sevenperforce

When you mix physics and philosophy, both end up muddy. Causality in philosophy is a little different from causality in physics. Causality in physics is a physical constant, known more generally as the speed of light. Causality propagates at roughly 3e8 m/s. If space and time are inverted, as in a black hole or at the start of the universe, causality becomes undefined. Not absent, but actually undefined.

This is still kind of possible. Barely, but can be a viable choice if payload mass must be conserved at all costs. Barely, yes. There's not much mass saved, though, really. A lifting body only needs ~600 m/s of dV to land on Mars, whereas a ballistic capsule needs 1000 m/s or more. You can add chutes to drive this down a little, but that increases your dry mass. 600 m/s of dV is nothing compared to the 4.1 km/s your MAV will need to get back to Martian orbit, so there's no sense doing an Apollo-style EDL stage separate from the ascent stage. Plus, your ballistic heat shield needs to be much beefier than the TPS on a lifting body, so that's extra mass to deal with. The problem is something like Columbia; even if they had identified the foam strike while the orbiter was still on the mission, staging a rescue would have been almost impossible. The inclination couldn't be reached by the Soyuz, and even if they could have pushed Atlantis ahead to do a rescue, it would have required a dizzying set of EVAs. They didn't have nearly enough dV to abort to the ISS, either. The rescue mission approach is only possible if you had a large enough fleet of RLVs and androgynous docking ports, so a rescue mission could launch almost immediately, rendezvous, and dock for the crew transfer. Of course, that assumes the rescue vehicle has enough seats for all the crew onboard the crippled vehicle. I agree that in an abort before reaching orbit, the crew cabin is the only thing you need to save. The only reason I was thinking of having the stage be able to independently re-enter is so that you could swap out the same upper stage for payload launches and crewed launches, since the upper stage will be re-entering by itself after payload launches. Of course, if you want downmass capability, then you don't build a swappable upper stage at all; you build two different vehicles with the same-size tanks: a crew version and a cargo bay version. You'd want to use something like methalox and intertanks, like the ITS has.

One of the things Elon Musk said in his September 2016 speech about the ITS was that the cost of getting to Mars is essentially infinite right now. In his powerpoint, he put it at $10 billion per person. Now, the logistics of the ITS aside, I wonder how accurate this is. How much WOULD it cost to get humans on Mars? What are the different cost breakdowns of different approaches, and which mission configuration would be best? And, most importantly, what happens if we Kerbal it? The challenge is to send a few Kerbals to Duna, using only currently-available propulsion methods, with enough supplies to live on for the trip, as cheaply as possible. Rules: Propulsion. No nukes and no airbreathers. SABRE isn't up and running, and NTR isn't likely any time soon, so your propulsion needs to be chemical only. No ion engines; we need to assume you're running against some kind of a deadline. ISRU. Nope, sorry. We can't wait around on Duna forever. Payload. Send up to 12 Kerbals to the surface of Duna and bring them back to Kerbin. But they need consumables, right? Let's be very Spartan and say that they each need a total of 0.2 tonnes of food and other consumables for each leg of the trip. You can pack that extra payload any way you want; that's approximately two Science Jrs per Kerbal, and you can ditch up to half of them (they're empty, after all) before you enter Duna's SOI. Prop transfer. This is not only permitted, but encouraged. I highly recommend it. Reuse. Recovery of components is encouraged by a cost reduction as outlined below. Scoring. Your total score is the total mission cost divided by the number of Kerbals you actually land on Duna and return safely. Kerbals which stay in Duna orbit do not count, and dead Kerbals do not count. Any recovered components (reusable launch vehicles, etc.) are counted at 30% of their full cost. You do not have to include the cost of whatever you use for dry mass payload for consumables. The winner is whoever has the lowest score. Mods. No part mods and nothing that would affect scoring, but anything else is fine. I only have the Demo, or I'd make my attempt, but obviously this can be done. I'm mostly interested in seeing HOW it is done, what mission architectures are used, and so forth. Good luck!

For LEO, the main advantage is that a big fluffy re-entry vehicle is much safer and endures lower peak heating and lower gees than a smaller vehicle. That's true whether the crew vehicle is a capsule or a lifting body; the version which retains the stage will have an easier entry than the version which doesn't. For BLEO, you pretty much have to have your capsule and your stage connected for EDL, since you can't re-integrate for relaunch. So that's an absolute requirement. You can get around it for the moon, since there is no atmosphere to make entry sticky, but it's automatically necessary for Mars. I mean, you can do a capsule on top of an MAV on top of a heat shield, but...egads. If you're going to go the route of having the crew cabin detach from the upper stage, then you have to ask yourself whether the cabin is going to detach with or without ECLSS, the aeroshell, and so forth. If it detaches with ECLSS and aeroshell, then it is essentially a separate capsule that can serve as a lifeboat, and so you have to ask whether it makes sense to give it an independent emergency re-entry heat shield. And so forth.

Isn't that essentially the Apollo missions' configuration when going from Earth orbit to Lunar orbit? Rocket stage, lander, crew capsule? Not to say this isn't complicated. but it's not that complicated. Though, I tend to agree with the idea that combining the upper stage and the payload/crew module, a la ITS, is a better configuration. Not at all; they can dock together just fine, but they obviously can't re-enter together. That's the part I was highlighting. The only issue with combining the upper stage and the crew cabin a la ITS is that launch abort is really unpleasant.

For LEO operations, using an integrated second stage crew vehicle is a bit of a tossup. Flexibility is actually a little more limited, because you cannot use (or reuse) the same stage or stage configuration for crewed launches and payload launches. This isn't necessarily a dealbreaker, though; as long as the tanks and engines have the same dimensions for manufacturing purposes, the supply-chain production side of things can benefit from economies of scale. The immediate advantage of an integrated reusable second stage vehicle is that you have only one orbital EDL to worry about. Of course, the final advantage is disputable. Having a single vehicle to refurbish doesn't necessarily make everything easier, a point often missed by SSTO advocates. It may be cheaper to refurbish a capsule and an upper stage separately, as both are highly specialized. But it is definitely attractive to be able to use the large volume of the upper stage to help gently decelerate the crew capsule. For BLEO operations, on the other hand, having refuelable, reusable upper stage integrated with the crew vehicle seems like a no-brainer. You've got to take tanks and engines with you for the transfer burn anyway, and high-velocity re-entries really benefit from the extra volume of an upper stage. The ideal configuration, I suppose, would be an independently-recoverable upper stage and crew vehicle which can dock together and enter together. But that seems ridiculously complicated.

I'd wager that a majority of post-v.10 R&D was recovery-oriented, but not all of it. Even the fully-expendable Falcon 9 FT is a far, far more capable launch vehicle than v1.0 was. Remember that v1.0 couldn't even launch satellites; it had no clamshell fairing, no octaweb, nothing. Falcon 9 FT can send more to GTO than Falcon 9 v1.0 ever sent to LEO.

As an example: I get 23.2 tonnes to a 185x185, 28.5 degree LEO orbit, flying F9 expendable in its current incarnation. With eight engines (core engine removed), I get 22.7 tonnes. With seven engines (one pair of engines removed), I get 21.2 tonnes. With six engines (core and one pair removed), I get 20.5 tonnes. With five engines (two pairs removed), TWR has dropped low enough that propellant loading needs to be reduced. Reducing to 330 tonnes of fuel on the first stage so it can get off the pad, I get 18.0 tonnes. If an expendable Falcon 5 could still get more payload to orbit than Falcon 9 flying reusable, it doesn't make sense to build a second smaller "mini-Falcon-9" for expendable payloads. Just reserve the landing propellant and be done with it.

Remembering Falcon 5... If SpaceX wanted to do a cheaper expendable Falcon 9, the fastest way for them to do so would be to mount fewer engines on the octaweb. They can drop the center engine or any pair of outer engines without affecting balance. Then, simply fill the tanks a little less full than normal, and launch. Does this make sense? Absolutely not. But it would cost them less than building an entirely new shorter stage and still throwing away all nine Merlins.

A larger crew, test systems for a surface hab, that sort of thing. Oh, and the upper stage/lander itself. Because reuse is the only way a moon base will ever be economical.

First of all, downmass isn't the issue; capsules are primarily volume-limited, not mass-limited. The point is to get high downvolume from the moon. Getting high down-volume from the moon doesn't take a spaceplane; it just requires a biconic re-entry. Integrate the lander with the upper stage and have the whole thing come back and re-enter on its belly, like the ITS Spaceship.

Exactly. Even when you consider the behavior of a single discrete photon, its interaction with the matrix is probabilistic rather than deterministic, as if it was not a single photon at all but a collection of them. That's the great underlying paradox of quantum mechanics: everything in the universe is quantized (meaning any substance comes in discrete, indivisible packets), and yet an individual quanta behaves as if it is a continuous wave when it interacts with other quanta. When you sit down and do the actual math with a single photon traveling through a medium, it gets REALLY weird. Like, we-all-live-in-a-simulation weird. Any time you have a photon (or a group of photons) hitting a medium like air or glass or water, there is a probability that it will be transmitted (absorption, phonon excitation, emission) and a probability that it will be scattered back (reflection). Suppose that for a group of photons passing from vacuum into something like glass, there is a 90% chance of transmission and a 10% chance of reflection Now, classically, we would expect that 10% of the photons are reflected, and 90% of the photons are transmitted. But on a quantum level, those probabilities apply individually to each photon itself. According to the math, each photon is 10% reflected and 90% transmitted. Yet the photon is a quanta, a discrete object; it cannot be broken into smaller pieces. So each photon is either transmitted or reflected; there is no middle ground. That's the paradox. The solution is that the interaction with the material matrix resolves the 10%-90% split. The photon is discrete, but there are actually two phonons created inside the matrix: one with 90% of the photon's energy, and one with 10% of the photon's energy. This is called a superposition of states; two different "outcomes" superimposed over each other. But this superposition of states isn't just division of energy; it's a division of probability. 90% of the time, the larger phonon "wins" and the photon is emitted out the other side for a transmission; 10% of the time, the smaller phonon "wins" and the photon is reflected. It is this superposition -- an interaction between two phonons of different probabilities -- which causes the time-delay in the apparent velocity of the photon. This interaction also is responsible for producing refraction. So there you have it. It's complicated as all hell, for sure.

They sort of look like pulsejets.

SES-10 was also originally intended to launch on Falcon Heavy, but they managed to put it on F9...and not only F9, but an ASDS-recoverable F9. It's a testament to how much improvement has been made since v1.0. Structural and aerodynamic load change from payload to payload on ascent, so that sort of variability is already factored in. Variance in payload, fuel consumption, and stuff like that is already adjusted in real-time based on exhaustive modeling, so it doesn't really matter whether you're lofting a heavier payload or you're lofting a lighter payload with more fuel reserved for RTLS. In contrast, changing the actual dry mass of the vehicle and altering the overall shape completely wrecks your aerodynamic modeling, along with a bunch of other stuff. I saw this too; it already looked sooty by the end of the boostback burn. That surprised me; I was expecting most of the soot to be deposited during the re-entry hypersonic retropropulsion phase. Or Falcon 5. It just makes way more sense to reserve extra propellant for a more gentle recovery.

This looks terrific! While I'm sure I could dig into your spreadsheet and get this information, I'm curious and impatient, so I'll ask here: What TWR are you projecting for the Raptor and the 1000 kN dev Raptor? Are you factoring the lower density of methalox into your upper stage tankage ratio? What method are you using to determine gravity drag and aerodynamic drag, and is it fixed or TWR-dependent? What first-stage reuse penalty are you using? What second-stage reuse penalty are you using (or is there second-stage reuse)?

The photons are converted into excitations, which travel through the medium slower than light and are then re-emitted as photons again on the other side.

The actual interaction is a little more than just absorption and re-emission by a single molecule. Transparent materials are a matrix of chemical bonds which have a collective wavefunction interaction with objects like photons. What happens is that the incident photon induces an excitation in that matrix, called a phonon. Although the phonon is a transient excitation, it is an excitation within that medium, which means it has relativistic mass. Objects with mass cannot travel at the speed of light, and so the phonon travels slower until it reaches the other side, and the phonon collapses and an identical photon is emitted. This can be modeled as sequential absorption and re-emission by each molecule in turn, but that interpretation misses the quantum wavefunction nature of the molecular matrix.

Technically, it is being absorbed and re-emitted by the glass molecules. But yes, you're right. Light travels at the speed of causality in spacetime; it cannot exist at a speed slower than that.

Tiles will work just fine.

It is nonsense, but I think it's instructive to explain why it's nonsense, and why the underlying question doesn't close. The speed of light isn't a speed limit; it's the relationship between space and causality. Time and space are linked dimensions, and you are only able to move through one of them at any given point. Light is something that essentially has infinite speed because it matches the rate of casuality; it's mathematically nonsensical to imagine something going faster than the rate of causality because it would then become its own cause.

AFAIK, BO has no plans to do a second-stage-as-payload approach, and their biconic orbiter cannot very well be expanded to a tanks-and-engines profile.

I factored in the extra dry mass for separate re-entry; you can use the same heat shield you'd be using anyway, and you need parachutes for the launch abort, so that's not a problem. There aren't really a lot of extra systems required. Columbia taught us that rescue missions aren't always possible; what if the vehicle is on an inclination other than the ISS? For landing abort, you can have the lower escape engine triggered a split second before the upper one, so it tilts you upward.

One thing to consider in looking at the economics of reuse is that the initial version of Falcon 9, v1.0, had a lower payload flying expendable than Falcon 9 FT has flying reusable. Granted, Falcon 9 v1.0 didn't have the extensive development investment that F9FT has behind it. But since SpaceX can almost certainly refurbish F9FT for MUCH cheaper than the cost of a new Falcon 9 v1.0 (or something like Falcon 5), I think the argument closes rather well.

The Shuttle approach of doing manned crew alongside unpressed cargo is a poor one, I think. Much better use can be made of pressurized cargo; send unpressed cargo up on the primary variant, the one with the unpressed cargo bay. The configuration can probably take a little bit of tweaking. The level of ECLSS in the nose cabin will depend somewhat on whether you want it to have lifeboat capability. In other words, if the back end of the vehicle is disabled by a micrometeoroid strike or some other major problem in orbit, you'd probably want the crew to be able to eject/decouple and be able to return to Earth safely. This means it may need to have, at the very minimum, a heating/cooling system, its own RCS/OMS, and CO2 scrubbers. You could probably also build it with an optional airlock in the payload compartment.

No, I really don't think it's much of a problem. Dragon and Soyuz can do all the pressurized downmass the ISS needs; nobody really seems to need unpressurized downmass a la Shuttle. Honestly, there's very little reason you'd need unpressurized downmass. I could see wanting to bring a large ISS system down for a failure investigation, maybe.