Author Topic: Refueling v Docking with Fuel Tanks  (Read 27914 times)

Offline intrepidpursuit

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Re: Refueling v Docking with Fuel Tanks
« Reply #20 on: 10/01/2019 05:48 pm »
Fuel depots NEVER make sense unless there are massive numbers of ships to the point that separate development of tanker specific hardware is worth the minor optimization gains.

You have to get the fuel up there, so you are launching a fully equipped ship no matter what. You have to station keep, so you are flying a fully equipped spacecraft. Once you are doing both of those things it is more efficient to just fly the spacecraft up there and use it as a tanker.

Although it has been framed that way, flying the mission starship up first makes no sense. They would put a refueling SS up there, fill it up, then fly the mission SS to meet it and take on the fuel load. It is zero extra steps and solves a few major concerns.

Now, if you left that SS up there then you could fly it with no fins or heat shield and thus a little more fuel. That only helps on the FIRST TRIP. After that it is just a SS that wastes most of its time loitering. Plus, this fuel depot SS needs to be positioned correctly to be useful to the mission SS. Moving it will cost at least as much fuel as was gained by flying it unrecoverable.

Fuel specific SS makes sense when doing regular interplanetary flights. It is just a SS optimized for no payload and maximum fuel reserves. Leaving one up indefinitely has no advantage in the big picture that I can see.

A tank (still a full space craft) that docks and carries its extra overhead on the mission is the worst of all worlds.

Offline edzieba

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Re: Refueling v Docking with Fuel Tanks
« Reply #21 on: 10/01/2019 06:22 pm »
The exception would be if you can store fuel in a deployable structure (anything from a big plastic bladder to an Atlas-like pressure-supported stainless tank that can be 'crumpled' and 'uncrumpled' once for package and then deployment). The structure needs only enough non-passive hardware to prevent uncontrolled tumbling (so an RCS system, for which venting propellent head volume may be sufficient). Dumping fuel from tankers to a Big Dumb Tank means you are not wasting ~$12m in Raptor engines at a time floating and doing no work, in exchange for the cost of launching the BDT. It also means you can potentially store more than one Starship's worth of propellant at a time, which helps for performing large launches with a smaller fleet.

Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #22 on: 10/22/2019 10:47 pm »
Twark_Main and I had a discussion over on an inappropriate thread (starting here) about something like this, and I got pretty excited when we combined a couple of things.  He started suggesting a "pusher" stage instead of an extra tank or even tanker, but we realized that a "pusher" can also be a "tanker" at the same time.

For human BEO spaceflight, keeping refueling rendezvous/dock operations to a minimum is important for risk reduction, and keeping quivering hunks of protoplasm from doing multiple passes through the Van Allen belts, or even leisurely drifting through the outer parts of VAB #2, is probably a requirement.  You also need as many abort points as possible.

So:  How to get to TLI with only one R/D for refueling in LEO, where the radiation environment is relatively benign?  Here's how:

First, we'll assume a pusher/tanker that's pretty large, but never lands.  Its properties:

a) It's stretched to hold more prop (somewhere between 1800 and 2400 t).  This should simply be a matter of welding a few more rings in place, and cannibalizing payload bay space.  It'll be a bit longer than a regular Starship, but not by much.  You could also go to a wider form factor and hammerhead the thing, but that sounds like a lot of work.
b) It has fewer Raptors (how many fewer is a topic of debate).
c) I launches with just enough prop to get its dry mass (with fewer Raptors) to LEO.
d) It never reenters, so no canards or rear winglets.
e) No heat shielding, for the same reason.
f) It docks nose-to-tail with the target Starship, and transfers fuel from the top, not the bottom.
g) Its nose (and docking latch system) is strong enough to transmit thrust to the target Starship.
h) I suspect you need an expendable nose cone on this thing to get it launched.
i) If we use the numbers from the Elon "Starkicker" tweet, I'd guess that, with the stretched tanks, this would have a dry mass of about 60 t.
j) We'll assume a fully expanded set of RaptorVacs here as opposed to the SL-safe set that got tweeted about recently with Isp=365 s, but let's be conservative and say Isp=375 s.

Now, here's a CONOPS for a human-capable mission that only refuels the crewed SS once, goes straight to TLI to minimize radiation exposure, and still lets the crewed SS carry enough prop to get home with no surface- or cis-lunar refueling:

1) We start with the pusher/tanker in LEO, empty.

2) Launch as many no-payload earth-to-LEO tankers as needed to do R/Ds and transfer prop to the P/T, per mission requirements.  (Whether these refuel tail-to-tail or nose-to-tail is an open question that's not very important.)

3) Now launch the crewed Starship to LEO.  (This works fine for cargo SSes as well, but crew has more stringent requirements.)

4) P/T does R/D with crewed Starship.  Docking crewed SS in front, with the P/T nose docked to the crewed SS tail. 
4a) Abort condition if R/D fails: Starship is in a low-prop state, and it's easy to deorbit and do EDL with what's available.

5) P/T now transfers prop from itself to the crewed SS until it's full.  Note that there is quite a bit of prop left in the P/T at the end of this process. 
5a) Abort condition if transfer fails: P/T simply detaches and Starship does deorbit and EDL.

6) P/T fires its reduced set of Raptors to impart somewhere between 1400 and 2000 m/s of delta-v. 
6a) Abort condition if P/T burn fails: Starship first jettisons the P/T, but it's in a high-prop state, and probably needs to do something to jettison prop before attempting EDL.  Whether it can dump prop safely or has to do some kind of meaningless burn is TBD.

7) At the end of the maneuver, P/T detaches from crewed SS.
7a) P/T has enough prop left to reverse the delta-v it just imparted and return to LEO, empty, for the next mission.
7b) Abort condition if P/T can't detach from crewed SS:  This is bad, but not fatal.  There's plenty of prop to abort to NRHO or LLO and await rescue.  I haven't looked at whether there's enough prop to do either a free-return or direct abort, but whatever happens has to result in a propulsively inserted LEO.  Reentry with the P/T on your rear end is likely fatal.

8) Now that it's free of the P/T, the crewed SS immediately (i.e., in seconds to minutes) starts its own burn to finish the TLI maneuver.  It should still be fairly near the perigee of its new P/T-imparted orbit, so this is a relatively efficient burn.

9) Off to the Moon, with plenty of prop.
9a) Plenty of prop for either a direct or free-return abort and direct EDL.

10) Land, do the mission, take off to TEI.
10a) By definition, there's enough prop to do an abort in any phase of this.

I'm assuming that the Isp=365 s RaptorSortaVacs will deliver 135 t of prop to LEO.  Vanilla-flavored no-payload cargo "tankers" launched from Earth can therefore transfer that amount of prop per launch.

With P/T dry mass=60 t and Isp=375 s (assuming true RaptorVacs), I worked up two scenarios:

A) Pure crewed mission with crew payload system mass = 25 t, both up to the surface and down back to earth.
Total P/T prop mass: 1780 t (13.2 tanker launches).
P/T prop transferred to crew SS: 1090 t (~110 t residual on crewed SS once in LEO, since payload is only 25 t)
P/T delta-v: 1410 m/s

B) Heavy cargo mission:  135 t payload up, 20 t payload down
Total P/T prop mass: 2400 t (17.8 tanker launches)
P/T prop transferred to cargo SS: 1200 t.
P/T delta-v: 2070 m/s.

For comparison with the heavy cargo scenario without the P/T, using just vanilla-flavored cargo SSes as tankers, if we do a full refueling of a cargo SS in LEO, then boost to an LEO+2000 HEEO and meet a tanker there, we need 17.4 tanker launches, so the P/T isn't quite as efficient, but an extra 54 tonnes of prop seems like a small price to pay for reduced risk to the cargo or crew, in terms of both R/Ds and radiation.

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

Mod the aforementioned fly, though, this sounds like a massive risk reduction, and fairly straightforward modification of a regular SS to create the P/T.

Offline SteveU

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Re: Refueling v Docking with Fuel Tanks
« Reply #23 on: 10/23/2019 12:00 am »

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

I may have missed it in your previous discussions, but why wouldn’t the nose of the P/T be the same as the nose of SH. In fact, why not simply make the P/T a SH variant instead of a SS variant?

edit - grammar
« Last Edit: 10/23/2019 12:08 am by SteveU »
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Offline Twark_Main

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Re: Refueling v Docking with Fuel Tanks
« Reply #24 on: 10/23/2019 12:03 am »
Thanks TheRadicalModerate, great summary. Everything seems perfectly well stated.

Merging in discussion from the O/T thread (hopefully this helps answer SteveU's question),

I'd be pretty surprised if SpaceX made two duplicate systems for performing very similar functions on a highly mass-optimized vehicle. "Null[ing] out misalignments and torques" sounds exactly like the type of system you'd need to stack the stages with an unassisted or minimally-assisted crane op.

As ZChris13 mentioned, SpaceX is transferring propellant through Super Heavy (no umbilical). So this is already a point of commonality with the SpaceX hardware roadmap.

No.  A crane is for all intents and purposes a four-degree-of-freedom system (z, zdot, zrot, zrotdot).  Beyond that, you can have guides and manual tweaks for wind gusts, or whatever.  By far the most important restricitions, though, are that gravity eliminates the xrot and yrot degrees of freedom, and simply knowing the position of the mount eliminates x and y.

In contrast, an on-orbit docking system is a 12-degree-of-freedom system (x, y, z, xrot, yrot, zrot, and all dots of the previous).  It needs much larger tolerances for soft capture, and then a way to convert that soft capture to an adequate hard capture.  Think of what you're proposing as a super-duper hard capture and you won't be far off.

Look, I think this idea is terrific.  But it definitely would require a much heftier docking system, with a lot more complexity, to work.  And it only gets heftier and more complex as you add thrust.  Complexity, especially when you're talking about docking something to a crew that might not undock properly, is a big deal.

That's a solid overview of the difference between the two. Docking in space is harder.

Since Starship needs the (more difficult) space docking hardware anyway for refueling, the crane op should be a cakewalk. :)



Which brings us to...

Quote
More than "slightly." At 626 kg/s (from livingjw's latest schematic), a 1500 tonne tank would take fully 40 minutes to empty. That's almost half an orbit, which has got to incur a substantial Oberth penalty. For comparison the Apollo TMI burn lasted less than 6 minutes, and Von Braun certainly did his homework.

I'd be looking for 3-6 (vacuum) engines, in the ideal case. But see also Elon's latest comment about needing SL engines for engine-out.

Here's a radical idea: perhaps have a set of 5 (yes, 5) vacuum engines, plus a single contingency central engine? In a nominal burn the central engine doesn't necessarily fire, but if a vacuum engine failed it would light and gimbal hard away from the failed engine to counteract the torque. Having an odd number of engines means the central engine has maximum gimbal range in the case with the highest torque asymmetry (ie 1 engine out).

This configuration might even be able to land on Earth, given the upgraded methox hot gas thrusters, and that Musk said the dual-bell Raptor Vacs would work at SL. Possible Starship evolution path?  ???

I'll buy the 40 minute burn with one engine for a large-sized cargo (I used 135 t up to the lunar surface and 25 t back down to Earth).  Maybe that needs 3 engines.  However, two things:

1) From a crew reliability standpoint, lower thrust is better on the docking system (see above).  For the 25 t up and down case, your pusher burn is about 25 minutes, which is still hefty, but...

Again, are we really expecting two separate independent docking systems in the same place on the vehicle? "The best part is no part." Seems better to just add a soft-capture mechanism to the existing SH pneumatic pusher and re-use the self-aligning latches and structural load carrying hard points.


2) You're starting out in a circular orbit, so there's no "Oberth effect" per se.  Instead, there are some efficiency losses from a non-impulsive burn.

It's equivalent really. Oberth effect says that burns down low in a gravity well are more efficient (because the spacecraft is going faster, having traded potential for kinetic energy). As soon as the burn starts the apoapsis rises, so by Oberth it's less efficient. The longer the burn the further it's moved away from the periapsis of the (now non-circular) orbit.


My math and tools aren't up to computing that but, intuitively, they ought to be pretty small. Essentially, you're bleeding off tangential velocity as your radial distance rises, which means that you add less energy than you would otherwise, as the burn progresses.  Maybe somebody has a handy numerical sim for a non-impulsive burn of about 2 MN with about a 3210 t coupled spacecraft with Isp=365 s, departing from a 200x200 orbit?

Can the GMAT software do this? AC in NC has done some killer simulations in the Moon Starship thread. https://forum.nasaspaceflight.com/index.php?topic=49144.msg2004908#msg2004908
« Last Edit: 10/23/2019 12:17 am by Twark_Main »

Offline WormPicker959

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Re: Refueling v Docking with Fuel Tanks
« Reply #25 on: 10/23/2019 01:56 am »
snip

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

snip

One could imagine a scenario where the nose of the P/T is actually a fairing, which upon jettisoning reveals a non-aerodynamic structure for docking/thrust transfer. It could be modeled on the rear end of a SS, using the normal docking/refueling infrastructure otherwise required (but simply reversed), and also the thrust structures of SS, but (obviously) no raptors. Essentially, make the P/T have two tails, with a fairing covering the "nose-tail" during ascent.

I see re-using the SS rear-end mainly as a benefit to not having to engineer some kind of one-off thrust structure - the drawback is that it's likely overkill, and will add significant dry mass to the P/T, reducing its overall utility - no idea to what extent, though.

Offline Twark_Main

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Re: Refueling v Docking with Fuel Tanks
« Reply #26 on: 10/23/2019 02:08 am »
snip

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

snip

One could imagine a scenario where the nose of the P/T is actually a fairing, which upon jettisoning reveals a non-aerodynamic structure for docking/thrust transfer. It could be modeled on the rear end of a SS, using the normal docking/refueling infrastructure otherwise required (but simply reversed), and also the thrust structures of SS, but (obviously) no raptors. Essentially, make the P/T have two tails, with a fairing covering the "nose-tail" during ascent.

I see re-using the SS rear-end mainly as a benefit to not having to engineer some kind of one-off thrust structure - the drawback is that it's likely overkill, and will add significant dry mass to the P/T, reducing its overall utility - no idea to what extent, though.

Bingo! That's exactly the idea.

Here's the original proposal:

The only way to avoid the extra two trips through the VAB and the long loiter at the edge of VAB #2 is to take the tanker all the way to TLI with the crewed Starship, then refuel in trans-lunar coast.

There is one way I can think of, but it may be too "radically moderate" for your tastes. ;) Have the Tanker push the Starship.

All it needs is a second docking port on the front (refueling plumbing not necessary, just the structural hard points and pneumatic separation collets). During TLI the Tanker engines fire, followed by a more-or-less conventional stage separation (hence "moderate"), then Starship's engines fire to finish the TLI burn. Afterwards the Tanker does a small perigee lowering burn and aerobrakes back to LEO. If the Starship engines fail to light you can abort to LEO (or abort-to-reentry) the same way. For launch it might have a nose cone to reduce drag, either disposable or reusable (your choice depending on your assumption about flight rate).

Reentry is probably out of the question, so such a vehicle would be reused on-orbit. Probably that means you'll want to throw some Hall effect thrusters on it too, for reboost and adjusting the orbit.

Too radical? Or just right?

This would be derived from the pared-down Starkicker architecture, so the second docking port would just be mounted above the propulsion section top dome.
« Last Edit: 10/23/2019 02:16 am by Twark_Main »

Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #27 on: 10/23/2019 04:35 am »

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

I may have missed it in your previous discussions, but why wouldn’t the nose of the P/T be the same as the nose of SH. In fact, why not simply make the P/T a SH variant instead of a SS variant?

edit - grammar

Twark had mentioned something similar on the other thread.  The big difference is going to be whether the interstage latch system can do double duty as a docking system.  I'm high skeptical of that, but hopefully some of the interstage thrust transmission and plumbing can be the same.

I don't think there's any value to starting with an SH.  It's not designed to go through max-q as the lead element, and its thrust structure would be swatting a fly with a nuclear weapon for the purposes of what's needed for the P/T.  Also, it's easy to to take stuff off of a Starship, then stretch it with some additional ring segments, to get a P/T.  With the exception of that top bulkhead/interstage (maybe), SH probably doesn't get you what you need there.

Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #28 on: 10/23/2019 04:39 am »
snip

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

snip

One could imagine a scenario where the nose of the P/T is actually a fairing, which upon jettisoning reveals a non-aerodynamic structure for docking/thrust transfer. It could be modeled on the rear end of a SS, using the normal docking/refueling infrastructure otherwise required (but simply reversed), and also the thrust structures of SS, but (obviously) no raptors. Essentially, make the P/T have two tails, with a fairing covering the "nose-tail" during ascent.

I see re-using the SS rear-end mainly as a benefit to not having to engineer some kind of one-off thrust structure - the drawback is that it's likely overkill, and will add significant dry mass to the P/T, reducing its overall utility - no idea to what extent, though.

Two tails isn't a bad idea, but I'm still grappling with whether what you use to convert a soft capture to a hard dock will be able to reliably transmit the thrust you're likely to need.  Docking is a lot different than having a big honkin' crane lower the stage into place and then snapping the latches shut under the supervision of humans.

Online Oersted

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Re: Refueling v Docking with Fuel Tanks
« Reply #29 on: 10/23/2019 09:03 am »
A tanker as depot makes the most sense to me. Launch one with special long term storage mods, fill it with regular tanker flights, then fill a Starship.
If there’s enough traffic, the storage tanker might just stay on orbit. Give it a permanent sun shield, solar array and radiator.
Why land and relaunch that tanker?
I am not understanding what advantage this is supposed to give, over just sending up tankers as needed. can someone explain?
I am only seeing disadvantages, such as needing that sun shield, having that tanker unavailable for sending up fuel for lengths of time and also such a depot might be less useful for missions that could profit from being tanked in some other orbit than the depot happens to be in.

You could fully fuel up the depot before launching the mars payload.  That way you would only have 1 docking event for the main mission.

Docking was solved in 1966. Several dockings do not pose a problem.

Offline SteveU

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Re: Refueling v Docking with Fuel Tanks
« Reply #30 on: 10/23/2019 10:56 am »

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

I may have missed it in your previous discussions, but why wouldn’t the nose of the P/T be the same as the nose of SH. In fact, why not simply make the P/T a SH variant instead of a SS variant?

edit - grammar

Twark had mentioned something similar on the other thread.  The big difference is going to be whether the interstage latch system can do double duty as a docking system.  I'm high skeptical of that, but hopefully some of the interstage thrust transmission and plumbing can be the same.


The interstage latch system would not be used as the alignment system used for docking/latching.  I think you're making this more difficult than it needs to be.

We all agree that they will be able to achieve a mating of two SS to transfer fuel.

In order to be able to have in-flight refueling, the two vessels will need to be docked - at some level of "hardness" - a "soft" dock won't cut it.  They might be able to adjust for a few cm of misalignment along the x, y and z axis's but i cannot see how any movement can be tolerated while transferring fuel.  The center of mass of the combined vessels will be constantly moving and any movement between the two ships at this time would be catastrophic (IMHO).

What that leaves us with is just how well aligned will the vessels need to be in order to be able to engage the interstage latching mechanism?  Since refueling will require cm accuracy, it isn't that difficult to imagine that the latching mechanism would be designed to accommodate a few cm of misalignment.  Once the two vessels are "Docked"  and all motion has ceased, engage the latches.  Isn't this the same as would be done when stacking at the launch pad?

What i'm envisioning for the docking mechanism - imagine a supersized Apollo Probe and Drogue. Using four of them on the SS located inboard of the  +x and -x leg positions, probes at +x, drogues at -x you have a system of alignment that should be able to give sub-cm accuracy. If sturdy enough, they could be used to assist in alignment during stacking as well.

PS - this brings us back to the discussions we had regarding popping the nose-cone off for cargo transfer!


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Offline ZChris13

Re: Refueling v Docking with Fuel Tanks
« Reply #31 on: 10/23/2019 04:05 pm »
I don't believe a crane will constrain the movement of the Starship for on-pad assembly as much as whoever it is thinks it will. Even a light wind against the massive surface area of Starship is going to move even the best rigged system. Steel lines have quite a bit of stretch in them at that scale. There's really no telling if the pad assembly equipment or a close facsimile will work and I find it unlikely that we'll ever find out.
Docking was solved in 1966. Several dockings do not pose a problem.
Reading this thread, and this is what stands out to me. All of these ideas to minimize rendezvous and docking maneuvers are cool, but it's additional hardware development in order to reduce something that SpaceX are planning to do thousands and more of.
« Last Edit: 10/23/2019 05:41 pm by ZChris13 »

Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #32 on: 10/23/2019 05:06 pm »
Docking was solved in 1966. Several dockings do not pose a problem.

Sure they do.  Every docking, fuel transfer, and undocking operation has an associated risk of failure.  A failed docking is a potential loss of mission.  Failed fuel transfers or especially undocking failures are potential loss-of-crew events.  High school probability tells you that if the combined dock/transfer/undock risk of failure is R, then n such operations have a combined risk of 1-(1-R)^n.

I'm willing to believe that R is small, but it's almost certainly non-trivial, and it's likely a lot higher for a pair of spacecraft that are 100 m long, instead of 18 m.  Minimizing n is an important goal.

Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #33 on: 10/23/2019 05:22 pm »
The interstage latch system would not be used as the alignment system used for docking/latching.  I think you're making this more difficult than it needs to be.

We all agree that they will be able to achieve a mating of two SS to transfer fuel.

In order to be able to have in-flight refueling, the two vessels will need to be docked - at some level of "hardness" - a "soft" dock won't cut it.  They might be able to adjust for a few cm of misalignment along the x, y and z axis's but i cannot see how any movement can be tolerated while transferring fuel.  The center of mass of the combined vessels will be constantly moving and any movement between the two ships at this time would be catastrophic (IMHO).

What that leaves us with is just how well aligned will the vessels need to be in order to be able to engage the interstage latching mechanism?  Since refueling will require cm accuracy, it isn't that difficult to imagine that the latching mechanism would be designed to accommodate a few cm of misalignment.  Once the two vessels are "Docked"  and all motion has ceased, engage the latches.  Isn't this the same as would be done when stacking at the launch pad?

What i'm envisioning for the docking mechanism - imagine a supersized Apollo Probe and Drogue. Using four of them on the SS located inboard of the  +x and -x leg positions, probes at +x, drogues at -x you have a system of alignment that should be able to give sub-cm accuracy. If sturdy enough, they could be used to assist in alignment during stacking as well.

PS - this brings us back to the discussions we had regarding popping the nose-cone off for cargo transfer!

I agree with most of that, but the details are important--and, since I've convinced myself that this P/T idea would be genuinely useful, it would require at least ensuring that the Starship thrust structure be retrofittable to accommodate the extra-hard version of docking.

Remember:  failure to undock on a crewed mission is a potential loss-of-crew event.  If you're going to free-orbit guide and align two spacecraft with a combined length of 100 m to the point where you can lock a set of load-bearing interstage latches, you're going to have to be awfully sure that they'll come unlatched after having thrust applied to them.  At the very least, that's a requirement that's substantially harder to meet than a sorta-kinda hard dock for refueling purposes.

I'm not sure I agree with you about the hardness of the dock for prop transfer.  I'd assume that the lines are pretty flexible, for this very reason.  You obviously need to be able to null out some torques and tension/compression caused by the ullage burns, but that's a far cry from dealing with 1.5-6 MN from a bunch of Raptors blasting away at one end of a 100 m vehicle.

Finally, the nose cone for such a P/T isn't the same as the pop-top proposal for a cargo Starship, because you can jettison the thing and forget about it.  A pop-top Starship had to actually extract (or insert) cargo through the opening, then re-latch the pop-top.

Offline Twark_Main

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Re: Refueling v Docking with Fuel Tanks
« Reply #34 on: 10/23/2019 05:26 pm »

The fly in the ointment here is this thrust-bearing nose-to-tail docking scheme.  The crew/cargo SS can obviously bear the load, because this is essentially how it gets launched, but the latch for an on-orbit docking that has to bear multiple meganewtons of thrust and gimbaling is a lot more complicated than one that just has to hold a pair of tail-to-tail tankers together under an ullage burn.  Twark and I have been having a spirited discussion about this, but I don't think we've gotten to an answer.  Any thoughts on the feasibility would be appreciated.

I may have missed it in your previous discussions, but why wouldn’t the nose of the P/T be the same as the nose of SH. In fact, why not simply make the P/T a SH variant instead of a SS variant?

edit - grammar

Twark had mentioned something similar on the other thread.  The big difference is going to be whether the interstage latch system can do double duty as a docking system.  I'm high skeptical of that, but hopefully some of the interstage thrust transmission and plumbing can be the same.


The interstage latch system would not be used as the alignment system used for docking/latching.  I think you're making this more difficult than it needs to be.

We all agree that they will be able to achieve a mating of two SS to transfer fuel.

In order to be able to have in-flight refueling, the two vessels will need to be docked - at some level of "hardness" - a "soft" dock won't cut it.  They might be able to adjust for a few cm of misalignment along the x, y and z axis's but i cannot see how any movement can be tolerated while transferring fuel.  The center of mass of the combined vessels will be constantly moving and any movement between the two ships at this time would be catastrophic (IMHO).

Yes, generally the next step immediately after soft docking is hard docking, via some sort of rigidizing retraction mechanism.

What that leaves us with is just how well aligned will the vessels need to be in order to be able to engage the interstage latching mechanism?  Since refueling will require cm accuracy, it isn't that difficult to imagine that the latching mechanism would be designed to accommodate a few cm of misalignment.  Once the two vessels are "Docked"  and all motion has ceased, engage the latches.

What you've just described in bold is exactly the same as "soft docking." ;)


Isn't this the same as would be done when stacking at the launch pad?

That's a point of contention between TheRadicalModerate and myself. I think there will be a single dual-purpose docking/latching system to save mass, and TheRadicalModerate thinks the functions are so divergent that an independent system is needed for both.


What i'm envisioning for the docking mechanism - imagine a supersized Apollo Probe and Drogue. Using four of them on the SS located inboard of the  +x and -x leg positions, probes at +x, drogues at -x you have a system of alignment that should be able to give sub-cm accuracy. If sturdy enough, they could be used to assist in alignment during stacking as well.

Sounds good to me. Not too many ways to make a 12-axis self aligning connection!

I expect there will be lessons learned from the Soyuz docking mechanism as well.


PS - this brings us back to the discussions we had regarding popping the nose-cone off for cargo transfer!

Yep. This would just be a disposable aerodynamic nose cone for launch.
« Last Edit: 10/23/2019 05:51 pm by Twark_Main »

Offline Twark_Main

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Re: Refueling v Docking with Fuel Tanks
« Reply #35 on: 10/23/2019 05:32 pm »
I don't believe a crane will constrain the movement of the Starship for on-pad assembly as much as you think it will, Twark_Main. Even a light wind against the massive surface area of Starship is going to move even the best rigged system. Steel lines have quite a bit of stretch in them at that scale. There's really no telling if the pad assembly equipment or a close facsimile will work and I find it unlikely that we'll ever find out.

I think you meant TheRadicalModerate, not me. My contention is that the docking hardware is helpful during stacking crane ops, to null out exactly the kind of movements you've described.

In another thread I did mention using a parallel robot crane (ie multiple lines to the load simultaneously). This would stabilize the load against side winds etc, and could conceivably be fully automated, including connecting the crane to the load.

Docking was solved in 1966. Several dockings do not pose a problem.
Reading this thread, and this is what stands out to me. All of these ideas to minimize rendezvous and docking maneuvers are cool, but it's additional hardware development in order to reduce something that SpaceX are planning to do thousands and more of.

Doing something thousands of times doesn't make it free, or risk-free. On the contrary it multiplies the cost thousands-fold, making the amortization of some hardware development a possibility.

Also Pusher Tanker was mainly designed to eliminate the triple-pass through the Van Allen Belts during HEEO refueling, cutting the radiation dose for passengers. Minimizing docking events is only a (welcome) secondary benefit imo.
« Last Edit: 10/23/2019 05:53 pm by Twark_Main »

Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #36 on: 10/23/2019 05:40 pm »
I don't believe a crane will constrain the movement of the Starship for on-pad assembly as much as you think it will, Twark_Main. Even a light wind against the massive surface area of Starship is going to move even the best rigged system. Steel lines have quite a bit of stretch in them at that scale. There's really no telling if the pad assembly equipment or a close facsimile will work and I find it unlikely that we'll ever find out.
Docking was solved in 1966. Several dockings do not pose a problem.
Reading this thread, and this is what stands out to me. All of these ideas to minimize rendezvous and docking maneuvers are cool, but it's additional hardware development in order to reduce something that SpaceX are planning to do thousands and more of.

However hard doing the crane-based alignment is, the point was that it's vastly easier than a free-flying hard dock that can tolerate high injection loads and still reliability undock later.

As for docking minimization: For cargo flights, I completely agree with you that tolerating the risk of a couple of R/D operations is well within vanilla-flavored mission risks (although at least aggregating the prop in a single tanker before approaching the payload is pretty much a no-brainer).  But for crewed flights, this is going to turn into a major term in your PLOC calculation.  There are already enough genuinely frightening things in the crewed mission profile (launch with less-than-stellar abort capability, doing hoverslams into unprepared regolith, and a largely unexplored EDL profile that has to work at ~11 km/s entry speeds being my three favorite hand-wringers) without adding additional major terms.

Offline SteveU

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Re: Refueling v Docking with Fuel Tanks
« Reply #37 on: 10/23/2019 06:18 pm »
...However hard doing the crane-based alignment is, the point was that it's vastly easier than a free-flying hard dock that can tolerate high injection loads and still reliability undock later.

I really disagree.  When the two ships are in refueling position they will need to be as close to a hard dock as possible. In this state, there is no reason that the same interstage latching mechanism between SH/SS could not be used between  P/T & SS.

I do not believe that there can be movement between the two ships while transferring tens of thousands pounds of fuel.  If you are concerned about a docking mechanism failing due to the masses involved, just how will fueling connectors hold up?

The intersatge latching mechanisms are designed to tolerate the dynamic loads of launch and disengage at stage separation. When stacking the ships on the pad there will have to be a way for the latches to "self cinch" to obtain a tight grip on SS - unless you believe that this will need to be a manual operation.

If the latching/cinching is automatic, I can't think of a reason why it wouldn't work in LEO.  And the forces involved will be much less than during launch.
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Online TheRadicalModerate

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Re: Refueling v Docking with Fuel Tanks
« Reply #38 on: 10/23/2019 07:34 pm »
...However hard doing the crane-based alignment is, the point was that it's vastly easier than a free-flying hard dock that can tolerate high injection loads and still reliability undock later.

I really disagree.  When the two ships are in refueling position they will need to be as close to a hard dock as possible. In this state, there is no reason that the same interstage latching mechanism between SH/SS could not be used between  P/T & SS.

I do not believe that there can be movement between the two ships while transferring tens of thousands pounds of fuel.  If you are concerned about a docking mechanism failing due to the masses involved, just how will fueling connectors hold up?

The intersatge latching mechanisms are designed to tolerate the dynamic loads of launch and disengage at stage separation. When stacking the ships on the pad there will have to be a way for the latches to "self cinch" to obtain a tight grip on SS - unless you believe that this will need to be a manual operation.

If the latching/cinching is automatic, I can't think of a reason why it wouldn't work in LEO.  And the forces involved will be much less than during launch.

Let's break this down.  I'll obviously agree that if you can completely replicate the conditions of stacking on-orbit, then we're done.  So:  what's different?  Three things:

1) Positional and momentum accuracy.  You can stack the SS onto the SH in a hurricane and you'll still have fewer degrees of freedom than you will on-orbit, and better guidance.

2) Pre-loading force before latching.  Set an empty Starship down on an SH and there will be about 1.2 MN of force loading the latch system before they engage.  I can't imagine a hard-dock retraction pre-loading a soft capture as evenly and reliably as good ol' gravity will.

3) Recovery from latching errors.  If things go screwy on the ground, a tech can inspect and, if necessary, reset the latch.  That's not happening on-orbit.  Even in the worst case of a jammed latch that can't be recovered during stacking, you can drill the silly thing out, pop the others, unstack, and repair.  Again, none of those are options on-orbit, and the lack of a recovery mode means that your crew will have to be rescued from a 100 m long, not-very-structurally-sound vehicle with now-limited ability to manage its own attitude (because the stack is AFU).

I completely agree that a hard dock of some quality is needed to do refueling ops.  However, it's that "of some quality" modifier that's in dispute.  The forces and torques incurred during refueling simply aren't anywhere near the forces and torques that would be required to allow a P/T to efficiently inject an SS into an orbit that's high enough energy for it to continue on to TLI--or TMI, if you want to do much-faster-than-Hohmann transfers.

I believe that this is a solvable problem.  However, I'm not sure it's solvable by reusing the latching system used for stacking, however desirable that might be.  I'm also not sure that it's easily solvable if it isn't planned-for ahead of time.  That might make a scheme like P/T non-viable later on--which would be a shame, because it solves some pretty important issues with crew safety and even cargo ops complexity quite elegantly.

Offline whitelancer64

Re: Refueling v Docking with Fuel Tanks
« Reply #39 on: 10/23/2019 07:54 pm »
...However hard doing the crane-based alignment is, the point was that it's vastly easier than a free-flying hard dock that can tolerate high injection loads and still reliability undock later.

I really disagree.  When the two ships are in refueling position they will need to be as close to a hard dock as possible. In this state, there is no reason that the same interstage latching mechanism between SH/SS could not be used between  P/T & SS.

I do not believe that there can be movement between the two ships while transferring tens of thousands pounds of fuel.  If you are concerned about a docking mechanism failing due to the masses involved, just how will fueling connectors hold up?

The intersatge latching mechanisms are designed to tolerate the dynamic loads of launch and disengage at stage separation. When stacking the ships on the pad there will have to be a way for the latches to "self cinch" to obtain a tight grip on SS - unless you believe that this will need to be a manual operation.

If the latching/cinching is automatic, I can't think of a reason why it wouldn't work in LEO.  And the forces involved will be much less than during launch.

Let's break this down.  I'll obviously agree that if you can completely replicate the conditions of stacking on-orbit, then we're done.  So:  what's different?  Three things:

1) Positional and momentum accuracy.  You can stack the SS onto the SH in a hurricane and you'll still have fewer degrees of freedom than you will on-orbit, and better guidance.

2) Pre-loading force before latching.  Set an empty Starship down on an SH and there will be about 1.2 MN of force loading the latch system before they engage.  I can't imagine a hard-dock retraction pre-loading a soft capture as evenly and reliably as good ol' gravity will.

3) Recovery from latching errors.  If things go screwy on the ground, a tech can inspect and, if necessary, reset the latch.  That's not happening on-orbit.  Even in the worst case of a jammed latch that can't be recovered during stacking, you can drill the silly thing out, pop the others, unstack, and repair.  Again, none of those are options on-orbit, and the lack of a recovery mode means that your crew will have to be rescued from a 100 m long, not-very-structurally-sound vehicle with now-limited ability to manage its own attitude (because the stack is AFU).

I completely agree that a hard dock of some quality is needed to do refueling ops.  However, it's that "of some quality" modifier that's in dispute.  The forces and torques incurred during refueling simply aren't anywhere near the forces and torques that would be required to allow a P/T to efficiently inject an SS into an orbit that's high enough energy for it to continue on to TLI--or TMI, if you want to do much-faster-than-Hohmann transfers.

I believe that this is a solvable problem.  However, I'm not sure it's solvable by reusing the latching system used for stacking, however desirable that might be.  I'm also not sure that it's easily solvable if it isn't planned-for ahead of time.  That might make a scheme like P/T non-viable later on--which would be a shame, because it solves some pretty important issues with crew safety and even cargo ops complexity quite elegantly.

1 & 2. are pretty much solved issues. SpaceX has been docking with the ISS for a while now. A similar positioning system / target will be needed on the aft end of Starship. Bonus, it would be dual-redundant because both vehicles would have it. Positioning systems are something SpaceX has been using for some time now. Both Starship and tanker would have positive control of themselves and will be able to accurately align themselves in space.

3. That would be a major issue for separation during staging in flight. A mechanism prone to jam or need drilling out while being stacked would never be used since it would be far too unreliable. This also resolves your previous concern about unlatching.

An androgynous latching system that is identical on the top of SH and the bottom of SS is a very SpaceX thing to do. Keep it simple. The best system is the one you don't have to design in the first place.
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