Starship On-orbit refueling - Options and Discussion

[Twark_Main]

Why would you do serial staging with a custom stage instead of parallel staging with a standard tanker?

...because serial staging lets you perform two burns at the (final) perigee, maximizing Oberth in the escape / TMI burn.

With "parallel staging" you have to wait for a fuel transfer process between burns, meaning there's not enough time to perform both burns down low in Earth's gravity well (LEO / perigee).

I thought we dropped discussion of HEO refueling when someone pointed out that SpaceX could just make the depot a little bigger and thus avoid problems with the Van Allen Belts.

The Van Allen belts didn't really "kill" the idea of HEO refueling. It's just yet another trade-off.

[Brigantine]

With "parallel staging" you have to wait for a fuel transfer process between burns, meaning there's not enough time to perform both burns down low in Earth's gravity well (LEO / perigee).

If you wanted to, you could have an entire orbit worth of time for fuel transfer (unless you're already hyperbolic)

[Greg Hullender]

The Van Allen belts didn't really "kill" the idea of HEO refueling. It's just yet another trade-off.

HEO refueling has other problems as well. It's a lot more complicated than simply refueling from a depot in a circular LEO. Not worth it if the only reason for it is to avoid making the depot and the HLS a little bit longer. Assuming that doesn't introduce intractable problems itself, of course.

Longer term, assuming there are ever going to be frequent flights to the moon, the right solution is probably a depot in NRHO or the moon itself.

[rsdavis9]

I believe that if you are in a very high earth orbit like/or L1 that to do interplanetary injection it is worth it to retro fire your engines to go into a highly elliptical orbit so you can perigee burn for max oberth effect. Is this true?

[Brigantine]

I believe that if you are in a very high earth orbit like/or L1 that to do interplanetary injection it is worth it to retro fire your engines to go into a highly elliptical orbit so you can perigee burn for max oberth effect. Is this true?

1) The NEA mission concept used it (see below)
2) Consider the extreme case, where your velocity at apogee tends to zero, so your ∆v to lower Perigee to the optimal height also tends to zero, and the benefit is non-zero

Earth Departure ∆v = 0.694 km/s
(0.242 km/s HEO Lowering and 0.452 km/s TNI)

[mikelepage]

Why would you do serial staging with a custom stage instead of parallel staging with a standard tanker?

...because serial staging lets you perform two burns at the (final) perigee, maximizing Oberth in the escape / TMI burn.

Wasn't meaning it to be a one-of-a-kind, non-reusable variant though. An idea I've been noodling over is to try to get two birds with one stone:
1) Rather than doing a "chomper" variant of SS for large/heavy payloads, where issues with operating an 8-meter hatch could make it more trouble than it's worth, it would be simpler to have twin fairings jettisoned and recovered like F9. So the first use of this variant would be delivering high mass/ 8m diameter payloads to LEO. The problem with this concept has always been: how do you reuse it?
2) I was thinking this "booster" reuse mode is the answer to that. Say you have mechanisms on the payload mounting plate that are able to mate to the base of a regular starship, you load propellant to both, then deliver that extra delta-V during the Oberth maneuver. Booster never leaves cis-lunar space, so save enough prop to brake into next rendezvous orbit, rinse and repeat.

Getting a bit Kerbal I'll admit, but serial staging in LEO seems like it could be the missing element that could provide that extra margin for round trips to the moon, or reduced trip times to Mars or NEOs. Also it occurs to me that once you have a booster stage in LEO, you can start to stratify variants on engine number (say 9 in the booster/heavy cargo variant - leave the crew/small-cargo/HSS variants with 6 engines). Or stratify on header tank size, for more optimal cryo propellant storage.

[Barley]

2) I was thinking this "booster" reuse mode is the answer to that. Say you have mechanisms on the payload mounting plate that are able to mate to the base of a regular starship, you load propellant to both, then deliver that extra delta-V during the Oberth maneuver. Booster never leaves cis-lunar space, so save enough prop to brake into next rendezvous orbit, rinse and repeat.

Getting a bit Kerbal

If you want Kerbal

Let the booster aerobrake after separation.  In principle you could be above escape velocity at separation and still get your booster back.  And of course the higher the speed at final separation the higher the total performance.

[Brigantine]

How feasible would it be to... basically do the same mission plan as above, but with the two ships docked side by side, and doing a high-flow propellant transfer under very high acceleration (full thrust for the heavier ship).

Potential issues:

1) How precisely could thrust in the lighter ship and gimbaling/balance be controlled to manage forces in the docking mechanism?
2) How much propellant could physically be transferred during the burn? For comparison, what's the combined cross-section of the propellant feeds to the 6 or 9 engines?

It probably wouldn't work, especially wrt (almost) fully depleting the tanker and having a full ship at separation - but for what reason and by what margin?

[Barley]

How feasible would it be to... basically do the same mission plan as above, but with the two ships docked side by side, and doing a high-flow propellant transfer under very high acceleration (full thrust for the heavier ship).

You may be overestimating how much higher thrust would help.  A few g for 9 minutes (a guess from Falcon 9) is not that far away from a pure impulse.  In a parabolic orbit the radius will vary by less than 10% over the burn.  The difference in V_infinity compared to a perfect impulsive burn should be somewhere below 200m/s.  Not nothing to be sure but it does mean you can't add much mass in the search of perfection.

I think I was conservative in my estimates, but I'd be interested if somebody with a simulator could evaluate the integrals.  I'd be even more interested if there was closed form solution, but it seems unlikely.

[Brigantine]

To clarify: the main advantage I was thinking of would be you can have a standard EDL-capable tanker w/ nosecone, and still get 2 stages worth of delta-v in one perigee.

Trying to make a counterfactual against which to compare the flat-top serial-staging orbital booster concept.
What sort of prop transfer rates are realistic?
what sort of acceleration can side-by-side docking safely accommodate?


I accept Barley's assessment of a very marginal benefit from having more than 9 engines burning at once.
TBH, I generally like a 2-perigee 2-burn concept better anyway (intermediate apogee TBD), but for now working on the premise of an impatient crew so single maneuver.

[Barley]

To clarify: the main advantage I was thinking of would be you can have a standard EDL-capable tanker w/ nosecone, and still get 2 stages worth of delta-v in one perigee.

... but for now working on the premise of an impatient crew so single maneuver.

As Twark_Main said the main advantage of a 2 stage burn at perigee is for the escape burn.  When you reach v_escape there will not be another perigee.  In solar orbit the next possibility would be a flyby and Oberth maneuver, but you have to wait years rather than days.

Why would you do serial staging with a custom stage instead of parallel staging with a standard tanker?

...because serial staging lets you perform two burns at the (final) perigee, maximizing Oberth in the escape / TMI burn.

With "parallel staging" you have to wait for a fuel transfer process between burns, meaning there's not enough time to perform both burns down low in Earth's gravity well (LEO / perigee).

[mikelepage]

2) I was thinking this "booster" reuse mode is the answer to that. Say you have mechanisms on the payload mounting plate that are able to mate to the base of a regular starship, you load propellant to both, then deliver that extra delta-V during the Oberth maneuver. Booster never leaves cis-lunar space, so save enough prop to brake into next rendezvous orbit, rinse and repeat.

Getting a bit Kerbal

If you want Kerbal

Let the booster aerobrake after separation.  In principle you could be above escape velocity at separation and still get your booster back.  And of course the higher the speed at final separation the higher the total performance.

Ha! (Although I'm curious now. How low can your perigee realistically go during TLI anyway?).

That said, by my BoE calcs I don't think the booster SS gets anywhere near escape velocity. (I know people have done similar calcs to this before - probably more accurately - but here's my work). A 1320 ton booster SS pushing a 1470 ton fully loaded SS (150 ton cargo, total mass 2790ton) gets us an extra ~2.1km/s delta-v at TLI.

So 8.4km/s delta-v to play with. After separation, SS has ~6.3km/s delta-v, of which 3.57km/s (680ton prop) is used to land on the moon. Once landed on the moon, we assume SS offloads most of its payload, with 20 tons payload returned back to Earth, so our liftoff delta-v remaining (140 ton starship, 520 tons prop) is 5.78 km/s. With 5.67km/s needed to return to Earth.

Even this seemed fairly tight to me, until I remembered someone upthread had worked out that as long as we had at least two HLS Starships tag-teaming the trip from LEO to NRHO and transferring the necessary prop to return to LEO (from NRHO) in NRHO, we could avoid lugging that extra prop down to the lunar surface and back (I think the difference is ~380 tons or so), and that gives us significant margin.

TLDR: Having a booster SS variant avoids having to send tanker starships anywhere but LEO, which seems much more efficient.

[Barley]

2) I was thinking this "booster" reuse mode is the answer to that. Say you have mechanisms on the payload mounting plate that are able to mate to the base of a regular starship, you load propellant to both, then deliver that extra delta-V during the Oberth maneuver. Booster never leaves cis-lunar space, so save enough prop to brake into next rendezvous orbit, rinse and repeat.

Getting a bit Kerbal

If you want Kerbal

Let the booster aerobrake after separation.  In principle you could be above escape velocity at separation and still get your booster back.  And of course the higher the speed at final separation the higher the total performance.

Ha! (Although I'm curious now. How low can your perigee realistically go during TLI anyway?).

That said, by my BoE calcs I don't think the booster SS gets anywhere near escape velocity. (I know people have done similar calcs to this before - probably more accurately - but here's my work). A 1320 ton booster SS pushing a 1470 ton fully loaded SS (150 ton cargo, total mass 2790ton) gets us an extra ~2.1km/s delta-v at TLI.

So 8.4km/s delta-v to play with. After separation, SS has ~6.3km/s delta-v, of which 3.57km/s (680ton prop) is used to land on the moon. Once landed on the moon, we assume SS offloads most of its payload, with 20 tons payload returned back to Earth, so our liftoff delta-v remaining (140 ton starship, 520 tons prop) is 5.78 km/s. With 5.67km/s needed to return to Earth.

Even this seemed fairly tight to me, until I remembered someone upthread had worked out that as long as we had at least two HLS Starships tag-teaming the trip from LEO to NRHO and transferring the necessary prop to return to LEO (from NRHO) in NRHO, we could avoid lugging that extra prop down to the lunar surface and back (I think the difference is ~380 tons or so), and that gives us significant margin.

TLDR: Having a booster SS variant avoids having to send tanker starships anywhere but LEO, which seems much more efficient.

We're probably thinking about two different things.  If you're staying in the Earth-Moon system, you don't want to exceed escape velocity.  If you exceed escape velocity, you will not remain in Earth-Moon system unless you slow down pretty soon.  (You would only do this if time was much more valuable than propellant.  You probably need a torch ship for this to make sense.)  If you are staying within the Moons orbit rendezvous and refueling is sufficient because all orbits repeat (more or less) in less than a month.  So whatever you need to do you can send out a fleet of ships, burn in unison at perigee (or elsewhere) then have time to refuel before the next burn. There might be operational reasons to prefer something different, but it's not strictly necessary.

The situation I was thinking of is something like Parker Solar probe.  You need to get beyond escape velocity by enough to get to Venus.  And if you can go faster you might avoid one or more of the gravity assists at Venus and get to the closest solar approach sooner.  If you can go fast enough (~18km/s!!) you could go directly to a close perihelion without a gravity assist.

For high energy escape like this you would start with the fueled booster and starship in say GEO transfer orbit.  That is only 773m/s from escape so the 2.1km/s burn of the booster puts both booster and SS on an escape trajectory.  Once they separate the SS burns to head fast for Venus, while the booster heads low enough into the Earth's atmosphere to scrub at least 1.4km/s to aerobraking so that it remains near Earth and can be recovered in days or months.

[Eer]

This discussion of multiple Ships being lashed together to fire simultaneously triggered the thought that though Kerbalesque in the extreme, a Falcon Heavy-like monster assembled in GEO or geo-transfer orbit could impart a huge amount of dv to the center core. The two side ships would be tankers full of propellant with 6-vac+3-gimbaled engines able to push through trans-Martian-or-wherever injection, and then boost back.  Core ship saves propellant for breaking at destination.

Obviously not a first generation configuration, but leverages overall commonality of design to hopefully achieve higher dv transit.

No, I have no factual basis for this idea, and I’ll probably feel better after a nap.

[Twark_Main]

Why would you do serial staging with a custom stage instead of parallel staging with a standard tanker?

...because serial staging lets you perform two burns at the (final) perigee, maximizing Oberth in the escape / TMI burn.

Wasn't meaning it to be a one-of-a-kind, non-reusable variant though.

Me neither.    IMO both reusable and non-reusable options are available.

You guys do seem to have figured out that StarKicker can do a flip-and-burn after staging so it remains within Earth's sphere-of-influence, then aerobrake back to LEO for reuse.

This basic idea was suggested by Robert Zubrin as one of several possible "improvements" upon the Starship architecture.

The Van Allen belts didn't really "kill" the idea of HEO refueling. It's just yet another trade-off.

HEO refueling has other problems as well. It's a lot more complicated than simply refueling from a depot in a circular LEO.

The incremental complexity isn't too great. There's no new incremental hardware R&D, for instance, which is a big win.

In the HEEO refilling plan you just have a standard tanker, which performs a rendezvous and fuel transfer in high orbit. SpaceX wil do lots of tanker missions, so we can expect they'll make that entire process extremely cheap, reliable, and routine. I expect nearly 100% automation here.

No need for hardware, just software. 

Not worth it if the only reason for it is to avoid making the depot and the HLS a little bit longer.

If you run the delta-v math on tandem HEEO refilling ladders, it offers a lot more than (the equivalent of) "a little bit longer" prop tanks.

The math (from way back on page 32!  ): https://forum.nasaspaceflight.com/index.php?topic=50157.msg2229093#msg2229093

[Twark_Main]

Also, don't think of StarKicker as (necessarily) some dinky little thing. Perhaps it shouldn't be surprising, but the optimal size for StarKicker is about the same size as Super Heavy. 

The main difference is you don't need as many Raptors. This frees up area on the thrust puck, and presumably this "free" area would be used to swap in some RVac engines, yielding a nice I_sp boost.

[Brigantine]

Empty mass 200t, actually that can reach orbit... so why aren't all the depots SH-sized? 3,400t fuel storage
(or even stretch to 3,600+margin for a nice round number capacity of 3 missions serviced on the trot)

Possibly something about aerodynamic stability/structural integrity on ascent... that's all I can think of? But that would equally affect starkicker.

(Though how much extra empty mass to add aerobraking capability? Or is it fully propulsive return to LEO?)

This discussion of multiple Ships being lashed together to fire simultaneously triggered the thought that though Kerbalesque in the extreme, a Falcon Heavy-like monster assembled in GEO or geo-transfer orbit could impart a huge amount of dv to the center core. The two side ships would be tankers full of propellant with 6-vac+3-gimbaled engines able to push through trans-Martian-or-wherever injection, and then boost back.  Core ship saves propellant for breaking at destination.

Obviously not a first generation configuration, but leverages overall commonality of design to hopefully achieve higher dv transit.

No, I have no factual basis for this idea, and I’ll probably feel better after a nap.

Until Starship actually goes to space, a Kerbal fantasy is all it's good for. All I want to know is, where is this Raptor-Heavy powered mission going? You might be carrying the centre core with you and end up doing a Callisto-Orbit-Rendezvous with it or something weird. Does that even reach?.
(I'm starting to see why electric propulsion is so popular for asteroid return missions. Also, there's probably a more appropriate thread for mission plans and staging methods that don't use propellant transfer)

[Greg Hullender]

If you run the delta-v math on tandem HEEO refilling ladders, it offers a lot more than (the equivalent of) "a little bit longer" prop tanks.

The math (from way back on page 32!  ): https://forum.nasaspaceflight.com/index.php?topic=50157.msg2229093#msg2229093

Not sure what I'm missing, but if I make a single change to your spreadsheet--raising mission Starship prop from 1200 to 1500 tons--then LEO direct works with 14 prop transfers, but tanker slam needs 16, and the rest effectively don't work at all (20 or more transfers).

[Twark_Main]

If you run the delta-v math on tandem HEEO refilling ladders, it offers a lot more than (the equivalent of) "a little bit longer" prop tanks.

The math (from way back on page 32!  ): https://forum.nasaspaceflight.com/index.php?topic=50157.msg2229093#msg2229093

Not sure what I'm missing, but if I make a single change to your spreadsheet--raising mission Starship prop from 1200 to 1500 tons--then LEO direct works with 14 prop transfers

I actually agree. If we're talking specifically about refueling for "standard" lunar missions (ie ~100 tonne payload), I expect that's what we'll see.

Honestly I forgot I had written the descriptions on that spreadsheet in terms of Moon missions. I really should make an updated version where the wording is more generic.

but tanker slam needs 16, and the rest effectively don't work at all (20 or more transfers).

I don't see why 20 tanker launches means "effectively doesn't work at all" in your book. Establishing a city on Mars is going to require a lot more than 20 tanker missions. 

You'll always need more launches when using tandem refilling, because lofting more tanker dry mass into higher orbits = more total impulse = more propellant. Can't cheat physics! 


To make the math very simple: tandem refilling allows any Starship-compatible vehicle configuration to gain (up to) 3.2 km/s of additional delta-v from LEO. Effectively, your vehicle begins its departure burn at (approaching) Earth escape velocity, instead of a ~circular orbit.


What's maybe getting lost is that this isn't necessarily an either/or choice. I think of it more like RTLS vs droneship landings — for missions that need additional delta-v (ie beyond whatever the "large" configuration offers), it becomes an optional value-add service SpaceX can upsell to the customer.   

[Greg Hullender]

but tanker slam needs 16, and the rest effectively don't work at all (20 or more transfers).

I don't see why 20 tanker launches means "effectively doesn't work at all" in your book. Establishing a city on Mars is going to require a lot more than 20 tanker missions. 

I just mean that, in comparison to a method that only needed 14 refuelings, something that needs 20+ effectively doesn't work. By that same logic, I suppose SLS effectively doesn't work either. :-)

You'll always need more launches when using tandem refilling, because lofting more tanker dry mass into higher orbits = more total impulse = more propellant. Can't cheat physics! 

Yeah, I get that part. But couldn't you also get that by just having a second depot in GEO? It seems that the motivation for HEEO refueling is just to satisfy NASA's requirement that a manned mission only refuel one time. Does it have any other advantage?

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. 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.

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.)

Contrast this with proposals to use an LEO depot and kick into HEEO to do the plane change. For those missions, you get to pick the orbit so you can be sure the perigee occurs right where you want it. (Or so it seems to me, anyway.) Everything except the mission itself is in LEO, so there's no special radiation protection required, and the mission itself only has to do two passes through the belts and it only does that once, so you have a better shot at timing things so as to avoid the worst parts of the belts.

Maybe I'm just failing to visualize things properly, but these seem like pretty big problems to me. Not small things that one might easily trade off.