Author Topic: Will SpaceX experiment with legless F9 Booster landing on a structure?  (Read 16078 times)

Offline Ludus

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The control aspects of the ITS BFR aren't much different from F9. If the BFR is expected to be legless and land directly back on the launch pad, wouldn't it make sense to work this out first with F9?

Obviously it wouldn't have to land at the launch pad, just on a structure built on one of the LZ pads. Perhaps they'd try a sort of legless grasshopper first.

If F9 could lose its legs and still be recovered (at least on land) it would drop a couple thousand kilos of dead weight that would go right to its lift capability.

Online Lars-J

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The control aspects of the ITS BFR aren't much different from F9. If the BFR is expected to be legless and land directly back on the launch pad, wouldn't it make sense to work this out first with F9?

Obviously it wouldn't have to land at the launch pad, just on a structure built on one of the LZ pads. Perhaps they'd try a sort of legless grasshopper first.

If F9 could lose its legs and still be recovered (at least on land) it would drop a couple thousand kilos of dead weight that would go right to its lift capability.

Maybe... But several years down the road IMO. Once F9 has shown the required precision repeatedly. (and it is nowhere close at the moment) But F9 also lacks bottom mounted thrusters that the BFR/ITS booster will have that will give it extra aiming ability.

But that extra leg weight doesn't really add much extra capability. (I seem to recall reading that taking off 1 ton of mass from the first stage would only add 100kg of payload, but don't quote me on it)

Offline Semmel

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I think the consensus to the question was that F9 would require thrusters at the bottom to achieve the landing precision. That would be a major redesign which they dont do. Also, it would require stronger thrusters than the cold gas thrusters which are used at the top of the stage at moment. Lots of no-goes all around.

@edit: Lars-J ninjad me ;)
« Last Edit: 07/11/2017 07:22 AM by Semmel »

Online AncientU

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More likely would be to use a spare F9 core and modify it for landing practice, maybe at Spaceport (unless they've completely given up on that site).  Modifying the F9 line at this point is a non-starter.  Block 5 is supposed to be the end of evolution for now.
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Online cppetrie

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I was also under the impression that the F9 couldn't throttle down enough to hover in the way that would be needed to align with a landing mount.

Offline RoboGoofers

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The control aspects of the ITS BFR aren't much different from F9. If the BFR is expected to be legless and land directly back on the launch pad, wouldn't it make sense to work this out first with F9?

Obviously it wouldn't have to land at the launch pad, just on a structure built on one of the LZ pads. Perhaps they'd try a sort of legless grasshopper first.

If F9 could lose its legs and still be recovered (at least on land) it would drop a couple thousand kilos of dead weight that would go right to its lift capability.

I imagine they will do the bolded part. They are, seemingly, spread a bit thin with all their projects and goals, but it would make sense to do this before they go too far down the path of engineering cradle landing into ITS.

The only reason they wouldn't is if they can't make ITS work without it and therefore it's do-or-die (unlikely), or if they are so confident in their simulations and F9 data so far that they don't feel it necessary, or they don't feel F9 scale will be illuminating enough.

Offline RoboGoofers

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I was also under the impression that the F9 couldn't throttle down enough to hover in the way that would be needed to align with a landing mount.

nothing i've seen suggests a hover. it'll still be a suicide burn, just with greater accuracy because of the extra, stronger thrusters.

Offline Doesitfloat

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On the other hand the only rocket that Spacex made that can't hover is the F9; Dragon2, Grasshopper 1&2 could.
There is nothing to suggest that a rocket that lands in a cradle can't hover or descend from a hover.
« Last Edit: 07/11/2017 05:52 PM by Doesitfloat »

Offline RoboGoofers

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On the other hand the only rocket that Spacex made that can't hover is the F9; Dragon2, Grasshopper 1&2 could.
There is nothing to suggest that a rocket that lands in a cradle can't hover or descend from a hover.

GH1&2 had specific fuel loads and ballast for the test flights they were flying, and could only "hover" in a specific window.

Dragon 2 did a hover test, but was suspended from a crane. it wasn't a free flight test, and there's no indication they'll hover to land it's when operational.

You could say "the only operational rocket SpaceX had made doesn't hover, so there's nothing to suggest a production rocket that lands in a cradle will hover."
« Last Edit: 07/11/2017 06:10 PM by RoboGoofers »

Offline rpapo

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Not only that, but hovering is incredibly expensive in fuel.  The more hover margin you build into a rocket, the less it can boost towards orbit.
An Apollo fanboy . . . fifty years ago.

Online Lars-J

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I was also under the impression that the F9 couldn't throttle down enough to hover in the way that would be needed to align with a landing mount.

nothing i've seen suggests a hover. it'll still be a suicide burn, just with greater accuracy because of the extra, stronger thrusters.

Exactly! One has nothing to do with the other. (hover != increased accuracy)

On the other hand the only rocket that Spacex made that can't hover is the F9; Dragon2, Grasshopper 1&2 could.
There is nothing to suggest that a rocket that lands in a cradle can't hover or descend from a hover.

Sure, but they are limited by the engines they use and basic physics... The M1D can only throttle down to 40% of its full thrust. And a leg-less stage is going to be ~2 tons lighter (no legs), which makes hovering even harder.

The point remains that landing accuracy is a separate thing from hover ability. One does not require the other. Would it be nice to do both? Sure... ITS will have more engines and should be able to hover more easily, but for an F9 it will be tricky.

« Last Edit: 07/11/2017 06:11 PM by Lars-J »

Offline Coastal Ron

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If F9 could lose its legs and still be recovered (at least on land) it would drop a couple thousand kilos of dead weight that would go right to its lift capability.

From what we know the Block 5 Falcon 9 is supposed to be the last iteration before they stop product development on the Falcon 9, and when they do that they are shifting a major amount of their engineering resources to the BFR/BFS.

The BFR & BFS are supposed to have a more squat profile, which would make them less "tippy" when they land, so it would make sense for SpaceX to investigate legless landing capabilities. But the Falcon 9 is very slender, and I just don't see that it is a good candidate for legless landings. Not until they can perfect it for wider base rockets at least.

So maybe in a decade, but at that point the market may have changed to just using the BFR/BFS architecture, which is supposed to be completely reusable, whereas the Falcon 9 architecture at this point is still dumping the 2nd stage. Which means we may be more likely to see a Falcon 9 retirement than a Falcon 9 re-engineering effort that provides little benefit.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline rakaydos

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More likely, they'll build a "Grasshopper 2" test vehical for Raptor and the hot methane RCS, to test cradle landings using  the hardware they'll be using on the actual vehical.

Offline dorkmo

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i think it would actually be pretty straight forward to design this. It would need to be big to accomidate current f9 abilites and have some massive hydraulic systems for agility and power. Maybe tweak the hooks on the falcon itself to be more forgivable.
maybe the roomba team will do this next.

Offline meekGee

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Modifying F9B5 to learn pinpoint landing does not contradict F9B5 being the "end if the line", since such a mode is part of BFR development.

Removed legs, attach cold thrusters and pressure tanks on same mount points, and that's all there is to the modification.

Fail cheaply.
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Offline Coastal Ron

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Modifying F9B5 to learn pinpoint landing does not contradict F9B5 being the "end if the line", since such a mode is part of BFR development.

Removed legs, attach cold thrusters and pressure tanks on same mount points, and that's all there is to the modification.

Fail cheaply.

My thinking is that legless Falcon 9 development would compete for the same resources that are needed for the BFR/BFS - both capital and people. And I don't yet understand what value such a capability would provide?

- Is this is supposed to address a market for Falcon 9 that is not currently served by planned Falcon 9 pricing, or not served by the upcoming Falcon Heavy capabilities? If so then today it must be pretty niche, so one would think it would be a gamble to go after this market.

- Is this supposed to help with BFR/BFS development? Not sure why Falcon 9 would be the best vehicle to test this since.

I've been known to be wrong, but I'm having trouble believing such a thing would make a lot of sense in the near term...
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline meekGee

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Modifying F9B5 to learn pinpoint landing does not contradict F9B5 being the "end if the line", since such a mode is part of BFR development.

Removed legs, attach cold thrusters and pressure tanks on same mount points, and that's all there is to the modification.

Fail cheaply.

My thinking is that legless Falcon 9 development would compete for the same resources that are needed for the BFR/BFS - both capital and people. And I don't yet understand what value such a capability would provide?

- Is this is supposed to address a market for Falcon 9 that is not currently served by planned Falcon 9 pricing, or not served by the upcoming Falcon Heavy capabilities? If so then today it must be pretty niche, so one would think it would be a gamble to go after this market.

- Is this supposed to help with BFR/BFS development? Not sure why Falcon 9 would be the best vehicle to test this since.

I've been known to be wrong, but I'm having trouble believing such a thing would make a lot of sense in the near term...

- I don't think it will help with commercial F9.
- I am on the fence on whether it will be good use for BFR development.

See if F9 wasn't already a working platform where "everything else" has checked out, then it would be wasted time and resources to build a small, incompatible, unrelated test platform.  It would be like making Grasshopper using different engines, fuel, and structure from F9.

But - F9 exists, flies, has flight rules that are well characterized, and there's plenty of them to spare.  So if they can learn anything from it - it's a small investment, and will cost less than "one more failure" at BFR scale.

So I don't think it's a non-starter.  It's a reasonable option, and they can go either way.
« Last Edit: 07/12/2017 05:00 AM by meekGee »
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Offline guckyfan

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I have no idea if it will be done. But it would secure the stage on the barge, better than the Roomba/Octocrab can. It would make turn around easier than with legs. They could be on the way back an hour or two after landing. It would save some weight. But can they come down precisely enough that the thrusters can do the fine tuning? The last two landings at the limits of what can be done, were not that precise. We will see if they can improve at the limits with practice.

I wonder if they could modify the He-pressurization for the legs for at least initial tests, before they install cold gas thruster in the thrust structure area.

Offline meekGee

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I have no idea if it will be done. But it would secure the stage on the barge, better than the Roomba/Octocrab can. It would make turn around easier than with legs. They could be on the way back an hour or two after landing. It would save some weight. But can they come down precisely enough that the thrusters can do the fine tuning? The last two landings at the limits of what can be done, were not that precise. We will see if they can improve at the limits with practice.

I wonder if they could modify the He-pressurization for the legs for at least initial tests, before they install cold gas thruster in the thrust structure area.

IMO the catching mechanism will have a substantial structure, both for last-inch alignment, and for arresting vertical motion (which the legs kinda do today).

That said, there are some advantages for doing it far off shore.
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Offline mikelepage

My thinking is that legless Falcon 9 development would compete for the same resources that are needed for the BFR/BFS - both capital and people. And I don't yet understand what value such a capability would provide?

- Is this is supposed to address a market for Falcon 9 that is not currently served by planned Falcon 9 pricing, or not served by the upcoming Falcon Heavy capabilities? If so then today it must be pretty niche, so one would think it would be a gamble to go after this market.

- Is this supposed to help with BFR/BFS development? Not sure why Falcon 9 would be the best vehicle to test this since.

I've been known to be wrong, but I'm having trouble believing such a thing would make a lot of sense in the near term...

I doubt we'll see a legless F9 booster anytime soon, but what about second stage?
To shamelessly quote myself (and also so you can reply in the appropriate thread):

Better idea (?): it should be easy enough to create a "cradle" for the second stage that could be temporarily installed on a the Vandenberg ASDS for Falcon Heavy launches from the Cape.  It would catch the second stage using the same attachments that the booster stage uses.  If you then install a pair/quartet of grid fins on any second stage where you have the margin, you could then practice cradle landings on a smaller scale.

From a bit of googling, I can see the F9 landing legs (collectively) are estimated at just under 2100kg, where as the grid fins are estimated at only 41kg each.  Even if that's off by an order of magnitude, it seems far more feasible to use grid fins on the second stage than landing legs.  A grid fin plus cradle arrangement seems like a likely "hail mary" attempt that they could perform on the Falcon heavy demo.

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