From Popular Mechanics
Engineers are designing a rocket engine powered by the sun.
The engine would use heated and pressurized hydrogen to achieve efficiencies three times greater than conventional rocket engines.
The researchers proposed using the sun to slingshot an experimental spacecraft into interstellar space.
Engineers at the Johns Hopkins University Applied Physics Laboratory are prototyping a previously theoretical rocket design that could someday take spacecraft to interstellar space. Their plan? Use heat from the sun, rather than combustion, to power a rocket engine.
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Unlike a traditional engine that’s mounted on the rear end of a rocket, the experimental solar-powered engine takes the shape of a flat shield made from black carbon foam. The engine would double as a heat shield, protecting the probe from the sun’s powerful rays, while coils of tubing filled with hydrogen lying beneath the surface absorb heat from the sun.
The hydrogen expands, becomes pressurized, and then explodes out from a nozzle, generating thrust. The scientists call it solar thermal propulsion.
“From a physics standpoint, it’s hard for me to imagine anything that’s going to beat solar thermal propulsion in terms of efficiency,” Jason Benkoski, a materials scientist at the Applied Physics Laboratory (APL), told WIRED. “But can you keep it from exploding?”
Benkoski and his colleagues from APL and NASA recently shared their design online at the 3rd Annual Interstellar Probe Exploration Workshop. According to Benkoski’s calculations, a real-life version of the engine could be three times more efficient than the most advanced chemical combustion engines used in today’s rockets.
In 2019, NASA partnered with APL to kick off its Interstellar Probe study. The study will determine missions that could be launched next decade to study science outside our sun’s sphere of influence. Where the solar system ends and where interstellar space begins isn’t completely agreed upon, but one metric is the boundary where the sun’s magnetic fields and solar winds that make up the heliosphere can no longer be detected—what scientists call the heliopause.
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APL is looking for a probe that can travel three times farther than the outermost reaches of the heliosphere in less than two decades time—a distance of 50 billion miles. To put that into perspective, let’s take a look at the current record holder of the farthest distance traveled.
In 2012, the Voyager 1 spacecraft became the first manmade object to leave the confines of our solar system. After lifting off from NASA’s Kennedy Space Center in 1977 aboard the Titan III rocket, the space probe embarked on a 2-year journey to Jupiter, where it was slingshotted by the gas giant’s massive gravity to continue its journey to Saturn, Uranus, and Neptune. (See the spacecraft’s full timeline here).
As of today, almost four decades since its launch, Voyager 1 is more than 14 billion miles from Earth and traveling at 38 thousand miles per hour (mph). The team at APL wants to shatter this record by accelerating its spacecraft to 200,000 mph and making the journey in half the time.
To pull it off, the spacecraft will have to accomplish another first: performing an Oberth maneuver a mere million miles from the fiery surface of the sun. Coined by one of the founders of modern rocketry, Hermann Oberth, the maneuver takes advantage of the gravitational pull of a celestial body by using a spacecraft’s engines to further accelerate its fall into a gravitational well, as seen here:
It’s a lot like running down a hill to gain momentum for the uphill. The steeper the hill, or the closer you get to a gravitational body like the sun, the easier it is to gain speed and maximize your energy. The problem? The sun is very, very hot.
In 2025, NASA’s Parker Solar Probe will perform its closest approach to the sun. It will come within 4 million miles of the sun’s surface, travel at speeds exceeding 400,000 mph, and experience temperatures as high as 2,500 degrees Fahrenheit. To fight off the giant nuclear furnace’s heat, NASA equipped its probe with a 4.5-inch-thick carbon-composite shield.
If APL plans to send its probe within a million miles of the sun, it will need to withstand temperatures around 4,500 degrees Fahrenheit for 2.5 hours as it performs its Oberth maneuver. That’s why NASA is finding new materials that could coat the spacecraft and reflect the sun’s heat. Additionally, the hydrogen flowing through the heat shield could act like a radiator, displacing the thermal energy as propellant.
“We want to make a spacecraft that will go faster, further, and get closer to the sun than anything has ever done before,” Benkoski told WIRED. “It’s like the hardest thing you could possibly do.”
Benksoski and his colleagues at APL plan to submit a report on findings from their experimental rocket design next year.
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