Ah, interesting. No such thing as a free lunch, I guess… or is it, premature optimization is the root of all evil?
I wonder how much of the tank gas actually gets used for attitude control?
Actually to a first order estimate, I don’t think it makes any difference. To balance a space elevator, you have to have outward “centrifugal” force (yes, I know physics purists hate that term!) You can get that by a large counterweight mass not too far above GEO, or a long extension beyond GEO, or indeed any combination of these.
But the force required at the GEO point to support the cable below it (and hence the tensile stregth required) is the same in either case. So if we were able to build the elevator in the first place, we may as well extend it out to provide a path to escape velocity. Now where’s my unobtanium?
I’d like to learn more about how this is supposed to work. Clearly the Raptors vaporize the fuel and oxidant as they enter the engines. But the tanks are quite a distance from the engines. Is there some heat transport mechanism from the engines to the tanks?
To be honest, I’m not sure of the details. I believe there’s a heat exchanger near the engines and that the hot gas is piped up, but I’m by no means certain of that. Pressurization also needs to work when the engines aren’t running, so there must be some kind of backup system (electrical? separate combustion unit?), but I don’t know what’s involved there.
Incidentally, the Shuttle and SLS (and Titan 34D) use(d) autogenous pressurization (I’d forgotten about those). So it’s not without flight heritage, though it’s still the exception rather than the rule. And never used with methane, of course, nor with subcooled propellants, nor for any long-duration use (as will be needed for Artemis). So there are plenty of unknowns here.
Is it heat exchange from the engines that’s the issue, or adiabatic expansion from the rapidly draining fuels causing the temperature to drop and condense the ullage?
Unknown. Musk didn’t provide many details in the Twitter Spaces thing.
I suspect (with a low degree of confidence) that they sized the heat exchangers and other infrastructure for a certain amount of volume, which is calculated based on the known rate of propellant use, and a predicted amount of condensation, plus some other safety factor. But what is unknown until they fly is how much slosh and other dynamic effects will add to the required volume, and possibly push it past the source limits.
It’s likely that there’s a very high upper bound here, say if the liquid aerosolizes due to some vibration effect. Super high surface area → fast condensation → ullage collapse. Potentially higher than any reasonable heat exchanger could provide. So, they hope that nothing like that happens, but this is the kind of thing that’s hard to predict before flying.
I wonder why Musk said “a few months” to next shot, given that they seem to have one or two more or less already-built vehicles? Just a manner of speech… or have they already seen something in the telemetry that is going to require significant design changes?
Just to make this thread make sense, we should mention: Starship cleared the launchpad (awesome!), but failed after about two minutes of excellent flight.
As I explained to my friends on Facebook just now:
“The headlines* are misleading. This was an amazing, successful launch: a HUGE, non-government-made rocket cleared the launch pad and flew just fine for several minutes. Separation of the booster from the next stage didn’t work as planned — but after the explosion, the crowd of technicians cheered. Anything after clearing the pad was “icing on the cake.”
Could ullage sloshing have been the cause? Or simply a problem with the separation itself?
*e.g., “SpaceX rocket explodes minutes after launch”
From the main thread on this launch, I understand the likely cause was the failure of about 5 of the 33 engines, soon after liftoff —> asymmetrical thrust —> tumbling —> deliberate cancellation of the stage separation —> deliberate destruction of the whole thing, probably not long before it would have destroyed itself anyway (but in a less controlled fashion)
A few months is not that much time to find and fix everything that went south during this launch (there are certainly undesirable things that occurred besides the fatal failure). That assume the problem can be fixed with minor changes to hardware, software, or procedures.
It will probably take a few months just to redesign and fix the launchpad [lots of damage].
Well, cost to which orbit… LEO or GEO (or indeed escape)?
I think we need to sit down and do some serious math here. Just in principle, a rocket launch system is wasting a lot of energy shoving up a big first stage which does not contribute to the payload.
Though of course it’s still apples and oranges: we don’t have any material which allows a space elevator.
The entire cloud of brown obscuration that enveloped the launch pad and that Starship climbed out of just a bit after clearing the launch tower was rocks and dirt. Not combustion products from the engines.
Which debris cloud massively enveloped the rest of the launch pad infrastructure.
Seems a bit short sighted not to have paved and cleaned that area. Obviously rocket exhaust can peel up pavement too. But I’d think a more engineered surface could more safely and surely be characterized and designed against than a natural one. Even a groomed natural one.
Well, I guess they were expending an order of magnitude at least more thrust than previous tests; perhaps they just hadn’t thought about it. Perhaps it’s swept now… 
I saw a video of a car getting pounded by the flying rocks. Seems like a few bad decisions leading to people and equipment being in dangerous areas.
Oh, PLEASE! Cue whines about how “dangerous it is and shouldn’t be allowed”, plus the usual rubbish about “spending all than money on space when there are so many problems on Earth”.
Nobody was forced to be close to the launch site.
It was paved with a significant amount of reinforced, high-temperature concrete. That proved insufficient, and it blasted through all that and into the compacted dirt below. It probably wouldn’t have been so bad if it was concrete all the way down. But once it got to the relatively soft dirt, suddenly it started displacing 10x the debris that it otherwise would have.
The area immediately near and under the OLM was of course not bare dirt; that’d have been silly.
But from the aerial footage taken pre-launch, it seemed obvious there were large expanses of bare dirt to within a couple hundred feet of the vehicle centerline. Maybe that stuff was real hard and groomed, or maybe it was just dirt / sand. But whatever it was, post ignition it was being moved around a lot. And seemingly not just by chunky debris blown from under the OLM; it appeared to me to be fast gas impingement.
It’ll be interesting to see some good pix of the whole pad area and any damaged equipment.
Yes, it may be time to rethink the lack of a deflective setup to direct the rocket exhaust at an angle - plus it’s time to surround the perimeter with a deflection berm or something?
Ok, fair enough. Still–I’m pretty sure the majority of the debris is from the massive hole the exhaust excavated below the OLM. That has to be hundreds of cubic meters. Some ungroomed rocks and stuff from farther out probably made little difference in comparison. Though, granted, maybe enough that it could cause problems even without the rest of it.