldn't be adequate either.)

If you add the O2 ullage the total available mass is a factor of 4X for mass over that of the CH4 alone.

But not the impulse.

[Second Update:  My first cut at this was completely wrong, with massive confusion of extensive and intensive properties.  Just to save space (and, yeah, a little bit of embarrassment), I'm just deleting it.  Here's the new version:]

Since O2 has exactly double the molecular weight of CH4, O2 cold gas exit speed vs. CH4 speed should be sqrt(1/2)v_ch4 = 0.71 of the CH4 cold gas speed.

The LOX tank should have 29% more volume than the LCH4 tank, and the same pressure.  So the O2 ullage mass should be 2*1.29 = 2.6x that of the CH4 mass.

Therefore, the total impulse of the LOX ullage (m_o2*v_o2) should be 1.8x that of the CH4 ullage, and the total impulse of both tanks is 2.8x what I calculated up-thread, which means that the acceleration should go from 57 microgee to 159 microgee.  Note that I'm assuming that the cold gas thrusters can accept two different mass flows at different times during the ullage acceleration.  Shouldn't be a problem.

159 microgee might be close to a viable ullage acceleration, but it's still a really small acceleration.  Still seems to me that cold gas ullage thrust is pretty iffy.

Update #3:  From the 2008 Kutter-Zegler paper on turning Centaur into a prop depot:

Through improved understanding of low-g fluid behavior Centaur has reduced the standard parking orbit settling from 10^-3 g to 10^-4 g realizing a significant performance enhancement while maintaining adequate propellant control. In the quest for even more performance and longer mission duration, Centaur has demonstrated effective propellant control at accelerations down to 10^-5 g...

Now, these numbers are for settling during storage, for the purpose of ensuring that they'd only vent gas instead of gas/liquid.  I can't think of a reason why prop transfer would have different properties, as long as vibration in the system is minimal.  If that's the case, then >10 microgee should be fine, and cold gas ullage acceleration will work.

[InterestedEngineer]

Can capillary action be used to transfer fuel?

https://m.facebook.com/Astro.Pettit/photos/a.241804595873073/998651346855057/

[OTV Booster]

But there's no reason to assume they'd do an long ullage burn in a stupid way for no good reason.

Agreed.  My point was that it doesn't look like the mass of CH4 in ullage space will be adequate for the ullage impulse.  (I didn't do the computation including the O2 ullage, but things were bad enough that roughly doubling the mass wouldn't be adequate either.)

If you add the O2 ullage the total available mass is a factor of 4X for mass over that of the CH4 alone.

But not the impulse.

[Second Update:  My first cut at this was completely wrong, with massive confusion of extensive and intensive properties.  Just to save space (and, yeah, a little bit of embarrassment), I'm just deleting it.  Here's the new version:]

Since O2 has exactly double the molecular weight of CH4, O2 cold gas exit speed vs. CH4 speed should be sqrt(1/2)v_ch4 = 0.71 of the CH4 cold gas speed.

The LOX tank should have 29% more volume than the LCH4 tank, and the same pressure.  So the O2 ullage mass should be 2*1.29 = 2.6x that of the CH4 mass.

Therefore, the total impulse of the LOX ullage (m_o2*v_o2) should be 1.8x that of the CH4 ullage, and the total impulse of both tanks is 2.8x what I calculated up-thread, which means that the acceleration should go from 57 microgee to 159 microgee.  Note that I'm assuming that the cold gas thrusters can accept two different mass flows at different times during the ullage acceleration.  Shouldn't be a problem.

159 microgee might be close to a viable ullage acceleration, but it's still a really small acceleration.  Still seems to me that cold gas ullage thrust is pretty iffy.

Update #3:  From the 2008 Kutter-Zegler paper on turning Centaur into a prop depot:

Through improved understanding of low-g fluid behavior Centaur has reduced the standard parking orbit settling from 10^-3 g to 10^-4 g realizing a significant performance enhancement while maintaining adequate propellant control. In the quest for even more performance and longer mission duration, Centaur has demonstrated effective propellant control at accelerations down to 10^-5 g...

Now, these numbers are for settling during storage, for the purpose of ensuring that they'd only vent gas instead of gas/liquid.  I can't think of a reason why prop transfer would have different properties, as long as vibration in the system is minimal.  If that's the case, then >10 microgee should be fine, and cold gas ullage acceleration will work.

ISTM that much depends on how transferred fluids are introduced into their new tank. For example, if the inlet is at the top of the tank the propellant will have any velocity imparted by the transfer force plus the acceleration of ullage thrust. When it hits the surface it splashes, and in the case of the LOX tank, the transfer tube forces it to hit asymmetrically and the ship picks up a wiggle.


If introduced at the bottom the transfer force will have to overcome whatever head pressure the already settled propellant will impose. On the first transfer, with little or no propellant already in the tank, where would be no pressure head. On the last transfer, there's a lot of head pressure.


Conclusion: bottom fill works best and whatever provides the transfer pressure needs to be variable.

[Greg Hullender]

Conclusion: bottom fill works best and whatever provides the transfer pressure needs to be variable.

Is this news, though? I thought it was already established that there's a large pipe at the bottom of each tank to fill/drain the cryogenic fluid and a small one at the top for the ullage gas. (This is reflected in the QD adaptor.) The system already works on Earth, and pressure head ought to be less in space.

[TheRadicalModerate]

Conclusion: bottom fill works best and whatever provides the transfer pressure needs to be variable.

This was why we stopped thinking very much about dorsal-to-dorsal, nose-to-tail configurations--or inline tail-to-tail configurations for that matter.

I still think that equalizing ullage pressures (by physically connecting the two ullage spaces) and using a very low power pump solves all the transfer pressure management problems.

One other problem, which is sort of related to your "don't splash" problem:  Prop will "geyser" into the receiving tank, even if it's pumped from the bottom.  At microgee accelerations, that geysering effect is going to cause all kinds of sloshing.  That may be OK, because uncovering the pump outlet on the receiving tank shouldn't be a problem.  But if you get blobs of prop bouncing off the receiving tank wallsı, they may mess up microgee ullage accelerations enough to cause sloshing on the sending side, which could uncover the inlet.

If you're using low-power pumps with equalized ullage pressures, the pump can probably be made robust enough to work with the sending side's inlet uncovered temporarily.  The pump will have to be able to re-prime itself with tiny head pressures, though.

Note that this is yet another reason not to use ullage pressure differences to transfer prop:  If you ever uncover the sending side inlet, the higher pressure ullage gas will instantly blow through the line, equalizing pressures, and then you have yourself a problem.  You can recover from this, but it'll require some combination of venting the receiving side and heating the sending side to restore the pressure differential.  It's wasteful, slow, and could potentially happen so often that the system wouldn't work at all.

_____________
ıYet another problem related to geysering:  If you have unsettled blobs on the receiving side, they'll occasionally get sucked into the ullage pressure equalization line.  That could be made to be OK, but you may wind up pumping some of the prop through the system multiple times.  Presumably, as the tank gets full it will geyser less.  That's important, because otherwise you could have a case where the recirculated prop problem gets worse just before you're completely full.

[DanClemmensen]

One other problem, which is sort of related to your "don't splash" problem:  Prop will "geyser" into the receiving tank, even if it's pumped from the bottom.  At microgee accelerations, that geysering effect is going to cause all kinds of sloshing.  That may be OK, because uncovering the pump outlet on the receiving tank shouldn't be a problem.  But if you get blobs of prop bouncing off the receiving tank wallsı, they may mess up microgee ullage accelerations enough to cause sloshing on the sending side, which could uncover the inlet.

If this is the only use for the fill pipe, they can put diverter such as a mushroom cap over the outlet.

[TheRadicalModerate]

One other problem, which is sort of related to your "don't splash" problem:  Prop will "geyser" into the receiving tank, even if it's pumped from the bottom.  At microgee accelerations, that geysering effect is going to cause all kinds of sloshing.  That may be OK, because uncovering the pump outlet on the receiving tank shouldn't be a problem.  But if you get blobs of prop bouncing off the receiving tank wallsı, they may mess up microgee ullage accelerations enough to cause sloshing on the sending side, which could uncover the inlet.

If this is the only use for the fill pipe, they can put diverter such as a mushroom cap over the outlet.

At the kind of accelerations and head pressures we're talking about, even turbulence after the diversion is likely to cause some slosh.  But it might be enough to prevent blobs of prop from slamming into stuff with enough force to cause slosh in the sending tank.  And a mushroom cap would probably help for the "nearly full" case, too.

[mikelepage]

If you intend to fill a depot for a specific mission, then number of tankers is really the right metric, and overall prop efficiency isn't very important.  But if you're continuously filling and partially drawing down a depot, as you would in very high cadence ops, then the efficiency becomes more important.  Note also that increasing the power of the transfer pumps, and thereby shortening the transfer time, can also change things.  As usual, it's a pretty rich trade space.

So, to answer your question:  In high cadence, there's probably a number where rotational settling makes more sense than ullage thrust.  But it doesn't make any sense early on, when low cadence will almost certainly boil the depot dry between missions.  And even at high cadence, you'll have to show your work on why the extra complexity is worth it.

I didn't respond to this earlier post, but seeing this, as well as your comment about 5mm/s² being conservative and finding the Kutter research paper had me all but give up on the spin-G prop transfer concept I was noodling with, because I think it probably only comes out on top of linear acceleration in scenarios where you actually need considerably more than 5mm/s² (like say 1% of G = 100mm/s²).

One other problem, which is sort of related to your "don't splash" problem:  Prop will "geyser" into the receiving tank, even if it's pumped from the bottom.  At microgee accelerations, that geysering effect is going to cause all kinds of sloshing.  That may be OK, because uncovering the pump outlet on the receiving tank shouldn't be a problem.  But if you get blobs of prop bouncing off the receiving tank wallsı, they may mess up microgee ullage accelerations enough to cause sloshing on the sending side, which could uncover the inlet.

If this is the only use for the fill pipe, they can put diverter such as a mushroom cap over the outlet.

At the kind of accelerations and head pressures we're talking about, even turbulence after the diversion is likely to cause some slosh.  But it might be enough to prevent blobs of prop from slamming into stuff with enough force to cause slosh in the sending tank.  And a mushroom cap would probably help for the "nearly full" case, too.

...But then I see these posts, and I wonder if 5mm/s² is really that conservative all things considered? The environment inside those prop tanks sounds like it will be quite dynamic if you want to pump propellant at any significant rate, and fluids are gonna fluid. In your earlier post you were assuming 150 ton of prop transferred in 2000s (75kg/s) which seems more than enough to disrupt surface tension if those 75kg only weigh ~40g, but still have the inertia of 75kg. You wouldn't want to get up there and find you can only pump at 15kg/s if you want to avoid chaotic effects. Playing with your spreadsheet to increase transfer time to 10,000s really does blow out the number of tanker trips.

My mental image was of one of those "incoming prop" geysers interacting with the receiver tank prop in such a way as to induce a vortex in the receiver tank, potentially torquing the entire structure in chaotic ways each time.  BoE says 1400 ton of prop swirling at 1 rpm is roughly equivalent to a reaction wheel with 1,800,000 Nms of angular momentum, so if that kind of induced swirling is a real problem, then that's a heck of a design challenge for your pump connectors. Presumably there are simple ways to baffle/disrupt these effects, but it could mean pumping slower or increasing tank mass. Maybe you have to pump slower anyway, and not expending a prohibitive amount of prop on linear G acceleration would be the problem a spin-G prop transfer scheme would be solving. I think I'll noodle a bit more.

[skyflyer81]

The tolerances would have to be pretty tight for something like this to work, but it is completely different than standard pumping in gravity or spinning ships around

[Greg Hullender]

The tolerances would have to be pretty tight for something like this to work, but it is completely different than standard pumping in gravity or spinning ships around

The image looks cool, but I can't quite make sense of it. Can you elaborate a little? :-)

[skyflyer81]

The tolerances would have to be pretty tight for something like this to work, but it is completely different than standard pumping in gravity or spinning ships around

The image looks cool, but I can't quite make sense of it. Can you elaborate a little? :-)

Just thinking that some baffles that start on one side of the interior of a tank could rotate around to the other side and "squeeze" out propellant