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Scientists Cook Up a Rocket Engine That Consumes Itself for Fuel
Future satellites could ride on a rocket that cannibalizes itself during flight.
The crew of The Ark (streaming now on Peacock) have a problem and it’s not figuring out how to survive the rest of their trip. Building and launching a spaceship large enough to support a robust human population capable of colonizing a new world would be a massive undertaking, and it’s not totally clear how you’d even get something that big into orbit, let alone fly it to another star system. In the end, the Ark One has the same problem all spacecraft have: balancing its mass with fuel requirements.
Getting a rocket and its payload into orbit requires generating enough thrust to lift it out of Earth’s gravity well. Thanks to Newton, we know that every action has an equal and opposite reaction, which means that throwing some of the mass of a rocket out of the back (in the form of spent fuel) pushes the rocket forward. The heavier a rocket and its payload, the more fuel you need to lift it, but adding fuel adds to the weight, requiring even more fuel. At a certain point, you max out what a rocket is capable of, creating an upper limit on what you can launch.
Traditional rockets get around this problem through staging, the practice of ditching part of the rocket’s mass during the flight, thereby making the rocket lighter and increasing the efficiency of the remaining fuel. It’s the point during flight when you see the first stage of a rocket disconnect and fall back to Earth. Staging is an effective way of making the launch calculations work in our favor, but it’s not necessarily the most efficient. Recently, a team of rocket scientists from the University of Glasgow and Kingston University London demonstrated an autophage (meaning self-eating) rocket which consumes itself for fuel during flight.
Ouroborous-3, the World’s First Self-Consuming Rocket
Autophage rockets were first dreamt up in 1938, but remained only a dream for 80 years. The main reason we haven't seen self-consuming rockets before is mostly because there wasn’t much of a need for them, according to Krzysztof Bzdyk, one of the engineers behind the Ouroborous-3 autophage rocket.
"Historically, we've been delivering really large payloads into low-Earth orbit. The space shuttle had something like 60,000 pounds worth of payload capacity, Saturn V even more than that. All of our launch vehicles historically have been able to deliver these large payloads to orbit. For those reasons, they never needed to go down this route of an infinitely staged self-eating rocket, because their trade off between the gains they could get and the risks and complexity that they would take on was never in favor of autophage,” Bzdyk told SYFY WIRE.
Over the last couple of decades, there has been increased interest in launching smaller payloads like CubeSats. Meanwhile, more and more nations are developing independent launch capabilities with a focus on smaller launch vehicles and payloads. Bzdyk and team think autophage might be the answer, providing a new kind of launch vehicle capable of lifting heavier payloads with less fuel. From a certain point of view, an autophage rocket is constantly staging, ditching mass out its back end by burning the fuselage itself during launch. And that means you can launch heavier payloads than you can with the same sized conventional rocket.
The Ouroborous-3 – which takes its name from the Ouroboros symbology, portraying a great dragon or serpent biting its own tail – uses conventional fuels (a mixture of propane and other hydrocarbon fuels) just like a typical rocket does, but it introduces an additional component in the form of a high-density plastic fuselage. Polyethylene plastics, like rocket fuels, are made of hydrocarbons which make them chemically suitable for burning as rocket fuel, you just need a way to melt it and fire it out the back.
“It’s important that we’re selecting polymer fuels that are based around that, we wouldn’t want to use something like teflon, which would release really toxic chemicals. We’re trying to limit any type of autophage systems to carbon-fiber reinforced polymers, but all sort of using carbon, hydrogen, and oxygen, because the exhaust will be mixtures of H2O, carbon in the form of soot, carbon dioxide, carbon monoxide, and more of the typical exhausts we see in other launch vehicles,” Bzdyk said.
Ouroborous-3 Test Firings at the MachLab Facility, Machrihanish Airbase
During the recent test firings, the team used high-density polyethylene, but future tests will incorporate plastics doped with other materials like graphite to help give them strength while maintaining the same byproduct profile as traditional fuels. “In the future we might look at some kind of composite plastic fuels where we might have a stronger skin, with an inside that’s easier to burn,” Bzdyk said. “Or it could be doped with aluminum which is often used to improve the thrust capability of solid rockets.”
In the current design, the fuselage is fed slowly into a combustion chamber where the heat of the rocket’s engine melts it and kicks it out the back with the rest of the fuel. The question researchers wanted to answer was whether a plastic fuselage could support the strain of being fed into the engine during firing. Their tests proved that it could. Moreover, they found that the fuselage was able to contribute about 15% of the total propellant mass, which is in line with the typical structural mass of a conventional rocket.
“Because you’re not going to be carrying all that deadweight with you throughout the whole launch, I would estimate a 10% to 20% decrease in vehicle size or increase in payload,” Bzdyk said.
During test firing, the team carried out a pulse test, wherein the engine fired and rested at regular intervals every second. Those tests are important for understanding how the rocket behaves and turning the engine off every once in a while, could actually improve the performance of the rocket overall.
“We have the pressure of the engine resisting insertion, so we thought if we could turn the engine off we should theoretically reduce the force we have to apply in order to push it into the engine. Can we pulse our engine and use the air resistance to push it into the engine, then keep operating? The other reason is that engines operate differently in pulse modes, they usually run hotter because pulsing influences cooling. We thought if we can heat up the engine a lot, we can feed more fuselage because it will melt faster. We wanted to see what limits we could push to,” Bzdyk said.
The team did see a huge increase in temperature during the pulse test that actually caused the engine itself to melt. They wanted to find the limit and they did. Temperatures inside the engine rose so high that the injector melted and got shoved out the back. In turn, the middle of the fuselage was exposed to the heat of the engine and buckled near the end of the test. That’s because melting the fuselage actually provides some passive cooling of the engine, and when the fuselage stopped feeding, that cooling effect vanished.
“As the fuselage melts down the sides of the combustion chamber it creates a boundary layer where the metal can’t exceed the temperature of the molten plastic. It’s a way to regulate the temperature, but since during that pulse mode we were no longer able to feed it in, that boundary layer evaporated. The engine overheated, melted the injector, and blew up spectacularly,” Bzdyk said.
Shrinking your rocket during flight has some other benefits, too. It reduces the overall drag profile, minimizing air resistance, and allowing the rocket to be even more efficient. However, it also means we’re unlikely to fly people on an autophage rocket, at least anytime soon. According to Bzdyk, as the rocket eats itself and shrinks, we’ll see a corresponding increase in the dynamic loading of the craft and everything inside. It will have much higher G-loads than conventional rockets, leading to an uncomfortable ride at best. Those G-loads would be only slightly more uncomfortable than the knowledge that the rocket you’re inside of is rapidly eating itself alive.
On second thought, maybe autophage isn’t the solution for The Ark, but they need to figure something out. Catch the complete first season of The Ark, streaming now on Peacock.