Another thought:What would be the largest payload that could be placed on top of a FH core+boosters stack?Using their existing heavy launcher, SpaceX could fly a subscale ITS stage as a reusable combined US/payload, and validate the OML. If adding CH4 to thr pad was possible, then it could also test fly the Raptor- otherwise it would use Merlins.This would get flight experience on a large lifting body, without needing to build the BFR and its pad first.
...Another thought is that the FH US/payload should total around 175t, and the dry mass of the BFS is supposed to be 150t- and the tanker presumably a bit less than that. So in fact it would be, in theory, possible to launch a short-fuelled full size BFS onto a suborbital trajectory. The aero loads for the poor little Falcon would perhaps be too much, and operating the BFS as a single stage suborbital (or even orbital) vehicle is probably more likely- but it's fun to run through the different options.
Neat, but the argument against is that it's a huge development project in its own right.
Coming back to a methane upper stage for F9/FH. Just one Raptor vac, scaled or full size. Make it the weight of the present upper stage, so slightly more diameter, but same height. How much can the TE take without shape modification? 50 or 60cm more diameter should do it. Carbon fiber body, autogenous pressurized. With the winglike extensions of ITS or without, but with the flaps. Nozzle extension small enough to fit into the existing interstage. This should give the reusable upper stage for F9 and FH without payload penalty for most orbits plus plenty of ITS related experience. It could even fit the end of next year timeline for reusable upper stage, or almost, if they have been working the idea for a while.
Hard to imagine that SpaceX would hesitate to modify the TE again, if they thought a wider diameter upper-stage had a market.
They've probably built at least 10 different variants of the TE. I remember that in the early days it seemed they used a new TE variant for every launch.
First, a dumb but full-diameter variant of the ITS-Orbiter created within 2yrs as a Grasshopper (more like a kangaroo) to test engines and landing systems, eventually to test the recovery cradle. This is the first vehicle that Musk referred to.
1) ITS boilerplate. Maybe aluminum. Purpose to test SL Raptors. Grasshopper style flights. Maybe 3 Raptors.Tests methalox engines in flight and some avionics.2) ITS prototype with design goal carbon-fiber tanks & skin. Rvac & SL Raptors, flies downrange & back RTLS.Completes Block 1 Raptor engine design validation for build out & qualifies basic tank & airframe technology.3) Booster prototype with ~21 engines. Inner ring & central cluster. Flies downrange & RTLS but not on cradle for initial flights; temporary "legs". Followed by cradle landing tests.The goal of this approach is to allow rapid prototyping and flight experience without large expense building lots of early prototype engines that get ECOed and discarded.4) Booster with ~21 engines with ITS (minimal "payload") 6 Rvac & 3 SL engines to LEO.I've run #s based on Musk's September reveal and a 21 Raptor booster lifts a low payload ITS to LEO.ITS orbital velocity re-entry qualified.5) All-up all engines testing to LEO with ITS tanker re-fueling. 5b: Possible crew config ITS around the moon flight ECLSS etc. qualification.6) Un-crewed cargo ITS re-fueled in LEO; lands on Mars with cargo, e.g. in-situ propellant equipment.
So are there going to be Grasshopper-style test flights for ITS?
Will they likely start at McGregor?