Author Topic: Barges, Tugs and Towing  (Read 12618 times)

Offline darkenfast

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Barges, Tugs and Towing
« on: 03/06/2018 06:37 am »
There has been some discussion on other threads about water-borne towing, speeds and such-like recently, following the decision by SpaceX not to attempt recovery of today's launch.  I thought it might be appropriate to bring it over here.

There was a good explanation elsewhere of how the typical displacement hull has a built-in speed limit that relates to the behavior of water and waves.  Basically, it goes like this: the Theoretical Hull Speed is the square-root of the waterline length in feet, times 1.34, equals THS in knots (I don't know the metric equivalent off the top of my head).  Like a lot of things related to boats and ships it should be taken as a ROUGH guide.  To go faster becomes rapidly pointless unless your hull has a shape that can plane over the top of the water (speedboats or Mr. Steven-types), or has a really slim shape that doesn't create much wave (catamarans or exotic submerged twin-hull types).  Someone calculated that Marmac-300s have a THS of 23 kts.  A giant oil tanker might have a THS of 48 kts. 

Notice, however, that one does not see a giant oil tanker, container ship or bulk-cargo barge performing at anywhere near that speed.  This is because it takes a ridiculous amount of power to get these big vessels to their THS.  Even the U.S. Navy's nuclear-powered aircraft carriers (260,000hp and up) cannot reach their THS, although they come a lot closer.  The big commercial vessels are the most economical way to move vast amounts of cargo around the world because they operate at a fraction of their hull speed, using far less horsepower and fuel.  There is a "sweet-spot" for every hull size, power-plant and mission that is aimed for by the designers.

Just as you probably wouldn't want to cross a road-less desert in a Lotus or enter a Formula race in a Land-Rover (even though they may have models with similar horsepower), you don't expect your vessel to do things it wasn't designed for.   The tug vessels that SpaceX charters are a case in point.   They have a lot of power for their size, but it is "geared" towards providing a lot of pull at slow speeds by the correct selection of engine and propeller.   A tug will not achieve planing speed when loaded light.  On the other hand, Mr. Stephen (the fairing-catcher on the West Coast), will.  It is designed for it.  It is also more expensive to operate than slower off-shore supply vessels that carry similar loads.

Another aspect is shape.  The barges are designed to carry a huge amount of bulk cargo for their length, at slow speeds, and with a low building cost.  They do this by being, basically, a box with slanted ends.  While I suppose, if you ballasted the stern of a Marmac to get the bow up higher, it theoretically could plane, it would be a ridiculous achievement and a great waste of fuel.

Hope this helps discussions regarding the small (but growing!), SpaceX Navy.
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Offline vanoord

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Re: Barges, Tugs and Towing
« Reply #1 on: 03/06/2018 10:36 am »
There's also the aspect of length to beam (width) ratio - effectively the same as the fineness ratio in aircraft.

Essentially, the narrower the boat is in relation to its length, the faster it can go (subject to a lot of provisos).

In the case of the ASDS barges, the lateral extensions to the deck (to make the barge squarer to accommodate the landing circle) are out of the water, so don't affect the shape of the hull.

If an ASDS were built with the hull that shape (approaching square), then it would be even slower. It may also be more difficult to tow it straight.

However, if the hull were made narrower (and the overall width maintained by building larger lateral extensions) there would be a loss of stability (needed to keep the deck vaguely level when the core lands) and a loss of buoyancy - which could be resolved by making it deeper, which in turn might create issues for bringing the barge into port.

Towing speed could be improved by making the bow and stern more 'pointy' but unless that was reflected in a significant lengthening of the barge, the stability/buoyancy issues rear their head again.

In short, those barges are the shape they are for a reason!

One alternative may be to switch to a catamaran hull, as two slender hulls would give speed benefits and - if they were widely spaced - also potentially improve stability.

There might then be an option to fit that with engines and a drive system to get the speed up, but that's a bottomless rabbit hole of speculation...

The issue with that is that it would require designing and building something bespoke: a possibility, but not necessarily a financially attractive option compared to just renting / buying something 'off the shelf'.


Going back to the point I usually make: what is the actual benefit in bringing an ASDS back to port with a transit time of (say) 24 hours rather than (say) 48?

Unless cores are being refurbished in a matter of days (we'll see whether that happens) and there is a very high launch cadence (which is currently restricted by pad turnaround) and there is an insufficient number of cores in existence (which seems unlikely, even if do keep dropping them into the sea) - then there's no real benefit in cutting a day or so off turnaround times.


Offline Ludus

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Re: Barges, Tugs and Towing
« Reply #2 on: 03/12/2018 10:43 pm »
Why does Blue Origin want to do Booster landings on a moving ship rather than a barge with thrusters to keep it a GPS position?

Offline Lar

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Re: Barges, Tugs and Towing
« Reply #3 on: 03/12/2018 11:49 pm »

Unless cores are being refurbished in a matter of days (we'll see whether that happens) and there is a very high launch cadence (which is currently restricted by pad turnaround) and there is an insufficient number of cores in existence (which seems unlikely, even if do keep dropping them into the sea) - then there's no real benefit in cutting a day or so off turnaround times.

Cadence is limited by the slowest operation divided by how many of them you are executing in parallel.

Cores can take a month to refurb and if you have 10 of them, you can do a 3 day cadence based on core refurb.

With only one ASDS, there is a limit to how close in you can do two ASDS (not RTLS) missions of a week or more. Faster transit/return times reduce this. But I suspect buying a whole second set offers a better reduction. Especially if you have to buy a second set anyway for FH.
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Offline CameronD

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Re: Barges, Tugs and Towing
« Reply #4 on: 03/13/2018 02:09 am »
Why does Blue Origin want to do Booster landings on a moving ship rather than a barge with thrusters to keep it a GPS position?

Theoretically, a moving displacement-hulled vessel is more stable than a stationary one.  This is one reason Sea Pilot vessels going alongside ships in the open ocean require the ship to be moving at around 5kts for the Pilot to safely board.

There are several reasons for this including (a) because the vessels mass will 'flatten' out (smooth) the ocean as it travels over it and (b) because the vessels own bow wave cancels out at least some of the ocean waves impacting the hull.. but it only works across a narrow range of vessel speed, size and sea state.

   
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline darkenfast

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Re: Barges, Tugs and Towing
« Reply #5 on: 03/13/2018 02:53 am »
Moving vessels can also use stabilizers to dampen out rolling.  These are fins that need a flow of water across them to work.
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Offline Robotbeat

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Re: Barges, Tugs and Towing
« Reply #6 on: 03/13/2018 03:13 am »
Or giant spinning flywheels!
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Offline deruch

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Re: Barges, Tugs and Towing
« Reply #7 on: 03/13/2018 05:01 pm »
Why does Blue Origin want to do Booster landings on a moving ship rather than a barge with thrusters to keep it a GPS position?

They might view it as an architecture which will enable faster cycling of landed cores.  Current speculation on their rocket plans shows recovery happening farther offshore than F9 landings.  Meaning that the downtime for towing a barge out and back will be significantly longer than SpaceX's current requirements.  As SpaceX is already finding this shorter time sink an inconvenience to maintaining a high cadence of launches with recovery, it's not too surprising that Blue may be considering alternatives.
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Offline Space Ghost 1962

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Re: Barges, Tugs and Towing
« Reply #8 on: 03/13/2018 05:19 pm »
Suggest it is to allow recovery irrespective of sea state, so that even in poor conditions the valuable, always meant to be recovered,  overbuilt to allow very large number of uses booster - will be recovered. Not the same as Falcon's "opportunistic' approach to recovery, with expendibility if things don't work out, which has happened.

Offline envy887

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Re: Barges, Tugs and Towing
« Reply #9 on: 03/13/2018 06:06 pm »
Suggest it is to allow recovery irrespective of sea state, so that even in poor conditions the valuable, always meant to be recovered,  overbuilt to allow very large number of uses booster - will be recovered. Not the same as Falcon's "opportunistic' approach to recovery, with expendibility if things don't work out, which has happened.

Also for faster return of the booster, which always lands downrange and never RTLS, and which always lands far downrange. It would take a towed barge well over a week to return.

Offline deruch

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Re: Barges, Tugs and Towing
« Reply #10 on: 03/13/2018 10:43 pm »
Suggest it is to allow recovery irrespective of sea state, so that even in poor conditions the valuable, always meant to be recovered,  overbuilt to allow very large number of uses booster - will be recovered. Not the same as Falcon's "opportunistic' approach to recovery, with expendibility if things don't work out, which has happened.

I don't doubt that that was also a consideration, but for the same reason I've just expected NG to only launch when recovery has good chances to succeed.  i.e. BO's Launch Commit Criteria will very explicitly include hard limits for landing area conditions.  With the possible exception of a rare mission with a hard launch window (like some planetary launches, etc).  If they're lucky, SpaceX will move toward including something along these lines so that the market has already had a chance to get used to the idea some by the time that NG starts launching.  Let SpaceX get all the flak and then come online and just say, "Well that's how everyone does reusable launches."  Gradatim, For The Win.

For SpaceX, it might be a progressive evolution.  Start with a sliding price scale where if you got a low price (only offered for missions where recovery is possible), then that has baked into it that SpaceX can delay in order to optimize for landing conditions but gives the customer an option to launch anyways for an added "non-standard service price " of $X.
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Offline Space Ghost 1962

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Re: Barges, Tugs and Towing
« Reply #11 on: 03/14/2018 12:05 am »
Also for faster return of the booster, which always lands downrange and never RTLS, and which always lands far downrange.
They've suggest a fast return ship that the booster is transferred to. As an refinement to operation.

Note - to keep up launch/recovery rate, keep in mind the "return to port, unload, return to downrange" considerable cycle time that limits launch frequency. Part of the reason for SX RTLS - the need for booster on site for quick relaunch.

Unsurprisingly, SX is finding with FH that with larger payloads RTLS is costly, and in the near future booster barge landings and disposable core are the best trade for maximum reasonable mission trades (if Block 5 earns its 10+ reuse rate, wouldn't be surprised if they advance the state of the art to downrange core recovery eventually).

Quote
It would take a towed barge well over a week to return.

Note also that a fast return ship also would work for SX too in the same sense.

I don't doubt that that was also a consideration, but for the same reason I've just expected NG to only launch when recovery has good chances to succeed.  i.e. BO's Launch Commit Criteria will very explicitly include hard limits for landing area conditions.
As "gradatim" would suggest. Again, they have to protect that reusable asset to the utmost. Evolve operations to gradually probe increasing flight rate through loosening the LCC.

Quote
With the possible exception of a rare mission with a hard launch window (like some planetary launches, etc).
When you have one pad and one recovery ship, everything slows down the chain of operations. Anything gradual gets even more gradual.

Quote
If they're lucky, SpaceX will move toward including something along these lines so that the market has already had a chance to get used to the idea some by the time that NG starts launching.  Let SpaceX get all the flak and then come online and just say, "Well that's how everyone does reusable launches."  Gradatim, For The Win.
Forgive me, but that sounds too accommodating for SX.

Please note that the immediate next step SX is taking is to drive hard on launch rate. Which suggests "non gradatim for the win"?

Perhaps if they clear the manifest fast, they then can factor in new launch payloads even faster, so "pipeline" the market, draining down next years payloads before rivals have a chance to bid on them? If they genuinely can get reuse to economically work for them, provoking a "launch desert" for a few years would put significant financial pressure on pure expendable rivals, and create a competitive lock up rush to get payloads on orbit ASAP. (Can happen because most are small payloads.)

You get there before anyone else.
 
Quote
For SpaceX, it might be a progressive evolution.  Start with a sliding price scale where if you got a low price (only offered for missions where recovery is possible), then that has baked into it that SpaceX can delay in order to optimize for landing conditions but gives the customer an option to launch anyways for an added "non-standard service price " of $X.
This might make sense if large payloads were more common. They're not.

And I think the SX response to all this is "to get a bigger hammer" ... er ... "bigger LV", so you can RTLS a fully reusable vehicle.

Which is why Musk talks of BFS flying in two years.

Keep in mind, you could likely field quite a number of barges for the cost of a ship. And just leave them there, serviced by fast ship(s), as an after the fact means to handle the recovered booster fleet.

Offline aero

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Re: Barges, Tugs and Towing
« Reply #12 on: 03/14/2018 12:56 am »
Quote
Keep in mind, you could likely field quite a number of barges for the cost of a ship. And just leave them there, serviced by fast ship(s), as an after the fact means to handle the recovered booster fleet.

That's not too much different than servicing an offshore launch/landing site for the BFS. Much farther out to sea, and booster stages instead of people but with different equipment the operations would be similar, ISTM. (It seems to me.)
« Last Edit: 03/14/2018 04:29 am by aero »
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Offline CameronD

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Re: Barges, Tugs and Towing
« Reply #13 on: 03/14/2018 03:26 am »
It would take a towed barge well over a week to return.

Note also that a fast return ship also would work for SX too in the same sense.

All I'll say is that SpaceX already know how hard it is to land on a (almost) stationary platform.. and landing on a moving one ups the difficulty to a whole other level never before seen by mankind.

If this really is what they intend (and not just marketing hype) I expect SpaceX will wait and see how it works out for BO (and what they have to do to overcome the not-inconsiderable challenges) before changing anything much to do with their current ASDS operations....  YMMV.
« Last Edit: 03/14/2018 03:27 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline guckyfan

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Re: Barges, Tugs and Towing
« Reply #14 on: 03/14/2018 09:28 am »
If they wait until BO has proven their approach SpaceX will be in an advanced state of developing BFR which will always do RTLS. I doubt at that time they will change their operating methods for Falcon.

In the mean time iff they need a faster turn around I would rather expect them to load stages onto a ship for fast return and keep the ASDS out there for extended periods.

Offline leetdan

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Re: Barges, Tugs and Towing
« Reply #15 on: 03/14/2018 10:11 am »
All I'll say is that SpaceX already know how hard it is to land on a (almost) stationary platform.. and landing on a moving one ups the difficulty to a whole other level never before seen by mankind.

I think you're getting carried away.  The precise landing time and location is known, and programming the ship's autopilot to intersect at both is a solved problem.

Offline speedevil

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Re: Barges, Tugs and Towing
« Reply #16 on: 03/14/2018 11:34 am »
I think you're getting carried away.  The precise landing time and location is known, and programming the ship's autopilot to intersect at both is a solved problem.

Arguably possibly easier due to the fact you can null out at least some of the wind.
(counter to this, a smaller vessel underway may have more vertical deck motion)

Offline CameronD

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Re: Barges, Tugs and Towing
« Reply #17 on: 03/15/2018 12:17 am »
All I'll say is that SpaceX already know how hard it is to land on a (almost) stationary platform.. and landing on a moving one ups the difficulty to a whole other level never before seen by mankind.

I think you're getting carried away.  The precise landing time and location is known, and programming the ship's autopilot to intersect at both is a solved problem.

Actually it isn't a "solved problem" because the ocean surface is rarely flat calm.  The current ASDS landing system works because (b) the ASDS is dynamically held 'stationary' at the target point and (b) offers a large target area in two dimensions.. but as soon as you start moving the vessel to intercept the target point, it is at the mercy of the ocean.

Ultimate success depends on the vessel's approach speed to the target, but, even then, holding to a precisely fixed speed and direction (to intercept the target position at a precise moment in time) on the open ocean in waves of random height, speed and direction is very difficult/impossible in a displacement hulled vessel as anyone who has travelled on an older-style fast ferry will tell you (I hope they were hanging on!).  As we've seen from video of the landings, the 'hover-slam' happens very quickly with very little room for error meaning (a) the vessel would need to have a large enough landing area to compensate (probably larger than it is now) and (b) the stage would need really, really good side thrusters to hold it upright against the movement of the vessel until it is safely on deck.

With reduced hull motion, it's arguably easier in a SWATH or similar high speed hull, however the mass and final position of the rocket's landing on the vessel would result in a significant near-instantaneous increase in displacement/trim and a corresponding near-instantaneous decrease in speed which could be enough to topple the stage.  It would feel like the ship hit a brick wall..

In summary: It's a nice idea that perhaps holds merit but, as I see it, SpaceX have far fewer variables to contend with in the present design.
 
« Last Edit: 03/15/2018 02:14 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline leetdan

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Re: Barges, Tugs and Towing
« Reply #18 on: 03/15/2018 04:45 am »
The ASDS has no control authority in the vertical direction, only X & Y translation.  The deck will be pitching as a function of wave period vs. hull dimensions.  A ship with stabilizer fins has the vertical control authority to null out this pitching.

A well-designed autopilot will be able to contend with instantaneous changes in current and wind, especially if controllable pitch propellers are used for propulsion.  This leaves the rate of travel itself, which is well within the capability of a gimballed rocket engine to match at touchdown.  The landing platform is more stable overall, at the cost of being installed on a larger and more expensive hull.

Offline CameronD

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Re: Barges, Tugs and Towing
« Reply #19 on: 03/15/2018 05:31 am »
The ASDS has no control authority in the vertical direction, only X & Y translation.  The deck will be pitching as a function of wave period vs. hull dimensions.

Yes, we've seen this.. but it isn't responding *quickly* which a moving craft will when it hits a wave.

A ship with stabilizer fins has the vertical control authority to null out this pitching.

Only within a set speed range which is dependent on sea state.  Too slow and they don't do anything.  Too fast and 'slamming' becomes an issue.

A well-designed autopilot will be able to contend with instantaneous changes in current and wind, especially if controllable pitch propellers are used for propulsion.

Actually, no, even a well-designed autopilot (and I'm not familiar with any that aren't) has it's limitations.  If what you are saying were true, you would no longer need hand-holds and narrow passage-ways on Azipod -equipped ocean liners... or any other large vessel for that matter.  Autopilots are designed to keep the ship on course and speed - not counteract every single wave that happens along.

This leaves the rate of travel itself, which is well within the capability of a gimballed rocket engine to match at touchdown.  The landing platform is more stable overall, at the cost of being installed on a larger and more expensive hull.

I never said it wasn't possible given similar parameters to those we see used successfully with the ASDS fleet.. only that it's more difficult (and expensive) than you think.  :)
« Last Edit: 03/15/2018 05:37 am by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline leetdan

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Re: Barges, Tugs and Towing
« Reply #20 on: 03/15/2018 06:34 am »
I think we mostly agree, my original point was that underway landing represented an incremental change as opposed to revolutionary.

I should've been clearer, by 'contend with' I wasn't implying feedback so perfect that handholds wouldn't be needed.  Assuming something in the Panamax neighborhood and assuming ASDS-compatible sea state, a capable autopilot should keep things well within the positional margin required for landing.

Offline vanoord

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Re: Barges, Tugs and Towing
« Reply #21 on: 03/15/2018 08:30 am »
Cadence is limited by the slowest operation divided by how many of them you are executing in parallel.

Cores can take a month to refurb and if you have 10 of them, you can do a 3 day cadence based on core refurb.

With only one ASDS, there is a limit to how close in you can do two ASDS (not RTLS) missions of a week or more. Faster transit/return times reduce this. But I suspect buying a whole second set offers a better reduction. Especially if you have to buy a second set anyway for FH.

Given the costs of ASDS recoveries, there are probably more benefits to be gained from maximising the proportion of RTLS recoveries.

The trick is going to be spacing out the recoveries on both East coast ASDSes between RTLS landings to ensure that cadence is maintained.

That said, they're a long way off that becoming a regular issue - payload availability / processing and S2 availability are likely to be more significant constraints.

Then again, adding the occasional Falcon Heavy dual offshore recovery into the mix is going to make things more interesting.

Offline Archibald

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Re: Barges, Tugs and Towing
« Reply #22 on: 03/15/2018 06:10 pm »
I remember that a looooooooooong time ago on this forum (SpaceX time: barely two years) we discussed one heck of an exciting idea. That is, a Falcon 9R stage that had landed on the drone ship could be automatically refueled with some kerolox and fly itself back to the Cape in a brief hop.
Did Musk ever considered that option ? and what would the FAA think about it ? can't be worse than the twin booster landing at the Cape...
« Last Edit: 03/15/2018 06:11 pm by Archibald »
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Offline envy887

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Re: Barges, Tugs and Towing
« Reply #23 on: 03/15/2018 06:55 pm »
I remember that a looooooooooong time ago on this forum (SpaceX time: barely two years) we discussed one heck of an exciting idea. That is, a Falcon 9R stage that had landed on the drone ship could be automatically refueled with some kerolox and fly itself back to the Cape in a brief hop.
Did Musk ever considered that option ? and what would the FAA think about it ? can't be worse than the twin booster landing at the Cape...

Musk originated that speculation with some rather off-the-cuff comments soon after the first barge landing. The idea is not quite that simple. The booster is not very aerodynamic with the open interstage and extended legs, so those would have to be capped and closed.

The booster needs more than just RP-1 and LOX: TEA/TEB, nitrogen, helium, at least. The barge would need to supply those, and also have a launch mount with hold-downs, and a flame deflector installed. All of this would have to be operated either automatically, or by crew from the support ships.

Offline CameronD

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Re: Barges, Tugs and Towing
« Reply #24 on: 03/15/2018 09:40 pm »
I remember that a looooooooooong time ago on this forum (SpaceX time: barely two years) we discussed one heck of an exciting idea. That is, a Falcon 9R stage that had landed on the drone ship could be automatically refueled with some kerolox and fly itself back to the Cape in a brief hop.
Did Musk ever considered that option ? and what would the FAA think about it ? can't be worse than the twin booster landing at the Cape...

Musk originated that speculation with some rather off-the-cuff comments soon after the first barge landing. The idea is not quite that simple. The booster is not very aerodynamic with the open interstage and extended legs, so those would have to be capped and closed.

The booster needs more than just RP-1 and LOX: TEA/TEB, nitrogen, helium, at least. The barge would need to supply those, and also have a launch mount with hold-downs, and a flame deflector installed. All of this would have to be operated either automatically, or by crew from the support ships.

..and, in the end, it was easier, simpler and safer to tow the entire thing back to shore and forget about it.
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Archibald

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Re: Barges, Tugs and Towing
« Reply #25 on: 03/16/2018 08:11 am »
Never bothered to ask how many hours does OCISLY and others needs to drive themselves back the from landing zone to Cape Canaveral. At some point in the future, with very high flight rates, all those hours lost in transit times might become an issue... or maybe not.

Quote
The booster needs more than just RP-1 and LOX: TEA/TEB, nitrogen, helium, at least. The barge would need to supply those, and also have a launch mount with hold-downs, and a flame deflector installed. All of this would have to be operated either automatically, or by crew from the support ships.

be creative ! hint: Sea Launch rebuild an entire burned-up oil rig to launch a rocket (of course it did not exactly ended well for them)

I remember the first time I red about Sea Launch, 1997, in the pre-history of the Internet. I thought the guys were completely nuts or insane. Launch a rocket from a rebuild oil rig, what can possibly go wrong ?
« Last Edit: 03/16/2018 08:13 am by Archibald »
Han shot first and Gwynne Shotwell !

Offline envy887

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Re: Barges, Tugs and Towing
« Reply #26 on: 03/16/2018 12:42 pm »
Never bothered to ask how many hours does OCISLY and others needs to drive themselves back the from landing zone to Cape Canaveral. At some point in the future, with very high flight rates, all those hours lost in transit times might become an issue... or maybe not.

Quote
The booster needs more than just RP-1 and LOX: TEA/TEB, nitrogen, helium, at least. The barge would need to supply those, and also have a launch mount with hold-downs, and a flame deflector installed. All of this would have to be operated either automatically, or by crew from the support ships.

be creative ! hint: Sea Launch rebuild an entire burned-up oil rig to launch a rocket (of course it did not exactly ended well for them)

I remember the first time I red about Sea Launch, 1997, in the pre-history of the Internet. I thought the guys were completely nuts or insane. Launch a rocket from a rebuild oil rig, what can possibly go wrong ?

The Sea Launch platform was basically an entire floating launch pad and cost hundreds of millions of dollars.

Of course it's possible to flyback the boosters, if they really wanted to do that. But it's way cheaper to just lease and outfit another barge for landings - which is in fact what they are doing.

Barges are a lot cheaper than floating launch facilities.

Offline RonM

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Re: Barges, Tugs and Towing
« Reply #27 on: 03/16/2018 01:54 pm »
I remember that a looooooooooong time ago on this forum (SpaceX time: barely two years) we discussed one heck of an exciting idea. That is, a Falcon 9R stage that had landed on the drone ship could be automatically refueled with some kerolox and fly itself back to the Cape in a brief hop.
Did Musk ever considered that option ? and what would the FAA think about it ? can't be worse than the twin booster landing at the Cape...

Musk originated that speculation with some rather off-the-cuff comments soon after the first barge landing. The idea is not quite that simple. The booster is not very aerodynamic with the open interstage and extended legs, so those would have to be capped and closed.

The booster needs more than just RP-1 and LOX: TEA/TEB, nitrogen, helium, at least. The barge would need to supply those, and also have a launch mount with hold-downs, and a flame deflector installed. All of this would have to be operated either automatically, or by crew from the support ships.

..and, in the end, it was easier, simpler and safer to tow the entire thing back to shore and forget about it.

Yes. Fly back from the barge is only necessary if that core needs to be launched again very soon. Cheaper and easier to have more cores in the launch queue.

Offline Kaputnik

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Re: Barges, Tugs and Towing
« Reply #28 on: 03/16/2018 05:48 pm »
Just to return to the opening post.
Theoretical hull speed is a recognised phenomenon, but not everyone understands why it exists.
As I understand it, when you move a vessel through the water you create a bow wave,, and the faster you go, the larger this wave becomes. Eventually you reach a point where the bow wave reaches all the way along the vessel, at which point to travel any faster you need to actually climb uphill onto the wave. This is THS. Clearly pushing a huge ship upwards requires a huge amount of energy and a level of thrust that most ships lack. In any case for huge vessels the structural loads imposed at this point would start to become just as big an issue as the sheer power.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Online OneSpeed

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Re: Barges, Tugs and Towing
« Reply #29 on: 03/19/2018 01:21 pm »
Just to return to the opening post.
Theoretical hull speed is a recognised phenomenon, but not everyone understands why it exists ...

The speed of waves is controlled by gravity, wavelength, and water depth. For deep ocean waves, water depth can be ignored, and the wave speed C = √gL/2Π.
E.g. for a 91m wavelength, the speed = √(9.8 * 91)/2Π = 11.9 m/s or about 23 knots.

For a 91m displacement hull to travel at 23 knots, it would need to be able to generate (and sit in the trough of) a wave travelling at the same speed, and therefore equal to the hull's waterline length. The first image from SSPA below shows a high fineness displacement hull in a towing tank generating a wave very close to its own waterline length. Note that the hull shape is also very close to the shape of the wave.

The Froude number (Fr) is a dimensionless number used in hydrodynamics to indicate the efficiency of a particular hull shape, where Fr = u/√gL.
For the example above, Fr = 11.9 / √(9.8 * 91) = 0.4.

The 91m ASDS hull is however nothing like the shape of a deep ocean wave, and so is not able to generate one that even approaches 91m in length. It is deeply inefficient under tow, which is usually at around 6 knots, or 3 m/s. This indicates a Froud number of 3 / √(9.8 * 91) = 0.1. Rearranging the wave speed equation, L = 2Π*C^2/g, giving a corresponding bow wave length of around 5.8m.

In order to exceed 23 knots, a 91m hull would need to generate a faster wave. E.g. to travel at √2 times the speed, 32.5 knots, it would need to somehow generate a wave twice as long, in other words, it would need to be capable of planing.

Theoretical hull speed is a bit like the speed of sound, only feasible to exceed with a suitable shape.

Offline Comga

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Re: Barges, Tugs and Towing
« Reply #30 on: 06/09/2018 04:52 am »
(Snip)
With reduced hull motion, it's arguably easier in a SWATH or similar high speed hull, however the mass and final position of the rocket's landing on the vessel would result in a significant near-instantaneous increase in displacement/trim and a corresponding near-instantaneous decrease in speed which could be enough to topple the stage.  It would feel like the ship hit a brick wall..

In summary: It's a nice idea that perhaps holds merit but, as I see it, SpaceX have far fewer variables to contend with in the present design.

I only regret that I have but one like to leave for my colleague. (To paraphrase Nathan Hale)
This commentary has been a long time coming.
Musk is a physicist and a “first principles” guy. He reduces problems to their simplest and peals away at the obstacles, the degrees of freedom. A stationary barge drives many DoFs to zero, or close enough in the case of vertical motion. A moving ship adds many more to that also need to arrive at zero, but now they have to do so at one precise moment.
The issue of a slow barge (ASDS if you wish) return is much simpler to “solve” than the very complex control issue.
« Last Edit: 06/09/2018 02:02 pm by Comga »
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline Nomadd

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Re: Barges, Tugs and Towing
« Reply #31 on: 06/10/2018 02:53 pm »
 I've been telling them for years to get Austal to build them a 400 foot cat.
Those who danced were thought to be quite insane by those who couldn't hear the music.

Offline Wolfram66

Re: Barges, Tugs and Towing
« Reply #32 on: 06/10/2018 04:30 pm »
(Snip)
With reduced hull motion, it's arguably easier in a SWATH or similar high speed hull, however the mass and final position of the rocket's landing on the vessel would result in a significant near-instantaneous increase in displacement/trim and a corresponding near-instantaneous decrease in speed which could be enough to topple the stage.  It would feel like the ship hit a brick wall..

In summary: It's a nice idea that perhaps holds merit but, as I see it, SpaceX have far fewer variables to contend with in the present design.

I only regret that I have but one like to leave for my colleague. (To paraphrase Nathan Hale)
This commentary has been a long time coming.
Musk is a physicist and a “first principles” guy. He reduces problems to their simplest and peals away at the obstacles, the degrees of freedom. A stationary barge drives many DoFs to zero, or close enough in the case of vertical motion. A moving ship adds many more to that also need to arrive at zero, but now they have to do so at one precise moment.
The issue of a slow barge (ASDS if you wish) return is much simpler to “solve” than the very complex control issue.

One could also take another note from history of the Glomar Explorer. Roll and heave compensators to maintain vertical stability. http://www.hnsa.org/resources/manuals-documents/single-topic/the-glomar-explorer-deep-ocean-working-vessel-technical-description-and-specification/

And use Nomadd’s Austral idea but one step better. Taking a cue from Star Trek. A Trimaran where the drive central section ballasts down and detaches becoming the support ship. Sort of like a semi-tractor trailer truck

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