Author Topic: SpaceX Falcon 9 - Eutelsat 117W B & ABS 2A - SLC-40 - June - DISCUSSION  (Read 220899 times)

Offline meithan

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We think the Merlin 1D (full thrust) now has a minimum throttle of about 40%, it could even be as low as 30%. Given the choice of lowering thrust below the Merlin 1D's rated minimum and not being able to land, it is quite possible that the guidance/code software has code to lower the Merlin 1D thrust.

Uhm, it's possible, but I haven't seen anything official-ish suggesting that's happening. You'd have to be able to throttle below 30% for TWR to be below 1.

But as someone said before, the strategy should be to master high-g landings since they're efficient propellant-wise, not give Falcon 9 the ability to hover. I'm not convinced by the "last resort" argument of such ability, but it's possible.

Offline John Alan

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Just to point out another advantage a lower available throttle setting for the M1D will provide...   ???

The core stage 9 engines on a FH...  :o

There is much to gain in throw weight if you can throttle those way back between say Max Q and Booster cutoff...
Then go back to max thrust with only G forces as you limiting item to watch, till MECO...
That acts just like cross-feed... but without the weight, complexity, and headache...  8)

I summary... there are TWO reasons SpaceX is likely working on a deeper throttle range...
ASDS landings AND FH core usage...

And being a pintle injector engine design...
My guess is the gas turbine in the turbo pump may be the area that things go wrong at low-low power settings...
Not optimized to go that slow and stalls out...
Remember a turboshaft engine at idle is sucking fuel like mad because it's also driving an air compressor...  :o
No such thing on a gas generator rocket engine... to 'load up" the power turbine above it's stall speeds and flows...
The moving pintle inside helps, by adding some back pressure to the prop flow and loading up the pump a bit..
But then at high throttle you got too much back pressure for the pump to overcome...
IF they could change the spring rate on that pintle on the fly... THEN...  :o
In hydraulic psi relief valves... there are ways to make that adjustable on the fly via a hydraulic or air override...
The pintle kinda serves that function as I understand it... sets back pressure to the pump... Hmmm...  :-\
Just my hand waving, Hydraulics and what I know about pumps... for what it's worth opinion...  ;)

The advantage of the deep throttle idea was covered in the past in FH threads... search for it if you want...
Just pointing that out, in case some here have missed that connection of need...  ;)
« Last Edit: 06/19/2016 12:22 am by John Alan »

Offline speedevil

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Just to point out another advantage a lower available throttle setting for the M1D will provide...   ???

The core stage 9 engines on a FH...  :o

There is much to gain in throw weight if you can throttle those way back between say Max Q and Booster cutoff...
Then go back to max thrust with only G forces as you limiting item to watch, till MECO...
That acts just like cross-feed... but without the weight, complexity, and headache...  8)

In principle, if you're confident enough on relights, you can put out the middle three engines too. In-flight restarts are another nice thing to learn.

Offline Barmaglot

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There is much to gain in throw weight if you can throttle those way back between say Max Q and Booster cutoff...
Then go back to max thrust with only G forces as you limiting item to watch, till MECO...
That acts just like cross-feed... but without the weight, complexity, and headache...  8)

Throttling the center would provide some benefits of cross-feed, but not all of them. With cross-feed, side boosters would consume their fuel 50% faster, which means they would stage at significantly lower velocity, which, simultaneously, would reduce the energy spent by the rocket on accelerating their structural mass, reduce energy required to decelerate them (which would directly translate into increased fraction of fuel spent on accelerating the payload) and leave the center fully fueled at booster separation, which would allow it to reserve a higher fuel fraction for deceleration and recovery after second stage separation.

Offline Craftyatom

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Just to point out another advantage a lower available throttle setting for the M1D will provide...   ???

The core stage 9 engines on a FH...  :o

There is much to gain in throw weight if you can throttle those way back between say Max Q and Booster cutoff...
Then go back to max thrust with only G forces as you limiting item to watch, till MECO...
That acts just like cross-feed... but without the weight, complexity, and headache...  8)

In principle, if you're confident enough on relights, you can put out the middle three engines too. In-flight restarts are another nice thing to learn.

Before anyone goes so far as to shut down engines and then re-start them later on in the flight, I think they'd pull a Titan and just not start the engines until further on in the ascent profile.  Sure, it increases gravity losses, but somehow I don't see the trade-off between the performance gain and safety going well.

Anyways, FH has plenty of performance margin at the moment for a number of payloads, and I don't think we'd see anyone agree to any of these "crazy" profiles just to lift their slightly-too-heavy bird.  All the same, I don't think SpaceX would try to get any "crazy" low throttle percentages with a payload on board.  It's likely that throttle capability will only benefit landing on the F9.

That said, it may help greatly with SpaceX's future developments.  We know they've got big plans for engines, and the more they learn about engine dynamics in unique and interesting situations, the better.  But that much has been said before.
All aboard the HSF hype train!  Choo Choo!

Offline LouScheffer

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For SpaceX, however, if the choice is running below the lower limit or losing the stage, surely it would be better to overstress the center engine in the hopes of saving everything else.  The center engine would then require a detailed inspection, or maybe even replacement, but that's still a big improvement over losing the whole stage.  So I would not be surprised if the SpaceX software will throttle below the lower limit if that's the only way to land.
If you're correct, why did the stage hover and crash?

It was because the engines couldn't throttle low enough to maintain a decent sink rate.

I don't think your argument matches the evidence.
This is a good point.  My suspicion is that the software is not sophisticated enough to do the right thing in an off-nominal situation.

Assume the engine is rated to throttle between 40-100% (actual numbers unimportant).  Then the landing, and the trajectory, will be designed to keep the throttle always in this range.  Going outside this range would mean using the engine in a regime it's not spec'ed for, meaning at least a detailed inspection and perhaps a replacement.  Since SpaceX wants minimal refurbishement,  I'd guess the initial landing software will try hard to stay within these limits.

Now in the off-nominal case that occurred (slow start on one engine) the software should in theory realize that the original plan is now fatally flawed, and create a new plan, running out of limits on the center engine, if need be. But that's a lot more complex.

So as a software developer, I'm guessing they made the software good enough to handle the expected range of variations.  After the slow start of one engine, it kept trying to converge to the original plan, not realizing it was doomed to failure.  When it reached 100 meters up, going down too slowly, it simply reduced the thrust to the programmed minimum.  What is should have done is said "Holy ****", I'm going to run out of fuel before I hit, ignore those engine limits, and let me drop.  I'll sacrifice the center engine if need be.  But software to correctly handle off-nominal conditions is MUCH more complex, both in development and testing, particularly since there is an almost inexhaustible number of possible conditions.

So I'm guessing what we are seeing is a side effect of software that (in this version) could not correctly handle this case.  Better software, at least for the "slow-start" case, will likely be the fix that Musk spoke of.

Offline CorvusCorax

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I have written controllers ( model aircraft, aka hobby UAV ) the most important lesson that keeps teaching itself is KISS. Don't make the controller any more complex than it needs to be.

SpaceX adheres to that. That cost the CRS-7 dragon - although a contingency routine in the software could have saved it. Such a contingency routine is additional risk ( could get triggered by accident if you combine a faulty sensor with an unlucky chain of events) and you don't want overly smart on board software deploy chutes just before Max-Q.

As such its very very unlikely they would have any logic in there that says " things went ooopsies, go to plan C: now the engines can be throttled beyond design ratings"

Because diagnosing the oopsies in the first place is very hard. Usually oopsies happen because either sensors or actuators behave outof spec. But then chances are, either your state estimation is unreliable, or your controller output will have "undefined" effects, both of which is very hard to handle. You can throw heuristics at it, classificators, even neural nets...
None of them are compatible with KISS.

SpaceX controllers are going to be complicated as hell already, since theres so many mode changes that affect the model. Leg deployment, engine starts and shutdowns, changes in weight and COG...

To keep this controllable ( the control system from a predictability pov), you need the simplest possible solution that gets the job done.

So I'd guess the lower and upper throttle limits are hard, and clipping, even if the controller wants a setting beyond the min/max

Same with engine gimbals or other actuator limits.

Offline douglas100

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I suppose it depends on the root cause of the crash. If it was basically down to pushing the envelope too far, then they've learned some valuable lessons. The solution, then, would be not to approach that edge on future flights. If the slow start of the engine was hardware related then there might be some small engineering mods which would fix it. Rather than making software changes to override a hardware problem, it seems sensible to deal with the root cause. I agree with CorvusCorax: you simply can't expect the software to deal with every possible contingency.
Douglas Clark

Offline hrissan

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The stage is descending with constant velocity for several seconds, so either Merlin throttles below 40% or the S1 landing weight is more than was estimated here... Or both, in any proportion.

A lucky soul might ask Elon in Twitter...

Offline Rocket Science

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Something to talk about... ;)

"The laws of physics are unforgiving"
~Rob: Physics instructor, Aviator, Vintage auto racer

Offline John Alan

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Question that has bugged me all night...  :-\

Where are the legs???   ???

Must of burned up in the post crash RP-1 fire...  :o

Need a crane to remove them otherwise... which they would not mess with at sea...  ???

Offline kevin-rf

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Aren't the legs carbon fiber? That can burn...
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Offline Rocket Science

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Question that has bugged me all night...  :-\

Where are the legs???   ???

Must of burned up in the post crash RP-1 fire...  :o

Need a crane to remove them otherwise... which they would not mess with at sea...  ???
I see two legs items underneath, one under octoweb another under the flattened tank and what looks like one actuator, piston)
"The laws of physics are unforgiving"
~Rob: Physics instructor, Aviator, Vintage auto racer

Offline macpacheco

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Aren't the legs carbon fiber? That can burn...
All areas that could catch fire during re-entry/landing is covered by a layer of SPAM (SpaceX Proprietary Ablative Material).
SPAM is designed to slowly combust when it gets very hot, its also good thermal insulator. Technically it isn't even a fire, as SPAM burns only as long as its very hot.
This has been discussed ad nauseum in other threads. Please google it.
Looking for companies doing great things for much more than money

Offline Jim

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Aren't the legs carbon fiber? That can burn...
All areas that could catch fire during re-entry/landing is covered by a layer of SPAM (SpaceX Proprietary Ablative Material).


a.  Legs are not covered with SPAM
b.  What "could catch fire during re-entry/landing" doesn't matter when there is released propellant

Online Doesitfloat

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Aren't the legs carbon fiber? That can burn...

Sort of
The fiber filaments don't burn. They are just carbon. The resin holding it together can burn. In addition resin can get hot and lose it's mechanical properties reverting back into liquid. IRRC the term for this is "Onset of Tg". Or in my terms the temperature where your carbon fiber part turns back into goo.

Online kdhilliard

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Also, this was the first launch I noticed that there seemed to be some sort of liquid running from the hold downs.  I'm guessing this was water and was meant to help protect them against damage from exhaust impingement but if so it was running well before ignition.

That flow through the hold downs is first visible in the Technical broadcast at T-9:40, a few seconds after the "M-1D Chill Down" call.  I suspect that it is intended to prevent the hold downs from freezing up.

~Kirk

Offline meekGee

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Aren't the legs carbon fiber? That can burn...

Sort of
The fiber filaments don't burn. They are just carbon. The resin holding it together can burn. In addition resin can get hot and lose it's mechanical properties reverting back into liquid. IRRC the term for this is "Onset of Tg". Or in my terms the temperature where your carbon fiber part turns back into goo.

That would be true of Fiberglass.  Carbon will burn just fine.   Whether that happens before or after the resin fails, I don't know.

The failure of the resin, btw, is not a "turn back into liquid" - that would be true of a thermoplastic material, which has a Tg (glass transition) temperature.  A two-part resin will generally just fail.  Burn, decompose, etc.
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Offline speedevil

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The failure of the resin, btw, is not a "turn back into liquid" - that would be true of a thermoplastic material, which has a Tg (glass transition) temperature.  A two-part resin will generally just fail.  Burn, decompose, etc.
Epoxy resin does not fully cure all the way.
It will cure to some point, and be solid under the glass transition temperature.
If it later exceeds the Tg, it can go significantly more flexible and gel-like, not solid-like, and cure more.

This can be in some cases going along with decomposition, but it depends how high the Tg is.

Offline Rocket Science

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FYI...

"The laws of physics are unforgiving"
~Rob: Physics instructor, Aviator, Vintage auto racer

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