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Raumfahrt - Was ist ein EM-Antrieb?

28.11.2016

NASA measured ‘impossible’ EM thrust – but is it really a big deal?


The copper bucket that supposedly breaks physics has finally passed peer-review. But, Cathal O'Connell writes, don't start packing your bags for Proxima b just yet.

The 'copper bucket' – or EM drive prototype – in a vacuum chamber.
NASA

Two years ago, NASA scientists announced they measured something that should not be possible – thrust from an engine that does not use propellant. 

Now, after a year-long review process, a paper describing the work has been published in the Journal of Propulsion and Power. It describes how the NASA team measured a thrust of about 1.2 millinewtons per kilowatt from the controversial electromagnetic or EM drive, though they are at a loss to explain how. 

The new work will add fresh impetus to the technology which has been touted by some as the future of space travel and by others as pseudoscientific hokum. But it is far from conclusive proof that the EM drive really works. 

What is an EM drive?

Down on Earth, we get around with the help of friction. A car’s wheels pushing on the road make it move forwards (or backwards if you’re in reverse). 

The problem in space is there’s nothing to push off – that’s why you need a propellant. According to Newton’s third law, where every action has an equal and opposite reaction, ejecting material in one direction will push the spacecraft in the other. In a rocket, the propellant is the exhaust gas fired out through thrusters at tremendous speed. 

The EM drive is a proposed new way of propelling without propellant – making spacecraft much lighter, cheaper and faster, because they don’t need to lug propellant around.

While only a handful of physics groups take the EM drive idea seriously, popular media has buoyed global interest in EM drive with reports so breathless they must have been written in a vacuum. 

If it works, the EM drive could take us to Mars in just 10 weeks (as opposed to about six months). Meanwhile, a trip to Alpha Centauri, the closest star system to our solar system and home to the possibly Earthlike planet Proxima-b, would take 92 years (as opposed to millennia using conventional thrusters).

Oh, and its inventor, the British electrical engineer Roger Shawyer, also claims it will give us flying cars, unlimited energy and solve the global warming crisis. His company, Satellite Propulsion Research Ltd, is currently seeking investment to commercialise the technology.

How is it supposed to work?

The basis of EM drive is a metal drum, empty but for the microwave photons bouncing around inside it millions of times a second. 

 

As the photons bounce back and forth, they generate a small pushing force on each end of the drum like ping pong balls hitting a wall. 

So far, this is nothing controversial. We know photons carry momentum – that’s how sunlight can propel spacecraft equipped with a solar sail.

If the drum is a cylinder, the forces on each end cancel out and nothing happens. But Shawyer’s idea, proposed around 2001, is to use a tapered drum, with one end wider than the other. 

Because of relativistic effects, he said, this shape would create an imbalance in the bouncing force and push the whole drum from the inside. This “pushing from within” is what breaks Newton’s law.

As John Baez, a mathematical physicist at the University of California, Riverside, puts it, “it's like sitting inside a car and making it roll forwards by pushing on the steering wheel”. 

Has the EM drive been tested before?

Yes, but not conclusively. 

In 2012, a team of Chinese scientists from Northwestern Polytechnical University in Xi’an reported measuring a large net thrust (720 millinewtons at 2,500 watts of input power). 

Later they realised that this measurement was an error stemming from a dodgy a power cable. They repeated their experiment in early 2016 and the measured thrust disappeared

Meanwhile, in July 2015, a German group led by Martin Tajmar at the University of Dresden also tested an EM drive device in a vacuum. 

They did measure a thrust, but in several directions besides the intended one. Tajmar concluded this to be a “null result”. 

So where does NASA come in?

Back in 2013, a small team at NASA’s Advanced Propulsion Physics Laboratory started to tinker around with the EM drive and another related technology called Cannae drive. 

The team is headed by Harold “Sonny” White, the scientist trying to warp spacetime using electromagnetic fields

In early EM drive work, White’s team did detect a net thrust, though they were criticised at first for performing the work only in air. 

That meant they couldn’t rule out the role of air molecules, kicked away from the heated device during operation, as the source of the anomalous thrust. 

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Now, the same team has published a paper describing their latest vacuum tests of their EM drive. The results seem to dispel the role of air because the team measures the same thrust in normal atmosphere and in a vacuum.

They also found that this force increased in rough proportion to the increased microwave power, which would be expected if microwaves were the source. 

In the experiment, White’s team sat their EM drive on the end of a torsion balance – an instrument that measures tiny forces. 

They measured a thrust of 1.2 millinewtons per kilowatt. That's the force you feel if you place seven grains of rice on your palm. A kilowatt is what a power-hungry household appliance, such as a vacuum cleaner, might use. 

The measured force is tiny but still more than 100 times stronger than that generated by a solar sail. If generated continuously, even one millinewton could be enough to propel a spacecraft to tremendous speeds, given enough time. 

The team noted nine possible sources of anomalous thrust in their set-up and accounted for as many as possible. 

For instance, to account for any systematic bias in their measuring instrument, they mounted the device on the swinging arm in both directions. In both cases, the thrust was towards the narrow end of the device. 

Puzzlingly, the original theory proposed by Shawyer seems to predict the thrust should go in the opposite direction than the NASA results show. Instead, White’s team suggests the device may work instead by pushing off the “quantum vacuum”, meaning the sea of virtual particles that fill even empty space. 

Does this mean NASA has ‘validated’ the EM drive? 

No. Getting a paper through peer-review is not conclusive proof that the EM drive really works. It just means that a bunch of independent scientists have pored over the methods and found nothing obviously wrong. 

Nothing in standard physics can explain how the EM drive might work. That is not necessarily a reason to reject the result out of hand – we know physics is not complete – but it is a reason to maintain scepticism. As Carl Sagan said, “extraordinary claims require extraordinary evidence”.

We can only learn from “anomalies” of the past. The faster-than-light neutrinos detected by CERN in 2011 turned out to be the fault of an improperly connected fibre optic cable. The source of the EM drive anomaly is most likely something just as mundane.

Caltech physicist Sean Carroll tweeted back in 2014: “The eagerness with which folks embrace sketchy claims about impossible space drives would make astrology fans blush.”

Quelle: COSMOS

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Update: 29.11.2016

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UA experts weigh-in on NASA's EM Drive

NASA’s peer-reviewed paper on the revolutionary Electromagnetic Drive— colloquially known as the EM Drive—has finally arrived, and it’s created quite a stir in the scientific community. 

NASA's Eagleworks Laboratory, the group that tested the EM Drive, released evidence claiming the EM Drive really does work, challenging Newton’s third law in the process.

“It’s an interesting story in that it seems to be an inexplicable technology development; nobody can really explain why it works,” said Tom Jones, former NASA astronaut and UA alumnus.

The EM Drive is a propulsion system which produces 1.2 millinewtons of force for every kilowatt of thrust. Propulsion normally works in concordance with Newton’s third law, meaning that for every action there is a reaction, said physicist Giorgio Torrieri, UA Ph.D. holder and professor at the State University of Campinas in Brazil. 

For example, a rocket expels exhaust in order to travel upwards. But this isn’t the case with the EM Drive.

“What [the researchers] claim they have found is movement without a reaction,” Torrieri said. “They found that with [applying electromagnetic radiation to a chamber of] a particular sort of shape, you get a movement without needing to burn any fuel. This violates conservation of energy and conservation of movement.”

According to Torrieri, the electromagnetic propulsion aspect of the EM Drive is not what causes it to defy the laws of physics. While electromagnetic propulsion may not be uncommon, the downside is it produces a small reaction and acceleration, making it much weaker than chemical-fueled rockets, Torrieri said. 

Propulsion systems can be ranked based on efficiency, starting with chemical propellant, moving on to more efficient electrical propulsion and ending with nuclear electric propulsion, Jones said. The EM Drive falls under a more exotic category, alongside systems such as solar sails and magnetohydrodynamic propulsion. 

“Assuming [the EM Drive] actually proves to be physically possible, it would fall into the category of a very efficient engine that could be used for very low thrust attitude control,” Jones said. 

Torrieri explained that, while the EM Drive may not need propellant, the amount of thrust it produces is very small. Though this may mean the thrust is weaker, the proposed electromagnetic propulsion could run for months. “You wouldn’t be able to leave Earth, but if you start from space, you’d be able to accelerate to very high velocities because you’re constantly accelerating."

While this low level of thrust may not be able to send humans to the Moon, it could potentially be used for a communications satellite, reconnaissance satellite or weather satellite, Jones said. The drive’s ability to stay on for long periods of time could also open up areas of the outer solar system that were previously out of reach for chemical-propelled rockets.

“This engine might fit into robot spacecraft powering around the inner solar system, to go and explore the planets,” Jones said. “And then if we develop a nuclear power source for use in orbit in space, then you could take that same technology and apply it to the outer solar system.”

In addition to thrust, energy efficiency is another issue to take into account when thinking about space travel.

“The biggest drawback seems to be that it eats up a lot of electricity for not a lot of thrust, and when you’re dealing with spacecraft powered by solar panels, electricity is not easy to come by,” Jones said. “So at this point, the demand seems to be a more robust power system.”

According to Jones, the key to developing this power system is nuclear power. And while nuclear propulsion has long since been a proposed route for space exploration, NASA is hesitant to experiment with it. Both the cost of development and the fact that the reactors would have to be tested here on Earth has caused NASA to be very reluctant in pursuing this, Jones said. 

“Maybe the developers will be able to make this more efficient in terms of power and more productive in terms of thrust,” Jones said. “Then over time they’ll develop a more practical engine that might be able to deal with larger spacecraft masses and real mission profile needs.”

Torrieri explained that from a physics perspective, scaling up the EM Drive so that it could power larger human-sized spacecraft is not the priority.

“I would say that before talking about spacecraft, one has to really make sure beyond reasonable doubt that this isn’t an experimental error, which it very likely is,” Torrieri said. "Physicists should be focused on finding every possible explanation for why this could occur without violating the laws of physics." 

Both Torrieri and Jones agree that the EM Drive needs to be tested further before its efficacy can be determined.

“There are these people who claim they see an effect, but considering how revolutionary it is, you need amazing evidence, and at the moment I don’t think this evidence is there,” Torrieri said. “Extraordinary claims require extraordinary measures.”

Jones remains more optimistic. “If this thing goes from a crude, experimental device to something practical in 20 or 30 years, even 10 years, then we might revolutionize the way we can explore the solar system.”

For more information on UA alumnus Tom Jones’ experiences in outer space, check out his latest book, "Ask the Astronaut." You can also find the original EM Drive paper published by NASA.

Quelle: The Daily Wildcat



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