Starship On-orbit refueling - Options and Discussion

[TheRadicalModerate]

Re number 2) HEEO-based refuelling: I think specifically, the conops where you fill up LSS in vLEO, then top it up during a single orbit in vLEO+1600-2000 m/s with a buddy tanker, rendezvousing shortly after the first burn - seems to me the most obvious way to handle that case, and as good a reason as any to set a lower limit on how slowly the system will pump propellant. In that case you would need to transfer 500-600t of prop during a 3-4 hour orbit (~50kg/s). So presumably the pump power can be less than 1kW, but not by very much.

VLEO+2000 is about a 5.5 hour orbit.  Being able to do RPOD, pre-transfer checkout, post-transfer prep, undocking/prox ops, and pre-insertion checkout in 2.5 hours seems pretty optimistic.

[Twark_Main]

Re number 2) HEEO-based refuelling: I think specifically, the conops where you fill up LSS in vLEO, then top it up during a single orbit in vLEO+1600-2000 m/s with a buddy tanker, rendezvousing shortly after the first burn - seems to me the most obvious way to handle that case, and as good a reason as any to set a lower limit on how slowly the system will pump propellant. In that case you would need to transfer 500-600t of prop during a 3-4 hour orbit (~50kg/s). So presumably the pump power can be less than 1kW, but not by very much.

VLEO+2000 is about a 5.5 hour orbit.  Being able to do RPOD, pre-transfer checkout, post-transfer prep, undocking/prox ops, and pre-insertion checkout in 2.5 hours seems pretty optimistic.

... it seems pretty SpaceX. I approve.

[TheRadicalModerate]

Random thought from another thread:

It seems reasonable always to point a depot's nose at the Sun.  That way, the ogive portion of the nose can act as a dewar for the LCH4 tank below it, and very little incident radiation should strike the LCH4 or LOX tank walls.  (The Sun has an angular diameter of 0.53º, so the total incident flux should be sin (0.53º/2) = 0.0046 * solarConstant = 6.3W/m², which is almost nothing.)

However, that leaves the albedo radiation reflected/emitted from Earth.  I believe the average flux at mid-latitude inclinations is about 250W/m².

Let's divide the sphere that the depot sees into three sections:

1) The Sun.  Dealt with via pointing at it.
2) The sunlit or nighttime Earth.  250W/m², over some variable viewing angle.
3) Empty space, which is presumably cold.

Inside the depot's tanks, we will have:

a) Conductive heating when a blob of prop touches a hot tank wall.  Tank walls should only be truly hot if they're facing the sunlit Earth.

b) Conductive/convective heating, where the ullage gas picks up heat from a tank wall and transmits it thermally to a blob of liquid prop.

c) Radiative heating, where a hot wall emits IR directly into a blob of prop (bad), or some other section of wall (considerably less bad).

Radiative heating ought to be fairly modest, because any particular spot on a tank wall ought to cycle between fairly hot while directly heated by Earth and cooling when pointed at the Earth nightside or at empty space.  flux = εσT⁴, so even modest declines in the temperature of the inside wall result in much less radiative heating.

So what if we apply continuous, extremely low acceleration toward the Earth?  That should put whatever prop is in the depot as far away from a hot wall as possible, minimizing conductive heating.  This won't help much if the depot is mostly full, but before it's more than half full, this should dramatically reduce heat transfer.

Does this make sense?

Note that continuous radial acceleration in a circular orbit doesn't actually change the shape of your orbit.  It just makes you go around faster, making your true anomaly larger than it would be if you were in freefall.  So the navigational consequences of this are easy to work around.

This also requires pretty fancy RCS management to achieve, since we're holding the attitude of the depot fixed by pointing it at the Sun.  Seems like a reasonable problem, though.  Also, I'd think that whatever boiloff does occur should be able to power cold gas thrusters, both for the radial ullage burn and the burns necessary to counteract Earth's tidal forces, which want the nose to point at the center of the Earth, rather than at the Sun.

[InterestedEngineer]

Random thought from another thread:

It seems reasonable always to point a depot's nose at the Sun.  That way, the ogive portion of the nose can act as a dewar for the LCH4 tank below it, and very little incident radiation should strike the LCH4 or LOX tank walls.  (The Sun has an angular diameter of 0.53º, so the total incident flux should be sin (0.53º/2) = 0.0046 * solarConstant = 6.3W/m², which is almost nothing.)

However, that leaves the albedo radiation reflected/emitted from Earth.  I believe the average flux at mid-latitude inclinations is about 250W/m².

Let's divide the sphere that the depot sees into three sections:

1) The Sun.  Dealt with via pointing at it.
2) The sunlit or nighttime Earth.  250W/m², over some variable viewing angle.
3) Empty space, which is presumably cold.

Inside the depot's tanks, we will have:

a) Conductive heating when a blob of prop touches a hot tank wall.  Tank walls should only be truly hot if they're facing the sunlit Earth.

b) Conductive/convective heating, where the ullage gas picks up heat from a tank wall and transmits it thermally to a blob of liquid prop.

c) Radiative heating, where a hot wall emits IR directly into a blob of prop (bad), or some other section of wall (considerably less bad).

Radiative heating ought to be fairly modest, because any particular spot on a tank wall ought to cycle between fairly hot while directly heated by Earth and cooling when pointed at the Earth nightside or at empty space.  flux = εσT⁴, so even modest declines in the temperature of the inside wall result in much less radiative heating.

So what if we apply continuous, extremely low acceleration toward the Earth?  That should put whatever prop is in the depot as far away from a hot wall as possible, minimizing conductive heating.  This won't help much if the depot is mostly full, but before it's more than half full, this should dramatically reduce heat transfer.

Does this make sense?

Note that continuous radial acceleration in a circular orbit doesn't actually change the shape of your orbit.  It just makes you go around faster, making your true anomaly larger than it would be if you were in freefall.  So the navigational consequences of this are easy to work around.

This also requires pretty fancy RCS management to achieve, since we're holding the attitude of the depot fixed by pointing it at the Sun.  Seems like a reasonable problem, though.  Also, I'd think that whatever boiloff does occur should be able to power cold gas thrusters, both for the radial ullage burn and the burns necessary to counteract Earth's tidal forces, which want the nose to point at the center of the Earth, rather than at the Sun.

Good and interesting hypothesis.  Now do the math.

There's also a heat shield that can be pointed at the Earth, so the Earth is only heating the tail and the heat shield.  The tiles should absorb all that heat and release it right back at the Earth, and the  insulation will keep the stainless steel cool on that side.

[Greg Hullender]

It seems reasonable always to point a depot's nose at the Sun.

Is it, though? As we've discussed upthread, if it is far away from Earth or the moon, a depot coated in Solar White tiles will get cold enough to freeze oxygen (or close to it), even though it's otherwise unshielded from the direct sun. What messes this up in LEO is the warmth from the Earth, which comes at inconvenient wavelengths.

In that case, why not make the depot long and thin and keep it pointed at the Earth, not the sun? In that case, if you want some small amount of microgravity, hang a counterweight on a long cable from the nose of the depot in the direction of the Earth. (I haven't figured out how much mass/cable you'd need to get, say, 50 micro-g; it seems to involve solving a cubic, and I'm too sleepy at the moment . . .)

[Robotbeat]

Problem is you can't "point" toward the Earth as it's nearly half the sky in LEO. So...

[TheRadicalModerate]

Good and interesting hypothesis.  Now do the math.

There's also a heat shield that can be pointed at the Earth, so the Earth is only heating the tail and the heat shield.  The tiles should absorb all that heat and release it right back at the Earth, and the  insulation will keep the stainless steel cool on that side.

The math is hard.

I doubt that a depot will have a heat shield.  And if it did, it's much more likely to be SOFI or some kind of bolt-on MLI (although I'm still not sure that works) than a refractory shield plus insulation.¹

It seems reasonable always to point a depot's nose at the Sun.

Is it, though? As we've discussed upthread, if it is far away from Earth or the moon, a depot coated in Solar White tiles will get cold enough to freeze oxygen (or close to it), even though it's otherwise unshielded from the direct sun. What messes this up in LEO is the warmth from the Earth, which comes at inconvenient wavelengths.

The depot is going to be in LEO, not in NRHO or some other BEO location.  So Earth radiation is at least a medium deal.

Also, when we say "Solar White", remember that we're talking about a coating that, last I saw, didn't actually exist.  The 90K equilibrium temperature is only achieved if the reflectance is 99% (i.e., the incident power absorbed is 1%) and the emissivity is 90%.  I suspect that there are coatings that are better than any currently in use, but they're unlikely to get down to the 90K level.

In that case, why not make the depot long and thin and keep it pointed at the Earth, not the sun? In that case, if you want some small amount of microgravity, hang a counterweight on a long cable from the nose of the depot in the direction of the Earth. (I haven't figured out how much mass/cable you'd need to get, say, 50 micro-g; it seems to involve solving a cubic, and I'm too sleepy at the moment . . .)

Because solar irradiance is about 1366W/m², and Earth irradiance maxes out at 250W/m².

_____________
¹Does anybody know if Centaur still uses the bolt-on/blow-off MLI panels?  And if so, what's on the outside of them to keep them aerodynamically intact?

Centaur used to blow those puppies off shortly after fairing jettison using pyrotechnics.  That would obviously be a problem for a vanilla-flavored Starship, but for a depot, you'd just keep them in place forever.  I'm just not sure that a vehicle as big as Starship could keep them in place reliably during launch.

[OTV Booster]

Random thought from another thread:

It seems reasonable always to point a depot's nose at the Sun.  That way, the ogive portion of the nose can act as a dewar for the LCH4 tank below it, and very little incident radiation should strike the LCH4 or LOX tank walls.  (The Sun has an angular diameter of 0.53º, so the total incident flux should be sin (0.53º/2) = 0.0046 * solarConstant = 6.3W/m², which is almost nothing.)

However, that leaves the albedo radiation reflected/emitted from Earth.  I believe the average flux at mid-latitude inclinations is about 250W/m².

Let's divide the sphere that the depot sees into three sections:

1) The Sun.  Dealt with via pointing at it.
2) The sunlit or nighttime Earth.  250W/m², over some variable viewing angle.
3) Empty space, which is presumably cold.

Inside the depot's tanks, we will have:

a) Conductive heating when a blob of prop touches a hot tank wall.  Tank walls should only be truly hot if they're facing the sunlit Earth.

b) Conductive/convective heating, where the ullage gas picks up heat from a tank wall and transmits it thermally to a blob of liquid prop.

c) Radiative heating, where a hot wall emits IR directly into a blob of prop (bad), or some other section of wall (considerably less bad).

Radiative heating ought to be fairly modest, because any particular spot on a tank wall ought to cycle between fairly hot while directly heated by Earth and cooling when pointed at the Earth nightside or at empty space.  flux = εσT⁴, so even modest declines in the temperature of the inside wall result in much less radiative heating.

So what if we apply continuous, extremely low acceleration toward the Earth?  That should put whatever prop is in the depot as far away from a hot wall as possible, minimizing conductive heating.  This won't help much if the depot is mostly full, but before it's more than half full, this should dramatically reduce heat transfer.

Does this make sense?

Note that continuous radial acceleration in a circular orbit doesn't actually change the shape of your orbit.  It just makes you go around faster, making your true anomaly larger than it would be if you were in freefall.  So the navigational consequences of this are easy to work around.

This also requires pretty fancy RCS management to achieve, since we're holding the attitude of the depot fixed by pointing it at the Sun.  Seems like a reasonable problem, though.  Also, I'd think that whatever boiloff does occur should be able to power cold gas thrusters, both for the radial ullage burn and the burns necessary to counteract Earth's tidal forces, which want the nose to point at the center of the Earth, rather than at the Sun.

I'm quite fond of CMG's (Control Moment Gyro) for attitude control. They weigh a bit but use propellant only when desaturating. Desaturation can be done as convenient, allowing boiloff time to build enough pressure to make hot gas more practical. With luck and some creative hardware, boiloff might be low enough that the higher ISP of hot gas would be welcome.

[Twark_Main]

It seems reasonable always to point a depot's nose at the Sun.

Is it, though? As we've discussed upthread, if it is far away from Earth or the moon, a depot coated in Solar White tiles will get cold enough to freeze oxygen (or close to it), even though it's otherwise unshielded from the direct sun.

Vera Rubin is going to love that....

One good approach is to keep the nose slightly "nose up" with respect to the Sun. Some additional insolation falls on the insulated tiles, but it's worth it to shade the uninsulated stainless backside.

Problem is you can't "point" toward the Earth as it's nearly half the sky in LEO. So...

You can do a pretty good job, regardless.

When the Sun is at a low angle, you point the black tiles at the Earth.

As the Sun climbs higher (and your nose pitches up to track it), you roll, keeping the black tiles aimed at the more sunlit half of the Earth—rolling clockwise when the the Sun is on the left side of the sky, anti-clock when it's on the right.

This attitude minimizes both thermal/albedo heating from Earth and nuisance reflections, because the brightest/warmest part of the Earth is seeing the black insulated tiles.

After half an orbit the Sun is now "behind" you, and the vehicle has rolled 180°, so the black tiles are again pointing at the Earth. The nose remains pointed at the Sun.



CMGs are great here. The ISS CMG would be more than sufficient, and (with decades-old technology) it masses about 1 metric ton.


You can use the same CMG during the trip to Mars. During a solar storm the main radiation direction "wiggles around" a lot, and if you can continuously align the vehicle in that direction you get improved radiation protection for the same mass. The motion is not fast (<0.1 °/s), so it shouldn't cause discomfort.

[OTV Booster]

It seems reasonable always to point a depot's nose at the Sun.

Is it, though? As we've discussed upthread, if it is far away from Earth or the moon, a depot coated in Solar White tiles will get cold enough to freeze oxygen (or close to it), even though it's otherwise unshielded from the direct sun.

Vera Rubin is going to love that....

One good approach is to keep the nose slightly "nose up" with respect to the Sun. Some additional insolation falls on the insulated tiles, but it's worth it to shade the uninsulated stainless backside.

Problem is you can't "point" toward the Earth as it's nearly half the sky in LEO. So...

You can do a pretty good job, regardless.

When the Sun is at a low angle, you point the black tiles at the Earth.

As the Sun climbs higher (and your nose pitches up to track it), you roll, keeping the black tiles aimed at the more sunlit half of the Earth—rolling clockwise when the the Sun is on the left side of the sky, anti-clock when it's on the right.

This attitude minimizes both thermal/albedo heating from Earth and nuisance reflections, because the brightest/warmest part of the Earth is seeing the black insulated tiles.

After half an orbit the Sun is now "behind" you, and the vehicle has rolled 180°, so the black tiles are again pointing at the Earth. The nose remains pointed at the Sun.



CMGs are great here. The ISS CMG would be more than sufficient, and (with decades-old technology) it masses about 1 metric ton.


You can use the same CMG during the trip to Mars. During a solar storm the main radiation direction "wiggles around" a lot, and if you can continuously align the vehicle in that direction you get improved radiation protection for the same mass. The motion is not fast (<0.1 °/s), so it shouldn't cause discomfort.

The ISS has four, so figure four tons. Ahh, but ISS masses 419t, and SS depot variant clocks on at 85t (dry) + 1600t propellant + 20t (guesstimate for cryo cooling, shades, PV and a hefty battery pack). Call it 1705t. Have I slipped a decimal somewhere? A loaded SS depot is 4x the mass of the ISS?

[Twark_Main]

The ISS has four, so figure four tons.

Each CMG is 600 lbs (of that, the spinning mass itself is 220 lb). Accounting for sig figs, that's a total system mass of 1.0-1.2 metric tons.

Ahh, but ISS masses 419t, and SS depot variant clocks on at 85t (dry) + 1600t propellant + 20t (guesstimate for cryo cooling, shades, PV and a hefty battery pack). Call it 1705t. Have I slipped a decimal somewhere? A loaded SS depot is 4x the mass of the ISS?

Angular momentum is what really matters, not mass. That's a product of the moment of inertia and the maximum rate of rotation.

The ISS is over 100 meters long, so it has a large moment of inertia per mass. For Starship most of the mass is concentrated in the tanks, so it's nice and compact.

A depot is all tanks, of course, but it's still half as long (you said ~1700 tonnes so no stretch tanker). The way moment of inertia scales, that means ~1/4 the moment of inertia per mass.

And of course we're rolling the Starship, which is the axis with the lowest moment of inertia.

[TheRadicalModerate]

The ISS has four, so figure four tons.

Each CMG is 600 lbs (of that, the spinning mass itself is 220 lb). Accounting for sig figs, that's a total system mass of 1.0-1.2 metric tons.

Ahh, but ISS masses 419t, and SS depot variant clocks on at 85t (dry) + 1600t propellant + 20t (guesstimate for cryo cooling, shades, PV and a hefty battery pack). Call it 1705t. Have I slipped a decimal somewhere? A loaded SS depot is 4x the mass of the ISS?

Angular momentum is what really matters, not mass. That's a product of the moment of inertia and the maximum rate of rotation.

The ISS is over 100 meters long, so it has a large angular momentum per mass. For Starship most of the mass is concentrated in the tanks, so it's nice and compact for a low moment of inertia.

A depot is all tanks, of course, but it's still half as long (you said ~1700 tonnes so no stretch tanker). The way moment of inertia scales, that means ~1/4 the moment of inertia per mass.

Can you use a CMG in a system with a center of mass that changes radically?  There's a pretty big difference between an empty depot and a full one.

[Twark_Main]

Can you use a CMG in a system with a center of mass that changes radically?

Three words: ISS construction sequence. 

[eriblo]

Can you use a CMG in a system with a center of mass that changes radically?

Three words: ISS construction sequence. 

Yes, a CMG does not care about the rest of the spacecraft - it will provide the same torques for the same durations regardless. Spacecraft mass properties will just determine how far that torque gets you with regard to rotations.

[OTV Booster]

The ISS has four, so figure four tons.

Each CMG is 600 lbs (of that, the spinning mass itself is 220 lb). Accounting for sig figs, that's a total system mass of 1.0-1.2 metric tons.

Ahh, but ISS masses 419t, and SS depot variant clocks on at 85t (dry) + 1600t propellant + 20t (guesstimate for cryo cooling, shades, PV and a hefty battery pack). Call it 1705t. Have I slipped a decimal somewhere? A loaded SS depot is 4x the mass of the ISS?

Angular momentum is what really matters, not mass. That's a product of the moment of inertia and the maximum rate of rotation.

The ISS is over 100 meters long, so it has a large moment of inertia per mass. For Starship most of the mass is concentrated in the tanks, so it's nice and compact.

A depot is all tanks, of course, but it's still half as long (you said ~1700 tonnes so no stretch tanker). The way moment of inertia scales, that means ~1/4 the moment of inertia per mass.

And of course we're rolling the Starship, which is the axis with the lowest moment of inertia.

Didn't realize you were quoting system mass, so yeah, ~a ton.

CMB's CMG,smight might work out during transfer ops too. How tight the two ships would be physically bound is an open question as is how RCS and ullage settling would be implemented. One ship to rule them all or a cooperative effort? If they're lashed up tight and only the depot supplies the off axis settling thrust, a set of CMB's CMG's would allow attitude control without thrusters. The hard 'blap' from the F9 cold gas RCS strikes me as a bit too impulsey for a lashup.

[OTV Booster]

The ISS has four, so figure four tons.

Each CMG is 600 lbs (of that, the spinning mass itself is 220 lb). Accounting for sig figs, that's a total system mass of 1.0-1.2 metric tons.

Ahh, but ISS masses 419t, and SS depot variant clocks on at 85t (dry) + 1600t propellant + 20t (guesstimate for cryo cooling, shades, PV and a hefty battery pack). Call it 1705t. Have I slipped a decimal somewhere? A loaded SS depot is 4x the mass of the ISS?

Angular momentum is what really matters, not mass. That's a product of the moment of inertia and the maximum rate of rotation.

The ISS is over 100 meters long, so it has a large angular momentum per mass. For Starship most of the mass is concentrated in the tanks, so it's nice and compact for a low moment of inertia.

A depot is all tanks, of course, but it's still half as long (you said ~1700 tonnes so no stretch tanker). The way moment of inertia scales, that means ~1/4 the moment of inertia per mass.

Can you use a CMG in a system with a center of mass that changes radically?  There's a pretty big difference between an empty depot and a full one.

Don't see why not. Just have to expect any specific input to not always give the same results.


With a continuously updated mass distribution model the depot (I'm assuming CMG's are depot specific) would have a rough idea of what input is needed and any inaccuracies in the model would only call for a bit of trim. Things might move slower as the depot fills up but that would be countered by the lashup center of mass migrating closer to the depots stand alone center of mass as it fills up.

[OTV Booster]

While writing the two posts above it struck me that if the depot and other ship are physically tightly bound the control systems had also better be tightly bound.


Would it be best if the Depot completely took over and controlled both ships? Every cooperative arrangement that I come up with for settling thrust and attitude control seems to get more complicated the deeper I get into it. The last thing we want is the two ships fighting each other because of minor calibration issues.

[robot_enthusiast]

While writing the two posts above it struck me that if the depot and other ship are physically tightly bound the control systems had also better be tightly bound.


Would it be best if the Depot completely took over and controlled both ships? Every cooperative arrangement that I come up with for settling thrust and attitude control seems to get more complicated the deeper I get into it. The last thing we want is the two ships fighting each other because of minor calibration issues.

It seems like it would be a relatively straightforward thing to use the IMU's and star trackers from one ship to feed the control computers in both. AFAIK, they use Ethernet protocol networking to communicate within the vehicle, getting one of the flight computers to poll the network on the other ship shouldn't be that difficult. They generally seem to prefer to keep the control computers as close as physically possible to the thing they'll be controlling (for example, the engine controller is mounted directly to the side of each engine), so I think it's unlikely they would want to send commands across the ship-ship link. For simplicity they could also just have one ship be operating dumb, just providing a fixed amount of ullage thrust while the other handles orientation.

[oldAtlas_Eguy]

A SpaceX example of the Master/Slaving of the Guidance computer sets is the F9/D2 stack. The Dragon is the master of everything until separation. Once a stage separates away it is then its own master.

The SS and SH will likely operate similar to F9 in the implementation of the software. The question would be which of the two vehicles Tanker/Depot would have all the added software loaded to run as a super master of a pair of SS with the other having software to be SS slave. The SS operating as a slave would not be a normal as it is currently software set on a SS. That software would probably look very similar to the software on the SH to enable it to be a slave to the SS.

SpaceX has successfully done it before on F9 so doing it for the Depot/Tanker case should not prove that difficult. Especially once they prove out the software for the Master/Slave full stack Starship launch software packages.

[TheRadicalModerate]

While writing the two posts above it struck me that if the depot and other ship are physically tightly bound the control systems had also better be tightly bound.

Would it be best if the Depot completely took over and controlled both ships? Every cooperative arrangement that I come up with for settling thrust and attitude control seems to get more complicated the deeper I get into it. The last thing we want is the two ships fighting each other because of minor calibration issues.

It will certainly be true that the coupled pair of ships (tanker/depot, depot/target, or even tanker/target) will form a single controlled system.  Whether one ship acts as the master or they operate cooperatively is too far in the weeds even for me.

However, this has a direct impact on the physical coupling of the pair.  I doubt that it's advisable to have the pair tandem free-flying and get this to work--the control loop is just too diffuse, and even the kinematics of the pair will get weird.  Even a loosely coupled but flexible berthing will induce unpredictable oscillations that may be hard to damp out.

This is why I expect the berthing to be capable of handling decent-sized loads between the two ships with minimal flex and oscillation.


Just to review the bidding on things we've talked about for attitude control, prox ops, and ullage acceleration.  From smallest torques/translations to largest:

1) CMGs (I think you guys have convinced me that this might be useful).
2) Cold gas RCS, fed from ullage pressure only.
3) Cold gas RCS, fed from supercritical COPVs.
4) Combusting gas RCS/thrusters, fed from supercritical COPVs.

I'm pretty sure you need #4 no matter what, if for no other reason than ullage acceleration needs more impulse than can be readily generated without the enthalpy of combustion, and SpaceX likely has to develop them for lunar landing and takeoff ops.  Then the question becomes what combination of #1-#3 is needed for fine attitude control, both coupled (during refueling) and uncoupled (during attitude maintenance to minimize boil-off).

Just remember that the minimum impulse bit for any thrusters can be relatively large for Starship, because the total mass and moments of inertia are so large.