Reusable rockets were partly a “bits” not “atoms” problem

The excellent Casey Handmer says you should be working on hardware. I mostly agree. Especially in biology, for example, we’re often limited by the unknown physical configuration of the system. That’s not something that can be solved by reasoning (software) alone. Often you have to deal directly with the physical world, to do what’s important. 

But progress driven by computing is not restricted to the virtual world. Consider SpaceX. Computers have given us better computer aided design (CAD) tools, and 3D printing, or more generally, computer aided machining (CAM) tools. These make it easier to sculpt materials, including metals, into a variety of shapes that would not be easy for a machinist of yore to make. It is thus easier than ever to design a rocket nozzle, or a whole rocket. Fast electronics and algorithms, meanwhile, let us control that rocket. But is this the difference that matters? Let’s look at what actually played out in the development of reusable rockets, building on a great explainer by Lars Blackmore. 

Since the 1950s, it had been clear that the cost of bringing objects (be they satellites or ships destined for Mars) into orbit would be greatly reduced if rockets were made reusable. If you had to build a new plane every time you wanted to take a flight, the cost of air travel would be enormous. Indeed, though we see huge fuel tanks taking up most of the mass of rockets, the cost of the fuel itself is not the dominant one. The largest factor that drives the cost is that we have to build or substantially rebuild our rocket each time we want to launch it.

In the late 1950s, Boeing and the Air Force collaborated on a project for a reusable space plane, aptly named the X-20 Dyna-Soar. This project in turn inspired the design of the Space Shuttle, which ran from 1981 to 2011. Alas, these approaches glide a human-piloted plane safely back to a landing site, but throw away the giant rocket boosters that put them into orbit in the first place. Such space planes are thus not in fact fully reusable, and they did not end up dramatically decreasing the cost of getting objects into orbit. 

In 2011, the same year the Space Shuttle program ended, the private company SpaceX announced its reusable rocket program, and showed successful landing and recovery, and later reuse, of its first stage booster around 2015. What enabled SpaceX to do what previous generations of NASA, Boeing and Air Force engineers could not?  Partly it was the agility of startups in general and the leadership of SpaceX in particular, of course, but it turns out that microcomputers and advanced programming were one of SpaceX’s secret sauces as well, enabling the reusable rocket. 

Consider what a reusable rocket booster has to do. Imagine it has just gone up, and floated around for a while, being buffeted by winds and atmospheric turbulence, and facing the consequences of inevitable errors in the timing, direction or strength of thrust since leaving the landing site. Now it needs to make its way back. Such a rocket is moving extremely fast, and subject to enormous friction by the atmosphere, so any small errors in direction, differences in wind resistance on one side or the other, and so on, can add up to huge deviations in where it ends up. Getting it back to a landing pad in the first place, let alone placing it back there at a low speed where it doesn’t hit the ground in a smashing explosion, not to mention landing right side up, is no small challenge. How did computers make the difference? 

Let’s return to our rocket. To start, it would be very helpful to know where it is located, up there. That’s possible (on the Earth, but not on Mars yet) because of the global positioning system, GPS. At any given time, the rocket has to calculate the series of vertical and sideways thrusts and tilts that it needs to make, to get back home to the landing pad. But if it has mis-estimated any of those parameters, quickly it will get off track. It can’t just blindly apply a fixed series of thrusts. It must frequently recalculate a new optimal series of actions to get back home, given what actually happened with its previous thrusts, and how conditions may have changed in the meantime. To do that quickly enough, it can’t rely on a supercomputer on the ground, it has to do those calculations on the rocket. To make that possible, SpaceX uses software created at the Stanford University lab of Stephen Boyd, which allows the rocket itself to rapidly re-compile, given new conditions, the fast code it will need to run to calculate the optimal path home at any given time, taking advantage of mathematical methods called “online convex optimization”. Fast computers, GPS (which depends on satellite communications, and on fast calculations, as well), and fancy programming tricks, therefore really did make full rocket reusability possible, in turn dramatically dropping the cost of getting physical material into orbit. 

That’s an example of how progress in computers can enable a non-linear leap in progress in the physical world. Now, SpaceX can deploy space based satellite internet at low cost and potentially dominate major aspects of the global telecommunications market, a move which may provide them with enough funding to make a serious go at Mars missions and even the creation of Mars colonies. Agile startup business models, digital markets, telecommunications, fast microchips and fancy computer science all were crucial in the SpaceX story so far.

See also:

A paper by Lars Blackmore of spacex on soft landing. Gives insight into the control logic used for soft landing.
byu/Paradoxical_Human inspacex