Author Topic: Starship On-orbit refueling - Options and Discussion  (Read 815132 times)

Offline Barley

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2220 on: 09/05/2023 02:20 am »

To be sure we're talking about the same thing, here's what I think we're discussing: The depot itself is in HEEO, with a period of a week or two. As a result, it likely only gets a refueling tanker to visit it once per orbit, so ten refuelings would take ten or twenty weeks.

Where you can send one ship you can send more than one.  In this case you could gather a fleet of ten tankers in LEO and have them burn in formation so they all rendezvous with the depot during the same orbit and transfer the fuel in less than a week.  You could fly the depot from LEO at the same time as the tankers.   You could also send five tankers at a time and take two orbits, etc.

There are tradeoffs between the number of ships used v. the length of time, but generally the fewer launch windows you have the more you want to send on each one of them.

Then there's the question of launching a mission from HEEO. Most likely, it has to rendezvous with the depot at perigee, do a plane change at apogee (since the HEEO is unlikely to be aligned with the mission target), and then do a big Oberth burn at perigee. But is that going to work? Unlike LEO, HEEO precesses rather slowly, so it seems to me that you'd be unlikely to be in the right place with respect to the Earth and the mission target. (The argument of periapsis will be wrong--and expensive to change.)
I suspect that for many years, and certainly for Artemis, the entire sequence of refueling will be preplanned for a particular mission.  The planning works backwards from the final goal, so everything is where you need it then you need it.  Depots will not usually be left in HEEO for the next mission, since they will be in the wrong orbit.  Depending on design and cost they will either be abandoned in orbit, disposed of by reentry or landed for reuse.

There might be contingencies to deal with occasional failures, such as scheduling 11 tankers when you need 10.  But if too many events miss their windows it's a mission fail, unless some clever clogs can figure an after the fact Hiten style rescue.


But couldn't you also get that by just having a second depot in GEO?
Did you mean GEO or GEO transfer orbit?  GEO itself is an awful place for a depot.  It's expensive to get to and the perigee is far too high for a good Oberth effect.  In the delta-V maps posted above you'll see GEO is a dead end.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2221 on: 09/05/2023 02:35 am »
I suspect that for many years, and certainly for Artemis, the entire sequence of refueling will be preplanned for a particular mission.  The planning works backwards from the final goal, so everything is where you need it then you need it.  Depots will not usually be left in HEEO for the next mission, since they will be in the wrong orbit.  Depending on design and cost they will either be abandoned in orbit, disposed of by reentry or landed for reuse.
Okay, this is the key point I missed; I assumed the depot was supposed to be reusable. If that's not the case, then I can see how HEEO could work.

Offline Brigantine

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2222 on: 09/05/2023 02:59 am »
Depots will not usually be left in HEEO for the next mission, since they will be in the wrong orbit.  Depending on design and cost they will either be abandoned in orbit, disposed of by reentry or landed for reuse.
Or perhaps (?) aerobraked into a vaguely ecliptic-aligned LEO, for re-use of the depot (though not of the high-energy orbit)

If a depot is able to EDL and re-use, what's the difference between that and a tanker?
(I guess it's still optimized to get to orbit with less fuel remaining and more empty tank, so still a valid variant)
« Last Edit: 09/05/2023 03:04 am by Brigantine »

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2223 on: 09/05/2023 10:47 am »
Barley did a great job addressing most of the other points. Just a couple things....


To be sure we're talking about the same thing, here's what I think we're discussing: The depot itself is in HEEO, with a period of a week or two. As a result, it likely only gets a refueling tanker to visit it once per orbit, so ten refuelings would take ten or twenty weeks. That could mean passive methods to reduce boiloff won't be adequate, so it'll need active cooling.

Why such a long orbital period? As you yourself point out, making that choice is causing problems for yourself.

Do the math on the extra delta-v gained by going from a 48-72 hour orbital period vs 2 weeks. It's only 0.1-0.2 km/s. So by choosing a more pragmatic (shorter) orbital period, effectively the tradeoff is that you "only" get a boost of 3.0-3.1 km/s, vs 3.2 km/s.


The depot will pass through the Van Allen belts over and over and over, so it'll need to be radiation hardened to some extent. The tankers will be exposed to a good bit more radiation as well (two passes each refueling), so they might need to be tougher as well.

Yet another reason why SpaceX was smart to make Starship so big.  :)

The penalty for radiation hardening gets smaller as the vehicle gets larger.
« Last Edit: 09/05/2023 11:08 am by Twark_Main »

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2224 on: 09/05/2023 11:01 am »
you could gather a fleet of ten tankers in LEO and have them burn in formation so they all rendezvous with the depot during the same orbit and transfer the fuel in less than a week

Ten tankers burning from LEO to HEEO?? Why so many?

It's very beneficial to consolidate fuel into one tanker in LEO before delivering it to HEEO. It's quite inefficient to have each individual tanker launch deliver directly to a higher orbit.
« Last Edit: 09/05/2023 11:16 am by Twark_Main »

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2225 on: 09/05/2023 11:14 am »
If a depot is able to EDL and re-use, what's the difference between that and a tanker?

Hence tandem refilling. :)
« Last Edit: 09/05/2023 11:17 am by Twark_Main »

Offline Barley

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2226 on: 09/05/2023 01:51 pm »
you could gather a fleet of ten tankers in LEO and have them burn in formation so they all rendezvous with the depot during the same orbit and transfer the fuel in less than a week

Ten tankers burning from LEO to HEEO?? Why so many?

It's very beneficial to consolidate fuel into one tanker in LEO before delivering it to HEEO. It's quite inefficient to have each individual tanker launch deliver directly to a higher orbit.
I'm just showing the scheduling.  He asked for ten tankers so that's what I gave him.  I agree with consolidating the fuel into a minimal number of tankers as low as practicable.

Offline Corey Mandler

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2227 on: 09/05/2023 01:55 pm »
raptor 3 for tankers makes more sense as higher twr means more efficiant

Offline Barley

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2228 on: 09/05/2023 02:07 pm »
If a depot is able to EDL and re-use, what's the difference between that and a tanker?
The role assigned.

The difference between the tanker and depot could be subtle.  Such as having extra capacity for some non-obvious consumable to enable longer on-orbit loiter.

Although they might go through phases.  During prototyping all the ships will be different, so some are slightly better for particular roles.  Then all the ships are the same as mass production kicks in.  Then they start to specialize again.


Note that there may be at least three different roles:
1) Earth to LEO.
2) LEO to HEEO.
3) Accumulation in HEEO.
The second and third roles may be pretty similar, although the last needs longer loiter.
« Last Edit: 09/05/2023 07:09 pm by Barley »

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2229 on: 09/05/2023 07:03 pm »
Why such a long orbital period? As you yourself point out, making that choice is causing problems for yourself.

Do the math on the extra delta-v gained by going from a 48-72 hour orbital period vs 2 weeks. It's only 0.1-0.2 km/s. So by choosing a more pragmatic (shorter) orbital period, effectively the tradeoff is that you "only" get a boost of 3.0-3.1 km/s, vs 3.2 km/s.
Interesting. Yeah, I get the same results, assuming a 200 km perigee. A two-day period gets you just under 3 k/s ∆v and a three-day period gets you just over it, while a 14-day period only gets you 3.15. The shorter periods make the depot spend more time in the Van Allen belts, but it's not as big a difference as I thought it would be; 60% of the time for a two-day orbit vs. 34% for a two-week orbit. (That's assuming the belts are actually spheres.) The absolute time in the belts is actually a good bit less for the shorter periods, which makes it a lot easier on the tankers and the mission. So, I agree, there's no good argument for an HEO with a period over three days.

The only reason I assumed HEO with a one or two-week period was that's what was used in papers I'd read about minimizing the cost of plane changes. Those papers assumed a depot in a circular LEO and a mission that would first boost itself into a two-week HEO, do the plane change at apogee, then do the big Oberth burn at perigee. But plane changes aren't the issue here, and even if they were, a two-week orbit only saves about 500 m/s off a 2-day orbit (300 m/s off a 3-day orbit), and that's in the most extreme case (where you have to cancel both the orbital inclination and the Earth's tilt).

So, given that you're willing to expend the depot after a single mission, it does all seem to work. If you want to reuse the depot, though, I think it pretty much has to be in LEO. Do you agree?

Offline Twark_Main

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2230 on: 09/05/2023 07:40 pm »
Huh, that's a very interesting result about the Van Allen belt. Thanks for running the numbers!

Sounds like those papers may have been talking about a Bi-elliptic type combination transfers, so in that case higher is better. I suspect 2 weeks is roughly the highest orbit where you can still ignore lunar perturbation and use "simple" (vs low-energy) analysis and planning.


So, given that you're willing to expend the depot after a single mission, it does all seem to work. If you want to reuse the depot, though, I think it pretty much has to be in LEO. Do you agree?

   A.) It doesn't need to be a specialized depot in HEEO. All you need is a standard tanker to deliver the fuel to HEEO. Afterwards the tanker just deorbits, reenters and gets reused — like normal.

   B.) Even if you want to use a specialized depot, the depot doesn't have to be expended after a single mission. It can be reused by aerobraking it back to LEO, after (optional) apogee burns to change orbital parameters if needed.

Offline Asteroza

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #2231 on: 09/05/2023 11:58 pm »
I'll just drop a link for Jon Goff's blog post on roving depots associated with HEEO for interplanetary departure, in the context of Jon Goff's 3 burn departure paper, as reference info...

https://selenianboondocks.com/2021/02/an-updated-propellant-depot-taxonomy-part-vi-roving-depots/

It'd be nice if Jon Goff could weigh in on this discussion, with respect to the HEEO orbital period in light of the sub-2 week option.

Offline Brigantine

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #2232 on: 09/06/2023 01:42 am »
I'll just drop a link for Jon Goff's blog post on roving depots associated with HEEO for interplanetary departure, in the context of Jon Goff's 3 burn departure paper, as reference info...

https://selenianboondocks.com/2021/02/an-updated-propellant-depot-taxonomy-part-vi-roving-depots/
He talks a lot about "the next time its plane lines up with the low-orbit"... is this based on orbits that naturally precess by a large amount? Is this a significant issue for elliptical earth orbits in the ecliptic plane?

Offline InterestedEngineer

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Re: Starship In-orbit refueling - Options and Discussion
« Reply #2233 on: 09/06/2023 02:02 am »
I'll just drop a link for Jon Goff's blog post on roving depots associated with HEEO for interplanetary departure, in the context of Jon Goff's 3 burn departure paper, as reference info...

https://selenianboondocks.com/2021/02/an-updated-propellant-depot-taxonomy-part-vi-roving-depots/

It'd be nice if Jon Goff could weigh in on this discussion, with respect to the HEEO orbital period in light of the sub-2 week option.

2 weeks seems like a long time.  A while back I calculated that 20,000km apogee was good enough (~6 hour period), most of the benefit is gained for the additional deltaV and for Mr Oberth and it's a lot less time to line up orbits for destinations and for refueling.
« Last Edit: 09/06/2023 02:07 am by InterestedEngineer »

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2234 on: 09/06/2023 04:13 pm »
   A.) It doesn't need to be a specialized depot in HEEO. All you need is a standard tanker to deliver the fuel to HEEO. Afterwards the tanker just deorbits, reenters and gets reused — like normal.

   B.) Even if you want to use a specialized depot, the depot doesn't have to be expended after a single mission. It can be reused by aerobraking it back to LEO, after (optional) apogee burns to change orbital parameters if needed.
The trouble, I think, is boiloff. To manage boiloff passively, you want to cover the depot with Solar White tiles. But if you do that, then I think you give up on recovering it; it'll burn up on reentry.

If you use a regular tanker, you can deorbit and reuse it, but those black reentry tiles seem guaranteed to warm up the interior fast.

Of course you could always use active cooling, but SpaceX has already said they plan to do it passively.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2235 on: 09/06/2023 05:08 pm »
2 weeks seems like a long time.  A while back I calculated that 20,000km apogee was good enough (~6 hour period), most of the benefit is gained for the additional deltaV and for Mr Oberth and it's a lot less time to line up orbits for destinations and for refueling.
I also get 6 hrs for a 20,000 km altitude, but the extra ∆v is just 2.0 km/s vs. the 3.0 km/s you get for a 2-day period. One kps seems like a lot to me. It also spends 60% of its time in the Van Allen belts, almost all of it in the inner belt.

As far as lining up goes, I figure it precesses once every 1.28 years vs. 45 days for a circular orbit 200 km up. To me, that seems like enough precession to be a problem but not enough to be useful.

Offline Greg Hullender

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2236 on: 09/06/2023 05:15 pm »
He talks a lot about "the next time its plane lines up with the low-orbit"... is this based on orbits that naturally precess by a large amount? Is this a significant issue for elliptical earth orbits in the ecliptic plane?
The trouble with precession is that it means that any Earth orbit "in the ecliptic plane" won't stay in the ecliptic plane. Over time, it'll move. How fast? Here's a table I worked out, assuming orbits are inclined 28.4 degrees to the Earth's equator (latitude of Kennedy Space Center).

Circular orbit (200 km up): 45.5 days to go around once. (Since 28.4 degrees is above the tropic of Cancer, these orbits are never exactly in the ecliptic plane, but they get very close to it once per 45.5-day cycle.)

Elliptic orbits with perigee 200 km up (by period):
6 hrs: 1.3 years
1 day: 2.8 years
2 days: 15 years
3 days: 23 years
1 week: 55 years
2 weeks: 111 years

Now if you had one vehicle in LEO and another in HEO, they should align with each other about once every 45 days--just because the LEO orbit is precessing so fast.

Caveat: someone should check my numbers.
« Last Edit: 09/06/2023 06:59 pm by Greg Hullender »

Offline InterestedEngineer

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2237 on: 09/06/2023 07:33 pm »
2 weeks seems like a long time.  A while back I calculated that 20,000km apogee was good enough (~6 hour period), most of the benefit is gained for the additional deltaV and for Mr Oberth and it's a lot less time to line up orbits for destinations and for refueling.
I also get 6 hrs for a 20,000 km altitude, but the extra ∆v is just 2.0 km/s vs. the 3.0 km/s you get for a 2-day period. One kps seems like a lot to me. It also spends 60% of its time in the Van Allen belts, almost all of it in the inner belt.

As far as lining up goes, I figure it precesses once every 1.28 years vs. 45 days for a circular orbit 200 km up. To me, that seems like enough precession to be a problem but not enough to be useful.

There's also the quantum of refueling to consider.  the 2.0km/sec gives you half full starships in the HEO so one full starship to do the boost at perigee.   [technically 2.2km/sec - 3650km/sec * ln(1320/720) ]

Anything else and the logistics get weird. 

3km/sec requires a mass ratio of  2.27 which uses 62% of the fuel.  You'd have to boost two refuelers to the HEO and that'd have to all be coordinated over 2 day orbits.  Yikes.
« Last Edit: 09/06/2023 07:39 pm by InterestedEngineer »

Offline Barley

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2238 on: 09/06/2023 09:29 pm »


3km/sec requires a mass ratio of  2.27 which uses 62% of the fuel.  You'd have to boost two refuelers to the HEO and that'd have to all be co is ordinated over 2 day orbits.  Yikes.

IMHO the yikes is completely inappropriate.  It's not like navigating a sailboat, where wind and tide are at least somewhat unpredictable.  If you do the orbital math right everything ends up exactly where you need it.  The math is absolutely trivial compared to every other part of a rocket program, such as finite element analysis, let alone computational fluid dynamics.  I am often amazed by how much money and effort engineers will expend to avoid simple calculations.

Offline InterestedEngineer

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Re: Starship On-orbit refueling - Options and Discussion
« Reply #2239 on: 09/06/2023 09:52 pm »


3km/sec requires a mass ratio of  2.27 which uses 62% of the fuel.  You'd have to boost two refuelers to the HEO and that'd have to all be co is ordinated over 2 day orbits.  Yikes.

IMHO the yikes is completely inappropriate.  It's not like navigating a sailboat, where wind and tide are at least somewhat unpredictable.  If you do the orbital math right everything ends up exactly where you need it.  The math is absolutely trivial compared to every other part of a rocket program, such as finite element analysis, let alone computational fluid dynamics.  I am often amazed by how much money and effort engineers will expend to avoid simple calculations.

Show the plan including all the 200t launches to LEO to make this work.   then tell me it's inappropriate.

Not to mention an additional $30M in fuel.  (1200t * $25k/ton = $30M).  That's an optimistic estimate.  It takes half of that to make a Mars trip, so wherever it is you want to go, it better be worth the extra 1-2 km/sec (it likely isn't).

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