RCgothic

This keeps getting repeated, but actually hydrolox is not necessarily the best choice for probes to high C3. The high ISP gets handicapped by poor mass fraction. Leaving aside the impossibilities, if Falcon Upper Stage were to be replaced with a notional 116t wet RL10 (462s ISP) stage with similar mass fraction to Centaur III, Falcon's maximum C3 would actually reduce by 290m/s, and that's discounting the likely less efficient trajectory due to lower thrust. For most probes that probably doesn't make too much difference, and the higher payloads to lesser C3 from hydrolox would probably be generally more useful. Probes to high C3 are a rare use case. But an equivalent methalox (380s) FUS would have 1407m/s more than the hydrolox version and have better DV for payloads 8.6t or less. The best use case for Hydrolox isn't small probes. It's massive payloads. The Hydrolox upper stage would increase falcon's ASDS recovery payloads from 15.6t to ~23t. The large stage size for Hydrolox also lends itself well to large payload fairings.

Atlas V's upcoming launches are: GEO, LEO, GTO, SSO, Heliocentric GTO, LEO, GEO, GEO, LEO, SSO, GEO, (Unknown air force orbit - unlikely beyond earth's SOI), GTO, GTO, LEO, LEO, LEO, LEO. Delta IV Heavy has only 3 NRO contracts remaining which will probably be to GEO or GTO. Vulcan currently has no missions beyond the earth-moon system on the public manifest. Missions beyond the earth-moon system are rare. Those that do exist, Falcon is getting an increasing share of. Most notably, it will almost certainly get Europa Clipper. It also has DART, PSYCH, and IMAP. That's not counting TLI missions.

Yes, "assy" is a shortened form of "assembly". Again, a common engineering abbreviation.

Construction stands. Fairly common practice. My work had some during our last project that we put multi-ton loads on.

And right on cue: 1 raptor every 2.x days.

This may be the cheapest rocket ever produced at this scale, but it'll still be cheaper to reuse it than not. Methane and LOX are inexpensive fuels. As long as the recovery cost is not more than the manufacturing cost, and the manufacturing cost is more than a small fraction of the fuel cost, then disposable rockets won't make sense from a cost perspective. Alternatively, if desired launch cadence exceeds manufacturing capability, then reuse still makes sense even if it's more expensive than building new. Not that I think it would be more expensive, but demand will certainly exceed supply for what spacex has planned.

The question arises why not just use guns. The standard sci-fi trope is flechette or shotguns or other low-penetrating munitions to avoid breaching the hull. And that's on the inside. Approaching a modern day spacecraft having an external armament would be insane. No current or planned spacecraft could weather a spray of machine gun bullets. Also, setting up an orbital rendezvous with a craft that doesn't want to be rendezvoused with is exceptionally difficult using conventional orbital mechanics.

Almost certainly the production will be both highly serialised, with a new engine immediately stepping into a manufacturing phase the moment the previous moves to the next, and massively parallel, with multiple engines being worked on at once at each phase of manufacturing.

An engine every two days doesn't mean the manufacturing process takes just two days. It means they're working on many at once. Assuming they eventually reach their goal of $250000 per engine and half of that is labour, then that's roughly a worker-year of labour per engine. If the engine's still costing ~$1m each then that's ~4 worker years per engine. Even with multiple people working on the engine at once it'll take months to make each engine from start to finish.

Worse, probably means all new tooling. This is the problem with building the factory before the rocket.

By the time they find out they can't get 2 of 3 engines to start, it'll be too late to do a one engine flip and even if it isn't they won't be carrying the extra fuel required. I believe cylinders experience more turbulence than shapes with sharper discontinuities because the vortex shedding from a cylinder is more irregular. And an open structure limits the pressure differential across the panels which help prevent the panels getting blown out. It's why skyscrapers are very rarely pure cylinders.

With a significant mass penalty due to increased gravity losses, yes. Which is why they don't. With hot gas thrusters they may be able to be even more aggressive.

Vostok 1 touched down West of its launch point. Gagarin is still first man in orbit. SN20's periapsis will probably be above earth's surface, or near enough.

I think it's fair to say it's a test of an orbital launch system even if it doesn't complete a complete revolution. The boundary between orbital and suborbital is quite fuzzy. It's actually quite difficult to stay in space for an entire revolution in an initially circular orbit just above the karman line, but that would definitely be an orbit despite the instability. Whereas an aerobraking trajectory from lunar return isn't really suborbital, but by strict definition it would be. The only reason there's a cat fight about it is because SLS fans still wants to claim they got there first and Starship fans also want to be first. But as SN21 and SN22 will probably make more ambitious flights before SLS launches anyway I'm fairly relaxed. I expect it will be heavily photographed by emplaced assets as it comes down, and that there'll be some sort of black box too.

There are different sizes of infinity. Not sure if they're the same magnitude in these instances.

If you're sticking with known physics, either a relativistic kinetic impactor, some sort of antimatter warhead, or a maybe a cleverly engineered virus bioweapon.

Depends on the inertia and velocity of the grab heads to Superheavy. If they design them to track Superheavy's motion before engaging, no shock. If Superheavy hovers motionless, no relative motion, no shock. If they design the active portion of the grabs with low inertia, low shock even with relative motion. The tower system can then progressively apply resistance so as not to cause excessive shock.

What doesn't make sense? Superheavy will come in pretty slowly. It can probably hover. Relative velocity will be low. We can see from F9 landings that accuracy will be good. Superheavy doesn't weigh that much, a small fraction of the ~1450t it has to support above. Suspending a weight is easier than supporting it. It already needs hardpoints for lifting purposes. The grid fins are already hardpoints. Mechanical stresses are well understood. It's not a fuzzy science like "where do the cavitations form when tons of cryogenic liquid in a long distribution system are suddenly rotated." Robotic grabs are also well understood, if not quite built at this scale before. All of these are just taking existing engineering knowledge and applying it to a new application. This is categorically not the hardest problem SpaceX has ever solved. I doubt it makes the top 10. And if it doesn't work, so what? Booster gets a little more dry mass for legs and takes a little longer to get back to the pad. No big deal. Doesn't break the system.

Believe it or not, engineering science is pretty good at calculating that sort of thing.

A free return probably requires a little more altitude and therefore DV than an exact match to lunar altitude, but probably only on the order of a couple 10s of m/s. So that's not where they're getting the margin from. Either they expend Superheavy for extra props to LEO (nominal time of MECO didn't seem to agree), Starship plus crew cabin outfitting weighs less than 120t, or Raptor has found some extra ISP.

I did the math for the Hoffman transfer to lunar altitude. Although the altitude required for free return is difficult to calculate, the difference in DV required to lunar apogee or lunar perigee is only 10m/s, 3125m/s minimum from a 200km reference orbit to lunar minimum of 363230 So they aren't gaining the extra margin from timing.

What's the absolute minimum DV required for a lunar free return trajectory insertion? I know we generally use 3200m/s for TLI, but it varies a bit. What's the absolute minimum?

Courtesy of xkcd/1235