sevenperforce

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.

Hey, looks like I was wrong (and I'm very happy about it):

Here's some lineart of my proposed crewed-version upper stage:

Eye candy:

This sort of thing could easily be a drop-in replacement for something like an upgraded/reworked Dragon 2 or the New Glenn equivalent. Having LES is a huge deal. Really huge. There were SO many failure modes for the Shuttle which resulted in LOCV.

Yeah, exactly. This sort of system could have enabled the Hubble servicing missions readily enough.

I think (I hope) that happens to all of us. Oh, it does.

Yes, exactly. The ISS could have been built without the Shuttle, for sure. Unpressed downmass was never used apart from ISS missions, I don't think. But if manned presence in space had been far, far greater than it was, then the Shuttle's capabilities would have been much more useful, and building a better version of it would have been a good idea.

My original post was more of a what-if, namely: what if the expectations in play when the Shuttle system was designed and funded (rapid launch cadence, need for frequent on-orbit servicing of payloads, and the need for downmass) had actually been accurate? Obviously they weren't, but if they had been, what systems could have performed the same tasks better than the Shuttle? Answers could be something as simple as "use liquid boosters with crossfeed on the Shuttle" or "fly with multiple booster configurations depending on payload".

There are so many tradeoffs. The idea of ISRU is really attractive, on the surface. You can give Tsiolkovsky the middle finger and use local resources. But actual utilization is challenging. In theory, the most straightforward ISRU approach is to land an empty MAV with the ISRU unit already attached and plumbed in, and let it collect its own propellant. For simplicity, you can just let it collect LOX in-situ and let it bring its (less massive) fuel along with it. But there are a lot of reasons why it's sub-optimal. The concept of having the crew leave their descent vehicle and head to a separate ascent vehicle is not attractive at all. You'll probably need a very sturdy rover as a primary mission survival requirement, just to allow the crew to get to the ascent vehicle, so that drives up mass. You have to have a separate airlock, cabin, and ECLSS in the ascent vehicle, and all its systems need to be able to be landed long in advance and remain perfectly ready for at least 2 years without maintenance or checkups. Are crew members in IVA suits or EVA suits? EVA suits take up much more space, but you need to bring at least one along in case there is some mission-critical problem that needs to be correct in order to allow ascent. Most disturbingly: no landing abort mode. None. From the moment you start entry, you're committed, and your survival depends not only on successful EDL, but landing in a site that will allow egress, and allow you to unload your rover, and allow you to make it to your MAV safely. Imagine how much riskier Apollo would have been if Buzz and Neil had been forced to head off to a separate ascent vehicle! Now, since the lander needs descent engines, you can size them a bit bigger and use your lander for ascent as well. Just go the ITS route and transfer propellant from a pre-launched ISRU unit to your sole crewed vehicle. But that requires an even greater landing precision, and I don't think anyone is comfortable with the idea of hooking up hoses for propellant transfer in an EVA on another planet for the first time. The only other option for ISRU (other than bringing the ISRU unit on the lander, which is a non-starter because of the time it takes to collect propellant even if dry mass wasn't an issue) is to have the ISRU vehicle autonomously land, collect propellant, and then return to Martian orbit on its own, to transfer prop to the manned vehicle prior to crew EDL. This is the lowest-risk option, because the manned vehicle has all the propellant it needs for ascent before committing to entry. It's also promising from a dry mass point of view; if your lander can drop up enough payload on Mars with a single direct ascent (projected mass to TMI from a fully-reusable Falcon Heavy with my mini-ITS upper stage is 4.5 tonnes plus the entire dry mass of the vehicle), then you can end up with about 35-40 tonnes of propellant available for transfer in low Martian orbit. Of course, you've wasted more than half of your fuel getting that fuel up into orbit, but you don't have to worry about finding a way to transfer propellant on the ground. And it would take much more than a single reusable Falcon Heavy launch to put 35 tonnes of propellant into a TMI; 35-40 tonnes of propellant is what the Falcon Heavy + mini-ITS tanker combination can manage for LEO. The biggest advantage of the last option is that your EDL and ascent double as tests for the parallel manned mission.