Over the past few months, Elon Musk has been revealing new details about SpaceX's upcoming launch system, called Starship.
Musk has said the new launch vehicle, which he and engineers recently redesigned , will eventually replace all of the company's rockets. It will be fully reusable and extremely cheap to launch, the thinking goes perhaps reducing the cost of access to space 100-fold. In time, Musk thinks such a system may lead to round-trip Mars tickets that cost less than $500,000 and " maybe even below $100k ."
But that assumes mundane debris like dust or even bird droppings don't cause Starship's giant spaceship, as it's currently designed, to burn up when landing on Mars or returning to Earth.
Musk has yet to release the full details of Starship he's said that may happen in March or April but he last described it as a 180-foot-tall spaceship that will ride into orbit atop Super Heavy, a rocket booster about 220 feet tall. The spaceship is designed to be refueled in low-Earth orbit in order to propel 100 passengers and more than 100 tons of cargo at a time to Mars.
One key design change is that SpaceX now plans to build Starship out of stainless-steel alloys instead of carbon-fiber composites, Musk said. The second change involves a "bleeding" heat shield that will leak liquid through small pores to keep the spaceship cool as it travels at high speeds.
However, more than a few aerospace experts are concerned about how SpaceX would build such a system without it getting clogged.
A heat shield is an absolute must for a spaceship.
When a vehicle returns to Earth from orbit, it can reach speeds of about 19,000 mph . That's so fast that molecules in front of a ship turn into superheated plasma that can corrode, melt, or even vaporize some of the toughest and most heat-resistant materials.
Musk has said the nose of Starship may be exposed to temperatures of about 2,700 degrees Fahrenheit. The type of of steel alloy SpaceX may use on Starship's outer skin, called 310S , melts at about 2,400 degrees .
Most spacecraft (including SpaceX's upcoming Crew Dragon vehicle ) use "ablative" heat shields that are designed to burn away and insulate a vehicle's hull during atmospheric entry. NASA used this type of technology in the Mercury, Gemini, and Apollo space capsules. To protect its space shuttle fleet from heat, NASA glued thousands of heavy ceramic tiles to the bottom of each ship.
But Starship, as Musk told Popular Mechanics , will have a transpirationally cooled heat shield.
The system would work similar to human skin, in which sweat absorbs the body's heat, evaporates to carry the warmth away, and ultimately cools off the body. With Starship, the "sweat" would be water or spare methane fuel, and the "pores" would be countless tiny holes in Starship's steel underbelly.
At least a few aerospace experts are concerned about the challenges that such a heat shield would face, though.
"You can imagine it wouldn't take much to clog something like that, if they were microscopic pores," Walt Engelund , an aerospace engineer and the director of the Space Technology and Exploration Directorate at NASA Langley, told Business Insider.
Dwayne Day , who helped investigate the loss of NASA's Columbia space shuttle and its crew, said such clogs could come any number of mundane factors.
"What if a bird poops on your rocket and it plugs up a few holes, and then when the thing is returning, no coolant comes out of those holes and that section of the vehicle overheats?" he said.
Dust on Mars, which recently killed NASA's Opportunity rover by blocking sunlight to its solar panels, could also pose a threat to small pores in Starship's steel skin.
Plus, Engelund said, the carbon in fuels like methane tend to "coke" or turn into solids when exposed to high temperatures, so that might also pose a clogging risk if Musk opts for methane as his coolant liquid.
Engelund said various liquid-cooling systems have been tested in NASA's hypersonic wind tunnels , which blow air at thousands of miles per hour to simulate incredible speeds in the upper atmosphere. In such tunnels, one blocked coolant channel can vaporize an aircraft model.
Looking for clogs in thousands of pores before a spaceship launches from Earth would be tough, but it would be much harder to do that on Mars, where there are no launch towers or gantries.
"How do you make sure that all the holes are open? Or at least that there are no significant obstructions?" Day said.
In an email to Business Insider, SpaceX emphasized that constant tweaks and changes to Starship's design are expected that's part of the company's larger design approach as engineers work to make the launch system a reality. Indeed, the system (previously called Big Falcon Rocket and other names before that) has already gone through several major redesigns since Musk first described it in detail in September 2016 .
"They've surprised a lot of people, and have a lot of smart people working for them, and Elon seems to be really committed and dedicated to this," Engelund said. "Perhaps there are some things that we could do with them. I suspect there will be."
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