Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 811694 times)

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2820 on: 12/18/2024 05:11 am »
multi-layer insolation shield like the one JWST uses

So basically, simplifing, you want the thermal micrometeoroid garment.

https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment

The ISS, Soyuz, and Shuttle have all used similar thermal blanket technology. The outer layer (a strong engineered textile like Beta cloth, Nomex, Ortho-Fabric, etc) acts as a MMOD bumper, the MLI acts as thermal insulation and also 'packs' the Whipple shield for higher efficacy, and optionally you can add an inner textile later (Kevlar or similar) for improved tank wall impact protection where it's most effective because the hypersonic cone has fanned out.

Online DanClemmensen

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2821 on: 12/18/2024 09:06 am »
multi-layer insolation shield like the one JWST uses

So basically, simplifing, you want the thermal micrometeoroid garment.

https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment

The ISS, Soyuz, and Shuttle have all used similar thermal blanket technology. The outer layer (a strong engineered textile like Beta cloth, Nomex, Ortho-Fabric, etc) acts as a MMOD bumper, the MLI acts as thermal insulation and also 'packs' the Whipple shield for higher efficacy, and optionally you can add an inner textile later (Kevlar or similar) for improved tank wall impact protection where it's most effective because the hypersonic cone has fanned out.
I don't have an opinion, because I know approximately zero about MMOD shielding. To respond to that post, I had to look up "Whipple shield" on Wikipedia:
    https://en.wikipedia.org/wiki/Whipple_shield

Offline LaunchedIn68

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2822 on: 12/18/2024 05:58 pm »
I wonder if Mr. Whipple asked people to Please Don't Squeeze his MMOD material?  Using TP as a shield?  IDK about that. ;)

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Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2823 on: 12/18/2024 08:39 pm »
Even a pinhole in a tank is a loss of mission
Is that true?

It's not. But impacts by hypersonic "pins" don't create pinhole leaks. So TRM's broader point is true. MMOD shielding will be vital.
This is another argument in favor of a depot. A ship hanging around waiting for tankers would need MMOD shielding. A ship or tanker spending minimum time in LEO would have a lesser need.


After discussion on the point we seem to have reached a consensus that where possible the depot should carry the heavy stuff so the tankers and outbound ships don't have to repeatedly haul it up to orbit. MMOD shielding is heavy.


Hmmm. A thought. MMOD shielding is a two layer affair with the outer layer taking the hit and turning the intruding particle into a spray of fine low energy particles that get stopped by the second layer. Just noodling here but could metallic heat shielding double as the outer MMOD shield?
I think you mean solar insolation shielding, not re-entry shielding. A Depot never needs to re-enter. I think a multi-layer insolation shield like the one JWST uses would make a fine Wipple shield. The problem is that the solar heat flux is not aligned with the MMOD threat.
The danger is greatest for the depot but not zero for the other ships, especially crew ships.


It would be a good thing if we had a handle on the expected kinetic impact probabilities at different altitudes. ABICT, we don't.
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Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2824 on: 12/18/2024 10:02 pm »
multi-layer insolation shield like the one JWST uses

So basically, simplifing, you want the thermal micrometeoroid garment.

https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment

The ISS, Soyuz, and Shuttle have all used similar thermal blanket technology. The outer layer (a strong engineered textile like Beta cloth, Nomex, Ortho-Fabric, etc) acts as a MMOD bumper, the MLI acts as thermal insulation and also 'packs' the Whipple shield for higher efficacy, and optionally you can add an inner textile later (Kevlar or similar) for improved tank wall impact protection where it's most effective because the hypersonic cone has fanned out.

I don't know how you attach a blanket to the exterior of the tanks so that:

1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.
2) It could expand/contract with prop loading and unloading.

The discussion I heard (see here) was talking about thermal tiles, not blankets.  I found this fairly surprising, but the source (the deputy HLS program manager) would seem to be fairly authoritative.

It's probably pretty easy to construct a tile that can mitigate MMOD collisions.  What I don't understand is how a tile helps very much in mitigating boiloff.  In equilibrium, insulation doesn't do anything, other than cause heat to pile up on the outside of the tile, which raises its temperature, which increases emissions.  However, unlike the TPS we've seen so far, insolation isn't going to jack up the temperature enough to get a lot of heat rejection by thermal emission.

Maybe you have a "tile" that is comprised, from the outside in, of:

1) A rigid, IR-transparent outer shell, that's just hard enough to break up MMOD.
2) A bunch of MLI layers.
3) A final barrier to catch the pieces of spall and the remaining MMOD debris.

Seems awful heavy, though.

Online DanClemmensen

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2825 on: 12/18/2024 10:12 pm »

I don't know how you attach a blanket to the exterior of the tanks so that:

1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.
2) It could expand/contract with prop loading and unloading.

I don't either. I would launch a multi-layer shield like the one for JWST as a separate launch, but with a conical shape as recommended on some thread here in the last two years. Unfold this piece of origami  and then carefully ease Depot into it. Depot would emerge for transfer operations and then sneak back into its cocoon and go back to sleep.

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2826 on: 12/18/2024 10:15 pm »
multi-layer insolation shield like the one JWST uses

So basically, simplifing, you want the thermal micrometeoroid garment.

https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment

The ISS, Soyuz, and Shuttle have all used similar thermal blanket technology. The outer layer (a strong engineered textile like Beta cloth, Nomex, Ortho-Fabric, etc) acts as a MMOD bumper, the MLI acts as thermal insulation and also 'packs' the Whipple shield for higher efficacy, and optionally you can add an inner textile later (Kevlar or similar) for improved tank wall impact protection where it's most effective because the hypersonic cone has fanned out.

I don't know how you attach a blanket to the exterior of the tanks so that:

1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.
2) It could expand/contract with prop loading and unloading.

On Shuttle they were literally just quilted and glued to the exterior. Undoubtedly SpaceX could do the same or better.

https://space1.com/Artifacts/Space_Shuttle_Artifacts/Other_Parts/AFRSI_Blanket/afrsi_blanket.html

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2827 on: 12/18/2024 10:16 pm »

I don't know how you attach a blanket to the exterior of the tanks so that:

1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.
2) It could expand/contract with prop loading and unloading.

I don't either. I would launch a multi-layer shield like the one for JWST as a separate launch, but with a conical shape as recommended on some thread here in the last two years. Unfold this piece of origami  and then carefully ease Depot into it. Depot would emerge for transfer operations and then sneak back into its cocoon and go back to sleep.

Removing complex deployment and associated failure points, unnecessary dimensional thickness, and geometries requiring constant attitude pointing? Those were all features, not bugs.  ;)
« Last Edit: 12/18/2024 10:19 pm by Twark_Main »

Offline andrewmcleod

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2828 on: 12/19/2024 07:53 am »
Good catch (although I haven't verified your results).  Two things jump out:

1) It's non-intuitive why mass loss from ullage gas would be less than from liquid prop.  Gas has much lower viscosity.  Of course, there's only so much gas available, but if the pressure drops to near-zero, the prop will boil like crazy.  Did you take this into account?  It's possible that the pressure is still very low at equilibrium, but that's a much more complicated calculation.

2) If your hole opens up on a gas pocket, the thrust from the escaping gas will accelerate the vehicle in the opposite direction, which will cause liquid prop to settle around the hole.  Given the outcome of the boiling calculation, that may be a good thing.  Or not...

1) Intuitively it makes sense to me I think? Something to do with the relationship between pressure, density and velocity. The pressure difference presumably drives the velocity through the hole in some way? You can have a relatively slow flow (low volume rate) of liquid through a hole (easily subsonic) and you get quite a high mass flow rate because the density is so much higher. With gas, you can have a quite high velocity (high volume rate) but relatively low mass flow rate because the density is so slow (and then, I think potentially quite quickly, the velocity becomes transonic and the flow chokes which is what the expansibility factor is about, but I've ignored that).

2) It's probably all complicated and depends exactly on the size of the hole as to what exactly happens when liquid approaches the hole. On one hand, it wants to boil as the pressure drops, but this takes energy so something else is presumably becoming cold - and will it boil before, during or after passing through the orifice? Does this slow or increase the mass flow rate? No idea.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2829 on: 12/19/2024 01:07 pm »
I'm concerned with how they'll balance the ullage thrust from the depot and the tanker to avoid imparting unwanted angular momentum. If you're venting gas to make the transfer happen, then it seems one side has a lot of ullage gas to work with while the other side has relatively little. If most of the mass is in the receiving vehicle, maybe this isn't a problem; a nearly full depot loading prop from a tanker could probably just angle the thrust a little. But in the case of transfer from a fully loaded depot to an empty tanker, I don't think that's possible.

So it seems like you'll need some amount of ullage thrust from both vehicles for the duration of the transfer, but I still don't see how you can do this and keep them balanced.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2830 on: 12/19/2024 05:15 pm »
I'm concerned with how they'll balance the ullage thrust from the depot and the tanker to avoid imparting unwanted angular momentum. If you're venting gas to make the transfer happen, then it seems one side has a lot of ullage gas to work with while the other side has relatively little. If most of the mass is in the receiving vehicle, maybe this isn't a problem; a nearly full depot loading prop from a tanker could probably just angle the thrust a little. But in the case of transfer from a fully loaded depot to an empty tanker, I don't think that's possible.

So it seems like you'll need some amount of ullage thrust from both vehicles for the duration of the transfer, but I still don't see how you can do this and keep them balanced.
If (definitely speculation) a depot can get by with one Raptor SL and 3 vacuum, it leaves a lot if room in the engine bay for small pressure fed hot ullage thrusters w/TVC. The plumbing's already there. A scant engine arrangement also opens up possibilities for depot specific hardware.


That leaves the question of how far out of line the assemblage can be before it interferes with transfer. Two extreme cases. 1) A near full depot and an almost empty tanker. 2) An empty depot and a tanker just starting transfer.


Case 1 doesn't look like much of a problem but I haven't run any numbers.


Case 2 looks like it could be a problem. A laterally firing cold gas thruster high up on the tanker is a possibility. Would it be enough? Would there be enough ullage gas throughout the transfer? Maybe one hot gas thruster up high on tankers for just this situation? Best part...no part.


If lateral cold gas on the tanker is close but no cigar, maybe trim the load for the first tanker to drop mass and open up more ullage gas space. Or, use tap off gas to fill a gas header during launch to augment the cold gas supply. Maybe vent the gas header directly and keep the main tank out of the process.


However gas inefficient these schemes are, the first moment of transfer from the first tanker is the worst. Efficiency needs to consider the entire transfer campaign, not the extreme case.


Worst case, a CMG (control moment gyro) or a reaction wheel on the depot for this and fine tuning alignment when docking.


ISTM there enough options to keep things aligned that it's not going to be a major problem.


Things we don't know that impact the problem:
- mass of R3
- mass delivery capabilities of the tanker
- tank size of both the tanker and the depot.


Oh yeah, one other thing. Transfer from the depot to a ship opens up the same can of worms with a different sized can and different sized worms.
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Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2831 on: 12/19/2024 08:05 pm »

1) Intuitively it makes sense to me I think? Something to do with the relationship between pressure, density and velocity. The pressure difference presumably drives the velocity through the hole in some way?

Worst-case (ie no boiling in the "nozzle") is given by Bernoulli. You're converting power from the pressure drop (volumetric flow × pressure drop) into power from kinetic energy (1/2 mass flow × velocity2), minus any friction losses.

Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2832 on: 12/19/2024 09:21 pm »
I'm concerned with how they'll balance the ullage thrust from the depot and the tanker to avoid imparting unwanted angular momentum. If you're venting gas to make the transfer happen, then it seems one side has a lot of ullage gas to work with while the other side has relatively little. If most of the mass is in the receiving vehicle, maybe this isn't a problem; a nearly full depot loading prop from a tanker could probably just angle the thrust a little. But in the case of transfer from a fully loaded depot to an empty tanker, I don't think that's possible.

So it seems like you'll need some amount of ullage thrust from both vehicles for the duration of the transfer, but I still don't see how you can do this and keep them balanced.

It's not really an issue if you're using combusting gas:  both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹  The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.

If you insist on using cold-to-warmish gas thrusters (which my back-of-napkin up-thread says is impossible without either stopping in mid-transfer to wait for the boiloff to build up, or actively heating prop to force-boil it), then using a pumped transfer with equalized ullage pressures might be simpler.

__________
¹I don't want to oversell combusting gas.  For a sustainable, long-duration system, you'll eventually need to vent the liquid COPVs, flow new LCH4 and LOX into them, and then create a separate reservoir of pressurized GCH4 and GO2 to drive them into the thrusters' combustion chambers.  It's quite a complex set of COPVs and manifolds, but it'll continue to provide on-demand thrust as long as there's LCH4 and LOX in the mains.

Offline Paul451

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2833 on: 12/19/2024 10:21 pm »
You're converting power from the pressure drop (volumetric flow × pressure drop) into power from kinetic energy (1/2 mass flow × velocity2), minus any friction losses.

Doesn't help them since it's velocity that they're asking for, without which volumetric flow, hence mass flow, is also unknown. The only variable they have is pressure drop (~6 bar.)

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2834 on: 12/19/2024 10:31 pm »
It's not really an issue if you're using combusting gas:  both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹  The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.
In this case, though, it seems as though they'd have to develop a whole new rocket engine. That seems to me to be a really big deal. And they can't just buy one off the shelf because, until recently, no one was developing methalox engines. Yeah, there's a lot written about throttling liquid-propellant engines, but, boy-oh-boy, talk about adding a complex new part!

It's possible that this is actually the biggest technical risk as far as making depots work goes.

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2835 on: 12/19/2024 10:47 pm »
You're converting power from the pressure drop (volumetric flow × pressure drop) into power from kinetic energy (1/2 mass flow × velocity2), minus any friction losses.

Doesn't help them since it's velocity that they're asking for, without which volumetric flow, hence mass flow, is also unknown. The only variable they have is pressure drop (~6 bar.)

One of the easiest solving methods is to express both mass flow and volumetric flow in terms of velocity, given some assumed area. Then solve for velocity, which also yields mass flow.

Briefly,


    Pressure drop × volumetric flow = 1/2 × mass flow × velocity2

    Pressure drop × area × velocity = 1/2 × density × area × velocity3

    Pressure drop = 1/2 × density × velocity2

    Velocity = sqrt( 2 × pressure drop / density )



    Mass flow = area × velocity × density

    Mass flow = area × sqrt( 2 × pressure drop × density )
« Last Edit: 12/20/2024 01:41 am by Twark_Main »

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2836 on: 12/20/2024 12:54 am »
It's not really an issue if you're using combusting gas:  both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹  The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.
In this case, though, it seems as though they'd have to develop a whole new rocket engine. That seems to me to be a really big deal. And they can't just buy one off the shelf because, until recently, no one was developing methalox engines. Yeah, there's a lot written about throttling liquid-propellant engines, but, boy-oh-boy, talk about adding a complex new part!

It's possible that this is actually the biggest technical risk as far as making depots work goes.
As rocket engines go, pressure fed is simple. The combustion chamber, igniter, throat, bell, cooling, TVC (maybe) and injector design is there but the turbopump and all its complexity goes away. And, they won't have to deal with crazy high combustion pressure or plumbing.

I do remember something on these forums about a small engine (exact size unknown to me) being tested at MacGregor maybe a year ago. What we're talking about here would be way less powerful than the HLS landing engines we haven't heard about in a while.

It doesn't have to be fancy. It doesn't even have to be super efficient for a first stab. It just has to work. If SX finds this to be a serious problem I will loose all faith in them.
« Last Edit: 12/20/2024 12:54 am by OTV Booster »
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Offline TheRadicalModerate

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2837 on: 12/20/2024 05:53 am »
It's not really an issue if you're using combusting gas:  both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹  The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.
In this case, though, it seems as though they'd have to develop a whole new rocket engine. That seems to me to be a really big deal. And they can't just buy one off the shelf because, until recently, no one was developing methalox engines. Yeah, there's a lot written about throttling liquid-propellant engines, but, boy-oh-boy, talk about adding a complex new part!

It's possible that this is actually the biggest technical risk as far as making depots work goes.

Well, they're going to have to develop at least one new rocket engine, because there's no way to land a Starship on the Moon without falling into the crater dug by exhaust impingement from Raptors.  That's what the waist thrusters are for on the LSS, and I assure you that they're combusting gas engines.

The real question is whether they need to develop a second new rocket engine.  That would be necessary if the waist thruster can't throttle down enough to support settling acceleration efficiently.  Let's model that.

In the most recent artwork of the LSS, it looks like there are 3 clusters of 3 thrusters around the waist, spanning about 120º, if I'm eyeballing correctly.  That would imply a total of 27 thrusters.  On landing, assuming that the LSS has a dry mass of 95t (no TPS, which may now be wrong) and let's arm-wave the crew module and its payload at 15t, for a total inert mass of 110t.

To get back to NRHO, we need about 170t of prop (just trust me on this, but note that I'm using v2 as a baseline, not v3), but let's goose it up to 200t just to be sure, for a gross landing mass of 310t.  If the max thrust required allows the LSS to hover, then in 1.62m/s² lunar gravity, we need a total of about 500kN of thrust.  Let's allow for a couple of thruster failures, and assume we have 25 working.  So each one needs to generate a max thrust of 20kN.

I'd guess that it's quite simple to throttle a pressure-fed methalox thruster down to 50%, so we can go as low as 10kN of settling thrust, using a single thruster.  (If one of them fails, just fire up another and wait for things to settle again.)

The combined mass of a depot should be 110t dry for the LSS, maybe the same for the depot with insulation, and let's say we need 1800t of prop in the combined system.  So combined wet mass is 2020t.  If we can throttle down one of the landing thrusters to 10kN, that should result in a system acceleration of 5mm/s².  That's more than we need, but not by a huge amount.  If the thruster has an Isp of 320s, it'll have 3.2kg/s of mass flow.

Up-thread, I estimated that if on-orbit prop transfer happens at a tenth the rate of pad prop transfer, moving 1500t of prop would take 9.3hrs.  That would imply that settling consumed 107t of prop--which ain't great.  It's way too much to put into COPVs at the start of the transfer operation.  It's even too much to put into header tanks in the depot.  (Note that there's no need for depot header tanks except maybe for this purpose, and the LSS can't have header tanks, because they'd block the docking tunnel.)

So:  either we need a second kind of combusting-gas thruster, or we need a thruster that can support much, much lower throttle settings--like maybe 10%, which would be 2kN thrust, 1mm/s², a mass flow of 0.6kg/s, and settling prop consumption of 21t, which is still pretty terrible.

So let's work the problem backwards.  Let's say we want to limit settling prop consumption to 5t, with 5kN thrust (throttle = 25%, which is aggressive but not unreasonable).  At Isp=320s, that'd be a mass flow of 1.6kg/s.  So we'd have to limit the prop transfer to about 55mins.  For 1500t, that's a prop transfer mass flow of 480kg/s.

That's very close to the fill rate on the pad.  We know the plumbing exists to handle that rate, but we have no hydrostatic pressure to play with, and we can't geyser or slosh a lot.¹

I'm not sure what conclusions to draw here.  Looks like we'd either need a second, smaller thruster (not particularly difficult, but guaranteed to stick in Elon's craw), or a really high prop transfer rate.


_______
¹This is where I'll put in a plug for Team Pump.  All things are possible with a self-priming pump and enough electrical power.

Online DanClemmensen

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2838 on: 12/20/2024 11:18 am »

In the most recent artwork of the LSS, it looks like there are 3 clusters of 3 thrusters around the waist, spanning about 120º, if I'm eyeballing correctly.  That would imply a total of 27 thrusters.  On landing, assuming that the LSS has a dry mass of 95t (no TPS, which may now be wrong) and let's arm-wave the crew module and its payload at 15t, for a total inert mass of 110t.

To get back to NRHO, we need about 170t of prop (just trust me on this, but note that I'm using v2 as a baseline, not v3), but let's goose it up to 200t just to be sure, for a gross landing mass of 310t.  If the max thrust required allows the LSS to hover, then in 1.62m/s² lunar gravity, we need a total of about 500kN of thrust.  Let's allow for a couple of thruster failures, and assume we have 25 working.  So each one needs to generate a max thrust of 20kN.
I'm not sure where you propose to mount the thrusters on Depot and/or Tanker. On HLS, they are at the waist, which means you need to compute the cosine loss. This affects the computation of the thrust needed at the Moon, and may also affect the system fuel efficiency for the transfer. I would also guess that there are 24 of them (or some other even number), not 27, because balancing the thrust is simpler if you use opposing pairs.

Offline OTV Booster

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2839 on: 12/20/2024 09:13 pm »
It's not really an issue if you're using combusting gas:  both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹  The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.
In this case, though, it seems as though they'd have to develop a whole new rocket engine. That seems to me to be a really big deal. And they can't just buy one off the shelf because, until recently, no one was developing methalox engines. Yeah, there's a lot written about throttling liquid-propellant engines, but, boy-oh-boy, talk about adding a complex new part!

It's possible that this is actually the biggest technical risk as far as making depots work goes.

Well, they're going to have to develop at least one new rocket engine, because there's no way to land a Starship on the Moon without falling into the crater dug by exhaust impingement from Raptors.  That's what the waist thrusters are for on the LSS, and I assure you that they're combusting gas engines.

The real question is whether they need to develop a second new rocket engine.  That would be necessary if the waist thruster can't throttle down enough to support settling acceleration efficiently.  Let's model that.

In the most recent artwork of the LSS, it looks like there are 3 clusters of 3 thrusters around the waist, spanning about 120º, if I'm eyeballing correctly.  That would imply a total of 27 thrusters.  On landing, assuming that the LSS has a dry mass of 95t (no TPS, which may now be wrong) and let's arm-wave the crew module and its payload at 15t, for a total inert mass of 110t.

To get back to NRHO, we need about 170t of prop (just trust me on this, but note that I'm using v2 as a baseline, not v3), but let's goose it up to 200t just to be sure, for a gross landing mass of 310t.  If the max thrust required allows the LSS to hover, then in 1.62m/s² lunar gravity, we need a total of about 500kN of thrust.  Let's allow for a couple of thruster failures, and assume we have 25 working.  So each one needs to generate a max thrust of 20kN.

I'd guess that it's quite simple to throttle a pressure-fed methalox thruster down to 50%, so we can go as low as 10kN of settling thrust, using a single thruster.  (If one of them fails, just fire up another and wait for things to settle again.)

The combined mass of a depot should be 110t dry for the LSS, maybe the same for the depot with insulation, and let's say we need 1800t of prop in the combined system.  So combined wet mass is 2020t.  If we can throttle down one of the landing thrusters to 10kN, that should result in a system acceleration of 5mm/s².  That's more than we need, but not by a huge amount.  If the thruster has an Isp of 320s, it'll have 3.2kg/s of mass flow.

Up-thread, I estimated that if on-orbit prop transfer happens at a tenth the rate of pad prop transfer, moving 1500t of prop would take 9.3hrs.  That would imply that settling consumed 107t of prop--which ain't great.  It's way too much to put into COPVs at the start of the transfer operation.  It's even too much to put into header tanks in the depot.  (Note that there's no need for depot header tanks except maybe for this purpose, and the LSS can't have header tanks, because they'd block the docking tunnel.)

So:  either we need a second kind of combusting-gas thruster, or we need a thruster that can support much, much lower throttle settings--like maybe 10%, which would be 2kN thrust, 1mm/s², a mass flow of 0.6kg/s, and settling prop consumption of 21t, which is still pretty terrible.

So let's work the problem backwards.  Let's say we want to limit settling prop consumption to 5t, with 5kN thrust (throttle = 25%, which is aggressive but not unreasonable).  At Isp=320s, that'd be a mass flow of 1.6kg/s.  So we'd have to limit the prop transfer to about 55mins.  For 1500t, that's a prop transfer mass flow of 480kg/s.

That's very close to the fill rate on the pad.  We know the plumbing exists to handle that rate, but we have no hydrostatic pressure to play with, and we can't geyser or slosh a lot.¹

I'm not sure what conclusions to draw here.  Looks like we'd either need a second, smaller thruster (not particularly difficult, but guaranteed to stick in Elon's craw), or a really high prop transfer rate.


_______
¹This is where I'll put in a plug for Team Pump.  All things are possible with a self-priming pump and enough electrical power.
Some thoughts.

Conditions change throughout a transfer campaign. Geysering is more of a problem with the first propellant to enter a tank. Calls for low transfer rate or a high settling acceleration. At the end of the last topoff geysering shouldn't be an issue. A high transfer rate or low settling thrust, albeit with a heavy load, works well.

The initial settling thrust needed to get all the props to the bottom end of the tank can be extremely low. The lower it is, the less slosh. Once the fluids are in the bottom higher acceleration will settle out bubbles and residual slosh faster.

Once everything is where it needs to be and is settled down, the question of geysering and acceleration tradeoffs becomes prominent. There may also an issue of outlet vortex leading to pump cavitation in the delivering craft - if pumps are used. Settling and geysering will change for each transfer.

The vortex issue brings on another thought. Just like the main engine pumps, delivering ship transfer pumps will require some combination of ullage pressure and liquid pressure head to avoid cavitation. Damn, another set of trades/limitations. A five or six dimension spreadsheet would be handy.

Elon may not like another thruster design but it's too handy a tool to ignore. He may find a workaround today but chances are that someday a small thruster will save another workaround or even keep him from dropping a handy idea because it would take more development than it warrants.

Edit to add: I think we have been ignoring something. We're dealing with two systems that can run in parallel - fuel and propellant oxidizer. Probably not a good idea if there's crew on board, but safe enough for routine ops?
« Last Edit: 12/22/2024 01:41 am by OTV Booster »
We are on the cusp of revolutionary access to space. One hallmark of a revolution is that there is a disjuncture through which projections do not work. The thread must be picked up anew and the tapestry of history woven with a fresh pattern.

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