I started to write something about subcooling not likely to be used in orbit, but then realized the whole situation is more complicated.
As mentioned, subcooling is cooling below the boiling point. But the boiling point depends on the pressure! And the pressure in space can be almost whatever you want, since it depends only on the pressure of the tanks.
So while I highly doubt that subcooling will be used outside of Earth in the strictest sense, ultimately the absolute temperature is more important than how close it is to the boiling point, since it’s the propellant density that you care about. So I think they’ll try to keep the temperatures low, on par with the subcooled temperatures on Earth.
In addition to ambient temperatures, Earth has the additional complication that the tanks have to be kept at a relatively high pressure for structural reasons. But that’s not so important for an orbital depot (once in orbit, that is), or hanging out on the Moon. So they can keep the temperatures low by keeping the tank pressure low (maybe only increasing it when it’s time to launch).
Actually achieving subcooling on Earth is rather tricky, using nitrogen gas that they pull a vacuum on to lower the temperature (venting the excess) and then having a heat exchanger to cool the propellant. Further, zero g makes all of this way more complicated–for example, the lack of convection makes it more likely to have hot/cold spots and possible ice formation. I don’t think they’ll use this in space. But a cryocooler for the depot and using pressure to adjust the boiling point is reasonable.
SpaceX does use subcooling for both Ship and Booster on Earth. They seem to have slightly more generous time allowances than with Falcon 9, possibly due to a combination of larger size (less surface area for the volume), less thermal conductivity for stainless steel, and less critical performance requirements, at least for now. But it’s still a matter of minutes, not hours.
Oxygen boiling point at atmospheric pressure being close to 90K.
(I haven’t found similar data for methane, but I’d expect a somewhat similar order of magnitude)?
We see that the density doesn’t change that rapidly at cooler temperatures (true of most liquids), but the vapor pressure drops dramatically.
Do we know by how many degrees they subcool?
If we assume 20 degrees, that’s about an 8% improvement in density.
Of course that’s not going to translate to an 8% improvement in payload since while they are packing in a bit more propellent they are also increasing the vehicle mass by much the same amount, since the fuel and oxidizer is the bulk of the vehicle mass.
8% increased density might not sound like a lot, but considering that the dry weight of the upper stage tankage also has to be covered in heat shielding any improvement can mean a lot. In fact if anything Starship is still not big enough to have a favorable ratio of fuel/oxidizer weight to tankage size; hence tentative plans that if Starship works out then it may someday be replaced by a ginormous 18-meter diameter version.
Thanks. I’m not actually seeing a graph or table for liquid methane density vs temperature there though? It may be available in the Rubber Company Handbook (if that still exists? I’ll have to look): I assume measurements have been made.
Aside: I wish people would use Kelvin rather than Celsius for this sort of data
I am surprised that Mr. Musk still has time for this stuff. And considering his approach to administration, respect of rules, rational planning and so on I wonder whether this can end well. I understand being fascinated by space, rockets and Mars, but are you sure your optimism is justified?
As others have remarked, although Musk founded SpaceX and as CEO ultimately dictates the goals of the Starship program, day-to-day control is in the hands of Gwynne Shotwell.
It’s especially notable when comparing the relative difficulty of storing liquid methane, oxygen, and hydrogen. Rather than the eye glancing over negative blah blah numbers, the difference becomes striking: hydrogen is a LOT harder to liquify than methane or oxygen.
Methane: 111.66 K
Oxygen: 90.188 K
Hydrogen: 20.271 K
So it is actually good news that he is distracted elsewhere and cannot screw up SpaceX for lack of time? OK, I grok that. Thanks for the info!
I think it will be better if I switch this thread back from Watching to Tracking.
Ruh-roh! Ship is spinning out of control! Video feed hasn’t cut out, though.
ETA: Well, the Ship Version 2 is now 0 for 2, while the Booster catch is 3 for 4. At least they’re getting good catch practice in.
Second mission of the day (the other one being the Intuitive Machines moon landing) that’s like half successful, but failed in a very similar way to the first failure.
Damn, it’s discouraging when something that never went wrong on any previous flight suddenly leads to a lost rocket. How many more times is this going to happen? At this rate we’re going to be waiting a long time before we ever see a tower catch of an upper stage.
Remember that Ship ver 2 is a very different design, despite looking similar (many changes on the inside). Not quite as much of a step back as it seems… just not a step forward.
Looks like one vacuum engine was lost, and then the center engines shut down quickly thereafter. Maybe some common cause, or maybe the failure propagated somehow. I’m sure we’ll find out soon. If it’s a propagating failure, they’ll definitely want to put in some resiliency measures. They should be able to handle the loss of one engine.
This little burny part on the nozzle base doesn’t look great:
But there’s something I don’t understand. In the pic above, it looks like there’s a gap in the nozzle (you can see blue behind it). But it also looks like there’s a gap on the other visible RVac:
So maybe there’s supposed to be a gap? Probably not a fire, though. There also seems to be some kind of thin attached to the side with a pipe going up. Not sure what all that’s about.
Funny how the guy is simultaneously responsible for all of SpaceX’s failures, while not responsible for any of their successes. Anyway, the footage is very cool.
I ran across a nice pic of the tail end:
You can see the pipe and manifold thing on the side of each Raptor Vac nozzle (pointing inward). At least one of these, and possibly two, were somehow the origin of a gap in the end of the nozzle. Don’t really know what to make of this.
They did put the Ship through some extensive static firing. That can be especially hard on vacuum engines due to flow separation–which might be concentrated near the end. Maybe all that static firing caused stress cracks or the like?
Another thing that is a bad problem is if you’re flying up to space and the parts start to fall off your space car in the wrong order. If that happens, it means you won’t go to space today, or maybe ever.
The thing about SpaceX that a lot of folks don’t get is that their failures are their successes. It takes a lot of failures to learn how to do something right.
