Sunday, February 20, 2022

What the Ukraine Thing Is Really All About

This is an opinion piece!

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Heard it live from the Russian ambassador’s mouth on CBS’s “Face the Nation”,  Sunday,  20 February!  The Ukraine crisis is not about the Ukraine,  it is about NATO!  They want NATO greatly-reduced in strength,  presumably to leave them freer to try rebuilding the old Soviet empire by re-annexing the breakaway territories.  

That is exactly why they demanded limits on NATO that we could not abide,  in order to “stop” the invasion.  To them,  it was worth a try to see if they could intimidate the West,  and get what they wanted without the cost of an invasion. 

This is almost no different from the way Hitler intimidated and extorted his way to start annexing parts of Europe,  which of course led to World War 2 in Europe.  The only real difference is that there were no nuclear weapons available at the start of World War 2.  (There were by its end,  and the role they played in its end is another quite-complicated story.)

Putin and the Russian oligarchs who support each other do not care about Western economic sanctions,  because no ambitious dictators ever really cared about the welfare of the people they oppress.  (I double-dog dare you to name one who did!)  But,  the real risk of revolution if things get bad enough,  is exactly why there is disinformation and false-flag activity to whip up support at home in Russia for this invasion.  Temporary though such support may be,  it will last long enough for the invasion.

With state-controlled media spewing propaganda,  this is easier to do in Russia than here.    But,  spewing that same propaganda to people in the countries of the opposition,  is a way to divide their populations and reduce their opposition,  to one extent or another.  We in the US are not immune to this nefarious activity. 

My fellow citizens,  you should be aware that there are recognized Russian state propaganda agencies broadcasting right here in the US,  masquerading as “foreign news services”,  and trying to interfere in our elections,  for several years now.  Are you familiar with Sputnik and RT (Russia Today)?  You should be!

And you should also know this:  I have lost count of how many times the infamous opinion-mongers at Fox News have quoted various outrageous claims,  that I was able to trace directly back to RT and Sputnik as the sources.  “Providing aid and comfort to the enemy” is in the official definition of treason,  or did you not know that?  That was the basis for the actual treason conviction of the US citizen who was on Japanese radio as “Tokyo Rose” during World War 2. 

As a result of this Russian focus on getting a reduction in the strength of NATO,  and the West refusing,  the invasion of the Ukraine is inevitable,  and there will be war there.  It will be lopsided,  and the Ukraine will quickly lose and be annexed into Russia,  or at least a Russian puppet placed in control.  The Western powers have decided to let this happen,  and punish Russia with economic sanctions,  not military force.  Russia’s post-Soviet economy is fragile and vulnerable,  but this strategy will take much time to have any effect that we in the West can see. 

In the longer term,  Putin may well fall to revolution,  as the Russian economy goes into complete collapse.  But he is quite likely to lash out at the West before he goes,  and that may well involve nuclear weapons.  Even without that,  the collapse of the Russian economy will cause problems in economies worldwide,  because they are all linked far closer than any economies were at the start of World War 2.  You need to be aware of that,  too.  Opposition in any form to ambitious dictators always has its price.  Often,  it is a very high price.  This one will be.

We learned during the lead-up to World War 2 that you cannot appease ambitious dictators,  you must oppose them,  no matter the price.  There is no other way.  I hope this slow-motion attempt to collapse the Russian economy is enough opposition to ward-off the start of a World War 3,  which otherwise might well start from Putin’s lashing-out at us with nuclear weapons,  when he does fall from power.  But it is only a hope!  That is how high this price might well turn out to be!

The stakes are very high.  The risks are very high.  But Putin and the oligarchs who support him,  must go!  There is no other choice.  Russia’s brief flirtation with democracy after the fall of the Soviet empire,  must resume.  

Why?  We in the West are going to need the Russians on our side to help oppose another ambitious dictator in China,  one with a whole lot more economic power behind him. 

Update 2-21-2022:  

Late breaking Monday 21 February:  Putin today begins the Russian invasion of the Ukraine,  using recognition of the Russian-backed separatists in Donbas and Luhansk as the excuse.  There is now no choice for the Western powers except to immediately maximize the economic sanctions!  The only possible goal now is to collapse the Russian economy as fast as is possible,  causing his fall by revolution at home.  It will take time.  Meanwhile,  the Ukraine will slowly cease to exist.

Update 2-24-2022:

Russia is really invading the Ukraine,  as events clearly show.  I told you so!  In the piece above!

So Putin and his government officials are proving to be the liars that I said they were in the piece above.  There is no substantive difference between the behavior of Putin and the behavior of Adolf Hitler.  You can believe nothing he says at face value,  same as was true with Hitler.  Instead,  you must parse his words against his prior words and behavior,  to try to predict what evil he will commit next.  

And,  beware of people who seem to admire him.  They are out there.  Putin has already threatened anybody who resists him militarily with nuclear weapons.  He didn't use those words,  but the meaning was quite clear!  You need no more proof of Putin's evil nature than that.  Putin's admirers would thus seem to be evil themselves.  Which is why I said beware of them.

The West must do more than simple economic sanctions;  they need to topple Putin as soon as possible.  It will take utterly-extreme sanctions,  plus cyberattacks,  plus assassination attempts to do this.  If he falls to a revolution at home,  be aware he is quite likely to lash out at the West as he falls.  I predict he will lash out with those same nuclear weapons.  Y'all have been warned!

You will note that Xi's China is not joining the sanctions against Putin's Russia,  but instead trying to subvert their effects.  Why would he do that?  Easy.  Xi needs Putin in Russia to divert the world's attention from his own ambitions of conquest. Y'all have been warned!

Next on Putin's target list are the other ex-Soviet republics,  and some of them are now members of NATO.  That means that war between Russia and NATO is now inevitable.  It's an Article 5 thing.  The inevitability of such war is precisely because Putin perceives no resistance to his conquering of the Ukraine (it was the same with Hitler).  As it says in the piece above,  Putin and his oligarch supporters in Russia do not care about the Russian people suffering as their economy collapses.  It will take time for them to turn on him.  The only remaining question is "how soon?",  because I rather doubt he can be toppled fast enough to avoid this war-with-NATO outcome.  Again,  I have warned y'all!

Update 3-7-2022I see no reason to change anything I have said,  especially the 2-24 update.  

Update 3-14-2022:  The beginnings of WW3 have already started,  whether any of you like it or not.  There will be nuclear weapons used before this is over.  The only real question now is how to prosecute this war without starting the massive nuclear exchange that will destroy most life on the planet. 

Prediction:  if we rely only on sanctions trying to topple Putin with a domestic revolution,  that takes time (months,  maybe a year or more).  During that time,  Putin with conquer Ukraine committing foul atrocities and genocide along the way.  Then he will invade other breakaway countries that had been part of the Soviet Union,  one after another.  He will not stop until militarily opposed.  He may not stop even then.  Same as it was with Hitler.  Thus WW3 between Russia and the West seems to be inevitable.

Observation:  something like the strike in Ukraine immediately adjacent to Poland is the opportunity to militarily oppose Putin without WW3 immediately escalating into total planetary destruction.  If he were to attack a NATO member (and he will,  Estonia Latvia and Lithuania were once Russian and are now NATO,  as is Poland,  and several more),  Article 5 triggers war as a response. Had Putin's army screwed up and actually hit Poland,  that's the excuse to push them back,  and push them out of Ukraine while we are doing it. Without that military opposition,  Putin WILL NEVER stop!  Neither did Hitler.

One more time:  whether we do anything or not,  after a while,  Putin's own people will eventually rise up and overthrow him.  As he topples,  HE WILL LASH OUT at the West!  That will be with nuclear weapons,  more likely tactical weapons used in Europe,  but there is a significant chance he might fling ICBM's at the US.  

It might be better to oppose him militarily,  earlier in this process.  That way we restrain him at least somewhat,  before he topples to that domestic revolution,  and lashes out with those nuclear weapons.  

Something to think about.  Think hard about it!

Update 3-23-2022I am surprised and pleased to see how stiff the Ukrainian resistance has proven to be.  As far as I am concerned,  it is no longer a certainty that Russia will conquer them.  Still likely,  but not certain.  The rest of my predictions,  including who-knows-what violence when Putin lashes out at us when he topples,  still stand.  

Update 4-13-2022:  I looked up whether napalm was still a legal weapon of war.  It is,  but its use against civilians is now considered to be a war crime!  So,  you may only use it against enemy military forces.  So be it!

The Russian military convoys strung out for miles along Ukrainian highways are a perfect target for air-dropped napalm bombs.  You just fly down the road,  pickling-off bomb after bomb after bomb. 

I suggest that the western allies make both the attack aircraft and a whole lot of napalm bombs available to the Ukrainians.  Some fighter aircraft to maintain a combat air patrol above the attack aircraft may also be necessary.  

Losses to Russian air defense will be inevitable.  So you send more of this stuff,  rather than less.

The same applies to the bombardment positions being established by Russian forces in eastern Ukraine.  Some of these are artillery,  and some are tactical bombardment missiles.  Doesn’t matter:  all would be destroyed by massive napalm attacks.  Without much civilian destruction,  if any.

This has to be done in a timely fashion,  something that seems to escape most politicians,  here and in other countries.  Time is of the essence.  Just get on with it!

The sooner Putin loses this war with massive casualties,  the sooner his people will rise up and topple him.  Is that not the ultimate objective here,  whether anyone wants to admit it or not?

Economic sanctions are taking too long.  Send the Ukrainians the napalm bombs,  and the aircraft needed to use them.

Update 4-16-22:  What the Russians are threatening the west about,  is exactly what scares them the most!  Therefore,  increase the heavy weapons sent to Ukraine,  including what I suggested in the 4-13-22 update just above,  about napalm bombs and the aircraft to carry them.  

The Russians are now worried about Finland and Sweden joining NATO,  which is why they are threatening everybody if such were to happen.  

So,  make that happen sooner,  rather than later!  It is the NATO Article 5 promise that an attack on one is an attack on all,  that scares the ever-loving piss out of Putin!  It is also EXACTLY why he doesn't want Ukraine to join NATO!  Because he knows that war with NATO leads to a defeat of Russia,  and the end of his autocratic reign.  That last is his REAL fear!  

Simple as that!  And just as messy,  if it comes to war with Russia.  I've already told you what is going to happen if/when Putin topples.  So,  how is THAT worse than any other overt war with Russia?  Quit being so f***ing timid dealing with Putin!

Think about it!  Then vote for somebody who will actually do something that makes any sense!  Few currently in office qualify,  from either party.  Party does NOT matter!  You want somebody who will actually DO something!  Period!  End of issue!

You have ALWAYS HAD TERM LIMITS!  No Constitutional amendment is necessary!

Stop voting for incumbents who won't do anything about the problems we face!  Once you know the incumbent won't do anything,  vote for his opponent.  Party doesn't matter.  Multiple terms in office leading to corruption does matter.  

You CANNOT do worse by voting for "the other guy"!  You MIGHT do better!  Keep this "throw-the-bastards-out" philosophy up,  until someone actually does what needs to be done!  Then keep him,  but only until he gets corrupted.  And he will!

Update 4-27-2022:  

Putin is now threatening to use nuclear weapons in Ukraine if "we" (The US and NATO) don't stop shipping weapons to Ukraine.  What he screams loudest about is what he fears most.  The words he uses do not matter,  it is the action:  the attempted extortion to stop the weapons shipments.  

Shipping them the really heavy stuff,  and in mass quantities,  is exactly what we need to do!  That will cause Putin's invasion to be a military failure,  and help speed his toppling by revolution at home.  (This could be over even quicker,  if we gave the Ukrainians some napalm to use on the supply convoys and the massed artillery and rocket-launcher positions.  As indicated above,  I checked on the legality of that under international law.) 

I have already predicted that Putin will lash out at the west with a few nuclear weapons as he topples.  That is inevitable.  He may also use a tactical nuke or two in Ukraine,  before his invasion fails there.  That is also almost inevitable now.  Putin has no qualms about using such weapons,  but he does have a longstanding policy of using them to prevent defeat. 

The "trick" here is distinguishing a small number of ballistic missiles from a mass attack.  That puts a razor edge on the possibility of this needless war turning into the all-out nuclear disaster we have always feared and avoided.  The cold war-era symmetrical reluctance to commit nuclear suicide no longer obtains.  And THAT is why Putin is so dangerous to us all!  This is a war warning!

As I said above,  there is no discernible difference between this situation and the beginnings of WW2 in Europe.  Putin is a ruthless clone of Adolf Hitler,  and he will not stop until opposed in force,  or until he is dead.  Learn from history,  or be destroyed by this madman's reckless actions.  

Simple as that!  And just as hard to learn as it was in 1939. 

Update 5-18-2022:  I see no reason to change the original article or any of the updates.  I see plenty of reason to change politicians serving in office who are screwing around not getting this job done,  by either impeachment or by election.  

Tuesday, February 1, 2022

A Concept for an On-Orbit Propellant Depot

This article describes a concept for an on-orbit propellant depot capable of refilling visiting craft,  which presumes the visiting craft have rendezvous and docking capabilities.  This is only a concept,  which has not had any design analysis. 

Such a depot needs orbit adjustment,  reboost,  and debris avoidance propulsion capability.  There is the option to propel the depot sufficiently that it might “go where the job is”,  instead of having the visiting craft come to it,  which could assist with some aspects of orbital debris removal.  But because the facility is so large,  a separate space tug is a far better option for such missions.

Classes of Propellants

There are fundamentally two completely different classes of propellants to handle.  These are the more-or-less room-temperature storable propellants,  now most commonly used in thruster and attitude-control systems,  and there are the cryogenic “main stage” propellants,  which do in fact include the storable material kerosene.  The thruster storables need to be supplied in relatively small quantities,  while the main stage cryogens (and the kerosene) need to be handled in very large quantities.

Zero-gravity propellant ullage problems have been thoroughly discussed in Ref. 1The solution for the storable propellants (and the storable kerosene) is the bladdered tank.  This could be a free bladder,  a side-everted bladder,  or an axially-everted bladder.  The exact bladder geometry to be selected is not the real issue here,  in a concept design description.  Those geometries are discussed in Ref. 1.

The solution for the cryogenic propellants has been rather difficult to find.  As discussed in Ref. 1,  the historical solution for various rocket stages requiring free-fall restart has been application of ullage thrust.  The same is said in Ref. 2.  There have been some ideas tested and flown that take advantage of surface tension effects,  as discussed in Ref. 2.  Those same references point out that application of ullage thrust can be expensive in terms of ullage motor propellant quantities,  if you have to do it many times.  The solution selected for this quandary is the spinning tank technology described in Ref. 1.

The other piece of this puzzle is the facility occupancy by a crew.  There is no need to continuously-man this facility!  You only need to send up a crew when there is a propellant transfer to be made.  Once done,  the crew returns to Earth.  There are already standard docking ports of the necessary types (plural) on the International Space Station.  You just equip this facility’s crew module with those same docking ports,  plus some room for a few future new designs.

What Propellants to Have In Stock

The small-quantity thruster propellants under consideration here are the four common hydrazine variants,  and hypergolic nitrogen tetroxide (NTO) as the oxidizer for all of them.  Those hydrazine variants are plain hydrazine (C2H4),  monomethyl hydrazine (MMH),  unsymmetrical dimethyl hydrazine (UDMH),  and the brand-name product Aerozine-50,  (a 50-50 blend of UDMH and plain hydrazine).  These are commonly stored at significant pressure,  for pressure-fed thruster systems.

The mostly-cryogenic combinations are (storable) rocket-grade kerosene (RP-1),  cryogenic liquid methane (LCH4,  which is not the same as liquified natural gas),  and cryogenic liquid hydrogen (LH2),  with cryogenic liquid oxygen (LOX) as the oxidizer for all of them.  These are commonly stored at rather low pressures,  only such that boiloff is slowed or prevented.  Such engines are usually fed by turbopumps.  List follows:

Fuel                       oxidizer                application

C2H4                     NTO                      small-quantity thrusters

MMH                    NTO                      small-quantity thrusters

UDMH                  NTO                      small-quantity thrusters

Aerozine-50          NTO                      small-quantity thrusters

RP1                       LOX                       main stage propellant

LCH4                     LOX                       main stage propellant

LH2                        LOX                       main stage propellant

There are two other storable oxidizers,  although neither has been used in spacecraft for some years now.  Those are inhibited red fuming nitric acid (IRFNA) and high-test hydrogen peroxide (H2O2).  If such should ever be demanded again,  you just add extra storable oxidizer modules to the facility for them.

Properly-Applying Spinning-Tank Technology to Solve Cryogen Ullage Problems

The “trick” is not to spin the cryogenic tanks while they are attached to the depot!  That introduces severe sealing problems and the resulting logistical problems. 

Instead,  the craft to be refueled docks to a tank,  detaches it from the depot,  and moves off some distance,  then spins up with the tank docked to the craft,  to effect the transfer tank-to-spacecraft with pumps.  Then it de-spins and returns the tank to the depot. You do this twice:  once for the fuel,  then again for oxidizer.  See Figure 1.  Vapor exchange is required along with liquid transfer,  per Ref. 1. 

Figure 1 – Spinning-Tank Technology Applied to Cryogenic Refill On-Orbit

Transfers to the depot from tanker craft work exactly the same way:  detach the empty tank to be filled,  move off and spin up the docked tank-and-spacecraft,  transfer the liquid (and vapor),  then de-spin and return the filled tank to the depot.  See again Figure 1

Spin-up/spin-down thruster propellants are minimized by the use of “rifle-bullet spin”,  and also very much by being limited to only the inertia of spacecraft plus tank,  not the entire depot or its entire assemblage of tanks!  It is end-to-end docking plus that “rifle-bullet spin”,  of only the tank and visiting craft,  so the moment of inertia,  and total impulse required of the spin thrusters,  is minimized.  This was well-discussed in Ref. 1.  At about 1 rpm and near 2 m radii,  the spin induces about 0.0022 gee (corrected value 2-4-22)

That's for the cryogenic materials only.  The storables are in the same sort of bladdered tanks as the spacecraft that use them.  You just dock with the depot module and pump,  from bladdered tank to bladdered tank.  Pressurant gases are involved to push on the bladders.  There is no spin needed.

The only design requirements imposed on the craft transferring-out cryogenic liquids,  are the need for propellant suction points out on the tank periphery,  as well as on the aft domes,  plus the presence of perforated radial baffles in the tank that speed the spin-up of the liquid globules inside.  See again Ref. 1.  Craft that only receive liquids do not need periphery drains or baffles,  spin or not.

Depot Station Layout

Excepting kerosene,  the bladdered storables are primarily for attitude/maneuvering thrusters.  Those quantities are far lower,  no matter which combination you desire.  So there are hydrazine tanks for around a 1 cubic meter each of hydrazine,  MMH,  UDMH,  and Aerozine-50,  plus tanks for about 5 cubic meters of NTO.   Those are 1-ton-class quantities of each fuel material,  and a 5-ton-class quantity of NTO to support any-and-all of them. 

The storable kerosene,  and the cryogenics LOX,  LCH4,  and LH2 are needed in far,  far larger quantities,  being main stage propellants.  The kerosene transfers just like the other storables,  except the tanks are far larger,  around 300 cubic meters for a nominal 250 ton quantity.  The LCH4 is near twice that volume,  and the LH2 around 11-12 times that volume.  This assumes very large receiving craft,  and roughly-equal frequency of demand for each fuel type,  which may or may not be actually true.  The shared LOX volume to support each of these is simply huge (at around 2300 cubic meters),  that being the largest volume stored on the depot station.

See Figure 2 for some crudely-estimated quantities.  These were done in an Excel spreadsheet.

Figure 2 – Crudely-Estimated Quantities for Storables and for Cryogens

Reboost and attitude control could be a combination of the storable propellants plus electric propulsion powered by some solar panels,  all located on (or in) a “power and propulsion module”.  This power and propulsion module would be at one end of the depot station.  The crew compartment would be at the other end.  You want the NTO tanks near one end,  as physically far from the 4 types of hydrazine tanks as possible,  to avoid any possibilities of explosions-upon-contact,  if there are ever any leaks.  These materials are hypergolic,  even in vacuum.

The large kerosene bladdered tanks should probably be at the same end as the hydrazine tanks,  just closer than the hydrazines to the center of mass.  The largest and heaviest item is the LOX tankage,  which should be near center of mass.  The LH2 tanks can be at the same end as the NTO storage,  just closer to the center of mass than the NTO.  I show the LCH4 tanks forward of the LOX,  just to better center the LOX on the center of gravity.  Because of the toxicity of NTO residues from any spills,  the crew compartment should be far away from the NTO storage.  That puts the propulsion and power module at the NTO end of the facility.  See Figures 3,  4,  and 5.

Figure 3 – The Bladdered-Tank Storable Modules

Those modules associated with bladdered tanks need an adjacent docking adapter for the visiting spacecraft,  with the tanks situated such that there is clearance to come in and dock.  Once docked,  you just hook up the transfer lines,  and do the fluid transfer.  This would also apply to tankers refilling the facility.  There is no spin,  and the pressurant gas supplies plus the plumbing and electrical,  are within the depot module truss.  The visiting craft undocks and then redocks elsewhere,  to switch from fuel to oxidizer.  There are manipulator arms on the modules to assist with the docking operations.

Figure 4 – The Cryogenic Tank Modules

Figure 5 – Recommended Facility Layout

The cryogenics are a different situation.  These large tanks must be perpendicular to the axis of the depot facility,  and must have appropriate docking gear on both ends of the tanks.  The visiting craft docks directly to the end of the tank,  and makes the fluid and vapor connections.  Then it detaches the tank from the depot,  and moves off a short distance (for safety).  The docked vehicle-and-tank gets spun up for the fluid transfer,  then de-spun once completed.  It redocks the tank with the station,  undoes the connections,  and finally undocks from the re-docked tank. 

If both fuel and oxidizer are cryogenic,  then one repeats this procedure with the appropriate other tanks.  There are a few relocatable manipulator arms that attach to the cryogenic tank docking structures,  which assist with all the docking operations.  Not every tank needs a manipulator arm.

Preventing Incorrect Connections

If the visiting vehicle is to be refilled from the facility,  its tanks need the plumbing connections to make the indicated hook-ups.  This is pretty simple for the storables,  including the kerosene.  For the cryogens,  the visiting vehicle does not necessarily need the perforated spin baffles or periphery fluid drains that are needed in the depot tanks.  But,  any tankers attempting to refill facility tanks will need these extra plumbing features,  as was indicated in Figure 1.

Since the oxidizer quantities are nearly always larger than the fuel quantities,  I recommend making the hose/plumbing connections and fittings for oxidizers about 1.5 to 2 times physically larger than the fuel fittings.  That prevents incorrect hook-ups,  something particularly important with the hypergolic hydrazines and NTO. 

However,  there also needs to be some sort of keying feature on all of the fuel connections,  so that only the correct fittings can be coupled,  even though they are of equal size.  You do not want to mix species among the hydrazines,  or get kerosene when you wanted a hydrazine,  or vice versa. 

You also need to key the oxidizer fittings by species,  just in case IRFNA or high-test H2O2 ever get added to the facility.  It would be harder,  but not impossible,  to mix up NTO and LOX,  just because of the difference in bladdered versus spinning-tank technologies.  But keying the fittings absolutely stops that.

These fitting sizes and keying features are a critical safety requirement,  and should be standardized “up front”,  so that everyone is using the same equipment!

Power and Propulsion Module

This item (also shown in Figure 5) has the thrusters and the solar panels required to power the entire facility,  and to move it as needed.  Those thrust applications would include reboost for orbital decay,  deorbit at end-of-life,  debris avoidance maneuvers,  and general attitude control.  I’d recommend something similar to the Space Shuttle “OMS” and attitude-thruster systems,  which were a hydrazine variant-NTO thruster design,  plus some sort of electric thruster,  for its far-higher specific impulse,  for the reboost operations and any other orbit changes. 

My own preference for the electric thruster would be an iodine thruster,  for the easy fuel storage and handling,  even though those are not yet very common,  or even in wide use.  The bigger the electric system,  the more solar panels would be needed on this module,  and there are geometric limits to that.

Crew Habitat and Docking Module

The layout in Figure 5 also shows a crew habitation module at the other end from the power and propulsion module.  There is attached to it a multi-port docking module for crew transfer vehicles.  Crews would be required during any refilling operations for visiting vessels,  or for tankers re-supplying the facility.  Otherwise,  there is no need for crew aboard,  not even during transits to other orbits.  Crews would be aboard relatively short-term for the necessary fluid transfer operations.  Long term occupancy (like the International Space Station) is not an issue here!

Options For Spinning-Up (and De-Spinning) Cryo-Tank and Vessel

The concepts selected here do minimize the thruster propellant quantities required for spin-up and de-spin,  assuming thrusters are used for this purpose.  That is the historically-proven way to do it. 

However,  adding a flywheel for electric torque spin-up and de-spin is another viable (if undemonstrated) option.  This flywheel should probably be within the docking adapter on the end of the tank adjacent to the vessel,  so as to be nearest the docked center of gravity. 

The flywheel will need to be rather massive,  and will spin quite fast,  so that its angular momentum magnitude equals the angular momentum magnitude of the docked cryo tank and vessel.

Adding a “Go-To-The-Job” Capability Is Unattractive

If you want this depot facility to take on the role of scavenging abandoned spent stages for their propellants,  before de-orbiting them,  then you need to add very considerable extra propulsive capability to the depot station.  It would have to go and rendezvous with every target spent stage. 

You do that with very much larger fuel tanks for the electric propulsion and for the storable-powered thruster,  both located in the power and propulsion module.  That option is also shown in Figure 5,  as extra tanks to be added surrounding the power and propulsion module.  The orbital changes to do this are made unmanned,  controlled from the ground.  You send up crews temporarily,  only when propellant transfers are to be made.

Otherwise,  there is just no need for a facility propulsive capacity that large!  That is why the option takes the form of added tanks.  Everything else is the same.  Although,  choosing instead to create a specialized vehicle to operate as a “space tug” to go get the spent stage,  and transport it to the facility,  is probably the better concept,  and by far.  Such a space tug is just not that massive,  while this propellant depot facility is simply enormous.

Thrust Sizing

But in any event,  the thrust-induced accelerations need to be quite low,  in order to avoid large bending loads in the cryogen tank attachments.  Accelerations on the order of 0.01 gee or less should be fine,  with the storable thruster.  It is quite likely the electric propulsion would provide accelerations more in the ballpark of 0.0001 gee.  While very low indeed,  that’s all right:  the electric propulsion would only be used while unmanned,  to make significant orbital changes that are not time critical.  More time-critical adjustments (such as debris avoidance),  would use the storable-propellant thruster.

References

#1. “Propellant Ullage Problem and Solutions”,  Gary W. Johnson,  posted 18 August 2021 to http://exrocketman.blogspot.com

#2. “A Detailed Historical Review of Propellant Management Devices for Low Gravity Propellant Acquisition”,  Jason W. Hartwig,  NASA Glenn Research Center, Cleveland, OH, 44135, USA; undated AIAA paper but ca. 2015;  (pretty much the same information was also in the Hartwig Ph.D. dissertation).

For articles posted on the “exrocketman” site,  use the navigation utility left side of page.  Click on the year,  then on the month,  then if need be on the title.