Commercial Rockets

DRAGON 2 — USA, 2020

The Dragon is, broadly speaking, a traditional capsule with a conical top and a heat shield on the bottom. The side taper is mild, only 15 degrees, making it rather blunt at the top. This gives it a tall profile, which makes sense because it incorporates into its interior a lot of parts which other capsules relegate to an attached service module, so they can be reused.

Relative to SpaceX’s original uncrewed Dragon, it has one major enhancement besides the passenger accommodations: a set of “SuperDraco” rockets around the base, firing at an angle over the rim of the heat shield. (Regular “Draco” is the name of the small thruster motors dotted around the capsule for maneuvering.) A SuperDraco is actually more powerful than the cheap non-reusable Kestrel engine they used for the second stage of their early Falcon 1 rocket, and the Dragon has eight of them. These would function as an escape system in the event of a launch mishap, and in the original plan, would allow the capsule to soft-land on pavement at any desired location, on hydraulic legs which pop out of the heat shield, thereby saving all the costs of having to send out a recovery team to pick it up. They burn hypergolics (monomethylhydrazine), which is unfortunate because this means that after landing, you have to wait a while before opening the door or approaching the capsule from outside, due to the toxicity. (Someone commented that “Hazmat suits might be standard attire for ground crew.”) They are pumpless, requiring the fuel to be pressurized with helium to about 70 bar. The parts are made from the same inconel alloy that most rocket engines use, but are 3D-printed — a technique which is becoming a trend in the industry.

With these built-in engines, it could in theory land without needing parachutes at all, keeping them onboard only as a backup. But NASA thought this sounded too risky for live passengers, and gradually others agreed, and the propulsive-landing idea was shelved. They are omitting the hydraulic legs, so the heat shield will be seamless, and now land it with an old-school splashdown into water, despite the inconvenience. Unfortunately a water landing means that the capsule needs more refurbishment than would be necessary if it landed dry, because the salt water might damage many parts, though they do have covers over the SuperDraco nozzles. But a rinse with seawater at least might help mitigate the issues with the toxicity of residual hypergolic propellant... though even after a dunk in the drink, the first astronauts to splash down in a Dragon had to wait extra time inside the capsule because the recovery crew detected a whiff of leaking N2O2 when the capsule was hoisted onto the boat. (The astronauts described riding in the Dragon as smoother than the Shuttle on ascent but rougher on descent, with the worst bit being the opening of the chutes.)

The refurbishment already includes work on the exterior. After a splashdown, the whole exterior looks scorched, and even the windows appear smoked and rather opaque. They don’t bother to put a fresh white coat on their used boosters, or even wash much of the soot off as far as I’ve ever seen, but they do make the crew capsules look shiny and fresh each time. How deeply into the protective coating that process goes, I have no idea, but it looks like it might be significant.

This disappointing increase in the refurbishment cost may be one reason why SpaceX is now eager to leave the Dragon design behind, and switch to an entirely reusable spacecraft which can land on pavement, or ideally, land directly into a perfectly fitted cradle. (That was, for a while, their stated plan for the super-heavy booster: to have it land right back into its own launchpad... but that sounds pretty risky: a mishap could put the whole launch tower out of commission. For a while they dropped that idea, but now it’s back: they want the tower to catch the booster by its grid fins, so it needs no legs.)

Even with the popout feet omitted, having these motors built-in gives one key advantage: because they aren’t jettisoned halfway up like a traditional launch escape tower, they can be used for an emergency abort at any time, bringing the crew safely back from any point in the flight, launchpad to orbit. They also weigh a lot less than an external tower. This is consistent with SpaceX’s general philosophy of putting into the reusable craft a lot of stuff which would ordinarily go into an expendable part. But this approach bit them pretty hard when the first Dragon 2 to visit the station was undergoing a static-fire test of its Draco and SuperDraco thrusters after being fished out of the water, as the entire capsule was destroyed in an explosion. This is the third time a SpaceX rocket has exploded; the other two were both due to failures of internal pressure vessels. This one, it turns out, was due to corrosive hypergolic oxidizer leaking through a valve, then being shot out at high speed when the engine was starting up. The fix is to add single-use burstable seals, which means that the SuperDraco engine may now be limited in its ability to shut off safely with fuel still in the tanks. With this fix, they were able to complete extensive retesting without incident, and fly live astronauts.

SpaceX hoped to get about ten flights out of a Dragon 2 capsule before it would need heavy refurbishing. What its lifespan might be with refurbishment is unstated — perhaps they won’t know until they wear some out. But if the water landing refurbishment cost is as substantial as some fear, there may be no point in keeping them in service after a single-digit number of uses. After SpaceX decided to put all of its effort into the Starship and have no further ambition for the Dragon 2 beyond bridging the transition, they dropped the number of expected flights per capsule to five. NASA’s plan was that refurbished Dragons would be used only for launching cargo, not crew, but once it started actually carrying astronauts, they relented and said a reflown one could be used by astronauts. (They said this much earlier for Starliner despite its poorer testing record.) The cargo flights will now be handled by separate capsules that do not have seats or SuperDracos. There are so many unneeded parts stripped out that its cargo capacity ends up being about double that of the first-gen Dragon.

Maybe there could be a compromise, like using the superdracos to splash down into a pool of fresh water when there are no people onboard. That would save some corrosion and some ocean travel. Or, as Musk has loosely speculated, perhaps it could land in the big shipborne net that they try to catch fairings with. But I doubt any of these will happen, because SpaceX is not interested anymore.

Until re-entry, it has a rudimentary service module on the back, which SpaceX calls a “trunk”. This is not much more than an interstage — a carbon fiber tube that you can look straight through when it’s detached — but it has solar panels and heat radiators, and can carry 14 cubic meters of unpressurized cargo tied to the inner walls. It has no propulsion. But the difference between a trunk and a true service module isn’t just rocket engines; a typical service module also contains a lot of tanks and batteries and other equipment which does not need to share breathable air with the crew. In the Dragon, this stuff is put under the floor, above the heat shield. The amount of stuff which gets thrown away with the trunk is kept to a minimum. For missions beyond low orbit, they have talked about a plan to build a stretched trunk with added capacity... though I don’t think they have any existing way yet to connect plumbing into it for extra air and water, which would be the main reason you’d want such a thing. The only fluid connection to the current trunk is for coolant. At this point, they are no longer pursuing any interplanetary expansion of the Dragon’s role, leaving that to the Starship.

The trunk has fins on it. If the launch escape rockets have to be used, the trunk stays attached in order to keep the nose of the craft stably pointed forward. In the atmosphere, once the trunk is jettisoned, the capsule tends to promptly swing around so it’s flying heat shield first. (All traditional space capsules behave this way, for safety during reentry.)

In the capsule, the available interior space for passengers is 9.3 cubic meters. (The Apollo command module had less than 6, with some of it inaccessible.) With a Falcon 9 under it, the cargo and passengers for a space station flight can weigh up to 3.3 metric tons. The limiting factor is probably the thrust of the escape rockets; for cargo flights I suspect they could lift a lot more if there was any need to. For returning to the ground, the limit is 2.5 tons, and it will probably be rare to bring back even half of that. ISS garbage can be put into the trunk to burn up, with a limit of 0.8 tons. The capsule’s dry mass is given as just 6.4 tons; with fuel for the Dracos it’s at least eight tons, and all the minor supplies bring it up to 8.9... but that doesn’t include the trunk, which pushes it to 12.5 tons at launch. That’s a good low number for something this roomy.

In passenger mode, it can seat up to seven, but space station flights will only seat two to four, which means that the back row of seats will be omitted to make room for cargo. For now, all NASA flights will use this setup. (There will always be at least four seats so the capsule can be used as an emergency lifeboat. There are always at least two crew capsules attached to the space station, so everyone will have a seat.) This is the first crew capsule to ever make a sales point of being designed for passenger comfort. Compared to most spacecraft, the interior looks as slick and shiny as an Apple store. Even the space suits that the passengers would wear for safety are designed for looking slick as well as for functionality. They are so tailored that I suspect there are compromises in freedom of movement.

The original design had five oval windows in the sides, including one in the door, but NASA allowed only two to stay. There’s also a little window in the docking hatch. This is covered during launch and reentry by a hinged fairing dome. They wanted five windows so they could use the Dragon for orbital tourism flights, and the plan now is to replace the docking hatch with a glass dome for wide views. The hatch would then be reinstalled for space station flights. The second flight of the Resilience, which was only the fourth crewed flight overall after two famous firsts by the Endeavour, was the first such tourist flight. So SpaceX put paying tourists into orbit just a couple of months after Virgin Galactic and Blue Origin managed their first brief suborbital rides. Then they added a third craft to the fleet, the Endurance, and a fourth as yet unnamed is coming soon, as there is no shortage of demand.

They’d said that once they got crewed flights going, they would put one of these atop a Falcon Heavy and take a pair of unnamed billionaires on a flight around the moon! That would be quite a showoff stunt, as well as proving that space tourism might sometimes be able to make real money. But then they dropped the idea because of the expense of getting the Heavy rated for human flight, when they hope to obsolete it in a few years anyway. They then said they’d put paying moon passengers in a Starship instead. A Japanese fashion billionaire paid for the flight, and apparently this is the same guy who had signed up for the original offer to go there in the Dragon. He recruited artists to come along. Anyway, yes, the Dragon 2 was designed to operate well beyond low Earth orbit. And speaking of stunts, SpaceX had also announced plans to land an uncrewed “Red Dragon” on Mars, using the SuperDracos with extra fuel tanks, as a preparatory test flight for their Interplanetary Transport System plans. But they dropped this idea too, for essentially the same reason: they want to put all their ambition into the Starship instead of into their older product lines. Since the Starship is large and light enough to do plenty of aerobraking in the thin Martian atmosphere, they felt that it would not be a useful learning exercise to attempt a more or less purely propulsive landing on Mars, as the Red Dragon would have done.

I would guess we won’t be seeing the stretched trunk either. Musk really doesn’t want to invest any more into anything other than the Starship. I’m not convinced that this is wise. I suspect there are still going to be lots of cases where a little Dragon makes a lot more sense than an airliner-sized Starship.