Discussion in 'Techforge' started by gturner, Dec 17, 2014.
'Tis but a scratch!
But it completely destroyed the Space X logo!
one more time! (landing attempt this Sunday June 28.)
THE WHY AND HOW OF LANDING ROCKETS
Some of you may have been following our recent attempts to vertically land the first stage of our Falcon 9 rocket back on Earth.
There was this attempt in January, followed by this one in April.
These landing attempts move us toward our goal of producing a fully and rapidly reusable rocket system, which will dramatically reduce the cost of space transport.
A jumbo jet costs about the same as one of our Falcon 9 rockets, but airlines don't junk a plane after a one-way trip from LA to New York. Yet when it comes to space travel, rockets fly only once—even though the rocket itself represents the majority of launch cost.
The Space Shuttle was technically reusable, but its giant fuel tank was discarded after each launch, and its side boosters parachuted into corrosive salt water every flight, beginning a long and involved process of retrieval and reprocessing. So, what if we could mitigate those factors by landing rockets gently and precisely on land? Refurbishment time and cost would be dramatically reduced.
Historically, most rockets have needed to use all of their available fuel in order to get their payload into space. SpaceX rockets were built from the beginning with reusability in mind—they have enough built-in fuel margin to deliver a Dragon to the space station and return the first-stage to Earth. That extra fuel is needed to reignite the engines a few times to slow the rocket down and ultimately land the first stage after it has sent the spacecraft on its way.
In addition to extra fuel, we’ve added a few critical features to our Falcon 9 first stage for reusability’s sake. Our rocket has small, foldable heat-resistant wings called grid fins needed for steering the first-stage as it plummets from the edge of space through Earth’s atmosphere, cold-gas thrusters on the top of the first-stage that are used to flip the rocket around as it begins its journey back to Earth, and strong but lightweight carbon fiber landing legs that deploy as it approaches touchdown. All of these systems, while built and programmed by humans, are totally automated once the rocket is launched—and are reacting and adjusting their behavior based on incoming, real-time data.
So, what have we learned from the most recent landing attempts?
The first attempt to land on a drone ship in the Atlantic was in January, and while we came close, the first stage prematurely ran out of the hydraulic fluid that is used to steer the small fins that help control the rocket’s descent. The vehicle has now been equipped with much more of that critical fluid for steering purposes.
Our second attempt was in April, and we came close to sticking this landing. Check out this previously unreleased, longer video from our tracking camera. It shows the stage’s descent through the atmosphere, when the vehicle is traveling faster than the speed of sound, all the way to touchdown.
That controlled descent was successful, but about 10 seconds before landing, a valve controlling the rocket’s engine power (thrust) temporarily stopped responding to commands as quickly as it should have. As a result, it throttled down a few seconds later than commanded, and—with the rocket weighing about 67,000 lbs and traveling nearly 200 mph at this point—a few seconds can be a very long time. With the throttle essentially stuck on “high” and the engine firing longer than it was supposed to, the vehicle temporarily lost control and was unable to recover in time for landing, eventually tipping over.
Last-second tilt aside, the landing attempt happened pretty much exactly as planned. Shortly after stage separation (when the second stage leaves the first stage behind and goes on to carry Dragon to orbit), cold gas thrusters fired to flip the stage to reorient it for reentry. Then, three engines lit for a “boostback burn” that slows the rocket and brings it toward the landing site.
The engines then re-lit to slow the stage for reentry through Earth’s atmosphere, and grid fins (this time with much more hydraulic fluid) extended to steer the lift produced by the stage. Our atmosphere is like molasses to an object traveling at Mach 4, and the grid fins are essential for landing with precision. The final landing burn ignited, and together the grid fins, cold gas thrusters and steerable engines controlled the vehicle, keeping the stage within 15 meters of its target trajectory throughout the landing burn. The vehicle’s legs deployed just before it reached our drone ship, “Just Read the Instructions”, where the stage landed within 10 meters of the target, albeit a bit too hard to stay upright.
Post-launch analysis has confirmed the throttle valve as the sole cause of this hard landing. The team has made changes to help prevent, and be able to rapidly recover from, similar issues for the next attempt, which will be on our next launch—the eighth Falcon 9 and Dragon cargo mission to the space station, currently scheduled for this Sunday.
Even given everything we’ve learned, the odds of succeeding on our third attempt to land on a drone ship (a new one named “Of Course I Still Love You”) are uncertain, but tune in here this Sunday as we try to get one step closer toward a fully and rapidly reusable rocket.
oops. that blowed up reeeal-good.
total loss of launch vehicle about 2 minutes into ascent.
I feel like a best friend just died.
I was reading the side-note about it being in Max-Q, the period of flight that exerts the most stress on the vehicle.
7:37 AM - 28 Jun 2015
Elon Musk @elonmusk 2m2 minutes ago
Falcon 9 experienced a problem shortly before first stage shutdown. Will provide more info as soon as we review the data.
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7:33 AM - 28 Jun 2015
SpaceX @SpaceX 7m7 minutes ago
The vehicle experienced an anomaly on ascent. Team is investigating. Updates to come.
1,320 retweets580 favorites
7:21 AM - 28 Jun 2015
SpaceX @SpaceX 18m18 minutes ago
8:51 AM - 28 Jun 2015
Elon Musk @elonmusk 10m10 minutes ago
That's all we can say with confidence right now. Will have more to say following a thorough fault tree analysis.
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8:48 AM - 28 Jun 2015
Elon Musk @elonmusk 13m13 minutes ago
There was an overpressure event in the upper stage liquid oxygen tank. Data suggests counterintuitive cause.
Overpressure event at Max Q?
It looked like a staging problem from what I could sea.
I don't think it's staging. To me it looks like the upper stage bursts open, while the 1st stage continues normally.
Elon Musk @elonmusk 4 hours ago
Cause still unknown after several thousand engineering-hours of review. Now parsing data with a hex editor to recover final milliseconds.
Should we send him an invite to TechForge™? I think you have it sussed.
A strut that holds the helium tank failed under a 2,000 lb load, when it should be able to hold a 10,000 lb load. The strut was from an outside supplier.
The helium tank apparently sits in the LOX tank, so when it broke loose it released a massive burst of helium that overpressurized the LOX tank.
I've advised Elon he should consult with WF TF @Paladin in future.
Here is a particularly forthright acknowledgement of failure and the return-to-flight plan.
CRS-7 Investigation Update
On June 28, 2015, following a nominal liftoff, Falcon 9 experienced an overpressure event in the upper stage liquid oxygen tank approximately 139 seconds into flight, resulting in loss of mission. This summary represents an initial assessment, but further investigation may reveal more over time.
Prior to the mishap, the first stage of the vehicle, including all nine Merlin 1D engines, operated nominally; the first stage actually continued to power through the overpressure event on the second stage for several seconds following the mishap. In addition, the Dragon spacecraft not only survived the second stage event, but also continued to communicate until the vehicle dropped below the horizon and out of range.
SpaceX has led the investigation efforts with oversight from the FAA and participation from NASA and the U.S. Air Force. Review of the flight data proved challenging both because of the volume of data —over 3,000 telemetry channels as well as video and physical debris—and because the key events happened very quickly.
From the first indication of an issue to loss of all telemetry was just 0.893 seconds. Over the last few weeks, engineering teams have spent thousands of hours going through the painstaking process of matching up data across rocket systems down to the millisecond to understand that final 0.893 seconds prior to loss of telemetry.
At this time, the investigation remains ongoing, as SpaceX and the investigation team continue analyzing significant amounts of data and conducting additional testing that must be completed in order to fully validate these conclusions. However, given the currently available data, we believe we have identified a potential cause.
Preliminary analysis suggests the overpressure event in the upper stage liquid oxygen tank was initiated by a flawed piece of support hardware (a “strut”) inside the second stage. Several hundred struts fly on every Falcon 9 vehicle, with a cumulative flight history of several thousand. The strut that we believe failed was designed and material certified to handle 10,000 lbs of force, but failed at 2,000 lbs, a five-fold difference. Detailed close-out photos of stage construction show no visible flaws or damage of any kind.
In the case of the CRS-7 mission, it appears that one of these supporting pieces inside the second stage failed approximately 138 seconds into flight. The pressurization system itself was performing nominally, but with the failure of this strut, the helium system integrity was breached. This caused a high pressure event inside the second stage within less than one second and the stage was no longer able to maintain its structural integrity.
Despite the fact that these struts have been used on all previous Falcon 9 flights and are certified to withstand well beyond the expected loads during flight, SpaceX will no longer use these particular struts for flight applications. In addition, SpaceX will implement additional hardware quality audits throughout the vehicle to further ensure all parts received perform as expected per their certification documentation.
As noted above, these conclusions are preliminary. Our investigation is ongoing until we exonerate all other aspects of the vehicle, but at this time, we expect to return to flight this fall and fly all the customers we intended to fly in 2015 by end of year.
While the CRS-7 loss is regrettable, this review process invariably will, in the end, yield a safer and more reliable launch vehicle for all of our customers, including NASA, the United States Air Force, and commercial purchasers of launch services. Critically, the vehicle will be even safer as we begin to carry U.S. astronauts to the International Space Station in 2017.
I think they might need to look deeper into slosh issues. There could be currents or pressure waves in the tank that were putting a lot more than 2,000 lbs force on the strut.
They have a smoking gun:
Data from more than 3,000 telemetry channels were analyzed, along with tracking camera footage and video from on-board cameras. When all was said and done, a strut failure was the most likely explanation for the mishap. And that was determined by acoustic triangulation -- locating the exact position of the break by analyzing sound from different sensors -- not from any definitive data in the telemetry stream.
"At approximately 3.2 Gs, the strut holding down one of the helium bottles appears to have snapped and as a result, releasing a lot of helium into the upper stage oxygen tank and causing an overpressure event quite quickly," Musk said.
The steel struts measure about two feet long and an inch or so thick. They are certified to handle 10,000 pounds of stress, but the failure occurred at a calculated load of just 2,000 pounds.
Musk said the data were confusing because they initially showed a drop in helium pressure, which would be expected in a breach, "and then, somewhat strangely, a rise in the helium system back to approximately its starting pressure."
"This is quite confusing, but we think what may have happened is that as the helium bottle broke free and twisted around, it may have pinched off that line to the helium manifold and restored pressure in the helium system but released enough helium into the liquid oxygen tank to cause the liquid oxygen tank to fail," he said.
It was not clear whether the buoyant helium bottle might have shot up to the top of the oxygen tank, possibly damaging the structure, or whether it simply released enough helium to cause it to rupture. Either way, engineers believe, the failure of the oxygen tank triggered the rocket's destruction.
Musk said engineers examined close-out photos to make sure the struts used in the rocket that failed had been installed correctly. No problems were found. But during tests to measure the actual strength of struts in the SpaceX inventory, one broke at less than 2,000 pounds. Microscopic inspection revealed abnormalities.
Almost time to try again: 8:29PM tonight. Watch it live.
This mission also marks the first time SpaceX will attempt to land the first stage of the Falcon 9 rocket on land.
Live coverage might also be here:
they've really kicked up their PR. The preflight show is informative and amusing...
wall to wall geeks.
Lots of talk about the lox. Do they burn kerosene or hydrogen?
kerosene, but not just any kerosene, hi-test rocket grade kerosene.
Erector retracting now.
max q. no pictures from the craft...
meco, separation, second stage ignition, pictures from 2nd stage.
first stage boostback ignition...
4 minutes to 1st stage "touchdown"
good trajectory on 2nd stage.
1st stage is...
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