Author Topic: Why not four pairs of SuperDracos in the F9 S1 interstage?  (Read 46380 times)

Offline speedevil

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #20 on: 01/13/2015 05:59 pm »
But the question is... is there an advantage of being able to hover the stage (F9 and BFR) to give more time to land accurately? More relevant to BFR since the Rapture is going to be a powerful bit of kit with I suspect a higher T2W ratio than the Merlin.

* I personally think the 'hoverslam' will work. I'm just wondering about hovering advantages.

If you're hovering, and have wind-loads, they will tend to push the stage over.
If the wind is from the left, they will tend to tilt the top of the rocket left.
If you keep the thrust through the centre of mass, then you drift left.
You need to then gimbal hard right to kick the stage over so it tilts right, followed by gimballing hard left as it comes to vertical.
During which time you can be hit with another gust from another direction.

I suspect the optimal trajectory is something like ~28s at 40% thrust, and 2s at 90%.

You want to really minimise the amount of hang-time, to avoid getting into back-and-forth battles with potentially gusty winds that you may never be able to win.

Helicopters can do this because they have at the least minutes of time available to land.


Offline mme

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #21 on: 01/13/2015 06:12 pm »
Geez, people, you're driving me crazy here!  Is everyone Rube Goldberg in disguise?

They have a system, it's going to work.  Let them tweak it.

They have a system, it's *probably* going to work. Fixed that for you. Unless you have a crystal ball and already know.

But the question is... is there an advantage of being able to hover the stage (F9 and BFR) to give more time to land accurately? More relevant to BFR since the Rapture is going to be a powerful bit of kit with I suspect a higher T2W ratio than the Merlin.

* I personally think the 'hoverslam' will work. I'm just wondering about hovering advantages.

No, there is virtually no advantage to hovering.  It wastes fuel, adds weight, adds complexity (and therefore failure modes),  it exposes the rocket to more risk not less.  During the ASDS landing attempt winds were 30 kmh.  How is "hovering" a 14 story building in a 30 kmh wind "safer?"  There is no reason the F9 won't be able to achieve sub-meter accuracy.  Therefore, hovering is a solution looking for a problem.
Space is not Highlander.  There can, and will, be more than one.

Offline nadreck

First of all, let me say that the comments by speedevil and cambrianera are most of the answer here. But to explain why people keep wanting to analogize to balancing broomsticks or landing things like helicopters, those are all things people can directly or vicariously identify with the difficulties of manually controlling, however slowing, hovering and gently settling on to a surface are methods used because people can't process quickly and methodically like a computer. However, a straight in, fast but accurately controlled landing is much easier for a computer than hovering and stabilizing above the landing surface.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline speedevil

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #23 on: 01/13/2015 06:53 pm »
However, a straight in, fast but accurately controlled landing is much easier for a computer than hovering and stabilizing above the landing surface.

Depending on if you have adequate control authority and target vector flexibility to ignore the gusts.

In principle though even for extreme gusts - 'brute force'' solutions - a thousand tiny quadcopters in rings 50m apart giving 5s warning of winds to allow some feed-forward would help _enormously_.

Offline hrissan

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #24 on: 01/13/2015 07:46 pm »
Geez, people, you're driving me crazy here!  Is everyone Rube Goldberg in disguise?

They have a system, it's going to work.  Let them tweak it.

They have a system, it's *probably* going to work. Fixed that for you. Unless you have a crystal ball and already know.

But the question is... is there an advantage of being able to hover the stage (F9 and BFR) to give more time to land accurately? More relevant to BFR since the Rapture is going to be a powerful bit of kit with I suspect a higher T2W ratio than the Merlin.

* I personally think the 'hoverslam' will work. I'm just wondering about hovering advantages.

To all posters: do not fix what is not broken...

Take a human walking for example: did you ever stumble upon something? How about third leg for you? May be better crawl instead of walking? Or hire 2 angels to support you whenever you walk? Shorten your legs so you fall from less altitude? Etc...

Offline nadreck

However, a straight in, fast but accurately controlled landing is much easier for a computer than hovering and stabilizing above the landing surface.

Depending on if you have adequate control authority and target vector flexibility to ignore the gusts.

In principle though even for extreme gusts - 'brute force'' solutions - a thousand tiny quadcopters in rings 50m apart giving 5s warning of winds to allow some feed-forward would help _enormously_.

But, what I am trying to point out, is that under automated control the slower you go, the harder it is to ignore the  gusts, the less control authority your aerodynamic surfaces have, the more ping ponging you will get with your gimballed engine, etc. with manual control you have no option but to go slow because a human can't do the fast/accurate decelerate to zero at zero in real time. So for a human to control the landing you need far more control authority to make a successful 'soft' landing of any type, for a machine, if only the last 2 seconds of the landing has effectively zero aerodynamic control authority as opposed to a human controlled one where there is maybe 10 or more seconds of that, then the automated landing needs 1/5th or less the control authority because there is 4/5th less possible deviation from when you had aerodynamic control authority.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline speedevil

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #26 on: 01/13/2015 09:06 pm »
But, what I am trying to point out, is that under automated control the slower you go, the harder it is to ignore the  gusts, the less control authority your aerodynamic surfaces have, the more ping ponging you will get with your gimballed engine, etc. with manual control you have no option but to go slow because a human can't do the fast/accurate decelerate to zero at zero in real time. So for a human to control the landing you need far more control authority to make a successful 'soft' landing of any type, for a machine, if only the last 2 seconds of the landing has effectively zero aerodynamic control authority as opposed to a human controlled one where there is maybe 10 or more seconds of that, then the automated landing needs 1/5th or less the control authority because there is 4/5th less possible deviation from when you had aerodynamic control authority.

I quite agree.
If, and only if the vehicle can't hover for a long time, and does not have the control authority to reliably fly through gusts at maximum safe deceleration speed.
If it can hover roughly around the pad waiting for a calm moment - that's quite another thing.
Also unlikely for near-term rockets for both of these to be true.

Offline Burninate

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #27 on: 01/14/2015 01:55 am »
But, what I am trying to point out, is that under automated control the slower you go, the harder it is to ignore the  gusts, the less control authority your aerodynamic surfaces have, the more ping ponging you will get with your gimballed engine, etc. with manual control you have no option but to go slow because a human can't do the fast/accurate decelerate to zero at zero in real time. So for a human to control the landing you need far more control authority to make a successful 'soft' landing of any type, for a machine, if only the last 2 seconds of the landing has effectively zero aerodynamic control authority as opposed to a human controlled one where there is maybe 10 or more seconds of that, then the automated landing needs 1/5th or less the control authority because there is 4/5th less possible deviation from when you had aerodynamic control authority.

I quite agree.
If, and only if the vehicle can't hover for a long time, and does not have the control authority to reliably fly through gusts at maximum safe deceleration speed.
If it can hover roughly around the pad waiting for a calm moment - that's quite another thing.
Also unlikely for near-term rockets for both of these to be true.
I don't see it that way.  The decision to hover or not would require foresight of gusts, and the sea is just inherently not very gusty;  Wind is steady and relatively undisturbed by turbulence.  The problem is it doesn't have the control authority to land in *steady state winds* above some level.  Hovering doesn't help.  More fuel doesn't help.  The grid fins don't help.  The barge will be positioned several hundred to several thousand (FH center core) kilometers downrange, far enough that launchsite weather conditions are no guarantee of landing site weather conditions;  And we're probably talking about not very many knots of wind in the first place.

I don't *know* the maximum wind criteria - maybe it won't affect this barge at all (as wind-driven swells might make the barge unusable before windspeed does), but once this is proven the next landing pad can be more seaworthy.

The issue is that the demands of stable flight in a moving airmass with fixed position *using only thrust from the business end of the rocket*, conflict with the demands of touching all four feet to the ground at nearly the same time without significant rotation rate or horizontal velocity.  You can squash any of the variables to zero, but doing so raises at least one of the other variables.
« Last Edit: 01/14/2015 02:02 am by Burninate »

Offline Robotbeat

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #28 on: 01/14/2015 02:00 am »
VTVL hovering rockets (F9R isn't going to be hovering, but whatever) can withstand pretty good side winds. Here's "rocket tug of war" from Armadillo Aerospace (or Masten?):


Jon Goff probably has some more war stories.
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Offline Burninate

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #29 on: 01/14/2015 02:14 am »
VTVL hovering rockets (F9R isn't going to be hovering, but whatever) can withstand pretty good side winds. Here's "rocket tug of war" from Armadillo Aerospace (or Masten?):


Jon Goff probably has some more war stories.
Yes, rockets can be made very effective at holding position *in the air*, but as soon as they touch down, the inherent structural stability and any dynamic momentum in the structure come into play, as well as any pressure from the wind;  The F9R first stage is a giant 18 ton hollow metal tube, with the top ~45 meters off the ground, standing on legs only ~18 meters wide.  A tiny little rocket like the one pictured, with wide, strong legs and a compact, fuel-filled tank, and a short, squat mass distribution, isn't directly comparable.

Maybe if the F9R's legs were actuated to be a dynamic structure which cushions impacts rather than a rigid fixed shape?
« Last Edit: 01/14/2015 02:25 am by Burninate »

Offline Lars-J

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #30 on: 01/14/2015 02:34 am »
VTVL hovering rockets (F9R isn't going to be hovering, but whatever) can withstand pretty good side winds. Here's "rocket tug of war" from Armadillo Aerospace (or Masten?):

Jon Goff probably has some more war stories.

Yes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come.

It is safest to land as quickly as possible, don't waste time.

Offline Robotbeat

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #31 on: 01/14/2015 02:58 am »
VTVL hovering rockets (F9R isn't going to be hovering, but whatever) can withstand pretty good side winds. Here's "rocket tug of war" from Armadillo Aerospace (or Masten?):

Jon Goff probably has some more war stories.

Yes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come.

It is safest to land as quickly as possible, don't waste time.
Agreed entirely. Just land the sucker. As I said, F9R isn't going to be hovering.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Microphobe

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #32 on: 01/14/2015 06:36 am »
https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy
Thanks for that, great explanation of a counter-intuitive concept.

Another reason not to mount engines, or parachutes or arresting hooks or whatever, to the top of the stage might be that it's built to withstand compressive forces (e.g. max-Q) but not necessarily tensile forces. I'm no engineer but I'd imagine that subjecting the stage to cycles of compressive, then tensile forces each launch would not be great for its lifespan unless significant re-engineering was done.

It's fun to speculate but I'm confident the F9 will get there very soon with only minor tweaks.

However if SpaceX are in the market for crazy suggestions from this forum, how about adding a large unmanned wooden rabbit?

"Look, if we built this    ::)

...later to develop into a crewed wooden badger once the wrinkles in the landing are smoothed out. Elon?

Offline eriblo

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #33 on: 01/14/2015 08:45 am »
Another reason not to mount engines, or parachutes or arresting hooks or whatever, to the top of the stage might be that it's built to withstand compressive forces (e.g. max-Q) but not necessarily tensile forces. I'm no engineer but I'd imagine that subjecting the stage to cycles of compressive, then tensile forces each launch would not be great for its lifespan unless significant re-engineering was done.

Another counter-intuitive concept: The stage is actually built more for tensile loads than compressive. Remember that it's pressurized in flight, if the internal pressure is 3 atm then the force on the forward bulkhead is ~300 tonnes. Some stages even use balloon tanks that can't support their own empty compressive loads against gravity :)
« Last Edit: 01/14/2015 08:50 am by eriblo »

Offline JamesH

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #34 on: 01/14/2015 09:23 am »
Quote from: Lars-J
Yes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come.

It is safest to land as quickly as possible, don't waste time.

I think I agree, but without the ability to hover you have precisely one chance to get it right - the accuracy of descent needs to be perfect, the engine needs to ignite at exactly the right time, the throttle response need to be perfect, the wind prediction need to be almost perfect (this should be feasible with the barge telling the rocket what the current deck wind conditions are). Hopefully it works every time. But without a low thrust engine, there are no second chances.

Point being, is it worth sacrificing a bit of performance (by having a lower thrust centre engine - no other changes) to enable a second chance? I suspect not.

Edit/CR: fixed quote tags
« Last Edit: 01/15/2015 07:32 am by CuddlyRocket »

Offline cambrianera

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #35 on: 01/14/2015 09:48 am »
without the ability to hover you have precisely one chance to get it right

Once your system is perfected (control algorithm enhancement, existing systems tweaking, addition of some auxiliary stuff like grid fins, landing gear with higher margins) you don't need a second chance.
Or better, your second chance for that single case every thousand is a new stage.
Oh to be young again. . .

Offline ChrisWilson68

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #36 on: 01/14/2015 10:28 am »
Quote from: Lars-J
Yes, but the main point of the argument (I think) is that hovering doesn't benefit you at all. Just set down directly. The longer you hover, the worse your throttling response will be (craft getting lighter and lighter), and you increase the time that something can go wrong. Waiting for calmer winds is not a good idea, it is just as likely that a stronger gust could come.

It is safest to land as quickly as possible, don't waste time.

I think I agree, but without the ability to hover you have precisely one chance to get it right - the accuracy of descent needs to be perfect, the engine needs to ignite at exactly the right time, the throttle response need to be perfect, the wind prediction need to be almost perfect (this should be feasible with the barge telling the rocket what the current deck wind conditions are). Hopefully it works every time. But without a low thrust engine, there are no second chances.

Point being, is it worth sacrificing a bit of performance (by having a lower thrust centre engine - no other changes) to enable a second chance? I suspect not.

No, none of those things has to be perfect.  They have time to continue making corrections during the landing burn.  They just have to be close enough initially that the needed corrections are within the range of throttle, gimbal, etc. that they have available.

Has everyone forgotten that they've done two successful water landings?  They already know how to get the stage to slow down the right amount at the right time with the landing burn.  The only question that was open was weather they could do positional accuracy.  Apparently they can.

Musk said 50% more hydraulic fluid is all they need to nail the landing.  Why should we doubt that?

Offline JamesH

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #37 on: 01/14/2015 11:14 am »
Musk said 50% more hydraulic fluid is all they need to nail the landing.  Why should we doubt that?

I'm more of a believe it when I see it type of chap. Not that I don't believe they can do it - I'm sure they can, but until they have actually successfully landed it on the barge it seems foolish to say it's a foregone conclusion as some are doing here.

Since no-one here knows exactly what happened on the water landings or the attempted barge landing, I'm going to wait before cracking the champagne. But I really do expect them to do it. They've had three tests so far. That is not a huge amount, but the progress has been rapid and impressive. Fingers crossed for the next one!

Online ugordan

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #38 on: 01/14/2015 11:19 am »
Musk said 50% more hydraulic fluid is all they need to nail the landing.

He never said that, certainly not as confidently as you make it sound.

"With the next flight we have 50% more hydraulic fluid margin. Something else could go wrong, certainly, but at least with respect to that, it should cover. So there's a real decent chance, within three weeks, of landing it."

Offline ChrisWilson68

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Re: Why not four pairs of SuperDracos in the F9 S1 interstage?
« Reply #39 on: 01/14/2015 11:26 am »
Musk said 50% more hydraulic fluid is all they need to nail the landing.

He never said that, certainly not as confidently as you make it sound.

"With the next flight we have 50% more hydraulic fluid margin. Something else could go wrong, certainly, but at least with respect to that, it should cover. So there's a real decent chance, within three weeks, of landing it."

He also said "Upcoming flight already has 50% more hydraulic fluid, so should have plenty of margin for landing attempt next month.", which was his original statement on the issue, on Twitter, and which didn't contain the qualifications.

Obviously something else could go wrong.  I figured on this forum readers are sophisticated enough it wasn't necessary to say.

Something could go wrong with all the other, much more elaborate, alternative solutions being proposed here.  There's no reason to think any of them is any better than Musk's solution.

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