Physics Tag

I’ve Just had an Intriguing Talk With my Pet Iguana, Rupert, About Elon Musk’s Rockets

Rupert: The whole idea is ridiculous! The very assumption that a plane is more complex than Elon Musk’s rockets. That’s just absurd and objectively not the case.

Wambua: …What!?

Rupert: Part of the reason is that Elon Musk’s rockets can not only climb up the atmosphere, but they can also come down to a predesignated spot in a desert or the ocean, without falling apart and becoming scrap metal or coral-reef anchoring, respectively. By doing so, Elon can theoretically save over 99% of the total cost of launching rockets.

Wambua: No, I mean, how come you are talking? Lizards don’t talk.

Rupert: And yet we are having a conversation. A fixed-wing plane is designed to generate lift in the easiest way possible, and then stay up in the air with the bare minimum struggle. This is because its aerodynamic profile makes it work with the atmosphere, rather than against it. Stick a plane in a wind tunnel, turn up the wind, and the plane will go up, lifted by the air itself. In fact, so aerodynamic are planes, that most can glide over 150 kilometres horizontally if their engines shut down at a 10 km altitude. For most planes, engine shut-off is rarely a fatal situation. In a sense, a fixed-wing craft is like a well-balanced kite in the wind: making it stay up in the air is easy. In fact, the most complicated part of aviation in fixed-wing planes is balancing the rolls, pitches and yaws of the body to maintain a stable flight. But staying up in the air? Easy peasy. Some things are not meant to defy sense.

Wambua: A talking iguana does. You peed in my stash of pot again, didn’t you? I’m hallucinating from inhaling your pee, aren’t I?

Rupert: A rocket, is a completely different animal. First, a rocket works against the atmosphere: the bulk of its fuel at launch is burnt just to crawl through the lower atmosphere. So, while the lower atmosphere lifts up an aeroplane, it creates a massive drag for a rocket. And the fuel needed to break through this barrier is often in the hundreds of tonnes, for even modest payloads. Secondly, while an aeroplane can glide idyllically with dead engines, a rocket with dead engines drops to the ground like dead weight. Engine shut-off in a plane is an inconvenience. In a rocket, it is either part of the flight profile or the beginning of a massive clusterfuck, that ends in a guaranteed total loss of hardware plus any payload: human or otherwise.

Wambua: Oddly, you make a lot of sense. Except for the whole talking lizard thing.

Rupert: Back to SpaceX, and the inimitable magic of landing a rocket safely back on a pad. Somewhere in the huge labs of SpaceX, a certain flight engineer once stumbled across a literal epiphany. He realized that, if the fuel for SpaceX rockets could be increased by a mere 3-4% above what was then deemed enough for a one-way flight, there theoretically would be enough fuel left in the rockets to check their descent and manoeuvre them to a safe landing, instead of the traditional splash landing in the Atlantic that used to scrap everything. So, a whole new family of tech was built from the ground up. The RCS (Reaction Control System) thrusters had to be reconfigured: given enough Delta-V to flip over the entire rocket at MECO (Main Engine Cut Off), often at flight apogee, to obtain a retrograde form. That is, the bottom part of the rocket, where the thrusters are, would be turned around to face the direction of rocket propagation, while the tip of the rocket now faced the direction from which the rocket was coming.

Wambua: Look, if we are going to effectively communicate, I have to get used to this … this whole madness. Tell me what it feels like to be a lizard.

Rupert: Feels like a human being without the cojones. So, at apogee, most SpaceX rockets are moving horizontally at about 7,000 km/hr. That’s about Mach 6. Hypersonic. When the flight path curves down, back towards the earth, this speed increases even further. Left unchecked, this increasing speed would result in irreparable damage to the rocket as it transited from the stratosphere to the troposphere. So, the on-deck computers have to fire some of the engines at the base to shave off some of the velocity. This firing is sophisticated in terms of timing and the retrograde thrust created. Too early, and the rocket exhausts its fuel, guaranteeing a crash landing. Too late, and aerodynamic buffeting tears the rocket apart. Too much thrust and the rocket not only stops mid-air but also reverses and rapidly climbs upwards, again: the fuel-thrust ratio in the Merlin-D engines is simply insane. The sweet spot hence is somewhere in between, and before SpaceX engineers got it just right, they blew quite a few rockets. The last major challenge in getting the rocket on the drone ship, downrange in the Atlantic, is twofold. First, the falling rocket and the drone ship have to rendezvous at the exact same point, or the rocket falls into the water and sinks. Grid fins and extra-articulate thrust gimbles kick in here, correcting flight path at microsecond timeframes. The tech to actuate these had to be built from the ground up. Secondly, at the last moment, just before the rocket connects with the drone ship’s surface, the rocket’s main thrusters fire for one last time, in what’s known in SpaceX as a “Hoverslam manoeuvre”, or as the “Suicide burn”. This last burn kills off all remaining downward velocities in the rocket, ensuring a soft impact with the landing legs. Done wrong, the rocket crashes on the deck, incinerating everything in sight.

Wambua: WAIT, wait. You don’t have cojones?

Rupert: I don’t. Back to rockets, it goes without saying that, before all the above manoeuvres were perfected, SpaceX engineers suffered a lot of cold sweats, and straight-up horror when any of a million variables went nuts: the weather around, the tolerance of some rocket parts giving up at the wrong time, some programming code hiccupping in the thick of things, etc. And in almost every single case of such misadventures, the cost would be the same: total, fiery loss of the rocket. Millions of dollars up in literal flames. By comparison, aviation, especially fixed-wing planes, is like a walk in the park.

Wambua: But if you don’t have cojones…

Rupert: I’m female, yeah. Stop looking at me like that, you twisted being.