sevenperforce

Elon explains the naming scheme for Starship and Super Heavy: So the vehicle is the Starship and it uses a booster sometimes. The booster is called a Super Heavy. Got it.

Elon said that it was burning off a little copper: Dunno where from. The combustion chamber is copper for the sake of thermal conductivity, so there was probably a little brazing inside that got blown out the nozzle. Nothing to worry about.

Short sequel. "Then the worlds became as refuse, and the sky as flame, and everyone died horribly. The end."

If they are both co-orbiting a dark matter "chunk" at those distances, then you end up with a chaotic three-body problem with no stable solution and the whole thing goes to hell.

Does Raptor use film cooling, regenerative cooling, or both? If not film cooling, that could be outlet flow separation due to the pressure differential. I suspect this nozzle is lip-optimized a la SSME.

Then the planets are pulled together and collide. You also run back into the Roche limit issues from before.

Holy crap. Guys guys guys. 257 bar. 257 bar. The RD-181 pulls 262.6 bar. The RD-171M pulls 250 bar. The RS-25 pulls 206.4 bar. This particular Raptor has achieved the second-highest chamber pressure of ANY rocket engine EVER. It is 2.2% away from being the highest-chamber-pressure engine of all time. Ahem. If the Starhopper design requires 170 metric tonnes (not tons, Elon!) and they have three engines, then the expected GLOW of Starhopper is 510 tonnes. On Jan 23, I predicted a propellant mass upper bound (based on pixel tracing) of 531 tonnes, so the assumptions were generous. Suppose Starhopper dries at 75 tonnes (which is ridiculously conservative). That gives 435 tonnes of props. Let's put Isp at 310 seconds to be more conservative. A realistic estimate of dV is 5.8 km/s. Hot damn. Longer than a few seconds, shorter than a mission-representative burn. It gets very warm after a few moments.

It's math. Given a proposed safety feature: if Σ(PF,PA)*PS > ΣPL (where PF is probability of vehicle failure, PA is probability of vehicle accident, PS is probability of the safety feature working properly, and ΣPL is the sum of probabilities of ways the safety feature could accidentally kill you), then you should have the safety feature. If not, then don't. For the safety feature of ejection seats on a jet fighter, it is very clear that Σ(PF,PA)*PS is greater than ΣPL, even though ΣPL is most definitely greater than zero. For an attack helicopter, they are pretty nearly equal. For a Chinook they are equal or worse; for a commercial airliner Σ(PF,PA)*PS is demonstrably less than ΣPL. The same math works for seat belts on various craft. For a passenger vehicle, Σ(PF,PA)*PS > ΣPL. For a speedboat, Σ(PF,PA)*PS = ΣPL. For a 4-wheeler, Σ(PF,PA)*PS < ΣPL. Then why don't commercial cargo planes have ejection seats? The problem is still that ΣPL can be shockingly large. Consider a full-envelope capsule-cabin LES with independent abort engines and heat shield. That's all very well and good...but you now have to sum a bunch of new probabilities, like "chance that new structural problems will case increased entry failure rate" and "chance that abort fuel will leak on orbit" and "chance of unintended fire during blackout trajectory" and so forth. That's because the STS used ginormous fire-belching tubes of bomb strapped on either side. Ejection seats on the Starship would have far less hazard to contend with and could be used all the way through MaxQ (on the way up) or for a landing failure (whether at engine ignition or at touchdown). The Starship can handle virtually any Superheavy failure.

Avionics!!

These deaths absolutely count. An installed, operational LES malfunctioned and caused a LOV that killed three people. No, it was not a nominal event, but LES is for off-nominal events. That is the point. Analogy to ejection seats is a good one. Ejection seats have malfunctioned and killed people before. It's not a high likelihood, so people still use them, but it has happened. But adding ejection seats to the cockpit of a Boeing 747 would make it objectively less safe, because the odds of ejection seat malfunction/accidental trigger are almost certainly higher than the odds of needing to eject from a commercial airliner. I would be a fan of ejection seats on Starship, at the very least. But there may be a point at which they make things less safe, not more safe, and at that point I would no longer be a fan.

It could be like a river. A space river. Spiraling flows or whatnot.

The gravipotential field would have a saddle point at the barycentre, so it is in unstable equilibrium at every point. I suppose that you COULD have some sort of wacky suborbital oceanic crossflow in conjunction with orbital eccentricity or something, but there's no way it would naturally be stable. Could possibly be constructed by a Kardashev-1.5 civilization.

Once tidal locking takes place, you'd have few sources of major drag, but you'd still have relativistic frame-dragging that would pull them together. We'd have to know the parameters in order to know what the timeline would look like. They could potentially have a shared exosphere, if it was indeed tidally locked. Depends on a LOT of factors. You could certainly have places where going world-to-world was easier than going point-to-point. The unknowns are huge. How large of worlds are we talking about? Does gravity need to be Earthlike? The lighter the worlds are, the easier it is for the whole thing to work. I had sketched out an idea for fiction like this...Earthlike gravity, 24-hour rotation period, daily eclipses. I had the magnetic fields of the two worlds coupled, so that depending on geomagnetic activity, you could occasionally have maglev "portals" open up where ferromagnetic material would become extraordinarily lightweight, enabling pre-rocket travel.

A hatch and personal chutes pose no threat. But once you talk about an entire cabin being peeled free of the integrated launch vehicle, things start to get messy. Do you use pyro bolts, frangible bolts, or pneumatics? They are, after all, different tech. Pyros have a nonzero chance of going off prematurely; frangibles have a chance of not separating at all. Pneumatics can go either way. RCS systems usually use COPVs, and I seem to recall a certain COPV-induced LOV event. Any new volatile system adds a new failure mode. Again, not denying the utility of LES on most vehicles, but this particular claim is simply untrue. During Soyuz 7K-OK No.1, the LES fired without warning nearly half am hour after a launch abort, igniting the second stage and killing a ground worker. Not nominal, I grant, but that's kind of the point. Seat belts have malfunctioned and killed people, and LES has malfunctioned and killed people. Not a reason to eschew any of the above out of hand, but it's still a valid question. At some point the math says no. Which is an easily-solved problem. The Ka-50 has a full-envelope 0-0 ejection system and it's been flying operationally for two and a half decades. Use pyros to blow the rotor, allowing rotor inertia to shred the blades and fling them in all directions, then kick the canopy up to deflect any debris before ejecting normally. Works like a charm. Of course, the risk lies in adding another system with new things to go wrong. Which is why the Chinook doesn't have it.

WAAAAAAAAAAAAAAY worse. Also, forget orbiting.

As I said before, fighter jets are not necessarily the best analogue here, for several reasons. That being said, there are still some points to be made. If my math is right, most non-combat jet ejections take place during air shows, which are NOT what the vehicle was designed for (and in which ejections are more likely because the vehicle is operating closer to the ground than typical, giving less time for correction). And ejections have killed pilots by mistake. RAF fighter pilot Sean Cunningham was killed in 2011 when his (poorly-installed) ejection seat handle was jostled and he was launched from the cockpit during preflight checks. That's not to say that ejection seats are a bad idea; they objectively save more lives than they take. But that's math. The equation (vehicle risk vs ejection system risk) is the same for an F-22 as it is for a Chinook; it's just a different solution. Contrast should be drawn between Starship and STS. The Shuttle would have been safer with full-envelope LES, but did not have the mass budget for it. Starship has the mass budget for full-envelope LES (seriously, it can carry multiple Dragons to orbit easily) but that would most likely make it LESS safe, not more safe. And this is where indefinite reuse comes in handy, because you can launch the same vehicle dozens of times at a fraction of conventional LV cost in order to prove complete reliability. That's where those orders of magnitude evaporate. The Space Shuttle had ample redundancy in a handful of systems, but zero redundancy in the systems that really mattered. Oh, I have low confidence in an actively-cooled transpiration heat shield. But we can test it. Once it tests, then we can talk. Elon recently said that the Starship will now use cold-gas thrusters for simplicity rather than meth-oxy RCS thrusters. If not for this, I would almost say "what if we put head-pressure tanks and RCS thrusters in the crew compartment...operational during a nominal mission, but always full so enough to pull it free during aborted launch/landing and enough for separation and pointing in an orbital RUD situation?" The problem is that if you make the entire crew compartment its own capsule, then at some point it just makes more sense to re-enter separately, which means you have a three-stage vehicle rather than a two-stage vehicle. And then complexity overwhelms safety. No argument on use case. I desperately want one, but...yeah. I'm talking more about the vehicle itself. Ejection seats (and all the pyros required to make them work) on a Chinook introduces more failure modes than the likelihood of needing the ejection seats; the same is true for an operational Starship. I think we all systematically overestimate the cost-per-launch with a fully-reusable LV.

I think the argument is more -- when Starship/Superheavy starts flying people, it will have enough of a reliability record to justify it. If it does not have a reliability record, it will not fly people. This is a conflation of reliability and accident risk. Fighter jets operate in an envelope where the risk of accident is far far greater than the reliability of the vehicle itself. Most fighter jet ejection scenarios happen because of operator error and/or pushing a vehicle beyond its limits, not because of a design or vehicle component failure. A more apt description would be a CH-47 troop transport. Are accidents possible? Yes, they are. However, any ejection seat mechanism capable of safely ejecting the pilots and passengers would prevent the Chinook from doing its job and introduce greater risk than benefit, and the Chinook is not typically pushed into a regime beyond its design limits. Starship and Superheavy are designed to be able to sustain the failure of any single major system and still recover. Engine-out? No worries, just push the other engines harder. Tank breach? No problem, just abort using the header tanks. Hydraulics lockup? Abort and then use engine gimbal to recover and land. Superheavy RUD? The Starship boosts itself to safety. That's not to say that I would want to fly without an LES right away, either. But it's an issue of math. If the risk of your LES system malfunctioning is greater than the chance of needing it, then you don't want it.

A pair of binary planets, mutually tidally locked, could be pretty darn close together. The Roche limit is good for looking at a very massive primary and a less massive secondary, but for the case where both bodies are equal in size, the variables cancel and the Roche limit d becomes constant at 1.26 planetary radii, which would have the bodies touching already. So the Roche approach is not useful. The limiting factor in this case is going to be a function of rotational period and centrifugal force. The closer the two bodies are, the greater their rotational rate must be in order to maintain orbit around a common barycentre. At some point, the rotational rate is going to be so immense that the ocean on the opposite sides of the worlds will get flung off into space like a slingshot.

Apart from the transpiration heat shield, the actual framework of the vehicle is ridiculously simple. They're packing all the complicated bits into the powerplant and letting autogenous pressurization and sheet metal take care of the rest. So if you need to repair on Mars or on LEO, NBD. Going pressure-fed would actually be MORE challenging. Harder to transfer propellant, harder to refuel, harder to repair if something goes bad. While I only agree to a degree, I think this definitely expresses the SpaceX view. They can test and retest and re-retest the same hardware over and over, and "flight-proven" launch vehicles will ultimately end up being considered more reliable than anything off the shelf. I mean, technically that's already true. SpaceX has reused launch vehicles 18 times with a 100% success rate. But yeah, I wager that people will never fly on a brand new Starship. Every vehicle needs to go "there and back again" once, to show everything is working properly. This is the part that people miss. At some point, the possibility of RUD falls so low that the possibility of being killed by your escape system is high by comparison, and then LES no longer makes any sense. Would I like a parachute if I'm flying on a brand new plane for the first time? Yes, I would...but not if it contains a grenade with the pin millimeters away from popping out unexpectedly.

Unless there was a problem. What is the rationale on preferring BE-4 over Raptor? BE-4 has a little more absolute thrust, but it has lower TWR, much lower thrust/area, and lower chamber pressure. Or is the lower chamber pressure why it would be preferred?

The problem is that pressure-feeding is not necessarily square-cube agnostic. When you figure in issues like buckling moments, etc., you can very easily run into a scenario where the tank dry mass grows faster than the square. Exactly. Don't let the "sheet-metal" rocket fool you. The Raptor engine is Elon's real "heart of gold" here; everything else is just window dressing.

No joke. I told my kids today that the engine is "this big" (arms out) and is powerful enough to pick up our house. Then I did the math. It actually is powerful enough to pick up our house. What the hell did they screw that test stand to?

ACES would conceivably be a major enabler in additional rendezvous options with its ICE and independent maneuvering capabilities.

Starship+Superheavy is so massively overpowered for any typical payload that they will never need the extra margin of downrange recovery.

It is definitely not dual bell; it would be twice as large if it was. Looks more like an altitude-compensating concavity nozzle a la SSME. Compare to https://airandspace.si.edu/sites/default/files/styles/slideshow_xlg/public/images/collection-objects/record-images/A20040205000CP06.jpg (Explanation: the bell expands wider than a SL engine close to the throat, leading to over-expansion at SL, but then tapers back at the edge so that the pressure at the rim is equal to atmospheric. Leads to low pressure (vacuum-optimized) at the center of the exhaust flow and high pressure (SL-optimized) at the edge of the exhaust flow. Not as much altitude-compensation as an aerospike, but not nearly as heavy or complex, either.)