Wired interviewed Elon Musk Elon talks about some improvements that have been and will be made by Spacex with its rocket technology.
[Elon] We needed to set rocket technology on a path of rapid improvement. In the course of trying to put together Mars Oasis, I had talked to a number of people in the space industry and got a sense of who was technically astute and who wasn’t. So I put together a team, and over a series of Saturdays I had them do a feasibility study about building rockets more efficiently. It became clear that there wasn’t anything to prevent us from doing it. Rocket technology had not materially improved since the ’60s—arguably it had gone backward! We decided to reverse that trend.
What’s different in your basic technology versus 50 years ago?
Musk: I can’t tell you much. We have essentially no patents in SpaceX. Our primary long-term competition is in China—if we published patents, it would be farcical, because the Chinese would just use them as a recipe book. But I can give you one example.
Anderson: What is it?
Musk: It involves the design of the airframe. If you think about it, a rocket is really just a container for the liquid oxygen and fuel—it’s a combination propellant tank and primary airframe. Traditionally, a rocket airframe is made by taking an aluminum plate perhaps a couple of inches thick and machining deep pockets into it. Then you’ll roll or form what’s left into the shape you want—usually sections of a cylinder, since rockets tend to be primarily cylindrical in shape. That’s how Boeing and Lockheed’s rockets are made, and most other rockets too. But it’s a pretty expensive way to do it, because you’re left with a tiny fraction of the plate’s original mass. You’re starting with a huge slab of material and then milling off what isn’t needed, so you get a huge loss of material. Plus, machining away all that metal takes a lot of time, and it’s very expensive.
What’s the alternative?
Musk: It’s similar to the way that most airplanes are made: The stiffness is provided by ribs and hoops that are added on.
Anderson: It’s basically aluminum origami—you’re cutting very precise grooves into it so it folds together into a stiff shape.
Musk: But there’s a catch, because you can’t rivet a rocket like you can an airplane. The pressure differential of an airplane—the difference between the internal and external pressure during flight—is perhaps 7 to 10 psi. But in the case of a rocket, it’s likely to be 80 psi. It’s a lot harder for rivets to withstand that pressure with no leaks.
Musk: So the approach used for aircraft is not exactly feasible for rockets. But there’s another way to do it, which is to use an advanced welding technology called stir welding. Instead of riveting the ribs and hoops, you use a special machine that softens the metal on both sides of the joint without penetrating it or melting it. Unlike traditional welding, which melts and potentially compromises some metals, this process works well with high-strength aluminum alloys. You wind up with a stiffer, lighter structure than was possible before. And your material loss is maybe 10 percent, just for trimming the edges. Instead of a ratio of purchased to flown material—what they call the “buy to fly” ratio—of maybe 10 to 20, you have a ratio of 1.1, 1.2 tops.
Anderson: Let’s talk about where all this is headed. You’ve brought the cost of rocket launches down by a factor of 10. Suppose you can bring it down even more. How does that change the game? It seems like when you radically reduce the price, you can discover a whole new market. It’s a form of exploration in itself.
Anderson: What glimpses of that new market have you seen?
Musk: A huge one is satellites. There are a lot of applications for satellites that suddenly begin to make sense if the transportation costs are low: more telecommunications, more broadcast, better weather mapping, more science experiments.
Anderson: So traditional satellite markets—but more of them, and cheaper.
Musk: There’s also likely to be a lot more private spaceflight.
Anderson: By that you mean tourism.
Musk: Yeah, but I think tourism is too pejorative a word. You could argue that much of our government spaceflight has been tourism. But the main thing—the goal I still believe in for the long term—is to make life multi-planetary.
Anderson: And Dragon, the spacecraft you berthed with the ISS in May, has features that might eventually prepare it for a manned Mars mission.
Musk: Eventually, yes. The thrusters on Dragon are sized so they’ll be able to do launch escape—which means being able to move away from the rocket at a force of approximately 6 g’s. That same thrust level happens to be kind of a good number for supersonic retro-propulsion for landing on Mars.
Anderson: Could you have sent Dragon to Mars instead of the ISS?
Musk: Well, it would have gone very slowly—and when it arrived, it couldn’t have landed. It would have made a crater.
Anderson: The issue is stopping once you get there.
Musk: Version two of Dragon, which should be ready in three years, should be able to do it. But really, if humanity is to become multi-planetary, the fundamental breakthrough that needs to occur in rocketry is a rapidly and completely reusable rocket. In the absence of that, space transportation will remain two orders of magnitude more expensive than it should be.
Musk: Imagine if you had to have a new plane for every flight. Very few people would fly.
Anderson: Isn’t the fuel a huge portion of the expense?
Musk: The cost of the propellant on Falcon 9 is only about 0.3 percent of the total price. So if the vehicle costs $60 million, the propellant is maybe a couple hundred thousand dollars. That’s with rocket propellant-grade jet fuel, which is three times the cost of normal jet fuel. That’s using helium as a pressurant, which is a very expensive pressurant. A next-generation rocket could use cheaper fuel and also be fully reusable.
Anderson: Are you making an announcement right now?
Musk: I hope we might unveil an [reusable rocket] architecture for that next year. I’d like to emphasize this is an aspiration for SpaceX—I’m not saying that we will do it. But I believe it can be done. And I believe that achieving it would be on a par with what the Wright brothers did. It’s the fundamental thing that’s necessary for humanity to become a space-faring civilization. America would never have been colonized if ships weren’t reusable.
Anderson: Wasn’t the space shuttle reusable?
Musk: A lot of people think it was reusable—but the main tank was thrown away every time. Even the parts that did come back were so difficult to refurbish that the shuttle cost four times more than an expendable rocket of equivalent payload capability.
Musk: We’ve begun testing reusability with something called the Grasshopper Project, which is a Falcon 9 first stage with landing gear that can take off and land vertically.