SpaceX vs BO re: Rocket Landing

I already left a bad doodoo in Verge’s post initially reporting the BO landing because they were really gung ho about that comparison. They have since posted a follow-up. And it’s still “wrong” in spirit. Let’s just get the facts here.

  1. SpaceX Falcon 9 re-useable test Stage 1 booster is the only thing they have tried to land, and is the “rocket” in question when the press talks about SpaceX’s landing experiment. It has 23-26 metric ton empty weight as guestimated. Maximum velocity the stage 1 booster achieves is estimated at Mach 10 and height of 97Km. I don’t know if there is official data but this sounds like what Falcon 9 v1.0 does rather than 1.1, which features a slower first stage (Mach 6) because the new Marlin engine can make up the difference. Dunno.
  2. Blue Origin’s New Shepard is a single ship. The whole rocket goes up and then comes back down. It reaches Mach ~3 on the ascent (every rocket that goes to space gets this fast basically), up to 100.5Km during her historic flight. No data on weight, but presumably much less than the Falcon 9 simply because it holds just a fraction of the fuel capacity.

Mach 10 versus 0 mph is where the focus is. Falcon 9’s booster is at Mach 10 at its highest point of ascent; the New Shepard at ~0 mph at its highest point of ascent. It doesn’t really matter if the stage 1 of Falcon 9 went only 5Km or 150Km or 200Km in altitude, the challenge is stopping, not dealing with the fall. And that’s kind of the point of space travel that the mass public don’t get. We understand both instinctively and in the mind what it means to fall down from a high place. On the other hand, we have no worldly or innate understanding of what it means to be going at Mach 10. Or Mach 30, which is what it takes to get to the Lower Earth Orbit. Or that it’s not that you go into space you stay there; it’s because you are orbiting earth at Mach 30 that you can stay up there. Going to space is what lets us to go as fast as Mach 30, as you’ll turn into hot ash if you tried going at that speed at 1Km altitude (Not to mention you have to go even faster if your orbit is closer to Earth too).

Yeah, the SpaceX Grasshopper already achieved the same thing New Shepard did albeit only at 0.25Km, so maybe it’s okay to knock it, but to rocket scientists there’s not that much of a difference between 0.25Km and 25Km or 100Km, versus a few hundred miles an hour and Mach 6 or Mach 10, which is a challenge in an entirely different class. Because it’s not the altitude that matters–free falling from 100Km only gets you to go so fast, thanks to the atmosphere capping your freefall velocity. Let’s just say you don’t even hit Mach 1 for this reason.

It’s the speed that you have to go from, to stop, that matters. In both tests the goal is to slow the rocket to relative speed of ground to as low as possible. Did y’all watch the Martian? Remember the final climatic scene? It’s about relative velocity, relative velocity, relative velocity.

And I’m not even going to get into what the Falcon 9 has to deal with–orientation, construction, weight, accuracy. It has to face a lot more challenges to do the “same thing.” Because, well, they are really different things, with different challenges, to accomplish different missions.

Take home: just read the Wired. Or remember middle school science class.