Starship On-orbit refueling - Options and Discussion

[Paul451]

so that's why you can round off the solar flux by pointing the nose at the sun.

Yeah, but MMOD says that not pointing the nose in the direction of orbit is a bad idea.

[Twark_Main]

so that's why you can round off the solar flux by pointing the nose at the sun.

Yeah, but MMOD says that not pointing the nose in the direction of orbit is a bad idea.

If you look at the MMOD spatial distribution there's a big flux from the "sidelobes" and from above. You'd ideally be oriented heatshield up, nose forward, and with the nose pitched up (ie down toward Earth) by ~30°. This orients the tiles to shadow the tank walls from the most common MMOD angles.


I don't think it matters too much, however. Starship mostly deals with MMOD by orbiting low and keeping missions short, eg Tankers and Pez dispensers. Mars- and Moon-bound ships leave orbit after tanking in LLEO.

Depots (and maybe lunar ships) stay in orbit for a significant length of time, but they are surrounded by an insulating system that usually doubles as a MMOD shield.

[Vultur]

If the idea is a quite low LEO, well below Starlink or ISS (like 200-250 km) is there much debris there?

[Roy_H]

If the idea is a quite low LEO, well below Starlink or ISS (like 200-250 km) is there much debris there?

About zero. At this low altitude air resistance removes any debris within about 6 months.

[Twark_Main]

If the idea is a quite low LEO, well below Starlink or ISS (like 200-250 km) is there much debris there?

About zero. At this low altitude air resistance removes any debris within about 6 months.

I mean, I suppose ~3 days is "within 6 months."   

Of course new debris is constantly being added as it decays from higher orbits, slowly "raining" down on those lower altitudes. So low orbits are safe not because all the debris has been cleared out, but because the debris moves though those altitudes quickly so it only has a short time to collide with anything.

You made me curious so I did the math. Every orbital lifespan calculation is based on the area to mass ratio of the debris. So technically you could have a debris object at 250 km that lasts 6 months. However, the object would have to be a solid sphere of tungsten 5 feet in diameter.   

[TheRadicalModerate]

You made me curious so I did the math. Every orbital lifespan calculation is based on the area to mass ratio of the debris. So technically you could have a debris object at 250 km that lasts 6 months. However, the object would have to be a solid sphere of tungsten 5 feet in diameter.

The lifespan calculation should really be based on ballistic coefficient.  It's probably fine to model all debris as having a particular coefficient of drag, which I assume is baked into the 0.01m²/kg value.  (A/m = 0.01 implies BC*Cd = 100.)  But we're also going to care about the drag on the depot, which likely has a substantially different Cd than your average hunk of space junk.  Just something to bear in mind.

Your solid sphere of tungsten will also have a much lower Cd than garden-variety space junk.  So its orbital life will be considerably longer than what you've calculated.

[Twark_Main]

You made me curious so I did the math. Every orbital lifespan calculation is based on the area to mass ratio of the debris. So technically you could have a debris object at 250 km that lasts 6 months. However, the object would have to be a solid sphere of tungsten 5 feet in diameter.

The lifespan calculation should really be based on ballistic coefficient.

Yes, that fact is exactly what I was calling attention to. That's why I wrote that fun little aside. 

It's probably fine to model all debris as having a particular coefficient of drag, which I assume is baked into the 0.01m²/kg value.  (A/m = 0.01 implies BC*Cd = 100.)

Yes, different ways to write the same thing.

But we're also going to care about the drag on the depot, which likely has a substantially different Cd than your average hunk of space junk.  Just something to bear in mind.

Your solid sphere of tungsten will also have a much lower Cd than garden-variety space junk.  So its orbital life will be considerably longer than what you've calculated.

Satellite Cd is typically assumed to be 2.2, and varies mostly with orbital height. This is already taken into account.

A sphere was more-or-less arbitrarily chosen because it same the same drag in any orientation, so we don't have to worry about it naturally re-orienting into some configuration that has higher or lower drag.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024SW003974

https://ui.adsabs.harvard.edu/abs/1965P&SS...13..929C/abstract

[envy887]

Does drag coefficient mean anything at 250 km? The mean free path should be about the size of the object.

[rsdavis9]

Does drag coefficient mean anything at 250 km? The mean free path should be about the size of the object.

Did he mean "ballistic coefficient"?
Just frontal area and weight should be all you need.

[TheRadicalModerate]

Does drag coefficient mean anything at 250 km? The mean free path should be about the size of the object.

There's still drag, so there's some scaling between the speed, the area, and the drag force. 

The real question is whether there's something like dynamic pressure.  But even there, if a big flat thing comes along and whacks a molecule, the molecule goes flying off in the direction it was whacked, and the whacker loses a corresponding amount of momentum.  Voilà!  Drag.

[TheRadicalModerate]

Does drag coefficient mean anything at 250 km? The mean free path should be about the size of the object.

Did he mean "ballistic coefficient"?
Just frontal area and weight should be all you need.

I'm picking at this because area per mass is a weird metric, and I don't understand why they didn't use a ballistic coefficient.  Somebody's normalizin' stuff without explaining why.

drag = ½ρv²C_dA, but you're usually interested in the deceleration, so you divide by m.  Instead, you can use the ballistic coefficient C_b = m/ (C_dA), and rewrite the drag as:

deceleration = ½ρv²/C_b

Big ballistic coefficient, small deceleration.  Small BC, large deceleration.

But notice that A/m, what they're using in their chart, is C_dC_b.  That's a... metric... but I don't know why you'd normalize to that.

[Twark_Main]

Does drag coefficient mean anything at 250 km? The mean free path should be about the size of the object.

Did he mean "ballistic coefficient"?
Just frontal area and weight should be all you need.

I'm picking at this because area per mass is a weird metric, and I don't understand why they didn't use a ballistic coefficient.  Somebody's normalizin' stuff without explaining why.

drag = ½ρv²C_dA, but you're usually interested in the deceleration, so you divide by m.  Instead, you can use the ballistic coefficient C_b = m/ (C_dA), and rewrite the drag as:

deceleration = ½ρv²/C_b

Big ballistic coefficient, small deceleration.  Small BC, large deceleration.

But notice that A/m, what they're using in their chart, is C_dC_b.  That's a... metric... but I don't know why you'd normalize to that.

Check out the links I sent. For orbits below 500 km, it's common practice for satellites to assume a C_D (usually 2.2), because for satellites the shape doesn't matter as much as it matters for aircraft, cars, etc.

[TheRadicalModerate]

Check out the links I sent. For orbits below 500 km, it's common practice for satellites to assume a C_D (usually 2.2), because for satellites the shape doesn't matter as much as it matters for aircraft, cars, etc.

Yeah, but A/m is still a weird metric.  They could have just used BC=4.5 for A/m=0.1  ( BC = 1/(0.1*2.2) ), and everybody would have understood what they were doing.

[Twark_Main]

Check out the links I sent. For orbits below 500 km, it's common practice for satellites to assume a C_D (usually 2.2), because for satellites the shape doesn't matter as much as it matters for aircraft, cars, etc.

Yeah, but A/m is still a weird metric.  They could have just used BC=4.5 for A/m=0.1  ( BC = 1/(0.1*2.2) ), and everybody would have understood what they were doing.

I will tell... checks source...  Johnson Space Center Space Physics Branch all about it. 

[rsdavis9]

Check out the links I sent. For orbits below 500 km, it's common practice for satellites to assume a C_D (usually 2.2), because for satellites the shape doesn't matter as much as it matters for aircraft, cars, etc.

Yeah, but A/m is still a weird metric.  They could have just used BC=4.5 for A/m=0.1  ( BC = 1/(0.1*2.2) ), and everybody would have understood what they were doing.

I will tell... checks source...  Johnson Space Center Space Physics Branch all about it. 

Ok stupid question:

At high altitude where it is just atoms/molecules hitting the surface is the impulse still normal to the surface? For example a 45deg barn door(inclined to direction of travel) gets an impulse from photons normal to its surface and not parallel to the direction of travel. How about the atoms/molecules?

Basically does it get lift?

[Chatskiy]

At high altitude where it is just atoms/molecules hitting the surface is the impulse still normal to the surface? For example a 45deg barn door(inclined to direction of travel) gets an impulse from photons normal to its surface and not parallel to the direction of travel. How about the atoms/molecules?

Basically does it get lift?

The barn door will still get impulse transfer normal to the surface - physics doesn't care if you are in orbit.
Unfortunately this does not generate "lift", quite the opposite.

The retrograde (backwards) part of the impulse slows the door and lowers the orbit, while anti-radial (up, away from the Earth) impulse "pivots" the orbit around the Earth moving apogee/perigee while not changing the overall size.

The overall effect would be a lower and more elliptical orbit.

[Striker58]

In terms of the depot itself, rather than using an existing would it be more effective to use an 'inflatable unit' such as Siera Space LIFE?  It would provide a larger volume for propelant than a fixed volume from an existing 2nd stage, and potential provide better thermal efficiency using multiple layes within the structure.

Orbital mobility could come from a tug such as Helios with extenible fueling umbilicals from a dedicted service module based of something like Haven to link with the standard fueling ports on Starship, removing the need for a close docking.

[TheRadicalModerate]

In terms of the depot itself, rather than using an existing would it be more effective to use an 'inflatable unit' such as Siera Space LIFE?  It would provide a larger volume for propelant than a fixed volume from an existing 2nd stage, and potential provide better thermal efficiency using multiple layes within the structure.

Orbital mobility could come from a tug such as Helios with extenible fueling umbilicals from a dedicted service module based of something like Haven to link with the standard fueling ports on Starship, removing the need for a close docking.

Vessels designed to hold air at room temperature don't do so well at holding cryogenic liquids.

[Twark_Main]

In terms of the depot itself, rather than using an existing would it be more effective to use an 'inflatable unit' such as Siera Space LIFE?  It would provide a larger volume for propelant than a fixed volume from an existing 2nd stage, and potential provide better thermal efficiency using multiple layes within the structure.

Orbital mobility could come from a tug such as Helios with extenible fueling umbilicals from a dedicted service module based of something like Haven to link with the standard fueling ports on Starship, removing the need for a close docking.

Vessels designed to hold air at room temperature don't do so well at holding cryogenic liquids.

Specifically, while the Kevlar straps tend to do just fine, the "bladder" layer that actually holds pressure turns to glass at cryogenic temperatures.

[Paul451]

Meanwhile, non-cryo propellants tend to be chemically aggressive to most flexible airtight materials. Solvents and oxidisers.