The future of manned missions to the International Space Station appears on solid footing after the successful demonstration of SpaceX's Crew Dragon spacecraft earlier this month, but the company is wasting no time making good on its promise to one day shuttle humans to Mars. The company will begin initial tests of its Starship spacecraft as soon as this week, according to founder and CEO Elon Musk.
"Always many issues integrating engine & stage," Musk tweeted about the stainless-steel prototype nicknamed "Starhopper," which is currently at the company's Boca Chica, Texas facility. "First hops will lift off, but only barely."
Built over the course of six weeks earlier this year, Starhopper is the first prototype of the Big Falcon Rocket (BFR) reusable launch vehicle and spacecraft system that SpaceX is developing to replace its Falcon 9 fleet. SpaceX tested a version of the new vehicle, but strong coastal winds in February ended up knocking it over and damaging the nosecone. Instead of dealing with weeks of repairs, the decision was made to move ahead with testing a more-squatty version of the prototype.
"We decided to skip building a new nosecone for Hopper. Don't need it," Musk tweeted. "What you see being built is the orbital Starship vehicle."
According to regulatory filings, SpaceX intends to incrementally test Starhopper's ability to take off and land from a variety of altitudes. These will range from several feet for the first tethered tests to as high as 16,000 feet for the final.
"Once we get through the hopper test campaign, we'll then be moving to orbital flight with Starship: getting up into Earth orbit and testing out systems on board and recovery," said Paul Wooster, principal Mars development engineer, during a March 17 presentation, according to Space.com.
Powering these initial tests will be a single, truck-sized Raptor rocket engine. In development for the last 10 years, the Raptor is a methane-fueled beast offering two times the thrust of the Merlin 1D engine that powers the Falcon 9. As I wrote back in February 2018, the Raptor is intended to be the force that gets humans to Mars.
Unlike the Merlin engine, which runs on a mixture of kerosene and liquid oxygen (LOX), the Raptor will make use of densified liquid methane and LOX. Not only does the switch to methane as a fuel allow for smaller tanks and a cleaner burn, it also enables SpaceX to harvest the one thing Mars has plenty of: carbon dioxide. Using the Sabatier process, which generates methane, oxygen and water from a reaction between hydrogen and CO2, Mars colonists would not only have the necessary elements to survive long-term on the planet, but the fuel to make return trips back to Earth.
You can see a static test fire demonstration of the Raptor engine in the video below.
According to SpaceX, the BFR launch vehicle will have no fewer than 31 Raptor engines. The orbital Starship/tanker by comparison will include four Raptors for propulsion and three for maneuvering in space.
"What they're trying to do sounds crazy to me and to many people in the industry," Marco Cáceres, a senior space analyst who studies the the aerospace and defense industry, told Business Insider, talking about the Raptor design. "They want to reuse these engines hundreds of times, which has never been done. These engines have to work like your car engine: You turn it on, it goes, and you never expect it to blow up."
As for the decision to use a special alloy of stainless steel for the Starship's exterior, Musk says the unusual move comes down to both cost and heat threshold. He's also confident the total weight of Starship will be lighter than if the company were to choose aluminum or carbon fiber, as was originally intended.
"The carbon fiber is $135 a kilogram, 35 percent scrap, so you’re starting to approach almost $200 a kilogram," he told Popular Mechanics. "The steel is $3 a kilogram."
Because SpaceX is intent on creating a spacecraft that can land back on Earth and immediately be sent back into space, it needs a material that can withstand the extreme temperatures of reentry without compromise. While carbon fiber has a steady-state temperature to about 300 degrees Fahrenheit (149 Celsius), according to Musk, it weakens when exposed to anything beyond that. Steel, meanwhile, with its extremely high melting point, can endure temperatures of 1600 degrees Fahrenheit (871 Celsius) without any compromise in strength.
"With steel, now you’ve got something where you can comfortably be at a 1500 F interface temperature instead of, say, a 300 F, so you have five times the temperature capability at interface point," he adds. "What that means is that for a steel structure, the leeward side of the back shell does not need any heat shielding."
Speaking of heat shielding, SpaceX wants to innovate on that front as well.
"On the windward side, what I want to do is have the first-ever regenerative heat shield," Musk said. "A double-walled stainless shell — like a stainless-steel sandwich, essentially, with two layers."
Flowing through those two layers would be a liquid of either water or methane that would enable "transpiration cooling" and effectively protect the heat shield from damage. Between flights, the heat shield reservoir would simply be refilled before launch. "Transpiration cooling will be added wherever we see erosion of the shield," tweeted Musk. "Starship needs to be ready to fly again immediately after landing. Zero refurbishment."
In a separate tweet, Musk showed off the heat shield tiles being tested at temperatures approaching reentry conditions of around 2,500 degrees F.
Testing Starship heatshield hex tiles pic.twitter.com/PycE9VthxQ— Elon Musk (@elonmusk) March 17, 2019
As you might expect, the hexagon shape of the tiles also plays a role in protecting the craft from burning up upon reentry. "No straight path for hot gas to accelerate through the gaps," Musk shared.
While SpaceX appears to be ahead of schedule with Starship's development, we're still years away from the first passengers boarding to take a trip to low-Earth orbit or around the moon. As the illustration above shows, however, the company is intent on making those journeys as comfortable and entertaining as possible.
As for finding the cash to settle in a SpaceX colony on Mars one day, that transaction could be as simple as selling your home on Earth.
"Very dependent on volume, but I'm confident moving to Mars (return ticket is free) will one day cost less than $500k & maybe even below $100k," Musk tweeted. "Low enough that most people in advanced economies could sell their home on Earth & move to Mars if they want."