Because the F-1 engine had its role ITTL. It might be less justifiable in, say, Eyes, but that's about five years in the past and to do anything about it now would require rewriting basically the whole TL. However, it is one reason this TL is focusing much more attention on the exact reasoning behind the alternate Shuttle decision. Given that PoD and the selected approach, the F-1's use is sort of inevitable, but we'll get more into that in this week's post.I had meant more the F-1 in general, and added the B by accident there (which I removed). Just a little of a pet peeve of mine how it is always the F-1 that survives. But it detracts from this TL (which is just going to be very fascinating to read through) and I would be happy to continue this kind of conversation with you over PM.
But why ballast with kerosene instead of say, water?
Patupi's got it. To quote the post: "A particularly revolutionary innovation in this study was the concept of “propellant ballasting.” By carrying more propellant than strictly necessary for lower-end payloads, and burning it off in a second post-staging burn of the first stage, reentry velocity could be reduced considerably for smaller payloads (like those needed to service a space station), extending stage life. Indeed, with sufficient ballasting, a payload of 25 tonnes could be delivered with such minimal heating on the booster that the existing aluminum skin of the S-IC would suffice for thermal protection."I think he's talking about using it as fuel, so Kerosene and oxidizer is required, not literal ballast. They use it to slow the 1st stage down after separation to reduce re-entry speeds. At least as I understand it from the chapter posted. True it's 'ballast' on launch because it's basically payload until separation, then it gets used.
Who said it had to glide?At any rate 180 tons is a less insane mass for Boeing to design to fly back; at 300 tons dry they are asking a Boeing 747 to be a glider!
Pictures....we want pictures....
Have no fear, both are coming. Images are waiting while Nixonshead is on a well-deserved vacation from the Blender mines, and specs are waiting to be posted until NASA ITTL has selected a plan and rolled it out. Stay tuned!Obviously! And I want specs!
Well, if it masses in the ballpark of 300 tons after all rocket burns (not to say "dry" because now we need fuel for jet engines) it will need some massive jet thrust installed to stay airborne, whether it cruises back supersonic or subsonic. The latter would probably be best for overall weight and fuel economy and make it more manageable landing. One might get away with less cruise thrust than a comparable subsonic long range transport, but highest thrust is needed for the trickiest maneuver any airplane performs--landing! However I suppose this need not necessarily be thrust entirely from the cruise engines, because you could install an auxiliary rocket. (Even a single F engine would be overkill, though a small LOX reserve with RP could enable the mighty 750+ ton thrust to save the stage from a crash in a dicey situation). So perhaps we can get away with say half the thrust installed in a 747, say two of the same Rolls Royce engines used on early 747s. They can't be installed in underslung pods of course, or can they? It would create drag, and risk ripping the things off, and they'd be exposed to reentry heating, so I would guess they have to be buried in the wing with a lot of ducting, as no doubt Boeing explored for the SST. The engines and perhaps their fuel (which could be RP-1 I suppose, and kept in the main rocket fuel tankage, though that's a waste of money since rocket fuel is an expensive grade of jet fuel) are yet more "dry" stage mass to consider and make it likelier the whole thing is heavier than 300 tons than lighter!...Patupi's got it. To quote the post: "A particularly revolutionary innovation in this study was the concept of “propellant ballasting.” By carrying more propellant than strictly necessary for lower-end payloads, and burning it off in a second post-staging burn of the first stage, reentry velocity could be reduced considerably for smaller payloads (like those needed to service a space station), extending stage life. Indeed, with sufficient ballasting, a payload of 25 tonnes could be delivered with such minimal heating on the booster that the existing aluminum skin of the S-IC would suffice for thermal protection."
This gets to one reason you might favor a 5-engine with more propellant ballasting (and thus more delta-v after separation and lower entry velocity) over a smaller version with fewer engines or a short-fueled booster: it dramatically simplifies the TPS problem. I think that addresses most of the rest of your post, Shevek, at least what I can without specs.
Who said it had to glide? ...
Have no fear, both are coming. Images are waiting while Nixonshead is on a well-deserved vacation from the Blender mines, and specs are waiting to be posted until NASA ITTL has selected a plan and rolled it out. Stay tuned!
You have to get down to 1.5 km/s or so, which isn't really an option if you're orbital.I'd be interested to know exactly how much delta V you need to spend to slow a craft from orbit to make the re-entry not need heat shielding. Obviously this would vary depending on starting orbit, and I'm assuming it'll be a slow burn (fewer engines firing), or a two stage burn, to put deceleration right up till it starts cutting air, but from a 200Km orbit roughly the kind of speed it needs to shed. It'd help a lot with my own TL to know what you can get away with.
Very interesting so far. Just so I'm clear, the RS-1C was being rolled into one of the High Bays, right?
It comes down in the end to just how much money per average launch the Lifter boosters will save, all things considered.
One of my earlier postponed remark/speculations hinged on the development of a later generation of smaller Lifters. Much discussion on space threads here asserts that a comprehensive space program can proceed indefinitely with maximum payload to LEO of 30 tons or so; if so, then the 100+ ton maximum payloads one can get from a Saturn V derived Lifter with 5 F class engines is clearly overkill. It may be economic compared even to EELVs to simply underuse the maximum capability of a standard big Lifter with recourse to "propellant ballasting," but surely then it would be more economic still to downsize the booster to optimize for the largest payloads typically demanded, and have just a few big ones on hand for the really big payloads one rather rarely wants to launch. But of course the more we can save per launch, the larger the overall demand for tonnage to orbit becomes and this justifies a somewhat bigger lifter--or even using the full capacity of the original big size by launching in batches.
But any savings achieved by downsizing the Lifters is offset by the cost of a whole new development program. So this evolution would wait for the day the first generation Lifters are nearing their end of life and must be replaced anyway--and it would still be cheaper, in first cost terms, to simply make another batch of the proven design.