SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION

[CraigLieb]

Barge is clearly winning by one metric, so far none of the damage has been more than "that'll buff right out", more or less. Meanwhile rockets are reduced to piles of scrap, every time.

Build a barge like a rocket and it'll crumple/explode first time you bring it into port and tie up in heavy seas.

Build a rocket like a barge and no matter the propulsion it won't get off the ground

“A ship in harbor is safe, but that is not what ships are built for.”
― John A Shedd

[cscott]

They were attempting a 3-engine landing out of necessity. They aren't trying to make an already difficult endeavor even more difficult. Repeatable landings on the ASDS with one engine first. Crawl before you walk.

Sure, but it takes development effort to perform the three-engine burn test: avionics, simulation, structures, etc.  They wouldn't be investing that effort if they didn't think it was worth learning about this regime; they would have just cut their losses and dumped the stage.  The fact that they are trying it indicates that it's an interesting point on their long term roadmap, even if they would have *preferred* to "crawl before they walk" as you say.

[John Alan]

Barge is clearly winning by one metric, so far none of the damage has been more than "that'll buff right out", more or less. Meanwhile rockets are reduced to piles of scrap, every time.

Build a barge like a rocket and it'll crumple/explode first time you bring it into port and tie up in heavy seas.

Build a rocket like a barge and no matter the propulsion it won't get off the ground

“A ship in harbor is safe, but that is not what ships are built for.”
― John A Shedd

Amen...
I look at modern cruise ships today and see a floating mobile city... a fair weather machine...
Queen Mary...United States... etc... now those are ships that can take on most any weather and laugh at it...
The barge by design is almost unsinkable... multi compartments... sturdy construction...
You would have to breach it in many places to scuttle it...

[LouScheffer]

Let's say it's a = 50m/s^2 (with respect to the surface, not freefall), and the stage has to be within v=2m/s of zero in order to land safely.  How accurate do you have to be within the z-direction?

a = v^2/(2*d) becomes: 2*d*a = v^2 becomes d = v^2/(2*a) = (2m/s)^2/(2*50m/s^2) = 4 centimeters (!)

You have to be within 4 centimeters in the z-direction in order to stay within your landing velocity constraint when you're hoverslamming with 3 engines. If something doesn't throttle up fast enough or you start too early or late, you're toast. This isn't impossible, but it's DANG challenging.

I think this analysis is too pessimistic.  It's OK for the ends of the legs to hit the ground faster, provided the body of the rocket reaches 0 vertical speed before the legs run out of travel (or the engine bell hits the ground, whichever comes first).  Assuming the legs can absorb one meter of bend before breaking, then you need the lower vertex of the parabola to be between the deck and a point one meter below. 

Is this practical?  With 3 engines, 30 tonnes mass, your acceleration varies from 3.8G at 70% throttle to 5.5G at 100%.  Assume you plan your burn for 4.5Gs so you have leeway in both directions.  If you are falling at 250 m/s (about what you'd guess from the one engine landings) you'd want the engine to start at 82% throttle at 5.5 seconds before impact, at a height of 694 meters.  You get about a 1/2 second of slop since as long as you start before 568 m you can still stop at full thrust.

Once (if) your engines start you are in good shape.   On this time scale the radar altimeter and calculations should be instantaneous, so you immediately know the desired acceleration to place the  vertex 50 cm below the landing pad (or whatever your target).  You don't know the exact mass of the stage, nor the actual thrust for a commanded amount, but measuring the achieved acceleration tells you the proportionality constant.   Now you start adjusting the commanded thrust to get the acceleration right.

At 1 second before landing at 4.5 Gs , you are 22.5 meters up.  A 1% acceleration error will move the vertex +- 22 cm.  That's about all you can afford, since it's already half your error budget.  So you need to have the acceleration right to the 1% level by 1 second to go.  You get 4.5 seconds of correction to do this.  If the initial error is 20% (say 10% for throttle and 10% for mass) the you need to reduce the error by a factor of 20.  Assuming a linear system, this level of correction requires 3 time constants (e^3 = 20) so if your time constant for throttle response is 1.5 seconds or less, it should be possible.  Given that the engine can get to (nearly) steady state during either a static fire or the short time before liftoff, such a time constant seems possible.

Now this analysis assumes you are coming straight down with the rocket vertical, no attempt to steer horizontally, no errors in the radar altimeter or IMU, etc.  But even given these errors, it seems possible to make this work.

[Semmel]

Let's say it's a = 50m/s^2 (with respect to the surface, not freefall), and the stage has to be within v=2m/s of zero in order to land safely.  How accurate do you have to be within the z-direction?

a = v^2/(2*d) becomes: 2*d*a = v^2 becomes d = v^2/(2*a) = (2m/s)^2/(2*50m/s^2) = 4 centimeters (!)

You have to be within 4 centimeters in the z-direction in order to stay within your landing velocity constraint when you're hoverslamming with 3 engines. If something doesn't throttle up fast enough or you start too early or late, you're toast. This isn't impossible, but it's DANG challenging.

I think this analysis is too pessimistic.  It's OK for the ends of the legs to hit the ground faster, provided the body of the rocket reaches 0 vertical speed before the legs run out of travel (or the engine bell hits the ground, whichever comes first).  Assuming the legs can absorb one meter of bend before breaking, then you need the lower vertex of the parabola to be between the deck and a point one meter below. 

Is this practical?  With 3 engines, 30 tonnes mass, your acceleration varies from 3.8G at 70% throttle to 5.5G at 100%.  Assume you plan your burn for 4.5Gs so you have leeway in both directions.  If you are falling at 250 m/s (about what you'd guess from the one engine landings) you'd want the engine to start at 82% throttle at 5.5 seconds before impact, at a height of 694 meters.  You get about a 1/2 second of slop since as long as you start before 568 m you can still stop at full thrust.

Once (if) your engines start you are in good shape.   On this time scale the radar altimeter and calculations should be instantaneous, so you immediately know the desired acceleration to place the  vertex 50 cm below the landing pad (or whatever your target).  You don't know the exact mass of the stage, nor the actual thrust for a commanded amount, but measuring the achieved acceleration tells you the proportionality constant.   Now you start adjusting the commanded thrust to get the acceleration right.

At 1 second before landing at 4.5 Gs , you are 22.5 meters up.  A 1% acceleration error will move the vertex +- 22 cm.  That's about all you can afford, since it's already half your error budget.  So you need to have the acceleration right to the 1% level by 1 second to go.  You get 4.5 seconds of correction to do this.  If the initial error is 20% (say 10% for throttle and 10% for mass) the you need to reduce the error by a factor of 20.  Assuming a linear system, this level of correction requires 3 time constants (e^3 = 20) so if your time constant for throttle response is 1.5 seconds or less, it should be possible.  Given that the engine can get to (nearly) steady state during either a static fire or the short time before liftoff, such a time constant seems possible.

Now this analysis assumes you are coming straight down with the rocket vertical, no attempt to steer horizontally, no errors in the radar altimeter or IMU, etc.  But even given these errors, it seems possible to make this work.

I like your approach. But I dont think they would set the target throttle to 82.5 %, since anything less than 100% will increase gravity losses. I think that they put the target throttle to something like 95%. With that little margin to 100%, they were unable to compensate for existing errors. Of course, it would be nice to know if my assumption is correct and we might be able to determine that from a video when it comes out. Keep your fingers crossed!

[dorkmo]

i wonder how the falcon's radar measurements respond to being near the edge of the barge. would they output the further distance (to the water) or the closer distance (to the barge deck). i assume they ignore the first few feet of nozzle interference. would you want to look for the next nearest thing?

[chrisking0997]

Im wondering if the landing legs were given enough time to fully deploy.  The Orbcomm video looks to me like they locked in place roughly at the height of the stage...if this one was coming in faster would they possibly have not gotten down in time (not that I think it matters based on the assumed landing spot over the hole)?  I give SpaceX the benefit of the doubt that they probably thought of that...it just popped into my head

[kevin-rf]

Im wondering if the landing legs were given enough time to fully deploy.  The Orbcomm video looks to me like they locked in place roughly at the height of the stage...if this one was coming in faster would they possibly have not gotten down in time (not that I think it matters based on the assumed landing spot over the hole)?  I give SpaceX the benefit of the doubt that they probably thought of that...it just popped into my head

I would agree, I don't think it matters if the legs where down and locked. But to punch a hole in 1" steel plate, I doubt the legs are beefy enough to take that. A WAG, but I am sure they broke all four legs.

First they break a leg, then they BBQ it!

[ZachS09]

Have they got landing footage from the ASDS yet? I heard they put one or two GoPros on the barge.

[chrisking0997]

actually I meant it doesnt matter because at that location it would be pretty likely that at least one of the legs would not have been on deck (either on the blast wall or over the side), meaning the landing would have not resulted in an intact stage.  But yeah, also highly likely legs were broken.   Eagerly awaiting video of the last 5 seconds of flight

[whitelancer64]

Have they got landing footage from the ASDS yet? I heard they put one or two GoPros on the barge.

They do have GoPros on the barge, and there is no footage available yet. Keep an eye on Elon Musk's Twitter, it will most likely be posted there first.

[the_other_Doug]

If I had to guess as to what happened with the landing, I'd say that one or two of the three engines didn't start up properly -- or failed to start entirely.  Considering the much hotter than normal entry and curtailed entry burn, any damage at all caused by entry heating, if it affected one of the three engines to be restarted, would result in exactly what we saw: a stage coming down with flames coming out of its end, but without enough thrust to slow the stage sufficiently to achieve a safe landing.

Since the stage survived entry, and since if all three engines lit up properly and on-time the thing would never be coming in fast enough to punch a hole in the deck, and since the leftover fuel and LOX explosion likely couldn't have both punched a hole in the barge and at the same time left the top of the stage fairly intact and aboard the barge (i.e., an explosion large enough to hole the barge wouldn't have left all the debris that we saw, most of that debris would have flown hundreds of yards out into the ocean), then the only explanation that both leaves the amount of debris on the deck that we saw and also holed the barge must have resulted from a high-velocity impact.  Ergo, one or two of the three engines failed to restart, shut down early, or failed to come up to the specified thrust level.

This might also explain why we haven't seen any video of the crash yet -- it might consist of the brightening we saw and then a very brief flash before the (literally) impacted GoPro had its existence ended in an untimely fashion.  Even a second GoPro may have only seen a flash and then nothing.  Unless there was a drone stationed a little distance away that caught the whole sequence, there may be next to nothing to see in any of the video captured from the barge itself.

[hkultala]

If I had to guess as to what happened with the landing, I'd say that one or two of the three engines didn't start up properly -- or failed to start entirely.  Considering the much hotter than normal entry and curtailed entry burn, any damage at all caused by entry heating, if it affected one of the three engines to be restarted, would result in exactly what we saw: a stage coming down with flames coming out of its end, but without enough thrust to slow the stage sufficiently to achieve a safe landing.

Only one engine is used for the final landing burn, so this cannot be the cause.

[the_other_Doug]

If I had to guess as to what happened with the landing, I'd say that one or two of the three engines didn't start up properly -- or failed to start entirely.  Considering the much hotter than normal entry and curtailed entry burn, any damage at all caused by entry heating, if it affected one of the three engines to be restarted, would result in exactly what we saw: a stage coming down with flames coming out of its end, but without enough thrust to slow the stage sufficiently to achieve a safe landing.

Only one engine is used for the final landing burn, so this cannot be the cause.

Normally, yes, but (as has been discussed at great length above), on this mission, they were going to try a three-engine landing burn.

So, yes -- it can be the cause.   

[Comga]

Have they got landing footage from the ASDS yet? I heard they put one or two GoPros on the barge.

You can be sure SpaceX has video of the landing.
The live video seems to cut out because of acoustic impact on the transmitter.  It is probably still being recorded.
It seems unlikely they use GoPros for the live footage. We know they put them on the fairings, so they might be anywhere, but their benefit is being autonomous.  ADSD cameras are more likely wired. That doesn't mean that there aren't GoPros on the ASDS, but....
You don't have to ask if the video has been released because once it has, it will be impossible to avoid.

[jcm]

Could they be working on inclination first? Don't you get the most bang for the buck when apogee speed is lowest? As you raise the perigee it will go up

SES-9 made its first burn today, to raise perigee to 5223 x     41606 and lower inc to 14.2 deg.
It's not unusual to check the sat out in initial orbit for a few days before starting the burns

[georgegassaway]

  Ergo, one or two of the three engines failed to restart, shut down early, or failed to come up to the specified thrust level.

Also possibly the burn began too low, for whatever reason(s) to be able to slow down enough even at full thrust on three engines.

Unless this was not a vertical impact, and it was a partial replay of the CRS-5 Kamikaze crash. With less of a horizontal vector than CRS-5 but still enough to cause a high speed impact.  Though the hole in the deck looks like a mostly vertical impact.  However,  that does not necessarily rule out a fast diagonal descent path and then a quick pitch to point vertical before it hit.

- George Gassaway

[cscott]

Nozzle damage to the outer engines could be another reason for lower-than-expected thrust; they've never been involved in the landing burn before.  You'd expect that sort of thing to have shown up in the inspection of the Orbcomm stage, though, and I have trouble seeing how nozzle problems wouldn't produce asymmetries severe enough to prevent the nice vertical Falcon punch we seem to have evidence of.

Slow engine start up, as the_other_Doug suggests, would leave more fuel than expected in the stage, so I'm not sure that's consistent with the number of stage pieces remaining on deck.

Re: impact taking out all video---impossible, in my opinion.  We know there are cameras on both sides of the ASDS, including the side untouched by the crash, in addition to onboard video.  SpaceX has also has lots of experience with F9s hitting an ASDS now: they are highly unlikely to make a rookie mistake that would allow an impact to destroy their video recordings.

[kevin-rf]

SpaceX has also has lots of experience with F9s hitting an ASDS now: they are highly unlikely to make a rookie mistake that would allow an impact to destroy their video recordings.

Maybe the Falcon doubles as an EMP

Honestly, If I was running PR, after a successful launch, I would wait until it's old news before releasing the video at 5:30 pm on a Friday. That way the new cycles will ignore it, and the only people interested are on NSF. You get zero bad press and kudo's from the fan crowd. Double Bonus!

[CyndyC]

There have been a lot of different ideas about why the video went out, when the answer is probably very simple. I just went back to copy Lauren's explanation verbatim, from around 19:00 min into the SpaceX hosted broadcast, "This is something we expect to happen on the drone ship, because as the stage is coming back down, it's vibrating the heck out of those cameras."

As for all the ideas about why we haven't seen a video of the landing yet and will we ever, that answer may be just as simple -- there isn't much if anything to discern amidst the chaos, or not that most of the general public could discern.