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The rocket motor of the future “breathes” air like a jet engine



Enlarge / The air intake on Mountain Aerospace Research Solution’s Fenris engine after its first hotfire last July. The lines around the cone feed kerosene and gaseous oxygen into a combustion chamber, where it is mixed with the air and ignited.

Aaron Davis | Mountain Aerospace Research Solutions

There’s a small airfield about a two-hour drive north of Los Angeles that sits on the edge of a vast expanse of desert and attracts aerospace mavericks like moths to a flame. The Mojave Air & Space Port is home to companies like Scaled Composites, the first to send a private astronaut to space, and Masten Space Systems, which is in the business of building lunar landers. It’s the proving ground for America’s most audacious space projects, and when Aaron Davis and Scott Stegman arrived at the hallowed tarmac last July, they knew they were in the right place.

The two men arrived at the airfield before dawn to set up the test stand for a prototype of their air-breathing rocket engine, a new kind of propulsion system that is a cross between a rocket motor and a jet engine. They call their unholy creation Fenris, and Davis believes that it’s the only way to make getting to space cheap enough for the rest of us. While a conventional rocket engine must carry giant tanks of fuel and oxidizer on its journey to space, an air-breathing rocket motor pulls most of its oxidizer directly from the atmosphere. This means that an air-breathing rocket can lift more stuff with less propellant and drastically lower the cost of space access—at least in theory.

The idea to combine the efficiency of a jet engine with the power of a rocket motor isn’t new, but historically these systems have only been combined in stages. Virgin Galactic and Virgin Orbit, for example, use jet aircraft to carry conventional rockets several miles into the atmosphere before releasing them for the final leg of the journey to space. In other cases, the order is reversed. The fastest aircraft ever flown, NASA’s X-43, used a rocket engine to provide an initial boost before an air-breathing hypersonic jet engine—known as a scramjet—took over and accelerated the vehicle to 7,300 mph, nearly 10 times the speed of sound.

But if these staged systems could be rolled up into one engine, the huge efficiency gains would dramatically lower the cost of getting to space. “The holy grail is a single-stage-to-orbit vehicle where you just take off from a runway, fly into space, and come back and reuse the system,” says Christopher Goyne, director of the University of Virginia’s Aerospace Research Laboratory and an expert in hypersonic flight.

The big challenge with a single-stage-to-orbit, or SSTO, rocket is that achieving the speeds necessary for orbit—around 17,000 mph—requires a lot of propellant. But adding more propellant makes a rocket heavier, which makes it harder to reach orbital velocity. This vicious circle is known as the “tyranny of the rocket equation,” and is why it takes a two-stage rocket the size of an office building to launch a satellite the size of a car. Staging a rocket helps because it can shed dead weight once the first stage’s propellant is used up, but it’s still pretty inefficient to have to burn all that propellant in the first place. This is where an SSTO rocket with air-breathing engines would provide a huge efficiency boost.

“The idea is to use air-breathing engines earlier in the launch to take advantage of efficiency gains from engines that don’t have to carry their own oxidizer,” says Goyne. “Once you get high enough in the atmosphere, you start to run out of air for the air-breathing system and you can use the rocket for that final boost to orbit.”

Big money, big challenge

When Davis founded Mountain Aerospace Research Solutions in 2018, no one had ever made a working air-breathing rocket engine before. NASA and aerospace giants like Rolls-Royce had tried, and all the projects fizzled out due to soaring costs and major technological challenges. But Davis, a former Aviation Ordnance technician in the Marines, had an idea for an air-breathing engine of his own and couldn’t shake the idea. “I hired Scott Stegman to prove to me it wouldn’t work,” Davis says. But Stegman, who previously worked as a mechanical engineer at Northrop Grumman, crunched the numbers and didn’t find any showstoppers. As far as physics was concerned, Davis’ engine seemed like it should work.

According to Stegman’s calculations, a full-scale Fenris engine could reduce the amount of oxidizer a rocket needs to carry by around 20 percent. That’s a huge efficiency gain, but first they had to demonstrate that Davis’ design would work. Davis lacked the funds needed to run detailed fluid dynamics simulations to model the engine on a computer, so the duo decided to build a physical engine instead. “At the end of the day, you can make really pretty simulations and nobody’s going to believe you,” Davis says. “It was cheaper to just go out and test whether my idea was valid or not.”

By the time Davis started the countdown at the Mojave Air & Space Port last July, he and Stegman had been working on the Fenris prototype for nearly a year and a half. Davis says he paid for the engine’s development entirely out of pocket and estimates that he’s spent about $500,000 on the project so far. The hourglass-shaped engine isn’t much larger than a toaster oven and is designed to passively pull in air from one end, combine the air with kerosene and some gaseous oxygen in a combustion chamber, and spit flames out the other end. And when Davis triggered the ignition last year, the Fenris engine worked.

Davis claims that test is the first and only time an air-breathing rocket motor has been successfully hotfired. It’s a big assertion and it comes with an important caveat: The Fenris engine wasn’t even close to powerful enough to send anything to space. The duo haven’t released any data about the engine’s performance, but in video of the engine firing it’s clear that the exhaust lacks the ordered structure you’d expect to see in a high-performance rocket engine. In fairness, Davis and Stegman weren’t trying to get to the final frontier. They just wanted to see if their engine could pull air in one end and belch flames out the other without blowing up. “It’s literally a rocket engine with holes at both ends,” Stegman says. “That’s not normal and it’s why we were really conservative for the first test.”

Later this year, Davis and Stegman will run some more advanced engine tests in a decommissioned missile silo in Wyoming. Unlike the first test run, these will be all about pushing Fenris to its limits and extracting as much power as possible from the experimental engine. Based on his computer models, Davis says he expects to achieve over 600 seconds of specific impulse during the tests, which is a measure of how efficiently a rocket engine uses its propellant. This would be a monumental achievement given that the world-record specific impulse—held by NASA—is 542 seconds, and most operating orbital rockets have specific impulses around 300 seconds. If the demos in Wyoming go well, the next big step would be to demonstrate the motor in flight. If he finds a launch partner, Davis says the Fenris engine could fly as soon as 2022.

The air intake on Mountain Aerospace Research Solution's Fenris engine after its first hotfire last July. The lines around the cone feed kerosene and gaseous oxygen into a combustion chamber, where it is mixed with the air and ignited.
Enlarge / The air intake on Mountain Aerospace Research Solution’s Fenris engine after its first hotfire last July. The lines around the cone feed kerosene and gaseous oxygen into a combustion chamber, where it is mixed with the air and ignited.

Aaron Davis | Mountain Aerospace Research Solutions

Historically speaking, Davis and Stegman are in good company. The birth of modern liquid-fueled rockets was driven by amateurs like Robert Goddard, Jack Parsons, and Werner von Braun who cleared the path for the massive state-run rocket programs that followed. But not everyone is convinced that Fenris is a game changer.

“I am skeptical of the entire concept,” says Dan Erwin, a professor of aerospace engineering at the University of Southern California and an expert in propulsion. One concern is that the atmosphere is mostly inert nitrogen—and in a rocket engine that nitrogen acts like a wet blanket. It gets heated by the combustion reaction between the oxygen and the kerosene without contributing to it, which lowers the combustion temperature and reduces thrust. And while nitrogen can contribute to an engine’s thrust—since it’s being heated in the combustion chamber and expelled through the nozzle—the exhaust speed must be greater than the spacecraft’s speed. Otherwise, Erwin says, the air is moving forward relative to the stationary atmosphere when it exits the engine, and this would detract from the rocket’s forward momentum. While such an engine isn’t impossible, it would have to be incredibly high performance.

Adonios Karpetis, an aerospace engineer at Texas A&M University and an expert in high-speed combustion, also has qualms about the feasibility of the Fenris engine. He points out that although rockets spend most of their time moving at supersonic or hypersonic speeds, the combustion chamber itself doesn’t experience those conditions. This is not the case with hypersonic air-breathing engines, which experience hypersonic airflow in the engine itself. This has been a major technical challenge for companies building hypersonic scramjet engines and would also be faced by an air-breathing engine like Fenris during flight. “The one static fire test of the Fenris device took place at zero speed,” says Karpetis. “What will happen when the Fenris device becomes truly supersonic and air is rushing into it through the inlet at high speeds? A simple guess would predict diminishing behavior, quickly reducing the 600 seconds specific impulse to some lesser value.”

It ain’t easy

There is a long history of organizations with lots of money and plenty of expertise that struggled to bring air-breathing rocket engines to life. In the 1980s, NASA and a partnership of British aerospace companies were both pursuing concepts for SSTO air-breathing space planes that could replace the space shuttle. NASA’s vehicle, known as the National Aero Space Plane, was designed to use an air-breathing scramjet to accelerate to 25 times the speed of sound [PDF] and reach orbit without a rocket engine. The British vehicle, called Horizontal Take-Off and Landing (or Hotol), was meant to have a hybrid engine that combined aspects of a jet engine and a rocket motor.

Budget constraints killed both spaceplane programs before they were ever built, but Alan Bond, one of Hotol’s lead engineers, couldn’t quit the idea. In 1989, Bond founded Reaction Engines to build a new air-breathing rocket engine based on Hotol’s designs. He envisioned using the engine on a conceptual space plane he called Skylon, which looks like a rocket outfitted with an air-breathing engine at the tips of its two narrow wings. Skylon’s engine is known as the Synergetic Air Breathing Rocket Engine, or Sabre, and although the spaceplane is still little more than a concept, the engine is very real.

The idea behind Sabre is to use the engine’s air-breathing mode to whip a spacecraft up to hypersonic speeds in the lower atmosphere and then switch to a full rocket mode at the edge of space. It’s conceptually simple, but the devil is in the details. For example, as the engine works the aircraft up to hypersonic speeds at low altitudes, the air temperature approaches 1,800 degrees Fahrenheit, which is hot enough to melt engine components. To overcome this challenge, Sabre uses a precooler to lower the air temperature by circulating hydrogen fuel through the engine. This lowers the air to ambient temperatures at altitude, which are around -200 degrees Fahrenheit. “Effectively the core engine does not know it is flying hypersonically,” says Shaun Driscoll, the programs director at Reaction Engines. “The precooler takes care of that.”

Once the air is lowered to a manageable temperature, it’s passed to a compressor to raise the gas pressure, much like in a conventional jet engine. Then it’s routed to a rocket combustion chamber where it is mixed with liquid hydrogen fuel and ignited to produce thrust. By the time the vehicle reaches hypersonic speeds, the atmosphere is too thin for an air-breathing engine and the system switches to its onboard oxidizer tank for the final leg of the journey to space.

Bond retired from Reaction Engines in 2017, but work on the Sabre engine continues apace. Over the past four years, the company has raised over $100 million to develop Sabre, and shortly after Bond stepped back from the company, Reaction Engines contracted with Darpa to develop a test facility for the engine’s precooler in Colorado. Late last year, the company demonstrated that its precooler could handle the extreme heat generated under hypersonic conditions, a major milestone on its path to a full engine demonstration. Around the same time, the European Space Agency concluded its design review of the engine and gave the company the green light to start testing its engine core.

Reaction Engines CEO Mark Thomas says the company expects to begin these tests next year. The Sabre engine core is the air-breathing heart of the propulsion system stripped of its exhaust nozzle and the precooler. “These tests will take place during next year and will be a significant step towards the world’s first air-breathing engine capable of accelerating from zero to Mach 5,” says Thomas. If these tests go well, Thomas says the next big step will be to integrate all the engine components and perform a high-speed flight demonstration with a custom airframe. Thomas says he anticipates the first demo flight to happen by the mid-2020s.

“In recent years commercial launch companies have delivered significant advances in reusability and reductions in launch costs, however, their approach is essentially utilizing existing chemical rocket technology that has been used for over 70 years,” Thomas says. “Only an air-breathing system will deliver a further quantum reduction in launch costs and reliability.”

Sabre is the culmination of more than 40 years of research and development backed by millions of dollars in government and industry funding. It’s about as far as you can get from two guys juicing a small prototype rocket engine in the desert, but Davis isn’t phased by the long odds. “This matters more than anything,” he says. “Only 600 people have ever been to outer space and I’m not going to quit until I’ve realized that ability for everybody.”

This story originally appeared on

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COVID was the leading cause of death in Americans aged 45-54 in 2021



Enlarge / A woman watches white flags on the National Mall on September 18, 2021 in Washington, DC. Over 660,000 white flags were installed here to honor Americans who have lost their lives to COVID-19 epidemic.

COVID-19 was the third leading cause of death in Americans between March 2020 and October 2021, accounting for one in every eight deaths.

In that time frame, COVID-19 ranked in the top five causes of death for every age group of people older than 15 years. Between January and October 2021, the pandemic disease was the leading cause of death among people 45 to 54 years old.

That’s all according to a study of national death certificate data, published Tuesday in JAMA Internal Medicine by researchers at the National Institutes of Health.

The study found COVID-19 caused roughly 700,000 deaths between March 2020 and October 2021. The pandemic disease trailed only heart disease and cancer, which caused roughly 2.15 million collectively in that time frame. The fourth and fifth deadliest afflictions in the US were accidental deaths—including car crashes, overdoses, and alcohol-related deaths—and stroke, which collectively caused around 624,000 deaths during that period.

The authors, led by Meredith Shiels, an expert in cancer epidemiology and genetics at the National Cancer Institute, broke up the time frame into two sections: the start of the pandemic in March 2020 to December 2020, and January 2021 to October 2021, the last month for which there was complete data. This revealed age-specific trends, likely driven partly by uptake of vaccines and other mitigation efforts.

In the 2020 period, COVID-19 was the second leading cause of death in people aged 85 and over, but, amid high vaccine uptake in this age group, it fell to the third leading cause of death from January to October 2021.

Younger adults saw the opposite trend. For those aged 45 to 54, COVID-19 was the fourth leading cause of death in the 2020 period but jumped to the leading cause of death in 2021. Likewise, in those aged 35 to 44, COVID-19 jumped from the fifth leading cause of death in 2020 to the second leading cause in 2021. And for those aged 15 to 24 and 25 to 34, COVID-19 wasn’t in the top five in 2020, but ranked as the fourth leading cause of death in both age groups in 2021.

For those aged 55 to 84, COVID-19 was the third leading cause of death in both time periods.

The study is limited by the potential for misclassifying deaths on death certificates. But the authors were careful to select a time cutoff that would limit provisional or incomplete data from skewing the findings. That meant, however, that the study did not include deaths from part of the delta wave or the towering omicron wave in January 2022. Since October 2021, around 300,000 additional people in the US have died from COVID-19.

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Russian astronauts use space station to promote anti-Ukraine propaganda



Enlarge / Cosmonauts Oleg Artemyev, Denis Matveyev, and Sergey Korsakov pose with a flag of the Luhansk People’s Republic on the International Space Station.

The Russian state space corporation responsible for spaceflight activities, Roscosmos, on Monday posted images to its official Telegram channel showing three cosmonauts with the tri-color flags of the Luhansk People’s Republic and the Donetsk People’s Republic.

The photos were taken recently on board the International Space Station and show smiling cosmonauts Oleg Artemyev, Denis Matveyev, and Sergey Korsakov posing with the flags.

“This is a long-awaited day that residents of the occupied areas of the Luhansk region have been waiting for eight years,” the Roscosmos message stated. “We are confident that July 3, 2022, will forever go down in the history of the republic.”

The images and social media posting represent the most blatant use of the International Space Station—which is operated by the United States, Russia, Japan, Canada, and the European Space Agency—for Russian propaganda purposes since the invasion of Ukraine.

Luhansk and Donetsk are two breakaway “quasi-states” in the eastern region of Ukraine known as the Donbas. Ukraine and Russia have battled over the two regions since 2014, as Russia has agitated separatists in the Ukrainian territory. The United Nations does not recognize the two “republics,” and Ukraine has designated them as “temporarily occupied territories.” Fighting has heated up since the Russian invasion of Ukraine in February 2022. This past weekend, Russian forces claimed to have established control over the entire Luhansk region.

A professional relationship

NASA and Roscosmos, as well as other space agencies, have continued cooperating on the International Space Station since the invasion began. Some US officials have suggested that NASA should consider breaking ties with Russia in space due to the atrocities in Ukraine. However, the space agency’s administrator has defended the partnership on the basis that the station flies above geopolitical tensions on Earth. NASA also wants to keep flying the station, as breaking the US segment from the Russian segment would be difficult and potentially fatal to the operation of the orbital facility.

In an interview published Monday in the German publication Der Spiegel, NASA Administrator Bill Nelson reiterated this stance.

“In the midst of the Cold War, when the Soviet Union and the United States were mortal enemies and their nuclear weapons could be used at any time, a US and a Soviet spacecraft met in space in 1975,” Nelson said. “Peaceful cooperation continued even after the collapse of the Soviet Union. Our space shuttle docked with the Russian space station Mir. And then we decided to build the International Space Station together. Both countries are needed for operations, the Russians for propulsion, the Americans for power. We will continue to have a very professional relationship between cosmonauts and astronauts to keep this station alive.”

Nevertheless the provocative actions this weekend by Roscosmos, with its cosmonauts celebrating the so-called liberation of Ukrainian territory, brings the bloody conflict on Earth into space. To some observers, such as former NASA astronaut Terry Virts, Russia’s use of the space station for propaganda purposes is unacceptable.

“I am incredibly disappointed to see cosmonauts and Roscosmos using the International Space Station as a platform to promote their illegal and immoral war, where civilians are being killed every day,” said Virts, who flew side by side with Russians and commanded the space station in 2015. “The space station is supposed to be a symbol of peace and cooperation.”

Virts said NASA has largely been trying to look the other way when it comes to Russian actions, most notably when it comes to Roscosmos chief Dmitry Rogozin, who has made numerous jingoistic statements about the war. But in this case, he said, the agency really cannot afford to.

Seat swap

NASA’s cooperation with Russia may come into greater public focus in a couple of months. At present, a NASA astronaut named Frank Rubio is scheduled to fly on a Russian Soyuz spacecraft to the station in September. Around the same time, a Russian cosmonaut named Anna Kikina is due to fly on a SpaceX Crew Dragon vehicle to the station as part of the seat swap. The arrangement has not been formally agreed to by the US and Russian government.

In his German interview, Nelson defended the swap, saying, “It makes a lot of sense for us. You need both Russians and Americans to operate the space station. What happens if something is wrong with one of our spacecraft? We need the other vehicle as a back-up. And that’s why we will continue to have crew exchanges.”

Such an argument may soon ring hollow, however. Boeing’s Starliner spacecraft may make its first crewed test flight before the end of this year, and if it is successful NASA will have two US spacecraft capable of reaching the station.

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How the Yurok Tribe is bringing back the California Condor



Enlarge / The California condor is a New World vulture, the largest North American land bird. This condor became extinct in the wild in 1987, but the species has been reintroduced in California and Arizona.

The first California condor to reach Yurok ancestral land in over a century arrived by plane and car in late March of 2022. The small plane that carried Condor 746 had a rough landing, and the bird was irritable. He rattled around in a large dog crate during the three-hour drive to the tribe’s newly built condor facility, in a remote location in Redwood National Park.

Once there, he hopped into the flight pen, a tall enclosure of wire mesh, furnished with log perches and a drinking pool. At 8 years old, Condor 746 is an adult, his naked head bright pink instead of the black found in younger birds. He’s on loan from the captive breeding program at the Peregrine Fund’s World Center for Birds of Prey in Boise, Idaho. His job is to act as the mentor for four juvenile birds who will become the founders of a reborn condor society in Yurok country.

“We have mentors because condors are so social,” says Joe Burnett, California Condor Recovery Program Manager at the Ventana Wildlife Society. Young birds in a pen with no adult will become unruly. “You get the Lord of the Flies syndrome,” says Burnett. He and his colleagues quickly learned that release programs need an adult to serve as a role model and enforce the social hierarchy that is crucial to the flock’s survival.

A few days after 746 arrived, Condor A0, age 2, entered the flight pen. The first thing she focused on was 746, lounging on a perch. Understanding that she was in a safe place, A0 checked out the food—the carcass of a stillborn calf—then flapped onto a perch and fluffed up her feathers, a sign of avian contentment. Three young male condors, tagged A1, A2, and A3, followed. The youngsters had been living together for months at other condor facilities in Boise, Idaho, and San Simeon, California, and they already felt at home with each other.

Condor, known as prey-go-neesh in the native language, is sacred to the Yurok people. The Yurok reservation lies along the Klamath River in northwest California, but much of the tribe’s ancestral land is now in the hands of government agencies or private landowners. The tribe has been working to bring back the California condor since 2003, when a group of elders identified the bird as a keystone species for both culture and ecology, and therefore the most important land-based creature in need of restoration.

Nineteen years after the Yurok made that bold decision, the condors arrived. Elders who had worked toward that pivotal moment watched as Tiana Williams-Claussen, director of the Yurok Wildlife Department, and her colleagues released each newcomer into the pen.

Williams-Claussen’s job is to understand the details of condor biology and to interpret Yurok culture for the wider world. A tribal member, she grew up on the coast near the mouth of the Klamath, and went off to Harvard University. She didn’t set out to be a condor biologist, but when she returned in 2007 with a degree in biochemical sciences, condor restoration was the work her people needed her to do. Williams-Claussen has since spent 14 years living and breathing condors, learning how to handle them, building partnerships with government agencies, and listening to what Yurok elders have to say about the great bird.

The California condor is a critically endangered species: In the 1980s, the total population dwindled to fewer than 30 individuals. Biologists concluded the species’ only chance of survival lay in capturing every living condor in order to breed the birds in captivity, safe from poisons and power lines.

Reintroducing condors to the wild proved difficult, however, and the process became a dramatic lesson for biologists on the importance of parenting and the slow pace of growing up among these long-lived, highly social birds. Scientists learned that time spent with adults was critical to the behavioral development of young condors. They also found that in a species where adults follow and protect their offspring for a year or more after the birds fledge, youngsters pioneering landscapes empty of condors require lots of human babysitting.

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