“On Saturday, April 20, 2019 at 18:13 UTC, SpaceX conducted a series of static fire engine tests of the Crew Dragon In-Flight Abort test vehicle on a test stand at SpaceX’s Landing Zone 1, Cape Canaveral Air Force Station in Florida.
Crew Dragon’s design includes two distinct propulsion systems – a low-pressure bi-propellant propulsion system with sixteen Draco thrusters for on-orbit maneuvering, and a high-pressure bi-propellant propulsion system with eight SuperDraco thrusters for use only in the event of a launch escape. After the vehicle’s successful demonstration mission to and from the International Space Station in March 2019, SpaceX performed additional tests of the vehicle’s propulsion systems to ensure functionality and detect any system-level issues prior to a planned In-Flight Abort test.
The initial tests of twelve Draco thrusters on the vehicle completed successfully, but the initiation of the final test of eight SuperDraco thrusters resulted in destruction of the vehicle. In accordance with pre-established safety protocols, the test area was clear and the team monitored winds and other factors to ensure public health and safety.
Following the anomaly, SpaceX convened an Accident Investigation Team that included officials from the National Aeronautics and Space Administration (NASA), and observers from the Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB), and began the systematic work on a comprehensive fault tree to determine probable cause. SpaceX also worked closely with the U.S. Air Force (USAF) to secure the test site, and collect and clean debris as part of the investigation. The site was operational prior to SpaceX’s Falcon Heavy launch of STP-2 and landing of two first stage side boosters at Landing Zones 1 and 2 on June 25, 2019.
Initial data reviews indicated that the anomaly occurred approximately 100 milliseconds prior to ignition of Crew Dragon’s eight SuperDraco thrusters and during pressurization of the vehicle’s propulsion systems. Evidence shows that a leaking component allowed liquid oxidizer – nitrogen tetroxide (NTO) – to enter high-pressure helium tubes during ground processing. A slug of this NTO was driven through a helium check valve at high speed during rapid initialization of the launch escape system, resulting in structural failure within the check valve. The failure of the titanium component in a high-pressure NTO environment was sufficient to cause ignition of the check valve and led to an explosion.
In order to understand the exact scenario, and characterize the flammability of the check valve’s titanium internal components and NTO, as well as other material used within the system, the accident investigation team performed a series of tests at SpaceX’s rocket development facility in McGregor, Texas. Debris collected from the test site in Florida, which identified burning within the check valve, informed the tests in Texas. Additionally, the SuperDraco thrusters recovered from the test site remained intact, underscoring their reliability.
It is worth noting that the reaction between titanium and NTO at high pressure was not expected. Titanium has been used safely over many decades and on many spacecraft from all around the world. Even so, the static fire test and anomaly provided a wealth of data. Lessons learned from the test – and others in our comprehensive test campaign – will lead to further improvements in the safety and reliability of SpaceX’s flight vehicles.
SpaceX has already initiated several actions, such as eliminating any flow path within the launch escape system for liquid propellant to enter the gaseous pressurization system. Instead of check valves, which typically allow liquid to flow in only one direction, burst disks, which seal completely until opened by high pressure, will mitigate the risk entirely. Thorough testing and analysis of these mitigations has already begun in close coordination with NASA, and will be completed well in advance of future flights.
With multiple Crew Dragon vehicles in various stages of production and testing, SpaceX has shifted the spacecraft assignments forward to stay on track for Commercial Crew Program flights. The Crew Dragon spacecraft originally assigned to SpaceX’s second demonstration mission to the International Space Station (Demo-2) will carry out the company’s In-Flight Abort test, and the spacecraft originally assigned to the first operational mission (Crew-1) will launch as part of Demo-2.”
SpaceX has announced via an official update and conference call the preliminary results of a failure investigation convened immediately after Crew Dragon capsule C201 exploded in the midsts of an April 20th static fire test.
Hosted by SpaceX Vice President of Mission Assurance Hans Koenigsmann and NASA Commercial Crew Program manager Kathy Lueders, the call provided some minor additional insight beyond a fairly extensive press release issued just prior. According to the preliminary results from SpaceX’s failure investigation, Crew Dragon’s explosion was unrelated to the spacecraft’s propellant tanks, Draco maneuvering thrusters, or SuperDraco abort engines. Rather, the cause lies in a more exotic and unanticipated chemical/material interaction between a plumbing valve, liquid oxidizer, and a helium-based pressurization system.
When metal burns
According to Hans Koenigsmann, SpaceX is approximately 80% of the way through what is known as the fault tree, essentially meaning that the failure investigation is 80% complete. That additional 20% could certainly throw some curveballs but the SpaceX executive was fairly confident that the results presented on July 15th would be representative of the final conclusion.
The ultimate (likely) cause of Crew Dragon’s extremely energetic and destructive explosion centers around the spacecraft’s extensive SuperDraco/Draco plumbing and its associated pressurization system, which uses helium to keep the pressure-fed engines, propellant tanks, and feed lines around 2400 psi (16.5 megapascals). Necessarily, this method of pressurization means that there is direct contact between the pressurant (helium) and the oxidizer/fuel, thus requiring some sort of valve preventing the pressurized fluid from flowing into the pressurization system.
During flight-proven Crew Dragon capsule C201’s April 20th static fire testing, that is reportedly exactly what happened. Over the course of ground testing, a “check valve” separating the pressurization system and oxidizer leaked what SpaceX described as a “slug” of nitrogen tetroxide oxidizer (NTO) into the helium pressurization lines. Around T-100 milliseconds to a planned ignition of the vehicle’s 8 SuperDraco abort engines, the pressurization system rapidly “initialized” (i.e. quickly pressurized the oxidizer and fuel to operational pressures, ~2400 psi).
To do this, helium is rapidly pushed through a check valve – designed with low-molecular-mass helium in mind – to physically pressurize the propellant systems. Unintentionally, the NTO that leaked ‘upstream’ through that valve effectively was taken along for the ride with the high-pressure burst of helium. In essence, picture that you crash your car, only to discover that your nice, fluffy airbag has accidentally been replaced with a bag of sand, and you might be able to visualize the unintended forces Dragon’s check valve (the metaphorical airbag) was subjected to when a “slug” of dense oxidizer was rammed into it at high speed.
In itself, this sort of failure mode is not hugely surprising and SpaceX may have even been aware of some sort of check valve leak(s) and accepted what it believed to be a minor risk in order to continue the test and perhaps examine Dragon’s performance under suboptimal conditions. What SpaceX says it did not realize was just how energetic the reaction between the NTO and the check valve could be. SpaceX’s understanding is that the high-speed slug of dense NTO was traveling so fast and at such a high pressure that, by impacting the titanium check valve, it quite literally broke the valve and may have chemically ignited the metal, thus introducing a slug of burning NTO into the liberated NTO system itself – effectively a match tossed into a powder keg.
It’s unclear if the ignition came from a chemical reaction between titanium (a technically flammable metal similar to magnesium) and NTO, or if the source came from the titanium valve being smashed apart, perhaps quite literally creating a spark as metal debris violently interacted. Either way, the solution – as SpaceX perceives it – is the same: instead of a mechanical check valve (simple but still not 100% passive), the barrier between pressurant and oxidizer (as well as fuel, most likely) will be replaced with something known as a burst disk. According to Koenigsmann, only a handful (~4) of those valves exist and thus need to be replaced by burst disks, a relatively fast and easy fix.
Burst disks are single-use and inherently unreusable, but they are also completely passive and simply do not leak until subjected to a specific amount of pressure. Because they are single-use, they can’t be directly tested prior to flight, limiting some of the in-principle reliability for the sake of an extremely leak-proof barrier.
Ultimately, both Koenigsmann and Lueders went out of their way to avoid answering any questions about SpaceX’s Crew Dragon upcoming test and launch schedule and what sort of delays the explosion will ultimately incur. Both individuals were nevertheless upbeat and by the sound of it, delays to Crew Dragon will be far less severe relative to delays caused by a pressure vessel or engine failure. For the time being, NASA has published a tentative target of mid-November 2019 for Crew Dragon’s first crewed launch to the International Space Station, while Lueders and Koenigsmann expressed hope in a 2019 launch but refused to give a specific estimate of the odds of that occurring.
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On June 25 this year, Elong Musk’s SpaceX launched one of the world’s most powerful rocket, the Falcon Heavy, into space. It carried 24 satellites to orbit to take up residence in space. However, some parts of the Falcon Heavy were destined to come back to Earth, including the payload fairing that protected the satellites from the extreme forces of a rocket launch. Recently, SpaceX released a stunning video showing the payload fairing of Falcon Heavy rocket plunging back into the atmosphere.
In a tweet, SpaceX proudly released a video giving rocket chasers a fairing-eye’s view of the return to Earth. Along with the video, it said, “View from the fairing during the STP-2 mission; when the fairing returns to Earth, friction heats up particles in the atmosphere, which appear bright blue in the video.”
Take a look:
View from the fairing during the STP-2 mission; when the fairing returns to Earth, friction heats up particles in the atmosphere, which appear bright blue in the video pic.twitter.com/P8dgaIfUbl
— SpaceX (@SpaceX) July 3, 2019
To catch one of its rocket nose cones during an actual mission is a particularly important milestone for SpaceX. The fairing acts like a nose cone for the rocket and protects the cargo being launched off-planet. Once the rocket has punched through Earth’s atmosphere, the fairing gets jettisoned and comes back to Earth. It’s a short life for the fairing, but it’s an expensive one.
According to cnet.com, the fairing costs around $6 million. Rather than manufacturing a new fairing every time, it’s better just to re-use them.
SpaceX’s fairing recovery boat, recently renamed from “Mr. Steven” to “Ms. Tree,” is fitted with a giant net to catch the rocket’s fairing shell as it descends under a parafoil.
The fairing catch June 25 showed there is promise for SpaceX’s preferred method of recovery. In another tweet, Elon Musk posted a video showing the fairing landing in Ms. Tree’s net.
Rocket fairing returning from space https://t.co/kundXPeslm
— Elon Musk (@elonmusk) July 4, 2019
The pressure to “go green” will soon travel outside of our horizon and into space.
NASA’s Green Propellant Infusion Mission (GPIM) is currently scheduled to launch on June 24 on a SpaceX Falcon Heavy rocket as part of a technology-testing mission dubbed STP-2. GPIM, a small, box-shaped spacecraft powered by green technology, will test out a low-toxicity propellant in space for the first time, according to NASA. The clean propellant, a hydroxyl ammonium nitrate fuel/oxidizer mix called AF-M315E, will serve as an alternative to hydrazine, a highly toxic compound used in rocket fuel to power satellites and spacecraft.
“It’s important that we develop technology that increases protections for launch personnel and the environment, and that has the potential to reduce costs,” Steve Jurczyk, associate administrator of NASA’s Space Technology Mission Directorate, said in a statement.
GPIM, which cost NASA a total of $65 million, has been in the works for years now and passed its first thruster pulsing test in 2013. This month marks another step toward the agency’s goal of providing a sustainable and efficient alternative fuel for spaceflight.
Right now, most spacecraft run on hydrazine, but NASA’s new fuel is nearly 50% more efficient, promising longer missions that use less propellant.
The fuel is also higher in density, meaning that more of it can be stored in less space, and it has a lower freezing point, and so requires less spacecraft power to maintain its temperature, according to NASA.
And compared with hydrazine, the fuel is much safer for humans. “It’s pretty benign, and we think that it can be loaded at universities or other environments where you’re not typically doing propellant-loading operations,” Dayna Ise, the technology demonstration missions program executive in NASA’s Space Technology Mission Directorate, said during a media call held on June 7. “Oh, and you can send it through FedEx, so it’s safe enough to be FedExed around the country.”
GPIM is one of four NASA technology missions among the payloads of the STP-2 mission that a SpaceX Falcon Heavy is scheduled to launch on June 24.
- The Amazing Triple Rocket Landing of SpaceX’s Falcon Heavy Arabsat-6A
- SpaceX Falcon Heavy Will Launch NASA Probe to Study Space Radiation
- Spacecraft Powered by ‘Green’ Propellant to Launch Soon
Stratolaunch’s giant aircraft on its first, and perhaps only, flight in April. (credit: Stratolaunch)
by Dwayne A. Day
Monday, June 3, 2019
Making predictions sometimes is not very enjoyable even—or perhaps especially—when they come true.
According to a Reuters article published Friday, Stratolaunch is about to cease operations and close up shop, selling off its assets. Whether this includes selling the record-setting Roc aircraft remains to be seen. It is hard to imagine any buyer for that aircraft, and it may prove too large for any museum. This is a sad end to an interesting project, but many people, myself included, never expected Stratolaunch to ever be successful. Stratolaunch seemed like the pet idea of a billionaire with so much money that he did not need to worry about market viability. When that billionaire, Paul Allen, died late last year, those of us skeptical about the company assumed that Allen’s trustees would finish and fly the aircraft, and then close up shop, and now it’s happening.
|When Allen’s plans to build the Stratolauncher were first made public in December 2011, the announcement was met with some degree of puzzlement. This led various people to try and develop a rationale for why Allen was really building this rocket, and soon some people started coming up with conspiracy theories.|
This has been a long time coming, and over the past seven years Stratolaunch has often seemed to be taking two steps forward and one, or even two steps back. In late 2012, Stratolaunch Systems parted ways with SpaceX, one of the original partners on the air-launch system (see “Stratolaunch: SpaceShipThree or Space Goose?”, The Space Review, December 19, 2011). SpaceX was supposed to build the rocket that would be dropped from the giant Stratolauncher airplane and carry payloads into Earth orbit. Stratolaunch then sought out SpaceX rival Orbital Sciences to investigate building a rocket for the project. A Stratolaunch executive announced that their separation from SpaceX had to do with the technical modifications required to adapt a rocket to the aircraft. While it is never a great idea to turn down somebody who wants to pay you money to do something, SpaceX was not short of customers. In addition, if they had been successful at building the air-launched rocket, this would have placed SpaceX in the awkward position of competing against itself. So the divorce made perfect sense, and Orbital, with its experience with the Pegasus rocket, knew more about air launch than anybody else.
But even that was not enough. Orbital could not come up with a viable rocket, and after years of study, what Stratolaunch planners eventually came up with was a plan to use their giant aircraft to launch up to three small Pegasus rockets. Considering that the Pegasus launched about once every couple of years at most, finding three Pegasus customers for a single launch opportunity seemed ridiculous. In the meantime, Orbital Sciences became Orbital ATK in 2014, then was acquired by Northrop Grumman in 2018. There were rumors of another rocket for Stratolaunch, and possibly even a human spacecraft, but those rumors made little sense.
When Allen’s plans to build the Stratolauncher were first made public in December 2011, the announcement was met with some degree of puzzlement by various news outlets and analysts. The private rocket, many realized, would face substantial technical challenges and a potentially huge price tag. It would also be pursuing a part of the launch market that was at best a niche. This led various people to try and develop a rationale for why Allen was really building this rocket, and soon some people started coming up with conspiracy theories.
|In addition to the lack of mission capability, there are the limitations of air launch itself. Air launch is simply not an attractive way to launch satellites. If it was, there would be more companies and countries developing this capability.|
The explanation that started to circulate in space circles was that the official story was a cover for a classified mission launching covert satellites into secret orbits. It’s akin to the Glomar Explorer, the very public “mining ship” that was actually funded by the CIA in the early 1970s to secretly recover a Soviet submarine on the bottom of the ocean floor. The problem with this theory for Stratolaunch was that it never made any sense. If the biggest airplane in the world took off with a rocket underneath and returned without that rocket, people would notice. It would not be “covert” at all. Instead, it would lead to much speculation about where the rocket had gone and what it had carried. The national security community could attract far less attention simply by launching on a conventional rocket, like they already do. Conspiracy-minded space enthusiasts claimed that the benefit of Stratolaunch was that nobody would know the orbit that the rocket had launched to, but this seemed like a thin argument. How important was such a capability when militarily-useful orbits are obvious? And besides, there has been a long history of this idea being explored and rejected.
During the early 1960s the CIA and the National Reconnaissance Office evaluated the possibilities of covert satellite launches. The earliest known scheme involved disguising a reconnaissance satellite as—no kidding—a test of an orbiting nuclear weapons platform launched atop an Atlas-Agena rocket. Details about this plan are scarce, but it seems rather odd to try and hide something secret behind the cloak of something that would have been dangerously provocative to the Soviet Union, and therefore would have attracted a lot of attention.
Declassified records indicate that by the mid-1960s the CIA evaluated several other options, including putting a satellite atop a Polaris missile launched via submarine, dropping a rocket out of the back of a C-130 cargo plane, and even launching a rocket from an A-12 OXCART spy plane. One program, designated “Town Hall,” would have used a B-58 Hustler medium bomber to launch a rocket. Instead of disguising the satellite, they sought to launch it out of sight. None of these options were pursued. The biggest problem was that none of the possible launch platforms could place a very large payload into orbit. Therefore, any “covert satellite” would be less capable than the conventional reconnaissance satellites then being launched every few weeks on Thor and Atlas, and later Titan, rockets. Instead of developing covert launch, the CIA and Air Force sought ways of rapidly launching existing satellites by reducing ground processing time. (See “Movements of fire and shadow: The X-23 PRIME reentry vehicle and American satellite reconnaissance,” The Space Review, March 5, 2018) They also increased the responsiveness of satellites on orbit. But these satellites were still visible from the ground using conventional radar and optical systems.
By the 1970s the spooks looked at another option: if they could not pass the satellite off as something else, and if they could not hide the launch, maybe they could make at least some satellites invisible to observation. The first stealth satellite research started in the 1970s with the launch of the Lincoln Experimental Satellite LES-8, equipped with a mirror that could make the satellite disappear to ground observers, and satellite stealth research and development increased substantially during the Reagan era. Stealth satellites are launched conventionally, but are not seen again by the amateur spy satellite trackers who search for them. They may be tracked by the Russians. Although little is known about them, we do know that stealth satellites are rare, and apparently insanely expensive. They may also have other operational limitations that explain why most intelligence satellites are more conventional, and indeed, visible from the ground. There is no legitimate reason to build a giant, non-covert launch system, just to launch at most one or two stealthy satellites a decade.
In addition to the lack of mission capability, there are the limitations of air launch itself. Air launch is simply not an attractive way to launch satellites. If it was, there would be more companies and countries developing this capability. Out of dozens of rockets developed over 60 years of spaceflight, you can count the number of operational air-launched versions on a single hand with four fingers missing, although Virgin Orbit is making a serious effort to begin air-launching satellites from a 747. But there have always been lots of proposals. In addition to the ones already mentioned, in the past couple of decades there was a proposal to launch a rocket from underneath a B-1 bomber, and a more recent DARPA project to launch a small rocket from underneath an F-15 fighter. Most proposals never got a go-ahead, and most that got a go-ahead were quickly canceled. This is true for foreign proposals as much as American ones. Air launch just doesn’t have obvious advantages.
|Outside observers looking for market logic or technical logic really only needed to consider the human (well, male) ego. Ego logic explained Stratolaunch.|
Some of the problems are obvious, while others are more subtle. For example, the gain in altitude from being launched from an aircraft is undercut by the limit to the size of the rocket, as well as other operational considerations like the boil off of cryogenics. Aircraft might be mobile, but satellites require ground support facilities that may not be. One former Air Force official has noted that an earlier (and now-canceled) air launching project had an inherent flaw: after being dropped from its carrier aircraft and falling toward the Earth, the rocket engines had to cancel the backwards velocity before they could start providing forward velocity. The performance hit was so great that it was worse than firing the rocket from the ground.
So, Paul Allen was building a big aircraft and a big rocket that seemed incredibly cool, but also not very logical unless you’re a Bond villain. But there are different types of logic when you’re insanely rich. The project was entirely logical if you accepted that it was never about profits but ego. If you’re massively rich, why shouldn’t you have very expensive toys to show off? Other rich men have the world’s biggest private jets, or the world’s most expensive antique cars. What Paul Allen wanted was the world’s biggest airplane that also happened to launch rockets. He got the former, a carbon-composite carrier plane, with a 117-meter wingspan and powered by six engines taken from 747s. But he never got to see it fly. Allen died last October, and the plane flew for the first time in April. Outside observers looking for market logic or technical logic really only needed to consider the human (well, male) ego. Ego logic explained Stratolaunch.
Indeed, we should be used to this by now. After all, there’s a joke that the best way to make a small fortune in the space business is to start with a large fortune, and we already have several examples of rich tycoons spending a small (or even moderate) amount of their net worth on space business ventures, most of which have not panned out even after many years of trying (Blue Origin was founded in 2000 and Bigelow Aerospace in 1998, neither one yet makes money.) They claim that they are also pursuing markets, but it’s worth asking if those markets are real or merely justifications that they use with their wives to explain why they don’t simply have a model train set in the basement, like normal middle-aged men. Sometimes business acumen gives way to daydreams, and if you have a lot of money, losing tens, or even billions of it on aerospace ventures that don’t return a profit is not that big a deal.
Paul Allen had other pursuits, and one of the great things he’s done—that also had no profit motive—was locate numerouslong-lostUS Navy ships from World War II such as the famed USS Hornet, which launched the Doolittle Raid on Tokyo. Allen did it because of his interest in history. And now it looks like Stratolaunch is also history. It’s too bad, because it would be much cooler to see that crazy project fly.
Dwayne Day can be reached at email@example.com.
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