Rune

Doing the nth enlargment of my venerable LackLuster, I figured out one I'm quite happy with, looks-wise. Good-looking for a rocket, right? Not the most efficient, because I used a lot of tankage with awful fuel fractions, but 2.5mT to LKO, like the original, with a bit more margin, a nicer-looking tail, and no finagling with the control surfaces to rotate it during reentry. In fact, it is 100% Vernor-powered, with no reaction wheels on the design, so quite realistic too. The flaps double as high-speed airbrakes, but they are disabled otherwise, so you don't have to reverse their controls once you flip it. And yes, it is stable in both configurations (tough it will dance a bit around retrograde until you hit the chutes), and can switch between 'feet first' and 'nose first' with vernors completely under autopilot, as well as also being stable in a weird ~90º attack angle, for some reason that I suspect is an off-center body lift on the payload bay, but that can come in handy to brake in a hurry. But I digress. Solar power and medium range comms allow self-ferry on interplanetary trajectories, and the >4km/s in the tanks when full means pretty much anywhere is a fuel depot or two away. You guys like? Rune. Took me quite a while to find one I liked as much as the old one.

Yup, they are Kerbin up-and-down SSTOs, getting only a couple hundred m/s once they are in orbit. But since I have a ~1000mT rock on a 100km orbit at all times with ISRU attached, from there they can take themselves anywhere, and serve as universal landers in my 100% reusable career infrastructure. If they can make Kerbin, they can make anything with a solid surface that isn't purple. If I need payload fraction in order to lift fuel (which I don't, really, but sometimes I build stuff too complicated to dock to my asteroid station, and it's more convenient to launch fuel), then the Claymore airbreathing SSTO version I use these days has been known to pull off >50mT to orbit, which is the weight of one of these things fully fuelled. Rune. I seriously need to update my thread one of these moons. In the meantime, KerbalX is 'only' one or two game versions out of date, and I've mostly only polished stuff since then.

This. When you actually play a lot, and like to do complex architectures with lots of moving parts, part count becomes a priority. If you can make it look almost as good with half the parts, you will pat yourself on the shoulder every time you fly it for a docking. Still, Mk3 adaptors kind of look really good for short rockets, and I always allow myself to hide some wing surfaces if it at least looks realistic. And if you build the rocket simple enough, with a decent payload on top and the tank in the middle, you don't actually need much to get the short-of-stable (I've noticed my Lackluster and derivatives with a payload bay find a stable attitude at almost 90º AoA that I don't know where is coming from). This thing here, my Heinlein, is actually built with no wing parts, and it can flip itself backwards with or without the airbrakes, being normally stable forwards ('cause the docking port heat tolerance is awesome). Considering it is a Kerbin SSTO, you could add a lot of fluff to a similar design, take out the short-of-cheaty side tanks (still visible and clickable, and only two! ), and whip up a good-looking analogue with a consistent color scheme and higher heat tolerance all around. Rune. And in case of doubt, enough Vernors will get anything pointed anywhere.

Yup, but in the same place he said that, he also said initially, there would not be a dedicated tanker, and they would just use an empty BFS in order to save that development for later. So, you know, while a tanker would have to be more efficient, the whole thing must work first without it. Sure, it should be less. But again, we don't know how much more, and we won't know for a while for sure, because a lot of other things have to happen earlier. Musk has been sufficiently vague. And frankly, until they actually start building the thing, he really can't be anything else, because everything is (or should be) subject to change if models don't pan out. And right now they must be using rough models for a lot of stuff. Mass shouldn't scale linearly with size, for example. And the payload section in whatever drawings we have seen is nothing more than concept art. Which is sensible, of course. To build a rocket, you first have to know what kind of legs you have to work with, and the Raptor is only now getting tested. Now that they know those key performance parameters (and it appears those 300bar were indeed a bit too much to shoot for, at least initially), they can work from there and figure out all the structures on top. At this stage is when mass growth usually happens, when you start figuring out subsystems and such. Rune. So if they freeze the size, perhaps we will see a more detailed version in next year's IAC.

A this point, that short of analysis is very close to pulling out the numbers of your metaphorical bottom, IMO. The only number I short of trust is the stated payload to LEO, and that is because even though it must be little more than a wish at this point, it's probably also a design goal the rest of the ship can be designed around. Rune. But if itmakes you happy to do such things, don't let me stop you.

Is nobody going to mention the cooling system for the focusing optics? Bringing up the nozzle was a good point, but that is the bigger elephant in the room, I think. I mean, at the initial reflectors, the energy density is low enough, but the final mirror focusing on the drum is going to be quite toasty in no time, if only because it is sitting right next to the 4,500K drum, never mind the operating requirements. Rune. Some parts of the engine do have to remain solid, after all.

Hey, I also want to the see that, 'cause sci-fi. But sadly, it is simpler to design stuff with a single axis of thrust. Rune. KISS.

That is not surprising at all. The thermal loading is, at the bare minimum, proportional to the ballistic coefficient, and an empty stage will always beat a capsule at that. Which is why Shuttle could reenter with non-ablative silica tiles, it had a low-ish ballistic coefficient at high angles of attack. A humongous empty upper stage (BFS), doubly so. Rune. Figuring out the thermal protection requirements requires so much more math, you'd need propietary software.

Well, a 37.5% decrease in payload is nothing to be sneered at, I think. Certainly not negligible. In any case, I'm glad I made you run the numbers and see for yourself that size does matter. In any case, the best argument about developing this is, well, development. Every dollar spent on this could be spent on its bigger, more efficient cousin, while F9 and F9h service the current and projected needs of pretty much everyone, at a sufficiently low cost. Sure, but the trajectory doesn't look the same, so you can't assume the same dV budget. My point was, basically, that if you have two rockets with the same TWR, one stages, and the other doesn't, then they will see a different dV to orbit. Just like two rockets with the same number of stages will see different dV if they have different TWR. Or, in general, how every design sees a different dV to orbit from every other design, and our '9,400m/s to orbit' is a very rough generalization. Rune. I never pack just 3,500m/s to lift from Kerbin.

The stock CoL indicator doesn't take into account body lift, or drag. And even the mod Correct CoL doesn't take into account the effects of drag, only lift (so it is not a true aerodynamic center). Having lots of wing and little drag, proportionally, hides this effect in most plane designs, so usually you don't notice it until you try flying rockets like planes, or mess with lifting bodies. Rune. So, you know, trust the flight test results over the indicators in the SPH.

But you ain't scaling up, you are scaling down. So ignore the structural advantages, because we can safely assume the BFR has already been calculated to be structurally sound, you gain nothing there by being stronger. Tank pressure can also be considered a constant, so you can't thin the walls either. What remains? A drop in volumetric efficiency. As long as we don't run into the structural limits of carbon-fiber composite tanks at BFR's flight pressures, yeah, bigger is always better. And we could go to ballon-like pressure-stabilized sizes like the old Atlas... Rune. As I said, the least believable thing in the IAC talks is seeing the same structural fraction in both versions of the thing, BFR and ITS.

Wait, what? Advantage? It is very much not an advantage, but a disadvantage! That comment convinces me you have no idea what you are talking about, or your brain just had its biggest fart in a long time. Rune. I was saying the exact opposite thing.

Per Elon's AMA the other day, initially there will be no tanker version, just a BFR with an empty payload bay. And reading a bit between the lines, if they eventually build a dedicated tanker, it would be a BFR without payload section, with the main tank stretched a bit and nothing on top. Rune. A pretty phallic picture, the dome-like nose.

That would lower the maximum pressure rating of said tank. An easier attack on my analysis would be to say that I also assumed more-or-less spherical tanks, while skinny cylinders would scale quite differently. But skinny cylinders are much less mass-efficient than BFR's almost-spherical ones anyhow. Rune. Back-of-the-napkin, as I said.

And for just that reason, the square/cube law applies like a ton of bricks. A cylindrical tank's area (A) is directly proportional to R^2, it's volume (V) is directly proportional to R^3. If you multiply V by 0.5 (you halve the tank's fuel capacity), you made R (0.5^1/3)=0,7937 times the initial radius, and A becomes 0.5^(1/2)=0.7071 times the initial area, not half. And the mass of an empty tank scales with area, but the tank's fuel capacity with volume. So yeah, I'd sooner believe the BFS can SSTO, rather than the tanks maintaining their structural fraction when scaled. (Note: I would still be healthily skeptical of both, tough ;)) As to other sources of error, did you take into consideration a SSTO has a rather humongous TWR at the end of the burn, lowering gravity losses? Just an example of why I never trust calculations about launch dV, there are a lot of things that can't be easily modeled with high school algebra. Rune. There are limits to back-of-the-napkin calculations.

As @FleshJeb says, the least believable thing in Elon's IAC talk was the fact that the scaled down BFR had the same structural fraction as the larger ITS. Real life doesn't work that way, 'cause square/cube law. But, if you actually believe every word Musk says as gospel, then you have to consider what he said in the AMA a few days later, and you'd have to believe the statement that it is "Worth noting that BFS is capable of reaching orbit by itself with low payload, but having the BF Booster increases payload by more than an order of magnitude ". So basically: ...what this guys says. Rune. But hey, interesting number-crunching.

Well, I guess that would make things easier and more repeatable. But, you totally don't need it. The only trick really is to leave the booster with the same-ish apoapsis every time, so you have the same time to put the spaceship on a long (>2min) coast to apoapsis that gives you time to switch back to the booster to land it while that is going on. Basically you just have to remember at which point to stage (I use '500m/s left on that stage' thanks to KER, I'll have to work out the fuel level for that in order to give the stock flight manual), then do a bunch of steps mechanically each time, with a nail-biting suicide burn required to land the booster. I think I can condense all that into a 'simple' flight manual of no more than ten discrete steps, using stock instrumentation. Note that I've never been able to do it without KER's suicide burn indicator, that is true, but I guess if you are Scott Manley and/or I work out the exact height to start it... Rune. Chutes would also probably help, but they wouldn't be accurate.

Descent was an awesome game. Rune. And I'm showing my age with that comment.

I've got a couple of takes on it, but I've yet to test them thoroughly: The thing is, I don't really like that cocodrile bay door, it doesn't fit the outline of the crewed version, just the one shot of the thing lunching satellites. So I made a crewed version, and it kinda works better because of the tank/weight distribution being much more sensible than in the real thing. That made the whole aerodynamic balancing act to reenter it belly-first and then flipping it close to the ground, well, much easier with much less wasted weight in wings. In the end, it has the same payload, but it has a crew capsule with abort capability on top, and a lower total weight: They are a bit tricky to launch in pure stock, but it is possible to recover the booster without resorting to quicksave-quickload in the same flight, I've done it several times now. It just takes a 'slightly' inefficient 4.5km/s launch (meaning I'm wasting around 1km/s), and a lot of switching around at the appropiate time, but hey, fully reusable TSTO ~20mT launcher. Not that I don't have lighter chemical SSTOs that can do the same thing... It is obviously scaled quite a bit down, but you know, KSP scale. I couldn't fit 31 engines in the first stage, six in the second, and make the staging make sense in kerbal scale, so I took out the outer ring of 12 on the booster, and scaled it to something that fit under the biggest stock tanks, turned out to fit the launch requirements (TSTO) quite well using the humble LV-T45 and 30 as Raptor analogs. The spaceship has the proper number of engines, at least, and the whole thing looks like the BFR if you squint, right? Rune. Coming Soon™.

Yup. Besides, 85mT+150mT payload, already 235mT. In order to keep about two and a half km/s to get back, you need a Mr of about two, so over 400mT when it touches down on the Moon. That assumes you take the cargo back of course, which is stupid, but still, a minimum of 310mT on touchdown (85*2+150). Rune. Add hoverslam software developed for F9, and it starts looking easy.

Yeah, but maximum payload would be the more favorable case in terms of mass efficiency, and therefore if you are building a base, the best bang for your buck. If you take a partially empty vehicle there, that means a smaller one could do the job just as well, and/or you will pay more per kg on the long run. Rune. Besides, the square/cube law kinda guarantees better results the bigger you go. Well yeah, if that's the case, maybe. But, again, how did you build that base in the first place? BFR is the thing that build the thing, because it needs no prior infrastucture, or can deploy it in more or less a single flight. Rune. Giving that out would be giving out its reason for existing in the first place.

LEO-Moon is less than 6km/s, so the BFR would have no trouble getting there, and it would in fact be easier than reaching Mars (<8 tanker flights). BUT. ISRU on the Moon is a very different beast than in Mars. You don't have an ubiquituous CO2 atmosphere, and you have much less water ice, and the ice is locked in hard-to-reach, permanently shadowed craters. Rune. So probably more difficult.

Fire up KSP, install RSS, try it out. Or, you know, have a little faith in the guy that did and commented a couple pages back (not me!), and quotes 9,1km/s as a likelier figure. Rune. It also works if you consider the fraction of kerbin's orbital velocity and kerbin's P2P requirements. Basically >90% of orbital speed.

I just posted this elsewhere, but I think you guys will appreciate the anlaysis better: So, I have run the numbers on that lunar mission. It's mighty interesting! This could be the real intent behind the whole 'paying for it' thing, finding the customer that foots the bill while feeling like he makes a great deal. Let me explain, and hold on to your hats, 'cause I dunno how long this will be: First, assumptions. I will assume that the numbers given in the presentation are true (Raptor Isp=375s, empty weight of the orbiter 85mT, max. payload 150mT, max. fuel load 1,100mT/1,250mT in the tanker version because payload is fuel), that the mission is staged from GTO (Geosynchronous transfer orbit, which is reasonable, analysis as to why later), and that the dV map in the wiki is close enough for government work. Why GTO? Well, according to the wiki, form there it's about 3.2km/s to the lunar surface. Add 2.3km/s to come back to an atmosphere-intercepting Earth orbit, and you get 5,5, well within the 6,4km/s my trusty excel sheet spits out for the BFR orbiter, with full fuel and 150mT in payload. Seems reasonable. That means that we need a full BFR orbiter at GTO, meaning we first have to refuel it in LEO (at least 8 trips of a tanker if a tanker can transfer 150mT of fuel and suffers little losses). Then, we have to refuel it again form fully-fueled BFR tankers, themselves refueled in LEO, each by its own eight tanker flights. How many tanker flights to GTO we need can increase the total number of launches pretty quickly. So, how much fuel can a BFR tanker get to GTO, if starting full at LEO? Well, that is easy. Massaging my sheet, I get that a payload of 590mT of fuel makes it to GTO. Coming back is easy, since perigee is still close to the planet, so the tanker can aerobrake. That means you need only slightly more than one tanker (at least!) to refill the ship with payload in GTO after it spends 660mT of fuel getting itself to GTO (again, that excel spreadsheet is neat). Since everything is a rough first-order approximation, let's call it two tanker flights, and the margin will take care of inefficiencies I haven't taken into account. So all in all, for one to reach the Moon and return, three BFRs must reach GTO, and in turn you have to launch 24 (!) to LEO to get those three up there. Well now, that proposition, at a decent enough launch rate, is starting to sound like a good business market to serve, a single lunar mission a year means two flights each month. There are the high flight rates that could make the whole thing economical, ginormous as it is. Rune. Imagine what that excel spreadsheet is meant for, and why I use it a lot. ^^'

That is a point for them, yeah. But even then, if you think about it, they need the commitment beforehand, right? I mean, they ain't going to be cheap toys to put together. You know, I am usually the one to bring up the "you guys are all a bunch of planetary chauvinists" spin to the conversation, but I had never though about using Aldrin cyclers as an argument and/or intermediate step. That could almost work, right, the two things justifying each other? But nah, on second thought, if you want to learn to live in space for realsies, you don't really want to go to Mars, and one thing distracts you from the other. Rune. Just to clarify, I truly believe that in, say, 2,000 years, living in a planet's surface full-time will be for the eccentric few.