sevenperforce

If you can tap the solar wind for reaction mass -- ideally, using a magnetic vortex to suck the stuff up -- you can use that. If 8.9% of c holds as your exhaust velocity, then a 250 tonne ship which needs 12 gees of acceleration (29.4 MN) need only scoop up 1.1 kilograms of solar-wind-mass every second. The solar wind has a bulk density of about 6 atoms per cubic centimeter, so your magnetic vortex need only vacuum up 1.1e11 cubic kilometers of solar wind each second...roughly five times the volume of the moon. Totally doable.

Elon says the notional lunar landing mission would involve propellant fill in elliptical Earth orbit, followed by departure, lunar EDL, and SSTO from the lunar surface to Earth entry interface. You need 2.74 km/s from the lunar surface to Earth entry, so let's say 2.94 km/s to allow for course correction and landing props. If this is a crewed mission we will say 30 tonnes payload off the moon, so dry mass is 150 tonnes for Starship and crew cabin. By the rocket equation, liftoff mass will be 432 tonnes. However, Starship needs to be able to deliver up to 100 tonnes of cargo, so let's add 70 tonnes of flatpacked something-or-other during descent. Touchdown mass is roughly 500 tonnes even. To get one gee of acceleration, you'd need fifty ten-tonne hot-gas thrusters, which is clearly prohibitive. However, the moon is not so tough a mistress. To hover, you need only nine and a half ten-tonne thrusters (accounting for 15% cosine losses). The Vacuum Raptors could be used to decelerate to a hover 50 meters above the surface, at which point the hot-gas thrusters would ignite for a gentle touchdown. You probably want a solid dozen for good measure. If you are going to set down on a dual thrust axis, you need more because you'll be needing much more creative differential throttling to maintain attitude.

At 10 metric tonnes of thrust each you would need 8-10 of them mounted on the dorsal aspect of Starship to do a dual-thrust-axis landing.

I am fairly certain the ejector duct also improves propulsive heat energy by mixing the untapped heat of the exhaust with the additional working mass. This is a (lesser) limiting factor on air augmentation, in fact; at maximum speed, the intake air is bringing more heat than you can add with exhaust mixing. The one thing an ejector duct does not accomplish is run a compressor turbine. For that, you'd need a turborocket. EDIT: Okay, I see this is what you were saying all along. To be clear, none of the mission profiles I presented above were 4+ year missions. They were all missions of less than 2 weeks. I was analyzing how to use the vehicle for the Earth escape burn and then brake back into orbit after payload jettison. No. 1 raptor has a thrust of 2000kN. 1 starship mass is 110 tonnes. Moon gravity is 1/6th of that of earth. Weight of empty starship is about 180kN. Raptor min throttle is about 50%. Or 1000kN. (Might be lower, I forgot). You're right, it's 50%. I have seen some nice ideas about using hot-gas thrusters as dedicated landing engines for the moon and perhaps Mars, damn the cosine losses. You don't need that many. If they are ten metric tonne thrusters then they are each more powerful than a SuperDraco. IIRC you need 6-8 to land a Starship on the moon with sufficient propellant for Earth return. I'd really like to see hot-gas thrusters arranged thus double as launch abort motors.

An operational Rod from God would place the "tungsten telephone poles" in a moderately eccentric Earth orbit synchronized to reach apogee slightly west of the probable target several times per day. They would feature a small RCS pack along with a solid propellant deorbit motor. With a few dozen of these spaced equally along the orbit, you have full target area coverage and can command a strike at virtually any time. Use RCS to line up and spin-stabilize, then burn the deorbit motor to lower your perigee low enough that your entry interface includes the target. You can also perform some inclination changes to widen the target range, since the cost of inclination changes at eccentric apogee is low. Then, kablooey. You would, however, need a different orbit (sometimes even a different orbital plane) for each collection of rods. It is also a rather significant deorbit risk......

Yeah, less heating than a Falcon 9 re-entry, actually. They are building two for the 20 km test in case they lose one, but ferocitering on with Mk3 and Mk4 because they want to be ready for the orbital attempt once they are confident about skydive and turn-burn. Air-augmented rocket engines are the lazy big brother of air-breathing rocket engines, also known as "Rocket Combined Cycle" engines (RCC). An RCC engine takes in air and burns it with fuel for at least the first part of the burn, reducing the amount of oxidizer (usually LOX) the vehicle must carry. Examples of this include SABRE and LACE, and a turbo/ramjet (the P&W J58 on the SR-71) or scramjet (X43A, X51) is the airbreathing-without-pure-rocket-mode version. It's much more propellant-efficient than a pure rocket, because you take your oxidizer from the air, but you run into problems as you accelerate faster and faster because you have less air and it is more difficult to slow it down, cool it, combust it, and get useable thrust out of it. Even hydrogen-fueled airbreathing engines cannot operate in airbreathing mode much beyond Mach 10 or 11, even theoretically, because the amount of air you need and its inlet speed gets too high to generate positive thrust. SABRE, with its novel dump-cycle-hydrogen precooler, shuts off the inlet just past Mach 5 and goes to pure-rocket mode because the thrust drops too low. An air-augmented rocket, on the other hand, recognizes that trying to burn air is a losing battle and so it doesn't even try. Instead, it does the same thing that a turbofan engine does: pull in and mix a bunch of inert air with the hot exhaust. Because kinetic energy is proportional to the square of exhaust velocity while momentum is proportional to exhaust velocity, "spreading out" the kinetic energy with more reaction mass (e.g., not just the exhaust, but the captured ambient air) produces greater thrust for the exact same amount of propellant use. If a duct is placed around the exhaust nozzle of a rocket engine, with an inlet at the front, air flowing into the inlet will mix with the supersonic exhaust and "push" against the walls of the duct, producing greater thrust: The only disadvantage is the weight of the duct. You get significantly more thrust with no added propellant consumption.

It may be that the process of bent-metal construction is much quicker than the process of actually outfitting and installing everything they need, and so Elon is expecting construction on Superheavy to be well underway by the time Mk1 is actually ready for its first flight. If they start bending metal for Mk3 in the next few weeks and they shave construction time in half (single-weld rings, lessons learned), Boca Chica could complete it and begin construction of their first Superheavy in as little as four months. Depending on when Mk1 flies, it may take that long to analyze the data and make any necessary adjustments to Mk3's software or hardware; meanwhile Superheavy will complete. The 10-meter "skirt" at the base is just begging to be modded into an airflow duct. I wonder if they would be able to open a flow path around the lower bulkhead such that air was uniformly directed into the gap between the core engines and the outer engines. Static entrainment would not be as significant because the inlet is too far from the exhaust. Part of me thinks that this configuration would effectively create a "virtual nozzle" with air trapped between the inner and outer exhaust plumes...but perhaps without a continued physical duct that added thrust would not be transferred to the base of Superheavy at all.

The outer engines are not throttleable anyway, so Superheavy will only ever use its six core engines for landing. Static entrainment in the air-augmentation configuration produces a 15% thrust boost, which gets you 379.5 s on the pad. It only goes up from there until about Mach 4, where it starts to drop off (but by that time you're out of the bulk of the atmosphere anyway). What dry mass numbers are you using for Superheavy? I posted the same image to NSF, and someone pointed out that the nozzle would be under compression and would require reinforcement against buckling, which is something I did not think about. Arguably the same approach could be used with an "inside-out" aerospike, where the nozzles point outward rather than inward and the nozzle is more like a hoopskirt around the base.

Agreed. Wished I'd heard more about the Starship itself but the geekspeak was great.

Well he basically said there would need to be a reason. This reduces mass, allows fitting more engines (not fewer) into the same space, and also allows for some potential air augmentation flow, which would raise Isp during early boost as well. Good reasons, even if not SSTO reasons.

Right, you need independent chambers so you have a nice well-understood spherical or cylindrical combustion zone. As far as thrust-to-area ratio is concerned...well, let's see. SL Raptor is 1.3 m at the bell but 0.47 meters just below the throat. They are 1.2 meters long from the top of the turbopump assembly to just below the throat. If truncated-bell Raptors were arranged in an inward-facing circle as I showed above, inscribing an outer diameter of 10 meters (the size of Superheavy's skirt), the throats would form a circle 23.9 meters in circumference, more than enough space to fit fifty Raptors. Granted, there's going to need to be some necessary spacing, but that's significant. Plus, if they were angled/canted down as would most likely be the case, you'd have even more space. I find it hard to believe only because I think they will fly Mk1 well before they have the first Superheavy built.

The 20 km test is not a hop. Starship doesn't have enough propellant to go to 20 km and back under constant thrust. The 20 km test will be a high-gee vertical launch, apogee 20 km, kill the engines, and skydive back to the landing site for the turn-and-burn to land. Elon is saying they will use welded steel rather than titanium (F9B5) or Al (F9B2-3). It likely will not. An abort at Max-Q is almost definitely not survivable for the booster, as the sudden aerodynamic shear will either frag it directly or cause it to tumble and break up. If it turns out that they have some aluminum grid fins, I suppose they will program a landing test routine in case by some miracle it survives.

Yeah here: https://www.theatlantic.com/science/archive/2019/10/elon-musk-jim-bridenstine-starship-commercial-crew/599218/ It's worth noting that in the planned schedule for Artemis, all the "commercial launch" deliveries are represented by single-stick rockets with F9 fairings, as opposed to Atlas-style rockets as in the old renders. On a related note... I keep thinking about Musk's commentary on aerospikes. I've been thinking about altitude compensation for Starship (or for a smaller SSTO shuttle), but what about Superheavy? Musk is planning on building cheaper, ungimballed SL Raptors with higher, constant thrust. What if they did something like this? You could pack in more engines in a ring. Not as heavy. Maximum efficiency all the way up. I think it's a solid solution. Basically Chrysler SERV's big brother.

I didn't know many middle class people with hundreds of thousands of dollars saved up for a half-hour tourist trip.

Precisely. The "weird action at a distance" behavior commonly associated with entangled particles is solely the result of applying the Copenhagen interpretation of uncollapsed superposed states. If you take the many-worlds hypothesis, for example, you end up "splitting the universe" whenever you open either box.

If you are launching vastly larger payloads BLEO -- like, two orders of magnitude larger than anything to date -- then it might make sense to launch a transfer vehicle with 3,300 tonnes of propellant. But at that point, you'd be better off using nukes and hydrogen cracked from the lunar surface.

Just read the full Bridenstine interview and this jumped out as hilarious.... Uh...if you want SpaceX to be as "credible" about cost and schedule as NASA has been with its internal projects, I think they are well ahead of the game.

Because that is the nature of quantum entanglement under the Copenhagen interpretation of quantum mechanics.

It would not break entanglement. Only opening the box would break entanglement. If you have only one box, you do not know whether the wavefunction has collapsed because you do not know whether the other box has been opened, but if you know that both boxes remain unopened then you know the wavefunction has not collapsed.

Quantum entanglement only produces the "weird spooky action at a distance" effect if you assume wavefunction collapse by the observer. It doesn't cause paradoxes in our ordinary experience. Consider the following. You place a quarter into a special machine. The machine has a mechanism which will slice the quarter exactly in half lengthwise, so one disc will be the "heads" disc and one disc will be the "tails" disc, and then drops each half into a separate box. The two boxes are unmarked, and you do not know which box holds which disc. The "slice" is such that the two halves weigh exactly the same, so you cannot do anything to determine which is which. These boxes are now "entangled". If you open one box and get tails, you know the other box has the heads, and vice versa. This is true regardless of the locations of the boxes. If you put one box in London and one box in New York City and open the one in New York City to see that it is tails, you know instantly that the box in London is heads. You can put one box on Earth and one box in space, one box on Earth and one box on the moon, one box on Mars and one box on Alpha Centauri, and the same relationship will hold true: you can gain information about the distant quarter instantaneously, simply by observing the quarter in the box you have. Similarly, a distant observer can gain information about your quarter instantaneously simply by observing the quarter in her box. There's nothing spooky here at all. No information is moving faster than the speed of light. It's not a paradox. What gets weird, however, is when we enter the world of quantum mechanics, where you can have an object in an "uncollapsed" wavefunction: the dead-and-alive cat of the Schrodinger's Cat thought experiment. You can have situations where two particles were produced in the same experiment and both have equal probability of having one value or another. It can be proven both mathematically and by actual experiment that it is not just an issue of uncertainty; both particles actually have both possible values until observed, at which point the wavefunction "collapses" into a state where one particle has one value and the other one has the corresponding one. And thus we have the weird situation where you "collapse" the distant particle instantly by observing the near one, in which it feels like information is moving faster than light. But you still cannot use this to transmit information.

Yes to the first question. No to the second, as entangled particles cannot be used to transmit information. Note that if you destroy one of the entangled particles, you collapse the wavefunction.

There's some math to be done here, I think. We don't know what the TPS will weigh, yet, but let's say it comes in close to the shuttle high-temp reusable surface insulation (the black stuff) which was 9.2 kg per square meter. Covering half of a 45-meter-high, 9-meter-wide cylinder with this would come to 6.44 tonnes. A little more if the Starship TPS is heavier; a little less if it isn't. We'll assume that a returning aerobraking pass would be high-altitude enough to control with hot-gas thrusters alone; 2-3 passes are fine if needed. Let's go with 80 tonnes and give it regular tanks. Some reference numbers (I'm using <> to indicate propulsive 1-way and >) to indicate aerobraking one-way): GTO-1 <> LEO = 2.27 km/s; GTO-1 >) LEO = 0.02 km/s EML-1 <> LEO = 3.77 km/s; EML-1 >) LEO = 0.77 km/s EML-2 <> LEO = 3.43 km/s; EML-2 >) LEO = 0.33 km/s LLO <> LEO = 4.04 km/s; LLO >) LEO = 1.31 km/s Earth escape <> LEO = 3.22 km/s; Earth escape >) LEO = 0.02 km/s GEO <> LEO = 4.33 km/s; GEO >) LEO = 2.06 km/s Jovian Transfer <> LEO = 8.8 km/s; Jovian Transfer >) LEO = 3.06 k/ms Thus round-trip payload capacity is as follows for going propulsive only: GTO: 1232 tonnes EML-1: 479 tonnes EML-2: 592 tonnes LLO: 402 tonnes EE: 674 tonnes GEO: 330 tonnes Jovian transfer: -4 tonnes And for using TPS (assuming 10 tonnes to be safe): GTO: 1268 tonnes EML-1: 532 tonnes EML-2: 645 tonnes LLO: 452 tonnes EE: 727 tonnes GEO: 373 tonnes Jovian transfer: 89 tonnes So adding 10 tonnes of TPS offers a distinct advantage to any destination, with a bigger advantage for higher-energy orbits. Obviously these payloads are vastly bigger than what would ordinarily be delivered, even with LEO orbital assembly, so the tug would almost never need to be fully fueled. But TPS wins in every situation...moreso if it is not quite 10 tonnes. The fact that the tug is SO overpowered for every single use inside the Earth-moon system is why I am not really so sure it is the best choice to begin with. It would be useful for BLEO payloads but those will be rarer (and anything going to Mars will need full TPS anyway) so why bother? Definitely not enough thrust with a 3-atm pressure-fed engine. If you add another Raptor up there, you're golden. Or you could put all the RCS bottles up there and use repurposed 10-tonne RCS thrusters for escape engines. Powered landing would have too many variables and require too much dV. Better to do a splashdown.

Or even filled on the pad. You could stack two five-meter DCSSes together with a single RL-10 and loft it into orbit easily, with up to 40 tonnes of terminal payload. Using excess props for a Starship elliptical kick burn before payload separation, you get the following performance: 40 tonnes: 3449 m/s 30 tonnes: 4280 m/s 15 tonnes: 6170 m/s 10 tonnes: 7176 m/s 4 tonnes: 8962 m/s And that is with a SINGLE launch priced at under $20M. (Elon says $2-10M but I am sandbagging.) Imagine what you could do with a 30-tonne spacecraft headed to the asteroid belt, or ten tonnes on a two-year transit to Jupiter, or a 4-tonne lander on freaking Triton. Indeed. It's all very well and good to pine for a 50-tonne gas-core thermal nuke for interplanetary missions, but you have to get into into space. Are you going to wait for SLS? There are a few questions still out there but I get the sense that most unanswered questions are still under dev at this point. Have we ever received any indication of the propellant load for the header tanks? I wouldn't assume Mk1 and Mk2 have the full header tanks, just because we don't know, but it could be. I would love to see some more detailed renders that account for having monocoque tanks in the nose and how the payload space will adapt. In that video, Elon called the interior of the fairing the "cargo bay" so it may be moving in that direction (again, despite those renders). Probably 75-85 tonnes, yeah. But that would only work for ferrying outward, not returning. It's a problem with a LOT of possible solutions. Not without LF engines.

My sense is that large-component actuation with hydraulics has so many clean COTS solutions that it was easier to just build it that way, and iterate to get direct-drive electric. I know! I feel like I could sit with good beer and talk to him for hours. Blasphemy! Seriously I loved hearing Elon talk about aerospikes just for the insight into his brain.

YESSSS I just watched it on high speed and came here to post it!!! This was spectacular. A few takeaways: Despite all the renders we saw, they WILL put header tanks in the nose permanently, and make them monocoque tanks with smaller nested bulkheads rather than suspended as in Mk1/2. Elon is a nerd. A HUGE nerd. He is also ridiculously smart. I doubt there are many science/engineering subjects where I know anything he doesn't know, other than the obvious stuff like earth science. Maybe a few edge cases, but not many. Elon has discussed aerospikes at SpaceX internally many times. He would happily develop them if he could find an area where they would be the best choice. He doesn't like them because they have poor combustion efficiency and aren't needed when you can optimize engines independently for SL and vacuum. Starship is using Tesla batteries and Model 3 motors. Mk1 and Mk2 are using the motors to drive a hydraulic reservoir for fin actuation, but starting with Mk3 they will go to direct-electric drive for reduced mass and simplicity. Elon talked about how school trains you to answer the professor's question rather than asking if it is the right question. Referenced HGTTG. Talked about how much efficiency is lost between departments because they are trying to optimize for the solution the other department gave them rather than synergizing. AND SO MUCH MORE